Investigation of type-I conformations of A2B4-[26]hexaphyrins(1.1.1.1.1.1) bearing strongly electron-withdrawing aryl substituents at 5,20-positions

Abstract

The dumbbell-shaped type-I conformation is the inherent structure of the [26]hexaphyrin(1.1.1.1.1.1) framework without any perturbations from its peripheral substituents. Conversely, the rectangular type-II conformation can be regarded as a more general structure seen in stable [26]hexaphyrins(1.1.1.1.1.1) because of not only the synthetic limitations but also the steric influences related to the meso-aryl substituents. In this study, the synthesis of [26]hexaphyrins(1.1.1.1.1.1) bearing a series of electron-withdrawing groups and their conformational characterizations were investigated. The 1H NMR spectra showed that [26]hexaphyrin with 3,5-dinitrophenyl groups at the 5,20-positions was obtained as the sole type-I conformer, whereas most other [26]hexaphyrins are in equilibrium between type-I, type-II, and type-II′ conformers. Moreover, UV-vis absorption measurements revealed that type-I conformations can utilize the electronic features of their 5,20-substituents owing to their relatively co-planar arrangements with the macrocycles.