σ-σ Stacked Supramolecular Junctions

The σ-σ stacking interactions between neighboring non-conjugated molecules can offer an efficient pathway for charge transport through supramolecular junctions, which provides a new guideline for the design and fabrication of organic materials and devices.
Published in Chemistry
σ-σ Stacked Supramolecular Junctions
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        The interaction between molecules is an important fundamental issue in the research of chemistry[1,2], materials[3,4], biology[5,6], and other related areas. For two conjugate molecules close to each other, there has been a lot of experimental evidence to prove that they can be stacked with each other through the intermolecular interaction between π electron clouds, which provides an efficient path for electron transport between molecules[7,8]. However, there is no experimental evidence showing that the σ-σ stacking interactions between two non-conjugated molecules can offer efficient charge transport through the supramolecular interactions between two monomers. This is mainly due to the generally underestimated weak intermolecular σ-σ stacking interactions compared with π-π stacking interactions. Recent theoretical studies have indicated that σ-σ stacking interactions and π-π stacking interactions may be equally important in stacked aromatic rings[9,10] and some pioneering studies using self-assembled monolayer (SAM) junctions demonstrated that lateral transport could occur via the through-space pathway among the adjacent hydrocarbon molecules within the SAM[11,12]. Single-molecule electrical characterization techniques provide us with a means to investigate charge transport through supramolecular junctions with controllable gap distance between the two electrodes[13,14].

         Here, we selected non-conjugated cyclohexanethiol and single-anchored adamantane molecules to fabricate and investigate charge transport through σ-σ stacked supramolecular junctions using the scanning tunneling microscope break junction (STM-BJ) technique. Our measurements demonstrated experimentally that the existence of σ-σ stacking interactions between neighboring non-conjugated cyclohexanethiol molecules is efficient enough to serve as a pathway for charge transport. We found that there are two different conductance states for σ-σ stacked cyclohexanethiol junctions formed during the break junction measurement, and the charge transport capacity of the stacked cyclohexanethiol dimer junction is comparable to that of the π-π stacked benzenethiol junction.

         Similar results were also obtained when we investigated single-anchored adamantane molecules, which have a highly symmetric cage structure consisting of four identical cyclohexane rings in the armchair configuration. The current-voltage characteristics demonstrate the existence of stacked molecular junctions with a symmetrical geometry configuration connected between the electrode pair, and the flicker noise analysis suggests that the pathway of charge transport through these σ-σ stacked molecular junctions has an obvious through-space characteristic. The specific configuration of these stacked molecular junctions is proposed, and the charge transport through σ-σ stacking intermolecular interactions is supported by theoretical calculations. Our findings open an avenue for the fabrication of supramolecular junctions using non-conjugated molecules, and this will increase the structural diversity of molecular devices and materials.

An abstract figure of this work. Left: the σ-σ stacked supramolecular junction formed with two cyclohexane rings. Right: the π-π stacked supramolecular junction formed with two benzene rings. The bluish-purple and pink regions between two molecules indicate the σ-σ interactions and π-π interactions, respectively. The red arrows with electrons show the charge transport from one molecule to the other one. The equal-arm balance with two ends balanced depicts that the charge transport capability of σ-σ interactions can be comparable to that of π-π interactions.

References

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[2] Olivo, G., Capocasa, G., Del Giudice, D., Lanzalunga, O. & Di Stefano, S. New horizons for catalysis disclosed by supramolecular chemistry. Chem. Soc. Rev. 50, 7681-7772 (2021).

[3]  de Greef, T. F. A. & Meijer, E. W. Supramolecular polymers. Nature 453, 171-173 (2008).

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[6] Kilchherr, F. et al. Single-molecule dissection of stacking forces in DNA. Science 353, aaf5508 (2016).

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[9] Grimme, S. Do special noncovalent π–π stacking interactions really exist? Angew. Chem. Int. Ed. 47, 3430-3434 (2008).

[10] Bloom, J. W. G. & Wheeler, S. E. Taking the aromaticity out of aromatic interactions. Angew. Chem. Int. Ed. 50, 7847-7849 (2011).

[11]  Slowinski, K., Chamberlain, R. V., Miller, C. J. & Majda, M. Through-bond and chain-to-chain coupling. Two pathways in electron tunneling through liquid alkanethiol monolayers on mercury electrodes. J. Am. Chem. Soc. 119, 11910-11919 (1997).

[12] Duati, M. et al. Electron transport across hexa-peri-hexabenzocoronene units in a metal-self-assembled monolayer-metal junction. Adv. Mater. 18, 329-333 (2006).

[13] Chen, H. & Fraser Stoddart, J. From molecular to supramolecular electronics. Nat. Rev. Mater. 6, 804-828 (2021).

[14] Liu, Y., Qiu, X., Soni, S. & Chiechi, R. C. Charge transport through molecular ensembles: Recent progress in molecular electronics. Chem. Phys. Rev. 2, 021303 (2021).

 

Read more about our work in Nature Chemistry:

https://www.nature.com/articles/s41557-022-01003-1

 

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