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Transient Electron Spin Polarization Imaging of Heterogeneous Charge-Separation Geometries at Bulk-Heterojunction Interfaces in Organic Solar Cells

  • Yasuhiro Kobori*
    Yasuhiro Kobori
    Molecular Photoscience Research Center,  Department of Chemistry, Graduate School of Science, , Kobe University, 1-1 Rokkodai-cho, Nada-ku, Kobe 657-8501, Japan
    *E-mail: [email protected]
    More by Yasuhiro Kobori
    Orcidhttp://orcid.org/0000-0001-8370-9362
    • , 
  • Takumi Ako
    Takumi Ako
    Department of Chemistry, Graduate School of Science, Kobe University, 1-1 Rokkodai-cho, Nada-ku, Kobe 657-8501, Japan
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    • , 
  • Shinya Oyama
    Shinya Oyama
    Department of Chemistry, Graduate School of Science, Kobe University, 1-1 Rokkodai-cho, Nada-ku, Kobe 657-8501, Japan
    More by Shinya Oyama
    • , 
  • Takashi Tachikawa
    Takashi Tachikawa
    Molecular Photoscience Research Center,  Department of Chemistry, Graduate School of Science, , Kobe University, 1-1 Rokkodai-cho, Nada-ku, Kobe 657-8501, Japan
    More by Takashi Tachikawa
    Orcidhttp://orcid.org/0000-0001-6465-5792
    • , and 
  • Kazuhiro Marumoto
    Kazuhiro Marumoto
    Division of Materials Science, University of Tsukuba, Tsukuba, Ibaraki 305-8573, Japan
    Tsukuba Research Center for Energy Materials Science (TREMS), University of Tsukuba, Tsukuba, Ibaraki 305-8571, Japan
    More by Kazuhiro Marumoto
    Orcidhttp://orcid.org/0000-0001-9792-0775
Cite this: J. Phys. Chem. C 2019, 123, 22, 13472–13481
Publication Date (Web):May 15, 2019
https://doi.org/10.1021/acs.jpcc.9b02672
Copyright © 2019 American Chemical Society
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Abstract

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Despite the importance of elucidating photoinduced charge-generation mechanisms for the development of efficient organic solar cells (OSC), it has been quite difficult to characterize molecular geometries, electronic couplings, and charge mobilities in the initial photoinduced charge-separated (CS) states for the heterogeneous molecular environments in bulk-heterojunction interfaces between electron donor–acceptor domains in the photoactive layers. In this study, we employed a time-resolved electron paramagnetic resonance method to characterize two kinds of electron spin polarizations (ESPs) of the photoinduced CS states as different geometries, exchange couplings, and spin-relaxation times of spin-correlated radical pairs in OSC blend films composed of regioregular poly(3-hexylthiophene-2,5-diyl) and [6,6]-phenyl-C61-butyric acid methyl ester by applying polarized light excitations (magnetophotoselection) with respect to an external magnetic field direction at a cryogenic temperature. From this, we performed analysis of mapping the ESPs to space directions to obtain image views of the molecular geometries in mobile and trapped CS states. We propose that the heterogeneities in the interfacial charge generations may be correlated with high- and low-frequency phonon modes leading to the mobile and weakly trapped charge pairs, respectively, denoting the great significance of such molecular motions for the efficient photocarriers overcoming the interfacial electrostatic binding potential.

Introduction

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The organic solar cells (OSC) have attracted great attention because of low cost, nontoxicity, and flexibility compared to the other types of solar cells. Recent remarkable progress in organic materials related to the organic semiconductors has achieved a power conversion efficiency (PCE) close to 15%(1,2) for single junction cells, although the PCE is still lower than the recent developments of 22–23% in the solar cells employing organolead halide perovskites.(3)
Solid-state photoactive layers have been fabricated by spin-coating of mixed solutions composed of the conjugated polymer as an electron donor (D) and an organic acceptor (A) material for generating the photocarrier in OSC. The photoactive layer of the blend film usually forms the bulk-heterojunction (BHJ) D:A interfaces by the phase segregation of self-organized domains.(4) This BHJ plays a key role in the initial photoinduced charge separation (CS) following exciton migrations to the domain interfaces. Despite recent progress in molecular designs tuning organic optoelectronic and structural properties for the higher PCEs,(1,5−7) the mechanism of the initial photocarrier generation is poorly understood even in the well-studied poly(alkylthiophene):PCBM systems (Scheme 1). Several studies discussed why and how the electron and hole escape from the charge transfer (CT) binding interaction at the domain interface.(8−15) It was pointed, as an example, that such a binding energy is overcome by entropy enhancement due to an increase in the possible sites of the separated charges after the CT state.(11,14) The importance of the electron–phonon (e–p) interactions was also discussed to promote the dissociations by molecular motions.(16−21) The orbital delocalization is another origin of the charge conductions.(13,15,18,22,23) Hodgkiss and co-workers(24) demonstrated distance distributions in photogenerated charge pairs using transient absorption spectroscopy at cryogenic temperature for blend films. It was shown that ca. 1 and 3–4 nm separated pairs were both generated at the interface, demonstrating the highly disordered interfacial molecular environment for the photocarrier generation in the blend materials. Kassal and co-workers(11) theoretically predicted that energetic disorder may enable the exergonic CS while delocalization by chromophore coupling would induce a barrier height for the CS.(13) However, the geometrical and energetic disorders are not experimentally characterized on the transient CS states.

Scheme 1

Scheme 1. Structure Formula of P3HT and PC61BM Utilized for OSC
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The time-resolved electron paramagnetic resonance (TREPR) method is powerful for characterizing distances and geometries in photoinduced charge pairs, as recently demonstrated for the primary CS state(25) in plant photosystem II. As shown in Figure 1, electron spin polarization (ESP) detected as microwave absorption (A) and emission (E) from spin-states of spin-correlated radical pairs (SCRP)(26−29) is often sensitive to directions (B0) of external magnetic field in a molecular-pair coordinate system because of the several spin anisotropic characters in the transient D+•···A–• states. Therefore, one may obtain the geometrical parameters in the transient charge pairs (Figure 1a) by analyzing the SCRP spin polarizations (Figure 1b,c). Recently, the electron spin polarization imaging (ESPI) method to map the ESP for several B0-directions has been presented for clear visualization of geometries of the transient CS state in a direct way.(25) From TREPR spectrum analysis of the SCRP polarization obtained in a frozen solution and in oriented multilayers of thylakoid membranes, the anisotropy of the spin–spin dipolar coupling was mapped to a three-dimensional B0-direction space to characterize the positions and orientation of the primary CS state in the membrane.

Figure 1

Figure 1. (a) Setting of the geometries in the interfacial CS state reflecting anisotropies of the g-tensors(35−37) and the spin–spin dipolar coupling in the present study. Principal axes and their values are depicted for the g-tensors. The principal axis of the dipolar interaction is represented by d as the inter-spin vector. The anisotropies in the hyperfine interactions of P3HT+• were also considered.(18,21)M denotes the direction of the transition dipole moment(38) memorized by the one-dimensional (1D) exciton diffusion after polarized laser excitation L. (b) Schematic SCRP model based on the singlet (S) and the triplet (+, 0, and −) sublevels of the radical pair. S–T0 mixing by the hyperfine interaction and by the Δg (=gP3HT – gPCBM) causes four EPR transitions (denoted by A and E) in the right because the S–T± forbidden transitions (dashed allows in the left) are mixed with the allowed T0–T± transitions.(39,40) (c) The EPR transition peaks by the two antiphase doublets at one specified B0 direction from the SCRP model. The peak splittings(29) denoted by the arrows reflect −4d + 2J, where d [=D(cos2 θD −1/3)/2] and J (=62 μT) are the dipolar and exchange interactions, respectively, with D < 0. kS and kT are the recombination rates to the ground state and to the excited triplet state (3P3HT*), respectively.

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In our previous studies,(16,18,21) geometries and electronic couplings of the CS states were estimated at the BHJ interfaces by analyses of the SCRP polarization (Figure 1b) for D:A blend thin films composed of regioregular poly(3-hexylthiophene-2,5-diyl) (P3HT) and [6,6]-phenyl-C61-butyric acid methyl ester (PC61BM) in Scheme 1. The disordered D/A interfacial molecular environment in the conformations and in the energies might, however, challenge the validities of the determinations of the magnetic parameters related to the CS geometries and their interactions,(30) even though these properties were regarded as ensemble averaged characteristics. Moreover, the ESP patterns (the E/A/E/A or A/E/A/E  in the EPR transitions from the lower B0 field) in Figure 1c varied with reports presented by several groups(18,21,30−34) even though the blend materials and their D:A blend ratios are fairly common. This would reflect the difference in the morphology of the fabricated film to influence the dominant ESP spectrum pattern because the ESP is sensitive to the CS geometry and to the exchange interaction (J) between the two spins as shown in Figure 1c.(30) Alternatively, EPR cavity conditions influenced by temperature, measurement materials, and intrinsic microwave power (B1 field) would alter the susceptibility of the ESP detections of those complex species depending on the heterogeneity in the phase memory times (T2) in these spin systems. Despite the importance of understanding the initial charge conduction mechanism, the heterogeneities in charge positions, orientations, and electronic couplings are not established together with the motions in the initial CS states. One may thus raise the following questions. (1) Can the heterogeneous geometries, motions, and electronic characters be experimentally distinguished for the interfacial CSs? (2) How are these characteristics related to the charge conduction in OSC?
It is also very important to obtain the geometries and the electronic coupling in the transient CS state not only in the blend films but also in the cell device. Marumoto and co-workers reported pioneering works on characterizing long-lived trapped carriers using the steady-state EPR method at the interfaces between the photoactive layers and the hole transport layers of PEDOT:PSS.(41) In this, the trapped charges were reported to be accumulated by continuous light irradiation in the device structures or device substrates, lowering the device performance. On the other hand, the geometries and electronic properties of the short-lived transient CS states have not yet been characterized in the OSC substrate.
In the present study, we have measured and analyzed the TREPR spectra from a substrate of a cell device composed of indium tin oxide (ITO)/PEDOT:PSS/P3HT:PC61BM/LiF/Al, where ITO and Al are cathode and aluminum anode, respectively. We characterized the interfacial CS geometries and their electronic couplings in the active layer by supporting detections of light polarization direction (L) effect of the excitation laser on the TREPR spectrum with respect to the B0 direction on the P3HT:PC61BM blend films applying L // B0 and L ⊥ B0. This L effect has been referred to as magnetophotoselection (MPS) and has long been demonstrated to be useful to determine the orientations of the spin–spin dipolar interactions.(42−46) We herein characterized two different TREPR components possessing different spin–spin geometries and relaxation times in the CS states due to the disorders.

Methods

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Chemicals and Sample Preparations

P3HT (Sigma-Aldrich), PC61BM (>99.5%, Tokyo Kasei), and 1,2-dichlorobenzene (>99%, Tokyo Kasei) were used as received without further purification. On the MPS measurements, the BHJ thin films were fabricated by the spin-coating of the solutions on thin cover glasses with 1 cm2 using a spin coater (Mikasa MS-A100). The films were then manually treated within a powder paper to produce a number of broken tips of ca. 0.5 mm2 and were placed in the EPR sample tubes (Figure S1). The films were then deaerated using a vacuum line under the vacuum pressures of 10–3 Pa to remove solvent and oxygen in the films. After evaporation, the 1:1 glycerol/water (v/v) mixture was added to the EPR tube to ensure the film temperature and to protect the films from the ablation damage by laser irradiation. The freeze–pump–thaw cycles were performed to deoxygenate the solutions using the vacuum line. The device fabrication was performed as reported previously.(41) No annealing procedures were performed before the TREPR measurements. The OSC device was sealed under dry a nitrogen atmosphere in an EPR sample tube before the TREPR measurements.

Time-Resolved EPR Measurements

The X-band TREPR measurements were carried out using a Bruker EMX Plus system without using the field modulations. In the microwave bridge, a modified wide-band preamplifier was equipped. Light excitations were performed by the second harmonics (532 nm) of an Nd:YAG laser (Continuum, Minilite II, fwhm ∼5 ns). A laser depolarizer (SIGMA KOKI, DEQ 1N) was placed between the laser exit and the microwave cavity for the depolarized TREPR data. Transient EPR signals were averaged by a Tektronix DPO3054 500 MHz digital phosphor oscilloscope at 201 different external magnetic field positions and were transferred to a personal computer via a USB communication to obtain the two-dimensional TREPR data. The sample tubes were placed in a liquid nitrogen Dewar flask filled with the liquid nitrogen and was fixed inside the EPR cavity for the MPS measurements for the spin-coated film. For the substrate of the OSC, temperature (98 K) was controlled using a nitrogen gas-flow system.

Numerical Simulations

Numerical calculations and the ESPI(25) mappings were performed using MATLAB (The MathWorks). The computation methods to obtain the delay time (td) dependence of the TREPR spectra(21,25,39) and the time profiles(18) of the transverse magnetizations were reported previously for the transient CS states. We also set a precursor CS state(30) in which density matrix elements of coherence terms (ρ0S and ρS0) in an S–T0 system developed within a nanosecond lifetime in the presence of a strong exchange coupling (Jpre = 0.4 mT). Then, such coherence terms together with the populations (ρSS and ρ00) were transferred to the corresponding density matrix elements of the two different SCRPs, as shown in Figure 1b, to generate the density matrix elements of the initial conditions.(47) This will produce an overlapping net “E” polarization at the P3HT+• resonance side and a net “A” at the higher field PC61BM–• side in Figure 1c, which is referred to as the chemically induced dynamic electron polarization (CIDEP) from the precursor radical pairs.(47−49) This CIDEP effect has, however, been revealed to be minor with respect to the SCRP contributions (Figure 1c) under the X-band TREPR measurements. For MPS analysis,(42) the ESP signals (ESPpara0 and ESPperp0) for B0 // L and B0⊥L were set to be proportional to the squares of direction cosines and to the halves of the squares of the sine components, respectively, between the B0 and M vectors as shown in Figure 1. A depolarization effect(46) (ξ = 0.38 in 0 ≤ ξ ≤ 1) was introduced to compute ESPpara = (1 – ξ)ESPpara0 + ξ(ESPpara0 + 2ESPperp0) and ESPperp = (1 – ξ)ESPperp0 + ξ(ESPpara0 + 2ESPperp0) for fitting the B0 // L and B0 ⊥ L spectra, respectively. A value of ξ = 0.35 was obtained to reproduce the MPS effect for another as-spun film (Figure S4). The ξ effect originates from depolarization of M by partially disordered exciton migrations, as detailed below.
The orientations of the g-tensors in PC61BM–• (Figure 1) were set by the Euler angles (Table S1) with respect to the principal axis system in P3HT+•, as reported previously.(18) In principle, one could find the angle parameter sets to eliminate the laser polarization effects on the higher-field antiphase doublets created by the PC61BM–• conformation. Thus, these Euler parameters might be treated as artifacts created during the spectrum fitting procedures because acceptor conformations are supposed to be highly disordered at the interface,(50) as described below.

Results and Discussion

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Samples and Crystallinity

We prepared the OSC with the substrate of ITO (150 nm thickness)/PEDOT:PSS (ca. 40 nm)/P3HT:PC61BM (ca. 160 nm)/LiF (0.6 nm)/Al (100 nm), as reported previously.(41) For the photoactive layer, ca. 3 wt % of P3HT:PC61BM = 1.0:0.8 (w/w) mixture solution in 1,2-dichlorobenzene was spin-coated on the PEDOT:PSS layer at 1000 rpm for 75 s to form ca. 160 nm thick films. We alternately prepared the P3HT:PC61BM film with the ca. 160 nm thickness using 1.0:0.7 (w/w) mixture solution in 1,2-dichlorobenzene fabricated by the spin-coating on cover glasses with 1000 rpm for 50 s. The present smaller spinning rates and the higher P3HT contents ensure higher crystallinity of the P3HT domain than in the spin-coated films of the previous TREPR report(21) on the 1:1 (w/w) blend films fabricated by the 3000 rpm rates. More details on the materials are described in the Method section and in Figure S1. Figure 2 shows the ground-state absorption spectrum of the as-spun blend films. The strong absorption bands at the 500–600 nm region correspond to the transitions of the self-organized crystalline phase of P3HT, as reported previously,(51) indicating the predominant crystallinity of the P3HT domain in the present study. To obtain the MPS effects, the light excitations were performed by the second harmonics (532 nm) of an Nd:YAG laser to dominantly photoexcite the crystalline phase of the P3HT domain of the blend film (Figure 2).

Figure 2

Figure 2. Ground-state absorption spectrum of the as-spun P3HT:PC61BM (1.0:0.7 weight ratio) blend film used for the MPS measurements. The 532 nm laser was employed to selectively pump the P3HT-crystalline domain. The interfacial CT band is also observed around 800 nm.

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TREPR Data of the OSC Substrate

Figure 3 shows the TREPR data obtained by the 532 nm laser excitation of the ITO/PEDOT:PSS/P3HT:PC61BM/LiF/Al substrate of the OSC at T = 98 K. In this experiment, depolarized light pulses were employed. It is evident that the TREPR spectrum is dramatically changed by the delay time (td) after the laser flash in Figure 3a–c and is similar to the previous TREPR study on 1:1 P3HT:PC61BM composites reported by Behrends and co-workers.(32) The TREPR spectrum exhibits an E/A/E/A pattern that was assigned to the transient interfacial CS state. On the other hand, an inverted A/E/A/E-polarized EPR spectrum emerges at td = 0.6 μs, as was predominantly observed in the previous study for the films spun at 3000 rpm.(21) These E/A/E/A and A/E/A/E-polarized EPR spectra were both interpreted by the SCRP model contributed by the spin–spin dipolar coupling (d) and by J as shown in Figure 1c.(18,32,33) This suggests that the two SCRP components possessing different combinations of D and J parameters are generated. Here, D represents the dipolar coupling constant in d = D (cos2 θD −1/3)/2 that determines the peak splitting depending on θD, i.e., the angle between B0 and d in Figure 1a. The A/E/A/E spectrum was explained by a strong D (≈−500 μT) with respect to J (≈10 μT).(21)Figure 3d shows the time profile of the TREPR intensity at the field position (337.2 mT) indicated by the arrow in Figure 3a. The emissive EPR signal initially rises within 0.1 μs but decays with ≈0.2 μs time constant. This quick component corresponds to the E/A/E/A spectrum as shown in Figure 3a. Subsequently, the slower rise in the component in the E signal is observed around 0.5 μs and decays at 2 μs, corresponding to the A/E/A/E spectrum as shown in Figure 3b. After the decay of this emissive polarization, an absorptive EPR spectrum was observed at td = 2.4 μs in Figure 3c, denoting that the spin–lattice relaxation time of T1 (=1.1 μs) is responsible for the decay of the E polarization resulting in the absorptive thermal equilibrium sublevel population in Figure 1b. Thus, the quick decay in the E/A/E/A component denotes that T1 (=0.3 μs) is small indicating that a mobile electron–hole pair is generated. In contrast, the A/E/A/E component with T1 = 1.1 μs in Figure 3b is attributable to a shallowly trapped charge pair with the separation distance of rCC ≈ 1.7 nm, as we concluded previously for the A/E/A/E polarization.(16,21)

Figure 3

Figure 3. (a–c) Delay time dependence of the TREPR spectrum by 532 nm laser irradiation of the substrate from ITO/PEDOT:PSS/P3HT:PC61BM/LiF/Al of the OSC obtained at 98 K, showing E/A/E/A (0.2 μs) and A/E/A/E (>0.6 μs) patterns. (d) Time profile of the TREPR signal at 337.2 mT as indicated by the arrow in (a). These data were obtained at open-circuit conditions. Solid red lines were obtained by stochastic-Liouville equation (SLE) analysis of the SCRP model using the two different CS state components as shown by the decomposed dotted profiles in (d).

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Magnetophotoselection of the TREPR Spectrum of Blend Films

For understanding the geometrical properties for the above two transient CS states, we observed the MPS effects for the blend film, as shown in Figure 4. The E/A/E/A polarizations were detected only at the early stage of td = 0.2 μs in Figure 4, which were very similar to the result in Figure 3. The E/A/E/A effect was most prominent when L is perpendicular to the B0 direction (Figure 4b), while the inner A/E contribution around the center field became weak when L is parallel to B0 (Figure 4c). Additionally, the emission peak position at the lower field indicated by the arrow in Figure 4b is situated to be lowest, while the corresponding peak is highest in Figure 4c among the spectra in Figure 4a–c. Because these resonance positions at the lower field reflect a P3HT+• resonance side in the SCRP due to gP3HT > gPCPM, as shown by the blue E/A transitions in Figure 1b,c, the MPS effect on the peak position is ascribed to the anisotropy in gP3HT. This implies that the P3HT+• conformations in the interfacial CS states memorize the L polarizations excited in the P3HT domains, i.e., one-dimensional singlet exciton diffusions to the domain interface (Figure 5b) and the subsequent interfacial CS cause the shifts in the peak positions in Figure 4a–c. The one-dimensional exciton (M in Figure 1a) migration was demonstrated by Ohkita and co-worker(51) in the spin-coated P3HT films possessing the crystallinity that is presently consistent with the intense absorption band around 600 nm by the crystalline phase as shown in Figure 2. Furthermore, P3HT-crystal/PCBM interfacial domains were clarified by grazing-incidence small-angle and wide-angle X-ray scattering measurements in the blend films.(50) These coincide with the above MPS results because the 1D exciton migration to the interface may result in the preferential generations of the ordered P3HT+• memorized by L in the interfacial CS states.

Figure 4

Figure 4. (a–c) Magnetophotoselection effects of the TREPR spectrum for the delay time of td = 0.2 μs by 532 nm laser irradiation of the blend film of P3HT:PC61BM at 77 K. The microwave power was 6.3 mW. (d–f) Computed EPR spectra of the SCRP for the field directions to the principal axes (gx, gy, gz) in P3HT+• by applying J = 62 μT, D = −100 μT, θ = 65°, and ϕ = 0° in Figure 1a. (g–l) MPS effects for the later delay times of td = 0.6 and 2.4 μs. (m, n) Time profiles of the TREPR signal at B0 = 337.30 mT for the depolarized conditions. Computed profiles are shown by the red solid lines obtained by SLE analysis. The red lines were decomposed to the mobile CS state (dotted blue lines computed with T1 = 0.4 μs and T2 < 0.2 μs) and to the trapped CS state (dotted red lines, T1 = 1.6 μs and T2 = 3.0 μs), respectively. Respective spectrum components are shown in Figure S2.

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Figure 5

Figure 5. Interfacial photoinduced charge-separation geometries (b, d) characterized by ESPIs (a, c) from the E/A/E/A pattern at 0.2 μs and from A/E/A/E at 0.6 μs, respectively. The ESPIs in (a) and (c) were obtained by distributing the transverse magnetization (ESP intensities in Figure 4d–f) at B0 = 337.25 mT as the color maps to the B0 space directions in the electron–hole pairs generated after the migrations of the exciton (M) to the P3HT-crystal/PCBM interfaces. The hole geometries in (b) and (d) are schematically drawn based on a tilted packing structure with short π–π stackings of 0.34 nm interplanar distances in the crystalline P3HT obtained by the electron diffraction analysis.(55) Inter-spin distances (rCC) and electronic couplings (VCR) were estimated from D and J, respectively.

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Because the optical transition dipole moment (M) is parallel to the long axis (gx direction in P3HT+•)(37,38) of the polymer as shown in Figure 1a, the orientations of the g tensor and of the d interaction are estimated from the SCRP model, as follows. The higher field position of the emission peak in the B0 // L situation (Figure 4c) denotes that the gx direction (gx = 2.0009) is parallel to M, while the gy = 2.0028 and gz = 2.0019 directions are perpendicular to the M vector, as in Figure 1a because M tends to be aligned with the L direction. When the inter-spin d vector is close to the gx direction and B0 is aligned to gx, the peak splitting determined by −4d + 2J in Figure 1c is greater than the splitting for the other B0 alignments because −4d ≈ −4D/3 (>0) is large and may exceed 2J (>0) due to θD ≈ 0 in d = D(cos2 θD – 1/3)/2. This larger E/A splitting in each antiphase doublet causes intensity cancellation in the inner A/E transitions of the E/A/E/A pattern, as calculated by the SCRP model in Figure 4f with J = 62 μT, D = −100 μT, θ = 65°, and ϕ = 0°. On the other hand, in the B0 ⊥ L situation, the d vector tends to be close to the magic angle of θD = 54.7° with respect to B0 lying mainly in the gy and gz directions in Figure 1a. In this case, the peak splitting is dominated by 2J resulting in the sharp E/A/E/A pattern, as computed in Figure 4d,e. It is noted that the sum of the spectra in Figure 4d,e is close to the spectrum of Figure 4b and that the spectrum Figure 4c is also very similar to the spectrum computed for the gx direction (Figure 4f). This denotes that the MPS effect is well explained by the d vector residing close to the gx axis, as in a situation in Figure 1a. From this geometry, the distant CS state is concluded to be generated via the intrachain hole conduction after the contact CT(52) at the domain interface. As for the higher field E/A component attributed to the PC61BM–• resonance (Figure 1c) in Figure 4a–c, the peak position indicated by the red arrow in Figure 4a is unchanged by L, suggesting that the PC61BM–• conformations (red axes system in Figure 1a) are disordered with respect to the P3HT+• coordinate system (blue axes in Figure 1a) residing at the crystalline domain. This probably reflects heterogeneous PC61BM interfacial conformations at the BHJ as opposed to the ordered polymer crystalline feature.
At the larger delay time (td = 0.6 μs in Figure 4g–i), no apparent MPS effect is observed on the A/E/A/E polarization overlapping to the E/A/E/A, as described below. However, the emissive peak position in the P3HT+• resonance side is still dependent on the L direction, suggesting that the P3HT+• part in the CS state has the memory of L polarization, as indicated by the arrows in Figure 4g–i. A similar trend was obtained for the lower-field peaks at td = 2.4 μs, although the absorptive signals by the thermal equilibrium spin population became dominant overlapping the A/E/A/E spin polarization in Figure 4j–l. Figure 4m,n display the time profiles of the TREPR signal at 337.25 mT indicated by the blue arrow in Figure 4a for different time scales. As observed in Figure 3d, the quick and slow rises were obtained and are attributable to the mobile (for the E/A/E/A signal) and shallowly trapped (A/E/A/E) CSs, respectively.

Modeling Electron Spin Polarization of the CS States

We presented matrix forms to analyze the TREPR data based on the stochastic-Liouville equation (SLE)(17,18,21,39) in the transient CS states with considering the charge-recombination kinetics (kS and kT in Figure 1b) and spin relaxations represented by (1) the relaxation time T23 between |2> and |3> levels (dashed arrows in Figure 1b) due to the J-modulation(53) and (2) the spin–lattice relaxation time T1 determined by D fluctuations.(25,54) We performed the spectrum computations for the two different CS species (Figure S2) having individual EPR parameter sets in Table 1, and summed to fit the MPS spectra (Figure 4) at the three different time-windows by powder-pattern integrations from all possible B0 directions by taking into account their angle distributions in Figure 1a with respect to M for the depolarized, B0 ⊥ L and B0 // L conditions,(42,43) as shown by the solid fitting lines in Figure 3 and in Figure 4. More details on the computations are described in the Method section and in Figure S2.
Table 1. Parameters for the Simulations of the Time-Dependent EPR Data (Figure 4) of the Two Photoinduced CS States in the as-spun P3HT:PC61BM Blend Film at 77 K
polarization patternD (μT)aJ (μT)adipolar anglesT1 (μs)T2 (μs)T23 (μs)kT (105 s–1)b|VCR| (cm–1)
E/A/E/A–100 (±33)62 (±17)θ = 65° ϕ = 0°0.4< 0.20.3 0.6 (±0.1)
A/E/A/E–530 (±90)9 (±20)θ = 45° ϕ = 35°1.63.0>102.50.2 (±0.1)
a

Errors in parentheses were evaluated from the line widths of 1/(2πT2d*) and 1/(2πT2J*) originating from heterogeneities of 3d and d – 2J in the T0–T± and S–T± transition energies,(17) respectively. (T2d*, T2J*) = (65, 75) and (55, 30) ns were employed for the E/A/E/A and A/E/A/E polarizations, respectively, in Figure 4.

b

The triplet CR rate to reproduce the spectrum shapes at td = 2.4 μs in the trap CS states.

Using J, D, and angle parameters, we also computed time profiles of the transverse magnetizations, solving a 16 × 16 matrix form(18) for time differential equations in 16 elements (ρ++, ρ+S, ρ+0, ρ+–, ρS+, ···) of the density matrix from the S, T+, T0, and T– basis functions. In this, we considered the microwave B1 strength (ω1 = 2.7 × 106 rad/s) to fit the profiles in Figure 4m,n. ω1 = 6 × 105 rad/s was utilized for Figure 3d. The spin-relaxation times were estimated for the electron–hole pairs with this analysis (Table 1). The E/A/E/A polarization was deduced to have the mobile charge characteristic from T1 = 0.4 μs, T23 = 0.3 μs, and T2 < 0.2 μs to reproduce the quick rise and decay of the TREPR signals, as shown by the dotted blue lines in Figure 3d,m. On the other hand, the slow rise and decay in the A/E/A/E component were explained by a larger T2 being 3.0 μs with T1 = 1.6 μs and T23 = 11 μs (red dotted lines in Figure 4m,n). These slower spin relaxation characteristics than those from the E/A/E/A contribution are evident from the spin nutation effect as the oscillatory feature for td > 1 μs in Figure 4n. We confirmed that, when the B1 field is very weak (microwave power =1.0 mW in Figure S3 vs 6.3 mW in Figure 4m,n), the A/E/A/E component decayed by more than 2 times longer time constants with the disappearance of the oscillatory feature, while the E/A/E/A component increased and decayed quickly, demonstrating the trap and mobile charge characteristics at the BHJ interface, respectively. Table 1 summarizes the EPR and kinetic parameters for the mobile (top) and trap (bottom) electron–hole pairs.

