The importance of vibrational motion and solvent diffusional motion in excited state intramolecular electron transfer reactions

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Abstract
Time resolved emission spectroscopy has been used to examine the dynamics of intramolecular charge transfer in dimethylaminobenzonitrile (DMABN) and diethylaminobenzonitrile (DEABN) in linear alcohol solutions as a function of temperature. For both DMABN and DEABN in methanol and DMABN in ethanol solutions, the population decay of the local excited (LE) state can be fit by a single exponential function. However, over the temperature range examined, 0 to −50 °C, the population decay of the local excited state in longer chain alcohol solutions (ethanol, propanol, butanol, pentanol, and hexanol) cannot be fit by a single exponential. The average survival probability of the LE state Q(t) is obtained by fitting the population decay to a multiexponential function. In all of the alcohol solvents studied, the average lifetime of Q(t) is faster than the solvent fluctuation rate gauged by the longitudinal relaxation time of the solvent τL(τDε∞/εs) corresponding to the slow collective hydrogen bonding dynamics. Comparison with recent dynamical solvation studies suggest that the multiexponential electron transfer kinetics reflected by Q(t) do not result from contributions of higher frequency responses of ε(ω). The experimentally observed dynamics of electron transfer are compared to a recent theoretical model of Marcus and co-workers which address the importance of contributions from both solvent fluctuations and intramolecular vibrational motions to the electron transfer rate. From this comparison it is concluded that fluctuations in the intramolecular coordinates for this reaction make a greater contribution to the rate of reaction in alcohol solutions than solvent diffusion.