Photophysical and photochemical aspects of curcumin

Curcumin not only exhibits numerous biomedical virtues, it also possesses many interesting photochemical and photophysical properties. In liquids, the optical absorption maximum of curcumin and even more so its emission maximum, are solvent sensitive and vary widely per solvent: ~408-434 nm for absorption and ~460-560 nm for emission. The fluorescence quantum yield of curcumin is low, generally speaking, and is reduced significantly in the presence of water. Due to its low water solubility, curcumin's excited-state photophysics has been investigated mainly in alcohols and micellar solutions. The time-resolved optical spectrum of curcumin was studied of late using a variety of ultrafast spectroscopy techniques by several groups. In this chapter we tackle the nature of the proton transfer and non-radiative decay processes of excited curcumin in solution responsible for its low quantum yield. From the temperature dependence of curcumin in ethanol and 1-propanol, it is found that the non-radiative rate constants at temperatures in the range of 175 to 250 K qualitatively follow the same trend as the viscosity and the dielectric relaxation times of both neat solvents. The non-radiative process is attributed to isomerization of the conjugate chains connecting the central diketone moiety with the peripheral methoxy phenols. Next, it is established that curcumin may act as a "photoacid" from two studies, in the first the effect of pressure was measured on its excited-state decay rate, using a diamond-anvil cell to build pressures of up to ~2.3 GPa, in the second the effect of acetate, a mild base, was inspected on the excited-state proton-transfer (ESPT) rate and inversely on the fluorescence of curcumin in both methanol and ethanol. It was found that at high acetate concentrations, a proton is transferred to the acetate and the curcumin fluorescence lifetime and intensity drops. Finally, it was found that in addition to its capability to act as a photoacid, curcumin possesses also a more pronounced photobasic potency. In the excited-state curcumin reacts effectively with either excess protons in solution or with weak organic acids like acetic or formic acids and forms an excited protonated form of curcumin that emits at long wavelength ~560 nm. In respect to the neutral ROH form, both charged forms: RO- and ROH₂⁺ are similarly red-shifted.