CHEMPHYSCHEM ARTICLES DOI: 10.1002/cphc.201402411

Mechanism of Back Electron Transfer in an Intermolecular Photoinduced Electron Transfer Reaction: Solvent as a Charge Mediator Sudhakar Narra, Yoshifumi Nishimura, Henryk A. Witek, and Shinsuke Shigeto*[a] Back electron transfer (BET) is one of the important processes that govern the decay of generated ion pairs in intermolecular photoinduced electron transfer reactions. Unfortunately, a detailed mechanism of BET reactions remains largely unknown in spite of their importance for the development of molecular photovoltaic structures. Here, we examine the BET reaction of pyrene (Py) and 1,4-dicyanobenzene (DCB) in acetonitrile (ACN) by using time-resolved near- and mid-IR spectroscopy. The Py

dimer radical cation (Py2C + ) and DCB radical anion (DCBC) generated after photoexcitation of Py show asynchronous decay kinetics. To account for this observation, we propose a reaction mechanism that involves electron transfer from DCBC to the solvent and charge recombination between the resulting ACN dimer anion and Py2C + . The unique role of ACN as a charge mediator revealed herein could have implications for strategies that retard charge recombination in dye-sensitized solar cells.

1. Introduction Intermolecular photoinduced electron transfer (PET), one of the most fundamental processes that are ubiquitous in photocatalytic and photosynthetic reactions, is a classical subject of photochemistry and photobiology.[1] Recent revolutionary developments in molecular photovoltaics driven by increasing demand for renewable energy has revived considerable interest in PET. In a PET reaction, forward electron transfer (FET) leads to charge formation, whereas back electron transfer (BET) leads to the decay of charges through, for example, recombination reactions between the induced ion pairs. The latter process is strongly favored thermodynamically. Preventing or impeding the charge recombination seems to be an important prerequisite for enhancing the performance of dye-sensitized solar cells (DSSCs)[2] and organic bulk heterojunction solar cells.[3] Although a variety of strategies have been developed and demonstrated, such as tuning the HOMO–LUMO band gap between the donor and the semiconductor material and changing orbital symmetry in donor–bridge–acceptor systems,[4] efficient retardation of charge recombination is still a challenging goal in solar-cell research. The major obstacle to this goal arises from the fact that BET is very often not a simple recombination of primary charge carriers but a multistep photochemical reaction that also involves other molecular species in the system. A detailed understanding of the mechanism of BET is thus of paramount importance for overcoming the conundrum of fast charge recombination [a] S. Narra, Dr. Y. Nishimura, Prof. Dr. H. A. Witek, Prof. Dr. S. Shigeto Department of Applied Chemistry and Institute of Molecular Science National Chiao Tung University 1001 Ta-Hsueh Road, Hsinchu 30010 (Taiwan) E-mail: [email protected] Supporting Information for this article is available on the WWW under http://dx.doi.org/10.1002/cphc.201402411.

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in all photovoltaic devices. Spectroscopic approaches provide a direct means to look at PET reactions and have proven powerful for such mechanistic studies. Much work with transient absorption spectroscopy has focused on ultrafast FET reactions.[5] In contrast, BET reactions, which proceed on a much slower timescale (typically on the ns–ms timescale), are yet to be more thoroughly investigated, particularly with regard to their reaction mechanisms. Herein, we used time-resolved near- and mid-IR spectroscopy[6] (see the Experimental Section for details) to unveil the mechanism of the BET dynamics in the intermolecular PET reaction between pyrene (Py) and 1,4-dicyanobenzene (DCB) dissolved in acetonitrile (ACN). Our method surpasses conventional transient absorption spectroscopy by being able to detect vibrational transitions in the mid-IR region and specific types of electronic transition (e.g., charge-transfer transitions) that appear in the near-IR region, which makes it feasible to identify and distinguish between coexisting molecular species. Py and DCB form a fundamental intermolecular PET system[7] suited for the mechanistic study of BET. Upon photoexcitation of Py in the presence of DCB in polar solvent (see Figure S1 in the Supporting Information for the UV/Vis spectra of Py and DCB in ACN), Py radical cation (PyC + ) and DCB radical anion (DCBC) are produced through intermolecular electron transfer from the first excited singlet state of Py. The efficiency of this FET reaction is expected to be considerable owing to the same LUMO symmetry of Py and DCB.[8] The generated PyC + reacts with ground-state Py to form a Py dimer radical cation (Py2C + ) in a diffusion-controlled manner within several ms.[9] The focus of this paper is on the subsequent slower dynamics of the free ions (Py2C + and DCBC) during which an electron is back-transferred and the ions are brought to the neutral state. This process in the Py–DCB system may be viewed as mirroring the dye regeneration by the iodide/triiodide redox couple in stanChemPhysChem 2014, 15, 2945 – 2950

