Solvent effects in optical spectra of ortho-aminobenzoic acid derivatives.

We investigated three amino derivatives of ortho-aminobenzoic or anthranilic acid (o-Abz): a) 2-Amino-benzamide (AbzNH2); b) 2-Amino-N-methyl-benzamide (AbzNHCH3) and c) 2-Amino-N-N'-dimethyl-bezamide (AbzNH(CH3)2), see Scheme 1. We describe the results of ab-initio calculations on the structural characteristics of the compounds and experimental studies about solvent effects in their absorption and steady-state and time-resolved emission properties. Ab-initio calculations showed higher stability for the rotameric conformation in which the oxygen of carbonyl is near to the nitrogen of ortho-amino group. The derivatives present decrease in the delocalization of pi electron, and absorption bands are blue shifted compared to the parent compound absorption, the extent of the effect increasing from to Abz-NH2 to Abz-NHCH3 Abz-NH(CH3)2. Measurements performed in several solvents have shown that the the dependence of Stokes shift of the derivatives with the orientational polarizability follows the Onsager-Lippert model for general effects of solvent. However deviation occurred in solvents with properties of Bronsted acids, or electron acceptor characteristics, so that hydrogen bonds formed with protic solvents predominates over intramolecular hydrogen bond. In most solvents the fluorescence decay of AbzNH2 and AbzNHCH3 was fitted to a single exponential with lifetimes around 7.0 ns and no correlation with polarity of the solvent was observed. The fluorescence decay of AbzN(CH3)2 showed lifetimes around 2.0 ns, consistent with low quantum yield of the compound. The spectroscopic properties of the monoamino derivative AbzNHCH3 are representative of the properties presented by Abz labelled peptides and fatty acids previously studied.

General and specific solvent effects in optical spectra of ortho-aminobenzoic acid.

We describe studies about solvent effects on the absorption and emission properties of o-aminobenzoic acid (o-Abz), interpreting the results within the framework of general and specific solute-solvent interactions. Measurements were performed in several solvents and analysis of the absorption and emission wavelengths were made based on Lippert's model for general solvent effects and on the use of different parameters to describe the ability of the solvent to promote specific interactions with the solute. We observed low sensitivity of the Stokes shift upon changes in the medium polarity, and large deviation from the linearity predicted by Lippert's equation when the solvents were characterized as Bronsted acid in the Kamlet-Taft pi* scale. Quantum yield and fluorescence lifetimes were best interpreted based on the AN+DN scale used to describe the electron donor/acceptor properties of the solvent. The results indicated that non-radiative deexcitation processes are favoured in solvents which promote the formation of intramolecular hydrogen bond, while interactions with electron acceptor solvents lead to enhancement of fluorescence.