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.