Nonradiative Electron Transfer Research Articles

A quantitative relationship between radiative and nonradiative electron transfer in radical-ion pairs

A quantitative relationship between radiative and nonradiative electron transfer in radical-ion pairs

Relationship between exciplex fluorescence and electron transfer in radical ion pairs

The relationship between non-radiative (thermal) return electron transfer within radical ion pairs and the fluorescence spectra of exciplexes is discussed. Those exciplexes in which charge transfer from the donor to the acceptor is nearly complete are essentially contact radical ion pairs, and their fluorescence is a return electron transfer process. In particular, the energy and width of the exciplex fluorescence spectrum are directly related to the electron tranfer reorganization parameters. From the emission spectra of the exciplexes formed by several common acceptors with alkylbenzenes as donors it is found that the reorganization energy for electron transfer decreases with increasing molecular size of the acceptor. The spectra also indicate that in the solvents used in this study, the reorganization parameters with dicyanoanthracene, tetracyanoanthracene, dicyanonaphthalene and dicyanobenzene as acceptors are essentially constant for different alkylbenzene donors. With tetracyanobenzene and, most strikingly, pyromellitic dianhydride, however, the reorganization energy is found to decrease with decreasing oxidation potential of the alkylbenzene donor. The relevance of these observations to the interpretation of the rates of return electron transfer in these system as a function of reaction exothermicity is discussed.

Adiabatic polaron theory of electron hopping in crystals: A reaction-rate approach

The paper presents an application of reaction-rate theory to nonradiative electron transitions in polar crystals based on a "two-site" model. For this purpose localized electronic states are described by using the adiabatic approximation. Both "large" polarons in ionic crystals and "small" polarons in molecular crystals are considered from a unified point of view. By assuming a single frequency of the lattice vibrations (Einstein model) one can calculate the transition probability for both the limits of adiabatic and nonadiabatic electron transfer as well as for the whole intermediate range. The relevant expressions, earlier derived in treating electron transfer in solution, are reviewed and discussed in order to give a justification of their application to polaron hopping in crystals. In a similar way, a review is made of the reaction-rate approach to electron transfer to obtain the appropriate rate equations for the low- and high-temperature limits and the intermediate temperature range as well. New expressions for the polaron-hopping rate are also derived. The conditions of its validity are discussed. This paper provides an essentially new approach to nonradiative electron transfer in polar crystals. It permits one to overcome the limitations of the usual methods, such as the multiphonon approach, entirely based on time-dependent perturbation theory and/or the Franck-Condon approximation which restrict their applicability only to nonadiabatic polaron hopping. The reaction-rate treatment exactly reproduces the results of the more rigorous multiphonon theory of nonadiabatic transitions. Moreover, it yields correct rate expressions for adiabatic transitions, in particular, in the high-temperature range, in which the classical occurrence-probability approach greatly overestimates the activation energy. Some problems concerning the Einstein one-frequency oscillator model assumed and the irreversibility of the electron transfer are discussed with some details from standpoint of both reaction-rate theory and multiphonon theory. The advantages of the reaction-rate approach are emphasized.