Preference for excited state imine–amine isomerisation over enol–keto isomerisation: spectroscopic exploration and quantum chemical calculations

Abstract

In this article, the photophysical properties of ethyl-3-(benzo[D]thiazol-2-yl)-5-chloro-2-hydroxybenzoate (EBTCH) and ethyl-3-(benzo[D]oxazol-2-yl)-5-chloro-2-hydroxybenzoate (EBOCH) have been explored spectroscopically along with quantum chemical calculations. From a structural viewpoint, both molecules have two proton acceptor sites (thiazole/oxazole N atom and ester O atom) and a common proton donor site (phenolic –OH) connected by a six-membered H-bonding ring capable of both imine–amine and enol–keto photoisomerisation. Steady state absorption and emission spectra and time-resolved fluorescence characteristics of EBTCH and EBOCH and a comparison with the spectral data of controlled compounds 2-(benzo[D]thiazol-2-yl)-4-chlorophenol (BTCP), 2-(benzo[D]oxazol-2-yl)-4-chlorophenol (BOCP) and ethyl 5-chloro-2-hydroxybenzoate (ECHB) support the preference for imine–amine isomerisation over enol–keto isomerization in the excited state. The computed structural parameters for the ground and excited states for both the molecules from the density functional theory (DFT) calculations are found to be in favour of the imine–amine isomerization process. Theoretical potential energy curves along the proton transfer coordinates of EBTCH and EBOCH support the existence of both isomers in the ground state (S₀). However, the formation of a tautomeric form involving imine–amine isomerisation obtained through asymmetric double-well potential is favoured over the enol–keto isomerisation in the excited state (S₁) due to lower barrier energy.