Letter Cite This: Nano Lett. XXXX, XXX, XXX−XXX
pubs.acs.org/NanoLett
Remarkable Multichannel Conductance of Novel Single-Molecule Wires Built on Through-Space Conjugated Hexaphenylbenzene Shijie Zhen,† Jin-Chuan Mao,‡ Long Chen,† Siyang Ding,† Wenwen Luo,† Xiao-Shun Zhou,*,‡ Anjun Qin,† Zujin Zhao,*,† and Ben Zhong Tang*,†,§
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†
Center for Aggregation-Induced Emission, State Key Laboratory of Luminescent Materials and Devices, South China University of Technology, Guangzhou 510640, China ‡ Key Laboratory of the Ministry of Education for Advanced Catalysis Materials, Institute of Physical Chemistry, Zhejiang Normal University, Jinhua, Zhejiang 321004, China § Department of Chemistry, Hong Kong Branch of Chinese National Engineering Research Center for Tissue Restoration and Reconstruction, The Hong Kong University of Science & Technology, Clear Water Bay, Kowloon, Hong Kong, China S Supporting Information *
ABSTRACT: Through-bond conjugated molecules are the major frameworks for traditional molecular wires, while through-space conjugated units are rarely utilized and studied although they have shown unique conducting potential. Herein, we present novel single-molecule wires built on through-space conjugated hexaphenylbenzene. Their conductance, measured by the scanning tunneling microscopy based break-junction technique, increases with the improvement of through-space conjugation and finally reaches a remarkable value (12.28 nS) which greatly exceeds that of conventional through-bond conjugated counterpart (2.45 nS). The multichannel conducting model by integrating through-space and through-bond conjugations could be a promising strategy for the further design of robust single-molecule wires with advanced conductance and stability. KEYWORDS: Single-molecule wire, through-space conjugation, multichannel conductance, scanning tunneling microscopy, hexaphenylbenzene
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on noncovalent through-space conjugation, in which the π-electrons spatially distributed within the interplane regions of closely stacked aromatic rings can serve charge transport.17−25 The involving through-space conjugation is the interaction that forms when two chromophores or aromatic rings are aligned sufficiently closely and there is an overlap of their electron density distributions, together with the longrang Coulombic (dipole−dipole) interactions.26 This unique through-space conducting manner draws increasing interests and demonstrates vital significance in various systems, including redox processes in biocomplexes,27 electronic signal delivering in DNA molecules,28 carrier transport in organic electronics and photovoltaics,29 etc. However, compared with through-bond single-molecule wires, through-space ones are far beyond explored, probably owing to the lack of appropriate organic platforms with strong through-space conjugation.20,30,31 Recently, we developed brand-new folded single-molecule wires with complementary through-bond and through-space
olecular electronics have attracted considerable attention because of the potential breakthrough of intrinsic limitation of silicon-based electronics.1 Single-molecule wires are the fundamental elements for molecular electronics,2 which are responsible for charge transport and electronic communication among different components at molecular level. Apart from the electrode−molecule interface,3,4 the internal electronic and geometrical structures of the molecule play a decisive role in the overall performance of a molecular wire in metal−molecule−metal junction.5−9 Consequently, the exploration of robust single-molecule wires with ideal conformations and unique electronic effects remains a great challenge to achieve high and stable conductance.10−13 So far, the widely studied molecular kernels for artificial single-molecule wires in the literatures are mainly confined to through-bond π-conjugated rod-like organic and organometallic molecules,5−7,14 in which the high-density delocalized π-electrons are favored for charge transport through the molecular backbone. This through-bond conjugation generally refers to the fact that aliphatic and aromatic units connect directly with carbon−carbon single, double, or triple bonds composed of sp, sp2, or sp3 carbon frameworks.15,16 Another kind of interesting single-molecule wire that is just emerging is based © XXXX American Chemical Society
Received: March 18, 2018 Revised: April 27, 2018
