Viscosity-responsive styryl benzoxazole probes for lipid droplet visualization

We report a rapid, solvent-minimized solid-state synthesis of a series of styryl benzoxazole fluorophores achieved within 40–180 seconds under ambient conditions, eliminating the need for chromatographic purification. The synthesized dyes exhibit strong visible-region absorption and efficient fluorescence emission arising from donor–π–acceptor architectures with pronounced intramolecular charge transfer (ICT) character. Systematic photophysical studies reveal minimal solvent effects on absorption but significant solvatochromic emission shifts and large Stokes shifts, indicating substantial excited-state reorganization. Notably, several derivatives display marked viscosity-dependent fluorescence enhancement due to restricted intramolecular rotation, with the magnitude of enhancement governed by donor strength and molecular rigidity. Live-cell imaging studies in COS-7 cells demonstrate that four derivatives selectively accumulate in lipid droplets, exhibiting strong co-localization with Nile red and negligible cytotoxicity. Furthermore, the probes effectively report oleic acid-induced lipid droplet biogenesis through increased fluorescence intensity and droplet number. Collectively, this work establishes clear structure–property–function relationships in styryl benzoxazole dyes and highlights their potential as efficient viscosity-responsive probes for lipid droplet imaging in live cells.