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Structure-Directed Exciton Dynamics in Templated Molecular Nanorings

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† Department of Physics, Clarendon Laboratory, University of Oxford, Parks Road, Oxford OX1 3PU, United Kingdom
‡ Department of Chemistry, Chemistry Research Laboratory, University of Oxford, Oxford OX1 3TA, United Kingdom
*E-mail: [email protected]
*E-mail: [email protected]
Cite this: J. Phys. Chem. C 2015, 119, 11, 6414–6420
Publication Date (Web):February 26, 2015
https://doi.org/10.1021/acs.jpcc.5b00210
Copyright © 2015 American Chemical Society
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Abstract

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Conjugated polymers with cyclic structures are interesting because their symmetry leads to unique electronic properties. Recent advances in Vernier templating now allow large shape-persistent fully conjugated porphyrin nanorings to be synthesized, exhibiting unique electronic properties. We examine the impact of different conformations on exciton delocalization and emission depolarization in a range of different porphyrin nanoring topologies with comparable spatial extent. Low photoluminescence anisotropy values are found to occur within the first few hundred femtoseconds after pulsed excitation, suggesting ultrafast delocalization of excitons across the nanoring structures. Molecular dynamics simulations show that further polarization memory loss is caused by out-of-plane distortions associated with twisting and bending of the templated nanoring topologies.

Introduction

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Conjugated polymers have been studied extensively because of their semiconducting and optical properties, which offer great potential for a diverse range of applications (1) such as organic light-emitting diodes, (2, 3) field-effect transistors, (4, 5) and polymer solar cells. (6, 7) Recently, macrocycles have attracted increasing attention as quasi-infinite π-conjugated systems exhibiting unique electronic and optical behavior. (8) Several studies have investigated electron or energy transfer, nonlinear optical phenomena, and topological effects in these structures. (9-14)
A promising approach to synthesis of nanosized fully π-conjugated macrocycles based on porphyrin units has been developed using template-directed self-assembly. (15, 16) However, the difficulty of synthesizing suitable templates has greatly limited the size of macrocycles accessible. To this end, Vernier effects were exploited to direct synthesis of large nanorings using multiple smaller templates. (17) Vernier templating not only proves to be a powerful new strategy to construct large nanorings, but also renders shape-persistent structures with diverse geometries. (18) Exciton delocalization and emission depolarization processes are expected to be strongly affected by the change of geometry in the structures. (19)
Conformational effects on photophysical properties have been examined extensively using both experimental (20, 21) and theoretical methods (22) on a variety of conjugated polymer systems, such as PPV, (23, 24) P3HT, (25, 26) and dendrimers. (27) In particular, effects on polarization memory loss (18, 23) and energy transfer (28, 29) have attracted much attention.
At the center of our study, we address the question of how deliberate distortion of a molecular nanoring influences the emissive properties of a molecule following excitation. We examine to what extent out-of-plane distortions of a particular nanoring topology affect the polarization memory of the transition dipole moment and whether such effects can be predicted by molecular mechanics simulations. In search for answers to these questions, a range of conjugated zinc porphyrin nanorings of similar size but different conformations are examined using ultrafast time-resolved spectroscopy and the results are compared to molecular dynamics simulations of ring structures.

Experimental Section

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Materials

The synthesis and characterization of the nanoring templates and complexes are described in detail elsewhere. (15, 17, 30-32) Figure 1 introduces the porphyrin nanorings under investigation, which comprise porphyrin units joined by butadiyne bridges. Through insertion of templates binding to the porphyrin units, a range of different molecular shapes are created with spatial extent on the nanometer scale. Untemplated nanorings c-P10 and c-P12 only differ slightly in ring diameter and have almost identical conformation and absorption/emission spectra (further information in the Supporting Information (SI)). Both samples are prepared in toluene/1% pyridine solution to prevent aggregation. Templating changes the geometry considerably by rigidifying the nanoring into two smaller porphyrin “loops” and introducing significant out-of-plane distortions, such as bending and twisting. Molecular mechanics simulations (vide infra) show that c-P10·(T5)2 and c-P10·(T6)2 have similar structures, with the main difference being in the shape of templates as shown in Figure 1b,c. The angle between the mean template planes in c-P10·(T5)2 is calculated to be θ = 72°. The twist in c-P12·(T6)2 is even more pronounced and has the form of a figure-of-eight, with θ = 28°, as shown from crystal structure data in a previous study. (17) On the other hand, c-P12·(T8)2 is planar and not expected to exhibit any substantial twist. These conformations have been deduced from molecular mechanics and dynamics simulation using HyperChem, which will be described later in detail. Samples with template complexes were prepared in pure toluene solution, as the templates would otherwise be displaced by competition with pyridine.

Figure 1

Figure 1. Chemical structures of the budadiyne-linked zinc porphyrin nanoring assemblies under investigation: (a) c-P10, (b) c-P10·(T5)2, (c) c-P10·(T6)2, (d) c-P12, (e) c-P12·(T6)2, and (f) c-P12·(T8)2. Templates are indicated in blue, and zinc atoms are denoted in red. All compounds have octyloxy side chains (Ar) which are omitted in the graphic for clarity.

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Time-Resolved Photoluminescence Spectroscopy

The photoluminescence (PL) upconversion technique was engaged to investigate PL dynamics of sample solutions held in quartz cuvettes as described in detail elsewhere. (18, 33) An excitation pulse was generated by a mode-locked Ti:sapphire laser with pulse duration of 100 fs and a repetition rate of 80 MHz. PL is collected and optically gated in a beta-barium-oxide (BBO) crystal by a vertically polarized time-delayed gate beam. The upconverted signal, which consists of sum-frequency photons from the gate pulse and the vertical component of the PL, was collected, dispersed in a monochromator, and detected using a nitrogen-cooled CCD. Using a combination of a half-wave plate and a Glan-Thompson polarizer, the polarization of the excitation pulse was varied and PL intensity dynamics were recorded separately for components polarized parallel (I∥) and perpendicular (I⊥) to the excitation pulse polarization. The PL anisotropy is defined using γ = (I∥ – I⊥) /(I∥ + 2I⊥) and calculated from the measured components. The full-width-half-maximum (FWHM) of the instrumental response function (IRF) was measured to be ∼270 fs, which gives the time-resolution limit of the system (further details in the SI). By recording the IRF with both polarizations, a temporal shift of ∼15 fs between I∥ and I⊥was found, and thus, the calculation of γ(t) was adjusted accordingly (further information in the SI). (34)
To investigate PL decay dynamics at longer delay time after excitation (>1 ns), electronic gating through time-correlated single-photon counting (TCSPC) technique was explored, using a Becker & Hickl module. Here, emission was detected with a silicon single-photon avalanche diode, yielding a temporal resolution of around 40 ps. By fitting the experimental data to a single exponential decay model IPL = A exp(−t/τ), the lifetime τ of the excitation was extracted.
Steady-state absorption and time-integrated PL spectra at room temperature were recorded using a PerkinElmer Lambda 1050 UV/Vis/NIR spectrometer and a Horiba FluoroLog fluorimeter, respectively.

Results and Discussion

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Absorption and Emission Spectra

In order to gain basic insight about the photophysical and electronic properties of the nanorings, steady state absorption and emission spectroscopy was performed. The resulting spectra for c-P12 and c-P12·(T6)2 are compared in Figure 2; spectra for further structures are provided in the SI. Zinc porphyrin monomers exhibit a strong S0 → S2 transition at ∼400 nm (Soret band) and a weaker S0 → S1 transition at ∼550 nm (Q-band). (35) For the nanorings, π-conjugation extends through the butadiyne bridges, lifting the degeneracy in both the lowest-energy Q band and Soret band of the porphyrin monomers. Transitions polarized parallel (Qx) and perpendicular (Qy) to the acetylenic backbone can therefore be observed (see Figure 2). When the nanorings bind to the templates, Qx is observed to be red-shifted with respect to the Soret band, while Qy shows no considerable change. In addition, a significant sharpening of features in both emission and absorption spectra can be observed for the templated complexes. Templates bring rigidity to the system; the molecules are less prone to interporphyrin torsional motions and the molecular backbone may be planarized, (18) resulting in the observed red shift of Qx. The four templated nanorings hence exhibit similar spectral features (see also further spectra in the SI).

Figure 2

Figure 2. Normalized steady-state absorption (blue lines) and time-integrated photoluminescence (green lines) spectra at 295 K for (a) c-P12 in toluene/1% pyridine and (b) c-P12·(T6)2 in toluene solution. The emission spectra were recorded after excitation at 520 nm (into the Soret band). The Qx and Qy transitions are indicated by red arrows.

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Photoluminescence Intensity and Polarization Anisotropy Dynamics

PL upconversion under ambient condition was performed to reveal the extent of emission depolarization following photoexcitation with a linearly polarized laser pulse. The templated nanorings were excited at 820 nm and the untemplated rings at 760 nm, with emission being monitored at the peak wavelength for each nanoring. To avoid PL quenching via exciton–exciton annihilation, (36) the excitation fluence was kept low (0.08 μJ cm–2). Figure 3 depicts the time-dependent PL of c-P12 and c-P12·(T6)2 over the first 15 ps after excitation with a pulse polarized either parallel (I∥) or perpendicular (I⊥) to the polarization of the detected emission. Corresponding data for all investigated molecules are given in the SI. For c-P12, the PL intensity remains constant over the experiment time window after the onset of the signal at t = 0, whereas c-P12·(T6)2 shows a slight decaying trend, in agreement with the shorter lifetime obtained using TCSPC (shown in Table 1).

