Effects of π-conjugation-substitution on ESIPT process for oxazoline-substituted hydroxyfluorenes

Abstract

Excited-state intramolecular proton transfer (ESIPT) molecules are broadly applied to UV absorbers, fluorescence sensing, and lighting materials. In previous work, the fluorescence colors of oxazoline-substituted hydroxyfluorenes and hydroxylated benzoxazole were diversified by adding the π-conjugation. There is intriguing that the mechanism of diversified fluorescence colors induced by ESIPT. Here, the density functional theory (DFT) and time-dependent DFT (TDDFT) are advised to identify the effects of π-conjugation on ESIPT and photophysical properties. The stabilized geometrical configurations, frontier molecular orbitals (FMOs) isosurfaces, and O–H stretching vibration frequency analysis demonstrate that PT processes are more active in S1 state. Constructing the minimum energy pathways of ESIPT processes, we find that the calculated peak of enol and keto fluorescence of naphthoxazole (NO–OH) is distinctly bathochromic-shift relative to the oxazoline-substituted hydroxyfluorenes (Oxa–OH) configuration when adding π-conjugation-substitution, and it means that π-conjugation-substitution can diversify the fluorescence color. We hope our studies can establish new channels to devise the ESIPT-based molecules.