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.

芳香性可以通过分子在磁场中维持环电流的能力来定义。Hückel 规则指出，具有 [4 n  + 2] π 电子的分子环是芳香的，其感应磁化强度与环内的外场相反，而具有 4 n π 电子的分子环是反芳香的，具有相反的磁化强度。该规则可靠地预测小分子的行为，通常少于 22 个 π 电子 ( n  = 5)。目前尚不清楚芳香性是否有大小限制，或者 Hückel 规则是否扩展到更大的大环。在这里，我们提出了具有高达 162 个 π 电子（n = 40); 芳香性是通过改变组成、氧化态和构象来控制的。每当观察到环流时，其方向都可以通过 Hückel 规则正确预测。当卟啉单元具有部分氧化态时，会出现最大的环电流。