Bookshelf liquid crystal materials and devices

Details Bookshelf liquid crystal materials and devices Bookshelf liquid crystal materials and devices

2001-06-20

2004-03-09

Wu, Shean C. (Department: 1756)

Stock material or miscellaneous articles

Liquid crystal optical display having layer of specified...

C252S299620, C252S299630, C252S299670

Reexamination Certificate

active

06703082

ABSTRACT:

BACKGROUND OF THE INVENTION
This invention is in the field of electrooptical devices and liquid crystal materials used in such devices. More particularly, the invention relates to bookshelf-type liquid crystal materials and devices employing such liquid crystal materials.
Liquid crystals have found use in a variety of electrooptical and display device applications, in particular those which require compact, energy-efficient, voltage-controlled light valves such as watch and calculator displays.
Thermotropic liquid crystal molecules typically possess structures which combine a rigid core coupled with two relatively “floppy” tails. Such LC molecules are generally rod-like in shape with the rigid core generally along the long axis of the molecule. Ferroelectric liquid crystal (FLC) materials have been prepared by the introduction of one or more chiral nonracemic LC molecules having one or more stereocenters in at least one of the tails to introduce chirality. The first FLC compound to be characterized was DOBAMBC which contains an (S)-2-methylbutyloxy chiral tail. Pure DOBAMBC exhibits a smectic C* phase with a ferroelectric polarization of −3 nC/cm
2
.
Electro-optic effects with sub-microsecond switching speeds can be achieved using the technology of N. A. Clark and S. T. Lagerwall(1980) Appl. Phys. Lett. 36:899 and U.S. Pat. No. 4,367,924. These investigators have reported display structures using FLC materials, the so-called Surface-Stabilized FLC (SSFLC) devices, having not only high speed, but which also exhibit bistable, threshold sensitive switching. Such properties make FLC-based devices excellent candidates for light modulation devices including matrix addressed light valves containing a large number of elements for passive displays of graphic and pictorial information, optical processing applications, as well as for high information content dichroic displays.
It is, however, well known in the art of FLC materials and devices that a typical FLC device does not exhibit true optical bistability, that is, the memory or the zero applied field orientation of the optic axis of the SSFLC device is typically different from that of its driven orientation. Descriptions of the construction and operation of a conventional bistable FLC device can be found, for example, in U.S. Pat. Nos. 5,748,164 and 5,808,800. The FLC materials used in these conventional devices exhibit smectic layer spacing shrinkage at the smectic A to smectic C transition and further into the smectic C phase. The most significant consequence of the decrease in smectic layer thickness is the formation of chevron smectic layer structures. In addition to inducing many defects, formation of such chevron structures, in effect, adds an extra interface at the chevron interface which is a nominally planar interface roughly parallel to the plane of the FLC film. This extra interface is internal to FLC materials, and together with the two surfaces bounding the FLC materials and the external electric field, determines the orientation of the optic axis of the FLC device. The added constraint imposed by the chevron interface is that the orientation of the optic axis of the FLC devices under an applied electric field depends on the strength of the applied field, and is, thus, different from the memory orientation of the device in the absence of the applied field. See, for example, Rieker, T. et al. (1987) Physical Rev. Letts. 59(23):2658 for a discussion of chevron layer structure in SSFLC cells.
FIG. 1A
schematically illustrates a typical electrooptical response (output light intensity as a function of applied voltage) of a conventional bistable FLC device. This conventional bistable device does not exhibit a true bistable switching and does not exhibit analog behavior. FLC compositions exhibiting bookshelf geometry will, in contrast, be substantially chevron-free when aligned in SSFLC devices and exhibit true bistable electrooptical response as schematically illustrated in FIG.
1
B.
Much attention has focused on the construction of FLC electrooptical devices with true optically bistability which are extremely desirable in practical applications to achieve stable memory performance, high contrast ratio, wide viewing angle and high speed response. However, only a few FLC materials have been identified which exhibit true bistability. A small class of naphthalene-based LCs were reported to be useful for preparation of FLC mixtures exhibiting optical bistability (Mochizuki et al. (1991) Ferroelectrics 122:37-51, U.S. Pat. No. 5,169,556, EP published application 405,868 (published Feb. 1, 1991) and U.S. Pat. No. 5,348,685). These FLC materials are said to have bookshelf geometry and to exhibit no smectic layer spacing shrinkage at the smectic A (SmA) to the chiral smectic C (SmC*) transition and into the SmC* phase range, unlike many conventional FLC materials. U.S. Pat. Nos. 5,568,299, 5,856,815 and 5,943,112 report applications of the naphthalene-based FLCs of U.S. Pat. Nos. 5,169,556 and 5,348,685. Additional naphthalene-core LCs are reported to provide improvement in response times and/or temperature dependency of response time in U.S. Pat. No. 5,861,108.
U.S. Pat. Nos. 5,262,082, 5,437,812 and 5,482,650 report achiral LC compounds having perfluoroether terminal groups exhibiting smectic phases or latent smectic phases that are said to provide “reduced temperature dependence of the smectic interlayer spacing” and “spontaneous generation of a bookshelf layer structure ideal for a ferroelectric liquid crystal device.” Preferred chiral LCs of these patents have a phenylpyrimidine core. A number of LC molecules have been reported to be useful in combination with these achiral bookshelf LCs.
U.S. Pat. Nos. 5,474,705, 5,702,637 and 5,972,241, as well as published EP application EP 736,078 (published Jun. 24, 1998) report chiral LC compounds also having a perfluoroether terminal portion or a chiral fluorinated terminal portion with preferred LC compounds having phenylpyrimidine cores. These patents report that the chiral LC molecules disclosed can be admixed with the achiral fluoroether-containing compounds of U.S. Pat. Nos. 5,262,082, 5,437,812 and 5,482,650 to exhibit “reduced temperature dependence of the smectic interlayer spacing” and “spontaneous generation of a bookshelf layer structure ideal for a ferroelectric liquid crystal device.
U.S. Pat. Nos. 5,658,491, 5,855,812 and 5,928,562 report a process for controlling cone tilt angle in tilted smectic FLC compositions. The compounds disclosed contain fluoroether or fluoroalkyl groups in the LC tail. The patents further report that the compounds useful in the invention can be admixed with the achiral fluoroether-containing compounds of U.S. Pat. Nos. 5,262,082, 5,437,812 and 5,482,650 to exhibit “reduced temperature dependence of the smectic interlayer spacing” and “spontaneous generation of a bookshelf layer structure.”
U.S. Pat. Nos. 4,886,619, 5,082,587, 5,399,291, 5,399,701 report chiral and achiral LC molecules having tilted smectic mesophases or latent tilted smectic mesophases and having fluorocarbon terminal portions.
U.S. Pat. Nos. 5,750,214 and 5,858,273 report liquid crystal devices with certain alignment control, which is said to be useful in improving a switching characteristic of a chiral smectic liquid crystal composition having bookshelf structure. The patents refer to the use of FLC compositions in the method in which at least one component of the FLC composition has a fluorocarbon terminal portion. The patents refer specifically to the use of compounds of bookshelf LCs of U.S. Pat. No. 5,262,082.
U.S. Pat. Nos. 6,019,911 and 6,007,737 report various liquid crystal compositions having structures related to the naphthalene and phenyl pyrimidines that are noted above to exhibit spontaneous generation of bookshelf structure. However, none of the LC compounds disclosed in these patents is identified as exhibiting bookshelf structure or as useful in the preparation of chevron-free FLCs.
Various naphthalene-containing liquid crystals of general formula:
where

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