Direct Observation Of Sequential Electron And Proton Transfer In Excited-State Et/Pt Reactions

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)(2)Ru(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 x 10(7) M-1 s(-1)), followed by proton transfer to MQ(0) 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.