A Family of Helical BN-Nanographenes: Systematic Synthesis, Structures, and Properties of Heterole-Fused Carbazole Azaborine Dimers.

Replacing two carbon atoms with neighboring boron-nitrogen (B-N) units in helical polycyclic aromatic hydrocarbons can substantially improve the photophysical properties, whereas the reports have remained scarce with underdeveloped structure-property relationships. We herein report the synthesis and physicochemical properties of a series of heterole-fused dimers of carbazole-containing azaborines as a new family of helical BN-nanographenes. We synthesized the carbazole azaborine dimers fused with N-butylpyrrole (1a), N-phenylpyrrole (1b), furan (2), thiophene (3a), selenophene (4), and thiophene S-dioxide (5) via double electrophilic borylation of rationally designed precursors as a key step. Thiophene-fused 3b and 3c bearing smaller terminal substituents than 3a were also synthesized. The central heterole had distinct effects on the electrochemical and photophysical properties, accompanied by strong emission of 1a, 1b, and 2 with fluorescence quantum yields of 0.51-0.56. The bulkiness of the terminal substituents had negligible effects on the properties in solution but markedly affected the crystal packing mode and fluorescence properties in the solid state. The structural effects on the electronic structures were also discussed with the aid of theoretical calculations.