Figure 3

Figure 3. Time-resolved PL dynamics of (a) c-P12 in toluene/1% pyridine (concentration 0.2 mM) and (b) c-P12·(T6)2 in toluene solution (concentration 0.4 mM). Samples were excited by excitation pulse polarized either parallel (blue dots) or perpendicular (red dots) to the detection polarization, as illustrated in the inset.

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Selective excitation near the Qx band of a rigid butadiyne-bridged porphyrin oligomer is expected to result in an excited state with a transition dipole moment along the molecular backbone. (18) In the absence of any depolarization mechanisms, emission will occur in the same polarization direction as the absorption, and for a randomly oriented distribution of such molecules in solution, an initial anisotropy value of 0.4 is expected. (37, 38) However, for a complete depolarization of the transition dipole moment within the 2D plane, as may be the case for the untemplated rings, a PL anisotropy value of 0.1 should be found. (37, 38) Other effects, such as out-of-plane distortion, could further lower the value toward zero. (38)
Figure 4 shows the temporal dependence of the anisotropy γ(t) for a number of the investigated nanoring topologies. Data for the remaining structures are provided in the SI. For all nanoring topologies investigated, the anisotropy is found to be static over the first 20 ps after excitation, within the temporal resolution of 270 fs. These dynamics were recorded for excitation near the middle of the Qx band. It is possible that excitation near the band edge (∼900 nm) may lead to creation of localized states that have higher anisotropy and/or slower PL depolarization; (19) however, we are unable to probe such effects here. These transients therefore indicate that while an initial ultrafast depolarization process may occur within the first hundred femtoseconds after excitation, slower effects such as molecular reorientation are absent over these time scales. (18) The values extracted in this way for the initial PL anisotropy γ are listed in Table 1 for all molecules investigated. These values have been independently determined multiple times at 5 ps after excitation, and an overall accuracy of ±0.02 was achieved. The untemplated rings c-P10 and c-P12 display anisotropy values around 0.1, as expected for a complete memory loss in the 2D plane and in the absence of significant out-of-plane distortions of the rings. (19) For all templated rings on the other hand, a significant reduction of γ below the value of 0.1 was observed.
These results suggest that the excitation can access any segment on the rings, both templated and untemplated, within the first few hundred femtoseconds after excitation. Such dynamics may be accounted for by either of two different mechanisms. In the first possible scenario, excitons are initially fully delocalized over the entire ring in the absorbing state, (18, 39) but may subsequently self-localize following geometric relaxation. (10, 40-44) In the second scenario, the absorbing state is only delocalized over a subset of monomers, but excitons are subject to ultrafast energy migration along the ring within hundreds of femtoseconds and thus beyond the time-resolution of the upconversion system, similar to observations reported in LH2 antenna complexes. (45) It has recently been shown that as the size of untemplated porphyrin nanorings increases, a shift from the former to the latter scenario may gradually occur. (14, 19)
We find that the measured anisotropy values below 0.1 for the templated nanorings can be explained by the presence of significant out-of-plane distortions. Molecular dynamics simulations discussed in detail below show that the measured γ is closer to zero for nanoring topologies with larger distortions. As illustrated in Figure 4, the rigid planar c-P12·(T8)2 has an appreciably higher anisotropy (γ = 0.07) than the other three templated nanorings which are found to display significant out-of-plane distortions.

Figure 4

Figure 4. PL polarization anisotropy as a function of time after excitation at 820 nm for c-P12 (blue circles, detection at 872 nm), c-P12·(T8)2 (red triangles, detection at 904 nm), and c-P12·(T6)2 (green squares, detection at 914 nm). The structures shown on the right are energy minimized geometries calculated using a modified form of MM+ force field in HyperChem. Side chains are not shown in the diagram but were included in the simulations.

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Molecular Dynamics and PL Depolarization Simulation

To quantitatively examine the distortions present in the different nanoring topologies, molecular dynamics simulations were conducted for the templated nanorings with side chains using HyperChem. The starting configurations were energy-minimized geometries (at 0 K) calculated using a modified form of HyperChem’s MM+ force field. Additional bond stretch, angle bend, and torsion terms were added to describe metalloporphyrins, (46, 47) alkynes, (48) and butadiynes. (49)
Molecular dynamics simulation were conducted assuming ambient vacuum at two constant temperatures, 300 and 600 K. Simulations ran in time steps of 1 fs for a time window of 500 ps, which gave a representative average. Since the templates are fairly rigid, it can be assumed that distortions mainly originate from motions between two template planes where two fundamental motions can be distinguished: twisting and bending, as illustrated in Figure 5a,b for the case of 300 K. Corresponding results for 600 K are shown in the SI. The corresponding angles α and β are introduced in the figure to quantitatively describe the deviation of the structure from a planar conformation. All starting configurations hold β(0) ∼ 0°. The distortion angles of the conformations in the 500 ps simulation window are plotted as a histogram, and the results are summarized in Figure 5c–f, with the root-mean-square (RMS) values listed in Table 1. Due to the nature of the simulations and the ambiguity in defining the distortion angles, these results only provide a good estimate of the overall trend. It is remarkable that, compared to the bending angle β, the spread in the twisting angle α is generally much narrower and the RMS remains closer to the energetically optimized values, which indicates that, in the molecular dynamics, thermal energy has a greater influence on the bending motion than the twisting motion. c-P10·(T5)2 and c-P10·(T6)2 show large out-of-plane distortions in twisting (around 65°) as well as bending motion at 300 K, consistent with the experimentally observed low PL anisotropy values of γ = 0.05. c-P12·(T6)2 and c-P12·(T8)2 both show similar distributions in α around ∼25°, much lower than that for the templated c-P10 topologies in comparison and in good agreement with structures obtained from crytallographic investigations. (17) These simulations explain the only slight decrease in γ from 0.1 to 0.07 experimentally observed for c-P12·(T8)2. However, c-P12·(T6)2 has a much lower PL anisotropy, which can be accounted for by its large bending angle similar to c-P10·(T5)2 and c-P10·(T6)2. In contrast, c-P12·(T8)2 remains rigid: β is narrowly distributed around the planar starting conformation.
In summary, the molecular dynamics simulations reveal that the templates provide an effective means to control the nanoring structure, as can be seen in the narrow distribution in Figure 5c–f as well as in the close resemblance between RMS values at 300 K and the energetically optimized values at 0 K for all nanorings. Furthermore, a good correspondence is evident between simulated conformations and measured PL anisotropy: the more pronounced the out-of-plane distortions in both twisting and bending motions, the further the anisotropy is found to be lowered from 0.1.
Table 1. Experimentally Obtained PL Lifetimes τ and Anisotropy γ Values for All Nanorings under Investigation, as Well as the Root-Mean-Squared Values of the Distortion angles α (Twisting) and β (Bending) Obtained for the Templated Rings from Molecular Dynamics Simulation using HyperChema
moleculeτ ± 5% (ps)γ ± 0.02γs(⟨α 2⟩)1/2(⟨β 2⟩)1/2
c-P106830.110.10––
c-P127920.100.10––
c-P10·(T5)24710.050.0368°54°
c-P10·(T6)24630.050.0360°62°
c-P12·(T6)24480.040.0624°44°
c-P12·(T8)25430.070.0827°14°
a

Using a simplified model based on the results from HyperChem as described in the main text, simulated anisotropy values γs are given.

To link the value experimentally obtained for the PL anisotropy with the calculated molecular structures, simple simulations were performed using a distributed point-dipole model. (38, 50, 51) The porphyrin units in each nanoring are assumed to be arranged in two smaller circles around each template as a result of the template’s rigidity, with their transition dipole moments aligned along the molecular backbone, as shown in the schematic in the SI. The angle between the two template planes in the calculation is taken to be the RMS values simulated by HyperChem at 300 K. Corresponding values for 600 K are given in the SI and exhibit similar trends. For simplicity, the emitting and absorbing states are assumed to extend over one monomer unit. This approximation seems sensible as the PL depolarization dynamics are observed to be sufficiently fast (<300 fs) to suggest that all sites are visited with similar probability. Therefore, the calculated average based on contributions from individual monomers will yield representative results, independent of the extent of exciton wave function delocalization. The polarization anisotropy is then calculated using (38)(1)with the only parameter being κ, the angle between absorbing and emitting dipole moments. Excitation can access any segment of a nanoring, and so each monomer can act as absorbing or emitting site with equal probability. Therefore, angles between dipoles of any two porphyrin monomers on the rings are considered and the average of γ is calculated according to eq 1. Even though this model is rather simplified and primarily focuses on the effect of bending and twisting distortions on the PL anisotropy, the results agree well with the experimental findings. The calculated anisotropy results γs are listed in Table 1 and lie within the error range of the experimental values. This correspondence confirms that the out-of-plane distortion is in fact the major contribution causing the lowered anisotropy value from 0.1. The modeled values tend to be slightly lower than the experimental values, but are surprisingly close, given the simplicity of the model. Deviations between experimental and simulated values may arise because simulations were carried out assuming vacuum conditions (screening of intermolecular interactions arising from the solvent was not taken into account) and because of some variation with the molecular mechanics potential chosen. In addition, a more complete description of exciton depolarization dynamics ought to include effects such as exciton localization dynamics, (41) for example, to sites that are preferred through specific local geometries. However, the good agreement between the experimental results and the simple simulation performed here ultimately results from the intimate link between the topology of the molecule to the polarization directions its emitting state can explore.

