Electrochemically Durable Thiophene Alkanethiol Self-Assembled

Article pubs.acs.org/Langmuir

Electrochemically Durable Thiophene Alkanethiol Self-Assembled Monolayers Taichi Ikeda,*,†,‡ Yuki Nagata,† Yijun Zheng,† Dian Liu,† Hans-Jürgen Butt,† and Masahiko Shimoda‡ †

Max Planck Institute for Polymer Research (MPI-P), Ackermannweg 10, D-55128 Mainz, Germany National Institute for Materials Science (NIMS), Namiki 1-1, Tsukuba, 305-0044 Japan

‡

S Supporting Information *

ABSTRACT: Thiophene-based redox-active self-assembled monolayers (SAMs) were prepared on gold substrates. The alkanethiol derivatives of 1TPh-OC12SH and ETPh-OC12SH contain thiophene (1T) and 3,4-ethylenedioxythiophene (ET) units, respectively, with unprotected (nonsubstituted) thiophene α-carbons. PhETPh-OC12SH contains the ET unit, and all thiophene carbons are protected. Using these thiophene alkanethiol derivatives, we characterized the effect of thiophene carbon protection on the redox behavior of the thiophene SAMs by cyclic voltammetry. The formation of SAMs was confirmed by X-ray photoelectron spectroscopy and reflective IR. The IR peaks in the fingerprint region were assigned with the help of DFT calculations. Although 1TPh-OC12SH and ETPh-OC12SH SAMs lost their electrochemical activity during the first anodic scan, PhETPh-OC12SH SAMs are stable and maintain their electrochemical activity for at least 1200 redox cycles.

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INTRODUCTION Since the first report in 1983,1 self-assembled monolayers (SAMs) have been intensively studied as a surface-modification method for metal surfaces.2−5 Recently, SAMs containing the redox-active unit (redox-active SAMs) have attracted much attention because of their potential applications in molecular electronics,6−9 solar cells,10−13 and sensors.14,15 Among the redox-active compounds, the thiophene derivative is one of the most important materials because of its well-established synthesis protocol and nice electrochemical and photophysical properties.16 For instance, thiophene SAM is a promising candidate for low-operating-voltage organic transisters.7 Therefore, thiophene SAMs have been studied more than other redox-active organic compounds. Tour et al. reported SAMs composed of molecular wires.17 In those examples, thiophene SAMs were also included, but no electrochemical property was reported. Liedberg et al. have conducted an electrochemical characterization of α-functionalized terthiophene SAMs.18 However, they could not obtain redox peaks of terthiophenes in CV because the experiments were conducted in 1.0 M HClO 4 aqueous solution. Michalitsch et al. prepared terthiophene alkanethiol SAMs on Pt and Au surfaces.19−21 They observed irreversible electrochemical reactions due to oxidation coupling between the surface-attached thiophenes (α−α coupling21 and β−β coupling20). A similar study on electrochemical polymerization in the thiophene SAMs was reported by Roncali et al.22 They obtained lower oxidation potentials after electrochemical polymerization as a result of the extended π-conjugation. Although they obtained reversible CV traces after the polymerization was complete, the chemical structure of the redox-active unit was unknown, for instance, the degree of polymerization, the ratio of α−α, α−β, and β−β © 2014 American Chemical Society

linkages, and so forth. Zhu et al. prepared thiophene SAMs in which oligothiophenes are attached via a tripod-shaped anchor unit.23,24 They reported that thiophene SAMs could improve the electroluminescence performance of organic light-emitting diodes. Tran et al. prepared thiophene SAMs in which quaterthiophene was doubly grafted on the surface.25 They introduced the ferrocene unit to the surface-attached quaterthiophene. In their CV analysis, the ferrocene unit showed intensive redox peaks, and the redox peak of the thiophene unit was broad and unclear. This result implied the difficulty of characterizing the redox property of thiophene SAMs as compared to that of ferrocene SAMs. Most of the thiophene derivatives in previous works have unprotected (nonsubstituted) thiophene α- and β-carbons. Unprotected thiophene carbons, especially α-carbons, are chemically active, and oxidation coupling between unprotected thiophene carbons takes places easily. These chemical reactions in the thiophene SAMs and the instability of the thiophene radical cation might be the reasons for broad and irreversible redox peaks in the CV analysis. In this study, we focused on the phenyl-capped thiophene derivatives in which thiophene αcarbons are protected by phenyl groups because the radical cation of phenyl-capped thiophene is quite stable.26 We synthesized an alkanethiol derivative containing a phenylcapped 3,4-ethylenedioxythiophene (ET) unit (PhETPhOC12SH, Figure 1) in which all thiophene carbons are protected by the substituents. In addition, we synthesized two kinds of model thiophene alkanethiols: 1TPh-OC12SH and Received: September 23, 2013 Revised: January 27, 2014 Published: February 6, 2014 1536

