Alignment of photoconductive self-assembled fibers composed of π-conjugated molecules under electric fields

Abstract

Aligned self-assembled fibers have been obtained for a phenylbithiophene derivative with a laterally fluorinated phenylene group by the application of electric fields. These fibrous aggregates exhibit photoconductive properties due to the stacking of the π-conjugated moiety. In contrast, a phenylbithiophene derivative without fluoro substituents forms unaligned fibers under electric fields. The dielectric anisotropy of the fluorinated molecule plays a key role in the alignment of photoconductive fibrous aggregates.

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Introduction

Self-assembly of π-conjugated molecules in solution can lead to the formation of one-dimensional supramolecular functional fibers.^1–8 These self-assembled fibers have received considerable attention because of great potentials for organic electronic materials. To develop these materials, the alignment of electro-active molecular fibers is of great importance.^2a–b However, it is difficult to arrange the fibers in an orderly manner by a simple solution casting due to the formation of webs of randomly entangled fibers. Self-assembled fibers have been aligned by several techniques such as the use of liquid crystals as anisotropic templetes,⁹ the applications of shear flow,¹⁰ magnetic fields,¹¹ and electric fields.¹² Among these methods, the alignment of fibrous aggregates under electric fields is considered to be versatile and useful for obtaining electro-active materials and devices. However, examples of aligned fibrous aggregates under electric fields^2a,4,12 are still limited mainly due to the thermal fluctuation of molecules in solvents. Stupp and coworkers prepared oligothiophene-based fibrous aggregates that aligned in electric fields and measured the conductivities of doped samples.^2a

Recently, we have succeeded⁴ in the electric field-assisted alignment of fibrous aggregates of hydrogen-bonded amide compounds containing a laterally fluorinated rod-shaped mesogen, a 2,3-difluoro-4-[4-(trans-4-pentylcyclohexyl)phenyl]phenyloxy derivative. These compounds exhibit negative dielectric anisotropy. The aligned fibers that bridged gold electrodes deposited on a glass substrate were obtained by applying alternating current (AC) electric fields in dodecylbenzene, while the randomly entangled fibers were formed without electric fields.

Our intention was to prepare electro-active aligned self-assembled fibers by using an electric field-assisted molecular self-assembly process. Our design was to incorporate lateral polar substituents onto the π-conjugated rigid rod part of a hydrogen-bonded molecule to facilitate electric field-assisted self-assembly. Herein, we report on the self-assembly of π-conjugated compound 1a in an organic solvent under electric fields to allow the formation of aligned photoconductive molecular fibers (Fig. 1).

Fig. 1 Molecular structures of phenylbithiophene derivatives 1a and b.

Results and discussion

Molecular design

Compound 1a has the fluoro-substituted phenylbithiophene unit as a π-conjugated moiety and the hydrogen-bonded N,N′-dialkyl urea moiety. Fluoro substitution for calamitic mesogens has been systematically studied to obtain polar materials.¹³ The urea moiety can form one-dimensional hydrogen-bonded assembled structures, which results in the formation of fibrous aggregates.¹⁴ The magnitude of the calculated dipole moment for 1a was 4.4 D to the direction of the molecular short axis (see ESI†). Compound 1b without fluoro substituents (calculated dipole moment: 2.9 D) was prepared to examine the effects of fluoro substituents on the molecular self-assembly under electric fields. The phenylbithiophene moiety was chosen as π-conjugated moiety. Intensive studies have focused on oligothiophenes for the preparation of organic semiconductors.^15–18 The formation of fibrous aggregates based on oligothiophenes and their charge transport properties have been reported.^2a,d To our knowledge, however, until now, there are few examples of electro-active self-assembled fibers consisting of π-conjugated molecules that have been aligned macroscopically and bridged electrodes under electric fields in the solvent.

Formation of fibrous aggregates

Compounds 1a and b exhibited gelation abilities for dodecylbenzene at the concentration of 50 g L⁻¹, while no gelation was observed in other common organic solvents (see ESI†). The fibrous aggregates of the xerogel of 1a were observed by scanning electron microscopy (SEM). The xerogel was prepared by immersing the dodecylbenzene gel (50 g L⁻¹) in hexane.^9a–b The diameters of the fibers were estimated to be in the range of 100–250 nm (Fig. 2).

Fig. 2 SEM image of fibrous aggregates of compound 1a. The scale bar indicates 3 μm.

