Development of novel bistolane-based liquid crystalline molecules with an alkylsulfanyl group for highly birefringent materials

Abstract

In order to generate high-birefringence liquid crystal (LC) materials, the introduction of highly polarisable groups into the terminal positions of the mesogen represents one of the most important design strategies. Even though the alkylsulfanyl group is a potentially interesting option for this purpose, it has so far been very difficult to obtain enantiotropic mesophases for LCs containing this group. Herein, we report novel high-birefringence LC molecules based on 1,4-bis(2-phenylethynyl)benzene (bistolane) with alkylsulfanyl groups in the terminal positions. The incorporation of a fluorine atom into the central benzene ring of an alkylsulfanyl-substituted bistolane led to the formation of a well-defined enantiotropic nematic phase. In contrast, the nonfluorinated analogue with alkylsulfanyl groups did not exhibit a mesophase. In comparison with an alkoxy-substituted derivative, the birefringence of the alkylsulfanyl-substituted analog was substantially higher (Δn = 0.42 at 550 nm). Furthermore, the birefringence properties of the alkylsulfanyl-substituted derivative were observed to be proportional to the order parameter and largely temperature-dependent over the entire temperature range. These results thus provide not only fundamental insights into structure–reactivity relations, but also furnish practical design principles for the synthesis of new sulfur-containing, rod-shaped LC materials with optical applications.

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Introduction

Recently, liquid-crystalline (LC) materials with high birefringence (Δn) have been introduced in a variety of optical applications, including LC display (LCD) technology,¹ cholesteric (Ch) LC films,² holographic materials,³ LC lenses,⁴ and polarisers.⁵ In these applications, nematic liquid crystals are the most important type of LC materials, owing to their desirable fluidity and the ability to easily control their alignment.

The conventional design of high-Δn materials is based on rod-shaped cores, which are generated by a combination of aromatic rings and unsaturated bonds. This strategy usually provides an improved refractive index (n) value along the molecular long axis, the so-called extraordinary refractive index (n_e). To date, a number of high-Δn materials, wherein multiple aromatic rings such as benzene, naphthalene, thiophene, or pyridine are joined by alkane, alkene, or azo bonds, have been synthesised.^3,5,6 Often, highly polarisable groups such as cyano and isothiocyanate groups are attached to the terminal positions of such materials, in order to accomplish large molecular polarisability anisotropies.^6c,d,g,h

In the context of the aforementioned molecular design strategy, we believe that the alkylsulfanyl (alkylthio or thioether) group should also represent a promising option, due to its high polarisability⁷ and the flexibility and/or small bend angle of its S–C bond,⁸ which lead to low transition temperatures relative to alkoxy groups. Hird et al. reported that LC biphenyl derivatives, bearing alkylsulfanyl groups with varying length of the carbon chain, exhibited high Δn, albeit that liquid crystallinity was scarcely observed.^7a–d Moreover, Seed et al. reported that naphthalene-based, rod-like materials with butylsulfanyl groups exhibited high Δn and nematic phases.^7e,f Very recently, we reported novel hydrogen-bonded, tolane-based LC compounds with hexylsulfanyl groups as high-Δn materials, which exhibited enantiotropic mesophases induced by the carboxylic dimeric form.⁹ Compared to derivatives with alkyl and alkoxy groups, these materials exhibited high Δn and highly correlated mesophases on account of the high polarisability of the sulfur atoms and the S–S interactions. However, additional smectic phases were eventually observed at relatively high temperatures (T > 200 °C), just below the nematic LC regime. It is generally assumed that sulfur-containing rod-like molecules, due to their strong molecular attractions, can generate stable nematic phases only with difficulty. Accordingly, reports on rod-shaped LC materials with alkylsulfanyl groups are scarce,^6–10 while an abundance of literature related to the mesomorphic properties of discotic-¹¹ and bend-type^7,12 LC materials is available.