Figure 5

Figure 5. (a,b) Definition of the distortion angles. (a) The twisting angle α between two template planes is defined using c-P12·(T6)2 as an example. Four atoms on the porphyrin ring (red) are chosen to form three vectors a⃗ , b⃗ , and c⃗, with a⃗ and c⃗ lying approximately parallel to each other when the molecule does not exhibit any twist. α is the torsional angle defined by the four atoms and is the angle between the planes formed by a⃗, b⃗ and b⃗, c⃗. α represents the twisting angle from a planar position. (b) The bending angle β is illustrated using c-P12·(T8)2 as an example. Four atoms (red) are chosen as shown. Two angles ϕ and δ are calculated, where β = ϕ + δ describes the deviation from a planar position. Simulations were carried out with octyloxy side chains, which are omitted in the diagram for clarity. (c–f) Area normalized histogram of twisting angle α (blue) and bending angle β (red) for (c) c-P10·(T5)2, (d) c-P10·(T6)2, (e) c-P12·(T6)2, (f) c-P12·(T8)2. Molecular dynamics simulation carried out using HyperChem at 300 K for 500 ps, as described in detail in the text.

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Conclusion

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π-Conjugated nanorings containing 10 or 12 porphyrin units have been investigated to explore the influence of molecular topology on exciton dynamics and polarization memory loss. It is found that excitations can access any part of the templated nanorings, despite their twisted conformations, thus showing similar exciton dynamics to their more planar untemplated counterparts. Time-dependent ultrafast spectroscopy reveals that all nanorings exhibit static PL anisotropy transients within the first 20 ps after excitation, but with an anisotropy much lower than the value of 0.1 found for untemplated rings. Molecular dynamics simulations show that this phenomenon can be fully explained and well described by the out-of-plane distortions of the rings from a planar conformation. Numerical simulations of the PL anisotropy taking into account such distortions yield values that correlate well with the experimentally obtained results. In summary, template-directed synthesis not only plays a vital role in the synthesis of large porphyrin rings, but it also rigidifies the nanorings and introduces a means to control their geometric structures while maintaining excellent π-conjugation. These findings open exciting new possibilities for fast and directional energy transfer in nanoscale molecular structures.

Supporting Information

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Absorption and emission spectra of all compounds. Characterization of PL upconversion response function. PL transients and anisotropy dynamics of all compounds. Details of anisotropy modeling. Molecular Dynamics and PL Depolarization at 600K. This material is available free of charge via the Internet at http://pubs.acs.org/.

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Author Information

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  • Corresponding Authors
    • Juliane Q. Gong - Department of Physics, Clarendon Laboratory, University of Oxford, Parks Road, Oxford OX1 3PU, United Kingdom;  Email: [email protected]
    • Laura M. Herz - Department of Physics, Clarendon Laboratory, University of Oxford, Parks Road, Oxford OX1 3PU, United Kingdom;  Email: [email protected]
  • Authors
    • Patrick Parkinson - Department of Physics, Clarendon Laboratory, University of Oxford, Parks Road, Oxford OX1 3PU, United Kingdom
    • Dmitry V. Kondratuk - Department of Chemistry, Chemistry Research Laboratory, University of Oxford, Oxford OX1 3TA, United Kingdom
    • Guzmán Gil-Ramírez - Department of Chemistry, Chemistry Research Laboratory, University of Oxford, Oxford OX1 3TA, United Kingdom
    • Harry L. Anderson - Department of Chemistry, Chemistry Research Laboratory, University of Oxford, Oxford OX1 3TA, United Kingdom
  • Notes
    The authors declare no competing financial interest.

Acknowledgment

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The authors thank Martin D. Peeks for modification of HyperChem MM+ force field for porphyrins. Support from European Research Council (COSUN) and the Engineering and Physical Sciences Research Council (EPSRC) is gratefully acknowledged.

