Extended conjugated mesogens: synthesis and mesomorphic properties of H-shaped mesogens based on 3,3ʹ,5,5ʹ-tetrasubstituted 2,2ʹ-bithiophene with oligo(1,4-phenyleneethynylene) arms

ABSTRACT A new class of extended conjugated mesogens, namely H-shaped mesogens based on 3,3ʹ,5,5ʹ-tetrasubstituted 2,2ʹ-bithiophene with oligo(1,4-phenyleneethynylene) arms, have been synthesised, and the relationships between molecular structures and mesomorphic properties investigated. Tetraalkyl, tetraalkoxy and dialkyldialkoxy derivatives, [R1C6H4CCC6H2(C2H5)2CC]2[R2C6H4CCC6H2(C2H5)2CC]2C8H2S2 where R1 and R2 = alkyl and alkoxy chains of different lengths, exhibit nematic phases. The length, number and position of the terminal chains strongly affect the mesomorphic properties. The tetraalkyl derivatives in which R1 = R2 = pentyl to heptyl exhibit enantiotropic mesophases, whereas the derivatives with octyl or nonyl chains exhibit monotropic mesophases. The tetraalkoxy derivatives in which R1 = R2 = pentyloxy to nonyloxy all exhibit enantiotropic nematic phases. The mesophase range increases with increasing alkoxy chain length, except that the octyloxy and nonyloxy derivatives have almost the same temperature range. The dialkyldialkoxy derivatives in which R1 = alkyl; R2 = alkoxy and in which R1 = alkoxy; R2 = alkyl (R1 and R2 = heptyl, nonyl, hexyloxy or nonyloxy) exhibit enantiotropic mesophases. The derivatives in which R1 = alkoxy have a significantly lower crystal–nematic transition temperature than the corresponding derivatives (R2 = alkoxy), although the two types of derivatives have a similar nematic–isotropic transition temperature. Graphical Abstract


Introduction
Extended conjugated mesogens have been the subject of considerable interest not only because of their optical and electrical properties [1][2][3][4][5] but also in terms of their mesomorphic properties (e.g. rod-like, [6][7][8][9][10] V-shaped, [11][12][13] star-shaped [14,15] and discotic [16][17][18][19][20] mesogens). The mesogens generally consist of a conjugated core and aliphatic chains. The shape and size of the core, and the length, number and position of the flexible chains are structural factors that affect or control the mesomorphic properties. However, the relationship between molecular structure and mesomorphic properties remains incompletely understood. Studies of the structure-property relationship have an important role to play in research on liquid crystalline conjugated materials.
To further our studies of extended conjugated mesogens and to investigate the effect of extending the 4PE2T core on mesomorphic behaviour, we have synthesised bithiophene derivatives that have oligo-(1,4-phenyleneethynylene) substituents instead of phenylethynyl groups. The extended core, that is, 3,3ʹ,5,5ʹtetrakis [4-(phenylethynyl)phenylethynyl]-2,2ʹ-bithiophene (4(PE) 2 2T), has four internal phenyl rings, and eight ethyl groups are attached to the rings ( Figure 3); that is, 4(PE) 2 2T derivatives contain four diethyl-substituted oligo(phenyleneethynylene) arms similarly to the (PE) 6 P derivatives shown in Figure 1. The ethylsubstituted 4(PE) 2 2T derivatives are designed to have a lower transition temperature and, moreover, to exhibit a higher solubility in a common organic solvent than 4(PE) 2 2T derivatives without side chains (or derivatives with only terminal chains) would do.
In this paper, we report the synthesis and mesomorphic properties of 4(PE) 2 2T derivatives 5a-e, 6ae, 7a, 7b, 8a and 8b. Derivatives 5a-e contain four terminal alkyl chains, whereas derivatives 6a-e have alkoxy chains instead. Derivatives 7a, 7b, 8a and 8b have two alkyl and two alkoxy chains at the terminal positions, but the positions of the alkyl and alkoxy chains in derivatives 7a and 8a are different from those in derivatives 7b and 8b. For tetraalkyl-, tetraalkoxy-and dialkyldialkoxy-substituted 4(PE) 2 2Ts, the effects of the length, number and position of the alkyl and alkoxy chains on the mesomorphic properties are discussed, and the structure-property relationships in the 4(PE) 2 2T derivatives are compared with those in the corresponding 4PE2T derivatives. Additionally, the entropy change of the N-I transition for the 4(PE) 2 2T mesogen is described in comparison with those found for the related mesogens.
The 4(PE) 2 2T derivatives are red-orange solids, soluble in common organic solvents such as chloroform, tetrahydrofuran or hexane. A hexane-ethanol or a chloroform-ethanol mixture is found to be a suitable recrystallisation solvent.