Herein, we report the synthesis and characterisation of the novel, conjugated, rod-like nematic LC material 6, which is based on 1,4-bis(2-phenylethynyl)benzene (bistolane), bearing alkylsulfanyl groups at the para-positions of the terminal benzene rings and a fluorine atom at the central benzene ring (Fig. 1). The use of bistolane offers two main advantages: (a) a high Δn due to the large anisotropic molecular structure, and (b) the opportunity to readily implement molecular derivatisation on account of 14 potential substitution positions on the three benzene rings.^6a,b,e,q In this study, we focused on bistolane derivatives with a fluorine atom on the central benzene ring (4–6). Such a fluorination should effectively lower the transition temperatures and expand the nematic phase.¹³ Furthermore, we speculated that the fluorination of bistolane derivatives with alkylsulfanyl groups could also lead to the formation of enantiotropic nematic phases. Indeed, by introducing fluorine at the central benzene moiety of alkylsulfanyl-substituted bistolane 3 that did not exhibit a mesophase, an enantiotropic nematic phase was successfully generated for fluorinated 6. Its phase transition behaviour and Δn were examined in detail by comparison with fluorinated (4 and 5) and nonfluorinated analogues (1–3) that contained alkyl, alkoxy, or alkylsulfanyl end groups. Moreover, the Δn, n_o, and n_e values in this study are actual values for the nematic phases of pure LC target compounds, obtained from our previous report.^6k In contrast, Δn values in the literatures represent almost always extrapolations of measurements on mixtures that contain nematic LC compounds, such as e.g. 5-cyanobiphenyl (5CB).

Fig. 1 Molecular structure of bistolane derivatives 1–6.

Experimental

General

¹H NMR and ¹³C NMR spectra were measured in CDCl₃ on a JEOL LNM-EX 400 or a Bruker DPX300S spectrometer using tetramethylsilane (TMS) as an internal standard. The transition behavior was investigated by polarizing optical microscopy (POM) using a Leica DM2500P microscope with a Mettler FP90 hot stage, and by differential scanning calorimetry (DSC) using a Perkin Elmer DSC7, whereby heating/cooling scans were carried with a temperature gradient of 10 °C min⁻¹. Measurement of n_e, n_o and Δn was achieved with uniaxially aligned nematic cells, which were purchased from EHC and contained an indium tin oxide (ITO) layer. Cell gaps (d) of 2–3 μm were measured according to the interferometric method.¹⁴ The transmittance of light under crossed nicol conditions was recorded as a function of wavelength by a spectroscopic microscopy method using a Nikon LV100 Pol optical microscope equipped with a USB4000 (Ocean photonics) spectrometer.

Materials

1-Ethynyl-4-hexylbenzene, 4-iodophenol, 4-bromobenzenethiol, trimethylsilylacetylene, 4-bromoiodobenzene, 4-bromo-2-fluoroiodobenzene, and Pd(PPh₃)₄ were purchased from TCI (Tokyo, Japan), while 6-bromohexane, triethylamine (TEA), and PPh₃ were purchased from Wako Pure Chem (Tokyo, Japan). CuI was purchased from Kanto Chemical (Tokyo, Japan). Unless otherwise noted, all other chemicals were purchased from common commercial suppliers and used as received. 1-Ethynyl-4-hexyloxybenzene,^6k 1-bromo-4-hexylsulfanylbenzene,⁹ 1-hexylsulfanyl-4-[2-(trimethylsilyl)ethynyl]benzene,⁹ and 1-ethynyl-4-hexylsulfanylbenzene⁹ were synthesized according to previously reported procedures.

The synthetic routes to 1–6 are shown in Scheme 1, and the procedures and characterisation data, including ¹H and ¹³C NMR spectra, as well as high-resolution mass spectroscopy (HRMS) data, are described below. Compounds 1–6 were prepared in a yield of ∼50% by Pd-catalyzed Sonogashira coupling reactions.

Scheme 1 Synthetic route to 1–6.

1,4-Bis[4-(hexyl)phenyl]ethynylbenzene (1) (general procedure for the Sonogashira coupling). In a two-neck flask, a mixture of 1-ethynyl-4-hexylbenzene (3.11 mmol, 0.58 g), TEA (5 mL), and THF (5 mL) was degassed with argon bubbling. Another two-way flask was charged with 1-bromo-4-iodobenzene (1.41 mmol, 0.40 g), CuI (71 μmol, 85 mg), and Pd(PPh₃)₄ (71 μmol, 14 mg) and filled with argon, before the aforementioned liquid mixture was added. The resulting mixture was stirred for 12 h at 60 °C. Subsequently, 2 M HCl aq. was added to quench any remaining TEA, and all inorganic salts were removed by filtration. Then, the reaction mixture was extracted with ethyl acetate, and the combined organic fractions were washed with water and brine, before being dried over MgSO₄. After filtration and evaporation of the solvents, the crude product was purified by column chromatography on silica gel (eluent: hexane), and recrystallization from methanol. Colorless solid (0.35 g, 55%). ¹H NMR (400 MHz, CDCl₃) δ 7.51–7.48 (brs, Ar–H, 4H), 7.45 (d, J = 8.0 Hz, Ar–H, 4H), 7.17 (d, J = 8.0 Hz, Ar–H, 4H), 2.62 (t, J = 7.8 Hz, –PhCH₂–, 4H), 1.62 (tt, J = 7.0 and 7.8 Hz, –PhCH₂CH₂–, 4H), 1.39–1.22 (m, –CH₂CH₂CH₂CH₃, 12H), 0.89 (t, J = 6.8 Hz, –CH₃, 6H) ppm.¹³C NMR (100 MHz, CDCl₃) δ 143.7, 131.5, 131.4, 128.5, 123.1, 120.1, 91.4, 88.5, 35.9, 31.7, 31.2, 28.9, 22.6, 14.1 ppm.

