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

We report the successful synthesis of tetramesityldiborane(4) (Mes₄ B₂ ) through the reductive coupling of a dimesitylborinium ion. Owing to the steric protection conferred by the mesityl groups, Mes₄ B₂ shows exceptional chemical stability and remains intact in water. Single-crystal X-ray analysis revealed that Mes₄ B₂ has an orthogonal geometry, where the B-B center is completely hidden by the mesityl groups. Remarkably, Mes₄ B₂ emits dual fluorescence at 460 and 620 nm, both in solution and in the solid state. Theoretical calculations showed that Mes₄ B₂ in the excited S₁ 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 Mes₄ B₂ has potential as a ratiometric viscosity sensor.