Effects of Cis-Trans Conformation Between Thiophene Rings on

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Surfaces, Interfaces, and Catalysis; Physical Properties of Nanomaterials and Materials

Effects of Cis-Trans Conformation Between Thiophene Rings on Conductance of Oligothiophenes Tatsuhiko Ohto, Takuya Inoue, Helen Stewart, Yuichi Numai, Yoshio Aso, Yutaka Ie, Ryo Yamada, and Hirokazu Tada J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.9b02059 • Publication Date (Web): 22 Aug 2019 Downloaded from pubs.acs.org on August 23, 2019

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The Journal of Physical Chemistry Letters

Effects of Cis-Trans Conformation between Thiophene Rings on Conductance of Oligothiophenes Tatsuhiko Ohto,†* Takuya Inoue,‡ Helen Stewart,† Yuichi Numai,† Yoshio Aso,‡ Yutaka Ie,‡* Ryo Yamada,†* and Hirokazu Tada†

†

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

Toyonaka, Osaka 560-8531, Japan

‡

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

Ibaraki, Osaka 567-0047, Japan

AUTHOR INFORMATION

ACS Paragon Plus Environment

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Corresponding Authors *[email protected], [email protected], [email protected].

ABSTRACT. Oligothiophenes have been established as important -conjugated frameworks in organic electronics and molecular electronics. Although oligothiophenes possess the rotational flexibility of thiophene rings, the effects of cis-trans conformations on their electrical conductance has not been investigated yet. To investigate the effects of cis-trans conformations between thiophene rings on the conductance of oligothiophenes, we performed first-principles transport calculations. The conductance of the cis-oligothiophene was calculated to be higher than that of trans-oligothiophenes because the highest occupied molecular orbital was closer to the Fermi level of the gold electrode in the cis isomer than the trans isomer. This prediction was confirmed through the mechanically controllable break junction measurement and fitting of the currentvoltage characteristics for the newly synthesized, insulated oligothiophenes with controlled cis-trans conformations. This study demonstrates that cis- and trans-


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conformations can affect the electrical properties of oligothiophene frameworks and potentially be used to control the electronic structure of long oligothiophene molecular wires.

TOC GRAPHICS

-conjugated molecules and their derivatives are promising materials for both organic semiconductors in organic thin-film electronics and molecular wires in molecular electronics, because of their high electrical conductivity and chemical tunability.1-2 Continuous extension of the -orbital to span across the whole molecule is a promising method of realizing highly conductive molecular wires, and various molecular wires


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have been developed through this method.3-4 However, it has recently been reported that charge mobilities of -conjugated molecular wires can be improved by intentionally introducing bends between -units.5 The regular bending of -units suppresses the fluctuation in the effective length of -segments in a molecular wire and, as a result, energy levels of the -segments in a molecular wire are aligned, which results in an improved intra-molecular charge transport.

The above result indicates that it is important to control conformations of -conjugated molecules and to understand their effects on intra-molecular charge transport in order to effectively tune the properties of single-molecule wires. Even for the charge transport in short -conjugated molecules, in which tunneling is the dominant form of transport, the conformation can affect the electrical conductance because the electronic structure and the molecular length depend on the conformation.6 Variation in the molecular conformations could provide an explanation for the large dispersion in measured tunneling conductance in previous studies.7-8 It could also explain the anomaly observed in the length-dependent conductance of non-substituted oligothiophenes in a previous


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study,9 where the conductance of the tetramer was reported to be higher than that of the trimer, even though the trimer is shorter than the tetramer. In contrast, we have demonstrated a simple exponential decay in the tunneling conductance as a function of molecular length for encapsulated and planar oligothiophenes.10-12

A nonequilibrium Green’s function method combined with density functional theory (NEGF-DFT) has been successfully employed to reveal the relationship between the structure and the tunneling conductance of oligothiophenes. Peng et al. reported an exponential decay in the conductance of oligothiophenes along the length of the oligomer.13 In addition to the length-dependent conductance, the effect of contact formation between the thiol anchor and the gold electrode on the tunneling conductance has been studied.14-15 However, mainly due to the large diversity of possible structures, all of these studies have assumed the trans-conformation and the effect of the cis-trans conformations between thiophene rings on the electrical conductance has not been investigated.


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Figure 1. Conformations of oligothiophenes employed for DFT calculations (upper) and measurements (lower). White, green, and yellow balls represent H, C, and S atoms, respectively. The difference between NCS-4Ttrans-SCN and NCS-4Tcis-SCN is the trans-

cis conformation in the middle of the quarterthiophene, and thus 4T, 4Ta, and 4Te are marked. Because the backbone of NCS-4Tcis-SCN is slightly different from 4Ta, 4Te was also computed.


