Controlling the thermodynamic stability of conformational isomers of bistricyclic aromatic enes by introducing boron and silicon atoms

Overcrowded bistricyclic aromatic enes (BAEs) have several conformational isomers, including twisted and anti-folded conformers. These compounds change color depending on their conformation because each isomer exhibits distinct absorption bands. In this study, we introduced several heteroatoms such as boron and silicon into BAEs to control the energy difference between the twisted and anti-folded conformers, thereby achieving chromic properties. The heteroatom-containing BAEs showed absorption bands attributable to both twisted and anti-folded conformers, suggesting the coexistence of these conformers in solution. Although the solution appeared bluish, yellowish crystals were obtained by recrystallization. Single-crystal X-ray diffraction analysis confirmed that the yellowish crystals existed in the anti-folded conformation. The color of these crystals changed from yellowish to bluish upon heating and grinding, demonstrating thermochromism and mechanochromism. In addition, when cyanide ions were added to the BAE solution, the color changed from bluish to colorless, indicating the chemochromic properties of the BAEs.