Thieno[3,2-b]thiophene derivatives exhibiting semiconducting liquid-crystalline phases at lower temperatures

Abstract

We synthesized novel liquid crystalline semiconductors bearing an extended π-conjugated mesogen based on thieno[3,2-b]thiophene, which exhibit high solubility and low phase transition temperature. In particular, lateral methyl substitution to the mesogenic unit led to the formation of efficiently self-organized LC domains at lower temperature than that of reported liquid crystalline thieno[3,2-b]thiophene derivatives. Smectic A and smectic E phases were observed by a polarizing optical microscope over a wide temperature range between 75 and 125 °C. Hole mobility was determined by the time-of-flight (TOF) method, and calculated up to 2 × 10⁻³ cm² V⁻¹ s⁻¹. Herein we report the synthesis, thermophysical and carrier transport properties of novel thieno[3,2-b]thiophene based liquid-crystalline semiconductors.

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Introduction

The development of organic semiconductors achieving both solution-processability and macroscopic homogeneity has been essential and is still highly challenging for organic field-effect transistors (OFETs) in printed electronics.¹ Obtaining liquid-crystalline (LC) phases from organic semiconductors can lead to solution-processability and self-organization into large area domains with a crystal-like structure and low defects.² Thus, they are promising candidates for the low-cost and high performance organic field-effect transistors.

Thiophene or fused thiophene based conjugated systems have been developed as prominent materials for OFETs due to their chemical stability, and versatile chemical modification to tune electronic properties.³ McCulloch and co-workers^4a have shown that the incorporation of heterocyclic thieno[3,2-b]thiophene (TT) into a polythiophene backbone led to the synthesis of liquid crystalline semiconducting polymers exhibiting charge carrier mobilities up to 0.15 cm² V⁻¹ s⁻¹ with lifetimes of several months. Indeed, the incorporation of rigid TT units in conjugated polymers has been developed to improve their electronic properties and to optimize the performance of the corresponding devices.⁴ However, difficulties are accompanied in controlling uniform molecular weight and molecular orientation compared to those of oligomers or small molecules.

High field effect mobilities over 1 cm² V⁻¹  s⁻¹ have been recently reported with single crystals or polycrystalline films based on TT and fused thienoacenes such as BTBT (2,7-didecylbenzothienobenzothiophene) derivatives.^5,6 Whereas extending π–π conjugation with highly fused aromatic structure are advantageous to achieve high charge transport properties, low solubility and scarce chance of exhibiting LC phase are usually inevitable due to the molecular rigidity.⁷ There are few reports on the liquid crystalline (LC) semiconducting TT oligomers, but most of them exhibit low solubility and high phase transition temperatures.⁸ Besides, exhibiting smectic phases are rather rare.^8c,d High hole mobilities have been observed only in their crystalline phases but not in their smectic phases.

In determining inter- and intra-molecular orientation and molecular packing characteristics, types and substitution position of solubilizing group (alkyl chain) does one of the important roles for organic semiconducting conjugated system.⁹ In this study, we synthesized new liquid crystalline TT derivatives by facile synthetic route, and all TT-derivatives consist of identical mesogenic part. Therefore we could elucidate the positional effect of alkyl substitution on conjugated rigid domain which possibly induces huge difference in thermal reorganization. Particularly, smectic phases were attainable from the molecules incorporated with lateral methyl group at reduced phase transition temperatures. Herein we report on the synthesis, thermal phase transition properties and charge transport properties in the smectic phases of the new thieno[3,2-b]thiophene based organic semiconductors.

Experimental

General methods

¹H NMR spectra were recorded with a JEOL ECX 400 MHz spectrometer. Absorption spectra were recorded with an Agilent 8453 spectrophotometer.

Phase transition characteristics of the compounds were investigated by micrographic optical textures obtained from optical microscope equipped with polarizer with a hot stage (Mettler Toledo F82HT), and thermograms measured by DSC (Differential Scanning Calorimetry, Netsch).

The charge carrier mobility was determined by the TOF method.^10,11 Each compound was melted and capillary-filled into a cell. consisting of two ITO-coated glass plates with a thickness of 25 μm. The cell was illuminated by the third harmonic generation of a Nd:YAG laser (wavelength = 356 nm, pulse duration = 2 ns). The transit time t_T was determined from the kink point in the transient photocurrent curves. The carrier mobility μ is expressed by eqn (1), where V is an applied voltage and d is the sample thickness (9 μm).

