Quantum Interference Effects in Charge Transport through Single

Article Cite This: Acc. Chem. Res. 2019, 52, 151−160

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Quantum Interference Effects in Charge Transport through Single-Molecule Junctions: Detection, Manipulation, and Application Junyang Liu, Xiaoyan Huang, Fei Wang, and Wenjing Hong*

Acc. Chem. Res. 2019.52:151-160. Downloaded from pubs.acs.org by IOWA STATE UNIV on 01/16/19. For personal use only.

State Key Laboratory of Physical Chemistry of Solid Surfaces, iChEM, NEL, Department of Chemical and Biochemical Engineering, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen 361005, China

CONSPECTUS: Quantum interference effects (QIEs), which offer unique opportunities for the fine-tuning of charge transport through molecular building blocks by constructive or destructive quantum interference, have become an emerging area in singlemolecule electronics. Benefiting from the QIEs, charge transport through molecular systems can be controlled through minor structural and environmental variations, which cause various charge transport states to be significantly changed from conductive to insulative states and offer promising applications in future functional single-molecule devices. Although QIEs were predicted by theoreticians more than two decades ago, only since 2011 have the challenges in ultralow conductance detection originating from destructive quantum interference been overcome experimentally. Currently, a series of single-molecule conductance investigations have been carried out experimentally to detect constructive and destructive QIEs in charge transport through various types of molecular junctions by altering molecular patterns and connectivities. Furthermore, the use of QIEs to tune the properties of charge transport through single-molecule junctions using external gating shows vital potential in future molecular electronic devices. The experimental and theoretical investigations of QIEs offer new fundamental understanding of the structural−electronic relationships in molecular devices and materials at the nanoscale. In this Account, we discuss our progress toward the experimental detection, manipulation, and further application of QIEs in charge transport through single-molecule junctions. These experiments were carried out continuously in our previous group at the University of Bern and in our lab at Xiamen University. As a result of the development of mechanically controllable break junction (MCBJ) and scanning tunneling microscope break junction (STM-BJ) techniques, we could detect ultralow charge transport through the cross-conjugated anthraquinone center, which was one of the earliest experimental studies of QIEs. In close cooperation with organic chemists and theoretical physicists, we systematically investigated charge transport through single-molecule junctions originating from QIEs in conjugated centers ranging from simple single benzene to polycyclic aromatic hydrocarbons (PAHs), heteroaromatics, and even complicated metalla-aromatics at room temperature. Then we further investigated the quantitative correlation between molecular structure and quantum interference by altering different molecular patterns and connectivities in homologous series of PAHs and heteroatom systems. Additionally, external chemical and electrochemical gating of single-molecule devices can be used for direct QIE manipulation via not only tuning molecular conjugation but also shifting the electrode Fermi level. Our study further suggested that distinguishable differences in conductance resulting from QIEs offer opportunities to detect photothermal reaction kinetics and to recognize isomers at the singlemolecule scale. These investigations demonstrate the universality of QIEs in charge transport through various molecular building blocks. Moreover, effective manipulation of QIEs leads to various novel phenomena and promising applications in molecular electronic devices.

1. INTRODUCTION Investigations of charge transport through single-molecule junctions offer fundamental understanding for the design of single-molecule devices and functional circuits using quantum effects.1−3 Recent investigations of quantum interference effects © 2018 American Chemical Society

(QIEs) provide promising potential for the manipulation of charge transport through single-molecule junctions.4−7 Additionally, Received: August 25, 2018 Published: November 30, 2018 151

DOI: 10.1021/acs.accounts.8b00429 Acc. Chem. Res. 2019, 52, 151−160

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Accounts of Chemical Research QIEs enable high-performance molecular switching between conductive constructive quantum interference (CQI) and insulative destructive quantum interference (DQI) states with large on/off ratios,2,8,9 suggesting future promising applications in molecular electronics. The QIEs in charge transport originate from interference among the quantum wave functions of electrons that coherently propagate through different discrete molecular orbitals (MOs) in single-molecule junctions. Thus, the transmission from all MOs contributes to the overall Green’s function (left part of eq 1),10,11 which describes patterns of QIEs by the phase difference strongly associated with Cik and Cjk, the kth MO coefficients at sites i and j, respectively, which act as connectivities anchoring onto the electrodes: Gij(E F) =

