Mesogenic materials with anomalous birefringence dispersion...

Details Mesogenic materials with anomalous birefringence dispersion... Mesogenic materials with anomalous birefringence dispersion...

2000-08-11

2003-05-27

Wu, Shean C. (Department: 1756)

Stock material or miscellaneous articles

Liquid crystal optical display having layer of specified...

C252S299010, C252S299200, C252S299600, C252S299630, C252S299610

Reexamination Certificate

active

06569504

ABSTRACT:

BACKGROUND OF THE INVENTION
This invention relates to compounds and liquid crystal compositions containing them which are useful in electooptical and non-linear optical applications.
Liquid crystal (LC) displays are now nearly ubiquitous in our culture, being used in both monochrome and color displays in a variety of products from watches to automobile gauges and from road signs to computer displays. It is most desirable that monochrome displays are simply black and white with no particular cast of color. Similarly, it is imperative for quality color displays that all colors be transmitted equally well. If a display is less transmissive for one wavelength compared to another, the display will not show true colors and will be less marketable than a display showing true colors.
LC displays rely on the birefringence (&Dgr;n) of the LC, i.e., the difference in refractive indices between different orientations of the LC. Birefringence, &Dgr;n=n
e
−n
0
, where n
e
is the index of refraction along the extraordinary axis of a birefringent material (parallel to the optic axis) and n
0
is the index of refraction its ordinary axis (perpendicular to the optic axis). The optimal thickness of an LC cell such that it behaves as a half-wave plate, to maximize contrast and true color transmission, at a given wavelength is proportional to the birefringence. The optimal birefringence for a fixed pathlength, i.e., thickness of LC, increases with increasing wavelength as shown in FIG.
1
. In contrast, birefringence of a given material generally decreases as a function of increasing wavelength (FIG.
1
). The change in birefringence of a material as a function of wavelength is called birefringence dispersion. (Herein, the term “positive birefringence dispersion” is used for birefringence that decreases with increasing wavelength and “negative birefringence dispersion” is used for birefringence dispersion that decreases with increasing wavelength.) Thus, if birefringence of an LC cell is optimized for transmission at one wavelength by optimization of cell thickness, it will not be the optimal birefringence at a second wavelength and as a consequence light transmission through the cell at the second wavelength will be lower.
Typically, in designing an LC device, a compromise is made by setting cell thickness for optimal transmission of a wavelength in the middle of the operational wavelength range (i.e., at the design wavelength). For LC devices used in the visible, cell thickness is chosen to optimize transmission of green light, giving a cell less than optimal, but useful, transmission in the red and blue. Such a cell has a slight yellow or green cast.
If the birefringence dispersion of an LC material were negative (increasing in slope as a function of increasing wavelength), cells made from this material would exhibit significantly less chromatic behavior. In general, LC materials, i.e. mesogenic compositions, which exhibit a lower positive (including zero) or negative birefringence dispersion than existing materials will be useful for decreasing the chromatic behavior of LC displays and related electrooptical devices. Such mesogenic materials will be useful in optical filters with improved color balance, larger free spectral range, maintaining high resolution with fewer filter stages and in tunable Fabry-Perot filters using liquid crystal spatial light modulators (SLMs).
Furthermore, ferroelectric liquid crystals (FLCs) used in displays often have quite high birefringence requiring the use of thin cells. When thin LC cells are used, small variances in cell thickness can have a significant effect on the cell's optical properties. For example, a 0.1 &mgr;m variance in thickness of a cell that is 1.1 &mgr;m thick results in a ±9% difference in transmission, while the same variance in a thicker 1.9 &mgr;m cell results in only a ±5% difference. Thinner LC cells also tend to suffer from non-uniform spacing, which can lead to shorts. Environmental contamination of LC cells, for example by inclusion of dust and other contaminants, has a more severe effect on thinner cells. Designs using thicker cells, for more stability, easier manufacturing and lower cost, require LC materials with generally lower birefringence (compared to presently available materials). There is a general need in the art for LC materials, particularly FLC materials, with decreased birefringence.
Ferroelectric liquid crystals (FLCs) are true fluids possessing thermodynamically stable polar order. As the liquid crystal cools from a normal isotropic liquid to a crystalline state, it passes through a series of mesogenic phases of increasing order. A typical phase sequence includes several phases, of which only the tilted smectic C* (S*
c
) phase possesses the thermodynamically stable polar order necessary to exhibit a net dipole moment. In the S*
c
phase the molecules self-assemble into layers, with the long axis of the molecules coherently tilted with respect to the layer normal. The single polar axis of the phase is normal to the tilt plane. For most such FLCs, a spontaneous macroscopic dipole density or spontaneous ferroelectric polarization P along the polar axis is easily measurable.
The ferroelectric nature of a C* phase affords a very strong coupling of the molecular orientation with external fields, leading to a high contrast electro-optic light valve with fast response relative to the well known nematic devices currently in use. The complicating factor of the C* helix was solved with the invention of the Surface Stabilized Ferroelectric Liquid Crystal (SSFLC) light valve. In the SSFLC geometry, the helix is spontaneously unwound due to surface interactions with bounding glass plates. In this case, when the director prefers a parallel orientation with respect to the surface plates, two states are allowed. In one state the molecules tilt right by tilt angle &thgr;, while in the other state they tilt left. In both cases, the ferroelectric polarization vector is pointing normal to the title plane (normal to the surface of the glass plates).
Due to the birefringence of FLC molecules, the two states have different optical characteristics. When the tilt angle &thgr;=22.5°, and the thickness of the cell is tuned correctly relative to the birefringence, then the cell behaves as a half wave plate, and can be aligned between crossed polarizers such that one state gives good transmission, while the other state shows good extinction, giving rise to the desired electro-optic effect.
SSFLC cells show very high contrast (1,500:1 demonstrated), low switching energy, bistability, high resolution (≡10
7
pixels/cm
2
demonstrated, 10
8
pixels/cm
2
possible) and other performance characteristics which make it particularly attractive for many optoelectronic applications.
Compounds which self-assemble into the smectic C phase are often termed C phase mesogens. While there is currently no detailed understanding of the relationship between molecular structure and the occurrence of LC phases, empirically, C phase mesogens generally possess a rigid core separating two “floppy” tails. The tails of chiral and achiral mesogens can include a variety of chemical functionalities, but components of commercial mixtures often have one or two alkyl or alkoxy tails. The tails often have similar lengths, and both are typically longer than four carbons. Many compounds of this type also exhibit a nematic phase. For C* mesogens generally one of the tails will possess one or more tetrahedral stereocenters.
In order to be useful in the many types of devices of interest, FLC materials must possess properties never achievable in a single compound, but the stable temperature range and other material parameters can in general be tuned by mixing components. Commercial LC mixtures are generally composed of at least eight components. FLC mixtures generally contain two types of components: 1) A smectic C host, designed to afford the required temperature range and other standard LC properties; and 2) Chiral components designed to

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