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 Cu2+, Ni2+, Co2+, Mn2+, Zn2+, Mg2+, and Ca2+ 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 Cu2+, Ni2+, Co2+, and Mn2+ coordinate the site between C(3)OH and C(4)O but Zn2+ prefers to bind to the site between C(5)OH and C(4)O. QCT–Cu2+, QCT–Ni2+, QCT–Co2+, and QCT–Mn2+ complexes exhibit S2 → S0 fluorescence only as S1 → S0 emission is absent due to the excited‐state intramolecular proton transfer (ESIPT) at the S1 state. As this ESIPT cannot occur at the QCT–Zn2+ due to the chelation of Zn2+ at a different site, QCT–Zn2+ can produce characteristic S2 → S0 and S1 → S0 dual fluorescence.