FREE-ENERGY DEPENDENCE OF PHOTOINDUCED CHARGE SEPARATION RATES IN PORPHYRIN DYADS

A series of covalently linked porphyrin dyads in which the thermodynam ic driving force for interporphyrin photoinduced charge separation spa ns a range of 1.13 eV has been prepared. Time-resolved fluorescence st udies have yielded 22 rate constants for photoinduced electron transfe r in dichloromethane solution ranging from 4.1 x 10(7) to 5.0 X 10(11) s(-1). The data are consistent with the theoretical treatments of Mar cus and Levich, although there is no evidence for inverted behavior. I n the normal region, electron transfer between free base porphyrin moi eties is about 4 times faster than transfer involving a zinc porphyrin and having the same thermodynamic driving force, based on electrochem ical measurements. Photoinduced electron transfer to an excited single t state and electron transfer from an excited singlet state have the s ame dependence upon free energy change.