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1.79 (m, 2H), 3.64 (t, 2H), 3.98 (t, 2H), 6.90 (d, J = 8.8 Hz, 2H), 7.05 (dd, 2H), 7.16−7.23 (m, 2H), 7.52 (d, J = 8.8 Hz, 2H). 13C NMR (62.5 MHz, CDCl3): δ 26.1, 26.4, 29.6, 29.7, 29.8, 29.9, 33.1, 63.4, 68.4, 115.2, 122.3, 124.1, 127.4, 127.5, 128.2, 144.8, 159.1. 1b. 12-(4-Bromophenoxy)dodecane-1-ol (1.6 g, 4.5 mmol), 2-tri-nbutylstannyl-3,4-ethylenedioxy-thiophene (2.8 g), and Pd(PPh3)4 (350 mg, 0.30 mmol) were mixed in dry DMF (10 mL). The solution was deaerated in vacuo and refilled with N2 three times. The reaction mixture was stirred at 70 °C overnight. The solvent was removed under reduced pressure at 70 °C. The residue was directly subjected to column chromatography (SiO2 containing 10% K2CO3,29 CH2Cl2/ acetone = 100:5) to afford a white solid. Yield: 0.94 g (50%). 1H NMR (250 MHz, CDCl3): δ 1.20−1.50 (m, 16H), 1.57 (m, 2H), 1.78 (m, 2H), 3.64 (t, J = 6.5 Hz, 2H), 3.96 (t, J = 6.5 Hz, 2H), 4.20−4.32 (m, 4H), 6.23 (s, 1H), 6.89 (d, J = 9.0 Hz, 2H), 7.61 (d, J = 9.0 Hz, 2H). 13 C NMR (62.5 MHz, CDCl3): δ 26.1, 26.4, 29.9, 33.2, 63.4, 64.8, 65.0, 68.4, 96.7, 115.0, 117.8, 126.0, 127.7, 137.4, 142.5, 158.3. 1c. Compound 1c was synthesized using the same procedure as for compound 1b. Yield: 0.90 g (50%). 1H NMR (250 MHz, CDCl3): δ 1.20−1.65 (m, 18H), 1.79 (m, 2H), 3.64 (t, J = 6.5 Hz, 2H), 3.97 (t, J = 6.5 Hz, 2H), 4.34 (s, 4H), 6.91 (d, J = 9.0 Hz, 2H), 7.20 (t, J = 7.5 Hz, 1H), 7.37 (t, J = 7.5 Hz, 2H), 7.67 (d, J = 9.0 Hz, 2H), 7.74 (d, J = 7.5 Hz, 2H). 13C NMR (62.5 MHz, CDCl3): δ 26.1, 26.4, 29.6, 29.7, 29.8, 29.9, 33.1, 63.4, 64.9, 68.4, 114.4, 115.0, 115.8, 125.8, 126.3, 126.7, 127.8, 128.9, 133.4, 137.9, 138.9, 158.3. 2a. 1a (1.5 g, 4.2 mmol), triethylamine (TEA, 0.70 mL, 5.0 mmol), and 4-dimethylaminopyridine (DMAP, 30 mg, 0.25 mmol) were dissolved in dry CHCl3 (40 mL). p-Toluenesulfonyl chloride (TsCl, 0.83 g, 4.4 mmol) was added to the solution. The reaction mixture was stirred at room temperature for 16 h. After CHCl3 (100 mL) was added, the solution was washed with a 0.5 N HCl aqueous solution, a 1 N NaHCO3 aqueous solution, and a 1 N NaCl aqueous solution. The organic layer was dried with MgSO4, filtered, and concentrated by evaporation. The crude product was purified by column chromatography (SiO2, CH2Cl2) to give a white solid. Yield: 1.9 g (89%). 1H NMR (250 MHz, CDCl3): δ 1.15−1.53 (m, 16H), 1.63 (m, 2H), 1.79 (m, 2H), 2.45 (s, 3H), 3.92−4.07 (m 8H), 6.90 (d, J = 8.8 Hz, 2H), 7.05 (dd, 1H), 7.16−7.24 (m, 2H), 7.34 (d, J = 8.3 Hz, 2H), 7.52 (d, J = 8.8 Hz, 2H), 7.79 (d, J = 8.3 Hz, 2H). 13C NMR (62.5 MHz, CDCl3): δ 22.0, 25.6, 26.4, 29.1, 29.2, 29.6, 29.7, 29.8, 68.4, 71.0, 115.1, 122.3, 124.0, 127.4, 127.5, 128.2, 130.1, 133.6, 144.7, 144.9, 159.1. 2b. Compound 2b was prepared using the same procedure as for compound 2a. Yield: 0.70 g (85%). 1H NMR (250 MHz, CDCl3): δ 1.16−1.52 (m, 16H), 1.63 (m, 2H), 1.78 (m, 2H), 2.45 (s, 3H), 3.92− 4.06 (m, 4H), 4.20−4.32 (m, 4H), 6.23 (s, 1H), 6.89 (d, J = 9.0 Hz, 2H), 7.34 (d, J = 8.5 Hz, 2H), 7.61 (d, J = 9.0 Hz, 2H), 7.79 (d, J = 8.5 Hz, 2H). 13C NMR (62.5 MHz, CDCl3): δ 22.0, 25.7, 26.4, 29.1, 29.2, 29.6, 29.7, 29.8, 29.9, 64.8, 65.0, 68.4, 71.0, 96.7, 115.0, 117.8, 126.0, 127.7, 128.2, 130.1, 133.6, 137.4, 142.5, 144.9, 158.3. 2c. Compound 2c was prepared using the same procedure as for compound 2a. Yield: 0.70 g (89%). 1H NMR (250 MHz, CDCl3): δ 1.16−1.52 (m, 16H), 1.63 (m, 2H), 1.79 (m, 2H), 2.44 (s, 3H), 3.92− 4.08 (m, 4H), 4.35 (s, 4H), 6.91 (d, J = 9.0 Hz, 2H), 7.20 (t, J = 7.5 Hz, 1H), 7.29−7.42 (m, 4H), 7.66 (d, J = 9.0 Hz, 2H), 7.70−7.84 (m, 4H). 13C NMR (62.5 MHz, CDCl3): δ 22.0, 25.7, 26.4, 29.1, 29.3, 29.7, 29.8, 64.9, 68.4, 71.0, 114.4, 115.0, 115.8, 125.8, 126.3, 126.7, 127.8, 128.2, 128.9, 130.1, 133.4, 133.6, 137.9, 139.0, 144.9, 158.3. 1TPh-OC12SH. 2a (1.0 g, 1.9 mmol) and potassium thioacetate (AcSK, 0.45 g, 3.9 mmol) were mixed in MeOH (30 mL). The solution was deaerated in vacuo and refilled with N2 three times. The reaction mixture was stirred at 70 °C for 3 h. After the mixture was cooled to room temperature, KOH (0.20 g) was added to the reaction mixture. The reaction mixture was stirred at 70 °C for another 2 h. After the mixture was cooled to room temperature, CH2Cl2 was added to the solution and washed with saturated NaCl aqueous solutions (three times). The organic layer was dried with MgSO4, filtered, and concentrated by evaporation. The crude product was purified by column chromatography (SiO2, CH2Cl2/hexane = 7:3) to afford white solid. Yield: 0.70 g (96%). 1H NMR (250 MHz, CDCl3): δ 1.15−1.53

Figure 1. Chemical structures of thiophene alkanethiol derivatives 1TPh-OC12SH, ETPh-OC12SH, PhETPh-OC12SH and methoxytetra(ethylene glycol)-substituted model compounds 1TPh-TEG, ETPh-TEG, and PhETPh-TEG. The electrochemically active unit is depicted in red. The labeling scheme for the NMR assignment is also depicted.

