BACKGROUND OF THE INVENTION

Field of the Invention

The present invention relates to liquid crystal materials and eutectic mixtures thereof for infrared and microwave applications. In particular, the invention is directed to a new class of diphenyl-diacetylene liquid crystal compounds and eutectic mixtures.

Description of the Related Art

Diphenyl-diacetylene liquid crystals are useful electro-optic media for modulating infrared radiation and for high speed light shutters. These liquid crystal materials possess not only high birefringence but also low rotational viscosity.

The symmetry and polarity of diphenyl-diacetylene liquid crystals are important to the overall properties of the liquid crystal. Properties, such as melting point 0 ^" mp)» birefringence (Δn), viscosity, threshold voltage ( h), dielectric anisotropy (Δε) and heat fusion enthalpy (ΔH), are influenced by the symmetry and polarity of the liquid crystal. These properties are important to the behavior of the liquid crystal in their applications as infrared spatial light modulators and polymer dispersed liquid crystal shutters. A high birefringence improves the light modulation efficiency; low viscosity shortens the response times; and low threshold voltage simplifies the driving electronics in these applications. Moreover, low threshold voltage is particularly attractive for polymer dispersed liquid crystal devices where the applied voltage is partially shielded by the polymer matrix so that the voltage drop across the liquid crystal droplets is far less than the applied voltage. Eutectic mixtures of diphenyl- diacetylene liquid crystals are essential to infrared and microwave applications. Both polar and nonpolar symmetrical diphenyl-diacetylene liquid crystals have been reported in articles by B. Grant, Mol. Cryst. Liq. Cryst., 4S, 175 (1978); S. T. Wu et al., J. Appl. Phys.,^5, 4372 (1989); and S. T. Wu et al., J.

Appl. Phys., 70, 3013 (1991). It is disclosed that the symmetrical diphenyl- diacetylene liquid crystals exhibit high melting temperatures (nonpolar T _m p >

80°C), narrow nematic temperature range (- 25 degrees), small dielectric anisotropy (Δε -0.8) and large heat fusion enthalpy ΔH relative to the ideal hosts for eutectic mixtures. High melting temperature is a result of long conjugation and small dielectric anisotropy is a result of high degree of symmetry of the liquid crystal molecules. Although increasing the alkyl chain length tends to reduce the melting point, disadvantageous^, the increase in chain length will increase the viscosity and decrease the dielectric anisotropy.

Polar asymmetrical diphenyl-diacetylene liquid crystals are reported by B. Grant et al., Mol. Cryst. Liq. Cryst, £1, 209 (1979). These liquid crystals have a cyano group attach to a phenyl group on one side and an alkoxy group attached to another phenyl group on the other side of the diacetylene triple- triple bonds. These compounds show a large dielectric anisotropy, but the melting temperatures of these cyano alkoxy diphenyl-diacetylene homologs are exceedingly high (greater than 150°C) and their nematic range is very narrow (only 5 degrees) relative to the ideal host for eutectic mixtures.

Fluorinated diphenyl-diacetylene liquid crystals are disclosed in German patent No. DE 4005882 A1. The German patent does not disclose experimental results or the properties of these fluorinated compounds. The melting temperatures are expected to be high and the nematic ranges thereof are expected to be narrow. Also the dielectric anisotropy (Δε) of laterally substituted fluoro- compounds is negative. The alignment for negative dielectric anisotropy Δε liquid crystals is different from the liquid crystal compounds with positive dielectric anisotropy (Δε). For infrared and microwave applications, the liquid crystal should have a positive dielectric anisotropy (Δε). Therefore, eutectic mixtures consisting of these fluorinated homologs alone are not expected to exhibit a wide nematic temperature range or be practical for infrared and microwave applications.