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Figure 1. Left: Time-resolved IR spectra in regions a) 3800–12000 cm1, b) 2000–2200 cm1, and c) 1120–1260 cm1 for Py and DCB dissolved in ACN (0.50 and 5.0 mm, respectively) excited at l = 355 nm with bubbled argon. The spectra in a) are masked at around 4400 cm1 due to interference from an overtone band of the solvent. Each time-resolved spectrum is offset by 4  105 for clarity of display. Right: Time profiles at d) 11000, e) 6800, f) 2100, g) 1216, and h) 1136 cm1. The smooth black curves are the result of the kinetic analysis based on the reaction mechanism shown in Scheme 1.

dard DSSCs.[10] The analogy here is not rigorous in the sense that dye regeneration in DSSCs is a part of sequential forward transfer reactions and not really back electron transfer. Nevertheless, as will be shown below, the BET in the Py–DCB system seems to resemble closely the process of the dye regaining an electron from anionic species in DSSC solvent.

2. Results and Discussion Time-resolved near- and mid-IR spectra of Py and DCB in ACN excited at l = 355 nm (Figure 1a–c) reveal four prominent transient absorption bands with maxima at 6800, 2100, 1216, and 1136 cm1 and a very broad feature that extends from ~ 9000 cm1 toward the visible region. The temporal behavior of the five transient features are displayed in Figure 1d–h. All of these time profiles have an instantaneous rise, although the transient at 11000 cm1 (Figure 1d) additionally exhibits a somewhat slower rise. The decay kinetics of the transients at 11000, 6800, 1216, and 1136 cm1 (Figure 1d, e, g, h) look similar but, interestingly, they are quite different from that of the transient at 2100 cm1 (Figure 1f). Because the FET reaction is complete within the time resolution of the apparatus (  80 ns),[7d, 11] the transient bands we observed must be predominantly due to either Py2C + or DCBC . It is well established that Py2C + shows a charge-resonance electronic band at around 6700 cm1 in polar solvent,[9a, 12] so the near-IR band at 6800 cm1 can be unambiguously assigned to Py2C + . The two mid-IR bands at 1216 and 1136 cm1 are vibrational bands that also originate from Py2C + because they appear to decay synchronously with the charge-resonance band of Py2C + . To assign these vibrational bands, we performed DFT calculations for gas-phase Py and PyC + monomers (see the  2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Experimental Section for computational details). Unfortunately, the dimer cation Py2C + proved to be too complex a system for DFT, which could not describe realistically either of the features responsible for bonding in Py2C + , that is, dispersion forces that glue the monomers together and the multireference character of its wave function responsible for the charge delocalization between both monomers. Note that the intramolecular portion of the vibrational spectra of Py2C + and PyC + is expected to display close structural similarity with possible minor spectral shifts of some of the features associated with the difference in effective charge (+ 1/2 vs. + 1) in each monomer. The FTIR spectrum of Py in ACN (Figure 2a) shows three prominent bands at 1244, 1186, and 1096 cm1, which are well reproduced in the calculated IR spectrum (Figure 2b) at 1235, 1182, and 1088 cm1, respectively. For the transient species, the agreement is less spectacular, but still the observed transient mid-IR spectrum at 0–5 ms (Figure 2c; same as the top trace of Figure 1c) shows close spectral resemblance to the calculated IR spectrum of PyC + (Figure 2d). Our calculation reveals three IR bands at 1234, 1201, and 1096 cm1 (Figure 2d), of which the 1234 and 1096 cm1 bands are likely to correspond to the two transient bands at 1216 and 1136 cm1 (Figure 2c). These bands are assigned to CH in-plane bending and in-plane ring deformation, respectively. Note that the deviations of 18 and 40 cm1 between the experimental and computed frequencies are within the typical error margin anticipated for DFT/B3LYP calculations.[13] The calculated band at 1201 cm1 appears to be missing in the experimental spectrum, and this could be because we have simulated the monomer spectrum and not the dimer spectrum. Although we have utilized the calculated result of PyC + solely for assignment purposes, its contribution to the mid-IR ChemPhysChem 2014, 15, 2945 – 2950

