Liquid crystalline doristerol-containing organosiloxanes

Details Liquid crystalline doristerol-containing organosiloxanes Liquid crystalline doristerol-containing organosiloxanes

1995-04-03

1997-12-09

Wu, Shean C.

Compositions

Liquid crystal compositions

25229962, 25229963, 25229964, 25229966, 349104, 428 1, 556445, C09K 1952, G02F 113, C07F 708

Patent

active

056956800

DESCRIPTION:

BRIEF SUMMARY
This application is a 371 of PCT/EP 93/02842 filed on Oct. 14, 1993.
The present invention relates to liquid crystalline organosiloxanes containing doristeryl radicals, a process for their preparation, their use, organosilanes which can be condensed to give organosiloxanes containing liquid crystalline doristeryl radicals and mixtures of doristerol-containing organosiloxanes with other liquid crystalline materials.
For some optical applications of liquid crystalline materials, such as e.g. in notch filters, it is necessary to have cholesteric phases having a right-handed helix and cholesteric phases having a left-handed helix in order to be able to reflect both left and right circularly polarized light.
For left-handed helical filters, resort is frequently made here of cholesterol compounds, i.e. steroid compounds, which apart from the chirality have an adequate mesogenicity, i.e. an adequate tendency to form liquid crystalline phases to produce a stable mesophase. The cyclic organosiloxanes containing cholesteryl radicals disclosed in U.S. Pat. No. 4,410,570 are suitable, for example, for this.
In the production of right-handed helical filters, use was until now made of nonsteroidal molecules which have various disadvantages.
From M. L. Tsai, S. H. Chen, S. D. Jacobs, Appl. Phys. Lett., 54, 2395, 1989, it is known that modified hydroxypropylcellulose does not have adequate mesophase stability and polybenzyl glutamates have too low a glass transition point of -25.degree. C. for long-term stability.
The only known sterol whose esters yield a right-handed helix is the cholest-8(14)-en-3.beta.-ol described in U.S. Pat. No. 3,907,406 and U.S. Pat. No. 3,888,892, of the formula 1 ##STR1## abbreviated as doristerol in the following. The compounds described are aliphatic esters, halo or carbonate derivatives of doristerol which on the one hand only have small mesophase widths and on the other hand can only be used at elevated temperature on account of the crystalline state at room temperature.
The object of the present invention was to make available right-handed helical liquid crystalline materials which at room temperature have a stable cholesteric phase, make possible a selective reflection of right-handed polarized light and whose reflection wavelength is largely temperature-independent. This material should additionally have a helical twisting power (htp) which is as high as possible, i.e. a high tendency to formhelical screw structures, in order to be able to keep the proportion of the expensive chiral component low.
The abovementioned objects are achieved by the present invention by means of liquid crystalline organosiloxanes which, per molecule, contain at least one doristeryl radical bonded via the 3.beta.-position.
The liquid crystalline organosiloxanes according to the invention show a low tendency towards crystallization, a stabilization and broadening of the cholesteric phase compared with the known doristerol derivatives and are present in the glass state at room temperature.
The cholesteric phase can be preserved in the glass state by quenching after alignment above the glass transition temperature and is stable at room temperature. The higher glass transition points of the liquid crystalline organosiloxanes according to the invention in comparison with known right-handed helical materials produce a higher stability in the frozen cholesteric phase at room temperature.
The liquid crystalline organosiloxanes according to the invention have distinctly wider mesophases than the known low molecular weight doristeryl compounds. For example, the average values in the liquid crystalline cyclic organosiloxanes according to the invention are about g30*130i, while doristeryl propionate, for example, has values of k80, 5n*83, 5i and doristeryl benzoate values of k111n*140.5i.
The liquid crystalline organosiloxanes according to the invention have a distinctly greater htp and thus a better optical power of rotation per doristeryl radical than the low molecular weight doristeryl derivatives, such that to achieve

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