Femtosecond Dynamics and Electrocatalysis of the Reduction of O₂:  Tetraruthenated Cobalt Porphyrins

Recently we reported the femtosecond dynamics of some porphyrin molecules. These studies elucidated the elementary steps involved in intramolecular relaxation and charge transfer (CT) in cobalt tetraphenylporphyrin (CoTPP) from the porphyrin π (a_(2u)) system to the Co(d_z^2). Our interest in the dynamics of intramolecular electron transfer stems from recent studies of cobalt porphyrins that were modified by attachment of Ru(II) or Os(II) complexes to the periphery of the porphyrin ring. These modified cobalt porphyrins exhibit high catalytic activity for the electroreduction of O_2 directly to H_2O, a process that most monomeric cobalt porphyrins are unable to accomplish. The mechanism by which the modified cobalt porphyrins achieve the direct, four-electron reduction of O_2 has been suggested to involve back-bonding interactions among the attached Ru(II) and Os(II) centers, the porphyrin ring, and the Co(II) ion at its center.