Investigation of excited-state intramolecular proton transfer (ESIPT) properties in dye compounds with 2-(2′-hydroxybenzofuran)benzoxazole (HBBO) core structure

Abstract

This study explores the family of dyes based on the 2-(2′-hydroxybenzofuran)benzoxazole (HBBO) core structure, including HBBO-CN, HBBO-Ph, and HBBO-NH₂, focusing on their Excited-State Intramolecular Proton Transfer (ESIPT) behavior. Quantum chemical calculations utilizing Density Functional Theory (DFT) and Time-Dependent Density Functional Theory (TDDFT) with the 6–311 + G(d, p) basis set were conducted to elucidate the electronic structures and stability of these compounds. The research reveals that the ESIPT reaction is significantly influenced by the nature of the substituents on the HBBO core. The electron-withdrawing group in HBBO-CN enhances the intramolecular hydrogen bond strength in the S₁ state, promoting ESIPT more effectively than in HBBO-Ph and HBBO-NH₂. This is supported by significant changes in bond lengths and angles, the most negative CVB index (indicating the strongest hydrogen bond interaction upon excitation), and infrared vibrational spectroscopy analysis showing enhanced hydrogen bonding in the excited state, with HBBO-CN exhibiting the smallest redshift, indicating a more favorable condition for ESIPT. Potential energy curve studies conducted under the TDDFT/B3LYP/TZVP framework depict the energy changes and barriers associated with the proton transfer process, where HBBO-CN shows the lowest barrier in the S₁ state, consistent with its propensity for ESIPT. These findings are of significant importance for the design and application of ESIPT-active compounds in photon devices and applications.