2-{3,5-Bis-[5-(3,4-didodecyloxyphenyl)thien-2-yl]phenyl}-5-(3,4-didodecyloxyphenyl)thiophene

Abstract

Star-shaped compounds are widely recognized as emerging materials for optical and electrical applications and as scaffolds of discotic liquid crystal. While the C₃-symmetrical tri(phenylthienyl)benzene is the core for several electroopotical materials, no liquid crystal with this scaffold has yet been reported. Acid-catalyzed cyclocondensation of bromoacetylthiophene gives a C₃-symmetrical star, threefold Suzuki coupling results in extension of the conjugated system. With 3,4-didodecylocyphenyl boronic acid, a star with a large rigid conjugated system and flexible aliphatic periphery is obtained. Differentials scanning calorimetry and polarized optical microscopy reveal an enantiotropic mesophase from 66 °C to 106 °C.

1. Introduction

Thiophene is a very prominent building block for the synthesis of organic electronic materials [1,2] and a large variety of oligo- and polythiophenes have been prepared and successfully applied as active material in electronic devices, e.g., OLEDs, OFETs, OVPs and sensors [3,4,5]. The alignment of the materials within the conductive layer is a highly competitive challenge in plastic electronics. Jiang et al. [6] presented the targeted electrochemical polymerization to optimally align the π-systems on the electrode. Discotic liquid crystals (DLCs) offer another strategy for orientation and targeted alignment, self-assembly of their large aromatic cores can form conductive columns [7,8,9]. Whereas a large number of star-shaped [10,11] and discotic [12,13] π-conjugated systems have been investigated, compounds with a thiophene based core appear only scarcely [13,14]. 1,3,5-Tris-(2-thienyl)benzene has been prepared via cyclocondensation of 2-acetylthiophenes [15], a recent approach is based on the addition of benzenetri(methylthiol) to diynes [16]. Extension of the conjugated system is possible via pd-catalyzed coupling reactions [17]. A few optoelectronic materials based on 1,3,5-trithienylbenzene have been extensively investigated [18,19,20,21,22]. Though this scaffold has even been used for the construction of catalysts [23] and gold nanoparticles [24], mesomorphous materials based on 1,3,5-trithienylbenzene are unknown. Here we report the synthesis of tris(didodecyloxyphenylthienyl)benzene, a π-conjugated star and the first discotic liquid crystal with this core.

2. Results

2.1. Synthesis

Retrosynthetic analysis of the title compound gives different possibilities, either the combination of the arms with formation of the central ring or the stepwise approach, construction of the arms in successive threefold coupling reactions. The first is an alkyne cyclotrimerization of ethynyl-substituted alkoxyphenyl thiophene. This generates the central ring, but with a notoriously poor regioselectivity. Starting with a 1,3,5-funtionalized benzene allows the second approach. This is a sequence of three three-fold reactions: a three-fold Suzuki-Miyaura coupling, three-fold bromination followed by a second threefold coupling reaction [25]. 1,3,5-Triarylbenzenes are easily prepared via cyclocondensation of acetylarenes [26]. Though this method is applicable to acetylthiophene [27], the cyclization of the analogous 5-bromo-2-acetylthiophene failed [28]. Nevertheless, Tokárová reported a 28% yield of the trimer when SiCl₄/ethanol was used as dehydrating agent [29]. In an extensive study, we could raise the yield up to 37%. The connection of the dialkoxyphenyl rings to the thiophenes was performed via Suzuki-Miyaura coupling of 3,4-di(dodecyloxy)phenylboronic acid [30] to the tribromo trithienyl benzene (Scheme 1).

2.2. Experimental Procedures: Synthesis of 1,3,5-Tris(5-bromothienyl)benzene 2

2-Acetyl-5-bromothiophene 1 (500 mg) was dissolved in dry ethanol (5 mL) at 0 °C in a N₂ atmosphere. SiCl₄ (0.7 mL, 2.5 eq) was added dropwise over 10 min to the stirred solution and stirring was continued for 24 h at ambient temperature. Two further portions SiCl₄ (each: 0.42 mL, 1.5 eq) were added after 24 and 48 h. the mixture was diluted with ice/water (5 mL) and extracted with toluene (3 × 5 mL). Washing the combined organic solutions with brine, drying with MgSO₄, evaporation of solvent and chromatography (SiO₂, petroleum ether/toluene = 5/1 over a short column yielded, after recrystallization from ethanol, the tribomide 2 (137 mg, 37 %) as yellow solid with m.p. = 176–178 °C. 1H NMR (300 MHz, CDCl₃, 298 K): δ = 7.52 (m, 3 H); 7.14 (m, 3 H), 7,97 (d, 3 H, J = 3.9 Hz). Whereas ¹H-NMR data are identical to the data reported from different groups, e.g., in ref. [28,31], the melting point of 2 is about 64° higher, as shown in the published data. As Rebourt et al. [32] reported a melting point of 183 °C, we assume polymorphism of 2.

