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) 2 Ru(4,4'-dhbpy)] 2+ (dpab = 4,4'-di( n -propyl)amido-2,2'-bipyridine, 4,4'-dhbpy = 4,4'-dihydroxy-2,2'-bipyridine) 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) 2 Ru(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 MQ 0 . 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.