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.

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.

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.

Highly twisted quasi-aza[8]circulene and planar triazaoxa[8]circulene have been prepared, which exhibited intriguing optical properties.