Specific salt effects on energy transfer reactions from [Tb(2,6-pyridinedicarboxylate)3]3− to Co(III) complexes in aqueous solutions

Specific cation and anion effects are investigated on reactions of energy transfer from [Tb(dpa)3]3− (dpa = 2,6-pyridinedicarboxylate) to [Co(bpy)3]3+ (bpy = 2,2′-bipyridine), [Co(en)3]3+ (en=ethylenediamine), [Co(pam)3]3+ (pam = 2-aminomethylpyridine) and [Co(NH3)6]3+ in aqueous solutions. Quenching rate constants of [Tb(dpa)3]3− by [Co(bpy)3]3+ are changed by the added anions in the following order, Cl− > Br− > ClO4−. However, the order of the quenching rate constants of [Tb(dpa)3]3− by [Co(en)3]3+, [Co(pam)3]3+ and [Co(NH3)6]3+ is different, Cl− ≈ Br− < ClO4−. The anion effect on the rate constants is discussed in terms of the variation of the distance between energy donor and acceptor in the encounter complexes surrounded by the indifferent ions. Both quenching reactions of [Tb(dpa)3]3−-[Co(bpy)3]3+ and −[Co(en)3]3+ are less affected by coexisting cations. The anion salt effect in the Tb-Co systems is discussed with the comparison of [Ru(bpy)3]2+-[Cr(CN)6]3− system.