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

: Star-shaped compounds are widely recognized as emerging materials for optical and electrical applications and as scaffolds of discotic liquid crystal. While the C 3 -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 C 3 -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.


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, selfassembly 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.

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 SiCl 4 /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). 2021, 2021, x FOR PEER REVIEW 2 of 5 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).

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 ∆T = 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 ∆H = 15.7 kJ/mol for melting and ∆H = 24.0 kJ/mol for clearing. A similar behavior has been observed for structural congeners with a tris-1,3,4-oxadiazolyltriazine center [33].
resulting from twofold Suzuki couplind and a hy calculated m/z = 1213.8313.

Mesomorphism
The combination of a rigid π-conjugated cor a flexible aliphatic periphery with six dodecyl ch lecular structure of 4. Polarized optical microsc mesophase (Figure 1). Differential scanning calo morphism. The second heating scan (10 K/min) 65.5 °C) and a clearing transition with onset at 9 S7). Both transitions appear at temperatures abo counterparts in the cooling scan. A peculiarity s thalpies are generally higher than the associated found for 4 are ΔH = 15.7 kJ/mol for melting and behavior has been observed for structural conge center [33].

Materials and Methods
General Information: Commercially availa

Materials and Methods
General Information: Commercially available reagents were used without further purification unless otherwise indicated; solvents and gases were dried by standard procedures. 1 H-and 13 C-NMR spectra: Bruker AC 300 (300 MHz) (Karlsruhe, Germany), Bruker