Electron Spin Polarization Imaging of Geometries of Mobile and Trapped CS States

Using the individual TREPR components (Figure S2) employed for the spectrum fittings in Figure 4 obtained by the powder-pattern averaging, one can distribute the ESP intensities of the transverse magnetization to all the field directions at a specified B0 strength to obtain ESP imaging maps for the mobile and trapped electron–hole pairs (Figure 5a,c, respectively). From Figure 4d–f, the ESP at B0 = 337.25 mT for the P3HT+• side is highly modulated by the field direction, as shown by the colored dotted arrows. This is because the E/A-resonance field position and its splitting are both changed by the combination of the gP3HT anisotropy and d-anisotropy effects; for the gy direction (Figure 5a), an A signal is obtained at the red dotted arrow in Figure 4e reflecting the largest gy principal value and the small −4d, while an E polarization is resulted (blue dotted arrow in Figure 4f) for the gx direction due to the higher resonance field by the small gx and due to the larger −4d splitting in the E/A pattern. Furthermore, around the field directions tilted by ca. 30° from the gy-axis in Figure 5a, it is expected that the absorptive signal intensities become strong because these directions may provide spectrum peak positions at B0 = 337.25 mT on the A polarization of the E/A pattern (Figure 4e). This means that one may identify the principal gy-axis from the center direction of the donut-shaped ring (Figure 5a) mapped with the transverse magnetization. Similar donut shapes were observed for the gx and gz directions, demonstrating that one may directly identify the orientation of P3HT+• in the CS state from this map. This is relevant to the principal axis assignment in the gP3HT tensor from the MPS effect of the peak position in Figure 4a–c, as described above.
As for the emissive polarization in the E/A/E/A pattern, the E signal in the hole side is expected to be strongest for the field direction parallel to the inter-spin vector because the E/A splitting by −4d + 2J is largest as described above. Thus, the E signal reaches a peak position at B0 = 337.25 mT in the field–swept E/A spectrum of Figure 4f. This leads to identifying the d vector from the blue mapping region, which corresponds to the direction of the strongest emissive magnetization in Figure 5a. As a result, from this ESPI obtained from the above-mentioned fittings of the MPS data, one may reliably identify the position of the counter PCBM–• from the d vector in the CS state,(25) as schematically drawn in Figure 5b for the mobile E/A/E/A polarization, demonstrating the one-dimensional intrachain hole conduction. Using the same procedure to obtain Figure 5a, we mapped the ESP (Figure 5c) of the A/E/A/E polarization to the several B0 directions to visualize the geometry of the CS state in Figure 5d. In this, the d vector is obtained from the polar direction of the blue emissive map(25) originating from the dipolar interaction because the A/E/A/E spectrum shape is dominated by D when J is weak (Table 1). This d vector in Figure 5c is tilted at angles of  c.a. 50° from the gx, gy, and gz axes equivalently and is coincident with the result (Figure 4g–i) that the EPR spectrum is independent of the L direction. This is consistent with the previous reports that the intermolecular charge separations via the contact CT generates the shallowly trapped state (Figure 5d) at 77 K.(16,21) The separations of the electrons might also take place in the PCBM domain(30) to several directions following the interfacial contact CT because the MPS effect was not observable for the A/E/A/E polarization.

Electronic Couplings and Trap Depths of the CS States

For the mobile electron–hole pairs that exhibited the E/A/E/A polarization, (J, D) = (62 μT, −100 μT) was essential to interpret the MPS effect, as explained above with Figure 4a–f. From this positive J, the electronic coupling (VCR) is estimated to be |VCR| ≈ 0.6 cm–1 using the configuration interaction(56) in 2J (=|VCR|2/ΔECR) where ΔECR (≈0.4 eV)(18) is the vertical energy gap for the kT recombination in Figure 1b, while the spin–spin separation (rCC) is estimated to be rCC = 3.0(±0.4) nm by the point-dipole approximation (D = −2780/rCC3) on the D parameter in the unit of μT. This distance is well consistent with a reported center separation in distributed CS distances obtained in the blend films at cryogenic temperatures by Hodgkiss and co-workers.(24) We also obtained (J, D) = (70 μT, −124 μT) by the spectrum simulation in Figure 3a, leading to |VCR| ≈0.6 cm–1 at rCC = 2.9(±0.3) nm in the OSC substrate.
The inter-spin distance and electronic coupling were also estimated for the trap CS state to be rCC = 1.7(±0.1) nm and |VCR| ≈ 0.2 cm–1, respectively, from (J, D) = (9 μT, −530 μT) as shown in Table 1. To comprehensibly understand how the electronic interactions are affected by the CS geometries and by the molecular motions in the present system, the above |VCR| values are plotted vs rCC by filled and open circles for the mobile and trapped charges, respectively, in the blend films including the previous reports(21) obtained by the TREPR analyses at the delay times td > 0.3 μs, as shown in Figure 6b. Several CS geometries including rCC were previously obtained at 77 K by varying the D:A blend ratio and by changing the alkyl side-chain length in poly(3-alkylthiophene), as summarized in Table S1.(18,21) These interfacial CSs were treated as the weakly trapped charges in the blend films.(16) For comparison, reported |VCR| values are also plotted (filled triangles in Figure 6a) on the primary photoinduced CS states in poly(3-hexylthiophene)–fullerene linked dyads bridged by rigid oligo-p-phenylene spacers (P3HT-Phm-C60, m = 2, 3 in Figure 6a)(17) as the long-range coupling via through-bond interactions. The larger electronic interactions of |VCR| ≈ 0.6 cm–1 (● in Figure 6b) for the separated charges (rCC ≈ 3 nm) than those (○ in Figure 6b) in the trapped charges in Figure 5d are explained by the intrachain interaction through the polymer backbone nearby the hole; the bridge mediated tunneling via the backbone which is directly π-stack on the PCBM–• moiety (as shown by the dotted line in Figure 5b) may induce the stronger electronic tunneling between the unpaired orbitals. Alternately, highly delocalized intrachain hole distribution in the backbone(30) would facilitate the strong electronic interaction between the charges while the spin–spin dipolar interaction becomes weak, because the one-dimensional hole delocalization after the photoinduced CS was observed in the P3HT-Phm-C60 linked systems.(17) In this respect, the highly strong electronic interactions |VCR| ≈ 3 cm–1 (▲ in Figure 6a) were explained by the strong through-bond coupling via the oligophenylene bridges, as reported previously.(17) The shorter spin-relaxation times in the mobile charges in Table 1 are thus very likely to be resulted from the one-dimensional intrachain charge-motion and are coincident with the reported CWEPR studies on the origins of small T1 and T2 of the polymer polarons in P3HT:PCBM composites at lower temperatures.(19,20)

Figure 6

Figure 6. (Left) Semi-log plots of the electronic couplings vs CS distances in poly(3-alkylthiophene-2,5-diyl)-fullerene systems including (a) P3HT–fullerene linked dyads(17) bridged by oligo-p-phenylenes (▲: P3HT-Phm-C60, m = 2, 3). (b) In the blend films, |VCR|s of the mobile CSs were obtained from the E/A/E/A patterns at 0.2 μs and are plotted by the filled circles (●). The |VCR|s of the trapped CSs were obtained from the SCRP analyses for the later delay times larger than 0.3 μs and are plotted by the open circles (○). Data sources are detailed in Table S1.(18,21) The error bars were derived from the line widths determined by 1/(2πT2d*) and 1/(2πT2J*) as described in Table 1. (Right) Origins of the heterogeneous charge generations are schematically drawn using arrows as the non-Condon effects by (c) high-frequency modes for the strongly coupled mobile CS states (●) and by (d) low-frequency phonon modes for the weakly coupled trapped CS states (○), respectively.

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As for the trapped CS states (|VCR| ≈ 0.2 cm–1 at rCC ≈ 1.7 nm as plotted by ○ in Figure 6), the weakened electronic interaction is explained by intermolecular couplings, as represented by Figure 5d; although the intervening polymer chains participate in mediating the tunneling interactions in VCR,(16) the involvement of the polymer–polymer through-space interactions (VBB) should result in the weaker |VCR| than in the case of Figure 5b. From the superexchange model, the distance dependence of |VCR| has been characterized by the attenuation factor (β) in |VCR| = Vd exp[−β (rCC – d)/2] where d denotes contact D:A separation. According to the McConnell’s electron-tunneling mechanism, the attenuation degree has been approximated as follows,(16,21,57)(1)where ΔEeff and VBB represent the trap depth (i.e., the tunneling barrier) and the bridge–bridge coupling at the dBB separation, respectively. The trap characteristics derived from the larger T1 and T2 in Table 1 may originate from the electrostatic attraction between the charges in Figure 5d;(16) the electron–hole pairs in Figure 5d are thus frozen at 77 K at rCC ≈ 1.7 nm. Such a binding potential is weaker for the 3 nm separated (or 1D delocalized) charges in Figure 5b than in the trapped CS, allowing the 1D hole motions in the polymer chains. This is well consistent with the shorter T1 and T2 values in the E/A/E/A polarization in Table 1. The line fit in Figure 6b to the group of the open circles denotes that the trapped CS states for td > 0.3 μs were explained by the above attenuation with β = 2 nm–1.(21) From this, ΔEeff ≈ 0.2 eV, which was comparable to VBB was estimated as the tunneling barrier from the unpaired orbital level in P3HT+• to the highest occupied molecular orbitals (HOMO) of the single P3HT chains.(16,21) This indicated that the trap depths by the Coulomb bindings are shallow enough to be detrapped(17,58) to contribute to the photocurrent at room temperature. This “shallowness” is also relevant to preferential triplet CR(59) deactivation in Table 1 with |VCR| ≈ 0.2 cm–1 from the trapped CS states,(18) because the CS state energy level is required to be as high as the triplet energy (1.65 eV) of 3P3HT*,(60) which is close to the P3HT-PC61BM HOMO–lowest unoccupied molecular orbital band offset,(52) denoting very weak electrostatic stabilization in the trapped CS.
In Figure 6b, the filled circles are deviated from the line on the trapped CSs and exhibit strong electronic interactions for the highly separated charge pairs, representing long-range hole-conducting characteristics through a single polymer chain(61) as described above. This is interpreted by a very small effective value in ΔEeff/VBB of eq 1 due to the strong intrachain coupling. The Coulomb binding with ΔEeff < 0.2 eV is thus conclusive for the mobile CS state and is consistent with the geometry in Figure 5b to be detrapped as was demonstrated in the P3HT-Phm-C60 systems at room temperature.(17)

Origin of the Heterogenous CS Geometries and Mobilities

In our previous reports, it was suggested that the e–p coupling may play a role for the distant CS; the coupling of the hole to the librations of the P3AT crystalline domains would facilitate the separated charge generations.(16,21) For understanding the present coexistence of the mobile and trapped CS states with different geometries, we propose that several molecular motions participate in creating the different CS states, as schematically drawn at the right panel in Figure 6 in terms of the non-Condon effect.(8,23,62) When a high-frequency C═C stretching vibration is coupled to regenerate the quinoid structure(62,63) in P3HT+• upon the hole conduction (Figure 6c), it is anticipated that the hole separation occurs within the aromatic backbone. On the other hand, when the polymer–polymer interchain motions are rather coupled, the interpolymer hole separations are also plausible to result in the CS state in Figure 6d. This interpretation is well consistent with the theoretical prediction by Vukmirović et al.(64) that the charge conductions were promoted by the e–p couplings in which several phonon modes were participating to result in the scattering behavior in the electronic attenuation (β) for the hole transfers, as is herein observed by the ● plots deviated from the straight line in Figure 6b. The involvement of the C═C stretching vibration for the hole generation was demonstrated by a recent ultrafast two-dimensional electronic spectroscopic observation of the vibration quantum coherence with a frequency of 1506 cm–1 in P3HT/PCBM aggregates, as reported by Song et al.(65) and correlates with the mobile charge pair geometry as shown in Figure 5b. Concerning the trap CS state (Figures 5d and 6d), the intermolecular libration modes and the disorder alkyl side-chain motions may also play roles for the interchain dissociations.(21) Although such low-frequency phonons seem not to be significant for the photocurrent at the present temperatures resulting in the frozen trap at 1.7 nm, these modes are crucial for the OSC operations at room temperature to cause detrapping(16) (vide supra). This is because such low-frequency modes numerously exist in the solid aggregates including the PC61BM clusters and may enhance the density of the states for the separated electron–hole pairs coupled with the dense phonon bath,(16) in contrast to the C═C high-frequency mode in the single polymer chain. Analysis of temperature dependence on the TREPR data would be informative to track detrapping in the blend films, as 1D charge-hopping dissociations were characterized in the P3HT-Phm-C60 systems at room temperature.(17)

Conclusions

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In conclusion, we have deduced the motional characters, the geometries, and the electronic couplings of the two different photoinduced electron–hole pairs simultaneously generated in the P3HT-crystal/PCBM interfaces of the blend films including the OSC substrate using TREPR analysis with the applications of the ESPI method at cryogenic temperature. The former E/A/E/A polarizations exhibited the MPS effect and originated from the mobile conducting charge pairs in which the holes would be delocalized one-dimensionally in the P3HT aromatic plane to which PC61BM–• is stuck. The latter A/E/A/E polarizations, on the other hand, possess the trap charge pair characteristics weakly bound by the electrostatic interactions at 77 K. The coexistence of intrachain and intermolecular photoinduced interfacial charge-motions are proposed to be resulted from the multiple phonon modes as the non-Condon effect in the aggregated organic materials.(62) In Figure 5b, vibrationally hot states could be involved in the 1D-mobile charges.(8,23) That is, high-frequency in-plane vibrations or reorganizations(66) within the aromatic backbone leading to the quinoid formation in P3HT+• would be associated with the photoinduced 1D charge separation as shown in Figure 6c, while the intermolecular libration modes and the disorder alkyl side-chain motions may play roles for the intermolecular dissociations as shown in Figure 6d.(21) The low-frequency phonons may be crucial for the OSC operations at room temperature because such low-frequency modes in Figure 5d and also in the PC61BM clusters may enhance the density of the states for the separated electron–hole pairs coupled with the dense phonon bath.(16) The present study thus implies that the three-dimensional imaging analysis of the spin polarization is powerful to understand the heterogeneous phonon-assisted photoinduced processes by mapping the transverse magnetizations by the transient spins based on SLE analysis that fully explains the MPS spectra and their time profiles in the anisotropic systems.

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.9b02672.

  • Sample conditions; decompositions of the computed transient EPR signals; time profile of the TREPR signal obtained at the microwave power of 1.0 mW; parameters for the simulations of the time-dependent EPR data; and MPS data for another blend film (PDF)

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Author Information

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  • Corresponding Author
  • Authors
    • Takumi Ako - Department of Chemistry, Graduate School of Science and , Kobe University, 1-1 Rokkodai-cho, Nada-ku, Kobe 657-8501, Japan
    • Shinya Oyama - Department of Chemistry, Graduate School of Science and , Kobe University, 1-1 Rokkodai-cho, Nada-ku, Kobe 657-8501, Japan
    • Takashi Tachikawa - Molecular Photoscience Research Center,  Department of Chemistry, Graduate School of Science,  and , Kobe University, 1-1 Rokkodai-cho, Nada-ku, Kobe 657-8501, Japan;  Orcidhttp://orcid.org/0000-0001-6465-5792
    • Kazuhiro Marumoto - Division of Materials Science, University of Tsukuba, Tsukuba, Ibaraki 305-8573, Japan;  Tsukuba Research Center for Energy Materials Science (TREMS), University of Tsukuba, Tsukuba, Ibaraki 305-8571, Japan;  Orcidhttp://orcid.org/0000-0001-9792-0775
  • Notes

    The authors declare no competing financial interest.

Acknowledgments

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This work is dedicated to the deceased former Prof. Kimio Akiyama (Tohoku University). This work was supported by a Grant-in-Aid for Scientific Research (Nos. JP17K19105, JP16H04097, and JP19H00888) from the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan.

