Diversity of Porphyrinoids

Porphyrin has fourfold symmetry, with the pyrrole rings connected by alternate meso bridges. About eight isomers can be sketched down by changing the sequence of the subunits, of which porphycene (37), hemiporphycene, corrphycene, and isoporphycene have been synthesized to date. Expanded porphyrins are, by definition, those possessing more than 16 atoms in the smallest circuit of the macrocycle. Sapphyrin (39) is the common expanded porphyrin derived by the insertion of an additional pyrrole ring into the macrocycle [40]. 

Unlike porphyrin, corroles form stable monoanions when treated with alkali, which is attributed to their increased acidity due to contraction of the ring, resulting in steric congestion [42]. 

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