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Unambiguous Identification of Möbius Aromaticity for meso -Aryl-Substituted [28]Hexaphyrins(1.1.1.1.1.1)

2008; American Chemical Society; Volume: 130; Issue: 41 Linguagem: Inglês

10.1021/ja801983d

1943-2984

Jeyaraman Sankar, Shigeki Mori, Shohei Saito, Harapriya Rath, Masaaki Suzuki, Yasuhide Inokuma, Hiroshi Shinokubo, Kil Suk Kim, Zin Seok Yoon, Jae‐Yoon Shin, Jong Min Lim, Yoichi Matsuzaki, Osamu Matsushita, Atsuya Muranaka, Nagao Kobayashi, Dongho Kim, Atsuhiro Osuka,

Photosynthetic Processes and Mechanisms

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.