New low polar tolane cholesterics designed for infrared applications

Abstract

We have designed, synthesized and evaluated the physical properties of new fluorinated phenyltolane based chiral liquid crystal materials with 2-methylbutyl terminal chain. The 2- or 2,6-fluorosubstitution in combination with the phenyltolane core brings surprising improvement of mesomorphic properties. The investigated compounds are characterized by a broad temperature range of the cholesteric (chiral nematic N*) phase, very low melting temperatures and easy thermal control of selective reflection in the near infrared region. The moderate helical twisting power β, high optical anisotropy and source of a blue phase make these compounds attractive for cholesteric mixture formulations.

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Introduction

Cholesteric structures attract much attention due to their number of unique applications.^1–5 Advanced studies on blue phase materials⁶ have also recently brought an increased interest in the chiral liquid crystal field. These fields provide fresh impact in the area of the design and synthesis of new cholesteric liquid crystals.⁷ Selective light reflection given by the cholesteric LCs may be placed in the UV, visible or IR wavelengths. Materials with reflection of infrared light and transparency in the visible region have already found many application areas such as an infrared-light reflective plates,⁸ films,⁹ reflectors,¹⁰ followed by numerous patent applications. They are used mainly to reduce the energy consumption or as a window heat-shielders (energy-saving smart windows).¹¹ These and other innovative applications create a need for broadband IR reflector, where characteristic reflection bandwidth Δλ should be wide. Δλ measured as a width of the reflection bandgap at half height is normally limited to ∼100 nm in the visible spectrum. This value strongly depends on the birefringence of the material used. Therefore, continuous efforts in broadening of the reflection band bring the need for increasing the birefringence Δn of cholesteric systems.¹² Increase of Δλ, obtained as a result of birefringence increase may be done only by the synthesizing tailored liquid crystal compounds. Molecules with large electron polarizability anisotropy Δα, which is induced by the presence of the long conjugated π electron bond systems, have high birefringence. The most promising are molecules with large length to breadth ratio and rigid cores composed of aromatic rings. The best ones comprise of benzene rings connected directly, or better, via the ethynyl (–C≡C–) group.¹³ Highly birefringent liquid crystals are often difficult to synthesize, moreover solubility of such structures is also usually poor. For these reasons, the fabrication of novel cholesteric liquid crystals with increased birefringence, hence broad bandgap is challenging. Typical commercially available chiral compounds show rather poor solubility in the liquid crystal host (several wt%), mainly because of structural incompatibility with widely used nematic hosts. Additionally, for a vast majority of cholesteric liquid crystals used nowadays Δn is less than 0.3.¹⁴ In this work we focused on synthesis of new class of liquid crystal materials based on phenyltolane core, which assures increased birefringence Δn > 0.3. Lateral fluorine substitution, 2,6-difluorophenylacetylene unit in particular brings unique and valuable nematic stability,^15–17 which in connection with a chiral centre broadens significantly the cholesteric (N*) phase range, even up to 100 K. As a chiral source (S)-2-methylbutyl alkyl chain was chosen, mainly because it's easily accessible. Although the selected chiral chain, due to lack of strong polar substituents in vicinity to asymmetric carbon, does not provide high helical twisting power (β < 20 μm⁻¹), still great solubility and miscibility with various achiral hosts make these kind of structures promising materials. Compounds were designed to show structural similarity to widely used nematic achiral hosts, in that way the solubility and interactions with hosts could be enhanced. As a result, solutions of different concentrations of chiral compound (5 ÷ 20 wt%) were composed, which shows reflection in the near infrared region.

Experimental

Synthesis

Synthetic approach described deeply in ref. 17 is generally based on the method of a copper (Li₂CuCl₄) catalysed cross coupling of an aryl Grignard reagents with alkyl bromides. This method is a simple modification of known procedure,¹⁸ gives much greater yields and besides of non-branched alkyl chains can be easily used for chiral branched (S-2-methylbutyl) alkyl chains incorporation as well. For the synthesis commercially available (S)-(+)-1-bromo-2-methylbutane (99% ee) was used. Following this one-pot procedure we obtained both alkyl fluorosubstituted benzene (1, 2) and 4-alkylbiphenyl (5) derivatives that were main starting materials to final mesogens' synthesis, see Fig. 1. Subsequent synthetic process was generally based on Sonogashira cross coupling reaction, after which purification of final compounds could be carried out.

Fig. 1 Synthetic route to investigated compounds. Synthetic conditions: (a) Mg, THF anhydrous; (b) (S)-(+)-1-bromo-2-methylbutane, Li₂CuCl₄; (c) n-BuLi or sec-BuLi, THF anhydrous, −70 °C, then iodine; (d) I₂, HIO₃, CH₃COOH, H₂SO₄, H₂O; (e) 2-methyl-3-butyn-2-ol, PdCl₂(PPh₃)₂, CuI, TEA, DBU, THF anhydrous; (f) NaH, toluene; (g) PdCl₂(PPh₃)₂, CuI, TEA, DBU, THF anhydrous.

