Aromatic and antiaromatic ring currents in a molecular nanoring

摘要

Aromatic and antiaromatic molecules exhibit ring currents around their perimeters. Similar persistent currents are observed in metal rings, when the coherence length of the electrons is longer than the circumference of the ring. The relationship between these quantum mechanical phenomena is poorly understood, partly because they are studied in different size regimes: the largest aromatic molecules have diameters of about 1 nm, whereas persistent currents are found in rings with diameters of 20–1000 nm. It is not clear whether aromaticity and antiaromaticity are confined to small molecules, or whether the necessary quantum coherence can be achieved in large macrocycles. Here we show, using 1H NMR spectroscopy and density functional theory, that a six-porphyrin nanoring template complex c-P6·T6, with a diameter of 2.4 nm, is antiaromatic in its 4+ oxidation state and aromatic in its 6+ oxidation state. These species have circuits of 80 and 78 π electrons, respectively, and they are the largest aromatic and antiaromatic systems yet reported. 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 nm, at room temperature, shows that quantum coherence can persist in surprisingly large molecular frameworks, and implies that it will be possible to exploit quantum interference effects, such as Aharonov-Bohm oscillations, in molecular electronic devices.