∑ k

≈

Anthraquinone (AQ) is a typical cross-conjugated system and is expected to be less conductive than linear-conjugated anthracene (AC) and broken-conjugated dihydroanthracene (AH), which have almost same length (Figure 1a; these molecules were

CikC*jk E F − εk ± iη

Ci HOMOC*jHOMO E F − εHOMO ± iη

+

Ci LUMOC*j LUMO E F − εLUMO ± iη

(1)

In eq 1, EF is the Fermi level, εk is the energy of the kth MO, and η is an infinitesimal positive number. Since the Fermi level of the electrode is located between the energies of the molecule’s highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO), while other MOs are energetically far away from the HOMO and LUMO and contribute much less to the transmission, the Green’s function approximately equals the right part of eq 1, which considers the transmission contributed by the dominant transporting orbital (either the HOMO or the LUMO) and the interference with the other. When the two terms of eq 1 have the same sign, the absolute value of the Green’s function increases, which leads to CQI; otherwise, DQI occurs when the two items have opposite signs.8,10−12 CQI has been observed through a parallel double-backbone molecular junction.13,14 Moreover, DQI with low conductance has attracted more attention because of the existence of a conspicuous sharp dip in the transmission spectra around the Fermi level. Nevertheless, the detection of DQI has remained experimentally challenging owing to the ultralow conductance.1,2,8,10 During the past years, by employing mechanically controllable break junction (MCBJ) and scanning tunneling microscope break junction (STM-BJ) techniques in Bern and Xiamen, we have investigated QIEs in charge transport through various molecular building blocks not only from single phenyl to polycyclic aromatic systems but also from conjugated hydrocarbons to heterocycles. The obtained phenomena unravel the structural− electronic relationships from the QIEs in single-molecule junctions. We have also demonstrated that chemical and electrochemical gating can be applied to directly manipulate the QIEs in charge transport through single-molecule junctions. Hence, this Account begins with a review of the early efforts in QIE detection and then summarizes our recent experimental results toward the manipulation of QIEs via structural tuning and external gating. Last, we demonstrate several potential applications through the adoption of QIEs.

Figure 1. (a) Molecular structures of AC, AQ, and AH and (b) their corresponding calculated transmission spectra. (c−e) Conductance histograms of (c) AC, (d) AQ, and (e) AH obtained from single-molecule conductance measurements using MCBJ techniques. (f) Comparison of the conductance trends from different measuring methods: CP-AFM, EGaIn, and MCBJ. Reproduced with permission from refs 19 and 15. Copyright 2011 Beilstein Institute and 2014 the PCCP Owner Societies, respectively.

synthesized in Prof. Hummelen’s group). The calculated transmission spectra of these molecules demonstrate that only AQ exhibits a prominent dip around the electrode Fermi level (Figure 1b).9,15 Several pioneering experimental investigations have focused on self-assembled monolayers (SAMs) of these molecules. Using eutectic gallium and indium (EGaIn),16 conducting probe atomic force microscopy (CP-AFM),17 and top-evaporated gold electrode18 methods, the AQ system was proved to exhibit the lowest current density. The single-molecule conductance value of AQ, however, remained unclear because those results exhibited an overall contribution from the sandwiched monolayer, including effects from defects and intermolecular interactions. To extract the single-molecule conductance, in 2011 we constructed an MCBJ setup with a home-built logarithmic I−V converter to access the ultralow conductance of AQ.19 With background noise close to the 10−9G0 level, the single-molecule conductance of AQ was statistically obtained as 10−7.0G0, which is significantly lower than those of AC (10−4.5G0) and AH (10−6.3G0) (Figure 1c−e). This effort was considered one of the first experimental observations of DQI in single-molecule junctions.20

2. DETECTION OF QIES IN CHARGE TRANSPORT THROUGH SINGLE-MOLECULE JUNCTIONS 2.1. Conjugation-Dependent QIEs

The cross-conjugated system is one of the first studied molecular building blocks theoretically predicted to have DQI.6,7 152


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Figure 2. (a) Molecular structures of the junctions with para and meta core units (groups 1 and 2, respectively). (b) Calculated transmission spectra of those molecules. (c, d) Conductance histograms of (c) group 1 and (d) group 2. Reproduced with permission from ref 26. Copyright 2015 Springer Nature.

