ESIPT mechanism and multi-stimuli-responsive of N-phenylanthranilic acid under AIE characteristics in solution

N -phenylanthranilic acid (NPAA) is weakly acidic compound that exhibits both excited-state intramolecular proton transfer (ESIPT) and aggregation-induced emission (AIE) characteristics. The molecular structure was confirmed, and its optical behavior was rationalized through a combined approach of quantum chemical calculations and experimental studies. At low concentrations in various solvents, NPAA exhibits dual emission, indicative of an ESIPT process. At higher concentrations, self-association leads to the formation of dimers, resulting in a single emission peak in the fluorescence spectrum. The emission intensity significantly increases with an increasing water fraction in methanol/H 2 O mixtures, demonstrating typical AIE behavior. Additionally, NPAA exhibits time-dependent enhancement of blue-shifted emission and pH-dependent fluorescence responses. Upon adding a large amount of water to the methanol solution (up to about 70 % water fraction), the mixture shows a gradual enhancement accompanied by a blue shift in fluorescence emission. Under alkaline conditions, the carboxyl group becomes negatively charged, which increases intermolecular electrostatic repulsion and weakens hydrogen bonding interactions. As a result, molecular aggregation is inhibited, leading to fluorescence quenching. In contrast, under acidic conditions, the molecules aggregate more readily, which enhances fluorescence emission, and the crystal morphology changes from needle-like to sheet-like. NPAA thus exhibits reversible acid-base responsive fluorescence switching behavior. Furthermore, NPAA films enable reversible fluorescence switching under acid/base vapor stimuli, demonstrating its potential in vapor-responsive luminescent sensing.