References

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    Wang, Chao; Nakano, Kyohei; Lee, Hsiao Fang; Chen, Yujiao; Hong, You-Lee; Nishiyama, Yusuke; Tajima, Keisuke
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    Precise control of the mol. arrangements at the interface between the electron donor and acceptor in mixed bulk heterojunctions (BHJs) remains challenging, despite the correlation between structural characteristics and efficiency in org. photovoltaics (OPVs). This study reveals that the substitution patterns of linear and branched alkyl side chains on electron-donating/-accepting alternating copolymers can control the positions of an acceptor mol. (C60) around the π-conjugated main chains in mixed BHJs. Two-dimensional solid-state NMR demonstrates a marked difference in the location of C60 in the blend films. A copolymer with an electron-accepting unit positioned in close proximity to C60 demonstrated higher OPV performance in combination with various fullerene derivs. This mol. design offers precise control over the interfacial mol. structure, thereby paving the way for overcoming the current limitations of OPVs comprising mixed BHJs.
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  8. 8
    Shimazaki, T.; Nakajima, T. Theoretical Study on the Cooperative Exciton Dissociation Process Based on Dimensional and Hot Charge-Transfer State Effects in an Organic Photocell. J. Chem. Phys. 2016, 144, 234906  DOI: 10.1063/1.4953905
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    8
    Theoretical study on the cooperative exciton dissociation process based on dimensional and hot charge-transfer state effects in an organic photocell
    Shimazaki, Tomomi; Nakajima, Takahito
    Journal of Chemical Physics (2016), 144 (23), 234906/1-234906/8CODEN: JCPSA6; ISSN:0021-9606. (American Institute of Physics)
    This paper discusses the exciton dissocn. process at the donor-acceptor interface in org. photocells. In our previous study, we introduced a local temp. to handle the hot charge-transfer (CT) state and calcd. the exciton dissocn. probability based on the 1D org. semiconductor model [T. Shimazaki and T. Nakajima, Phys. Chem. Chem. Phys. 17, 12538 (2015)]. Although the hot CT state plays an essential role in exciton dissocns., the probabilities calcd. are not high enough to efficiently sep. bound electron-hole pairs. This paper focuses on the dimensional (entropy) effect together with the hot CT state effect and shows that cooperative behavior between both effects can improve the exciton dissocn. process. In addn., we discuss cooperative effects with site-disorders and external-elec.-fields. (c) 2016 American Institute of Physics.
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  9. 9
    Grancini, G.; Maiuri, M.; Fazzi, D.; Petrozza, A.; Egelhaaf, H. J.; Brida, D.; Cerullo, G.; Lanzani, G. Hot Exciton Dissociation in Polymer Solar Cells. Nat. Mater. 2013, 12, 29– 33,  DOI: 10.1038/nmat3502
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    9
    Hot exciton dissociation in polymer solar cells
    Grancini, G.; Maiuri, M.; Fazzi, D.; Petrozza, A.; Egelhaaf, H-J.; Brida, D.; Cerullo, G.; Lanzani, G.
    Nature Materials (2013), 12 (1), 29-33CODEN: NMAACR; ISSN:1476-1122. (Nature Publishing Group)
    The std. picture of photovoltaic conversion in all-org. bulk heterojunction solar cells predicts that the initial excitation dissocs. at the donor/acceptor interface after thermalization. Accordingly, on above-gap excitation, the excess photon energy is quickly lost by internal dissipation. Here we directly target the interfacial physics of an efficient low-bandgap polymer/PC60BM system. Exciton splitting occurs within the first 50 fs, creating both interfacial charge transfer states and polaron species. On high-energy excitation, higher-lying singlet states convert into hot interfacial charge transfer states that effectively contribute to free-polaron generation. We rationalize these findings in terms of a higher degree of delocalization of the hot charge transfer states with respect to the relaxed ones, which enhances the probability of charge dissocn. in the first 200 fs. Thus, the hot charge transfer state dissocn. produces an overall increase in the charge generation yield.
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  10. 10
    Vandewal, K.; Albrecht, S.; Hoke, E. T.; Graham, K. R.; Widmer, J.; Douglas, J. D.; Schubert, M.; Mateker, W. R.; Bloking, J. T.; Burkhard, G. F. Efficient Charge Generation by Relaxed Charge-Transfer States at Organic Interfaces. Nat. Mater. 2014, 13, 63– 68,  DOI: 10.1038/nmat3807
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    10
    Efficient charge generation by relaxed charge-transfer states at organic interfaces
    Vandewal, Koen; Albrecht, Steve; Hoke, Eric T.; Graham, Kenneth R.; Widmer, Johannes; Douglas, Jessica D.; Schubert, Marcel; Mateker, William R.; Bloking, Jason T.; Burkhard, George F.; Sellinger, Alan; Frechet, Jean M. J.; Amassian, Aram; Riede, Moritz K.; McGehee, Michael D.; Neher, Dieter; Salleo, Alberto
    Nature Materials (2014), 13 (1), 63-68CODEN: NMAACR; ISSN:1476-1122. (Nature Publishing Group)
    Interfaces between org. electron-donating (D) and electron-accepting (A) materials have the ability to generate charge carriers on illumination. Efficient org. solar cells require a high yield for this process, combined with a min. of energy losses. Here, the authors study the role of the lowest energy emissive interfacial charge-transfer state (CT1) in the charge generation process. The authors measure the quantum yield and the elec. field dependence of charge generation on excitation of the charge-transfer (CT) state manifold via weakly allowed, low-energy optical transitions. For a wide range of photovoltaic devices based on polymer:fullerene, small-mol.:C60 and polymer:polymer blends, the authors' study reveals that the internal quantum efficiency (IQE) is essentially independent of whether or not D, A or CT states with an energy higher than that of CT1 are excited. The best materials systems show an IQE >90̂ without the need for excess electronic or vibrational energy.
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  11. 11
    Hood, S. N.; Kassal, I. Entropy and Disorder Enable Charge Separation in Organic Solar Cells. J. Phys. Chem. Lett. 2016, 7, 4495– 4500,  DOI: 10.1021/acs.jpclett.6b02178
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    11
    Entropy and Disorder Enable Charge Separation in Organic Solar Cells
    Hood, Samantha N.; Kassal, Ivan
    Journal of Physical Chemistry Letters (2016), 7 (22), 4495-4500CODEN: JPCLCD; ISSN:1948-7185. (American Chemical Society)
    Although org. heterojunctions can sep. charges with near-unity efficiency and on a subpicosecond time scale, the full details of the charge-sepn. process remain unclear. In typical models, the Coulomb binding between the electron and the hole can exceed the thermal energy kBT by an order of magnitude, suggesting that it is impossible for the charges to sep. before recombining. Here, we consider the entropic contribution to charge sepn. in the presence of disorder and find that even modest amts. of disorder have a decisive effect, reducing the charge-sepn. barrier to about kBT or eliminating it altogether. Therefore, the charges are usually not thermodynamically bound at all and could sep. spontaneously if the kinetics otherwise allowed it. Our conclusion holds despite the worst-case assumption of localized, thermalized carriers and is only strengthened if mechanisms like delocalization or "hot" states are also present.
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  12. 12
    Janković, V.; Vukmirovic, N. Combination of Charge Delocalization and Disorder Enables Efficient Charge Separation at Photoexcited Organic Bilayers. J. Phys. Chem. C 2018, 122, 10343– 10359,  DOI: 10.1021/acs.jpcc.8b03114
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    12
    Combination of Charge Delocalization and Disorder Enables Efficient Charge Separation at Photoexcited Organic Bilayers
    Jankovic, Veljko; Vukmirovic, Nenad
    Journal of Physical Chemistry C (2018), 122 (19), 10343-10359CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
    The authors study incoherent charge sepn. in a lattice model of an all-org. bilayer. Charge delocalization is taken into account by working in the basis of electron-hole pair eigenstates, and the sepn. is described as incoherent hops between these states. Relatively weak energetic disorder, in combination with good charge delocalization, can account for efficient and weakly field- and temp.-dependent sepn. of the strongly bound charge transfer (CT) state. The sepn. efficiency is detd. by the competition between the recombination from the initial CT state and the escape toward intermediate CT states, from which free-charge states can be reached with certainty. The sepn. of donor excitons also exhibits quite high yields, less bound excitons sepg. more efficiently. The results support the notion that efficient charge sepn. can be achieved even out of strongly bound pair states without invoking coherent effects.
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  13. 13
    Pace, N. A.; Reid, O. G.; Rumbles, G. Delocalization Drives Free Charge Generation in Conjugated Polymer Films. ACS Energy Lett. 2018, 3, 735– 741,  DOI: 10.1021/acsenergylett.8b00108
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    13
    Delocalization drives free charge generation in conjugated polymer films
    Pace, Natalie A.; Reid, Obadiah G.; Rumbles, Garry
    ACS Energy Letters (2018), 3 (3), 735-741CODEN: AELCCP; ISSN:2380-8195. (American Chemical Society)
    The authors demonstrate that the product of photoinduced electron transfer between a conjugated polymer host and a dil. mol. sensitizer is controlled by the structural state of the polymer. Ordered semicryst. solids exhibit free charge generation, while disordered polymers in the melt phase do not. The authors use photoluminescence (PL) and time-resolved microwave cond. (TRMC) measurements to sweep through polymer melt transitions in situ. Free charge generation measured by TRMC turns off upon melting, whereas PL quenching of the mol. sensitizers remains const., implying unchanged electron transfer efficiency. The key difference is the intermol. order of the polymer host in the solid state compared to the melt. The authors propose that this order-disorder transition modulates the localization length of the initial charge-transfer state, which controls the probability of free charge formation.
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  14. 14
    Clarke, T. M.; Durrant, J. R. Charge Photogeneration in Organic Solar Cells. Chem. Rev. 2010, 110, 6736– 6767,  DOI: 10.1021/cr900271s
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    14
    Charge Photogeneration in Organic Solar Cells
    Clarke, Tracey M.; Durrant, James R.
    Chemical Reviews (Washington, DC, United States) (2010), 110 (11), 6736-6767CODEN: CHREAY; ISSN:0009-2665. (American Chemical Society)
    A review. Org. solar cells utilizing π-conjugated polymers have attracted interest in the academic and com. communities. These polymers are promising in terms of their electronic properties, low cost, versatility of functionalization, thin film flexibility, and ease of processing. These factors indicate that org. solar cells, although currently having relatively low power conversion efficiencies of 5 to 7%, compared to inorg. solar cells, they can compete effectively with alternative solar cell technologies. The efficiency of org. solar cells need further improvement.
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  15. 15
    Guo, J. M.; Ohkita, H.; Benten, H.; Ito, S. Charge Generation and Recombination Dynamics in Poly(3-hexylthiophene)/Fullerene Blend Films with Different Regioregularities and Morphologies. J. Am. Chem. Soc. 2010, 132, 6154– 6164,  DOI: 10.1021/ja100302p
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    15
    Charge Generation and Recombination Dynamics in Poly(3-hexylthiophene)/Fullerene Blend Films with Different Regioregularities and Morphologies
    Guo, Jiamo; Ohkita, Hideo; Benten, Hiroaki; Ito, Shinzaburo
    Journal of the American Chemical Society (2010), 132 (17), 6154-6164CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    The charge generation and recombination dynamics in blend films of a poly(3-hexylthiophene) (P3HT) and a methanofullerene deriv. (PCBM) were comprehensively studied by transient absorption spectroscopy in the wavelength region from 450 to 1650 nm under various excitation intensities and different excitation wavelengths. In homogeneously mixed blend films of regiorandom P3HT (RRa-P3HT) and PCBM, virtually all the excitons can reach the interface of RRa-P3HT/PCBM and then form bound radical pairs. However, 2/3 of them geminately recombine to the ground state, and only 1/3 of them can be dissocd. into free polarons that survive up to milliseconds. In phase-sepd. blend films of regioregular P3HT (RR-P3HT) and PCBM, almost all the excitons can reach the interface of RR-P3HT/PCBM, where most of them can be dissocd. into free polarons efficiently and the rest of them form bound radical pairs. There are two pathways for the polaron generation: the prompt formation from hot excitons generated near the interface on a time scale of <100 fs and the delayed formation via the exciton migration to the interface on a time scale of ∼10 ps. The thermal annealing improves the charge dissocn. efficiency of bound radical pairs. From such spectroscopic data, fundamental photovoltaic conversion processes are quant. analyzed. Consequently, there is not much difference in the charge generation yield between RRa-P3HT/PCBM(50%) and RR-P3HT/PCBM(50%) blend films. Rather, the charge dissocn. and collection have a crit. impact on the overall device performance of P3HT/PCBM solar cells, where the phase-sepd. blend structures have a high tendency to form free carriers and transport these free carriers to the electrode.
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  16. 16
    Kobori, Y.; Miura, T. Overcoming Coulombic Traps: Geometry and Electronic Characterizations of Light-Induced Separated Spins at the Bulk Heterojunction Interface. J. Phys. Chem. Lett. 2015, 6, 113– 123,  DOI: 10.1021/jz5023202
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    16
    Overcoming Coulombic Traps: Geometry and Electronic Characterizations of Light-Induced Separated Spins at the Bulk Heterojunction Interface
    Kobori, Yasuhiro; Miura, Taku
    Journal of Physical Chemistry Letters (2015), 6 (1), 113-123CODEN: JPCLCD; ISSN:1948-7185. (American Chemical Society)
    A review. Recent progress is overviewed on exptl. elucidations of fundamental mol. functions of the light-energy conversions by the photoactive layers of the org. photovoltalic (OPV) cells by the time-resolved ESR spectroscopy. Positions and orientations of the unpaired electrons and electronic coupling matrix elements are clarified in photoinduced, primary charge-sepd. (CS) states. Connections between the mol. geometries and the electronic couplings were characterized for the initial CS states to elucidate how the structure, orbital delocalization, and mol. libration play roles on exothermic carrier dissocn. via a vibrationally relaxed charge-transfer complex with prevention of the energy-wasting charge recombination. Superior functions to biol. mols. are presented for the efficient photocurrent generations induced by orbital delocalization and by shallow trap depths at polymer-stacking domains. The above structural and electronic characteristics of the primary electron-hole pairs are essential to evaluations, designs, and developments of the efficient solar cells using org. mols.
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  17. 17
    Miura, T.; Tao, R.; Shibata, S.; Umeyama, T.; Tachikawa, T.; Imahori, H.; Kobori, Y. Geometries, Electronic Couplings, and Hole Dissociation Dynamics of Photoinduced Electron–Hole Pairs in Polyhexylthiophene–Fullerene Dyads Rigidly Linked by Oligophenylenes. J. Am. Chem. Soc. 2016, 138, 5879– 5885,  DOI: 10.1021/jacs.5b13414
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    17
    Geometries, Electronic Couplings, and Hole Dissociation Dynamics of Photoinduced Electron-Hole Pairs in Polyhexylthiophene-Fullerene Dyads Rigidly Linked by Oligophenylenes
    Miura, Taku; Tao, Ran; Shibata, Sho; Umeyama, Tomokazu; Tachikawa, Takashi; Imahori, Hiroshi; Kobori, Yasuhiro
    Journal of the American Chemical Society (2016), 138 (18), 5879-5885CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    To shed a light on fundamental mol. functions of photoinduced charge conductions by org. photovoltaic materials, it is important to directly observe mol. geometries of the intermediate charges just after the photoinduced electron-transfer reactions. However, highly inhomogeneous mol. environments at the bulk heterojunction interfaces in the photoactive layers have prevented us from understanding the mechanism of the charge conductions. We have herein investigated orbital geometries, electronic couplings and hole-dissocn. dynamics of photoinduced charge-sepd. (CS) states in a series of poly(3-hexylthiophene)-fullerene linked dyads bridged by rigid oligo-p-phenylene spacers by using time resolved EPR spectroscopy. It has been revealed that one-dimensional intramol. hole-dissocns. exothermically take place from localized holes in initial CS states, following bridge-mediated, photoinduced charge-sepns. via triplet exciton diffusions in the conjugated polymer-backbones. This mol. wire property of the photoinduced charges in soln. at room temp. demonstrates the potential utility of the covalently bridged polymer mols. applied for the mol. devices.
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  18. 18
    Kobori, Y.; Noji, R.; Tsuganezawa, S. Initial Molecular Photocurrent: Nanostructure and Motion of Weakly Bound Charge-Separated State in Organic Photovoltaic Interface. J. Phys. Chem. C 2013, 117, 1589– 1599,  DOI: 10.1021/jp309421s
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    18
    Initial Molecular Photocurrent: Nanostructure and Motion of Weakly Bound Charge-Separated State in Organic Photovoltaic Interface
    Kobori, Yasuhiro; Noji, Ryohei; Tsuganezawa, Shuhei
    Journal of Physical Chemistry C (2013), 117 (4), 1589-1599CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
    To elucidate mechanism of the efficient photocarrier generation by the photoactive bulk heterojunction layers of the org. photovoltaic (OPV) devices, the time-resolved ESR method is employed on solid blends composed of [6,6]-C61-butyric acid Me ester (PCBM) and of poly(3-hexylthiophene-2,5-diyl) (P3HT) with different regioregularities. The photoinduced charge-sepd. (CS) states were detected at the boundary regions between the P3HT and PCBM domains at T = 77 K. Mol. geometries, electronic couplings, and mol. motions of the long-range CS states are characterized. From the CS structure, it is indicated that the pentagonal or hexagonal arom. rings of the buckyball in PCBM directly face the arom. plane of the π-stacked P3HT surfaces. The distant CS states are produced via fast hole-delocalization process from the contact charge-transfer (CT) states. Such hole dynamics is explained by a coupling of the hole to librations of chains in the conjugated polymer. Both the enthalpy stabilization and the enhancement of the entropy occur through the orbital delocalization by the electron-phonon coupling, overcoming the initial CT binding to generate the mol. photocurrent.
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  19. 19
    Krinichnyi, V. I.; Yudanova, E. I.; Spitsina, N. G. Light-Induced Electron Paramagnetic Resonance Study of Poly(3-alkylthiophene)/Fullerene Composites. J. Phys. Chem. C 2010, 114, 16756– 16766,  DOI: 10.1021/jp105873r
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    19
    Light-Induced Electron Paramagnetic Resonance Study of Poly(3-alkylthiophene)/Fullerene Composites
    Krinichnyi, Victor I.; Yudanova, Eugenia I.; Spitsina, Natalia G.
    Journal of Physical Chemistry C (2010), 114 (39), 16756-16766CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
    Radical pairs, polarons, and fullerene anion radicals photoinduced by photons with an energy of 1.98-2.73 eV in bulk heterojunctions formed by poly(3-hexylthiophene) (P3HT) and poly(3-dodecylthiophene) (P3DDT) with [6,6]-Ph C61-butyric acid Me ester (PCBM) and 2-(azahomo[60]fullereno)-5-nitropyrimidine (AFNP) fullerene derivs. were studied by a direct light-induced ESR (LEPR) method in a wide temp. range. LEPR spectra of the polymer/fullerene composites consist of contributions of mobile and trapped charge carriers. Concn. and magnetic resonance parameters of these charge carriers were found to depend on the energy of initiated photons. Spin-lattice and spin-spin relaxation times of polarons and fullerene anion radicals were detd. by the steady-state satn. method. The interaction of most charge carriers with the lattice is characterized by monotonic temp. dependence, whereas the spin-lattice relaxation time of fullerene anion radicals trapped in the P3DDT matrix demonstrates sharper temp. dependence. Spin-spin interaction is shown to be nearly temp. independent and to be governed by structural properties of polymer/fullerene composites. Longitudinal diffusion of polarons and pseudorotation of fullerene derivs. was shown to follow the activation Elliot hopping model. The replacement of the P3HT matrix by P3DDT accelerates polaron dynamics and increases its anisotropy. The energetic barrier required for polaron interchain hopping mainly prevails upon that of its intrachain diffusion in all composites except P3DDT/AFNP one. Spin dynamics becomes easier when the PCBM fullerene deriv. is replaced by the AFNP one.
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  20. 20
    Krinichnyi, V. I.; Yudanova, E. I. Light-Induced EPR Study of Charge Transfer in P3HT/PC71BM Bulk Heterojunctions. J. Phys. Chem. C 2012, 116, 9189– 9195,  DOI: 10.1021/jp2120516
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    20
    Light-Induced EPR Study of Charge Transfer in P3HT/PC71BM Bulk Heterojunctions
    Krinichnyi, Victor I.; Yudanova, Eugenia I.
    Journal of Physical Chemistry C (2012), 116 (16), 9189-9195CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
    Radical pairs, polarons, and fullerene anion radicals photoinduced by photons with energy of 1.98-2.73 eV in bulk heterojunctions formed by regioregular poly(3-hexylthiophene) (P3HT) with (6,6)-phenyl-C71-butyric acid ester (PC71BM) methanofullerene have been studied by the direct light-induced EPR (LEPR) method at a wide temp. region. LEPR spectra of the P3HT/PC71BM composite were deconvoluted, and the main magnetic resonance parameters of these charge carriers have been detd. A part of photoinduced polarons is pinned by large-depth traps in the polymer matrix. It was shown that magnetic resonance, relaxation, and dynamics parameters of photoinduced charge carriers depend extremely on the energy of initiated photons. Relaxation and dynamics parameters of both the charge carriers were detd. sep. by the steady-state satn. method. Longitudinal diffusion of polarons was analyzed in terms of spin interaction with the lattice phonons of cryst. domains embedded into an amorphous polymer matrix. The interchain spin hopping is detd. by the no. and depth of the traps photoinitiated in the polymer matrix. Pseudorotation of methanofullerene mols. in a polymer matrix was shown to follow the activation Pike model. The replacement in the composite of PC61BM acceptors by PC71BM ones accelerates electron relaxation, hinders the formation of spin traps, and favors more ordered (cryst.) structure of bulk heterojunction that facilitates charge transfer in the P3HT/PC71BM composite.
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  21. 21
    Miura, T.; Aikawa, M.; Kobori, Y. Time-Resolved EPR Study of Electron-Hole Dissociations Influenced by Alkyl Side Chains at the Photovoltaic Polyalkylthiophene:PCBM Interface. J. Phys. Chem. Lett. 2014, 5, 30– 35,  DOI: 10.1021/jz402300m
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    21
    Time-Resolved EPR Study of Electron-Hole Dissociations Influenced by Alkyl Side Chains at the Photovoltaic Polyalkylthiophene:PCBM Interface
    Miura, Taku; Aikawa, Motoko; Kobori, Yasuhiro
    Journal of Physical Chemistry Letters (2014), 5 (1), 30-35CODEN: JPCLCD; ISSN:1948-7185. (American Chemical Society)
    Nanosecond time-resolved ESR (TREPR) spectroscopy has been utilized at T = 77 K to characterize alkyl side-chain effects on geometries and on the electronic couplings (VCR) of transient charge-sepd. (CS) states in the photoactive layers fabricated by the spin-coating of mixed solns. of regioregular polyalkylthiophenes (RR-P3AT) and [6,6]-C61-butyric acid Me ester (PCBM). By increasing the alkyl side-chain no. from 6 to 12 in P3AT, a highly distant and long-lived CS state has been obtained. This result is explained by a coupling of the hole dissocn. to the polymer librations by the side-chains. From an exponential decay of VCR with respect to the CS distance, the attenuation factor (βe) has been detd. to be βe = 0.2 Å-1. Such a long-range tunneling feature is explained by the generations of the shallowly trapped, delocalized electron-hole pairs by the dissocn. of the hole toward π-stacking directions at the org. photovoltaic interface.
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  22. 22
    Bakulin, A. A.; Rao, A.; Pavelyev, V. G.; van Loosdrecht, P. H. M.; Pshenichnikov, M. S.; Niedzialek, D.; Cornil, J.; Beljonne, D.; Friend, R. H. The Role of Driving Energy and Delocalized States for Charge Separation in Organic Semiconductors. Science 2012, 335, 1340– 1344,  DOI: 10.1126/science.1217745
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    22
    The Role of Driving Energy and Delocalized States for Charge Separation in Organic Semiconductors
    Bakulin, Artem A.; Rao, Akshay; Pavelyev, Vlad G.; van Loosdrecht, Paul H. M.; Pshenichnikov, Maxim S.; Niedzialek, Dorota; Cornil, Jerome; Beljonne, David; Friend, Richard H.
    Science (Washington, DC, United States) (2012), 335 (6074), 1340-1344CODEN: SCIEAS; ISSN:0036-8075. (American Association for the Advancement of Science)
    The electron-hole pair created via photon absorption in org. photoconversion systems must overcome the Coulomb attraction to achieve long-range charge sepn. This process is facilitated through the formation of excited, delocalized band states. In the authors' expts. on org. photovoltaic cells, these states were accessed for a short time (<1 ps) via IR optical excitation of electron-hole pairs bound at the heterojunction. Atomistic modeling showed that the IR photons promote bound charge pairs to delocalized band states, similar to those formed just after singlet exciton dissocn., which indicates that such states act as the gateway for charge sepn. The authors' results suggest that charge sepn. in efficient org. photoconversion systems occurs through hot-state charge delocalization rather than energy-gradient-driven intermol. hopping.
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    Tamura, H.; Burghardt, I. Ultrafast Charge Separation in Organic Photovoltaics Enhanced by Charge Delocalization and Vibronically Hot Exciton Dissociation. J. Am. Chem. Soc. 2013, 135, 16364– 16367,  DOI: 10.1021/ja4093874
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    23
    Ultrafast Charge Separation in Organic Photovoltaics Enhanced by Charge Delocalization and Vibronically Hot Exciton Dissociation
    Tamura, Hiroyuki; Burghardt, Irene
    Journal of the American Chemical Society (2013), 135 (44), 16364-16367CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    In org. photovoltaics, the mechanism by which free electrons and holes are generated, overcoming the Coulomb attraction, is a currently much debated topic. To elucidate this mechanism at a mol. level, a combined electronic structure and quantum dynamic anal. is carried out that captures the elementary events from the exciton dissocn. to the free carrier generation at polymer/fullerene donor/acceptor heterojunctions. The calcns. show that exptl. obsd. efficient charge sepns. can be explained by a combination of two effects: first, the delocalization of charges which substantially reduces the Coulomb barrier, and second, the vibronically hot nature of the charge-transfer state which promotes charge dissocn. beyond the barrier. These effects facilitate an ultrafast charge sepn. even at low-band-offset heterojunctions.
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    Barker, A. J.; Chen, K.; Hodgkiss, J. M. Distance Distributions of Photogenerated Charge Pairs in Organic Photovoltaic Cells. J. Am. Chem. Soc. 2014, 136, 12018– 12026,  DOI: 10.1021/ja505380j
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    24
    Distance Distributions of Photogenerated Charge Pairs in Organic Photovoltaic Cells
    Barker, Alex J.; Chen, Kai; Hodgkiss, Justin M.
    Journal of the American Chemical Society (2014), 136 (34), 12018-12026CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    Strong Coulomb interactions in org. photovoltaic cells dictate that charges must sep. over relatively long distances in order to circumvent geminate recombination and produce photocurrent. In this article, we measure the distance distributions of thermalized charge pairs by accessing a regime at low temp. where charge pairs are frozen out following the primary charge sepn. step and recombine monomolecularly via tunneling. The exponential attenuation of tunneling rate with distance provides a sensitive probe of the distance distribution of primary charge pairs, reminiscent of electron transfer studies in proteins. By fitting recombination dynamics to distributions of recombination rates, we identified populations of charge-transfer states and well-sepd. charge pairs. For the wide range of materials we studied, the yield of sepd. charges in the tunneling regime is strongly correlated with the yield of free charges measured via their intensity-dependent bimol. recombination dynamics at room temp. We therefore conclude that populations of free charges are established via long-range charge sepn. within the thermalization time scale, thus invoking early branching between free and bound charges across an energetic barrier. Subject to assumed values of the electron tunneling attenuation const., we est. crit. charge sepn. distances of ∼3-4 nm in all materials. In some blends, large fullerene crystals can enhance charge sepn. yields; however, the important role of the polymers is also highlighted in blends that achieved significant charge sepn. with minimal fullerene concn. We expect that our approach of isolating the intrinsic properties of primary charge pairs will be of considerable value in guiding new material development and testing the validity of proposed mechanisms for long-range charge sepn.
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    Hasegawa, M.; Nagashima, H.; Minobe, R.; Tachikawa, T.; Mino, H.; Kobori, Y. Regulated Electron Tunneling of Photoinduced Primary Charge-Separated State in the Photosystem II Reaction Center. J. Phys. Chem. Lett. 2017, 8, 1179– 1184,  DOI: 10.1021/acs.jpclett.7b00044
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    Regulated electron tunneling of photoinduced primary charge-separated state in the photosystem II reaction center
    Hasegawa, Masashi; Nagashima, Hiroki; Minobe, Reina; Tachikawa, Takashi; Mino, Hiroyuki; Kobori, Yasuhiro
    Journal of Physical Chemistry Letters (2017), 8 (6), 1179-1184CODEN: JPCLCD; ISSN:1948-7185. (American Chemical Society)
    In initial events of the photosynthesis by higher plants, the photosystem II (PSII) generates photoinduced primary charge-sepd. (CS) state composed of reduced pheophytin (PheoD1-•) and oxidized special pair (P+•) in chlorophyll a (Chla) PD1/PD2 in the D1/D2 heterodimer, ultimately leading to the water oxidn. at the oxygen-evolving Mn4CaO5 cluster by P+•. To understand the mechanism of the efficient generation of initially localized CS state (PD1+• PheoD1-•), we have characterized cofactor geometries and electronic coupling of the photoinduced primary CS state in quinone prereduced membrane of PSII from spinach using the time-resolved ESR method. It has been revealed that the electronic coupling between the charges is significantly weak in the CS state sepd. by 1.5 nm, showing the importance of regulated cofactor-cofactor electronic interaction between a vinyl substituent in PheoD1 and an accessory chlorophyll to inhibit the energy-wasting charge recombination after the primary electron-transfer processes.
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    Closs, G. L.; Forbes, M. D. E.; Norris, J. R. Spin-Polarized Electron-Paramagnetic Resonance-Spectra of Radical Pairs in Micelles - Observation of Electron-Spin Spin Interactions. J. Phys. Chem. 1987, 91, 3592– 3599,  DOI: 10.1021/j100297a026
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    Spin-polarized electron paramagnetic resonance spectra of radical pairs in micelles: observation of electron spin-spin interactions
    Closs, Gerhard L.; Forbes, Malcolm D. E.; Norris, James R., Jr.
    Journal of Physical Chemistry (1987), 91 (13), 3592-9CODEN: JPCHAX; ISSN:0022-3654.
    A model for polarized EPR spectra generated in radical pair reactions in micelles is proposed. The model is based on electron spin-spin interactions which remain observable because of limited diffusion in micelles. The exptl. observable is a doubling of hyperfine transitions, split by the magnitude of the interaction. The polarization is generated by the nonadiabatic generation of the radical pair with a triplet or singlet precursor. The time evolution of the wave function leads to a non-Boltzmann distribution of the populations among the 4 energy levels. The theory is tested by comparison with expts., previously reported and repeated in this lab., obtained by laser flash photolysis of benzophenone in sodium dodecyl sulfate micelles. Simulations of the shape of the spectra and their time dependence agree with expt. The model is further supported by expts. in micelles modified by different salt concns. as well as different chain lengths of the micelle-forming mols.
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    Hore, P. J.; Hunter, D. A.; McKie, C. D.; Hoff, A. J. Electron Paramagnetic Resonance of Spin-Correlated Radical Pairs in Photosynthetic Reactions. Chem. Phys. Lett. 1987, 137, 495– 500,  DOI: 10.1016/0009-2614(87)80617-6
