Charge recombination reactions in self-assembled porphyrin-based ion-pair complexes

Abstract

The dynamics of charge recombination (CR) reactions after photoinduced charge separation (CS) in self-assembled ion pairs of charged porphyrins and oppositely charged electron acceptors 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 the multipicosecond time scale. The CR reactions show an invariance of rate constant (ca. 10 12 s −1) with driving force at Δ G ⪅ 1.2 eV. Moreover, the CR processes exhibited a strongly inverted dependence on driving force when Δ G > 1.3 eV. The 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 rate in the driving force independent region is limited by vibrational relaxation in the ground state of the complex.