Theoretical modelling for the ground state rotamerisation and excited state intramolecular proton transfer of 2-(2'-hydroxyphenyl)oxazole, 2-(2'-hydroxyphenyl)imidazole, 2-(2'-hydroxyphenyl)thiazole and their benzo analogues

Abstract

Two series of compounds, one comprising of 2-(2'-hydroxyphenyl)benzoxazole (HBO), 2-(2'-hydroxyphenyl)benzimidazole (HBI), 2-(2'-hydroxyphenyl)benzothiazole (HBT), and the other of 2-(2'-hydroxyphenyl)oxazole (HPO), 2-2'-hydroxyphenyl)imidazole (HPI) and 2-(2'-hydroxyphenyl)thiazole (HPT) are susceptible to ground state rotamerization as well as excited state intramolecular proton transfer (ESIPT) reactions. Some of these compounds show experimental evidence of the existence of two ground state conformers. Out of these two one undergoes ESIPT reaction leading to the formation of the tautomer. The two photophysical processes, in combination, result in the production of a number of fluorescence bands each one of which corresponding to a particular species. Semiempirical AM1-SCI calculations have been performed to rationalize the photophysical behaviour of the compounds. The calculations suggest that for the first series of compounds, two rotational isomers are present in the ground state of HBO and HBI while HBT has a single conformer under similar circumstances. For the molecules of the other series existence of rotamers depends very much on the polarity of the environment. The potential energy curves (PEC) for the ESIPT process in different electronic states of the molecules have been generated theoretically. The simulated PECs reveal that for all these systems the IPT reaction is unfavourable in the ground state but feasible, both kinetically and thermodynamically, in the S₁ as well as T₁ states.