Peculiarities of exited state intramolecular proton transfer in 2-amino-3-(2′-benzazolyl)-quinolines

It has been shown that a low fluorescence quantum yield of 2-amino-3-(2′-benzoxazolyl)-quinoline (ABO) and 2-amino-3-(2′-benzothiazolyl)-quinoline (ABT) is due to the relaxation process caused by excited state intramolecular proton transfer (ESIPT). Quantum-chemical calculations have revealed that ESIPT in these compounds is characterized by overcoming a potential barrier, with the lower basicity of the proton-accepting moiety in ABO resulting in a higher barrier than in ABT and, thereby, determining a substantial difference in their fluorescence quantum yields.