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

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 an d transient absorption techniques having pico and nanosecond time resolutio n, in addition to steady-state absorption and fluorescence spectroscopic te chniques. Steady-state absorption and fluorescence characteristics of curcu min have been found to be sensitive to the solvent characteristics. Large c hange (Delta mu = 6.1 Debye) in dipole moments due to photoexcitation to th e excited singlet state (S-1) indicates strong intramolecular charge transf er character of the latter. Curcumin is a weakly fluorescent molecule and t he fluorescence decay properties in most of the solvents could be fitted we ll to a double-exponential decay function. The shorter component having lif etime in the range 50-350 ps and percent contribution of amplitude more tha n 90% in different solvents may be assigned to the enol form, whereas the l onger 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-D-6 also supports the fact that the enol form is present In the solution by more than about 95% in these solvents. Excited singlet (S-1) and triplet (T-1) absorption s pectrum and decay kinetics have been characterized by pico and nanosecond l aser Rash photolysis. Quantum yield of the triplet Is low (phi (r) less tha n or equal to 0.12), Both the fluorescence and triplet quantum yields being low (phi (r) + phi (r) < 0.18), the photophysics of curcumin is dominated by the energy relaxation mechanism via the internal conversion process.