Tetraalkyl-substituted 4(PE) 2 2Ts
Derivatives 5a-e have four terminal alkyl chains of the same length, from pentyl to nonyl. These derivatives exhibit enantiotropic or monotropic nematic phases. Nematic Schlieren textures are observed for the derivatives ( Figure 4). Their thermotropic data are summarised in Table 1.
Derivatives 5a-c, with pentyl, hexyl and heptyl chains, respectively, exhibit enantiotropic mesophases. The N-I transition temperature decreases significantly   with increasing terminal chain length (5a: 157°C; 5b: 139°C; 5c: 134°C). The change in the Cr-N transition temperature (5a: 133°C; 5b: 122°C; 5c: 123°C) is slightly different from that in the N-I transition temperature. The replacement of the pentyl chains with hexyl chains causes a significant decrease in the Cr-N transition temperature, but the transition temperatures of the hexyl and heptyl derivatives are almost the same. As a result of these changes, a significant decrease is observed in the mesophase range (5a: 24°C; 5b: 17°C; 5c: 11°C). Derivatives 5d and 5e, with octyl and nonyl chains, respectively, exhibit monotropic mesophases. For the octyl derivative, the Cr-I transition occurs at 118°C, and the mesophase is observed from 118°C to 62°C on cooling. For the nonyl derivative, the Cr-I transition occurs at 122°C, and the mesophase is found between 113°C and 81°C.
These results show that the 4(PE) 2 2T unit is a mesogenic core, and that the mesomorphic properties of the tetraalkyl-substituted 4(PE) 2 2Ts are strongly affected by the length of the alkyl chains. The derivatives with pentyl, hexyl or heptyl chains, 5a-c, exhibit enantiotropic mesophases, whereas the derivatives with octyl or nonyl chains, 5d and 5e, exhibit monotropic mesophases. For derivatives 5a-c, the mesophase range decreases significantly with increasing alkyl chain length. Interestingly, the enantiotropic or monotropic behaviour of derivatives 5a-e is the same as that of the corresponding 4PE2T derivatives, 1a-e, except that hexyl derivative 1b exhibit a monotropic nematic phase ( Figure 5). Both of pentyl derivatives 1a and 5a and both of heptyl derivatives 1c and 5c exhibit enantiotropic nematic phases. The replacement of the heptyl chains of derivatives 1c and 5c by octyl or nonyl chains results in monotropic nematic phases (derivatives 1d, 1e, 5d and 5e). This is probably because the longer chains stabilise further the crystalline phase by additional Van der Waals interactions.
Interestingly, 1,2,4,5-tetrakis(4ʹ-alkyl-biphenyl-4ethynyl)benzenes where alkyl = butyl, hexyl, octyl and decyl), X-shaped nematogens that have a molecular structure similar to these of derivatives 5a-e, exhibit enantiotropic nematic phases with a wide temperature range (15°C, 40°C, 36°C and 41°C, respectively). [15] The longer alkyl chains of the X-shaped mesogens do not stabilise the crystalline phase but assist the formation of the mesophase. The X-shaped core, based on 1,2,4,5-tetrasubstituted benzene, has a high symmetry, whereas the 4(PE) 2 2T core, containing 2,2ʹ-bithiophene, has a skewed H-shape. It appears that the difference of the shape of the core causes the difference of the behaviour of the longer chains between the X-and H-shaped mesogens. For a better understanding of these structure-property relationships, it will be necessary to investigate the crystal structures of the 4(PE) 2 2T derivatives.

