Thiophene-based Tripodal Anchor Units for Hole Transport in Single

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Thiophene-Based Tripodal Anchor Units for Hole Transport in Single-Molecule Junctions with Gold Electrodes Yutaka Ie, Kazunari Tanaka, Aya Tashiro, See Kei Lee, Henrique Rosa Testai, Ryo Yamada, Hirokazu Tada, and Yoshio Aso J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.5b01662 • Publication Date (Web): 03 Sep 2015 Downloaded from http://pubs.acs.org on September 8, 2015

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The Journal of Physical Chemistry Letters

Thiophene-Based Tripodal Anchor Units for Hole Transport in Single-Molecule Junctions with Gold Electrodes Yutaka Ie,*,† Kazunari Tanaka,† Aya Tashiro,† See Kei Lee,‡ Henrique Rosa Testai,§ Ryo Yamada,*,‡ Hirokazu Tada,*,‡ and Yoshio Aso*,† †

The Institute of Scientific and Industrial Research (ISIR), Osaka University, 8-1 Mihogaoka,

Ibaraki, Osaka 567-0047, Japan ‡

Graduate School of Engineering Science, Osaka University, 1-3 Machikaneyama, Toyonaka,

Osaka 560-8531, Japan §

Institute of Mechanical Engineering (IEM), Universidade Federal de Itajubá, 1303 Bairro

Pinheirinho, Itajubá, Minas Gerais 37500 903, Brasil

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]; [email protected]; [email protected]; [email protected]

ACS Paragon Plus Environment

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ABSTRACT: Molecule-metal junctions are inevitable for the realization of single-molecule electronics. In this study, we developed new tripodal anchors with electron-rich aromatic rings to achieve robust contact with gold electrodes, an effective hybridization of the  orbital with gold electrodes ( channel), and hole transport through -channel hybridization. Cyclic voltammetry and X-ray photoelectron spectroscopy measurements of the monolayers indicated that the thiophene-based tripodal molecule exhibits anchoring characteristics as expected. The electrical conductance of thiophene-anchored bistripodal molecules using the scanning tunneling microscope (STM)-based break junction technique confirmed the formation of molecular junctions. The Seebeck coefficient of this compound estimated from thermoelectric voltage measurements using a STM was determined to be a positive value, which indicates that the charge carriers are holes. On the other hand, the corresponding pyridine-anchored molecules showed electron transport. These results reveal the versatility of π-channel tripodal anchors for the control of charge-carrier type in single-molecule electronics.

Table of Contents Graphic

KEYWORDS single-molecule electronics · tripodal anchor · structure-property relationship · -channel hybridization · surface analysis · single-molecule junction · Seebeck coefficient


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Single-molecule electronics have attracted much interest in terms of their bottom-up construction and potential for device miniaturization.1-8 Concerning these devices, investigations regarding the interface between organic molecules and metal electrodes are of great importance for elucidating the relationships that exist among the structure and other various properties of the device.9,10 In light of the fact that novel techniques using scanning tunneling microscope (STM) and mechanically controllable break junction methods have recently been established,9-15 it is clear that the introduction of anchor units to organic molecules is essential for forming metalmolecule-metal junctions for single-molecule conductance measurements.11 For the construction of single-molecule electronics, the anchor units should facilitate not only electronic communication between the molecules and metal electrodes, but also robust attachment to the electrodes that allows for control over the orientation of molecules. Moreover, the identity of the charge carriers – whether they be holes or electrons – is dependent on the type of anchor used.16 Recently, a new anchoring strategy that utilizes direct hybridization between the  orbital of a conjugated molecule and metal electrodes has been reported to show high conductance.17-22 We successfully developed this strategy to a rationalized molecular design of the anchor units extended to a tripodal tetraphenylmethane framework.23,24 The tripodal arms – each bearing an anchoring functional group – are advantageous for both the formation of robust junctions and the perpendicular orientation of the fourth arm against metal electrodes,25-32 although their singlemolecule conductances are still veiling.23,32 We found that the tripodal anchor substituted with electron-deficient pyridines, 3Py-Fc (Figure 1(a)), could form a junction with a gold electrode using the  orbital of pyridine, leading to high conductance for bistripodal-anchored molecules


