Photoinduced trans-to-cis Isomerization of Cyclocurcumin

The photophysical properties of cyclocurcumin in various solvents are studied for the first time to shed light on the nonradiative processes of the parent compound, curcumin, which has a range of medicinal properties. Steady-state fluorescence and fluorescence–excitation spectra of cyclocurcumin in polar aprotic solvents are strongly dependent on excitation (λ_ex) and emission (λ_em) wavelength, respectively. The fluorescence quantum yield also depends on λ_ex and increases with the viscosity of the medium. Time-resolved studies show nonexponential fluorescence decay in all solvents studied. The two fluorescence decay components of cyclocurcumin in alcohols exhibit a strong dependence on viscosity and temperature. NMR spectroscopy indicates that cyclocurcumin is entirely in the trans form with respect to the C6–C7 double bond in methanol, chloroform, and acetone. It is suggested that at least two conformational isomers about another single bond (C5–C6 or C7–C1″ or both) and that trans-to-cis isomerization about the C6–C7 double bond of these isomers provide the most likely means of rationalizing the steady-state spectra and the nonradiative decay mechanisms in both protic and aprotic polar solvents.