Dual Emission of a Free‐Base 5‐Oxaporphyrinium Cation from its cis‐ and trans‐NH Tautomers

Replacement of the  meso  methine carbon atoms of porphyrins with heteroatoms represents a powerful strategy for tuning their optical and electronic properties. In particular, 5-oxaporphyrin is an attractive target due to its importance as an intermediate in heme catabolism. In this work, we describe the synthesis and properties of a free-base 5-oxaporphyrinium cation, which was prepared by the ring-closure of a bilindione with trifluoromethanesulfonic anhydride. This free-base 5-oxaporphyrinium cation exhibits dual fluorescence originating from its unique NH tautomerism. In contrast to normal porphyrins, the  cis  form of the 5-oxaporphyrinium cation is more stable than the  trans  form due to the effective delocalization of its positive charge. We thus demonstrate here that  meso -modified  heteroporphyrins enable the manipulation of NH tautomerism in porphyrinic macrocycles.