Experimental Procedure: Synthesis of 2.3.2-{3,5-Bis-[5-(3,4-didodecyloxyphenyl)thien-2- yl]phenyl}-5-(3,4-didodecyloxyphenyl)thiophene 4

A Schlenk tube was charged with tris- 1,3,5-(5′ bromothien 2 yl)benzene 2 (72 mg, 128 µmol, 1.0 eq), 3,4 didodecyloxypenylbronic acid 3 (208 mg, 424 µmol, 1.0 eq), and K₂CO₃ (80 mg, 577 µmol, 4.5 eq). After a mixture of toluene and water (2.5/1.25 mL) had been added, the mixture was degassed with nitrogen at 50 °C and tetrakistriphenylphosphin palladium(0) (8.0 mg, 7 µmol, 5.4 mol-%) was added and the mixture refluxed for 48 h. After dilution of the mixture with water (20 mL) and toluene (20 mL), the organic layer was separated and the aqueous layer was extracted with toluene (3 × 5 mL). The pooled organic solution was washed with hydrochloric acid (2 M, 10 mL) and water (15 mL), dried over MgSO₄ and the solvent was evaporated. The remaining brownish wax was purified by column chormatography on silica with toluene/petroleum ether as eluent. A gradient from 0:1 to 1:0 was applied. The main fraction was recrystallized from propanol-2 and a second chromatography on alumina with cyclohexane/toluene gradient and, finally, recrystallization from propanol-2 gave 31 mg (15%) of the title compound 4 with m.p. = 68 °C. R_f = 0.74 (SiO₂, CH₂Cl₂/petroleum ether 1:1). ¹H NMR (300 MHz, CDCl₃, 298 K, HSQC, HMBC): δ = 7.74 (s, 3H, H1), 7.39 (d, ³J = 3.67 Hz, 3H, H4), 7.17–7.22 (m, 9H, H5,8,12), 6.96 (d, ³J = 9.29 Hz, 3H, H11), 4.01–4.11 (m, 12H, H13,25), 1.79–1.91 (m, 12H, H14,26), 1.27–1.54 (m, 118H, H15-23,27–35), 0.84–0.90 (m, 18H, H24-36). (Figure S2). ¹³C NMR (75 MHz, CDCl₃, 298 K, HSQC, HMBC): δ = 149.52 (C10), 149.33 (C9), 144.67 (C6), 141.80 (C2), 135.83 (C3), 127.48 (C7), 124.77 (C4), 123.22 (C5), 121.70 (C1), 118.70 (C12), 114.15 (C11), 111.86 (C8), 69.61 (C13/21), 69.52 (C13/21), 32.09, 29.87, 29.81, 29.62, 29.61, 29.54, 29.51, 29.45, 26.24, 26.20, 22.86 (C14-23,26–35), 14.29(C24,36). (Figure S3). IR (ATR)/[cm⁻¹]: 2919, 2849, 1586, 1516, 1464, 1389, 1322, 1251, 1139, 1019, 794, 721, 671. HR-MS (APCI): calcd.: 1657.2003; found: 1657.2313 [M]⁺; 1658,2371 [M + H]⁺. Mass spectrometry reveald traces of an impurity with m/z = 1213,8109. This corresponds to a product resulting from twofold Suzuki couplind and a hydro-debromination on the third branch, calculated m/z = 1213.8313.

2.3. Mesomorphism

The combination of a rigid π-conjugated core composed of seven aromatic rings and a flexible aliphatic periphery with six dodecyl chains results in an anisotropy of the molecular structure of 4. Polarized optical microscopy shows birefringence of the mobile mesophase (Figure 1). Differential scanning calorimetry reveals an enantiotropic mesomorphism. The second heating scan (10 K/min) shows a melting peak at 68.0 °C (onset: 65.5 °C) and a clearing transition with onset at 99.9 °C and maximum at 106.6 °C (Figure S7). Both transitions appear at temperatures about ΔT = 10 °C higher than their reverse counterparts in the cooling scan. A peculiarity should be mentioned: While melting enthalpies are generally higher than the associated clearing transition enthalpies, the values found for 4 are ΔH = 15.7 kJ/mol for melting and ΔH = 24.0 kJ/mol for clearing. A similar behavior has been observed for structural congeners with a tris-1,3,4-oxadiazolyltriazine center [33].

3. Materials and Methods

General Information: Commercially available reagents were used without further purification unless otherwise indicated; solvents and gases were dried by standard procedures. ¹H- and ¹³C-NMR spectra: Bruker AC 300 (300 MHz) (Karlsruhe, Germany), Bruker AV 400 (400 MHz) (Karlsruhe, Germany), and Bruker ARX 400 (400 MHz) (Karlsruhe, Germany), solvents CDCl₃. Chemical shifts are expressed as δ values in ppm, coupling constants are given in Hz. Assignments of ¹H and ¹³C signals on the basis of HSQC, and HMBC experiments. Melting points: Stuart Scientific SMP3. DSC: Perkin Elmer (Rodgau, Germany), DSC 7, IR: JASCO 4100 FT-IR (ATR) (Pfungstadt, Germany), HR-ESI: Thermo QExactive Orbitrap MS NaICsI as reference. Polarized microscopy: Olympus BX51, Color View Olympus camera (Shinjuku, Japan), heating table Linkam LTS 350 for temperature regulation.

4. Conclusions

A combination of cyclocondensation and Suzuki-Miyaura coupling gives a short access to the first discotic liquid crystal with a 1,3,5-tris(phenylthienyl)benzene core.