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In this work, we performed NEGF-DFT calculations for oligothiophenes (trans-nT and

nTx, which contains cis-bonds, n = 1~8, Figure 1) sandwiched between gold electrodes to investigate the effect of cis-trans conformations on the tunneling conductance. While the trans-oligothiophenes showed an exponential decay in the conductance, oligothiophenes containing cis-bonds showed a higher conductance than their transequivalents, because of the slight energy shift of the highest occupied molecular orbital (HOMO). To support these calculations, we synthesized two conformationally planar and structurally well-defined quarterthiophenes NCS-4Ttrans-SCN and NCS-4Tcis-SCN (Figure 1). It should be noted that the cyclopentene-annelated thiophene and methylene-bridged bithiophene components completely retain their trans and cis conformations, respectively. Since the orthogonal substitution with bis(2ethylhexyl)fluorene functions as an insulating unit, the influence of intermolecular - interactions can be considered as negligible for these molecules. The experimental trend in the conductance values of NCS-4Ttrans-SCN and NCS-4Tcis-SCN molecules,


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determined using the mechanically controllable break junction (MCBJ) method, is in good agreement with the theoretical results.

Theoretical Results. First, we discuss the length-dependent tunneling conductance of

trans-nT (n = 1–8). The conductance values could be fitted by a single exponential curve, Aexp(-βl), where A is a constant and l is the molecular length, respectively. The value of the decay constant, β, for the conductance ranges from 1.4 to 1.9 nm–1, depending on the computational method (Figure 2). The self-interaction correction (SIC) method gives a larger β value than the local density approximation (LDA) and PerdewBurke-Ernzerhof (PBE) methods because the SIC lowers the energy level of the HOMO, which dominates the transport. The larger gap between the HOMO level and the Fermi level of the electrode raises the β value.16 The β value predicted by SIC is similar to the experimentally reported value (2.0 nm–1)12, which indicates that correction methods such as SIC, GW,17 DFT-18 or hybrid functionals19 are desirable to simulate the length dependent conductance of oligothiophenes. Note that the simple exponential decay of the conductance for trans-nT has been calculated with NEGF-DFT,13 although the


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conductance decay constant (2.11 nm–1) is slightly larger compared to our values. This difference originates from the difference in the morphology of the electrode surface considered.

Figure 2. (a) Length-dependent conductance of Au-S-trans-nT-S-Au junctions by three computational methods. The unit cell structure of the Au-S-4T-S-Au junction is also shown. (b) Conductance of Au-S-nTx-S-Au junctions together with that of Au-S-trans-


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nT-S-Au junctions (shown in red), calculated using PBE. The red line is the fitted exponential curve for the conductance values of Au-S-nT-S-Au.

We also note that the variety of cis-trans conformations induces the dispersion of the tunneling conductance. For 5T and 6T, the range of the dispersion is about one order of magnitude. Further, the large dispersion in the measurements shown later would be explained by the configurations between the thiol anchor and the electrodes, which are not considered in the first-principles calculations. The conductance dispersion of 4T is smaller than 5T and 6T, because the change in the molecular length along with the cistrans conformations is smaller than that of 5T and 6T. This tendency agrees well with the widths of conductance histograms measured experimentally in a previous report.9


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Figure 3. Transmission coefficient of the Au-S-4T-S-Au, Au-S-4Ta-S-Au and Au-S-4TeS-Au junctions with PBE (top) and SIC (bottom) methods.

Next, we calculated a series of oligothiophenes nTx structures by using the PBE functional to estimate the influence of the cis conformation on the conductance. The conductance value becomes larger than the fitted exponential curve for nT when the oligothiophene has at least one cis-bond, as shown in Figure 2 (see Table S1 for the conductance values). The origin of the higher conductance value of cis-nT is illustrated in Figure 3. The PBE and SIC methods are qualitatively similar, but SIC corrects the


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high HOMO level predicted by PBE.20 The order of the conductance values calculated using SIC (Table S1) agree well with the measured values shown later. The transmission functions are very similar in shape, but the HOMO level of 4Ta is slightly closer to the Fermi level than that of 4T, which reflect the better -conjugation in 4Ta than in 4T. The HOMO level of 4Te is closer to the Fermi level than that of 4Ta due to the rigid connection between thiophene rings. These shifts affect the transmission coefficient at the Fermi level, which corresponds to the conductance value that can be measured by the break junction method.