Temperature of the cells was in situ controlled from that of each liquid crystalline phase to room temperature during the measurement. The mesomorphic properties were characterized by the observation of optical textures using a polarizing optical microscope (OLYMPUS BX-60) equipped with a SONY DXC-950 3CCD camera and differential scanning calorimetry (DSC). Scanning rate; 10.0 K min⁻¹.

Materials

All purchased chemicals were used as obtained without further purification.

2-(4-Alkylphenyl)-2,3-thieno[3,2-b]thiophene (5). To a THF solution of thieno[3,2-b]thiophene-2-boronic acid (10.1 mmol; Wako) and p-bromohexylbenzene or p-bromododecylbenzene (10 mmol), THF (25 mL), 10 mL of 2 M sodium carbonate solution was added and degassed using a flow of argon. After tetrakis(triphenylphosphine)palladium(0) (0.1 mmol, TCI) was added, the solution was refluxed overnight under Ar. Then the solution was cooled to room temperature, and water (50 mL) was added. The organic layer was separated, washed with brine, and dried over MgSO₄. After evaporation of solvent, it was purified by column chromatography (silica gel, hexane as the eluent).

5a (white solid, 75% yield) ¹H NMR (400 MHz, CDCl₃): δ (ppm) 7.55 (d, 2H), 7.45 (s, 1H), 7.34 (d, 1H), 7.24–7.20 (t, 3H), 2.63 (t, 2H), 1.64 (m, 2H), 1.38–1.27 (m, 6H), 0.89 (t, 3H) ¹³C NMR (400 MHz, CDCl₃): δ (ppm) 146.07, 143.24, 140.24, 138.23, 132.32, 129.12, 126.59, 125.87, 119.70, 114.86, 35.77, 31.82, 31.46, 29.07, 22.72, 14.24. MALDI-TOF-MS: m/z = 300.49 (M + H); calcd for C₁₂H₁₄OS₂ = 299.09.

5b (white solid, 87% yield) ¹H NMR (400 MHz, CDCl3): δ (ppm) 7.53 (d, 2H), 7.44 (s, 1H), 7.33 (d, 1H), 7.24–7.19 (dd, 3H), 2.61 (t, 2H), 1.62 (m, 2H), 1.26–1.23 (m, 18H), 0.87 (t, 3H) ¹³C NMR (400 MHz, CDCl₃): δ (ppm) 143.24, 142.99, 140.09, 138.23, 132.32, 129.12, 126.59, 125.87, 119.70, 114.86, 35.78, 32.04, 31.53, 29.78, 29.71, 29.63, 29.47, 29.42, 22.81, 14.24. MALDI-TOF-MS: m/z. 385.21 (M + H); calcd for C₂₄H₃₂S₂ = 384.19.

2-Bromo-5-(4-alkylphenyl)thieno[3,2-b]thiophenes (6). N-Bromosuccinimide (7.7 mmol) was added to the THF solution of 5a or 5b (7.5 mmol), at 0 °C under Ar atmosphere. After overnight stirring at room temperature, distilled water was added and the solution was extracted with CH₂Cl₂. The organic layers were washed with brine and dried over MgSO₄. After solvent removal in vacuo, the residue was recrystallized from ethanol to afford the title compound.

6a (white solid, 95% yield) ¹H NMR (400 MHz, CDCl₃): δ (ppm) 7.50 (d, 2H), 7.33 (s, 1H), 7.23–7.19 (dd, 3H), 2.61 (t, 2H), 1.62 (m, 2H), 1.31 (m, 6H), 0.88 (t, 3H) ¹³C NMR (400 MHz, CDCl₃): δ (ppm) 146.07, 143.24, 140.24, 136.94, 131.79, 129.19, 125.83, 122.39, 114.47, 35.77, 31.82, 31.46, 29.07, 22.72, 14.22. MALDI-TOF-MS: m/z = 379.93 (M⁺); calcd for C₁₈H₁₉BrS₂ = 380.01.

