Excited state intramolecular proton transfer (ESIPT) to nitrogen in heterocyclic compounds

Abstract

Excited state intramolecular proton transfer (ESIPT) reactions have aroused considerable interest in the last 10-15 years. The ESIPT reaction is normally extremely rapid and yields excited species of considerably lower energy than the initial Franck-Condon excited state. ESIPT has therefore been used for the rapid dissipation of energy (e. g. for polymer protection against UV degradation) and to produce fluorophores with large Stokes shifts.
Approximately half of the compounds studied to date involve ESIPT to nitrogen but there has been no real attempt at a coherent study of ESIPT to nitrogen. In this work we have tried to remedy this deficiency by studying a range of compounds of increasing complexity in order to characterise ESIPT and subsequent reactions and to evaluate how these properties vary with molecular structure. The molecules selected for this studyf all into two categories:
- The azole group includes the 2-(2'-hydroxyphenyl)-oxazole (HPO) and
-thiazole (HPT) whose study complements their well known benzo counterparts (HPBT and HPBO) and the related imidazoles.
- The compounds of the pyridine group are related to the basic structure of 2-(2'-hydroxyphenyl)-pyridine (HPP). Structural variations involve benzo fusion (quinolines) and addition of a 3,3' bridge. A complementary compound is the well studied 2,2'-bipyridyl-3,3'-diol (BP(OH)2) which undergoes double proton transfer. Whenever possible, the methoxy counterparts were prepared in order to study the photochemistry of these compounds when ESIPT is not possible. The absorption and fluorescence properties of these systems were studied as a function of solvent, temperature and pH conditions. Lifetimes, quantum yields and pKa data were determined under these various conditions and semi-empirical quantum chemical
calculations were performed on each system.