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compared with that for corresponding single-armed anchor.23 Theoretical analysis revealed that the electron-accepting * orbital of the pyridine component directly interacts with the electrode, thereby achieving -channel electron transport. On the basis of these results and the fact that the -channel anchoring groups are limited to electron-deficient aromatic compounds,20-23 we anticipated that, by taking advantage of electron-rich aromatic anchoring groups, the electrondonating-channel hybridization would lead to hole conduction; such a phenomenon would be highly desirable because most -conjugated systems applicable for single-molecule electronics (e.g., molecular wires) possess a high-lying energy level for the highest occupied molecular orbital (HOMO) and thus the hole-transporting characteristics are also highly energetic.33-35 Therefore, in this study, we have developed new tripodal compounds that use electron-rich aromatic rings as anchoring functional groups (Figure 1(a)) and investigated the adsorption behavior of these compounds to gold electrodes, which revealed that the thiophene-based tripodal unit showed benefits as an anchor. Electrical conductance and thermoelectric voltage (TEV) measurements were employed for 3Th-Ph-3Th and the reference compound 3Py-Ph-3Py (Figure 1(b)) with gold electrodes to confirm the formation of molecular junctions and experimentally identify the charge carrier of each molecular junction.


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Figure 1. Chemical structures used in this study.

A series of tripodal compounds – e.g., 3Ph-Fc, 3Fu-Fc, 3Th-Fc, 3(-Np)-Fc, 3(-Np)-Fc, 3(9-Ant)-Fc, and 3(2-Ant)-Fc – were synthesized by the Suzuki coupling reaction from ethynylferrocene-containing tribromotetraphenylmethane 124 (Scheme S1) to introduce the ferrocene unit which plays an important role as a redox-active component for cyclic voltammetry (CV) and as an internal standard for X-ray photoelectron spectroscopy (XPS) measurements. The chemical structures of these compounds were characterized using nuclear magnetic resonance (NMR) spectroscopy, mass spectroscopy (MS), and elemental analysis. The details of synthesis and characterization are summarized in the Electronic Supporting Information (ESI). To investigate the influence of the anchoring functional groups on the adsorption behavior, gold electrodes modified with the synthesized tripodal compounds were evaluated using the CV technique. The modified electrode were prepared by immersing Au(111)/mica substrates in dichloromethane (CH2Cl2) solutions (500 M) for 24 h at room temperature, followed by washing with CH2Cl2 and drying under a nitrogen gas flow. As shown in Figure 2(a), the cyclic voltammogram of the 3Th-Fc-modified gold electrodes exhibits a reversible one-electron redox wave that corresponds to the oxidation process of the ferrocene moiety. The peak currents associated with the oxidation process were found to increase linearly with the scan rate (Figure S1), which is in good agreement with the electrochemical response of the adsorbed species.36 The surface coverage () of 3Th-Fc was determined to be 32 ± 3 pmol cm−2, based on the integrated charge estimated from the anodic peak area of Figure 2(a) and the equation  = Q (nFA)−1, where Q, n, F, and A are the integrated charge, the number of participant electrons, the


