Anion Sensing with a Blue Fluorescent Triarylboron-Functionalized Bisbenzimidazole and Its Bisbenzimidazolium Salt

A blue fluorescent p -dimesitylboryl-phenyl-functionalized 1,3-bisbenzimidazolyl benzene molecule ( 1 ) has been synthesized in high yield by Stille coupling of bisbenzimidazolyl bromobenzene with p -BMes 2 -SnBu 3 -benzene. Methylation of 1 led to the formation of the bisbenzimidazolium salt ( 2 ). The utility of both 1 and 2 in sensing CN - and halide (F - , Cl - , Br - , and I - ) was examined, and it was found that only the small fluoride and cyanide anions were able to bind to the boron atom with binding constants in the range of 2.9 × 10 4 to 5 × 10 5 M -1 . Computational studies provided insight into the photophysical properties of the molecules and verified that a charge-transfer process is quenched in these "turn-off" molecular sensors.