Bimolecular Excited-State Proton-Coupled Electron Transfer within Encounter Complexes

Bimolecular excited-state proton-coupled electron transfer (PCET*) was observed for reaction of the triplet MLCT state of [(dpab)2Ru(4,4 '-dhbpy)]2+ (dpab = 4,4 '-di(n-propyl)-amido-2,2 '-bipyridine, 4,4 '-dhbpy = 4,4 '-dihydroxy-2,2 '-bipyri-dine) with N-methyl-4,4 '-bipyridinium (MQ+) and N-benzyl-4,4 '- bipyridinium (BMQ+) in dry acetonitrile solutions. The PCET* reaction products, the oxidized and deprotonated Ru complex, and the reduced protonated MQ+ can be distinguished from the excited state electron transfer (ET*) and the excited state proton transfer (PT*) products by the difference in the visible absorption spectrum of the species emerging from the encounter complex. The observed behavior differs from that of reaction of the MLCT state of [(bpy)2Ru(4,4 '-dhbpy)]2+ (bpy = 2,2 '-bipyridine) with MQ+, where initial ET* is followed by diffusion-limited proton transfer from the coordinated 4,4 '-dhbpy to MQ0. The difference in observed behavior can be rationalized based on changes in the free energies of ET* and PT*. Substitution of bpy with dpab results in the ET* process becoming significantly more endergonic and the PT* reaction becoming somewhat less endergonic.