Water-Soluble Triarylborane Chromophores for One- and Two-Photon Excited Fluorescence Imaging of Mitochondria in Cells

Three water‐soluble tetracationic quadrupolar chromophores comprising two three‐coordinate boron π‐acceptor groups bridged by thiophene‐containing moieties were synthesised for biological imaging applications. Compound 3 containing the bulkier 5‐(3,5‐Me2C6H2)‐2,2′‐(C4H2S)2‐5′‐(3,5‐Me2C6H2) bridge is stable over a long period of time, exhibits a high fluorescence quantum yield and strong one‐ and two‐photon absorption (TPA), and has a TPA cross section of 268 GM at 800 nm in water. Confocal laser scanning fluorescence microscopy studies in live cells indicated localisation of the chromophore at the mitochondria; moreover, cytotoxicity measurements proved biocompatibility. Thus, chromophore 3 has excellent potential for one‐ and two‐photon‐excited fluorescence imaging of mitochondrial function in cells. Very positive image of boron: A new tetracationic, quadrupolar, three‐coordinate boron compound is water‐soluble and stable, and has a large two‐photon absorption cross section σ2 and fluorescence quantum yield, even in water. It localises at mitochondria and is non‐toxic to cells at concentrations required for one‐ and two‐photon excited fluorescence imaging of mitochondrial function (see figure).