Fibrous aggregates formed under electric fields

The electric field-assisted alignment of fibrous aggregates of 1a was successfully achieved for the dodecylbenzene mixture (50 g L⁻¹) as shown in Fig. 3. The fibers aligned in the cells of A and B by applying AC electric fields. Cell A was composed of a glass plate with comb-shaped gold electrodes with the thickness of 0.8 μm. Fig. 3a shows a part of the electrodes on the substrate. Upon application of the AC electric field (0.5 V μm⁻¹, 1 kHz), the dodecylbenzene solution was cooled (Fig. 3a), which resulted in the aligned aggregation of fibers. The formation of fibers occurred in seconds. The fibers exhibited a birefringence image under crossed polarizers (Fig. 3b and c). The microscopic image of the fibers changed from bright to dark on each rotation by 45° (Fig. 3b and c). This observation suggests that the phenylbithiophene moieties are macroscopically aligned in the fibrous aggregates. The aligned fibrous aggregates of 1a were formed under applying AC electric fields when the concentration of 1a in dodecylbenzene was in the range of 20–50 g L⁻¹ (see ESI†). The minimum value of the magnitude of electric fields for obtaining aligned fibrous aggregates was 0.3 V μm⁻¹ (see ESI†).

Fig. 3 Schematic illustrations and polarized optical microscopic images of the mixture of 1a (50 g L⁻¹) (grey spheres) and dodecylbenzene (light grey shading) in cells A and B. Cell A: (a) isotropic liquid state at 135 °C (left) and aligned fibers of 1a formed between the two gold electrodes by applying the electric field (0.5 V μm⁻¹, 1 kHz) on cooling (right). (b and c) Microscopic images of the aligned fibers in cell A under crossed analyzer (A) and polarizer (P) at room temperature. In image (c), the sample was rotated by 45° from the position in image (b). Cell B: (d) isotropic liquid state at 135 °C in the ITO cell (left) and aligned fibers of 1a formed between the ITO electrodes under the electric field (1.0 V μm⁻¹, 1 kHz) on cooling (right). (e) Microscopic image of aligned fibers on the ITO substrate. The scale bar indicates 50 μm.

Cell B consists of a pair of indium tin oxide (ITO) electrodes. The thickness between the two electrodes was fixed by a spacer of 25 μm. When the dodecylbenzene solution of 1a was cooled to room temperature under the AC electric field (1.0 V μm⁻¹, 1 kHz) (Fig. 3d), the aligned fibers were obtained (Fig. 3e).

In contrast, randomly entangled fibrous aggregates of 1a were formed without electric fields in the cells of A and B. Moreover, for compound 1b without fluoro substituents, no aligned fibers were obtained under the AC electric fields (see ESI†). These results show that the fluoro substituents on the phenylbithiophene unit should play a key role in the formation of aligned fibrous aggregates under the electric fields.

Self-assembled structures in fibrous aggregates

The X-ray diffraction pattern of the xerogel of 1a (Fig. 4) shows five peaks at 30.7, 15.5, 10.3, 7.8, and 6.2 Å with a reciprocal d-spacing ratio of 1 : 2 : 3 : 4 : 5, indicating a layered structure. The peak at 4.6 Å corresponds to the short-range order of the self-assembled structure.¹⁹ The molecular length of 1a was calculated to be approximately 37 Å. The xerogel of 1b gives two peaks with a d-spacing of 38.5 Å (100) and 18.1 Å (200) in the X-ray diffraction pattern (see ESI†). These results show the formation of the layered structure has not been affected by the fluoro substitution of the π-conjugated moiety. The FT-IR spectra of the mixture of 1a and dodecylbenzene in the gel and sol states were obtained to examine the hydrogen-bonded states of the urea group of 1a. In the gel state at room temperature, the N–H and C=O stretching bands of the urea group of 1a were observed at 3326 and 1614 cm⁻¹, respectively. In contrast, in the sol state at 135 °C, these stretching bands shifted to 3361 and 1644 cm⁻¹ (see ESI†). These results suggest that in the gel state the urea moieties form one-dimensional bifurcated hydrogen-bonded arrays parallel to the plane of interdigitated monolayers consisting of nanosegregated π-conjugated and aliphatic domains (Fig. 5).

Fig. 4 An X-ray diffraction pattern of the xerogel of 1a at room temperature. The xerogel sample was prepared by immersing the dodecylbenzene gel of 1a (50 g L⁻¹) in hexane.

Fig. 5 A schematic illustration of (a) fibrous aggregates formed under electric fields and (b and c) molecular arrangement of 1a in the fibers. (b) View along the y axis, showing the stacking of π-conjugated moieties and the hydrogen bonding of urea moieties. (c) View along the z axis, indicating the formation of interdigitated monolayer structure consisting of nanosegregated π-conjugated and aliphatic domains.