investigated charge transport through a series of oligo(phenylene ethynylene) (OPE)-type molecules with pyridyl anchors. The molecules varied the para (p), meta (m), or ortho (o) connectivity of the anchors and the para or meta connectivity of the central phenyl core (Figure 2a). The calculated transmission spectra predicted sharp dips when the meta configuration existed in both the phenyl core and the anchors (Figure 2b). As confirmed experimentally in Figure 2c,d, meta connectivity shows significantly lower conductance in both the core and anchors.26 Nevertheless, the DQIs were still suppressed with the existence of a parallel conductance pathway associated with coupling between the pyridyl anchor and the gold electrode. Further calculations demonstrated that the conductance of the m−p−m molecule was significantly reduced without consideration of metal−ring interactions (see Figure 5 in ref 26). Therefore, the effect of nitrogen position variations in the anchors becomes much weaker, and variations in the connectivity of the central ring account for the major changes in molecular conductance. Moreover, because the central ring is weakly coupled to the anchor ring via acetylene spacers, the quantitative conductance measurement demonstrated a quantum circuit rule of Gp−p−p × Gm−m−m = Gp−m−p × Gm−p−m, which revealed the independent charge transport contributions from the core and anchor units. As a larger aromatic system, a polycyclic aromatic hydrocarbon (PAH) system provides more connection site options than the smallest system, benzene. Since the Green’s function of a conjugated PAH is an overall contribution of frontier MOs

Figure 1f shows comparisons between the calculated differential conductance values of SAM junctions summarized from refs 16 and 17 and the single-molecule conductance values from ref 19. All three techniques show the same conductance trend.15 Nevertheless, the difference in conductance between single-molecule junctions with/without QIEs is significantly larger,15,19 suggesting that QIEs become more prominent in coherent transport through single-molecule junctions without intramolecular interactions. 2.2. Connectivity-Dependent QIEs

As described in eq 1, QIEs are strongly correlated to the connectivity patterns. Therefore, a series of aromatic systems with different connectivities were investigated to explore connectivitydependent interference. As the simplest building block in molecular electronics, benzenedithiol with meta connectivity is theoretically predicted to be less conductive than those with para and ortho connectivity by DQI,7−10,21 and these results have been confirmed by several previous experimental reports.22,23 Meanwhile, Arroyo et al.24 altered the connectivity of thienylethynyl-anchored benzene and observed smaller conductance below 10−5G0 with meta connectivity, which was lower than the value of 10−3.1G0 with para connectivity. Another pioneering work investigated the stilbene system with para- and meta-terminated methylthiophenyl using STM-BJ and found that the latter is more insulated than the former because of DQIs.25 To evaluate the synergistic connectivity contributions by both the anchors and the core units in the molecular junction, we 153


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Figure 3. (a, d) Molecular structures of (a) 2,5′-connected (red) and 1′,5-connected pyrene (blue) and (d) 1,5′-connected (red) and 7,2′-connected anthanthrene (blue) and (b, e) the corresponding conductance histograms. (c, f) Derived integer tables of the matrix M for the (c) pyrene and (f) anthanthrene lattices, showing that the measured conductance ratio equals the square of the magic number ratio, as highlighted by the corresponding colors. Reproduced from refs 30 and 31. Copyright 2015 American Chemical Society.

Figure 4. (a) Exchange of a carbon atom with a nitrogen atom in the m-OPE system and (b) measured conductance histograms of the molecular system in (a). (c) Calculated transmission spectra of those molecules. Reproduced with permission from ref 35. Copyright 2016 Wiley.

ratio of |gij(E)|2 values, where gij(E) is the Green’s function of the isolated central moiety, which is proportional to the overall Green’s function Gij(E). For a bipartite lattice with equal numbers of primed and unprimed sites (as labeled in Figure 3a,d), the connectivity matrix C with i in rows and i′ in columns is constructed, where Cii′ is equal to 1 when i and i′ are connected and equal to 0 otherwise. Hence, the integer table of matrix M, defined as the transpose of the cofactor matrix of C, equals d × C−1, where d is the determinant of C. According to the tightbinding model, gij(E) ≈ (−1/d)Mij, suggesting that the conductance ratio of σij(E) for different connectivities simply equals the ratio of Mij2 values.28,30 To validate the hypothesis, we measured the conductances of 2,5′- and 1′,5-connected pyrene (Figure 3a) and 1,5′- and 7,2′-connected anthanthrene (Figure 3d). The conductance ratios between the pyrenes and between the anthanthrenes were approximately 8 and 79, respectively (Figure 3b,e), which agree with the predicted ratios of the Mij2 values for the pyrenes (32 = 9) and the anthanthrenes (92 = 81) (Figure 3c,f).30,31 The magic ratio rule further fitted 1,4- and 2,6-connected naphthalene, 5,10- and 2,7-connected anthracene, and even the

