Highly Planar and Completely Insulated Oligothiophenes: Effects of Ï

Letter Cite This: J. Phys. Chem. Lett. 2019, 10, 3197−3204

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Highly Planar and Completely Insulated Oligothiophenes: Effects of π‑Conjugation on Hopping Charge Transport Yutaka Ie,*,† Yuji Okamoto,† Takuya Inoue,† Saori Tone,† Takuji Seo,† Yasushi Honda,‡ Shoji Tanaka,§ See Kei Lee,∥ Tatsuhiko Ohto,*,∥ 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 West Japan Office, HPC Systems Inc., 646 Nijohanjikicho, Shimogyo-ku, Kyoto 600-8412, Japan § Research Center for Molecular Scale Nanoscience, Institute for Molecular Science, Okazaki, Aichi 444-8585, Japan ∥ Graduate School of Engineering Science, Osaka University, 1-3 Machikaneyama, Toyonaka, Osaka 560-8531, Japan Downloaded by UNIV AUTONOMA DE COAHUILA at 06:14:41:217 on May 31, 2019 from https://pubs.acs.org/doi/10.1021/acs.jpclett.9b00747.

‡

S Supporting Information *

ABSTRACT: Elucidating the nature of long-range intramolecular charge transport in π-conjugated molecules is of great importance for the development of organic electronic materials. However, the effects of the degree of π-conjugation on the hopping charge transport have not been experimentally explored so far owing to the lack of π-conjugated backbones with different conjugation degrees and severalnanometer lengths. Here we develop highly planar and completely insulated oligothiophenes between 0.85 and 9.64 nm in length. As compared to distorted oligothiophenes, single-molecule conductance measurements of the planar molecules show (i) a smaller activation energy and larger electrical conductance in the hopping transport regime and (ii) a shift in crossover between tunneling and hopping conduction toward a short molecular length. Theoretical calculations indicate that small reorganization energies and narrow energy gaps derived from the planar backbones result in these superior characteristics. This study reveals that the planarity of π-conjugation has significant advantages for hopping charge transport. he long-range intramolecular charge transport in πconjugated systems has attracted considerable interest not only because it forms the basis for new strategies to realize novel functions for single-molecule devices but also because it can provide valuable knowledge for correlating the transport mechanisms of single molecules and organic electronic materials.1−3 Recent advances in experimental and theoretical methods have enabled researchers to investigate the chargetransport mechanisms and electronic functions of single molecules connected to metal electrodes (single-molecule junctions).4−8 It has been recognized that the charge-transport characteristics are governed by coherent tunneling over a short distance and incoherent hopping over a long distance.2,9−17 The crossover between tunneling and hopping charge transport was observed by using a series of π-conjugated systems.9−13 The degree of effective conjugation on biaryl systems, which is strongly affected by the dihedral angle, is known to affect molecular conductance in the tunneling regime.18−20 On the other hand, the influence of effective conjugation on the hopping charge transport has not been explored so far owing to the lack of π-conjugated backbones with different conformations and lengths on the order of several nanometers. Oligothiophenes are among the most promising candidates to elucidate such a structure−charge transport relationship.21−25 Thus, various single-molecule characteristics of charge transport

T

© XXXX American Chemical Society

in the tunneling regime have been reported by utilizing oligothiophenes shorter than thiophene 6-mers.16,26−31 In contrast, hopping charge-transport characteristics including crossover have only been investigated for NCSTed(T12TNT12)mTed-SCN (Figure 1a), wherein bulky silyl substituents prevent intermolecular charge transport.32−35 As shown in Figures 1b and S1 (in the Supporting Information (SI)), density functional theory (DFT) calculations indicate that the conjugation in (T1TNT1)m is distorted largely because of the bulkiness of the silyl substituents. As a result, energy levels of the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) of (T1TNT1)m differ from those of the corresponding unsubstituted and planar oligothiophenes (nT) (Figure 1c and Table S1). Thus, the charge-transport characteristics of (T1TNT1)m cannot be considered as intrinsic characteristics of oligothiophenes. In order to investigate the intrinsic charge-transport characteristics of oligothiophenes, conformationally highly planar π-conjugation and structurally well-defined long oligomers are required. Insulation of π-conjugation is also essential to avoid Received: March 18, 2019 Accepted: April 17, 2019 Published: May 27, 2019 3197

