Peripheral Fusion of Carbon-Based Aromatic Rings to BN-Heteropentalene Leading to Close π-Stacking in the Solid State.

π-Electronic molecules with a BN-heterocyclic and carbon-based aromatic hybrid ring system (h-CBN) are interesting in that they potentially exhibit synergistic properties arising from the two different π-systems. Here we report the synthesis and properties of a h-CBN-type molecule (1) having a bicyclic BN-heteropentalene core fused with extended aromatic rings. This molecule exhibits excellent chemical stability despite the absence of bulky substituents for kinetic protection, which in turn provides effective stacking of the π-system upon crystallization. Depending on the crystallization solvent, 1 forms two polymorphs, i. e., the α- and β-phases. While both phases have one-dimensional columnar structures, the π-stacking geometries associated with the transfer integrals of the frontier orbitals are different, resulting in a twofold difference in the electrical conducting properties. We also found that upon thermal vacuum deposition, 1 gives an amorphous film, which serves as a host material for a red phosphorescent OLED device (maximum external quantum efficiency: 15.5 and 13.3 % at 0.1 and 2.5 mA, respectively).