Exothermic rate restrictions on electron transfer in a rigid medium

Abstract

Many highly exothermic (2–3 eV) electron transfer reactions are shown to be slower than moderately exothermic reactions by factors as large as 105. The decrease occurs in a regular way with increasing exothermicity, tending to confirm theoretical predictions of Franck–Condon restrictions on strongly exothermic electron transfer reactions. Deviations from the above trends occur if the reaction product, a molecular anion, has a low-lying electronic excited state into which the reaction may occur with more moderate vibrational exothermicity. Then greatly enhanced rates are found. The rates are enhanced to a lesser extent for acceptors likely to undergo configurational changes upon accepting an electron. These effects are found in measurements of rates of electron tunneling reactions between trapped electrons and 48 organic electron acceptors in rigid 2-methyltetrahydrofuran glass at 77 K. Electron tunneling rates were observed from 10−6 to 102 s. Measured tunneling distances were 15–40 Å. In most cases the observed kinetic decay curves are well simulated by a theory in which the only variable parameter is the effective Franck–Condon factor (F), which is a constant characteristic of the acceptor. For the various acceptors F ranges from 1 to 10−5, and scales the reaction rate at each distance. However in reactions of small vibration exothermicity, the Franck–Condon factors are expected to be very sensitive to small changes in reaction exothermicity caused by relaxations of trapped electrons, which deepen their trap depths with time and, possibly, a dispersion of trap depths. These effects cause F to change with time leading to changes in the shapes of the decay curves. The relationship between the shapes of the decay curves, Franck–Condon factors, and exothermicity allows a thorough and consistent semiquantitative interpretation of the present results and much of the earlier tunneling data on electron transfer reactions involving other solvents and solutes.

Transient Effect in Fluorescence Quenching by Electron Transfer. 3. Distribution of Electron Transfer Distance in Liquid and Solid Solutions

“…In their experimental studies of electron transfer in solid solutions, Miller et al 11,12 have shown that the results can be very well analyzed by assuming the following form for k(r).…”

Dielectric Asymmetry in the Photosynthetic Reaction Center

“…This possibility has received only limited discussion in the photosynthesis field (62) and in electron transfer work in general (63)(64)(65)(66). The simple concept is that the barrier height for electron tunneling may decrease with increasing dielectric constant, thereby increasing orbital overlap and enhancing the electronic coupling between reactant and product states (65,66).…”

Excited States in Electron-Transfer Reaction Products: Ultrafast Relaxation Dynamics of an Isolated Acceptor Radical Anion

“…[8][9][10][11] However, many systems predicted to exhibit inverted region behavior do not and have ET rates that exceed expected rates by orders of magnitude. 10,[12][13][14][15][16] One mechanism used to explain these deviations is the participation of excited states in the ET products, of either vibrational or electronic nature. 14,[17][18][19][20] Both effectively reduce the exergonicity of the ET process; the case of an electronically excited state is illustrated in Figure 1, in which the barrier can be bypassed by accessing an excited state of the radical anion of the acceptor, A• -*.…”