2,1,3-benzothiadiazole-based probe exhibits inverted fluorosolvatochromism and aggregation-induced emission

Fluorosolvatochromic compounds are widely employed as probes for analyzing pure and mixed solvents, as their fluorescence emission wavelength and intensity vary with solvent environment. The 2,1,3-benzothiadiazole ( BTD ) nucleus is an attractive fluorophore for designing such probes, yet its fluorosolvatochromic behavior has not been systematically explored. Here, we report the synthesis and photophysical characterization of a novel BTD -based donor–π–acceptor probe (compound 1 ), featuring a triphenylamino (TPA) substituent and a terminal alkynyl group. Optical properties were examined in a broad set of pure organic solvents and alcohol–water mixtures. The probe displays reverse fluorosolvatochromism, with an initial bathochromic shift in less polar solvents followed by a hypsochromic shift as polarity increases. This reversal is influenced by the terminal alkynyl group, which interacts with oxygenated solvents through hydrogen bonding, modulating excited-state stabilization and the emission response. Theoretical calculations indicate that the visible band arises from π–π* electronic transitions. The probe is fluorescent in the solid state, shows negligible emission in ethanol, is insoluble in water, and exhibits strong fluorescence in ethanol/water mixtures. These features support the aggregation-induced emission (AIE), promoted by the TPA unit. These findings provide new insights into the interplay between solvent environment, molecular structure, and aggregation in BTD -based systems, highlighting their potential for sensing, imaging, and materials science applications. • BTD -based probe exhibits reverse fluorosolvatochromism. • Alkynyl substituent controls fluorosolvatochromic behavior. • Solid-state fluorescence observed. • Ethanol/water mixtures induce AIE. • Probe enables a reversible on–off AIE device.