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 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-CAH2) and the single anionic form (p-CAH−) at low pH (pKna ≈ 4.9), and between the single anionic and the double anionic form (p-CA2−) at high pH (pKaa ≈ 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 ϕF ≈ 1.4 × 10−4 for the neutral and the single anionic form, while it is ϕF ≈ 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-CA2−, solvent-assisted intra-molecular charge-transfer or ππ∗ to nπ∗ transfer and internal conversion for p-CAH2 and p-CAH−). The solvent dependence of the first ππ∗ electronic transition frequency and of the fluorescence Stokes shift of p-CAH2 is discussed in terms of polar solute-solvent interaction effects. Thereby the ground-state and excite-state molecular dipole moments are extracted.