Specific cation effect on quenching reactions of excited tris(α,α ′-diimine)ruthenium(II) and tris(2,2 ′-bipyridine)chromium(III) by tris(oxalato)- and tris(malnato)chromates(III) in aqueous solutions

Abstract

Specific salt effects were studied on the quenching reaction of excited [Ru(N–N) 3] 2+ (N–N=2,2 ′-bipyridine (bpy), 1,10-phenanthrorine (phen)) and [Cr(bpy) 3] 3+ by [Cr(ox) 3] 3− (ox=oxalate ion) and [Cr(mal) 3] 3− (mal=malonate ion) in aqueous solutions as a function of alkali metal ions which were added for adjustment of ionic strength. The value of k q, quenching rate constants, and k 1, energy transfer rate constant in encounter complex, is changed by the cations as the order of Li + > Na + > K + ≈ Rb + ⩽ Cs +, although diffusion rate constants are not changed by the co-existing cations. Among the quenching reactions investigated in this work, a ratio of k 1 values in the aqueous solutions whose ionic strength was adjusted with LiCl and KCl, k 1 LiCl/ k 1 KCl, is larger for quenching systems of closely approached donor-acceptor pair than loosely bounded pair. These results indicate that co-existing alkali cation tunes the distance between donor and acceptor in encounter complex where energy transfer occurs.