期刊
JOURNAL OF THE AMERICAN CHEMICAL SOCIETY
卷 130, 期 41, 页码 13568-13579出版社
AMER CHEMICAL SOC
DOI: 10.1021/ja801983d
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资金
- Ministry of Education, Culture, Sports, Science and Technology of Japan [19205006]
- Ministry of Education and Human Resources of Korea
- National Research Foundation of Korea [R16-1997-011-01001-0, 과06A1503] Funding Source: Korea Institute of Science & Technology Information (KISTI), National Science & Technology Information Service (NTIS)
meso-Aryl-substituted [28]hexaphyrins(1.1.1.1.1.1) have been examined by H-1, C-13, and F-19 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 Mobius 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 Mobius-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 Mobius conformations are frozen, allowing the detection of a single [28]hexaphyrin(1.1.1.1.1.1) species having a Mobius conformation. Detailed analyses of the solid-state Mobius 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 Mobius aromatic systems consisting of free-base expanded porphyrins without the assistance of metal coordination.
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