ETPh-OC12SH (Figure 1). 1TPh-OC12SH has a thiophene (1T) unit with unprotected α- and β-carbons. ETPh-OC12SH contains a 3,4-ethylenedioxythiophene (ET) unit with an unprotected α-carbon. The redox behavior of these thiophene alkanethiol SAMs was characterized by CV. We confirmed the durability of the PhETPh-OC12SH SAMs by repetitive redox cycles over 1000 times.

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EXPERIMENTAL SECTION

General. All of chemicals were purchased and used without further purification. 12-(4-Bromophenoxy)dodecane-1-ol,27 2-phenyl-3,4-ethylenedioxythiophene,26 2-tri-n-butylstannyl-3,4-ethylenedioxythiophene,28 and 2-phenyl-5-tri-n-butylstannyl-3,4-ethylenedioxythiophene26 were synthesized according to the literature. The synthesis of the model compounds (1TPh-TEG, ETPh-TEG, and PhETPhTEG, Figure 1) is described in the Supporting Information. NMR spectra were recorded on an Avance 250 FT NMR (Bruker; 250 and 62.5 MHz for 1H and 13C nuclei, respectively) with residual solvent as the internal standard. Gold substrates were prepared on a silicon wafer (Si-Mat) by Ar plasma sputtering of a 5 nm titanium adhesion layer followed by 50 nm of gold using a MED 020 coating system (BALTEC). The surface roughness (root-mean-square value) of the gold substrate was confirmed to be 2.4 nm by atomic force microscopy measurements (Dimension 3100 scanning probe microscope, Veeco). XPS spectra were measured with a VG ESCALAB MkII spectrometer using Mg Kα radiation (hν = 1253.6 eV). The peak intensity was normalized on the basis of the C 1s peak (binding energy = 284.6 eV). Reflective IR spectra were recorded on a Magna 850 FTIR spectrometer (Nicolet Instrument) at room temperature under a dry nitrogen atmosphere. Synthesis. 1a. 12-(4-Bromophenoxy)dodecane-1-ol (2.2 g, 6.2 mmol), thiophene-2-boronic acid (1.1 g, 8.6 mmol), Pd(PPh3)4 (0.40 g, 0.35 mmol), and K2CO3 (3.6 g, 26 mmol) were mixed in dry DMF (20 mL). The solution was deaerated in vacuo and refilled with N2 three times. The reaction mixture was stirred at 80 °C for 16 h. After the mixture was cooled to room temperature, CH2Cl2 was added. The solution was washed with 1 N NaCl aqueous solution (three times). The organic layer was dried with MgSO4, filtered, and concentrated by evaporation. The crude product was purified by column chromatography (SiO2, CH2Cl2/acetone = 100:6) to afford a white solid. Yield: 2.1 g (92%). 1H NMR (250 MHz, CDCl3): δ 1.20−1.64 (m, 18H), 1537

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(m, 16H), 1.62 (m, 2H), 1.80 (m, 2H), 2.52 (dd, J = 14.6, 7.4, 2H), 3.98 (t, J = 6.5 Hz, 2H), 6.90 (d, J = 8.8 Hz, 2H), 7.05 (dd, J = 5.0, 3.7 Hz, 1H), 7.17−7.23 (m, 2H), 7.53 (d, J = 8.8 Hz, 2H). 13C NMR (62.5 MHz, CDCl3): δ 25.0, 26.4, 28.7, 29.4, 29.6, 29.7, 29.8, 29.9, 34.4, 68.4, 115.2, 122.3, 124.1, 127.4, 127.5, 128.2, 144.8, 159.1. ETPh-OC12SH. This compound was prepared using the same procedure as 1TPh-OC12SH. Yield: 0.35 g (92%). 1H NMR (250 MHz, CDCl3): δ 1.24−1.52 (m, 16H), 1.61 (m, 2H), 1.78 (m, 2H), 2.52 (q, J = 7.3 Hz, 2H), 3.96 (t, J = 6.5 Hz, 2H), 4.20−4.32 (m, 4H), 6.23 (s, 1H), 6.89 (d, J = 9.0 Hz, 2H), 7.61 (d, J = 9.0 Hz, 2H). 13C NMR (62.5 MHz, CDCl3): δ 25.0, 26.4, 28.7, 29.4, 29.6, 29.7, 29.8, 29.9, 34.4, 64.8, 65.0, 68.4, 96.7, 115.0, 117.8, 126.0, 127.7, 137.4, 142.5, 158.3. PhETPh-OC12SH. This compound was prepared using the same procedure as for 1TPh-OC12SH, but EtOH was used as a solvent. Yield: 0.36 g (76%). 1H NMR (250 MHz, CDCl3): δ 1.20−1.68 (m, 18H), 1.79 (m, 2H), 2.53 (q, J = 7.5 Hz, 2H), 3.98 (t, J = 6.5 Hz, 2H), 4.35 (s, 4H), 6.91 (d, J = 9.0 Hz, 2H), 7.21 (t, J = 7.5 Hz, 1H), 7.37 (t, J = 7.5 Hz, 2H), 7.66 (d, J = 9.0 Hz, 2H), 7.74 (d, J = 7.5 Hz, 2H). 13C NMR (62.5 MHz, CDCl3): δ 25.0, 26.4, 28.7, 29.4, 29.6, 29.7, 29.9, 34.4, 64.9, 65.0, 68.4, 114.4, 115.0, 115.8, 125.8, 126.3, 126.7, 127.8, 128.9, 133.4, 137.9, 139.0, 158.4. SAM Preparation. Gold substrates were cleaned with a UV-ozone cleaner for 30 min just before functionalization. The gold substrate was immersed in a 1 mM thiophene alkanethiol solution for 24 h. Toluene was chosen as a solvent. Afterward, the functionalized gold substrate was rinsed with toluene, CHCl3, and ethanol in order to remove excess adsorbate. Then it was dried in a stream of N2 gas to remove residual solvent. DFT Calculation for IR Assignment. DFT calculations were performed by using the Gaussian 03 package program, and the frequencies of the molecules were calculated at the B3LYP/cc-pVTZ level of theory. To reduce the computational cost, we used 1TPh-OC2, ETPh-OC2, and PhETPh-OC2 instead of 1TPh-OC12SH, ETPhOC12SH, and PhETPh-OC12SH, respectively. Electrochemical Analysis. Electrochemical measurements were carried out at room temperature with a PGSTAT12 (Metrohm Autolab) potentiostat. In the case of the model compounds, CV was performed in argon-purged MeCN solution containing a 1.0 mM sample and a 0.1 M supporting electrolyte. The working electrode was a glassy carbon electrode (surface area = 0.07 cm2, ALS Co.). Its surface was polished routinely with a 0.05 μm alumina/water slurry on a felt surface before use. In the case of thiophene alkanethiol SAMs, a functionalized gold substrate was used as a working electrode and characterized in the argon-purged solution containing 0.1 M supporting electrolyte using a plate material evaluating cell (ALS Co.). The counter and reference electrodes were a platinum wire and a saturated calomel electrode (SCE), respectively (ALS Co.). Tetrabutylammonium hexafluorophosphate (TBA·PF6, electrochemistry grade, Sigma-Aldrich Co.) was used as a supporting electrolyte. The half-wave potential (E1/2) was calculated from the differential pulse voltammetry (DPV) by using E1/2 = Emax + ΔE/2.30 Emax and ΔE are the DPV peak top value and the pulse height, respectively. ΔE was set to 50 mV.

