Solvatochromism, prototropism and complexation of para-aminobenzoic acid

Effect of solvents, buffer solutions of different pH and beta-cyclodextrin (beta-CD) on the absorption and fluorescence spectra of p-aminobenzoic acid (pABA) have been investigated. The inclusion complex of pABA with beta-CD is investigated by UV-visible, fluorimetry, semiempirical quantum calculations (AMI), H-1 NMR and Scanning Electron Microscope (SEM). The thermodynamic parameters (Delta H, Delta G and Delta S) of the inclusion process are also determined. The experimental results indicated that the inclusion processes is an exothermic and spontaneous. The large Stokes shift emission in solvents with pABA are correlated with different solvent polarity scales. The increase in the excited dipole moment values suggest that pABA molecule is more polar in the S-1 state. Solvent and beta-CD studies indicates intramolecular charge transfer in pABA is less than ortho and meta isomers. Acidity constants for different prototropic equilibria of pABA in the S-0 and S-1 states are calculated. beta-cyclodextrin studies shows that pABA forms a 1:1 inclusion complex with beta-CD. A mechanism is proposed to explain the inclusion process.