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    Electron paramagnetic resonance of spin-correlated radical pairs in photosynthetic reactions
    Hore, P. J.; Hunter, D. A.; McKie, C. D.; Hoff, A. J.
    Chemical Physics Letters (1987), 137 (6), 495-500CODEN: CHPLBC; ISSN:0009-2614.
    Previous attempts to interpret the time-resolved EPR spectra of photosynthetic bacteria have been based on the premise that electron spin polarization arises in the primary radical pair (P+I-) formed by photoinduced charge sepn. The obsd. spectrum is assumed to be the sum of the EPR spectra of P+ and X-, the radical produced from I- by electron transfer. Here it is argued that P+I- may be too short-lived to give rise to significant polarization and that the exptl. spectrum is consistent with the rapid formation of a spin-correlated secondary radical pair.
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    Avdievich, N. I.; Forbes, M. D. E. Dynamic Effects in Spin-Correlated Radical Pair Theory: J Modulation and a New Look at the Phenomenon of Alternating Line Widths in the EPR Spectra of Flexible Biradicals. J. Phys. Chem. 1995, 99, 9660– 9667,  DOI: 10.1021/j100024a004
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    Dynamic Effects in Spin-Correlated Radical Pair Theory: J Modulation and a New Look at the Phenomenon of Alternating Line Widths in the EPR Spectra of Flexible Biradicals
    Avdievich, Nikolai I.; Forbes, Malcolm D. E.
    Journal of Physical Chemistry (1995), 99 (24), 9660-7CODEN: JPCHAX; ISSN:0022-3654. (American Chemical Society)
    Expressions for contributions to T1 and T2 from modulation of the exchange interaction (J) in spin-correlated radical pairs (SCRPs) are derived and incorporated into SCRP theory for simulation of direct detection EPR spectra of alkane chain biradicals. The T2 term is consistent with that previously derived by Luckhurst to explain the temp.-dependent alternating line widths obsd. in EPR spectra of stable nitroxide biradicals. Alternating line-width patterns are obsd. for sym. bis(alkyl) biradicals at high temps., where the rapid modulation of J is caused by conformational jumping. However, in unsym. systems such as acyl-alkyl biradicals, J modulation manifests itself at low temps. as an increased broadening of the spectral lines with increasing distance of the transition from the center of the spectrum, with no alternation. Several exptl. examples are presented, simulated, and discussed. Contrary to a suggestion by Maeda et al. (1991), J modulation is shown to have only a minimal effect on T1 in SCRP theory for the case of biradicals undergoing rapid conformational interconversion. A connection is proposed between the magnitude of the relaxation matrix elements and the biradical conformational distribution and chain dynamics.
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    Weber, S.; Biskup, T.; Okafuji, A.; Marino, A. R.; Berthold, T.; Link, G.; Hitomi, K.; Getzoff, E. D.; Schleicher, E.; Norris, J. R. Origin of Light-Induced Spin-Correlated Radical Pairs in Cryptochrome. J. Phys. Chem. B 2010, 114, 14745– 14754,  DOI: 10.1021/jp103401u
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    Origin of Light-Induced Spin-Correlated Radical Pairs in Cryptochrome
    Weber, Stefan; Biskup, Till; Okafuji, Asako; Marino, Anthony R.; Berthold, Thomas; Link, Gerhard; Hitomi, Kenichi; Getzoff, Elizabeth D.; Schleicher, Erik; Norris, James R., Jr.
    Journal of Physical Chemistry B (2010), 114 (45), 14745-14754CODEN: JPCBFK; ISSN:1520-6106. (American Chemical Society)
    Blue-light excitation of cryptochromes and homologues uniformly triggers electron transfer (ET) from the protein surface to the FAD cofactor. A cascade of three conserved tryptophan residues has been considered to be critically involved in this photoreaction. If the FAD is initially in its fully oxidized (diamagnetic) redox state, light-induced ET via the tryptophan triad generates a series of short-lived spin-correlated radical pairs comprising an FAD radical and a tryptophan radical. Coupled doublet-pair species of this type have been proposed as the basis, for example, of a biol. magnetic compass in migratory birds, and were found crit. for some cryptochrome functions in vivo. In this contribution, a cryptochrome-like protein (CRYD) derived from Xenopus laevis has been examd. as a representative system. The terminal radical-pair state FAD····W324· of X. laevis CRYD has been characterized in detail by time-resolved electron-paramagnetic resonance (TREPR) at X-band microwave frequency (9.68 GHz) and magnetic fields around 345 mT, and at Q-band (34.08 GHz) at around 1215 mT. Different precursor states, singlet vs. triplet, of radical-pair formation have been considered in spectral simulations of the exptl. electron-spin polarized TREPR signals. Conclusively, we present evidence for a singlet-state precursor of FAD····W324· radical-pair generation because at both magnetic fields, where radical pairs were studied by TREPR, net-zero electron-spin polarization has been detected. Neither a spin-polarized triplet precursor nor a triplet at thermal equil. can explain such an electron-spin polarization. It turns out that a two-microwave-frequency TREPR approach is essential to draw conclusions on the nature of the precursor electronic states in light-induced spin-correlated radical pair formations.
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    Niklas, J.; Poluektov, O. G. Charge Transfer Processes in OPV Materials as Revealed by EPR Spectroscopy. Adv. Energy Mater. 2017, 7, 1602226  DOI: 10.1002/aenm.201602226
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    Uvarov, M. N.; Kulik, L. Electron Spin Echo of Photoinduced Spin-Correlated Polaron Pairs in P3HT:PCBM Composite. Appl. Magn. Reson. 2013, 97– 106,  DOI: 10.1007/s00723-012-0401-2
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    Electron Spin Echo of Photoinduced Spin-Correlated Polaron Pairs in P3HT:PCBM Composite
    Uvarov, Mikhail N.; Kulik, Leonid V.
    Applied Magnetic Resonance (2013), 44 (1-2), 97-106CODEN: APMREI; ISSN:0937-9347. (SpringerWienNewYork)
    A variation of the electron spin echo (ESE) signal caused by laser pulse in a blend of [6,6]-Ph C61 butyric acid Me ester and poly(3-hexylthiophene) (P3HT:PCBM) was detected. This variation was attributed to light-generated paramagnetic species in P3HT:PCBM blend, with non-equil. spin polarization. The echo-detected ESR (EPR) spectrum of these species closely resembles the time-resolved EPR spectrum of spin-correlated polaron pair PCBM-/P3HT+ (Behrends et al. in Phys. Rev. B 85:125206, 2012) and was assigned to this pair. The characteristic times for polarization and coherence decay (9 ± 1 and 1.0 ± 0.2 μs, resp.) were measured for the PCBM-/P3HT+ pair at 77 K. These times are long enough, which shows the possibility of the application of the ESE technique for studying spin evolution of light-generated charge transfer intermediates in composites of fullerenes and conductive polymers.
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    Behrends, J.; Sperlich, A.; Schnegg, A.; Biskup, T.; Teutloff, C.; Lips, K.; Dyakonov, V.; Bittl, R. Direct Detection of Photoinduced Charge Transfer Complexes in Polymer Fullerene Blends. Phys. Rev. B 2012, 85, 125206  DOI: 10.1103/PhysRevB.85.125206
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    Direct detection of photoinduced charge transfer complexes in polymer fullerene blends
    Behrends, Jan; Sperlich, Andreas; Schnegg, Alexander; Biskup, Till; Teutloff, Christian; Lips, Klaus; Dyakonov, Vladimir; Bittl, Robert
    Physical Review B: Condensed Matter and Materials Physics (2012), 85 (12), 125206/1-125206/6CODEN: PRBMDO; ISSN:1098-0121. (American Physical Society)
    We report transient ESR (trEPR) measurements with submicrosecond time resoln. performed on a polymer:fullerene blend consisting of poly(3-hexylthiophene) (P3HT) and [6,6]-Ph C61-butyric acid Me ester (PCBM) at low temps. The trEPR spectrum immediately following photoexcitation reveals signatures of spin-correlated polaron pairs. The pair partners (pos. polarons in P3HT and neg. polarons in PCBM) can be identified by their characteristic g values. The fact that the polaron pair states exhibit strong non-Boltzmann population unambiguously shows that the constituents of each pair are geminate, i.e., originate from one exciton. We demonstrate that coupled polaron pairs are present even several microseconds after charge transfer and suggest that they embody the intermediate charge transfer complexes that form at the donor/acceptor interface and mediate the conversion from excitons into free charge carriers.
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    Kraffert, F.; Behrends, J. Spin-Correlated Doublet Pairs as Intermediate States in Charge Separation Processes. Mol. Phys. 2017, 115, 2373– 2386,  DOI: 10.1080/00268976.2016.1278479
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    Spin-correlated doublet pairs as intermediate states in charge separation processes
    Kraffert, Felix; Behrends, Jan
    Molecular Physics (2017), 115 (19), 2373-2386CODEN: MOPHAM; ISSN:0026-8976. (Taylor & Francis Ltd.)
    Spin-correlated charge-carrier pairs play a crucial role as intermediate states in charge sepn. both in natural photosynthesis as well as in solar cells. Using transient ESR (trEPR) spectroscopy in combination with spectral simulations, we study spin-correlated polaron pairs in polymer:fullerene blends as org. solar cells materials. The semi-anal. simulations presented here are based on the well-established theor. description of spin-correlated radical pairs in biol. systems, however, explicitly considering the disordered nature of polymer:fullerene blends. The large degree of disorder leads to the fact that many different relative orientations between both polarons forming the spin-correlated pairs have to be taken into account. This has important implications for the spectra, which differ significantly from those of spin-correlated radical pairs with a fixed relative orientation. We systematically study the influence of exchange and dipolar couplings on the trEPR spectra and compare the simulation results to measured X- and Q-band trEPR spectra. Our results demonstrate that assuming dipolar couplings alone does not allow us to reproduce the exptl. spectra. Due to the rather delocalised nature of polarons in conjugated org. semiconductors, a significant isotropic exchange coupling needs to be included to achieve good agreement between expts. and simulations.
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    Kraffert, F.; Steyrleuthner, R.; Albrecht, S.; Neher, D.; Scharber, M. C.; Bittl, R.; Behrends, J. Charge Separation in PCPDTBT:PCBM Blends from an EPR Perspective. J. Phys. Chem. C 2014, 118, 28482– 28493,  DOI: 10.1021/jp509650v
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    Charge Separation in PCPDTBT:PCBM Blends from an EPR Perspective
    Kraffert, Felix; Steyrleuthner, Robert; Albrecht, Steve; Neher, Dieter; Scharber, Markus C.; Bittl, Robert; Behrends, Jan
    Journal of Physical Chemistry C (2014), 118 (49), 28482-28493CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
    Using time-resolved ESR (EPR) spectroscopy in conjunction with optical excitation we study charge sepn. in conjugated polymers blended with [6,6]-Ph C61-butyric acid Me ester (PCBM). A direct comparison between samples comprising poly[2,6-(4,4-bis(2-ethylhexyl)-4H-cyclopenta[2,1-b;3,4-b']-dithiophene)-alt-4,7-(2,1,3-benzothiadiazole)] (C-PCPDTBT) and their analogs contg. poly[(4,4'-bis(2-ethylhexyl)dithieno[3,2-b:2',3'-d]silole)-2,6-diyl-alt-(4,7-bis(2-thienyl)-2,1,3-benzothiadiazole)-5,5'-diyl] (Si-PCPDTBT) reveals a remarkable influence of the bridging atom (carbon vs silicon) in the polymer on the EPR spectra. While the EPR signatures of photogenerated pos. polarons in C- and Si-bridged PCPDTBT are virtually identical, significant differences are obsd. with respect to the spin-relaxation behavior. The spin-lattice relaxation time of pos. polarons in C-PCPDTBT at low temp. (T = 80 K) is found to be more than two orders or magnitude longer than in the Si-bridged polymer deriv. This surprisingly slow relaxation can be rationalized by polarons trapped in defect states that seem to be absent (or are present in a substantially smaller concn.) in blends comprising Si-PCPDTBT. Transient EPR signals attributed to charge transfer (CT) states and sepd. polarons are smaller in the blends with C-PCPDTBT as compared to those with the silicon-bridged polymer. We propose that triplet formation occurs via the CT state, thus diminishing the probability that the CT state forms free charge carriers in blends of C-PCPDTBT with PCBM. This hypothesis is confirmed by direct detection of triplet excitons in C-PCPDTBT:PCBM blends. The shape of the transient EPR spectra reveals that the triplet excitons are, in contrast to those formed in pristine polymer films, not generated by direct intersystem crossing but result from back electron transfer through CT state recombination. The strong triplet signal is not obsd. in blends contg. the Si-bridged polymer, indicating efficient singlet exciton splitting and subsequent charge carrier sepn. at the Si-PCPDTBT/PCBM interface.
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    Aguirre, A.; Gast, P.; Orlinskii, S.; Akimoto, I.; Groenen, E. J. J.; El Mkami, H.; Goovaerts, E.; Van Doorslaer, S. Multifrequency EPR analysis of the positive polaron in I-2-doped poly(3-hexylthiophene) and in poly[2-methoxy-5-(3,7-dimethyloctyloxy)]-1,4-phenylenevinylene. Phys. Chem. Chem. Phys. 2008, 10, 7129– 7138,  DOI: 10.1039/b811419f
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    Multifrequency EPR analysis of the positive polaron in I2-doped poly(3-hexylthiophene) and in poly[2-methoxy-5-(3,7-dimethyloctyloxy)]-1,4-phenylenevinylene
    Aguirre, Aranzazu; Gast, Peter; Orlinskii, Sergey; Akimoto, Ikuko; Groenen, Edgar J. J.; El Mkami, Hassane; Goovaerts, Etienne; Van Doorslaer, Sabine
    Physical Chemistry Chemical Physics (2008), 10 (47), 7129-7138CODEN: PPCPFQ; ISSN:1463-9076. (Royal Society of Chemistry)
    The W-band continuous-wave ESR (EPR) anal. of chem. induced polarons in drop-cast and spin-coated polyphenylenevinylene-type and polythiophene-type polymer films reveals rhombic g tensors in both cases. The dependence of the W-band EPR signals on the orientation of the spin-coated films with respect to the magnetic field indicates a high degree of backbone alignment with the substrate and allows a partial assignment of the g tensor orientation. The derived mol. orientations of the polymer chains in the spin-coated films show clear differences between the two types of polymers. The proton hyperfine interactions obtained from X-band HYSCORE (hyperfine sublevel correlation) and Q- and W-band pulsed ENDOR (electron-nuclear double resonance) expts. are interpreted in terms of earlier theor. studies on the extension of the polarons.
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    De Ceuster, J.; Goovaerts, E.; Bouwen, A.; Hummelen, J. C.; Dyakonov, V. High-Frequency (95 GHz) Electron Paramagnetic Resonance Study of the Photoinduced Charge Transfer in Conjugated Polymer-Fullerene Composites. Phys. Rev. B 2001, 64, 195206  DOI: 10.1103/PhysRevB.64.195206
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    High-frequency (95 GHz) electron paramagnetic resonance study of the photoinduced charge transfer in conjugated polymer-fullerene composites
    De Ceuster, J.; Goovaerts, E.; Bouwen, A.; Hummelen, J. C.; Dyakonov, V.
    Physical Review B: Condensed Matter and Materials Physics (2001), 64 (19), 195206/1-195206/6CODEN: PRBMDO; ISSN:0163-1829. (American Physical Society)
    Light-induced ESR (LEPR) measurements are reported in composites of poly(2-methoxy-5-(3-,7-dimethyloctyloxy)-1,4-phenylenevinylene) (MDMO-PPV) and [6,6]-phenyl-C61-butyric acid Me ester (PCBM), a sol. deriv. of C60. Under illumination of the sample, two paramagnetic species are formed due to photoinduced charge transfer between conjugated polymer and fullerene. One is the pos. polaron P+ on the polymer backbone and the other is the radical anion on the methanofullerene. Using high-frequency (95 GHz) LEPR it was possible to sep. these two contributions to the spectrum on the basis of their g factors, and moreover to resolve the g anisotropy for both radicals. The pos. polaron on the conjugated polymer chain possesses axial symmetry with g values g‖=2.0034(1) and g.perp.=2.0024(1). EPR on low doped polymer gave extra proof for the assignment to the pos. polaron. The neg. charged methanofullerene has a lower, rhombic symmetry with gx=2.0003(1), gy=2.0001(1), and gz=1.9982(1). Different spin-lattice relaxation of both species gives rise to a rapid passage effect for the pos. polaron spectrum.
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    Watanabe, S.-i.; Tanaka, H.; Kuroda, S.-i.; Toda, A.; Nagano, S.; Seki, T.; Kimoto, A.; Abe, J. Electron Spin Resonance Observation of Field-Induced Charge Carriers in Ultrathin-Film Transistors of Regioregular Poly(3-hexylthiophene) with Controlled In-Plane Chain Orientation. Appl. Phys. Lett. 2010, 96, 173302  DOI: 10.1063/1.3421538
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    Electron spin resonance observation of field-induced charge carriers in ultrathin-film transistors of regioregular poly(3-hexylthiophene) with controlled in-plane chain orientation
    Watanabe, Shun-ichiro; Tanaka, Hisaaki; Kuroda, Shin-ichi; Toda, Akio; Nagano, Shusaku; Seki, Takahiro; Kimoto, Atsushi; Abe, Jiro
    Applied Physics Letters (2010), 96 (17), 173302/1-173302/3CODEN: APPLAB; ISSN:0003-6951. (American Institute of Physics)
    Ultrathin-film polymeric transistors with controlled in-plane chain orientation were fabricated based on Langmuir-Blodgett technique by cospreading liq. crystal mol. with regioregular poly(3-hexylthiophene). The mobilities parallel to the chain direction reached 0.001-0.01 cm2/V s for 1-5 monolayer thick transistors. Mobility ratio was about 2 for the parallel and perpendicular directions. ESR signals of the field-induced polarons exhibited clear in-plane anisotropies due to unpaired π-electrons. The anisotropic ESR spectra together with the optical dichroism det. the detailed mol. orientations in the channel of such ultrathin transistors. (c) 2010 American Institute of Physics.
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    Kouki, F.; Spearman, P.; Valat, P.; Horowitz, G.; Garnier, F. Experimental Determination of Excitonic Levels in Alpha-Oligothiophenes. J. Chem. Phys. 2000, 113, 385– 391,  DOI: 10.1063/1.481804
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    Experimental determination of excitonic levels in α-oligothiophenes
    Kouki, Faycal; Spearman, Peter; Valat, Pierre; Horowitz, Gilles; Garnier, Francis
    Journal of Chemical Physics (2000), 113 (1), 385-391CODEN: JCPSA6; ISSN:0021-9606. (American Institute of Physics)
    The effects of intermol. interactions on the optical spectra of oligothiophenes are examd. Absorption spectra of isolated mols. are calcd. and exptl. recorded in a rigid host matrix whereby mols. are distributed randomly or organized unidirectionally. Absorption spectra of thin films in an ordered and disordered state are given. Ordered films have three principal spectral regions which are discussed in terms of classical exciton theory. Absorption spectra in transmission of single crystals of quinquethiophene and sexithiophene are analyzed. The lowest optically allowed transition in the crystal corresponds to the lowest Davydov component. It consists of a sharp peak that is obsd. for even-numbered oligothiophenes in b polarization, and absent for odd-numbered rings due to the perfect alignment of the transition dipole moment with the long mol. axis. The upper Davydov component is viewed in both thin film and single crystal spectra. In between the two principal Davydov components lies a broadband that is tentatively attributed to charge-transfer states or to a noninteracting mol. transition reminiscent of isolated matrix spectra. The Davydov splitting of the first optically allowed transition is detd. to be about 10 000 cm-1 and increases slightly with chain length. The Herzberg-Teller region in 6T reveals a dominant coupling mode of 340 cm-1. Vibronic structure in excitation and emission spectra shows peaks of 1460 cm-1 spacing.
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    Kobori, Y.; Ponomarenko, N.; Norris, J. R. Time-Resolved Electron Paramagnetic Resonance Study on Cofactor Geometries and Electronic Couplings after Primary Charge Separations in the Photosynthetic Reaction Center. J. Phys. Chem. C 2015, 119, 8078– 8088,  DOI: 10.1021/acs.jpcc.5b01294
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    Time-Resolved Electron Paramagnetic Resonance Study on Cofactor Geometries and Electronic Couplings after Primary Charge Separations in the Photosynthetic Reaction Center
    Kobori, Yasuhiro; Ponomarenko, Nina; Norris, James R.
    Journal of Physical Chemistry C (2015), 119 (15), 8078-8088CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
    To elucidate how cofactor geometries after photoinduced primary charge-sepns. influence electronic couplings (VCR) for primary charge-recombination (CR) processes in the photosynthetic reaction center, we have analyzed time-resolved ESR (TREPR) spectra both of the primary charge-sepd. (CS) state (P+•HA-•) and its charge-recombined triplet state (3P*) of the special pair in Rhodobacter sphaeroides R26. To det. the CS-state geometry, quantum mech. modeling has been performed on the spin polarization of the 3P* generated by the spin dynamics due to the anisotropies of the hyperfine and the spin-spin dipolar interactions in the primary CS state. From transverse magnetizations of the primary CS state, we have also detd. the VCR value leading to the singlet excited state (1P*) of the special pair. The above analyses have revealed that while the primary charge sepn. does not largely modulate the cofactor conformations, it leads to significant enhancement in the VCR of the singlet recombination processes with respect to the triplet CR. This enhanced coupling is demonstrated by the larger orbital overlap between 1P* and an electron-accepting orbital of chlorophyll (BA) than between 3P* and BA, caused by the charge-transfer electronic character of 1P* in which the electron is locally distributed at a side of the bacteriochlorophyll (PM) situated in close proximity to BA.
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    Till, U.; Klenina, I. B.; Proskuryakov, I. I.; Hoff, A. J.; Hore, P. J. Recombination Dynamics and EPR Spectra of the Primary Radical Pair in Bacterila Photosynthetic Reaction Centers with Blocked Electron Transfer to the Primary Acceptor. J. Phys. Chem. B 1997, 101, 10939– 10948,  DOI: 10.1021/jp970686q
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    Recombination dynamics and EPR spectra of the primary radical pair in bacterial photosynthetic reaction centers with blocked electron transfer to the primary acceptor
    Till, U.; Klenina, I. B.; Proskuryakov, I. I.; Hoff, A. J.; Hore, P. J.
    Journal of Physical Chemistry B (1997), 101 (50), 10939-10948CODEN: JPCBFK; ISSN:1089-5647. (American Chemical Society)
    Recent ESR expts. on the primary radical pair in bacterial reaction centers are interpreted in detail. The spin-correlated radical pair model is extended to include the time dependence of the EPR intensities, differential lifetime broadening of the basic four-line spectrum, and averaging over the inhomogeneous distributions of hyperfine interactions in the two radicals. Values of the singlet and triplet recombination rate consts. and the magnitude and sign of the exchange interaction of the radical pair are obtained from the shape and linewidth of the spin-polarized EPR spectra and the kinetics of formation of the triplet state of the primary electron donor. For Rhodobacter sphaeroides at 70 K, kS = (1.2 ± 0.3) × 107 s-1, kT = (5.7 - 8.8) × 108, -J = 8 - 6 G; and for Rhodopseudomonas viridis at 190 K, kS = (8 ± 2) × 107 s-1, kT = (6.5 - 10.0) × 108 s-1, -J = 17 - 16 G. The reliability of these values is discussed in the light of the energetic heterogeneity of the radical pair energies and of the sensitivity and selectivity of the EPR measurements toward the three parameters.
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    Nagamori, T.; Marumoto, K. Direct Observation of Hole Accumulation in Polymer Solar Cells During Device Operation using Light-Induced Electron Spin Resonance. Adv. Mater. 2013, 25, 2362– 2367,  DOI: 10.1002/adma.201204015
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    Direct Observation of Hole Accumulation in Polymer Solar Cells During Device Operation using Light-Induced Electron Spin Resonance
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    The hole accumulation during operation was studied of a heterojunction solar cell prepd. from P3HT, PEDOT-PSS, Al, and PCBM. The hole accumulation is obsd. in P3HT.
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    Kobori, Y.; Fuki, M. Protein–Ligand Structure and Electronic Coupling of Photoinduced Charge-Separated State: 9,10-Anthraquinone-1-sulfonate Bound to Human Serum Albumin. J. Am. Chem. Soc. 2011, 133, 16770– 16773,  DOI: 10.1021/ja206898j
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    Protein-Ligand Structure and Electronic Coupling of Photoinduced Charge-Separated State: 9,10-Anthraquinone-1-sulfonate Bound to Human Serum Albumin
    Kobori, Yasuhiro; Fuki, Masaaki
    Journal of the American Chemical Society (2011), 133 (42), 16770-16773CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    To elucidate how the protein-ligand docking structure affects electronic interactions in the electron-transfer process, we have analyzed time-resolved ESR spectra of photoinduced charge-sepd. (CS) states generated by light excitation of 9,10-anthraquinone-1-sulfonate (AQ1S-) bound to human serum albumin at a hydrophobic drug-binding region. The spectra have been explained in terms of the triplet-triplet electron spin polarization transfer model to det. both the geometries and the exchange couplings of the CS states of AQ1S2-·-histidine-242 radical cation (H242+·) and AQ1S2-·-tryptophan-214 radical cation (W214+·). For the CS state of the former, it has been revealed that, due to the orthogonal relationship between the singly occupied MOs of AQ1S2-· and H242+·, the electronic coupling (5.4 cm-1) is very weak, contributing to the prevention of energy-wasting charge recombination, even at a contact edge-to-edge sepn.
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    Fuki, M.; Murai, H.; Tachikawa, T.; Kobori, Y. Time Resolved EPR Study on the Photoinduced Long-Range Charge-Separated State in Protein: Electron Tunneling Mediated by Arginine Residue in Human Serum Albumin. J. Phys. Chem. B 2016, 120, 4365– 4372,  DOI: 10.1021/acs.jpcb.6b01072
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    Time Resolved EPR Study on the Photoinduced Long-Range Charge-Separated State in Protein: Electron Tunneling Mediated by Arginine Residue in Human Serum Albumin
    Fuki, Masaaki; Murai, Hisao; Tachikawa, Takashi; Kobori, Yasuhiro
    Journal of Physical Chemistry B (2016), 120 (19), 4365-4372CODEN: JPCBFK; ISSN:1520-5207. (American Chemical Society)
    To elucidate how local mol. conformations play a role on electronic couplings for the long-range photoinduced charge-sepd. (CS) states in protein systems, we have analyzed time-resolved ESR (TREPR) spectra by polarized laser irradiations of 9,10-anthraquinone-1-sulfonate (AQ1S-) bound to human serum albumin (HSA). Analyses of the magnetophotoselection effects on the EPR spectra and a docking simulation clarified the mol. geometry and the electronic coupling of the long-range CS states of AQ1S•2--tryptophan214 radical cation (W214•+) sepd. by 1.2 nm. The ligand of AQ1S- has been demonstrated to be bound to the drug site I in HSA. Mol. conformations of the binding region were estd. by the docking simulations, indicating that an arginine218 (R218+) residue bound to AQ1S•2- mediates the long-range electron-transfer. The energetics of triad states of AQ1S•2--R218+-W214•+ and AQ1S--R218•-W214•+ have been computed on the basis of the d. functional MO calcns., providing the clear evidence for the long-range electronic couplings of the CS states in terms of the superexchange tunneling model through the arginine residue.
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    Ema, F.; Tanabe, M.; Saito, S.; Yoneda, T.; Sugisaki, K.; Tachikawa, T.; Akimoto, S.; Yamauchi, S.; Sato, K.; Osuka, A. Charge-Transfer Character Drives Möbius Antiaromaticity in the Excited Triplet State of Twisted [28]Hexaphyrin. J. Phys. Chem. Lett. 2018, 9, 2685– 2690,  DOI: 10.1021/acs.jpclett.8b00740
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    Charge-Transfer Character Drives M.ovrddot.obius Antiaromaticity in the Excited Triplet State of Twisted [28]Hexaphyrin
    Ema, Fumitoshi; Tanabe, Mana; Saito, Shohei; Yoneda, Tomoki; Sugisaki, Kenji; Tachikawa, Takashi; Akimoto, Seiji; Yamauchi, Seigo; Sato, Kazunobu; Osuka, Atsuhiro; Takui, Takeji; Kobori, Yasuhiro
    Journal of Physical Chemistry Letters (2018), 9 (10), 2685-2690CODEN: JPCLCD; ISSN:1948-7185. (American Chemical Society)
    Mobius arom. mols. have attracted great attention as new functional materials because of their π-orbital cyclic conjugations lying along the twisted Mobius topol. To elucidate the electronic character of the lowest excited triplet (T1) state of a Mobius arom. [28]hexaphyrin, we employed a time-resolved ESR (TREPR) method with applied magnetophotoselection measurements at 77 K. Analyses of the EPR parameters have revealed that the T1 state possesses intramol. charge-transfer (CT) character together with local excitation character residing at one side in the Mobius strip ring. We have also demonstrated that the CT character between orthogonal unpaired orbitals triggers quick triplet deactivation by spin-orbit coupling. This deactivation can be an important barometer to represent the "antiaromaticity" because of a connection between the orthogonal CT character and instability by a weakened spin-spin exchange coupling in the T1 state.
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    Borovykh, I. V.; Proskuryakov, I. I.; Klenina, I. B.; Gast, P.; Hoff, A. J. Magnetophotoselection Study of the Lowest Excited Triplet State of the Primary Donor in Photosynthetic Bacteria. J. Phys. Chem. B 2000, 104, 4222– 4228,  DOI: 10.1021/jp993780a
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    Magnetophotoselection Study of the Lowest Excited Triplet State of the Primary Donor in Photosynthetic Bacteria
    Borovykh, Igor V.; Proskuryakov, Ivan I.; Klenina, Irina B.; Gast, Peter; Hoff, Arnold J.
    Journal of Physical Chemistry B (2000), 104 (17), 4222-4228CODEN: JPCBFK; ISSN:1089-5647. (American Chemical Society)
    The first time-resolved magnetophotoselection study of the primary donor triplet state (3P) in bacterial reaction centers from Rhodobacter sphaeroides R26 and Rhodopseudomonas viridis is reported. With direct excitation of the primary donor, this approach provides the orientation (spherical coordinates δ, colatitude, and γ, longitude) of the excited optical transition moment in the principal triplet axis system. It is essentially free from the effects of spin-lattice relaxation of 3P, enabling magnetophotoselection measurements over a wide temp. range. Two independently measured triplet-state spectra, excited with light polarized parallel and perpendicular to the EPR magnetic field, are simulated with the same set of parameters. This procedure results in a high precision (∼±5° with sufficient signal/noise ratio) of the obtained spherical coordinates of the optical transition moment vector. We find δ = 80±5°, γ = 70±5° and δ = 75±5°, γ = 70±5° for Rb. sphaeroides R26 and Rps. viridis, resp. We demonstrate that excitation of the sample with nonpolarized light is essentially nonisotropic. Neglect of this effect in spectral simulations of light-induced signals may lead to considerable error in the parameters detd.
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    Regev, A.; Michaeli, S.; Levanon, H.; Cyr, M.; Sessler, J. L. Solvent Effect in Randomly and Partially Oriented Triplets of the Sapphyrin Dication: Optical and Fast EPR-Magnetophotoselection Measurements. J. Phys. Chem. 1991, 95, 9121– 9129,  DOI: 10.1021/j100176a019
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    Solvent effect in randomly and partially oriented triplets of the sapphyrin dication: optical and fast EPR-magnetophotoselection measurements
    Regev, Ayelet; Michaeli, Shalom; Levanon, Haim; Cyr, Mike; Sessler, Jonathan L.
    Journal of Physical Chemistry (1991), 95 (23), 9121-9CODEN: JPCHAX; ISSN:0022-3654.
    The photoexcited triplet state of the sapphyrin dication, H5.Sap2+, was investigated by optical absorption and fast EPR-magnetophotoselection (MPS) spectroscopies. Results indicate that H5.Sap2+ exists as a monomer in nonpolar isotropic solvents (chloroform and toluene) and nematic liq. crystals, while in a polar solvent, e.g., ethanol, the stable form exists as a face-to-face dimer via specific solvent-mediated interactions. Anal. of the MPS-triplet EPR and optical absorption results provides the following conclusions: (1) relation between the mol. and magnetic frames of refs.; (2) out-of-plane location of the optical transition moment for both the monomer, H5.Sap2+, and the dimer, (H5.Sap2+)2, suggesting a possible admixt. of (n,π*) and (π,π*) states; (3) in-plane triplet state alignment and planar mol. structure of the monomer; (i.v.) a neg. value of the zero-field splitting parameter, D, for the monomer and the dimer.
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    Kobori, Y.; Yamauchi, S.; Akiyama, K.; Tero-Kubota, S.; Imahori, H.; Fukuzumi, S.; Norris, J. R. Primary Charge-Recombination in an Artificial Photosynthetic Reaction Center. Proc. Natl. Acad. Sci. U.S.A. 2005, 102, 10017– 10022,  DOI: 10.1073/pnas.0504598102
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    Primary charge-recombination in an artificial photosynthetic reaction center
    Kobori, Yasuhiro; Yamauchi, Seigo; Akiyama, Kimio; Tero-Kubota, Shozo; Imahori, Hiroshi; Fukuzumi, Shunichi; Norris, James R., Jr.
    Proceedings of the National Academy of Sciences of the United States of America (2005), 102 (29), 10017-10022CODEN: PNASA6; ISSN:0027-8424. (National Academy of Sciences)
