Photoinduced electron transfer from excited [tris(2,2′-bipyridine)ruthenium(II)]2+ to a series of anthraquinones with small positive or negative Gibbs energy of reaction. Marcus behavior and negative activation enthalpies

Abstract

In the electron transfer (ET) quenching reactions of electronically excited *Ru(bpy)₃²⁺ in acetonitrile an increase of the rate constant k_q is observed in the series of 2-methyl-, 1-chloro-, and 1-nitro-anthraquinone as quenchers. If alkali salts are used as supporting electrolytes the AQ^- radical anions are found to form specific associates with the alkali cations. In the presence of non-associating tetraalkylammonium salts the system follows the predictions of Marcus theory. Numerical methods are developed which allow the determination of the rate constants of the conventional reaction scheme. This analysis shows that the quantum yield of free AQ^- radical anion formation is governed by the interplay of forward, reverse and back ET. Negative activation enthalpies are found for the activation controlled quenching reactions. From the numerical analysis of the system of rate constants it is inferred that this phenomenon is due to the elementary ET step in the reaction sequence. We discuss the pre-equilibrium and elementary reaction models for reactions with negative activation enthalpy and present, to our knowledge, the first example of successful discrimination between them.