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 solvent s. A nanosecond laser flash photolysis apparatus permitted the observa tion of the porphyrin triplet state decay, with the quinone fully redu ced or with it fully oxidized to allow enhanced quenching of the porph yrin 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 methyl ene chloride. It is shown that the available Gibbs energy and the elec tron-transfer rate constants, determined in various solvents over a 40 degrees C temperature range, do not exhibit the relationship put fort h by Marcus electron-transfer theory. An alternative hypothesis of a f ast equilibrium being established between the triplet porphyrin and an intermediate state before the molecule reaches a radical-ion-pair sta te is supported by the observation of negative activation energies in benzonitrile and methylene chloride. Since neither the radical ion pai r nor the intermediate was observed as a spectroscopic entity, it is n ot possible to identify conclusively the pathway of de-excitation of t he porphyrin triplet.