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

In contrast to the UV-photoinduced ligand photoionization of the flavonoid complexes of Fe-III, redox reactions initiated in ligand-to-metal charge-transfer excited states were observed on irradiation of the quercetin (1) and rutin (2) complexes of Cu-II. Solutions of complexes with stoichiometries [(CuL2)-L-II] (L=quercetin, rutin) and [(Cu2Ln)-L-II] (n=1, L=quercetin; n=3, L=rutin) 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 Cu-II to Cu-I. The radical ligands remained coordinated to the Cu-I centers, and the substitution reactions replacing them by solvent occurred with lifetimes tau < 350 ns. These are lifetimes shorter than the known lifetimes (tau > 1 ms) of the quercetin and rutin radical's decay.