Synthesis and characterization of novel fluorescent BOPIM dyes with large Stokes shift

A novel BOPIM (boron 2-(2-pyridyl)imidazole complex) dye 1 was facilely synthesized by treatment of previously reported 2-(2′-pyridyl)imidazole with BF3·Et2O under basic condition. The bromination of BOPIM dye 1 by NBS gives an unexpected product 2-(2′-pyridyl)-4,5-dibromoimidazole (L2) with no BF2 group. The desired brominated boron complex 2 was obtained by treating L2 with BF3·Et2O. The photophysical properties of these two compounds are thoroughly studied in various solvents. Compound 1 formed aggregates in non-polar solvents, inducing abnormal emission in long-wavelength region. Both 1 and 2 show moderate fluorescent intensity and comparatively large Stokes shift, especially for compound 2 (fluorescent quantum yield is more than 0.30, and Stokes shift is over 70 nm in all adopted solvents) due to its p, π-conjugated effect, which makes BOPIM a valuable building block for synthesis of multi-functional materials.