The ideal host for eutectic mixtures should have properties, such as low viscosity (less than about 30 centipoises), low melting temperature (below 40°C, and preferably about room temperature), wide nematic temperature range (ranging from about -40°C to +80°C), low heat fusion enthalpy (less than about 5 kcal/mol) and high birefringence (greater than about 0.25). It would be desirable to have host liquid crystals which have these ideal properties for formulating eutectic mixtures for infrared and microwave applications.

SUMMARY OF THE INVENTION

In accordance with the invention, a new class of liquid crystal compounds are provided which exhibit ideal properties for formulating eutectic mixtures suitable for use in infrared and microwave applications. The new class of liquid crystals compounds are based on an asymmetrical diphenyl- diacetylene structure with nonpolar end groups providing the asymmetry. These materials exhibit low melting points, wide nematic temperature range and low heat fusion enthalpy, as described above for an ideal host candidate for eutectic mixtures. Eutectic mixtures containing entirely diphenyl-diacetylene homologs using the new compounds according to the invention have high birefringence, low viscosity and wide nematic range. In the preferred embodiment, the new liquid crystal compounds comprise the following basic structure:

Rl -C6H4-C=C-C=C-C6H4-R2

wherein R-) is an alkyl, alkenyl, alkoxy or alkenyloxy end group and R2 is alkyl, alkenyl or an alkenyloxy end group. When R-j is an alkyigroup, R-j has the general formula (C _n H2n+ 1); as an alkoxy group, the general formula (OC _n H2n+ 1); as an alkenyl group, the general formula (C _n H2n-i); and as an alkenyloxy group, the general formula (OC _n H2n-ι)- When R2 is an alkyl group, 2 has the formula (C _m H2m+ 1); as an alkoxy group, the general formula (OC _m H2 _m + 1); as an alkenyl group, the general formula (C _m H2 _m -i); and as an alkenyloxy group, the general formula (OC _m H2m-l)» and wherein n is not equal to m. In another embodiment, the liquid crystal compound can comprise an alkenyl group having the general formula C _x H2 + ιCH=CH-

(CH2)n-2-x ' ^{n e} ^- ^{er the R} 1 ^{or R} 2 locations, so long as R-j does not equal R2.

Eutectic mixtures according to the invention comprise at least one asymmetrical diphenyl-diacetylene liquid crystal compound having the structure described above.

Diphenyl-diacetylene liquid crystal compounds and eutectic mixtures according to the invention exhibit low viscosity of less than about 30 centipoises, high birefringence greater than about 0.25, wide nematic temperature range of about -40°C to +80°C, low melting temperatures less than about 40°C, and low heat of fusion enthalpy of less than about 5 kcal/mole. These and other features and advantages of the present invention will be apparent from the following more detailed description of the preferred

embodiments, taken in conjunction with the accompanying drawings, which illustrate by way of example, the principles of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 is a graphical illustration of wavelength dependent birefringence of a eutectic mixture according to the present invention.

Figure 2 is a graphical illustration of reduced temperature dependent dielectric constants of a eutectic mixture according to the present invention.

Figure 3 is a graphical illustration of temperature dependent splay elastic constant of eutectic mixtures according to the invention.

Figure 4 is a graphical illustration of reduced temperature dependent rotational viscosity of eutectic mixtures according to the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

At least 20 asymmetrical alkyl-alkyl, 5 alkyl-alkoxy, and 5 polar diphenyl- diacetylene liquid crystals were synthesized and their properties were compared to 5 symmetrical diphenyl-diacetylene liquid crystals.

A general procedure for the preparation of the liquid crystal compounds according to the invention is known in the art and can be found, for example, in

