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Validating a Density Functional Theory Approach for Predicting the Redox Potentials Associated with Charge Carriers and Excitons in Polymeric Photocatalysts
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Department of Chemistry, University College London, 20 Gordon Street, London WC1H 0AJ, U.K.
*E-mail: [email protected]
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The Journal of Physical Chemistry C

Cite this: J. Phys. Chem. C 2017, 121, 3, 1498–1506
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https://doi.org/10.1021/acs.jpcc.6b11133
Published December 22, 2016

Copyright © 2016 American Chemical Society. This publication is licensed under CC-BY.

Abstract

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We compare, for a range of conjugated polymers relevant to water-splitting photocatalysis, the predictions for the redox potentials associated with charge carriers and excitons by a total-energy ΔDFT approach to those measured experimentally. For solid-state potentials, of the different classes of potentials available experimentally for conjugated polymers, the class measured under conditions which are the most similar to those during water splitting, we find a good fit between the ionization potentials predicted using ΔB3LYP and those measured experimentally using photoemission spectroscopy (PES). We also observe a reasonable fit to the more limited data sets of excited-state ionization potentials, obtained from two-photon PES, and electron affinities, measured by inverse PES, respectively. Through a comparison of solid-state potentials with gas phase and solution potentials for a range of oligomers, we demonstrate how the quality of the fit to experimental solid-state data is probably the result of benign error cancellation. We discuss that the good fit for solid-state potentials in vacuum suggests that a similar accuracy can be expected for calculations on solid-state polymers interfaced with water. We also analyze the quality of approximating the ΔB3LYP potentials by orbital energies. Finally, we discuss what a comparison between experimental and predicted potentials teaches us about conjugated polymers as photocatalysts, focusing specifically on the large exciton-binding energy in these systems and the mechanism of free charge carrier generation.

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Introduction

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Oligomeric and polymeric photocatalysts (1-13) that can drive the redox half-reactions that underlie photocatalytic water splitting, i.e., the reduction of protons to molecular hydrogen and/or oxidation of water to oxygen gas, or CO2 photoreduction, are currently a very active area of research. (14, 15) The attraction of these materials arises from the fact that they are based on earth abundant elements, that their properties can easily be tuned through copolymerization, and that, compared to related carbon nitride (16-27) and graphene oxide (28, 29) photocatalysts, their molecular structure is generally well understood.
One of the crucial requirements for a potential photocatalyst, polymeric or not, to meet is the thermodynamic ability to drive both of the water splitting or CO2 photoreduction solution half-reactions. This prerequisite, at least for colloidal photocatalysts, translates into the constraint that the adiabatic potentials associated with the charge carriers, the ionization potential (IP), and electron affinity (EA) for holes and electrons, respectively, and the excited state, IP* and EA*, in a materials straddle those of the solution half-reactions, as shown in Figure 1. Knowledge of these potentials is hence crucial when trying to understand the activity of known photocatalysts or developing new ones. Reliably measuring such potentials, especially under operating conditions, however, is far from easy. For example, cyclic voltammetry (CV) on solid polymer films on an electrode in contact with an aprotic polar solvent system, the closest measurements in practice come to a solid polymer in contact with water, is generally hard to interpret because the voltammograms tend to be broad, show signs of irreversibility, (30-32) and involve the incorporation of counterions and solvent molecules into the film, (33) something not expected under photocatalysis conditions. A related problem with CV and other electrochemical measurements is that measuring the electron affinity values of polymers is complicated by the fact that these often lie outside the stability window of common solvents and that one thus has to be cautious not to confuse a signal due to solvent oxidation with the signature of electron affinity. The ability to calculate the redox potentials of charge carriers and excitons, therefore, is very useful, especially as it moreover allows one to predict such potentials of hypothetical materials that have not been synthesized as yet and thus to screen computationally for promising photocatalysts. The latter is an especially attractive proposition for polymeric materials, for which the synthesis, purification, and characterization of both the polymer and constituting monomers can be very time-consuming.

Figure 1

Figure 1. Scheme illustrating how, in the case of water splitting, the redox potentials associated with charge carriers (IP and EA) and excitons (EA* and IP*) of a photocatalyst must straddle the proton reduction and water oxidation potentials (blue and red broken lines, respectively) for both processes to be thermodynamically favorable. The vertical axis on the right of the figure shows the different possible alignments with vacuum, the result of the different experimental SHEAP values.

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In our group, we developed a density functional theory (DFT) approach specifically aimed at predicting the potentials of polymeric and nanoparticulate photocatalysts (34-38) and applied it, among other things, to linear polymers, (11, 34) including the first ever reported polymer photocatalyst poly(p-phenylene), (1-4) polymer networks, (8) and carbon nitride materials. (35) The main difference between our approach and that developed by others (39-45) for describing photocatalysts is that it revolves around molecular calculations in combination with a continuum dielectric screening model (46) to describe the material and its aqueous environment rather than calculations using periodic boundary conditions (PBC). The focus in the case of polymeric materials on a single polymer strand and the assumption that all intermolecular interaction, be it with other polymer strands or water, can be described in terms of an isotropic dielectric response, is related to the fact that such materials are often amorphous or only poorly crystallized. Hence, we do not have good experimental structural models for polymers to use in PBC calculations and, even if we did construct artificially periodic models, they probably would not necessarily be representative for the solid. Another difference between our approach and that of many others is that we calculate the potentials using a total-energy ΔDFT approach, rather than equate IP and EA with (generalized) Kohn−Sham−(G)KS−orbital energies, which at least conceptually is problematic, especially for EA, (37, 47) as well as, calculate potentials associated with the exciton. Finally, we also take into account the (self-)trapping of charge carriers and excitons, i.e., the formation of polarons and polaronic excitons, by considering adiabatic rather than vertical potentials.
Our approach predicts potentials that are in line with observed activity of polymeric photocatalysts; e.g., many polymers are predicted to have no thermodynamic driving force for water oxidation, their ionization potential is more negative than the potential associated with the water oxidation half-reaction, while sacrificial electron donor oxidation is predicted to be exergonic. However, we never previously compared the explicit predicted polymer and oligomer potentials with those measured experimentally. Here, we correct this omission and compare predicted potentials, in the absence of data for polymeric solids in contact with water, to those measured from three distinctly different alternative data sets, gas phase photoemission spectroscopy (PES) for oligomers of poly(p-phenylene), (48) solution electrochemistry data in aprotic polar solvents for oligomers of poly(p-phenylene) (49) and poly(fluorene) (50) derivatized with solubilizing alkyl chains, and solid-state (inverse) PES data (44, 48, 51-60) for a range of conjugated polymer solids in vacuum. The differences between this and previous work validating the use of hybrid DFT for predicting redox potentials in the literature (61-73) are a combination of (i) our focus on oligomers/polymers rather than small molecules or metal complexes, (ii) our emphasis on potentials in condensed phases, including the solid-state, (iii) the fact that we consider adiabatic rather than vertical potentials, and/or (iv) because we also include excited state potentials in our comparison. We show that, for conjugated oligomers and polymers, (TD-)DFT calculations using the standard B3LYP (74, 75) density functional yield reasonable gas and solution phase potentials and rather consistently good solid-state potentials, the latter in line with previous work for the IP and EA of small molecules. (65-67) We further demonstrate that these good solid-state potentials appear to be the result of rather benign error cancellation. We discuss that the good fit for solid-state potentials in vacuum suggests that a similar accuracy can be expected for calculations on solid-state polymers interfaced with water. We will also briefly touch upon the merits of the orbital approximation mentioned above, as well as discuss the requirements on a density functional to consistently calculate the potentials associated with charge carriers and excitons in polymeric materials. Finally, we will examine what a comparison of experimental and computationally predicted potentials teaches us about conjugated polymers as water-splitting photocatalysts.

Methodology

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We calculate the adiabatic IP, EA, IP*, and EA* potentials of a polymer P in a ΔDFT fashion from the Gibbs free energy difference of the following four redox reactions, written, in line with convention, as reductions(1)(2)(3)(4)where P+, P–, and P* are the polymer with a hole, excess electron and exciton, respectively. The Gibbs free energy differences ΔGr are converted to reduction potentials E via(5)where F is the Faraday constant and n the number of electrons involved in the reaction, typically one. In our calculations, we furthermore equate the Gibbs free energy difference to the total energy difference, neglecting the vibrational, translational and rotational contribution to the free energy. In our previous work on poly(para-phenylene) oligomers, (34) this was found to be a generally good approximation because of the relative similarity of the structures of P+, P–, P*, and P. Effects of solvation and going to condensed phases in general are described using the COSMO (76) dielectric screening model, where we generally neglect the outlying charge correction. P* energies were obtained by a TD-DFT energy minimization in vacuum, followed, if needs be, by a single point TD-DFT calculation using COSMO, where all TD-DFT calculations make the Tamm–Dancoff approximation. (77) All (TD-)DFT calculations, finally, except where otherwise indicated, were performed using the Turbomole 6.6 code, (78, 79) the B3LYP functional, the double-ζ DZP (80) basis-set, and the m3 medium integration grid.
Potentials, be them computationally predicted or experimentally measured, are always expressed with respect to a reference, typically vacuum in the case of photoelectron spectroscopy and a reference electrode, for example, the standard hydrogen electrode (SHE), for liquid electrochemistry. A key parameter in the conversion from one potential reference to another is the value of the SHE absolute potential (SHEAP), for which a range of experimental values are proposed, something that is partly related to different possible choices for thermodynamic standard states and partly due to extra-thermodynamic assumptions. (81) In this paper, we consider two values of the SHEAP (see Figure 1), 4.44 V, the original IUPAC recommended value (82) we used in our previous work, and, a more recently proposed value of 4.28 V. (81) We will present results where possible for both SHEAP choices. However, if in the text only one value is mentioned for a given system, this will be based on the use of 4.44 V for SHEAP.

Results and Discussion

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IP

When an oligomer or polymer, or more generally a molecule, is taken from the gas phase into a solution, the solid state or a solid in contact with a solution, the IP of the molecule gets reduced and becomes shallower, through two different mechanisms. (83, 84) First, prior to ionization, in solids, where the molecules are densely packed, hybridization raises the energy of the highest energy occupied orbital, from which an electron will get removed upon ionization. Second, after ionization, the dielectric nature of the environment will screen the generated charge by polarization and stabilize the charged species formed energetically. The difference between the gas and condensed IP values is commonly referred to as the polarization energy, even if only part of this difference is the result of dielectric polarization and, in contrast to what the names suggest, it also contains a contribution due to hybridization.
Table 1 shows experimental and DFT predicted values of the IP of oligomers and polymers of p-phenylene in the gas phase, in a dichloromethane (DCM) solution, and as a solid. We will first concentrate on the experimental values taken from the literature. (48, 49) While one has to be slightly careful with comparing these experimental potentials as they are measured using two fundamentally different methods, ultraviolet PES (UV-PES) and CV, and because of the need to convert between different potential references, vacuum for UV-PES and the standard calomel electrode (SCE)/SHE and the ferrocene/ferrocenium redox couple in the case of CV, such a comparison is very insightful.
Table 1. Experimentally Measured and Computationally Predicted Ionization Potentials of Oligomers and Polymers of p-Phenylene in Different Environments vs the Standard Hydrogen Electrode (All Values in Volts)
 gas phaseDCMbsolid
 UV-PESa, (48)B3LYPaCVc, (49)B3LYPa εr 8.93UV-PESa, (48)B3LYPa εr 2
23.72 (3.88)3.26 (3.42)1.911.42 (1.58)  
33.36 (3.52)2.71 (2.87)1.781.22 (1.38)1.66 (1.82)2.09 (2.25)
4  1.731.13 (1.29)  
5  1.691.09 (1.25)  
62.76 (2.92)2.08 (2.24)1.661.07 (1.23)1.46 (1.62)1.63 (1.79)
7  1.651.05 (1.21)  
8  1.65   
∞  1.5d0.9d1.21/1.36e (1.37/1.52)1.45 (1.61)f
a

Absolute IP vs vacuum converted to SHE scale by a shift of 4.44 and 4.28 (inside parentheses), respectively.

b

Oligomers with branched iso-alkyl chains at the terminal p-carbon atoms.

c

Measured in DCM in the presence of 0.2 M n-Bu4NPF6 supporting electrolyte against SCE, values converted to SHE scale by application of a shift of +0.244.

d

Obtained through linear extrapolation vs 1/n.

e

Value obtained by two different extrapolation methods in the original experimental paper.

f

Modeled using an oligomer of 12 units.

As expected based on the literature, the gas phase IPs are the deepest, with the solid-state and DCM solution values being 1–2 V smaller than the corresponding gas phase values. More interestingly, the polarization energies for the different oligomers in the solid state and DCM, which can be extracted from the IP values in Table 1, are very similar. As the dielectric permittivity of the solid-state oligomer/polymer phase is likely to be smaller than that of DCM, especially given the fact that the supporting electrolyte is likely to increase the effective dielectric permittivity of the DCM solution through ion-pair formation, (85) this suggests that the smaller dielectric contribution to the permittivity and thus screening of the formed charge in the solid state relative to the solution is more than compensated by the larger contribution due to hybridization in the former. On the basis of the numbers, it is hard to extract an exact value for the effect of hybridization, but it is likely to be at least a few tenths of volts. The experimental polarization energy values measured for the oligomers, finally, are similar in magnitude to those measured for organic crystals (83, 84) and appear to decrease with oligomer length.
Moving on to the computational predictions, where we approximate the alkyl side chain by an isopropyl group (see Figure 2), inspection of Table 1 shows that, for the gas and solution phases, ΔDFT calculations using the B3LYP functional yield IP values that are 0.5–0.6 V shallower, less positive, than those measured experimentally. A similar shift was reported by Baik and Friesner when calculating EA values of small molecules in solution using B3LYP. (61) Calculations, discussed in the Supporting Information (ESI-1), using a slightly higher dielectric permittivity for the DCM solution than that of pure DCM in order to take into account that the supporting electrolyte will likely increase the effective dielectric permittivity, as well as calculations with larger basis-sets (ESI-2), do not sufficiently change this observation. Moreover, a comparison between similar ΔB3LYP calculations and experimental data for the ionization potentials of oligo(fluorene) in solution in the Supporting Information (ESI-1) finds a similar 0.5–0.6 V shift, suggesting this to be a quite general feature. In contrast to for the gas and solution phases, in calculations for the solid state, in which as discussed above we neglect hybridization, the predicted IPs are 0.2–0.4 V more shallow, less positive, than those measured experimentally. For the dissolved oligomers, the calculations reproduce the experimentally obtained polarization energy values rather well, but the solid-state polarization energies are underestimated by ∼1 V. If the isotropic dielectric screening model used in our calculation correctly reproduces the dielectric component of the polarization energy, this would suggest that the effect of hybridization is ∼65% of the polarization energy and far from negligible. The absolute values of the solid-state IPs, most relevant for our work, however, are, as discussed above, reasonably well reproduced. As the inherent density functional related error and the error introduced by neglecting hybridization have similar magnitudes and opposite signs, the solid-state values by benign error cancelation lie close to their experimental counterparts.

Figure 2

Figure 2. Structures of oligomer models studied computationally.

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This success in reproducing experimental values of solid-state IP values for polymers appears to be not limited to oligomers and the polymer of p-phenylene. Figure 3 and Table S5 in the Supporting Information show a comparison of IPs of a range of conjugated polymers measured experimentally by UV-PES (48, 51-54, 56, 60) and our B3LYP calculations, again using εr 2, for oligomers of 12 units (see Figure 4 for the structures of different polymers studied). Concentrating on the polymers without side chains, for all materials, except perhaps poly(pyrrole), the match is quite good (maximum deviation of −0.44 V, for poly(pyrrole), and a mean absolute deviation of 0.20 V) and the DFT predictions correctly recover the relative ordering of the IPs of the different polymers. Since poly(pyrrole) is easily oxidizable, (54) the deviation observed for this material can in part find its origin in experiment. Use of a slightly higher dielectric permittivity than 2 for the heteroatom containing polymers, to account for the fact that such polymers probably have a higher dielectric permittivity than pure hydrocarbon polymers (see Table S8 in the Supporting Information), if anything worsens the fit. In this context, it is important to remember that photoemission is likely to mostly involve molecules near the polymer–vacuum interface due to the surface sensitivity of UV-PES and that such molecules as a result will be less screened than in the bulk. (86) These results also suggest that the single oligomer embedded in a dielectric continuum approximation, which ignores details of molecular packing, works very well for these amorphous/quasi-crystalline polymers, even if for fully crystalline materials there are cases known where nonisotropic packing effects are large and one has to go beyond the continuum approach. (87)

Figure 3

Figure 3. Comparison between the potentials predicted using (TD-)B3LYP and εr 2, thick lines, and measured experimentally, thin lines, for a range of conjugated polymers.

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Figure 4

Figure 4. Structures of the polymers studied; the n used in the calculations was 12 for all structures except PF, where an n of 6 was used instead.

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Overall, on the basis of these results and the fact that the (dielectric) effect of going from an interface with vacuum to water is most likely additive, we feel confident that the computational setup used will also provide decent predictions for the IP of a polymer interfaced with water. Protonation of a polymer might induce an additional shift in the potentials, but this is, even for the nitrogen containing polymers, unlikely to be an issue at (near) neutral pH.

EA

In contrast to the relative multitude of reference data on the IP of oligomers and polymers, there is very little experimental data on the EA of oligomers and polymers, especially in the solid state. Most reported electron affinities are obtained by adding the optical gap, the onset of light absorption, to the value of the IP, but this is a questionable approach. More theoretically justified values can be obtained from either inverse PES or the high kinetic energy edge of two-photon PES (2PPE) spectra. Such data are only available, as far as we are aware, for three of the polymers discussed above. for the alkyl-chain derivatized version of poly(thiophene): poly(3-hexylthiophene) (P3HT), (59, 60) poly(fluorene): poly(9,9-dioctylfluorene) (57) (PF8), and poly(2-methoxy-5-(2-ethyl-hexyloxy)-p-phenylene vinylene) (58) (MEH-PPV). As can be seen from Figure 3 and Table S6 in the Supporting Information, the fit is good for P3HT and PPV, and slightly less good for PF8. This is for calculations neglecting the alkyl side chains. However, calculations that take these side chains into account, included in Table S6 in the Supporting Information, show that these side chains are predicted to only have a small effect on the electron affinity. While these three data points are not sufficient to properly test our approach in terms of its ability to correctly predict electron affinities of polymers in the solid state, it at least give some confidence in our approach. Similarly as for the IP, we expect that the difference in the EA for a polymer in contact with vacuum and water is adequately described by changing the dielectric permittivity from 2 to that of water.

Excited State Potentials: IP* and EA*

Experimental benchmark data for the excited state potentials of relevant polymers are as rare as data for EA, if not rarer. We are aware of 2PPE data, where the process of ionization of the intermediate state by the second photon can be hypothesized to correspond to the ionization of a (self-trapped) exciton and thus IP*, for only two polymers: P3HT, (59) and MEH-PPV. (58) As can be seen from Figure 3 and Table S7 in the Supporting Information, the experimentally measured values appear in line with the values predicted by our computational approach for the polymers without side chains. Moreover, for both P3HT and MEH-PPV, EA and IP* are split by ∼0.7 (e)V, the adiabatic exciton-binding energy, in experiment and by ∼1 (e)V in our calculations. Similarly as for EA, while these two data points for IP* are not sufficient to properly test our approach in terms of its ability to correctly predict electron affinities of polymers in the solid state, it does give confidence in our approach. We are not aware of any experimental data that would allow us to directly validate our EA* predictions; this would require a 2PPE equivalent inverse photoemission spectroscopy. However, as by definition, the splitting between IP and EA* is the same as between EA and IP*, the fit between predicted IP* and 2PPE values in combination with the fit between experimental and computational IP values suggests that EA* might be as well described as IP*. Again, just as for IP and EA, the effect of water is likely to be additive for IP* and EA*.

Orbital Approximation

IP and EA are often approximated in the literature by the negative of the energy of the highest energy occupied (G)KS molecular orbital (HOMO or valence band maximum) and lowest energy unoccupied (G)KS molecular orbital (LUMO or conduction band minimum). This approximation is especially problematic on conceptual grounds for EA as, at least for pure density dependent functionals, the unoccupied orbitals are too strongly bound and lie too deep. (37, 47) The (G)KS gap, the energy difference between LUMO and HOMO, as a result lies often closer to the vertical optical gap than the vertical fundamental gap. A comparison between IP/EA and −HOMO/–LUMO for the polymers in Figure 5, however, shows a surprisingly decent fit between both and by extension with experimental IP/EA values. The difference between the orbital energies and potentials is an approximately constant small shift of a couple of tenths of volts. In line with what was previously observed by Schwenn and co-workers (67) in the case of small molecules, the reason again appears to be benign error cancellation. The underestimation of the vertical fundamental gap by the HOMO–LUMO energy difference is similar in size to a shift in IP and EA due to adiabatic and polarization effects not included in orbital energies, and both effects cancel each other. However, while we observe a decent fit between IP/EA and −HOMO/–LUMO for the solid-state oligomers studied, the accuracy of unshifted orbital energies will be different for different dielectric constants (see the Supporting Information) and hence different environments, as well as, we expect, when using different density functionals or for the case of PBC calculations on solids.

Figure 5

Figure 5. Comparison between IP and −HOMO, red open squares, and EA and −LUMO, open blue triangles, calculated with B3LYP and εr 2.

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Perspective

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Methodology

Consistently calculating the set of potentials associated with the charge carriers and exciton is rather a demanding application. Very likely other density functionals than B3LYP, for example optimally tuned range-separated density functionals, (69, 72, 88) might yield more accurate values for the ionization potential or electron affinity of oligomers in the gas phase or in solution. However, to be useful in this context, use of such a functional should simultaneously allow for the calculations of the other potentials, including the excited state potentials, and hence the optical properties of a system, and the potentials of solution reactions, and all to a similar consistent standard. Additionally, while our dependence on error cancelation to predict decent solid-state values is unsatisfying from a theoretical point of view, it saves one from having to do calculations on extended models of the solid. If our estimate of the contribution of hybridization to the polarization energy for PPP is correct and a general feature of conjugated polymers, then calculations with functionals that give better gas phase values might require calculations on explicit stacks to obtain solid-state values or ad-hoc shifts. The latter is probably as conceptually unsatisfying as relying on error cancelation, while the former is, at least for polymers that are amorphous or poorly crystallized, difficult to achieve due to, as discussed in the Introduction, the lack of meaningful structural models. An additional complication with calculations on explicit stacks is the fact that the percentage of Hartree–Fock exchange included the density functional fixes the intermolecular dielectric screening inside the stack. (89) As a result, the effective dielectric constant inside the stack might be different from its desired value and different from that used in the external continuum dielectric screening model.
We see the use of orbital energies to predominantly lie in fast screening of materials before calculating the potentials explicitly with ΔDFT for promising materials. Even if the −HOMO/–LUMO values would be exactly identical to IP/EA, the fact that within the orbital formalism one cannot calculate the excited state potentials or estimate adiabatic or polarization effects makes it, in our opinion, at least for polymers, more of a high throughput screening tool than an in-depth analysis method.

Materials

Besides validating our computational methodology, the comparison between experimentally measured and computationally predicted potentials also gives us additional insight into polymeric materials and their ability to act as water-splitting photocatalysts. Specifically, the good match between the predicted and experimentally measured ionization potentials, as well as the specific experimental values of the ionization potentials and electron affinities for the polymers considered, supports our previous observation (34, 38) that conjugated polymers generally have a large thermodynamic driving force for proton reduction and a small(er) driving force, if at all, for water oxidation, i.e., that the electron affinities and ionization potentials of most polymers are relatively shallow. The experimental data are similarly in line with our prediction (38) that the incorporation of heteroatoms should drive down the ionization potential of a polymer and improve the driving force for water oxidation if it results in a polymer with an electron-poor π-system, e.g., poly(pyridine), and reduce this driving force if the resulting polymer is electron-rich, i.e., poly(pyrrole) and poly(thiophene).
Perhaps even more interestingly, our estimate of the experimental adiabatic exciton-binding energy in substituted poly(thiophene) P3HT and poly(phenylvinylene) PPV of ∼0.7 (e)V, obtained above from the difference in the measured electron affinity (58-60) and 2PPE intermediate state energy (58, 59) discussed above, as well as the fact that it is similar to our theoretical estimate, confirms our previous observation (38) that excitons in conjugated polymers should be so strongly bound that they are unlikely to spontaneously fall apart in the bulk. The generation of free charge carriers, where free signifies that the charge carriers are not bound in the form of an exciton, thus has to be mediated by either a reduction of the adiabatic exciton-binding energy through induced dielectric screening by close contact with water or dissociation of the exciton at the polymer–water (or heteropolymer) interface. Calculations (38) predict that the former indeed takes place but that the resulting adiabatic exciton-binding energies still will be larger than kB*T at room temperature. Free charge carrier generation thus appears mediated by interface dissociation of excitons, similar to exciton dissociation in organic photovoltaics on the donor–acceptor interface. (90)

Conclusions

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We have for a range of polymers relevant to photocatalysis compared the predictions of density functional theory for the redox potentials associated with charge carriers and excitons to those measured experimentally. We find for the ionization potentials of solid-state polymers in contact with vacuum, of the different classes of potentials available experimentally for conjugated polymers, the set measured under conditions which are the most similar to those during water splitting, a good fit between the values predicted using ΔB3LYP and those measured experimentally. Experimental data measured under similar conditions for the electron affinity and excited state ionization potential are much more limited, but we find a good fit to ΔB3LYP in the latter case and a decent fit in the former. Overall, the comparison with experimental data gives good confidence in the use of ΔB3LYP to predict polymer potentials for solids and suggests that, if the effect of replacing the interface with vacuum by an interface with water is largely dielectric in nature, the here used approach should also give accurate predictions under water-splitting conditions.
In contrast to the case of solid-state polymers, the ΔB3LYP predicted ionization potentials for oligomers of p-phenylene in the gas phase and solutions and oligomers of fluorene are off by 0.5–0.6 V with respect to experiment. A combination of this observation and comparison of experimental and theoretical estimates of the polarization energy suggests that the consistently good fit for solid polymers may be the result of benign error cancellation. We lack similar data for the electron affinity and excited state potentials, but it stands to reason that the decent description of these potentials is similarly the result of error cancelation.
Besides validating our computational approach, the comparison between experimental and computational results, among other things, also confirms our previous prediction that generally excitons in conjugated polymers are so strongly bound that they do not spontaneously fall apart in the bulk. The generation of free charge carriers instead thus must be mediated by the dissociation of excitons on the polymer–solution interface or polymer–polymer interface in heterogeneous materials.

Supporting Information

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The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.6b11133.

  • Tables of potentials for alternative εr values, tables of solid-state potentials vs vacuum, results with larger basis sets, and orbital approximation data for εr 80.1 (PDF)

  • xyz coordinates of relevant DFT optimized structures (ZIP)

Terms & Conditions

Most electronic Supporting Information files are available without a subscription to ACS Web Editions. Such files may be downloaded by article for research use (if there is a public use license linked to the relevant article, that license may permit other uses). Permission may be obtained from ACS for other uses through requests via the RightsLink permission system: http://pubs.acs.org/page/copyright/permissions.html.

Author Information

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  • Corresponding Author
  • Authors
    • Pierre Guiglion - Department of Chemistry, University College London, 20 Gordon Street, London WC1H 0AJ, U.K.
    • Adriano Monti - Department of Chemistry, University College London, 20 Gordon Street, London WC1H 0AJ, U.K.
  • Notes
    The authors declare no competing financial interest.

Acknowledgment

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We kindly acknowledge Prof. Dave Adams, Dr. Enrico Berardo, Prof. Richard Buchner, Dr. Cristina Butchosa, Prof. Andrew Cooper, Dr. Sam Chong, Dr. Warren Duffy, Prof. Furio Cora, Prof. Iain McCulloch, Prof. Chad Risko, Dr. Sebastian Sprick, and Lei Yuang for stimulating discussions. M.A.Z. acknowledges the UK Engineering and Physical Sciences Research Council (EPSRC) for a Career Acceleration Fellowship (Grant EP/I004424/1), as well as additional funding (EP/N004884/1). Computational time on Archer, the UK’s national high-performance computing service (via our membership of the UK’s HPC Materials Chemistry Consortium, which is funded by EPSRC grant EP/L000202/1), and at the EPSRC National Centre for Computational Chemistry Software (NCCCS) is gratefully acknowledged.