(eq 1), each term of eq 1 possesses the product of source and drain electron wave functions. Therefore, different amplitudes and signs demonstrate different degrees of interference from different source and drain connectivities, leading to intraorbital QIEs in a single MO and interorbital QIEs in different MOs. Subsequently, the availability of multiple connections offers chances to use multiple interference patterns inside the PAH systems for the quantitative evaluation of QIEs.8,10−12,27,28 Previous experimental efforts indicated that the difference in conductance of naphthalenedithiols resulted from the connectivity difference.29 To reveal the origin of the difference in conductance, in cooperation with Prof. Colin Lambert for theory together with Prof. Silvio Decurtins and Dr. Shi-Xia Liu for synthesis, we proposed a magic ratio rule by calculating the integer table of matrix M originating from connection sites. The conductance σij(E) of an electron with energy E for the molecular junction with connection sites i and j is proportional to |Gij(E)|2. When the coupling between the connection and the central moiety is sufficiently weak because of the acetylene spacer, the ratio of conductances σij(E) is proportional to the 154


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Accounts of Chemical Research nonalternant aromatic azulene system, demonstrating the generality of the magic ratio rule in PAH systems.31,32 The connectivity-driven charge transport caused by different degrees of interference in PAH systems provides the unique opportunity to further design logical functional units with quantifiable transport states. The stretched displacement distributions of these molecules with different connectivities were also investigated, and the characterized molecular lengths were found to be in good agreement with the theoretical lengths,30,31 suggesting that the molecular junctions are sufficiently stable for the charge transport investigations and thus that the conductance differences originated from the QIEs in different conjugation systems with different connectivities. 2.3. QIEs with Heteroatom Substitution

Inserting heteroatoms into aromatic hydrocarbons enriches the molecular species and provides the opportunity for the realization of multiple degrees of interference.33,34 A change in heteroatom substitution will lead to totally different QIE patterns in charge transport through heterocyclic aromatics. As shown in Figure 4a, starting from the p−m−p (m-OPE) configuration in Figure 2a but using SAc anchors, we found that substituted nitrogen at the M1 position exhibited the lowest conductance of 10−5.55G0, suggesting that DQI remains. Moreover, the DQI was alleviated when nitrogen was placed at the other two positions (M2 and M3), resulting in conductance values of 10−4.35G0 and 10−5.1G0, respectively (Figure 4b). The results were confirmed by the calculated transmission spectra (Figure 4c), indicating that QIEs could be tuned via single-atom gating.35 Furthermore, the experimental ratios agreed with the calculated M1/M3 and M2/M3 ratios based on the magic ratio theory, again demonstrating the generality of this rule to quantify interference in charge transport through single-molecule junctions.

Figure 5. (a) Series of azulene molecules before (black) and after (red) protonation and (b−d) the corresponding differences in conductance for (b) 1,3-Az, (c) 4,7-Az, and (d) 5,7-Az (black for neutral states and red for protonated states). (e, f) Calculated conductances of (e) the initial state and (f) the protonated state for 1,3-Az (red), 4,7-Az (blue), and 5,7-Az (green). The dashed lines represent the Fermi level. Reproduced with permission from ref 37. Copyright 2017 Royal Society of Chemistry.

3. MANIPULATION OF QUANTUM INTERFERENCE Toward single-molecule functional devices, direct manipulation of QIEs to tune the properties of charge transport through single-molecule junctions will be the next important step. Further direct interference pattern manipulation through gating methods currently remains an experimental challenge.