B. Grant, Mol. Cryst. Liq. Cryst., 48 175 (1978). For the preparation of 4-n- alkyiphenylacetylene (shown below as formula 00). a suspension of 33.1 grams (0.1 mole) carbon tetrabromide (CBr.4), 6.5 grams (0.1 mole) zinc (Zn) powder and 26.2 grams (0.1 mole) of triphenylphosphine in 340 milliliters of methylene chloride (CH2CI2) was stirred at room temperature for 48 hours (the suspension was purple in color) . To the suspension, 0.05 mole 4-n- alkylbenzaldehyde was added and stirred for an additional 1.5 hours. The suspension was filtrated and the filtrate was evaporated resulting in a crude dibromoolefin (3,/3-dibromostyrene, formulas (1) or (III)), as shown in Equation (1a) below. The j3,S-dibromostyrene (I) was purified by chromatography on silica gel with a solvent, preferably hexane, using a purification procedure disclosed in B. Grant, Mol. Cryst. Liq. Cryst, 48 175 (1978) and also see the Thesis of Yong-Hong Lu, "Synthesis of Side-Chain Liquid Crystalline Polysiloxanes Containing trans-Cyclohexane or Diacetylene Based Mesogenic

Side Groups", submitted to the Institute of Applied Chemistry, National Chiao Tung University, Hsinchu, Taiwan, Republic of China, June 1991 for Master of Science in Applied Chemistry, Oncorporated herein by reference). The purified olefin (/3,/S-dibromostyrene (I)) in 50 milliliters of dry tetrahydrofuran (THF) at - 78°C under N2 atmosphere was treated with butyl lithium (2 equivalents) and stirred at -78°C for an hour, followed by one hour at room temperature. The reaction was quenched with water, and the ethereal layer was separated. After re-extraction of the aqueous layer with ether, the combined extracts were washed with water, dried in MgSO evaporated to dryness and purified by chromatography, as stated above, to give the 4-n-alkylphenylacetylene (formula II), as shown in Equation (1b) below:

R-C6H4-CHO — > R-C6H4-CH=CBr ₂ (1a)

(I) or (III) where R = Ri (I) or R ₂ (III).

Rl-C6H4-CH=CBr2 — > Rl~C6H4-C=CH (1b)

0) (")

R2-C6H4-CH=CBr2 > R2~C6H4-C=CBr (1c)

(III) (IV)

The 0,/3-dibromostyrene (formula III) in a quantity of 5.74 grams (0.02 mole) and 2.24 grams (0.02 mole) potassium t-butoxide in 75 milliliters of toluene were heated at reflux for 4 hours. The reaction was cooled to room temperature, filtered and the filtrate was evaporated to dryness and purified by chromatography on silica gel (with the purification procedure mentioned above) to give the bromoacetyϋde (formula IV), as shown in Equation (1c) above.

The bromoacetylide (IV) in an amount of 3.2 grams (0.015 mole) was stirred in 50 milliliters of ethanol and added portionwise to a stirred solution containing 747 milligrams NH2OH HCI, 14.9 milliliters n-butylamine, 10 milligrams cuprous chloride and 0.015 mole 4-n-alkylphenylacetylene (II) in 50 milliliters ethanol. After 1.5 hours, the product was removed by filtration and purified by chromatography on silica gel, as mentioned above, to give the diphenyl-diacetylene product (formula V), as shown in Equation 2, according to the invention:

Equation (2)

Rl-C6H4-C≡CH + R2-C6H4-C≡CBr — > Rι-C6H4-C≡C-C≡C-C6H4-R2 (II) (IV) (V)

Hereinafter, diphenyl-diacetylene liquid crystals will be abbreviated

PTTP-nm, in which P stands for phenyl ring, T stands for triple bond and n and m refer to the number of carbons in the respective alkyl group. For example,

PTTP-24 refers to a two carbon alkyl group on one end of the phenyl-triple bond-triple bond-phenyl and a four carbon alkyl group on the other end.

Alternatively, PTTP-n'm or PTTP-n'm" refer to -alkenyl groups. For example,

PTTP-2 ^, 4' refers to a two carbon alkenyl group on one end of the phenyl-triple bond-triple bond-phenyl and a four carbon alkenyl group on the other end.

Likewise, -n" and/or -m" refers to alkoxy and -n" and/or -rrf refer to alkenyloxy groups. Table 1 lists the combination of nonpolar end groups for R-j and R2, according to the invention.

