A Resonating B, N Covalent Bond and Coordination Bond in Aromatic Compounds and Conjugated Polymers.

We herein report an organoboron compound (SBN-1) based on N-B←N units with a balanced resonance hybrid of B, N single bond (B-N) and B, N coordination bond (B←N). Despite the different chemical environments of the two nitrogen atoms in the N-B←N unit, the single-crystal structure reveals that the two B, N bonds have similar bond lengths of ca. 1.55 Å and are almost equivalent. The resonance nature of the two B, N bonds contributes to the excellent chemical stability and photostability of SBN-1. The N-B←N units in SBN-1 show a strong electron-withdrawing capability and large dipole, resulting in a red-shifted absorption spectrum, downshifted LUMO/HOMO energy levels, and drastically altered molecular packing behaviors in solid state. SBN-1 can be used as a building block to construct small band gap conjugated polymers. The resonance of B, N bonds discovered in this work provides new insight into the design of stable organoboron small molecules and polymers for practical applications.