Ethanol Concentration Dependence of Photoinduced Charge Separation Reaction between Zinc Tetraphenylporphyrin and Duroquinone Studied by Laser Flash Photolysis Tomoaki Yago,  Masao Gohdo, and Masanobu Wakasa Department of Chemistry, Graduate School of Science and Engineering, Saitama University, 255 Shimo-ohkubo, Sakura-ku, Saitama 338-8870 (Received June 22, 2009; CL-090584; E-mail: yago@chem.saitama-u.ac.jp) Ethanol concentration dependence of photoinduced charge separation between zinc tetraphenylporphyrin (ZnTPP) and duro- quinone (DQ) in benzonitrile was studied by nanosecond laser flash photolysis. Acceleration of the photoinduced charge sepa- ration reaction rate by hydrogen bonding between DQ anion rad- ical and ethanol was observed. A simple analysis in the frame- work of the Marcus theory indicated that the observed elec- tron-transfer reaction rate was affected not only by the decrease of the reaction free energy but also an increase of the reorgani- zation energy in the presence of hydrogen bonding. Electron transfer (ET) is one of the most fundamental chemi- cal reactions and has been studied extensively over the last six decades. Particular attention has been focused on biological ET systems where the reactions proceed with high efficiency. 1 Hydro- gen-bonding interactions are ubiquitous to biological ET systems and have been believed to contribute to efficient ET reactions by stabilizing the charge-separated states. Actually, Fukuzumi et al. have reported the acceleration of charge separation (CS) re- actions in the presence of hydrogen-bonding interactions. 2 Ac- cording to the Marcus theory, the activation barrier for the ET reactions depends on reaction free energy (ÁG) and reorganiza- tion energy ( ). 3,4 The stabilization of the charge-separated state by hydrogen bonding causes the modulation of the ÁG value for the CS and charge recombination reactions. 5–11 Recent time-re- solved electron paramagnetic resonance (EPR) studies on sol- vent-separated radical ion pairs generated by photoinduced CS reactions suggest that  values are also increased by solute–sol- vent hydrogen-bonding interactions in polar solvents. 12 The re- sults were interpreted with a change of chemical equilibrium for the hydrogen-bonding complex during the ET reactions; 13 however, the ET reaction rates have not been evaluated directly. It is, therefore, desirable to evaluate ET reaction rates directly in the presence of hydrogen bonding to clarify their role. In this contribution, we study ethanol concentration depend- ence of photoinduced CS between zinc tetraphenylporphyrin (ZnTPP) and duroquinone (DQ) in benzonitrile (PhCN) to clar- ify how the ET reaction rates are affected by the modulation of ÁG and  in the presence of hydrogen bonding. For this pur- pose, nanosecond laser flash photolysis experiments were car- ried out with an apparatus that was essentially the same as an ap- paratus described elsewhere. 14 The second harmonic (532 nm) of a Nd:YAG laser was used for the selective excitation of ZnTPP (1  10 4 M) in the sample solutions. The concentrations of DQ were 0.1–2 mM. To investigate the effect of hydrogen bonding on the CS reaction rates, ethanol (EtOH), which has been known to form hydrogen-bonding complexes with the DQ anion (DQ  ), was added to the sample solution. Since EtOH has a similar viscosity ( ¼ 1:08 cP) 15 and dielectric constant (" ¼ 24:6) 15 as PhCN ( ¼ 1:24 cP, " ¼ 25:2), 15 the diffusion coeffi- cients of solutes and macroscopic dielectric properties of solu- tions were unchanged under the experimental conditions. There- fore, the hydrogen-bonding effects on the CS reactions can be studied with the present photoinduced CS reaction system. The previous time-resolved EPR study showed that the pho- toinduced CS reactions proceed between the photoexcited triplet state of ZnTPP and DQ in PhCN. 16 In the PhCN solution, semi- quinone radical of DQ is not produced even in the presence of EtOH. 12b We can, therefore, describe the photoinduced CS reac- tions in the present system as follows: ZnTPP þ hvð532 nmÞ! 1 ZnTPP  ! 3 ZnTPP  ð1Þ 3 ZnTPP  þ DQ ! ZnTPP þ þ DQ  ð2Þ Here, 1 ZnTPP  , 3 ZnTPP  , and ZnTPP þ represent the singlet and triplet excited states of ZnTPP, and ZnTPP cation radical, re- spectively. Upon irradiation of PhCN solution containing ZnTPP and DQ, a strong T–T absorption band of 3 ZnTPP  was observed around 470 nm. 17 DQ concentration dependence on the time pro- file of the transient absorption (AðtÞ) was observed at 450 nm. 18 In the presence of DQ, AðtÞ curves had a fast decay component and an almost constant one. The fast component was assigned to the decay of the T–T absorption of 3 ZnTPP  , and the almost constant one can be assigned to the transient absorption of ZnTPP þ and DQ  produced by the photoinduced CS reac- tions. 19,20 The DQ concentration dependences of the decay rate constants (k) of 3 ZnTPP  were observed at 450 nm in the pres- ence of the various concentrations of EtOH. 18 In each EtOH con- centration, a good linear relationship was observed between k and DQ concentration. From the slopes of these plots, the sec- ond-order rate constants (k q ) for the CS reactions of 3 ZnTPP  with DQ were determined. The determined k q value for each EtOH concentration was smaller than the reaction rate constant for the diffusion-controlled reactions (5:3  10 9 M 1 s 1 ), indi- cating that the rate-determining step for CS is the ET and not the encounter process promoted by solute diffusion motions. The results were analyzed with the classical Marcus equa- tion combined with the diffusion rate constant (k dif ), 21 k CS ¼ Z exp  ðÁG þ  Þ 2 4 k B T   ð3Þ 1 k q ¼ 1 k dif þ 1 k CS ð4Þ where k CS is CS reaction rate constant, and Z is a pre-exponential factor. Figure 1 shows EtOH concentration dependence of k CS de- termined for the ZnTPP–DQ system in PhCN with the parameters of k dif ¼ 5:3  10 9 M 1 s 1 . 22 The CS reaction was accelerated by the addition of EtOH. Since  for PhCN and EtOH are quite similar, the addition of EtOH to the sample solutions has little 880 Chemistry Letters Vol.38, No.9 (2009) Copyright Ó 2009 The Chemical Society of Japan