6b (white solid, 94% yield) ¹H NMR (400 MHz, CDCl₃): δ (ppm) 7.50 (d, 2H), 7.33 (s, 1H), 7.23–7.18 (dd, 3H), 2.61 (t, 2H), 1.62 (m, 2H), 1.26–1.23 (m, 18H), 0.87 (t, 3H) ¹³C NMR (400 MHz, CDCl₃): δ (ppm) 143.25, 140.25, 136.95, 131.79, 129.19, 125.83, 122.39, 114.47, 112.69, 35.78, 32.03, 31.49, 29.77, 29.70, 29.61, 29.47, 29.40, 22.80, 14.24. MALDI-TOF-MS: m/z = 464.25 (M⁺); calcd for C₂₄H₃₁BrS₂ = 464.10.

2-(4-Alkylphenyl)-5-(3-methylthiophen-2-yl)thieno[3,2-b]thiophenes (1). 3-Methylthiophene-2-boronic acid pinacol ester (7.5 mmol) and 6a or 6b (7.0 mmol) were dissolved in THF (10 mL) and was added 2 M sodium carbonate aqueous solution (3 mL). The mixture was degassed and then tetrakis(triphenylphosphine)palladium(0) (0.05, mmol, TCI) was added. The solution was refluxed overnight under Ar. The solution was cooled to room temperature, and water (50 mL) was added. The organic layer was separated, washed with brine, and dried over MgSO₄. After evaporation of solvent, it was purified by column chromatography (silica gel, hexane as the eluent).

1a (white solid, 85%) ¹H NMR (400 MHz, CDCl₃): δ (ppm) 7.54 (d, 2H), 7.41 (s, 1H), 7.26 (s, 1H), 7.22–7.17 (dd, 3H), 6.91 (d, 2H), 2.62 (t, 2H), 2.43 (s, 3H), 1.62 (m, 2H), 1.31 (m, 6H), 0.89 (t, 3H). ¹³C NMR (400 MHz, CDCl₃): δ (ppm) 146.18, 143.03, 131.57, 130.67, 129.14, 125.82, 125.39, 123.87, 121.69, 117.93, 114.81, 113.48, 35.78, 31.83, 31.49, 22.72, 22.24, 15.53, 14.22. MALDI-TOF-MS: m/z. 397.44 [M⁺ + H] (M); 396.10 calcd for C₂₃H₂₄S₃.

1b (white solid, 85%) ¹H NMR (400 MHz, CDCl3): δ (ppm) 7.54 (d, 2H), 7.41 (s, 1H), 7.26 (s, 1H), 7.22–7.17 (dd, 3H), 6.91 (d, 2H), 2.62 (t, 2H), 2.43 (s, 3H), 1.62 (m, 2H), 1.26–1.23 (m, 18H), 0.87 (t, 3H). ¹³C NMR (400 MHz, CDCl₃): δ (ppm) 143.04, 138.47, 135.42, 134.52, 132.18, 131.57, 129.14, 125.81, 125.39, 123.87, 120.00, 117.93, 114.80, 35.70, 32.03, 31.52, 29.78, 29.70, 29.62, 29.47, 29.42, 26.32, 22.72, 22.80, 15.53, 14.24. MALDI-TOF-MS: m/z. 481.01 [M⁺ + H] (M); 480.20 calcd for C₂₉H₃₆S₃.

2-(4-Alkylphenyl)-5-(thiophen-2-yl)thieno[3,2-b]thiophenes (2). It was synthesized by the same procedure introduced for compound 3 using 6a (7 mmol) and thiophene-2-boronic acid pinacol ester (7.5 mmol) or 2-hexylthiophene-2-boronic acid pinacol ester for compound 2a or 2b, respectively. Compounds were purified by column chromatography (silica gel, hexane and DCM as the eluent).

2a (yellowish solid, 68%). ¹H NMR (400 MHz, CDCl₃): δ (ppm) 7.53 (d, 2H), 7.39 (s, 1H), 7.32 (s, 1H), 7.24–7.19 (m, 4H), 7.03 (dd, 2H), 2.62 (t, 2H), 1.62 (m, 2H), 1.31 (m, 6H), 0.88 (t, 3H). ¹³C NMR (400 MHz, CDCl₃): δ (ppm) 146.38, 143.06, 138.88, 138.42, 132.12, 129.15, 128.01, 125.76, 124.74, 123.90, 116.11, 114.87, 35.79, 31.84, 31.48, 29.09, 22.73, 14.23. MALDI-TOF-MS: m/z 382.21 (M⁺); 382.09 calcd for C₂₂H₂₂S₃.