Synthesis and Properties. To confirm the above theoretical prediction, we synthesized NCS-4Tcis-SCN and NCS-4Ttrans-SCN (Figure 1), whose core-structures correspond to 4T and 4Te, respectively. These molecules are synthesized using Stille coupling reactions as a key step for C-C bond formation. Synthetic experimental details are summarized in the Supporting Information. Energy gaps and energy levels of the molecules were investigated by UV-vis absorption spectra and cyclic voltammetry (CV). Figure 4(a) shows UV-vis absorption spectra in CH2Cl2 solutions. These molecules


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showed strong absorption bands between 300 and 500 nm, which is attributed to the – * transition of the oligothiophene backbones. The absorption maximum of NCS-4TcisSCN (447 nm) is red-shifted compared to that of NCS-4Ttrans-SCN (430 nm), which indicates that the introduction of the cis conformation led to a narrow optical energy gap. Figure 4(b) shows the CV curves of NCS-4Tcis-SCN and NCS-4Ttrans-SCN measured in CH2Cl2/CH3CN (10/1) solution containing 0.1 M tetrabultylammonium hexafluorophosphate (TBAPF6). The ferrocene/ferrocenium (Fc/Fc+) redox couple was used as the internal standard for the calibration of redox potentials. Both NCS-4TcisSCN and NCS-4Ttrans-SCN showed a reversible oxidation wave, and the first oxidation potential of NCS-4Ttrans-SCN (0.59 V) is positively shifted compared to that of NCS4Tcis-SCN (0.48 V). This result indicates that the HOMO energy level of NCS-4TtransSCN is lower than that of NCS-4Tcis-SCN, which is consistent with DFT calculations for isolated 4T and 4Te molecules. (Table S2).


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Figure 4. (a) UV-vis absorption spectra and (b) CV curves of NCS-4Ttrans-SCN (red) and NCS-4Tcis-SCN (black).

Electrical Measurements. To compare the experimentally observed electrical characteristics and the theoretical predictions, the measured current–voltage (I–V) curves were analyzed on the basis of a single-level resonant transport model using the following formula21

𝑡cis/trans(𝐸) =

4𝛤𝑅cis/trans𝛤Lcis/trans

(1)

(𝛤𝑅cis/trans + 𝛤Lcis/trans)2 + 4(𝐸 ― 𝜀0cis/trans)2

where tcis/trans (E), 𝜀0cis/trans , 𝛤 Rcis/trans and 𝛤 Lcis/trans represent transmission probability, the


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energy level of molecular orbitals responsible for charge transport, electronic coupling between the energy level of a molecule and the left and right electrodes in NCS4Tcis/trans-SCN, respectively. By fitting experimental I-V curves with eq. (1), the values for 𝜀 0cis/trans, 𝛤 Rcis/trans and 𝛤 Lcis/trans can be obtained. To compare results with the theoretical model, where both anchors were symmetrically attached to the electrodes, I-V curves for a symmetrical junction, i.e., 𝛤 Rcis/trans ~= 𝛤 Lcis/trans should be used. Therefore, only fitting results with 0.8≦ 𝛤 Rcis/trans/ 𝛤 Lcis/trans ≦ 1.2 were included. In order to assess the fitting quality, the p-value of the fitted function was used. Only results that show a correlation with the fitting model with a p-value ≦ 0.5 were used for the analysis.

Figure 5(a) shows histograms of the conductance of NCS-4Tcis/trans-SCN (Gcis/trans). Note that Gcis/trans were calculated from the gradient of the I-V curves between 0.05 V and 0.07 V. The peak values in the histogram, determined by fitting a Gaussian function to the distribution, were Gcis = (1.4±0.09) × 10-2 G0 and Gtrans = (8.0±0.3) × 10-3 G0. As expected from the theoretical calculation, NCS-4Tcis-SCN showed a higher conductance than NCS-4Ttrans-SCN.


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Figure 5. Histogram of Gcis/trans (a) 𝜀0𝑐𝑖𝑠/𝑡𝑟𝑎𝑛𝑠 (b) 𝛤Lcis/trans (c). Results for cis- and transmolecules are shown in the top (black) and bottom (red) panels, respectively.


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Figures 5(b) and 5(c) show histograms of 𝜀0cis/trans and 𝛤Lcis/trans, respectively. Because the distribution of 𝛤Lcis/trans was asymmetric, the peak value was determined by fitting a lognormal distribution function to the histogram. Also, because of the condition that the coupling to the right and left electrodes is symmetrical, where 0.8≦ 𝛤Rcis/trans / 𝛤Lcis/trans ≦ 1.2, only the histogram for 𝛤 Lcis/trans is shown. As a result of this analysis the following values were determined: 𝛤Lcis = (4.0±0.1) × 10-2 eV, 𝛤Ltrans = (2.6±0.1) × 10-2 eV, 𝜀0cis = (9.7±0.2) × 10-1 eV and 𝜀0trans = 1.4±0.01 eV. The HOMO level of NCS-4Tcis-SCN was closer to the Fermi level than that of NCS-4Ttrans-SCN, which supports the prediction from the first-principles calculation.