The photophysical properties of the mixture of 1a and dodecylbenzene in the gel and sol states were studied (see ESI†). The UV-vis absorption spectrum of the mixture in the sol state showed the π–π* absorption maximum at 346 nm. In the gel state, the absorption peak was blue-shifted by 24 nm compared to that in the sol state. The fluorescence spectrum of the dodecylbenzene gel of 1a exhibited the emission peaks at 451 and 477 nm, while the emission peak at 429 nm was observed in the sol state. These spectral changes can be ascribed to the formation of π-stacked aggregates with H-type stacking mode.²⁰

Photoconductive properties of fibrous aggregates

The π-stacked structures of phenylbithiophene moieties in the fibrous aggregates of 1a were expected to function as hole transport pathways. Upon irradiation of ultraviolet light using a high pressure mercury lamp (ca. 50 mW cm⁻²), a photocurrent of 300 nA under the direct current electric field of 10.0 V μm⁻¹ (Fig. 6) was generated for the aligned fibers in the ITO cell with the thickness of 9 μm (Fig. 3d, right). The photocurrent of 300 nA was also generated for the randomly entangled fibers of 1a (see ESI†). The significant differences of the photocurrent values have not been observed. The charge transport properties of the aligned fibrous aggregates in the ITO cell (Fig. 3d, right) were also examined by a time-of-flight (TOF) method.¹⁸ A transient photocurrent induced by a YAG pulse laser (λ = 355 nm) showed a dispersive nature of positive charge transport (see ESI†), which suggests that fibers acted as hole transport pathways. However, the photogenerated holes in the fibers may be trapped by disordered assemblies of π-conjugated moieties in the fibers or by the defects formed at the interface between the fibers and the ITO electrodes. Future investigation will be directed to the studies on the surface modification of electrodes for efficient photocurrent generation in π-conjugated fibers.

Fig. 6 Photocurrent responses of the fibrous aggregates of 1a in dodecylbenzene (50 g L⁻¹) at room temperature by the on–off irradiation of ultraviolet light (λ = 365 nm) from a high-pressure mercury lamp. The fibers were aligned between ITO electrodes. The sample thickness was 9 μm. The applied voltage was 10 V μm⁻¹ for positive carriers.

Conclusions

The self-assembly of the laterally fluorinated π-conjugated molecules in dodecylbenzene under the AC electric fields has led to the formation of fibrous aggregates between electrodes. The molecular design exhibiting negative dielectric anisotropy was essential for the alignment of fibers under electric fields. These aligned fibers showed photoconductive properties. They may be promising candidates for semiconducting organic fibrous aggregates.

Experimental section

General

All reagents and solvents were purchased from Tokyo Kasei, Aldrich, Kanto, or Wako and were used without further purification. All reactions were carried out under an Ar atmosphere. Analytical thin-layer chromatography (TLC) was performed on silica gel plates of E. Merck (Silica Gel F₂₅₄). Silica gel column chromatography was carried out with silica gel 60 (spherical 40–50 μm) from Kanto. Recycling preparative GPC was conducted with a Japan Analytical Industry LC-9201. ¹H and ¹³C NMR spectra were obtained using a JEOL JNM-LA400 at 400 MHz and 100 MHz, respectively. Chemical shifts of ¹H and ¹³C NMR signals were expressed in ppm (δ) with TMS as the internal standard. Mass spectra (MALDI-TOF-MS) were recorded on a PerSeptive Biosystems Voyager-DE STR spectrometer. Elemental analysis was carried out on a Yanako MT6-CHN autocorder. An Olympus BX-51 optical polarizing microscope equipped with a Mettler FP82HT hot-stage was used for visual observation. SEM measurements were performed on a KEYENCE VE-9800 at an accelerating voltage of 1 kV. X-Ray diffraction measurements were carried out on a Rigaku RINT-2500 system with Cu Kα radiation. Fourier transform infrared spectroscopy (FT-IR) spectra were recorded on a JASCO FT/IR-660 plus spectrometer equipped with a JASCO Irtron IRT-30 microscope. UV-vis absorption spectra and fluorescence spectra were recorded on a JASCO V-670 and JASCO FP-6300, respectively. The density functional theory (DFT) calculations were carried out using Wavefunction SPARTAN'04 (v. 1.0.3) programs. The ground-state geometries were optimized at the B3LYP/6-31G* level of theory.

