SOLVENT EFFECTS ON THE ENERGETICS AND DYNAMICS OF ULTRAFAST ELECTRON-TRANSFER IN CHLOROPHYLL-PORPHYRIN-ACCEPTOR TRIADS

The photochemistry and electron transfer kinetics of two fixed distanc e and fixed orientation triad systems are studied. The two molecules a re a zinc-chlorophyll-zinc-porphyrin-naphthoquinone (ZCZPNQ) system an d a nc-por-phyrin-3,4,9,10-perylenetetracarboxydiimide (ZCZPPER) syste m. Optical excitation of ZC with a 190 fs pulse at 585 nm results in c harge transfer and the formation of ZC+ZPNQ- (PER-). The chromophores are chosen to minimize spectral overlap of the radical ion products, s o that the analysis of the kinetic data is unambiguous. The kinetics o f charge transfer are monitored by ultrafast pump-probe spectroscopy, and the role that the central porphyrin plays in mediating electron tr ansfer is discussed. We find that solvent relaxation also plays a role in determining the rates of both charge separation and recombination in polar solvents. Differing mechanisms for solvent relaxation are dis cussed.