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

Abstract

Replacement of the meso methine carbon atoms of porphyrins with heteroatoms is 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.

Synthesis, Structure, and Redox and Optical Properties of 5,10,15,20‐Tetraaryl‐5‐azaporphyrinium Salts

“…In contrast to DAPs, 5‐monoazaporphyrins (MAPs) have received little attention and their structure‐property relationships have not been fully elucidated [10–12] . Recently, Shinokubo and Takiguchi reported the free base of 2,3,7,8,12,13,17,18‐octaethyl‐5‐azaporphyrinium salt P2 as the first example of a MAP bearing a substituent on the meso ‐N atom, which can be obtained from the corresponding 5‐oxaporphyrinium ion [13] via sequential ring‐opening and ‐closing reactions [14] . However, there is no report on the synthesis of MAPs bearing four meso ‐substituents.…”