Gelation test

The compounds of 5.0 mg were added to 100 μL of organic solvents in sealed test tubes. The tubes were heated until clear solutions were obtained, and then the mixtures were cooled to room temperature. When a tube could be inverted without any flow, it was determined to be a “gel” (see ESI†). When the compound could not be dissolved completely even if it was heated, it was determined to be “insoluble”. In the case when reprecipitation occurred on cooling, it was determined as “precipitation”.

Alignment of fibrous aggregates by applying electric fields

The samples in sol states (at 135 °C for 1a and at 150 °C for 1b) were filled between comb-shaped gold electrodes or ITO coated glass cell. In the presence of alternating electric fields (NF Corporation WF1943A 1CH as a power source and NF Corporation HSA 4011 as an amplifier), the sample was cooled from the sol state to the gel state. The cooling rate was 5 °C min⁻¹.

Measurement of photocurrents

The mixture of 1a and dodecylbenzene was placed between two pieces of glass with ITO electrodes on their surfaces. The area of electrodes was 0.16 cm². The thickness was 9 μm. A DC voltage of 90 V was applied for these cells. Photocurrrents were observed as voltage drop across a serial resistor (10 kΩ) by means of a digital oscilloscope. A high pressure mercury lamp (Ushio, 500 W) with a glass filter (Asahi Technoglass, UV-D36C) was employed as an irradiation source.

Syntheses of phenylbithiophene derivatives 1a and b.

Phenylbithiophene derivatives 1a and b were synthesized as shown in Scheme 1. Compound 2 was prepared by similar procedures reported in the literature.²¹

Scheme 1 Syntheses of phenylbithiophene derivatives 1a,b.

5-(2,3-Difluoro-4-isopropoxyphenyl)-5′-hexyl-2,2′-bithiophene (3a). To a solution of 2 (2.62 g, 7.96 mmol) and 2-[2,3-difluoro-(4-isopropoxy)phenyl]-4,4,5,5-tetramethyl-1,3,2-dioxaborolane (3.02 g, 10.1 mmol) in THF (60 mL), Pd(PPh₃)₄ (620 mg, 0.54 mmol) and degassed aqueous solution of K₂CO₃ (1.58 M, 30 mL) were added. The reaction mixture was stirred and refluxed for 15 h at 60 °C. The reaction mixture was cooled to room temperature and poured into water and extracted with hexane and ethyl acetate. The organic phase was dried over anhydrous MgSO₄ and filtered. The solvent was removed in vacuo, and the residue was purified by column chromatography (eluent: CHCl₃–hexane = 1 : 3, v/v) to give 3a as a light-yellow solid (2.52 g, 75%). ¹H NMR (400 MHz, CDCl₃): δ = 7.27–7.25 (m, 1H), 7.25–7.19 (m, 1H), 7.07 (d, J = 4.0 Hz, 1H), 7.01 (d, J = 3.6 Hz, 1H), 6.78–6.73 (m, 1H), 6.69 (d, J = 3.6 Hz, 1H), 4.62–4.54 (m, 1H), 2.79 (t, J = 7.6 Hz, 2H), 1.72–1.64 (m, 2H), 1.38 (d, J = 6.0 Hz, 6H), 1.36–1.28 (m, 6H), 0.89 (t, J = 6.8 Hz, 3H). ¹³C NMR (CDCl₃): δ = 145.7, 137.7, 134.4, 134.4, 126.4, 126.4, 124.8, 123.5, 123.5, 123.5, 121.6, 121.5, 121.5, 112.2, 73.0, 31.6, 30.2, 28.7, 22.6, 22.0, 14.1. MS (MALDI-TOF): calcd. for [M]˙⁺, 420.14; found, 420.10. Elemental analysis calcd. for C₂₃H₂₆F₂OS₂: C, 65.68; H, 6.23%. Found C, 65.73; H, 6.43%. IR (KBr): 3059, 2983, 2960, 2927, 2857, 1630, 1578, 1543, 1505, 1463, 1387, 1377, 1336, 1302, 1243, 1222, 1200, 1180, 1141, 1112, 1083, 1064, 1022 cm⁻¹.

