Two-dimensionally extended, polycyclic heteroaromatic molecules (heterocyclic nanographenes) are a highly versatile class of organic materials, applicable as functional chromophores and organic semiconductors. In this Review, we discuss the rich chemistry of large heteroaromatics, focusing on their synthesis, electronic properties, and applications in materials science. This Review summarizes the historical development and current state of the art in this rapidly expanding field of research, which has become one of the key exploration areas of modern heterocyclic chemistry.

The aromatic character of porphyrins, which has significant chemical and biological consequences, can be substantially altered by judicious modifications of the parent ring system. Expansion of the macrocycle, which is achieved by introducing additional subunits, usually increases the so-called free curvature of the ring, leading to larger angular strain. This strain is reduced by a variety of conformational changes, most notably by subunit inversion and p surface twisting. The latter effect creates a particularly convenient access to Möbius aromatic molecules, whose properties, predicted over 40 years ago, are of considerable theoretical importance. The conformational processes occurring in porphyrin analogues are often coupled to other chemical phenomena, and can thus be exploited as a means of constructing functional molecular devices. In this Review, the structural chemistry of porphyrinoids is discussed in the context of their conformational dynamics and p-electron conjugation.

Dual identity: It takes a single phenylene twist to reveal the dichotomous nature of a di‐para‐benzihexaphyrin (see picture; phenylene rings highlighted in red). This expanded porphyrinoid switches between Hückel and Möbius topologies in an unusual solvent‐ and temperature‐dependent equilibrium. Each of the two incarnations of the macrocycle has its own unmistakable spectral signature.

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The four expanded p-benziporphyrins A,C-di-p-benzi[24]pentaphyrin(1.1.1.1.1), N-fused A-p-benzi[24]pentaphyrin, A,D-di-p-benzi[28]hexaphyrin(1.1.1.1.1.1), and A,C-di-p-benzi[28]hexaphyrin(1.1.1.1.1.1) were obtained in three-component Lindsey-type macrocyclizations. These compounds were explored as macrocyclic ligands and as potential aromaticity switches. A BODIPY-like difluoroboron complex was obtained from the A,C-di-p-benzi[24]pentaphyrin, whereas A,C-di-p-benzi[28]hexaphyrin yielded a Möbius-aromatic Pd(II) complex containing fused pyrrole and phenylene subunits. Conformational behavior, tautomerism, and acid-base chemistry of the new macrocycles were characterized by means of NMR spectroscopy and DFT calculations. Free base N-fused A-p-benzi[24]pentaphyrin showed temperature-dependent Hückel-Möbius aromaticity switching, whereas the A,C-di-p-benzi[28]hexaphyrin formed a Möbius-aromatic dication.