Definitions

• the present invention relates to liquid crystal materials and eutectic mixtures thereof.
• the invention is directed to a new class of diphenyldiacetylene liquid crystal compounds and eutectic liquid crystal mixtures with infrared and microwave applications.
• Nematic liquid crystals that have high birefringence, low viscosity, low threshold voltage, and are capable of maintaining the nematic phase over a broad temperature range are desirable in electro-optic phase and amplitude modulation applications such as infrared light valves and polymer-dispersed liquid crystal displays.
• High birefringence, or optical anisotropy ( ⁇ n) improves the efficiency of light modulation, and low rotational viscosity serves to shorten the response times.
• Low threshold voltage which is inversely related to the dielectric anisotropy of the liquid crystal material, simplifies the electronics that drive the application.
• Nematic diphenyldiacetylene liquid crystals as illustrated in Structure 1 have been shown to be useful electro-optic media for infrared and microwave modulation.
• FM is the figure-of-merit
• K is the elastic constant
• ⁇ n is the optical anisotropy or birefringence
• ⁇ i is the rotational viscosity of the compound.
• R m is an alkyl group, an alkenyl group, an alkoxy group or an alkenoxy group and R n is an alkyl group, an alkenyl group, or an alkenoxy group
• R m is an alkyl group, an alkenyl group, an alkoxy group or an alkenoxy group
• R n is an alkyl group, an alkenyl group, or an alkenoxy group
• Diphenyldiacetylenes as shown in Structure 3 below, with an alkyl, alkoxy, or alkenoxy side chain on one end (Ry) and a fluoro side group at the other end have been prepared for this purpose. [Wu et al, Applied Physics Letters, 61 , 2275 (1992) and 64, 1204 (1994); Opt. Eng. 32, 1792 (1993)]
• R ⁇ is an alkyl group
• X is a different alkyl group, an alkoxy group, a vinyl group, or a polar group such as cyano, chloro, or fluoro.
• Y position is substituted, it is either a fluoro or chloro group.
• the dielectric anisotropy values for these compounds are about an order of magnitude higher than their non-polar asymmetric counte ⁇ arts.
• the melting points of these compounds tend to be quite high and nematic ranges are also quite narrow, so the uiltity of these materials in liquid crystal applications is quite limited.
• a new class of liquid crystal compounds which has properties ideal to light modulation applications, particularly as components of eutectic mixtures.
• This new class of compounds is based on an asymmetrical diphenyldiacetylene structure with an alkenyl group on one end and a polar substituent on the other end, with optional additional polar side groups on the phenyl rings, as shown generally in Structure 5:
• Ri is an alkenyl group having the general formula (C 2 H 2n _ ⁇ )
• X is a polar group, namely F, CN, CF , OCF 3 , or Cl
• Yi and Y 2 are independently H, F, Cl, or a methyl group
• Z is H, F, or a methyl group.
• FIG. 1 is a plot comparing the figure of merit data for a mixture of alkyl-alkyl substituted and alkyl-fluoro diphenyldiacetylenes and a mixture of two polar diphenyldiacetylenes according to the present invention
• FIG. 2 is a plot comparing the figure-of-merit data for a mixture of alkyl-alkyl substituted and alkyl-fluoro substituted diphenyldiacetylenes and a mixture of several asymmetric non-polar diphenyldiacetylenes and polar diphenyldiacetylenes according to the present invention.
• the present invention relates to a new class of liquid crystal materials and eutectic mixtures thereof and a method for their synthesis.
• the following description is presented to enable one of ordinary skill in the art to make and use the invention and to inco ⁇ orate it in the context of particular applications. It is anticipated that the invention may be tailored to a variety of other applications. Various modifications, as well as a variety of uses in different applications, will be readily apparent to those skilled in the art, and the general principles defined herein may be applied to a wide range of embodiments. Thus, the present invention is not intended to be limited to the embodiments presented, but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.
• the present invention relates to polar, asymmetric diphenyldiacetylene liquid crystal compounds with the basic structure shown in Structure 5:
• Ri is an alkenyl group of the general structure (C 2 H 2n - ⁇ )
• X is a polar side group, namely F, CN, CF , OCF 3 , or Cl
• Yi and Y 2 are each either unsubstituted or a fluoro (F), chloro (Cl), or methyl (CH 3 ) group
• Zi and Z 2 is either unsubstituted or fluoro (F) or methyl (CH 3 ) group .
• this alkenyl side group has the general formula wherein n ranges from 2 to 12 and y ranges from 0 to 10.
