A dual-responsive fluorescent naphthalimide probe shows strong fluorogenic sensitivity to solvent polarity and viscosity. It enables precise imaging of lipid droplets in live cells, with enhanced fluorescence under high-viscosity conditions, offering a tool for visualizing cellular microenvironments.

This study reports thecharacterization and photophysical properties of 4-(diphenylamino)phenyl-ethynyl naphthalimide, which is synthesized via Sonogashira coupling reactions. This compound exhibits sensitive fluorogenic responses to solvent polarity and viscosity, with fluorescence intensities and quantum yields decreasing as solvent polarity increases. Computational calculations indicate a significant increase in the excited-state dipole moment when transitioning from toluene to dimethyl sulfoxide, suggesting enhanced stabilization in polar solvents. Additionally, fluorescence enhancement is observed in glycerol-ethanol mixtures, particularly at glycerol concentrations of 80% and above, where increased viscosity extends fluorescence lifetimes and reduces particle sizes. The compound also demonstrates low cytotoxicity, high biocompatibility, and selective lipid droplet affinity in live cells, enabling precise visualization of cellular viscosity changes. These findings highlight the potential of this dual-function fluorescent probe as a valuable tool for investigating the microenvironmental properties of lipid droplets and other cellular structures.