On the Ligand-to-Metal Charge-Transfer Photochemistry of the Copper(II) Complexes of Quercetin and Rutin

Abstract

In contrast to the UV-photoinduced ligand photoionization of the flavonoid complexes of FeIII, redox reactions initiated in ligand-to-metal charge-transfer excited states were observed on irradiation of the quercetin (1) and rutin (2) complexes of CuII. Solutions of complexes with stoichiometries [CuIIL2] (L¼quercetin, rutin) and [CuII 2Ln] (n¼1, L¼quercetin; n¼3, L¼rutin) were flash-irradiated at 351 nm. Transient spectra observed in these experiments showed the formation of radical ligands corresponding to the one-electron oxidation of L and the reduction of CuII to CuI. The radical ligands remained coordinated to the CuI centers, and the substitution reactions replacing them by solvent occurred with lifetimes t<350 ns. These are lifetimes shorter than the known lifetimes (t>1 ms) of the quercetin and rutin radical s decay.