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Faraday constant, and the electrode surface area, respectively. These results indicate the formation of densely packed monolayers. The immersion time dependence of  showed that the adsorption of 3Th-Fc reached saturation within 1 h (Figure S2). As shown in Figure 2(b), the plot of C −1 against C, where C is the concentration of the immersion solution, is quite linear in the range of 1–50 M. Thus, on the basis of the Langmuir isothermal adsorption model,37 the adsorption equilibrium constant (K) of 3Th-Fc was estimated to be 5.9 × 105 M−1. The free energy of adsorption (G) was therefore determined to be −32.9 kJ mol−1, according to the equation ∆𝐺 = −𝑅𝑇 ln 𝐾 Interestingly, in spite of the physical adsorption (vide infra), the G value of 3Th-Fc is comparable to that reported for chemically adsorbed self-assembled thiols.38 The observed large negative G for 3Th-Fc might be ascribed to the synergy adsorption effect of the tripodal structure. On the other hand, modified electrodes using 3Ph-Fc, 3Fu-Fc, 3(-Np)-Fc, 3(-Np)Fc, 3(9-Ant)-Fc, and 3(2-Ant)-Fc showed a rather indistinct redox wave under the same measurement conditions (Figure S3), resulting in  values of less than 10 pmol cm−2 (Figure 2(c)). The relative intensity of the integrated Fe 2p3/2 peaks in the XPS spectra obtained from the modified electrodes qualitatively agreed with the relative  values. It is worth noting that not only sterically crowded 3(-Np)-Fc and 3(9-Ant)-Fc, but also the less crowded 3(-Np)-Fc and 3(2-Ant)-Fc, showed a similar adsorption trend, indicating that these low-adsorption characteristics originate from weak electronic interactions between gold electrodes and the aromatic hydrocarbon anchoring groups. In contrast to the thiophene anchoring group in 3Th-Fc, the five-membered aromatic heterocycle of furan in 3Fu-Fc has a weak tendency to be adsorbed


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on gold. This result implies that the presence of a sulfur atom in the aromatic ring contributes to increasing the interactions.

Figure 2. (a) Cyclic voltammogram of 3Th-Fc-modified gold electrode in CH2Cl2 containing 0.1 M tetrabutylammonium hexafluorophosphate. V vs. ferrocene/ferrocenium (Fc/Fc+). (b) Plot of C

−1 vs. C for 3Th-Fc. (c) Summary of on Au(111) estimated from CV (red) and relative intensity of Fe 2p3/2 obtained from XPS (blue). To characterize the adsorbed state of 3Th-Fc on the gold surface, XPS measurements of the monolayers were conducted at room temperature in an ultrahigh vacuum chamber. As shown in Figure 3, two photoemission peaks at 165.1 and 163.7 eV were observed in the S 2p region, and these are assigned to the S 2p1/2 and S 2p3/2 peaks, respectively. These values are relatively


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higher than those for perpendicularly oriented (i.e., “standing-up”) thiophenes (163.2 and 162.0 eV), and are rather consistent with those reported for parallel (i.e., “flat-lying”) thiophenes (164.6 and 163.4 eV).39 This result supports the assumption that the parallel adsorption of the thiophene anchoring groups on the gold surface via the  orbital is preferable for 3Th-Fc (Figure S4).

Figure 3. S 2p XPS spectrum of 3Th-Fc on Au(111). In order to perform the electrical conductance measurement of single molecules, as well as to investigate the Seebeck coefficient for the thiophene and pyridine tripodal anchors, we synthesized the bistripodal 1,4-phenylene compounds 3Th-Ph-3Th and 3Py-Ph-3Py (Figure 1(b)). Because the six-fold cross-coupling reaction for the introduction of anchor aromatic rings resulted in the formation of a complex mixture, we applied the sequential Suzuki coupling reactions to obtain 3Th-Ph-3Th and 3Py-Ph-3Py (Scheme S2). The structure of 3Th-Ph-3Th was unambiguously identified by an X-ray crystallographic analysis (see Figure S5).40 These compounds showed moderate solubility in chlorinated solvents, such as chloroform (CHCl3) and o-chlorobenzene. The synthetic details and compound-identification data including NMR, MS, and elemental analysis are summarized in the ESI.


