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 transfer 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 systems as a function of reaction exothermicity is discussed.