TABLE 1: 1

Properties such as phase transition temperature, melting point and molar heat fusion enthalpy of the synthesized liquid crystals are reported in Table 2 below. The corresponding values for the known symmetrical liquid crystals (highlighted in bold print) were obtained from B. Grant, Mol. Cryst. Liq. Cryst, 48, 175 (1978) and S. T. Wu et al., J. Appl. Phys.,IQ, 3013 (1991) (cited previously) and incorporated herein by reference.

Common to the nonpolar symmetrical liquid crystals are melting temperatures greater than about 80°C, while the polar symmetrical liquid crystals have melting temperatures greater than about 150°C. Moreover, the

polar PTTP-6CN and -8CN, upon melting, are monotropic in that they transition to the isotropic liquid phase before reaching the liquid crystal phase. The melting temperatures for PTTP-6CN and PTTP-8CN, therefore, are listed in parentheses in Table 2 to distinguish these melting temperatures from the other liquid crystals which transition from the solid phase-to-liquid crystal-to-isotropic liquid phase upon melting.

Also evident from Table 2 is that increasing the alkyl chain length will tend to reduce the melting temperature, but not shown in Table 2 is that the increasing alkyl chain length will increase the viscosity and decrease the dielectric anisotropy to unacceptable levels.

The asymmetrical nonpolar diphenyl-diacetylene liquid crystal compounds according to the invention unexpectedly showed a lower melting point and wider nematic temperature range (T _c - T _m p) than the symmetrical liquid crystals with the same total chain length. Moreover, the melting temperatures of PTTP-46 and -48 were unexpectedly as low as about 25°C.

Even more unexpected and quite advantageous is that three asymmetrical dialkyl PTTP samples, PTTP -24, -36, and -68 (highlighted by an asterisk * in Table 2), show modest melting temperatures (less than about

45°C) and unexpectedly low molar heat of fusion enthalpy (less than about 3 kcal/mole).

The asymmetrical nonpolar liquid crystal compounds according to the invention have the basic structure:

Rl-C6H4-C≡C-C≡C-C6H -R2

wherein R-j and R2 are nonpolar end groups and R1 does not equal R2, thereby making the diphenyl-diacetylene liquid crystal asymmetrical. According to a first embodiment, R-j and R2 are taken from Table 1. When R1 is an alkyl group, R1 preferably has the general formula (C _n H2n+ 1)- As an alkoxy group, R-| preferably has the general formula (OC _n H2n+ι). As an alkenyl group, R-j preferably has the general formula (C _n H2n-i); and as an alkenyloxy group, R1 preferably has the general formula (OC _n H2n-l)- When R2 is an alkyl group, R2 preferably has the formula (C _m H2m+ 1); the general formula (C _m H2 _m --|) as an alkenyl group; and as an alkenyloxy group, R2 preferably has the general formula (OC _m H2m-l)- ^ln each case above for R-| and R2, n is not equal to m. For the invention, n ranges from 1 to 12 and m ranges from 1 to 12.

In a second embodiment, R-j is an alkenyl group having the general formula C H2χ+ -jCH =CH-(CH2)n-2-x where n ranges from 2 to 12 and x ranges from 0 to 10. R2 is the same as described above for the first embodiment.

In a third embodiment, both R-j and R2 are alkenyl end groups, wherein

R-l is as described above for the second embodiment and R2 has the general formula CyH2y + -|CH =CH-(CH2)n-2-y' where x is not equal to y and y ranges from 0 to 10. Table 2 has data for the first embodiment of the invention and other liquid crystal compounds as a comparison.

It is known from the Schroder-van Laar equation (reported in E. C. H. Hsu et al., Mol. Cryst. Liq. Cryst., 2D, 177 (1973) and incorporated herein by reference) that low melting temperature and small molar heat of fusion enthalpy of an individual liquid crystal component of a eutectic mixture play equally important roles in determining the melting point of the eutectic mixture. Therefore, the asymmetrical FTTPs are excellent host candidates for forming eutectic mixtures with wide nematic range.

