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

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 S ₁ 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.