Effect of the meso-substituent in the Hückel-to-Möbius topological switches.

Expanded porphyrins have emerged as a new promising class of molecules for the creation of new Hückel-to-Möbius topological switches with distinct aromaticities and magnetic and electric properties. In this work, we report a theoretical investigation of the conformational switch between the Hückel planar and the singly twisted Möbius structure for eight different meso-substituted [28]-hexaphyrins (with different steric effects and electron-withdrawing and -releasing character). Our results show how a change in the nature of the meso-substituent is able to turn an endothermic interconversion process with a high energy barrier into an exothermic and almost barrierless Hückel-Möbius transition. We also provide a thorough analysis of the main factors (aromaticity, intramolecular hydrogen bonds, ring strain, and steric effects) that play a role in this interconversion process. Overall, these results are very relevant to find new ways to control the thermochemistry and kinetics of these topological switches and even "freeze" the switch in the desired Möbius or Hückel conformation.