Red Emissive Biocompatible Nanoparticles from Tetraphenylethene-Decorated BODIPY Luminogens for Two-Photon Excited Fluorescence Cellular Imaging and Mouse Brain Blood Vascular Visualization

BODIPY (4,4-difluoro-4-bora-3a,4a-diaza-s-indacene) is an emissive chromophore in solutions but suffers from fluorescence quenching when aggregated due to its flat molecular conformation and small Stokes shift. To create aggregate-state emissive BODIPY luminogens, tetraphenylethene (TPE), which is a popular luminogen with intriguing aggregation-induced emission (AIE) characteristic, is introduced as periphery to a methylated BODIPY core. Three TPE-BODIPY adducts are synthesized and characterized, and their photophysical properties and electronic structures are investigated. The incorporation of AIE-active TPE units alleviates aggregation-caused quenching of BODIPY core, furnishing emissive nanoparticles based on TPE-BODIPY adducts. Significantly, the two-photon absorption (TPA) and two-photon excited fluorescence (TPEF) properties are improved as more TPE units are attached. The luminogens with 3TPE units (3TPE-BODIPY) shows the strongest TPA and TPEF in the wavelength range of 750-830 nm, with cross-section values of 264 and 116 GM at 810 nm, respectively. Red emissive nanoparticles with a Stokes shift of 60 nm and a fluorescence quantum yield of 16% are attained by encapsulating 3TPE-BODIPY with 1,2-sistearoyl-sn-glycero-3-phosphoethanolamine-N-[methoxy(polyethylene glycol)-2000]. The nanoparticles are biocompatible and function well in TPEF cellular imaging and mouse brain blood vascular visualization.