Cycloazulenylene

Nonalternant hydrocarbons differ significantly from their alternant analogs due to their nonsymmetric electronic structure, which manifests in distinct (anti)aromaticity, optical behavior, and redox properties. Subjecting such systems to strain and macrocyclic conjugation further changes these characteristics, leading to materials with fundamentally new properties. Azulene, a nonalternant hydrocarbon known for its anti-Kasha fluorescence and intense blue coloration, is herein strained into a macrocycle along its 2,6-positions. Alternating 2,2' and 6,6' linkages of the subunits form the symmetric all-azulene analogue of [6]cycloparaphenylene, termed [6]cycloparaazulenylene ([6]CPA). Its structure was elucidated via single-crystal X-ray diffraction, revealing pronounced dihedral torsion between adjacent azulene subunits and an unusual tubular packing motif. The macrocycle exhibits electronic delocalization along the macrocyclic perimeter, a remarkably narrow HOMO-LUMO gap, and intense absorption in the visible range. Computational analyses of aromaticity show that, despite the 60 π-electron (4n) macrocyclic perimeter, the overall electronic structure is dominated by the local aromaticity of the 10 π-electron (4n+2) azulene subunits, while a weak paratropic ring current is induced upon application of an external magnetic field.