Heteroarene-Fused Benzo[b]arsoles: Structure, Photophysical Properties, and Effects of the Bridging Element.

Heteroarene-fused heteroles have attracted considerable attention owing to their unique electronic and photophysical properties. The bridging element plays a crucial role in determining the electronic characteristics of the resulting π-conjugated molecules. In this study, we synthesized a series of heteroarene-fused benzo[b]arsoles and investigated their structures and photophysical properties. All the synthesized arsoles exhibited phosphorescence at 77 K, whereas arsole oxide did not. The phosphorescence intensities of the pnictogen-containing heteroles (phosphorus and arsenic) were significantly higher than those of the tetrel-containing heteroles (silicon and germanium). This trend was analyzed through theoretical calculations, focusing on the energy levels of the excited singlet and triplet states as well as the spin-orbit coupling matrix elements. Furthermore, the indole-fused benzoarsole oxide exhibited a mechanically induced emission color change. The removal of crystalline water triggered a change in the molecular packing that reduced the excimer emission.