Controlled Core‐Modification of a Porphyrin into an Antiaromatic Isophlorin

Partial core‐modification of a porphyrin can be employed to synthesize the 20π antiaromatic isophlorin. Unlike the tetra‐, tri‐, and dipyrrole derivatives of a porphyrin, a monopyrrole porphyrin exhibits antiaromatic characteristics. It undergoes a two‐electron reversible ring oxidation to yield the 18π aromatic dication. 1 H NMR analysis provides distinct evidence of the altered electronic characteristics through typical paratropic and diatropic ring current effects for the 4n and the (4n+2) π‐electron systems, respectively.