A study on the spectroscopy and photophysics of 4-hydroxy-3-methoxybenzoic acid in different solvents, pH and β-cyclodextrin

Effect of solvents, buffer solutions of different pH and β-cyclodextrin (β-CD) on the absorption and fluorescence spectra of 4-hydroxy-3-methoxybenzoic acid (vanillic acid, HMB) have been investigated and compared with 4-hydroxy-3,5-dimethoxybenzoic acid (HDMB). The inclusion complex of β-CD with HMB is investigated by UV–vis, fluorimetry, FT-IR, 1H NMR, scanning electron microscope (SEM) and semiempirical methods. The thermodynamic parameters (ΔG, ΔH and ΔS) of inclusion process are also determined. Solvent study shows in the S0 state, HMB is more polar than HDMB, whereas the polarity is same in the S1 state. pH studies suggest proton transfer reactions follow the same trend in HMB and HDMB molecules. β-CD studies indicates (i) HMB forms 1:1 inclusion complex with β-CD and (ii) unusual blue shift of hydroxyl ion (dianion) in the β-CD medium confirms OH group present in the interior part of the β-CD cavity and COOH group present in the upper part of the β-CD cavity. A mechanism is proposed to explain the inclusion process.