5-(2,3-Difluoro-4-hydroxyphenyl)-5′-hexyl-2,2′-bithiophene (4a). To a solution of 3a (2.52 g, 6.00 mmol) in CH₂Cl₂ (100 mL), BBr₃ (1.0 M in CH₂Cl₂, 7.5 mL) was added dropwise at 0 °C. The reaction mixture was stirred at room temperature for 3 h. The reaction mixture was poured into water and extracted with CHCl₃. The combined organic extracts were washed with brine. The organic phase was dried over anhydrous MgSO₄ and filtered. The solvent was removed in vacuo, and the residue was purified by column chromatography (eluent: CHCl₃) to give 4a as a white solid (1.62 g, 72%). ¹H NMR (400 MHz, CDCl₃): δ = 7.25 (d, J = 4.0 Hz, 1H), 7.25–7.24 (m, 1H), 7.07 (d, J = 4.0 Hz, 1H), 7.00 (d, J = 4.0 Hz, 1H), 6.83–6.77 (m, 1H), 6.69 (d, J = 4.0 Hz, 1H), 5.46 (br, 1H), 2.79 (t, J = 7.6 Hz, 2H), 1.71–1.64 (m, 2H), 1.40–1.29 (m, 6H), 0.89 (t, J = 6.8 Hz, 3H). ¹³C NMR (CDCl₃): δ = 145.8, 137.8, 134.3, 126.4, 126.3, 124.8, 123.5, 122.4, 112.4, 31.6, 30.2, 28.8, 22.6, 14.1. MS (MALDI-TOF): calcd. for [M]˙⁺, 378.09; found, 378.44. Elemental analysis calcd. for C₂₀H₂₀F₂OS₂: C, 63.46; H, 5.33%. Found C, 63.28; H, 5.44%. IR (KBr): 3373, 2956, 2925, 2857, 1628, 1598, 1510, 1492, 1464, 1395, 1324, 1300, 1255, 1221, 1078, 1047, 1008 cm⁻¹.

5-{2,3-Difluoro-4-[5-(N-phthalimido)pentyloxy]phenyl}-5′-hexyl-2,2′-bithiophene (5a). A mixture of 4a (302 mg, 0.88 mmol), K₂CO₃ (365 mg, 2.64 mmol), and N-(5-bromopentyl)phthalimide (311 mg, 1.05 mmol) in DMF (30 mL) was stirred for 14 h at 80 °C. The reaction mixture was poured into a sat. NH₄Cl aqueous solution and extracted with ethyl acetate three times. The combined organic extracts were washed with brine. The organic phase was dried over anhydrous MgSO₄ and filtered. The solvent was removed in vacuo, and the residue was purified by column chromatography (eluent: CHCl₃) and GPC to give 5a as a yellow solid (420 mg, 80%). ¹H NMR (400 MHz, CDCl₃): δ = 7.85 (dd, J = 5.2, 3.6 Hz, 2H), 7.71 (dd, J = 5.2, 3.6 Hz, 2H), 7.25–7.23 (m, 2H), 7.07 (d, J = 3.6 Hz, 1H), 7.01 (d, J = 3.6 Hz, 1H), 6.75–6.71 (m, 1H), 6.69 (d, J = 6.4 Hz, 1H), 4.06 (t, J = 6.4 Hz, 2H), 3.75–3.71 (m, 2H), 3.40 (t, J = 6.4 Hz, 2H), 2.80 (d, J = 8.0 Hz, 2H), 1.94–1.90 (m, 2H), 1.82–1.78 (m, 2H), 1.72–1.69 (m, 2H), 1.57–1.54 (m, 2H), 1.50–1.34 (m, 6H), 0.90 (t, J = 6.8 Hz, 3H). ¹³C NMR (CDCl₃): δ = 168.5, 145.7, 137.7, 137.6, 134.4, 134.0, 132.1, 126.4, 126.3, 124.8, 123.5, 123.2, 121.6, 116.4, 116.3, 109.6, 69.5, 37.7, 31.6, 30.2, 28.8, 28.6, 28.2, 23.1, 22.6, 14.1. MS (MALDI-TOF): calcd. for [M]˙⁺, 593.19; found, 593.12. Elemental analysis calcd. for C₃₃H₃₃F₂NO₃S₂: C, 66.75; H, 5.60; N, 2.36%. Found C, 66.54; H, 5.66; N, 2.37%. IR (KBr): 3074, 2924, 2854, 1768, 1712, 1624, 1508, 1488, 1467, 1448, 1433, 1398, 1371, 1333, 1294, 1220, 1187, 1137, 1101, 1078, 1064, 1030 cm⁻¹.

