Department of Chemistry, University of Oxford, Oxford, UK.
Department of Chemistry, University of Zurich, Zurich, Switzerland.
Nat Chem. 2020 Mar;12(3):236-241. doi: 10.1038/s41557-019-0398-3. Epub 2020 Jan 20.
Aromaticity can be defined by the ability of a molecule to sustain a ring current when placed in a magnetic field. Hückel's rule states that molecular rings with [4n + 2] π-electrons are aromatic, with an induced magnetization that opposes the external field inside the ring, whereas those with 4n π-electrons are antiaromatic, with the opposite magnetization. This rule reliably predicts the behaviour of small molecules, typically with fewer than 22 π-electrons (n = 5). It is not clear whether aromaticity has a size limit, or whether Hückel's rule extends to much larger macrocycles. Here, we present evidence for global aromaticity in porphyrin nanorings with circuits of up to 162 π-electrons (n = 40); aromaticity is controlled by changing the constitution, oxidation state and conformation. Whenever a ring current is observed, its direction is correctly predicted by Hückel's rule. The largest ring currents occur when the porphyrin units have fractional oxidation states.
芳香性可以定义为分子在磁场中时保持环电流的能力。休克尔规则指出，具有[4n + 2]π电子的分子环是芳香性的，其感应磁化与环内的外磁场相反，而具有 4nπ电子的分子环是反芳香性的，其磁化方向相反。该规则可靠地预测了具有少于 22π电子（n = 5）的小分子的行为。目前尚不清楚芳香性是否有限制，或者休克尔规则是否适用于更大的大环。在这里，我们提出了具有多达 162π电子（n = 40）的卟啉纳米环中存在整体芳香性的证据；芳香性通过改变构成、氧化态和构象来控制。只要观察到环电流，其方向就可以通过休克尔规则正确预测。当卟啉单元具有分数氧化态时，会出现最大的环电流。