    Photoinduced primary charge-sepn. and charge-recombination are characterized by a combination of time-resolved optical and EPR measurements of a fullerene-porphyrin-linked triad that undergoes fast, stepwise charge-sepn. processes. The electronic coupling for the energy-wasting charge recombination is evaluated from the singlet-triplet electronic energy gap in the short-lived, primary charge-sepd. state. The electronic coupling is found to be smaller by ≈40% than that for the primary charge-sepn. This inhibition of the electronic interaction for the charge-recombination to excited triplet state largely results from a symmetry-broken electronic structure modulated by CI between 3(b1u,b3g) and 3(au, b3g) electronic states of the free-base porphyrin.
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    Norris, J. R.; Morris, A. L.; Thurnauer, M. C.; Tang, J. A General-Model of Electron-Spin Polarization Arizing From the Interactions within Radical Pairs. J. Chem. Phys. 1990, 92, 4239– 4249,  DOI: 10.1063/1.457782
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    A general description is given of electron spin polarization obsd. for interacting radical pairs. Unlike previous treatments, both chem. induced dynamic electron polarization and correlated radical pair polarization are included for ST0 mixing. A key feature of this model is that the members of the pair can remain interacting throughout the time sequence of electron spin polarization. Explanations of the time-dependent evolution of the electron spin polarization are presented using the d. matrix and its assocd. vector diagram. A new vector method is formulated for calcg. and visualizing chem. induced electron spin polarization. The approach is based on the vector, Δρ(t) which represents the displacement of the d. vector ρ(t) from its direction at the time of its "birth". In addn., these electron spin polarization phenomena are described in spectroscopic terms as well as in the usual math. manner.
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    Morris, A. L.; Snyder, S. W.; Zhang, Y. N.; Tang, J.; Thurnauer, M. C.; Dutton, P. L.; Robertson, D. E.; Gunner, M. R. Electron-Spin Polarization Model Applied to Sequential Electron-Transfer in Iron-Containing Photosynthetic Bacterial Reaction Centers with Different Quinones as Q(a). J. Phys. Chem. 1995, 99, 3854– 3866,  DOI: 10.1021/j100011a063
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    Electron Spin Polarization Model Applied to Sequential Electron Transfer in Iron-Containing Photosynthetic Bacterial Reaction Centers with Different Quinones as QA
    Morris, Andrea L.; Snyder, Seth W.; Zhang, Yuenian; Tang, Jau; Thurnauer, Marion C.; Dutton, P. Leslie; Robertson, Dan E.; Gunner, M. R.
    Journal of Physical Chemistry (1995), 99 (11), 3854-66CODEN: JPCHAX; ISSN:0022-3654. (American Chemical Society)
    Electron spin polarization develops on P+[QFe2+]- in iron-contg. photosynthetic bacterial reaction centers (RC) of Rhodobacter sphaeroides. The spin-polarized ESR spectra of the oxidized primary donor (P+) depend on τH-, the lifetime of the radical pair P+H- formed prior to P+[QFe2+]-. The polarized EPR signal can be described by the sequential electron transfer polarization (SETP) model in which the chem. induced dynamic electron polarization (CIDEP) developed in P+H- is projected onto the correlated radical pair polarization (CRPP) developed in P+[QFe2+]-. Replacing the native ubiquinone-10 with various anthraquinones and naphthoquinones alters both the free energy and rate of electron transfer from H- to QFe2+, which in turn modifies τH-. At long τH- the polarized P+ EPR signal is dominated by the CIDEP component of SETP. At short τH- the signal is dominated by the CRPP component, while at intermediate τH-'s the signal can only be described using the full SETP model. The ranges of τH- where polarization is dominated by interactions on the prior or obsd. radical pair are influenced by the EPR microwave frequency and RC isotopic compn. Exptl. spectra of spin-polarized P+ from a series of Rb. sphaeroides RCs having τH-'s ranging from 0.33 to 25 ns are modeled with SETP. The model accounts for differences in the polarization line shape with deuteration of the RCs or increase in the EPR microwave frequency.
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    Huang, Y.-C.; Tsao, C.-S.; Chuang, C.-M.; Lee, C.-H.; Hsu, F.-H.; Cha, H.-C.; Chen, C.-Y.; Lin, T.-H.; Su, C.-J.; Jeng, U. S. Small- and Wide-Angle X-ray Scattering Characterization of Bulk Heterojunction Polymer Solar Cells with Different Fullerene Derivatives. J. Phys. Chem. C 2012, 116, 10238– 10244,  DOI: 10.1021/jp210140j
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    Small- and Wide-Angle X-ray Scattering Characterization of Bulk Heterojunction Polymer Solar Cells with Different Fullerene Derivatives
    Huang, Yu-Ching; Tsao, Cheng-Si; Chuang, Chih-Min; Lee, Chia-Hsin; Hsu, Fan-Hsuan; Cha, Hou-Chin; Chen, Charn-Ying; Lin, Tsung-Han; Su, Chun-Jen; Jeng, U-Ser; Su, Wei-Fang
    Journal of Physical Chemistry C (2012), 116 (18), 10238-10244CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
    The aim of this study is to quant. investigate the effect of different fullerene type (PC60BM and PC70BM) on various morphol. structures and power conversion efficiency in the bulk heterojunction poly(3-hexylthiophene) (P3HT)/PCxBM solar cells. The solar cells are fabricated by spin coating without thermal annealing. The quant. investigations of three-dimensional self-organized nanostructures are performed by using combined grazing-incidence small- and wide-angle x-ray scattering technique (GISAXS/GIWAXS). Two types of nanostructures are obsd. due to the phase sepn. in the bulk heterojunction films during the processing. They include (a) intercalated PCxBM mols. around boundary of P3HT cryst. domain and within amorphous domain and (b) aggregated PCxBM clusters in PCxBM domains. The lamellar spacing of P3HT cryst. domains in P3HT/PC70BM is larger than that in P3HT/PC60BM. This result indicates that more interfacial areas are generated between PC70BM and P3HT at the mol. scale for more efficient charge sepn. On the other hand, the size, vol. fraction, partial attachment, and spatial distribution of PC60BM clusters are larger than that of PC70BM clusters, which reveals more efficient electron transport in P3HT/PC60BM. We deduce the correlation between nanostructures and power conversion efficiency (3.25% and 2.64% for P3HT/PC70BM and P3HT/PC60BM, resp.). The structure of fullerene intercalated with P3HT rather than the size of fullerene cluster plays a major role in the power conversion efficiency performance of bulk heterojunction solar cell without thermal annealing.
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    Tamai, Y.; Matsuura, Y.; Ohkita, H.; Benten, H.; Ito, S. One-Dimensional Singlet Exciton Diffusion in Poly(3-hexylthiophene) Crystalline Domains. J. Phys. Chem. Lett. 2014, 5, 399– 403,  DOI: 10.1021/jz402299a
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    51
    One-Dimensional Singlet Exciton Diffusion in Poly(3-hexylthiophene) Crystalline Domains
    Tamai, Yasunari; Matsuura, Yuu; Ohkita, Hideo; Benten, Hiroaki; Ito, Shinzaburo
    Journal of Physical Chemistry Letters (2014), 5 (2), 399-403CODEN: JPCLCD; ISSN:1948-7185. (American Chemical Society)
    Singlet exciton dynamics in cryst. domains of regioregular poly(3-hexylthiophene) (P3HT) films was studied by transient absorption spectroscopy. Upon the selective excitation of cryst. P3HT at the absorption edge, no red shift of the singlet exciton band was obsd. with an elapse of time, suggesting singlet exciton dynamics in relatively homogeneous P3HT cryst. domains without downhill relaxation in the energetic disorder. Even under such selective excitation conditions, the annihilation rate coeff. γ-(t) was still dependent on time, γ-(t) .varies. t-1/2, which is attributed to anisotropic exciton diffusion in P3HT cryst. domains. From the annihilation rate coeff., the singlet exciton diffusion coeff. D and exciton diffusion length LD in the cryst. domains were evaluated to be 7.9 × 10-3 cm2 s-1 and 20 nm, resp. The origin of the time-dependent exciton dynamics is discussed in terms of dimensionality.
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    Lee, J.; Vandewal, K.; Yost, S. R.; Bahlke, M. E.; Goris, L.; Baldo, M. A.; Manca, J. V.; Van Voorhis, T. Charge Transfer State Versus Hot Exciton Dissociation in Polymer-Fullerene Blended Solar Cells. J. Am. Chem. Soc. 2010, 132, 11878– 11880,  DOI: 10.1021/ja1045742
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    Charge Transfer State Versus Hot Exciton Dissociation in Polymer-Fullerene Blended Solar Cells
    Lee, Jiye; Vandewal, Koen; Yost, Shane R.; Bahlke, Matthias E.; Goris, Ludwig; Baldo, Marc A.; Manca, Jean V.; Van Voorhis, Troy
    Journal of the American Chemical Society (2010), 132 (34), 11878-11880CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    We examine the significance of hot exciton dissocn. in two archetypical polymer-fullerene blend solar cells. Rather than evolving through a bound charge transfer state, hot processes are proposed to convert excitons directly into free charges. But we find that the internal quantum yields of carrier photogeneration are similar for both excitons and direct excitation of charge transfer states. The internal quantum yield, together with the temp. dependence of the current-voltage characteristics, is consistent with negligible impact from hot exciton dissocn.
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    Fukuju, T.; Yashiro, H.; Maeda, K.; Murai, H.; Azumi, T. Singlet-Born SCRP Observed in the Photolysis of Tetraphenylhydrazine in an SDS Micelle: Time Dependence of the Population of the Spin States. J. Phys. Chem. A 1997, 101, 7783– 7786,  DOI: 10.1021/jp971620y
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    Singlet-Born SCRP Observed in the Photolysis of Tetraphenylhydrazine in an SDS Micelle: Time Dependence of the Population of the Spin States
    Fukuju, Tadahiro; Yashiro, Haruhiko; Maeda, Kiminori; Murai, Hisao; Azumi, Tohru
    Journal of Physical Chemistry A (1997), 101 (42), 7783-7786CODEN: JPCAFH; ISSN:1089-5639. (American Chemical Society)
    The spectrum of the singlet-born spin-correlated radical pair generated in the photolysis of tetraphenylhydrazine in a micelle is analyzed. The time evolution of the spectral pattern directly reflects the time dependence of the population of the individual spin states of the radical pair. The relaxation times between these states and the reaction rates of the geminate process are estd., and the relaxation of the middle two states of the radical pairs is discussed.
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    Miura, T.; Murai, H. Effect of Molecular Diffusion on the Spin Dynamics of a Micellized Radical Pair in Low Magnetic Fields Studied by Monte Carlo Simulation. J. Phys. Chem. A 2015, 119, 5534– 5544,  DOI: 10.1021/acs.jpca.5b02183
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    Effect of Molecular Diffusion on the Spin Dynamics of a Micellized Radical Pair in Low Magnetic Fields Studied by Monte Carlo Simulation
    Miura, Tomoaki; Murai, Hisao
    Journal of Physical Chemistry A (2015), 119 (22), 5534-5544CODEN: JPCAFH; ISSN:1089-5639. (American Chemical Society)
    Magnetic field effect is a powerful tool to study dynamics and kinetics of radical pairs (RPs), which are one of the most important intermediates for org. photon-energy conversion reactions. However, quant. discussion regarding the relationship between the modulation of interelectron interactions and spin dynamics at low magnetic fields (<10 mT) is still an open question. We have studied the spin dynamics of a long-lived RP in a micelle by newly developed Monte Carlo simulation, in which fluctuations of the exchange and magnetic dipolar interactions by in-cage diffusion are directly introduced to the time-domain spin dynamics calcn. State-dependent relaxation/dephasing times of a few to a few tens of nanoseconds are obtained by simulations without hyperfine interactions (HFIs) as a function of the mutual diffusion const. (∼10-6 cm2/s). Simulations with the HFIs exhibit incoherent singlet-triplet (S-T) mixings resulting from interplay between the HFIs and the fluctuating spin-spin interactions. The exptl. obsd. incoherent S-T mixing of ∼20 ns at 3 mT for a singlet-born RP in a sodium dodecyl sulfate micelle is reproduced by the simulation with reasonable diffusion coeffs. The computational method developed here contributes to quant. detection of mol. motion that governs the recombination efficiency of RPs.
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    Kayunkid, N.; Uttiya, S.; Brinkmann, M. Structural Model of Regioregular Poly(3-hexylthiophene) Obtained by Electron Diffraction Analysis. Macromolecules 2010, 43, 4961– 4967,  DOI: 10.1021/ma100551m
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    Structural Model of Regioregular Poly(3-hexylthiophene) Obtained by Electron Diffraction Analysis
    Kayunkid, Navaphun; Uttiya, Sureeporn; Brinkmann, Martin
    Macromolecules (Washington, DC, United States) (2010), 43 (11), 4961-4967CODEN: MAMOBX; ISSN:0024-9297. (American Chemical Society)
    This study presents a structural anal. of regioregular poly(3-hexylthiophene) (P3HT) based on electron diffraction from epitaxied thin films. Epitaxial orientation of the hexane fraction of P3HT was performed by slow rate directional solidification in 1,3,5-trichlorobenzene leading to highly oriented and cryst. P3HT films with different contact planes. Representative electron diffraction patterns corresponding to different zone axes were obtained by the rotation-tilt electron diffraction method. A trial-and-error method based on mol. modeling and calcn. of the electron diffraction patterns for the different zone axes was used to det. the crystal structure of P3HT. The unit cell is monoclinic with space group P21/c and two chains per cell (a = 1.60 nm, b = 0.78 nm, c = 0.78 nm and γ = 86.5 deg). The stacking period of successive polythiophene backbones along the b axis is 0.39 nm but short interplanar distances of 0.34 nm are obsd. because the conjugated polythiophene backbones are tilted to the b axis. The n-hexyl side groups crystallize in an orthogonal subcell with parameters as = 0.7 nm and bs = 0.78 nm. The present structural model highlights the essential role of the linear side chain crystn. on the supra-macromol. packing of "hairy-rod" polymers like P3HT.
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    Kobori, Y.; Sekiguchi, S.; Akiyama, K.; Tero-Kubota, S. Chemically Induced Dynamic Electron Polarization Study on the Mechanism of Exchange Interaction in Radical Ion Pairs Generated by Photoinduced Electron Transfer Reactions. J. Phys. Chem. A 1999, 103, 5416– 5424,  DOI: 10.1021/jp990359d
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    Chemically induced dynamic electron polarization study on the mechanism of exchange interaction in radical ion pairs generated by photoinduced electron transfer reactions
    Kobori, Yasuhiro; Sekiguchi, Shinji; Akiyama, Kimio; Tero-Kubota, Shozo
    Journal of Physical Chemistry A (1999), 103 (28), 5416-5424CODEN: JPCAFH; ISSN:1089-5639. (American Chemical Society)
    Photoinduced electron transfer reactions were studied by using the continuous wave time-resolved ESR and Fourier-transformed ESR spectroscopy in polar solvents. The chem. induced dynamic electron polarization was investigated in both singlet and triplet precursor intermol. electron transfer systems. The signs of the exchange interaction, which are defined by the energy differences between the singlet and triplet radical ion pairs, depended on the free energy changes for the charge recombination processes. The results are interpreted in terms of the mechanism that the spin selective stabilization and destabilization are caused by the perturbation through the electronic coupling from the ground state and the locally excited triplet state of the donor-acceptor pair at the equil. nuclear coordinate. In the singlet precursor electron transfer systems, the pos. exchange interaction resulted from the selective triplet stabilization in the radical ion pair, when the ion pair state crossed with the locally excited triplet state at the normal region. In the triplet precursor electron transfer systems, the neg. exchange interaction resulted from the selective singlet stabilization when the ion pair state crossed with the singlet ground state at the normal region. When the free energy change is larger than about 1.8 eV, the pos. exchange interaction resulted from the spin-selective destabilization in the singlet ion pair, since the level crossing occurs at the inverted region.
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    Mcconnell, H. Intramolecular Charge Transfer in Aromatic Free Radicals. J. Chem. Phys. 1961, 35, 508– 515,  DOI: 10.1063/1.1731961
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    Intramolecular charge transfer in aromatic free radicals
    McConnell, Harden M.
    Journal of Chemical Physics (1961), 35 (), 508-15CODEN: JCPSA6; ISSN:0021-9606.
    A theoretical analysis was made of the rate of intramol. transfer of the odd electron between 2 Ph groups in the mononeg. ions of α,ω-diphenylalkanes. The essential features of the calcns. were given. (a) The polymethylene chain could be replaced by a pseudopotential corresponding to an effective direct transfer between the rings. (b) There was a strong tendency for self-trapping of the odd electron on one Ph ring or the other, due to solvent polarization and bond distortions in the rings. The self-trapping greatly reduced the rate of intramol. charge transfer. (c) The intramol. charge transfer occurred as an electronic resonance effect when a short-lived thermally activated mol. state was formed in which the 2 rings appeared to the odd electron to be equiv. The activation energy was estd. to be of the order of 1000 cm.-1 (d) The rate of intramol. charge transfer decreased exponentially with the length of the polymethylene chain, the decrease being as much or more than a factor of 10 for each added CH2 group.
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    Goldsmith, R. H.; Sinks, L. E.; Kelley, R. F.; Betzen, L. J.; Liu, W. H.; Weiss, E. A.; Ratner, M. A.; Wasielewski, M. R. Wire-Like Charge Transport at Near Constant Bridge Energy through Fluorene Oligomers. Proc. Natl. Acad. Sci. U.S.A. 2005, 102, 3540– 3545,  DOI: 10.1073/pnas.0408940102
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    Wire-like charge transport at near constant bridge energy through fluorene oligomers
    Goldsmith, Randall H.; Sinks, Louise E.; Kelley, Richard F.; Betzen, Laura J.; Liu, Wenhao; Weiss, Emily A.; Ratner, Mark A.; Wasielewski, Michael R.
    Proceedings of the National Academy of Sciences of the United States of America (2005), 102 (10), 3540-3545CODEN: PNASA6; ISSN:0027-8424. (National Academy of Sciences)
    The study of photoinitiated electron transfer in donor-bridge-acceptor mols. has helped elucidate the distance dependence of electron transfer rates and behavior of various electron transfer mechanisms. In all reported cases, the energies of the bridge electronic states involved in the electron transfer change dramatically as the length of the bridge is varied. The authors report here, in contrast, an instance in which the length of the bridge, and therefore the distance over which the electron is transferred, can be varied without significantly changing the energies of the relevant bridge states. A series of donor-bridge-acceptor mols. having phenothiazine (PTZ) donors, 2,7-oligofluorene (FLn) bridges, and perylene-3,4:9,10-bis(dicarboximide) (PDI) acceptors was studied. Photoexcitation of PDI to its lowest excited singlet state results in oxidn. of PTZ via the FLn bridge. In toluene, the rate consts. for both charge sepn. and recombination as well as the energy levels of the relevant FLn+• bridge states for n = 1-4 are only weakly distance dependent. After the initial photogeneration of 1(PTZ+•-FLn-PDI-•), radical pair intersystem crossing results in formation of 3(PTZ+•-FLn-PDI-•) that recombines to give 3*PDI. The dependence of the 3*PDI yield on an applied magnetic field shows a resonance, which gives the singlet-triplet splitting, 2J, of the radical ion pair. The magnitude of 2J directly monitors the contribution of coherent charge transfer (superexchange) to the overall electron transfer rate. These data show that charge recombination through FLn is dominated by incoherent hopping at long distances.
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    Shoji, R.; Omori, T.; Wakikawa, Y.; Miura, T.; Ikoma, T. Magnetoconductance Study on Nongeminate Recombination in Solar Cell Using Poly(3-hexylthiophene) and [6,6]-Phenyl-C-61-butyric Acid Methyl Ester. ACS Omega 2018, 3, 9369– 9377,  DOI: 10.1021/acsomega.8b01746
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    59
    Magnetoconductance Study on Nongeminate Recombination in Solar Cell Using Poly(3-hexylthiophene) and [6,6]-Phenyl-C61-butyric Acid Methyl Ester
    Shoji, Ryota; Omori, Takuya; Wakikawa, Yusuke; Miura, Tomoaki; Ikoma, Tadaaki
    ACS Omega (2018), 3 (8), 9369-9377CODEN: ACSODF; ISSN:2470-1343. (American Chemical Society)
    The magnetoconductance (MC) effect was investigated for two types of org. solar cells with single junction (SJ) and bulk junction (BJ) of poly(3-hexylthiophene) (P3HT) as donor (D) and [6,6]-phenyl-C61-butyric acid Me ester (PCBM) as acceptor (A). Three components with different half-field-at-half-max. (B1/2) of 4±1, 20±15 and >400 mT, hereafter referred to as MCs,m,b in a sequence, were obsd. in the magnetic field dependence of the MC effects measured under dark and light conditions. The magnitude of the MCs,m,b components is sensitive to not only the junction structure of the cell, but also the presence or absence of incident light. The bias voltage (V) dependence of the MC effect in the dark for the SJ-cell is maximized around the turn-on voltage (Von) of the dark current, where a flat band condition of the active layer is achieved. The B1/2 for the MCm component of the SJ-cell increases with V beyond Von. In light, the BJ-cell exhibits the MC effect, whereas no effect is detected for the SJ-cell. The MCs,m components for the BJ-cell in light increase with the incident light power. The transient MCs,m components for the BJ-cell measured using a nanosecond pulse laser increases with the delay time after the flash. By integrating these phenomena and the phase of the MC effect, it is concluded that all the MC components arise from the magnetic field effect on the spin conversion of nongeminate electron (e)-hole (h) pairs with spin-dependent charge recombinations at the D/A-interface. The B1/2 values for MCs,m,b are resp. understood by the spin conversion due to the hyperfine interaction, the spin relaxation and the g-factor difference for e (PCBM-) and h (P3HT+). Kinetic simulations of the MCs,m components for the BJ-cell obsd. at the short-circuit condition in light yield an efficiency of ca. 40% for the nongeminate recombination, which is accompanied by the generation of triplet excitons as well as relaxation to a ground singlet state. The loss mechanism of moderate triplet recombination suggests an important possibility to improve the power conversion efficiency by harvesting of the triplet excitons.
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    Monkman, A. P.; Burrows, H. D.; Hartwell, L. J.; Horsburgh, L. E.; Hamblett, I.; Navaratnam, S. Triplet Energies of pi-Conjugated Polymers. Phys. Rev. Lett. 2001, 86, 1358– 1361,  DOI: 10.1103/PhysRevLett.86.1358
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    Triplet Energies of π-Conjugated Polymers
    Monkman, A. P.; Burrows, H. D.; Hartwell, L. J.; Horsburgh, L. E.; Hamblett, I.; Navaratnam, S.
    Physical Review Letters (2001), 86 (7), 1358-1361CODEN: PRLTAO; ISSN:0031-9007. (American Physical Society)
    Using pulse radiolysis and triplet energy transfer has enabled us to measure the triplet energies in a broad range of different π-conjugated polymers. In all cases we find that the 1 3 Bu is of order 0.6 to 1 eV below the 1 1 Bu, indicative of localized triplet states with strong electron-electron correlation. We also observe that the 1 1 Ag-1 3Bu gap decreases linearly as the 1 1 Ag-1 1Bu gap decreases even though polymers with very different structure have been studied. This surprising result suggests that polymers with singlet gap <1.3 eV will have a triplet ground state.
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    Li, B.; Yu, H.; Montoto, E. C.; Liu, Y.; Li, S.; Schwieter, K.; Rodríguez-López, J.; Moore, J. S.; Schroeder, C. M. Intrachain Charge Transport through Conjugated Donor–Acceptor Oligomers. ACS Appl. Electron. Mater. 2019, 1, 7– 12,  DOI: 10.1021/acsaelm.8b00050
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    Intrachain Charge Transport through Conjugated Donor-Acceptor Oligomers
    Li, Bo; Yu, Hao; Montoto, Elena C.; Liu, Yun; Li, Songsong; Schwieter, Kenneth; Rodriguez-Lopez, Joaquin; Moore, Jeffrey S.; Schroeder, Charles M.
    ACS Applied Electronic Materials (2019), 1 (1), 7-12CODEN: AAEMBP; ISSN:2637-6113. (American Chemical Society)
    Donor-acceptor (D-A) polymers are promising materials for org. electronics because of high charge carrier mobilities and narrow band gaps. Despite recent progress, there is incomplete understanding of the mechanisms underlying charge transport in these materials. In this work, we use single mol. techniques to study intrachain charge transport in D-A oligomers contg. alternating diketopyrrolopyrrole (DPP) acceptor and bithiophene donor units. Interestingly, at high applied bias, longer DPPTT oligomers exhibit substantially higher conductance compared to shorter oligomers, which is interpreted using d. functional theory (DFT) simulations. Overall, this work provides an increased understanding of intrachain charge transport along D-A oligomers.
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    Kilina, S.; Kilin, D.; Tretiak, S. Light-Driven and Phonon-Assisted Dynamics in Organic and Semiconductor Nanostructures. Chem. Rev. 2015, 115, 5929– 5978,  DOI: 10.1021/acs.chemrev.5b00012
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    Light-Driven and Phonon-Assisted Dynamics in Organic and Semiconductor Nanostructures
    Kilina, Svetlana; Kilin, Dmitri; Tretiak, Sergei
    Chemical Reviews (Washington, DC, United States) (2015), 115 (12), 5929-5978CODEN: CHREAY; ISSN:0009-2665. (American Chemical Society)
    A review. This Review targets the goal of helping exptl. scientists and computational chemists to synergistically address open challenges in the field of applied nanoscience. Computational modeling cannot provide universally exhaustive and accurate information for all cases involving large mol. systems by acting as a black box tool. Instead, specific model chem. has to be applied to a well-defined and focused problem with a deep understanding of manifold of approxns. that were used to reach a desired compromise between accuracy and numerical cost. This frequently provides only narrow-range qual. descriptions of a targeted process in extended nanosystems. While direct comparison of computational results with their exptl. counterparts constitutes an important task of benchmarking and validation of specific theor. technique(e.g., DFT model), it is usually only feasible only for simple well-defined systems. The authors have outlined multiple examples demonstrating to the reader that the main benefit of reported computational approaches for realistic complex structures is recognition of phys. mechanisms, structure-property relationships, and trends in observables. In this way, quantum chem. simulations have potentials to get insight into the most important processes taking place at atomistic and mol. levels of realistic materials and bring applied and fundamental research closer together.
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    Luo, J.; Piette, B. M. A. G. Directed Polaron Propagation in Linear Polypeptides Induced by Intramolecular Vibrations and External Electric Pulses. Phys. Rev. E 2018, 98, 012401  DOI: 10.1103/PhysRevE.98.012401
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    Directed polaron propagation in linear polypeptides induced by intramolecular vibrations and external electric pulses
    Luo, J.; Piette, B. M. A. G.
    Physical Review E (2018), 98 (1), 012401CODEN: PREHBM; ISSN:2470-0053. (American Physical Society)
    We study the propagation of α-helix polarons in a model describing the nonadiabatic interaction between an electron and a lattice of quantum mech. oscillators at physiol. temp. We show that when excited by a subpicosecond elec. pulse, as induced by exptl. obsd. subpicosecond charge sepn., the polaron is displaced by up to hundreds of lattice sites before the electron becomes delocalised. We discuss biophys. implications of our results.
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    Vukmirović, N.; Wang, L. W. Carrier Hopping in Disordered Semiconducting Polymers: How Accurate is the Miller–Abrahams Model?. Appl. Phys. Lett. 2010, 97, 043305  DOI: 10.1063/1.3474618
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    Carrier hopping in disordered semiconducting polymers. How accurate is the Miller-Abrahams model?
    Vukmirovic, Nenad; Wang, Lin-Wang
    Applied Physics Letters (2010), 97 (4), 043305/1-043305/3CODEN: APPLAB; ISSN:0003-6951. (American Institute of Physics)
    We performed direct calcns. of carrier hopping rates in strongly disordered conjugated polymers based on the at. structure of the system, the corresponding electronic states and their coupling to all phonon modes. The dependence of hopping rates on distance and the dependence of the mobility on temp. are significantly different than the ones stemming from the simple Miller-Abrahams model, regardless of the choice of the parameters in the model. A model that satisfactorily describes the hopping rates in the system and avoids the explicit calcn. of electron-phonon coupling consts. was then proposed and verified. In addn. to electronic d. of states, the phonon d. of states and the spatial overlap of the wave functions are the quantities necessary to properly describe carrier hopping in disordered conjugated polymers. (c) 2010 American Institute of Physics.
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    Song, Y.; Clafton, S. N.; Pensack, R. D.; Kee, T. W.; Scholes, G. D. Vibrational Coherence Probes the Mechanism of Ultrafast Electron Transfer in Polymer-Fullerene Blends. Nat. Commun. 2014, 5, 4933  DOI: 10.1038/ncomms5933
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    Vibrational coherence probes the mechanism of ultrafast electron transfer in polymer-fullerene blends
    Song, Yin; Clafton, Scott N.; Pensack, Ryan D.; Kee, Tak W.; Scholes, Gregory D.
    Nature Communications (2014), 5 (), 4933pp.CODEN: NCAOBW; ISSN:2041-1723. (Nature Publishing Group)
    The conversion of photoexcitations into charge carriers in org. solar cells is facilitated by the dissocn. of excitons at the donor/acceptor interface. The ultrafast timescale of charge sepn. demands sophisticated theor. models and raises questions about the role of coherence in the charge-transfer mechanism. Here, we applied two-dimensional electronic spectroscopy to study the electron transfer process in poly(3-hexylthiophene)/PCBM (P3HT/PCBM) blends. We reported dynamics maps showing the pathways of charge transfer that clearly expose the significance of hot electron transfer. During this ultrafast electron transfer, vibrational coherence was directly transferred from the P3HT exciton to the P3HT hole polaron in the cryst. domain. This result revealed that the exciton converts to a hole with a similar spatial extent on a timescale far exceeding other photophys. dynamics including vibrational relaxation.
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    Nagasawa, Y.; Yartsev, A. P.; Tominaga, K.; Bisht, P. B.; Johnson, A. E.; Yoshihara, K. Dynamical Aspects of Ultrafast Intermolecular Electron-Transfer Faster Than Solvation Process - Substituent Effects and Energy-Gap Dependence. J. Phys. Chem. 1995, 99, 653– 662,  DOI: 10.1021/j100002a033
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    Dynamic Aspects of Ultrafast Intermolecular Electron Transfer Faster Than Solvation Process: Substituent Effects and Energy Gap Dependence
    Nagasawa, Yutaka; Yartsev, Arkadiy P.; Tominaga, Keisuke; Bisht, Prem B.; Johnson, Alan E.; Yoshihara, Keitaro
    Journal of Physical Chemistry (1995), 99 (2), 653-62CODEN: JPCHAX; ISSN:0022-3654. (American Chemical Society)
    We have investigated intermol. electron transfer (ET) from electron-donating solvents (aniline and N,N-dimethylaniline) to coumarins in the excited state by means of the femtosecond fluorescence up-conversion technique. The coumarins we studied have a variety of structures with different substituents in the 4- and 7-positions. The ET occurs on a time scale ranging from a few nanoseconds to a couple of hundred femtoseconds depending on the structure of the coumarins and solvent. As for the 7-position, as the length of the alkyl chain on the amino group is longer, the ET is slower, and when the amino group is fixed by a double-hexagonal ring, it is slowest. When the electron-accepting ability of the substituent in the 4-position is increased, the reaction occurs faster. The origin of this substituent effect is mainly attributed to the variation of the energy gap between the reactant and product states. This is confirmed by theor. calcns. in terms of the extended Sumi-Marcus two-dimensional model. A good agreement between the expt. and calcn. indicates that some of the reactions take place from the relaxed vibrational state of reactant to the excited vibrational states of high-frequency modes of product states. The simulated population decays for nonequil. configuration of solvents agreed well with exptl. data. In the steady-state fluorescence spectra was also obsd. an effect of very fast fluorescence quenching due to ET; i.e., the amt. of fluorescence Stokes shift depends on the rate of ET because the excited state is quenched in competition with thermal equilibration of the solvent configuration. We regard this spectral shift as the result of the "chem. timing" effect in soln.
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  • Abstract

