Photoinduced electron transfer reactions of ruthenium(II) complexes. 2. Oxidative quenching of excited tris(2,2'-bipyridine)ruthenium(2+) by neutral organic electron acceptors

Oxidative quenching of *Ru(bpy){sub 3}{sup 2+} (bpy = 2,2{prime}-bipyridine) by neutral organic electron acceptors such as quinones, nitroaromatics, and aromatic esters in acetonitrile was studied in detail. The quenching exhibited an anomalous negative temperature dependence at {Delta}G{sub 23} (free energy change of an electron-transfer step, k{sub 23}) > {minus}3 kcal/mol, which accompanied large and negative activation entropies ({minus}18 to {minus}44 eu). The abnormal behavior was explained satisfactorily by the large contribution of back electron transfer regenerating the excited-state reactants (k{sub 32}). In a narrow {Delta}G{sub 23} region between {minus}2 and 0 kcal/mol, a bell-shaped Eyring plot was observed for several quenchers. The activation parameters for the forward (k{sub 23}) and backward (k{sub 32}) electron-transfer processes were determined on the basis of the bell-shaped Eyring plot and the standard enthalpy and entropy changes of the process. Although the temperature dependence of the quenching is anomalous, the activation parameters for the elementary process, k{sub 23} or k{sub 32}, are quite normal. The mechanism of the quenching was discussed on the basis of the rate constant as well as of the activation parameters relevant to k{sub 23}, k{sub 32}, and k{sub 30} (deactivation from the product ion pairs).