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)Ru(4,4'-dhbpy)] (dpab = 4,4'-di(-propyl)amido-2,2'-bipyridine, 4,4'-dhbpy = 4,4'-dihydroxy-2,2'-bipyridine) with -methyl-4,4'-bipyridinium (MQ) and -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)Ru(4,4'-dhbpy)] (bpy = 2,2'-bipyridine) with MQ, where initial ET* is followed by diffusion-limited proton transfer from the coordinated 4,4'-dhbpy to MQ. 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.