Intramolecular charge transfer associated with hydrogen bonding effects on 2-aminobenzoic acid

Abstract

Effect of solvents, buffer solutions of different pH and β-cyclodextrin (β-CD) on the absorption and fluorescence spectra of 2-aminobenzoic acid (2ABA) have been investigated. The inclusion complex of 2ABA with β-CD is discussed by semiempirical quantum calculations (AM1), absorption, emission, FT-IR, 1H NMR and scanning electron microscope (SEM). The Stokes shifts in 2ABA is correlated with different polarity scales suggest that 2ABA molecule is more polar in the S 1 state. The increase in the excited state dipole moment values and β-CD studies confirm that the presence of intramolecular charge transfer (ICT) in 2ABA. Acidity constants for different prototropic equilibria of 2ABA in the S 0 and S 1 states are calculated. β-CD studies shows that (i) at pH ∼ 1, 2ABA forms 1:1 inclusion complex with β-CD, whereas at pH ∼ 7, it forms mixture of 1:1 and 1:2 inclusion complex and (ii) at pH ∼ 1, appearance of dual luminescence in higher β-CD solutions indicates carboxyl and amino groups present in the hydrophobic part of the β-CD. A mechanism is proposed to explain the inclusion process.