SOLVENT EFFECT AND ACTIVATION PARAMETERS IN THE OXIDATIVE QUENCHING OF EXCITED TRIS(2,2'-BIPYRIDINIUM)RUTHENIUM(II) BY ORGANIC ACCEPTORS

The solvent effect on the electron transfer quenching of excited tris(2,2'-bipyridinium)ruthenium(II) by nitrobenzenes and duroquinone has been investigated. The rate constants and activation parameters were determined in acetonitrile (ACN), methanol (MeOH) and dimethylformamide (DMF). The activation enthalpies are negative in ACN while they are near zero or positive in MeOH and DMF. The activation entropies are much more negative for ACN than for MeOH or DMF. The results are discussed in terms of the Rehm-Weller formalism for excited-state electron transfer and the Marcus-Sutin theory. The negative activation enthalpies can be explained by the reversibility of the electron transfer step or by transition state theory if quantum-state-dependent activated complexes are considered. In the light of the results the second alternative is considered more probable.