INTRAMOLECULAR PHOTOCHEMICAL ELECTRON-TRANSFER .8. DECAY OF THE TRIPLET-STATE IN A PORPHYRIN-QUINONE MOLECULE

The rate constant for the quenching of the porphyrin triplet state in a covalently linked porphyrin-amide-quinone molecule has been measured in several solvents and as a function of temperature in three solvents. A nanosecond laser flash photolysis apparatus permitted the observation of the porphyrin triplet state decay, with the quinone fully reduced or with it fully oxidized to allow enhanced quenching of the porphyrin triplet via electron transfer. A difference of rate constants in the two cases yielded the electron-transfer rate constant which ranged from 1.0 x 10(4) s(-1) in acetonitrile to 2.8 X 10(5) s(-1) in methylene chloride. It is shown that the available Gibbs energy and the electron-transfer rate constants, determined in various solvents over a 40 degrees C temperature range, do not exhibit the relationship put forth by Marcus electron-transfer theory. An alternative hypothesis of a fast equilibrium being established between the triplet porphyrin and an intermediate state before the molecule reaches a radical-ion-pair state is supported by the observation of negative activation energies in benzonitrile and methylene chloride. Since neither the radical ion pair nor the intermediate was observed as a spectroscopic entity, it is not possible to identify conclusively the pathway of de-excitation of the porphyrin triplet.