DOI: 10.1021/acs.jpclett.9b00747 J. Phys. Chem. Lett. 2019, 10, 3197−3204

Letter

The Journal of Physical Chemistry Letters

Figure 1. (a) Chemical structures of oligothiophenes used in this study. (b) Top and side views of optimized structures for 12TMe and (T1TNT1)4. (c) Energy levels and molecular orbitals of (T1TNT1)2, 6TMe, and 6T. DFT calculations were performed at the B3LYP/6-31G(d,p) level.

Scheme 1. Synthetic Route

nTOct was limited to the thiophene 6-mer due to solubility issues; consequently, investigation of the charge-transport mechanism was restricted to the tunneling regime.28 In the present work, we developed new molecules, nTEH and NCS-nTEH-SCN (Figure 1a; see also Figure S3) in which the octyl groups in the fluorene units of nTOct are replaced with 2ethylhexyl groups to enhance solubility. Thiocyanate (SCN) groups are attached at both ends of the molecule as the anchors to gold electrodes.32 This molecular design enables us to compare electrical conductance (G) against NCSTed(T12TNT12)mTed-SCN, directly. The length and temperature dependence of G of single NCS-nTEH-SCN molecule junctions were measured by the scanning tunneling microscope-break junction (STM-BJ) method using gold electrodes.

intermolecular charge transport between molecules through intermolecular π−π interactions.36−38 We previously revealed that thiophenes containing a 3,4-fused cyclopentene ring did not disrupt the effective conjugation when incorporated into an oligothiophene.28,39−42 The oligothiophenes nTOct afforded both highly effective conjugation and complete insulation of the π-conjugated backbones; thus, in the absence of intermolecular interactions, the tunneling decay constant (ß) for the single-molecule conductance of HSPhnTOctPh-SH was determined to be 1.9 nm−1 (Figure 1a).28 As shown in Figure 1c, DFT calculation showed that the molecular orbitals of model compound 6TMe are delocalized entirely over the π-conjugated backbone, and the HOMO− LUMO energy gaps of nTMe are nearly superimposable upon nT (Figure S2; see also Table S1). However, the molecular length of 3198


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Figure 2. (a) UV−vis absorption spectra of nTEH in CH2Cl2. (b) Correlation between E and n−1 of nTEH and nT. (c) Cyclic voltammograms of nTEH in CH2Cl2/CH3CN (10/1) containing 0.1 M TBAPF6.

On the basis of our established synthesis of nTOct and 1TEHBr,28,42 we obtained nTEH (n = 2−6) (Scheme S1). For the longer oligomers, our synthetic strategy relied on the blockcoupling of building components to ease the isolation and purification of the products by exploiting the molecular size differences. To perform the single-molecule conductance measurements of nTEH in a relatively small interval of molecular length, we selected the quaterthiophene 4TEH as a building component. The Stille coupling reaction between Br-4TEH-Br and Sn-4TEH-Sn in the presence of the conventional catalyst Pd(PPh3)4 gave only thiophene 8-mers (X-8TEH-X (X = H or Br or SnMe3)) in less than 10% yield. This unexpectedly low reactivity was due to the bulkiness of the substrates and correspondingly increased transmetalation barrier (Figure S4). Ultimately, the catalytic system of del Pozo et al. (PdCl2(PPh3)2/AuCl(AsPh3) afforded a mixture of nTEH (n = 8, 16, 20, 24, and ≥28, Scheme 1).43 The crude products were treated with silica gel to remove the trimethylstannyl groups of nTEH and then subjected to preparative gel-permeation liquid chromatography (GPLC) to isolate the oligomers (Figure S5). Finally, SCN groups were incorporated into the terminal positions of nTEH to give the corresponding NCS-nTEH-SCN. The electronic absorption spectra of nTEH and NCS-nTEHSCN in CH2Cl2 solutions were measured to investigate the photophysical properties of the oligomers. All compounds showed two absorption bands corresponding to the π−π* transition of fluorene units at around 280 nm and that of oligothiophene backbones in the visible region, similarly to nTOct and the TEH-containing oligothiophenes (Figure 2a).28,42 Upon increasing the number of thiophene rings, the absorption band (λabs) originating from the oligothiophene backbone is red-

