Exploring Aromaticity in Expanded Porphyrins: A Multidimensional Approach to Structure–Property Relationships

Expanded porphyrins offer a unique platform for probing how (anti)aromaticity influences molecular properties. These macrocycles are remarkably flexible, capable of adopting diverse π ‐conjugation topologies, including Möbius strip‐like and twisted Hückel structures, which are difficult to realize with regular porphyrins. Their rich redox chemistry facilitates the formation of congeneric macrocycles with (4 n + 2) and (4 n ) π ‐electrons, making them ideal systems for testing the practical limits of Hückel, Möbius, and Baird aromaticity rules. Spectroscopic properties are commonly employed to experimentally probe the ground‐ and excited‐state aromaticity of expanded porphyrins. Nevertheless, quantifying aromaticity remains challenging from both experimental and theoretical standpoints due to the intricate interplay between local and macrocyclic ring currents, which often leads to discrepancies between descriptors based on different criteria. This review summarizes our efforts to unravel the aromaticity fingerprint on the photophysical and nonlinear optical properties of expanded porphyrins. A multidimensional framework to quantify Hückel and Möbius aromaticity is first introduced, integrating global and local descriptors derived from the energetic, reactivity, magnetic, electronic, and structural criteria. The complex structure–property relationships between aromaticity and spectroscopic features across redox‐ and topology‐controlled expanded porphyrins are examined. Lastly, innovative approaches to uncover the driving forces governing the spectroscopic properties of diverse hexaphyrin databases are introduced.