1,4-Bis[[4-(hexyloxy)phenyl]ethynylbenzene (2). Compound 2 was synthesised according to the previously described Sonogashira coupling method. 1-Bromo-4-iodobenzene (0.400 g, 1.41 mmol), Pd(PPh₃)₄ (81.0 g, 71.0 μmol), CuI (14.0 mg, 71.0 μmol), 1-ethynyl-4-hexyloxybenzene (0.630 mg, 3.11 mmol), TEA (5 mL) and THF (5 mL) were used. Colorless solid (0.371 g, 55%). ¹H NMR (400 MHz, CDCl₃) δ 7.43 (d, J = 8.4 Hz, Ar–H, 8H), 6.85 (d, J = 8.4 Hz, Ar–H, 4H), 3.97 (t, J = 6.6 Hz, –OCH₂–, 4H), 1.83 (tt, J = 6.6 and 7.4 Hz, –OCH₂CH₂–, 4H), 1.50–1.32 (m, –CH₂CH₂CH₂CH₃, 12H), 0.91 (t, J = 6.6 Hz, –CH₃, 6H) ppm. ¹³C NMR (100 MHz, CDCl₃) δ 13.9, 22.6, 25.7, 29.2, 31.6, 68.3, 88.0, 91.4, 114.7, 115.2, 123.3, 131.3, 133.1, 159.5 ppm. HRMS-FAB⁺ (m/z): [M] calcd for C₃₄H₃₈O₂, 478.2872; found, 478.2878.

1,4-Bis[4-(hexylsulfanyl)phenyl]ethynylbenzene (3). Compound 3 was synthesised according to the previously described Sonogashira coupling method. 1-Ethynyl-4-hexylsulfanylbenzene (0.33 mmol, 1.50 g), 1-bromo-4-iodobenzene (0.71 mmol, 0.20 g), TEA (5 mL), THF (5 mL), Pd(PPh₃)₄ (75 μmol, 87 mg) and CuI (75 μmol, 14 mg) were used. Colorless solid (0.34 g, 47%). ¹H NMR (400 MHz, CDCl₃) δ 7.51–7.46 (brs, Ar–H, 4H), 7.43 (d, J = 8.0 Hz, Ar–H, 4H), 7.29–7.24 (m, Ar–H, 4H), 2.95 (t, J = 7.2 Hz, –SCH₂–, 4H), 1.68 (tt, J = 7.2 and 7.8 Hz, –SCH₂CH₂–, 4H), 1.44 (tt, J = 6.8 and 7.8 Hz, –SCH₂CH₂CH₂–, 4H), 1.36–1.24 (m, –CH₂CH₂CH₃, 8H), 0.89 (t, J = 7.0 Hz,–CH₃, 6H) ppm. ¹³C NMR (75 MHz, CDCl₃) δ 139.0, 133.9, 132.2, 131.8, 128.5, 120.5, 91.5, 89.8, 33.7, 31.7, 29.4, 28.8, 22.8, 14.2 ppm.

