Substitution by tert-butyl groups facilitates excited state proton transfer in hydroxylated triphenylimidazole frameworks more than it does for oxazole and thiazole analogs

Abstract

Excited state intramolecular proton transfer (ESIPT) processes typically occur when photoexcited molecules relax via a tautomeric proton transfer event in the excited state, and it is known to depend on the properties of donor and acceptor groups. In this work, we employed ground state and Time Dependent Density Functional Theory aiming at studying the ESIPT viability on triphenyl-substituted heterocyclic (imidazole, oxazole, and thiazole) tert-butyl derivatives. For all compounds, the formation of a keto tautomer is favored once an excited state is generated from a ground state enol tautomer. The expected tert-butyl electron donor effect and enol form stabilization is verified in the ground state, but the opposite is observed for the excited state. We found that tert-butyl substituted compounds have reduced HOMO-LUMO gap, exhibit barrierless proton transfer in the excited state and stronger Hydrogen bond after light excitation.