osmapentalene derivatives was enhanced from 10−6.3G0 to 10−4.8G0 when the phosphonium group was introduced (Figure 6).38 An additional control experiment excluded the possible PPh3− electrode interaction since there are no lone-pair electrons on the phosphorus atom to coordinate to electrode (see Figure S23 in ref 38). Therefore, to rationalize the difference in conductance between complexes 2 and 3, UV−vis spectroscopy and X-ray diffraction demonstrated a more delocalized osmapentalene unit in complex 2 because of the phosphonium group, suggesting the formation of resonance structures (2a and 2b in Figure 6a). Those resonance structures change the cross-conjugation pattern in 3 compared with 2 (dashed circle in Figure 6a). Therefore, because of the DQI, charge transport is facilitated through the C7−Os double bond in complex 2 and blocked in the C7−C6 bond (blue pathway in 2a and 2b). Complex 3, however, demonstrates a longer transport route (red pathway) through the C7−C6 bond while the C7−Os bond is blocked (directed by the black arrows in Figure 6a),6,8,10 leading to a smaller conductance than that of complex 2 (Figure 6b). This phenomenon was also observed in a dithiafulvene-incorporated OPE system: the structure changed into a cross-conjugated quinoid form possessing DQI because of charge delocalization, leading to lower conductance.39

3.1. Chemical Gating of QIEs

The nonalternant azulene system was demonstrated to exhibit connectivity-dependent QIEs.36 Meanwhile, azulene can be protonated by trifluoroacetic acid (TFA), leading to further chemical gating of the QIE pattern.37 Azulenium derivatives exhibited a prominent increase in conductance compared with the neutral state. The conductance values of 1,3-Az, 4,7-Az, and 5,7-Az were enhanced from 10−3.8G0 to 10−2.7G0, from 10−4.2G0 to 10−3.7G0, and from 10−4.9G0 to 10−3.6G0, respectively, after protonation (Figure 5).37 Combined DFT calculations suggested that the protonation introduces a negative electrostatic potential to act as chemical gate, which shifted the HOMO level closer to the Fermi energy (vertical dashed lines in Figure 5e,f) and reduced the HOMO−LUMO gap (Figure 5e,f), leading to the significant conductance enhancement. The highest enhancement was observed for 5,7-Az because pristine 5,7-Az with DQI presents a sharp transmission dip near the Fermi energy, so the shift of the HOMO toward the Fermi energy and the disappearance of the DQI feature after protonation produce the largest increase compared with the other two molecules without DQI. The quantum interference pattern can also be manipulated through the introduction of side groups in a dπ−pπ conjugated metalla-aromatic system. The single-molecule conductance of

3.2. Electrochemical Gating of QIEs

The third gating electrode in both electrochemical gating (reference electrode) and electrostatic gating (bottom gate electrode) provides efficient tuning of conjugation patterns and 155


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Figure 6. (a) Osmacycle complexes for single-molecule conductance measurements. The resonance structure of 2 was changed by the delocalization effect of the phosphonium group into 2a and 2b, which prefer the blue charge transport pathway, while complex 3 prefers the red pathway, as directed by arrows in the cross-conjugated configuration labeled by dashed circles. (b) Conductance histograms for the two complexes with different colors corresponding to (a). Reproduced from ref 38. Copyright 2017 American Chemical Society.

shifting of energy levels in molecular devices. Tao’s group used electrochemical gating to reduce the AQ-centered molecules into the H2AQ form by the STM-BJ method and observed a significant enhancement in charge transport properties.40 Moreover, electrostatic gating has also been introduced to create a single-molecule transistor through an aluminum bottom gate to tune the electromigration-fabricated AQ junction, demonstrating electric-field-induced differential conductance of the charged AQ2− by breaking of the cross-conjugation.41 To tune both the Fermi energy and the redox state of the molecule, we tried to evaluate the electrochemical behavior of AQ containing both redox and QIE centers (Figure 7a) by applying electrochemical STM-BJ.42 For 1,5-AQ, this crossconjugated connectivity with DQI exhibited ultralow conductance below the detection limit (10−6G0; see the transmission spectrum in Figure 2a in ref 42) but showed a much higher conductance of 10−5.0G0 for the reduced state with reversible switching (Figure 7c). Meanwhile, the linearconjugated 1,4-AQ is more conductive than the crossconjugated 1,5-AQ in both the oxidized state (10−5.0G0) and the reduced state (10−4.0G0) (Figure 7d). The conductance varied with the gate potential and changed abruptly during the redox process (Figure 7e,f), and this was confirmed by theoretical simulations (Figure 7g,h), suggesting direct tuning of both the electrode Fermi energy and the redox states of the molecule by electrochemical gating. 3.3. Mechanical Manipulation of QIEs

The QIEs in charge transport through the π−π stacking system can be fine-tuned by mechanically adjusting the relative positions of two adjacent molecules. Using a nanolithographic MCBJ chip, van der Zant’s group was able to shift the π-stacked single-ended OPE3-SAc molecule attached to each electrode with subangstrom scale movement.43 Periodic conductance decreases in the conductance−distance curve were found during the stretching, resulting from periodic DQIs that occurred when the opposite signs of the frontier orbitals of those two stacked molecules were periodically coupled.

