Aromaticity in Fully -Conjugated Multicyclic Macrocycles

The research on aromaticity has mainlyfocused on monocyclic [n]-annulene-likesystems or polycyclic aromatic hydrocarbons. For fully pi-conjugatedmulticyclic macrocycles (MMCs), the electronic coupling between theindividual constitutional macrocycles would lead to unique electronicstructures and aromaticity. The studies on MMCs, however, are quitelimited, presumably due to the great challenges to design and synthesizea fully pi-conjugated MMC molecule. Herein, we report the facilesynthesis of two MMCs (2TMC and 3TMC) inwhich two and three thiophene-based macrocycles are fused togetherby employing both intramolecular and intermolecular Yamamoto couplingreactions of a properly designed precursor (7). The monocyclicmacrocycle (1TMC) was also synthesized as a model compound.The geometry, aromaticity, and electronic properties of these macrocyclesat different oxidation states were investigated by X-ray crystallographicanalysis, NMR, and theoretical calculations, which disclosed how theconstitutional macrocycles interplay with each other and lead to uniquearomatic/antiaromatic character. This study provides new insightsinto the complex aromaticity in MMC systems.