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 (delta mu = 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 (phi T < or = 0.12). Both the fluorescence and triplet quantum yields being low (phi f + phi T < 0.18), the photophysics of curcumin is dominated by the energy relaxation mechanism via the internal conversion process.

Excited-State Intramolecular Hydrogen Atom Transfer of Curcumin in Surfactant Micelles

“…It appears that the fast ESIHT times observed in these micelles compare well with those in the nonpolar solvent cyclohexane. 21,22 However, as indicated earlier, the ESIHT process of curcumin in cyclohexane is most likely faster than what was measured with TCSPC due to limited temporal resolution. In comparison to curcumin in the TX-100 micelle, the ESIHT process occurs faster in the DTAB and SDS micelles.…”

“…[21][22][23] Nardo et al and Khopde et al independently reported that there is a ∼50 ps decay component in the fluorescence lifetime (89 and 64 ps, as measured by the two groups, respectively) of curcumin in cyclohexane using TCSPC. 21,22 This decay component is, however, limited by the time resolution of the TCSPC apparatus.…”

“…In short, in the case of the TX-100 micelle, our results indicate that hydrogen bonding between curcumin and the micellar environment is similar to that between curcumin and bulk methanol, which is consistent with results from a previous report. 22 In the DTAB and SDS micelles, the ESIHT time constants are approximately 50 ps, as shown in Table 1. It appears that the fast ESIHT times observed in these micelles compare well with those in the nonpolar solvent cyclohexane.…”

Excited-State Intramolecular Hydrogen Atom Transfer and Solvation Dynamics of the Medicinal Pigment Curcumin

“…In addition to ESIHT, the fluorescence upconversion results also reveal that there is a short-lived component with a time scale of 12 ps in methanol, which is likely to be undetectable in TCSPC because of insufficient time resolution. Solvation, in addition to ESIHT, is expected to play a role in the excited-state relaxation dynamics due to the significant dipole moment change reported for curcumin: ∆µ ) 6.1 D. 31 Such a ∆µ is comparable to that of Coumarin 153 (∆µ ≈ 8 D), 42,49 which is a standard probe molecule for solvation dynamics. Additionally the fluorescence spectra exhibit a Stokes shift that is largely dependent on solvent polarity, implying that solvation may play a major role in the relaxation of the excited state in addition to ESIHT.…”

“…12,[31][32][33][34] In all cases, these studies focused on excited-state intramolecular hydrogen atom transfer (ESIHT) of curcumin because this phenomenon has been associated with the medicinal properties of other pigment molecules, including hypericin and hypocrellin. [35][36][37][38][39] Two of these studies agree that the fluorescence lifetime of curcumin in methanol has a dominant component with a time constant of roughly 130 ps.…”