Fluorine-tuned dual-state luminogens for versatile applications in mechanofluorochromism, trace water analysis, and live cell imaging

Dual-state luminogens (DSEgens) are organic luminescent molecules that exhibit strong emissions in both molecularly dispersed and aggregated states. The introduction of fluorine into organic conjugates has been shown to influence their photophysical properties, but its effect on DSEgens remains unexplored. Herein, we synthesized three naphthalimide-arylimidazole luminogens, namely NIBH, NIBPF, and NIBOF, with fluorinated or nonfluorinated phenyl groups on the pyrrole-like nitrogen. We systematically investigated their photophysical properties using spectroscopic studies, theoretical calculations, and single crystallography. Our results demonstrate that fluorine substitution in DSEgens leads to shortened electronic conjugation, highly twisted molecular conformations, and loose three-dimensional packing arrangements. Consequently, fluorinated DSEgens, such as NIBPF and NIBOF, exhibited a significant spectral blueshift and more pronounced mechanofluorochromic properties compared to the non-fluorinated NIBH. Intriguingly, NIBPF with a para-position fluorine substituent displayed relatively redshifted and stronger emission in tetrahydrofuran solutions, while showing a blueshifted and lower emission in the solid state compared to NIBOF, which has an ortho-position fluorine substituent. These findings confirm the efficacy of fluorine as a small substituent to tune the photophysical properties of DSEgens in both dilute solutions and the solid state. Furthermore, the three bright DSEgens were employed for mechanofluorochromism, trace water analysis, and live-cell imaging applications.