Synthesis, Structures, and Optical Properties of Azahelicene Derivatives and Unexpected Formation of Azahepta[8]circulenes

Polycyclic heteroaromatic compounds including pyrrole units are promising functional scaffolds owing to their electron‐rich nature, bright fluorescence, and applicability to anion recognition at the pyrrolic hydrogen atom. We report herein the effective synthesis of pseudo‐aza[5]helicene and aza[7]helicene derivatives, and unexpected formation of azahepta[8]circulenes by oxidative fusion reactions. By choosing reaction conditions and peripheral substituents attached at the terminal indole moieties, we obtained aza[7]helicenes 10 a–c and azahepta[8]circulenes 11 a,c selectively in moderate to good yields. Their solid‐state structures have been revealed by X‐ray diffraction analysis. UV/Vis absorption, emission, and cyclic voltammetry of these compounds were studied in comparison with those of previously reported tetraaza[8]circulene (TA8C), a symmetric and planar molecule. Furthermore, the enantiomeric separation of dimethyl‐substituted aza[7]helicene 10 b was achieved, and the racemization kinetics have been elucidated both theoretically and experimentally. This work illustrates a wealth of advantages of pyrrole incorporation into the polycyclic aromatic scaffolds in terms of synthetic aspects, structural variation, and optical tuning. Fused‐ring systems: Dibenzotriaza[5]helicene, dibenzotriaza[7]helicene, and dibenzotriazahepta[8]circulene derivatives have been synthesized by oxidative fusion reactions in a selective manner. Structures have been revealed by X‐ray diffraction analysis and their optical and electrochemical properties investigated (see figure)