Solvent dependent photophysical properties of dimethoxy curcumin

Dimethoxy curcumin (DMC) is a methylated derivative of curcumin. In order to know the effect of ring substitution on photophysical properties of curcumin, steady state absorption and fluorescence spectra of DMC were recorded in organic solvents with different polarity and compared with those of curcumin. The absorption and fluorescence spectra of DMC, like curcumin, are strongly dependent on solvent polarity and the maxima of DMC showed red shift with increase in solvent polarity function (Δf), but the above effect is prominently observed in case of fluorescence maxima. From the dependence of Stokes’ shift on solvent polarity function the difference between the excited state and ground state dipole moment was estimated as 4.9 D. Fluorescence quantum yield (ϕf) and fluorescence lifetime (τf) of DMC were also measured in different solvents at room temperature. The results indicated that with increasing solvent polarity, ϕf increased linearly, which has been accounted for the decrease in non-radiative rate by intersystem crossing (ISC) processes.

A significantly large Stokes’ shift of 3513 cm−1 and 4524 cm−1 was observed for dimethoxy curcumin (DMC) in cyclohexane and acetonitrile respectively. The excited state dipole moment of DMC is higher than the ground state by 4.9 D..Figure optionsDownload as PowerPoint slideHighlights
► Photophysical properties of DMC were studied in solvents of different polarity.
► Absorption and fluorescence spectra of DMC are strongly dependent on solvent polarity.
► Difference between the excited state and ground state dipole moment of DMC was 4.9 D.
► Fluorescence quantum yield, lifetime of DMC increased linearly with solvent polarity.
► Higher the solvent polarity, lower the rate of intersystem crossing for DMC.