References

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    Journal of the Chemical Society, Chemical Communications (1985), (8), 474-5CODEN: JCCCAT; ISSN:0022-4936.
    Irradn. (λ >290 nm) of an aq. suspension of poly(p-phenylene) in the presence of an electron donor (Et3N, Et2NH) leads to photocatalytic H evolution. This is the 1st report of H evolution photocatalyzed by an org. semiconductor in sacrificial systems.
  2. 2
    Shibata, T.; Kabumoto, A.; Shiragami, T.; Ishitani, O.; Pac, C.; Yanagida, S. Novel Visible-Light-Driven Photocatalyst - Poly(Para-Phenylene)-Catalyzed Photoreductions of Water, Carbonyl-Compounds, and Olefins J. Phys. Chem. 1990, 94, 2068– 2076 DOI: 10.1021/j100368a063
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    There is no corresponding record for this reference.
  3. 3
    Matsuoka, S.; Fujii, H.; Pac, C.; Yanagida, S. Photocatalysis of Oligo(Para-Phenylene) Leading to Reductive Formation of Hydrogen and Ethanol from Triethylamine in Aqueous Organic-Solvent Chem. Lett. 1990, 19, 1501– 1502 DOI: 10.1246/cl.1990.1501
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    There is no corresponding record for this reference.
  4. 4
    Matsuoka, S.; Kohzuki, T.; Pac, C.; Ishida, A.; Takamuku, S.; Kusaba, M.; Nakashima, N.; Yanagida, S. Photocatalysis of Oligo(Para-Phenylenes) - Photochemical Reduction of Carbon-Dioxide with Triethylamine J. Phys. Chem. 1992, 96, 4437– 4442 DOI: 10.1021/j100190a057
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    There is no corresponding record for this reference.
  5. 5
    Schwab, M. G.; Hamburger, M.; Feng, X. L.; Shu, J.; Spiess, H. W.; Wang, X. C.; Antonietti, M.; Mullen, K. Photocatalytic Hydrogen Evolution through Fully Conjugated Poly(Azomethine) Networks Chem. Commun. 2010, 46, 8932– 8934 DOI: 10.1039/c0cc04057f
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    5
    Photocatalytic hydrogen evolution through fully conjugated poly(azomethine) networks
    Schwab, Matthias Georg; Hamburger, Manuel; Feng, Xinliang; Shu, Jie; Spiess, Hans Wolfgang; Wang, Xinchen; Antonietti, Markus; Muellen, Klaus
    Chemical Communications (Cambridge, United Kingdom) (2010), 46 (47), 8932-8934CODEN: CHCOFS; ISSN:1359-7345. (Royal Society of Chemistry)
    Three-dimensional conjugated poly(azomethine) networks were found to be promising candidates for applications in photocatalytic water splitting. Straightforward synthetic protocols lead to fully org. photocatalysts that showed enhanced long-time stability. Furthermore, the catalytic performance of these materials was correlated to the mol. compn. and the optoelectronic properties of the samples.
  6. 6
    Zhang, Z. Z.; Long, J. L.; Yang, L. F.; Chen, W. K.; Dai, W. X.; Fu, X. Z.; Wang, X. X. Organic Semiconductor for Artificial Photosynthesis: Water Splitting into Hydrogen by a Bioinspired C3N3S3 Polymer under Visible Light Irradiation Chem. Sci. 2011, 2, 1826– 1830 DOI: 10.1039/C1SC00257K
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    6
    Organic semiconductor for artificial photosynthesis: water splitting into hydrogen by a bioinspired C3N3S3 polymer under visible light irradiation
    Zhang, Zizhong; Long, Jinlin; Yang, Lifang; Chen, Wenkai; Dai, Wenxin; Fu, Xianzhi; Wang, Xuxu
    Chemical Science (2011), 2 (9), 1826-1830CODEN: CSHCCN; ISSN:2041-6520. (Royal Society of Chemistry)
    A novel org. semiconductor photocatalyst mimicking natural light-harvesting antenna complexes in photosynthetic organisms, a disulfide (-S-S-) bridged C3N3S3 polymer, was designed and developed to generate hydrogen from water under visible light irradn. The artificial conjugated polymer shows high H2-producing activity from the half-reaction of water splitting without the aid of a sacrificial electron donor. The H2-producing efficiency and photo-stability of the catalyst could be improved greatly using Ru and single-wall carbon nanotubes as cocatalysts or by adding a sacrificial donor. The results represent a potential and prospective application of the C3N3S3 polymer in solar energy conversion and offer significant guidance to develop more stable and efficient photocatalytic systems based on org. semiconductors.
  7. 7
    Chu, S.; Wang, Y.; Guo, Y.; Zhou, P.; Yu, H.; Luo, L. L.; Kong, F.; Zou, Z. G. Facile Green Synthesis of Crystalline Polyimide Photocatalyst for Hydrogen Generation from Water J. Mater. Chem. 2012, 22, 15519– 15521 DOI: 10.1039/c2jm32595k
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    7
    Facile green synthesis of crystalline polyimide photocatalyst for hydrogen generation from water
    Chu, Sheng; Wang, Ying; Guo, Yong; Zhou, Peng; Yu, He; Luo, Leilei; Kong, Fei; Zou, Zhigang
    Journal of Materials Chemistry (2012), 22 (31), 15519-15521CODEN: JMACEP; ISSN:0959-9428. (Royal Society of Chemistry)
    A simple green process based on thermal condensation is developed to synthesize polyimide. The obtained product possesses a high degree of crystallinity and exhibits efficient photocatalytic activity for hydrogen prodn. from water under visible light.
  8. 8
    Sprick, R. S.; Jiang, J. X.; Bonillo, B.; Ren, S. J.; Ratvijitvech, T.; Guiglion, P.; Zwijnenburg, M. A.; Adams, D. J.; Cooper, A. I. Tunable Organic Photocatalysts for Visible-Light-Driven Hydrogen Evolution J. Am. Chem. Soc. 2015, 137, 3265– 3270 DOI: 10.1021/ja511552k
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    8
    Tunable organic photocatalysts for visible-light-driven hydrogen evolution
    Sprick, Reiner Sebastian; Jiang, Jia-Xing; Bonillo, Baltasar; Ren, Shijie; Ratvijitvech, Thanchanok; Guiglion, Pierre; Zwijnenburg, Martijn A.; Adams, Dave J.; Cooper, Andrew I.
    Journal of the American Chemical Society (2015), 137 (9), 3265-3270CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    Photocatalytic hydrogen prodn. from water offers an abundant, clean fuel source, but it is challenging to produce photocatalysts that use the solar spectrum effectively. Many hydrogen-evolving photocatalysts are active in the UV range, but UV light accounts for only 3% of the energy available in the solar spectrum at ground level. Solid-state cryst. photocatalysts have light absorption profiles that are a discrete function of their cryst. phase and that are not always tunable. Here, we prep. a series of amorphous, microporous org. polymers with exquisite synthetic control over the optical gap in the range 1.94-2.95 eV. Specific monomer compns. give polymers that are robust and effective photocatalysts for the evolution of hydrogen from water in the presence of a sacrificial electron donor, without the apparent need for an added metal cocatalyst. Remarkably, unlike other org. systems, the best performing polymer is only photoactive under visible rather than UV irradn.
  9. 9
    Bornoz, P.; Prevot, M. S.; Yu, X. Y.; Guijarro, N.; Sivula, K. Direct Light-Driven Water Oxidation by a Ladder-Type Conjugated Polymer Photoanode J. Am. Chem. Soc. 2015, 137, 15338– 15341 DOI: 10.1021/jacs.5b05724
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    9
    Direct Light-Driven Water Oxidation by a Ladder-Type Conjugated Polymer Photoanode
    Bornoz, Pauline; Prevot, Mathieu S.; Yu, Xiaoyun; Guijarro, Nestor; Sivula, Kevin
    Journal of the American Chemical Society (2015), 137 (49), 15338-15341CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    A conjugated polymer known for high stability, poly[benzimidazobenzophenanthroline], BBL, is examd. as a photoanode for direct solar H2O oxidn. In aq. electrolyte with a sacrificial hole acceptor (SO32-), photoelectrodes show a morphol.-dependent performance. Films prepd. by a dispersion-spray method with a nanostructured surface (feature size of ∼20 nm) gave photocurrents up to 0.23 ± 0.02 mA cm-2 at 1.23 VRHE under std. simulated solar illumination. Electrochem. impedance spectroscopy reveals a const. flat-band potential over a wide pH range at +0.31 VNHE. The solar H2O oxidn. photocurrent with bare BBL electrodes increases with increasing pH, and no evidence of semiconductor oxidn. was obsd. over a 30 min testing time. Characterization of the photooxidn. reaction suggests H2O2 or •OH prodn. with the bare film, while functionalization of the interface with 1 nm of TiO2 followed by a Ni-Co catalyst gave solar photocurrents of 20-30 μA cm-2, corresponding with O2 evolution. Limitations to photocurrent prodn. are discussed.
  10. 10
    Schwinghammer, K.; Hug, S.; Mesch, M. B.; Senker, J.; Lotsch, B. V. Phenyl-Triazine Oligomers for Light-Driven Hydrogen Evolution Energy Environ. Sci. 2015, 8, 3345– 3353 DOI: 10.1039/C5EE02574E
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    10
    Phenyl-triazine oligomers for light-driven hydrogen evolution
    Schwinghammer, K.; Hug, S.; Mesch, M. B.; Senker, J.; Lotsch, B. V.
    Energy & Environmental Science (2015), 8 (11), 3345-3353CODEN: EESNBY; ISSN:1754-5706. (Royal Society of Chemistry)
    The design of stable, yet highly tunable org. photocatalysts which orchestrate multi-step electron transfer reactions is at the heart of the newly emerging field of polymer photocatalysis. Covalent triazine frameworks such as the archetypal CTF-1 have been theorized to constitute a new class of photocatalytically active polymers for light-driven water splitting. Here, we revisit the ionothermal synthesis of CTF-1 by trimerization of 1,4-dicyanobenzene catalyzed by the Lewis acid zinc chloride and demonstrate that the microporous black polymer CTF-1 is essentially inactive for hydrogen evolution. Instead, highly photoactive phenyl-triazine oligomers (PTOs) with higher crystallinity as compared to CTF-1 are obtained by lowering the reaction temp. to 300 °C and prolonging the reaction time to >150 h. The low reaction temp. of the PTOs largely prevents incipient carbonization and thus results in a carbon-to-nitrogen wt. ratio close to the theor. value of 3.43. The oligomers were characterized by MALDI-TOF and quant. solid-state NMR spectroscopy, revealing variations in size, connectivity and thus nitrile-to-triazine ratios depending on the initial precursor diln. The most active PTO samples efficiently and stably reduce water to hydrogen with an av. rate of 1076 (±278) μmol h-1 g-1 under simulated sunlight illumination, which is competitive with the best carbon nitride-based and purely org. photocatalysts. The photocatalytic activity of the PTOs is found to sensitively depend on the polymn. degree, thus suggesting a prominent role of the unreacted nitrile moieties in the photocatalytic process. Notably, PTOs even show moderate hydrogen prodn. without the addn. of any co-catalyst.
  11. 11
    Sprick, R. S.; Bonillo, B.; Clowes, R.; Guiglion, P.; Brownbill, N. J.; Slater, B. J.; Blanc, F.; Zwijnenburg, M. A.; Adams, D. J.; Cooper, A. I. Visible-Light-Driven Hydrogen Evolution Using Planarized Conjugated Polymer Photocatalysts Angew. Chem. 2016, 128, 1824– 1828 DOI: 10.1002/ange.201510542
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    11
    Visible-Light-Driven Hydrogen Evolution Using Planarized Conjugated Polymer Photocatalysts
    Sprick Reiner Sebastian; Bonillo Baltasar; Clowes Rob; Brownbill Nick J; Slater Benjamin J; Adams Dave J; Cooper Andrew I; Guiglion Pierre; Zwijnenburg Martijn A; Blanc Frederic
    Angewandte Chemie (Weinheim an der Bergstrasse, Germany) (2016), 128 (5), 1824-1828 ISSN:0044-8249.
    Linear poly(p-phenylene)s are modestly active UV photocatalysts for hydrogen production in the presence of a sacrificial electron donor. Introduction of planarized fluorene, carbazole, dibenzo[b,d]thiophene or dibenzo[b,d]thiophene sulfone units greatly enhances the H2 evolution rate. The most active dibenzo[b,d]thiophene sulfone co-polymer has a UV photocatalytic activity that rivals TiO2, but is much more active under visible light. The dibenzo[b,d]thiophene sulfone co-polymer has an apparent quantum yield of 2.3 % at 420 nm, as compared to 0.1 % for platinized commercial pristine carbon nitride.
  12. 12
    Yang, C.; Ma, B. C.; Zhang, L. Z.; Lin, S.; Ghasimi, S.; Landfester, K.; Zhang, K. A. I.; Wang, X. C. Molecular Engineering of Conjugated Polybenzothiadiazoles for Enhanced Hydrogen Production by Photosynthesis Angew. Chem., Int. Ed. 2016, 55, 9202– 9206 DOI: 10.1002/anie.201603532
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    12
    Molecular Engineering of Conjugated Polybenzothiadiazoles for Enhanced Hydrogen Production by Photosynthesis
    Yang, Can; Ma, Beatriz Chiyin; Zhang, Linzhu; Lin, Sen; Ghasimi, Saman; Landfester, Katharina; Zhang, Kai A. I.; Wang, Xinchen
    Angewandte Chemie, International Edition (2016), 55 (32), 9202-9206CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
    The search for metal-free org. photocatalysts for H2 prodn. from water using visible light remains a key challenge. Reported herein is a mol. structural design of pure org. photocatalysts, derived from conjugated polybenzothiadiazoles, for photocatalytic H2 evolution using visible light. By alternating the substitution position of the electron-withdrawing benzothiadizole unit on the Ph unit as a comonomer, various polymers with either one- or three-dimensional structures were synthesized and the effect of the mol. structure on their catalytic activity was investigated. Photocatalytic H2 evolution efficiencies up to 116 μmol h-1 were obsd. by employing the linear polymer based on a phenyl-benzothiadiazole alternating main chain, with an apparent quantum yield (AQY) of 4.01% at 420 nm using triethanolamine as the sacrificial agent.
  13. 13
    Li, L. W.; Cai, Z. X.; Wu, Q. H.; Lo, W. Y.; Zhang, N.; Chen, L. X.; Yu, L. P. Rational Design of Porous Conjugated Polymers and Roles of Residual Palladium for Photocatalytic Hydrogen Production J. Am. Chem. Soc. 2016, 138, 7681– 7686 DOI: 10.1021/jacs.6b03472
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    13
    Rational Design of Porous Conjugated Polymers and Roles of Residual Palladium for Photocatalytic Hydrogen Production
    Li, Lianwei; Cai, Zhengxu; Wu, Qinghe; Lo, Wai-Yip; Zhang, Na; Chen, Lin X.; Yu, Luping
    Journal of the American Chemical Society (2016), 138 (24), 7681-7686CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    Developing highly efficient photocatalysts for water splitting is one of the grand challenges in solar energy conversion. Here, we report the rational design and synthesis of porous conjugated polymer (PCP) that photocatalytically generates hydrogen from water splitting. The design mimics natural photosynthetics systems with conjugated polymer component to harvest photons and the transition metal part to facilitate catalytic activities. A series of PCPs have been synthesized with different light harvesting chromophores and transition metal binding bipyridyl (bpy) sites. The photocatalytic activity of these bpy-contg. PCPs can be greatly enhanced due to the improved light absorption, better wettability, local ordering structure, and the improved charge sepn. process. The PCP made of strong and fully conjugated donor chromophore DBD (M4) shows the highest hydrogen prodn. rate at ∼33 μmol/h. The results indicate that copolymn. between a strong electron donor and weak electron acceptor into the same polymer chain is a useful strategy for developing efficient photocatalysts. This study also reveals that the residual palladium in the PCP networks plays a key role for the catalytic performance. The hydrogen generation activity of PCP photocatalyst can be further enhanced to 164 μmol/h with an apparent quantum yield of 1.8% at 350 nm by loading 2 wt % of extra platinum cocatalyst.
  14. 14
    Vyas, V. S.; Lau, V. W. H.; Lotsch, B. V. Soft Photocatalysis: Organic Polymers for Solar Fuel Productions Chem. Mater. 2016, 28, 5191– 5204 DOI: 10.1021/acs.chemmater.6b01894
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  15. 15
    Zhang, G.; Lan, Z.-A.; Wang, X. Organic Conjugated Semiconductors for Photocatalytic Hydrogen Evolution with Visible Light Angew. Chem., Int. Ed. 2016, 55, 15712– 15727 DOI: 10.1002/anie.201607375
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    15
    Organic Conjugated Semiconductors for Photocatalytic Hydrogen Evolution with Visible Light
    Wang, Xinchen; Zhang, Guigang; Lan, Zhi-An
    Angewandte Chemie, International Edition (2016), 55 (51), 15712-15727CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
    Conjugated polymers, constituted with fully -conjugated systems, present a new generation of heterogeneous photocatalysts for solar energy utilization. They bare three key features for photocatalytic process, namely robust, non-toxic and visible-light-active, making them appealing candidates for scale up when integrating with their intrinsic soft features. In this minireview, we present a brief summary on the recent development of various promising polymer photocatalysts for hydrogen evolution from aq. solns., including linear polymers, planarized polymers, triazine/heptazine polymers, and other related org. conjugated semiconductors, with particular focus on the rational manipulation in the compn., architectures, optical and electronic properties that are closely relevant with photophysic and photochem. properties. Some future trends and prospects of org. conjugated photocatalysts for water splitting and its coupling with CO2 redn. are also envisaged.
  16. 16
    Wang, X. C.; Maeda, K.; Thomas, A.; Takanabe, K.; Xin, G.; Carlsson, J. M.; Domen, K.; Antonietti, M. A Metal-Free Polymeric Photocatalyst for Hydrogen Production from Water under Visible Light Nat. Mater. 2009, 8, 76– 80 DOI: 10.1038/nmat2317
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    16
    A metal-free polymeric photocatalyst for hydrogen production from water under visible light
    Wang, Xinchen; Maeda, Kazuhiko; Thomas, Arne; Takanabe, Kazuhiro; Xin, Gang; Carlsson, Johan M.; Domen, Kazunari; Antonietti, Markus
    Nature Materials (2009), 8 (1), 76-80CODEN: NMAACR; ISSN:1476-1122. (Nature Publishing Group)
    The prodn. of hydrogen from water using a catalyst and solar energy is an ideal future energy source, independent of fossil reserves. For an economical use of water and solar energy, catalysts that are sufficiently efficient, stable, inexpensive and capable of harvesting light are required. Here, we show that an abundant material, polymeric carbon nitride, can produce hydrogen from water under visible-light irradn. in the presence of a sacrificial donor. Contrary to other conducting polymer semiconductors, carbon nitride is chem. and thermally stable and does not rely on complicated device manufg. The results represent an important first step towards photosynthesis in general where artificial conjugated polymer semiconductors can be used as energy transducers.
  17. 17
    Wang, X. C.; Maeda, K.; Chen, X. F.; Takanabe, K.; Domen, K.; Hou, Y. D.; Fu, X. Z.; Antonietti, M. Polymer Semiconductors for Artificial Photosynthesis: Hydrogen Evolution by Mesoporous Graphitic Carbon Nitride with Visible Light J. Am. Chem. Soc. 2009, 131, 1680– 1681 DOI: 10.1021/ja809307s
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    17
    Polymer semiconductors for artificial photosynthesis: hydrogen evolution by mesoporous graphitic carbon nitride with visible light
    Wang, Xinchen; Maeda, Kazuhiko; Chen, Xiufang; Takanabe, Kazuhiro; Domen, Kazunari; Hou, Yidong; Fu, Xianzhi; Antonietti, Markus
    Journal of the American Chemical Society (2009), 131 (5), 1680-1681CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    We investigated semiconductor characteristics for polymeric carbon nitride as a metal-free photocatalyst working with visible light and have shown that the efficiency of hydrogen prodn. by photochem. water redn. can be improved by ∼1 order of magnitude by introducing the right type of mesoporosity into polymeric C3N4. We anticipate a wide range of potential application of C3N4 as energy transducers for artificial photosynthesis in general, esp. with a 3D continuous nanoarchitecture. Moreover, the results of finding photoactivity for carbon nitride nanoparticles can enrich the discussion on prebiotic chem. of the Earth, as HCN polymer clusters are unequivocal in the solar system.
  18. 18
    Maeda, K.; Wang, X. C.; Nishihara, Y.; Lu, D. L.; Antonietti, M.; Domen, K. Photocatalytic Activities of Graphitic Carbon Nitride Powder for Water Reduction and Oxidation under Visible Light J. Phys. Chem. C 2009, 113, 4940– 4947 DOI: 10.1021/jp809119m
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    18
    Photocatalytic Activities of Graphitic Carbon Nitride Powder for Water Reduction and Oxidation under Visible Light
    Maeda, Kazuhiko; Wang, Xinchen; Nishihara, Yasushi; Lu, Daling; Antonietti, Markus; Domen, Kazunari
    Journal of Physical Chemistry C (2009), 113 (12), 4940-4947CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
    Graphitic carbon nitride (g-C3N4) with a band gap of 2.7 eV is studied as a nonmetallic photocatalyst for H2 or O2 evolution from water under UV and visible light. The g-C3N4 catalyst exhibits activities for water redn. into H2 or water oxidn. into O2 in the presence of a proper sacrificial electron donor or acceptor, resp., even without the need for precious metal co-catalysts. When bis(1,5-cyclooctadiene)platinum complex [Pt(cod)2] (a nonionic complex) is used as a precursor of Pt co-catalyst instead of H2PtCl6 (an ionic complex), enhanced H2 evolution activity is acquired. This difference in activity is primarily due to the better dispersion of Pt nanoparticles on g-C3N4, which is considered to originate from the better access of Pt(cod)2 to the g-C3N4 surface, as compared to that of H2PtCl6 in the prepn. process. Unmodified g-C3N4 produces O2 from an aq. silver nitrate soln. upon UV irradn. (λ > 300 nm), although N2 release due to self-decompn. of g-C3N4 by photogenerated holes takes place. Modification of g-C3N4 with RuO2 improves not only O2 evolution activity but also stability against the self-decompn., resulting in stable visible-light-driven O2 evolution (λ > 420 nm).
  19. 19
    Zhang, J. S.; Chen, X. F.; Takanabe, K.; Maeda, K.; Domen, K.; Epping, J. D.; Fu, X. Z.; Antonietti, M.; Wang, X. C. Synthesis of a Carbon Nitride Structure for Visible-Light Catalysis by Copolymerization Angew. Chem., Int. Ed. 2010, 49, 441– 444 DOI: 10.1002/anie.200903886
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    19
    Synthesis of a Carbon Nitride Structure for Visible-Light Catalysis by Copolymerization
    Zhang, Jinshui; Chen, Xiufang; Takanabe, Kazuhiro; Maeda, Kazuhiko; Domen, Kazunari; Epping, Jan Dirk; Fu, Xianzhi; Antonietti, Markus; Wang, Xinchen
    Angewandte Chemie, International Edition (2010), 49 (2), 441-444, S441/1-S441/4CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
    The authors have shown that the optical and electronic properties of carbon nitride polymer can be easily modified by std. org. protocols, here exemplified by copolymn. with barbituric acid. The authors believe that the diversity of org. chem. will allow the rational design of a much broader set of carbon nitride polymers with controlled functions, opening a new pathway for the development of light-harvesting semiconductors. Physicochem. properties and photocatalytic activity of Pt/carbon nitride polymer for H2 generation from H2O/triethanolamine under UV and visible light were studied.
  20. 20
    Zhang, G. G.; Lan, Z. A.; Lin, L. H.; Lin, S.; Wang, X. C. Overall Water Splitting by Pt/G-C3N4 Photocatalysts without Using Sacrificial Agents Chem. Sci. 2016, 7, 3062– 3066 DOI: 10.1039/C5SC04572J
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    20
    Overall water splitting by Pt/g-C3N4 photocatalysts without using sacrificial agents
    Zhang, Guigang; Lan, Zhi-An; Lin, Lihua; Lin, Sen; Wang, Xinchen
    Chemical Science (2016), 7 (5), 3062-3066CODEN: CSHCCN; ISSN:2041-6520. (Royal Society of Chemistry)
    We report the direct splitting of pure water by light-excited graphitic carbon nitride (g-C3N4) modified with Pt, PtOx, and CoOx as redox cocatalysts, while pure g-C3N4 is virtually inactive for overall water splitting by photocatalysis. The novelty is in the selective creation of both H2 and O2 cocatalysts on surface active sites of g-C3N4via photodeposition triggering the splitting of water for the simultaneous evolution of H2 and O2 gases in a stoichiometric ratio of 2:1, irradiated with light, without using any sacrificial reagents. The photocatalyst was stable for 510 h of reaction.
  21. 21
    Sui, Y.; Liu, J. H.; Zhang, Y. W.; Tian, X. K.; Chen, W. Dispersed Conductive Polymer Nanoparticles on Graphitic Carbon Nitride for Enhanced Solar-Driven Hydrogen Evolution from Pure Water Nanoscale 2013, 5, 9150– 9155 DOI: 10.1039/c3nr02413j
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    21
    Dispersed conductive polymer nanoparticles on graphitic carbon nitride for enhanced solar-driven hydrogen evolution from pure water
    Sui, Yi; Liu, Jinghai; Zhang, Yuewei; Tian, Xike; Chen, Wei
    Nanoscale (2013), 5 (19), 9150-9155CODEN: NANOHL; ISSN:2040-3372. (Royal Society of Chemistry)
    Developing new methods to improve the photocatalytic activity of graphitic carbon nitride (g-C3N4) for hydrogen (H2) evolution has attracted intensive research interests. Here, we report that the g-C3N4 exhibits photocatalytic activity for H2 evolution from pure water. And, the activity is dramatically improved by loading highly dispersed conductive polymer nanoparticles. The H2 evolution rate increases up to 50 times for g-C3N4 with 1.5 wt% polypyrrole (PPy) nanoparticles on the surface. The reaction proceeding in a pure water system excludes the need for sacrificial agents. The role of the highly conductive PPy in enhancing H2 evolution is as a surface junction to increase the no. of photoinduced electrons, and to facilitate electron transfer to the interface.
  22. 22
    Schwinghammer, K.; Tuffy, B.; Mesch, M. B.; Wirnhier, E.; Martineau, C.; Taulelle, F.; Schnick, W.; Senker, J.; Lotsch, B. V. Triazine-Based Carbon Nitrides for Visible-Light-Driven Hydrogen Evolution Angew. Chem., Int. Ed. 2013, 52, 2435– 2439 DOI: 10.1002/anie.201206817
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    22
    Triazine-based Carbon Nitrides for Visible-Light-Driven Hydrogen Evolution
    Schwinghammer, Katharina; Tuffy, Brian; Mesch, Maria B.; Wirnhier, Eva; Martineau, Charlotte; Taulelle, Francis; Schnick, Wolfgang; Senker, Juergen; Lotsch, Bettina V.
    Angewandte Chemie, International Edition (2013), 52 (9), 2435-2439CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
    Carbon nitride (CN) photocatalysts based on triazine building blocks are developed and their enhanced photocatalytic activity demonstrated in comparison to heptazine-based CNs. Moreover, it is shown that their performance can be amplified by small-mol. doping, thus rendering them the most active nonmetal photocatalysts for the hydrogen evolution reaction that are reported to date. The 2D triazine-based carbon nitrides show substantial visible light-induced hydrogen prodn. from water, and in this regard rival the benchmark heptazine-derived photocatalysts. With external quantum efficiencies as high as 3.4%, the amorphous carbon- and oxygen-enriched poly(triazine imide) species not only outperform melon-type photocatalysts, but also cryst. poly(triazine imide) by 5-6 times. Consistent with results of X. Wang et al.(2009), it is demonstrated that a rather low level of structural definition and the introduction of defects up to a certain doping level (16% for 4-amino-2,6-dihydroxypyrimidine) tend to enhance the photoactivity of the catalysts. The diverse range of available org. and inorg. dopants will allow the rational design of a broad set of triazine-based CN polymers with controlled functions, thus opening new avenues for the development of light-harvesting semiconductors. The. easily adjustable structural and electronic properties of CN polymers render them particularly versatile for solar energy applications.
  23. 23
    Martin, D. J.; Qiu, K. P.; Shevlin, S. A.; Handoko, A. D.; Chen, X. W.; Guo, Z. X.; Tang, J. W. Highly Efficient Photocatalytic H2 Evolution from Water Using Visible Light and Structure-Controlled Graphitic Carbon Nitride Angew. Chem., Int. Ed. 2014, 53, 9240– 9245 DOI: 10.1002/anie.201403375
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    23
    Highly Efficient Photocatalytic H2 Evolution from Water using Visible Light and Structure-Controlled Graphitic Carbon Nitride
    Martin, David James; Qiu, Kaipei; Shevlin, Stephen Andrew; Handoko, Albertus Denny; Chen, Xiaowei; Guo, Zhengxiao; Tang, Junwang
    Angewandte Chemie, International Edition (2014), 53 (35), 9240-9245CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
    The major challenge of photocatalytic water splitting, the prototypical reaction for the direct prodn. of hydrogen by using solar energy, is to develop low-cost yet highly efficient and stable semiconductor photocatalysts. Herein, an effective strategy for synthesizing extremely active graphitic carbon nitride (g-C3N4) from a low-cost precursor, urea, is reported. The g-C3N4 exhibits an extraordinary hydrogen-evolution rate (ca. 20 000 μmol h-1 g-1 under full arc), which leads to a high turnover no. (TON) of over 641 after 6 h. The reaction proceeds for more than 30 h without activity loss and results in an internal quantum yield of 26.5 % under visible light, which is nearly an order of magnitude higher than that obsd. for any other existing g-C3N4 photocatalysts. Furthermore, it was found by exptl. anal. and DFT calcns. that as the d.p. increases and the proton concn. decreases, the hydrogen-evolution rate is significantly enhanced.
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    Schwinghammer, K.; Mesch, M. B.; Duppel, V.; Ziegler, C.; Senker, J.; Lotsch, B. V. Crystalline Carbon Nitride Nanosheets for Improved Visible-Light Hydrogen Evolution J. Am. Chem. Soc. 2014, 136, 1730– 1733 DOI: 10.1021/ja411321s
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    Crystalline Carbon Nitride Nanosheets for Improved Visible-Light Hydrogen Evolution
    Schwinghammer, Katharina; Mesch, Maria B.; Duppel, Viola; Ziegler, Christian; Senker, Juergen; Lotsch, Bettina V.
    Journal of the American Chemical Society (2014), 136 (5), 1730-1733CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    Nanosheets of a cryst. 2D carbon nitride were obtained by ionothermal synthesis of the layered bulk material poly(triazine imide), PTI, followed by one-step liq. exfoliation in water. Triazine-based nanosheets are 1-2 nm in height and afford chem. and colloidally stable suspensions under both basic and acidic conditions. We use solid-state NMR spectroscopy of isotopically enriched, restacked nanosheets as a tool to indirectly monitor the exfoliation process and carve out the chem. changes occurring upon exfoliation, as well as to det. the nanosheet thickness. PTI nanosheets show significantly enhanced visible-light driven photocatalytic activity toward hydrogen evolution compared to their bulk counterpart, which highlights the crucial role of morphol. and surface area on the photocatalytic performance of carbon nitride materials.
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    Liu, J.; Liu, Y.; Liu, N. Y.; Han, Y. Z.; Zhang, X.; Huang, H.; Lifshitz, Y.; Lee, S. T.; Zhong, J.; Kang, Z. H. Metal-Free Efficient Photocatalyst for Stable Visible Water Splitting Via a Two-Electron Pathway Science 2015, 347, 970– 974 DOI: 10.1126/science.aaa3145
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    Metal-free efficient photocatalyst for stable visible water splitting via a two-electron pathway
    Liu, Juan; Liu, Yang; Liu, Naiyun; Han, Yuzhi; Zhang, Xing; Huang, Hui; Lifshitz, Yeshayahu; Lee, Shuit-Tong; Zhong, Jun; Kang, Zhenhui
    Science (Washington, DC, United States) (2015), 347 (6225), 970-974CODEN: SCIEAS; ISSN:0036-8075. (American Association for the Advancement of Science)
    The use of solar energy to produce mol. hydrogen and oxygen (H2 and O2) from overall water splitting is a promising means of renewable energy storage. In the past 40 years, various inorg. and org. systems have been developed as photocatalysts for water splitting driven by visible light. These photocatalysts, however, still suffer from low quantum efficiency and/or poor stability. We report the design and fabrication of a metal-free carbon nanodot-carbon nitride (C3N4) nanocomposite and demonstrate its impressive performance for photocatalytic solar water splitting. We measured quantum efficiencies of 16% for wavelength λ = 420 ± 20 nm, 6.29% for λ = 580 ± 15 nm, and 4.42% for λ = 600 ± 10 nm, and detd. an overall solar energy conversion efficiency of 2.0%. The catalyst comprises low-cost, Earth-abundant, environmentally friendly materials and shows excellent stability.
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    Lau, V. W. H.; Mesch, M. B.; Duppel, V.; Blum, V.; Senker, J.; Lotsch, B. V. Low-Molecular-Weight Carbon Nitrides for Solar Hydrogen Evolution J. Am. Chem. Soc. 2015, 137, 1064– 1072 DOI: 10.1021/ja511802c
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    Low-Molecular-Weight Carbon Nitrides for Solar Hydrogen Evolution
    Lau, Vincent Wing-hei; Mesch, Maria B.; Duppel, Viola; Blum, Volker; Senker, Juergen; Lotsch, Bettina V.
    Journal of the American Chemical Society (2015), 137 (3), 1064-1072CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    This work focuses on the control of the polymn. process for melon ("graphitic carbon nitride"), with the aim of improving its photocatalytic activity intrinsically. We demonstrate here that redn. of the synthesis temp. leads to a mixt. of the monomer melem and its higher condensates. We show that this mixt. can be sepd. and provide evidence that the higher condensates are isolated oligomers of melem. On evaluating their photocatalytic activity for hydrogen evolution, the oligomers were found to be the most active species, having up to twice the activity of the monomer/oligomer mixt. of the as-synthesized material, which in turn has 3 times the activity of the polymer melon, the literature benchmark. These results highlight the role of "defects", i.e., chain terminations, in increasing the catalytic activity of carbon nitrides and at the same time point to the ample potential of intrinsically improving the photocatalytic activity of "carbon nitride", esp. through the selective synthesis of the active phase.
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    Lau, V. W. H.; Moudrakovski, I.; Botari, T.; Weinberger, S.; Mesch, M. B.; Duppel, V.; Senker, J.; Blum, V.; Lotsch, B. V. Rational Design of Carbon Nitride Photocatalysts by Identification of Cyanamide Defects as Catalytically Relevant Sites Nat. Commun. 2016, 7, 12165 DOI: 10.1038/ncomms12165
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    Rational design of carbon nitride photocatalysts by identification of cyanamide defects as catalytically relevant sites
    Lau, Vincent Wing-hei; Moudrakovski, Igor; Botari, Tiago; Weinberger, Simon; Mesch, Maria B.; Duppel, Viola; Senker, Juergen; Blum, Volker; Lotsch, Bettina V.
    Nature Communications (2016), 7 (), 12165CODEN: NCAOBW; ISSN:2041-1723. (Nature Publishing Group)
    The heptazine-based polymer melon (also known as graphitic carbon nitride, g-C3N4) is a promising photocatalyst for hydrogen evolution. Nonetheless, attempts to improve its inherently low activity are rarely based on rational approaches because of a lack of fundamental understanding of its mechanistic operation. Here we employ mol. heptazine-based model catalysts to identify the cyanamide moiety as a photocatalytically relevant 'defect'. We exploit this knowledge for the rational design of a carbon nitride polymer populated with cyanamide groups, yielding a material with 12 and 16 times the hydrogen evolution rate and apparent quantum efficiency (400 nm), resp., compared with the unmodified melon. Computational modeling and material characterization suggest that this moiety improves coordination (and, in turn, charge transfer kinetics) to the platinum co-catalyst and enhances the sepn. of the photogenerated charge carriers. The demonstrated knowledge transfer for rational catalyst design presented here provides the conceptual framework for engineering high-performance heptazine-based photocatalysts.
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    Yeh, T. F.; Syu, J. M.; Cheng, C.; Chang, T. H.; Teng, H. S. Graphite Oxide as a Photocatalyst for Hydrogen Production from Water Adv. Funct. Mater. 2010, 20, 2255– 2262 DOI: 10.1002/adfm.201000274
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    Graphite Oxide as a Photocatalyst for Hydrogen Production from Water
    Yeh, Te-Fu; Syu, Jhih-Ming; Cheng, Ching; Chang, Ting-Hsiang; Teng, Hsisheng
    Advanced Functional Materials (2010), 20 (14), 2255-2262CODEN: AFMDC6; ISSN:1616-301X. (Wiley-VCH Verlag GmbH & Co. KGaA)
    A graphite oxide (GO) semiconductor photocatalyst with an apparent bandgap of 2.4-4.3 eV is synthesized by a modified Hummers' procedure. The as-synthesized GO photocatalyst has an interlayer spacing of 0.42 nm because of its moderate oxidn. level. Under irradn. with UV or visible light, this GO photocatalyst steadily catalyzes H2 generation from a 20 vol % aq. methanol soln. and pure water. As the GO sheets extensively disperse in water, a cocatalyst is not required for H2 generation over the GO photocatalyst. During photocatalytic reaction, the GO loses some oxygen functional groups, leading to bandgap redn. and increased cond. This structural variation does not affect the stable H2 generation over the GO. The encouraging results presented in this study demonstrate the potential of graphitic materials as a medium for water splitting under solar illumination.
  29. 29
    Yeh, T. F.; Teng, C. Y.; Chen, S. J.; Teng, H. S. Nitrogen-Doped Graphene Oxide Quantum Dots as Photocatalysts for Overall Water-Splitting under Visible Light Illumination Adv. Mater. 2014, 26, 3297– 3303 DOI: 10.1002/adma.201305299
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    Nitrogen-Doped Graphene Oxide Quantum Dots as Photocatalysts for Overall Water-Splitting under Visible Light Illumination
    Yeh, Te-Fu; Teng, Chiao-Yi; Chen, Shean-Jen; Teng, Hsisheng
    Advanced Materials (Weinheim, Germany) (2014), 26 (20), 3297-3303CODEN: ADVMEW; ISSN:0935-9648. (Wiley-VCH Verlag GmbH & Co. KGaA)
    Based on the structural characteristics required for photocatalytic water-splitting, we synthesized nitrogen-doped graphene oxide-quantum dots (NGO-QDs) as the catalyst. The NGO-QDs exhibited both p- and n-type conductivities, based on the results of the electrochem. Mott-Schottky anal. The prominent photo-luminescence emission indicated that photochem. p-n diodes constituted the NGO-QDs. The diode configuration resulted in an internal Z-scheme charge transfer for effective reaction at the QD interface. Visible light (>420 nm) irradn. on the NGO-QDs resulted in simultaneous H2 and O2 evolution from pure water at an H2:O2 molar ratio of 2:1. This paper demonstrated that graphene species are promising materials for synthesizing metal-free, cost-effective, and environmentally-friendly catalysts for overall water-splitting under solar illumination.