shifted, as summarized in Table 1, with a concomitant increase in the absorption coefficient. In contrast to the typical alkylTable 1. Photophysical and Electrochemical Properties of nTEH compound

L/nma

λmax/nmb

λonset/nmb

E/eVc

Eonset/Vd

2TEH 3TEH 4TEH 6TEH 8TEH 12TEH 16TEH 20TEH 24TEH

0.85 1.25 1.64 2.43 3.23 4.81 6.38 7.97 9.64

314 371 419 467 496, 529 521, 551 529, 562 533, 568 534, 569

349 415 491 554 583 624 629 633 640

3.95 3.34 2.96 2.66 2.50 2.38 2.34 2.33 2.32

0.74e 0.43e 0.23 0.14 0.05 0.02 0.02 0.01 0.01

a The molecular length (L) of each structure (Figure S3) was estimated from the optimized structures of nTMe. The calculated length of 2TEH is in very close agreement with the distance obtained by single-crystal X-ray diffraction (0.83 nm). See ref 28. bIn CH2Cl2. c E = 1240/λmax. dIn CH2Cl2/CH3CN containing 0.1 M TBAPF6 V. vs Fc/Fc+. eIrreversible.

substituted long oligothiophenes,44 the absorption spectra of nTEH showed well-resolved vibronic bands, reflecting their rigid structures. The π−π* transition energies (E) of nTEH were plotted as a function of the inverse of the number of thiophene units (n−1) (Figure 2b). A linear relationship given by E (eV) = 1.63 + 3.85n−1 was estimated within the short oligomeric range between 2TEH and 12TEH. Its slope (3.85) is steeper than that of unsubstituted oligothiophenes (3.73)45 and in good agreement with that of nTOct (3.85).28 These results clearly indicate that 3199


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Figure 3. (a) Typical conductance transients observed for NCS-nTEH-SCN. (b) 2D conductance histograms constructed for NCS-nTEH-SCN (n = 12, 16, 20, and 24). The corresponding conductance histograms of each molecule are shown beside the 2D histograms. Results of fitting of the Gaussian function (blue line) to determine G values are also shown in the histograms with peak values. Note that the conductance histogram for NCS-16TEHSCN is constructed from the subset of the data between dashed lines indicated in the figure to pick up the tail of the conductance plateaus. See the SI for details. (c) Semilog plots of G as a function of the estimated molecular length (L) for NCS-nTEH-SCN (blue circles) and NCS-Ted(T12TNT12)mTedSCN (black circles). The data of Ted(T12TNT12)mTed-SCN were extracted from ref 33. (d) Resistance (R) against L for NCS-nTEH-SCN (blue circles) and NCS-Ted(T12TNT12)mTed-SCN (black circles). (e) Arrhenius plots of G for NCS-8TEH-SCN (orange circles) and NCS-12TEH-SCN (green circles) with standard error bars (red lines). Gray lines in the figures are obtained by linear regression. Note that standard error bars are shown in the dots, indicating the data points with red lines in (c−e).

oligothiophenes composed of TEH units have similar effective conjugations irrespective of the bulky 2-ethylhexyl groups incorporated in the fluorene units. The absorption maxima of NCS-nTEH-SCN in the short oligomeric range (n = 2−6) are slightly blue-shifted compared to those of nTEH because of the

electron-withdrawing nature of the SCN groups (Figure S6, Table S2). The oxidation processes of nTEH and NCS-nTEH-SCN were measured using cyclic voltammetry (CV) in CH2Cl2/CH3CN (10/1) solution containing 0.1 M tetrabutylammonium 3200


