Optimization of Aqueous Stability versus π-Conjugation in Tetracationic Bis(triarylborane) Chromophores: Applications in Live-Cell Fluorescence Imaging.

The stability of tetracationic triarylboranes in dilute aqueous solution was investigated by tuning the steric demand of the linker in a (para-(N,N,N-trimethylammonio)xylyl) B-(linker)-B(para-(N,N,N-trimethylammonio)xylyl) structure. With increasing steric bulk of the linker, namely 1,4-phenylene, 2,2'''-(3,3'''-dimethyl)-5,2':5',2'':5'',5'''-quaterthiophene, 9,10-anthracenylene, and 4,4'''-(5'-(3,5-dimethylphenyl))(5''-(3''',5'''-dimethylphenyl))-2',2''-bithiophene, the stability of the compounds increased. The anthracene-based chromophore, compound 3M is water-stable for at least 48 h, is nontoxic to cells and exhibits an exceedingly high fluorescence quantum yield of 0.86 in water making it an ideal candidate for confocal live-cell imaging of lysosomes.