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Highly-Conducting Molecular Circuits Based on Antiaromaticity

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Highly-Conducting Molecular Circuits Based on Antiaromaticity ARTICLE Received 24 Jan 2017 | Accepted 17 May 2017 | Published 19 Jul 2017 DOI: 10.1038/ncomms15984 OPEN Highly-conducting molecular circuits based on antiaromaticity Shintaro Fujii1, Santiago Marque´s-Gonza´lez1, Ji-Young Shin2, Hiroshi Shinokubo2, Takuya Masuda3, Tomoaki Nishino1, Narendra P. Arasu4,He´ctor Va´zquez4 & Manabu Kiguchi1 Aromaticity is a fundamental concept in chemistry. It is described by Hu¨ckel’s rule that states that a cyclic planar p-system is aromatic when it shares 4n þ 2 p-electrons and antiaromatic when it possesses 4n p-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. 1 Department of Chemistry, Graduate School of Science and Engineering, Tokyo Institute of Technology, Ookayama, Meguro-ku, Tokyo 152-8551, Japan. 2 Department of Molecular and Macromolecular Chemistry, Graduate School of Engineering, Nagoya University, Aichi 464-8603, Japan. 3 Global Research Center for Environment and Energy Based on Nanomaterials Science (GREEN), National Institute for Materials Science (NIMS), Tsukuba 305-0044, Japan. 4 Institute of Physics, Academy of Sciences of the Czech Republic, Cukrovarnicka 10, Prague CZ-162 00, Czech Republic. Correspondence and requests for materials should be addressed to S.F. (email: [email protected]) or to H.S. (email: [email protected]) or to H.V. (email: [email protected]) or to M.K. (email: [email protected]). NATURE COMMUNICATIONS | 8:15984 | DOI: 10.1038/ncomms15984 | www.nature.com/naturecommunications 1 ARTICLE NATURE COMMUNICATIONS | DOI: 10.1038/ncomms15984 he concept of aromaticity1–3 is of fundamental importance charge transport properties at the single-molecule scale. To that in the chemistry of p-electrons that play an essential role in end, a recently developed, antiaromatic norcorrole-based Ni(II) Tmany organic materials relevant for nanoscale chemical, complex is studied (Fig. 1). Ni(nor) features a chemically stable, physical and biological studies. The stability and electronic structurally rigid antiaromatic porphyrinoid moiety20–22 where properties of the p-system depend sensitively on the geometry antiaromaticity can be preserved due in no small part to its and connectivity of the molecule. For instance, when a linear molecular rigidity. The electronic properties of Ni(nor) are p-conjugated system adopts a cyclic and planar framework, the benchmarked against a porphyrin-based aromatic counterpart, molecule gains additional energetic or resonance stabilization. This Ni(porph) (Fig. 1). The antiaromatic molecule Ni(nor) exhibits resonance or aromatic stabilization energy represents the energy enhanced charge transport properties in which single-molecule difference with the best canonical structure that can be drawn for conductance is more than one order of magnitude higher than the molecule and is thus a measure of molecular stability. The that of its aromatic counterpart. This makes Ni(nor) the most greater this energy, the more stable the compound is. In addition to conductive organometallic complex reported to date, to the best topology, this quantity also depends on the number of p-electrons of our knowledge. The ab initio transport calculations show that in the cyclic conjugated system. Cyclic and planar molecules with the remarkable conductance of Ni(nor) originates from the 4n þ 2 p-electrons present the highest resonance energy (stability) smaller gap and more favourable frontier orbital alignment with and are referred to as aromatic within Hu¨ckel’s rule4.Onthe respect to the aromatic species. Through electrochemical gating, other hand, cyclic and planar molecules sharing 4n p-electrons the conductance of the antiaromatic molecule Ni(nor) is further feature a remarkable energetic destabilization and are referred modulated by more than a factor of five. This work ultimately to as antiaromatic1,5. For example, the lower resonance energy in expands the current synthetic guidelines for the design of cyclobutadiene makes it more energetic (and far more reactive) highly conducting and functional organic electronic materials than its open-chain counterpart 1,3-butadiene. Higher stability using antiaromaticity. and lower reactivity are reflected in larger molecular gaps and aromatic molecules generally have larger gaps than antiaromatic