Tetraalkoxy-substituted 4(PE) 2 2Ts
Derivatives 6a-e contain four terminal alkoxyl chains of the same length, from pentyloxy to nonyloxy. All the alkoxy derivatives exhibit enantiotropic nematic phases. Optical textures characteristic of a nematic phase are observed both on heating and cooling ( Figure 6). The thermotropic data are summarised in Table 2, and the changes in the transition temperatures are illustrated in Figure 7.
The N-I transition temperature decreases significantly with increasing alkoxy chain length (190°C, 183°C, 170°C, 165°C and 154°C for 6a-e, respectively). The Cr-N transition temperature also decreases; however, the replacement of the heptyloxy chains with octyloxy chains causes a great decrease (184°C, 172°C, 154°C, 114°C and 105°C for 6a-e, respectively). As a result of these changes, the mesophase range increases with increasing alkoxy chain length, except that the ranges of the octyloxy and nonyloxy derivatives, 6d and 6e, are almost the same (6°C, 11°C, 16°C, 51°C and 49°C for 6a-e, respectively). In addition, the ranges of derivatives 6d and 6e are much broader than those of derivatives 6a-c.
These results show that the alkoxy chains are important constituents of the 4(PE) 2 2T mesogen. All the alkoxy derivatives exhibit enantiotropic nematic phases, unlike the corresponding alkyl derivatives. The results also show that length of the alkoxy chains strongly affects the mesomorphic properties. The Cr-N and N-I transition temperatures decrease with increasing alkoxy chain length, and the use of octyloxy or nonyloxy chains gives rise to a wide mesophase range. Interestingly, these effects of the alkoxy chain length on the mesomorphic properties are quite different from the observations for the alkyl derivatives. For alkyl derivatives 5a-c, the mesophase range decreases with increasing terminal chain length; for alkoxy derivatives 6a-c, the mesophase range increases, although the Cr-N and N-I transition temperatures of the alkoxy derivatives are much higher than those of the corresponding alkyl derivatives. Alkyl derivatives 5d and 5e exhibit monotropic mesophases; alkoxy derivatives 6d and 6e exhibit enantiotropic mesophases with a wide temperature range.
The mesomorphic properties of derivatives 6a-e are very similar to those of the corresponding 4PE2T derivatives, 2a-e, except that the Cr-N transition temperatures of derivatives 6a-c are much higher than those of the corresponding derivatives (derivatives 6a-c have a narrower mesophase range than derivatives 2a-c). Both  the octyloxy derivatives, 2d and 6d, and both the nonyloxy derivatives, 2e and 6e, exhibit enantiotropic mesophases with a wide temperature range. This is quite different from the corresponding alkyl derivatives, 1d, 1e, 5d and 5e, which exhibit monotropic mesophases. Interestingly, the mesomorphic properties of tetraalkoxy derivatives 6a-e are similar to those of the X-shaped mesogens mentioned in the previous section. In both cases, the longer chains assist the formation of the mesophase with a wide temperature range. This indicates that the O atoms attached to the H-shaped core have an important role to play in stabilising the mesophase. For a better understanding of the structure-property relationship in 4(PE) 2 2T derivatives, it will be necessary to investigate the mesomorphic properties of derivatives with two alkyl and two alkoxy chains. The mesomorphic properties of 4(PE) 2 2T derivatives that have the same terminal chains as 4PE2T derivatives 3a, 3b, 4a and 4b are discussed in the following section.