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Scheme 1. Reaction Scheme of Thiophene Alkanethiol Derivatives 1TPh-OC12SH, ETPh-OC12SH, and PhETPhOC12SH

Figure 2. 1H NMR spectra of (a) 1TPh-OC12SH, (b) ETPh-OC12SH, and (c) PhETPh-OC12SH. Aromatic peaks are highlighted in red.

compounds are shown in Figure 2. All of the aromatic protons and methylene protons were assignable. In the case of ETPhOC12SH (Figure 2b), the methylene protons of the ET unit were split into two peaks (4.3 ppm) because of the unsymmetrical structure of the compound. However, these protons of PhETPh-OC12SH gave a singlet peak because these protons experience similar local magnetic fields from the phenyl rings attached on both sides of the thiophene ring. The model compounds having the same aromatic unit were also synthesized (1TPh-TEG, ETPh-TEG, and PhETPh-TEG; Figure 1). In the case of the model compounds, methoxyterminated tetra(ethylene glycol) is attached to the phenylthiophene units. Tetra(ethylene glycol) suppresses the physical adhesion to the surface of the glassy carbon electrode. Electrochemical Properties of Model Compounds. Figure 3a shows cyclic voltammograms of the model compounds. 1TPh-TEG and ETPh-TEG show one irreversible peak at around +1.3 and +1.0 V, respectively. In the case of

RESULTS AND DISCUSSION

Synthesis. The reaction scheme for the thiophene alkanethiol compounds (1TPh-OC12SH, ETPh-OC12SH, and PhETPh-OC12SH) is shown in Scheme 1. The 1T and ET units were introduced into 12-(4-bromophenoxy)dodecane-1-ol by Suzuki and Stille couplings, respectively. After the conversion of the hydroxyl group to the tosyl group, the thiophene compounds were reacted with potassium thioacetate. After hydrolysis, we could remove the disulfide compound (R−S−S− R) by SiO2 column chromatography. We failed to synthesize a PhETPh-OC12SH analogue with a 1T unit instead of an ET unit (Ph1TPh-OC12SH) because of the low solubility of the product. 1H NMR spectra of the thiophene alkanethiol 1538

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Figure 3. (a) Cyclic voltammograms of the solutions containing model compounds 1TPh-TEG, ETPh-TEG, and PhETPh-TEG. Sample concentration, 1 mM; electrolyte solution, 0.1 M TBA·ClO4 in MeCN; scan rate, 0.1 V s−1; reference electrode, SCE. (b) Scan rate dependency of PhETPh-TEG cyclic voltammograms (0.05, 0.10. 0.15, 0.20, 0.25, and 0.30 V s−1). (Inset) Relationship between the square root of the scan rate and the current intensity of the anodic peak.

PhETPh-TEG, we observed one reversible peak at around +1.0 V and another irreversible peak at around +1.4 V. The halfwave potentials of the first and second redox processes were determined to be +0.95 and +1.34 V, respectively, by DPV. This result indicates that the radical cation of the PhETPh unit is stable whereas the dication is unstable. Figure 3b shows the scan rate dependency of PhETPh-TEG cyclic voltammogram. We confirmed a linear relationship between the first anodic peak intensity and the square root of the scan rate. This result indicates that the current is limited by the diffusion of PhETPhTEG in the solution.30 XPS Analysis of SAMs. The binding energies of S 2p, O 1s, and C 1s core-level photoemission peaks of 1TPh-OC12SH, ETPh-OC12SH, PhETPh-OC12SH, and C12H25SH SAMs are summarized in Table 1. Figure 4a shows the XPS spectrum of

Figure 4. XPS spectra for (a) the S 2p region of 1TPh-OC12SH SAMs (decomposed peaks: red, thiophene sulfur; green, gold-attached alkanethiol sulfur). (b) O 1s region of ETPh-OC12SH (red) and 1TPh-OC12SH (blue) SAMs. (c) C 1s region of 1TPh-OC12SH (blue) and PhETPh-OC12SH SAMs (green).