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    Highly π-Extended Copolymers with Diketopyrrolopyrrole Moieties for High-Performance Field-Effect Transistors
    Chen, Huajie; Guo, Yunlong; Yu, Gui; Zhao, Yan; Zhang, Ji; Gao, Dong; Liu, Hongtao; Liu, Yunqi
    Advanced Materials (Weinheim, Germany) (2012), 24 (34), 4618-4622, S4618/1-S4618/13CODEN: ADVMEW; ISSN:0935-9648. (Wiley-VCH Verlag GmbH & Co. KGaA)
    The authors synthesized two D-A type copolymers PDVTs with a highly π-extended conjugation backbone. Although the polymers with different length sidechains exhibit almost, same absorption maxima and HOMO energy levels, they show different film-forming ability, intermol. interaction force, and charge-carrier transporting characteristics. The longer sidechains (2-decyltetradecyl chains) endow PDVT-10 with more uniform thin films and closer n-n stacking distances compared to those of PDVT-8. The mobility higher than 8.0 cm2 V-1s-1 as well as high shelf storage and operation stability have been demonstrated for the FET devices based on PDVT-1O, which is much higher than amorphous silicon FETs. These results provide important progress of soln.-processed polymeric FETs. Such FETs fabricated from polymer semiconductors can be competitive with small mol. counterparts and amorphous silicon semiconductors.
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  6. 6
    Boudreault, P.-L. T.; Najari, A.; Leclerc, M. Processable Low-Bandgap Polymers for Photovoltaic Applications Chem. Mater. 2011, 23, 456– 469
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    6
    Processable Low-Bandgap Polymers for Photovoltaic Applications
    Boudreault, Pierre-Luc T.; Najari, Ahmed; Leclerc, Mario
    Chemistry of Materials (2011), 23 (3), 456-469CODEN: CMATEX; ISSN:0897-4756. (American Chemical Society)
    A review of the latest conjugated polymers used in bulk heterojunction solar cells. We will mainly focus on electron-donating (p-type) polymers based on thiophenes, 1,3,2-benzodiathiazoles, pyrrolo[3,4-c]pyrrole-1,4-diones, benzo[1,2-b;3,4-b]dithiophenes, and few other materials with more exotic structures. The bulk heterojunction architecture, in which the photoactive layer consists of a bicontinuous blend of an electron donor and an electron acceptor, has allowed power conversion efficiencies around 8%. This review should be helpful to evaluate which are the most promising materials and where this research field is going in the years to come.
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  7. 7
    Blouin, N.; Michaud, A.; Gendron, D.; Wakim, S.; Blair, E.; Neagu-Plesu, R.; Belletête, M.; Durocher, G.; Tao, Y.; Leclerc, M. Toward a Rational Design of Poly(2,7-carbazole) Derivatives for Solar Cells J. Am. Chem. Soc. 2008, 130, 732– 742
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    7
    Toward a Rational Design of Poly(2,7-Carbazole) Derivatives for Solar Cells
    Blouin, Nicolas; Michaud, Alexandre; Gendron, David; Wakim, Salem; Blair, Emily; Neagu-Plesu, Rodica; Belletete, Michel; Durocher, Gilles; Tao, Ye; Leclerc, Mario
    Journal of the American Chemical Society (2008), 130 (2), 732-742CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    From theor. models and calcns., several alternating polymeric structures were studied to develop optimized poly(2,7-carbazole) derivs. for solar cell applications. Selected low band gap alternating copolymers were obtained via a Suzuki coupling reaction. A good correlation between DFT theor. calcns. performed on model compds. and the exptl. HOMO, LUMO, and band gap energies of the corresponding polymers was obtained. This study reveals that the alternating copolymer HOMO energy level is mainly fixed by the carbazole moiety, whereas the LUMO energy level is mainly related to the nature of the electron-withdrawing comonomer. However, solar cell performances are not solely driven by the energy levels of the materials. Clearly, the mol. wt. and the overall organization of the polymers are other important key parameters to consider when developing new polymers for solar cells. Preliminary measurements revealed hole mobilities of ∼1 × 10-3 cm2/V-s and a power conversion efficiency (PCE) up to 3.6%. Further improvements are anticipated through a rational design of new sym. low band gap poly(2,7-carbazole) derivs.
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  8. 8
    Iyoda, M.; Yamakawa, J.; Rahman, M. J. Conjugated Macrocycles: Concepts and Applications Angew. Chem., Int. Ed. Engl. 2011, 50, 10522– 10553
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  9. 9
    Aratani, N.; Kim, D.; Osuka, A. Discrete Cyclic Porphyrin Arrays as Artificial Light-Harvesting Antenna Acc. Chem. Res. 2009, 42, 1922– 1934
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    9
    Discrete Cyclic Porphyrin Arrays as Artificial Light-Harvesting Antenna
    Aratani, Naoki; Kim, Dongho; Osuka, Atsuhiro
    Accounts of Chemical Research (2009), 42 (12), 1922-1934CODEN: ACHRE4; ISSN:0001-4842. (American Chemical Society)
    A review. The importance of photosynthesis has driven researchers to seek ways to mimic its fundamental features in simplified systems. The absorption of a photon by light-harvesting (antenna) complexes made up of a large no. of protein-embedded pigments initiates photosynthesis. Subsequently the many pigments within the antenna system shuttle that photon via an efficient excitation energy transfer (EET) until it encounters a reaction center. Since the 1995 discovery of the circularly arranged chromophoric assemblies in the crystal structure of light-harvesting antenna complex LH2 of purple bacteria Rps. Acidophila, many designs of light-harvesting antenna systems have focused on cyclic porphyrin wheels that allow for efficient EET. In this Account, the authors review recent research in their labs. in the synthesis of covalently and noncovalently linked discrete cyclic porphyrin arrays as models of the photosynthetic light-harvesting antenna complexes. On the basis of the silver(I)-promoted oxidative coupling strategy, the authors have prepd. a series of extremely long yet discrete meso-meso-linked porphyrin arrays and covalently linked large porphyrin rings. The authors examd. the photophys. properties of these mols. using steady-state absorption, fluorescence, fluorescence lifetime, fluorescence anisotropy decay, and transient absorption measurements. Both the pump-power dependence on the femtosecond transient absorption and the transient absorption anisotropy decay profiles are directly related to the EET processes within the porphyrin rings. Within these structures, the exciton-exciton annihilation time and the polarization anisotropy rise time are well-described in terms of the Forster-type incoherent energy hopping model. In noncoordinating solvents such as CHCl3, meso-pyridine-appended zinc(II) porphyrins and their meso-meso-linked dimers spontaneously assemble to form tetrameric porphyrin squares and porphyrin boxes, resp. In the latter case, the authors have demonstrated the rigorous homochiral self-sorting process and efficient EET along these cyclic porphyrin arrays. The meso-cinchomeronimide appended zinc(II) porphyrin forms a cyclic trimer. The authors have also shown that the corresponding meso-meso-linked diporphyrins undergo high-fidelity self-sorting assembling to form discrete cyclic trimer, tetramer, and pentamer with large assocn. consts. through perfect discrimination of enantiomeric and conformational differences of the meso-cinchomeronimide substituents. Collectively, these studies of covalently and noncovalently linked discrete cyclic porphyrin arrays aid in the understanding of the structural requirements for such very fast EET in natural light-harvesting complexes.
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  10. 10
    Aggarwal, A. V.; Thiessen, A.; Idelson, A.; Kalle, D.; Würsch, D.; Stangl, T.; Steiner, F.; Jester, S.-S.; Vogelsang, J.; Höger, S. Fluctuating Exciton Localization in Giant π-Conjugated Spoked-Wheel Macrocycles Nat. Chem. 2013, 5, 964– 970
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    10
    Fluctuating exciton localization in giant π-conjugated spoked-wheel macrocycles
    Aggarwal, A. Vikas; Thiessen, Alexander; Idelson, Alissa; Kalle, Daniel; Wuersch, Dominik; Stangl, Thomas; Steiner, Florian; Jester, Stefan-S.; Vogelsang, Jan; Hoeger, Sigurd; Lupton, John M.
    Nature Chemistry (2013), 5 (11), 964-970CODEN: NCAHBB; ISSN:1755-4330. (Nature Publishing Group)
    A structurally rigid mol. spoked wheel with 6 nm diam. was prepd. as a model for understanding the localization of elementary photoexcitations (excitons) in conjugated polymers with extended π conjugation (where an exciton is generated and if it is always generated in the same repeat units) and its time dependence; related linear oligomers were also prepd. for comparison. Single-mol. fluorescence revealed random exciton localization, which leads to temporally varying emission polarization. Initially, the random localization arose after every photon absorption event because of temp.-independent spontaneous symmetry breaking; the fast fluctuations slowed to millisecond timescales after prolonged illumination. Intramol. heterogeneity was revealed in cryogenic spectroscopy by jumps in transition energy, but emission polarization also switched without a spectral jump occurring, which implied long-range homogeneity in the local dielec. environment.
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    Yamamoto, T.; Tezuka, Y. Topological Polymer Chemistry: A Cyclic Approach Toward Novel Polymer Properties and Functions Polym. Chem. 2011, 2, 1930– 1941
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    11
    Topological polymer chemistry: a cyclic approach toward novel polymer properties and functions
    Yamamoto, Takuya; Tezuka, Yasuyuki
    Polymer Chemistry (2011), 2 (9), 1930-1941CODEN: PCOHC2; ISSN:1759-9962. (Royal Society of Chemistry)
    A review. We describe first the synthetic developments for cyclic polymers. Recent progress obsd. in Topol. Polymer Chem. is outlined with particular emphasis on single-cyclic (ring) and multi-cyclic polymers having programmed chem. structures. By making use of these topol. polymers, unprecedented opportunities have now been realized to provide new insights on fundamental polymer properties either in soln. or bulk, in static or dynamic states, or in self-assemblies. Moreover, unusual properties and functions for polymer materials have now been revealed based on their cyclic topologies, i.e., topol. effects, unattainable either by linear or branched counterparts.
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  12. 12
    Yang, J.; Yoon, M.-C.; Yoo, H.; Kim, P.; Kim, D. Excitation Energy Transfer in Multiporphyrin Arrays with Cyclic Architectures: Towards Artificial Light-Harvesting Antenna Complexes Chem. Soc. Rev. 2012, 41, 4808– 4826
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    12
    Excitation energy transfer in multiporphyrin arrays with cyclic architectures: towards artificial light-harvesting antenna complexes
    Yang, Jaesung; Yoon, Min-Chul; Yoo, Hyejin; Kim, Pyosang; Kim, Dongho
    Chemical Society Reviews (2012), 41 (14), 4808-4826CODEN: CSRVBR; ISSN:0306-0012. (Royal Society of Chemistry)
    A review. Since highly sym. cyclic architecture of light-harvesting antenna complex LH2 in purple bacteria was revealed in 1995, there has been a renaissance in developing cyclic porphyrin arrays to duplicate natural systems in terms of high efficiency, in particular, in transferring excitation energy. This tutorial review highlights the mechanisms and rates of excitation energy transfer (EET) in a variety of synthetic cyclic porphyrin arrays on the basis of time-resolved spectroscopic measurements performed at both ensemble and single-mol. levels. Subtle change in structural parameters such as connectivity, distance, and orientation between neighboring porphyrin moieties exquisitely modulates not only the nature of interchromophoric interactions but also the rates and efficiencies of EET. The relationship between the structure and EET dynamics described here should assist a rational design of novel cyclic porphyrin arrays, more contiguous to real applications in artificial photosynthesis.
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  13. 13
    Parkinson, P.; Knappke, C. E. I.; Kamonsutthipaijit, N.; Sirithip, K.; Matichak, J. D.; Anderson, H. L.; Herz, L. M. Ultrafast Energy Transfer in Biomimetic Multistrand Nanorings J. Am. Chem. Soc. 2014, 136, 8217– 8220
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    13
    Ultrafast Energy Transfer in Biomimetic Multistrand Nanorings
    Parkinson, Patrick; Knappke, Christiane E. I.; Kamonsutthipaijit, Nuntaporn; Sirithip, Kanokkorn; Matichak, Jonathan D.; Anderson, Harry L.; Herz, Laura M.
    Journal of the American Chemical Society (2014), 136 (23), 8217-8220CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    The authors report the synthesis of LH2-like supramol. double- and triple-stranded complexes based upon porphyrin nanorings. Energy transfer from the antenna dimers to the π-conjugated nanoring occurs on a subpicosecond time scale, rivaling transfer rates in natural light-harvesting systems. The presence of a second nanoring acceptor doubles the transfer rate, providing strong evidence for multidirectional energy funneling. The behavior of these systems is particularly intriguing because the local nature of the interaction may allow energy transfer into states that are, for cyclic nanorings, symmetry-forbidden in the far field. These complexes are versatile synthetic models for natural light-harvesting systems.
