Impact of benzannulation on ESIPT in 2-(2′-hydroxyphenyl)-oxazoles: a unified perspective in terms of excited-state aromaticity and intramolecular charge transfer

BAUMGARTNER, MARÍA T.; VERA, D. M. A.; MENA, LEANDRO D.

RSC Advances

Royal Society of Chemistry

Lugar: London; Año: 2020 vol. 10 p. 39049 - 39059

Hydroxyphenyl-azoles are among the most popular ESIPT (Excited State Intramolecular Proton Transfer) scaffolds and as such, they have been thoroughly studied. Nevertheless, some aspects regarding theinterplay between the emissive properties of these fluorophores and the size of theirp-conjugatedframework remain controversial. Previous studies have demonstrated that benzannulation of 20-hydroxyphenyl-oxazole at the phenol group of the molecule can lead to either red- or blue-shiftedfluorescence emission, depending on the site where it occurs. In this report, benzannulation at theheterocyclic unit (the oxazole site) is analysed in order to get the whole picture. The extension of p-conjugation does not significantly affect the ESIPT emission wavelength, but it leads instead to higherenergy barriers for proton transfer in thefirst excited singlet state, as a consequence of dramatic changes in the charge transfer character of excitation caused by successive benzannulation. Theoretical calculations revealed an interesting connection between intramolecular charge transfer and excited-state aromaticity in the S1state. The theoretical approach presented herein allows the behaviour of hydroxyphenyl-oxazoles in the excited state to be rationalized and, more generally, a deeper understanding of the factors governing the ESIPT process to be obtained, a crucial point in the design ofnew and efficient fluorophores.