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    Tsai, E. W.; Basak, S.; Ruiz, J. P.; Reynolds, J. R.; Rajeshwar, K. Electrochemistry of Some Beta-Substituted Polythiophenes - Anodic-Oxidation, Electrochromism, and Electrochemical Deactivation J. Electrochem. Soc. 1989, 136, 3683– 3689 DOI: 10.1149/1.2096530
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    Novak, P.; Rasch, B.; Vielstich, W. Overoxidation of Polypyrrole in Propylene Carbonate - an Insitu FTIR Study J. Electrochem. Soc. 1991, 138, 3300– 3304 DOI: 10.1149/1.2085406
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    Zotti, G.; Schiavon, G.; Zecchin, S. Irreversible-Processes in the Electrochemical Reduction of Polythiophenes - Chemical Modifications of the Polymer and Charge-Trapping Phenomena Synth. Met. 1995, 72, 275– 281 DOI: 10.1016/0379-6779(95)03280-0
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    Irreversible processes in the electrochemical reduction of polythiophenes. Chemical modifications of the polymer and charge-trapping phenomena
    Zotti, G.; Schiavon, G.; Zecchin, S.
    Synthetic Metals (1995), 72 (3), 275-81CODEN: SYMEDZ; ISSN:0379-6779. (Elsevier)
    Irreversible changes occurring in the electrochem. redn. of some polythiophenes in MeCN have been investigated by cyclic voltammetry (CV), UV-visible and IR spectroscopy, and the electrochem. quartz crystal microbalance. Redn. in the presence of alkali metal cations (Li+, Na+, K+, and Cs+) does not result in n-doping but in hydrogenation of the alkene moieties. The promotion of double-bond satn. by alkali metal cations following the order Li+, Na+ > K+ > Cs+ is attributed to their charge-pinning action which decreases as the ion radius is increased. Reversible redn. in tetralkylammonium electrolyte (n-doping) is accompanied by prodn. of hydroxide ions within the polymer which are thereby inserted by subsequent oxidn. The resulting quinone-like groups are responsible for the CV pre-peaks usually obsd. in conducting polymers after n-doping and previously assigned to trapped charges.
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    Vyas, R. N.; Wang, B. Electrochemical Analysis of Conducting Polymer Thin Films Int. J. Mol. Sci. 2010, 11, 1956– 1972 DOI: 10.3390/ijms11041956
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    Electrochemical analysis of conducting polymer thin films
    Vyas, Ritesh N.; Wang, Bin
    International Journal of Molecular Sciences (2010), 11 (), 1956-1972CODEN: IJMCFK; ISSN:1422-0067. (Molecular Diversity Preservation International)
    Polyelectrolyte multilayers built via the layer-by-layer (LbL) method has been one of the most promising systems in the field of materials science. Layered structures can be constructed by the adsorption of various polyelectrolyte species onto the surface of a solid or liq. material by means of electrostatic interaction. The thickness of the adsorbed layers can be tuned precisely in the nanometer range. Stable, semiconducting thin films are interesting research subjects. We use a conducting polymer, poly(p-phenylene vinylene) (PPV), in the prepn. of a stable thin film via the LbL method. Cyclic voltammetry and electrochem. impedance spectroscopy have been used to characterize the ionic cond. of the PPV multilayer films. The ionic cond. of the films has been found to be dependent on the polymn. temp. The film cond. can be fitted to a modified Randle's circuit. The circuit equiv. calcns. are performed to provide the diffusion coeff. values.
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    Guiglion, P.; Butchosa, C.; Zwijnenburg, M. A. Polymeric Watersplitting Photocatalysts; a Computational Perspective on the Water Oxidation Conundrum J. Mater. Chem. A 2014, 2, 11996– 12004 DOI: 10.1039/C4TA02044H
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    Polymeric water splitting photocatalysts; computational perspective on water oxidation conundrum
    Guiglion, Pierre; Butchosa, Cristina; Zwijnenburg, Martijn A.
    Journal of Materials Chemistry A: Materials for Energy and Sustainability (2014), 2 (30), 11996-12004CODEN: JMCAET; ISSN:2050-7496. (Royal Society of Chemistry)
    A computational scheme to predict the thermodn. ability of photocatalysts to drive both of the water splitting half reactions, proton redn. and water oxidn., is discussed, and applied to a no. of polymeric systems to explain their apparent inability to oxidize water. We predict that the poly(p-phenylene) (PPP) is thermodynamically unable to oxidize water and that PPP is hence unlikely to split water in the absence of an external elec. bias. For other polymers, however, for example carbon nitride, the lack of oxygen evolution activity appears kinetic in origin and hence a suitable co-catalyst could potentially transform them into true water splitting photocatalysts.
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    Butchosa, C.; Guiglion, P.; Zwijnenburg, M. A. Carbon Nitride Photocatalysts for Water Splitting: A Computational Perspective J. Phys. Chem. C 2014, 118, 24833– 24842 DOI: 10.1021/jp507372n
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    Carbon nitride photocatalysts for water splitting: A computational perspective
    Butchosa, Cristina; Guiglion, Pierre; Zwijnenburg, Martijn A.
    Journal of Physical Chemistry C (2014), 118 (43), 24833-24842CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
    We study the thermodn. ability of carbon nitride materials to act as water splitting photocatalysts using a computational approach that involves a combination of d. functional theory (DFT) and time-dependent DFT (TD-DFT) calcns. on cluster models of both triazine- and heptazine-based structures. We first use TD-DFT to calc. the absorption spectra of the different cluster models and compare these spectra to those measured exptl. and then calc. using DFT and TD-DFT the redn. potentials of the free electron, free hole, and exciton in these models. We predict that all classes of carbon nitride structures considered should thermodynamically be able to reduce protons and oxidize water. We further provide evidence for the hypothesis that the exptl. lack of overall water splitting activity for pure carbon nitride arises from the fact that water oxidn. is a four-hole reaction and hence very susceptible to competition with electron-hole recombination. Finally, we propose that the recently reported overall water splitting activity of carbon nitride loaded with polypyrrole nanoparticles arises from a junction formed at the interface of both materials, which assists in keeping electrons and holes apart.
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    Berardo, E.; Zwijnenburg, M. A. Modeling the Water Splitting Activity of a Tio2 Rutile Nanoparticle J. Phys. Chem. C 2015, 119, 13384– 13393 DOI: 10.1021/acs.jpcc.5b01512
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    Modeling the Water Splitting Activity of a TiO2 Rutile Nanoparticle
    Berardo, Enrico; Zwijnenburg, Martijn A.
    Journal of Physical Chemistry C (2015), 119 (24), 13384-13393CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
    We explore, from a theor. perspective, the effect of particle size on the photocatalytic water splitting activity of TiO2 rutile (nano)particles by a combination of explicit quantum chem. calcns. on a hydroxylated rutile nanoparticle in a realistic solvation environment and a comparison with the calcd. properties of bulk rutile (surfaces) from the literature. Specifically, we use d. functional theory (DFT) and time-dependent DFT to calc. the nanoparticle thermodn. driving force for the water splitting half-reactions and identify in the process the crucial role of self-trapping of the free charge carriers responsible for proton redn. and water oxidn.
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    Guiglion, P.; Berardo, E.; Butchosa, C.; Wobbe, M. C. C.; Zwijnenburg, M. A. Modelling Materials for Solar Fuel Synthesis by Artificial Photosynthesis; Predicting the Optical, Electronic and Redox Properties of Photocatalysts J. Phys.: Condens. Matter 2016, 28, 074001 DOI: 10.1088/0953-8984/28/7/074001
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    Modelling materials for solar fuel synthesis by artificial photosynthesis; predicting the optical, electronic and redox properties of photocatalysts
    Guiglion, Pierre; Berardo, Enrico; Butchosa, Cristina; Wobbe, Milena C. C.; Zwijnenburg, Martijn A.
    Journal of Physics: Condensed Matter (2016), 28 (7), 074001/1-074001/11CODEN: JCOMEL; ISSN:0953-8984. (IOP Publishing Ltd.)
    A review. In this mini-review, we discuss what insight computational modeling can provide into the working of photocatalysts for solar fuel synthesis and how calcns. can be used to screen for new promising materials for photocatalytic water splitting and carbon dioxide redn. We will extensively discuss the different relevant (material) properties and the computational approaches (DFT, TD-DFT, GW/BSE) available to model them. We illustrate this with examples from the literature, focussing on polymeric and nanoparticle photocatalysts. We finish with a perspective on the outstanding conceptual and computational challenges.
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    Guiglion, P.; Butchosa, C.; Zwijnenburg, M. A. Polymer Photocatalysts for Water Splitting: Insights from Computational Modeling Macromol. Chem. Phys. 2016, 217, 344– 353 DOI: 10.1002/macp.201500432
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    Polymer Photocatalysts for Water Splitting: Insights from Computational Modeling
    Guiglion, Pierre; Butchosa, Cristina; Zwijnenburg, Martijn A.
    Macromolecular Chemistry and Physics (2016), 217 (3), 344-353CODEN: MCHPES; ISSN:1022-1352. (Wiley-VCH Verlag GmbH & Co. KGaA)
    A review. Based on insights from computational chem. calcns., the ability of polymers to act as water splitting photocatalysts for the prodn. of renewable hydrogen from water and sunlight is discussed. Specifically, the important role of exciton dissocn. in these materials is highlighted, as well as the possible microscopic origins of the exptl. obsd. changes in the photocatalytic activity of a polymer with increasing chain length or changing chem. compn. The reason why water oxidn., with polymeric photocatalysts, is difficult, and which polymer properties to target when developing new polymers for water splitting photocatalysis are, finally, also discussed.
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    Toroker, M. C.; Kanan, D. K.; Alidoust, N.; Isseroff, L. Y.; Liao, P. L.; Carter, E. A. First Principles Scheme to Evaluate Band Edge Positions in Potential Transition Metal Oxide Photocatalysts and Photoelectrodes Phys. Chem. Chem. Phys. 2011, 13, 16644– 16654 DOI: 10.1039/c1cp22128k
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    First principles scheme to evaluate band edge positions in potential transition metal oxide photocatalysts and photoelectrodes
    Toroker, Maytal Caspary; Kanan, Dalal K.; Alidoust, Nima; Isseroff, Leah Y.; Liao, Pei-Lin; Carter, Emily A.
    Physical Chemistry Chemical Physics (2011), 13 (37), 16644-16654CODEN: PPCPFQ; ISSN:1463-9076. (Royal Society of Chemistry)
    The positions of electronic band edges are one important metric for detg. a material's capability to function in a solar energy conversion device that produces fuels from sunlight. In particular, the position of the valence band max. (conduction band min.) must lie lower (higher) in energy than the oxidn. (redn.) reaction free energy in order for these reactions to be thermodynamically favorable. First principles quantum mechanics calcns. are presented of the band edge positions in five transition metal oxides and the feasibility is discussed of using these materials in photoelectrochem. cells that produce fuels, including hydrogen, methane, methanol, and formic acid. The band gap center is detd. within the framework of d. functional theory + intra-at. Coulomb (DFT + U) theory. The valence band max. (conduction band min.) is found by subtracting (adding) half of the quasiparticle gap obtained from a non-self-consistent GW calcn. The calcns. are validated against exptl. data where possible; results for several materials including manganese(ii) oxide, iron(ii) oxide, iron(iii) oxide, copper(i) oxide, and nickel(ii) oxide are presented.
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    Castelli, I. E.; Olsen, T.; Datta, S.; Landis, D. D.; Dahl, S.; Thygesen, K. S.; Jacobsen, K. W. Computational Screening of Perovskite Metal Oxides for Optimal Solar Light Capture Energy Environ. Sci. 2012, 5, 5814– 5819 DOI: 10.1039/C1EE02717D
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    Computational screening of perovskite metal oxides for optimal solar light capture
    Castelli, Ivano E.; Olsen, Thomas; Datta, Soumendu; Landis, David D.; Dahl, Soren; Thygesen, Kristian S.; Jacobsen, Karsten W.
    Energy & Environmental Science (2012), 5 (2), 5814-5819CODEN: EESNBY; ISSN:1754-5706. (Royal Society of Chemistry)
    One of the possible solns. to the world's rapidly increasing energy demand is the development of new photoelectrochem. cells with improved light absorption. This requires development of semiconductor materials which have appropriate bandgaps to absorb a large part of the solar spectrum at the same time as being stable in aq. environments. Here we demonstrate an efficient, computational screening of relevant oxide and oxynitride materials based on electronic structure calcns. resulting in the redn. of a vast space of 5400 different materials to only 15 promising candidates. The screening is based on an efficient and reliable way of calcg. semiconductor band gaps. The outcome of the screening includes all already known successful materials of the types investigated plus some new ones which warrant further exptl. investigation.
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    Persson, K. A.; Waldwick, B.; Lazic, P.; Ceder, G. Prediction of Solid-Aqueous Equilibria: Scheme to Combine First-Principles Calculations of Solids with Experimental Aqueous States Phys. Rev. B: Condens. Matter Mater. Phys. 2012, 85, 235438 DOI: 10.1103/PhysRevB.85.235438
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    Prediction of solid-aqueous equilibria: scheme to combine first-principles calculations of solids with experimental aqueous states
    Persson, Kristin A.; Waldwick, Bryn; Lazic, Predrag; Ceder, Gerbrand
    Physical Review B: Condensed Matter and Materials Physics (2012), 85 (23), 235438/1-235438/12CODEN: PRBMDO; ISSN:1098-0121. (American Physical Society)
    We present an efficient scheme for combining ab initio calcd. solid states with exptl. aq. states through a framework of consistent ref. energies. Our work enables accurate prediction of phase stability and dissoln. in equil. with water, which has many important application areas. We formally outline the thermodn. principles of the scheme and show examples of successful applications of the proposed framework on (1) the evaluation of the water-splitting photocatalytic material Ta3N5 for aq. stability, (2) the stability of small nanoparticle Pt in acid water, and (3) the prediction of particle morphol. and facet stabilization of olivine LiFePO4 as a function of aq. conditions.
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    Zhuang, H. L. L.; Hennig, R. G. Single-Layer Group-Iii Monochalcogenide Photocatalysts for Water Splitting Chem. Mater. 2013, 25, 3232– 3238 DOI: 10.1021/cm401661x
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    Single-layer group-III monochalcogenide photocatalysts for water splitting
    Zhuang, Houlong L.; Hennig, Richard G.
    Chemistry of Materials (2013), 25 (15), 3232-3238CODEN: CMATEX; ISSN:0897-4756. (American Chemical Society)
    The recent synthesis of single-layer GaS and GaSe opens the question of stability for other single-layer group-III monochalcogenides (MX, M = Ga and In, X = S, Se, and Te) and how the dimension redn. affects the properties of these materials. Using a first-principles design approach, we det. that the single-layer group-III monochalcogenides exhibit low formation energies and are suitable for photocatalytic water splitting. First, d.-functional calcns. using a van der Waals functional reveal that the monochalcogenides have formation energies similar to that of single-layer MoS2, implying the ease of mech. extg. single-layer monochalcogenides from their layered bulk counterparts. Next, calcns. using a hybrid d. functional and the quasiparticle many-body G0W0 approxn. det. the conduction and valence band edge positions. Comparing the band edge positions with the redox potentials of water shows that single-layer monochalcogenides are potential photocatalysts for water splitting. Moreover, the bandgaps, band edge positions, and optical absorption of the single-layer monochalcogenides can be tuned by biaxial strain to increase the efficiency of solar energy conversion. Finally, calcns. of the enthalpy of solvation of the single-layer monochalcogenides suggest their stability in aq. soln.
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    Stevanovic, V.; Lany, S.; Ginley, D. S.; Tumas, W.; Zunger, A. Assessing Capability of Semiconductors to Split Water Using Ionization Potentials and Electron Affinities Only Phys. Chem. Chem. Phys. 2014, 16, 3706– 3714 DOI: 10.1039/c3cp54589j
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    Assessing capability of semiconductors to split water using ionization potentials and electron affinities only
    Stevanovic, Vladan; Lany, Stephan; Ginley, David S.; Tumas, Willam; Zunger, Alex
    Physical Chemistry Chemical Physics (2014), 16 (8), 3706-3714CODEN: PPCPFQ; ISSN:1463-9076. (Royal Society of Chemistry)
    We show in this article that the position of semiconductor band edges relative to the water redn. and oxidn. levels can be reliably predicted from the ionization potentials (IP) and electron affinities (AE) only. Using a set of 17 materials, including transition metal compds., we show that accurate surface dependent IPs and EAs of semiconductors can be computed by combining d. functional theory and many-body GW calcns. From the extensive comparison of calcd. IPs and EAs with available exptl. data, both from photoemission and electrochem. measurements, we show that it is possible to sort candidate materials solely from IPs and EAs thereby eliminating explicit treatment of semiconductor/water interfaces. We find that at pH values corresponding to the point of zero charge there is on av. a 0.5 eV shift of IPs and EAs closer to the vacuum due to the dipoles formed at material/water interfaces.
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    Jiang, X.; Wang, P.; Zhao, J. J. 2d Covalent Triazine Framework: A New Class of Organic Photocatalyst for Water Splitting J. Mater. Chem. A 2015, 3, 7750– 7758 DOI: 10.1039/C4TA03438D
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    2D covalent triazine framework: a new class of organic photocatalyst for water splitting
    Jiang, Xue; Wang, Peng; Zhao, Jijun
    Journal of Materials Chemistry A: Materials for Energy and Sustainability (2015), 3 (15), 7750-7758CODEN: JMCAET; ISSN:2050-7496. (Royal Society of Chemistry)
    Since the graphene boom, great efforts have been devoted to two-dimensional (2D) monolayer materials with exciting possibilities for applications. Most known 2D materials are inorg. Using the covalent triazine framework (CTF) as a representative, we explored 2D org. semiconductors using first-principles calcns. From a systematic study of the electronic band structures, work functions, CBM/VBM positions, and optical absorption spectra, we identified the CTF as a new class of 2D visible-light-driven organocatalyst for water splitting. Controllable construction of such CTFs from suitable org. subunits paves the way to correlate band alignment and chem. composites. In addn., multilayer CTFs have enhanced visible-light absorption compared to monolayer CTFs due to interlayer coupling. Our theor. prediction not only has fulfilled the search for org. counterparts of inorg. photocatalysts for water splitting, but also would motivate scientists to further search for novel 2D org. materials with other technol. applications.
  45. 45
    Buckeridge, J.; Butler, K. T.; Catlow, C. R. A.; Logsdail, A. J.; Scanlon, D. O.; Shevlin, S. A.; Woodley, S. M.; Sokol, A. A.; Walsh, A. Polymorph Engineering of Tio2: Demonstrating How Absolute Reference Potentials Are Determined by Local Coordination Chem. Mater. 2015, 27, 3844– 3851 DOI: 10.1021/acs.chemmater.5b00230
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    Polymorph Engineering of TiO2: Demonstrating How Absolute Reference Potentials Are Determined by Local Coordination
    Buckeridge, John; Butler, Keith T.; Catlow, C. Richard A.; Logsdail, Andrew J.; Scanlon, David O.; Shevlin, Stephen A.; Woodley, Scott M.; Sokol, Alexey A.; Walsh, Aron
    Chemistry of Materials (2015), 27 (11), 3844-3851CODEN: CMATEX; ISSN:0897-4756. (American Chemical Society)
    We report that the valence and conduction band energies of TiO2 can be tuned over a 4 eV range by varying the local coordination environments of Ti and O. We examine the electronic structure of eight known polymorphs and align their ionization potential and electron affinity relative to an abs. energy ref., using an accurate multiscale quantum-chem. approach. For applications in photocatalysis, we identify the optimal combination of phases to enhance activity in the visible spectrum. The results provide a coherent explanation for a wide range of phenomena, including the performance of TiO2 as an anode material for Li-ion batteries, allow us to pinpoint hollandite TiO2 as a new candidate transparent conducting oxide, and serve as a guide to improving the efficiency of photo-electrochem. water splitting through polymorph engineering of TiO2.
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    Tomasi, J.; Mennucci, B.; Cammi, R. Quantum Mechanical Continuum Solvation Models Chem. Rev. 2005, 105, 2999– 3093 DOI: 10.1021/cr9904009
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    Quantum Mechanical Continuum Solvation Models
    Tomasi, Jacopo; Mennucci, Benedetta; Cammi, Roberto
    Chemical Reviews (Washington, DC, United States) (2005), 105 (8), 2999-3093CODEN: CHREAY; ISSN:0009-2665. (American Chemical Society)
    A review.
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    Baerends, E. J.; Gritsenko, O. V.; van Meer, R. The Kohn-Sham Gap, the Fundamental Gap and the Optical Gap: The Physical Meaning of Occupied and Virtual Kohn-Sham Orbital Energies Phys. Chem. Chem. Phys. 2013, 15, 16408– 16425 DOI: 10.1039/c3cp52547c
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    The Kohn-Sham gap, the fundamental gap and the optical gap: the physical meaning of occupied and virtual Kohn-Sham orbital energies
    Baerends, E. J.; Gritsenko, O. V.; van Meer, R.
    Physical Chemistry Chemical Physics (2013), 15 (39), 16408-16425CODEN: PPCPFQ; ISSN:1463-9076. (Royal Society of Chemistry)
    A no. of consequences of the presence of the exchange-correlation hole potential in the Kohn-Sham potential are elucidated. One consequence is that the HOMO-LUMO orbital energy difference in the KS-DFT model (the KS gap) is not "underestimated" or even "wrong", but that it is phys. expected to be an approxn. to the excitation energy if electrons and holes are close, and numerically proves to be so rather accurately. It is phys. not an approxn. to the difference between ionization energy and electron affinity I - A (fundamental gap or chem. hardness) and also numerically differs considerably from this quantity. The KS virtual orbitals do not possess the notorious diffuseness of the Hartree-Fock virtual orbitals, they often describe excited states much more closely as simple orbital transitions. The Hartree-Fock model does yield an approxn. to I - A as the HOMO-LUMO orbital energy difference (in Koopmans' frozen orbital approxn.), if the anion is bound, which is often not the case. We stress the spurious nature of HF LUMOs if the orbital energy is pos. One may prefer Hartree-Fock, or mix Hartree-Fock and (approx.) KS operators to obtain a HOMO-LUMO gap as a Koopmans' approxn. to I - A (in cases where A exists). That is a different one-electron model, which exists in its own right. But it is not an "improvement" of the KS model, it necessarily deteriorates the (approx.) excitation energy property of the KS gap in mols., and deteriorates the good shape of the KS virtual orbitals.
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    Seki, K.; Karlsson, U. O.; Engelhardt, R.; Koch, E. E.; Schmidt, W. Intramolecular Band Mapping of Poly(Para-Phenylene) Via Uv Photoelectron-Spectroscopy of Finite Polyphenyls Chem. Phys. 1984, 91, 459– 470 DOI: 10.1016/0301-0104(84)80078-6
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    Banerjee, M.; Shukla, R.; Rathore, R. Synthesis, Optical, and Electronic Properties of Soluble Poly-P-Phenylene Oligomers as Models for Molecular Wires J. Am. Chem. Soc. 2009, 131, 1780– 1786 DOI: 10.1021/ja805102d
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    Synthesis, Optical, and Electronic Properties of Soluble Poly-p-phenylene Oligomers as Models for Molecular Wires
    Banerjee, Moloy; Shukla, Ruchi; Rathore, Rajendra
    Journal of the American Chemical Society (2009), 131 (5), 1780-1786CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    A homologous series of sol. poly-p-phenylenes contg. up to eight phenylene moieties (PP2-PP8) with branched iso-alkyl (or bis-n-alkylmethyl) groups has been synthesized and the structure-property relationship with regards to various optical and electronic properties established. All electronic and optoelectronic properties of poly-p-phenylenes followed a 1/n relationship (where n is no. of phenylene moieties) with the increasing no. of phenylene moieties. The low-energy electronic transition of the PP2-PP7 cation radicals, generated either by laser-flash photolysis or by chem. oxidn., varied as well according to the inverse (1/n) relationship. The obsd. inverse relationship of all measured electronic and optoelectronic properties against the increasing no. of phenylene units in various PPn's, as well as X-ray crystallog. of both neutral and a cation-radical salt of a representative tetra-p-phenylene oligomer allows us to demonstrate that the effective conjugation length in poly-p-phenylenes is, in part, controlled by the increasing no. of interactions of ortho hydrogens which may prevent simultaneous planarization of the continuous arrays of a large no. of phenylene moieties.
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    Chi, C. Y.; Wegner, G. Chain-Length Dependence of the Electrochemical Properties of Conjugated Oligofluorenes Macromol. Rapid Commun. 2005, 26, 1532– 1537 DOI: 10.1002/marc.200500437
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    Chain-length dependence of the electrochemical properties of conjugated oligofluorenes
    Chi, Chunyan; Wegner, Gerhard
    Macromolecular Rapid Communications (2005), 26 (19), 1532-1537CODEN: MRCOE3; ISSN:1022-1336. (Wiley-VCH Verlag GmbH & Co. KGaA)
    Electrochem. properties of monodisperse oligofluorenes (OFn, n = 2 to 7) and corresponding polyfluorene were studied by cyclic and differential pulse voltammetry. In combination with data of UV-vis absorption spectra, a series of linear relations such as the band gap, the oxidn. potential, the ionization potential, and the electron affinity with the reciprocal no. of the fluorene units (1/n) were deduced. When a chain length of ca. 14 repeat units is reached, a stable structure of about one pos. charge per 3.5 repeat units is obtained.
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    Nagashima, U.; Fujimoto, H.; Inokuchi, H.; Seki, K. Electronic and Geometric Structures of Oligothiophenes J. Mol. Struct. 1989, 197, 265– 289 DOI: 10.1016/0022-2860(89)85169-5
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    Seki, K.; Asada, S.; Mori, T.; Inokuchi, H.; Murase, I.; Ohnishi, T.; Noguchi, T. Uv Photoemission Spectroscopy of Poly(P-Phenylene Vinylene) (PPV) Solid State Commun. 1990, 74, 677– 680 DOI: 10.1016/0038-1098(90)90700-L
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    Fujimoto, H.; Nagashima, U.; Inokuchi, H.; Seki, K.; Nakahara, N.; Nakayama, J.; Hoshino, M.; Fukuda, K. Electronic and Geometric Structures of Oligothiophenes Studied by Ups and Mndo - π-Band Evolution and Effect of Disorder Phys. Scr. 1990, 41, 105– 109 DOI: 10.1088/0031-8949/41/1/026
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    Electronic and geometric structures of oligothiophenes studied by UPS and MNDO: π-band evolution and effect of disorder
    Fujimoto, H.; Nagashima, U.; Inokuchi, H.; Seki, K.; Nakahara, N.; Nakayama, J.; Hoshino, M.; Fukuda, K.
    Physica Scripta (1990), 41 (1), 105-9CODEN: PHSTBO; ISSN:0031-8949.
    UPS was applied to the investigation of the electronic structure of oligothiophenes with 4-8 thiophene ring. In a series of α-linked oligomers (αn, with the no. of rings n = 4-8), a systematic evolution of the π band forming levels are obsd. in the region of 0.7-3 eV below the Fermi level (EF) and the bandwidth becomes broader with the n. The non-bonding π band is obsd. at 3.5 eV below EF and its energy is almost independent of N. UPS spectra of α7 and α8 are fairly similar to the spectra of polythiophene, showing that these oligomers are good model compds. of the polymer. The effect of irregularity on the π-electron system was also studied by using oligomers which contain a β linkage or a vinylene group in the middle of the mol. The UPS spectra showed that the β linkages significantly affect the electronic structure of polythiophene, but vinylene does not. In order to analyze the UPS spectra and to investigate the electronic structures of oligomers, the orbital energies and the geometry of these oligomers are calcd. by MNDO-SCF MO method. The calcd. spectra using the obtained orbital energies agree well with the obsd. ones, particularly in the π region. The optimized geometry showed that (1) αn's have planar structures and π electrons can be delocalized, (2) the oligomer with β-linkages have nonplanar structure leading to limited delocalization of π-electrons, and (3) to oligomers with a vinylene group are almost planar and the disturbance by the vinylene group on the delocalization is small.
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    Hino, S.; Iwasaki, K.; Tatematsu, H.; Matsumoto, K. Photoelectron-Spectra of Polypyrrole - the Effect of the Ambient Atmosphere to the Spectra Bull. Chem. Soc. Jpn. 1990, 63, 2199– 2205 DOI: 10.1246/bcsj.63.2199
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    Miyamae, T.; Yoshimura, D.; Ishii, H.; Ouchi, Y.; Seki, K.; Miyazaki, T.; Koike, T.; Yamamoto, T. Ultraviolet Photoelectron-Spectroscopy of Poly(Pyridine-2,5-Diyl), Poly(2,2′-Bipyridine-5,5′-Diyl), and Their K-Doped States J. Chem. Phys. 1995, 103, 2738– 2744 DOI: 10.1063/1.470508
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    Liao, L. S.; Fung, M. K.; Lee, C. S.; Lee, S. T.; Inbasekaran, M.; Woo, E. P.; Wu, W. W. Electronic Structure and Energy Band Gap of Poly (9,9-Dioctylfluorene) Investigated by Photoelectron Spectroscopy Appl. Phys. Lett. 2000, 76, 3582– 3584 DOI: 10.1063/1.126713
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    Electronic structure and energy band gap of poly (9,9-dioctylfluorene) investigated by photoelectron spectroscopy
    Liao, L. S.; Fung, M. K.; Lee, C. S.; Lee, S. T.; Inbasekaran, M.; Woo, E. P.; Wu, W. W.
    Applied Physics Letters (2000), 76 (24), 3582-3584CODEN: APPLAB; ISSN:0003-6951. (American Institute of Physics)
    The electronic structure of poly(9,9-dioctylfluorene) (PFO) film on a Au-coated Si substrate was investigated by UPS and XPS. From the UPS measurement, we obtained the ionization potential (Ip) of the PFO film, Ip = 5.60 ± 0.05 eV. From the XPS shake-up peaks of the C1s core level, we estd. the electron energy band gap (Eg) of the film, Eg = 3.10 ± 0.10 eV. By comparing the Eg with the optical absorption gap, we found that the value of Eg is closer to the optical absorption max. than to the optical absorption edge. Therefore, we suggest that the optical absorption max. may be a better approxn. than the optical absorption edge in estg. Eg.
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    Hwang, J.; Kim, E. G.; Liu, J.; Bredas, J. L.; Duggal, A.; Kahn, A. Photoelectron Spectroscopic Study of the Electronic Band Structure of Polyfluorene and Fluorene-Arylamine Copolymers at Interfaces J. Phys. Chem. C 2007, 111, 1378– 1384 DOI: 10.1021/jp067004w
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    Photoelectron Spectroscopic Study of the Electronic Band Structure of Polyfluorene and Fluorene-Arylamine Copolymers at Interfaces
    Hwang, Jaehyung; Kim, Eung-Gun; Liu, Jie; Bredas, Jean-Luc; Duggal, Anil; Kahn, Antoine
    Journal of Physical Chemistry C (2007), 111 (3), 1378-1384CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
    The occupied and unoccupied states of poly(9,9'-dioctylfluorene) (F8) and poly(9,9'-dioctylfluorene-co-bis-N,N'-(4-butylphenyl)diphenylamine) (TFB) are investigated using UV photoelectron and inverse photoemission spectroscopies, cyclic voltammetry, and d. functional theory calcns. Hole injection barriers are detd. for interfaces between substrates with work function ranging from 4.3 to 5.1 eV and these two polymers as well as poly(9,9'-dioctylfluorene-co-bis-N,N'-(4-butylphenyl)-bis-N,N'-phenyl-1,4-phenylenediamine) (PFB). Vacuum level alignment with flat bands away from the interface is found when the interface hole barrier is 0.6 eV or larger. Band bending away from the Fermi level occurs when the hole barrier is smaller than 0.4 eV. This is due to the accumulation of excess interface charges on the polymer when the barrier is small. The resulting field shifts the polymer levels to limit charge penetration in the bulk of the film.
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    Sohn, Y.; Stuckless, J. T. Bimolecular Recombination Kinetics and Interfacial Electronic Structures of Poly 2-Methoxy-5-(2-Ethyl-Hexyloxy)-P-Phenylene Vinylene on Gold Studied Using Two-Photon Photoemission Spectroscopy J. Chem. Phys. 2007, 126, 174901 DOI: 10.1063/1.2727473
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    Sohn, Y.; Stuckless, J. T. Characteristics of Photoexcitations and Interfacial Energy Levels of Regioregular Poly(3-Hexythiophene-2,5-Diyl) on Gold ChemPhysChem 2007, 8, 1937– 1942 DOI: 10.1002/cphc.200700348
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    Kanai, K.; Miyazaki, T.; Suzuki, H.; Inaba, M.; Ouchi, Y.; Seki, K. Effect of Annealing on the Electronic Structure of Poly(3-Hexylthiophene) Thin Film Phys. Chem. Chem. Phys. 2010, 12, 273– 282 DOI: 10.1039/B914100F
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    Effect of annealing on the electronic structure of poly(3-hexylthiophene) thin film
    Kanai, Kaname; Miyazaki, Takahiro; Suzuki, Hiroyuki; Inaba, Mina; Ouchi, Yukio; Seki, Kazuhiko
    Physical Chemistry Chemical Physics (2010), 12 (1), 273-282CODEN: PPCPFQ; ISSN:1463-9076. (Royal Society of Chemistry)
    The electronic structure and film structure of poly(3-hexylthiophene) (P3HT) have been studied by X-ray diffraction (XRD) measurements, UV-visible (UV-vis) absorption spectroscopy, near-edge X-ray absorption fine structure (NEXAFS) measurements, UV photoemission spectroscopy (UPS) and inverse photoemission spectroscopy (IPES). As reported in previous works, XRD results show that the crystallinity of the film with regioregular P3HT is significantly improved by annealing at 170 °C. The effects of annealing on the electronic structure strongly depend on the substrate and the degree of regioregularity of the P3HT polymer backbone. Even in the case of the regiorandom sample, annealing considerably changes the vacuum level energy, which is the result of changes in the conformation of the hexyl groups at the free surface of the film. The π- and π*-band onsets uniformly shift downward by the annealing resulting in an increased hole-injection barrier at the electrode interface. The effects of annealing on the electronic structure of regioregular samples are more complex and depend on multiple factors. It is necessary to take into account variations in the π- and π*-band widths and the polarization energy to det. the effects of annealing. The former is assocd. with the conformation of the backbones of the polymer chains, and the latter is assocd. with the packing d. of the conjugated polymer planes. The combination of these variations dets. the effects of annealing on the electronic structure of the regioregular film. This is a possible reason for the strong dependence of the effects of annealing on the surface roughness of the substrate, since substrate roughness has a considerable effect on the morphol. and crystallinity of regioregular P3HT films.
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    Baik, M. H.; Friesner, R. A. Computing Redox Potentials in Solution: Density Functional Theory as a Tool for Rational Design of Redox Agents J. Phys. Chem. A 2002, 106, 7407– 7412 DOI: 10.1021/jp025853n
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    Computing Redox Potentials in Solution: Density Functional Theory as A Tool for Rational Design of Redox Agents
    Baik, Mu-Hyun; Friesner, Richard A.
    Journal of Physical Chemistry A (2002), 106 (32), 7407-7412CODEN: JPCAFH; ISSN:1089-5639. (American Chemical Society)
    High-level d. functional theory in combination with a continuum solvation model was employed to compute std. redox potentials in soln. phase for three different classes of electrochem. active mols.: small org. mols., metallocenes, and M(bpy)3x (M = Fe, Ru, Os; x = +3, +2, +1, 0, -1). Excellent agreement with exptl. detd. redox potentials is found with an av. deviation of ∼150 mV when four different solvents commonly in use for electrochem. measurements were included. To obtain quant. agreement between theory and expt., the use of a large basis set is crucial esp. when the redox couple includes anionic species. Whereas the addn. of diffuse functions improved the results notably, vibrational zero-point-energy corrections and addn. of entropy effects are less important. The computational protocol for computing redox potentials in soln., which was benchmarked, is a powerful and novel tool that will allow a mol.-level understanding of the features dictating the properties of redox-active species.
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    Uudsemaa, M.; Tamm, T. Density-Functional Theory Calculations of Aqueous Redox Potentials of Fourth-Period Transition Metals J. Phys. Chem. A 2003, 107, 9997– 10003 DOI: 10.1021/jp0362741
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    Density-functional theory calculations of aqueous redox potentials of fourth-period transition metals
    Uudsemaa, Merle; Tamm, Toomas
    Journal of Physical Chemistry A (2003), 107 (46), 9997-10003CODEN: JPCAFH; ISSN:1089-5639. (American Chemical Society)