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Figure 3. Time-resolved near- and mid-IR spectra of Py (0.50 mm) and DCB (5.0 mm) dissolved in ACN, measured at 0–5 ms with bubbled argon (top) and oxygen (bottom). The spectrum taken with argon bubbling is offset by 4  105 for clarity of display.

Figure 2. a) FTIR spectrum of Py (30 mm) in CD3CN. b) DFT/B3LYP-calculated IR spectrum of the gas-phase Py molecule. c) The transient IR spectrum at 0–5 ms, the same as the top trace in Figure 1c. d) DFT/B3LYP-calculated IR spectrum of gas-phase PyC + .

transient bands should be negligibly small compared with Py2C + . The ionic species of polycyclic aromatic hydrocarbons tend to dimerize in solution and dimer cations are energetically more favored than their monomer counterparts.[11, 14] In addition to the difference in population, the reactivity of monomeric and dimeric species is known to differ typically by an order of magnitude.[9b, 11] Nevertheless, we find no noticeable difference in decay kinetics between the 6800 cm1 band (Py2C + ) and the 1216 and 1136 cm1 bands. Taken together, the negligible contribution of PyC + is deemed physically sound. The 2100 cm1 band is characteristic of the C  N stretching mode of nitriles and is safely attributable to DCBC . Our DFT/ B3LYP calculation of gas-phase DCBC (see the Experimental Section) indeed predicts a very intense IR band at 2112 cm1, in excellent agreement with the observed peak position. This band arises from the asymmetric combination of the two C  N stretches. The spectral feature above 9000 cm1 can be associated with neither Py2C + nor DCBC . To obtain more clues about its origin, we performed oxygen quenching experiments in which the sample solution was continuously bubbled with oxygen gas. Anions are usually more efficiently quenched by molecular oxygen than cations. Despite the overall intensity drop, presumably due to quenching of the excited-state population,[15] the intensities of the transient bands at 11000 and 2100 cm1 are drastically quenched with oxygen bubbling (Figure 3, bottom) to an almost undetectable level compared with those at 6800, 1216, and 1136 cm1 (Figure 3, top). This result indicates that the transient band at 11000 cm1 comes from an anionic species. It is most likely the ACN dimer anion ACN2 , in which the excess electron forms a covalent bond between the  2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

