Excited-state intramolecular proton transfer (ESIPT) in 2-(2′-hydroxyphenyl)pyridine and some carbon-bridged derivatives

The absorption and emission properties of 2-(2'-hydroxyphenyl)pyridine (HPP), 10-hydroxy-5,6-dihydrobenzo[h]quinoline (HdihBQ) and 10-hydroxybenzo[h]quinoline (HBQ), are reported in non-polar, alcoholic and aqueous environments. Ground and excited state pK values are presented, and compared with those of related derivatives. The important role of intramolecular rotational mechanisms and structural flexibility in the behaviour of these systems is suggested by (1) the temperature dependence of the fluorescence and (2) the increase in the quantum yields of the tautomeric emission across the series HPP - HdihBQ - HBQ. The existence of conformers of HPP is proposed and their energy as a function of the torsional angle is estimated by quantum chemical calulations. It is concluded that two nonradiative channels operate in HPP tautomer, one connected to twisting and the other related to n pi* deactivation. (C) 1998 Elsevier Science S.A. All rights reserved.