    Aq. M3+/M2+ redox potentials for 9 of the 10 4th-period transition metals, M, were calcd. using DFT methodol. in combination with the COSMO continuum model. Entropy contributions to the potentials are taken from expts. The model introduces no adjustable parameters beyond those present in the underlying theor. models. Inclusion of 2 solvation spheres (18 H2O mols.) is necessary. For the ions studied, the av. abs. difference from exptl. values is 0.29 V, with 4 out of 9 potentials (those of V, Cr, Fe, Cu) reproduced with better than 0.1 V accuracy.
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    Shimodaira, Y.; Miura, T.; Kudo, A.; Kobayashi, H. Dft Method Estimation of Standard Redox Potential of Metal Ions and Metal Complexes J. Chem. Theory Comput. 2007, 3, 789– 795 DOI: 10.1021/ct700015t
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    DFT Method Estimation of Standard Redox Potential of Metal Ions and Metal Complexes
    Shimodaira, Yoshiki; Miura, Toshiko; Kudo, Akihiko; Kobayashi, Hisayoshi
    Journal of Chemical Theory and Computation (2007), 3 (3), 789-795CODEN: JCTCCE; ISSN:1549-9618. (American Chemical Society)
    The DFT calcn. was carried out to evaluate std. redox potential (SRP) for metal-to-metal cation and metal-to-metal complex systems. With the Born-Haber cycle, std. redox potential was composed of the cohesive energy, ionization energy, and solvation energy. The ligand exchange energy was added in case of metal complex. The solvent effects were incorporated by the self-consistent reaction field theory at the level of the polarized continuum model (PCM). At the highest level of calcns., the geometry optimization and harmonic frequency anal. were evaluated under the PCM. Utilizing exptl. values of the cohesive energy of metals, the std. deviations between the calcd. and exptl. SRP values were 0.20-0.27 V depending on the calcn. levels and basis sets used. For three Ag complexes with CN-, S2O32-, and NH3 ligands, the discrepancy was within 0.3 V.
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    Roy, L. E.; Jakubikova, E.; Guthrie, M. G.; Batista, E. R. Calculation of One-Electron Redox Potentials Revisited. Is It Possible to Calculate Accurate Potentials with Density Functional Methods? J. Phys. Chem. A 2009, 113, 6745– 6750 DOI: 10.1021/jp811388w
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    Calculation of One-Electron Redox Potentials Revisited. Is It Possible to Calculate Accurate Potentials with Density Functional Methods?
    Roy, Lindsay E.; Jakubikova, Elena; Guthrie, M. Graham; Batista, Enrique R.
    Journal of Physical Chemistry A (2009), 113 (24), 6745-6750CODEN: JPCAFH; ISSN:1089-5639. (American Chemical Society)
    D. Functional calcns. have been performed to calc. the one-electron oxidn. potential for ferrocene and the redox couples for a series of small transition metal compds. of the first-, second-, and third-row elements. The solvation effects are incorporated via a self-consistent reaction field (SCRF), using the polarized continuum model (PCM). From our study of seven different d. functionals combined with three different basis sets for ferrocene, we find that no d. functional method can reproduce the redox trends from expt. when referencing our results to the exptl. abs. std. hydrogen electrode (SHE) potential. Including addnl. necessary assumptions such as solvation effects does not lead to any conclusion regarding the appropriate functional. However, we propose that if one refs. their transition metal compds. results to the calcd. abs. half-cell potential of ferrocene, they can circumvent the addnl. assumptions necessary to predict a redox couple. Upon employing this method on several organometallic and inorg. complexes, we obtained very good correlation between calcd. and exptl. values (R2 = 0.97), making it possible to predict trends with a high level of confidence. The hybrid functional B3LYP systematically underestimates the redox potential; however, the linear correlation between DFT and expt. is good (R2 = 0.96) when including a baseline shift. This protocol is a powerful tool that allows theor. chemists to predict the redox potential in soln. of several transition metal complexes a priori and aids in the rational design of redox-active catalysts.
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    Nayak, P. K.; Periasamy, N. Calculation of Ionization Potential of Amorphous Organic Thin-Films Using Solvation Model and DFT Org. Electron. 2009, 10, 532– 535 DOI: 10.1016/j.orgel.2009.01.011
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    Calculation of ionization potential of amorphous organic thin-films using solvation model and DFT
    Nayak, Pabitra K.; Periasamy, N.
    Organic Electronics (2009), 10 (3), 532-535CODEN: OERLAU; ISSN:1566-1199. (Elsevier B.V.)
    D. functional theory and polarizable continuum model are used to calc. ionization potential of thin-films of 12 org. mols. Computed values are compared with exptl. values obtained from UV photoemission spectroscopy. The excellent correlation shows that it is possible to det. the ionization potential of org. mols. in solid-state within ±0.15 eV of the exptl. value. This method is useful for chemists in designing mols. for org. electronics.
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    Nayak, P. K.; Periasamy, N. Calculation of Electron Affinity, Ionization Potential, Transport. Gap, Optical Band Gap and Exciton Binding Energy of Organic Solids Using ’Solvation’ Model and DFT Org. Electron. 2009, 10, 1396– 1400 DOI: 10.1016/j.orgel.2009.06.011
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    Calculation of electron affinity, ionization potential, transport gap, optical band gap and exciton binding energy of organic solids using solvation' model and DFT
    Nayak, Pabitra K.; Periasamy, N.
    Organic Electronics (2009), 10 (7), 1396-1400CODEN: OERLAU; ISSN:1566-1199. (Elsevier B.V.)
    Electron affinity of org. mols. in solids is calcd. using d. functional theory (DFT) and solvation' correction using polarizable continuum model. Computed values for 13 mols. show excellent correlation with exptl. values obtained from inverse photoemission spectroscopy. The same method was used to calc. ionization potential and transport gap of the 13 org. mols. in solid state. Optical band gap was calcd. for the 13 mols. using time-dependent DFT and polarizable continuum model, which in turn was used to calc. exciton binding energy of the mols. in solid state. Calcd. and exptl. values show good agreement that it is possible to det. the electron affinity, ionization potential, transport gap and optical band gap of org. mols. in solid state using mol. structure as the only input required for the calcn.
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    Schwenn, P. E.; Burn, P. L.; Powell, B. J. Calculation of Solid State Molecular Ionisation Energies and Electron Affinities for Organic Semiconductors Org. Electron. 2011, 12, 394– 403 DOI: 10.1016/j.orgel.2010.11.025
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    Calculation of solid state molecular ionisation energies and electron affinities for organic semiconductors
    Schwenn, P. E.; Burn, P. L.; Powell, B. J.
    Organic Electronics (2011), 12 (2), 394-403CODEN: OERLAU; ISSN:1566-1199. (Elsevier B.V.)
    Energy level engineering has become one of the central ideas in org. optoelectronics; particularly in the field of org. photovoltaics. However, this paradigm has led to significant ambiguities and errors in the literature. To investigate and address these issues we calc. solid state ionisation energies and electron affinities of a range of org. mol. semiconductors from d. functional theory and the polarizable continuum model. We show that the differences between these results and measurements from (inverse) photoemission are on the same scale as the differences between the measured values reported by different groups. We compare our results with in vacuo calcns. and ests. of the ionisation energies and electron affinities from the eigenvalues of the Kohn-Sham equations. In vacuo calcns. overestimate the ionisation energies measured in the solid state, but underestimate solid state electron affinities. However, the Kohn-Sham eigenvalues predict the measured ionisation energies nearly as well as the full calcn. However, we show that the apparent accuracy of the Kohn-Sham eigenvalues is fortuitous and arises from the cancellation of the errors due to the use of Kohn-Sham MO energies as predictions of ionisation energies and electron affinities and the neglect of the polarizable solid state environment. These results stress the importance of descriptions based on mol. states rather than MOs when designing and characterizing materials for org. electronic and optoelectronic devices.
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    Hughes, T. F.; Friesner, R. A. Development of Accurate Dft Methods for Computing Redox Potentials of Transition Metal Complexes: Results for Model Complexes and Application to Cytochrome P450 J. Chem. Theory Comput. 2012, 8, 442– 459 DOI: 10.1021/ct2006693
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    Development of Accurate DFT Methods for Computing Redox Potentials of Transition Metal Complexes: Results for Model Complexes and Application to Cytochrome P450
    Hughes, Thomas F.; Friesner, Richard A.
    Journal of Chemical Theory and Computation (2012), 8 (2), 442-459CODEN: JCTCCE; ISSN:1549-9618. (American Chemical Society)
    Single-electron redn. half potentials of 95 octahedral fourth-row transition metal complexes binding a diverse set of ligands have been calcd. at the unrestricted pseudospectral B3LYP/LACV3P level of theory in a continuum solvent. Through systematic comparison of exptl. and calcd. potentials, it is detd. that B3LYP strongly overbinds the d-manifold when the metal coordinates strongly interacting ligands and strongly underbinds the d-manifold when the metal coordinates weakly interacting ligands. These error patterns give rise to an extension of the localized orbital correction (LOC) scheme previously developed for org. mols. and which was recently extended to the spin-splitting properties of organometallic complexes. Mean unsigned errors in B3LYP redox potentials are reduced from 0.40 ± 0.20 V (0.88 V max. error) to 0.12 ± 0.09 V (0.34 V max. error) using a simple seven-parameter model. Although the focus of this article is on redox properties of transition metal complexes, we have found that applying our previous spin-splitting LOC model to an independent test set of oxidized and reduced complexes that are also spin-crossover complexes correctly reverses the ordering of spin states obtained with B3LYP. Interesting connections are made between redox and spin-splitting parameters with regard to the spectrochem. series and in their combined predictive power for properly closing the thermodn. cycle of d-electron transitions in a transition metal complex. Results obtained from our large and diverse databases of spin-splitting and redox properties suggest that, while the error introduced by single ref. B3LYP for simple multireference systems, like mononuclear transition metal complexes, remains significant, at around 2-5 kcal/mol, the dominant error, at around 10-20 kcal/mol, is in B3LYP's prediction of metal-ligand binding. Application of the LOC scheme to the rate-detg. hydrogen atom transfer step in substrate hydroxylation by cytochrome P 450 shows that this approach is able to correct the B3LYP barriers in comparison to recent kinetics expts.
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    Tamblyn, I.; Refaely-Abramson, S.; Neaton, J. B.; Kronik, L. Simultaneous Determination of Structures, Vibrations, and Frontier Orbital Energies from a Self-Consistent Range-Separated Hybrid Functional J. Phys. Chem. Lett. 2014, 5, 2734– 2741 DOI: 10.1021/jz5010939
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    Simultaneous Determination of Structures, Vibrations, and Frontier Orbital Energies from a Self-Consistent Range-Separated Hybrid Functional
    Tamblyn, Isaac; Refaely-Abramson, Sivan; Neaton, Jeffrey B.; Kronik, Leeor
    Journal of Physical Chemistry Letters (2014), 5 (15), 2734-2741CODEN: JPCLCD; ISSN:1948-7185. (American Chemical Society)
    A self-consistent optimally tuned range-sepd. hybrid d. functional (scOT-RSH) approach is developed. It can simultaneously predict accurate geometries, vibrational modes, and frontier orbital energies. This is achieved by optimizing the range-sepn. parameter, γ, to both satisfy the ionization energy theorem and minimize interat. forces. We benchmark our approach against an established hybrid functional, B3LYP, using the G2 test set. scOT-RSH greatly improves the accuracy of occupied frontier orbital energies, with a mean abs. error (MAE) of only 0.2 eV relative to exptl. ionization energies compared to 2.96 eV with B3LYP. Geometries do not change significantly compared to those obtained from B3LYP, with a bond length MAE of 0.012 Å compared to 0.008 Å for B3LYP, and a 6.5% MAE for zero-point energies, slightly larger than that of B3LYP (3.1%). scOT-RSH represents a new paradigm in which accurate geometries and ionization energies can be predicted simultaneously from a single functional approach.
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    Phillips, H.; Zheng, Z. L.; Geva, E.; Dunietz, B. D. Orbital Gap Predictions for Rational Design of Organic Photovoltaic Materials Org. Electron. 2014, 15, 1509– 1520 DOI: 10.1016/j.orgel.2014.03.040
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    Orbital gap predictions for rational design of organic photovoltaic materials
    Phillips, Heidi; Zheng, Zilong; Geva, Eitan; Dunietz, Barry D.
    Organic Electronics (2014), 15 (7), 1509-1520CODEN: OERLAU; ISSN:1566-1199. (Elsevier B.V.)
    Ionization potentials (IP) and electron affinities (EA) of org. mols. with applications in photovoltaic devices are calcd. using modern d. functional theory (DFT). Calcd. frontier orbital energies are compared to exptl. detd. IPs and EAs at gas phase and thin film environments. Gas phase frontier orbital energies calcd. with widely-used DFT functionals accidentally coincide with thin film measurements, reproducing condensed phase results for the wrong reasons. Recently developed range sepd. hybrid (RSH) functionals, on the other hand, provide gas phase frontier orbital energies that correspond properly to measured IPs and EAs. We also employ a polarizable continuum model to address the effects of the electrostatic environment in the solid state. We find that the environmentally-cor. RSH orbital energies compare well with thin film exptl. measurements.
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    Richard, R. M.; Marshall, M. S.; Dolgounitcheva, O.; Ortiz, J. V.; Bredas, J. L.; Marom, N.; Sherrill, C. D. Accurate Ionization Potentials and Electron Affinities of Acceptor Molecules I. Reference Data at the CCSD(T) Complete Basis Set Limit J. Chem. Theory Comput. 2016, 12, 595– 604 DOI: 10.1021/acs.jctc.5b00875
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    Accurate Ionization Potentials and Electron Affinities of Acceptor Molecules I. Reference Data at the CCSD(T) Complete Basis Set Limit
    Richard, Ryan M.; Marshall, Michael S.; Dolgounitcheva, O.; Ortiz, J. V.; Bredas, Jean-Luc; Marom, Noa; Sherrill, C. David
    Journal of Chemical Theory and Computation (2016), 12 (2), 595-604CODEN: JCTCCE; ISSN:1549-9618. (American Chemical Society)
    In designing org. materials for electronics applications, particularly for org. photovoltaics (OPV), the ionization potential (IP) of the donor and the electron affinity (EA) of the acceptor play key roles. This makes OPV design an appealing application for computational chem. since IPs and EAs are readily calculable from most electronic structure methods. Unfortunately reliable, high-accuracy wave function methods, such as coupled cluster theory with single, double, and perturbative triples [CCSD(T)] in the complete basis set (CBS) limit are too expensive for routine applications to this problem for any but the smallest of systems. One soln. is to calibrate approx., less computationally expensive methods against a database of high-accuracy IP/EA values; however, to our knowledge, no such database exists for systems related to OPV design. The present work is the first of a multipart study whose overarching goal is to det. which computational methods can be used to reliably compute IPs and EAs of electron acceptors. This part introduces a database of 24 known org. electron acceptors and provides high-accuracy vertical IP and EA values expected to be within ±0.03 eV of the true non-relativistic, vertical CCSD(T)/CBS limit. Convergence of IP and EA values toward the CBS limit is studied systematically for the Hartree-Fock, MP2 correlation, and beyond-MP2 coupled cluster contributions to the focal point ests.
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    Gallandi, L.; Marom, N.; Rinke, P.; Korzdorfer, T. Accurate Ionization Potentials and Electron Affinities of Acceptor Molecules II: Non-Empirically Tuned Long-Range Corrected Hybrid Functionals J. Chem. Theory Comput. 2016, 12, 605– 614 DOI: 10.1021/acs.jctc.5b00873
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    Accurate Ionization Potentials and Electron Affinities of Acceptor Molecules II: Non-Empirically Tuned Long-Range Corrected Hybrid Functionals
    Gallandi, Lukas; Marom, Noa; Rinke, Patrick; Koerzdoerfer, Thomas
    Journal of Chemical Theory and Computation (2016), 12 (2), 605-614CODEN: JCTCCE; ISSN:1549-9618. (American Chemical Society)
    The performance of non-empirically tuned long-range cor. hybrid functionals for the prediction of vertical ionization potentials (IPs) and electron affinities (EAs) is assessed for a set of 24 org. acceptor mols. Basis set-extrapolated coupled cluster singles, doubles, and perturbative triples [CCSD(T)] calcns. serve as a ref. for this study. Compared to std. exchange-correlation functionals, tuned long-range cor. hybrid functionals produce highly reliable results for vertical IPs and EAs, yielding mean abs. errors on par with computationally more demanding GW calcns. In particular, it is demonstrated that long-range cor. hybrid functionals serve as ideal starting points for non-self-consistent GW calcns.
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    Rangel, T.; Hamed, S. M.; Bruneval, F.; Neaton, J. B. Evaluating the Gw Approximation with CCSD(T) for Charged Excitations across the Oligoacenes J. Chem. Theory Comput. 2016, 12, 2834– 2842 DOI: 10.1021/acs.jctc.6b00163
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    Evaluating the GW Approximation with CCSD(T) for Charged Excitations Across the Oligoacenes
    Rangel, Tonatiuh; Hamed, Samia M.; Bruneval, Fabien; Neaton, Jeffrey B.
    Journal of Chemical Theory and Computation (2016), 12 (6), 2834-2842CODEN: JCTCCE; ISSN:1549-9618. (American Chemical Society)
    Charged excitations of the oligoacene family of mols., relevant for astrophysics and technol. applications, are widely studied and therefore provide an excellent system for benchmarking theor. methods. In this work, we evaluate the performance of many-body perturbation theory within the GW approxn. relative to new high-quality CCSD(T) ref. data for charged excitations of the acenes. We compare GW calcns. with a no. of hybrid d. functional theory starting points and with eigenvalue self-consistency. Special focus is given to elucidating the trend of GW-predicted excitations with mol. length increasing from benzene to hexacene. We find that GW calcns. with starting points based on an optimally tuned range-sepd. hybrid (OTRSH) d. functional and eigenvalue self-consistency can yield quant. ionization potentials for the acenes. However, for larger acenes, the predicted electron affinities can deviate considerably from ref. values. Our work paves the way for predictive and cost-effective GW calcns. of charged excitations of mols. and identifies certain limitations of current GW methods used in practice for larger mols.
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    Density-functional thermochemistry. III. The role of exact exchange
    Becke, Axel D.
    Journal of Chemical Physics (1993), 98 (7), 5648-52CODEN: JCPSA6; ISSN:0021-9606.
    Despite the remarkable thermochem. accuracy of Kohn-Sham d.-functional theories with gradient corrections for exchange-correlation, the author believes that further improvements are unlikely unless exact-exchange information is considered. Arguments to support this view are presented, and a semiempirical exchange-correlation functional (contg. local-spin-d., gradient, and exact-exchange terms) is tested for 56 atomization energies, 42 ionization potentials, 8 proton affinities, and 10 total at. energies of first- and second-row systems. This functional performs better than previous functionals with gradient corrections only, and fits expt. atomization energies with an impressively small av. abs. deviation of 2.4 kcal/mol.
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    Stephens, P. J.; Devlin, F. J.; Chabalowski, C. F.; Frisch, M. J. Ab-Initio Calculation of Vibrational Absorption and Circular-Dichroism Spectra Using Density-Functional Force-Fields J. Phys. Chem. 1994, 98, 11623– 11627 DOI: 10.1021/j100096a001
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    Ab Initio Calculation of Vibrational Absorption and Circular Dichroism Spectra Using Density Functional Force Fields
    Stephens, P. J.; Devlin, F. J.; Chabalowski, C. F.; Frisch, M. J.
    Journal of Physical Chemistry (1994), 98 (45), 11623-7CODEN: JPCHAX; ISSN:0022-3654.
    The unpolarized absorption and CD spectra of the fundamental vibrational transitions of the chiral mol. 4-methyl-2-oxetanone are calcd. ab initio. Harmonic force fields are obtained using d. functional theory (DFT), MP2 and SCF methodologies, and a [5s4p2d/3s2p] (TZ2P) basis set. DFT calcns. use the LSDA, BLYP, and Becke3LYP (B3LYP) d. functionals. Mid-IR spectra predicted using LSDA, BLYP, and B3LYP force fields are of significantly different quality, the B3LYP force field yielding spectra in clearly superior, and overall excellent, agreement with expt. The MP2 force field yields spectra in slightly worse agreement with expt. than the B3LYP force field. The SCF force field yields spectra in poor agreement with expt. The basis set dependence of B3LYP force fields is also explored: the 6-31G* and TZ2P basis sets give very similar results while the 3-21G basis set yields spectra in substantially worse agreement with expt.
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    Klamt, A.; Schuurmann, G. Cosmo - a New Approach to Dielectric Screening in Solvents with Explicit Expressions for the Screening Energy and Its Gradient J. Chem. Soc., Perkin Trans. 2 1993, 799– 805 DOI: 10.1039/P29930000799
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    COSMO: a new approach to dielectric screening in solvents with explicit expressions for the screening energy and its gradient
    Klamt, A.; Schueuermann, G.
    Journal of the Chemical Society, Perkin Transactions 2: Physical Organic Chemistry (1972-1999) (1993), (5), 799-805CODEN: JCPKBH; ISSN:0300-9580.
    Starting from the screening in conductors, an algorithm for the accurate calcn. of dielec. screening effects in solvents is presented, which leads to rather simple explicit expressions for the screening energy and its analytic gradient with respect to the solute coordinates. Thus geometry optimization of a solute within a realistic dielec. continuum model becomes practicable for the first time. The algorithm is suited for mol. mechanics as well as for any MO algorithm. The implementation into MOPAC and some example applications are reported.
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    Hirata, S.; Head-Gordon, M. Time-Dependent Density Functional Theory within the Tamm-Dancoff Approximation Chem. Phys. Lett. 1999, 314, 291– 299 DOI: 10.1016/S0009-2614(99)01149-5
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    Time-dependent density functional theory within the Tamm-Dancoff approximation
    Hirata, S.; Head-Gordon, M.
    Chemical Physics Letters (1999), 314 (3,4), 291-299CODEN: CHPLBC; ISSN:0009-2614. (Elsevier Science B.V.)
    A computationally simple method for mol. excited states, namely, the Tamm-Dancoff approxn. to time-dependent d. functional theory, is proposed and implemented. This method yields excitation energies for several closed- and open-shell mols. that are essentially of the same quality as those obtained from time-dependent d. functional theory itself, when the same exchange-correlation functional is used.
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    Furche, F.; Ahlrichs, R.; Hattig, C.; Klopper, W.; Sierka, M.; Weigend, F. Turbomole Wiley Interdiscip. Rev. Comput. Mol. Sci. 2014, 4, 91– 100 DOI: 10.1002/wcms.1162
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    Turbomole
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    Wiley Interdisciplinary Reviews: Computational Molecular Science (2014), 4 (2), 91-100CODEN: WIRCAH; ISSN:1759-0884. (Wiley-Blackwell)
    Turbomole is a highly optimized software package for large-scale quantum chem. simulations of mols., clusters, and periodic solids. Turbomole uses Gaussian basis sets and specializes on predictive electronic structure methods with excellent cost to performance characteristics, such as (time-dependent) d. functional theory (TDDFT), second-order Moller-Plesset theory, and explicitly correlated coupled cluster (CC) methods. These methods are combined with ultraefficient and numerically stable algorithms such as integral-direct and Laplace transform methods, resoln.-of-the-identity, pair natural orbitals, fast multipole, and low-order scaling techniques. Apart from energies and structures, a variety of optical, elec., and magnetic properties are accessible from anal. energy derivs. for electronic ground and excited states. Recent addns. include post-Kohn-Sham calcns. within the RPA, periodic calcns., spin-orbit couplings, explicitly correlated CC singles doubles and perturbative triples methods, CC singles doubles excitation energies, and nonadiabatic mol. dynamics simulations using TDDFT. A dedicated graphical user interface and a user support network are also available.
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    Ahlrichs, R.; Bar, M.; Haser, M.; Horn, H.; Kolmel, C. Electronic-Structure Calculations on Workstation Computers - the Program System Turbomole Chem. Phys. Lett. 1989, 162, 165– 169 DOI: 10.1016/0009-2614(89)85118-8
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    Electronic structure calculations on workstation computers: the program system TURBOMOLE
    Ahlrichs, Reinhart; Baer, Michael; Haeser, Marco; Horn, Hans; Koelmel, Christoph
    Chemical Physics Letters (1989), 162 (3), 165-9CODEN: CHPLBC; ISSN:0009-2614.
    The basic structure of the program system TURBOMOLE for SCF - including first and second anal. derivs. with respect to nuclear coordinates - and MP2 calcns. is briefly described. The program takes full advantage of all discrete point group symmetries and has only modest - and (partially) adjustable - I/O and background storage requirements. The performance of TURBOMOLE is documented for demonstrative applications.
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    Schafer, A.; Horn, H.; Ahlrichs, R. Fully Optimized Contracted Gaussian-Basis Sets for Atoms Li to Kr J. Chem. Phys. 1992, 97, 2571– 2577 DOI: 10.1063/1.463096
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    Isse, A. A.; Gennaro, A. Absolute Potential of the Standard Hydrogen Electrode and the Problem of Interconversion of Potentials in Different Solvents J. Phys. Chem. B 2010, 114, 7894– 7899 DOI: 10.1021/jp100402x
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    Absolute Potential of the Standard Hydrogen Electrode and the Problem of Interconversion of Potentials in Different Solvents
    Isse, Abdirisak A.; Gennaro, Armando
    Journal of Physical Chemistry B (2010), 114 (23), 7894-7899CODEN: JPCBFK; ISSN:1520-6106. (American Chemical Society)
    The abs. potential of the std. hydrogen electrode, SHE, was calcd. from a thermodn. cycle involving H2(g) atomization, ionization of H·(g) to H+(g)+, and hydration of H+. The most up-to-date literature values on the free energies of these reactions were selected and, when necessary, adjusted to the electron convention Fermi-Dirac statistics since both e- and H+ are fermions. As a ref. state for the electron, the authors have chosen the electron at 0 K, which is the one used in computational chem. Unlike almost all previous estns. of SHE, ΔG0aq-(H+) was used instead of the real potential, αaq(H+). This choice was made to obtain a SHE value based on the chem. potential, which is the appropriate ref. to be used in theor. computations of std. redn. potentials. The result of this new estn. is a value of 4.281 V for the abs. potential of SHE. The problem of conversion of std. redn. potentials (SRPs) measured or estd. in H2O to the corresponding values in nonaqeuous solvents also was addressed. In fact, thermochem. cycles are often used to calc. SRPs in H2O vs. SHE, and it is extremely important to have conversion factors enabling estn. of SRPs in nonaq. solvents. A general equation relating E0 of a generic redox couple in H2O vs. the SHE to the value of E0 in an org. solvent vs. the aq. SCE is reported.
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    Trasatti, S. The Absolute Electrode Potential - an Explanatory Note (Recommendations 1986) Pure Appl. Chem. 1986, 58, 955– 966 DOI: 10.1351/pac198658070955
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    The absolute electrode potential: an explanatory note. Recommendations 1986
    Trasatti, Sergio
    Pure and Applied Chemistry (1986), 58 (7), 955-66CODEN: PACHAS; ISSN:0033-4545.
    An illustration is given of the most widespread misunderstandings in the literature about the phys. meaning of abs. electrode potential. The correct expression for this quantity is then derived by a thermodn. anal. of the components of the emf. of an electrochem. cell. In principle 3 ref. levels can be chosen to measure an abs. value of the electrode potential. Only 1 of these possess all the requisites for a meaningful comparison on a common energy scale between electrochem. and phys. parameters. Such a comparison is the main problem for which the adoption of a correct scale for abs. electrode potentials is a prerequisite. The document concludes with the recommendation of a crit. evaluated value for the abs. potential of the std. H electrode in H2O and in a few other protic solvents.
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    Sato, N.; Seki, K.; Inokuchi, H. Polarization Energies of Organic-Solids Determined by Ultraviolet Photoelectron-Spectroscopy J. Chem. Soc., Faraday Trans. 2 1981, 77, 1621– 1633 DOI: 10.1039/f29817701621
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    Polarization energies of organic solids determined by ultraviolet photoelectron spectroscopy
    Sato, Naoki; Seki, Kazuhiko; Inokuchi, Hiroo
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    The polarization energies of 44 org. solids were detd. by UPS in the gaseous and solid states. Condensed polycyclic arom. hydrocarbons with planar mol. structures have a common value, 1.7eV, independent of their mol. sizes and crystal structures. The common value is approx. interpreted by the 1st-order expression for the polarization energy. A large variation of values in the range 0.9-3.0 eV was obtained for several compds. Among them, mols. with intricate structures have smaller values and those with large mol. polarizabilities have larger values than the common value. The polarization energy of an org. solid is mainly detd. by 2 factors, its mol. polarizability and the mol. packing in the solid. Intermol. interactions in the solid, other than the van der Waals force, also contribute to the value.
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    Inokuchi, H.; Seki, K.; Sato, N. Uv Photoelectron-Spectroscopy of Organic Molecular Materials Phys. Scr. 1987, T17, 93– 103 DOI: 10.1088/0031-8949/1987/T17/011
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    UV photoelectron spectroscopy of organic molecular materials
    Inokuchi, Hiroo; Seki, Kazuhiko; Sato, Naoki
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    A review with 101 refs., on the recent progress in UPS studies of org. mol. materials taking the examples mainly from the results obtained in the authors' lab. Mol solids with and without strong intermol. interactions, mol complexes and polymers are discussed.
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    Tsiper, E. V.; Soos, Z. G.; Gao, W.; Kahn, A. Electronic Polarization at Surfaces and Thin Films of Organic Molecular Crystals: Ptcda Chem. Phys. Lett. 2002, 360, 47– 52 DOI: 10.1016/S0009-2614(02)00774-1
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    Electronic polarization at surfaces and thin films of organic molecular crystals: PTCDA
    Tsiper, E. V.; Soos, Z. G.; Gao, W.; Kahn, A.
    Chemical Physics Letters (2002), 360 (1,2), 47-52CODEN: CHPLBC; ISSN:0009-2614. (Elsevier Science B.V.)
    The electronic polarization energies, P=P++P-, of a perylenetetracarboxylic acid dianhydride (PTCDA) cation and anion in a cryst. thin film on a metallic substrate are computed and compared with measurements of the PTCDA transport gap on gold and silver. Both expts. and theory show that P is 500 meV larger in a PTCDA monolayer than in 50 Å films. Electronic polarization in systems with surfaces and interfaces are obtained self-consistently in terms of charge redistribution within mols.
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    Ryno, S. M.; Risko, C.; Bredas, J. L. Impact of Molecular Packing on Electronic Polarization in Organic Crystals: The Case of Pentacene Vs Tips-Pentacene J. Am. Chem. Soc. 2014, 136, 6421– 6427 DOI: 10.1021/ja501725s
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    Impact of Molecular Packing on Electronic Polarization in Organic Crystals: The Case of Pentacene vs TIPS-Pentacene
    Ryno, Sean M.; Risko, Chad; Bredas, Jean-Luc
    Journal of the American Chemical Society (2014), 136 (17), 6421-6427CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    Polarization energy corresponds to the stabilization of the cation or anion state of an atom or mol. when going from the gas phase to the solid state. The decrease in ionization energy and increase in electron affinity in the solid state are related to the (electronic and nuclear) polarization of the surrounding atoms and mols. in the presence of a charged entity. Here, through a combination of mol. mechanics and quantum mechanics calcns., the authors evaluate the polarization energies in two prototypical org. semiconductors, pentacene and 6,13-bis[2-(triisopropylsilyl)ethynyl]pentacene (TIPS-pentacene). Comparison of the results for the two systems reveals the crit. role played by the mol. packing configurations in the detn. of the polarization energies and provides phys. insight into the exptl. data reported by Lichtenberger and co-workers (J. Amer. Chem. Soc.2010, 132, 580; J. Phys. Chem. C2010, 114, 13838). Results underline that the impact of packing configurations, well established in the case of the charge-transport properties, also extends to the polarization properties of π-conjugated materials.
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    Stein, Tamar; Eisenberg, Helen; Kronik, Leeor; Baer, Roi
    Physical Review Letters (2010), 105 (26), 266802/1-266802/4CODEN: PRLTAO; ISSN:0031-9007. (American Physical Society)
    We present a broadly applicable, phys. motivated, first-principles approach to detg. the fundamental gap of finite systems from single-electron orbital energies. The approach is based on using a range-sepd. hybrid functional within the generalized Kohn-Sham approach to d. functional theory. Its key element is the choice of a range-sepn. parameter such that Koopmans' theorem for both neutral and anion is obeyed as closely as possible. We demonstrate the validity, accuracy, and advantages of this approach on first, second and third row atoms, the oligoacene family of mols., and a set of hydrogen-passivated silicon nanocrystals. This extends the quant. usage of d. functional theory to an area long believed to be outside its reach.
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    Refaely-Abramson, S.; Sharifzadeh, S.; Jain, M.; Baer, R.; Neaton, J. B.; Kronik, L. Gap Renormalization of Molecular Crystals from Density-Functional Theory Phys. Rev. B: Condens. Matter Mater. Phys. 2013, 88, 081204 DOI: 10.1103/PhysRevB.88.081204
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    Gap renormalization of molecular crystals from density-functional theory
    Refaely-Abramson, Sivan; Sharifzadeh, Sahar; Jain, Manish; Baer, Roi; Neaton, Jeffrey B.; Kronik, Leeor
    Physical Review B: Condensed Matter and Materials Physics (2013), 88 (8), 081204/1-081204/5CODEN: PRBMDO; ISSN:1098-0121. (American Physical Society)
    Fundamental gap renormalization due to electronic polarization is a basic phenomenon in mol. crystals. Despite its ubiquity and importance, all conventional approaches within d.-functional theory completely fail to capture it, even qual. Here, we present a new screened range-sepd. hybrid functional, which, through judicious introduction of the scalar dielec. const., quant. captures polarization-induced gap renormalization, as demonstrated on the prototypical org. mol. crystals of benzene, pentacene, and C60. This functional is predictive, as it contains system-specific adjustable parameters that are detd. from first principles, rather than from empirical considerations.
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    Few, S.; Frost, J. M.; Nelson, J. Models of Charge Pair Generation in Organic Solar Cells Phys. Chem. Chem. Phys. 2015, 17, 2311– 2325 DOI: 10.1039/C4CP03663H
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    Models of charge pair generation in organic solar cells
    Few, Sheridan; Frost, Jarvist M.; Nelson, Jenny
    Physical Chemistry Chemical Physics (2015), 17 (4), 2311-2325CODEN: PPCPFQ; ISSN:1463-9076. (Royal Society of Chemistry)
    A review. Efficient charge pair generation is obsd. in many org. photovoltaic (OPV) heterojunctions, despite nominal electron-hole binding energies which greatly exceed the av. thermal energy. Empirically, the efficiency of this process appears to be related to the choice of donor and acceptor materials, the resulting sequence of excited state energy levels and the structure of the interface. In order to establish a suitable phys. model for the process, a range of different theor. studies have addressed the nature and energies of the interfacial states, the energetic profile close to the heterojunction and the dynamics of excited state transitions. In this paper, we review recent developments underpinning the theory of charge pair generation and phenomena, focussing on electronic structure calcns., electrostatic models and approaches to excited state dynamics. We discuss the remaining challenges in achieving a predictive approach to charge generation efficiency.

