Tetrabenzotetraaza[8]circulene (1) has been synthesized in good yield by a "fold-in" oxidative fusion reaction of a 1,2-phenylene-bridged cyclic tetrapyrrole. X-ray diffraction analysis of 1 has revealed a planar square structure with a central cyclooctatetraene (COT) core that shows little alternation of the bond lengths. Despite these structural features, 1 shows aromatic-like character, such as sharp absorption bands, high fluorescence quantum yields (Φ(F)=0.55 in THF), and a single exponential fluorescence decay with τ(F)=3.8 ns. These observations indicate a dominant contribution of an [8]radialene-like π conjugation and hence aromatic character of the local aromatic segments in 1.

Oxidative fusion reactions of ortho-phenylene-bridged cyclic hexapyrroles and hexathiophenes furnished novel closed helicenes in a selective manner. X-Ray diffraction analysis unambiguously revealed the structures to be a closed pentaaza[9]helicene, the longest azahelicene reported so far, and an unexpected double-helical structure of hexathia[9]/[5]helicene, whose formation was assumed to result from multiple oxidative fusion along with a 1,2-aryl shift. The pentaaza[9]helicene exhibited well-defined emission with high fluorescence quantum yield (Φ =0.31) among the known [9]helicenes. Chiral resolution of the racemic pentaaza[9]helicene and hexathia[9]/[5]helicene were achieved by chiral-phase HPLC and the enantiomers were characterized by circular dichroism spectra and DFT calculations.

Tetrabenzotetraaza[8]circulene (1)has been synthesized in good yield by a"fold-in" oxidative fusion reaction of a1 ,2-phenylene-bridged cyclic tetrapyrrole.X -rayd iffraction analysis of 1 has revealed ap lanar square structure with ac entral cyclooctatetraene (COT) core that shows little alternation of the bond lengths.D espite these structural features, 1 shows aromatic-like character,s uch as sharp absorption bands,h igh fluorescence quantum yields (F F = 0.55 in THF), and as ingle exponential fluorescence decay with t F = 3.8 ns.T hese observations indicate ad ominant contribution of an [8]radialene-like p conjugation and hence aromatic character of the local aromatic segments in 1.

Oxidative fusion reactions of ortho‐phenylene‐bridged cyclic hexapyrroles and hexathiophenes furnished novel closed helicenes in a selective manner. X‐Ray diffraction analysis unambiguously revealed the structures to be a closed pentaaza[9]helicene, the longest azahelicene reported so far, and an unexpected double‐helical structure of hexathia[9]/[5]helicene, whose formation was assumed to result from multiple oxidative fusion along with a 1,2‐aryl shift. The pentaaza[9]helicene exhibited well‐defined emission with high fluorescence quantum yield (ΦF=0.31) among the known [9]helicenes. Chiral resolution of the racemic pentaaza[9]helicene and hexathia[9]/[5]helicene were achieved by chiral‐phase HPLC and the enantiomers were characterized by circular dichroism spectra and DFT calculations.

Sequential N‐alkylations of tetrabenzotetraaza[8]circulene provided mono‐, di‐, tri, and tetra‐N‐alkylated products in a controlled manner. Curiously, only opp isomer was obtained as a di‐N‐alkylated product. Upon increase of the N‐alkyl groups, the absorption and emission spectra exhibit continuous red‐shifts, and the excited‐state lifetimes become shortened, probably because of increased steric congestion at the nitrogen atoms that causes the central core to deviate from planarity. Mixed N‐substituted [8]circulenes have been also prepared.