Intersystem crossing in charge-transfer excited states

Absolute rate constants for intersystem crossing (ISC) in exciplexes formed between cyanoanthracenes as acceptors and alkylbenzenes as donors in solvents with low to moderate polarity have been measured. The extent of charge transfer in these particular exciplexes has been determined previously, and thus the dependence of the ISC rate constant on this parameter is obtained. A smooth transition of exciplexes with high locally excited state character to those with essentially pure charge-transfer character argues in favor of a mechanism of intersystem crossing involving a spin-forbidden return electron transfer process to form the locally excited triplet directly. The proposed mechanism is supported by the observed energy gap dependence of the rate constant, and by deuterium isotope effects. External heavy-atom effects in chlorinated solvents are observed. Analysis of the rate constant for intersystem crossing for the pure charge-transfer exciplexes using electron-transfer theory allows determination of the matrix element for the spin-forbidden electron-transfer reaction. It is found to be roughly 3000 times smaller than the matrix element for corresponding spin-allowed electron transfer in similar systems. General principles for the factors that control the mechanisms of intersystem crossing in excited charge-transfer states are discussed.