Excited state proton transfer (ESPT) from phenol to nitrogen and carbon in (2-hydroxyphenyl)pyridines

Abstract

ESIPT and ESPT are the only photochemical deactivation pathways of the singlet excited states of 2-, 3- and 4-(2′-hydroxyphenyl)pyridines (1a–c). Due to the existence of an intramolecular hydrogen bond in 2-(2′-hydroxyphenyl)pyridine (1a), ESIPT leads only to protonation of the pyridine nitrogen. On the other hand, the singlet excited states of 3- and 4-(2′-hydroxyphenyl)pyridine (1b,c) undergo protonation of both nitrogen and carbon atoms of the pyridine ring, via ESIPT or ESPT. The extent of ESIPT to carbon (as measured by extent of deuterium incorporation) can be controlled by the amount of water in the solvent system.