Two Approaches to the Synthesis of Redox-Switchable 5,10,20-Triaryl-5,15-diazaporphyrinoids

Despite the significant development and extensive application of phthalocyanine and related azaporphyrins, little attention has been paid to meso-N-substituted azaporphyrinoids. Here, we report new derivatives of 5,10,20-triaryl-5,15-diazaporphyrinoids (Ar₃DAP), which are reversibly redox-switchable between the 18π- and 19π-electron state. Four kinds of metal(II) complexes and free bases of Ar₃DAP were prepared by metal-templated cyclization of metal(II) complexes of 5,10,15-triaryl-10-azabiladiene-ac with sodium azide or copper-catalyzed N-phenylation of 10,20-diaryl-5,15-diazaporphyrins (Ar₂DAP) with diphenyliodonium hexafluorophosphate. In particular, regioselective N-phenylation of covalently linked Ar₂DAP dimer afforded two kinds of dimers bearing one or two meso-N-phenyl groups. Cyclic voltammetry revealed that attaching one aryl group onto the meso-nitrogen atom markedly changed the redox potentials of the DAP ring. The aromaticity and optical properties of the new Ar₃DAP π-electron systems were assessed using various spectroscopic measurements and density functional theory calculations. The distribution pattern of an electron spin in the neutral Ar₂DAP–Ar₃DAP radical revealed that the meso-N-phenyl group had a remarkable effect on spin delocalization of DAP π-radical. The present study provides valuable information for understanding the effects of the number of meso-N-substituents on azaporphyrin-based π-electron systems.