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  14. 14
    Parkinson, P.; Kondratuk, D. V.; Menelaou, C.; Gong, J. Q.; Anderson, H. L.; Herz, L. M. Chromophores in Molecular Nanorings: When Is a Ring a Ring? J. Phys. Chem. Lett. 2014, 5, 4356– 4361
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    14
    Chromophores in Molecular Nanorings: When Is a Ring a Ring?
    Parkinson, Patrick; Kondratuk, Dmitry V.; Menelaou, Christopher; Gong, Juliane Q.; Anderson, Harry L.; Herz, Laura M.
    Journal of Physical Chemistry Letters (2014), 5 (24), 4356-4361CODEN: JPCLCD; ISSN:1948-7185. (American Chemical Society)
    The topol. of a conjugated mol. plays a significant role in controlling both the electronic properties and the conformational manifold that the mol. may explore. Fully π-conjugated mol. nanorings are of particular interest, as their lowest electronic transition may be strongly suppressed as a result of symmetry constraints. In contrast, the simple Kasha model predicts an enhancement in the radiative rate for corresponding linear oligomers. Here we investigate such effects in linear and cyclic conjugated mols. contg. between 6 and 42 butadiyne-linked porphyrin units (corresponding to 600 C-C bonds) as pure monodisperse oligomers. We demonstrate that as the diam. of the nanorings increases beyond ∼10 nm, its electronic properties tend toward those of a similarly sized linear mol. as a result of excitation localization on a subsegment of the ring. However, significant differences persist in the nature of the emitting dipole polarization even beyond this limit, arising from variations in mol. curvature and conformation.
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  15. 15
    Hoffmann, M.; Wilson, C. J.; Odell, B.; Anderson, H. L. Template-Directed Synthesis of a Pi-Conjugated Porphyrin Nanoring Angew. Chem., Int. Ed. 2007, 46, 3122– 3125
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  16. 16
    Hoffmann, M.; Kärnbratt, J.; Chang, M. H.; Herz, L. M.; Albinsson, B.; Anderson, H. L. Enhanced Pi Conjugation Around a Porphyrin[6] Nanoring Angew. Chem., Int. Ed. 2008, 47, 4993– 4996
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    Kondratuk, D. V.; Sprafke, J. K.; O’Sullivan, M. C.; Perdigao, L. M.; Saywell, A.; Malfois, M.; O’Shea, J. N.; Beton, P. H.; Thompson, A. L.; Anderson, H. L. Vernier-Templated Synthesis, Crystal Structure, and Supramolecular Chemistry of a 12-Porphyrin Nanoring Chem.—Eur. J. 2014, 20, 12826– 12843
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    17
    Vernier-Templated Synthesis, Crystal Structure, and Supramolecular Chemistry of a 12-Porphyrin Nanoring
    Kondratuk, Dmitry V.; Sprafke, Johannes K.; O'Sullivan, Melanie C.; Perdigao, Luis M. A.; Saywell, Alex; Malfois, Marc; O'Shea, James N.; Beton, Peter H.; Thompson, Amber L.; Anderson, Harry L.
    Chemistry - A European Journal (2014), 20 (40), 12826-12834CODEN: CEUJED; ISSN:0947-6539. (Wiley-VCH Verlag GmbH & Co. KGaA)
    Vernier templating exploits a mismatch between the no. of binding sites in a template and a reactant to direct the formation of a product that is large enough to bind several template units. Here, the authors present a detailed study of the Vernier-templated synthesis of a 12-porphyrin nanoring. NMR and small-angle x-ray scattering (SAXS) analyses show that Vernier complexes are formed as intermediates in the cyclo-oligomerization reaction. UV/visible/NIR titrns. show that the three-component assembly of the 12-porphyrin nanoring figure-of-eight template complex displays high allosteric cooperativity and chelate cooperativity. This nanoring-template 1:2 complex is among the largest synthetic mols. to have been characterized by single-crystal anal. It crystallizes as a racemate, with an angle of 27° between the planes of the two template units. The crystal structure reveals many unexpected intramol. C-H···N contacts involving the tert-Bu side chains. Scanning tunneling microscopy (STM) expts. show that mols. of the 12-porphyrin template complex can remain intact on the gold surface, although the majority of the material unfolds into the free nanoring during electrospray deposition.
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  18. 18
    Chang, M.-H.; Hoffmann, M.; Anderson, H. L.; Herz, L. M. Dynamics of Excited-State Conformational Relaxation and Electronic Delocalization in Conjugated Porphyrin Oligomers J. Am. Chem. Soc. 2008, 130, 10171– 10178
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    18
    Dynamics of Excited-State Conformational Relaxation and Electronic Delocalization in Conjugated Porphyrin Oligomers
    Chang, Ming-Hua; Hoffmann, Markus; Anderson, Harry L.; Herz, Laura M.
    Journal of the American Chemical Society (2008), 130 (31), 10171-10178CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    The authors have investigated the influence of nuclear geometric relaxation on the extent of the excited-state electronic delocalization in conjugated zinc porphyrin oligomers using ultrafast transient photoluminescence spectroscopy. By use of metal-coordinating templates that force the oligomers into specific geometries in soln. the authors are able to distinguish clearly between relaxation effects arising from the two vibrational modes that preferentially couple to the electronic transitions in such materials, i.e., carbon-carbon bond stretches and inter-ring torsions. The authors find that light absorption generates an excited state that is initially strongly delocalized along the oligomer but contracts rapidly following vibrational relaxation of the nuclei along C-C stretch coordinates on the subpicosecond time scale. The authors are able to monitor such excitonic self-trapping effects by observing the extent to which the concomitant ultrafast rotation of the transition dipole moment is found to correlate with the degree of bending induced in the mol. backbone. The authors further demonstrate that interporphyrin torsional relaxation leads to a subsequent increase in the excited-state electronic delocalization on a longer time scale (∼100 ps). Such dynamic planarization of the mol. backbone is evident from the time-dependent increase in the overall emission intensity and red-shift in the peak emission energy that can be obsd. for wormlike flexible porphyrin octamers but not for torsionally rigidified cyclic or double-strand octamer complexes. These results therefore indicate that, following excitation, the initially highly delocalized excited-state wave function first contracts and then expands again along the conjugated backbone in accordance with the time periods for the vibrational modes coupled to the electronic transition.
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  19. 19
    Yong, C.-K.; Parkinson, P.; Kondratuk, D. V.; Chen, W.-H.; Stannard, A.; Summerfield, A.; Sprafke, J. K.; O’Sullivan, M. C.; Beton, P. H.; Anderson, H. L. Ultrafast Delocalization of Excitation in Synthetic Light-Harvesting Nanorings Chem. Sci. 2015, 6, 181– 189
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    19
    Ultrafast delocalization of excitation in synthetic light-harvesting nanorings
    Yong, Chaw-Keong; Parkinson, Patrick; Kondratuk, Dmitry V.; Chen, Wei-Hsin; Stannard, Andrew; Summerfield, Alex; Sprafke, Johannes K.; O'Sullivan, Melanie C.; Beton, Peter H.; Anderson, Harry L.; Herz, Laura M.
    Chemical Science (2015), 6 (1), 181-189CODEN: CSHCCN; ISSN:2041-6520. (Royal Society of Chemistry)
    Rings of chlorophyll mols. harvest sunlight remarkably efficiently during photosynthesis in purple bacteria. The key to their efficiency lies in their highly delocalized excited states that allow for ultrafast energy migration. Here we show that a family of synthetic nanorings mimic the ultrafast energy transfer and delocalization obsd. in nature. π-Conjugated nanorings with diams. of up to 10 nm, consisting of up to 24 porphyrin units, are found to exhibit excitation delocalization within the first 200 fs of light absorption. Transitions from the first singlet excited state of the circular nanorings are dipole-forbidden as a result of symmetry constraints, but these selection rules can be lifted through static and dynamic distortions of the rings. The increase in the radiative emission rate in the larger nanorings correlates with an increase in static disorder expected from Monte Carlo simulations. For highly sym. rings, the radiative rate is found to increase with increasing temp. Although this type of thermally activated superradiance has been theor. predicted in circular chromophore arrays, it has not previously been obsd. in any natural or synthetic systems. As expected, the activation energy for emission increases when a nanoring is fixed in a circular conformation by coordination to a radial template. These nanorings offer extended chromophores with high excitation delocalization that is remarkably stable against thermally induced disorder. Such findings open new opportunities for exploring coherence effects in nanometer mol. rings and for implementing these biomimetic light-harvesters in man-made devices.
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  20. 20
    Kanimozhi, C.; Naik, M.; Yaacobi-Gross, N.; Burnett, E. K.; Briseno, A. L.; Anthopoulos, T. D.; Patil, S. Controlling Conformations of Diketopyrrolopyrrole-Based Conjugated Polymers: Role of Torsional Angle J. Phys. Chem. C 2014, 118, 11536– 11544
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    Adachi, T.; Vogelsang, J.; Lupton, J. M. Chromophore Bending Controls Fluorescence Lifetime in Single Conjugated Polymer Chains J. Phys. Chem. Lett. 2014, 5, 2165– 2170
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    21
    Chromophore Bending Controls Fluorescence Lifetime in Single Conjugated Polymer Chains
    Adachi, Takuji; Vogelsang, Jan; Lupton, John M.
    Journal of Physical Chemistry Letters (2014), 5 (12), 2165-2170CODEN: JPCLCD; ISSN:1948-7185. (American Chemical Society)
    Single-mol. spectroscopy of conjugated polymers offers unique insight into the interplay between the spatial arrangement of monomer units-twisting and bending-and the characteristics of the primary excitonic photoexcitation, provided that a single conjugated segment can be isolated. β-phase polyfluorene constitutes an ideal model to study variations in intermonomeric coupling, detd. by nanoscale mol. shape, on the fundamental optical transition. If structural relaxation in the excited state is weak, exciton self-trapping occurs stochastically along the conjugated segment. Bending of the π-system is then revealed by a redn. in single-photon polarization anisotropy, correlating directly with increased fluorescence lifetime. Strong relaxation raises emission anisotropy because of deterministic exciton localization, decelerating fluorescence decay due to a decrease in exciton coherence length.
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  22. 22
    Hestand, N. J.; Spano, F. C. The Effect of Chain Bending on the Photophysical Properties of Conjugated Polymers J. Phys. Chem. B 2014, 118, 8352– 8363
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    22
    The Effect of Chain Bending on the Photophysical Properties of Conjugated Polymers
    Hestand, Nicholas J.; Spano, Frank C.
    Journal of Physical Chemistry B (2014), 118 (28), 8352-8363CODEN: JPCBFK; ISSN:1520-5207. (American Chemical Society)
    The impact of chain bending on the photophys. properties of emissive conjugated polymers (CPs) is studied theor. using Holstein-style Hamiltonians which treat vibronic coupling involving the ubiquitous vinyl/ring stretching mode nonadiabatically. The photophys. impact of chain bending is already evident at the level of an effective Frenkel Hamiltonian, where the pos. exciton band curvature in CPs translates to neg. excitonic coupling between monomeric units, as in J-aggregates. The absorption and luminescence (PL) spectral line shapes respond very differently to chain bending. The misalignment of monomeric transition dipole moments with bending selectively attenuates the 0-0 PL peak intensity while leaving the 0-1 intensity practically unchanged, a property which is ultimately due to the uniquely coherent nature of the 0-0 peak. Hence, the 0-0/0-1 PL ratio, as well as the radiative decay rate, decrease with chain bending, effects that are more pronounced at lower temps. where exciton coherence extends over a larger portion of the chain. Increasing temp. and/or static disorder reduces the exciton coherence no., Ncoh, thereby reducing the sensitivity to bending. In marked contrast, the absorption vibronic progression is far less sensitive to morphol. changes, even at low temps., and is mainly responsive to the exciton bandwidth. The above results also hold when using a more accurate 1D semiconductor Hamiltonian which allows for electron-hole sepn. along the CP chain. The findings may suggest unique ways of controlling the radiative properties of conjugated polymer chains useful in applications such as org. LEDs and low-temp. sensors.
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  23. 23
    Dykstra, T. E.; Hennebicq, E.; Beljonne, D.; Gierschner, J.; Claudio, G.; Bittner, E. R.; Knoester, J.; Scholes, G. D. Conformational Disorder and Ultrafast Exciton Relaxation in PPV-Family Conjugated Polymers J. Phys. Chem. B 2009, 113, 656– 667
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  • Abstract