1,4-Bis[4-(hexyl)phenyl]ethynyl-2-fluorobenzene (4). Compound 4 was synthesised according to the previously described Sonogashira coupling method. 1-Ethynyl-4-hexylbenzene (3.11 mmol, 0.58 g), 1-bromo-3-fluoro-4-iodobenzene (1.41 mmol, 0.40 g), TEA (5 mL), THF (5 mL), Pd(PPh₃)₄ (71 μmol, 85 mg) and CuI (71 μmol, 14 mg). Colorless solid (0.30 g, 46%). ¹H NMR (400 MHz, CDCl₃) δ 7.47 (d, J = 8.0 Hz, Ar–H, 2H), 7.44 (d, J = 8.0 Hz, Ar–H, 2H), 7.44 (s, Ar–H, 1H), 7.29–7.22 (m, Ar–H, 2H), 7.17 (d, J = 8.0 Hz, Ar–H, 4H), 2.62 (t, J = 7.8 Hz, –PhCH₂–, 4H), 1.61 (tt, J = 7.0 and 7.8 Hz, –PhCH₂CH₂–, 4H), 1.38–1.21 (m, –CH₂CH₂CH₂CH₃, 12H), 0.88 (t, J = 6.6 Hz, –CH₃, 6H) ppm.

1,4-Bis[4-(hexyloxy)phenyl]ethynyl-2-fluorobenzene (5). Compound 5 was synthesised according to the previously described Sonogashira coupling method. 1-Ethynyl-4-hexyloxybenzene (0.630 g, 3.11 mmol), 1-bromo-3-fluoro-4-iodobenzene (0.420 g, 1.41 mmol), Pd(PPh₃)₄ (81.0 g, 71.0 μmol), CuI (14.0 mg, 71.0 μmol), TEA (5 mL) and THF (5 mL) were used. Pale yellow solid (0.101 g, 72%). ¹H NMR (400 MHz, CDCl₃) δ 7.52–7.39 (m, Ar–H, 5H), 7.28–7.18 (m, Ar–H, 2H), 6.87 (d, J = 8.8 Hz, 4H), 3.97 (t, J = 6.6 Hz, –OCH₂–, 4H), 1.79 (tt, J = 6.6 and 7.6 Hz, –OCH₂CH₂–, 4H), 1.52–1.24 (m, –CH₂CH₂CH₂CH₃, 12H), 0.91 (t, J = 6.6 Hz, –CH₃, 6H) ppm. HRMS-FAB⁺ (m/z): [M] calcd for C₃₄H₃₇FO₂, 496.2778; found, 496.2772.

1,4-Bis[4-(hexylsulfanyl)phenyl]ethynyl-2-fluorobenzene (6). Compound 6 was synthesised according to the previously described Sonogashira coupling method. 1-Ethynyl-4-hexylsulfanylbenzene (1.0 mmol, 0.30 g), 1-bromo-3-fluoro-4-iodobenzene (2.1 mmol, 0.46 g), Pd(PPh₃)₄ (50 μmol, 58 mg), CuI (50 μmol, 10 mg), TEA (5 mL) and THF (5 mL) were used. Colorless solid (0.45 g, 85%). ¹H NMR (400 MHz, CDCl₃) δ 7.49–7.41 (m, Ar–H, 5H), 7.29–7.22 (m, Ar–H, 6H), 2.95 (t, J = 7.4 Hz, –SCH₂–, 4H), 1.68 (tt, J = 7.4 and 7.4 Hz, –SH₂CH₂–, 4H), 1.44 (tt, J = 6.6 and 7.2 Hz, –SCH₂CH₂–, 4H), 1.23–1.27 (m, –CH₂CH₂CH₂CH₃, 12H), 0.89 (t, J = 6.8 Hz,–CH₃, 6H) ppm. HRMS-FAB⁺ (m/z): [M] calcd for C₃₄H₃₇FS₂, 528.2321; found, 528.2330.

Results and discussion

Thermal properties of 1–6

The phase transition behaviour of the bistolane derivatives (1–6) was characterised by polarising optical microscopy (POM) and differential scanning calorimetry (DSC). The POM images and DSC curves of all compounds, except those described in the main text, are shown in the ESI.† Phase transition temperatures and enthalpy changes were determined and mesophases classified; a summary can be found in Table 1. Nonfluorinated bistolanes with alkyl (1) and alkoxy tails (2) were observed to form enantiotropic nematic (N) phases. However, alkylsulfanyl-substituted 3 did not exhibit mesophases, but displayed a melting point (T_m = 171.7 °C). Fluorinated 4 and 5, which contain alkyl and alkoxy tails, respectively, also exhibited enantiotropic N phases, similar to nonfluorinated 1 and 2. It is noteworthy that fluorinated 6, with an alkylsulfanyl tail, eventually exhibited an enantiotropic N phase.

