Probing the Dynamics of Solvation and Structure of the OH(-) Ion in Aqueous Solution from Picosecond Transient Absorption Measurements

The reaction of intracomplex proton transfer (44BPY(-.)...HO-H) → 44BPYH(.) + OH(-) that follows the photoreduction of 4,4’-bipyridine (44BPY) into its anion radical 44BPY(-) in the presence of 1,4-diazabicyclo[2.2.2]octane (DABCO) is investigated in acetonitrile-water mixtures by using picosecond transient absorption. The dependence of the appearance kinetics of the 44BPYH(.) radical on the water content reveals a highly diffusional proton transfer process that is controlled by the dynamics of solvation of the released hydroxide ion. The results are interpreted on the basis of a two-step mechanism where an intermediate solvation complex (44BPYH(.))OH(-)(H(2)O)(3) is formed first before evolving toward a final four-water hydration structure OH(-)(H(2)O)(4).