cyano carbon atoms of two antiparallel ACN molecules. Previous studies[16] showed that this species has an extremely broad absorption centered in the visible region, the red edge of which has been observed in the present study. In those studies, another type of excess electron stabilization in ACN is discussed, that is, traditional solvated electrons. However, the reported lifetime of the solvated electron in ACN (  80 ps)[16] is simply too short to be detected with our apparatus. We thus attribute the spectral feature above 9000 cm1 to ACN2 . Having assigned the five transient bands to Py2C + , DCBC , and ACN2 , we now examine their time profiles (Figure 1d–h) in more depth. The observation that Py2C + and DCBC show markedly different decay kinetics (e.g., compare Figure 1e and f) seems counterintuitive because it is compelling to think that those carriers with opposite charges should decay in unison, irrespective of whether geminate (first-order reaction) or nongeminate (second-order reaction) recombinations take place, to maintain charge balance. To account for the observed asynchronous decay kinetics in the Py–DCB system, we propose here a mechanism of the BET reaction between Py2C + and DCBC in which ACN plays a pivotal role as a charge mediator (Scheme 1). There are two possible decay channels for DCBC : recombination with Py2C + or ejection of the electron into the solvent, which leads to the formation of ACN2 (Scheme 1, top). In the first case, DCBC would decay concurrently with Py2C + . The observed time profiles contradict this prediction and suggest that the second case, that is, electron ejection into the solvent, is the major route of DCBC decay. It follows a pseudo-first-order reaction with rate constant k1’ (= k1[ACN]2). The preference for electron ejection into ACN over the Py2C + /DCBC recombination may well be due to several factors, such as orbital symmetry mismatch between the HOMO of Py and the LUMO of DCB,[8] the high capability of ACN to solvate electrons, and the abundance of ACN as solvent molecules ([Py]:[DCB]:[ACN] = 1:10:40 000). The formation of ACN2 is clearly manifested as the initial rise of the transient at 11 000 cm1 with a maximum at approximately 6 ms (see Figure 1d). Because DCBC decays preferentially through the reaction with ACN, the only available ChemPhysChem 2014, 15, 2945 – 2950

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curve fitting analysis of the timeresolved spectra shown in Figure 1a–c by using Equations (4) and (5). The parameters used in the fitting are the amplitudes of the two contributions [Py2C + ] and [DCBC] in linear combinations (see b1 and b2 in Equation (7) in the Experimental Section), k1’, and a. The global analysis gives k1’ = (1.1  0.2)  105 s1 and a = 0.4  0.1. The smooth curves in Figure 1d–h are best Scheme 1. Top: DCBC ejects an electron into the ACN solvent, which results in the formation of neutral DCB and ACN2 . Bottom: Py2C + captures the excess electron in ACN2 . fits obtained with these parameters, from which it is clear that the kinetic analysis based on our proposed reaction mechanism (Scheme 1) does an excellent decay pathway for Py2C + that retains the charge neutrality of job of reproducing the observed time profiles of the five tranthe system is an interaction with ACN2 , which obeys sient bands. A good agreement between experiment and sima second-order rate law with rate constant k2 (Scheme 1, ulation is also found in the entire spectral window studied, as bottom). can be seen in Figure S2 in the Supporting Information. The The resulting rate equations governing the kinetics of the above global fitting alone does not allow us to determine C0 system are given by Equations (1)–(3): and k2 independently because they appear together as paramð1Þ d½DCBC  =dt ¼ k1 0 ½DCBC   eter a in the fitting functions [Eqns (4) and (5)]. However, we are able to provide estimates for the values of C0 and k2. Apþ þ  C C ð2Þ d½Py2 =dt ¼ k 2 ½Py2 ½ACN2  proximately 1 % of Py molecules are calculated to be photoexð3Þ d½ACN2  =dt ¼ k1 0 ½DCBC  k 2 ½Py2 C þ ½ACN2   cited under the present excitation conditions, so the upper limit of initial concentration C0 is 5  106 m. Given the reported + C quantum yield of 0.38 of the ion-pair (PyC + and DCBC) formaBy solving Equations (1)–(3) with the initial conditions [Py2   C tion in ACN,[7b] the value of C0 is most likely of the order of ] = [DCB ] = C0 and [ACN2 ] = 0 at t = 0 and with the charge  +  C C neutrality condition [Py2 ] = [DCB ] + [ACN2 ] at any given 106 m, which results in k2  1010 m1 s1. time t, we are able to obtain the time-dependent changes in We have shown that the unique ability of ACN to solvate the concentrations of Py2C + and DCBC as follows: electrons dictates the BET dynamics between Py and DCB. It would be natural to expect that the entire picture would be ð4Þ ½DCBC   ¼ C 0 expðk1 0 tÞ changed drastically in different solvents. To show that this is 0 really the case, we measured the time-resolved IR spectra of Py  þ  C0 exp½að1  expðk1 tÞÞ ð5Þ Py2 ¼ and DCB in benzene (Figure 4). In sharp contrast to the ACN 0 1 þ a expðaÞ½EiðaÞ  Eiða expðk1 tÞÞ case, the decay of the transients seen in Figure 4 is extremely in which a = C0k2/k1’ and Ei(x) represents the exponential integral defined as Rx t EiðxÞ ¼ 1 e =tdt. These solutions can be verified by substitution into Equations (1) and (2). Owing to the charge-neutrality condition, the concentrations of the three ionic species are linearly dependent. Thus, the time-dependent change in absorbance at any wavenumber in Figure 1a–c should be reproduced by a linear combination of solely [Py2C + ] and [DCBC] (see the Experimental Section). To test the validity of our model, we performed a global