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    Scheme 1

    Scheme 1. Structure Formula of P3HT and PC61BM Utilized for OSC
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    Figure 1

    Figure 1. (a) Setting of the geometries in the interfacial CS state reflecting anisotropies of the g-tensors(35−37) and the spin–spin dipolar coupling in the present study. Principal axes and their values are depicted for the g-tensors. The principal axis of the dipolar interaction is represented by d as the inter-spin vector. The anisotropies in the hyperfine interactions of P3HT+• were also considered.(18,21)M denotes the direction of the transition dipole moment(38) memorized by the one-dimensional (1D) exciton diffusion after polarized laser excitation L. (b) Schematic SCRP model based on the singlet (S) and the triplet (+, 0, and −) sublevels of the radical pair. S–T0 mixing by the hyperfine interaction and by the Δg (=gP3HT – gPCBM) causes four EPR transitions (denoted by A and E) in the right because the S–T± forbidden transitions (dashed allows in the left) are mixed with the allowed T0–T± transitions.(39,40) (c) The EPR transition peaks by the two antiphase doublets at one specified B0 direction from the SCRP model. The peak splittings(29) denoted by the arrows reflect −4d + 2J, where d [=D(cos2 θD −1/3)/2] and J (=62 μT) are the dipolar and exchange interactions, respectively, with D < 0. kS and kT are the recombination rates to the ground state and to the excited triplet state (3P3HT*), respectively.

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    Figure 2

    Figure 2. Ground-state absorption spectrum of the as-spun P3HT:PC61BM (1.0:0.7 weight ratio) blend film used for the MPS measurements. The 532 nm laser was employed to selectively pump the P3HT-crystalline domain. The interfacial CT band is also observed around 800 nm.

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    Figure 3

    Figure 3. (a–c) Delay time dependence of the TREPR spectrum by 532 nm laser irradiation of the substrate from ITO/PEDOT:PSS/P3HT:PC61BM/LiF/Al of the OSC obtained at 98 K, showing E/A/E/A (0.2 μs) and A/E/A/E (>0.6 μs) patterns. (d) Time profile of the TREPR signal at 337.2 mT as indicated by the arrow in (a). These data were obtained at open-circuit conditions. Solid red lines were obtained by stochastic-Liouville equation (SLE) analysis of the SCRP model using the two different CS state components as shown by the decomposed dotted profiles in (d).

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    Figure 4

    Figure 4. (a–c) Magnetophotoselection effects of the TREPR spectrum for the delay time of td = 0.2 μs by 532 nm laser irradiation of the blend film of P3HT:PC61BM at 77 K. The microwave power was 6.3 mW. (d–f) Computed EPR spectra of the SCRP for the field directions to the principal axes (gx, gy, gz) in P3HT+• by applying J = 62 μT, D = −100 μT, θ = 65°, and ϕ = 0° in Figure 1a. (g–l) MPS effects for the later delay times of td = 0.6 and 2.4 μs. (m, n) Time profiles of the TREPR signal at B0 = 337.30 mT for the depolarized conditions. Computed profiles are shown by the red solid lines obtained by SLE analysis. The red lines were decomposed to the mobile CS state (dotted blue lines computed with T1 = 0.4 μs and T2 < 0.2 μs) and to the trapped CS state (dotted red lines, T1 = 1.6 μs and T2 = 3.0 μs), respectively. Respective spectrum components are shown in Figure S2.

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    Figure 5

    Figure 5. Interfacial photoinduced charge-separation geometries (b, d) characterized by ESPIs (a, c) from the E/A/E/A pattern at 0.2 μs and from A/E/A/E at 0.6 μs, respectively. The ESPIs in (a) and (c) were obtained by distributing the transverse magnetization (ESP intensities in Figure 4d–f) at B0 = 337.25 mT as the color maps to the B0 space directions in the electron–hole pairs generated after the migrations of the exciton (M) to the P3HT-crystal/PCBM interfaces. The hole geometries in (b) and (d) are schematically drawn based on a tilted packing structure with short π–π stackings of 0.34 nm interplanar distances in the crystalline P3HT obtained by the electron diffraction analysis.(55) Inter-spin distances (rCC) and electronic couplings (VCR) were estimated from D and J, respectively.

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    Figure 6

    Figure 6. (Left) Semi-log plots of the electronic couplings vs CS distances in poly(3-alkylthiophene-2,5-diyl)-fullerene systems including (a) P3HT–fullerene linked dyads(17) bridged by oligo-p-phenylenes (▲: P3HT-Phm-C60, m = 2, 3). (b) In the blend films, |VCR|s of the mobile CSs were obtained from the E/A/E/A patterns at 0.2 μs and are plotted by the filled circles (●). The |VCR|s of the trapped CSs were obtained from the SCRP analyses for the later delay times larger than 0.3 μs and are plotted by the open circles (○). Data sources are detailed in Table S1.(18,21) The error bars were derived from the line widths determined by 1/(2πT2d*) and 1/(2πT2J*) as described in Table 1. (Right) Origins of the heterogeneous charge generations are schematically drawn using arrows as the non-Condon effects by (c) high-frequency modes for the strongly coupled mobile CS states (●) and by (d) low-frequency phonon modes for the weakly coupled trapped CS states (○), respectively.

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  • References

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    This article references 66 other publications.