5-[4-(5-Aminopentyloxy)-2,3-difluorophenyl]-5′-hexyl-2,2′-bithiophene (6a). A solution of 5a (420 mg, 0.71 mmol) and hydrazine monohydrate (500 μL, 10 mmol) in ethanol (100 mL) was refluxed for 3 h. The reaction mixture was cooled to room temperature. Resulting precipitate was filtrated and washed with hexane and cold CHCl₃ to give 6a as a yellow solid (322 mg, 98%). Compound 6a was used without further purification. ¹H NMR (400 MHz, CDCl₃): δ = 7.28–7.24 (m, 2H), 7.09 (d, J = 4.0 Hz, 1H), 7.03 (d, J = 4.0 Hz, 1H), 6.76 (m, 1H), 6.71 (d, J = 3.6 Hz, 1H), 4.10 (t, J = 6.4 Hz, 2H), 2.82 (t, J = 8.0 Hz, 2H), 1.88–1.87 (m, 2H), 1.71 (m, 2H), 1.60–1.35 (m, 14H), 1.35–1.30 (m, 2H), 0.92 (t, J = 6.4 Hz, 3H). ¹³C NMR (CDCl₃): δ = 145.8, 134.5, 126.5, 126.4, 124.9, 123.6, 123.6, 121.7, 110.0, 70.0, 31.6, 31.6, 30.2, 29.1, 28.8, 23.3, 22.6, 14.0. MS (MALDI-TOF): calcd. for [M]˙⁺, 463.18; found, 462.97. IR (KBr): 3334, 3180, 3033, 2974, 2925, 1661, 1602, 1594, 1494, 1461, 1443, 1378, 1349, 1330, 1307, 1262, 1221, 1082, 1023 cm⁻¹.

5-Pentyl-5′-{2,3-difluoro-4-[5-(N′-hexylureido)-N-pentyloxy]phenyl}-2,2′-bithiophene (1a). To a solution of 6a (153 mg, 0.28 mmol) in CH₂Cl₂ (80 mL) and THF (80 mL), hexyl isocyanate (50 μL, 0.35 mmol) was added. The reaction mixture was stirred for 6 h at 60 °C. The solvent was removed in vacuo and the residue was purified by column chromatography (eluent: CHCl₃–MeOH = 10 : 1, v/v) and GPC to give 1a as a yellow solid (50.0 mg, 30%). ¹H NMR (400 MHz, CDCl₃): δ = 7.26–7.20 (m, 2H), 7.07 (d, J = 3.6 Hz, 1H), 7.01 (d, J = 3.6 Hz, 1H), 6.80–6.70 (m, 1H), 6.69 (d, J = 3.2 Hz, 1H), 4.18–4.13 (m, 2H), 4.08 (t, J = 6.4 Hz, 2H), 3.22 (m, 2H), 3.15 (m, 2H), 2.80 (t, J = 8.0 Hz, 2H), 1.90–1.80 (m, 2H), 1.70–1.60 (m, 2H), 1.60–1.40 (m, 18H), 0.90–0.80 (m, 6H). ¹³C NMR (CDCl₃): δ = 206.6, 134.4, 126.4, 124.8, 123.6, 121.7, 110.0, 69.9, 40.8, 40.4, 31.6, 31.5, 30.8, 30.2, 30.0, 28.8, 28.7, 26.6, 23.3, 22.6, 14.0, 13.9. MS (MALDI-TOF): calcd. for [M]˙⁺, 590.28; found, 590.07. Elemental analysis calcd. for C₃₂H₄₄F₂N₂O₂S₂: C, 65.05; H, 7.51; N, 4.74%. Found C, 64.81; H, 7.78; N, 4.47%.

5-(4-Isopropoxyphenyl)-5′-hexyl-2,2′-bithiophene (3b). The procedure used to obtain 3a was applied with 2 (3.02 g, 9.17 mmol) and 2-(4-isopropoxyphenyl)-4,4,5,5-tetramethyl-1,3,2-dioxaborolane (2.88 g, 11.0 mmol) to give 3b as a yellow solid (2.37 g, 68%). ¹H NMR (400 MHz, CDCl₃): δ = 7.50 (d, J = 8.8 Hz, 2H), 7.08 (d, J = 4.0 Hz, 1H), 7.03 (d, J = 3.6 Hz, 1H), 6.98 (d, J = 4.0 Hz, 1H), 6.89 (d, J = 8.8 Hz, 2H), 6.68 (d, J = 3.6 Hz, 1H), 4.59–4.56 (m, 1H), 2.79 (t, J = 7.2 Hz, 2H), 1.72–1.67 (m, 2H), 1.39–1.32 (m, 12H), 0.90 (t, J = 6.8 Hz, 3H). ¹³C NMR (CDCl₃): δ = 157.5, 145.2, 142.5, 136.2, 135.0, 126.8, 126.8, 124.7, 123.8, 123.0, 122.5, 116.2, 70.0, 31.6, 30.2, 28.8, 22.6, 22.0, 14.1. MS (MALDI-TOF): calcd. for [M]˙⁺, 384.16; found, 384.39. Elemental analysis calcd. for C₂₃H₂₈OS₂: C, 71.83; H, 7.34%. Found C, 71.74; H, 7.43%. IR (KBr): 3066, 2972, 2951, 2921, 2848, 1604, 1557, 1533, 1508, 1478, 1467, 1447, 1372, 1335, 1310, 1286, 1254, 1224, 1181, 1121, 1109, 1068, 1038 cm⁻¹.

