The prospect of a photoexcited chromophore undergoing a reaction that involves transfer of an electron and a proton within an encounter complex with an appropriate acceptor is examined in this work, employing [(bpy)2Ru­(4,4′-dhbpy)]2+ (4,4′-dhbpy = 4,4′-dihydroxy-2,2′-bipyridine, [Ru­(II)­OH]2+) as a chromophore and MQ+ (N-methyl-4,4′-bipyridinium) as an acceptor. The use of an aprotic solvent (CH3CN) allowed unambiguous evaluation of the photoproducts emerging from the encounter complex by using transient absorption spectroscopy in the visible region. While the overall photoproducts reflect transfer of an electron and a proton from [Ru­(II)­OH]2+ to MQ+, detailed analysis suggests that the process occurs via excited-state electron transfer (k q = 7 × 107 M–1 s–1), followed by proton transfer to MQ0 from [Ru­(II)­OH]2+ in solution. The proton transfer process follows pseudo-first order kinetics in the Ru complex and has overall second-order rate constants that are near the diffusion limit. The results are discussed in terms of the free energies for the reactions involved.

No abstract

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.

No abstract