Solvatochromic Near-Infrared Aggregation-Induced Emission-Active Acrylonitriles by Acceptor Modulation for Low-Power Stimulated Emission Depletion Nanoscopy

Stimulated emission depletion (STED) nanoscopy has broadened our horizons to unravel mysterious functions of cellular structures on an unparalleled nanometer scale. Nevertheless, an intense depletion laser power is a general prerequisite for STED super-resolution imaging with a satisfactory resolution, inevitably leading to severe photobleaching of fluorophores, irreparable photodamage to biosamples, and impaired imaging quality. Herein, by modulating distinct acceptor units, a series of donor-acceptor-acceptor structured fluorescent acrylonitriles featured with aggregation-induced emission (AIE) for STED super-resolution imaging at a low depletion power were systematically developed. These AIE luminogens (AIEgens) exhibited tunable near-infrared emissions (650-733 nm) and high fluorescence quantum yields (QYs of up to 26.8%) in the solid state, significant Stokes shifts, and large two-photon absorption cross-sections. They also exhibited a typical solvatochromic effect and ultrahigh QYs of up to 98.4% in low-polarity solvents. Furthermore, these lipophilic solvatochromic acrylonitriles specifically lit up lipid droplets (LDs) with exceptionally high photostability in a wash-free manner. By taking TPA-BT-ANBI as an example, the STED super-resolution imaging of LDs with excellent resolution of 62 and 80 nm for cytosolic and nuclear LDs, respectively, at a low saturation depletion power of 0.83 MW/cm² and extended time-lapse imaging of LD dynamics was achieved. Subsequent use of TPA-BT-ANBI for the optical discrimination of fatty liver tissues and two-photon deep-tissue imaging was also demonstrated. This study opens new avenues for versatile photostable materials at low depletion powers for target-specific STED super-resolution imaging.