C12H25SH SAMs (162.2 and 163.4 eV).31 Therefore, this peak pair was assigned to the thiol chemisorbed on the gold surface. Another peak pair at 163.9 and 165.1 was assigned to the sulfur in the thiophene ring.18,25,32 The intensity ratio of the two components, thiophene and thiol sulfurs, was calculated to be 3.2:1.0, which does not match the chemical composition. This result is caused by the inelastic scattering of photoelectrons, which gives rise to a larger attenuation of the photoemission signal from the atoms in deeper sites. The result is consistent with the fact that the thiophene and thiol sulfurs stay on the surface and at the bottom of the SAMs, respectively.18 Figure 4b shows the XPS spectra of the O 1s region for 1TPh-OC12SH and ETPh-OC12SH. The difference in peak intensities of these SAMs corresponds to the difference in the chemical composition. XPS did not afford different binding energies for the oxygens of phenol and ET units. We confirmed that C12H25SH SAMs gave no XPS peak in O 1s region. Figure 4c shows XPS spectra of the C 1s region for 1TPhOC12SH and PhETPh-OC12SH SAMs. The XPS peak of 1TPhOC12SH SAMs had a small shoulder at higher binding energy. This shoulder was more pronounced in the case of PhETPhOC12SH SAMs. It was assigned to the carbons next to the oxygen.33,34 C12H25SH SAMs showed a single peak without a

Table 1. Summary of XPS Data of SAMs on a Au Substrate binding energy, eV compound 1TPh-OC12SH ETPh-OC12SH PhETPhOC12SH C12H25SH

S 2p3/21

S 2p1/21

S 2p3/22

S 2p1/22

C 1s-1

O 1s

163.9 163.9 164.0

165.1 165.1 165.2

162.1 161.9 161.8

163.3 163.1 163.0

284.6 284.6 284.6

532.9 532.9 532.7

162.2

163.4

284.6

the S 2p region for 1TPh-OC12SH SAMs. The observed spectrum can be decomposed into four individual peaks. Because the S 2p peak shows spin−orbit splitting consisting of a high-intensity S 2p3/2 peak at lower energy and a low-intensity S 2p1/2 peak at higher energy with an intensity ratio of 2:1, four peaks suggest that XPS detected two components corresponding to two kinds of sulfur atoms.17,18,25,31,32 The peak pair at 162.1 and 163.3 eV is consistent with that observed in 1539

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shoulder in the C 1s region. All of these XPS data support the formation of thiophene alkanethiol SAMs. Reflective IR Measurement of SAMs. Although XPS provides information on the elemental composition of SAMs, reflective IR measurements combined with DFT calculations help to confirm the chemical structures of the compounds in SAMs.35,36 Table 2 summarizes the vibrational frequencies of Table 2. Experimental and Calculated Vibrational Frequencies of Reflective IR Peaks for Thiophene Alkanethiol SAMs on Gold PhETPhOC12SH

1TPh-OC12SH

ETPh-OC12SH

exptl

calcd

exptl

calcd

exptl

calcd

1180

1204

1182

1209

1182 1254b

1208 1261

1255

1276 1254

1278

1254b

1279

1292

1321

1365

1393

1292 1286 1360

1320 1323 1392

1392 1416 1442

1428 1449 1461

1394 1416 1435

1427 1449 1461

1450 1475 1502 1524

1517 1525 1537 1554

1574 1600 1608 2850 2922

1622 1642 1652

1263 1290

1394

1426

1466

1473

1475

1524

1491 1473

1520 1527

1504 1535

1544 1607

1520 1572

1553 1602

1608 2850 2922 a b

1285 1321

1652

1608 2854 2927

1653

Figure 5. Reflective IR spectra of (a) the high-frequency region and (b) the fingerprint region: blue, 1TPh-OC12SH SAMs; red, ETPhOC12SH SAMs; and green, PhETPh-OC12SH SAMs.

assignment vibrating unit phenol phenol + phenyl phenol + 1T phenol phenol + 1T all aromatic all aromatic ET methylene alkyl phenol + ET phenol + ET phenol + 1T all aromatic alkyl phenyl phenol phenol all aromatic phenyl phenol alkyl alkyl

νia

and 2915 cm−1) and the liquid state (2856 and 2928 cm−1).40,41 Therefore, the alkyl chain conformation in 1TPh-OC12SH SAMs is considered to be partially disordered. The frequencies of νs and νas for ETPh-OC12SH SAMs were 2854 and 2927 cm−1, respectively. These frequencies are close to those for the liquid state, suggesting that the alkyl chain conformation in ETPh-OC12SH SAMs is completely disordered. This is because the 3,4-ethylenedioxy substituent of the ET unit disturbs the lateral molecular packing in SAMs. The frequencies of νs and νas for PhETPh-OC12SH SAMs were 2850 and 2922 cm−1, respectively, which are comparable to those in 1TPh-OC12SH SAMs. The thiophene alkanethiol SAMs in this study have lower crystallinity than conventional alkanethiol SAMs because of the bulkiness of the thiophene units.38,39 Figure 5b shows partial reflective IR spectra in the fingerprint region. We assigned IR peaks on the basis of the vibration mode of the alkyl chain. All of the thiophene alkanethiol SAMs in our experiment had peaks at 1392−1394 (ν9) and 1473− 1475 cm−1 (ν14). These peaks are comparable to those observed at 1383 and 1468 cm−1 for alkanethiol SAMs.38 The number of observed peaks agrees with the number of the calculated peaks with large IR intensities. These results support the formation of thiophene alkanethiol SAMs. Electrochemical Analysis of SAMs. We conducted CV measurement using a thiophene alkanethiol-functionalized gold substrate as a working electrode. In the case of 1TPh-OC12SH SAMs (Figure 6a), a large oxidation peak was observed in the first anodic scan. This redox reaction was irreversible, and the oxidation peak disappeared in the second anodic scan. The peak top potential in the first anodic scan was +1.23 V, which is close to the peak top potential of model compound 1TPh-TEG observed in the solution (+1.26 V). Therefore, this oxidation

ν1 ν2 ν3 ν4 ν5 ν6 ν7 ν8 ν9 ν10 ν11 ν12 ν13 ν14 ν15 ν16 ν17 ν18 ν19 ν20 νas νs

See the Supporting Information for the vibrational modes in detail. Overlap of the peaks.

