Design principles for ESIPT-based fluorophores: effects of heteroatoms, substituents and solvent polarity in benzazole derivatives

Abstract Excited-state intramolecular proton transfer (ESIPT) plays a pivotal role in governing the photophysical behaviour of benzazole-based fluorophores. However, the synergistic effects of heteroatom identity, substituent electronic properties and solvent polarity remain insufficiently understood. Here, we use density functional theory (DFT) and time-dependent DFT calculations to investigate the ESIPT activity of a series of 2-(2′-hydroxyphenyl)benzazole derivatives containing different heteroatoms (NH, O, S and Se) and substituents (H, NO2 and N(CH3)2) in heptane, chloroform and water. The calculations show that heavier chalcogen atoms promote ESIPT by strengthening the intramolecular hydrogen bond, reducing the frontier-orbital gap and shifting absorption to longer wavelengths. In the unsubstituted series, replacing NH with Se decreases the highest occupied molecular orbital−lowest unoccupied molecular orbital gap and produces a clear bathochromic shift in the calculated absorption maximum. Substituent effects are similarly pronounced: electron-withdrawing NO2 enhances charge transfer, stabilizes the keto tautomer and red-shifts emission, whereas N(CH3)2 suppresses ESIPT. Solvent polarity further modulates these behaviours, with non-polar media generally favouring stronger intramolecular hydrogen bonding and more facile ESIPT than polar environments. Atoms-in-molecule analysis, potential energy profiles and transition-state-based kinetic calculations performed in chloroform as a representative medium support these trends. The results provide a mechanistic framework for understanding how heteroatom identity, substituent type and solvent polarity govern ESIPT in 2-(2′-hydroxyphenyl)benzazole-based fluorophores.