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

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 kq, quenching rate constants, and k1, 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 k1 values in the aqueous solutions whose ionic strength was adjusted with LiCl and KCl, k1LiCl/k1KCl, 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.