SOLVENT DEPENDENCE OF PHOTOINDUCED ELECTRON-TRANSFER IN PORPHYRIN DYADS

Photoinduced electron transfer in four dyads consisting of electron do nating porphyrins covalently linked to electron accepting porphyrins ( P-D-P-A) has been studied in 17 solvents. Excitation of a porphyrin mo iety is followed by singlet singlet energy transfer to the attached po rphyrin and/or photoinduced electron transfer to yield the P-D(+)-P-A (-) charge-separated state. Reasonable correlation of the electron-tr ansfer rate constants with the solvent static dielectric constant was observed using the Marcus theory of electron transfer and the Weller d ielectric continuum model for the variation of thermodynamic driving f orce with dielectric constant. Little or no improvement in the correla tion was introduced by inclusion of a solvent dependence of the total reorganization energy. These results differ significantly from those r eported for porphyrin-quinone dyads. The differences may be due to a r educed sensitivity to specific solvent interactions by the large, diff use porphyrin ions. Comparisons among the dyads suggest that inclusion of a meso-aryl group in the linkage joining the donor and acceptor po rphyrins attenuates electron transfer rates only weakly.