Photophysical behaviour of 4-hydroxy-3,5-dimethoxybenzoic acid in different solvents, pH and β-cyclodextrin

Effect of solvents, buffer solutions of different pH and β-cyclodextrin on the absorption and fluorescence spectra of 4-hydroxy-3,5-dimethoxybenzoic acid [HDMB] have been investigated. The inclusion complex of β-CD with HDMB is investigated by UV–vis, fluorometry, FTIR, 1H NMR, scanning electron microscope (SEM) and semiempirical methods. The thermodynamic parameters (ΔG, ΔH and ΔS) of inclusion process are also determined. The Stokes shifts of HDMB correlated with various solvent polarity scales, suggest that the HDMB molecule is more polar and the change in dipole moment is large in the S1 state. The pKa and pKa* values of neutral-monoanion and monoanion–dianion equilibria have been determined with fluorimetric titrations and Förster cycle methods and compared. β-Cyclodextrin studies shows that HDMB forms a 1:1 inclusion complex with β-CD. A mechanism is proposed to explain the inclusion process.