2b (yellowish solid, 85%). ¹H NMR (400 MHz, CDCl₃): δ (ppm) 7.52 (d, 2H), 7.37 (s, 1H), 7.22–7.18 (t, 3H), 7.00 (d, 1H), 6.69 (d, 1H), 2.81 (d, 2H), 2.61 (d, 2H), 1.62 (m, 4H), 1.38–1.31 (m, 12H), 0.88 (t, 6H). ¹³C NMR (400 MHz, CDCl₃): δ (ppm) 145.92, 142.94, 139.08, 138.68, 138.42, 135.16, 132.20, 129.12, 125.71, 124.94, 123.58, 115.28, 114.89, 35.78, 31, 84, 31.67, 31.48, 30.31, 29.09, 28.87, 22.72, 22.69, 14.20. MALDI-TOF-MS: m/z. 466.24 (M⁺); 466.18 calcd for C₂₈H₃₄S₃.

2-(5-Bromothiophen-2-yl)-5-(4-hexylphenyl)thieno[3,2-b]thiophene (2c). Bromination of 2a was carried out as described for the compound 3 using NBS (yellow solid, 87% yield). ¹H NMR (400 MHz, CDCl₃): δ (ppm) 7.52 (d, 2H), 7.38 (s, 1H), 7.24–7.19 (m, 3H), 6.99 (d, 1H), 6.94 (d, 1H), 2.61 (t, 2H), 1.62 (m, 2H), 1.31 (m, 6H), 0.88 (t, 3H). ¹³C NMR (400 MHz, CDCl₃): δ (ppm) 146.09, 143.60, 137.10, 131.97, 130.81, 129.17, 127.97, 125.79, 125.24, 123.93, 119.25, 116.39, 114.80, 35.79, 31.82, 31.47, 29.07, 22.72, 14.21. MALDI-TOF-MS: 460.66 m/z (M⁺); 460.00 calcd for C₂₂H₂₁BrS₃.

2-(4-Hexylphenyl)-5-(5-(tetradecynyl)thiophen-2-yl)thieno[3,2-b]thiophene (3). To a mixed THF and triethylamine (5 : 1 vol. ratio) solution of 1-tetradecyne and 2c, CuI (5 mol%) and Pd(PPh₃)₄ (10 mol%) were introduced at Ar atmosphere followed by reflux overnight. The solution was cooled to room temperature, and water (50 mL) was added. The organic layer was separated, washed with brine, and dried over MgSO₄. After evaporation of solvent, it was purified by column chromatography (silica gel, hexane and DCM as the eluent) (orange solid, 52% yield). ¹H NMR (400 MHz, CDCl₃): δ (ppm) 7.52 (d, 2H), 7.38 (s, 1H), 7.28 (s, 1H), 7.2 (d, 2H) 7.01 (s, 2H), 2.62 (t, 2H), 2.43 (t, 2H), 1.61 (m, 4H), 1.38–1.26 (m, 24H), 0.87 (tt, 6H). ¹³C NMR (400 MHz, CDCl₃): δ (ppm) 143.14, 138.85, 137.90, 137.78, 132.05, 131.88, 129.16, 125.76, 123.40, 123.36, 120.12, 116.20, 114.84, 96.31, 85.09, 35.78, 32.04, 31.83, 31.47, 29.81, 29.78, 29.74, 29.63, 29.48, 29.25, 29.08, 28.64, 22.81, 22.72, 19.22, 14.24, 14.22. MALDI-TOF-MS: m/z 575.40 (M + H); 574.28 calcd for C₁₂H₁₄OS₂.

2-(4-Alkylphenyl)-5-(4-methylthiophen-2-yl)thieno[3,2-b]thiophene (4). Compound 4 synthesized by following the same procedure described for compound 1 by introducing 6a and 4-methylthiophene-2-boronic acid pinacol ester, respectively.

4a (white solid, 80%). ¹H NMR (400 MHz, CDCl₃): δ (ppm) 7.52 (d, 2H), 7.38 (s, 1H), 7.28 (s, 1H), 7.20 (d, 2H), 7.01 (d, 1H), 6.81 (t, 1H), 2.62 (t, 2H), 2.27 (s, 3H), 1.63 (m, 2H), 1.31 (m, 6H), 0.88 (t, 3H) ¹³C NMR (400 MHz, CDCl₃): δ (ppm) 146.25, 143.02, 138.73, 138.65, 137.50, 132.14, 129.14, 126.18, 125.74, 120.08, 115.79, 114.88, 35.78, 31.83, 31.48, 29.08, 22.72, 15.86, 14.22. MALDI-TOF-MS: m/z. 397.44 g mol⁻¹ [M + H⁺]; 396.10 calcd for C₁₂H₁₄OS₂.