Figure 4. Time-resolved near- and mid-IR spectra of Py (5.0 mm) and DCB (50 mm) dissolved in benzene, excited at l = 355 nm with bubbled argon. Each spectrum is offset by 1  104 for clarity. Asterisks indicate interference from a solvent band in the mid-IR region and the stimulated emission of Py in the near-IR region.

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CHEMPHYSCHEM ARTICLES fast and almost complete within 500 ns. Additionally, the nearIR transient in benzene shows a much broader absorption with some structure on it compared with Py2C + in ACN (see Figure 1a). These observations suggest that the BET in benzene occurs through a different mechanism, such as direct recombination of PyC + and DCBC .

3. Conclusion We have shown that the reaction mechanism we propose (Scheme 1) accounts well for the experimental results, in particular the asynchronous decay of Py2C + and DCBC . We have revealed a dual role of ACN as a solvent and a reactant that mediates BET from DCBC to Py2C + . This report is the first, to our knowledge, to elucidate the mechanism of BET dynamics in intermolecular PET reactions in a detailed manner. Although one could devise other mechanisms that require more adjustable parameters to be included in kinetic analysis, our scenario is one of the simplest mechanisms with a minimum set of parameters. It is important for a holistic understanding of the BET reaction to further study Py concentration dependence and solvent dependence, and such studies are in progress in our laboratory. The present study of the BET reaction between Py and DCB in ACN is not merely a case study in photochemistry, but could also have significant implications for DSSCs. Baheti and co-workers[17] recently developed pyrene-based organic dyes for DSSC applications. By using pyrene as a model dye, Maggio and co-workers[4] theoretically illustrated the importance of orbital symmetry for minimizing charge recombination in DSSCs. Furthermore, ACN is a commonly used solvent for the redox couple in DSSCs. Our conclusion that ACN is a key player in the BET reaction that involves the relevant dye will thus call new attention to the importance of the solvent for optimizing the photovoltaic performance of DSSCs.

www.chemphyschem.org tinuously circulated by a gear pump through a 500 mm flow cell that consisted of two CaF2 windows. The sample solution in the reservoir was normally bubbled with argon gas. In the oxygen quenching experiment, however, it was saturated with oxygen gas. Near-IR light (> 4000 cm1) derived from a tungsten–halogen lamp and mid-IR light (< 4000 cm1) from a ceramic IR emitter were used to probe the IR absorbance difference (DA) after photoexcitation. The probe light transmitted through the sample was dispersed with a monochromator and then detected by using a photovoltaic HgCdTe detector for the spectral region below 2000 cm1 or an InSb detector for the region above 2000 cm1. The AC-coupled output of the detector was amplified, followed by signal processing with a high-speed digitizer mounted on a computer to give a time profile of the DA signal at a given wavenumber. The spectral resolution was set to 200 cm1 for the near-IR and 16 cm1 for the mid-IR. The time resolution of the apparatus was limited by the temporal response of the IR detectors; it was about 80 ns in the present case. The FTIR spectrum of Py in CD3CN (Figure 2a) was recorded by using a JASCO FT/IR-6100 spectrometer equipped with a sample cell composed of two CaF2 windows and a 100 mm lead spacer. A spectral resolution of 2 cm1 was used in the FTIR measurement. All measurements were done at RT.