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The electrical conductance of the molecular junctions was determined using an STM break junction (BJ) method.11,41 The STM-BJ method was performed with the use of mechanically cut Au tip, which was repeatedly brought into contact with and retracted from Au(111) substrates that were covered with the molecules. The conductance of the molecules was determined from the conductance histogram created from five hundred to thousands of conductance traces measured during the tip retracting process. Curves showing only exponential decay, which was attributed to a tunneling decay without forming molecular junctions, were not chosen to construct the histogram. The thermopower, or the Seebeck coefficient, of the single-molecule junction is used to identify the charge carrier.42,43 A positive (negative) thermopower indicates that the charge carrier is a hole (electron). The setup for the TEV measurement is schematically shown in Figure 4.44 The STM tip was brought close to the substrate with a bias voltage of 50 mV until a certain threshold current value (i.e., larger than the conductance of the molecular junctions) was reached. Subsequently, the bias voltage source and current amplifier were disconnected, and the voltage amplifier was connected instead to measure the TEV induced by a temperature difference ΔT (Figure 4). After the voltage measurement, the electrical conductance of the junction was measured again to confirm the stability of the junction. If the formation of the junction was confirmed, the TEV was measured again. These steps were usually repeated three times. The voltage of the contact was determined from the voltage histogram created from the data obtained. The relation between the Seebeck coefficient of the junction, Sjunction, and the measured TEV, ΔV, is given as the equation


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∆𝑉 = (𝑆junction − 𝑆Cu )∆𝑇 where SCu is the Seebeck coefficient of bulk Cu, which is ~1.85 V K−1 at 300 K, as reported by Segalman et al.45

Figure 4. Schematic diagram for the conductance and TEV measurement setup. Measurements were performed under an argon (Ar) atmosphere. Note that in the experiment, in order to align the sign of the thermoelectric voltage and Seebeck coefficient, the equation S = ΔV ΔT−1 was used, wherein ΔV = (Vcold − Vhot) and ΔT = (Thot − Tcold). Figure 5(a) shows the conductance histogram for 3Th-Ph-3Th with Au electrodes taken by the STM-BJ method. The arrow in the figure indicates the peak position assigned as the molecular conductance value, which was found to be 2 × 10−5 G0 where 1 G0 = 2e2 h−1. Additionally, two peaks were found at higher conductance below 1 × 10−4 G0. These peaks should be attributed to molecular junctions with different contact geometries.46 Figure 5(b) shows the voltage histograms for 3Th-Ph-3Th with Au electrodes. The threshold conductance used to measure the TEV was 0.001 G0, where all the possible structures observed in the conductance histogram


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could be probed. Positive voltage values were observed for the molecule at all different values of ΔT.

Figure 5. (a) Conductance and (b) voltage histograms of 3Th-Ph-3Th junctions. Figure 6 shows the peak values of the TEV in the voltage histograms as a function of ΔT. The thermopower, SAu-3Th-Ph-3Th-Au, was calculated to be +22.4 ± 2.4 V K−1 (Figure 6(a)). The positive S indicates that the Fermi level is located close to the HOMO level, and that the charge carriers are holes.43,44 The thermopower of 3Py-Ph-3Py was also measured, and was shown to be −5.7 ± 1.0 V K−1 (Figure 6(b), see also Figures S6 and S7). This result supports electron transport, which coincides with the theoretical calculation of the previous report.23


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Figure 6. Peak values of the TEV as a function of ΔT for (a) 3Th-Ph-3Th and (b) 3Py-Ph-3Py junctions. Seebeck coefficients shown in the figure are calculated from the slope obtained by least squares method and SCu as explained in the text. In summary, we investigated the tripodal structure in order to construct a robust singlemolecule junction with a gold electrode. Furthermore, in order to accomplish a hybridization of the  orbital with gold electrodes and hole transport through this hybridization, we designed and synthesized a family of tripodal anchors with electron-rich aromatic rings to be used as anchoring functional groups. The adsorption behavior with respect to gold electrodes is largely dependent on the type of anchoring aromatic rings, and was investigated by CV measurements. The thiophene rings showed the best adsorption tendency among the examined anchors. This feature can be explained by the contribution of the multiplier effect of the tripodal structure and


