Electron tunneling in a cofacial zinc porphyrin-quinone cage molecule: novel temperature and solvent dependence

• Published in: Journal of the American Chemical Society 1990, Vol.112 (3), p.957-963
• Main Authors: Delaney, John K, Mauzerall, David C, Lindsey, Jonathan S
• Format: Article
• Language: English
• Subjects: 400500 - Photochemistry; 550500 - Metabolism; AROMATICS; Atomic and molecular physics; BASIC BIOLOGICAL SCIENCES; CARBOXYLIC ACIDS; DATA; DATA ANALYSIS; ELEMENTS; Exact sciences and technology; EXPERIMENTAL DATA; Fluorescence and phosphorescence; radiationless transitions, quenching (intersystem crossing, internal conversion); HETEROCYCLIC ACIDS; HETEROCYCLIC COMPOUNDS; INFORMATION; INORGANIC, ORGANIC, PHYSICAL AND ANALYTICAL CHEMISTRY; KINETICS; MEASURING INSTRUMENTS; MEASURING METHODS; METALS; Molecular properties and interactions with photons; NUMERICAL DATA; ORGANIC ACIDS; ORGANIC COMPOUNDS; ORGANIC NITROGEN COMPOUNDS; ORGANIC OXYGEN COMPOUNDS; PHOTOSYNTHETIC REACTION CENTERS; Physics; PORPHYRINS; QUINONES; REACTION KINETICS; SOLVENT PROPERTIES; TEMPERATURE DEPENDENCE; TUNNELING; ZINC
• ISSN: 0002-7863; 1520-5126
• DOI: 10.1021/ja00159a008

The zinc porphyrin-quinone cage molecule ZnPQ(Ac){sub 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, PQ{sup a}, 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 {minus}1 and +2 kJ/mol. The quenching rates of the singlet state of the more distant conformer PQ{sub b} vary over a somewhat greater range (10-fold) in these solvents with activation energies of +4 kJ/mol. The activation energies of the forward 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){sub 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.