COMPETITIVE ENERGY-TRANSFER AND REDUCTIVE QUENCHING OF THE CT EXCITED-STATES OF COPPER(I) PHENANTHROLINES

For the charge-transfer excited states of three different copper phena nthrolines, decamethyl ferrocene is a significantly better quencher th an ferrocene itself In a more detailed investigation we have studied t he quenching of photoexcited Cu(dpp)(2)(+), where dpp denotes 2,9-diph enyl-1,10-phenanthroline, by a series of ferrocene derivatives with va rying numbers of methyl substituents. When the ferrocenes have relativ ely positive reduction potentials, the quenching rate is consistently around 10(8) M(-1) s(-1) due to an energy-transfer process. For the st rongest reducing agents, the quenching rate constants are larger, but electron-transfer quenching does not become dominant until the driving force is about 0.3 V. An innersphere reorganizational energy requirem ent of the copper system inhibits reductive quenching such that the ef fective self-exchange rate of the system is about 10(5) M(-1) s(-1). I n contrast, with oxidative quenchers the corresponding rate constant i s about 10(10) M(-1) s(-1). The former value is comparable to estimate s obtained in previous studies of ground state processes that involve the population of the analogous d sigma molecular orbitals. Compariso ns with literature data suggest that a significant Franck-Condon barri er inhibits energy-transfer quenching as well. It should be possible t o avoid the various kinetic limitations by using phenanthroline ligand s with bulkier substituents in the 2,9 positions; however, to develop more efficient photooxidants, it will also be necessary to enhance the excited-state reduction potential.