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      Although org. heterojunctions can sep. charges with near-unity efficiency and on a subpicosecond time scale, the full details of the charge-sepn. process remain unclear. In typical models, the Coulomb binding between the electron and the hole can exceed the thermal energy kBT by an order of magnitude, suggesting that it is impossible for the charges to sep. before recombining. Here, we consider the entropic contribution to charge sepn. in the presence of disorder and find that even modest amts. of disorder have a decisive effect, reducing the charge-sepn. barrier to about kBT or eliminating it altogether. Therefore, the charges are usually not thermodynamically bound at all and could sep. spontaneously if the kinetics otherwise allowed it. Our conclusion holds despite the worst-case assumption of localized, thermalized carriers and is only strengthened if mechanisms like delocalization or "hot" states are also present.
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      Janković, V.; Vukmirovic, N. Combination of Charge Delocalization and Disorder Enables Efficient Charge Separation at Photoexcited Organic Bilayers. J. Phys. Chem. C 2018, 122, 10343– 10359,  DOI: 10.1021/acs.jpcc.8b03114
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      12
      Combination of Charge Delocalization and Disorder Enables Efficient Charge Separation at Photoexcited Organic Bilayers
      Jankovic, Veljko; Vukmirovic, Nenad
      Journal of Physical Chemistry C (2018), 122 (19), 10343-10359CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
      The authors study incoherent charge sepn. in a lattice model of an all-org. bilayer. Charge delocalization is taken into account by working in the basis of electron-hole pair eigenstates, and the sepn. is described as incoherent hops between these states. Relatively weak energetic disorder, in combination with good charge delocalization, can account for efficient and weakly field- and temp.-dependent sepn. of the strongly bound charge transfer (CT) state. The sepn. efficiency is detd. by the competition between the recombination from the initial CT state and the escape toward intermediate CT states, from which free-charge states can be reached with certainty. The sepn. of donor excitons also exhibits quite high yields, less bound excitons sepg. more efficiently. The results support the notion that efficient charge sepn. can be achieved even out of strongly bound pair states without invoking coherent effects.
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      Pace, N. A.; Reid, O. G.; Rumbles, G. Delocalization Drives Free Charge Generation in Conjugated Polymer Films. ACS Energy Lett. 2018, 3, 735– 741,  DOI: 10.1021/acsenergylett.8b00108
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      13
      Delocalization drives free charge generation in conjugated polymer films
      Pace, Natalie A.; Reid, Obadiah G.; Rumbles, Garry
      ACS Energy Letters (2018), 3 (3), 735-741CODEN: AELCCP; ISSN:2380-8195. (American Chemical Society)
      The authors demonstrate that the product of photoinduced electron transfer between a conjugated polymer host and a dil. mol. sensitizer is controlled by the structural state of the polymer. Ordered semicryst. solids exhibit free charge generation, while disordered polymers in the melt phase do not. The authors use photoluminescence (PL) and time-resolved microwave cond. (TRMC) measurements to sweep through polymer melt transitions in situ. Free charge generation measured by TRMC turns off upon melting, whereas PL quenching of the mol. sensitizers remains const., implying unchanged electron transfer efficiency. The key difference is the intermol. order of the polymer host in the solid state compared to the melt. The authors propose that this order-disorder transition modulates the localization length of the initial charge-transfer state, which controls the probability of free charge formation.
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    14. 14
      Clarke, T. M.; Durrant, J. R. Charge Photogeneration in Organic Solar Cells. Chem. Rev. 2010, 110, 6736– 6767,  DOI: 10.1021/cr900271s
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      Charge Photogeneration in Organic Solar Cells
      Clarke, Tracey M.; Durrant, James R.
      Chemical Reviews (Washington, DC, United States) (2010), 110 (11), 6736-6767CODEN: CHREAY; ISSN:0009-2665. (American Chemical Society)
      A review. Org. solar cells utilizing π-conjugated polymers have attracted interest in the academic and com. communities. These polymers are promising in terms of their electronic properties, low cost, versatility of functionalization, thin film flexibility, and ease of processing. These factors indicate that org. solar cells, although currently having relatively low power conversion efficiencies of 5 to 7%, compared to inorg. solar cells, they can compete effectively with alternative solar cell technologies. The efficiency of org. solar cells need further improvement.
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    15. 15
      Guo, J. M.; Ohkita, H.; Benten, H.; Ito, S. Charge Generation and Recombination Dynamics in Poly(3-hexylthiophene)/Fullerene Blend Films with Different Regioregularities and Morphologies. J. Am. Chem. Soc. 2010, 132, 6154– 6164,  DOI: 10.1021/ja100302p
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      15
      Charge Generation and Recombination Dynamics in Poly(3-hexylthiophene)/Fullerene Blend Films with Different Regioregularities and Morphologies
      Guo, Jiamo; Ohkita, Hideo; Benten, Hiroaki; Ito, Shinzaburo
      Journal of the American Chemical Society (2010), 132 (17), 6154-6164CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
      The charge generation and recombination dynamics in blend films of a poly(3-hexylthiophene) (P3HT) and a methanofullerene deriv. (PCBM) were comprehensively studied by transient absorption spectroscopy in the wavelength region from 450 to 1650 nm under various excitation intensities and different excitation wavelengths. In homogeneously mixed blend films of regiorandom P3HT (RRa-P3HT) and PCBM, virtually all the excitons can reach the interface of RRa-P3HT/PCBM and then form bound radical pairs. However, 2/3 of them geminately recombine to the ground state, and only 1/3 of them can be dissocd. into free polarons that survive up to milliseconds. In phase-sepd. blend films of regioregular P3HT (RR-P3HT) and PCBM, almost all the excitons can reach the interface of RR-P3HT/PCBM, where most of them can be dissocd. into free polarons efficiently and the rest of them form bound radical pairs. There are two pathways for the polaron generation: the prompt formation from hot excitons generated near the interface on a time scale of <100 fs and the delayed formation via the exciton migration to the interface on a time scale of ∼10 ps. The thermal annealing improves the charge dissocn. efficiency of bound radical pairs. From such spectroscopic data, fundamental photovoltaic conversion processes are quant. analyzed. Consequently, there is not much difference in the charge generation yield between RRa-P3HT/PCBM(50%) and RR-P3HT/PCBM(50%) blend films. Rather, the charge dissocn. and collection have a crit. impact on the overall device performance of P3HT/PCBM solar cells, where the phase-sepd. blend structures have a high tendency to form free carriers and transport these free carriers to the electrode.
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    16. 16
      Kobori, Y.; Miura, T. Overcoming Coulombic Traps: Geometry and Electronic Characterizations of Light-Induced Separated Spins at the Bulk Heterojunction Interface. J. Phys. Chem. Lett. 2015, 6, 113– 123,  DOI: 10.1021/jz5023202
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      16
      Overcoming Coulombic Traps: Geometry and Electronic Characterizations of Light-Induced Separated Spins at the Bulk Heterojunction Interface
      Kobori, Yasuhiro; Miura, Taku
      Journal of Physical Chemistry Letters (2015), 6 (1), 113-123CODEN: JPCLCD; ISSN:1948-7185. (American Chemical Society)
      A review. Recent progress is overviewed on exptl. elucidations of fundamental mol. functions of the light-energy conversions by the photoactive layers of the org. photovoltalic (OPV) cells by the time-resolved ESR spectroscopy. Positions and orientations of the unpaired electrons and electronic coupling matrix elements are clarified in photoinduced, primary charge-sepd. (CS) states. Connections between the mol. geometries and the electronic couplings were characterized for the initial CS states to elucidate how the structure, orbital delocalization, and mol. libration play roles on exothermic carrier dissocn. via a vibrationally relaxed charge-transfer complex with prevention of the energy-wasting charge recombination. Superior functions to biol. mols. are presented for the efficient photocurrent generations induced by orbital delocalization and by shallow trap depths at polymer-stacking domains. The above structural and electronic characteristics of the primary electron-hole pairs are essential to evaluations, designs, and developments of the efficient solar cells using org. mols.
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    17. 17
      Miura, T.; Tao, R.; Shibata, S.; Umeyama, T.; Tachikawa, T.; Imahori, H.; Kobori, Y. Geometries, Electronic Couplings, and Hole Dissociation Dynamics of Photoinduced Electron–Hole Pairs in Polyhexylthiophene–Fullerene Dyads Rigidly Linked by Oligophenylenes. J. Am. Chem. Soc. 2016, 138, 5879– 5885,  DOI: 10.1021/jacs.5b13414
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      17
      Geometries, Electronic Couplings, and Hole Dissociation Dynamics of Photoinduced Electron-Hole Pairs in Polyhexylthiophene-Fullerene Dyads Rigidly Linked by Oligophenylenes
      Miura, Taku; Tao, Ran; Shibata, Sho; Umeyama, Tomokazu; Tachikawa, Takashi; Imahori, Hiroshi; Kobori, Yasuhiro
      Journal of the American Chemical Society (2016), 138 (18), 5879-5885CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
      To shed a light on fundamental mol. functions of photoinduced charge conductions by org. photovoltaic materials, it is important to directly observe mol. geometries of the intermediate charges just after the photoinduced electron-transfer reactions. However, highly inhomogeneous mol. environments at the bulk heterojunction interfaces in the photoactive layers have prevented us from understanding the mechanism of the charge conductions. We have herein investigated orbital geometries, electronic couplings and hole-dissocn. dynamics of photoinduced charge-sepd. (CS) states in a series of poly(3-hexylthiophene)-fullerene linked dyads bridged by rigid oligo-p-phenylene spacers by using time resolved EPR spectroscopy. It has been revealed that one-dimensional intramol. hole-dissocns. exothermically take place from localized holes in initial CS states, following bridge-mediated, photoinduced charge-sepns. via triplet exciton diffusions in the conjugated polymer-backbones. This mol. wire property of the photoinduced charges in soln. at room temp. demonstrates the potential utility of the covalently bridged polymer mols. applied for the mol. devices.
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    18. 18
      Kobori, Y.; Noji, R.; Tsuganezawa, S. Initial Molecular Photocurrent: Nanostructure and Motion of Weakly Bound Charge-Separated State in Organic Photovoltaic Interface. J. Phys. Chem. C 2013, 117, 1589– 1599,  DOI: 10.1021/jp309421s
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      18
      Initial Molecular Photocurrent: Nanostructure and Motion of Weakly Bound Charge-Separated State in Organic Photovoltaic Interface
      Kobori, Yasuhiro; Noji, Ryohei; Tsuganezawa, Shuhei
      Journal of Physical Chemistry C (2013), 117 (4), 1589-1599CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
      To elucidate mechanism of the efficient photocarrier generation by the photoactive bulk heterojunction layers of the org. photovoltaic (OPV) devices, the time-resolved ESR method is employed on solid blends composed of [6,6]-C61-butyric acid Me ester (PCBM) and of poly(3-hexylthiophene-2,5-diyl) (P3HT) with different regioregularities. The photoinduced charge-sepd. (CS) states were detected at the boundary regions between the P3HT and PCBM domains at T = 77 K. Mol. geometries, electronic couplings, and mol. motions of the long-range CS states are characterized. From the CS structure, it is indicated that the pentagonal or hexagonal arom. rings of the buckyball in PCBM directly face the arom. plane of the π-stacked P3HT surfaces. The distant CS states are produced via fast hole-delocalization process from the contact charge-transfer (CT) states. Such hole dynamics is explained by a coupling of the hole to librations of chains in the conjugated polymer. Both the enthalpy stabilization and the enhancement of the entropy occur through the orbital delocalization by the electron-phonon coupling, overcoming the initial CT binding to generate the mol. photocurrent.
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    19. 19
      Krinichnyi, V. I.; Yudanova, E. I.; Spitsina, N. G. Light-Induced Electron Paramagnetic Resonance Study of Poly(3-alkylthiophene)/Fullerene Composites. J. Phys. Chem. C 2010, 114, 16756– 16766,  DOI: 10.1021/jp105873r
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      19
      Light-Induced Electron Paramagnetic Resonance Study of Poly(3-alkylthiophene)/Fullerene Composites
      Krinichnyi, Victor I.; Yudanova, Eugenia I.; Spitsina, Natalia G.
      Journal of Physical Chemistry C (2010), 114 (39), 16756-16766CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
      Radical pairs, polarons, and fullerene anion radicals photoinduced by photons with an energy of 1.98-2.73 eV in bulk heterojunctions formed by poly(3-hexylthiophene) (P3HT) and poly(3-dodecylthiophene) (P3DDT) with [6,6]-Ph C61-butyric acid Me ester (PCBM) and 2-(azahomo[60]fullereno)-5-nitropyrimidine (AFNP) fullerene derivs. were studied by a direct light-induced ESR (LEPR) method in a wide temp. range. LEPR spectra of the polymer/fullerene composites consist of contributions of mobile and trapped charge carriers. Concn. and magnetic resonance parameters of these charge carriers were found to depend on the energy of initiated photons. Spin-lattice and spin-spin relaxation times of polarons and fullerene anion radicals were detd. by the steady-state satn. method. The interaction of most charge carriers with the lattice is characterized by monotonic temp. dependence, whereas the spin-lattice relaxation time of fullerene anion radicals trapped in the P3DDT matrix demonstrates sharper temp. dependence. Spin-spin interaction is shown to be nearly temp. independent and to be governed by structural properties of polymer/fullerene composites. Longitudinal diffusion of polarons and pseudorotation of fullerene derivs. was shown to follow the activation Elliot hopping model. The replacement of the P3HT matrix by P3DDT accelerates polaron dynamics and increases its anisotropy. The energetic barrier required for polaron interchain hopping mainly prevails upon that of its intrachain diffusion in all composites except P3DDT/AFNP one. Spin dynamics becomes easier when the PCBM fullerene deriv. is replaced by the AFNP one.
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      Krinichnyi, V. I.; Yudanova, E. I. Light-Induced EPR Study of Charge Transfer in P3HT/PC71BM Bulk Heterojunctions. J. Phys. Chem. C 2012, 116, 9189– 9195,  DOI: 10.1021/jp2120516
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      20
      Light-Induced EPR Study of Charge Transfer in P3HT/PC71BM Bulk Heterojunctions
      Krinichnyi, Victor I.; Yudanova, Eugenia I.
      Journal of Physical Chemistry C (2012), 116 (16), 9189-9195CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
      Radical pairs, polarons, and fullerene anion radicals photoinduced by photons with energy of 1.98-2.73 eV in bulk heterojunctions formed by regioregular poly(3-hexylthiophene) (P3HT) with (6,6)-phenyl-C71-butyric acid ester (PC71BM) methanofullerene have been studied by the direct light-induced EPR (LEPR) method at a wide temp. region. LEPR spectra of the P3HT/PC71BM composite were deconvoluted, and the main magnetic resonance parameters of these charge carriers have been detd. A part of photoinduced polarons is pinned by large-depth traps in the polymer matrix. It was shown that magnetic resonance, relaxation, and dynamics parameters of photoinduced charge carriers depend extremely on the energy of initiated photons. Relaxation and dynamics parameters of both the charge carriers were detd. sep. by the steady-state satn. method. Longitudinal diffusion of polarons was analyzed in terms of spin interaction with the lattice phonons of cryst. domains embedded into an amorphous polymer matrix. The interchain spin hopping is detd. by the no. and depth of the traps photoinitiated in the polymer matrix. Pseudorotation of methanofullerene mols. in a polymer matrix was shown to follow the activation Pike model. The replacement in the composite of PC61BM acceptors by PC71BM ones accelerates electron relaxation, hinders the formation of spin traps, and favors more ordered (cryst.) structure of bulk heterojunction that facilitates charge transfer in the P3HT/PC71BM composite.
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      Miura, T.; Aikawa, M.; Kobori, Y. Time-Resolved EPR Study of Electron-Hole Dissociations Influenced by Alkyl Side Chains at the Photovoltaic Polyalkylthiophene:PCBM Interface. J. Phys. Chem. Lett. 2014, 5, 30– 35,  DOI: 10.1021/jz402300m
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      21
      Time-Resolved EPR Study of Electron-Hole Dissociations Influenced by Alkyl Side Chains at the Photovoltaic Polyalkylthiophene:PCBM Interface
      Miura, Taku; Aikawa, Motoko; Kobori, Yasuhiro
      Journal of Physical Chemistry Letters (2014), 5 (1), 30-35CODEN: JPCLCD; ISSN:1948-7185. (American Chemical Society)
      Nanosecond time-resolved ESR (TREPR) spectroscopy has been utilized at T = 77 K to characterize alkyl side-chain effects on geometries and on the electronic couplings (VCR) of transient charge-sepd. (CS) states in the photoactive layers fabricated by the spin-coating of mixed solns. of regioregular polyalkylthiophenes (RR-P3AT) and [6,6]-C61-butyric acid Me ester (PCBM). By increasing the alkyl side-chain no. from 6 to 12 in P3AT, a highly distant and long-lived CS state has been obtained. This result is explained by a coupling of the hole dissocn. to the polymer librations by the side-chains. From an exponential decay of VCR with respect to the CS distance, the attenuation factor (βe) has been detd. to be βe = 0.2 Å-1. Such a long-range tunneling feature is explained by the generations of the shallowly trapped, delocalized electron-hole pairs by the dissocn. of the hole toward π-stacking directions at the org. photovoltaic interface.
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    22. 22
      Bakulin, A. A.; Rao, A.; Pavelyev, V. G.; van Loosdrecht, P. H. M.; Pshenichnikov, M. S.; Niedzialek, D.; Cornil, J.; Beljonne, D.; Friend, R. H. The Role of Driving Energy and Delocalized States for Charge Separation in Organic Semiconductors. Science 2012, 335, 1340– 1344,  DOI: 10.1126/science.1217745
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      The Role of Driving Energy and Delocalized States for Charge Separation in Organic Semiconductors
      Bakulin, Artem A.; Rao, Akshay; Pavelyev, Vlad G.; van Loosdrecht, Paul H. M.; Pshenichnikov, Maxim S.; Niedzialek, Dorota; Cornil, Jerome; Beljonne, David; Friend, Richard H.
      Science (Washington, DC, United States) (2012), 335 (6074), 1340-1344CODEN: SCIEAS; ISSN:0036-8075. (American Association for the Advancement of Science)
      The electron-hole pair created via photon absorption in org. photoconversion systems must overcome the Coulomb attraction to achieve long-range charge sepn. This process is facilitated through the formation of excited, delocalized band states. In the authors' expts. on org. photovoltaic cells, these states were accessed for a short time (<1 ps) via IR optical excitation of electron-hole pairs bound at the heterojunction. Atomistic modeling showed that the IR photons promote bound charge pairs to delocalized band states, similar to those formed just after singlet exciton dissocn., which indicates that such states act as the gateway for charge sepn. The authors' results suggest that charge sepn. in efficient org. photoconversion systems occurs through hot-state charge delocalization rather than energy-gradient-driven intermol. hopping.
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      Tamura, H.; Burghardt, I. Ultrafast Charge Separation in Organic Photovoltaics Enhanced by Charge Delocalization and Vibronically Hot Exciton Dissociation. J. Am. Chem. Soc. 2013, 135, 16364– 16367,  DOI: 10.1021/ja4093874
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      23
      Ultrafast Charge Separation in Organic Photovoltaics Enhanced by Charge Delocalization and Vibronically Hot Exciton Dissociation
      Tamura, Hiroyuki; Burghardt, Irene
      Journal of the American Chemical Society (2013), 135 (44), 16364-16367CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
      In org. photovoltaics, the mechanism by which free electrons and holes are generated, overcoming the Coulomb attraction, is a currently much debated topic. To elucidate this mechanism at a mol. level, a combined electronic structure and quantum dynamic anal. is carried out that captures the elementary events from the exciton dissocn. to the free carrier generation at polymer/fullerene donor/acceptor heterojunctions. The calcns. show that exptl. obsd. efficient charge sepns. can be explained by a combination of two effects: first, the delocalization of charges which substantially reduces the Coulomb barrier, and second, the vibronically hot nature of the charge-transfer state which promotes charge dissocn. beyond the barrier. These effects facilitate an ultrafast charge sepn. even at low-band-offset heterojunctions.
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      Barker, A. J.; Chen, K.; Hodgkiss, J. M. Distance Distributions of Photogenerated Charge Pairs in Organic Photovoltaic Cells. J. Am. Chem. Soc. 2014, 136, 12018– 12026,  DOI: 10.1021/ja505380j
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      Distance Distributions of Photogenerated Charge Pairs in Organic Photovoltaic Cells
      Barker, Alex J.; Chen, Kai; Hodgkiss, Justin M.
      Journal of the American Chemical Society (2014), 136 (34), 12018-12026CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
      Strong Coulomb interactions in org. photovoltaic cells dictate that charges must sep. over relatively long distances in order to circumvent geminate recombination and produce photocurrent. In this article, we measure the distance distributions of thermalized charge pairs by accessing a regime at low temp. where charge pairs are frozen out following the primary charge sepn. step and recombine monomolecularly via tunneling. The exponential attenuation of tunneling rate with distance provides a sensitive probe of the distance distribution of primary charge pairs, reminiscent of electron transfer studies in proteins. By fitting recombination dynamics to distributions of recombination rates, we identified populations of charge-transfer states and well-sepd. charge pairs. For the wide range of materials we studied, the yield of sepd. charges in the tunneling regime is strongly correlated with the yield of free charges measured via their intensity-dependent bimol. recombination dynamics at room temp. We therefore conclude that populations of free charges are established via long-range charge sepn. within the thermalization time scale, thus invoking early branching between free and bound charges across an energetic barrier. Subject to assumed values of the electron tunneling attenuation const., we est. crit. charge sepn. distances of ∼3-4 nm in all materials. In some blends, large fullerene crystals can enhance charge sepn. yields; however, the important role of the polymers is also highlighted in blends that achieved significant charge sepn. with minimal fullerene concn. We expect that our approach of isolating the intrinsic properties of primary charge pairs will be of considerable value in guiding new material development and testing the validity of proposed mechanisms for long-range charge sepn.
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      Hasegawa, M.; Nagashima, H.; Minobe, R.; Tachikawa, T.; Mino, H.; Kobori, Y. Regulated Electron Tunneling of Photoinduced Primary Charge-Separated State in the Photosystem II Reaction Center. J. Phys. Chem. Lett. 2017, 8, 1179– 1184,  DOI: 10.1021/acs.jpclett.7b00044
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      Regulated electron tunneling of photoinduced primary charge-separated state in the photosystem II reaction center
      Hasegawa, Masashi; Nagashima, Hiroki; Minobe, Reina; Tachikawa, Takashi; Mino, Hiroyuki; Kobori, Yasuhiro
      Journal of Physical Chemistry Letters (2017), 8 (6), 1179-1184CODEN: JPCLCD; ISSN:1948-7185. (American Chemical Society)
      In initial events of the photosynthesis by higher plants, the photosystem II (PSII) generates photoinduced primary charge-sepd. (CS) state composed of reduced pheophytin (PheoD1-•) and oxidized special pair (P+•) in chlorophyll a (Chla) PD1/PD2 in the D1/D2 heterodimer, ultimately leading to the water oxidn. at the oxygen-evolving Mn4CaO5 cluster by P+•. To understand the mechanism of the efficient generation of initially localized CS state (PD1+• PheoD1-•), we have characterized cofactor geometries and electronic coupling of the photoinduced primary CS state in quinone prereduced membrane of PSII from spinach using the time-resolved ESR method. It has been revealed that the electronic coupling between the charges is significantly weak in the CS state sepd. by 1.5 nm, showing the importance of regulated cofactor-cofactor electronic interaction between a vinyl substituent in PheoD1 and an accessory chlorophyll to inhibit the energy-wasting charge recombination after the primary electron-transfer processes.
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      Closs, G. L.; Forbes, M. D. E.; Norris, J. R. Spin-Polarized Electron-Paramagnetic Resonance-Spectra of Radical Pairs in Micelles - Observation of Electron-Spin Spin Interactions. J. Phys. Chem. 1987, 91, 3592– 3599,  DOI: 10.1021/j100297a026
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      Spin-polarized electron paramagnetic resonance spectra of radical pairs in micelles: observation of electron spin-spin interactions
      Closs, Gerhard L.; Forbes, Malcolm D. E.; Norris, James R., Jr.
      Journal of Physical Chemistry (1987), 91 (13), 3592-9CODEN: JPCHAX; ISSN:0022-3654.
      A model for polarized EPR spectra generated in radical pair reactions in micelles is proposed. The model is based on electron spin-spin interactions which remain observable because of limited diffusion in micelles. The exptl. observable is a doubling of hyperfine transitions, split by the magnitude of the interaction. The polarization is generated by the nonadiabatic generation of the radical pair with a triplet or singlet precursor. The time evolution of the wave function leads to a non-Boltzmann distribution of the populations among the 4 energy levels. The theory is tested by comparison with expts., previously reported and repeated in this lab., obtained by laser flash photolysis of benzophenone in sodium dodecyl sulfate micelles. Simulations of the shape of the spectra and their time dependence agree with expt. The model is further supported by expts. in micelles modified by different salt concns. as well as different chain lengths of the micelle-forming mols.
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    27. 27
      Hore, P. J.; Hunter, D. A.; McKie, C. D.; Hoff, A. J. Electron Paramagnetic Resonance of Spin-Correlated Radical Pairs in Photosynthetic Reactions. Chem. Phys. Lett. 1987, 137, 495– 500,  DOI: 10.1016/0009-2614(87)80617-6
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      Electron paramagnetic resonance of spin-correlated radical pairs in photosynthetic reactions
      Hore, P. J.; Hunter, D. A.; McKie, C. D.; Hoff, A. J.
      Chemical Physics Letters (1987), 137 (6), 495-500CODEN: CHPLBC; ISSN:0009-2614.
      Previous attempts to interpret the time-resolved EPR spectra of photosynthetic bacteria have been based on the premise that electron spin polarization arises in the primary radical pair (P+I-) formed by photoinduced charge sepn. The obsd. spectrum is assumed to be the sum of the EPR spectra of P+ and X-, the radical produced from I- by electron transfer. Here it is argued that P+I- may be too short-lived to give rise to significant polarization and that the exptl. spectrum is consistent with the rapid formation of a spin-correlated secondary radical pair.
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      Avdievich, N. I.; Forbes, M. D. E. Dynamic Effects in Spin-Correlated Radical Pair Theory: J Modulation and a New Look at the Phenomenon of Alternating Line Widths in the EPR Spectra of Flexible Biradicals. J. Phys. Chem. 1995, 99, 9660– 9667,  DOI: 10.1021/j100024a004
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      Dynamic Effects in Spin-Correlated Radical Pair Theory: J Modulation and a New Look at the Phenomenon of Alternating Line Widths in the EPR Spectra of Flexible Biradicals
      Avdievich, Nikolai I.; Forbes, Malcolm D. E.
      Journal of Physical Chemistry (1995), 99 (24), 9660-7CODEN: JPCHAX; ISSN:0022-3654. (American Chemical Society)
      Expressions for contributions to T1 and T2 from modulation of the exchange interaction (J) in spin-correlated radical pairs (SCRPs) are derived and incorporated into SCRP theory for simulation of direct detection EPR spectra of alkane chain biradicals. The T2 term is consistent with that previously derived by Luckhurst to explain the temp.-dependent alternating line widths obsd. in EPR spectra of stable nitroxide biradicals. Alternating line-width patterns are obsd. for sym. bis(alkyl) biradicals at high temps., where the rapid modulation of J is caused by conformational jumping. However, in unsym. systems such as acyl-alkyl biradicals, J modulation manifests itself at low temps. as an increased broadening of the spectral lines with increasing distance of the transition from the center of the spectrum, with no alternation. Several exptl. examples are presented, simulated, and discussed. Contrary to a suggestion by Maeda et al. (1991), J modulation is shown to have only a minimal effect on T1 in SCRP theory for the case of biradicals undergoing rapid conformational interconversion. A connection is proposed between the magnitude of the relaxation matrix elements and the biradical conformational distribution and chain dynamics.
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      Weber, S.; Biskup, T.; Okafuji, A.; Marino, A. R.; Berthold, T.; Link, G.; Hitomi, K.; Getzoff, E. D.; Schleicher, E.; Norris, J. R. Origin of Light-Induced Spin-Correlated Radical Pairs in Cryptochrome. J. Phys. Chem. B 2010, 114, 14745– 14754,  DOI: 10.1021/jp103401u
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      Origin of Light-Induced Spin-Correlated Radical Pairs in Cryptochrome
      Weber, Stefan; Biskup, Till; Okafuji, Asako; Marino, Anthony R.; Berthold, Thomas; Link, Gerhard; Hitomi, Kenichi; Getzoff, Elizabeth D.; Schleicher, Erik; Norris, James R., Jr.
      Journal of Physical Chemistry B (2010), 114 (45), 14745-14754CODEN: JPCBFK; ISSN:1520-6106. (American Chemical Society)
      Blue-light excitation of cryptochromes and homologues uniformly triggers electron transfer (ET) from the protein surface to the FAD cofactor. A cascade of three conserved tryptophan residues has been considered to be critically involved in this photoreaction. If the FAD is initially in its fully oxidized (diamagnetic) redox state, light-induced ET via the tryptophan triad generates a series of short-lived spin-correlated radical pairs comprising an FAD radical and a tryptophan radical. Coupled doublet-pair species of this type have been proposed as the basis, for example, of a biol. magnetic compass in migratory birds, and were found crit. for some cryptochrome functions in vivo. In this contribution, a cryptochrome-like protein (CRYD) derived from Xenopus laevis has been examd. as a representative system. The terminal radical-pair state FAD····W324· of X. laevis CRYD has been characterized in detail by time-resolved electron-paramagnetic resonance (TREPR) at X-band microwave frequency (9.68 GHz) and magnetic fields around 345 mT, and at Q-band (34.08 GHz) at around 1215 mT. Different precursor states, singlet vs. triplet, of radical-pair formation have been considered in spectral simulations of the exptl. electron-spin polarized TREPR signals. Conclusively, we present evidence for a singlet-state precursor of FAD····W324· radical-pair generation because at both magnetic fields, where radical pairs were studied by TREPR, net-zero electron-spin polarization has been detected. Neither a spin-polarized triplet precursor nor a triplet at thermal equil. can explain such an electron-spin polarization. It turns out that a two-microwave-frequency TREPR approach is essential to draw conclusions on the nature of the precursor electronic states in light-induced spin-correlated radical pair formations.
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      Niklas, J.; Poluektov, O. G. Charge Transfer Processes in OPV Materials as Revealed by EPR Spectroscopy. Adv. Energy Mater. 2017, 7, 1602226  DOI: 10.1002/aenm.201602226
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      Uvarov, M. N.; Kulik, L. Electron Spin Echo of Photoinduced Spin-Correlated Polaron Pairs in P3HT:PCBM Composite. Appl. Magn. Reson. 2013, 97– 106,  DOI: 10.1007/s00723-012-0401-2
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      Electron Spin Echo of Photoinduced Spin-Correlated Polaron Pairs in P3HT:PCBM Composite
      Uvarov, Mikhail N.; Kulik, Leonid V.
      Applied Magnetic Resonance (2013), 44 (1-2), 97-106CODEN: APMREI; ISSN:0937-9347. (SpringerWienNewYork)
      A variation of the electron spin echo (ESE) signal caused by laser pulse in a blend of [6,6]-Ph C61 butyric acid Me ester and poly(3-hexylthiophene) (P3HT:PCBM) was detected. This variation was attributed to light-generated paramagnetic species in P3HT:PCBM blend, with non-equil. spin polarization. The echo-detected ESR (EPR) spectrum of these species closely resembles the time-resolved EPR spectrum of spin-correlated polaron pair PCBM-/P3HT+ (Behrends et al. in Phys. Rev. B 85:125206, 2012) and was assigned to this pair. The characteristic times for polarization and coherence decay (9 ± 1 and 1.0 ± 0.2 μs, resp.) were measured for the PCBM-/P3HT+ pair at 77 K. These times are long enough, which shows the possibility of the application of the ESE technique for studying spin evolution of light-generated charge transfer intermediates in composites of fullerenes and conductive polymers.
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      Behrends, J.; Sperlich, A.; Schnegg, A.; Biskup, T.; Teutloff, C.; Lips, K.; Dyakonov, V.; Bittl, R. Direct Detection of Photoinduced Charge Transfer Complexes in Polymer Fullerene Blends. Phys. Rev. B 2012, 85, 125206  DOI: 10.1103/PhysRevB.85.125206
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      Direct detection of photoinduced charge transfer complexes in polymer fullerene blends
      Behrends, Jan; Sperlich, Andreas; Schnegg, Alexander; Biskup, Till; Teutloff, Christian; Lips, Klaus; Dyakonov, Vladimir; Bittl, Robert
      Physical Review B: Condensed Matter and Materials Physics (2012), 85 (12), 125206/1-125206/6CODEN: PRBMDO; ISSN:1098-0121. (American Physical Society)
      We report transient ESR (trEPR) measurements with submicrosecond time resoln. performed on a polymer:fullerene blend consisting of poly(3-hexylthiophene) (P3HT) and [6,6]-Ph C61-butyric acid Me ester (PCBM) at low temps. The trEPR spectrum immediately following photoexcitation reveals signatures of spin-correlated polaron pairs. The pair partners (pos. polarons in P3HT and neg. polarons in PCBM) can be identified by their characteristic g values. The fact that the polaron pair states exhibit strong non-Boltzmann population unambiguously shows that the constituents of each pair are geminate, i.e., originate from one exciton. We demonstrate that coupled polaron pairs are present even several microseconds after charge transfer and suggest that they embody the intermediate charge transfer complexes that form at the donor/acceptor interface and mediate the conversion from excitons into free charge carriers.
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      Kraffert, F.; Behrends, J. Spin-Correlated Doublet Pairs as Intermediate States in Charge Separation Processes. Mol. Phys. 2017, 115, 2373– 2386,  DOI: 10.1080/00268976.2016.1278479
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      Spin-correlated doublet pairs as intermediate states in charge separation processes
      Kraffert, Felix; Behrends, Jan
      Molecular Physics (2017), 115 (19), 2373-2386CODEN: MOPHAM; ISSN:0026-8976. (Taylor & Francis Ltd.)
      Spin-correlated charge-carrier pairs play a crucial role as intermediate states in charge sepn. both in natural photosynthesis as well as in solar cells. Using transient ESR (trEPR) spectroscopy in combination with spectral simulations, we study spin-correlated polaron pairs in polymer:fullerene blends as org. solar cells materials. The semi-anal. simulations presented here are based on the well-established theor. description of spin-correlated radical pairs in biol. systems, however, explicitly considering the disordered nature of polymer:fullerene blends. The large degree of disorder leads to the fact that many different relative orientations between both polarons forming the spin-correlated pairs have to be taken into account. This has important implications for the spectra, which differ significantly from those of spin-correlated radical pairs with a fixed relative orientation. We systematically study the influence of exchange and dipolar couplings on the trEPR spectra and compare the simulation results to measured X- and Q-band trEPR spectra. Our results demonstrate that assuming dipolar couplings alone does not allow us to reproduce the exptl. spectra. Due to the rather delocalised nature of polarons in conjugated org. semiconductors, a significant isotropic exchange coupling needs to be included to achieve good agreement between expts. and simulations.
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      Kraffert, F.; Steyrleuthner, R.; Albrecht, S.; Neher, D.; Scharber, M. C.; Bittl, R.; Behrends, J. Charge Separation in PCPDTBT:PCBM Blends from an EPR Perspective. J. Phys. Chem. C 2014, 118, 28482– 28493,  DOI: 10.1021/jp509650v
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      Charge Separation in PCPDTBT:PCBM Blends from an EPR Perspective
      Kraffert, Felix; Steyrleuthner, Robert; Albrecht, Steve; Neher, Dieter; Scharber, Markus C.; Bittl, Robert; Behrends, Jan
      Journal of Physical Chemistry C (2014), 118 (49), 28482-28493CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
      Using time-resolved ESR (EPR) spectroscopy in conjunction with optical excitation we study charge sepn. in conjugated polymers blended with [6,6]-Ph C61-butyric acid Me ester (PCBM). A direct comparison between samples comprising poly[2,6-(4,4-bis(2-ethylhexyl)-4H-cyclopenta[2,1-b;3,4-b']-dithiophene)-alt-4,7-(2,1,3-benzothiadiazole)] (C-PCPDTBT) and their analogs contg. poly[(4,4'-bis(2-ethylhexyl)dithieno[3,2-b:2',3'-d]silole)-2,6-diyl-alt-(4,7-bis(2-thienyl)-2,1,3-benzothiadiazole)-5,5'-diyl] (Si-PCPDTBT) reveals a remarkable influence of the bridging atom (carbon vs silicon) in the polymer on the EPR spectra. While the EPR signatures of photogenerated pos. polarons in C- and Si-bridged PCPDTBT are virtually identical, significant differences are obsd. with respect to the spin-relaxation behavior. The spin-lattice relaxation time of pos. polarons in C-PCPDTBT at low temp. (T = 80 K) is found to be more than two orders or magnitude longer than in the Si-bridged polymer deriv. This surprisingly slow relaxation can be rationalized by polarons trapped in defect states that seem to be absent (or are present in a substantially smaller concn.) in blends comprising Si-PCPDTBT. Transient EPR signals attributed to charge transfer (CT) states and sepd. polarons are smaller in the blends with C-PCPDTBT as compared to those with the silicon-bridged polymer. We propose that triplet formation occurs via the CT state, thus diminishing the probability that the CT state forms free charge carriers in blends of C-PCPDTBT with PCBM. This hypothesis is confirmed by direct detection of triplet excitons in C-PCPDTBT:PCBM blends. The shape of the transient EPR spectra reveals that the triplet excitons are, in contrast to those formed in pristine polymer films, not generated by direct intersystem crossing but result from back electron transfer through CT state recombination. The strong triplet signal is not obsd. in blends contg. the Si-bridged polymer, indicating efficient singlet exciton splitting and subsequent charge carrier sepn. at the Si-PCPDTBT/PCBM interface.
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      Aguirre, A.; Gast, P.; Orlinskii, S.; Akimoto, I.; Groenen, E. J. J.; El Mkami, H.; Goovaerts, E.; Van Doorslaer, S. Multifrequency EPR analysis of the positive polaron in I-2-doped poly(3-hexylthiophene) and in poly[2-methoxy-5-(3,7-dimethyloctyloxy)]-1,4-phenylenevinylene. Phys. Chem. Chem. Phys. 2008, 10, 7129– 7138,  DOI: 10.1039/b811419f
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      Multifrequency EPR analysis of the positive polaron in I2-doped poly(3-hexylthiophene) and in poly[2-methoxy-5-(3,7-dimethyloctyloxy)]-1,4-phenylenevinylene
      Aguirre, Aranzazu; Gast, Peter; Orlinskii, Sergey; Akimoto, Ikuko; Groenen, Edgar J. J.; El Mkami, Hassane; Goovaerts, Etienne; Van Doorslaer, Sabine
      Physical Chemistry Chemical Physics (2008), 10 (47), 7129-7138CODEN: PPCPFQ; ISSN:1463-9076. (Royal Society of Chemistry)
      The W-band continuous-wave ESR (EPR) anal. of chem. induced polarons in drop-cast and spin-coated polyphenylenevinylene-type and polythiophene-type polymer films reveals rhombic g tensors in both cases. The dependence of the W-band EPR signals on the orientation of the spin-coated films with respect to the magnetic field indicates a high degree of backbone alignment with the substrate and allows a partial assignment of the g tensor orientation. The derived mol. orientations of the polymer chains in the spin-coated films show clear differences between the two types of polymers. The proton hyperfine interactions obtained from X-band HYSCORE (hyperfine sublevel correlation) and Q- and W-band pulsed ENDOR (electron-nuclear double resonance) expts. are interpreted in terms of earlier theor. studies on the extension of the polarons.
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      De Ceuster, J.; Goovaerts, E.; Bouwen, A.; Hummelen, J. C.; Dyakonov, V. High-Frequency (95 GHz) Electron Paramagnetic Resonance Study of the Photoinduced Charge Transfer in Conjugated Polymer-Fullerene Composites. Phys. Rev. B 2001, 64, 195206  DOI: 10.1103/PhysRevB.64.195206
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      High-frequency (95 GHz) electron paramagnetic resonance study of the photoinduced charge transfer in conjugated polymer-fullerene composites
      De Ceuster, J.; Goovaerts, E.; Bouwen, A.; Hummelen, J. C.; Dyakonov, V.
      Physical Review B: Condensed Matter and Materials Physics (2001), 64 (19), 195206/1-195206/6CODEN: PRBMDO; ISSN:0163-1829. (American Physical Society)
      Light-induced ESR (LEPR) measurements are reported in composites of poly(2-methoxy-5-(3-,7-dimethyloctyloxy)-1,4-phenylenevinylene) (MDMO-PPV) and [6,6]-phenyl-C61-butyric acid Me ester (PCBM), a sol. deriv. of C60. Under illumination of the sample, two paramagnetic species are formed due to photoinduced charge transfer between conjugated polymer and fullerene. One is the pos. polaron P+ on the polymer backbone and the other is the radical anion on the methanofullerene. Using high-frequency (95 GHz) LEPR it was possible to sep. these two contributions to the spectrum on the basis of their g factors, and moreover to resolve the g anisotropy for both radicals. The pos. polaron on the conjugated polymer chain possesses axial symmetry with g values g‖=2.0034(1) and g.perp.=2.0024(1). EPR on low doped polymer gave extra proof for the assignment to the pos. polaron. The neg. charged methanofullerene has a lower, rhombic symmetry with gx=2.0003(1), gy=2.0001(1), and gz=1.9982(1). Different spin-lattice relaxation of both species gives rise to a rapid passage effect for the pos. polaron spectrum.
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      Watanabe, S.-i.; Tanaka, H.; Kuroda, S.-i.; Toda, A.; Nagano, S.; Seki, T.; Kimoto, A.; Abe, J. Electron Spin Resonance Observation of Field-Induced Charge Carriers in Ultrathin-Film Transistors of Regioregular Poly(3-hexylthiophene) with Controlled In-Plane Chain Orientation. Appl. Phys. Lett. 2010, 96, 173302  DOI: 10.1063/1.3421538
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      Electron spin resonance observation of field-induced charge carriers in ultrathin-film transistors of regioregular poly(3-hexylthiophene) with controlled in-plane chain orientation
      Watanabe, Shun-ichiro; Tanaka, Hisaaki; Kuroda, Shin-ichi; Toda, Akio; Nagano, Shusaku; Seki, Takahiro; Kimoto, Atsushi; Abe, Jiro
      Applied Physics Letters (2010), 96 (17), 173302/1-173302/3CODEN: APPLAB; ISSN:0003-6951. (American Institute of Physics)
      Ultrathin-film polymeric transistors with controlled in-plane chain orientation were fabricated based on Langmuir-Blodgett technique by cospreading liq. crystal mol. with regioregular poly(3-hexylthiophene). The mobilities parallel to the chain direction reached 0.001-0.01 cm2/V s for 1-5 monolayer thick transistors. Mobility ratio was about 2 for the parallel and perpendicular directions. ESR signals of the field-induced polarons exhibited clear in-plane anisotropies due to unpaired π-electrons. The anisotropic ESR spectra together with the optical dichroism det. the detailed mol. orientations in the channel of such ultrathin transistors. (c) 2010 American Institute of Physics.
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      Kouki, F.; Spearman, P.; Valat, P.; Horowitz, G.; Garnier, F. Experimental Determination of Excitonic Levels in Alpha-Oligothiophenes. J. Chem. Phys. 2000, 113, 385– 391,  DOI: 10.1063/1.481804
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      Experimental determination of excitonic levels in α-oligothiophenes
      Kouki, Faycal; Spearman, Peter; Valat, Pierre; Horowitz, Gilles; Garnier, Francis
      Journal of Chemical Physics (2000), 113 (1), 385-391CODEN: JCPSA6; ISSN:0021-9606. (American Institute of Physics)
      The effects of intermol. interactions on the optical spectra of oligothiophenes are examd. Absorption spectra of isolated mols. are calcd. and exptl. recorded in a rigid host matrix whereby mols. are distributed randomly or organized unidirectionally. Absorption spectra of thin films in an ordered and disordered state are given. Ordered films have three principal spectral regions which are discussed in terms of classical exciton theory. Absorption spectra in transmission of single crystals of quinquethiophene and sexithiophene are analyzed. The lowest optically allowed transition in the crystal corresponds to the lowest Davydov component. It consists of a sharp peak that is obsd. for even-numbered oligothiophenes in b polarization, and absent for odd-numbered rings due to the perfect alignment of the transition dipole moment with the long mol. axis. The upper Davydov component is viewed in both thin film and single crystal spectra. In between the two principal Davydov components lies a broadband that is tentatively attributed to charge-transfer states or to a noninteracting mol. transition reminiscent of isolated matrix spectra. The Davydov splitting of the first optically allowed transition is detd. to be about 10 000 cm-1 and increases slightly with chain length. The Herzberg-Teller region in 6T reveals a dominant coupling mode of 340 cm-1. Vibronic structure in excitation and emission spectra shows peaks of 1460 cm-1 spacing.
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      Kobori, Y.; Ponomarenko, N.; Norris, J. R. Time-Resolved Electron Paramagnetic Resonance Study on Cofactor Geometries and Electronic Couplings after Primary Charge Separations in the Photosynthetic Reaction Center. J. Phys. Chem. C 2015, 119, 8078– 8088,  DOI: 10.1021/acs.jpcc.5b01294
