ELECTRON-TUNNELING IN A COFACIAL ZINC PORPHYRIN-QUINONE CAGE MOLECULE - NOVEL TEMPERATURE AND SOLVENT DEPENDENCE

The zinc porphyrin-quinone cage molecule ZnPQ(Ac)4 exists as two slowly equilibrating conformers differing in the interplanar porphyrin-quinone distance and thus in the size of the cavity. The close conformer, PQa, shows only a 4-fold change in rate of quenching of fluorescence in 21 solvents of widely varying properties. Rate constants have been measured in four solvents over wide ranges of temperature: 300-80 K. The activation energies in five solvents are small, varying between -1 and +2 kJ/mol. The quenching rates of the singlet state of the more distant conformer PQb vary over a somewhat greater range (10-fold) in these solvents with activation energies of +4 kJ/mol. The activation energies of the foward and reverse electron transfer from the triplet state average +7 kJ/mol. The rates are little affected by viscosity including transition to the glassy state. The weak temperature and solvent dependence of electron transfer in ZnPQ(Ac)4 can best be explained by nonadiabatic electron tunneling. The triplet reaction stores >90% of the energy of the excited state in the free energy of the products. The interesting observation was made that increasing the molar volume of the solvent decreased the conformer equilibrium constant. © 1990, American Chemical Society. All rights reserved.