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observe any anomalously large conductance for the 4-mer, which had been reported for oligothiophene molecules without any functional groups along the backbone and attributed to cisoid conformational effects.29 The length dependence of the conductance becomes weaker for molecules longer than NCS8TEH-SCN. For molecules longer than NCS-12TEH-SCN, the resistance (R = G−1) increases linearly against the molecular length (Figure 3d). Because the change in the length dependence of the resistance could be due to the transition of the charge-transport mechanism from tunneling to hopping, we measured the electrical conductance of NCS-8TEH-SCN and NCS-12TEHSCN at 273, 283, 298, 310, and 323 K, as shown in Figure 3e. See Figures S11−S14 for conductance histograms and other details. Above 323 K, conductance plateaus were hardly observed. Within the temperature range examined, the conductance of NCS-12TEH-SCN increased, whereas that of NCS-8TEH-SCN did not significantly change as the temperature increased, indicating that hopping transport becomes dominant from NCS-12TEH-SCN (Figure 3e). The activation energy for hopping charge transport in NCS-12TEH-SCN, obtained from the Arrhenius plot, was 0.18 eV. This activation energy is smaller than that for NCS-Ted(T12TNT12)mTed-SCN (m = 5−7), where previous studies33,35 showed hopping transport with an activation energy of 0.36 eV. As shown in Figure 3d, G of NCS-nTEH-SCN (n = 12, 16, 20, and 24) was larger than that of NCS-Ted(T12TNT12)mTed-SCN in the hopping transport regime. Furthermore, the crossover length of nTEH (between the 8-mer (3.23 nm) and 12-mer (4.81 nm)) is shorter than that observed for (T12TNT12)m (thiophene 17-mer (5.69 nm); Figure 3c) and is comparable to that of other rigid π-conjugated molecular wires (2.5−6.0 nm) at 298 K.2,9−13 The activation energy is smaller than the values (0.55−0.65 eV) reported for other rigid π-conjugated molecular wires.11−13 These trends can be qualitatively understood from the longer effective conjugation length and positive shift of the HOMO level in nTEH (Figure 1b, Table S1) compared to (T12TNT12)m. To better understand the characteristics of the origins of the shorter crossover length observed for NCS-nTEH-SCN, we carried out theoretical investigations as follows. We considered why nTEH molecules exhibit hopping transport at a much shorter length than Ted(T12TNT12)mTed. For hopping transport, interfacial charge transfer is the ratelimiting process, with a rate constant given by47,48 ÅÄÅ ÑÉ ∞ ÅÅ (λ + δ − E)2 ÑÑÑ 2π 1 → 2 ÅÅ− ÑÑ ( ) exp k int V E = | | ÅÅ ÑÑ 4λkBT ℏ (4πλkBT )1/2 −∞ ÅÇ ÑÖ ρ(E)fFD (E) dE (1)

