Temperature dependence of the fluorescence properties of curcumin.

TLDR: A model in which the excited-state proton transfer breaks the planar hexagonal structure of the keto-enol center of the molecule enhances the nonradiative process driven by the twist angle between the two phenol moieties is proposed.

Abstract: Steady-state and time-resolved techniques were employed to study the nonradiative process of curcumin dissolved in ethanol and 1-propanol in a wide range of temperatures. We found that the nonradiative rate constants at temperatures between 175–250 K qualitatively follow the same trend as the dielectric relaxation times of both neat solvents. We attribute the nonradiative process to solvent-controlled proton transfer. We also found a kinetic isotope effect on the nonradiative process rate constant of ∼2. We propose a model in which the excited-state proton transfer breaks the planar hexagonal structure of the keto–enol center of the molecule. This, in turn, enhances the nonradiative process driven by the twist angle between the two phenol moieties.