Theoretical Study of the Switching between Hückel and Möbius Topologies for Expanded Porphyrins

The expanded porphyrins have become a useful tool to synthesize new Hückel-to-Möbius topological switches. Only applying small changes in the external conditions (temperature, solvent, redox potential) or in the structure of the ring allows the expanded porphyrins to achieve Hückel and Möbius topologies with distinct aromaticities and magnetic and electric properties. In this work, we report a theoretical investigation of the reaction mechanism of the conformational switch between the Hückel planar and the singly twisted Möbius structures for two different meso-substituted [28]­hexaphyrins(1.1.1.1.1.1); that is, the meso-substituents are the hydrogen atoms and pentafluorophenyls. Calculations are performed with the B3LYP, BH&HLYP, CAM-B3LYP, M05-2X, and MP2 methodologies along with the 6-31G and 6-311G­(d,p) basis sets. For benchmarking purposes, single-point energies are calculated at the CCSD­(T)/6-31G level. Our results conclude that, in overall, CAM-B3LYP and M05-2X are the methods that provide the most equilibrated results for the different steps of the topological switch. In addition, we point out that the ring strain and the steric effect of the meso-substituents play a relevant role in the thermochemistry and kinetics of the Hückel-to-Möbius topological switches.