4b (white solid, 80%). ¹H NMR (400 MHz, CDCl₃): δ (ppm) 7.52 (d, 2H), 7.38 (s, 1H), 7.28 (s, 1H), 7.20 (d, 3H), 7.01 (d, 1H), 6.81 (t, 1H), 2.62 (t, 2H), 2.27 (s, 3H), 1.63 (m, 2H), 1.31 (m, 18H), 0.87 (t, 3H) ¹³C NMR (400 MHz, CDCl₃): δ (ppm) 143.03, 138.73, 138.65, 137.96, 132.15, 129.14, 126.18, 125.81, 120.08, 117.33, 115.80, 114.87, 112.69, 35.78, 32.03, 31.52, 29.78, 29.70, 29.62, 29.47, 22.80, 15.87, 14.24. MALDI-TOF-MS: m/z. 482.61 [M + H] (M); 480.20 calcd for C₂₉H₃₆S₃.

Results and discussion

Design and synthesis

We designed new semiconducting materials from 1 to 4 bearing TT-based aromatic core and alkyl chains associated with carrier transport, self-organization and solubility of the molecules. Increasing the planarity of the π-conjugated systems in organic semiconductors is known to lead to better stacking and better charge transport properties. However, increasing rigidity in thiophenes, and is also known to decrease solubility and environmental stability.¹² At the same time, fused thiophene compounds are known for their very good environmental stability owing to their relatively large band gaps.^12,13 Thus, here we introduced thieno[3,2-b]thiophenes as the fused thiophene unit for better stacking and charge transport properties, and substituted with alkyl chains to increase their solubility. Compounds were synthesized almost in three steps consisting of simple bromination and Pd-catalyzed Suzuki coupling or Sonogashira (3) coupling reactions as shown in Scheme 1 with moderately high yields.

Scheme 1 Synthetic routes to molecules 1 to 4.

1 and 4 were substituted with lateral methyl group that would lower the liquid-crystalline temperatures. Compound 2b and 3 were substituted with parallel hexyl chains, and decynyl group was introduced to one-end of 3 to reduce the rigidity from mesogen part. All compounds were soluble in various organic solvents such as dichloromethane (DCM), chloroform, and tetrahydrofuran (THF). The solubility varies depending on the alkyl substitution position, lateral 3-methylthiophene substituted 1b shows the highest solubility in THF (>400 mg mL⁻¹) and DCM (250 mg mL⁻¹) with heating up to the solvent boiling temperatures among the compounds followed by the parallel decynyl substituted 3 (80 mg mL⁻¹) in THF and 4-methylthiophene substituted 4a (50 mg mL⁻¹) in THF.

Alkyl chain substitution to thieno[3,2-b]thiophenes moiety (Scheme 1) led to a novel family of LC semiconductors. From design point of view this family of materials has the potential to lead to good backbone π–π stacking combined with good solubility and stability.

Thermal phase transition properties

The mesomorphic properties of TT-derivatives were characterized by differential scanning calorimetry (DSC) measurements and optical-texture observation. By using a polarized microscope equipped with heating stage, mesomorphic phase image was attainable. It revealed different thermal phase transition characteristics of TT derivatives which might indicate a different influence of the side chains on the thermal molecular alignment.

In the thermogram of 3-methyl thiophene substituted 1b (Fig. 1a), it shows a wide temperature range of liquid crystalline phases for around 50 degree (ΔT) and low isotropic–LC phase transition temperature at 125 °C. An optical texture typical of a smectic A (SmA) phase was observed at 123 °C as shown in Fig. 2a for the compound 1b. Its metastable smectic E phase^2b was confirmed on cooling (Fig. 2b and magnified inset at 80 °C) followed by arced focal conic (filament-like) texture at 70 °C (Fig. 2c). Then, an amorphous crystalline state was observed at room temperature (Fig. 2d). 3-Methyl thiophene substituted 1a with shorter alkyl chain length (hexyl) gives similar phase transition temperatures (Fig. S1a†) and textures (Fig. S2†) to those of 1b.

