Anion-complexation-induced stabilization of charge separation

A supramolecular oligochromophoric system possessing exclusive binding sites for both a guest electron acceptor and an anionic cofactor species is developed, and anion-binding-induced stabilization of the charge-separated (CS) state is demonstrated. Toward this, intramolecular and intermolecular photochemical processes of a supramolecular complex of a bis-porphyrinyl- substituted oxoporphyrinogen with a bis(4-pyridyl)-substituted fullerene were investigated by using femtosecond and nanosecond laser flash photolysis measurements. Transient absorption spectra of the supramolecular complex obtained by femtosecond laser flash photolysis indicate that efficient electron transfer occurs from the porphyrin moiety to the fullerene moiety, followed by faster back electron transfer to the ground state. Binding of several different anionic species at the pyrrole amine groups of an oxoporphyrinogen unit within the supramolecular complex was found to improve the rate of the photoinduced electron transfer due to the favorable structural change. The anion binding also improves persistence of the photoinduced CS state between the anion-bound oxoporphyrinogen and fullerene moieties, which is produced by intermolecular electron transfer from the triplet excited state of free porphyrin molecules to free fullerene molecules, as indicated by the nanosecond laser flash photolysis measurements. In the case of fluoride anion binding, anion-complexation-induced stabilization of charge separation gave a 90-fold elongation of the CS state lifetime from 163 ns to 14 μs. Complexation with other anions (acetate or dihydrogen phosphate) also resulted in stabilization of the CS state, whereas weakly bound perchlorate anions gave no improvement. Complexation of anions to the oxoporphyrinogen center lowers its oxidation potential by nearly 600 mV, creating an intermediate energy state for charge migration from the ZnP·⁺ to the oxoporphyrinogen:anion complex. An increase in reorganizational energy of electron transfer combined with the decrease in charge recombination driving force caused by anion binding results in an increase in the lifetime of the CS state.