the experimental and calculated IR spectra, and the normal modes of the vibrations are depicted in the Supporting Information. Because high-frequency modes are sensitive to the element of the chemical structures, we focused on the vibrational modes above 1150 cm−1. A slight deviation is detectable between the calculated and the experimentally observed values in the intermediate frequency region (∼1600 cm−1). It is known that the B3LYP level of the theory overestimates the frequencies in this region. Therefore, the frequency scale factor of 0.9691 has been proposed for B3LYP/ cc-pVTZ-level calculations.37 The scaled frequencies obtained from the DFT calculation are in good agreement with the experiments. Figure 5a shows partial reflective IR spectra in the highfrequency region. Two peaks in this region were assignable to the symmetric (νs) and asymmetric (νas) C−H stretching modes of the alkyl groups (Table 2).18,38,39 The frequencies of these modes relate to the conformation of the alkyl chain.18,40,41 The frequencies of νs and νas for 1TPh-OC12SH SAMs were 2850 and 2922 cm−1, respectively. These are between the expected frequencies in the crystalline state (2846

Figure 6. Cyclic voltammograms of (a) 1TPh-OC12SH SAMs (first scan, solid curve; second scan, dot curve) and (b) ETPh-OC12SH SAMs (first scan, solid curve; second scan, dotted curve). Electrolyte solution, 0.1 M TBA·ClO4 in MeCN; reference electrode, SCE; scan rate, 0.1 V s−1; area of SAMs, 0.636 cm2. 1540

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solution (+0.95 V). The reason for the lower oxidation potential in SAMs is attributable to the change in the effective conjugation length as mentioned above. As long as we confined the scan range between 0.0 and +1.0 V, PhETPh-OC12SH SAMs were stable. We confirmed no deterioration of the redox activity after 1200 redox cycles (Figure S1 in Supporting Information). When we increased the maximum potential to +1.4 V, we observed an irreversible oxidation peak above +1.3 V, and the redox peak of the PhETPh units disappeared in successive scans. This result indicates that the oxidation desorption of the PhETPh-OC12SH SAMs takes place above +1.3 V.42 The first anodic peak intensity increased linearly with the scan rate (Figure 7b). This result indicates that the electrochemically active PhETPh unit is attached to the gold surface.30 From the average value of the anodic and cathodic peak areas, the surface coverage of PhETPh-OC12SH was calculated to be (1.5 ± 0.2) × 10−10 mol cm−2 (9.0 × 1013 molecules cm−2, 111 Å2 per molecule). This surface coverage is much smaller than that reported for the terthiophene-alkanethiol SAMs on Pt (4.5 × 10−10 mol cm−2).19 The bulky 3,4-ethylenedioxy substituent and the twisted conformation of the aromatic rings might result in a low surface density of the PhETPh-OC12SH SAMs.

peak is assignable to the oxidation of the 1TPh unit in the 1TPh-OC12SH SAMs. No oxidation peak in the second scan indicates that the 1TPh unit has decomposed during the first scan. The redox behavior of ETPh-OC12SH SAMs was similar to that of 1TPh-OC12SH SAMs except for the fact that the oxidation took place at a lower potential in the first anodic scan (Figure 6b). The peak top potential in the first anodic scan was +0.85 V, which is significantly smaller than that of model compound ETPh-TEG observed in the solution (+1.05 V). Compared to the electrochemical reaction in solution, a lower peak top potential in SAMs had been reported by other researchers.20 Because the redox reaction of the compound dissolved in the solution is limited by diffusion, its peak top potential shifts to a higher oxidation potential as compared to that of the species adsorbed on the surface.30 In addition, the change in the effective conjugation length of the aromatic rings could also affect the oxidation potential. The thiophene and phenyl rings do not form a coplanar conformation because of the steric hindrance between the phenyl hydrogen and 3,4ethylenedioxy substituent (Supporting Information). The dihedral angle between the planes of the thiophene and phenyl rings would be fixed to a smaller value in SAMs because of the molecular interaction with the surrounding molecules, which may lead to the extension of π-conjugation and a lower oxidation potential. In the case of PhETPh-OC12SH SAMs, we could observe reversible redox peaks (Figure 7a). The first and second CV traces were almost identical. From the average peak top values of anodic and cathodic scans, the half-wave potential of PhETPh-OC12SH in SAMs was calculated to be +0.69 V. This value is much smaller than that of the first oxidation potential of the model compound PhETPh-TEG observed in the

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CONCLUSIONS We have observed a reversible redox peak in the CV measurement for PhETPh-OC12SH SAMs. PhETPh-OC12SH SAMs showed no deterioration after 1200 redox cycles. This result is in contrast to the results obtained for 1TPh-OC12SH and ETPh-OC12SH SAMs, in which they lost their electrochemical activity during the first scan. These results indicate that the protection (substitution) of the thiophene carbons is effective for obtaining durable electrochemically active thiophene SAMs. However, our molecular design presented here decreases the surface density of SAMs because of the bulky side group and twisted conformation between the aromatic rings. Optimizing the molecular design to fulfill two conditions (i.e., the electrochemical durability and high surface density) will be the theme of a future study.

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ASSOCIATED CONTENT

S Supporting Information *

Synthesis of model compounds, Cartesian coordinates of all optimized structures, vibrational modes obtained by DFT calculations, and results of the durability test. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Fax: +81-29-860-4721. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Helma Burg and Uwe Rietzler at the Max Planck Institute for Polymer Research and Katsumi Ohno at the National Institute for Materials Science for the support of gold substrate preparation.

Figure 7. (a) Cyclic voltammograms of PhETPh-OC12SH SAMs (first scan, solid curve; second scan, dot curve). Electrolyte solution, 0.1 M TBA·ClO4 in CH2Cl2; reference electrode, SCE; scan rate, 0.1 V s−1; area of SAMs, 0.636 cm2. (b) Scan rate dependency of cyclic voltammograms for PhETPh-OC12SH SAMs (0.05, 0.10. 0.15, 0.20, 0.25, and 0.30 V s−1). (Inset) Relationship between the scan rate and current intensity of the anodic peak.