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    Figure 1

    Figure 1. Chemical structures of the budadiyne-linked zinc porphyrin nanoring assemblies under investigation: (a) c-P10, (b) c-P10·(T5)2, (c) c-P10·(T6)2, (d) c-P12, (e) c-P12·(T6)2, and (f) c-P12·(T8)2. Templates are indicated in blue, and zinc atoms are denoted in red. All compounds have octyloxy side chains (Ar) which are omitted in the graphic for clarity.

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    Figure 2

    Figure 2. Normalized steady-state absorption (blue lines) and time-integrated photoluminescence (green lines) spectra at 295 K for (a) c-P12 in toluene/1% pyridine and (b) c-P12·(T6)2 in toluene solution. The emission spectra were recorded after excitation at 520 nm (into the Soret band). The Qx and Qy transitions are indicated by red arrows.

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    Figure 3

    Figure 3. Time-resolved PL dynamics of (a) c-P12 in toluene/1% pyridine (concentration 0.2 mM) and (b) c-P12·(T6)2 in toluene solution (concentration 0.4 mM). Samples were excited by excitation pulse polarized either parallel (blue dots) or perpendicular (red dots) to the detection polarization, as illustrated in the inset.

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    Figure 4

    Figure 4. PL polarization anisotropy as a function of time after excitation at 820 nm for c-P12 (blue circles, detection at 872 nm), c-P12·(T8)2 (red triangles, detection at 904 nm), and c-P12·(T6)2 (green squares, detection at 914 nm). The structures shown on the right are energy minimized geometries calculated using a modified form of MM+ force field in HyperChem. Side chains are not shown in the diagram but were included in the simulations.

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    Figure 5

    Figure 5. (a,b) Definition of the distortion angles. (a) The twisting angle α between two template planes is defined using c-P12·(T6)2 as an example. Four atoms on the porphyrin ring (red) are chosen to form three vectors a⃗ , b⃗ , and c⃗, with a⃗ and c⃗ lying approximately parallel to each other when the molecule does not exhibit any twist. α is the torsional angle defined by the four atoms and is the angle between the planes formed by a⃗, b⃗ and b⃗, c⃗. α represents the twisting angle from a planar position. (b) The bending angle β is illustrated using c-P12·(T8)2 as an example. Four atoms (red) are chosen as shown. Two angles ϕ and δ are calculated, where β = ϕ + δ describes the deviation from a planar position. Simulations were carried out with octyloxy side chains, which are omitted in the diagram for clarity. (c–f) Area normalized histogram of twisting angle α (blue) and bending angle β (red) for (c) c-P10·(T5)2, (d) c-P10·(T6)2, (e) c-P12·(T6)2, (f) c-P12·(T8)2. Molecular dynamics simulation carried out using HyperChem at 300 K for 500 ps, as described in detail in the text.

