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 [Cu II L 2 ] (L=quercetin, rutin) and [Cu II 2 L n ] ( 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 τ <350 ns. These are lifetimes shorter than the known lifetimes ( τ >1 ms) of the quercetin and rutin radical's decay.