DYNAMICS OF CHARGE-TRANSFER AND RECOMBINATION IN A COVALENTLY-LINKED, FACE-TO-FACE ELECTRON DONOR-ACCEPTOR COMPLEX

The photophysical properties of a covalently-linked, electron donor-acceptor (EDA) complex, containing 1,4-dialkoxybenzene and N,N'-dialkyl-4,4'-bipyridinium dication subunits, have been studied in polar solvents. The structure of the compound has been elucidated by X-ray crystallography, 2D NMR, and resonance Raman spectroscopy. Excitation of the ground-state EDA complex with a short laser pulse results in formation of the corresponding excited charge-transfer state, which is weakly fluorescent. The excited state decays, at least in part, via charge transfer to form an intimate radical ion pair (RIP) within a few picoseconds. The free energy content of the RIP decreases with increasing polarity of the solvent, and there is a concomitant increase in the rate of charge recombination, which also occurs on the picosecond time scale. For this system, the dynamics for deactivation of the RIP can be described satisfactorily in terms of both conventional nonadiabatic electron-transfer theory and an exponential energy gap law.