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Published December 22, 2016

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  • Abstract

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    Figure 1

    Figure 1. Scheme illustrating how, in the case of water splitting, the redox potentials associated with charge carriers (IP and EA) and excitons (EA* and IP*) of a photocatalyst must straddle the proton reduction and water oxidation potentials (blue and red broken lines, respectively) for both processes to be thermodynamically favorable. The vertical axis on the right of the figure shows the different possible alignments with vacuum, the result of the different experimental SHEAP values.

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    Figure 2

    Figure 2. Structures of oligomer models studied computationally.

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    Figure 3

    Figure 3. Comparison between the potentials predicted using (TD-)B3LYP and εr 2, thick lines, and measured experimentally, thin lines, for a range of conjugated polymers.

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    Figure 4

    Figure 4. Structures of the polymers studied; the n used in the calculations was 12 for all structures except PF, where an n of 6 was used instead.

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    Figure 5

    Figure 5. Comparison between IP and −HOMO, red open squares, and EA and −LUMO, open blue triangles, calculated with B3LYP and εr 2.

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  • References

    This article references 90 other publications.

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      Schwab, Matthias Georg; Hamburger, Manuel; Feng, Xinliang; Shu, Jie; Spiess, Hans Wolfgang; Wang, Xinchen; Antonietti, Markus; Muellen, Klaus
      Chemical Communications (Cambridge, United Kingdom) (2010), 46 (47), 8932-8934CODEN: CHCOFS; ISSN:1359-7345. (Royal Society of Chemistry)
      Three-dimensional conjugated poly(azomethine) networks were found to be promising candidates for applications in photocatalytic water splitting. Straightforward synthetic protocols lead to fully org. photocatalysts that showed enhanced long-time stability. Furthermore, the catalytic performance of these materials was correlated to the mol. compn. and the optoelectronic properties of the samples.
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      Zhang, Z. Z.; Long, J. L.; Yang, L. F.; Chen, W. K.; Dai, W. X.; Fu, X. Z.; Wang, X. X. Organic Semiconductor for Artificial Photosynthesis: Water Splitting into Hydrogen by a Bioinspired C3N3S3 Polymer under Visible Light Irradiation Chem. Sci. 2011, 2, 1826– 1830 DOI: 10.1039/C1SC00257K
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      Organic semiconductor for artificial photosynthesis: water splitting into hydrogen by a bioinspired C3N3S3 polymer under visible light irradiation
      Zhang, Zizhong; Long, Jinlin; Yang, Lifang; Chen, Wenkai; Dai, Wenxin; Fu, Xianzhi; Wang, Xuxu
      Chemical Science (2011), 2 (9), 1826-1830CODEN: CSHCCN; ISSN:2041-6520. (Royal Society of Chemistry)
      A novel org. semiconductor photocatalyst mimicking natural light-harvesting antenna complexes in photosynthetic organisms, a disulfide (-S-S-) bridged C3N3S3 polymer, was designed and developed to generate hydrogen from water under visible light irradn. The artificial conjugated polymer shows high H2-producing activity from the half-reaction of water splitting without the aid of a sacrificial electron donor. The H2-producing efficiency and photo-stability of the catalyst could be improved greatly using Ru and single-wall carbon nanotubes as cocatalysts or by adding a sacrificial donor. The results represent a potential and prospective application of the C3N3S3 polymer in solar energy conversion and offer significant guidance to develop more stable and efficient photocatalytic systems based on org. semiconductors.
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      Chu, S.; Wang, Y.; Guo, Y.; Zhou, P.; Yu, H.; Luo, L. L.; Kong, F.; Zou, Z. G. Facile Green Synthesis of Crystalline Polyimide Photocatalyst for Hydrogen Generation from Water J. Mater. Chem. 2012, 22, 15519– 15521 DOI: 10.1039/c2jm32595k
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      Facile green synthesis of crystalline polyimide photocatalyst for hydrogen generation from water
      Chu, Sheng; Wang, Ying; Guo, Yong; Zhou, Peng; Yu, He; Luo, Leilei; Kong, Fei; Zou, Zhigang
      Journal of Materials Chemistry (2012), 22 (31), 15519-15521CODEN: JMACEP; ISSN:0959-9428. (Royal Society of Chemistry)
      A simple green process based on thermal condensation is developed to synthesize polyimide. The obtained product possesses a high degree of crystallinity and exhibits efficient photocatalytic activity for hydrogen prodn. from water under visible light.
    8. 8
      Sprick, R. S.; Jiang, J. X.; Bonillo, B.; Ren, S. J.; Ratvijitvech, T.; Guiglion, P.; Zwijnenburg, M. A.; Adams, D. J.; Cooper, A. I. Tunable Organic Photocatalysts for Visible-Light-Driven Hydrogen Evolution J. Am. Chem. Soc. 2015, 137, 3265– 3270 DOI: 10.1021/ja511552k
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      Tunable organic photocatalysts for visible-light-driven hydrogen evolution
      Sprick, Reiner Sebastian; Jiang, Jia-Xing; Bonillo, Baltasar; Ren, Shijie; Ratvijitvech, Thanchanok; Guiglion, Pierre; Zwijnenburg, Martijn A.; Adams, Dave J.; Cooper, Andrew I.
      Journal of the American Chemical Society (2015), 137 (9), 3265-3270CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
      Photocatalytic hydrogen prodn. from water offers an abundant, clean fuel source, but it is challenging to produce photocatalysts that use the solar spectrum effectively. Many hydrogen-evolving photocatalysts are active in the UV range, but UV light accounts for only 3% of the energy available in the solar spectrum at ground level. Solid-state cryst. photocatalysts have light absorption profiles that are a discrete function of their cryst. phase and that are not always tunable. Here, we prep. a series of amorphous, microporous org. polymers with exquisite synthetic control over the optical gap in the range 1.94-2.95 eV. Specific monomer compns. give polymers that are robust and effective photocatalysts for the evolution of hydrogen from water in the presence of a sacrificial electron donor, without the apparent need for an added metal cocatalyst. Remarkably, unlike other org. systems, the best performing polymer is only photoactive under visible rather than UV irradn.
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      Bornoz, P.; Prevot, M. S.; Yu, X. Y.; Guijarro, N.; Sivula, K. Direct Light-Driven Water Oxidation by a Ladder-Type Conjugated Polymer Photoanode J. Am. Chem. Soc. 2015, 137, 15338– 15341 DOI: 10.1021/jacs.5b05724
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      Direct Light-Driven Water Oxidation by a Ladder-Type Conjugated Polymer Photoanode
      Bornoz, Pauline; Prevot, Mathieu S.; Yu, Xiaoyun; Guijarro, Nestor; Sivula, Kevin
      Journal of the American Chemical Society (2015), 137 (49), 15338-15341CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
      A conjugated polymer known for high stability, poly[benzimidazobenzophenanthroline], BBL, is examd. as a photoanode for direct solar H2O oxidn. In aq. electrolyte with a sacrificial hole acceptor (SO32-), photoelectrodes show a morphol.-dependent performance. Films prepd. by a dispersion-spray method with a nanostructured surface (feature size of ∼20 nm) gave photocurrents up to 0.23 ± 0.02 mA cm-2 at 1.23 VRHE under std. simulated solar illumination. Electrochem. impedance spectroscopy reveals a const. flat-band potential over a wide pH range at +0.31 VNHE. The solar H2O oxidn. photocurrent with bare BBL electrodes increases with increasing pH, and no evidence of semiconductor oxidn. was obsd. over a 30 min testing time. Characterization of the photooxidn. reaction suggests H2O2 or •OH prodn. with the bare film, while functionalization of the interface with 1 nm of TiO2 followed by a Ni-Co catalyst gave solar photocurrents of 20-30 μA cm-2, corresponding with O2 evolution. Limitations to photocurrent prodn. are discussed.
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      Schwinghammer, K.; Hug, S.; Mesch, M. B.; Senker, J.; Lotsch, B. V. Phenyl-Triazine Oligomers for Light-Driven Hydrogen Evolution Energy Environ. Sci. 2015, 8, 3345– 3353 DOI: 10.1039/C5EE02574E
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      Phenyl-triazine oligomers for light-driven hydrogen evolution
      Schwinghammer, K.; Hug, S.; Mesch, M. B.; Senker, J.; Lotsch, B. V.
      Energy & Environmental Science (2015), 8 (11), 3345-3353CODEN: EESNBY; ISSN:1754-5706. (Royal Society of Chemistry)
      The design of stable, yet highly tunable org. photocatalysts which orchestrate multi-step electron transfer reactions is at the heart of the newly emerging field of polymer photocatalysis. Covalent triazine frameworks such as the archetypal CTF-1 have been theorized to constitute a new class of photocatalytically active polymers for light-driven water splitting. Here, we revisit the ionothermal synthesis of CTF-1 by trimerization of 1,4-dicyanobenzene catalyzed by the Lewis acid zinc chloride and demonstrate that the microporous black polymer CTF-1 is essentially inactive for hydrogen evolution. Instead, highly photoactive phenyl-triazine oligomers (PTOs) with higher crystallinity as compared to CTF-1 are obtained by lowering the reaction temp. to 300 °C and prolonging the reaction time to >150 h. The low reaction temp. of the PTOs largely prevents incipient carbonization and thus results in a carbon-to-nitrogen wt. ratio close to the theor. value of 3.43. The oligomers were characterized by MALDI-TOF and quant. solid-state NMR spectroscopy, revealing variations in size, connectivity and thus nitrile-to-triazine ratios depending on the initial precursor diln. The most active PTO samples efficiently and stably reduce water to hydrogen with an av. rate of 1076 (±278) μmol h-1 g-1 under simulated sunlight illumination, which is competitive with the best carbon nitride-based and purely org. photocatalysts. The photocatalytic activity of the PTOs is found to sensitively depend on the polymn. degree, thus suggesting a prominent role of the unreacted nitrile moieties in the photocatalytic process. Notably, PTOs even show moderate hydrogen prodn. without the addn. of any co-catalyst.
    11. 11
      Sprick, R. S.; Bonillo, B.; Clowes, R.; Guiglion, P.; Brownbill, N. J.; Slater, B. J.; Blanc, F.; Zwijnenburg, M. A.; Adams, D. J.; Cooper, A. I. Visible-Light-Driven Hydrogen Evolution Using Planarized Conjugated Polymer Photocatalysts Angew. Chem. 2016, 128, 1824– 1828 DOI: 10.1002/ange.201510542
      11
      Visible-Light-Driven Hydrogen Evolution Using Planarized Conjugated Polymer Photocatalysts
      Sprick Reiner Sebastian; Bonillo Baltasar; Clowes Rob; Brownbill Nick J; Slater Benjamin J; Adams Dave J; Cooper Andrew I; Guiglion Pierre; Zwijnenburg Martijn A; Blanc Frederic
      Angewandte Chemie (Weinheim an der Bergstrasse, Germany) (2016), 128 (5), 1824-1828 ISSN:0044-8249.
      Linear poly(p-phenylene)s are modestly active UV photocatalysts for hydrogen production in the presence of a sacrificial electron donor. Introduction of planarized fluorene, carbazole, dibenzo[b,d]thiophene or dibenzo[b,d]thiophene sulfone units greatly enhances the H2 evolution rate. The most active dibenzo[b,d]thiophene sulfone co-polymer has a UV photocatalytic activity that rivals TiO2, but is much more active under visible light. The dibenzo[b,d]thiophene sulfone co-polymer has an apparent quantum yield of 2.3 % at 420 nm, as compared to 0.1 % for platinized commercial pristine carbon nitride.
    12. 12
      Yang, C.; Ma, B. C.; Zhang, L. Z.; Lin, S.; Ghasimi, S.; Landfester, K.; Zhang, K. A. I.; Wang, X. C. Molecular Engineering of Conjugated Polybenzothiadiazoles for Enhanced Hydrogen Production by Photosynthesis Angew. Chem., Int. Ed. 2016, 55, 9202– 9206 DOI: 10.1002/anie.201603532
      12
      Molecular Engineering of Conjugated Polybenzothiadiazoles for Enhanced Hydrogen Production by Photosynthesis
      Yang, Can; Ma, Beatriz Chiyin; Zhang, Linzhu; Lin, Sen; Ghasimi, Saman; Landfester, Katharina; Zhang, Kai A. I.; Wang, Xinchen
      Angewandte Chemie, International Edition (2016), 55 (32), 9202-9206CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
      The search for metal-free org. photocatalysts for H2 prodn. from water using visible light remains a key challenge. Reported herein is a mol. structural design of pure org. photocatalysts, derived from conjugated polybenzothiadiazoles, for photocatalytic H2 evolution using visible light. By alternating the substitution position of the electron-withdrawing benzothiadizole unit on the Ph unit as a comonomer, various polymers with either one- or three-dimensional structures were synthesized and the effect of the mol. structure on their catalytic activity was investigated. Photocatalytic H2 evolution efficiencies up to 116 μmol h-1 were obsd. by employing the linear polymer based on a phenyl-benzothiadiazole alternating main chain, with an apparent quantum yield (AQY) of 4.01% at 420 nm using triethanolamine as the sacrificial agent.
    13. 13
      Li, L. W.; Cai, Z. X.; Wu, Q. H.; Lo, W. Y.; Zhang, N.; Chen, L. X.; Yu, L. P. Rational Design of Porous Conjugated Polymers and Roles of Residual Palladium for Photocatalytic Hydrogen Production J. Am. Chem. Soc. 2016, 138, 7681– 7686 DOI: 10.1021/jacs.6b03472
      13
      Rational Design of Porous Conjugated Polymers and Roles of Residual Palladium for Photocatalytic Hydrogen Production
      Li, Lianwei; Cai, Zhengxu; Wu, Qinghe; Lo, Wai-Yip; Zhang, Na; Chen, Lin X.; Yu, Luping
      Journal of the American Chemical Society (2016), 138 (24), 7681-7686CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
      Developing highly efficient photocatalysts for water splitting is one of the grand challenges in solar energy conversion. Here, we report the rational design and synthesis of porous conjugated polymer (PCP) that photocatalytically generates hydrogen from water splitting. The design mimics natural photosynthetics systems with conjugated polymer component to harvest photons and the transition metal part to facilitate catalytic activities. A series of PCPs have been synthesized with different light harvesting chromophores and transition metal binding bipyridyl (bpy) sites. The photocatalytic activity of these bpy-contg. PCPs can be greatly enhanced due to the improved light absorption, better wettability, local ordering structure, and the improved charge sepn. process. The PCP made of strong and fully conjugated donor chromophore DBD (M4) shows the highest hydrogen prodn. rate at ∼33 μmol/h. The results indicate that copolymn. between a strong electron donor and weak electron acceptor into the same polymer chain is a useful strategy for developing efficient photocatalysts. This study also reveals that the residual palladium in the PCP networks plays a key role for the catalytic performance. The hydrogen generation activity of PCP photocatalyst can be further enhanced to 164 μmol/h with an apparent quantum yield of 1.8% at 350 nm by loading 2 wt % of extra platinum cocatalyst.
    14. 14
      Vyas, V. S.; Lau, V. W. H.; Lotsch, B. V. Soft Photocatalysis: Organic Polymers for Solar Fuel Productions Chem. Mater. 2016, 28, 5191– 5204 DOI: 10.1021/acs.chemmater.6b01894
      There is no corresponding record for this reference.
    15. 15
      Zhang, G.; Lan, Z.-A.; Wang, X. Organic Conjugated Semiconductors for Photocatalytic Hydrogen Evolution with Visible Light Angew. Chem., Int. Ed. 2016, 55, 15712– 15727 DOI: 10.1002/anie.201607375
      15
      Organic Conjugated Semiconductors for Photocatalytic Hydrogen Evolution with Visible Light
      Wang, Xinchen; Zhang, Guigang; Lan, Zhi-An
      Angewandte Chemie, International Edition (2016), 55 (51), 15712-15727CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
      Conjugated polymers, constituted with fully -conjugated systems, present a new generation of heterogeneous photocatalysts for solar energy utilization. They bare three key features for photocatalytic process, namely robust, non-toxic and visible-light-active, making them appealing candidates for scale up when integrating with their intrinsic soft features. In this minireview, we present a brief summary on the recent development of various promising polymer photocatalysts for hydrogen evolution from aq. solns., including linear polymers, planarized polymers, triazine/heptazine polymers, and other related org. conjugated semiconductors, with particular focus on the rational manipulation in the compn., architectures, optical and electronic properties that are closely relevant with photophysic and photochem. properties. Some future trends and prospects of org. conjugated photocatalysts for water splitting and its coupling with CO2 redn. are also envisaged.
    16. 16
      Wang, X. C.; Maeda, K.; Thomas, A.; Takanabe, K.; Xin, G.; Carlsson, J. M.; Domen, K.; Antonietti, M. A Metal-Free Polymeric Photocatalyst for Hydrogen Production from Water under Visible Light Nat. Mater. 2009, 8, 76– 80 DOI: 10.1038/nmat2317
      16
      A metal-free polymeric photocatalyst for hydrogen production from water under visible light
      Wang, Xinchen; Maeda, Kazuhiko; Thomas, Arne; Takanabe, Kazuhiro; Xin, Gang; Carlsson, Johan M.; Domen, Kazunari; Antonietti, Markus
      Nature Materials (2009), 8 (1), 76-80CODEN: NMAACR; ISSN:1476-1122. (Nature Publishing Group)
      The prodn. of hydrogen from water using a catalyst and solar energy is an ideal future energy source, independent of fossil reserves. For an economical use of water and solar energy, catalysts that are sufficiently efficient, stable, inexpensive and capable of harvesting light are required. Here, we show that an abundant material, polymeric carbon nitride, can produce hydrogen from water under visible-light irradn. in the presence of a sacrificial donor. Contrary to other conducting polymer semiconductors, carbon nitride is chem. and thermally stable and does not rely on complicated device manufg. The results represent an important first step towards photosynthesis in general where artificial conjugated polymer semiconductors can be used as energy transducers.
    17. 17
      Wang, X. C.; Maeda, K.; Chen, X. F.; Takanabe, K.; Domen, K.; Hou, Y. D.; Fu, X. Z.; Antonietti, M. Polymer Semiconductors for Artificial Photosynthesis: Hydrogen Evolution by Mesoporous Graphitic Carbon Nitride with Visible Light J. Am. Chem. Soc. 2009, 131, 1680– 1681 DOI: 10.1021/ja809307s
      17
      Polymer semiconductors for artificial photosynthesis: hydrogen evolution by mesoporous graphitic carbon nitride with visible light
      Wang, Xinchen; Maeda, Kazuhiko; Chen, Xiufang; Takanabe, Kazuhiro; Domen, Kazunari; Hou, Yidong; Fu, Xianzhi; Antonietti, Markus
      Journal of the American Chemical Society (2009), 131 (5), 1680-1681CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
      We investigated semiconductor characteristics for polymeric carbon nitride as a metal-free photocatalyst working with visible light and have shown that the efficiency of hydrogen prodn. by photochem. water redn. can be improved by ∼1 order of magnitude by introducing the right type of mesoporosity into polymeric C3N4. We anticipate a wide range of potential application of C3N4 as energy transducers for artificial photosynthesis in general, esp. with a 3D continuous nanoarchitecture. Moreover, the results of finding photoactivity for carbon nitride nanoparticles can enrich the discussion on prebiotic chem. of the Earth, as HCN polymer clusters are unequivocal in the solar system.
    18. 18
      Maeda, K.; Wang, X. C.; Nishihara, Y.; Lu, D. L.; Antonietti, M.; Domen, K. Photocatalytic Activities of Graphitic Carbon Nitride Powder for Water Reduction and Oxidation under Visible Light J. Phys. Chem. C 2009, 113, 4940– 4947 DOI: 10.1021/jp809119m
      18
      Photocatalytic Activities of Graphitic Carbon Nitride Powder for Water Reduction and Oxidation under Visible Light
      Maeda, Kazuhiko; Wang, Xinchen; Nishihara, Yasushi; Lu, Daling; Antonietti, Markus; Domen, Kazunari
      Journal of Physical Chemistry C (2009), 113 (12), 4940-4947CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
      Graphitic carbon nitride (g-C3N4) with a band gap of 2.7 eV is studied as a nonmetallic photocatalyst for H2 or O2 evolution from water under UV and visible light. The g-C3N4 catalyst exhibits activities for water redn. into H2 or water oxidn. into O2 in the presence of a proper sacrificial electron donor or acceptor, resp., even without the need for precious metal co-catalysts. When bis(1,5-cyclooctadiene)platinum complex [Pt(cod)2] (a nonionic complex) is used as a precursor of Pt co-catalyst instead of H2PtCl6 (an ionic complex), enhanced H2 evolution activity is acquired. This difference in activity is primarily due to the better dispersion of Pt nanoparticles on g-C3N4, which is considered to originate from the better access of Pt(cod)2 to the g-C3N4 surface, as compared to that of H2PtCl6 in the prepn. process. Unmodified g-C3N4 produces O2 from an aq. silver nitrate soln. upon UV irradn. (λ > 300 nm), although N2 release due to self-decompn. of g-C3N4 by photogenerated holes takes place. Modification of g-C3N4 with RuO2 improves not only O2 evolution activity but also stability against the self-decompn., resulting in stable visible-light-driven O2 evolution (λ > 420 nm).
    19. 19
      Zhang, J. S.; Chen, X. F.; Takanabe, K.; Maeda, K.; Domen, K.; Epping, J. D.; Fu, X. Z.; Antonietti, M.; Wang, X. C. Synthesis of a Carbon Nitride Structure for Visible-Light Catalysis by Copolymerization Angew. Chem., Int. Ed. 2010, 49, 441– 444 DOI: 10.1002/anie.200903886
      19
      Synthesis of a Carbon Nitride Structure for Visible-Light Catalysis by Copolymerization
      Zhang, Jinshui; Chen, Xiufang; Takanabe, Kazuhiro; Maeda, Kazuhiko; Domen, Kazunari; Epping, Jan Dirk; Fu, Xianzhi; Antonietti, Markus; Wang, Xinchen
      Angewandte Chemie, International Edition (2010), 49 (2), 441-444, S441/1-S441/4CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
      The authors have shown that the optical and electronic properties of carbon nitride polymer can be easily modified by std. org. protocols, here exemplified by copolymn. with barbituric acid. The authors believe that the diversity of org. chem. will allow the rational design of a much broader set of carbon nitride polymers with controlled functions, opening a new pathway for the development of light-harvesting semiconductors. Physicochem. properties and photocatalytic activity of Pt/carbon nitride polymer for H2 generation from H2O/triethanolamine under UV and visible light were studied.
    20. 20
      Zhang, G. G.; Lan, Z. A.; Lin, L. H.; Lin, S.; Wang, X. C. Overall Water Splitting by Pt/G-C3N4 Photocatalysts without Using Sacrificial Agents Chem. Sci. 2016, 7, 3062– 3066 DOI: 10.1039/C5SC04572J
      20
      Overall water splitting by Pt/g-C3N4 photocatalysts without using sacrificial agents
      Zhang, Guigang; Lan, Zhi-An; Lin, Lihua; Lin, Sen; Wang, Xinchen
      Chemical Science (2016), 7 (5), 3062-3066CODEN: CSHCCN; ISSN:2041-6520. (Royal Society of Chemistry)
      We report the direct splitting of pure water by light-excited graphitic carbon nitride (g-C3N4) modified with Pt, PtOx, and CoOx as redox cocatalysts, while pure g-C3N4 is virtually inactive for overall water splitting by photocatalysis. The novelty is in the selective creation of both H2 and O2 cocatalysts on surface active sites of g-C3N4via photodeposition triggering the splitting of water for the simultaneous evolution of H2 and O2 gases in a stoichiometric ratio of 2:1, irradiated with light, without using any sacrificial reagents. The photocatalyst was stable for 510 h of reaction.
    21. 21
      Sui, Y.; Liu, J. H.; Zhang, Y. W.; Tian, X. K.; Chen, W. Dispersed Conductive Polymer Nanoparticles on Graphitic Carbon Nitride for Enhanced Solar-Driven Hydrogen Evolution from Pure Water Nanoscale 2013, 5, 9150– 9155 DOI: 10.1039/c3nr02413j
      21
      Dispersed conductive polymer nanoparticles on graphitic carbon nitride for enhanced solar-driven hydrogen evolution from pure water
      Sui, Yi; Liu, Jinghai; Zhang, Yuewei; Tian, Xike; Chen, Wei
      Nanoscale (2013), 5 (19), 9150-9155CODEN: NANOHL; ISSN:2040-3372. (Royal Society of Chemistry)
      Developing new methods to improve the photocatalytic activity of graphitic carbon nitride (g-C3N4) for hydrogen (H2) evolution has attracted intensive research interests. Here, we report that the g-C3N4 exhibits photocatalytic activity for H2 evolution from pure water. And, the activity is dramatically improved by loading highly dispersed conductive polymer nanoparticles. The H2 evolution rate increases up to 50 times for g-C3N4 with 1.5 wt% polypyrrole (PPy) nanoparticles on the surface. The reaction proceeding in a pure water system excludes the need for sacrificial agents. The role of the highly conductive PPy in enhancing H2 evolution is as a surface junction to increase the no. of photoinduced electrons, and to facilitate electron transfer to the interface.
    22. 22
      Schwinghammer, K.; Tuffy, B.; Mesch, M. B.; Wirnhier, E.; Martineau, C.; Taulelle, F.; Schnick, W.; Senker, J.; Lotsch, B. V. Triazine-Based Carbon Nitrides for Visible-Light-Driven Hydrogen Evolution Angew. Chem., Int. Ed. 2013, 52, 2435– 2439 DOI: 10.1002/anie.201206817
      22
      Triazine-based Carbon Nitrides for Visible-Light-Driven Hydrogen Evolution
      Schwinghammer, Katharina; Tuffy, Brian; Mesch, Maria B.; Wirnhier, Eva; Martineau, Charlotte; Taulelle, Francis; Schnick, Wolfgang; Senker, Juergen; Lotsch, Bettina V.
      Angewandte Chemie, International Edition (2013), 52 (9), 2435-2439CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
      Carbon nitride (CN) photocatalysts based on triazine building blocks are developed and their enhanced photocatalytic activity demonstrated in comparison to heptazine-based CNs. Moreover, it is shown that their performance can be amplified by small-mol. doping, thus rendering them the most active nonmetal photocatalysts for the hydrogen evolution reaction that are reported to date. The 2D triazine-based carbon nitrides show substantial visible light-induced hydrogen prodn. from water, and in this regard rival the benchmark heptazine-derived photocatalysts. With external quantum efficiencies as high as 3.4%, the amorphous carbon- and oxygen-enriched poly(triazine imide) species not only outperform melon-type photocatalysts, but also cryst. poly(triazine imide) by 5-6 times. Consistent with results of X. Wang et al.(2009), it is demonstrated that a rather low level of structural definition and the introduction of defects up to a certain doping level (16% for 4-amino-2,6-dihydroxypyrimidine) tend to enhance the photoactivity of the catalysts. The diverse range of available org. and inorg. dopants will allow the rational design of a broad set of triazine-based CN polymers with controlled functions, thus opening new avenues for the development of light-harvesting semiconductors. The. easily adjustable structural and electronic properties of CN polymers render them particularly versatile for solar energy applications.
    23. 23
      Martin, D. J.; Qiu, K. P.; Shevlin, S. A.; Handoko, A. D.; Chen, X. W.; Guo, Z. X.; Tang, J. W. Highly Efficient Photocatalytic H2 Evolution from Water Using Visible Light and Structure-Controlled Graphitic Carbon Nitride Angew. Chem., Int. Ed. 2014, 53, 9240– 9245 DOI: 10.1002/anie.201403375
      23
      Highly Efficient Photocatalytic H2 Evolution from Water using Visible Light and Structure-Controlled Graphitic Carbon Nitride
      Martin, David James; Qiu, Kaipei; Shevlin, Stephen Andrew; Handoko, Albertus Denny; Chen, Xiaowei; Guo, Zhengxiao; Tang, Junwang
      Angewandte Chemie, International Edition (2014), 53 (35), 9240-9245CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
      The major challenge of photocatalytic water splitting, the prototypical reaction for the direct prodn. of hydrogen by using solar energy, is to develop low-cost yet highly efficient and stable semiconductor photocatalysts. Herein, an effective strategy for synthesizing extremely active graphitic carbon nitride (g-C3N4) from a low-cost precursor, urea, is reported. The g-C3N4 exhibits an extraordinary hydrogen-evolution rate (ca. 20 000 μmol h-1 g-1 under full arc), which leads to a high turnover no. (TON) of over 641 after 6 h. The reaction proceeds for more than 30 h without activity loss and results in an internal quantum yield of 26.5 % under visible light, which is nearly an order of magnitude higher than that obsd. for any other existing g-C3N4 photocatalysts. Furthermore, it was found by exptl. anal. and DFT calcns. that as the d.p. increases and the proton concn. decreases, the hydrogen-evolution rate is significantly enhanced.