• Ri is an alkenyl side chain with a double bond in the second position
• X is a fluoro group
• Yi, Y 2 , and Z are all unsubstituted.
• the total carbon chain length for the alkenyl side chain is preferably 4, 5, or 6 carbons, and most preferably 4 or 5 carbons.
• the figure-of-merit for the resulting compound is the unexpectedly high value of 95 at 95°C.
• the nematic range of this material is about 71°C.
• a similar compound wherein the alkenyl group is replaced by a 4-carbon alkyl chain has a nematic range of only 2.3 °C.
• the carbon chain in the alkenyl side group of this embodiment contains 5 carbons, the resulting compound has an unexpected value of 90 at 95°C and a nematic range of approximately 71 °C.
• the comparable 5-carbon alkyl-substituted analog has a nematic range of only 7.2°C.
• FM is the figure-of-merit
• T is the operating temperature
• T c is the clearing temperature
• k is the Boltzmann constant
• ⁇ is the activation energy of the rotational viscosity
• both X and Yi or X, Yi, and Y 2 as shown in Structure 5 are fluoro groups.
• the alkenyl chain Ri as described above preferably has a double bond in the second position, and contains 4, 5, or 6 carbons in the chain. Addition of the fluoro groups in the Yi position, and possibly also the Y 2 position, serves to decrease the melting point and increase the dielectric anisotropy relative to that of the first embodiment presented wherein X only is a fluoro group and the Y positions are unsubstituted.
• nematic ranges of the difluoro and trifluoro compounds are appreciably wider, at approximately 40°C and 22°C, respectively, compared to the alkyl variant, which has a nematic range of only 7.2°C.
• the compounds used have an alkenyl Ri group with the double bond in the second position and containing 4 or 5 carbons as described previously.
• the preferred compounds additionally have a single fluoro group substitution at the X position and are unsubstituted at the Yi, Y 2 , and Z positions.
• Figure 1 shows the figure-of-merit plots for different eutectic mixtures of diphenyldiacetylene liquid crystal compounds.
• the mixture referred to as IR-93 in Figure 1 is combination of previously developed diphenyldiacetylenes with the basic structures of Structure 2 and Structure 3 as redrawn below:
• R m and R n for Structure 2 and R ⁇ ⁇ for Structure 3 are all alkyl substitutents. This mixture reaches an optimal figure-of-merit value of about 20 at about 70°C.
• homologues of the present invention as shown in the basic Structure 5 are combined with non-polar asymmetric diphenyldiacetylenes, as were disclosed in US Pat. No. 5,338,481, and have the basic structure shown in Structure 2 wherein R m is either an alkyl group having the general formula (C m H 2m+ ⁇ ), an alkoxy group having the general formula (OC m H 2m+ ⁇ ), wherein m ranges from 1 to 12, an alkenyl group having the general formula (C m H 2rn - ⁇ ), or an alkenoxy group having the general formula (OC m H 2m _ ⁇ ) wherein m ranges from 1 to 12 and, R n is independently selected from the group consisting of an alkyl group having the general formula (C n H 2n+ ⁇ ), an alkoxy group having the general formula (OC n H 2n+ ⁇ ), wherein n ranges from 1 to 12, an alkenyl group having the general formula (C n H 2n
• homologues of the present invention used in the current embodiments preferably have a 4- or 5-carbon chain alkenyl R] group with the double bond in the second position as described above.
• the preferred compounds additionally have a single fluoro group substitution at the X position and are unsubstituted at the Yi, Y , and Z positions.
• the mixture IR-93 in the figure is a combination of diphenyldiacetylenes as shown in Structures 2 and 3 wherein R m , R n and Rj- are all alkyl substitutents.
• This mixture reaches an optimal figure-of-merit value of about 20 at about 70°C.
• the mixture referred to as IR-97 in FIG. 2 is a combination of multiple preferred alkenyl-fluoro homologues of the present invention as shown in Structure 5 and also multiple non-polar asymmetric diphenyldiacetylenes as described above and shown in Structure 2.
• This mixture has a high optimal figure-of-merit of approximately 60 at about 100°C and achieves improved figure-of-merit values versus the IR-93 mixture at temperatures of about 50°C and higher.
• Bromoacetylene compounds VTI were prepared using the methods already described in: H.-H.B Meng, L.R. Dalton and S.-T. Wu, Mol.Cryst. Liq. Cryst. 250, 303(1994), S.-T.

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