Figure 7. (a) Molecular structures of 1,5-AQ and 1,4-AQ. (b) Cyclic voltammograms of 1,5-AQ (black) and 1,4-AQ (gray). (c, d) Conductance histograms of (c) 1,5-AQ (blue) and 1,5-H2AQ (red) and (d) 1,4-AQ (blue) and 1,4-H2AQ (red). The inset of (c) shows three sequential reversible switches between 1,5-AQ and 1,5-H2AQ. (e−h) Experimental and calculated molecular conductances as functions of the applied gating potential for (e, g) 1,5-AQ and (f, h) 1,4-AQ. Reproduced from ref 42. Copyright 2014 American Chemical Society. 156


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Figure 8. (a) Reaction route from dha-7 to vhf and then reversibly to dha-6. (b) Calculated transmission spectra of each state during the photo/ thermal transition. (c) Evolution of the 2D conductance histogram in going from dha-7 to vhf and finally to dha-6. (d) Calculated proportion of dha-6 vs the reaction time extracted from the plateau length distribution of each component. The solid and dashed fitted curves present kinetic information on the photothermal transition process. Reproduced with permission from ref 52. Copyright 2017 Springer Nature.

2,6-connectivity, which could be converted back to vhf reversibly by illumination (Figure 8a). Theoretical simulations revealed that the conductance of dha-7 with DQIs is prominently smaller than those of vhf with shifted interference and dha-6 with no interference (Figure 8b). Figure 8c shows the evolution of the measured single-molecule conductance, with distinct conductance features from molecular junctions at each state during the photo/thermal transition. Since the junction formation probability of the reactant and product highly depends on their relative concentration in the solution, the time-dependent proportion of reactant during the in situ process (upper panel of Figure 8d) could be determined from the integral area ratio of the relative displacement distributions of the reactant and product (lower panel of Figure 8d), suggesting that the reaction kinetics between vhf and dha-6 could be extracted through conductance measurements. This work demonstrates that distinct differences in the conductance and displacement distributions originating from DQIs can be applied to investigate the reaction kinetics at the single-molecule scale.

Although several ideas were proposed from the theoretical predictions, experimental efforts were still required, such as structural tuning by substituting atoms or functional groups into a bigger PAH system44 or cross-conjugated system,45 localized gating through single atoms on benzene46 or spintronic graphene nanoribbon.20 Furthermore, directly and reversibly revealing the existence of DQIs by mapping the sharp transmission profile through electrostatic/electrochemical gating is fundamentally important.

4. POTENTIAL APPLICATIONS OF QUANTUM INTERFERENCE The experimental detection and manipulation of QIEs in charge transport through molecular building blocks leads to various potential applications, such as the design of single-molecule switches,47 phase-coherent interferometers and logic gates,31 organic dielectric materials with low leakage currents,48 a proofof-principle single-molecule insulator,49 and a strategy to avoid charge recombination in dye-sensitized solar cells.50 Recently, Tao’s group implanted an AQ unit into a double-helical DNA molecule.51 They found that the structural change between DNA redox states leads to a prominent change in conductance, providing an opportunity to study the kinetics of this redox reaction through conductance monitoring.

4.2. Recognition of Isomers

Recently, we further used DQI for isomer recognition through single-molecule conductance measurments.53 Diketopyrrolopyrrole (DPP) is a typical dye material that is widely used in organic semiconductors for optoelectronics. During alkylation of the DPP units, two types of isomers called SDPP and SPPO (upper panel of Figure 9a) are simultaneously produced and coexist in DPP-containing materials. Understanding and differentiating the charge transport properties through these two isomers are important for future materials design. The charge transport properties of these two isomers detected through the

4.1. Reaction Kinetics

We designed a photo/thermal transition system based on the photochromic dihydroazulene (dha) and vinylheptafulvene (vhf) structures.52 Starting from dha-7 with 2,7-connectivity, the molecule was irreversibly converted to vhf under UV illumination and then subsequently heated to form dha-6 with 157


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Figure 9. (a) Upper panel: two isomer structures of DPP dye materials. Lower panel: resonance structures between linear conjugation and crossconjugation of SPPO-H+. (b) Typical conductance histograms of SDPP, SPPO, and their corresponded acidified states. Reproduced from ref 53. Copyright 2018 American Chemical Society.

