Effect of solvent on the excited-state photophysical properties of curcumin

Abstract

Photophysical properties of curcumin, 1,7-bis-(4-hydroxy-3-methoxy phenyl)-1,6-heptadiene-2,5-dione, a pigment found in the rhizomes of Curcuma longa (turmeric) have been studied in different kinds of organic solvent and also in Triton X-100 aqueous micellar media using time-resolved fluorescence and transient absorption techniques having pico and nanosecond time resolution, in addition to steady-state absorption and fluorescence spectroscopic techniques. Steady-state absorption and fluorescence characteristics of curcumin have been found to be sensitive to the solvent characteristics. Large change (Δμ= 6.1 Debye) in dipole moments due to photoexcitation to the excited singlet state (S₁) indicates strong intramolecular charge transfer character of the latter. Curcumin is a weakly fluorescent molecule and the fluorescence decay properties in most of the solvents could be fitted well to a double-exponential decay function. The shorter component having lifetime in the range 50–350 ps and percent contribution of amplitude more than 90% in different solvents may be assigned to the enol form, whereas the longer component, having lifetime in the range 500–1180 ps with less than 10% contribution may be assigned to the di-keto form of curcumin. Our nuclear magnetic resonance study in CDCl₃ and dimethyl sulfoxide-D₆ also supports the fact that the enol form is present in the solution by more than about 95% in these solvents. Excited singlet (S₁) and triplet (T₁) absorption spectrum and decay kinetics have been characterized by pico and nanosecond laser flash photolysis. Quantum yield of the triplet is low (φ_T≤0.12). Both the fluorescence and triplet quantum yields being low (φ_f+φ_T<0.18), the photophysics of curcumin is dominated by the energy relaxation mechanism via the internal conversion process.