Expanded Porphyrins

Expanded porphyrins are porphyrin analogues consisting of more than five pyrroles. Among these, meso -aryl-substituted expanded porphyrins have emerged as novel π-conjugated macrocycles in light of their intriguing optical and electrochemical properties, structural diversities, and coordination abilities. Their first one-pot synthesis was developed in 2001, and since then, various modified synthetic protocols have allowed the isolation of unique expanded porphyrin congeners. This chapter summarizes the syntheses and properties of meso -aryl-substituted expanded porphyrins with a particular focus on the relationship between aromaticity and molecular topology. An intriguing feature is that meso -aryl-substituted expanded porphyrins have been shown to serve as a unique platform for realizing Möbius aromatic and antiaromatic molecules upon suitable stimuli such as metal coordination, protonation, and intramolecular fusion reaction. Another attractive attribute is the possibility of realizing giant aromatic molecules with distinct global aromaticities. Large meso -aryl-substituted expanded porphyrins in a neutral state tend to adopt coiled structures due to effective intramolecular hydrogen bonding interactions, but, upon protonation or metalation, their conformations are changed to yield global aromatic networks.