SELF-ASSEMBLED ION-PAIR COMPLEXES BETWEEN PORPHYRINS AND BIPYRIDINIUM SPECIES - PICOSECOND DYNAMICS OF CHARGE RECOMBINATION

Spontaneous ion pairing occurs between soluble anionic porphyrins (TPPS and Up) and cationic bipyridinium salts in water. After photoexcitation (532 nm) of the porphyrin component, nonfluorescent ion pairs are formed by fast electron transfer (<100 fs) from the porphyrin donor to the bipyridinium acceptor. The system returns to the ground state by reverse electron transfer without dissociation into separated radical ions. The reverse electron transfer reactions occur over hundreds of picoseconds, and the rate constants are shown to form a homogeneous set of data which demonstrate an inverted region in the driving force range 0.8-1.7 eV. These results are discussed in terms of adiabatic and nonadiabatic electron-transfer theory.