In summary, in order to investigate the influence of cis-trans conformations on the electrical conductance, we performed first-principles transport calculations for Auoligothiophene-Au junctions with various lengths and conformational patterns. The

trans-oligothiophenes showed a simple exponential decay in conductance, while cisoligothiophenes gave a higher conductance than the exponential curve. The origin of


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this higher conductance is attributed to the higher HOMO level of the cis-oligothiophene. This prediction was confirmed by MCBJ measurements of NCS-4Ttrans-SCN and NCS4Tcis-SCN. In addition to the conductance histogram, we measured the I-V characteristics, which enabled us to extract the energy level and coupling parameters of the molecular orbital. These results clearly show that the HOMO of NCS-4Tcis-SCN is closer to the Fermi level than that of NCS-4Ttrans-SCN. This study demonstrates that the fine-tuning of oligothiophene conformations is required for precise design of their electronic functionality.

EXPERIMENTAL SECTION

Theoretical Calculation. In this study, we have used the SMEAGOL code22-24 based on the SIESTA program.25 The LDA, the SIC20, 26 with LDA, and the PBE27 were used for the computational methods. The electrode was modeled as an Au(111) slab having a p(4×4) surface periodicity with a 2×2×1 k-point sampling grid. An 8×8×1 k-point mesh was used to compute the transmission coefficient. Double/single zeta plus polarization


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basis sets were used for molecules/Au. The top two layers were taken as a plateau to model the tip-like structure so as to avoid any interaction between the Au surface and the oligothiophene molecules containing cis-bonds. The S atom in the oligothiophene was in the hollow site of the top layer. We neglected the covering parts because they do not significantly affect the dihedral angle between thiophene rings.

Synthesis. Microwave irradiation was performed using a Biotage Initiator Ver. 2.5. Column chromatography was performed on KANTO Chemical silica gel 60N (40–50 μm). GPLC was performed on a Japan Analytical Industry LC-9210 NEXT system equipped with a JAI-GEL 1HH/2HH or JAI-GEL 2.5H/3H column. 1H and 13C NMR spectra were recorded on a JEOL ECS-400 spectrometer in CDCl3 with tetramethylsilane (TMS) as an internal standard. The data is reported as follows: chemical shift in ppm (), multiplicity (s = singlet, d = doublet, t = triple, m = multiplet), coupling constant (Hz), and integration. High-resolution mass spectroscopy (HR-MS) was obtained using a Bruker ultraflex III. Elemental analyses were performed on


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PerkinElmer LS-50B by the elemental analysis section of the Comprehensive Analysis Center of ISIR, Osaka University.

Properties. UV-vis absorption spectra were recorded on a Shimadzu UV-3600 spectrophotometer. Cyclic voltammetry was carried out on a BAS CV-620C voltammetric analyzer using a platinum disk as the working electrode, a platinum wire as the counter electrode, and Ag/AgNO3 as the reference electrode at a scan rate of 100 mV s–1.

MCBJ measurements. I–V characteristics of single-molecule junctions were measured using a MCBJ method at 80 K (see reference 28 for details). In brief, a small Au contact was prepared on an elastic substrate made of phosphorous copper covered with polyimide. A pushing rod was moved to bend the substrate, which was fixed in a threepoint bending geometry. As the substrate was bent, the Au contact was elongated and, finally, broken. To form molecular junctions, a droplet of a 0.1 M solution of molecules in trichloromethane was placed on the substrate so that the Au contact was covered. Oligothiophenes were spontaneously adsorbed onto the Au electrodes via Au–SCN


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bond formation.29-30 During the experiment, the distance between the electrodes was changed by approximately 0.04 nm in a stepwise manner using a piezoelectric stepper motor. I–V characteristics were measured after every step of the piezoelectric stepper motor.

ASSOCIATED CONTENT

AUTHOR INFORMATION

Notes The authors declare no competing financial interests.

ACKNOWLEDGMENT

This work was supported by Grants-in-Aid for Scientific Research Nos. JP19K15505, JP17K19104, and JP18H03899, and a Grant-in-Aid for Scientific Research on Innovative Areas (Molecular Architectonics) from the Ministry of Education, Culture,


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Sports, Science and Technology, Japan. We acknowledge Mr. Takuji Seo for synthesis support.

Supporting Information Available. The results of calculations and the details of synthesis.

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Effects of Cis-Trans Conformation Between Thiophene Rings on

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