Fused Thiophene‑S,S‑dioxide-Based Super-Photostable Fluorescent Marker for Lipid Droplets

Lipid droplets (LDs) are essential organelles in most eukaryotes, and tracking intracellular LD dynamics using synthetic small molecules is crucial for biological studies. However, only a limited number of fluorescent markers that satisfy all requirements, such as the selective staining of LDs, high photostability, and sufficient biocompatibility, have been developed. Herein, we report a series of donor-π-acceptor dyes based on the thiophene-containing fused polycyclic scaffold [1]­benzothieno­[3,2-b]­[1]­benzothiophene (BTBT), in which either or both thiophene rings are oxidized into thiophene-S,S-dioxide to form an electron-accepting building block. Among these dyes, LAQ1 satisfied all the aforementioned requirements and allowed us to capture ultrasmall LDs on the endoplasmic reticulum (ER) membrane by stimulation emission depletion (STED) microscopy with a super-resolution below the diffraction limit of light. Moreover, the extremely high photostability of LAQ1 enabled recording the lipolysis of LDs and the concomitant lipogenesis, as well as long-term trajectory analysis of micro-LDs at the single-particle level in living cells.