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the increase of the interaction between sulfur atoms in the thiophene ring and the gold electrodes. XPS measurements indicated that the  orbital of thiophene contributes to the physical adsorption of the tripodal anchor unit to the gold surface. After successfully synthesizing the bistripodal compound, which contained thiophene rings as anchoring functional groups, the single-molecule conductance was measured using STM-BJ techniques, and was successfully determined to be 2 × 10−5 G0. Most importantly, TEV measurements showed the Seebeck coefficient to be positive, thereby unambiguously demonstrating that the charge carriers of thiophene-anchored tripodal molecules are holes; on the other hand, a negative Seebeck coefficient – indicative of electron transport – was confirmed for the pyridine-anchored tripodal molecules. To our knowledge, this is the first anchor unit that contributes to hole transport through -channel hybridization. Therefore, we can conclude that our newly developed tripodal thiophene anchors pave the way for achieving a versatile molecule–metal junction and that charge-carrier types are controllable by the π-channel hybridizations. Further investigation to develop new molecular systems containing hole- and/or electron transporting tripodal anchors applicable as single-molecule electronics is currently underway in our group.

ACKNOWLEDGMENT This work was supported by a Grant-in-Aid for Scientific Research from the Ministry of Education, Culture, Sports, Science and Technology (Molecular Architectonics), Japan. We acknowledge Dr. Shin-ichiro Kato for the X-ray analysis. We also acknowledge Dr. Hisao Nakamura for the helpful discussion. Thanks are extended to the CAC and Center for Scientific


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Instrument Renovation and Manufacturing Support, Osaka University, for assistance in obtaining elemental analyses and XPS measurements. Supporting Information. Synthetic details of 3Ar-Fc (Ar = Ph, Fu, Th, -Np, -Np, 9-Ant, and 2-Ant), compound 2, and 3Py-Ph-3Py, supplementary data of CV, crystal structure of 3ThPh-3Th, conductance histogram of 3Py-Ph-3Py, and voltage histogram of 3Py-Ph-3Py are available free of charge via the Internet at http://pubs.acs.org.

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30 Ie, Y.; Hirose, T.; Yao, A.; Yamada, T.; Takagi, N.; Kawai, M.; Aso, Y. Synthesis of Tripodal Anchor Units Bearing Selenium Functional Groups and Their Adsorption Behaviour on Gold. Phys. Chem. Chem. Phys. 2009, 11, 4949−4951. 31 Valášek, M.; Edelmann, K.; Gerhard, L.; Fuhr, O.; Lukas, M.; Mayor, M. Synthesis of Molecular Tripods Based on a Rigid 9,9'-Spirobifluorene Scaffold. J. Org. Chem. 2014, 79, 7342−7357. 32 Lukas, M.; Dössel, K.; Schramm, A.; Fuhr, O.; Stroh, C.; Mayor, M.; Fink, K.; Löhneysen, H. v. A Tripodal Molecule on a Gold Surface: Orientation-Dependent Coupling and Electronic Properties of the Molecular Legs. ACS Nano 2013, 7, 6170−6180. 33 Xu, B. Q.; Li, X. L.; Xiao, X. Y.; Sakaguchi, H.; Tao, N. J. Electromechanical and Conductance Switching Properties of Single Oligothiophene Molecules. Nano Lett. 2005, 5, 1491−1495. 34 Malen, J. A.; Doak, P.; Baheti, K.; Tilley, T. D.; Segalman, R. A.; Majumdar, A. Identifying the Length Dependence of Orbital Alignment and Contact Coupling in Molecular Heterojunctions. Nano Lett. 2009, 9, 1164−1169. 35 Ie, Y.; Endou, M.; Lee, S. K.; Yamada, R.; Tada, H.; Aso, Y. Completely Encapsulated Oligothiophenes: Synthesis, Properties, and Single-Molecule Conductance. Angew. Chem. Int. Ed. 2011, 50, 11980−11984. 36 Bard, A. J.; Faulkner, L. R. In Electrochemical Methods–Fundamentals and Applications: Wiley: New York, 1984; Chapter 14.