    24. 24
      Schwinghammer, K.; Mesch, M. B.; Duppel, V.; Ziegler, C.; Senker, J.; Lotsch, B. V. Crystalline Carbon Nitride Nanosheets for Improved Visible-Light Hydrogen Evolution J. Am. Chem. Soc. 2014, 136, 1730– 1733 DOI: 10.1021/ja411321s
      24
      Crystalline Carbon Nitride Nanosheets for Improved Visible-Light Hydrogen Evolution
      Schwinghammer, Katharina; Mesch, Maria B.; Duppel, Viola; Ziegler, Christian; Senker, Juergen; Lotsch, Bettina V.
      Journal of the American Chemical Society (2014), 136 (5), 1730-1733CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
      Nanosheets of a cryst. 2D carbon nitride were obtained by ionothermal synthesis of the layered bulk material poly(triazine imide), PTI, followed by one-step liq. exfoliation in water. Triazine-based nanosheets are 1-2 nm in height and afford chem. and colloidally stable suspensions under both basic and acidic conditions. We use solid-state NMR spectroscopy of isotopically enriched, restacked nanosheets as a tool to indirectly monitor the exfoliation process and carve out the chem. changes occurring upon exfoliation, as well as to det. the nanosheet thickness. PTI nanosheets show significantly enhanced visible-light driven photocatalytic activity toward hydrogen evolution compared to their bulk counterpart, which highlights the crucial role of morphol. and surface area on the photocatalytic performance of carbon nitride materials.
    25. 25
      Liu, J.; Liu, Y.; Liu, N. Y.; Han, Y. Z.; Zhang, X.; Huang, H.; Lifshitz, Y.; Lee, S. T.; Zhong, J.; Kang, Z. H. Metal-Free Efficient Photocatalyst for Stable Visible Water Splitting Via a Two-Electron Pathway Science 2015, 347, 970– 974 DOI: 10.1126/science.aaa3145
      25
      Metal-free efficient photocatalyst for stable visible water splitting via a two-electron pathway
      Liu, Juan; Liu, Yang; Liu, Naiyun; Han, Yuzhi; Zhang, Xing; Huang, Hui; Lifshitz, Yeshayahu; Lee, Shuit-Tong; Zhong, Jun; Kang, Zhenhui
      Science (Washington, DC, United States) (2015), 347 (6225), 970-974CODEN: SCIEAS; ISSN:0036-8075. (American Association for the Advancement of Science)
      The use of solar energy to produce mol. hydrogen and oxygen (H2 and O2) from overall water splitting is a promising means of renewable energy storage. In the past 40 years, various inorg. and org. systems have been developed as photocatalysts for water splitting driven by visible light. These photocatalysts, however, still suffer from low quantum efficiency and/or poor stability. We report the design and fabrication of a metal-free carbon nanodot-carbon nitride (C3N4) nanocomposite and demonstrate its impressive performance for photocatalytic solar water splitting. We measured quantum efficiencies of 16% for wavelength λ = 420 ± 20 nm, 6.29% for λ = 580 ± 15 nm, and 4.42% for λ = 600 ± 10 nm, and detd. an overall solar energy conversion efficiency of 2.0%. The catalyst comprises low-cost, Earth-abundant, environmentally friendly materials and shows excellent stability.
    26. 26
      Lau, V. W. H.; Mesch, M. B.; Duppel, V.; Blum, V.; Senker, J.; Lotsch, B. V. Low-Molecular-Weight Carbon Nitrides for Solar Hydrogen Evolution J. Am. Chem. Soc. 2015, 137, 1064– 1072 DOI: 10.1021/ja511802c
      26
      Low-Molecular-Weight Carbon Nitrides for Solar Hydrogen Evolution
      Lau, Vincent Wing-hei; Mesch, Maria B.; Duppel, Viola; Blum, Volker; Senker, Juergen; Lotsch, Bettina V.
      Journal of the American Chemical Society (2015), 137 (3), 1064-1072CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
      This work focuses on the control of the polymn. process for melon ("graphitic carbon nitride"), with the aim of improving its photocatalytic activity intrinsically. We demonstrate here that redn. of the synthesis temp. leads to a mixt. of the monomer melem and its higher condensates. We show that this mixt. can be sepd. and provide evidence that the higher condensates are isolated oligomers of melem. On evaluating their photocatalytic activity for hydrogen evolution, the oligomers were found to be the most active species, having up to twice the activity of the monomer/oligomer mixt. of the as-synthesized material, which in turn has 3 times the activity of the polymer melon, the literature benchmark. These results highlight the role of "defects", i.e., chain terminations, in increasing the catalytic activity of carbon nitrides and at the same time point to the ample potential of intrinsically improving the photocatalytic activity of "carbon nitride", esp. through the selective synthesis of the active phase.
    27. 27
      Lau, V. W. H.; Moudrakovski, I.; Botari, T.; Weinberger, S.; Mesch, M. B.; Duppel, V.; Senker, J.; Blum, V.; Lotsch, B. V. Rational Design of Carbon Nitride Photocatalysts by Identification of Cyanamide Defects as Catalytically Relevant Sites Nat. Commun. 2016, 7, 12165 DOI: 10.1038/ncomms12165
      27
      Rational design of carbon nitride photocatalysts by identification of cyanamide defects as catalytically relevant sites
      Lau, Vincent Wing-hei; Moudrakovski, Igor; Botari, Tiago; Weinberger, Simon; Mesch, Maria B.; Duppel, Viola; Senker, Juergen; Blum, Volker; Lotsch, Bettina V.
      Nature Communications (2016), 7 (), 12165CODEN: NCAOBW; ISSN:2041-1723. (Nature Publishing Group)
      The heptazine-based polymer melon (also known as graphitic carbon nitride, g-C3N4) is a promising photocatalyst for hydrogen evolution. Nonetheless, attempts to improve its inherently low activity are rarely based on rational approaches because of a lack of fundamental understanding of its mechanistic operation. Here we employ mol. heptazine-based model catalysts to identify the cyanamide moiety as a photocatalytically relevant 'defect'. We exploit this knowledge for the rational design of a carbon nitride polymer populated with cyanamide groups, yielding a material with 12 and 16 times the hydrogen evolution rate and apparent quantum efficiency (400 nm), resp., compared with the unmodified melon. Computational modeling and material characterization suggest that this moiety improves coordination (and, in turn, charge transfer kinetics) to the platinum co-catalyst and enhances the sepn. of the photogenerated charge carriers. The demonstrated knowledge transfer for rational catalyst design presented here provides the conceptual framework for engineering high-performance heptazine-based photocatalysts.
    28. 28
      Yeh, T. F.; Syu, J. M.; Cheng, C.; Chang, T. H.; Teng, H. S. Graphite Oxide as a Photocatalyst for Hydrogen Production from Water Adv. Funct. Mater. 2010, 20, 2255– 2262 DOI: 10.1002/adfm.201000274
      28
      Graphite Oxide as a Photocatalyst for Hydrogen Production from Water
      Yeh, Te-Fu; Syu, Jhih-Ming; Cheng, Ching; Chang, Ting-Hsiang; Teng, Hsisheng
      Advanced Functional Materials (2010), 20 (14), 2255-2262CODEN: AFMDC6; ISSN:1616-301X. (Wiley-VCH Verlag GmbH & Co. KGaA)
      A graphite oxide (GO) semiconductor photocatalyst with an apparent bandgap of 2.4-4.3 eV is synthesized by a modified Hummers' procedure. The as-synthesized GO photocatalyst has an interlayer spacing of 0.42 nm because of its moderate oxidn. level. Under irradn. with UV or visible light, this GO photocatalyst steadily catalyzes H2 generation from a 20 vol % aq. methanol soln. and pure water. As the GO sheets extensively disperse in water, a cocatalyst is not required for H2 generation over the GO photocatalyst. During photocatalytic reaction, the GO loses some oxygen functional groups, leading to bandgap redn. and increased cond. This structural variation does not affect the stable H2 generation over the GO. The encouraging results presented in this study demonstrate the potential of graphitic materials as a medium for water splitting under solar illumination.
    29. 29
      Yeh, T. F.; Teng, C. Y.; Chen, S. J.; Teng, H. S. Nitrogen-Doped Graphene Oxide Quantum Dots as Photocatalysts for Overall Water-Splitting under Visible Light Illumination Adv. Mater. 2014, 26, 3297– 3303 DOI: 10.1002/adma.201305299
      29
      Nitrogen-Doped Graphene Oxide Quantum Dots as Photocatalysts for Overall Water-Splitting under Visible Light Illumination
      Yeh, Te-Fu; Teng, Chiao-Yi; Chen, Shean-Jen; Teng, Hsisheng
      Advanced Materials (Weinheim, Germany) (2014), 26 (20), 3297-3303CODEN: ADVMEW; ISSN:0935-9648. (Wiley-VCH Verlag GmbH & Co. KGaA)
      Based on the structural characteristics required for photocatalytic water-splitting, we synthesized nitrogen-doped graphene oxide-quantum dots (NGO-QDs) as the catalyst. The NGO-QDs exhibited both p- and n-type conductivities, based on the results of the electrochem. Mott-Schottky anal. The prominent photo-luminescence emission indicated that photochem. p-n diodes constituted the NGO-QDs. The diode configuration resulted in an internal Z-scheme charge transfer for effective reaction at the QD interface. Visible light (>420 nm) irradn. on the NGO-QDs resulted in simultaneous H2 and O2 evolution from pure water at an H2:O2 molar ratio of 2:1. This paper demonstrated that graphene species are promising materials for synthesizing metal-free, cost-effective, and environmentally-friendly catalysts for overall water-splitting under solar illumination.
    30. 30
      Tsai, E. W.; Basak, S.; Ruiz, J. P.; Reynolds, J. R.; Rajeshwar, K. Electrochemistry of Some Beta-Substituted Polythiophenes - Anodic-Oxidation, Electrochromism, and Electrochemical Deactivation J. Electrochem. Soc. 1989, 136, 3683– 3689 DOI: 10.1149/1.2096530
      There is no corresponding record for this reference.
    31. 31
      Novak, P.; Rasch, B.; Vielstich, W. Overoxidation of Polypyrrole in Propylene Carbonate - an Insitu FTIR Study J. Electrochem. Soc. 1991, 138, 3300– 3304 DOI: 10.1149/1.2085406
      There is no corresponding record for this reference.
    32. 32
      Zotti, G.; Schiavon, G.; Zecchin, S. Irreversible-Processes in the Electrochemical Reduction of Polythiophenes - Chemical Modifications of the Polymer and Charge-Trapping Phenomena Synth. Met. 1995, 72, 275– 281 DOI: 10.1016/0379-6779(95)03280-0
      32
      Irreversible processes in the electrochemical reduction of polythiophenes. Chemical modifications of the polymer and charge-trapping phenomena
      Zotti, G.; Schiavon, G.; Zecchin, S.
      Synthetic Metals (1995), 72 (3), 275-81CODEN: SYMEDZ; ISSN:0379-6779. (Elsevier)
      Irreversible changes occurring in the electrochem. redn. of some polythiophenes in MeCN have been investigated by cyclic voltammetry (CV), UV-visible and IR spectroscopy, and the electrochem. quartz crystal microbalance. Redn. in the presence of alkali metal cations (Li+, Na+, K+, and Cs+) does not result in n-doping but in hydrogenation of the alkene moieties. The promotion of double-bond satn. by alkali metal cations following the order Li+, Na+ > K+ > Cs+ is attributed to their charge-pinning action which decreases as the ion radius is increased. Reversible redn. in tetralkylammonium electrolyte (n-doping) is accompanied by prodn. of hydroxide ions within the polymer which are thereby inserted by subsequent oxidn. The resulting quinone-like groups are responsible for the CV pre-peaks usually obsd. in conducting polymers after n-doping and previously assigned to trapped charges.
    33. 33
      Vyas, R. N.; Wang, B. Electrochemical Analysis of Conducting Polymer Thin Films Int. J. Mol. Sci. 2010, 11, 1956– 1972 DOI: 10.3390/ijms11041956
      33
      Electrochemical analysis of conducting polymer thin films
      Vyas, Ritesh N.; Wang, Bin
      International Journal of Molecular Sciences (2010), 11 (), 1956-1972CODEN: IJMCFK; ISSN:1422-0067. (Molecular Diversity Preservation International)
      Polyelectrolyte multilayers built via the layer-by-layer (LbL) method has been one of the most promising systems in the field of materials science. Layered structures can be constructed by the adsorption of various polyelectrolyte species onto the surface of a solid or liq. material by means of electrostatic interaction. The thickness of the adsorbed layers can be tuned precisely in the nanometer range. Stable, semiconducting thin films are interesting research subjects. We use a conducting polymer, poly(p-phenylene vinylene) (PPV), in the prepn. of a stable thin film via the LbL method. Cyclic voltammetry and electrochem. impedance spectroscopy have been used to characterize the ionic cond. of the PPV multilayer films. The ionic cond. of the films has been found to be dependent on the polymn. temp. The film cond. can be fitted to a modified Randle's circuit. The circuit equiv. calcns. are performed to provide the diffusion coeff. values.
    34. 34
      Guiglion, P.; Butchosa, C.; Zwijnenburg, M. A. Polymeric Watersplitting Photocatalysts; a Computational Perspective on the Water Oxidation Conundrum J. Mater. Chem. A 2014, 2, 11996– 12004 DOI: 10.1039/C4TA02044H
      34
      Polymeric water splitting photocatalysts; computational perspective on water oxidation conundrum
      Guiglion, Pierre; Butchosa, Cristina; Zwijnenburg, Martijn A.
      Journal of Materials Chemistry A: Materials for Energy and Sustainability (2014), 2 (30), 11996-12004CODEN: JMCAET; ISSN:2050-7496. (Royal Society of Chemistry)
      A computational scheme to predict the thermodn. ability of photocatalysts to drive both of the water splitting half reactions, proton redn. and water oxidn., is discussed, and applied to a no. of polymeric systems to explain their apparent inability to oxidize water. We predict that the poly(p-phenylene) (PPP) is thermodynamically unable to oxidize water and that PPP is hence unlikely to split water in the absence of an external elec. bias. For other polymers, however, for example carbon nitride, the lack of oxygen evolution activity appears kinetic in origin and hence a suitable co-catalyst could potentially transform them into true water splitting photocatalysts.
    35. 35
      Butchosa, C.; Guiglion, P.; Zwijnenburg, M. A. Carbon Nitride Photocatalysts for Water Splitting: A Computational Perspective J. Phys. Chem. C 2014, 118, 24833– 24842 DOI: 10.1021/jp507372n
      35
      Carbon nitride photocatalysts for water splitting: A computational perspective
      Butchosa, Cristina; Guiglion, Pierre; Zwijnenburg, Martijn A.
      Journal of Physical Chemistry C (2014), 118 (43), 24833-24842CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
      We study the thermodn. ability of carbon nitride materials to act as water splitting photocatalysts using a computational approach that involves a combination of d. functional theory (DFT) and time-dependent DFT (TD-DFT) calcns. on cluster models of both triazine- and heptazine-based structures. We first use TD-DFT to calc. the absorption spectra of the different cluster models and compare these spectra to those measured exptl. and then calc. using DFT and TD-DFT the redn. potentials of the free electron, free hole, and exciton in these models. We predict that all classes of carbon nitride structures considered should thermodynamically be able to reduce protons and oxidize water. We further provide evidence for the hypothesis that the exptl. lack of overall water splitting activity for pure carbon nitride arises from the fact that water oxidn. is a four-hole reaction and hence very susceptible to competition with electron-hole recombination. Finally, we propose that the recently reported overall water splitting activity of carbon nitride loaded with polypyrrole nanoparticles arises from a junction formed at the interface of both materials, which assists in keeping electrons and holes apart.
    36. 36
      Berardo, E.; Zwijnenburg, M. A. Modeling the Water Splitting Activity of a Tio2 Rutile Nanoparticle J. Phys. Chem. C 2015, 119, 13384– 13393 DOI: 10.1021/acs.jpcc.5b01512
      36
      Modeling the Water Splitting Activity of a TiO2 Rutile Nanoparticle
      Berardo, Enrico; Zwijnenburg, Martijn A.
      Journal of Physical Chemistry C (2015), 119 (24), 13384-13393CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
      We explore, from a theor. perspective, the effect of particle size on the photocatalytic water splitting activity of TiO2 rutile (nano)particles by a combination of explicit quantum chem. calcns. on a hydroxylated rutile nanoparticle in a realistic solvation environment and a comparison with the calcd. properties of bulk rutile (surfaces) from the literature. Specifically, we use d. functional theory (DFT) and time-dependent DFT to calc. the nanoparticle thermodn. driving force for the water splitting half-reactions and identify in the process the crucial role of self-trapping of the free charge carriers responsible for proton redn. and water oxidn.
    37. 37
      Guiglion, P.; Berardo, E.; Butchosa, C.; Wobbe, M. C. C.; Zwijnenburg, M. A. Modelling Materials for Solar Fuel Synthesis by Artificial Photosynthesis; Predicting the Optical, Electronic and Redox Properties of Photocatalysts J. Phys.: Condens. Matter 2016, 28, 074001 DOI: 10.1088/0953-8984/28/7/074001
      37
      Modelling materials for solar fuel synthesis by artificial photosynthesis; predicting the optical, electronic and redox properties of photocatalysts
      Guiglion, Pierre; Berardo, Enrico; Butchosa, Cristina; Wobbe, Milena C. C.; Zwijnenburg, Martijn A.
      Journal of Physics: Condensed Matter (2016), 28 (7), 074001/1-074001/11CODEN: JCOMEL; ISSN:0953-8984. (IOP Publishing Ltd.)
      A review. In this mini-review, we discuss what insight computational modeling can provide into the working of photocatalysts for solar fuel synthesis and how calcns. can be used to screen for new promising materials for photocatalytic water splitting and carbon dioxide redn. We will extensively discuss the different relevant (material) properties and the computational approaches (DFT, TD-DFT, GW/BSE) available to model them. We illustrate this with examples from the literature, focussing on polymeric and nanoparticle photocatalysts. We finish with a perspective on the outstanding conceptual and computational challenges.
    38. 38
      Guiglion, P.; Butchosa, C.; Zwijnenburg, M. A. Polymer Photocatalysts for Water Splitting: Insights from Computational Modeling Macromol. Chem. Phys. 2016, 217, 344– 353 DOI: 10.1002/macp.201500432
      38
      Polymer Photocatalysts for Water Splitting: Insights from Computational Modeling
      Guiglion, Pierre; Butchosa, Cristina; Zwijnenburg, Martijn A.
      Macromolecular Chemistry and Physics (2016), 217 (3), 344-353CODEN: MCHPES; ISSN:1022-1352. (Wiley-VCH Verlag GmbH & Co. KGaA)
      A review. Based on insights from computational chem. calcns., the ability of polymers to act as water splitting photocatalysts for the prodn. of renewable hydrogen from water and sunlight is discussed. Specifically, the important role of exciton dissocn. in these materials is highlighted, as well as the possible microscopic origins of the exptl. obsd. changes in the photocatalytic activity of a polymer with increasing chain length or changing chem. compn. The reason why water oxidn., with polymeric photocatalysts, is difficult, and which polymer properties to target when developing new polymers for water splitting photocatalysis are, finally, also discussed.
    39. 39
      Toroker, M. C.; Kanan, D. K.; Alidoust, N.; Isseroff, L. Y.; Liao, P. L.; Carter, E. A. First Principles Scheme to Evaluate Band Edge Positions in Potential Transition Metal Oxide Photocatalysts and Photoelectrodes Phys. Chem. Chem. Phys. 2011, 13, 16644– 16654 DOI: 10.1039/c1cp22128k
      39
      First principles scheme to evaluate band edge positions in potential transition metal oxide photocatalysts and photoelectrodes
      Toroker, Maytal Caspary; Kanan, Dalal K.; Alidoust, Nima; Isseroff, Leah Y.; Liao, Pei-Lin; Carter, Emily A.
      Physical Chemistry Chemical Physics (2011), 13 (37), 16644-16654CODEN: PPCPFQ; ISSN:1463-9076. (Royal Society of Chemistry)
      The positions of electronic band edges are one important metric for detg. a material's capability to function in a solar energy conversion device that produces fuels from sunlight. In particular, the position of the valence band max. (conduction band min.) must lie lower (higher) in energy than the oxidn. (redn.) reaction free energy in order for these reactions to be thermodynamically favorable. First principles quantum mechanics calcns. are presented of the band edge positions in five transition metal oxides and the feasibility is discussed of using these materials in photoelectrochem. cells that produce fuels, including hydrogen, methane, methanol, and formic acid. The band gap center is detd. within the framework of d. functional theory + intra-at. Coulomb (DFT + U) theory. The valence band max. (conduction band min.) is found by subtracting (adding) half of the quasiparticle gap obtained from a non-self-consistent GW calcn. The calcns. are validated against exptl. data where possible; results for several materials including manganese(ii) oxide, iron(ii) oxide, iron(iii) oxide, copper(i) oxide, and nickel(ii) oxide are presented.
    40. 40
      Castelli, I. E.; Olsen, T.; Datta, S.; Landis, D. D.; Dahl, S.; Thygesen, K. S.; Jacobsen, K. W. Computational Screening of Perovskite Metal Oxides for Optimal Solar Light Capture Energy Environ. Sci. 2012, 5, 5814– 5819 DOI: 10.1039/C1EE02717D
      40
      Computational screening of perovskite metal oxides for optimal solar light capture
      Castelli, Ivano E.; Olsen, Thomas; Datta, Soumendu; Landis, David D.; Dahl, Soren; Thygesen, Kristian S.; Jacobsen, Karsten W.
      Energy & Environmental Science (2012), 5 (2), 5814-5819CODEN: EESNBY; ISSN:1754-5706. (Royal Society of Chemistry)
      One of the possible solns. to the world's rapidly increasing energy demand is the development of new photoelectrochem. cells with improved light absorption. This requires development of semiconductor materials which have appropriate bandgaps to absorb a large part of the solar spectrum at the same time as being stable in aq. environments. Here we demonstrate an efficient, computational screening of relevant oxide and oxynitride materials based on electronic structure calcns. resulting in the redn. of a vast space of 5400 different materials to only 15 promising candidates. The screening is based on an efficient and reliable way of calcg. semiconductor band gaps. The outcome of the screening includes all already known successful materials of the types investigated plus some new ones which warrant further exptl. investigation.
    41. 41
      Persson, K. A.; Waldwick, B.; Lazic, P.; Ceder, G. Prediction of Solid-Aqueous Equilibria: Scheme to Combine First-Principles Calculations of Solids with Experimental Aqueous States Phys. Rev. B: Condens. Matter Mater. Phys. 2012, 85, 235438 DOI: 10.1103/PhysRevB.85.235438
      41
      Prediction of solid-aqueous equilibria: scheme to combine first-principles calculations of solids with experimental aqueous states
      Persson, Kristin A.; Waldwick, Bryn; Lazic, Predrag; Ceder, Gerbrand
      Physical Review B: Condensed Matter and Materials Physics (2012), 85 (23), 235438/1-235438/12CODEN: PRBMDO; ISSN:1098-0121. (American Physical Society)
      We present an efficient scheme for combining ab initio calcd. solid states with exptl. aq. states through a framework of consistent ref. energies. Our work enables accurate prediction of phase stability and dissoln. in equil. with water, which has many important application areas. We formally outline the thermodn. principles of the scheme and show examples of successful applications of the proposed framework on (1) the evaluation of the water-splitting photocatalytic material Ta3N5 for aq. stability, (2) the stability of small nanoparticle Pt in acid water, and (3) the prediction of particle morphol. and facet stabilization of olivine LiFePO4 as a function of aq. conditions.
    42. 42
      Zhuang, H. L. L.; Hennig, R. G. Single-Layer Group-Iii Monochalcogenide Photocatalysts for Water Splitting Chem. Mater. 2013, 25, 3232– 3238 DOI: 10.1021/cm401661x
      42
      Single-layer group-III monochalcogenide photocatalysts for water splitting
      Zhuang, Houlong L.; Hennig, Richard G.
      Chemistry of Materials (2013), 25 (15), 3232-3238CODEN: CMATEX; ISSN:0897-4756. (American Chemical Society)
      The recent synthesis of single-layer GaS and GaSe opens the question of stability for other single-layer group-III monochalcogenides (MX, M = Ga and In, X = S, Se, and Te) and how the dimension redn. affects the properties of these materials. Using a first-principles design approach, we det. that the single-layer group-III monochalcogenides exhibit low formation energies and are suitable for photocatalytic water splitting. First, d.-functional calcns. using a van der Waals functional reveal that the monochalcogenides have formation energies similar to that of single-layer MoS2, implying the ease of mech. extg. single-layer monochalcogenides from their layered bulk counterparts. Next, calcns. using a hybrid d. functional and the quasiparticle many-body G0W0 approxn. det. the conduction and valence band edge positions. Comparing the band edge positions with the redox potentials of water shows that single-layer monochalcogenides are potential photocatalysts for water splitting. Moreover, the bandgaps, band edge positions, and optical absorption of the single-layer monochalcogenides can be tuned by biaxial strain to increase the efficiency of solar energy conversion. Finally, calcns. of the enthalpy of solvation of the single-layer monochalcogenides suggest their stability in aq. soln.
    43. 43
      Stevanovic, V.; Lany, S.; Ginley, D. S.; Tumas, W.; Zunger, A. Assessing Capability of Semiconductors to Split Water Using Ionization Potentials and Electron Affinities Only Phys. Chem. Chem. Phys. 2014, 16, 3706– 3714 DOI: 10.1039/c3cp54589j
      43
      Assessing capability of semiconductors to split water using ionization potentials and electron affinities only
      Stevanovic, Vladan; Lany, Stephan; Ginley, David S.; Tumas, Willam; Zunger, Alex
      Physical Chemistry Chemical Physics (2014), 16 (8), 3706-3714CODEN: PPCPFQ; ISSN:1463-9076. (Royal Society of Chemistry)
      We show in this article that the position of semiconductor band edges relative to the water redn. and oxidn. levels can be reliably predicted from the ionization potentials (IP) and electron affinities (AE) only. Using a set of 17 materials, including transition metal compds., we show that accurate surface dependent IPs and EAs of semiconductors can be computed by combining d. functional theory and many-body GW calcns. From the extensive comparison of calcd. IPs and EAs with available exptl. data, both from photoemission and electrochem. measurements, we show that it is possible to sort candidate materials solely from IPs and EAs thereby eliminating explicit treatment of semiconductor/water interfaces. We find that at pH values corresponding to the point of zero charge there is on av. a 0.5 eV shift of IPs and EAs closer to the vacuum due to the dipoles formed at material/water interfaces.
    44. 44
      Jiang, X.; Wang, P.; Zhao, J. J. 2d Covalent Triazine Framework: A New Class of Organic Photocatalyst for Water Splitting J. Mater. Chem. A 2015, 3, 7750– 7758 DOI: 10.1039/C4TA03438D
      44
      2D covalent triazine framework: a new class of organic photocatalyst for water splitting
      Jiang, Xue; Wang, Peng; Zhao, Jijun
      Journal of Materials Chemistry A: Materials for Energy and Sustainability (2015), 3 (15), 7750-7758CODEN: JMCAET; ISSN:2050-7496. (Royal Society of Chemistry)
      Since the graphene boom, great efforts have been devoted to two-dimensional (2D) monolayer materials with exciting possibilities for applications. Most known 2D materials are inorg. Using the covalent triazine framework (CTF) as a representative, we explored 2D org. semiconductors using first-principles calcns. From a systematic study of the electronic band structures, work functions, CBM/VBM positions, and optical absorption spectra, we identified the CTF as a new class of 2D visible-light-driven organocatalyst for water splitting. Controllable construction of such CTFs from suitable org. subunits paves the way to correlate band alignment and chem. composites. In addn., multilayer CTFs have enhanced visible-light absorption compared to monolayer CTFs due to interlayer coupling. Our theor. prediction not only has fulfilled the search for org. counterparts of inorg. photocatalysts for water splitting, but also would motivate scientists to further search for novel 2D org. materials with other technol. applications.