Fig. 1 DSC profiles: (a) 1b and (b) 3 (solid line) and 2b (dashed line).

Fig. 2 Optical textures of 1b at 123 °C (a), 80 °C (b), 70 °C (c), and at room temperature (d).

Parallel alkyl or alkynyl substitution to TT-based mesogen moieties resulted in high isotropic temperatures, and their mesomorphic phases were hardly observed by DSC as shown in Fig. 1b. Only a pair of peaks during heating and cooling cycle was found at 233 °C which corresponds to the isotropic–crystalline transition for the parallel bis-hexyl substituted 2b. Alkynyl substituted compound 3 shows two pairs of partially overlapped peaks at around 210 °C (Fig. 1b) – isotropic and crystalline phase transitions accompanying very short intermediate mesomorphic phase and additional small glass transition peaks below crystalline temperature. For the compound 2b, a flake-like mesomorphic texture (Fig. 3a) was observed which was not detected by DSC in between isotropic–crystalline transition at 235 °C. Compound 3 shows a fan-shape LC texture (Fig. 3b) at 202 °C. However, crystalline phases of both compounds were much predominantly observed compared to mesomorphic phases. Parallel alkyl or alkynyl substitution to mesogen moieties probably induces strong tendency to form π–π stacking of the molecules and dense molecular packing.

Fig. 3 Optical texture of liquid crystalline phases from 2b (a), 3 (b), and 4a (c) and needle-like mesophase from 4b (d).

On the other hand, change in the lateral methyl substitution position: a 4-methylthiophene substitution to TT (4) resulted in somewhat intermediate thermal phase transition properties between 3-methylthiophene substitution (1) and parallel alkyl substituted to TT (2 and 3) as shown in Table 1 and Fig. S1.† Although much higher isotropic temperatures compared to those of compound 1 were exhibited, quite different thermal phase transitions and textures were attainable by changing parallel alkyl chain length. Clear smectic A phase was maintained over a wide range of temperature (ΔT ∼ 120 °C) for the hexyl substitution (4a), and needle-like mesophase was exhibited for the dodecyl substitution (4b) as shown in Fig. 3c and d, respectively.

Table 1 Phase transition temperature^a,b of TT derivatives

	Heating	Cooling
a Phase transition temperatures (°C) recorded during the second heating and cooling cycle.b Phase assignment; Cr: crystalline phase; M: ordered mesophase; SmA: smectic A phase; SmE: smectic E phase; Iso: isotropic liquid.
1a	Cr 105 SmA 125 Iso	Iso 123 SmA 93 SmE 73 Cr
1b	Cr 73 SmA129 Iso	Iso 126 SmA 78 SmE 64 Cr
2b	Cr 232 M 239 Iso	Iso 233 M 226 Cr
3	Cr 212 M 220 Iso	Iso 212 M 206 Cr
4a	Cr 53 SmA 199 Iso	Iso 196 SmA 78 Cr
4b	Cr 227 M 231 Iso	Iso 226 M 222 Cr

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A mesogen unit bearing long rigid segment in the central region and flexible chains at its two ends is a typical liquid crystal molecule aligns well with a lamellar structure. As the one of the spacer comes close to central region of the rigid segment, it can interrupt mesogens from being densely stacked into aligned domains, but enough to form moderately orientational packing structure resulting in lower phase transition temperature with smectic A phase than that of reported liquid crystalline thieno[3,2-b]thiophene derivatives.⁸

Optical properties

The absorption spectra of all TT-derivatives are depicted in Fig. 4. As clearly shown in the figure, they exhibit spectral shift with change in alkyl substitution to the thiophene unit. Parallel hexyl and decynyl substituted 3 was found to have most red-shifted absorption maximum (λ_max) at 392 nm with shoulder at around 414 nm due to the slightly extended conjugation by acetylene group, followed by parallel bis-hexyl substituted 2b (λ_max = 368 nm).

Fig. 4 Absorption spectra of TT-derivatives in THF solutions.

