Synthesis, Structure, and Properties of Stable Twist‐Induced Nonaromatic Expanded Porphyrins

Abstract Porphyrins are 18π electron aromatic macrocycles containing four pyrrole rings connected via four meso‐carbon bridges and expanded porphyrinoids contain more than four pyrrole rings, multiple meso‐carbon bridges, and more than 18π‐delocalized electrons in a macrocyclic conjugation circuit. Porphyrins are aromatic and follow Huckel's [4n + 2] π‐electron aromaticity, whereas expanded porphyrins exhibit both Huckel's [4n + 2] and Mobius [4n] π‐electron aromatic topologies. Porphyrinoids are, in general aromatic, highly stable, and exhibit very interesting physicochemical properties. Over the years, different types of porphyrinoids have been synthesized because of their diverse applications in a wide range of fields that include biology, materials, and medicine. In recent times, very stable nonaromatic porphyrinoids, which absorb in visible to NIR region, have been synthesized and explored for their structure, spectral, and redox properties. The nonaromatic expanded porphyrinoids are fully conjugated but their π‐electron delocalisation has been hampered due to which these macrocycles are nonaromatic. This review describes an overview of nonaromatic expanded porphyrins in terms of their synthesis, key structural aspects, and properties.