    45. 45
      Buckeridge, J.; Butler, K. T.; Catlow, C. R. A.; Logsdail, A. J.; Scanlon, D. O.; Shevlin, S. A.; Woodley, S. M.; Sokol, A. A.; Walsh, A. Polymorph Engineering of Tio2: Demonstrating How Absolute Reference Potentials Are Determined by Local Coordination Chem. Mater. 2015, 27, 3844– 3851 DOI: 10.1021/acs.chemmater.5b00230
      45
      Polymorph Engineering of TiO2: Demonstrating How Absolute Reference Potentials Are Determined by Local Coordination
      Buckeridge, John; Butler, Keith T.; Catlow, C. Richard A.; Logsdail, Andrew J.; Scanlon, David O.; Shevlin, Stephen A.; Woodley, Scott M.; Sokol, Alexey A.; Walsh, Aron
      Chemistry of Materials (2015), 27 (11), 3844-3851CODEN: CMATEX; ISSN:0897-4756. (American Chemical Society)
      We report that the valence and conduction band energies of TiO2 can be tuned over a 4 eV range by varying the local coordination environments of Ti and O. We examine the electronic structure of eight known polymorphs and align their ionization potential and electron affinity relative to an abs. energy ref., using an accurate multiscale quantum-chem. approach. For applications in photocatalysis, we identify the optimal combination of phases to enhance activity in the visible spectrum. The results provide a coherent explanation for a wide range of phenomena, including the performance of TiO2 as an anode material for Li-ion batteries, allow us to pinpoint hollandite TiO2 as a new candidate transparent conducting oxide, and serve as a guide to improving the efficiency of photo-electrochem. water splitting through polymorph engineering of TiO2.
    46. 46
      Tomasi, J.; Mennucci, B.; Cammi, R. Quantum Mechanical Continuum Solvation Models Chem. Rev. 2005, 105, 2999– 3093 DOI: 10.1021/cr9904009
      46
      Quantum Mechanical Continuum Solvation Models
      Tomasi, Jacopo; Mennucci, Benedetta; Cammi, Roberto
      Chemical Reviews (Washington, DC, United States) (2005), 105 (8), 2999-3093CODEN: CHREAY; ISSN:0009-2665. (American Chemical Society)
      A review.
    47. 47
      Baerends, E. J.; Gritsenko, O. V.; van Meer, R. The Kohn-Sham Gap, the Fundamental Gap and the Optical Gap: The Physical Meaning of Occupied and Virtual Kohn-Sham Orbital Energies Phys. Chem. Chem. Phys. 2013, 15, 16408– 16425 DOI: 10.1039/c3cp52547c
      47
      The Kohn-Sham gap, the fundamental gap and the optical gap: the physical meaning of occupied and virtual Kohn-Sham orbital energies
      Baerends, E. J.; Gritsenko, O. V.; van Meer, R.
      Physical Chemistry Chemical Physics (2013), 15 (39), 16408-16425CODEN: PPCPFQ; ISSN:1463-9076. (Royal Society of Chemistry)
      A no. of consequences of the presence of the exchange-correlation hole potential in the Kohn-Sham potential are elucidated. One consequence is that the HOMO-LUMO orbital energy difference in the KS-DFT model (the KS gap) is not "underestimated" or even "wrong", but that it is phys. expected to be an approxn. to the excitation energy if electrons and holes are close, and numerically proves to be so rather accurately. It is phys. not an approxn. to the difference between ionization energy and electron affinity I - A (fundamental gap or chem. hardness) and also numerically differs considerably from this quantity. The KS virtual orbitals do not possess the notorious diffuseness of the Hartree-Fock virtual orbitals, they often describe excited states much more closely as simple orbital transitions. The Hartree-Fock model does yield an approxn. to I - A as the HOMO-LUMO orbital energy difference (in Koopmans' frozen orbital approxn.), if the anion is bound, which is often not the case. We stress the spurious nature of HF LUMOs if the orbital energy is pos. One may prefer Hartree-Fock, or mix Hartree-Fock and (approx.) KS operators to obtain a HOMO-LUMO gap as a Koopmans' approxn. to I - A (in cases where A exists). That is a different one-electron model, which exists in its own right. But it is not an "improvement" of the KS model, it necessarily deteriorates the (approx.) excitation energy property of the KS gap in mols., and deteriorates the good shape of the KS virtual orbitals.
    48. 48
      Seki, K.; Karlsson, U. O.; Engelhardt, R.; Koch, E. E.; Schmidt, W. Intramolecular Band Mapping of Poly(Para-Phenylene) Via Uv Photoelectron-Spectroscopy of Finite Polyphenyls Chem. Phys. 1984, 91, 459– 470 DOI: 10.1016/0301-0104(84)80078-6
      There is no corresponding record for this reference.
    49. 49
      Banerjee, M.; Shukla, R.; Rathore, R. Synthesis, Optical, and Electronic Properties of Soluble Poly-P-Phenylene Oligomers as Models for Molecular Wires J. Am. Chem. Soc. 2009, 131, 1780– 1786 DOI: 10.1021/ja805102d
      49
      Synthesis, Optical, and Electronic Properties of Soluble Poly-p-phenylene Oligomers as Models for Molecular Wires
      Banerjee, Moloy; Shukla, Ruchi; Rathore, Rajendra
      Journal of the American Chemical Society (2009), 131 (5), 1780-1786CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
      A homologous series of sol. poly-p-phenylenes contg. up to eight phenylene moieties (PP2-PP8) with branched iso-alkyl (or bis-n-alkylmethyl) groups has been synthesized and the structure-property relationship with regards to various optical and electronic properties established. All electronic and optoelectronic properties of poly-p-phenylenes followed a 1/n relationship (where n is no. of phenylene moieties) with the increasing no. of phenylene moieties. The low-energy electronic transition of the PP2-PP7 cation radicals, generated either by laser-flash photolysis or by chem. oxidn., varied as well according to the inverse (1/n) relationship. The obsd. inverse relationship of all measured electronic and optoelectronic properties against the increasing no. of phenylene units in various PPn's, as well as X-ray crystallog. of both neutral and a cation-radical salt of a representative tetra-p-phenylene oligomer allows us to demonstrate that the effective conjugation length in poly-p-phenylenes is, in part, controlled by the increasing no. of interactions of ortho hydrogens which may prevent simultaneous planarization of the continuous arrays of a large no. of phenylene moieties.
    50. 50
      Chi, C. Y.; Wegner, G. Chain-Length Dependence of the Electrochemical Properties of Conjugated Oligofluorenes Macromol. Rapid Commun. 2005, 26, 1532– 1537 DOI: 10.1002/marc.200500437
      50
      Chain-length dependence of the electrochemical properties of conjugated oligofluorenes
      Chi, Chunyan; Wegner, Gerhard
      Macromolecular Rapid Communications (2005), 26 (19), 1532-1537CODEN: MRCOE3; ISSN:1022-1336. (Wiley-VCH Verlag GmbH & Co. KGaA)
      Electrochem. properties of monodisperse oligofluorenes (OFn, n = 2 to 7) and corresponding polyfluorene were studied by cyclic and differential pulse voltammetry. In combination with data of UV-vis absorption spectra, a series of linear relations such as the band gap, the oxidn. potential, the ionization potential, and the electron affinity with the reciprocal no. of the fluorene units (1/n) were deduced. When a chain length of ca. 14 repeat units is reached, a stable structure of about one pos. charge per 3.5 repeat units is obtained.
    51. 51
      Nagashima, U.; Fujimoto, H.; Inokuchi, H.; Seki, K. Electronic and Geometric Structures of Oligothiophenes J. Mol. Struct. 1989, 197, 265– 289 DOI: 10.1016/0022-2860(89)85169-5
      There is no corresponding record for this reference.
    52. 52
      Seki, K.; Asada, S.; Mori, T.; Inokuchi, H.; Murase, I.; Ohnishi, T.; Noguchi, T. Uv Photoemission Spectroscopy of Poly(P-Phenylene Vinylene) (PPV) Solid State Commun. 1990, 74, 677– 680 DOI: 10.1016/0038-1098(90)90700-L
      There is no corresponding record for this reference.
    53. 53
      Fujimoto, H.; Nagashima, U.; Inokuchi, H.; Seki, K.; Nakahara, N.; Nakayama, J.; Hoshino, M.; Fukuda, K. Electronic and Geometric Structures of Oligothiophenes Studied by Ups and Mndo - π-Band Evolution and Effect of Disorder Phys. Scr. 1990, 41, 105– 109 DOI: 10.1088/0031-8949/41/1/026
      53
      Electronic and geometric structures of oligothiophenes studied by UPS and MNDO: π-band evolution and effect of disorder
      Fujimoto, H.; Nagashima, U.; Inokuchi, H.; Seki, K.; Nakahara, N.; Nakayama, J.; Hoshino, M.; Fukuda, K.
      Physica Scripta (1990), 41 (1), 105-9CODEN: PHSTBO; ISSN:0031-8949.
      UPS was applied to the investigation of the electronic structure of oligothiophenes with 4-8 thiophene ring. In a series of α-linked oligomers (αn, with the no. of rings n = 4-8), a systematic evolution of the π band forming levels are obsd. in the region of 0.7-3 eV below the Fermi level (EF) and the bandwidth becomes broader with the n. The non-bonding π band is obsd. at 3.5 eV below EF and its energy is almost independent of N. UPS spectra of α7 and α8 are fairly similar to the spectra of polythiophene, showing that these oligomers are good model compds. of the polymer. The effect of irregularity on the π-electron system was also studied by using oligomers which contain a β linkage or a vinylene group in the middle of the mol. The UPS spectra showed that the β linkages significantly affect the electronic structure of polythiophene, but vinylene does not. In order to analyze the UPS spectra and to investigate the electronic structures of oligomers, the orbital energies and the geometry of these oligomers are calcd. by MNDO-SCF MO method. The calcd. spectra using the obtained orbital energies agree well with the obsd. ones, particularly in the π region. The optimized geometry showed that (1) αn's have planar structures and π electrons can be delocalized, (2) the oligomer with β-linkages have nonplanar structure leading to limited delocalization of π-electrons, and (3) to oligomers with a vinylene group are almost planar and the disturbance by the vinylene group on the delocalization is small.
    54. 54
      Hino, S.; Iwasaki, K.; Tatematsu, H.; Matsumoto, K. Photoelectron-Spectra of Polypyrrole - the Effect of the Ambient Atmosphere to the Spectra Bull. Chem. Soc. Jpn. 1990, 63, 2199– 2205 DOI: 10.1246/bcsj.63.2199
      There is no corresponding record for this reference.
    55. 55
      Miyamae, T.; Yoshimura, D.; Ishii, H.; Ouchi, Y.; Seki, K.; Miyazaki, T.; Koike, T.; Yamamoto, T. Ultraviolet Photoelectron-Spectroscopy of Poly(Pyridine-2,5-Diyl), Poly(2,2′-Bipyridine-5,5′-Diyl), and Their K-Doped States J. Chem. Phys. 1995, 103, 2738– 2744 DOI: 10.1063/1.470508
      There is no corresponding record for this reference.
    56. 56
      Liao, L. S.; Fung, M. K.; Lee, C. S.; Lee, S. T.; Inbasekaran, M.; Woo, E. P.; Wu, W. W. Electronic Structure and Energy Band Gap of Poly (9,9-Dioctylfluorene) Investigated by Photoelectron Spectroscopy Appl. Phys. Lett. 2000, 76, 3582– 3584 DOI: 10.1063/1.126713
      56
      Electronic structure and energy band gap of poly (9,9-dioctylfluorene) investigated by photoelectron spectroscopy
      Liao, L. S.; Fung, M. K.; Lee, C. S.; Lee, S. T.; Inbasekaran, M.; Woo, E. P.; Wu, W. W.
      Applied Physics Letters (2000), 76 (24), 3582-3584CODEN: APPLAB; ISSN:0003-6951. (American Institute of Physics)
      The electronic structure of poly(9,9-dioctylfluorene) (PFO) film on a Au-coated Si substrate was investigated by UPS and XPS. From the UPS measurement, we obtained the ionization potential (Ip) of the PFO film, Ip = 5.60 ± 0.05 eV. From the XPS shake-up peaks of the C1s core level, we estd. the electron energy band gap (Eg) of the film, Eg = 3.10 ± 0.10 eV. By comparing the Eg with the optical absorption gap, we found that the value of Eg is closer to the optical absorption max. than to the optical absorption edge. Therefore, we suggest that the optical absorption max. may be a better approxn. than the optical absorption edge in estg. Eg.
    57. 57
      Hwang, J.; Kim, E. G.; Liu, J.; Bredas, J. L.; Duggal, A.; Kahn, A. Photoelectron Spectroscopic Study of the Electronic Band Structure of Polyfluorene and Fluorene-Arylamine Copolymers at Interfaces J. Phys. Chem. C 2007, 111, 1378– 1384 DOI: 10.1021/jp067004w
      57
      Photoelectron Spectroscopic Study of the Electronic Band Structure of Polyfluorene and Fluorene-Arylamine Copolymers at Interfaces
      Hwang, Jaehyung; Kim, Eung-Gun; Liu, Jie; Bredas, Jean-Luc; Duggal, Anil; Kahn, Antoine
      Journal of Physical Chemistry C (2007), 111 (3), 1378-1384CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
      The occupied and unoccupied states of poly(9,9'-dioctylfluorene) (F8) and poly(9,9'-dioctylfluorene-co-bis-N,N'-(4-butylphenyl)diphenylamine) (TFB) are investigated using UV photoelectron and inverse photoemission spectroscopies, cyclic voltammetry, and d. functional theory calcns. Hole injection barriers are detd. for interfaces between substrates with work function ranging from 4.3 to 5.1 eV and these two polymers as well as poly(9,9'-dioctylfluorene-co-bis-N,N'-(4-butylphenyl)-bis-N,N'-phenyl-1,4-phenylenediamine) (PFB). Vacuum level alignment with flat bands away from the interface is found when the interface hole barrier is 0.6 eV or larger. Band bending away from the Fermi level occurs when the hole barrier is smaller than 0.4 eV. This is due to the accumulation of excess interface charges on the polymer when the barrier is small. The resulting field shifts the polymer levels to limit charge penetration in the bulk of the film.
    58. 58
      Sohn, Y.; Stuckless, J. T. Bimolecular Recombination Kinetics and Interfacial Electronic Structures of Poly 2-Methoxy-5-(2-Ethyl-Hexyloxy)-P-Phenylene Vinylene on Gold Studied Using Two-Photon Photoemission Spectroscopy J. Chem. Phys. 2007, 126, 174901 DOI: 10.1063/1.2727473
      There is no corresponding record for this reference.
    59. 59
      Sohn, Y.; Stuckless, J. T. Characteristics of Photoexcitations and Interfacial Energy Levels of Regioregular Poly(3-Hexythiophene-2,5-Diyl) on Gold ChemPhysChem 2007, 8, 1937– 1942 DOI: 10.1002/cphc.200700348
      There is no corresponding record for this reference.
    60. 60
      Kanai, K.; Miyazaki, T.; Suzuki, H.; Inaba, M.; Ouchi, Y.; Seki, K. Effect of Annealing on the Electronic Structure of Poly(3-Hexylthiophene) Thin Film Phys. Chem. Chem. Phys. 2010, 12, 273– 282 DOI: 10.1039/B914100F
      60
      Effect of annealing on the electronic structure of poly(3-hexylthiophene) thin film
      Kanai, Kaname; Miyazaki, Takahiro; Suzuki, Hiroyuki; Inaba, Mina; Ouchi, Yukio; Seki, Kazuhiko
      Physical Chemistry Chemical Physics (2010), 12 (1), 273-282CODEN: PPCPFQ; ISSN:1463-9076. (Royal Society of Chemistry)
      The electronic structure and film structure of poly(3-hexylthiophene) (P3HT) have been studied by X-ray diffraction (XRD) measurements, UV-visible (UV-vis) absorption spectroscopy, near-edge X-ray absorption fine structure (NEXAFS) measurements, UV photoemission spectroscopy (UPS) and inverse photoemission spectroscopy (IPES). As reported in previous works, XRD results show that the crystallinity of the film with regioregular P3HT is significantly improved by annealing at 170 °C. The effects of annealing on the electronic structure strongly depend on the substrate and the degree of regioregularity of the P3HT polymer backbone. Even in the case of the regiorandom sample, annealing considerably changes the vacuum level energy, which is the result of changes in the conformation of the hexyl groups at the free surface of the film. The π- and π*-band onsets uniformly shift downward by the annealing resulting in an increased hole-injection barrier at the electrode interface. The effects of annealing on the electronic structure of regioregular samples are more complex and depend on multiple factors. It is necessary to take into account variations in the π- and π*-band widths and the polarization energy to det. the effects of annealing. The former is assocd. with the conformation of the backbones of the polymer chains, and the latter is assocd. with the packing d. of the conjugated polymer planes. The combination of these variations dets. the effects of annealing on the electronic structure of the regioregular film. This is a possible reason for the strong dependence of the effects of annealing on the surface roughness of the substrate, since substrate roughness has a considerable effect on the morphol. and crystallinity of regioregular P3HT films.
    61. 61
      Baik, M. H.; Friesner, R. A. Computing Redox Potentials in Solution: Density Functional Theory as a Tool for Rational Design of Redox Agents J. Phys. Chem. A 2002, 106, 7407– 7412 DOI: 10.1021/jp025853n
      61
      Computing Redox Potentials in Solution: Density Functional Theory as A Tool for Rational Design of Redox Agents
      Baik, Mu-Hyun; Friesner, Richard A.
      Journal of Physical Chemistry A (2002), 106 (32), 7407-7412CODEN: JPCAFH; ISSN:1089-5639. (American Chemical Society)
      High-level d. functional theory in combination with a continuum solvation model was employed to compute std. redox potentials in soln. phase for three different classes of electrochem. active mols.: small org. mols., metallocenes, and M(bpy)3x (M = Fe, Ru, Os; x = +3, +2, +1, 0, -1). Excellent agreement with exptl. detd. redox potentials is found with an av. deviation of ∼150 mV when four different solvents commonly in use for electrochem. measurements were included. To obtain quant. agreement between theory and expt., the use of a large basis set is crucial esp. when the redox couple includes anionic species. Whereas the addn. of diffuse functions improved the results notably, vibrational zero-point-energy corrections and addn. of entropy effects are less important. The computational protocol for computing redox potentials in soln., which was benchmarked, is a powerful and novel tool that will allow a mol.-level understanding of the features dictating the properties of redox-active species.
    62. 62
      Uudsemaa, M.; Tamm, T. Density-Functional Theory Calculations of Aqueous Redox Potentials of Fourth-Period Transition Metals J. Phys. Chem. A 2003, 107, 9997– 10003 DOI: 10.1021/jp0362741
      62
      Density-functional theory calculations of aqueous redox potentials of fourth-period transition metals
      Uudsemaa, Merle; Tamm, Toomas
      Journal of Physical Chemistry A (2003), 107 (46), 9997-10003CODEN: JPCAFH; ISSN:1089-5639. (American Chemical Society)
      Aq. M3+/M2+ redox potentials for 9 of the 10 4th-period transition metals, M, were calcd. using DFT methodol. in combination with the COSMO continuum model. Entropy contributions to the potentials are taken from expts. The model introduces no adjustable parameters beyond those present in the underlying theor. models. Inclusion of 2 solvation spheres (18 H2O mols.) is necessary. For the ions studied, the av. abs. difference from exptl. values is 0.29 V, with 4 out of 9 potentials (those of V, Cr, Fe, Cu) reproduced with better than 0.1 V accuracy.
    63. 63
      Shimodaira, Y.; Miura, T.; Kudo, A.; Kobayashi, H. Dft Method Estimation of Standard Redox Potential of Metal Ions and Metal Complexes J. Chem. Theory Comput. 2007, 3, 789– 795 DOI: 10.1021/ct700015t
      63
      DFT Method Estimation of Standard Redox Potential of Metal Ions and Metal Complexes
      Shimodaira, Yoshiki; Miura, Toshiko; Kudo, Akihiko; Kobayashi, Hisayoshi
      Journal of Chemical Theory and Computation (2007), 3 (3), 789-795CODEN: JCTCCE; ISSN:1549-9618. (American Chemical Society)
      The DFT calcn. was carried out to evaluate std. redox potential (SRP) for metal-to-metal cation and metal-to-metal complex systems. With the Born-Haber cycle, std. redox potential was composed of the cohesive energy, ionization energy, and solvation energy. The ligand exchange energy was added in case of metal complex. The solvent effects were incorporated by the self-consistent reaction field theory at the level of the polarized continuum model (PCM). At the highest level of calcns., the geometry optimization and harmonic frequency anal. were evaluated under the PCM. Utilizing exptl. values of the cohesive energy of metals, the std. deviations between the calcd. and exptl. SRP values were 0.20-0.27 V depending on the calcn. levels and basis sets used. For three Ag complexes with CN-, S2O32-, and NH3 ligands, the discrepancy was within 0.3 V.
    64. 64
      Roy, L. E.; Jakubikova, E.; Guthrie, M. G.; Batista, E. R. Calculation of One-Electron Redox Potentials Revisited. Is It Possible to Calculate Accurate Potentials with Density Functional Methods? J. Phys. Chem. A 2009, 113, 6745– 6750 DOI: 10.1021/jp811388w
      64
      Calculation of One-Electron Redox Potentials Revisited. Is It Possible to Calculate Accurate Potentials with Density Functional Methods?
      Roy, Lindsay E.; Jakubikova, Elena; Guthrie, M. Graham; Batista, Enrique R.
      Journal of Physical Chemistry A (2009), 113 (24), 6745-6750CODEN: JPCAFH; ISSN:1089-5639. (American Chemical Society)
      D. Functional calcns. have been performed to calc. the one-electron oxidn. potential for ferrocene and the redox couples for a series of small transition metal compds. of the first-, second-, and third-row elements. The solvation effects are incorporated via a self-consistent reaction field (SCRF), using the polarized continuum model (PCM). From our study of seven different d. functionals combined with three different basis sets for ferrocene, we find that no d. functional method can reproduce the redox trends from expt. when referencing our results to the exptl. abs. std. hydrogen electrode (SHE) potential. Including addnl. necessary assumptions such as solvation effects does not lead to any conclusion regarding the appropriate functional. However, we propose that if one refs. their transition metal compds. results to the calcd. abs. half-cell potential of ferrocene, they can circumvent the addnl. assumptions necessary to predict a redox couple. Upon employing this method on several organometallic and inorg. complexes, we obtained very good correlation between calcd. and exptl. values (R2 = 0.97), making it possible to predict trends with a high level of confidence. The hybrid functional B3LYP systematically underestimates the redox potential; however, the linear correlation between DFT and expt. is good (R2 = 0.96) when including a baseline shift. This protocol is a powerful tool that allows theor. chemists to predict the redox potential in soln. of several transition metal complexes a priori and aids in the rational design of redox-active catalysts.
    65. 65
      Nayak, P. K.; Periasamy, N. Calculation of Ionization Potential of Amorphous Organic Thin-Films Using Solvation Model and DFT Org. Electron. 2009, 10, 532– 535 DOI: 10.1016/j.orgel.2009.01.011
      65
      Calculation of ionization potential of amorphous organic thin-films using solvation model and DFT
      Nayak, Pabitra K.; Periasamy, N.
      Organic Electronics (2009), 10 (3), 532-535CODEN: OERLAU; ISSN:1566-1199. (Elsevier B.V.)
      D. functional theory and polarizable continuum model are used to calc. ionization potential of thin-films of 12 org. mols. Computed values are compared with exptl. values obtained from UV photoemission spectroscopy. The excellent correlation shows that it is possible to det. the ionization potential of org. mols. in solid-state within ±0.15 eV of the exptl. value. This method is useful for chemists in designing mols. for org. electronics.
    66. 66
      Nayak, P. K.; Periasamy, N. Calculation of Electron Affinity, Ionization Potential, Transport. Gap, Optical Band Gap and Exciton Binding Energy of Organic Solids Using ’Solvation’ Model and DFT Org. Electron. 2009, 10, 1396– 1400 DOI: 10.1016/j.orgel.2009.06.011
      66
      Calculation of electron affinity, ionization potential, transport gap, optical band gap and exciton binding energy of organic solids using solvation' model and DFT
      Nayak, Pabitra K.; Periasamy, N.
      Organic Electronics (2009), 10 (7), 1396-1400CODEN: OERLAU; ISSN:1566-1199. (Elsevier B.V.)
      Electron affinity of org. mols. in solids is calcd. using d. functional theory (DFT) and solvation' correction using polarizable continuum model. Computed values for 13 mols. show excellent correlation with exptl. values obtained from inverse photoemission spectroscopy. The same method was used to calc. ionization potential and transport gap of the 13 org. mols. in solid state. Optical band gap was calcd. for the 13 mols. using time-dependent DFT and polarizable continuum model, which in turn was used to calc. exciton binding energy of the mols. in solid state. Calcd. and exptl. values show good agreement that it is possible to det. the electron affinity, ionization potential, transport gap and optical band gap of org. mols. in solid state using mol. structure as the only input required for the calcn.
    67. 67
      Schwenn, P. E.; Burn, P. L.; Powell, B. J. Calculation of Solid State Molecular Ionisation Energies and Electron Affinities for Organic Semiconductors Org. Electron. 2011, 12, 394– 403 DOI: 10.1016/j.orgel.2010.11.025
      67
      Calculation of solid state molecular ionisation energies and electron affinities for organic semiconductors
      Schwenn, P. E.; Burn, P. L.; Powell, B. J.
      Organic Electronics (2011), 12 (2), 394-403CODEN: OERLAU; ISSN:1566-1199. (Elsevier B.V.)
      Energy level engineering has become one of the central ideas in org. optoelectronics; particularly in the field of org. photovoltaics. However, this paradigm has led to significant ambiguities and errors in the literature. To investigate and address these issues we calc. solid state ionisation energies and electron affinities of a range of org. mol. semiconductors from d. functional theory and the polarizable continuum model. We show that the differences between these results and measurements from (inverse) photoemission are on the same scale as the differences between the measured values reported by different groups. We compare our results with in vacuo calcns. and ests. of the ionisation energies and electron affinities from the eigenvalues of the Kohn-Sham equations. In vacuo calcns. overestimate the ionisation energies measured in the solid state, but underestimate solid state electron affinities. However, the Kohn-Sham eigenvalues predict the measured ionisation energies nearly as well as the full calcn. However, we show that the apparent accuracy of the Kohn-Sham eigenvalues is fortuitous and arises from the cancellation of the errors due to the use of Kohn-Sham MO energies as predictions of ionisation energies and electron affinities and the neglect of the polarizable solid state environment. These results stress the importance of descriptions based on mol. states rather than MOs when designing and characterizing materials for org. electronic and optoelectronic devices.
    68. 68
      Hughes, T. F.; Friesner, R. A. Development of Accurate Dft Methods for Computing Redox Potentials of Transition Metal Complexes: Results for Model Complexes and Application to Cytochrome P450 J. Chem. Theory Comput. 2012, 8, 442– 459 DOI: 10.1021/ct2006693
      68
      Development of Accurate DFT Methods for Computing Redox Potentials of Transition Metal Complexes: Results for Model Complexes and Application to Cytochrome P450
      Hughes, Thomas F.; Friesner, Richard A.
      Journal of Chemical Theory and Computation (2012), 8 (2), 442-459CODEN: JCTCCE; ISSN:1549-9618. (American Chemical Society)
      Single-electron redn. half potentials of 95 octahedral fourth-row transition metal complexes binding a diverse set of ligands have been calcd. at the unrestricted pseudospectral B3LYP/LACV3P level of theory in a continuum solvent. Through systematic comparison of exptl. and calcd. potentials, it is detd. that B3LYP strongly overbinds the d-manifold when the metal coordinates strongly interacting ligands and strongly underbinds the d-manifold when the metal coordinates weakly interacting ligands. These error patterns give rise to an extension of the localized orbital correction (LOC) scheme previously developed for org. mols. and which was recently extended to the spin-splitting properties of organometallic complexes. Mean unsigned errors in B3LYP redox potentials are reduced from 0.40 ± 0.20 V (0.88 V max. error) to 0.12 ± 0.09 V (0.34 V max. error) using a simple seven-parameter model. Although the focus of this article is on redox properties of transition metal complexes, we have found that applying our previous spin-splitting LOC model to an independent test set of oxidized and reduced complexes that are also spin-crossover complexes correctly reverses the ordering of spin states obtained with B3LYP. Interesting connections are made between redox and spin-splitting parameters with regard to the spectrochem. series and in their combined predictive power for properly closing the thermodn. cycle of d-electron transitions in a transition metal complex. Results obtained from our large and diverse databases of spin-splitting and redox properties suggest that, while the error introduced by single ref. B3LYP for simple multireference systems, like mononuclear transition metal complexes, remains significant, at around 2-5 kcal/mol, the dominant error, at around 10-20 kcal/mol, is in B3LYP's prediction of metal-ligand binding. Application of the LOC scheme to the rate-detg. hydrogen atom transfer step in substrate hydroxylation by cytochrome P 450 shows that this approach is able to correct the B3LYP barriers in comparison to recent kinetics expts.