Purity and structural analysis

The purity of intermediates and final products was confirmed by thin-layer chromatography (TLC), gas chromatography equipped with flame ionization and mass selective detector GC-FID/MS (Shimadzu GCMS-QP2010S), and by high-performance liquid chromatography HPLC-PDA-MS (APCI-ESI-dual source) (Shimadzu LCMS-2010 EV, column Kinetex C18 and PFP; 50 mm × 3.00 mm × 2.6 μm) in water/acetonitrile or water/methanol. Products were purified by crystallization and column liquid chromatography (or in the case of liquid products column chromatography only) to confirm final purity greater than 99.5%. Optical purity: 99% ee for compounds 10, 11, 12 and 13 and 98% ee for compounds 8, 9 respectively.

Mesomorphic properties

Phase transition temperatures and enthalpy data were determined by polarising optical microscopy (POM) with an OLYMPUS BX51 equipped with a Linkam hot stage THMS-600 and differential scanning calorimeter (SETARAM DSC 141) during heating/cooling cycles (rate 2 K min⁻¹).

Selective light reflection measurements and helical twisting power calculations

Selective reflection measurements were conducted with UV-3600 Shimadzu spectrophotometer using Peltier's temperature controller.

The helical twisting power β [μm⁻¹] was determined using a selective light reflection method¹⁹ and evaluated according to eqn (1):

β [μm⁻¹] = (p × ee × x_e)⁻¹, (1)

where x_e is the mole fraction of chiral compound and p is the helical pitch, ee is the enantiomeric excess.¹⁹ The helical pitch as a linear function of selectively reflected light wavelength λ_max is defined by the relationship (2):

λ_max = p × n_average, (2)

where n_average is the average refractive index, approximately 1.5 for liquid crystals. β values were determined at several different weight solutions: 5 ÷ 10 wt% in three different nematic hosts (see Table 1) by the method described in ref. 7b. First host mixture 1980 is based on structurally similar to the investigated in this article compounds: fluoro-substituted alkyl–alkyl phenyltolanes, described in ref. 16, with the following properties: T_N-Iso = 106 °C, melting point T_m < −10 °C, birefringence Δn = 0.31 measured at 20 °C for sodium line D, Δε = 2.04 measured at 1 kHz and 20 °C. Helical twisting power values measured in this host are indicated β₁. The second host mixture 1816, the dielectrically positive nematic, six component material containing two- and three-ring isothiocyanates and cyanates has a nematic range T_N-Iso = 87.6 °C, melting point T_m < −10 °C, and birefringence Δn = 0.22 measured at 20 °C for sodium line D. Helical twisting power values measured in this host are indicated β₂. The third host composition was the dielectrically negative nematic mixture 1754 (Δε = −1.82 measured at 1 kHz and 20 °C), containing fluorinated terphenyls. Temperatures of phase transitions are the following T_m = 16.7 °C, T_N-Iso = 114.2 °C, optical anisotropy is Δn = 0.25 measured at 20 °C for sodium line D. Twisting power values measured in this host are indicated β₃.

Table 1 Nematic host mixtures for helical twisting power measurements

Indicator	Nematic host	Dielectric anisotropy (Δε)	Structure
β1	1980	+2.5
β2	1816	+15.0
β3	1754	−1.8

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Results and discussion

Mesomorphic properties

Characterized liquid crystal compounds possess (S)-2-methylbutyl chain as a chiral source either at one or both terminal sides of molecule. In case where only one branched chain is presented, the second side of molecule is ended with carbon atom equivalent, n-pentyl alkyl chain. Temperatures and enthalpies of phase transitions are listed in Table 2. Compounds are distinguished by a broad N* phase temperature range. Additionally, homologues with two fluorine atoms (9, 11, 13) have much lower melting and clearing temperatures than compounds mono fluoro-substituted (8, 10, 12), however N* phase range is broader for difluoro homologues. This is the phenomena of 2,6-difluorophenylacetylene unit, which strongly stabilizes nematic phase. Melting temperatures below 30 °C allow to broaden the range of N* phase even up to 100 K, which is still difficult to obtain for single liquid crystal chiral compounds. It is also worth to add that BP-N* transitions were observable in DSC thermograms with its fine enthalpies. This basic property hardly ever met in liquid crystal systems, in our case was obtained only for dichiral homologues 8 and 9 (see Fig. 2).

