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. 2017 Jul 19:8:15984.
doi: 10.1038/ncomms15984.

Highly-conducting molecular circuits based on antiaromaticity

Shintaro Fujii  1 , Santiago Marqués-González  1 , Ji-Young Shin  2 , Hiroshi Shinokubo  2 , Takuya Masuda  3 , Tomoaki Nishino  1 , Narendra P Arasu  4 , Héctor Vázquez  4 , Manabu Kiguchi  1
Affiliations

Highly-conducting molecular circuits based on antiaromaticity

Shintaro Fujii et al. Nat Commun. .

Abstract

Aromaticity is a fundamental concept in chemistry. It is described by Hückel's rule that states that a cyclic planar π-system is aromatic when it shares 4n+2 π-electrons and antiaromatic when it possesses 4n π-electrons. Antiaromatic compounds are predicted to exhibit remarkable charge transport properties and high redox activities. However, it has so far only been possible to measure compounds with reduced aromaticity but not antiaromatic species due to their energetic instability. Here, we address these issues by investigating the single-molecule charge transport properties of a genuinely antiaromatic compound, showing that antiaromaticity results in an order of magnitude increase in conductance compared with the aromatic counterpart. Single-molecule current-voltage measurements and ab initio transport calculations reveal that this results from a reduced energy gap and a frontier molecular resonance closer to the Fermi level in the antiaromatic species. The conductance of the antiaromatic complex is further modulated electrochemically, demonstrating its potential as a high-conductance transistor.

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Conflict of interest statement

The authors declare no competing financial interests.

Figures

Figure 1
Figure 1. Antiaromatic and aromatic molecules.
Antiaromatic Ni norcorrole Ni(nor) and its porphyrin-based aromatic counterpart Ni(porph) (Supplementary Note 1).
Figure 2
Figure 2. Measured single-molecule conducting properties.
(a) Logarithmically binned conductance histograms (111 bins per decade) of Ni(nor) and Ni(porph) at 100 mV in air. The peaks at 4.2 × 10−4 and 1.7 × 10−5 G0 denote the most probable molecular conductance values of Ni(nor) and Ni(porph), respectively. Histograms were built from 2,500 conductance traces with no data selection. Inset: representative examples of STM-BJ conductance–distance traces of Ni(nor) and Ni(porph) featuring molecular conductance plateaus. Traces were laterally offset for clarity. (b) Two-dimensional (2D) conductance–distance histogram of Ni(nor). Traces showing a conventional tunnelling decay were removed for clarity. For additional conductance measurements and histograms see Supplementary Figs 4–11.
Figure 3
Figure 3. Calculated transport properties.
(a) DFT transmission spectra of Ni(nor) and Ni(porph). Peaks at 0.1 and 0.6 eV are indicated by arrows. (b) Real part of the most conducting channels of Ni(nor) at 0.1 eV and Ni(porph) at 0.6 eV. In both cases, current is carried by a single channel.
Figure 4
Figure 4. Experimental determination of transmission resonance positions and electronic couplings.
(a) Two-dimensional (2D) histogram built from 216 I–V profiles collected from single-molecule junctions of Ni(nor). (b) Current–voltage histogram constructed with 168 profiles of Ni(porph). Overlaid lines show the most probable I–V profile. All current–voltage profiles were recorded during single-molecule conductance plateaus in the correlated BJ trace. Bin sizes of 0.02 V and 0.075 nA were employed. (c) Statistical distribution of transmission peak position Ni(nor) and Ni(porph) extracted from the fitting of each individual I–V response to a single-level tunnelling model. (d) Statistical distribution of metal–molecule coupling for Ni(nor) and Ni(porph) single-molecule junctions. (e) Schematic representation of the orbital alignment and metal–molecule electronic coupling for Ni(nor) and Ni(porph) assuming a single-level model.
Figure 5
Figure 5. Electrochemical gating of the charge transport properties.
(a) Logarithmically binned conductance histograms of Ni(nor) built from STM-BJ trances at different electrochemical potentials. An approximately fivefold conductance modulation was attained for the antiaromatic complex Ni(nor) under electrochemical environment. Histograms were built from STM-BJ traces collected at a different electrochemical potential (E), while holding a constant tip–substrate bias of 0.1 V. Experiments were performed using a Ag/AgCl reference (ΔEref=±1 mV). (b) Ni(nor) conductance as a function of electrochemical potential. For details on the experimental setup, individual histograms and reproducibility see Supplementary Figs 20–24.
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References

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