Norcorrole: A Stable Porphyrinoid for Exploration of Antiaromaticity

Antiaromatic molecules are generally reactive and unstable due to their high highest occupied molecular orbital (HOMO) and low lowest unoccupied molecular orbital (LUMO) energy levels, which hinder their handling under ambient conditions and their use in functional materials. Nevertheless, antiaromatic molecules possess unique electronic features, including narrow HOMO-LUMO gaps, stable redox behavior, and distinctive magnetic properties, making them attractive building blocks for advanced functional materials. In this regard, the development of stable antiaromatic π-conjugated molecules is therefore an important challenge in contemporary molecular science. Norcorrole is a porphyrin analogue that lacks two bridging carbon atoms between the pyrrole subunits. Recent advances in norcorrole synthesis have established norcorroles as a unique platform for exploring antiaromaticity. Despite their pronounced antiaromatic character, norcorroles exhibit sufficient stability under ambient conditions and display a range of unusual properties that are different from conventional porphyrins. This Account details the development of the chemistry of norcorroles, focusing mainly on our research journey, covering the synthesis, structural characteristics, electronic properties, reactivity, and emerging functions of norcorrole derivatives. These include stacked-ring aromaticity arising from closely π-stacked architectures, reversible redox behavior for rechargeable batteries, intense near-infrared absorption for efficient photothermal conversion, high single-molecule conductivity, and the ability to form well-defined supramolecular assemblies. Through these investigations, norcorrole derivatives have evolved from fundamental targets in the chemistry of antiaromatic compounds into versatile functional materials for future applications.