Heptaphyrins: Expanded porphyrins with seven heterocyclic rings

Expanded porphyrins containing seven pyrrole/heterocyclic rings linked in a cyclic fashion are termed heptaphyrins. The number of p-electrons in heptaphyrins depends on the number of meso carbon bridges used to link the heterocyclic rings, accordingly heptaphyrins with 28p-electrons and 30 p-electrons are reported to date. Both condensation reactions of the appropriate precursors and acid-catalysed oxidative coupling reactions have been utilized to synthesise the heptaphyrins. The 30p heptaphyrins exhibit rich structural diversity where some of the heterocyclic rings in the acrocycle undergo a 180Cring flipping. An overview of the synthetic methods employed for the synthesis of heptaphyrins, their spectroscopic properties, structural behaviour and aromatic properties are highlighted in this paper.