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REFERENCES

(1) Nuzzo, R. G.; Allara, D. L. Adsorption of bifunctional organic disulfides on gold surfaces. J. Am. Chem. Soc. 1983, 105, 4481−4483.

1541

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derivatives, through electrochemical intermolecular coupling. J. Electroanal. Chem. 1998, 457, 129−139. (21) Michalitsch, R.; Nogues, C.; Najari, A.; El Kassmi, A.; Yassar, A.; Lang, P.; Garnier, F. β-Functionalized oligothiophenes for molecular self-assembly. Synth. Met. 1999, 101, 5−6. (22) Oçafrain, M.; Tran, T. K.; Blanchard, P.; Lenfant, S.; Godey, S.; Vuillaume, D.; Roncali, J. Electropolymerized self-assembled monolayers of a 3,4-ethylenedioxythiophene-thiophene hybrid system. Adv. Funct. Mater. 2008, 18, 2163−2171. (23) Zhu, L. H.; Tang, H. Q.; Harima, Y.; Yamashita, K.; Hirayama, D.; Aso, Y.; Otsubo, T. Electrochemical properties of self-assembled monolayers of tripod-shaped molecules and their applications to organic light-emitting diodes. Chem. Commun. 2001, 1830−1831. (24) Zhu, L. H.; Tang, H. Q.; Harima, Y.; Yamashita, K.; Aso, Y.; Otsubo, T. Enhanced hole injection in organic light-emitting diodes consisting of self-assembled monolayer of tripod-shaped pi-conjugated thiols. J. Mater. Chem. 2002, 12, 2250−2254. (25) Tran, T. K.; Bricaud, Q.; Ocafrain, M.; Blanchard, P.; Roncali, J.; Lenfant, S.; Godey, S.; Vuillaume, D.; Rondeau, D. Thiolate chemistry: a powerful and versatile synthetic tool for immobilization/functionalization of oligothiophenes on a gold surface. Chem.Eur. J. 2011, 17, 5628−5640. (26) Apperloo, J. J.; Groenendaal, L.; Verheyen, H.; Jayakannan, M.; Janssen, R. A. J.; Dkhissi, A.; Beljonne, D.; Lazzaroni, R.; Bredas, J. L. Optical and redox properties of a series of 3,4-ethylenedioxythiophene oligomers. Chem.Eur. J. 2002, 8, 2384−2396. (27) Zheng, Y.; Ohto, T.; Liu, D.; Butt, H.-J.; Ikeda, T. Optical and redox properties of phenyl-capped cyclohexa[c]-oligothiophenes. Synth. Met. 2013, 181, 1−9. (28) Ikeda, T.; Higuchi, M.; Kurth, D. G. From thiophene [2]rotaxane to polythiophene polyrotaxane. J. Am. Chem. Soc. 2009, 131, 9158−9159. (29) Harrowven, D. C.; Curran, D. P.; Kostiuk, S. L.; Wallis-Guy, I. L.; Whiting, S.; Stenning, K. J.; Tang, B. C.; Packard, E.; Nanson, L. Potassium carbonate-silica: a highly effective stationary phase for the chromatographic removal of organotin impurities. Chem. Commun. 2010, 46, 6335−6337. (30) Bard, A. J.; Faulkner, L. R. Electrochemical Methods: Fundamentals and Applications, 2nd ed.; Wiley: New York, 2002. (31) Castner, D. G.; Hinds, K.; Grainger, D. W. X-ray photoelectron spectroscopy sulfur 2p study of organic thiol and disulfide binding interactions with gold surfaces. Langmuir 1996, 12, 5083−5086. (32) Ahn, H.; Kim, M.; Sandman, D. J.; Whitten, J. E. Self-assembled monolayers of ω-(3-thienyl)alkanethiols on gold. Langmuir 2003, 19, 5303−5310. (33) Schmidt, R.; McNellis, E.; Freyer, W.; Brete, D.; Giessel, T.; Gahl, C.; Reuter, K.; Weinelt, M. Azobenzene-functionalized alkanethiols in self-assembled monolayers on gold. Appl. Phys. A 2008, 93, 267−275. (34) Okpalugo, T. I. T.; Papakonstantinou, P.; Murphy, H.; McLaughlin, J.; Brown, N. M. D. High resolution XPS characterization of chemical functionalised MWCNTs and SWCNTs. Carbon 2005, 43, 153−161. (35) Lambie, B.; Ramaekers, R.; Maes, G. Conformational behavior of serine: An experimental matrix-isolation FT-IR and theoretical DFT(B3LYP)/6-31++G** study. J. Phys. Chem. A 2004, 108, 10426− 10433. (36) Abbott, L. C.; Batchelor, S. N.; Oakes, J.; Gilbert, B. C.; Whitwood, A. C.; Smith, J. R. L.; Moore, J. N. Experimental and computational studies of structure and bonding in parent and reduced forms of the azo dye orange II. J. Phys. Chem. A 2005, 109, 2894− 2905. (37) Sinha, P.; Boesch, S. E.; Gu, C. M.; Wheeler, R. A.; Wilson, A. K. Harmonic vibrational frequencies: scaling factors for HF, B3LYP, and MP2 methods in combination with correlation consistent basis sets. J. Phys. Chem. A 2004, 108, 9213−9217. (38) Nuzzo, R. G.; Dubois, L. H.; Allara, D. L. Fundamental studies of microscopic wetting on organic surfaces. 1. Formation and

