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-BMes2-SnBu3-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 × 104 to 5 × 105 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.