Synthesis and Properties of Benzo[d]dithieno[b,f]borepins

Borepin is a seven-membered unsaturated ring system containing a tricoordinate boron. Although many borepin compounds annulated with aromatic systems have been reported to date, only one [b,d,f]-annulated tetracyclic borepins has been synthesized. In the present study, we synthesized benzo[d]-dithieno[b,f]borepins as a new class of building blocks for functional materials motivated by the following structural features: (1) the high chemical stability of dithieno[b,f]borepin species; (2) the highly conjugated tetracyclic system with a borepin ring; and (3) electronic structures that are easily tuned by the introduction of functional groups on the annulated benzene ring. The prepared borepins were stable in air both in solution and as solids, and the electronic states could be finely tuned by changing the substituents on the benzene ring. Interestingly, DFT calculations revealed that the LUMO and LUMO+1 energy levels were strongly affected by the benzo[d]-annulation, depending on the boron-bridged positions on the fused thiophene rings. NICS and HOMA methods proved that the aromaticity of the borepin ring in the benzo[d]dithieno[b,f]borepin system is relatively small. We also report an unexpected red emission observed for the first time in borepin compounds for benzo[d]dithieno[b,f]borepin in the solid state.