Mechanisms of Exciplex Formation. Roles of Superexchange, Solvent Polarity, and Driving Force for Electron Transfer

The efficiencies (a) with which exciplexes or excited charge-transfer (CT) complexes (collectively termed Ex's) are formed in bimolecular electron-transfer quenching reactions of excited electron acceptors (A∗) by donors (D) are determined. The acceptors are 9,10-dicyanoanthracene (DCA) and 2,6,9,10-tetracyanoanthracene (TCA). The donors are simple alkyl-substituted benzenes. The solvents vary in polarity, from cyclohexane to acetonitrile. The lifetimes of the Ex's (τ) and the efficiencies with which bimolecular quenching leads to Ex emission (Φ_f) are determined. The ratio Φ_f/τ corresponds to αk_f, where k_f is the radiative rate constant of the Ex. αk_f is found to decrease with increasing solvent polarity, as observed previously for other acceptor/donor systems. With only a few exceptions, this is the result of a decrease in k_f, rather than a decrease in a. The rate constant k_f also decreases with decreasing redox energy of the A/D pair. These changes in k_f are caused by varying contributions of locally excited and pure ion-pair states to the electronic structure of the Ex. Values for α of less than unity are found, however, for some quenching reactions in acetonitrile, as a result of direct formation of solvent-separated radical-ion pairs (SSRIP) from the A∗/D encounter pair. The competition between SSRIP and Ex formation is determined by the rate of SSRIP formation, which is driving force dependent and exhibits Marcus normal-region behavior. The Ex's are also bypassed in the quenching reactions of TCA∗ in o-dichlorobenzene. In this case, superexchange interactions in the encounter pair facilitate direct formation of the SSRIP.