Kinetic Analysis Rate equations (1)–(3) were solved manually and by using Mathematica 7 (Wolfram Research). The IR absorbance difference at time t and wavenumber v~, DAðv~; tÞ, can be expressed as a linear combination of [DCBC], [Py2C + ], and [ACN2]:  ~ DAðv~; tÞ ¼ a1 ðv~Þ½DCB  þ a2 ðv~Þ½Py 2  þ a3 ðvÞ½ACN2 

ð6Þ

in which a1 ðv~Þ, a2 ðv~Þ, and a3 ðv~Þ represent the amplitudes of the time-dependent concentrations of DCBC , Py2C + , and ACN2 , respectively. Due to the charge neutrality condition, Equation (6) leads to Equation (7):   ~ DAðv~; tÞ ¼ a1 ðv~Þ½DCB  þ a2 ðv~Þ½Py 2  þ a3 ðvÞð½Py2   ½DCB Þ

Experimental Section

¼ b1 ðv~Þ½DCB  þ b2 ðv~Þ½Py 2  ð7Þ

Materials Pyrene ( 99 %) and 1,4-dicyanobenzene (98 %) were purchased from Sigma Aldrich. HPLC-grade acetonitrile was purchased from J. T. Baker. Prior to spectroscopic measurements, pyrene and 1,4-dicyanobenzene were purified by sublimation and subsequent recrystallization from ethanol (anhydrous). Acetonitrile was used as received. Pyrene and 1,4-dicyanobenzene were dissolved in acetonitrile at 0.50 and 5.0 mm, respectively.

Nanosecond Time-Resolved IR Spectroscopy The laboratory-built nanosecond time-resolved IR spectrometer used herein has been described previously.[6] The unique feature of this apparatus is that it employs alternating current (AC)-coupled detection in combination with a dispersive monochromator[18] rather than the FTIR method. At the expense of the long time required to scan the grating to acquire the whole spectrum, the dispersive method can reach a sensitivity as high as DA  106, which is still difficult to achieve by using the conventional FTIR method. A l = 355 nm pulse (7 ns duration; 0.4 mJ energy; 500 Hz repetition rate) photoexcited Py in the sample solution, which was con 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

in which b1 ðv~Þ ¼ a1 ðv~Þ  a3 ðv~Þ and b2 ðv~Þ ¼ a2 ðv~Þ þ a3 ðv~Þ. The time-resolved spectral data were globally fit by using Equation (7) together with Equations (4) and (5). The global fitting was performed on Igor Pro 6 (WaveMetrics).

Computational Details DFT calculations of isolated Py monomer, Py monomer cation (PyC + ), and DCBC were carried out by using the B3LYP exchange-correlation functional[19] in conjunction with an empirical dispersion correction recently introduced by Grimme and co-workers[20] (B3LYPD3). The aug-cc-pVTZ basis set[21] was adopted in this work. Equilibrium geometries, harmonic vibrational frequencies, and IR intensities were determined by using the Gaussian 09 program package.[22] To facilitate comparison with experimental IR spectra, calculated harmonic frequencies were uniformly scaled by a factor of 0.97 in accordance with a reported B3LYP/aug-cc-VDZ scaling factor of 0.9698 for frequencies higher than 1000 cm1.[23] Theoretical IR spectra were plotted by using a Gaussian band envelop with a full-width at half-maximum of 10 cm1. ChemPhysChem 2014, 15, 2945 – 2950