Table 1 Phase transition temperatures and enthalpy changes from the 2nd heating (top line) and 1st cooling (bottom line) in DSC thermograms (temperature gradient: 10 °C min⁻¹)

Compound	Cr	T (°C)	ΔH (kJ mol^−1)	N	T (°C)	ΔH (kJ mol^−1)	Iso	LC range (°C)
1	•	155.3	19.6	•	187.9	1.5	•	32.6
	•	148.9	18.8	•	183.8	1.6	•	34.9
2	•	179.5	27.3	•	239.4	2.5	•	59.9
	•	173.3	25	•	235.6	2.7	•	62.3
3	•	171.7	25.9				•	—
	•	165.8	25.7				•	—
4	•	87.9	37.9	•	165	1.5	•	77
	•	66	18.7	•	161.2	1.6	•	95.2
5	•	123.4	30.2	•	216.7	2.6	•	93.3
	•	110.6	30.3	•	211.9	2.7	•	101.3
6	•	112.2	32.6	•	143.6	1.3	•	31.3
	•	100	32.6	•	139.6	1.4	•	39.6

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A representative DSC curve and POM image of 6 are shown in Fig. 2. Here, T_m and T_i (the transition temperature from the N to the isotropic phase) values of 112.2 °C and 143.6 °C were observed, respectively. These values, which were determined from the heating scan, indicated a relatively wide N (ΔT > 30 °C). The T_m value of 6 was found to be approximately 60 °C lower than that of 3. Accordingly, the introduction of a fluorine atom at the central benzene ring may weaken the intermolecular interactions due to the expanded rotational volume, and may thus contribute to the formation of mesophases in 6. This result suggests that fluorine substitution at the central benzene ring of conjugated, rod-like molecules effectively contributes to the formation of mesophases. During the cooling scan, crystallization temperatures (T_c), which indicate a phase transition from the N to the crystalline phase, were found to be significantly decreased, and a nematic temperature range of ∼40 °C was observed for 6 (Table 1). In general, the introduction of a fluorine atom on the central benzene ring resulted in a significant decrease of the T_m values relative to the T_i values, i.e. a wider mesophase temperature range was observed for each fluorinated bistolanes relative to its corresponding nonfluorinated analog.¹³ The lowest T_m value (88 °C) was observed for 4. Interestingly, a slightly lower T_m value was observed for 6 (112 °C) relative to 5 (123 °C). This is consistent with previously reported results, which implies that this might be a universal trend (cf. 2 and 3 in Table 1).^7a,9 The observed results should be ascribed to the increased flexibility of the S–C bond in the alkylsulfanyl group compared to the C–O bond in the alkoxy group.⁸ The ΔH values for the transition from mesophase to isotropic phase (ΔH_i) increased in the order of 6 < 4 < 5. Accordingly, the T_i value for 6 (144 °C) is considerably lower than those of 4 (165 °C) and 5 (217 °C), resulting in a comparatively small LC range for 6 (∼30 °C upon heating and ∼40 °C upon cooling).

Fig. 2 DSC curve and POM image of 6 at 137 °C (N phase).

Optical properties of 5 and 6

As the optical anisotropy depends both on the polarisability and the order parameters, the understanding of the temperature dependence of LC molecules is important for developing advanced optical materials. Therefore, the following discussion refers to the measured temperature (T) as well as to the reduced temperature (T_IN − T) values, wherein T_IN represents the transition temperature from isotropic to namatic phase upon cooling process.

To evaluate the effect of the introduction of a sulfur atom on the refractive index, the ordinary (n_o) and extraordinary refractive index (n_e), as well as the birefringence (Δn = n_e − n_o) of 5 and 6 were compared.‡.

The actual method for determining the n_e, n_o, and Δn values of single-component systems is described in our previous reports.^6k,14 The dependence of the obtained refractive indices (n_e and n_o) and Δn at 550 nm on the reduced temperature (T_IN − T) for 5 and 6 are compared in Fig. 3. With an increase of the T_IN − T values, n_e values were found to increase for both compounds, while n_o values were found to decrease (Fig. 3(a)). This is commensurate with an increase of Δn upon decreasing the temperature (Fig. 3(b)), which is due to the enhanced order parameters at lower temperatures. Over the entire reduced temperature region, the n_e values of 6 were higher than those of 5, while the corresponding n_o values remained comparable. This should be predominantly attributed to the significantly greater polarisability of the sulfur atom relative to that of the oxygen atom. Therefore, Δn values of 6 are clearly higher than those of 5 at each reduced temperature over the entire reduced temperature region (Fig. 3(b)). The n_e, n_o, and Δn values of 5 and 6 were compared at T_IN − T = 26 °C in Table 2. The n_e value of 6 (1.99) was greater than that of 5 (1.93), resulting in a higher Δn value of 6 (0.41) relative to that of 5 (0.34). Furthermore, the temperature dependence of the Δn values for sulfur-containing 6 was found to be higher than that for 5 (cf. the slope of both curves in Fig. 3(b)), which is consistent with the trend that we observed in our recent report.⁹ This may be attributed to the strong attractive interactions derived from the high polarisability of sulfur atoms and the S–S interactions, and may be a universal trend for sulfur-containing, rod-like nematic LC molecules. Even though the nematic region of 6 is much narrower than that of 5, the maximum Δn value of 6 (0.42) was higher than that of 5 (0.41) when measured at the respective minimal temperature. This result indicates that alkylsulfanyl groups are suitable substituents for the generation of novel highly birefringent nematogens.

