Free energy dependence of photoinduced charge separation rates in porphyrin dyads

A series of covalently linked porphyrin dyads in which the thermodynamic driving force for interporphyrin photoinduced charge separation spans a range of 1.13 eV has been prepared. Time-resolved fluorescence studies have yielded 22 rate constants for photoinduced electron transfer in dichloromethane solution ranging from 4.1 × 10⁷ to 5.0 × 10¹¹ s^-1. The data are consistent with the theoretical treatments of Marcus and Levich, although there is no evidence for inverted behavior. In the normal region, electron transfer between free base porphyrin moieties is about 4 times faster than transfer involving a zinc porphyrin and having the same thermodynamic driving force, based on electrochemical measurements. Photoinduced electron transfer to an excited singlet state and electron transfer from an excited singlet state have the same dependence upon free energy change.