hexafluorophosphate (TBAPF6). The cyclic voltammograms, calibrated versus the ferrocene/ferrocenium (Fc/Fc+) couple, are shown in Figures 2c and S7, and the onset points of the first oxidation waves (Eonset) are listed in Tables 1 and S2. For 2TEH and 3TEH, only irreversible oxidation processes occurred. Whereas one reversible and one quasi-reversible oxidation processes were observed for 4TEH, multiple reversible oxidation waves could be seen in 6TEH, 8TEH, and 12TEH. This phenomenon indicates that the formation of cationic species becomes kinetically feasible as the π-conjugated backbone is extended. In contrast, the CVs of 16TEH, 20TEH, and 24TEH show undefined multiple oxidation waves due to the superimposition of multiredox processes. This behavior is characteristic of longer oligothiophenes.44 The introduction of SCN groups causes a positive shift in the Eonset values. To evaluate the encapsulating behavior,28,42 we measured the UV−vis−NIR spectra of the polaronic species derived from 6TEH and FeCl3 as an oxidant in CH2Cl2 solution; no spectral change was observed when the solution was cooled to 223 K (Figure S8). This result confirms that π-dimer formation was completely inhibited by the presence of the insulating substituents.28,40,42 G of single-molecule junctions for NCS-nTEH-SCN was measured using the STM-BJ method.46 In this technique, a gold STM tip is repeatedly brought into contact with and retracted from a gold substrate modified with the molecules. When the STM tip is retracted after contact with the substrate, G changes in a stepwise manner at integer multiples of the quantum conductance G0 (G0 = 2e2h−1, where e and h are the elementary charge and Planck constant, respectively) owing to the formation of gold atomic contacts. When molecules are inserted between the STM tip and substrate, conductance steps or plateaus, which are attributed to the formation of molecular junctions, are observed below 1G0 upon further retraction of the STM tip. See the SI for the experimental details. As shown in Figures 3a and S9, conductance plateaus were observed for all of the molecules. Figure 3b shows the two-dimensional conductance−displacement (2D) histograms for NCS-nTEH-SCN (n = 12, 16, 20, and 24; see Figure S10 for other molecules). Detailed information such as the number of traces used to create 2D histogram and results of Gaussian fitting used to determine G are shown beside each 2D histogram and in Table S3. Major plateau lengths observed for some molecules are significantly shorter than the estimated molecular length. A similar trend was also reported by Tao et al.13,17 The plateau length can be shorter than the molecular length because single-molecule junctions can be broken during the pulling process before forming the fully stretched structure. In our measurement, the length of the plateau varied from experiment to experiment probably due to the uncontrollable conditions of the STM tip and surface. Therefore, we did not conduct quantitative analysis for the plateau length. Figure 3c shows the length dependence of G observed for NCS-nTEH-SCN together with that observed for NCS(T1TNT1)m-SCN recreated from the previous data33 using the estimated molecular length based on the optimized structures of nTMe and (T1TNT1)m (Figures S1 and S3). For NCS-nTEHSCN, an exponential relationship, G = Gc exp(−ß L), where Gc is a constant, ß is the tunneling decay constant, and L is the molecular length, is evident from NCS-2TEH-SCN to NCS8TEH-SCN, indicating that tunneling transport occurs for these molecules. ß is determined to be 2.0 nm−1 based on the estimated molecular lengths shown in Table 1. This ß value is almost the same as that of HS-PhnTOctPh-SH.28 We did not

∫

where V(E) is the interaction energy between the metal and the molecule, ρ(Ε) is the density of states of the electrode, λ is the reorganization energy, δ is the energy gap between the conducting orbital and the Fermi level of the electrode, i.e., the injection barrier, and f FD(E) is the Fermi−Dirac distribution function. Assuming that V(E) and ρ(Ε) are independent of energy, eq 1 can be recast as ÄÅ É ∞ ÅÅ (λ + δ − E)2 ÑÑÑ 1 → Å ÑÑf (E) dE expÅÅÅ− k int = c ÑÑ FD Å ÑÑÖ −∞ (4πλk T )1/2 4 λ k T Å B B Ç → ≡ crint (2)

∫

3201


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Figure 4. (a) Transmission coefficients of (T1TNT1)3, (T1TNT1)4, 8TMe, and 8T. (b) Side views of the calculated polaronic structures for 8TMe (top) → depending on δ and λ. and (T1TNT1)4 (bottom). The corresponding neutral structures are semitransparently superimposed. (c) 3D plot of rint

