Solvent dependence of photoinduced electron transfer in porphyrin dyads

Photoinduced electron transfer in four dyads consisting of electron donating porphyrins covalently linked to electron accepting porphyrins (PD-PA) has been studied in 17 solvents. Excitation of a porphyrin moiety is followed by singlet-singlet energy transfer to the attached porphyrin and/or photoinduced electron transfer to yield the PD+-P,\~ charge-separated state. Reasonable correlation of the electron-transfer rate constants with the solvent static dielectric constant was observed using the Marcus theory of electron transfer and the Weller dielectric continuum model for the variation of thermodynamic driving force with dielectric constant. Little or no improvement in the correlation was introduced by inclusion of a solvent dependence of the total reorganization energy. These results differ significantly from those reported for porphyrin-quinone dyads. The differences may be due to a reduced sensitivity to specific solvent interactions by the large, diffuse porphyrin ions. Comparisons among the dyads suggest that inclusion of a meso-aryl group in the linkage joining the donor and acceptor porphyrins attenuates electron transfer rates only weakly.