A binary eutectic mixture according to the invention was formulated containing 52 weight percent of PTTP-24 and 48 weight percent PTTP-36 (PTTP-24/36). The melting point of the mixture dropped to 10°C and the clearing point remained at 97.7°C. The nematic temperature range, calculated as in J.D. Margerum et al., Mol. Cryst. Liq. Cryst, m, 103 (1984) Oncorporated herein by reference) ranged from 0 to 98°C. The melting point of the eutectic mixture can be lowered further by formulating multi-component PTTP eutectic mixtures using carefully chosen PTTP liquid crystal homologs.

The wavelength dependent birefringence of PTTP-24/36, as illustrated in Figure 1, was measured at 22.8°C using tunable Ar ⁺ and HeNe lasers. The solid line 10 represents the fitting with the single-band birefringence dispersion model obtained from Wu, Phys. Rev. A..3Q, 1270 (1986), incorporated herein by reference:

Δn = G (T) ( λ ² λ*2)/ (λ ² - λ* ² ) (3)

where G is a proportionality constant which determines the temperature effect, T is the temperature, λ* is the mean electronic resonance wavelength and λ is the wavelength of measurement. From Figure 1, G= 4.108 x 10 ^"6 nm ² and λ* = 263.5 nm were obtained. Substituting these parameters back to Eq. (3), the birefringence Δn in the infrared region ( where λ > > λ*) was calculated (-G λ* ² ) to be 0.285. Such a high birefringence Δn makes these

materials particularly attractive for modulating IR and microwave radiation where the photostability is not a problem.

Temperature dependent birefringence of PTTP-24/36 and PTTP-48 were measured at λ = 632.8 nm. Results (not shown) fit well with Halter's equation, as disclosed in I. Haller , Prog. Solid State Chem.,lQ, 103 (1975) and incorporated herein by reference, except near the phase transition region where Halter's equation is invalid:

Δn = Δn ₀ S (4a)

S = [1-T _r ]0 (4b)

where Δn ₀ is the birefringence at T _r = 0 or S = 1 , as if the liquid crystal were in its completely ordered state; S is the order parameter of the second rank; T _r = T/T _c is the reduced temperature, where T _c is the clearing point, and β is an exponent which is dependent on an individual material. From the experimental results, the [Δn ₀ , β] values for PTTP-24/36 and PTTP-48 were found to be [0.521, 0.241] and [0.434, 0.211], respectively. From β, the order parameter S was computed from Equation (4b) for different reduced temperatures fTc).

The dielectric constants of PTTP-24/36 and PTTP-48 were measured by the single cell method reported in S . Wu et al., Liq. Cryst, JQ, 635 (1991), incorporated herein by reference, and the results are illustrated in Figure 2. In general, the dielectric constants and the anisotropy (Δε) of the nonpolar liquid crystals are small. Small (Δε) results in a large Freedericksz transition threshold voltage (V h) (obtained from V. Freedericksz and V. Zolina, Trans.

Faraday Soc, 29, 919 (1933)), incorporated herein by reference, as:

V _t h = τ [K ₁₁ /ε ₀ Δε]V ² (5)

wherein K-| -j is the splay elastic constant and ε ₀ is the permittivity of vacuum. The data illustrated in Figure 2 includes both the parallel (ε n) and perpendicular (ε\) dielectric constants for PTTP-24/36 (20) and (22), respectively, and for PTTP-48 (21) and (23), respectively. From the data shown in Figure 2, the dielectric anisotropy of PTTP-24/36 (Δε is approximately 1) is slightly larger than that of PTTP-48 (Δε is approximately 0.8) at room

temperature which corresponds to a reduced temperature T _r of approximately 0.84 owing to the shorter chain length.

