Tetraaryldiborane(4) Can Emit Dual Fluorescence Responding to the Structural Change around the B–B Bond

Abstract

We report the successful synthesis of tetramesityldiborane(4) (Mes 4 B 2 ) through the reductive coupling of a dimesitylborinium ion. Due to the steric protection conferred by the mesityl groups, Mes 4 B 2 shows exceptional chemical stability and remains intact in water. Single-crystal X-ray analysis revealed that Mes 4 B 2 has an orthogonal geometry, where the B–B center is completely hidden by the mesityl groups. Remarkably, Mes 4 B 2 emits dual fluorescence at 460 and 620 nm, both in solution and in the solid state. Theoretical calculations showed that Mes 4 B 2 in the excited S 1 state adopts a twisted or planar geometry, which is responsible for the shorter- or longer-wavelength fluorescence, respectively. The intensity ratio of the dual fluorescence is sensitive to the viscosity of the medium, which suggests that Mes 4 B 2 has potential as a ratiometric viscosity sensor.