Fluorescence of curcumin in alkaline dimethyl sulfoxide and the effects of alkali metal cations on it

Abstract

Earlier, we reported our finding of the fluorescence emission derived from the triple-deprotonated form of curcumin (Cur3–) in dimethyl sulfoxide (DMSO) in the presence of NaOH. Herein, we disclose the results of the studies on the luminescent properties of Cur3– in the presence of LiOH, NaOH and KOH, which are indicative of the cation-dependent behavior of the Cur3– fluorescence excitation and the emission spectra. The fluorescence quantum yield in the presence of LiOH and NaOH significantly increases compared to that of neutral curcumin. Studying the fluorescence polarization properties revealed that the observed increase in the quantum yield is accounted for by the decreased rate of the nonradiative excited-state deactivation. We assume that the origin of the cation-dependent fluorescence properties refers to the formation of coordination bonds between the alkali metal cations and the oxygen atoms of the β-diketone (keto-enol) group of curcumin, which is substantiated by the experimental results and supported by the semiempirical modeling.