where c is a constant that includes all of the prefactors. The → , are δ and parameters required to evaluate the integral value, rint λ. To estimate δ for (T12TNT12)n, nTEH, and nT, we calculated their transmission coefficients (see the SI for computational details).49−51 The slightly shorter lengths that exhibit hopping transport, or conversely, the maximum lengths that exhibit tunneling transport, can be regarded as the units of charge hopping. From the experimental results, these units are (T1TNT1)3−433 and 8TMe. We also calculated the δ value of 8T for comparison from the difference between the Fermi level and the peak top of the HOMO. As shown in Figure 4a, the δ for 8TMe is 0.29 eV, which is nearly identical to that of unsubstituted 8T. The δ’s of (T1TNT1)3 and (T1TNT1)4 are 0.72 and 0.76 eV, respectively, which are larger than that of 8TMe. This is attributed to the large HOMO−LUMO gaps of (T1TNT1)n (n = 3 and 4), which arise from their highly disordered molecular conformations and lower effective conjugation (Figure 1b). The λ values were estimated from gas-phase calculations. The λ of 8TMe is 0.22 eV, and that of (T1TNT1)4 is 0.73 eV. Although δ varies according to the molecular length, this difference is derived from the degree of structural change between the neutral and polaronic states: (T1TNT1)4 tends to become planarized under one-electron oxidation, whereas 8TMe retains its planar → , considering the structure (Figure 4b). To estimate rint fluctuation of δ and λ when the length of the hopping unit → values for wide ranges of δ and λ, where changes, we plotted rint → we assumed T = 300 K (Figure 4c). The data show that rint decreases by more than 6 orders of magnitude when δ increases from 0.29 to 0.76 eV. It is also worth mentioning that the activation energy can be extracted from eq 1 via (λ + δ)2/4λ. The smaller δ and λ of 8TMe, as compared to those of (T1TNT1)4, can explain the decreased activation energy for NCS-nTEH-SCN observed in the experiment. The foregoing results indicate that the stiff and planar π-conjugated structures of nTEH realize narrow HOMO−LUMO energy gaps and small reorganization energies, leading to the lower injection barrier and higher hopping conductance. In conclusion, we successfully synthesized highly planar and completely insulated oligothiophenes with molecular lengths of up to ca. 10 nm. Photophysical and electrochemical measurements indicated that these oligomers possess high planarity and

completely insulated backbones. The electrical conductance of single-molecule junctions, determined using the STM-BJ method, showed that the conformational planarity of oligothiophene backbones led to significant differences in hopping charge transport: a smaller activation energy and larger electrical conductance in the hopping transport regime were observed as compared to those of the distorted oligothiophenes. As a result, crossover between tunneling and hopping conduction took place within a short length compared with NCS-Ted(T12TNT12)mTed-SCN, i.e., between the thiophene 8mer and 12-mer at 298 K. Theoretical studies indicated that the small reorganization energies and narrow HOMO−LUMO energy gaps derived from the planar oligothiophene backbones contribute to these superior characteristics, which provided new insights into the methodology for tuning the crossover length. These results demonstrate not only the intrinsic electrical characteristics of the oligothiophenes in the hopping conduction regime but also the potential of planar π-conjugated systems for application to both single-molecule and thin-film organic electronics. The realization of ideal 10 nm scale π-conjugated oligothiophene systems in the present study enables us to reveal the precise structure−conductance relationship and experimental determination of the localization length in the hopping regime in single-molecule junctions, which will be reported by our group in due course.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpclett.9b00747.

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Results of calculation, synthesis, GPLC, UV−vis, CV, conductance histograms, as well as calculated details (PDF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. *E-mail: [email protected]. *E-mail: [email protected]. *E-mail: [email protected]. 3202


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Yutaka Ie: 0000-0003-0208-4298 Tatsuhiko Ohto: 0000-0001-8681-3800 Ryo Yamada: 0000-0003-1761-0713 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by Grants-in-Aid for Scientific Research (B) (JP24350025) and Grant-in-Aid for Scientific Research on Innovative Areas (Molecular Architectonics, JP25110004, JP25110012) from the Ministry of Education, Culture, Sports, Science and Technology, Japan. We acknowledge Ms. Juan Juan Pang for synthesis support. Y.I. and R.Y. are grateful to the Tokuyama Science Foundation and the Murata Science Foundation, respectively, for the partial support of this study. Thanks are extended to the Comprehensive Analysis Center, ISIR, for assistance in obtaining elemental analyses.

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