<div><p>Aromaticity is an important concept for predicting
electronic delocalisation in molecules, particularly for designing organic
semiconductors and single-molecule electronic devices. It is most simply
defined by the ability of a cyclic molecule to sustain a ring current when
placed in a magnetic field. Hückel’s rule states that if a ring has [4n+2]
π-electrons, it will be aromatic with an induced magnetisation that opposes the
external field inside the ring, whereas if it has 4n π-electrons, it will be antiaromatic
with the opposite magnetisation. This rule reliably predicts the behaviour of
small molecules, typically with circuits of less than about 22 π-electrons (n =
5). It is not clear whether aromaticity has a size limit and whether Hückel’s
rule is valid in much larger macrocycles. Here, we present evidence for global
aromaticity in a wide variety of porphyrin nanorings, with circuits of up to
162 π-electrons (n = 40; diameter 5 nm). We show that aromaticity can be
controlled by changing the molecular structure, oxidation state and
three-dimensional conformation. Whenever a global ring current is observed, its
direction is correctly predicted by Hückel’s rule. The magnitude of the current
is maximised when the average oxidation state of the porphyrin units is around
0.5–0.7, when the system starts to resemble a conductor with a partially filled
valence band. Our results show that aromaticity can arise in large macrocycles,
bridging the size gap between ring currents in molecular and mesoscopic rings.</p></div>
Global Aromaticity at the Nanoscale
