Möbius-Strip Topology of Expanded Porphyrins: A Minireview on EPR, ENDOR and DFT MO Studies

Abstract The one-sided Möbius strip with its characteristic 180° twist in the loop has inspired philosophers, artists and scientists since hundreds of years continues to do so. On molecular level, only last 15 have some groups succeeded synthesizing new expanded porphyrin compounds large enough adopt Möbius-strip topology aromaticity, first being Lechosław Latos-Grażyński collaborators Wroclaw (2007) Atsuhiro Osuka Kyoto (2008). We report on studies porphyrins either or Hückel that were synthesized these laboratories. In this minireview, we focus recent continuous-wave time-resolved EPR, ENDOR DFT MO open-shell states di- p -benzi[28]hexaphyrin(1.1.1.1.1.1), specifically, ground-state radical cation doublet state (total electron spin S = 1/2) excited triplet ( 1). review is largely based results discussions two previous publications: et al. (Appl Magn Reson 47:757–780, 2016) Ema (J Phys Chem Lett 9:2685–2690, 2018). systems, besides electron-nuclear hyperfine couplings also zero-field interaction tensor turned out be a viable sensor for electronic structure changes between topologies. Outlook section, address cyclotides, class natural circular mini-proteins, usually less than 100 amino acids long. They are distinguished by exceptional chemical biological stability. This due topological constraints imposed threefoil knot formation. As result, their physical qualities “topologically protected”. makes them highly interesting medical agricultural applications, example as novel active ingredients against autoimmune diseases, viral infections, agents insect damage crops.