    69. 69
      Tamblyn, I.; Refaely-Abramson, S.; Neaton, J. B.; Kronik, L. Simultaneous Determination of Structures, Vibrations, and Frontier Orbital Energies from a Self-Consistent Range-Separated Hybrid Functional J. Phys. Chem. Lett. 2014, 5, 2734– 2741 DOI: 10.1021/jz5010939
      69
      Simultaneous Determination of Structures, Vibrations, and Frontier Orbital Energies from a Self-Consistent Range-Separated Hybrid Functional
      Tamblyn, Isaac; Refaely-Abramson, Sivan; Neaton, Jeffrey B.; Kronik, Leeor
      Journal of Physical Chemistry Letters (2014), 5 (15), 2734-2741CODEN: JPCLCD; ISSN:1948-7185. (American Chemical Society)
      A self-consistent optimally tuned range-sepd. hybrid d. functional (scOT-RSH) approach is developed. It can simultaneously predict accurate geometries, vibrational modes, and frontier orbital energies. This is achieved by optimizing the range-sepn. parameter, γ, to both satisfy the ionization energy theorem and minimize interat. forces. We benchmark our approach against an established hybrid functional, B3LYP, using the G2 test set. scOT-RSH greatly improves the accuracy of occupied frontier orbital energies, with a mean abs. error (MAE) of only 0.2 eV relative to exptl. ionization energies compared to 2.96 eV with B3LYP. Geometries do not change significantly compared to those obtained from B3LYP, with a bond length MAE of 0.012 Å compared to 0.008 Å for B3LYP, and a 6.5% MAE for zero-point energies, slightly larger than that of B3LYP (3.1%). scOT-RSH represents a new paradigm in which accurate geometries and ionization energies can be predicted simultaneously from a single functional approach.
    70. 70
      Phillips, H.; Zheng, Z. L.; Geva, E.; Dunietz, B. D. Orbital Gap Predictions for Rational Design of Organic Photovoltaic Materials Org. Electron. 2014, 15, 1509– 1520 DOI: 10.1016/j.orgel.2014.03.040
      70
      Orbital gap predictions for rational design of organic photovoltaic materials
      Phillips, Heidi; Zheng, Zilong; Geva, Eitan; Dunietz, Barry D.
      Organic Electronics (2014), 15 (7), 1509-1520CODEN: OERLAU; ISSN:1566-1199. (Elsevier B.V.)
      Ionization potentials (IP) and electron affinities (EA) of org. mols. with applications in photovoltaic devices are calcd. using modern d. functional theory (DFT). Calcd. frontier orbital energies are compared to exptl. detd. IPs and EAs at gas phase and thin film environments. Gas phase frontier orbital energies calcd. with widely-used DFT functionals accidentally coincide with thin film measurements, reproducing condensed phase results for the wrong reasons. Recently developed range sepd. hybrid (RSH) functionals, on the other hand, provide gas phase frontier orbital energies that correspond properly to measured IPs and EAs. We also employ a polarizable continuum model to address the effects of the electrostatic environment in the solid state. We find that the environmentally-cor. RSH orbital energies compare well with thin film exptl. measurements.
    71. 71
      Richard, R. M.; Marshall, M. S.; Dolgounitcheva, O.; Ortiz, J. V.; Bredas, J. L.; Marom, N.; Sherrill, C. D. Accurate Ionization Potentials and Electron Affinities of Acceptor Molecules I. Reference Data at the CCSD(T) Complete Basis Set Limit J. Chem. Theory Comput. 2016, 12, 595– 604 DOI: 10.1021/acs.jctc.5b00875
      71
      Accurate Ionization Potentials and Electron Affinities of Acceptor Molecules I. Reference Data at the CCSD(T) Complete Basis Set Limit
      Richard, Ryan M.; Marshall, Michael S.; Dolgounitcheva, O.; Ortiz, J. V.; Bredas, Jean-Luc; Marom, Noa; Sherrill, C. David
      Journal of Chemical Theory and Computation (2016), 12 (2), 595-604CODEN: JCTCCE; ISSN:1549-9618. (American Chemical Society)
      In designing org. materials for electronics applications, particularly for org. photovoltaics (OPV), the ionization potential (IP) of the donor and the electron affinity (EA) of the acceptor play key roles. This makes OPV design an appealing application for computational chem. since IPs and EAs are readily calculable from most electronic structure methods. Unfortunately reliable, high-accuracy wave function methods, such as coupled cluster theory with single, double, and perturbative triples [CCSD(T)] in the complete basis set (CBS) limit are too expensive for routine applications to this problem for any but the smallest of systems. One soln. is to calibrate approx., less computationally expensive methods against a database of high-accuracy IP/EA values; however, to our knowledge, no such database exists for systems related to OPV design. The present work is the first of a multipart study whose overarching goal is to det. which computational methods can be used to reliably compute IPs and EAs of electron acceptors. This part introduces a database of 24 known org. electron acceptors and provides high-accuracy vertical IP and EA values expected to be within ±0.03 eV of the true non-relativistic, vertical CCSD(T)/CBS limit. Convergence of IP and EA values toward the CBS limit is studied systematically for the Hartree-Fock, MP2 correlation, and beyond-MP2 coupled cluster contributions to the focal point ests.
    72. 72
      Gallandi, L.; Marom, N.; Rinke, P.; Korzdorfer, T. Accurate Ionization Potentials and Electron Affinities of Acceptor Molecules II: Non-Empirically Tuned Long-Range Corrected Hybrid Functionals J. Chem. Theory Comput. 2016, 12, 605– 614 DOI: 10.1021/acs.jctc.5b00873
      72
      Accurate Ionization Potentials and Electron Affinities of Acceptor Molecules II: Non-Empirically Tuned Long-Range Corrected Hybrid Functionals
      Gallandi, Lukas; Marom, Noa; Rinke, Patrick; Koerzdoerfer, Thomas
      Journal of Chemical Theory and Computation (2016), 12 (2), 605-614CODEN: JCTCCE; ISSN:1549-9618. (American Chemical Society)
      The performance of non-empirically tuned long-range cor. hybrid functionals for the prediction of vertical ionization potentials (IPs) and electron affinities (EAs) is assessed for a set of 24 org. acceptor mols. Basis set-extrapolated coupled cluster singles, doubles, and perturbative triples [CCSD(T)] calcns. serve as a ref. for this study. Compared to std. exchange-correlation functionals, tuned long-range cor. hybrid functionals produce highly reliable results for vertical IPs and EAs, yielding mean abs. errors on par with computationally more demanding GW calcns. In particular, it is demonstrated that long-range cor. hybrid functionals serve as ideal starting points for non-self-consistent GW calcns.
    73. 73
      Rangel, T.; Hamed, S. M.; Bruneval, F.; Neaton, J. B. Evaluating the Gw Approximation with CCSD(T) for Charged Excitations across the Oligoacenes J. Chem. Theory Comput. 2016, 12, 2834– 2842 DOI: 10.1021/acs.jctc.6b00163
      73
      Evaluating the GW Approximation with CCSD(T) for Charged Excitations Across the Oligoacenes
      Rangel, Tonatiuh; Hamed, Samia M.; Bruneval, Fabien; Neaton, Jeffrey B.
      Journal of Chemical Theory and Computation (2016), 12 (6), 2834-2842CODEN: JCTCCE; ISSN:1549-9618. (American Chemical Society)
      Charged excitations of the oligoacene family of mols., relevant for astrophysics and technol. applications, are widely studied and therefore provide an excellent system for benchmarking theor. methods. In this work, we evaluate the performance of many-body perturbation theory within the GW approxn. relative to new high-quality CCSD(T) ref. data for charged excitations of the acenes. We compare GW calcns. with a no. of hybrid d. functional theory starting points and with eigenvalue self-consistency. Special focus is given to elucidating the trend of GW-predicted excitations with mol. length increasing from benzene to hexacene. We find that GW calcns. with starting points based on an optimally tuned range-sepd. hybrid (OTRSH) d. functional and eigenvalue self-consistency can yield quant. ionization potentials for the acenes. However, for larger acenes, the predicted electron affinities can deviate considerably from ref. values. Our work paves the way for predictive and cost-effective GW calcns. of charged excitations of mols. and identifies certain limitations of current GW methods used in practice for larger mols.
    74. 74
      Becke, A. D. Density-Functional Thermochemistry 0.3. The Role of Exact Exchange J. Chem. Phys. 1993, 98, 5648– 5652 DOI: 10.1063/1.464913
      74
      Density-functional thermochemistry. III. The role of exact exchange
      Becke, Axel D.
      Journal of Chemical Physics (1993), 98 (7), 5648-52CODEN: JCPSA6; ISSN:0021-9606.
      Despite the remarkable thermochem. accuracy of Kohn-Sham d.-functional theories with gradient corrections for exchange-correlation, the author believes that further improvements are unlikely unless exact-exchange information is considered. Arguments to support this view are presented, and a semiempirical exchange-correlation functional (contg. local-spin-d., gradient, and exact-exchange terms) is tested for 56 atomization energies, 42 ionization potentials, 8 proton affinities, and 10 total at. energies of first- and second-row systems. This functional performs better than previous functionals with gradient corrections only, and fits expt. atomization energies with an impressively small av. abs. deviation of 2.4 kcal/mol.
    75. 75
      Stephens, P. J.; Devlin, F. J.; Chabalowski, C. F.; Frisch, M. J. Ab-Initio Calculation of Vibrational Absorption and Circular-Dichroism Spectra Using Density-Functional Force-Fields J. Phys. Chem. 1994, 98, 11623– 11627 DOI: 10.1021/j100096a001
      75
      Ab Initio Calculation of Vibrational Absorption and Circular Dichroism Spectra Using Density Functional Force Fields
      Stephens, P. J.; Devlin, F. J.; Chabalowski, C. F.; Frisch, M. J.
      Journal of Physical Chemistry (1994), 98 (45), 11623-7CODEN: JPCHAX; ISSN:0022-3654.
      The unpolarized absorption and CD spectra of the fundamental vibrational transitions of the chiral mol. 4-methyl-2-oxetanone are calcd. ab initio. Harmonic force fields are obtained using d. functional theory (DFT), MP2 and SCF methodologies, and a [5s4p2d/3s2p] (TZ2P) basis set. DFT calcns. use the LSDA, BLYP, and Becke3LYP (B3LYP) d. functionals. Mid-IR spectra predicted using LSDA, BLYP, and B3LYP force fields are of significantly different quality, the B3LYP force field yielding spectra in clearly superior, and overall excellent, agreement with expt. The MP2 force field yields spectra in slightly worse agreement with expt. than the B3LYP force field. The SCF force field yields spectra in poor agreement with expt. The basis set dependence of B3LYP force fields is also explored: the 6-31G* and TZ2P basis sets give very similar results while the 3-21G basis set yields spectra in substantially worse agreement with expt.
    76. 76
      Klamt, A.; Schuurmann, G. Cosmo - a New Approach to Dielectric Screening in Solvents with Explicit Expressions for the Screening Energy and Its Gradient J. Chem. Soc., Perkin Trans. 2 1993, 799– 805 DOI: 10.1039/P29930000799
      76
      COSMO: a new approach to dielectric screening in solvents with explicit expressions for the screening energy and its gradient
      Klamt, A.; Schueuermann, G.
      Journal of the Chemical Society, Perkin Transactions 2: Physical Organic Chemistry (1972-1999) (1993), (5), 799-805CODEN: JCPKBH; ISSN:0300-9580.
      Starting from the screening in conductors, an algorithm for the accurate calcn. of dielec. screening effects in solvents is presented, which leads to rather simple explicit expressions for the screening energy and its analytic gradient with respect to the solute coordinates. Thus geometry optimization of a solute within a realistic dielec. continuum model becomes practicable for the first time. The algorithm is suited for mol. mechanics as well as for any MO algorithm. The implementation into MOPAC and some example applications are reported.
    77. 77
      Hirata, S.; Head-Gordon, M. Time-Dependent Density Functional Theory within the Tamm-Dancoff Approximation Chem. Phys. Lett. 1999, 314, 291– 299 DOI: 10.1016/S0009-2614(99)01149-5
      77
      Time-dependent density functional theory within the Tamm-Dancoff approximation
      Hirata, S.; Head-Gordon, M.
      Chemical Physics Letters (1999), 314 (3,4), 291-299CODEN: CHPLBC; ISSN:0009-2614. (Elsevier Science B.V.)
      A computationally simple method for mol. excited states, namely, the Tamm-Dancoff approxn. to time-dependent d. functional theory, is proposed and implemented. This method yields excitation energies for several closed- and open-shell mols. that are essentially of the same quality as those obtained from time-dependent d. functional theory itself, when the same exchange-correlation functional is used.
    78. 78
      Furche, F.; Ahlrichs, R.; Hattig, C.; Klopper, W.; Sierka, M.; Weigend, F. Turbomole Wiley Interdiscip. Rev. Comput. Mol. Sci. 2014, 4, 91– 100 DOI: 10.1002/wcms.1162
      78
      Turbomole
      Furche, Filipp; Ahlrichs, Reinhart; Haettig, Christof; Klopper, Wim; Sierka, Marek; Weigend, Florian
      Wiley Interdisciplinary Reviews: Computational Molecular Science (2014), 4 (2), 91-100CODEN: WIRCAH; ISSN:1759-0884. (Wiley-Blackwell)
      Turbomole is a highly optimized software package for large-scale quantum chem. simulations of mols., clusters, and periodic solids. Turbomole uses Gaussian basis sets and specializes on predictive electronic structure methods with excellent cost to performance characteristics, such as (time-dependent) d. functional theory (TDDFT), second-order Moller-Plesset theory, and explicitly correlated coupled cluster (CC) methods. These methods are combined with ultraefficient and numerically stable algorithms such as integral-direct and Laplace transform methods, resoln.-of-the-identity, pair natural orbitals, fast multipole, and low-order scaling techniques. Apart from energies and structures, a variety of optical, elec., and magnetic properties are accessible from anal. energy derivs. for electronic ground and excited states. Recent addns. include post-Kohn-Sham calcns. within the RPA, periodic calcns., spin-orbit couplings, explicitly correlated CC singles doubles and perturbative triples methods, CC singles doubles excitation energies, and nonadiabatic mol. dynamics simulations using TDDFT. A dedicated graphical user interface and a user support network are also available.
    79. 79
      Ahlrichs, R.; Bar, M.; Haser, M.; Horn, H.; Kolmel, C. Electronic-Structure Calculations on Workstation Computers - the Program System Turbomole Chem. Phys. Lett. 1989, 162, 165– 169 DOI: 10.1016/0009-2614(89)85118-8
      79
      Electronic structure calculations on workstation computers: the program system TURBOMOLE
      Ahlrichs, Reinhart; Baer, Michael; Haeser, Marco; Horn, Hans; Koelmel, Christoph
      Chemical Physics Letters (1989), 162 (3), 165-9CODEN: CHPLBC; ISSN:0009-2614.
      The basic structure of the program system TURBOMOLE for SCF - including first and second anal. derivs. with respect to nuclear coordinates - and MP2 calcns. is briefly described. The program takes full advantage of all discrete point group symmetries and has only modest - and (partially) adjustable - I/O and background storage requirements. The performance of TURBOMOLE is documented for demonstrative applications.
    80. 80
      Schafer, A.; Horn, H.; Ahlrichs, R. Fully Optimized Contracted Gaussian-Basis Sets for Atoms Li to Kr J. Chem. Phys. 1992, 97, 2571– 2577 DOI: 10.1063/1.463096
      There is no corresponding record for this reference.
    81. 81
      Isse, A. A.; Gennaro, A. Absolute Potential of the Standard Hydrogen Electrode and the Problem of Interconversion of Potentials in Different Solvents J. Phys. Chem. B 2010, 114, 7894– 7899 DOI: 10.1021/jp100402x
      81
      Absolute Potential of the Standard Hydrogen Electrode and the Problem of Interconversion of Potentials in Different Solvents
      Isse, Abdirisak A.; Gennaro, Armando
      Journal of Physical Chemistry B (2010), 114 (23), 7894-7899CODEN: JPCBFK; ISSN:1520-6106. (American Chemical Society)
      The abs. potential of the std. hydrogen electrode, SHE, was calcd. from a thermodn. cycle involving H2(g) atomization, ionization of H·(g) to H+(g)+, and hydration of H+. The most up-to-date literature values on the free energies of these reactions were selected and, when necessary, adjusted to the electron convention Fermi-Dirac statistics since both e- and H+ are fermions. As a ref. state for the electron, the authors have chosen the electron at 0 K, which is the one used in computational chem. Unlike almost all previous estns. of SHE, ΔG0aq-(H+) was used instead of the real potential, αaq(H+). This choice was made to obtain a SHE value based on the chem. potential, which is the appropriate ref. to be used in theor. computations of std. redn. potentials. The result of this new estn. is a value of 4.281 V for the abs. potential of SHE. The problem of conversion of std. redn. potentials (SRPs) measured or estd. in H2O to the corresponding values in nonaqeuous solvents also was addressed. In fact, thermochem. cycles are often used to calc. SRPs in H2O vs. SHE, and it is extremely important to have conversion factors enabling estn. of SRPs in nonaq. solvents. A general equation relating E0 of a generic redox couple in H2O vs. the SHE to the value of E0 in an org. solvent vs. the aq. SCE is reported.
    82. 82
      Trasatti, S. The Absolute Electrode Potential - an Explanatory Note (Recommendations 1986) Pure Appl. Chem. 1986, 58, 955– 966 DOI: 10.1351/pac198658070955
      82
      The absolute electrode potential: an explanatory note. Recommendations 1986
      Trasatti, Sergio
      Pure and Applied Chemistry (1986), 58 (7), 955-66CODEN: PACHAS; ISSN:0033-4545.
      An illustration is given of the most widespread misunderstandings in the literature about the phys. meaning of abs. electrode potential. The correct expression for this quantity is then derived by a thermodn. anal. of the components of the emf. of an electrochem. cell. In principle 3 ref. levels can be chosen to measure an abs. value of the electrode potential. Only 1 of these possess all the requisites for a meaningful comparison on a common energy scale between electrochem. and phys. parameters. Such a comparison is the main problem for which the adoption of a correct scale for abs. electrode potentials is a prerequisite. The document concludes with the recommendation of a crit. evaluated value for the abs. potential of the std. H electrode in H2O and in a few other protic solvents.
    83. 83
      Sato, N.; Seki, K.; Inokuchi, H. Polarization Energies of Organic-Solids Determined by Ultraviolet Photoelectron-Spectroscopy J. Chem. Soc., Faraday Trans. 2 1981, 77, 1621– 1633 DOI: 10.1039/f29817701621
      83
      Polarization energies of organic solids determined by ultraviolet photoelectron spectroscopy
      Sato, Naoki; Seki, Kazuhiko; Inokuchi, Hiroo
      Journal of the Chemical Society, Faraday Transactions 2: Molecular and Chemical Physics (1981), 77 (9), 1621-33CODEN: JCFTBS; ISSN:0300-9238.
      The polarization energies of 44 org. solids were detd. by UPS in the gaseous and solid states. Condensed polycyclic arom. hydrocarbons with planar mol. structures have a common value, 1.7eV, independent of their mol. sizes and crystal structures. The common value is approx. interpreted by the 1st-order expression for the polarization energy. A large variation of values in the range 0.9-3.0 eV was obtained for several compds. Among them, mols. with intricate structures have smaller values and those with large mol. polarizabilities have larger values than the common value. The polarization energy of an org. solid is mainly detd. by 2 factors, its mol. polarizability and the mol. packing in the solid. Intermol. interactions in the solid, other than the van der Waals force, also contribute to the value.
    84. 84
      Inokuchi, H.; Seki, K.; Sato, N. Uv Photoelectron-Spectroscopy of Organic Molecular Materials Phys. Scr. 1987, T17, 93– 103 DOI: 10.1088/0031-8949/1987/T17/011
      84
      UV photoelectron spectroscopy of organic molecular materials
      Inokuchi, Hiroo; Seki, Kazuhiko; Sato, Naoki
      Physica Scripta (1987), T17 (Proc. Int. Conf. Vac. Ultraviolet Radiat. Phys., 8th, 1986), 93-103CODEN: PHSTBO; ISSN:0031-8949.
      A review with 101 refs., on the recent progress in UPS studies of org. mol. materials taking the examples mainly from the results obtained in the authors' lab. Mol solids with and without strong intermol. interactions, mol complexes and polymers are discussed.
    85. 85
      Gestblom, B.; Songstad, J. Solvent Properties of Dichloromethane 0.6. Dielectric-Properties of Electrolytes in Dichloromethane Acta Chem. Scand. 1987, 41, 396– 409 DOI: 10.3891/acta.chem.scand.41b-0396
      There is no corresponding record for this reference.
    86. 86
      Tsiper, E. V.; Soos, Z. G.; Gao, W.; Kahn, A. Electronic Polarization at Surfaces and Thin Films of Organic Molecular Crystals: Ptcda Chem. Phys. Lett. 2002, 360, 47– 52 DOI: 10.1016/S0009-2614(02)00774-1
      86
      Electronic polarization at surfaces and thin films of organic molecular crystals: PTCDA
      Tsiper, E. V.; Soos, Z. G.; Gao, W.; Kahn, A.
      Chemical Physics Letters (2002), 360 (1,2), 47-52CODEN: CHPLBC; ISSN:0009-2614. (Elsevier Science B.V.)
      The electronic polarization energies, P=P++P-, of a perylenetetracarboxylic acid dianhydride (PTCDA) cation and anion in a cryst. thin film on a metallic substrate are computed and compared with measurements of the PTCDA transport gap on gold and silver. Both expts. and theory show that P is 500 meV larger in a PTCDA monolayer than in 50 Å films. Electronic polarization in systems with surfaces and interfaces are obtained self-consistently in terms of charge redistribution within mols.
    87. 87
      Ryno, S. M.; Risko, C.; Bredas, J. L. Impact of Molecular Packing on Electronic Polarization in Organic Crystals: The Case of Pentacene Vs Tips-Pentacene J. Am. Chem. Soc. 2014, 136, 6421– 6427 DOI: 10.1021/ja501725s
      87
      Impact of Molecular Packing on Electronic Polarization in Organic Crystals: The Case of Pentacene vs TIPS-Pentacene
      Ryno, Sean M.; Risko, Chad; Bredas, Jean-Luc
      Journal of the American Chemical Society (2014), 136 (17), 6421-6427CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
      Polarization energy corresponds to the stabilization of the cation or anion state of an atom or mol. when going from the gas phase to the solid state. The decrease in ionization energy and increase in electron affinity in the solid state are related to the (electronic and nuclear) polarization of the surrounding atoms and mols. in the presence of a charged entity. Here, through a combination of mol. mechanics and quantum mechanics calcns., the authors evaluate the polarization energies in two prototypical org. semiconductors, pentacene and 6,13-bis[2-(triisopropylsilyl)ethynyl]pentacene (TIPS-pentacene). Comparison of the results for the two systems reveals the crit. role played by the mol. packing configurations in the detn. of the polarization energies and provides phys. insight into the exptl. data reported by Lichtenberger and co-workers (J. Amer. Chem. Soc.2010, 132, 580; J. Phys. Chem. C2010, 114, 13838). Results underline that the impact of packing configurations, well established in the case of the charge-transport properties, also extends to the polarization properties of π-conjugated materials.
    88. 88
      Stein, T.; Eisenberg, H.; Kronik, L.; Baer, R. Fundamental Gaps in Finite Systems from Eigenvalues of a Generalized Kohn-Sham Method Phys. Rev. Lett. 2010, 105, 266802 DOI: 10.1103/PhysRevLett.105.266802
      88
      Fundamental Gaps in Finite Systems from Eigenvalues of a Generalized Kohn-Sham Method
      Stein, Tamar; Eisenberg, Helen; Kronik, Leeor; Baer, Roi
      Physical Review Letters (2010), 105 (26), 266802/1-266802/4CODEN: PRLTAO; ISSN:0031-9007. (American Physical Society)
      We present a broadly applicable, phys. motivated, first-principles approach to detg. the fundamental gap of finite systems from single-electron orbital energies. The approach is based on using a range-sepd. hybrid functional within the generalized Kohn-Sham approach to d. functional theory. Its key element is the choice of a range-sepn. parameter such that Koopmans' theorem for both neutral and anion is obeyed as closely as possible. We demonstrate the validity, accuracy, and advantages of this approach on first, second and third row atoms, the oligoacene family of mols., and a set of hydrogen-passivated silicon nanocrystals. This extends the quant. usage of d. functional theory to an area long believed to be outside its reach.
    89. 89
      Refaely-Abramson, S.; Sharifzadeh, S.; Jain, M.; Baer, R.; Neaton, J. B.; Kronik, L. Gap Renormalization of Molecular Crystals from Density-Functional Theory Phys. Rev. B: Condens. Matter Mater. Phys. 2013, 88, 081204 DOI: 10.1103/PhysRevB.88.081204
      89
      Gap renormalization of molecular crystals from density-functional theory
      Refaely-Abramson, Sivan; Sharifzadeh, Sahar; Jain, Manish; Baer, Roi; Neaton, Jeffrey B.; Kronik, Leeor
      Physical Review B: Condensed Matter and Materials Physics (2013), 88 (8), 081204/1-081204/5CODEN: PRBMDO; ISSN:1098-0121. (American Physical Society)
      Fundamental gap renormalization due to electronic polarization is a basic phenomenon in mol. crystals. Despite its ubiquity and importance, all conventional approaches within d.-functional theory completely fail to capture it, even qual. Here, we present a new screened range-sepd. hybrid functional, which, through judicious introduction of the scalar dielec. const., quant. captures polarization-induced gap renormalization, as demonstrated on the prototypical org. mol. crystals of benzene, pentacene, and C60. This functional is predictive, as it contains system-specific adjustable parameters that are detd. from first principles, rather than from empirical considerations.
    90. 90
      Few, S.; Frost, J. M.; Nelson, J. Models of Charge Pair Generation in Organic Solar Cells Phys. Chem. Chem. Phys. 2015, 17, 2311– 2325 DOI: 10.1039/C4CP03663H
      90
      Models of charge pair generation in organic solar cells
      Few, Sheridan; Frost, Jarvist M.; Nelson, Jenny
      Physical Chemistry Chemical Physics (2015), 17 (4), 2311-2325CODEN: PPCPFQ; ISSN:1463-9076. (Royal Society of Chemistry)
      A review. Efficient charge pair generation is obsd. in many org. photovoltaic (OPV) heterojunctions, despite nominal electron-hole binding energies which greatly exceed the av. thermal energy. Empirically, the efficiency of this process appears to be related to the choice of donor and acceptor materials, the resulting sequence of excited state energy levels and the structure of the interface. In order to establish a suitable phys. model for the process, a range of different theor. studies have addressed the nature and energies of the interfacial states, the energetic profile close to the heterojunction and the dynamics of excited state transitions. In this paper, we review recent developments underpinning the theory of charge pair generation and phenomena, focussing on electronic structure calcns., electrostatic models and approaches to excited state dynamics. We discuss the remaining challenges in achieving a predictive approach to charge generation efficiency.

  • Supporting Information

    Supporting Information

    The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.6b11133.

    • Tables of potentials for alternative εr values, tables of solid-state potentials vs vacuum, results with larger basis sets, and orbital approximation data for εr 80.1 (PDF)

    • xyz coordinates of relevant DFT optimized structures (ZIP)


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