Specifics of photoinduced excited-state intramolecular proton transfer in o-tosylaminobenzoic and o-acetylaminobenzoic acids

In the ground state, o-tosylaminobenzoic and o-acetylaminobenzoic acids exist in the form of two rotamers with intramolecular hydrogen bonds N-H...O=C (cis) and N-H...O(OH)-C (trans). In nonpolar solvents, the formation of dimers with hydrogen bonding between carboxyl groups takes place. Efficient barrierless excited state intramolecular proton transfer (ESIPT) occurs along the N-H...O=C hydrogen bond upon excitation of o-tosylaminobenzoic acid. The efficiency of ESIPT in o-acetylaminobenzoic acid is lower because of the low acidity of the substituted amino group.