Studies on curcumin and curcuminoids. XXXIX. Photophysical properties of bisdemethoxycurcumin

Abstract

The steady-state absorption and fluorescence, as well as the time-resolved fluorescence properties of bisdemethoxycurcumin dissolved in several solvents differing in polarity and H-bonding capability were measured. The photodegradation quantum yield of the compound in acetonitrile and methanol was determined. The bisdemethoxycurcumin decay mechanisms from the S(1) state were discussed and compared with those of curcumin. The differences in S(1) dynamics observed between bisdemethoxy-curcumin and curcumin could be ascribed to a difference in H-bond acceptor/donor properties of the phenolic OH and a difference in strength of the intramolecular H-bond in the keto-enol moiety within the two molecules.

Fig. 1

Naturally occurring curcuminoids

Fig. 2

Upper panel: fluorescence decay (dots) of bisDMC in DMSO, single exponential fit (dark-gray solid line) and double exponential fit (light-gray solid line); lower panel: residuals corresponding to the single exponential (dark full dots) and double exponential (light empty circles) fits of above

Fig. 3

a) Absorption and b) fluorescence emission spectra of bisDMC in cyclohexane (solid black line), acetone (solid grey line), DMSO (full dots) and methanol (empty circles). In panel b) inset the emission spectrum of bisDMC in cyclohexane obtained upon excitation at 420 nm (full line) is compared with that obtained upon excitation at 350 nm (triangles)

Fig. 4

a) Enol conformers and b) diketo conformers of the investigated curcuminoids. For R-structures see Fig. 1

Fig. 5

Scheme of the excited-state intra-molecular proton transfer (ESIPT) undergone by the H-bonded cis enol conformer, see Fig. 4a