Chiral additives for cholesteric displays

Details Chiral additives for cholesteric displays Chiral additives for cholesteric displays

2001-09-24

2004-12-14

Wu, Shean C. (Department: 1756)

Stock material or miscellaneous articles

Liquid crystal optical display having layer of specified...

With viewing layer of specified composition

C252S299010, C428S001100, C549S430000, C549S453000

Reexamination Certificate

active

06830789

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a unique class of chiral compounds for liquid crystals, and to liquid-crystalline mixtures containing such chiral compounds, and to their use for cholesteric displays and the resulting devices.
2. Description of the Related Art
Cholesteric flat-panel displays are currently under development because of their low power consumption, bright viewing characteristics at wide angles, and high-resolution capability at low cost. Their low-power consumption is a result of the bistable memory characteristic inherent in the technology. As described in the first U.S. patents on this technology (See U.S. Pat. Nos. 5,251,048, 5,384,067, 5,437,811, and 5,453,863), each pixel of the display can exist in a stable color reflective state with any desired reflective intensity or brightness (gray-scale) without any applied power. The degree of brightness is electronically selected by a pulse. A unique feature described in those inventions, is the existence of a threshold in the electronic response to a pulse such that a matrix of pixels can be multiplexed to achieve a high resolution display at low cost without the need of transistor elements (active matrix) at each pixel. Because of the low power and reflective brightness characteristics, cholesteric displays are used in portable or handheld applications where long battery life and versatile viewing capabilities are important.
The reflective properties of cholesteric liquid crystals have been generally known for many years. Sometimes called a chiral nematic, a cholesteric liquid crystal achieves its color reflective property because the molecules are arranged in a helical twist pattern with a periodicity (pitch length) equal to the wavelength of light in the material. The first materials explored with this property were the cholesterol esters. These materials are not only chiral but also liquid crystalline and reflect iridescent colors when the periodicity of the twist corresponds to a reflective wavelength from 400 nm to about 700 nm [“Cholesteric Structure—II: Chemical Significance”, p105-119, J. L. Fergason, N. N. Goldberg, R. J. Nadalin,
Liquid Crystals
, Ed. G. H. Brown, G. J. Dienes, M. M. Labes, Gordon and Breach Science Publishers, New York (1966)]. The materials were therefore called cholesteric liquid crystals, a name still used today even though cholesterol materials are seldom used today. Instead, a mixture of chiral and achiral compounds is used as discussed by Gottarelli et al. [“Induced Cholesteric Mesophases: Origin and Application, G. Gottarelli, G. P. Spada, Mol. Cryst. Liq. Cryst., 123, 377-388 (1985)]. Achiral liquid crystalline compounds make up a nematic liquid crystalline host mixture, which has no helical twist by itself. To this host nematic is added a chiral compound to twist up the nematic material into one of a cholesteric structure, hence the name chiral nematic.
The helical arrangement of the molecules provides a self-assembled stack of dielectric layers because of the anisotropy of the refractive index of the molecules. The index of refraction continuously varies along the stack by as much as 0.25 depending upon the nematic material. Because of the helical nature of the refractive index in the layer, the stack will reflect one circular component of a selected bandwidth of colored light. A right handed twisted planar texture will therefore decompose incident unpolarized white light into its right and left components by reflecting the right hand component and transmitting the left. A left-handed twisted material will do the opposite. A left-hand display cell stacked on top of a right-hand cell, both with the same pitch length, will reflect all of the incident light.
According to Bragg's law, the wavelength &lgr;, of the selective reflection is given by the equation: &lgr;=np where p is the pitch of the helical structure and n is the average refractive index of the liquid crystal mixture. In mixtures of a nematic liquid crystal with the chiral additive, the reciprocal of the pitch length is approximately proportional to the concentration X, of the chiral compound, p
−1
=&bgr;
X
with &bgr; being the helical twisting power (HTP). Conventional chiral additives available today have twisting powers typically of &bgr;<15 &mgr;m
−1
when X is measured in weight percent.
Certain dimethanoldioxolane derivatives have been described in the literature as possessing large HTP values. E.g., “TADDOLs with Unprecedented Helical Twisting Power in Liquid Crystals”, H. G. Kuball, B. Weiss, A. K. Beck, D. Seebach,
Helvetica Chimica Acta,
80, 2507-2514 (1997); “TADDOLs Under Closer Scrutiny—Why Bulky Substituents Make it All Different”, A. K. Beck, M. Dobbler, D. A. Plattner,
Helvetica Chimica Acta,
80, 2073-2083 (1997); and in U.S. Pat. No. 5,637,255. Like other chiral additives, they are also known to generally possess large temperature dependent values of dp/dT and, hence, d&lgr;/dT. Their temperature dependencies tend to be positive in that the pitch length increases with w increasing temperature, causing a cholesteric material to change from blue to red reflecting. Furthermore, the temperature dependence dp/dT has been shown to depend on the material of the host nematic. See “TADDOLs with Unprecedented Helical Twisting Power in Liquid Crystals”, H. G. Kuball, B. Weiss, A. K. Beck, D. Seebach,
Helvetica Chimica Acta,
80, 2507-2514 (1997).
In order for flat-panel displays to be useful for portable applications, it is necessary for the display be operable over a wide range of temperatures. Outdoor temperatures can range from −20° C. to +50° C. depending on the environment. It would be advantageous if the reflected colors did not change over this temperature range. Implementing this desirable characteristic is not a straightforward task, since nearly all cholesteric liquid crystalline materials are well known to exhibit reflective colors that vary strongly with changes in temperature. Depending on the shape or chemical structure of the chiral molecule, the pitch length p and, hence, the peak reflective wavelength &lgr;, can increase with temperature (+d&lgr;)/dT) or decrease with temperature (−d&lgr;/dT). Also, in many cases, d&lgr;/dT is not linear over the temperature range of the cholesteric phase.
It should be mentioned at this point that the measurement for the temperature dependence of the pitch is performed using test cells, each of which is 5 &mgr;m thick, and has a hard coat layer on both substrates. There is also an unrubbed polyimide surface on top of the hard coat layers. The measurement for the temperature dependence of the pitch is performed in the following manner. A collimated light is incident on the display at surface normal and the reflected light is detected at 45°. The display is scanned in the wavelength band that includes the peak-reflected wavelength, e.g., 400 nm to 700 nm, or 700 nm to 1500 nm. The cell is switched to the planar texture at each test temperature. The measurement is performed from −20° C. to +70° C. in 10° C. intervals. It should be noted that, for the purposes of a flat temperature dependence, only a portion of the test temperature range, i.e., temperatures between +10° C. to +50° C., are considered. In any event, the maximum change in peak reflected wavelength is then recorded. A mixture is considered to exhibit temperature independent color behavior if the maximum change in the peak reflection wavelength is 30 nm or less across the temperature range of +10° C. to +50° C.
It has been shown that cholesteric displays fabricated using a mixture of two or more chiral compounds as additives can be made to produce a helical twist power and, hence, reflective wavelength that is independent of temperature by combining a chiral compound that has a +d&lgr;/dT with one with a −d&lgr;/dT. U.S. Pat. No. 5,309,265 describes a means of achieving a temperature independent &lgr; by combining a plurality o

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