Conjugated porphyrin arrays that possess delocalised electronic networks have, for the most part, been assembled by using alkene or alkyne type bridging units or by directly connecting individual porphyrin chromophores with multiple bonds to form fused porphyrin arrays. Throughout the last two decades, such conjugated porphyrin arrays have been actively explored due to their attractive electronic, optical and electrochemical properties. This review aims to cover the multitude of synthetic methodologies that have been developed for the construction of conjugated porphyrin arrays as well as to summarise their structure-property relationships and use in various applications such as near infrared (NIR) dyes, nonlinear optical materials and electron-conducting molecular wires.

Since the discovery of its facile synthesis in 2001, meso-aryl-substituted expanded porphyrins have been developed as a new class of azaannulenes in light of their facile redox interconversions, conformational flexibilities involving flipping of the constitutional pyrroles, rich metal coordination behaviors, unprecedented chemical reactivities, effective platforms to realize versatile electronic states including Möbius aromatic and antiaromatic species, and abilities to stabilize organic radicals. In this Review, the syntheses, structures, and optical, electronic, and magnetic properties of meso-aryl-substituted expanded porphyrins and their metal complexes have been updated with a particular focus on the relationship between "aromaticity and molecular twist (molecular topology)". While the importance of the interplay of these two characteristics has been long recognized from the theoretical viewpoint, meso-aryl-substituted expanded porphyrins offered solid experimental evidence to provide Möbius aromatic and antiaromatic molecules with distinct diatropic and paratropic ring currents, respectively. This attribute is not shared with β-alkylated expanded porphyrin counterparts, underlining the importance and uniqueness of meso-aryl-substituted expanded porphyrins.

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.