ones6,7. From a fundamental and simplified viewpoint, cyclic Results molecules with 4n þ 1and4n þ 3 p-electrons obviously possess Conductance studies. To assess the impact of antiaromaticity in unpaired electron(s) and become nonaromatic. These compounds molecular conductance, single-molecule transport studies were exhibit open shell character and cannot be chemically isolated in a performed on two structurally similar complexes, Ni(nor) and controlled manner. Ni(porph) (Fig. 1, Supplementary Figs 1,2, and Supplementary Aromatic and antiaromatic p-systems have attracted wide Note 1). The thioacetate groups bind to Au electrodes through the attention in the field of the organic synthesis and materials S lone pair, a strategy that has been shown to result in selective science8–11. Aromatic molecules such as oligoacenes8, porphyrins9, bonding and clear experimental conductance signatures (see tetrathiafulvalenes10,11 and related compounds play an important Supplementary Note 3 for further details)23. The scanning role in organic electronic components such as electroluminescent tunnelling microscope-based break-junction (STM-BJ) technique devices and organic photovoltaic devices. In contrast, the characte- was used to obtain 2,500 conductance–distance traces for each ristic instability and smaller gap of antiaromatic molecules brings to molecule (see Methods and Supplementary Fig. 3). Measurements the p-system a great ability to donate and accept charge carriers were repeated on analogue samples of both complexes to ensure and eases electrochemical tunability of the molecular orbitals, reproducibility. All recorded traces were employed to build making antiaromatic compounds well suited for organic electronic logarithmically binned conductance histograms with no data components. Indeed, antiaromatic compounds had been predicted selection (Fig. 2a). Conductance histograms display well-defined by Breslow to be more conducting. Early electrochemical peaks denoting the most probable molecular conductance value at –4 –5 measurements of oxidation/reduction potentials had shown that 4.2 Â 10 and 1.7 Â 10 G0 for Ni(nor) and Ni(porph), electron transfer from donor to acceptor groups in a molecule was respectively. No additional molecular signatures were observed in 12,13 –2 amplified when connected through an antiaromatic structure . the high conductance regime 10 –1 G0 (Supplementary Fig. 5). The Advances in electronic characterization make it possible to measure measured conductance of the antiaromatic complex Ni(nor) was an electronic current through individual molecules, probing found to be an order of magnitude higher than that of Ni(porph). their inherent charge transport properties at the single-molecule The comparatively low conductance of the aromatic counterpart is scale14–16. Recent single-molecule electronics studies of a series of in good agreement with previous studies on closely related molecules with varying degrees of aromaticity established a porphyrin-based complexes24,25. The structural similarities negative relation between aromaticity and conductance. More between both complexes suggest that the enhanced conductance aromatic rings with higher resonance energies were seen to of Ni(nor) arises from the antiaromatic nature of the norcorrole correlate with HOMO (highest occupied molecular orbital) moiety. A two dimensional conductance–distance histogram positions further from the Fermi level and lower conductance constructed with traces featuring Ni(nor) molecular plateaus (which in these molecules is HOMO dominated)17. Subsequent extracted from the same data set reveals junction lengths transport calculations verified this trend and reported smaller of B0.8–1.0 nm (Fig. 2b; Supplementary Figs 6 and 7), ruling molecular gaps and HOMO positions closer to the Fermi level out the possibility of molecular junctions being formed with decreasing aromaticity18. However, the chemical instability perpendicularly to the norcorrole plane26. Slightly longer of the antiaromatic molecules and unfavourable intermolecular junction dimensions of B1.0–1.2 nm were typically observed interaction in the bulk has restricted transport studies to aromatic for Ni(porph) (Supplementary Fig. 8) in good agreement with systems or species with no aromaticity. While these works previous reports25. Similar junction lengths were obtained for clearly established the negative relationship of aromaticity
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