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      Time-Resolved Electron Paramagnetic Resonance Study on Cofactor Geometries and Electronic Couplings after Primary Charge Separations in the Photosynthetic Reaction Center
      Kobori, Yasuhiro; Ponomarenko, Nina; Norris, James R.
      Journal of Physical Chemistry C (2015), 119 (15), 8078-8088CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
      To elucidate how cofactor geometries after photoinduced primary charge-sepns. influence electronic couplings (VCR) for primary charge-recombination (CR) processes in the photosynthetic reaction center, we have analyzed time-resolved ESR (TREPR) spectra both of the primary charge-sepd. (CS) state (P+•HA-•) and its charge-recombined triplet state (3P*) of the special pair in Rhodobacter sphaeroides R26. To det. the CS-state geometry, quantum mech. modeling has been performed on the spin polarization of the 3P* generated by the spin dynamics due to the anisotropies of the hyperfine and the spin-spin dipolar interactions in the primary CS state. From transverse magnetizations of the primary CS state, we have also detd. the VCR value leading to the singlet excited state (1P*) of the special pair. The above analyses have revealed that while the primary charge sepn. does not largely modulate the cofactor conformations, it leads to significant enhancement in the VCR of the singlet recombination processes with respect to the triplet CR. This enhanced coupling is demonstrated by the larger orbital overlap between 1P* and an electron-accepting orbital of chlorophyll (BA) than between 3P* and BA, caused by the charge-transfer electronic character of 1P* in which the electron is locally distributed at a side of the bacteriochlorophyll (PM) situated in close proximity to BA.
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      Till, U.; Klenina, I. B.; Proskuryakov, I. I.; Hoff, A. J.; Hore, P. J. Recombination Dynamics and EPR Spectra of the Primary Radical Pair in Bacterila Photosynthetic Reaction Centers with Blocked Electron Transfer to the Primary Acceptor. J. Phys. Chem. B 1997, 101, 10939– 10948,  DOI: 10.1021/jp970686q
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      Recombination dynamics and EPR spectra of the primary radical pair in bacterial photosynthetic reaction centers with blocked electron transfer to the primary acceptor
      Till, U.; Klenina, I. B.; Proskuryakov, I. I.; Hoff, A. J.; Hore, P. J.
      Journal of Physical Chemistry B (1997), 101 (50), 10939-10948CODEN: JPCBFK; ISSN:1089-5647. (American Chemical Society)
      Recent ESR expts. on the primary radical pair in bacterial reaction centers are interpreted in detail. The spin-correlated radical pair model is extended to include the time dependence of the EPR intensities, differential lifetime broadening of the basic four-line spectrum, and averaging over the inhomogeneous distributions of hyperfine interactions in the two radicals. Values of the singlet and triplet recombination rate consts. and the magnitude and sign of the exchange interaction of the radical pair are obtained from the shape and linewidth of the spin-polarized EPR spectra and the kinetics of formation of the triplet state of the primary electron donor. For Rhodobacter sphaeroides at 70 K, kS = (1.2 ± 0.3) × 107 s-1, kT = (5.7 - 8.8) × 108, -J = 8 - 6 G; and for Rhodopseudomonas viridis at 190 K, kS = (8 ± 2) × 107 s-1, kT = (6.5 - 10.0) × 108 s-1, -J = 17 - 16 G. The reliability of these values is discussed in the light of the energetic heterogeneity of the radical pair energies and of the sensitivity and selectivity of the EPR measurements toward the three parameters.
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      Nagamori, T.; Marumoto, K. Direct Observation of Hole Accumulation in Polymer Solar Cells During Device Operation using Light-Induced Electron Spin Resonance. Adv. Mater. 2013, 25, 2362– 2367,  DOI: 10.1002/adma.201204015
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      Direct Observation of Hole Accumulation in Polymer Solar Cells During Device Operation using Light-Induced Electron Spin Resonance
      Nagamori, Tatsuya; Marumoto, Kazuhiro
      Advanced Materials (Weinheim, Germany) (2013), 25 (16), 2362-2367CODEN: ADVMEW; ISSN:0935-9648. (Wiley-VCH Verlag GmbH & Co. KGaA)
      The hole accumulation during operation was studied of a heterojunction solar cell prepd. from P3HT, PEDOT-PSS, Al, and PCBM. The hole accumulation is obsd. in P3HT.
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      Kobori, Y.; Fuki, M. Protein–Ligand Structure and Electronic Coupling of Photoinduced Charge-Separated State: 9,10-Anthraquinone-1-sulfonate Bound to Human Serum Albumin. J. Am. Chem. Soc. 2011, 133, 16770– 16773,  DOI: 10.1021/ja206898j
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      Protein-Ligand Structure and Electronic Coupling of Photoinduced Charge-Separated State: 9,10-Anthraquinone-1-sulfonate Bound to Human Serum Albumin
      Kobori, Yasuhiro; Fuki, Masaaki
      Journal of the American Chemical Society (2011), 133 (42), 16770-16773CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
      To elucidate how the protein-ligand docking structure affects electronic interactions in the electron-transfer process, we have analyzed time-resolved ESR spectra of photoinduced charge-sepd. (CS) states generated by light excitation of 9,10-anthraquinone-1-sulfonate (AQ1S-) bound to human serum albumin at a hydrophobic drug-binding region. The spectra have been explained in terms of the triplet-triplet electron spin polarization transfer model to det. both the geometries and the exchange couplings of the CS states of AQ1S2-·-histidine-242 radical cation (H242+·) and AQ1S2-·-tryptophan-214 radical cation (W214+·). For the CS state of the former, it has been revealed that, due to the orthogonal relationship between the singly occupied MOs of AQ1S2-· and H242+·, the electronic coupling (5.4 cm-1) is very weak, contributing to the prevention of energy-wasting charge recombination, even at a contact edge-to-edge sepn.
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      Fuki, M.; Murai, H.; Tachikawa, T.; Kobori, Y. Time Resolved EPR Study on the Photoinduced Long-Range Charge-Separated State in Protein: Electron Tunneling Mediated by Arginine Residue in Human Serum Albumin. J. Phys. Chem. B 2016, 120, 4365– 4372,  DOI: 10.1021/acs.jpcb.6b01072
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      Time Resolved EPR Study on the Photoinduced Long-Range Charge-Separated State in Protein: Electron Tunneling Mediated by Arginine Residue in Human Serum Albumin
      Fuki, Masaaki; Murai, Hisao; Tachikawa, Takashi; Kobori, Yasuhiro
      Journal of Physical Chemistry B (2016), 120 (19), 4365-4372CODEN: JPCBFK; ISSN:1520-5207. (American Chemical Society)
      To elucidate how local mol. conformations play a role on electronic couplings for the long-range photoinduced charge-sepd. (CS) states in protein systems, we have analyzed time-resolved ESR (TREPR) spectra by polarized laser irradiations of 9,10-anthraquinone-1-sulfonate (AQ1S-) bound to human serum albumin (HSA). Analyses of the magnetophotoselection effects on the EPR spectra and a docking simulation clarified the mol. geometry and the electronic coupling of the long-range CS states of AQ1S•2--tryptophan214 radical cation (W214•+) sepd. by 1.2 nm. The ligand of AQ1S- has been demonstrated to be bound to the drug site I in HSA. Mol. conformations of the binding region were estd. by the docking simulations, indicating that an arginine218 (R218+) residue bound to AQ1S•2- mediates the long-range electron-transfer. The energetics of triad states of AQ1S•2--R218+-W214•+ and AQ1S--R218•-W214•+ have been computed on the basis of the d. functional MO calcns., providing the clear evidence for the long-range electronic couplings of the CS states in terms of the superexchange tunneling model through the arginine residue.
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      Ema, F.; Tanabe, M.; Saito, S.; Yoneda, T.; Sugisaki, K.; Tachikawa, T.; Akimoto, S.; Yamauchi, S.; Sato, K.; Osuka, A. Charge-Transfer Character Drives Möbius Antiaromaticity in the Excited Triplet State of Twisted [28]Hexaphyrin. J. Phys. Chem. Lett. 2018, 9, 2685– 2690,  DOI: 10.1021/acs.jpclett.8b00740
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      Charge-Transfer Character Drives M.ovrddot.obius Antiaromaticity in the Excited Triplet State of Twisted [28]Hexaphyrin
      Ema, Fumitoshi; Tanabe, Mana; Saito, Shohei; Yoneda, Tomoki; Sugisaki, Kenji; Tachikawa, Takashi; Akimoto, Seiji; Yamauchi, Seigo; Sato, Kazunobu; Osuka, Atsuhiro; Takui, Takeji; Kobori, Yasuhiro
      Journal of Physical Chemistry Letters (2018), 9 (10), 2685-2690CODEN: JPCLCD; ISSN:1948-7185. (American Chemical Society)
      Mobius arom. mols. have attracted great attention as new functional materials because of their π-orbital cyclic conjugations lying along the twisted Mobius topol. To elucidate the electronic character of the lowest excited triplet (T1) state of a Mobius arom. [28]hexaphyrin, we employed a time-resolved ESR (TREPR) method with applied magnetophotoselection measurements at 77 K. Analyses of the EPR parameters have revealed that the T1 state possesses intramol. charge-transfer (CT) character together with local excitation character residing at one side in the Mobius strip ring. We have also demonstrated that the CT character between orthogonal unpaired orbitals triggers quick triplet deactivation by spin-orbit coupling. This deactivation can be an important barometer to represent the "antiaromaticity" because of a connection between the orthogonal CT character and instability by a weakened spin-spin exchange coupling in the T1 state.
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      Borovykh, I. V.; Proskuryakov, I. I.; Klenina, I. B.; Gast, P.; Hoff, A. J. Magnetophotoselection Study of the Lowest Excited Triplet State of the Primary Donor in Photosynthetic Bacteria. J. Phys. Chem. B 2000, 104, 4222– 4228,  DOI: 10.1021/jp993780a
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      Magnetophotoselection Study of the Lowest Excited Triplet State of the Primary Donor in Photosynthetic Bacteria
      Borovykh, Igor V.; Proskuryakov, Ivan I.; Klenina, Irina B.; Gast, Peter; Hoff, Arnold J.
      Journal of Physical Chemistry B (2000), 104 (17), 4222-4228CODEN: JPCBFK; ISSN:1089-5647. (American Chemical Society)
      The first time-resolved magnetophotoselection study of the primary donor triplet state (3P) in bacterial reaction centers from Rhodobacter sphaeroides R26 and Rhodopseudomonas viridis is reported. With direct excitation of the primary donor, this approach provides the orientation (spherical coordinates δ, colatitude, and γ, longitude) of the excited optical transition moment in the principal triplet axis system. It is essentially free from the effects of spin-lattice relaxation of 3P, enabling magnetophotoselection measurements over a wide temp. range. Two independently measured triplet-state spectra, excited with light polarized parallel and perpendicular to the EPR magnetic field, are simulated with the same set of parameters. This procedure results in a high precision (∼±5° with sufficient signal/noise ratio) of the obtained spherical coordinates of the optical transition moment vector. We find δ = 80±5°, γ = 70±5° and δ = 75±5°, γ = 70±5° for Rb. sphaeroides R26 and Rps. viridis, resp. We demonstrate that excitation of the sample with nonpolarized light is essentially nonisotropic. Neglect of this effect in spectral simulations of light-induced signals may lead to considerable error in the parameters detd.
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      Regev, A.; Michaeli, S.; Levanon, H.; Cyr, M.; Sessler, J. L. Solvent Effect in Randomly and Partially Oriented Triplets of the Sapphyrin Dication: Optical and Fast EPR-Magnetophotoselection Measurements. J. Phys. Chem. 1991, 95, 9121– 9129,  DOI: 10.1021/j100176a019
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      Solvent effect in randomly and partially oriented triplets of the sapphyrin dication: optical and fast EPR-magnetophotoselection measurements
      Regev, Ayelet; Michaeli, Shalom; Levanon, Haim; Cyr, Mike; Sessler, Jonathan L.
      Journal of Physical Chemistry (1991), 95 (23), 9121-9CODEN: JPCHAX; ISSN:0022-3654.
      The photoexcited triplet state of the sapphyrin dication, H5.Sap2+, was investigated by optical absorption and fast EPR-magnetophotoselection (MPS) spectroscopies. Results indicate that H5.Sap2+ exists as a monomer in nonpolar isotropic solvents (chloroform and toluene) and nematic liq. crystals, while in a polar solvent, e.g., ethanol, the stable form exists as a face-to-face dimer via specific solvent-mediated interactions. Anal. of the MPS-triplet EPR and optical absorption results provides the following conclusions: (1) relation between the mol. and magnetic frames of refs.; (2) out-of-plane location of the optical transition moment for both the monomer, H5.Sap2+, and the dimer, (H5.Sap2+)2, suggesting a possible admixt. of (n,π*) and (π,π*) states; (3) in-plane triplet state alignment and planar mol. structure of the monomer; (i.v.) a neg. value of the zero-field splitting parameter, D, for the monomer and the dimer.
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      Kobori, Y.; Yamauchi, S.; Akiyama, K.; Tero-Kubota, S.; Imahori, H.; Fukuzumi, S.; Norris, J. R. Primary Charge-Recombination in an Artificial Photosynthetic Reaction Center. Proc. Natl. Acad. Sci. U.S.A. 2005, 102, 10017– 10022,  DOI: 10.1073/pnas.0504598102
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      Primary charge-recombination in an artificial photosynthetic reaction center
      Kobori, Yasuhiro; Yamauchi, Seigo; Akiyama, Kimio; Tero-Kubota, Shozo; Imahori, Hiroshi; Fukuzumi, Shunichi; Norris, James R., Jr.
      Proceedings of the National Academy of Sciences of the United States of America (2005), 102 (29), 10017-10022CODEN: PNASA6; ISSN:0027-8424. (National Academy of Sciences)
      Photoinduced primary charge-sepn. and charge-recombination are characterized by a combination of time-resolved optical and EPR measurements of a fullerene-porphyrin-linked triad that undergoes fast, stepwise charge-sepn. processes. The electronic coupling for the energy-wasting charge recombination is evaluated from the singlet-triplet electronic energy gap in the short-lived, primary charge-sepd. state. The electronic coupling is found to be smaller by ≈40% than that for the primary charge-sepn. This inhibition of the electronic interaction for the charge-recombination to excited triplet state largely results from a symmetry-broken electronic structure modulated by CI between 3(b1u,b3g) and 3(au, b3g) electronic states of the free-base porphyrin.
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      Norris, J. R.; Morris, A. L.; Thurnauer, M. C.; Tang, J. A General-Model of Electron-Spin Polarization Arizing From the Interactions within Radical Pairs. J. Chem. Phys. 1990, 92, 4239– 4249,  DOI: 10.1063/1.457782
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      A general model of electron spin polarization arising from the interactions within radical pairs
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      Journal of Chemical Physics (1990), 92 (7), 4239-49CODEN: JCPSA6; ISSN:0021-9606.
      A general description is given of electron spin polarization obsd. for interacting radical pairs. Unlike previous treatments, both chem. induced dynamic electron polarization and correlated radical pair polarization are included for ST0 mixing. A key feature of this model is that the members of the pair can remain interacting throughout the time sequence of electron spin polarization. Explanations of the time-dependent evolution of the electron spin polarization are presented using the d. matrix and its assocd. vector diagram. A new vector method is formulated for calcg. and visualizing chem. induced electron spin polarization. The approach is based on the vector, Δρ(t) which represents the displacement of the d. vector ρ(t) from its direction at the time of its "birth". In addn., these electron spin polarization phenomena are described in spectroscopic terms as well as in the usual math. manner.
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      Morris, A. L.; Snyder, S. W.; Zhang, Y. N.; Tang, J.; Thurnauer, M. C.; Dutton, P. L.; Robertson, D. E.; Gunner, M. R. Electron-Spin Polarization Model Applied to Sequential Electron-Transfer in Iron-Containing Photosynthetic Bacterial Reaction Centers with Different Quinones as Q(a). J. Phys. Chem. 1995, 99, 3854– 3866,  DOI: 10.1021/j100011a063
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      Electron Spin Polarization Model Applied to Sequential Electron Transfer in Iron-Containing Photosynthetic Bacterial Reaction Centers with Different Quinones as QA
      Morris, Andrea L.; Snyder, Seth W.; Zhang, Yuenian; Tang, Jau; Thurnauer, Marion C.; Dutton, P. Leslie; Robertson, Dan E.; Gunner, M. R.
      Journal of Physical Chemistry (1995), 99 (11), 3854-66CODEN: JPCHAX; ISSN:0022-3654. (American Chemical Society)
      Electron spin polarization develops on P+[QFe2+]- in iron-contg. photosynthetic bacterial reaction centers (RC) of Rhodobacter sphaeroides. The spin-polarized ESR spectra of the oxidized primary donor (P+) depend on τH-, the lifetime of the radical pair P+H- formed prior to P+[QFe2+]-. The polarized EPR signal can be described by the sequential electron transfer polarization (SETP) model in which the chem. induced dynamic electron polarization (CIDEP) developed in P+H- is projected onto the correlated radical pair polarization (CRPP) developed in P+[QFe2+]-. Replacing the native ubiquinone-10 with various anthraquinones and naphthoquinones alters both the free energy and rate of electron transfer from H- to QFe2+, which in turn modifies τH-. At long τH- the polarized P+ EPR signal is dominated by the CIDEP component of SETP. At short τH- the signal is dominated by the CRPP component, while at intermediate τH-'s the signal can only be described using the full SETP model. The ranges of τH- where polarization is dominated by interactions on the prior or obsd. radical pair are influenced by the EPR microwave frequency and RC isotopic compn. Exptl. spectra of spin-polarized P+ from a series of Rb. sphaeroides RCs having τH-'s ranging from 0.33 to 25 ns are modeled with SETP. The model accounts for differences in the polarization line shape with deuteration of the RCs or increase in the EPR microwave frequency.
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      Huang, Y.-C.; Tsao, C.-S.; Chuang, C.-M.; Lee, C.-H.; Hsu, F.-H.; Cha, H.-C.; Chen, C.-Y.; Lin, T.-H.; Su, C.-J.; Jeng, U. S. Small- and Wide-Angle X-ray Scattering Characterization of Bulk Heterojunction Polymer Solar Cells with Different Fullerene Derivatives. J. Phys. Chem. C 2012, 116, 10238– 10244,  DOI: 10.1021/jp210140j
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      Small- and Wide-Angle X-ray Scattering Characterization of Bulk Heterojunction Polymer Solar Cells with Different Fullerene Derivatives
      Huang, Yu-Ching; Tsao, Cheng-Si; Chuang, Chih-Min; Lee, Chia-Hsin; Hsu, Fan-Hsuan; Cha, Hou-Chin; Chen, Charn-Ying; Lin, Tsung-Han; Su, Chun-Jen; Jeng, U-Ser; Su, Wei-Fang
      Journal of Physical Chemistry C (2012), 116 (18), 10238-10244CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
      The aim of this study is to quant. investigate the effect of different fullerene type (PC60BM and PC70BM) on various morphol. structures and power conversion efficiency in the bulk heterojunction poly(3-hexylthiophene) (P3HT)/PCxBM solar cells. The solar cells are fabricated by spin coating without thermal annealing. The quant. investigations of three-dimensional self-organized nanostructures are performed by using combined grazing-incidence small- and wide-angle x-ray scattering technique (GISAXS/GIWAXS). Two types of nanostructures are obsd. due to the phase sepn. in the bulk heterojunction films during the processing. They include (a) intercalated PCxBM mols. around boundary of P3HT cryst. domain and within amorphous domain and (b) aggregated PCxBM clusters in PCxBM domains. The lamellar spacing of P3HT cryst. domains in P3HT/PC70BM is larger than that in P3HT/PC60BM. This result indicates that more interfacial areas are generated between PC70BM and P3HT at the mol. scale for more efficient charge sepn. On the other hand, the size, vol. fraction, partial attachment, and spatial distribution of PC60BM clusters are larger than that of PC70BM clusters, which reveals more efficient electron transport in P3HT/PC60BM. We deduce the correlation between nanostructures and power conversion efficiency (3.25% and 2.64% for P3HT/PC70BM and P3HT/PC60BM, resp.). The structure of fullerene intercalated with P3HT rather than the size of fullerene cluster plays a major role in the power conversion efficiency performance of bulk heterojunction solar cell without thermal annealing.
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      Tamai, Y.; Matsuura, Y.; Ohkita, H.; Benten, H.; Ito, S. One-Dimensional Singlet Exciton Diffusion in Poly(3-hexylthiophene) Crystalline Domains. J. Phys. Chem. Lett. 2014, 5, 399– 403,  DOI: 10.1021/jz402299a
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      One-Dimensional Singlet Exciton Diffusion in Poly(3-hexylthiophene) Crystalline Domains
      Tamai, Yasunari; Matsuura, Yuu; Ohkita, Hideo; Benten, Hiroaki; Ito, Shinzaburo
      Journal of Physical Chemistry Letters (2014), 5 (2), 399-403CODEN: JPCLCD; ISSN:1948-7185. (American Chemical Society)
      Singlet exciton dynamics in cryst. domains of regioregular poly(3-hexylthiophene) (P3HT) films was studied by transient absorption spectroscopy. Upon the selective excitation of cryst. P3HT at the absorption edge, no red shift of the singlet exciton band was obsd. with an elapse of time, suggesting singlet exciton dynamics in relatively homogeneous P3HT cryst. domains without downhill relaxation in the energetic disorder. Even under such selective excitation conditions, the annihilation rate coeff. γ-(t) was still dependent on time, γ-(t) .varies. t-1/2, which is attributed to anisotropic exciton diffusion in P3HT cryst. domains. From the annihilation rate coeff., the singlet exciton diffusion coeff. D and exciton diffusion length LD in the cryst. domains were evaluated to be 7.9 × 10-3 cm2 s-1 and 20 nm, resp. The origin of the time-dependent exciton dynamics is discussed in terms of dimensionality.
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      Lee, J.; Vandewal, K.; Yost, S. R.; Bahlke, M. E.; Goris, L.; Baldo, M. A.; Manca, J. V.; Van Voorhis, T. Charge Transfer State Versus Hot Exciton Dissociation in Polymer-Fullerene Blended Solar Cells. J. Am. Chem. Soc. 2010, 132, 11878– 11880,  DOI: 10.1021/ja1045742
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      Charge Transfer State Versus Hot Exciton Dissociation in Polymer-Fullerene Blended Solar Cells
      Lee, Jiye; Vandewal, Koen; Yost, Shane R.; Bahlke, Matthias E.; Goris, Ludwig; Baldo, Marc A.; Manca, Jean V.; Van Voorhis, Troy
      Journal of the American Chemical Society (2010), 132 (34), 11878-11880CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
      We examine the significance of hot exciton dissocn. in two archetypical polymer-fullerene blend solar cells. Rather than evolving through a bound charge transfer state, hot processes are proposed to convert excitons directly into free charges. But we find that the internal quantum yields of carrier photogeneration are similar for both excitons and direct excitation of charge transfer states. The internal quantum yield, together with the temp. dependence of the current-voltage characteristics, is consistent with negligible impact from hot exciton dissocn.
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      Fukuju, T.; Yashiro, H.; Maeda, K.; Murai, H.; Azumi, T. Singlet-Born SCRP Observed in the Photolysis of Tetraphenylhydrazine in an SDS Micelle: Time Dependence of the Population of the Spin States. J. Phys. Chem. A 1997, 101, 7783– 7786,  DOI: 10.1021/jp971620y
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      Singlet-Born SCRP Observed in the Photolysis of Tetraphenylhydrazine in an SDS Micelle: Time Dependence of the Population of the Spin States
      Fukuju, Tadahiro; Yashiro, Haruhiko; Maeda, Kiminori; Murai, Hisao; Azumi, Tohru
      Journal of Physical Chemistry A (1997), 101 (42), 7783-7786CODEN: JPCAFH; ISSN:1089-5639. (American Chemical Society)
      The spectrum of the singlet-born spin-correlated radical pair generated in the photolysis of tetraphenylhydrazine in a micelle is analyzed. The time evolution of the spectral pattern directly reflects the time dependence of the population of the individual spin states of the radical pair. The relaxation times between these states and the reaction rates of the geminate process are estd., and the relaxation of the middle two states of the radical pairs is discussed.
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      Miura, T.; Murai, H. Effect of Molecular Diffusion on the Spin Dynamics of a Micellized Radical Pair in Low Magnetic Fields Studied by Monte Carlo Simulation. J. Phys. Chem. A 2015, 119, 5534– 5544,  DOI: 10.1021/acs.jpca.5b02183
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      Effect of Molecular Diffusion on the Spin Dynamics of a Micellized Radical Pair in Low Magnetic Fields Studied by Monte Carlo Simulation
      Miura, Tomoaki; Murai, Hisao
      Journal of Physical Chemistry A (2015), 119 (22), 5534-5544CODEN: JPCAFH; ISSN:1089-5639. (American Chemical Society)
      Magnetic field effect is a powerful tool to study dynamics and kinetics of radical pairs (RPs), which are one of the most important intermediates for org. photon-energy conversion reactions. However, quant. discussion regarding the relationship between the modulation of interelectron interactions and spin dynamics at low magnetic fields (<10 mT) is still an open question. We have studied the spin dynamics of a long-lived RP in a micelle by newly developed Monte Carlo simulation, in which fluctuations of the exchange and magnetic dipolar interactions by in-cage diffusion are directly introduced to the time-domain spin dynamics calcn. State-dependent relaxation/dephasing times of a few to a few tens of nanoseconds are obtained by simulations without hyperfine interactions (HFIs) as a function of the mutual diffusion const. (∼10-6 cm2/s). Simulations with the HFIs exhibit incoherent singlet-triplet (S-T) mixings resulting from interplay between the HFIs and the fluctuating spin-spin interactions. The exptl. obsd. incoherent S-T mixing of ∼20 ns at 3 mT for a singlet-born RP in a sodium dodecyl sulfate micelle is reproduced by the simulation with reasonable diffusion coeffs. The computational method developed here contributes to quant. detection of mol. motion that governs the recombination efficiency of RPs.
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      Kayunkid, N.; Uttiya, S.; Brinkmann, M. Structural Model of Regioregular Poly(3-hexylthiophene) Obtained by Electron Diffraction Analysis. Macromolecules 2010, 43, 4961– 4967,  DOI: 10.1021/ma100551m
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      Structural Model of Regioregular Poly(3-hexylthiophene) Obtained by Electron Diffraction Analysis
      Kayunkid, Navaphun; Uttiya, Sureeporn; Brinkmann, Martin
      Macromolecules (Washington, DC, United States) (2010), 43 (11), 4961-4967CODEN: MAMOBX; ISSN:0024-9297. (American Chemical Society)
      This study presents a structural anal. of regioregular poly(3-hexylthiophene) (P3HT) based on electron diffraction from epitaxied thin films. Epitaxial orientation of the hexane fraction of P3HT was performed by slow rate directional solidification in 1,3,5-trichlorobenzene leading to highly oriented and cryst. P3HT films with different contact planes. Representative electron diffraction patterns corresponding to different zone axes were obtained by the rotation-tilt electron diffraction method. A trial-and-error method based on mol. modeling and calcn. of the electron diffraction patterns for the different zone axes was used to det. the crystal structure of P3HT. The unit cell is monoclinic with space group P21/c and two chains per cell (a = 1.60 nm, b = 0.78 nm, c = 0.78 nm and γ = 86.5 deg). The stacking period of successive polythiophene backbones along the b axis is 0.39 nm but short interplanar distances of 0.34 nm are obsd. because the conjugated polythiophene backbones are tilted to the b axis. The n-hexyl side groups crystallize in an orthogonal subcell with parameters as = 0.7 nm and bs = 0.78 nm. The present structural model highlights the essential role of the linear side chain crystn. on the supra-macromol. packing of "hairy-rod" polymers like P3HT.
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      Kobori, Y.; Sekiguchi, S.; Akiyama, K.; Tero-Kubota, S. Chemically Induced Dynamic Electron Polarization Study on the Mechanism of Exchange Interaction in Radical Ion Pairs Generated by Photoinduced Electron Transfer Reactions. J. Phys. Chem. A 1999, 103, 5416– 5424,  DOI: 10.1021/jp990359d
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      Chemically induced dynamic electron polarization study on the mechanism of exchange interaction in radical ion pairs generated by photoinduced electron transfer reactions
      Kobori, Yasuhiro; Sekiguchi, Shinji; Akiyama, Kimio; Tero-Kubota, Shozo
      Journal of Physical Chemistry A (1999), 103 (28), 5416-5424CODEN: JPCAFH; ISSN:1089-5639. (American Chemical Society)
      Photoinduced electron transfer reactions were studied by using the continuous wave time-resolved ESR and Fourier-transformed ESR spectroscopy in polar solvents. The chem. induced dynamic electron polarization was investigated in both singlet and triplet precursor intermol. electron transfer systems. The signs of the exchange interaction, which are defined by the energy differences between the singlet and triplet radical ion pairs, depended on the free energy changes for the charge recombination processes. The results are interpreted in terms of the mechanism that the spin selective stabilization and destabilization are caused by the perturbation through the electronic coupling from the ground state and the locally excited triplet state of the donor-acceptor pair at the equil. nuclear coordinate. In the singlet precursor electron transfer systems, the pos. exchange interaction resulted from the selective triplet stabilization in the radical ion pair, when the ion pair state crossed with the locally excited triplet state at the normal region. In the triplet precursor electron transfer systems, the neg. exchange interaction resulted from the selective singlet stabilization when the ion pair state crossed with the singlet ground state at the normal region. When the free energy change is larger than about 1.8 eV, the pos. exchange interaction resulted from the spin-selective destabilization in the singlet ion pair, since the level crossing occurs at the inverted region.
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      Mcconnell, H. Intramolecular Charge Transfer in Aromatic Free Radicals. J. Chem. Phys. 1961, 35, 508– 515,  DOI: 10.1063/1.1731961
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      Intramolecular charge transfer in aromatic free radicals
      McConnell, Harden M.
      Journal of Chemical Physics (1961), 35 (), 508-15CODEN: JCPSA6; ISSN:0021-9606.
      A theoretical analysis was made of the rate of intramol. transfer of the odd electron between 2 Ph groups in the mononeg. ions of α,ω-diphenylalkanes. The essential features of the calcns. were given. (a) The polymethylene chain could be replaced by a pseudopotential corresponding to an effective direct transfer between the rings. (b) There was a strong tendency for self-trapping of the odd electron on one Ph ring or the other, due to solvent polarization and bond distortions in the rings. The self-trapping greatly reduced the rate of intramol. charge transfer. (c) The intramol. charge transfer occurred as an electronic resonance effect when a short-lived thermally activated mol. state was formed in which the 2 rings appeared to the odd electron to be equiv. The activation energy was estd. to be of the order of 1000 cm.-1 (d) The rate of intramol. charge transfer decreased exponentially with the length of the polymethylene chain, the decrease being as much or more than a factor of 10 for each added CH2 group.
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      Goldsmith, R. H.; Sinks, L. E.; Kelley, R. F.; Betzen, L. J.; Liu, W. H.; Weiss, E. A.; Ratner, M. A.; Wasielewski, M. R. Wire-Like Charge Transport at Near Constant Bridge Energy through Fluorene Oligomers. Proc. Natl. Acad. Sci. U.S.A. 2005, 102, 3540– 3545,  DOI: 10.1073/pnas.0408940102
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      Wire-like charge transport at near constant bridge energy through fluorene oligomers
      Goldsmith, Randall H.; Sinks, Louise E.; Kelley, Richard F.; Betzen, Laura J.; Liu, Wenhao; Weiss, Emily A.; Ratner, Mark A.; Wasielewski, Michael R.
      Proceedings of the National Academy of Sciences of the United States of America (2005), 102 (10), 3540-3545CODEN: PNASA6; ISSN:0027-8424. (National Academy of Sciences)
      The study of photoinitiated electron transfer in donor-bridge-acceptor mols. has helped elucidate the distance dependence of electron transfer rates and behavior of various electron transfer mechanisms. In all reported cases, the energies of the bridge electronic states involved in the electron transfer change dramatically as the length of the bridge is varied. The authors report here, in contrast, an instance in which the length of the bridge, and therefore the distance over which the electron is transferred, can be varied without significantly changing the energies of the relevant bridge states. A series of donor-bridge-acceptor mols. having phenothiazine (PTZ) donors, 2,7-oligofluorene (FLn) bridges, and perylene-3,4:9,10-bis(dicarboximide) (PDI) acceptors was studied. Photoexcitation of PDI to its lowest excited singlet state results in oxidn. of PTZ via the FLn bridge. In toluene, the rate consts. for both charge sepn. and recombination as well as the energy levels of the relevant FLn+• bridge states for n = 1-4 are only weakly distance dependent. After the initial photogeneration of 1(PTZ+•-FLn-PDI-•), radical pair intersystem crossing results in formation of 3(PTZ+•-FLn-PDI-•) that recombines to give 3*PDI. The dependence of the 3*PDI yield on an applied magnetic field shows a resonance, which gives the singlet-triplet splitting, 2J, of the radical ion pair. The magnitude of 2J directly monitors the contribution of coherent charge transfer (superexchange) to the overall electron transfer rate. These data show that charge recombination through FLn is dominated by incoherent hopping at long distances.
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      Shoji, R.; Omori, T.; Wakikawa, Y.; Miura, T.; Ikoma, T. Magnetoconductance Study on Nongeminate Recombination in Solar Cell Using Poly(3-hexylthiophene) and [6,6]-Phenyl-C-61-butyric Acid Methyl Ester. ACS Omega 2018, 3, 9369– 9377,  DOI: 10.1021/acsomega.8b01746
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      59
      Magnetoconductance Study on Nongeminate Recombination in Solar Cell Using Poly(3-hexylthiophene) and [6,6]-Phenyl-C61-butyric Acid Methyl Ester
      Shoji, Ryota; Omori, Takuya; Wakikawa, Yusuke; Miura, Tomoaki; Ikoma, Tadaaki
      ACS Omega (2018), 3 (8), 9369-9377CODEN: ACSODF; ISSN:2470-1343. (American Chemical Society)
      The magnetoconductance (MC) effect was investigated for two types of org. solar cells with single junction (SJ) and bulk junction (BJ) of poly(3-hexylthiophene) (P3HT) as donor (D) and [6,6]-phenyl-C61-butyric acid Me ester (PCBM) as acceptor (A). Three components with different half-field-at-half-max. (B1/2) of 4±1, 20±15 and >400 mT, hereafter referred to as MCs,m,b in a sequence, were obsd. in the magnetic field dependence of the MC effects measured under dark and light conditions. The magnitude of the MCs,m,b components is sensitive to not only the junction structure of the cell, but also the presence or absence of incident light. The bias voltage (V) dependence of the MC effect in the dark for the SJ-cell is maximized around the turn-on voltage (Von) of the dark current, where a flat band condition of the active layer is achieved. The B1/2 for the MCm component of the SJ-cell increases with V beyond Von. In light, the BJ-cell exhibits the MC effect, whereas no effect is detected for the SJ-cell. The MCs,m components for the BJ-cell in light increase with the incident light power. The transient MCs,m components for the BJ-cell measured using a nanosecond pulse laser increases with the delay time after the flash. By integrating these phenomena and the phase of the MC effect, it is concluded that all the MC components arise from the magnetic field effect on the spin conversion of nongeminate electron (e)-hole (h) pairs with spin-dependent charge recombinations at the D/A-interface. The B1/2 values for MCs,m,b are resp. understood by the spin conversion due to the hyperfine interaction, the spin relaxation and the g-factor difference for e (PCBM-) and h (P3HT+). Kinetic simulations of the MCs,m components for the BJ-cell obsd. at the short-circuit condition in light yield an efficiency of ca. 40% for the nongeminate recombination, which is accompanied by the generation of triplet excitons as well as relaxation to a ground singlet state. The loss mechanism of moderate triplet recombination suggests an important possibility to improve the power conversion efficiency by harvesting of the triplet excitons.
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      Monkman, A. P.; Burrows, H. D.; Hartwell, L. J.; Horsburgh, L. E.; Hamblett, I.; Navaratnam, S. Triplet Energies of pi-Conjugated Polymers. Phys. Rev. Lett. 2001, 86, 1358– 1361,  DOI: 10.1103/PhysRevLett.86.1358
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      Triplet Energies of π-Conjugated Polymers
      Monkman, A. P.; Burrows, H. D.; Hartwell, L. J.; Horsburgh, L. E.; Hamblett, I.; Navaratnam, S.
      Physical Review Letters (2001), 86 (7), 1358-1361CODEN: PRLTAO; ISSN:0031-9007. (American Physical Society)
      Using pulse radiolysis and triplet energy transfer has enabled us to measure the triplet energies in a broad range of different π-conjugated polymers. In all cases we find that the 1 3 Bu is of order 0.6 to 1 eV below the 1 1 Bu, indicative of localized triplet states with strong electron-electron correlation. We also observe that the 1 1 Ag-1 3Bu gap decreases linearly as the 1 1 Ag-1 1Bu gap decreases even though polymers with very different structure have been studied. This surprising result suggests that polymers with singlet gap <1.3 eV will have a triplet ground state.
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      Li, B.; Yu, H.; Montoto, E. C.; Liu, Y.; Li, S.; Schwieter, K.; Rodríguez-López, J.; Moore, J. S.; Schroeder, C. M. Intrachain Charge Transport through Conjugated Donor–Acceptor Oligomers. ACS Appl. Electron. Mater. 2019, 1, 7– 12,  DOI: 10.1021/acsaelm.8b00050
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      Intrachain Charge Transport through Conjugated Donor-Acceptor Oligomers
      Li, Bo; Yu, Hao; Montoto, Elena C.; Liu, Yun; Li, Songsong; Schwieter, Kenneth; Rodriguez-Lopez, Joaquin; Moore, Jeffrey S.; Schroeder, Charles M.
      ACS Applied Electronic Materials (2019), 1 (1), 7-12CODEN: AAEMBP; ISSN:2637-6113. (American Chemical Society)
      Donor-acceptor (D-A) polymers are promising materials for org. electronics because of high charge carrier mobilities and narrow band gaps. Despite recent progress, there is incomplete understanding of the mechanisms underlying charge transport in these materials. In this work, we use single mol. techniques to study intrachain charge transport in D-A oligomers contg. alternating diketopyrrolopyrrole (DPP) acceptor and bithiophene donor units. Interestingly, at high applied bias, longer DPPTT oligomers exhibit substantially higher conductance compared to shorter oligomers, which is interpreted using d. functional theory (DFT) simulations. Overall, this work provides an increased understanding of intrachain charge transport along D-A oligomers.
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      Kilina, S.; Kilin, D.; Tretiak, S. Light-Driven and Phonon-Assisted Dynamics in Organic and Semiconductor Nanostructures. Chem. Rev. 2015, 115, 5929– 5978,  DOI: 10.1021/acs.chemrev.5b00012
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      Light-Driven and Phonon-Assisted Dynamics in Organic and Semiconductor Nanostructures
      Kilina, Svetlana; Kilin, Dmitri; Tretiak, Sergei
      Chemical Reviews (Washington, DC, United States) (2015), 115 (12), 5929-5978CODEN: CHREAY; ISSN:0009-2665. (American Chemical Society)
      A review. This Review targets the goal of helping exptl. scientists and computational chemists to synergistically address open challenges in the field of applied nanoscience. Computational modeling cannot provide universally exhaustive and accurate information for all cases involving large mol. systems by acting as a black box tool. Instead, specific model chem. has to be applied to a well-defined and focused problem with a deep understanding of manifold of approxns. that were used to reach a desired compromise between accuracy and numerical cost. This frequently provides only narrow-range qual. descriptions of a targeted process in extended nanosystems. While direct comparison of computational results with their exptl. counterparts constitutes an important task of benchmarking and validation of specific theor. technique(e.g., DFT model), it is usually only feasible only for simple well-defined systems. The authors have outlined multiple examples demonstrating to the reader that the main benefit of reported computational approaches for realistic complex structures is recognition of phys. mechanisms, structure-property relationships, and trends in observables. In this way, quantum chem. simulations have potentials to get insight into the most important processes taking place at atomistic and mol. levels of realistic materials and bring applied and fundamental research closer together.
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      Luo, J.; Piette, B. M. A. G. Directed Polaron Propagation in Linear Polypeptides Induced by Intramolecular Vibrations and External Electric Pulses. Phys. Rev. E 2018, 98, 012401  DOI: 10.1103/PhysRevE.98.012401
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      Directed polaron propagation in linear polypeptides induced by intramolecular vibrations and external electric pulses
      Luo, J.; Piette, B. M. A. G.
      Physical Review E (2018), 98 (1), 012401CODEN: PREHBM; ISSN:2470-0053. (American Physical Society)
      We study the propagation of α-helix polarons in a model describing the nonadiabatic interaction between an electron and a lattice of quantum mech. oscillators at physiol. temp. We show that when excited by a subpicosecond elec. pulse, as induced by exptl. obsd. subpicosecond charge sepn., the polaron is displaced by up to hundreds of lattice sites before the electron becomes delocalised. We discuss biophys. implications of our results.
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      Vukmirović, N.; Wang, L. W. Carrier Hopping in Disordered Semiconducting Polymers: How Accurate is the Miller–Abrahams Model?. Appl. Phys. Lett. 2010, 97, 043305  DOI: 10.1063/1.3474618
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      Carrier hopping in disordered semiconducting polymers. How accurate is the Miller-Abrahams model?
      Vukmirovic, Nenad; Wang, Lin-Wang
      Applied Physics Letters (2010), 97 (4), 043305/1-043305/3CODEN: APPLAB; ISSN:0003-6951. (American Institute of Physics)
      We performed direct calcns. of carrier hopping rates in strongly disordered conjugated polymers based on the at. structure of the system, the corresponding electronic states and their coupling to all phonon modes. The dependence of hopping rates on distance and the dependence of the mobility on temp. are significantly different than the ones stemming from the simple Miller-Abrahams model, regardless of the choice of the parameters in the model. A model that satisfactorily describes the hopping rates in the system and avoids the explicit calcn. of electron-phonon coupling consts. was then proposed and verified. In addn. to electronic d. of states, the phonon d. of states and the spatial overlap of the wave functions are the quantities necessary to properly describe carrier hopping in disordered conjugated polymers. (c) 2010 American Institute of Physics.
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      Song, Y.; Clafton, S. N.; Pensack, R. D.; Kee, T. W.; Scholes, G. D. Vibrational Coherence Probes the Mechanism of Ultrafast Electron Transfer in Polymer-Fullerene Blends. Nat. Commun. 2014, 5, 4933  DOI: 10.1038/ncomms5933
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      Vibrational coherence probes the mechanism of ultrafast electron transfer in polymer-fullerene blends
      Song, Yin; Clafton, Scott N.; Pensack, Ryan D.; Kee, Tak W.; Scholes, Gregory D.
      Nature Communications (2014), 5 (), 4933pp.CODEN: NCAOBW; ISSN:2041-1723. (Nature Publishing Group)
      The conversion of photoexcitations into charge carriers in org. solar cells is facilitated by the dissocn. of excitons at the donor/acceptor interface. The ultrafast timescale of charge sepn. demands sophisticated theor. models and raises questions about the role of coherence in the charge-transfer mechanism. Here, we applied two-dimensional electronic spectroscopy to study the electron transfer process in poly(3-hexylthiophene)/PCBM (P3HT/PCBM) blends. We reported dynamics maps showing the pathways of charge transfer that clearly expose the significance of hot electron transfer. During this ultrafast electron transfer, vibrational coherence was directly transferred from the P3HT exciton to the P3HT hole polaron in the cryst. domain. This result revealed that the exciton converts to a hole with a similar spatial extent on a timescale far exceeding other photophys. dynamics including vibrational relaxation.
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      Nagasawa, Y.; Yartsev, A. P.; Tominaga, K.; Bisht, P. B.; Johnson, A. E.; Yoshihara, K. Dynamical Aspects of Ultrafast Intermolecular Electron-Transfer Faster Than Solvation Process - Substituent Effects and Energy-Gap Dependence. J. Phys. Chem. 1995, 99, 653– 662,  DOI: 10.1021/j100002a033
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      66
      Dynamic Aspects of Ultrafast Intermolecular Electron Transfer Faster Than Solvation Process: Substituent Effects and Energy Gap Dependence
      Nagasawa, Yutaka; Yartsev, Arkadiy P.; Tominaga, Keisuke; Bisht, Prem B.; Johnson, Alan E.; Yoshihara, Keitaro
      Journal of Physical Chemistry (1995), 99 (2), 653-62CODEN: JPCHAX; ISSN:0022-3654. (American Chemical Society)
      We have investigated intermol. electron transfer (ET) from electron-donating solvents (aniline and N,N-dimethylaniline) to coumarins in the excited state by means of the femtosecond fluorescence up-conversion technique. The coumarins we studied have a variety of structures with different substituents in the 4- and 7-positions. The ET occurs on a time scale ranging from a few nanoseconds to a couple of hundred femtoseconds depending on the structure of the coumarins and solvent. As for the 7-position, as the length of the alkyl chain on the amino group is longer, the ET is slower, and when the amino group is fixed by a double-hexagonal ring, it is slowest. When the electron-accepting ability of the substituent in the 4-position is increased, the reaction occurs faster. The origin of this substituent effect is mainly attributed to the variation of the energy gap between the reactant and product states. This is confirmed by theor. calcns. in terms of the extended Sumi-Marcus two-dimensional model. A good agreement between the expt. and calcn. indicates that some of the reactions take place from the relaxed vibrational state of reactant to the excited vibrational states of high-frequency modes of product states. The simulated population decays for nonequil. configuration of solvents agreed well with exptl. data. In the steady-state fluorescence spectra was also obsd. an effect of very fast fluorescence quenching due to ET; i.e., the amt. of fluorescence Stokes shift depends on the rate of ET because the excited state is quenched in competition with thermal equilibration of the solvent configuration. We regard this spectral shift as the result of the "chem. timing" effect in soln.
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