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.

Publication Type

Journal contribution

Authors

Frank, R.; Greiner, G.; Rau, H.

Year of publication

1999

Published in

Phys. Chem. Chem. Phys.

Band/Volume

1/15

ISBN / ISSN / eISSN

1463-9076

Page (from - to)

3481-3490

In the electron transfer (ET) quenching reactions of electronically excited *Ru(bpy)32+ in acetonitrile an increase of the rate const. kq is obsd. in the series of 2-methyl-, 1-chloro-, and 1-nitroanthraquinone as quenchers. If alkali salts are used as supporting electrolytes the AQ-radical anions are found to form specific assocs. with the alkali cations. In the presence of non-assocg. tetraalkylammonium salts the system follows the predictions of Marcus theory. Numerical methods are developed which allow the detn. of the rate consts. of the conventional reaction scheme. This anal. shows that the quantum yield of free AQ- radical anion formation is governed by the interplay of forward, reverse and back ET. Neg. activation enthalpies are found for the activation controlled quenching reactions. From the numerical anal. of the system of rate consts. it is inferred that this phenomenon is due to the elementary ET step in the reaction sequence. The authors discuss the pre-equil. and elementary reaction models for reactions with neg. activation enthalpy and present, to our knowledge, the first example of successful discrimination between them.