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

Abstract

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 1 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 (p K na ≈ 4.9), and between the single anionic and the double anionic form ( p-CA 2−) at high pH (p K aa ≈ 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 ϕ F ≈ 1.4 × 10 −4 for the neutral and the single anionic form, while it is ϕ F ≈ 1.3 × 10 −3 for the double anionic form. For trans- p-coumaric acid in organic solvents fluorescence quantum yields in the range from 4.8 × 10 −5 (acetonitrile) to 1.5 × 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 ππ ∗ to nπ ∗ transfer and internal conversion for p-CAH 2 and p-CAH −). The solvent dependence of the first ππ ∗ 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.