5-(4-Hydroxyphenyl)-5′-hexyl-2,2′-bithiophene (4b). The synthesis procedure used to obtain 4a was applied with 3b (2.37 g, 6.2 mmol) and BBr₃ (7.5 mL, 7.54 mmol) to give 4b as a white solid (1.29 g, 61%). ¹H NMR (400 MHz, CDCl₃): δ = 7.47 (d, J = 8.8 Hz, 2H), 7.07 (d, J = 3.6 Hz, 1H), 7.03 (d, J = 4.0 Hz, 1H), 6.98 (d, J = 3.6 Hz, 1H), 6.84 (d, J = 8.8 Hz, 2H), 6.68 (d, J = 4.0 Hz, 1H), 4.83 (s, 1H), 2.79 (t, J = 7.2 Hz, 2H), 1.72–1.64 (m, 2H), 1.52 (s, 2H), 1.38–1.31 (m, 8H), 0.89 (t, J = 6.8 Hz, 3H). ¹³C NMR (CDCl₃): δ = 155.2, 145.3, 127.1, 124.7, 123.8, 123.1, 122.7, 115.9, 31.5, 30.2, 28.8, 22.6, 14.0. MS (MALDI-TOF): calcd. for [M]˙⁺, 342.11; found, 342.41. Elemental analysis calcd. for C₂₀H₂₂OS₂: C, 70.13; H, 6.47%. Found C, 70.03; H, 6.57%. IR (KBr): 3399, 3068, 2953, 2918, 2872, 2854, 1609, 1560, 1535, 1509, 1450, 1438, 1371, 1263, 1177, 1107, 1071, 1041, 1010 cm⁻¹.

5-{4-[5-(N-Phthalimido)pentyloxy]-pentyl}-5′-hexyl-2,2′-bithiophene (5b). The synthesis procedure used to obtain 5a was applied with 4b (197 mg, 0.58 mmol), K₂CO₃ (171 mg, 1.24 mmol), and N-(5-bromopentyl)phthalimide (213 mg, 0.72 mmol) to give 5b as a white solid (258 mg, 80%). ¹H NMR (400 MHz, CDCl₃): δ = 7.84 (m, 2H), 7.71 (m, 2H), 7.48 (d, J = 8.8 Hz, 2H), 7.07 (d, J = 3.6 Hz, 1H), 7.03 (d, J = 3.6 Hz, 1H), 6.97 (d, J = 3.2 Hz, 1H), 6.87 (d, J = 8.8 Hz, 2H), 6.68 (d, J = 3.2 Hz, 1H), 3.98 (t, J = 6.4 Hz, 2H), 3.73 (m, 2H), 2.79 (t, J = 7.2 Hz, 2H), 1.85–1.31 (m, 14H), 0.90 (t, J = 6.8 Hz, 3H). ¹³C NMR (CDCl₃): δ = 168.4, 158.6, 145.1, 142.5, 136.2, 133.9, 132.1, 126.9, 126.8, 124.7, 123.7, 123.2, 123.0, 122.5, 114.8, 67.7, 37.8, 31.5, 30.2, 28.8, 28.7, 28.3, 23.4, 22.6, 14.1. MS (MALDI-TOF): calcd. for [M]˙⁺, 557.21; found, 557.12. Elemental analysis calcd. for C₃₃H₃₅NO₃S₂: C, 71.06; H, 6.32; N, 2.51%. Found C, 70.89; H, 6.42; N, 2.47%. IR (KBr): 3080, 2929, 2854, 1773, 1707, 1604, 1572, 1531, 1508, 1467, 1450, 1435, 1398, 1374, 1338, 1282, 1247, 1180, 1114, 1054 cm⁻¹.

