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Aromatic and antiaromatic ring currents in a molecular nanoring

机译：分子纳米环中的芳香和反芳香环电流

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Aromatic and antiaromatic molecules-which have delocalized circuits of [4n + 2] or [4n] electrons, respectively-exhibit ring currents around their perimeters1-4. The direction of the ring current in an aromatic molecule is such as to generate a magnetic field that opposes the external field inside the ring (a 'diatropic' current), while the ring current in an antiaromatic molecule flows in the reverse direction ('paratropic')(5). Similar persistent currents occur in metal or semiconductor rings, when the phase coherence of the electronic wavefunction is preserved around the ring(6,7). Persistent currents in non-molecular rings switch direction as a function of the magnetic flux passing through the ring, so that they can be changed from diatropic ('aromatic') to paratropic ('antiaromatic') simply by changing the external magnetic field. As in molecular systems, the direction of the persistent current also depends on the number of electrons(8). The relationship between ring currents in molecular and non-molecular rings is poorly understood, partly because they are studied in different size regimes: the largest aromatic molecules have diameters of about one nanometre, whereas persistent currents are observed in microfabricated rings with diameters of 20-1,000 nanometres. Understanding the connection between aromaticity and quantum-coherence effects in mesoscopic rings provides a motivation for investigating ring currents in molecules of an intermediate size(9). Here we show, using nuclear magnetic resonance spectroscopy and density functional theory, that a six-porphyrin nanoring template complex, with a diameter of 2.4 nanometres, is antiaromatic in its 4+ oxidation state (80 pi electrons) and aromatic in its 6+ oxidation state (78 pi electrons). The antiaromatic state has a huge paramagnetic susceptibility, despite having no unpaired electrons. This work demonstrates that a global ring current can be promoted in a macrocycle by adjusting its oxidation state to suppress the local ring currents of its components. The discovery of ring currents around a molecule with a circumference of 7.5 nanometres, at room temperature, shows that quantum coherence can persist in surprisingly large molecular frameworks.
机译：具有[4n + 2]或[4n]电子离域电路的芳香和抗芳香分子分别在其周长1-4周围表现出环电流。芳族分子中环电流的方向会产生与环内部的外场相反的磁场（“ diatropic”电流），而抗芳族分子中的环电流会沿相反方向（“ paratropic”）流动'）（5）。当电子波函数的相位相干性保持在环周围时，在金属或半导体环中会发生类似的持续电流（6,7）。非分子环中的持久电流根据通过环的磁通量来切换方向，因此只需改变外部磁场，就可以将它们从变径（“芳香族”）变为顺磁性（“反芳香族”）。就像在分子系统中一样，持续电流的方向也取决于电子的数量（8）。人们对分子环和非分子环中环电流之间的关系了解得很少，部分原因是它们是在不同尺寸范围内进行研究的：最大的芳族分子的直径约为一纳米，而在直径为20- 1,000纳米了解介观环中芳香性和量子相干效应之间的联系为研究中等大小分子中的环电流提供了动力（9）。在这里，我们使用核磁共振波谱学和密度泛函理论表明，直径为2.4纳米的六卟啉纳米环模板复合物在其4+氧化态（80 pi电子）中是抗芳族化合物，在其6+氧化中是芳族化合物。状态（78 pi电子）。尽管没有不成对的电子，但抗芳族态具有很大的顺磁敏感性。这项工作表明，通过调节环的氧化态以抑制其组件的局部环电流，可以在大环中促进全局环电流。在室温下，发现围绕着一个7.5纳米圆周的分子周围的环电流，表明量子相干性可以在出乎意料的大分子框架中持续存在。

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  • 来源
    《Nature》 |2017年第7636期|200-203|共4页
  • 作者

    Peeks Martin D.; Claridge Timothy D. W.; Anderson Harry L.;

  • 作者单位

    Univ Oxford, Dept Chem, Chem Res Lab, Oxford OX1 3TA, England;

    Univ Oxford, Dept Chem, Chem Res Lab, Oxford OX1 3TA, England;

    Univ Oxford, Dept Chem, Chem Res Lab, Oxford OX1 3TA, England;

  • 收录信息 美国《科学引文索引》(SCI);美国《工程索引》(EI);美国《生物学医学文摘》(MEDLINE);美国《化学文摘》(CA);
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