Aromatic and antiaromatic ring currents in a molecular nanoring

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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, 2, 3, 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 ring6, 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 electrons8. 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 size9. 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 π electrons) and aromatic in its 6+ oxidation state (78 π 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.

At a glance


  1. Molecular structures of the butadiyne-linked porphyrin oligomers used in this study. l-PN, c-PN and c-P6·T6.
    Figure 1: Molecular structures of the butadiyne-linked porphyrin oligomers used in this study. l-PN, c-PN and c-P6·T6.

    Ar = (3,5-bis(trihexylsilyl))phenyl as shown for l-PN.

  2. Computational data supporting aromaticity and antiaromaticity.
    Figure 2: Computational data supporting aromaticity and antiaromaticity.

    ac, NICS(0)iso grids in the x–y plane of c-P6 (a), c-P64+ (b) and c-P66+ (c). The colour axis is truncated to compare the grids on the same scale; see Supplementary Fig. 8 for grids with individual scales. df, ACID plots for each oxidation state. The yellow iso-surface depicts the anisotropy of the induced current density (isovalue 0.1 a.u.). The neutral oxidation state (a, d) shows ring current effects local to each porphyrin subunit. In contrast, the 4+ and 6+ oxidation states (b, e and c, f) show global ring currents, manifest by sign-reversal of the NICS inside/outside the ring.

  3. Square-wave voltammetry of c-P6·T6.
    Figure 3: Square-wave voltammetry of c-P6·T6.

    The solvent was CH2Cl2 (0.1 M Bu4NPF6). The arrows show the first reduction potential of each oxidant20: ferrocene, diacetylferrocenium, tris(4-bromophenyl)aminium hexafluoroantimonate (BAHAF), thianthrenium hexafluoroantimonate (Thn) and tris(2,4-dibromophenyl)aminium hexafluoroantimonate (DIBAHAF). There are six oxidations in a first manifold, generating oxidation states up to the hexacation (6+). A second manifold contains only a single oxidation wave generating the dodecacation (12+).

  4. NMR spectra of neutral and oxidised c-P6·T6.
    Figure 4: NMR spectra of neutral and oxidised c-P6·T6.

    ad, 1H NMR (500 MHz, CD2Cl2) of neutral c-P6·T6 at 298 K (a); c-P6·T64+ generated by titration with DIBAHAF at 223 K (b); c-P6·T66+ generated by titration with AgSbF6/I2 at 223 K (c); and c-P6·T612+ generated by oxidation with excess DIBAHAF at 223 K (d). The inset shows the molecular structure of the repeat unit of the sixfold symmetric c-P6·T6. The peaks labelled # and * arise from CHDCl2 and neutral oxidant (tris(2,4-dibromophenyl)amine), respectively. Unlabelled resonances are not assigned. is an unidentified impurity. In the neutral state (a), the template resonances (α–δ) probe the local aromaticity of each porphyrin. This aromaticity is reversed in the dodecacation (d), where the template protons report local antiaromaticity. The global aromaticity and antiaromaticity of the tetracation (b) and hexacation (c) are revealed by the large chemical shift difference between similar protons inside and outside the ring. The full spectra, without truncated peaks, are shown in Supplementary Fig. 27.


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  1. University of Oxford, Department of Chemistry, Chemistry Research Laboratory, Oxford OX1 3TA, UK

    • Martin D. Peeks,
    • Timothy D. W. Claridge &
    • Harry L. Anderson


M.D.P. synthesized the compounds, performed the calculations, collected and analysed the spectroscopic data. T.D.W.C. assisted with NMR data collection and interpretation. H.L.A. devised the project. M.D.P. and H.L.A. wrote the paper. All authors discussed the results and edited the manuscript.

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    This file contains Supplementary Tables 1-5 and Supplementary Figures 1-27. This file was updated on 11 January 2017 to correct the DOI number.

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