In case of lateral methyl substitution, the other end-alkyl length change induced no difference in absorbance as shown for the hexyl and dodecyl substitution between 1a and 1b, and between 4a and 4b, respectively. However, blue-shift around 15 nm was measured by changing the substitution position from 4-methyl (λ_max = 367 nm) to 3-methyl (λ_max = 352 nm) thiophene. Since a decreased planarity is achieved in the latter when shifting the alkyl chain along the conjugated backbone as side chain.^4a In particular, 3-methyl thiophene substituted compounds (1) have higher steric hindrance to the mesogen core compared to those of 4-methyl thiophene (4) or parallel spacer substituted (2a, 2b and 3) system, leading to less planar conformation exhibiting larger band gaps than other organic semiconductors.

Charge transport properties characterization

The charge transport characteristics of the LC compounds¹⁰ (1 and 3) were investigated by the time-of-flight technique, and their charge carrier mobilities were determined from the kink point in the transient photocurrent curve in double logarithmic plots as summarized in Table 2. The transient photocurrents for the holes were examined at smectic and crystalline phases for compound 1. Non-dispersive transient photocurrents were observed for holes at temperatures exhibiting LC phase. Each curve was found to have a clear shoulder indicating a transit time when the positive carriers arrive at the counter electrode i.e. shown in Fig. 5a and b for 1a and 1b with the highest hole mobilities of 2 × 10⁻⁴ cm² V⁻¹ s⁻¹ and 3.4 × 10⁻⁴ cm² V⁻¹ s⁻¹ at 115 °C and 125 °C, respectively.

Table 2 Charge carrier mobility values of positive carriers determined by TOF for TT derivatives in different phases

	Temp./°C	Phase	μ/cm^2 V^−1 s^−1
1a	115	SmA	2 × 10^−4
1b	125	SmA	3.4 × 10^−4
3	158	Cr	1.9 × 10^−3

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Fig. 5 Transient photocurrent signals for holes of 1a at 115 °C (a) and 1b at 125 °C (b) in the SmA phases and of 3 at 158 °C (c) in crystalline phase.

Although it was not possible to determine the hole mobility of 3 at its mesophase because the cell temperature was not able to be elevated upto that temperature, soft crystals showed similar phenomena to amorphous semiconductor having the highest hole mobility of 2 × 10⁻³ cm² V⁻¹ s⁻¹ at 158 °C. Despite the lack of a liquid crystalline transition, 3 yields highly ordered films resulting in a higher hole mobility compared to those of 1.

Fig. 6 shows the hole mobilities of 1b and 3 as a function of a square root of electric field. Hole mobilities were proportional to the square root of electric fields which are similar to those of the amorphous organic semiconductors whose carrier transport characteristics can be described by the Gaussian^10a or correlation disorder models.^10b Besides, decreasing behavior of hole mobility was observed by decreasing temperature to 80 °C (μ = 1.2 × 10⁻⁴ cm² V⁻¹ s⁻¹ at smectic E phase) of 1b (Fig. S3a†). At 70 °C exhibiting arced focal conic (filament-like) texture and at room temperature (Fig. 2c and d), it was not able to distinguish the kink point from the photocurrent curves for both holes and electrons (Fig. S4†) probably originated from the formation of grain boundaries during the crystalline process which trap the charge carriers. Unlikely to 1b, 3 contains temperature-independent mobility region between at 110 °C and 130 °C where small glass transition peaks were observed by DSC (Fig. S3b†).

Fig. 6 Hole mobilities of 1b (a) and 3 (b) as a function of the square root of electric field.

As revealed from the hole transport properties and thermal molecular reorientation behaviours of the TT derivatives, lateral alkyl substitution rather affects the self-organization behavior of the molecules than the chain length change at a parallel-end. Although one order reduction in the maximum hole mobility was inevitable, lateral methyl group substitution to rigid mesogen unit provided a relatively low temperature LC processability compared to that of reported TT derivatives⁸ with a moderate hole mobility by a facile synthetic route. It is of an efficient way to impede highly crystallizing and stacking behaviors of the rigid mesogens dramatically lowers the phase transition temperatures.

Conclusions

In conclusion, this study indicates that engineering of partial solubilizing group can be an efficient approach to tune the thermal transition and charge transport properties of highly conjugated crystalline or liquid crystalline semiconductors. Particularly, it would offer a perspective for highly performing active layers under mild fabrication condition for the OFETs.

Acknowledgements

This study was financially supported by F3 project of Hokkaido University.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: DSC profiles, images of LC phases, hole mobilities as a function of temperature, and photocurrent curves at room temperature. See DOI: 10.1039/c4ra11326h

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