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  • References

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      Parkinson, Patrick; Kondratuk, Dmitry V.; Menelaou, Christopher; Gong, Juliane Q.; Anderson, Harry L.; Herz, Laura M.
      Journal of Physical Chemistry Letters (2014), 5 (24), 4356-4361CODEN: JPCLCD; ISSN:1948-7185. (American Chemical Society)
      The topol. of a conjugated mol. plays a significant role in controlling both the electronic properties and the conformational manifold that the mol. may explore. Fully π-conjugated mol. nanorings are of particular interest, as their lowest electronic transition may be strongly suppressed as a result of symmetry constraints. In contrast, the simple Kasha model predicts an enhancement in the radiative rate for corresponding linear oligomers. Here we investigate such effects in linear and cyclic conjugated mols. contg. between 6 and 42 butadiyne-linked porphyrin units (corresponding to 600 C-C bonds) as pure monodisperse oligomers. We demonstrate that as the diam. of the nanorings increases beyond ∼10 nm, its electronic properties tend toward those of a similarly sized linear mol. as a result of excitation localization on a subsegment of the ring. However, significant differences persist in the nature of the emitting dipole polarization even beyond this limit, arising from variations in mol. curvature and conformation.
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    15. 15
      Hoffmann, M.; Wilson, C. J.; Odell, B.; Anderson, H. L. Template-Directed Synthesis of a Pi-Conjugated Porphyrin Nanoring Angew. Chem., Int. Ed. 2007, 46, 3122– 3125
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    16. 16
      Hoffmann, M.; Kärnbratt, J.; Chang, M. H.; Herz, L. M.; Albinsson, B.; Anderson, H. L. Enhanced Pi Conjugation Around a Porphyrin[6] Nanoring Angew. Chem., Int. Ed. 2008, 47, 4993– 4996
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      Kondratuk, D. V.; Sprafke, J. K.; O’Sullivan, M. C.; Perdigao, L. M.; Saywell, A.; Malfois, M.; O’Shea, J. N.; Beton, P. H.; Thompson, A. L.; Anderson, H. L. Vernier-Templated Synthesis, Crystal Structure, and Supramolecular Chemistry of a 12-Porphyrin Nanoring Chem.—Eur. J. 2014, 20, 12826– 12843
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      17
      Vernier-Templated Synthesis, Crystal Structure, and Supramolecular Chemistry of a 12-Porphyrin Nanoring
      Kondratuk, Dmitry V.; Sprafke, Johannes K.; O'Sullivan, Melanie C.; Perdigao, Luis M. A.; Saywell, Alex; Malfois, Marc; O'Shea, James N.; Beton, Peter H.; Thompson, Amber L.; Anderson, Harry L.
      Chemistry - A European Journal (2014), 20 (40), 12826-12834CODEN: CEUJED; ISSN:0947-6539. (Wiley-VCH Verlag GmbH & Co. KGaA)
      Vernier templating exploits a mismatch between the no. of binding sites in a template and a reactant to direct the formation of a product that is large enough to bind several template units. Here, the authors present a detailed study of the Vernier-templated synthesis of a 12-porphyrin nanoring. NMR and small-angle x-ray scattering (SAXS) analyses show that Vernier complexes are formed as intermediates in the cyclo-oligomerization reaction. UV/visible/NIR titrns. show that the three-component assembly of the 12-porphyrin nanoring figure-of-eight template complex displays high allosteric cooperativity and chelate cooperativity. This nanoring-template 1:2 complex is among the largest synthetic mols. to have been characterized by single-crystal anal. It crystallizes as a racemate, with an angle of 27° between the planes of the two template units. The crystal structure reveals many unexpected intramol. C-H···N contacts involving the tert-Bu side chains. Scanning tunneling microscopy (STM) expts. show that mols. of the 12-porphyrin template complex can remain intact on the gold surface, although the majority of the material unfolds into the free nanoring during electrospray deposition.
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    18. 18
      Chang, M.-H.; Hoffmann, M.; Anderson, H. L.; Herz, L. M. Dynamics of Excited-State Conformational Relaxation and Electronic Delocalization in Conjugated Porphyrin Oligomers J. Am. Chem. Soc. 2008, 130, 10171– 10178
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      18
      Dynamics of Excited-State Conformational Relaxation and Electronic Delocalization in Conjugated Porphyrin Oligomers
      Chang, Ming-Hua; Hoffmann, Markus; Anderson, Harry L.; Herz, Laura M.
      Journal of the American Chemical Society (2008), 130 (31), 10171-10178CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
      The authors have investigated the influence of nuclear geometric relaxation on the extent of the excited-state electronic delocalization in conjugated zinc porphyrin oligomers using ultrafast transient photoluminescence spectroscopy. By use of metal-coordinating templates that force the oligomers into specific geometries in soln. the authors are able to distinguish clearly between relaxation effects arising from the two vibrational modes that preferentially couple to the electronic transitions in such materials, i.e., carbon-carbon bond stretches and inter-ring torsions. The authors find that light absorption generates an excited state that is initially strongly delocalized along the oligomer but contracts rapidly following vibrational relaxation of the nuclei along C-C stretch coordinates on the subpicosecond time scale. The authors are able to monitor such excitonic self-trapping effects by observing the extent to which the concomitant ultrafast rotation of the transition dipole moment is found to correlate with the degree of bending induced in the mol. backbone. The authors further demonstrate that interporphyrin torsional relaxation leads to a subsequent increase in the excited-state electronic delocalization on a longer time scale (∼100 ps). Such dynamic planarization of the mol. backbone is evident from the time-dependent increase in the overall emission intensity and red-shift in the peak emission energy that can be obsd. for wormlike flexible porphyrin octamers but not for torsionally rigidified cyclic or double-strand octamer complexes. These results therefore indicate that, following excitation, the initially highly delocalized excited-state wave function first contracts and then expands again along the conjugated backbone in accordance with the time periods for the vibrational modes coupled to the electronic transition.
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    19. 19
      Yong, C.-K.; Parkinson, P.; Kondratuk, D. V.; Chen, W.-H.; Stannard, A.; Summerfield, A.; Sprafke, J. K.; O’Sullivan, M. C.; Beton, P. H.; Anderson, H. L. Ultrafast Delocalization of Excitation in Synthetic Light-Harvesting Nanorings Chem. Sci. 2015, 6, 181– 189
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      19
      Ultrafast delocalization of excitation in synthetic light-harvesting nanorings
      Yong, Chaw-Keong; Parkinson, Patrick; Kondratuk, Dmitry V.; Chen, Wei-Hsin; Stannard, Andrew; Summerfield, Alex; Sprafke, Johannes K.; O'Sullivan, Melanie C.; Beton, Peter H.; Anderson, Harry L.; Herz, Laura M.
      Chemical Science (2015), 6 (1), 181-189CODEN: CSHCCN; ISSN:2041-6520. (Royal Society of Chemistry)
      Rings of chlorophyll mols. harvest sunlight remarkably efficiently during photosynthesis in purple bacteria. The key to their efficiency lies in their highly delocalized excited states that allow for ultrafast energy migration. Here we show that a family of synthetic nanorings mimic the ultrafast energy transfer and delocalization obsd. in nature. π-Conjugated nanorings with diams. of up to 10 nm, consisting of up to 24 porphyrin units, are found to exhibit excitation delocalization within the first 200 fs of light absorption. Transitions from the first singlet excited state of the circular nanorings are dipole-forbidden as a result of symmetry constraints, but these selection rules can be lifted through static and dynamic distortions of the rings. The increase in the radiative emission rate in the larger nanorings correlates with an increase in static disorder expected from Monte Carlo simulations. For highly sym. rings, the radiative rate is found to increase with increasing temp. Although this type of thermally activated superradiance has been theor. predicted in circular chromophore arrays, it has not previously been obsd. in any natural or synthetic systems. As expected, the activation energy for emission increases when a nanoring is fixed in a circular conformation by coordination to a radial template. These nanorings offer extended chromophores with high excitation delocalization that is remarkably stable against thermally induced disorder. Such findings open new opportunities for exploring coherence effects in nanometer mol. rings and for implementing these biomimetic light-harvesters in man-made devices.
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    20. 20
      Kanimozhi, C.; Naik, M.; Yaacobi-Gross, N.; Burnett, E. K.; Briseno, A. L.; Anthopoulos, T. D.; Patil, S. Controlling Conformations of Diketopyrrolopyrrole-Based Conjugated Polymers: Role of Torsional Angle J. Phys. Chem. C 2014, 118, 11536– 11544
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    21. 21
      Adachi, T.; Vogelsang, J.; Lupton, J. M. Chromophore Bending Controls Fluorescence Lifetime in Single Conjugated Polymer Chains J. Phys. Chem. Lett. 2014, 5, 2165– 2170
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      21
      Chromophore Bending Controls Fluorescence Lifetime in Single Conjugated Polymer Chains
      Adachi, Takuji; Vogelsang, Jan; Lupton, John M.
      Journal of Physical Chemistry Letters (2014), 5 (12), 2165-2170CODEN: JPCLCD; ISSN:1948-7185. (American Chemical Society)
      Single-mol. spectroscopy of conjugated polymers offers unique insight into the interplay between the spatial arrangement of monomer units-twisting and bending-and the characteristics of the primary excitonic photoexcitation, provided that a single conjugated segment can be isolated. β-phase polyfluorene constitutes an ideal model to study variations in intermonomeric coupling, detd. by nanoscale mol. shape, on the fundamental optical transition. If structural relaxation in the excited state is weak, exciton self-trapping occurs stochastically along the conjugated segment. Bending of the π-system is then revealed by a redn. in single-photon polarization anisotropy, correlating directly with increased fluorescence lifetime. Strong relaxation raises emission anisotropy because of deterministic exciton localization, decelerating fluorescence decay due to a decrease in exciton coherence length.
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    22. 22
      Hestand, N. J.; Spano, F. C. The Effect of Chain Bending on the Photophysical Properties of Conjugated Polymers J. Phys. Chem. B 2014, 118, 8352– 8363
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      22
      The Effect of Chain Bending on the Photophysical Properties of Conjugated Polymers
      Hestand, Nicholas J.; Spano, Frank C.
      Journal of Physical Chemistry B (2014), 118 (28), 8352-8363CODEN: JPCBFK; ISSN:1520-5207. (American Chemical Society)
      The impact of chain bending on the photophys. properties of emissive conjugated polymers (CPs) is studied theor. using Holstein-style Hamiltonians which treat vibronic coupling involving the ubiquitous vinyl/ring stretching mode nonadiabatically. The photophys. impact of chain bending is already evident at the level of an effective Frenkel Hamiltonian, where the pos. exciton band curvature in CPs translates to neg. excitonic coupling between monomeric units, as in J-aggregates. The absorption and luminescence (PL) spectral line shapes respond very differently to chain bending. The misalignment of monomeric transition dipole moments with bending selectively attenuates the 0-0 PL peak intensity while leaving the 0-1 intensity practically unchanged, a property which is ultimately due to the uniquely coherent nature of the 0-0 peak. Hence, the 0-0/0-1 PL ratio, as well as the radiative decay rate, decrease with chain bending, effects that are more pronounced at lower temps. where exciton coherence extends over a larger portion of the chain. Increasing temp. and/or static disorder reduces the exciton coherence no., Ncoh, thereby reducing the sensitivity to bending. In marked contrast, the absorption vibronic progression is far less sensitive to morphol. changes, even at low temps., and is mainly responsive to the exciton bandwidth. The above results also hold when using a more accurate 1D semiconductor Hamiltonian which allows for electron-hole sepn. along the CP chain. The findings may suggest unique ways of controlling the radiative properties of conjugated polymer chains useful in applications such as org. LEDs and low-temp. sensors.
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    23. 23
      Dykstra, T. E.; Hennebicq, E.; Beljonne, D.; Gierschner, J.; Claudio, G.; Bittner, E. R.; Knoester, J.; Scholes, G. D. Conformational Disorder and Ultrafast Exciton Relaxation in PPV-Family Conjugated Polymers J. Phys. Chem. B 2009, 113, 656– 667
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      Huser, T.; Yan, M.; Rothberg, L. J. Single Chain Spectroscopy of Conformational Dependence of Conjugated Polymer Photophysics Proc. Natl. Acad. Sci. U. S. A. 2000, 97, 11187– 11191
