Temperature-dependent conformational change of weso- Hexakis(pentafluorophenyl) [28]Hexaphyrins(1.1.1.1.1.1) into mobius structures

At room temperature, meso-hexaaryl-substituted [28]hexaphyrins(1.1.1.1.1.1) in solution exist largely as an equilibrium between planar antiaromatic and distorted Mobius aromatic conformers. As the temperature decreases, the molecular structure changes into the distorted Mobius topology that commonly occurs in [28]hexaphyrins, which gives rise to longer excited singlet and triplet state lifetimes than planar antiaromatic [28]hexaphyrins. Temperature-dependent two-photon absorption measurements of [28]hexaphyrin indicate that the degree of aromaticity of Mobius [28]hexaphyrin is large, comparable to that of Huckel aromatic planar [26]hexaphyrin. Through our spectroscopic investigations, we have demonstrated that a subtle balance between the strains induced by the size of the [28]hexaphyrin macrocyclic ring and the energy stabilization contributed by electron delocalization in the formation of distorted Mobius [28]hexaphyrin leads to the molecular structure change into the Mobius topology as the temperature decreases.