Dialkyldialkoxy-substituted 4(PE) 2 2Ts
Derivatives 7a and 7b have two heptyl and two hexyloxy chains at the terminal positions similarly to derivatives 3a and 3b, respectively. Derivatives 8a and 8b have nonyl and nonyloxy chains similarly to derivative 4a and 4b, respectively. With the two types of derivatives, that is, 7a and 8a (R 1 = alkyl; R 2 = alkoxy) and 7b and 8b (R 1 = alkoxy; R 2 = alkyl), the effects of the number and position of the alkoxy chains on the mesomorphic properties have been investigated. The thermotropic data of these derivatives are listed in Table 3. Derivatives 7a and 7b exhibit enantiotropic nematic phases with a wide temperature range. The N-I Table 2. Phase transition temperatures (°C) and transition enthalpies (in square brackets, kJ mol -1 ) of compounds 6a-e.  transition temperatures of derivatives 7a and 7b are similar (157°C and 164°C, respectively), whereas the Cr-N transition temperature of derivative 7b (114°C) is significantly lower than that of derivative 7a (131°C). As a result, derivative 7b has a wider mesophase range than derivative 7a (7a: 26°C; 7b: 50°C). Derivatives 8a and 8b also exhibit enantiotropic nematic phases with a wide temperature range (8a: 113-136°C; 8b: 100-141°C). The effects of the position of the nonyloxy chains on the Cr-N and N-I transition temperatures are the same as the observations for derivatives 7a and 7b, and derivative 8b has a wider mesophase range than the other derivative. As shown in Figure 8, the Cr-N transition temperatures of 7a and 7b are significantly lower than the average of those of derivatives 5c and 6b, although the N-I transition temperatures of derivatives 7a and 7b are almost the same as the average of those of derivatives 5c and 6b. Similar features are found for a series of derivatives 8a, 8b, 5e and 6e. Derivative 8b has the lowest Cr-N transition temperature and exhibits almost the same mesophase range as derivative 6e.
These results indicate that the alkoxy chains of the dialkyldialkoxy derivative have a crucial role to play in their mesomorphic behaviour. The replacement of the two alkyl chains of the tetraalkyl derivative by alkoxy chains causes an enantiotropic mesophase with a wide temperature range. Thanks to the two alkoxy chains,  Figure 8. Comparisons of the transition temperatures of dialkyldialkoxy derivatives 7a, 7b, 8a and 8b, tetraalkyl derivatives 5c and 5e and tetraalkoxy derivatives 6b and 6e.
the derivative has a significantly lower N-I transition temperature than the corresponding tetraalkoxy derivative. The results also show that the position of the alkoxy chains (R 1 or R 2 = alkoxy) is an important factor in mesomorphic behaviour. Although the N-I transition temperatures of the derivatives are hardly affected by the position, the Cr-N transition temperatures are strongly affected. The Cr-N temperature of the derivative in which R 1 = alkoxy is significantly lower than those of the other derivative. These effects of the number and position of the alkoxy chains on the transition temperatures are the same as the observations for the corresponding 4PE2T mesogens. For the both H-shaped mesogens, the attachment of alkyl and alkoxy chains in the R 1 and R 2 positions is a useful method for controlling the mesomorphic properties, and the attachment of alkoxy chains in the R 1 positions is more favourable for an increase in the mesophase range. The mesogenic core has a skewed H-shape. The long rod includes the terminal (5-and 5ʹ-) positions of 2,2ʹ-bithiophene and therefore is linear, whereas the short rod, including the lateral (3-and 3ʹ-) positions, is not linear. The observations indicate that the attachment of alkoxy chains at the ends of the long rod causes a wider mesophase range. For a better understanding of the effects of such attachment, it will be necessary to investigate the mesomorphic properties of different H-shaped mesogens, for example, derivatives of 3,3ʹ-bis(phenylethynyl)-5,5ʹ-bis[4-(phenylethynyl)phenylethynyl]-2,2ʹbithiophene (2PE2(PE) 2 2T) and 3,3ʹ-bis[4-(phenylethynyl)phenylethynyl]-5,5ʹ-bis(phenylethynyl)-2,2ʹ-bithiophene (2(PE) 2 2PE2T).

