Spectroscopic Properties of the Quercetin–Divalent Metal Complexes in Hydro‐Organic Mixed Solvent

The formation and spectroscopic properties of quercetin (QCT)–divalent metal complexes were studied using Cu 2+ , Ni 2+ , Co 2+ , Mn 2+ , Zn 2+ , Mg 2+ , and Ca 2+ in a hydro‐organic mixed solvent. The change of UV/visible absorption spectra of QCT due to addition of a metal showed the complex formation. The intensity of fluorescence spectra increased gradually with titration of the metal. The experimental data and theoretical calculation suggest that Cu 2+ , Ni 2+ , Co 2+ , and Mn 2+ coordinate the site between C(3)OH and C(4)O but Zn 2+ prefers to bind to the site between C(5)OH and C(4)O. QCT–Cu 2+ , QCT–Ni 2+ , QCT–Co 2+ , and QCT–Mn 2+ complexes exhibit S 2  → S 0 fluorescence only as S 1  → S 0 emission is absent due to the excited‐state intramolecular proton transfer (ESIPT) at the S 1 state. As this ESIPT cannot occur at the QCT–Zn 2+ due to the chelation of Zn 2+ at a different site, QCT–Zn 2+ can produce characteristic S 2  → S 0 and S 1  → S 0 dual fluorescence.