Absorption and emission behaviour of trans-p-coumaric acid in aqueous solutions and some organic solvents

The absorption and fluorescence behaviour of trans-p-coumaric acid (trans-4-hydroxycinnamic acid) is investigated in buffered aqueous solution over a wide range from pH I to pH 12, in un-buffered water, and in some organic solvents. Absorption cross-section spectra, fluorescence quantum distributions, fluorescence quantum yields, and degrees of fluorescence polarisation are measured. p-Coumaric acid exists in different ionic forms in aqueous solution depending on the pH. There is an equilibrium between the neutral form (p-CAH(2)) and the single anionic form (p-CAH(-)) at low pH (pK(na) approximate to 4.9), and between the single anionic and the double anionic form (p-CA(2-)) at high pH (pK(a) approximate to 9.35). In the organic solvents studied trans-p-coumaric acid is dissolved in its neutral form. The fluorescence quantum yield of trans-p-coumaric acid in aqueous solution is phi(F) approximate to 1.4 x 10(-4) for the neutral and the single anionic form, while it is phi(F) approximate to 1.3 x 10(-3) for the double anionic form. For trans-p-coumaric acid in organic solvents fluorescence quantum yields in the range from 4.8 x 10(-5) (acetonitrile) to 1.5 x 10(-4) (glycerol) were measured. The fluorescence spectra are 7700-10,000 cm(-1) Stokes shifted in aqueous solution, and 5400-8200 cm(-1) Stokes shifted in the studied organic solvents. Decay paths responsible for the low fluorescence quantum yields are discussed (photo-isomerisation and internal conversion for p-CA(2-), solvent-assisted intra-molecular charge-transfer or pi pi* to pi pi* transfer and internal conversion for p-CAH(2) and p-CAH(-)). The solvent dependence of the first pi pi* electronic transition frequency and of the fluorescence Stokes shift of p-CAH(2) is discussed in terms of polar solute-solvent interaction effects. Thereby the ground-state and excite-state molecular dipole moments are extracted. (C) 2007 Elsevier B.V. All rights reserved.