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37 Karpovich, D. S.; Blanchard, G. J. Direct Measurement of the Adsorption Kinetics of Alkanethiolate Self-Assembled Monolayers on a Microcrystalline Gold Surface. Langmuir 1994, 10, 3315−3322. 38 Jakubowicz, A.; Jia, H.; Wallace, R. M.; Gnade, B. E. Adsorption Kinetics of pNitrobenzenethiol Self-Assembled Monolayers on a Gold Surface. Langmuir 2005, 21, 950−955. 39 Noh, J.; Ito, E.; Araki, T.; Hara, M. Adsorption of Thiophene and 2,5-Dimethylthiophene on Au(111) from Ethanol Solutions. Surf. Sci. 2003, 535, 1116−1120. 40 CCDC-1061094 contains the supplementary crystallographic data. Single crystal for X-ray crystallography was obtained by recrystallization from chlorobenzene. 41 Tao, N. J. Electron Transport in Molecular Junctions. Nat. Nanotechnol. 2006, 1, 173−181. 42 Reddy, P.; Jang. S.-Y.; Segalman, R. A.; Majumdar, A. Thermoelectricity in Molecular Junctions. Science 2007, 315, 1568−1571. 43 Malen, J. A.; Yee, S. K.; Majumdar, A.; Segalman, R. A. Fundamentals of Energy Transport, Energy Conversion, and Thermal Properties in Organic-Inorganic Heterojunctions. Chem. Phys. Lett. 2010, 491, 109−122. 44 Lee, S. K.; Ohto, T.; Yamada. R.; Tada, H. Thermopower of Benzenedithiol and C60 Molecular Junctions with Ni and Au Electrodes. Nano. Lett. 2014, 14, 5276−5280. 45 Yee, S. K.; Malen, J. A.; Majumdar, A.; Segalman, R. A. Thermoelectricity in Fullerene– Metal Heterojunction. Nano Lett. 2011, 11, 4089−4094.


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46 Li, C.; Pobelov, I.; Wandlowski, T.; Bagrets, A.; Arnold, A.; Evers, F. Charge Transport in Single Au | Alkanedithiol | Au Junctions: Coordination Geometries and Conformational Degrees of Freedom. J. Am. Chem. Soc.,2008, 130, 318−326.


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Figure 1. Chemical structures used in this study. 176x54mm (300 x 300 DPI)



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Figure 2. (a) Cyclic voltammogram of 3Th-Fc-modified gold electrode in CH2Cl2 containing 0.1 M tetrabutylammonium hexafluorophosphate. V vs. ferrocene/ferrocenium (Fc/Fc+). (b) Plot of C Γ−1 vs. C for 3Th-Fc. (c) Summary of Γ on Au(111) estimated from CV (red) and relative intensity of Fe 2p3/2 obtained from XPS (blue). 88x112mm (300 x 300 DPI)


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Figure 3. S 2p XPS spectrum of 3Th-Fc on Au(111). 75x55mm (300 x 300 DPI)



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Figure 4. Schematic diagram for the conductance and TEV measurement setup. Measurements were performed under an argon (Ar) atmosphere. Note that in the experiment, in order to align the sign of the thermoelectric voltage and Seebeck coefficient, the equation S = ∆V ∆T−1 was used, wherein ∆V = (Vcold − Vhot) and ∆T = (Thot − Tcold). 225x126mm (96 x 96 DPI)


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Figure 5. (a) Conductance and (b) voltage histograms of 3Th-Ph-3Th junctions. 75x118mm (300 x 300 DPI)



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Figure 6. Peak values of the TEV as a function of ∆T for (a) 3Th-Ph-3Th and (b) 3Py-Ph-3Py junctions. Seebeck coefficients shown in the figure are calculated from the slope obtained by least squares method and SCu as explained in the text. 83x112mm (300 x 300 DPI)


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Thiophene-based Tripodal Anchor Units for Hole Transport in Single

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