Design of bond-cleavage-induced intramolecular charge transfer emission with dibenzoboroles and their application to ratiometric sensors for discriminating chain lengths of alkanes

Dibenzoborole derivatives containing four-coordinated boron were designed to realize emission from the bond-cleavage-induced intramolecular charge transfer (BICT) transition. A series of electrondonating and accepting groups were introduced into dibenzoborole through regioselective bromination and Suzuki-Miyaura coupling reaction in high yields. The synthesized dibenzoboroles showed dual-emissive properties composed of emission from the pi-pi* transition with four-coordinated boron and the BICT transition with a three-coordinated state. The influence of the substituents on the optical properties was experimentally and theoretically analyzed. Next, on the basis of the dual-emissive properties, it was demonstrated that dibenzoborole was feasible as a ratiometric fluorescent sensor for discriminating the viscosity of solvent alkanes including the length of alkane chains with a wide detection range. Finally, it was demonstrated that this detection system with a ratiometric output was applicable for evaluating viscosity with mixture samples and various kinds of natural oils.