To improve the dielectric anisotropy Δε, polar PTTPs can be added to the mixture. For example, the binary mixture of PTTP-6CN and -8CN (with 1:1 ratio) shows a nematic range from 114 to 135°C. The dielectric anisotropy Δε of this mixture was measured by the guest-host method to be Δε = 17 at 1 KHz sine wave frequency and the birefringence Δn = 0.438 (where the refractive index of the ordinary ray (n ₀ ) = 1.495 and the refractive index of the extraordinary ray (n _e ) = 1.933 at a wavelength (λ) = 589 nm and temperature

(T) = 22°C). According to this method, approximately 10 percent of the compound to be studied was dissolved in a host liquid crystal. The host mixture employed was ZLI- 132 (Merck, Germany), because it exhibited a reasonably wide nematic range (from -40 to +71°C) so that adding 10 percent of a guest host compound cause little change on the clearing point of the mixture.

The threshold voltages of PTTP-24/36 and -48 were measured by voltage dependent capacitance and voltage dependent birefringence methods

(see Wu et al., Liq. Cryst, Jβ, 635 (1991) which is incorporated herein by reference). Results obtained from both methods agree to within 2 percent. The threshold voltage V^ at room temperature (or T _r = 0.84) and 1 KHz sine wave frequency is 4.34 and 3.83 Vrms for PTTP-48 and PTTP-24/36, respectively. As temperature increases, the threshold voltage decreases.

From the threshold voltage data and the dielectric constants shown in Figure 2, the temperature dependent splay elastic constant K-j 1 is obtained for PTTP-

24/36 and PTTP-48, as illustrated in Figure 3. The data for PTTP-24/36 and PTTP-48 fall close to solid line 30 in Figure 3. These results are used in the mean-field theory, as discussed in W. Maier et al., Z. Naturforsch Teil A J4, 882 (1959), incorporated herein by reference, where:

Kn = AoS ² (6)

From Figure 3, the proportionality constant A _Q is found to be 3x10' ¹¹

Newton for both PTTP-24/36 and PTTP-48.

To evaluate the rotational viscosity (y\), the viscoelastic coefficient (71 ^/K 11) was obtained from the decay time of a liquid crystal cell. See for example S. T. Wu et al., Phys. Rev. A 42, 2219 (1990), which is incorporated herein by reference. Once the viscoelastic coefficient (γi/K-j 1) was obtained,

the rotational viscosity ( 1) was evaluated by using the K-j-j results illustrated in

Figure 3. Experimental results of the temperature dependent rotational viscosity γ-j (T) are illustrated in Figure 4 for PTTP-24/36 and PTTP-48. The rotational viscosity γ j is a complex function of temperature, as described in Wu et al., Phys. Rev. A, 42, 2219 (1990) and M. Osipov et al., Z. Naturforsch. Teil A 44, 785 (1989), both incorporated herein by reference, because the rotational viscosity 1 depends not only on the absolute temperature, but also on the reduced temperature (T _r ). From Figure 4, PTTP-48 exhibits a larger rotational viscosity than PTTP-24/36 mixture at a given reduced temperature (T _r j. This is because PTTP-48 possesses a larger moment of inertia due to its longer chain length.

There has been disclosed a new class of liquid crystal compounds, asymmetrical nonpolar diphenyl-diacetylene liquid crystal compounds, which have high birefringence and low viscosity and unexpectedly possess low melting temperature, wide nematic range and small heat of fusion enthalpy. These compounds contain either dialkyl, alkylalkenyl, dialkenyl, alkylalkenyloxy, alkoxyalkenyl, alkoxyalkenyloxy, alkenylalkenyioxy or dialkenyloxy end groups. These compounds are excellent host candidates for eutectic mixtures and the dielectric anisotropy of the mixtures can be enhanced by adding polar PTTP compounds, such as the polar liquid crystals from Table 2. The compounds and mixtures according to the invention are expected to be very useful media for IR and microwave modulators where photostability is not a problem. Changes and modifications may be made to the invention which may be readily apparent to those skilled in the art without going beyond the intended scope of the invention, as defined in the appended claims.