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

We report the successful synthesis of tetramesityldiborane(4) (Mes 4 B 2 ) through the reductive coupling of a dimesitylborinium ion. Owing 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.