title: Unambiguous identification of Möbius aromaticity for meso-aryl-substituted [28]hexaphyrins(1.1.1.1.1.1).
abstract: meso-Aryl-substituted [28]hexaphyrins(1.1.1.1.1.1) have been examined by (1)H, (13)C, and (19)F NMR spectroscopies, UV-vis absorption spectroscopy, magnetic circular dichroism spectroscopy, and single-crystal X-ray diffraction analysis. All of these data consistently indicate that [28]hexaphyrins(1.1.1.1.1.1) in solution at 25 degrees 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, [28]hexaphyrins(1.1.1.1.1.1) 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 degrees C in THF, these rapid interconversions among Möbius conformations are frozen, allowing the detection of a single [28]hexaphyrin(1.1.1.1.1.1) 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 pi-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.
authors.affiliation: Department of Chemistry, Graduate School of Science, Kyoto University, Sakyo-ku, Kyoto 606-8502, Japan.
authors.forename: Jeyaraman
authors.initials: J
authors.lastname: Sankar
authors.forename: Shigeki
authors.initials: S
authors.lastname: Mori
authors.forename: Shohei
authors.initials: S
authors.lastname: Saito
authors.forename: Harapriya
authors.initials: H
authors.lastname: Rath
authors.forename: Masaaki
authors.initials: M
authors.lastname: Suzuki
authors.forename: Yasuhide
authors.initials: Y
authors.lastname: Inokuma
authors.forename: Hiroshi
authors.initials: H
authors.lastname: Shinokubo
authors.forename: Kil Suk
authors.initials: KS
authors.lastname: Kim
authors.forename: Zin Seok
authors.initials: ZS
authors.lastname: Yoon
authors.forename: Jae-Yoon
authors.initials: JY
authors.lastname: Shin
authors.forename: Jong Min
authors.initials: JM
authors.lastname: Lim
authors.forename: Yoichi
authors.initials: Y
authors.lastname: Matsuzaki
authors.forename: Osamu
authors.initials: O
authors.lastname: Matsushita
authors.forename: Atsuya
authors.initials: A
authors.lastname: Muranaka
authors.forename: Nagao
authors.initials: N
authors.lastname: Kobayashi
authors.forename: Dongho
authors.initials: D
authors.lastname: Kim
authors.forename: Atsuhiro
authors.initials: A
authors.lastname: Osuka