5-[4-(5-Aminopentyloxy)phenyl]-5′-hexyl-2,2′-bithiophene (6b). The synthesis procedure used to obtain 6a was applied with 5b (258 mg, 0.46 mmol) and hydrazine monohydrate (2 mL, 41 mmol) to give 6b as a white solid (135 mg, 68%). ¹H NMR (400 MHz, CDCl₃): δ = 7.49 (d, J = 8.8 Hz, 2H), 7.06 (d, J = 4.0 Hz, 1H), 7.02 (d, J = 3.2 Hz, 1H), 6.97 (d, J = 3.6 Hz, 1H), 6.89 (d, J = 8.8 Hz, 2H), 6.67 (d, J = 3.2 Hz, 1H), 3.99 (t, J = 6.4 Hz, 2H), 3.71 (q, J = 6.8 Hz, 2H), 2.81–2.72 (m, 6H), 1.81–1.21 (m, 12H), 0.90 (t, J = 6.4 Hz, 3H). MS (MALDI-TOF): calcd. for [M]˙⁺, 427.20; found, 427.31. IR (KBr): 3328, 3286, 3153, 3088, 2935, 2782, 2726, 1664, 1638, 1606, 1577, 1471, 1443, 1365, 1249, 1217, 1155, 1096, 1067, 1023 cm⁻¹.

5-Pentyl-5′-{4-[5-(N′-hexylureido)-N-pentyloxy]phenyl}-2,2′-bithiophene (1b). The synthesis procedure used to obtain 1a was applied with 6b (135 mg, 0.32 mmol) and hexyl isocyanate (110 μL, 0.76 mmol) to give 1b as a white solid (30 mg, 17%). ¹H NMR (400 MHz, CDCl₃): δ = 7.50 (d, J = 8.8 Hz, 2H), 7.08 (d, J = 4.0 Hz, 1H), 7.03 (d, J = 3.6 Hz, 1H), 6.98 (d, J = 3.6 Hz, 1H), 6.89 (d, J = 8.8 Hz, 2H), 6.68 (d, J = 4.0 Hz, 1H), 4.20–4.10 (m, 2H), 3.99 (t, J = 6.2 Hz, 2H), 3.23–3.12 (m, 4H), 2.79 (t, J = 7.6 Hz, 2H), 1.84–1.80 (m, 2H), 1.70–1.65 (m, 2H), 1.59–1.25 (m, 18H), 0.90–0.87 (m, 6H). ¹³C NMR (CDCl₃): δ = 158.6, 145.2, 142.4, 127.0, 126.8, 124.7, 123.8, 123.0, 122.5, 114.8, 67.8, 40.7, 40.5, 31.6, 31.5, 30.2, 30.1, 30.0, 28.9, 28.7, 26.6, 23.4, 22.6, 14.1, 14.0. MS (MALDI-TOF): calcd. for [M]˙⁺, 554.30; found, 554.50. Elemental analysis calcd. for C₃₂H₄₆N₂O₂S₂: C, 69.27; H, 8.36; N, 5.05%. Found C, 69.15; H, 8.48; N, 4.83%. IR (KBr): 3341, 2954, 2922, 2856, 1621, 1589, 1509, 1472, 1462, 1377, 1309, 1292, 1253, 1179, 1112, 1036 cm⁻¹.

Acknowledgements

We are grateful to Prof. H. Ohno (Department of Biotechnology, Tokyo University of Agriculture and Technology) for supplying us with comb-shaped gold electrodes. This study was financially supported by Grant-in-Aid for Encouragement of Young Scientists B (No. 18750111) (M.Y.) from the Ministry of Education, Culture, Sports, Science and Technology and by Grant-in-Aid for Global COE Program of Chemistry Innovation (T.K. and Y.S.), Creative Scientific Research of “Invention of Conjugated Electronic Structures and Novel Function” (No. 16GS0209) (T.K.) from the Japan Society for the Promotion of Science, and Grant-in-Aid for Scientific Research (A) of “Development of Highly Functional Ion-Conductive Materials Using Nanostructured Liquid Crystals” (No. 19205017) (T.K. and M.Y.).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: The dipole moment of 1a, gelation properties of 1a and b, self-assembly of 1a and b in dodecylbenzene under electric fields, molecular model of 1a, X-ray diffraction pattern of the xerogel of 1b at room temperature, FT-IR spectra of the mixture of 1a and dodecylbenzene, UV-vis and fluorescence spectra of the mixture of 1a and dodecylbenzene, photocurrent responses of the randomly entangled fibrous aggregates of 1a in dodecylbenzene, transit photocurrent curves of the fibers of 1a in dodecylbenzene. See DOI: 10.1039/b915822g

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