Ethanol Concentration Dependence of Photoinduced Charge Separation Reaction between Zinc Tetraphenylporphyrin and Duroquinone Studied by Laser Flash Photolysis

Abstract

Ethanol concentration dependence of photoinduced charge separation between zinc tetraphenylporphyrin (ZnTPP) and duroquinone (DQ) in benzonitrile was studied by nanosecond laser flash photolysis. Acceleration of the photoinduced charge separation reaction rate by hydrogen bonding between DQ anion radical and ethanol was observed. A simple analysis in the framework of the Marcus theory indicated that the observed electron-transfer reaction rate was affected not only by the decrease of the reaction free energy but also an increase of the reorganization energy in the presence of hydrogen bonding.Electron transfer (ET) is one of the most fundamental chemical 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 Hydrogen-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) reactions in the presence of hydrogen-bonding interactions.2 According to the Marcus theory, the activation barrier for the ET reactions depends on reaction free energy (ÁG) and reorganization 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-resolved electron paramagnetic resonance (EPR) studies on solvent-separated radical ion pairs generated by photoinduced CS reactions suggest that values are also increased by solute-solvent hydrogen-bonding interactions in polar solvents.12 The results 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 dependence of photoinduced CS between zinc tetraphenylporphyrin (ZnTPP) and duroquinone (DQ) in benzonitrile (PhCN) to clarify how the ET reaction rates are affected by the modulation of ÁG and in the presence of hydrogen bonding. For this purpose, nanosecond laser flash photolysis experiments were carried out with an apparatus that was essentially the same as an apparatus described elsewhere.14 The second harmonic (532 nm) of a Nd:YAG laser was used for the selective excitation of ZnTPP (1 Â 10 À4 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 À ), 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 coefficients of solutes and macroscopic dielectric proper...

Hydrogen Bonding Effects on the Reorganization Energy for Photoinduced Charge Separation Reaction between Porphyrin and Quinone Studied by Nanosecond Laser Flash Photolysis

“…The parameters affected by the addition of alcohols are Δ G and λ. In the previous study, we demonstrated that the alcohol concentration dependence of k CS was not explained by the change of Δ G alone . We have proposed the increase in λ with the addition of alcohol in the present photoinduced CS reaction system.…”

“…For the EtOH concentration dependence of k CS , the reported chemical equilibrium constants ( K ≈ 0 (mol dm −3 ) −1.3 and K − = 85 (mol dm −3 ) −1.3 ) are used. The constant parameters used are Z = 5 × 10 11 mol −1 dm 3 s −1 , λ° = 0.9 eV, and −0.05 eV for Δ G in the absence of alcohols. The CS process in the present ET system is in the Marcus normal region (λ > −Δ G ).…”

Hydrogen-Bonding Effects on the Formation and Lifetimes of Charge-Separated States in Molecular Triads

“…Prior experimental studies on electron transfer in hydrogen-bonded systems have come to the conclusion that hydrogen-bond formation and breakage can contribute substantially to the reorganization energy (λ) accompanying the electron-transfer event. − Therefore we explored the temperature dependence of τ CR in order to obtain information about the activation energies ( E A ) for thermal charge recombination. Figure shows Arrhenius plots based on lifetime measurements in CH 3 CN between 5 and 70 °C (Supporting Information Figure S3 for TAA-Ru II -AQ and Figure S4 for TAA-Os II -AQ) and in HFIP between 5 and 50 °C (Supporting Information Figure S5 for TAA-Ru II -AQ and Figure S6 for TAA-Os II -AQ).…”