(2) Ulman, A. Formation and structure of self-assembled monolayers. Chem. Rev. 1996, 96, 1533−1554. (3) Love, J. C.; Estroff, L. A.; Kriebel, J. K.; Nuzzo, R. G.; Whitesides, G. M. Self-assembled monolayers of thiolates on metals as a form of nanotechnology. Chem. Rev. 2005, 105, 1103−1169. (4) Vericat, C.; Vela, M. E.; Benitez, G.; Carro, P.; Salvarezza, R. C. Self-assembled monolayers of thiols and dithiols on gold: new challenges for a well-known system. Chem. Soc. Rev. 2010, 39, 1805− 1834. (5) Gooding, J. J.; Ciampi, S. The molecular level modification of surfaces: from self-assembled monolayers to complex molecular assemblies. Chem. Soc. Rev. 2011, 40, 2704−2718. (6) Aswal, D. K.; Lenfant, S.; Guerin, D.; Yakhmi, J. V.; Vuillaume, D. Self assembled monolayers on silicon for molecular electronics. Anal. Chim. Acta 2006, 568, 84−108. (7) Mottaghi, M.; Lang, P.; Rodriguez, F.; Rumyantseva, A.; Yassar, A.; Horowitz, G.; Lenfant, S.; Tondelier, D.; Vuillaume, D. Lowoperating-voltage organic transistors made of bifunctional selfassembled monolayers. Adv. Funct. Mater. 2007, 17, 597−604. (8) Smits, E. C. P.; Mathijssen, S. G. J.; van Hal, P. A.; Setayesh, S.; Geuns, T. C. T.; Mutsaers, K.; Cantatore, E.; Wondergem, H. J.; Werzer, O.; Resel, R.; Kemerink, M.; Kirchmeyer, S.; Muzafarov, A. M.; Ponomarenko, S. A.; de Boer, B.; Blom, P. W. M.; de Leeuw, D. M. Bottom-up organic integrated circuits. Nature 2008, 455, 956−959. (9) Halik, M.; Hirsch, A. The potential of molecular self-assembled monolayers in organic electronic devices. Adv. Mater. 2011, 23, 2689− 2695. (10) Imahori, H.; Norieda, H.; Yamada, H.; Nishimura, Y.; Yamazaki, I.; Sakata, Y.; Fukuzumi, S. Light-harvesting and photocurrent generation by cold electrodes modified with mixed self-assembled monolayers of boron-dipyrrin and ferrocene-porphyrin-fullerene triad. J. Am. Chem. Soc. 2001, 123, 100−110. (11) Hirayama, D.; Takimiya, K.; Aso, Y.; Otsubo, T.; Hasobe, T.; Yamada, H.; Imahori, H.; Fukuzumi, S.; Sakata, Y. Large photocurrent generation of gold electrodes modified with [60]fullerene-linked oligothiophenes bearing a tripodal rigid anchor. J. Am. Chem. Soc. 2002, 124, 532−533. (12) Kim, K. S.; Kang, M. S.; Ma, H.; Jen, A. K. Y. Highly efficient photocurrent generation from a self-assembled monolayer film of a novel C-60-tethered 2,5-dithienylpyrrole triad. Chem. Mater. 2004, 16, 5058−5062. (13) Shirai, Y.; Cheng, L.; Chen, B.; Tour, J. M. Characterization of self-assembled monolayers of fullerene derivatives on gold surfaces: implications for device evaluations. J. Am. Chem. Soc. 2006, 128, 13479−13489. (14) Ronkainen, N. J.; Halsall, H. B.; Heineman, W. R. Electrochemical biosensors. Chem. Soc. Rev. 2010, 39, 1747−1763. (15) Samanta, D.; Sarkar, A. Immobilization of bio-macromolecules on self-assembled monolayers: methods and sensor applications. Chem. Soc. Rev. 2011, 40, 2567−2592. (16) Igor, F. P.; Dmitrri, F. P. Handbook of Thiophene-Based Materials Applications in Organic Electronics and Photonics, John Wiley & Sons: Chichester, U.K., 2009. (17) Tour, J. M.; Jones, L.; Pearson, D. L.; Lamba, J. J. S.; Burgin, T. P.; Whitesides, G. M.; Allara, D. L.; Parikh, A. N.; Atre, S. V. Selfassembled monolayers and multilayers of conjugated thiols, α,ωdithiols, and thioacetyl-containing adsorbates − understanding attachments between potential molecular wires and gold surfaces. J. Am. Chem. Soc. 1995, 117, 9529−9534. (18) Liedberg, B.; Yang, Z.; Engquist, I.; Wirde, M.; Gelius, U.; Gotz, G.; Bauerle, P.; Rummel, R. M.; Ziegler, C.; Gopel, W. Self-assembly of α-functionalized terthiophenes on gold. J. Phys. Chem. B 1997, 101, 5951−5962. (19) Michalitsch, R.; Lang, P.; Yassar, A.; Nauer, G.; Garnier, F. Properties of self-assembled monolayers (SAMs) from thiol-functionalized oligothiophenes. Adv. Mater. 1997, 9, 321−326. (20) Michalitsch, R.; El Kassmi, A.; Yassar, A.; Lang, P.; Garnier, F. Increase of conjugation length within SAMs of terthiophene1542

dx.doi.org/10.1021/la403689e | Langmuir 2014, 30, 1536−1543

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Article

structural characterization of a self-consistent series of polyfunctional organic monolayers. J. Am. Chem. Soc. 1990, 112, 558−569. (39) Laibinis, P. E.; Whitesides, G. M.; Allara, D. L.; Tao, Y. T.; Parikh, A. N.; Nuzzo, R. G. Comparison of the structures and wetting properties of self-assembled monolayers of n-alkanethiols on the metal-surfaces, Cu, Ag, Au. J. Am. Chem. Soc. 1991, 113, 7152−7167. (40) Snyder, R. G.; Strauss, H. L.; Elliger, C. A. C-H stretching modes and the structure of n-alkyl chains. 1. Long, disordered chains. J. Phys. Chem. 1982, 86, 5145−5150. (41) Macphail, R. A.; Strauss, H. L.; Snyder, R. G.; Elliger, C. A. C-H stretching modes and the structure of n-alkyl chains. 2. Long, all-trans chains. J. Phys. Chem. 1984, 88, 334−341. (42) Widrig, C. A.; Chung, C.; Porter, M. D. The electrochemical desorption of n-alkanethiol monolayers from polycrystalline Au and Ag electrodes. J. Electroanal. Chem. 1991, 310, 335−359.

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