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 distance and fixed orientation triad systems are studied. The two molecules are a zinc-chlorophyll-zinc-porphyrin-naphthoquinone (ZCZPNQ) system and a zinc-chlorophyll-zinc-porphyrin-3,4,9,10-perylenetetracarboxydiimide (ZCZPPER) system. Optical excitation of ZC with a 190 fs pulse at 585 nm results in charge transfer and the formation of ZC⁺ZPNQ^- (PER^-). The chromophores are chosen to minimize spectral overlap of the radical ion products, so that the analysis of the kinetic data is unambiguous. The kinetics of charge transfer are monitored by ultrafast pump-probe spectroscopy, and the role that the central porphyrin plays in mediating electron transfer 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 discussed.