Effect of acid on the ultraviolet-visible absorption and emission properties of curcumin

Yuval Erez, Ron Simkovitch, Shay Shomer, Rinat Gepshtein, Dan Huppert*

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

41 Scopus citations

Abstract

Steady-state and time-resolved emission techniques were employed to study the acid-base effects on the UV-vis spectrum of curcumin in several organic solvents. The fluorescence-decay rate of curcumin increases with increasing acid concentration in all of the solvents studied. In methanol and ethanol solutions containing about 1 M HCl, the short-wavelength fluorescence (λ < 560 nm) decreases by more than an order of magnitude. (The peak fluorescence intensity of curcumin in these solvents is at 540 nm.) At longer wavelengths (λ ≥ 560 nm) the fluorescence quenching is smaller by a factor of ∼3. A new fluorescence band with a peak at about 620 nm appears at an acid concentration of about 0.2 M in both methanol and ethanol. The 620 nm/530 nm band intensity ratio increases with an increase in the acid concentration. In trifluoroethanol and also in acetic acid in the presence of formic acid, the steady-state emission of curcumin shows an emission band at 620 nm. We attribute this new emission band in hydrogen-bond-donating solvents to a protonated curcumin ROH2+ form. At high acid concentrations in acetic acid and in trifluoroethanol, the ground state of curcumin is also transformed to ROH2+ which absorbs at longer wavelengths with a band peak at ∼530 nm compared to 420 nm in neutral-pH samples or 480 nm in basic solutions. In hydrogen-bond-accepting solvents such as dimethyl sulfoxide and also in methanol and ethanol, curcumin does not accept a proton to form the ground-state ROH2+

Original languageEnglish
Pages (from-to)872-884
Number of pages13
JournalJournal of Physical Chemistry A
Volume118
Issue number5
DOIs
StatePublished - 6 Feb 2014

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