Fig. 3 (a) Temperature dependence of the n_e and n_o values of 5 (blue squares) and 6 (red circles). (b) Temperature dependence of Δn for 5 (blue squares) and 6 (red circles).

Table 2 Experimental n_e, n_o, and Δn values, as well as extrapolated parameters for 5 and 6 at T_IN − T = 26 °C

Compound	ne^a	no^a	Δn^a	Δno^b	To^b	β^b
a At TIN − T = 26 °C.b Extrapolated by fitting to Haller's equation.
5	1.93	1.60	0.34	0.59	218.1	0.20
6	1.99	1.58	0.41	0.69	142.8	0.20

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We also attempted to estimate the Δn_o values, i.e. the approximated Δn values for perfectly aligned materials (S = 1), by using the temperature dependence of Δn via Δn = Δn_o(1 − T/T_i)^β, wherein β refers to a characteristic material constant for nematic LC molecules.¹⁵ Here, we employed T_IN values alternative to T_i values in the above equation. The Δn values could be fitted well with the aforementioned equation, and the extrapolated values are listed in Table 2. The obtained β values (0.20) are consistent with typical values for nematic phases. Extrapolated Δn_o values of 0.69 and 0.59 were estimated for 6 and 5, respectively. These results suggest that an improvement of the Δn_o values should be expected upon replacement of the oxygen with sulfur atoms, due to the large temperature dependence of the perfectly aligned sulfur-containing molecules, as well as on account of the dependence of Δn on (T_IN − T).⁹

Conclusion

In this article, we describe the design and synthesis of bistolane-based LC molecules with alkylsulfanyl groups as novel sulfur-containing, conjugated, rod-like LC molecules for materials with high birefringence. Nonfluorinated alkylsulfanyl derivative 3 did not exhibit a mesophase, which is in marked contrast to the nonfluorinated analogues with alkyl (1) and alkoxy groups (2) that exhibit well-defined enantiotropic nematic phases. Nevertheless, we succeeded in obtaining an enantiotropic nematic phase for alkylsulfanyl-substituted 6 by introducing fluorine atoms at the lateral position on the central benzene ring. A comparison of the refractive index properties revealed higher n_e and Δn values for 6 relative to alkoxy-substituted 5. Furthermore, a calculation of Δn_o for 5 and 6 suggested a higher optical performance for the latter with respect to the former, given that the same order parameter is achieved. The birefringence is significantly influenced by temperature, due to the large intermolecular interactions arising from the greater polarisability of the sulfur atoms and the S–S interactions, which is consistent with previously reported results.⁹ Consequently, the maximum Δn value of 0.42, which was observed for 6 at 550 nm implies that alkylsulfanyl-substituted 6 may find applications in novel materials with high birefringence. Compared to our previous reports,^6l the obtained Δn values are moderate, which is due to the moderate Δn of the bistolane mesogen itself. The observed mesomorphism and optical performance of 6 might in conclusion be attributed to the combined effects of the fluorination, resulting in the formation of a nematic phase, and the introduction of terminal alkylsulfanyl groups, which should contribute to the molecular interactions and polarizability. Thus, the fluorination of the central benzene ring, combined with the incorporation of polar terminal groups seems to represent a desirable solution for the generation of highly birefringent LC materials.

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Footnotes

† Electronic supplementary information (ESI) available: DSC charts and POM images. See DOI: 10.1039/c5ra25122b

‡ As the refractive indices and the birefringence of alkoxy-substituted derivatives are significantly higher than those of alkyl-derivatives,^6k those of 4 are omitted from this comparison.

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