Table 2 Phase transition temperatures and enthalpies (J mol⁻¹) of investigated phenyltolanes. Temperatures of melting are taken from heating, while N*-BP and clearing temperatures are listed from cooling

No.	Temperature [°C] and enthalpy [J mol^−1] of phase transition
a Obtained from DSC.b Obtained from polarizing thermomicroscope.
8^a	Cr	102.2 (18 329)	N*	124.8 (13.1)	BP	126.6 (366.2)	Iso
9^a	Cr	56.5 (17 988)	N*	105.1 (10.0)	BP	106.5 (339.9)	Iso
10^b	Cr	89.2 (18 134)	N*	156.7 (831.5)			Iso
11^b	Cr	30.9 (18 715)	N*	133.5 (841.3)			Iso
12^b	Cr	75.2 (16 497)	N*	151.0 (1108)			Iso
13^b	Cr	27.4 (24 530)	N*	130.8 (889)			Iso

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Fig. 2 (a) DSC thermogram of compound 8. (b) DSC thermogram of compound 9.

Selective light reflection and helical twisting power

Synthesized compounds and their mixtures in achiral nematics show selective reflection in the near infrared region 0.7 ÷ 2.5 μm (see Fig. 3). Additionally, compounds are characterized by an excellent miscibility with nematic hosts – primarily due to low enthalpies of crystalline–nematic transitions (17 ÷ 24 kJ mol⁻¹ – Table 2) and their structural compatibility with nematic hosts, we were able to prepare even up to 20 weight% solutions. By the formulation of different concentrations, it is shown in Fig. 3, that selective reflection is still placed in NIR region. Therefore compounds should stand as perfect ingredients of cholesteric mixtures, especially when precisely fixed reflection band is needed.

Fig. 3 Transmission spectra of thin layers of compound 9 in 1980 nematic host mixture recorded at 25 °C for three different concentrations: green: 18.5 wt%; red: 14 wt%; black: 7 wt%.

For all investigated compounds helical twisting power was determined every 5 K step in temperature range: 10 °C to 90 °C. β₁ values were obtained for all synthesized homologues, while β₂ and β₃ only for dichiral compounds 8 and 9. Results for two main temperatures 20 °C and 50 °C are listed in Table 3. Fig. 4 and 5 show temperature dependence of experimental results over the whole range. Helical twisting power, as one of the parameters that informs us of chiral dopants ability to induce the helical structure in achiral liquid crystalline host, helps to categorize chiral dopants.²⁰ It is also proven that achiral nematic medium has strong influence on helical twisting power values for chiral liquid crystalline compounds.²¹ We decided to select three structurally different achiral nematic hosts, to sufficiently describe synthesized phenyltolanes. Investigated compounds are characterized by moderate values of the helical twisting power indicator. It is clearly seen that compounds 8 and 9 with two chiral chains have much higher values of β₁ than single-chiral homologues 10–13 (see Fig. 4). Results for 1816 (β₂) and 1754 (β₃) do not bring significant difference. Twisting power is higher for compound 9 than for compounds 8 in three different hosts. We can also find, that twisting power values decrease with temperature for 1980 and 1816 hosts (β₁ and β₂), whereas for dielectrically negative nematic 1754 host, β₃ values increase, especially at lower temperature regions.

Table 3 Comparison of helical twisting power β values for compounds 8 and 9 in different nematic achiral hosts, obtained at 20 °C and 50 °C

Helical twisting power β [μm^−1]		Compound number
		8	9
β1	20 °C	9.17	9.53
	50 °C	8.68	8.97
β2	20 °C	11.10	11.98
	50 °C	11.04	11.64
β3	20 °C	10.19	12.08
	50 °C	10.33	12.01

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Fig. 4 Temperature dependence of β₁ values for compounds 8–13.

Fig. 5 Temperature dependence of helical twisting power values β₂ (red symbols) and β₃ (blue symbols) for compounds 8 and 9.

Summary

We have synthesized and evaluated physical properties of new fluorinated phenyltolane based chiral liquid crystal materials with (S)-2-methylbutyl chains. Investigated compounds are characterized by a broad temperature range of N* phase, very low melting temperatures and easy to achieve selective reflection in the near infrared region. That makes them excellent candidates for a cholesteric mixture formulation, designed for example for energy-saving exploitation. High stability of the chiral nematic mesophase originates mainly from the rigid core which consists of 2,6-difluorophenyl acetylene structural substructure, investigated in detail.^15–18 Its phenomenon leads to a perfect miscibility of investigated compounds in different achiral nematics. Moderate values of twisting power come from the origin of chiral source used here, but its accessibility should be found as a huge advantage.

Acknowledgements

Work was supported by the PBS 23-651 grant and Military University of Technology grant 08-794.

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