meso-Aryl-substituted hexaphyrins(126.96.36.199.1.1) have been examined by 1H, 13C, and 19F NMR spectroscopies, UV-vis absorption spectroscopy, magnetic circular dichroism spectroscopy, and single-crystal X-ray diffraction analysis. All of these data consistently indicate that hexaphyrins(188.8.131.52.1.1) in solution at 25°C exist largely as an equilibrium among several rapidly interconverting twisted Möbius conformations with distinct aromaticities, with a small contribution from a planar rectangular conformation with antiaromatic character at slightly higher energy. In the solid state, hexaphyrins(184.108.40.206.1.1) take either planar or Möbius-twisted conformations, depending upon the meso-aryl substituents and crystallization conditions, indicating a small energy difference between the two conformers. Importantly, when the temperature is decreased to -100°C in THF, these rapid interconversions among Möbius conformations are frozen, allowing the detection of a single hexaphyrin(220.127.116.11.1.1) species having a Möbius conformation. Detailed analyses of the solid-state Möbius structures of compounds 2b, 2c, and 2f showed that singly twisted structures are achieved without serious strain and that cyclic π-conjugation is well-preserved, as needed for exhibiting strong diatropic ring currents. Actually, the harmonic-oscillator model for aromaticity (HOMA) values of these structures are significantly large (0.85, 0.69, and 0.71, respectively), confirming the first demonstration of stable Möbius aromatic systems consisting of free-base expanded porphyrins without the assistance of metal coordination.
ASJC Scopus subject areas
- Colloid and Surface Chemistry