MECHANISMS OF EXCIPLEX FORMATION - ROLES OF SUPEREXCHANGE, SOLVENT POLARITY, AND DRIVING-FORCE FOR ELECTRON-TRANSFER

The efficiencies (alpha) with which exciplexes or excited charge-transfer (CT) complexes (collectively termed Ex's) are formed in bimolecular electron-transfer quenching reactions of excited electron accepters (A*) by donors (D) are determined. The accepters 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 (tau) and the efficiencies with which bimolecular quenching leads to Ex emission (Phi(f)) are determined. The ratio Phi(f)/tau corresponds to alpha k(f), where k(f) is the radiative rate constant of the Ex. alpha k(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 alpha. 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 a 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.