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DOI: 10.1021/acs.nanolett.8b01082 Nano Lett. XXXX, XXX, XXX−XXX
Letter
Nano Letters
shorter than the typical distance for π−π interaction (3.5 Å), implying strong electronic coupling interaction exists among these phenyls. The peripheral phenyls in HPB(OM)3-SM and HPB(OM)-SM are connected to the central benzene core in a nearly perpendicular manner, with large torsion angles of ∼86° and ∼81°, respectively, suggesting that the toroidal throughspace conjugation is strong but the through-bond conjugation on the linear backbones of both molecules is greatly weakened. The torsion angle of peripheral phenyls in HPB-SM without the methoxy, however, is reduced to ∼66°, disclosing that HPB-SM has relatively weaker through-space conjugation but stronger through-bond conjugation than HPB(OM)3-SM and HPB(OM)-SM. The optimized structures of all the molecules are further calculated for systematical comparison. As illustrated in Figure S1, the torsion angles are increased from HPB-SM (59°) to HPB(OM)-SM (67°) and then to HPB(OM)3-SM (87°), revealing that the through-bond conjugation of the molecular backbone are decreased progressively, consistent with the results from crystallographic analysis. However, the linear counterpart PP-SM has a much more planar and better-conjugated structure as evidenced by the small torsion angle (32°). These findings are also supported by their absorption properties. As illustrated in Figure 3a, HPB(OM)3-SM shows absorption maximum at 300 nm, which is the same as that of HPB(OM)-SM (300 nm), but the spectral profile of HPB(OM)3-SM is broader relative to that of HPB(OM)-SM. The absorption maximum of HPB-SM is slightly red-shifted to 306 nm because of its apparently decreased torsion angles in the backbone. However, all of these absorption wavelengths are much shorter than that of better-conjugated PP-SM (321 nm). Because the conductance is closely associated with the geometry and conjugation of the molecule,5 it is envisioned that the through-bond conductance of HPB-based molecules, particularly HPB(OM)3-SM should be significantly inferior to that of PP-SM. The through-space conjugation in HPB is highly sensitive to the electronic density of the peripheral phenyls.32−36 The addition of methoxy groups can enrich the π-electron cloud and promote the electronic coupling interaction between adjacent phenyls. To confirm this, the electrochemical behaviors of the HPB-based molecules are investigated. As expected, HPB(OM)3-SM that has most methoxy groups apparently exhibits strong multiple oxidation waves, which indicates that the removal of first electron from molecule affects the removal of further electrons and thus validates that the circularly arrayed peripheral phenyls in HPB(OM)3-SM are strongly electronically coupled.36,37 With the decrease of methoxy groups, such feature becomes weak in HPB(OM)-SM and even indiscernible in HPB-SM (Figure 3b). These results clearly manifest that the through-space conjugation gets stronger as the increase of methoxy groups. However, because the conductance property of single-molecule wires with methyl sulfide anchoring groups is generally dominated by its highest occupied molecular orbital (HOMO), a closer HOMO energy level to the Fermi level of the Au electrode (−5.2 eV)38 is more conducive to conductance. The HOMO energy levels of HPB-SM, HPB(OM)-SM, and HPB(OM)3-SM are calculated to be −5.67, −5.60, and −5.51 eV from the onset oxidation potentials, respectively, which are increased as the addition of methoxy groups. The closer HOMO energy level of HPB(OM)3-SM to the Fermi level of Au electrode also ensures a better conductance of HPB(OM)3-SM than HPB-SM and HPB(OM)-SM.
conducting channels30 and found that multiple conducting channels were conceptually feasible to improve conductance of single-molecule wires. However, owing to insufficient contribution from through-space conducting channel, folded single-molecule wires only provided close conductance in comparison with conventional linear counterparts. In view of this, we herein report a series of novel single-molecule wires created by melting a hexaphenylbenzene (HPB) core and an oligophenylene backbone. Our interest in HPB is inspired by its strong toroidal through-space conjugation among the six face-to-face aligned phenyls around the central benzene (Figure 1),32−35 and the prominent through-space conjugation
Figure 1. (a) Molecular structure, (b) illustration of toroidal delocalization of π electrons, and (c) frontier molecular orbitals ranging from HOMO−2 to LUMO+2 of hexaphenylbenzene (HPB).