Biographies

MCBJ method were found to be almost the same (blue and red conductance histograms in Figure 9b). Afterward, they became distinguishable when camphorsulfonic acid was added (orange and magenta conductance histograms in Figure 9b). The conductance of acidified SPPO dropped more than 1 order of magnitude, while that of SDPP remained the same. This significant conductance drop of SPPO originates from protonation of the nitrogen atom, thus forming resonance structures transitioning from linear conjugation with CQI to cross-conjugation with DQI (lower panel of Figure 9a), while SDPP cannot be protonated and remains unchanged. Therefore, the acidification process enables the recognition of two DPP isomers, which pushes the capability of isomer recognition to the single-molecule level.

Junyang Liu received his Ph.D. from Xiamen University in 2016 under the supervision of Prof. Zhong-Qun Tian and is currently a postdoctoral fellow in Prof. Wenjing Hong’s group. His research interests include single-molecule electronics, nanofabrication, and Raman spectroscopy. Xiaoyan Huang is currently a Master’s student in Prof. Wenjing Hong’s group, focusing on single-molecule reactions. Fei Wang is currently an undergraduate student in Prof. Wenjing Hong’s group, focusing on charge transport through single-molecule wires. Wenjing Hong received his Ph.D. (summa cum laude) in 2013 from the University of Bern in Switzerland under the supervision of Prof. Thomas Wandlowski, and from 2014 he led the molecular electronics group in Prof. Wandlowski’s lab. In 2015 he became a full professor at Xiamen University and was awarded a Min-jiang Chair Professorship in 2016. His research interests include single-molecule electronics, scientific instruments, and artificial intelligence.

5. CONCLUSION AND PERSPECTIVE We experimentally detected and manipulated the QIEs in charge transport through various molecular building blocks not only from single benzene to polycyclic aromatics but also from hydrocarbons to conjugated heterocyclic systems. Chemical and electrochemical gating were employed for the manipulation of QIEs, and we further adopted QIEs to monitor reaction kinetics and isomer recognition at the single-molecule scale. Our studies demonstrate that the magnified changes in electronic structure result from minor chemical structural changes through tuning of the interference patterns, and the changes in electronic structure indicate the universality of this effect in molecular devices and materials. Beyond these findings, molecular building blocks with QIEs have a high potential to exhibit great thermopower because of the abrupt change in the transmission coefficient around the Fermi level.8,54 Since QIEs may also exist in molecular junctions with weak interactions,43,55,56 their relative weak phonon transport can facilitate high figures of merit in thermoelectricity,8,54 therefore indicating that molecules with QIEs will be promising materials for future molecular thermoelectric devices.

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ACKNOWLEDGMENTS This work was supported by the National Key R&D Program of China (2017YFA0204902), the National Natural Science Foundation of China (21673195 and 21703188), and the China Postdoctoral Science Foundation (2017M622060). We thank all of our collaborators who contributed to the works described here, including Prof. J. C. (Kees) Hummelen (Groningen, The Netherlands), Prof. Martin R. Bryce (Durham, U.K.), Prof. Silvio Decurtins and Dr. Shi-Xia Liu (Bern, Switzerland), Prof. Mogens Brøndsted Nielsen (Copenhagen, Denmark), Prof. Deqing Zhang and Prof. Zitong Liu (ICCAS, China), Prof. Haiping Xia (Xiamen, China), and Prof. Hao-Li Zhang (Lanzhou, China) for molecular design and synthesis and Prof. Colin J. Lambert (Lancaster, U.K.), Prof. Kristian S. Thygesen (Lyngby, Denmark), and Prof. Gemma Solomon (Copenhagen, Denmark) for theory.

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AUTHOR INFORMATION

Corresponding Author

DEDICATION This work is in memory of Prof. Thomas Wandlowski of the University of Bern, Switzerland.

*E-mail: [email protected]. ORCID

Junyang Liu: 0000-0002-7252-1900 Wenjing Hong: 0000-0003-4080-6175

REFERENCES

(1) Huang, C.; Rudnev, A. V.; Hong, W.; Wandlowski, T. Break Junction under Electrochemical Gating: Testbed for Single-Molecule Electronics. Chem. Soc. Rev. 2015, 44, 889−901.

Notes

The authors declare no competing financial interest. 158


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