Time-resolved spectroscopic characterization of photo-induced valence tautomerism for a cobalt-dioxolene complex

The valence tautomerism of low-spin Co-III (Cat-N-BQ)(Cat-N-SQ) (where Cat-N-BQ is 2-(2-hydroxy-3,5-di-tert-butylphenylimino)-4,6-di-tert-butylcyclohexa-3,5-dienone and Cat-N-SQ is the dianionic radical analogue) was investigated by means of UV-vis pump-probe transient absorption spectroscopy and H-1 NMR technique in chloroform and dichloromethane. By exciting the CT transition of the complex at 480 nm, an intramolecular electron transfer process is selectively triggered. The photo-induced charge transfer is pursued by a cascade of two main molecular events characterized by the ultrafast transient absorption spectroscopy: the first gives rise to the metastable high-spin Co-II(Cat-N-BQ), that, secondly, reaches the chemical equilibrium with the reactant species. The rate constant of back valence tautomerization estimated by measuring the lifetime of high-spin Co-II(Cat-N-BQ)2 species and the equilibrium constant for the Co-III(Cat-N-BQ)(Cat-N-SQ) reversible arrow Co-II(Cat-N-BQ)2 interconversion, is significantly large (on the order of 10(9) s(-1)). It is interpreted under the point of view of the theory formulated by Jortner and Buhks et al. for non-adiabatic radiationless processes. (c) 2005 Elsevier B.V. All rights reserved.