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

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 S1 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 S0 and S1 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.