CHARGE RECOMBINATION REACTIONS IN SELF-ASSEMBLED PORPHYRIN-BASED ION-PAIR COMPLEXES

The dynamics of charge recombination (CR) reactions after photoinduced charge separation (CS) in self-assembled ion pairs of charged porphyr ins and oppositely charged electron accepters were studied in aqueous solution. In most cases the CS reaction occurs in times less than 0.1 ps after the photoexcitation event, whereas the CR reaction is slower, on che multipicosecond time scale. The CR reactions show an invarianc e of rate constant (ca. 10(12) s(-1)) with driving force at Delta G le ss than or similar to 1.2 eV. Moreover, the CR processes exhibited a s trongly inverted dependence on driving force when Delta G > 1.3 eV. Th e rate constants of CR reactions in the region independent of driving force were insensitive to changes in the longitudinal relaxation time of the solvent, as determined by changing the solvent from neat water to alcohol-water mixtures, which may indicate that the CR reaction rat e in the driving force independent region is limited by vibrational re laxation in the ground state of the complex.