Macrocyclic conjugation in N-fused porphyrins and related species.

Macrocyclic aromaticity is the most important concept in porphyrin chemistry. We propose a general graph-theoretical procedure for predicting the main macrocyclic conjugation pathway in porphyrinoids. This procedure, based on calculated bond resonance energies (BREs), can be applied not only to natural and expanded porphyrins but also to porphyrinoids with fused rings. Main macrocyclic conjugation pathways predicted with this procedure are exactly the same as those proposed by porphyrin chemists. Macrocyclic aromaticity can be estimated readily from the BRE for any of the pi-bonds linking adjacent pyrrolic rings. It was found that N-fusion often gives rise to anti-aromatic tripentacyclic subunits with negative BREs. Thus, our procedure properly characterizes macrocyclic conjugation and macrocyclic aromaticity in a wide variety of porphyrinoids.