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.
 Neel, A. J., Hilton, M. J., Sigman, M. S. & Toste, F. D. Exploiting non-covalent π interactions for catalyst design. Nature 543, 637-646 (2017).
 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).
 de Greef, T. F. A. & Meijer, E. W. Supramolecular polymers. Nature 453, 171-173 (2008).
 Amabilino, D. B., Smith, D. K. & Steed, J. W. Supramolecular materials. Chem. Soc. Rev. 46, 2404-2420 (2017).
 Kim, T., Park, J. Y., Hwang, J., Seo, G. & Kim, Y. Supramolecular two-dimensional systems and their biological applications. Adv. Mater. 32, 2002405 (2020).
 Kilchherr, F. et al. Single-molecule dissection of stacking forces in DNA. Science 353, aaf5508 (2016).
 Wu, S. et al. Molecular junctions based on aromatic coupling. Nat. Nanotechnol. 3, 569-574 (2008).
 Li, X. et al. Structure-independent conductance of thiophene-based single-stacking junctions. Angew. Chem. Int. Ed. 59, 3280-3286 (2020).
 Grimme, S. Do special noncovalent π–π stacking interactions really exist? Angew. Chem. Int. Ed. 47, 3430-3434 (2008).
 Bloom, J. W. G. & Wheeler, S. E. Taking the aromaticity out of aromatic interactions. Angew. Chem. Int. Ed. 50, 7847-7849 (2011).
 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).
 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).
 Chen, H. & Fraser Stoddart, J. From molecular to supramolecular electronics. Nat. Rev. Mater. 6, 804-828 (2021).
 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: