Mechanism of fast intramolecular electron-transfer reactions

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Abstract

Solvent motion controls the rate of fast intramolecular electron transfer. This conclusion is based on the finding that τ1'[ = (∊op/∊s1] for solvents correlates well (slope = 1) with the relaxation time, τfl,) for intramolecular electron transfers within the initial S1 states of TNSDMA [6-((4-methylphenyl)amino)-2-naphthalenesulfonyl dimethylamide] and DMAB [4-(dimethylamino)benzonitrile]. Two approaches toward understanding the result are used. A simplified molecular model [microscopic steps: (a) electron transfer, (b) solvent motion (including that which may have preceded step a: only one solvent molecule is shown in motion), and (c) hydrogen motion] illustrates the molecular rearrangements involved in the electron-transfer process. A simplified dipole interaction model [microscopic steps: (a) substrate dipole change in the Franck—Condon state, (b) solvent dipole motion, and (c) electron transfer] shows the changes in electrostatic interactions in the course of the electron-transfer process. Solvent dipolar motion in the absence of the reaction field should be characterized by τ1', corresponding to the constant charge case outlined by Friedman (Friedman, H. L. J. Chem. Soc., Faraday Trans. 2 1983, 79, 1465–1467). The movement of the solvent molecule is favored by an energy term proportional to (l/∊op)(ΔΔG/∊op) and retarded by interaction of the moving solvent molecule with the reaction field of the stationary solvent molecules (l/∊s)(ΔΔG/∊s). The kinetic constants are not easily related to these energy terms. The molecular model shows the close relationship between two kinetically similar systems (6,2-ANS and DMAB) and appears to fit additional less-well-studied systems. Explanations for the photophysical behavior of the N-methyl-6,2-ANS case and the very fast quenching of the S1 excited state of a complex rhodamine derivative can be derived from the model. “Slower” electron-transfer processes (e.g., S1,ct → S0,np in TNSDMA) exhibit relaxation times that are greater than τ1 by a factor which varies somewhat with the strength of the solvent-S1,Ct interactions (6.8 for ethanol to 3.5 for 1-decanol). The model suggests that this reflects the strength of the organization induced by the ion pair in the solvent and possibly some internal reorganization (bond length and/or angle changes). The model also leads to some insights about the details of the long distance electron transfers reported by Miller, Calcaterra, and Closs (Miller, J. R.; Calcaterra, L. T.; Closs, G. L. J. Am. Chem. Soc. 1984, 106, 3047–3049).

Original languageEnglish
Pages (from-to)1114-1118
Number of pages5
JournalJournal of the American Chemical Society
Volume107
Issue number5
DOIs
StatePublished - 1 Mar 1985

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