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Acknowledgements We acknowledge financial support from the Ministry of Science and Technology (grants NSC102-2113-M-009-012 to S.S. and NSC102-2113-M-009-015-MY3 to H.A.W.) and the ATU plan of National Chiao Tung University and the Ministry of Education, Taiwan. We are also grateful to Professor Hiro-o Hamaguchi for helpful discussion, Dr. Rintaro Shimada for performing the global fitting analysis, and Dr. Hajime Okajima for solving the differential equations. Keywords: charge recombination · electrons · ion pairs · photochemistry · time-resolved spectroscopy [1] a) Dynamics and Mechanisms of Photoinduced Electron Transfer and Related Phenomena (Eds.: N. Mataga, T. Okada, H. Masuhara), Elsevier, Amsterdam, 1992; b) Electron Transfer in Chemistry (Ed.: V. Balzani), WileyVCH, Weinheim, 2001; c) G. J. Kavarnos, N. J. Turro, Chem. Rev. 1986, 86, 401 – 449. [2] a) S. Handa, H. Wietasch, M. Thelakkat, J. R. Durrant, S. A. Haque, Chem. Commun. 2007, 1725 – 1727; b) A. Hagfeldt, G. Boschloo, L. Sun, L. Kloo, H. Pettersson, Chem. Rev. 2010, 110, 6595 – 6663. [3] S. R. Cowan, N. Banerji, W. L. Leong, A. J. Heeger, Adv. Funct. Mater. 2012, 22, 1116 – 1128. [4] E. Maggio, N. Martsinovich, A. Troisi, Angew. Chem. Int. Ed. 2013, 52, 973 – 975; Angew. Chem. 2013, 125, 1007 – 1009. [5] a) Y. Tachibana, J. E. Moser, M. Grtzel, D. R. Klug, J. R. Durrant, J. Phys. Chem. 1996, 100, 20056 – 20062; b) J. B. Asbury, R. J. Ellingson, H. N. Ghosh, S. Ferrere, A. J. Nozik, T. Lian, J. Phys. Chem. B 1999, 103, 3110 – 3119; c) M. Wielopolski, J.-H. Kim, Y.-S. Jung, Y.-J. Yu, K.-Y. Kay, T. W. Holcombe, S. M. Zakeeruddin, M. Grtzel, J.-E. Moser, J. Phys. Chem. C 2013, 117, 13805 – 13815; d) G. Grancini, D. Polli, D. Fazzi, J. Cabanillas-Gonzalez, G. Cerullo, G. Lanzani, J. Phys. Chem. Lett. 2011, 2, 1099 – 1105; e) H. Ohkita, S. Ito, Polymer 2011, 52, 4397 – 4417; f) D. A. Vithanage, A. Devizˇis, V. Abramavicˇius, Y. Infahsaeng, D. Abramavicˇius, R. C. I. MacKenzie, P. E. Keivanidis, A. Yartsev, D. Hertel, J. Nelson, V. Sundstrçm, V. Gulbinas, Nat. Commun. 2013, 4, 2475. [6] a) S. Yabumoto, S. Shigeto, Y.-P. Lee, H. Hamaguchi, Angew. Chem. Int. Ed. 2010, 49, 9201 – 9205; Angew. Chem. 2010, 122, 9387 – 9391; b) S. Narra, S.-W. Chang, H. A. Witek, S. Shigeto, Chem. Eur. J. 2012, 18, 2543 – 2550. [7] a) K. H. Grellmann, A. R. Watkins, A. Weller, J. Phys. Chem. 1972, 76, 469 – 473; b) T. Hino, H. Masuhara, N. Mataga, Bull. Chem. Soc. Jpn. 1976, 49, 394 – 396; c) H. Masuhara, N. Mataga, Acc. Chem. Res. 1981, 14, 312 – 318; d) Y. Hirata, Y. Kanda, N. Mataga, J. Phys. Chem. 1983, 87, 1659 – 1662. [8] a) M. Baba, M. Saitoh, Y. Kowaka, K. Taguma, K. Yoshida, Y. Semba, S. Kasahara, T. Yamanaka, Y. Ohshima, Y.-C. Hsu, S. H. Lin, J. Chem. Phys. 2009,

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[9]

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Received: June 8, 2014 Published online on July 17, 2014

ChemPhysChem 2014, 15, 2945 – 2950

2950

Mechanism of back electron transfer in an intermolecular photoinduced electron transfer reaction: solvent as a charge mediator.

Back electron transfer (BET) is one of the important processes that govern the decay of generated ion pairs in intermolecular photoinduced electron tr...
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