Effects of the Solvent Dynamics and Vibrational Motions in Electron Transfer

Abstract

Recent theoretical and experimental progress concerning electron transfer (ET) in solution is reviewed by focusing on the mechanism of ET, which occurs much faster than solvation dynamics. Theories of ET in solution are briefly reviewed with particular emphasis placed on the relation to solvent dynamics. Experimental methods to investigate solvent polarization relaxation are described. Ultrafast intramolecular ET, which is found in back ET from the photo-induced charge-transfer state to the ground state, is described concerning highly polar betaines and mixed-valence compounds. Ultrafast intermolecular ET has been observed for the systems of various dyes in electron-donating solvents. A non-exponential process with a significant temperature dependence was observed in aniline. A faster ET with a single exponential decay as fast as 1013 s−1 was observed with no temperature dependence in a system of oxazine 1 in N,N-dimethylaniline. The ET rate constants of excited coumarins in electron-donating solvents drastically depend on the substituent groups of the coumarin. Relatively small Stokes shifts in steady-state fluorescence spectra of ultrafast reacting molecules in solution are evidence of a “chemical timing” effect; namely, the reaction occurs in a non-equilibrium configuration of the solvent. These experimental observations are explained in terms of the extended Sumi–Marcus theory, in which the effect of solvation dynamics and low- and high-frequency vibrational modes are taken into account.

Solvent as Electron Donor: Donor/Acceptor Electronic Coupling Is a Dynamical Variable

“…We report on four DMA/coumarin pairs not previously studied: coumarins 120, 343, 500, and 503. Qualitatively, we agree strongly with the picture of electron transfer with solvent as donor put forward by the Yoshihara group, [37][38][39][40][41][42][43][44][45][46][47][48][49][50][51][52][53] as well as more recently by Walker et al 92 and the Zinth group. 95,96 The Yoshihara group studied a series of substituted coumarins, where the substituent at the 4-position was chosen from R ) {H, CH 3 , CF 3 }, and the 7-amino functional group NX 2 , with X chosen from H, CH 3 , C 2 H 5 , fused ring, and julolidyl double-ring.…”

“…This area has been explored in great detail by Yoshihara and co-workers. [37][38][39][40][41][42][43][44][45][46][47][48][49][50][51][52][53] We find excellent agreement between our C152/DMA results and theirs, and good agreement with their earlier results on C151/DMA. 40 More recent work by Shirota et al indicates that the C151/DMA photoreaction is also slightly biexponential in rate.…”

“…77 Yoshihara and co-workers have used several classes of organic laser dye molecules to study the ultrafast bimolecular electron-transfer between solvent and excited-state chromophore. [37][38][39][40][41][42][43][44][45][46][47][48][49][50][51][52][53] In the experimental parts of this paper, we present studies on the photoinduced bimolecular electron-transfer rates from solvent donors to substituted coumarin excited-state acceptors. The experiments may be categorized using three paradigms.…”

Theoretical Investigation of the Ground and Excited States of Coumarin 151 and Coumarin 120

“…30,31 The Yoshihara group has studied solvent to solute et, [32][33][34][35][36][37][38][39][40][41][42][43] using coumarins or related species as electron acceptorssthe coumarin is photoexcited, and the hole in the HOMO is then a good oxidant for many solvents.…”

Solvent dynamics: Modified Rice–Ramsperger–Kassel–Marcus theory. II. Vibrationally assisted case

“…Both types of coordinates can contribute to the reaction coordinate by assisting the system in its crossing of the transition state for the reaction, and this role for the fast coordinates has been termed vibrational assistance. For constant temperature systems it has been treated for heme-ligand binding kinetics, 6 electron transfer reactions, [7][8][9][10] isomerization reactions, 11,12 and for model and other systems. [13][14][15][16] A brief review of some of this history has been given elsewhere.…”