Conclusion
A new class of extended conjugated mesogens that exhibit nematic phases, that is, H-shaped mesogens based on 3,3ʹ,5,5ʹ-tetrasubstituted 2,2ʹ-bithiophene with oligo(1,4-phenyleneethynylene) arms, have been synthesised, and the relationship between molecular structure and mesomorphic properties has been studied. The results show that the length, number and position of the alkyl and alkoxy terminal chains strongly affect the mesomorphic properties.
Tetraalkyl derivatives 5a-c, which have pentyl, hexyl and heptyl chains, respectively, exhibit enantiotropic mesophases, whereas derivatives 5d and 5e, with octyl and nonyl chains, respectively, exhibit monotropic mesophases. For the C5-C7 derivatives, the mesophase range decreases with increasing alkyl chain length. The replacement of the heptyl chains of derivative 5c with octyl or nonyl chains destroys the mesophase on heating.
Tetraalkoxy derivatives 6a-e, with alkoxyl chains from pentyloxy to nonyloxy, respectively, exhibit enantiotropic mesophases. The mesophase range increases with increasing alkoxy chain length, except that the octyloxy and nonyloxy derivatives, 6d and 6e, have almost the same temperature range. The ranges of the two derivatives are much broader than those of derivatives 6a-c.
Diheptyldihexyloxy derivatives 7a and 7b and dinonyldinonyloxy derivatives 8a and 8b exhibit enantiotropic mesophases. Derivatives 7b and 8b (R 2 = alkyl; R 1 = alkoxy) have a significantly lower Cr-N transition temperature than derivatives 7a and 8a (R 1 = alkyl; R 2 = alkoxy), respectively. The two types of derivatives, 7a and 7b; and 8a and 8b, have a similar N-I transition temperature, and the temperatures are significant lower than those of the corresponding tetraalkoxy derivatives.
The structure-property relationships of the newly synthesised mesogens should prove useful for the molecular design of both H-shaped mesogens and other types of mesogens with extended conjugations. We believe that systematic studies of these conjugated mesogens will play an important role in research on liquid crystalline conjugated materials.

Methods
Silica gel column chromatography was carried out using Wakogel C-200 (0.075-0.15 mm, Wako Pure Chemical Industries, Ltd (Wako)). Recycling gel permeation chromatography (GPC) was performed using an HPLC system equipped with a HPLC pump (JASCO PU-2086), two columns in series (Shodex K-2002 and K-2002.5), a refractive index detector (JASCO RI-2031) and a recycle valve (GL Science HPV-Rc). Chloroform was used as eluent at a flow rate of 3.0 ml min -1 .
The chemical structures of the compounds were confirmed by their 1 H and 13 CNMR spectra, recorded on a Bruker Avance 500 MHz NMR spectrometer (Fällanden, Switzerland) in CDCl 3 with TMS as internal standard. Matrix-assisted laser desorption/ionisation time-of-flight mass spectrometry (MALDI-TOF-MS) was performed on a Bruker Daltonics Autoflex Speed MALDI-TOF/TOF mass spectrometer (Billerica, MA, USA), using 2,5-dihydroxybenzoic acid as the matrix. Elemental analyses were carried out on a CE Instruments EA 1110.
The mesomorphic properties of the compounds were investigated by differential scanning calorimetry (DSC) and polarising optical microscopy (POM). Phase transition temperatures and enthalpies were determined on a Perkin-Elmer DSC 7 instrument (Norwalk, CT, USA), operated at a scanning rate of 5°C min -1 for heating and cooling cycles. Optical textures were observed using a Leitz Orthoplan-Pol polarising microscope (Wetzlar, Germany) equipped with a Mettler FP82HT hot stage. Photomicrographs were taken using a Nikon Coolpix 5000 digital camera.

Synthesis
Synthetic procedures and characterisation data for compound 9a, 12a, 12b, 13a and 13b are described in the Supplemental Information.

Compounds 5a-e
To a solution of compound 9a and the corresponding phenyleneethynylene dimer in i-Pr 2 NH (25 ml) were added Pd(PhCN) 2 Cl 2 , PPh 3 and CuI. The mixture was stirred for 18 h at room temperature. MeOH (80 ml) was added to the reaction mixture, and the solid was filtered and washed with MeOH (20 ml). The resulting solid was chromatographed on silica gel using hexane-CH 2 Cl 2 (4:1). Recrystallisation from hexane-EtOH gave the desired product. An analytical sample was obtained by recycling GPC followed by recrystallisation from CHCl 3 -EtOH.  13