character among these phenyl rings is dominative in the frontier orbitals (e.g., LUMO+2), as revealed by calculation, which is anticipated to serve as an additional conducting channel in single-molecule wires. The conductance of these novel singlemolecule wires, measured in single-molecule junctions using the scanning tunneling microscopy based break-junction (STM-BJ) technique, is as high as 10−3.8 G0 (12.28 nS; G0 = 2e2 h−1, in which G0 is the quantum conductance, e is the charge on an electron, and h is Planck’s constant),39 which greatly outstrips that of its conventional through-bond conjugated counterpart (2.45 nS). It is demonstrated that intriguing multiple conducting channels from through-space and through-bond conjugations simultaneously contribute to the superb conductance. Specifically, the strong through-space conjugation can sufficiently offset theloss in through-bond conjugation, enabling these HPB-based molecular wires to outperform conventional ones. The molecular structures of HPB-based single-molecule wires (HPB-SM, HPB(OM)-SM, and HPB(OM)3-SM) as well as a conventional counterpart (PP-SM) are displayed in Figure 2a, and the detailed synthetic procedures and characterization data are described in the Supporting Information. Grafting two 4-(methylthio)phenyls as terminal anchors onto HPB enables the molecular wires to efficiently interact with gold electrodes to form a stable metal−molecule−metal junction. Meanwhile, methoxy groups are introduced in four peripheral phenyls in HPB(OM)-SM and HPB(OM)3-SM to enhance the electron density and, thus, the through-space conjugation degree. Single crystals of HPB-SM, HPB(OM)-SM, and HPB(OM)3-SM are obtained from THF−hexane mixtures and analyzed by X-ray diffraction crystallography. The crystal structures reveal that the peripheral phenyls in HPB are face-to-face aligned with the shortest distances of 2.87−2.95 Å between neighboring phenyls (Figure 2b). The distances are much B
DOI: 10.1021/acs.nanolett.8b01082 Nano Lett. XXXX, XXX, XXX−XXX
Letter
Nano Letters
Figure 2. (a) Molecular structures of HPB-SM, HPB(OM)-SM, HPB(OM)3-SM, and linear PP-SM with indicated torsion angles (φ) between the central benzene and its adjacent one on the backbone in optimized molecular structures. (b) ORTEP drawings of crystal structures of HPB-SM, HPB(OM)-SM, and HPB(OM)3-SM with indicated shortest distances between face-to-face aligned neighboring phenyls. Hydrogen atoms are omitted for clarity.
provides a remarkable conductance of 10−3.8 G0 (12.28 nS). These results are in good agreement with those observed in two-dimensional conductance histograms (Figure S3). Although HPB(OM)-SM has lower energy levels and weaker throughbond conjugation than linear PP-SM, the conductance value of HPB-SM (10−4.3 G0 and 3.88 nS) is larger than that of PP-SM (10−4.5 G0 and 2.45 nS), manifesting the conductance in HPBSM is benefited from through-space conjugation indeed. The conductance of HPB-based molecules is increased along with the addition of methoxy groups that result in stronger throughspace conjugation as well as slightly increased energy levels.
The conductance of these molecules is measured in singlemolecule junction using STM-BJ technique. Figure 4 shows the dependence of conductance with the displacement and one-dimensional conductance histograms of these molecules. HPB(OM)3-SM, HPB(OM)-SM, and HPB-SM exhibit plateaus at the individual trace during the elongation below G0, indicative of the formation of Au−molecule−Au junctions. One-dimensional conductance histogram constructed from about 1000 individual traces giving a reproducible conductance peak at 10−4.3 G0 (3.88 nS) for HPB-SM, which is enhanced to 10−4.0 G0 (7.75 nS) for HPB(OM)-SM. Delightfully, HPB(OM)3-SM C
DOI: 10.1021/acs.nanolett.8b01082 Nano Lett. XXXX, XXX, XXX−XXX
Letter
Nano Letters
Figure 3. (a) UV absorption spectra in THF solutions and (b) cyclic voltammograms of HPB-SM, HPB(OM)-SM, and HPB(OM)3-SM.
Figure 4. Conductance distance curves and one-dimensional conductance histograms of HPB-SM (panels a and e), HPB(OM)-SM (panels b and f), HPB(OM)3-SM (panels c and g), and PP-SM (panels d and h). Each histogram is built from thousands of traces obtained during the STM breakjunction measurement at a finite bias voltage of 100 mV.
molecular wires. To further deepen understanding of this interesting finding and validate the through-space conducting channel, the theoretical calculation of molecular frontier orbitals is carried out. As shown in Figure 5a, through-space conjugation character among the six face-to-face aligned phenyls in HPB core is obvious and becomes more dominative as the increase of methoxy groups, which is consist with the electrochemical results. Thus, it is reasonable that the contribution from the strong toroidal through-space conjugation in HPB-based molecular wires should have exceedingly compensated the conductance loss because of weakened through-bond conjugation as well as degraded match of the HOMO energy level with an Au electrode. This further corroborates the statement that increasing through-space conjugation indeed results in the improvement of single-molecule conductance. The synergistic effect of through-space and through-bond conjugations endows HPB(OM)3-SM with significantly superior conductance to that of traditional linear PP-SM. In view of this, we propose a novel model of multichannel conductance of HPB(OM)3-SM in a single-molecule junction (Figure 5b),
Moreover, the conductance of HPB(OM)3-SM is not only greatly advanced relative to those of conventional throughbond better-conjugated molecules with similar lengths (
