LASER SPECTROSCOPY OF PORPHYRIN-AZAAROMATIC ION-PAIRS - INFLUENCE OF MOLECULAR DIMENSIONS ON THE RATE OF CHARGE RECOMBINATION

Spontaneous (DELTA-G-degrees almost-equal-to 30 kJ mol-1) ion pairing occurs between zinc meso-tetrakis(4-sulfonatophenyl)porphyrin (ZnTSPP4-) and polycyclic quaternary salts in water. Upon excitation with visible light, these nonfluorescent ion pairs undergo rapid (< 10 ps) electron transfer to form radical ion pairs which do not dissociate into separated radical ions. The rate of reverse electron transfer within a radical ion pair decreases with increasing size of the quaternary salt. This effect is sufficiently pronounced, in the extreme case, to produce a long-lived (tau almost-equal-to 70 ns) radical ion pair in which the reactants may be held together less tightly due to solvent penetration.