Synthesis of a Blue‐Emissive Azaborathia[9]helicene by Silicon‐Boron Exchange from Unusual Atropisomeric Teraryls.

We report the concise synthesis and chir(optical) properties of an azaborathia[9]helicene consisting of two thienoazaborole motifs. The key intermediate, a highly congested teraryl with nearly parallel isoquinoline moieties, was generated as a mixture of atropisomers upon fusion of the central thiophene ring of the dithienothiophene moiety. These diastereomers were characterized by single crystal X‐ray analysis revealing intriguing interactions in the solid state. Subsequent insertion of boron into the aromatic scaffold via silicon‐boron exchange involving triisopropylsilyl groups fixed the helical geometry, thereby establishing a novel method for the preparation of azaboroles. The ligand exchange at boron in the final step afforded the blue emitter displaying a fluorescence quantum yield of 0.17 in CH2Cl2 and excellent configurational stability. Detailed structural and theoretical investigation of unusual atropisomers and the helicene provide insights into their isomerization processes. [ABSTRACT FROM AUTHOR]