Role of explicit hydrogen bonding in solvation calculation on the photophysics of 2-(2′-Hydroxyphenyl)oxazolo[4,5-b]pyridine

Abstract

The photophysics of 2-(2′-hydroxyphenyl)oxazolo[4,5-b]pyridine (HPOP) was theoretically examined by employing the integral equation formalism variant of polarizable continuum model (PCM) at the level of density functional theory with M06 functional. Polar protic methanol and water and aprotic acetonitrile and tetrahydrofuran, which have varied dielectric constants, were considered to study the role of polarity on the photophysics of HPOP and was compared with the results obtained in vacuum. Six different structural forms of HPOP were observed: cis and trans-closed enol, cis and trans-open enol, and cis and trans-keto. cis-closed enol is the most stable and trans-keto the least in the ground state. In the S1 state, cis-keto is the most stable and cis-open enol the most unstable. The relative stabilities of the different structures increase with the polarity of the solvent. The calculations were extended to vacuum and methanol environments by adding two methanol molecules, and similarly done with two water molecules in vacuum and water environments which are explicitly hydrogen bonded to HPOP at different hydrogen bonding centres of HPOP to study the role of solute–solvent hydrogen bond on its photophysics and the changes in results obtained under the PCM formalism. The hydrogen bond between the solute and solvent molecules further increases the relative stabilities of the different hydrogen bonded clusters. The calculations show that the intramolecular proton transfer (IPT) between cis-closed enol and cis-keto is a barrierless process with the transition state displaying a submerged barrier both in S0 and S1 states where the IPT occurs within a single OH stretching vibration. The IPT trajectory is shorter in the S1 state.