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      24
      Single chain spectroscopy of conformational dependence of conjugated polymer photophysics
      Huser, Thomas; Yan, Ming; Rothberg, Lewis J.
      Proceedings of the National Academy of Sciences of the United States of America (2000), 97 (21), 11187-11191CODEN: PNASA6; ISSN:0027-8424. (National Academy of Sciences)
      Single mol. confocal fluorescence microscopy was used to perform photoluminescence spectroscopy on single, isolated mols. of the conjugated polymer poly[2-methoxy,5-(2'-ethylhexyloxy)-p-phenylene-vinylene] (MEH-PPV). We show that the fluorescence from single chains of this electroluminescent polymer depends strongly on chain conformation. The time evolution of the spectra, emission intensity, and polarization all provide direct evidence that memory of the chain conformation in soln. is retained after solvent evapn. Chains cast from toluene soln. are highly folded and show memory of the excitation polarization. Exciton funneling to highly aggregated low energy regions causes the chain to mimic the photophys. behavior of a single chromophore. Chains cast from chloroform, however, behave as multichromophore systems, and no sudden discrete spectral or intensity jumps are obsd. These also exhibit different spectroscopy from the folded chromophores.
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    25. 25
      Adachi, T.; Lakhwani, G.; Traub, M. C.; Ono, R. J.; Bielawski, C. W.; Barbara, P. F.; Vanden Bout, D. A. Conformational Effect on Energy Transfer in Single Polythiophene Chains J. Phys. Chem. B 2012, 116, 9866– 9872
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      25
      Conformational Effect on Energy Transfer in Single Polythiophene Chains
      Adachi, Takuji; Lakhwani, Girish; Traub, Matthew C.; Ono, Robert J.; Bielawski, Christopher W.; Barbara, Paul F.; Vanden Bout, David A.
      Journal of Physical Chemistry B (2012), 116 (32), 9866-9872CODEN: JPCBFK; ISSN:1520-5207. (American Chemical Society)
      Herein we describe the use of regioregular (rr-) and regiorandom (rra-) P3HT as models to study energy transfer in ordered and disordered single conjugated polymer chains. Single mol. fluorescence spectra and excitation/emission polarization measurements were compared with a Foerster resonance energy transfer (FRET) model simulation. An increase in the mean single chain polarization anisotropy from excitation to emission was obsd. for both rr- and rra-P3HT. The peak emission wavelengths of rr-P3HT were at substantially lower energies than those of rra-P3HT. A simulation based on FRET in single polymer chain conformations successfully reproduced the exptl. observations. These studies showed that ordered conformations facilitated efficient energy transfer to a small no. of low-energy sites compared to disordered conformations. As a result, the histograms of spectral peak wavelengths for ordered conformations were centered at much lower energies than those obtained for disordered conformations. Collectively, these exptl. and simulated results provide the basis for quant. describing energy transfer in an important class of conjugated polymers commonly used in a variety of org. electronics applications.
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      Parkinson, P.; Müller, C.; Stingelin, N.; Johnston, M. B.; Herz, L. M. Role of Ultrafast Torsional Relaxation in the Emission from Polythiophene Aggregates J. Phys. Chem. Lett. 2010, 2788– 2792
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      26
      Role of Ultrafast Torsional Relaxation in the Emission from Polythiophene Aggregates
      Parkinson, Patrick; Muller, Christian; Stingelin, Natalie; Johnston, Michael B.; Herz, Laura M.
      Journal of Physical Chemistry Letters (2010), 1 (19), 2788-2792CODEN: JPCLCD; ISSN:1948-7185. (American Chemical Society)
      An understanding of aggregation effects in semiconducting polymers is essential for their use in optoelectronic devices; the dynamic evolution of such interchain states is not well understood. A blend of semiconducting poly(3-hexylthiophene) (P3HT) with an electronically inert ultrahigh-mol.-wt. polyethylene (UHMW-PE) matrix that allows precise control over the extent to which the P3HT chains aggregate was studied. The singlet exciton population within isolated and aggregated P3HT regions was detd. using fs time-resolved luminescence measurements, and a strong ultrafast decay pathway was found in the aggregated case only. Comparison of the emission from the 2 lowest vibronic bands demonstrates a changeover from an initial vibrationally hot photoexcited state to a geometrically relaxed aggregate state within ∼13 ps, corresponding to time scales for torsional relaxation in these materials. Formation of an aggregate excited state in conjugated polymers is mediated by vibrational relaxation from a low-symmetry to a high-symmetry ordered state for the ensemble.
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    27. 27
      Maus, M.; Mitra, S.; Lor, M.; Hofkens, J.; Weil, T.; Herrmann, A.; Müllen, K.; De Schryver, F. C. Intramolecular Energy Hopping in Polyphenylene Dendrimers with an Increasing Number of Peryleneimide Chromophores J. Phys. Chem. A 2001, 105, 3961– 3966
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      Bolinger, J. C.; Traub, M. C.; Brazard, J.; Adachi, T.; Barbara, P. F.; Vanden Bout, D. A. Conformation and Energy Transfer in Single Conjugated Polymers Acc. Chem. Res. 2012, 45, 1992– 2001
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      Conformation and Energy Transfer in Single Conjugated Polymers
      Bolinger, Joshua C.; Traub, Matthew C.; Brazard, Johanna; Adachi, Takuji; Barbara, Paul F.; Vanden Bout, David A.
      Accounts of Chemical Research (2012), 45 (11), 1992-2001CODEN: ACHRE4; ISSN:0001-4842. (American Chemical Society)
      In this account, we examine several photophys. properties of different conjugated polymers using single-mol. spectroscopy. In these expts., we probed the relation between the conformation of single conjugated polymer chains and the distance scale and efficiency of energy transfer within the polymer. This account summarizes the effects of monomer regioregularity and backbone rigidity, by comparing a regiorandom phenylene vinylene (MEH-PPV) with both a regiorandom and regioregular thiophene (P3HT). Synthesis of novel polymers allowed us to explore the role of different conformation-directing inclusions in a PPV backbone. We showed that these inclusions control the conformation of individual chains and that mol. dynamics can predict these structural effects. In situ solvent vapor annealing studies explored the dynamics of polymer chains and the effect of solvent evapn. on the structural equil. of the polymer. We obsd. that a slower rate of solvent evapn. results in a narrow population of highly ordered polymer chains. These highly ordered single chains serve as a model system to probe the effect of conformation on energy transfer following excitation in single MEH-PPV polymer chains in two distinct expts. In the first, we correlated the anisotropy of the fluorescence emission of individual chains with the anisotropy of their fluorescence excitation. Using this data, we derived a model for energy transfer in a conjugated polymer, simulating chromophores along a chain, coupled via Foerster energy transfer. In the second expt., super-resoln. measurements demonstrated the ability of single-mol. spectroscopy to directly visualize energy transfer along a polymer chain embedded in a model device environment. A capacitive device allowed for controlled localization of hole polarons onto the polymer chain. These pos. charges subsequently quenched local excitations, providing insight into the range of energy transfer in these single polymer mols. As researchers continue to characterize conjugated polymer films and develop methods for creating multichain systems, single-mol. techniques will provide a greater understanding of how polymer morphol. influences interchain interactions and will lead to a richer description of the electronic properties of bulk conjugated polymer films.
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      Larsen, J.; Brüggemann, B.; Khoury, T.; Sly, J.; Crossley, M. J.; Sundström, V.; Akesson, E. Structural Induced Control of Energy Transfer within Zn(II)-Porphyrin Dendrimers J. Phys. Chem. A 2007, 111, 10589– 10597
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      Liu, S.; Kondratuk, D. V.; Rousseaux, S. A. L.; Gil-Ramírez, G.; O’Sullivan, M. C.; Cremers, J.; Claridge, T. D. W.; Anderson, H. L. Caterpillar Track Complexes in Template-Directed Synthesis and Correlated Molecular Motion Angew. Chem., Int. Ed. 2015,  DOI: 10.1002/anie.201412293
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      Hogben, H. J.; Sprafke, J. K.; Hoffmann, M.; Pawlicki, M.; Anderson, H. L. Stepwise Effective Molarities in Porphyrin Oligomer Complexes: Preorganization Results in Exceptionally Strong Chelate Cooperativity J. Am. Chem. Soc. 2011, 133, 20962– 20969
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      O’Sullivan, M. C.; Sprafke, J. K.; Kondratuk, D. V.; Rinfray, C.; Claridge, T. D. W.; Saywell, A.; Blunt, M. O.; O’Shea, J. N.; Beton, P. H.; Malfois, M. Vernier Templating and Synthesis of a 12-Porphyrin Nano-Ring Nature 2011, 469, 72– 75
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      Chang, M.; Hoeben, F.; Jonkheijm, P.; Schenning, A.; Meijer, E.; Silva, C.; Herz, L. Influence of Mesoscopic Ordering on the Photoexcitation Transfer Dynamics in Supramolecular Assemblies of Oligo-p-Phenylenevinylene Chem. Phys. Lett. 2006, 418, 196– 201
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      Montgomery, N. A.; Hedley, G. J.; Ruseckas, A.; Denis, J.-C.; Schumacher, S.; Kanibolotsky, A. L.; Skabara, P. J.; Galbraith, I.; Turnbull, G. A.; Samuel, I. D. W. Dynamics of Fluorescence Depolarisation in Star-Shaped Oligofluorene-Truxene Molecules Phys. Chem. Chem. Phys. 2012, 14, 9176– 9184
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      Anderson, H. L. Building Molecular Wires from the Colours of Life: Conjugated Porphyrin Oligomers Chem. Commun. 1999, 2323– 2330
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      Building molecular wires from the colours of life: conjugated porphyrin oligomers
      Anderson, Harry L.
      Chemical Communications (Cambridge) (1999), (23), 2323-2330CODEN: CHCOFS; ISSN:1359-7345. (Royal Society of Chemistry)
      A review with 65 refs. reports on conjugated porphyrin polymers that are giant supramol. chromophores with extraordinary electrooptical and nonlinear optical properties. Close collaboration between synthetic org. chemists, chem. physicists and theoreticians has yielded new insights into the electronic structure of these remarkable materials. Alkyne-linked oligomers were most intensively studied. Several strands of independent work show that the unusual electronic behavior of these materials can be attributed to strong ground state interporphyrin conjugation, which is amplified in the excited states, in the oxidized and reduced forms.
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      Herz, L. M.; Daniel, C.; Silva, C.; Hoeben, F. J. M.; Schenning, A. P. H. J.; Meijer, E. W.; Friend, R. H.; Phillips, R. T. Exciton Dynamics in Supramolecular Assemblies of p-Phenylenevinylene Oligomers Synth. Met. 2003, 139, 839– 842
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      Exciton dynamics in supramolecular assemblies of p-phenylenevinylene oligomers
      Herz, L. M.; Daniel, C.; Silva, C.; Hoeben, F. J. M.; Schenning, A. P. H. J.; Meijer, E. W.; Friend, R. H.; Phillips, R. T.
      Synthetic Metals (2003), 139 (3), 839-842CODEN: SYMEDZ; ISSN:0379-6779. (Elsevier Science B.V.)
      The authors have studied the dynamics of photoexcitations in chiral assemblies of p-phenylenevinylene oligomers functionalized with H-bonding motifs. In the regime of low excitation densities, the luminescence transients are influenced by the migration of excitons to defect sites, indicative of fast diffusivity of excitons along the mol. assemblies. At high excitation densities, bimol. exciton annihilation results in the fast depopulation of the stacks' excitonic energy levels.
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      Sprafke, J. K.; Kondratuk, D. V.; Wykes, M.; Thompson, A. L.; Ho, M.; Drevinskas, R.; Chen, W.; Yong, C. K.; Joakim, K.; Bullock, J. E. Belt-Shaped π -Systems: Relating Geometry to Electronic Structure in a Six-Porphyrin Nanoring J. Am. Chem. Soc. 2011, 133, 17262– 17273
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      Ruseckas, A.; Wood, P.; Samuel, I. D. W.; Webster, G. R.; Mitchell, W. J.; Burn, P. L.; Sundström, V. Ultrafast Depolarization of the Fluorescence in a Conjugated Polymer Phys. Rev. B 2005, 72, 115214 1– 5
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    Absorption and emission spectra of all compounds. Characterization of PL upconversion response function. PL transients and anisotropy dynamics of all compounds. Details of anisotropy modeling. Molecular Dynamics and PL Depolarization at 600K. This material is available free of charge via the Internet at http://pubs.acs.org/.


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