Geminal dimethylalkyl compounds, process for their preparation a

Details Geminal dimethylalkyl compounds, process for their preparation a Geminal dimethylalkyl compounds, process for their preparation a

1992-07-24

1994-11-22

Stoll, Robert L.

Compositions

Liquid crystal compositions

Containing nonsteryl liquid crystalline compound of...

25229961, 25229966, 25229967, 544298, 548136, C09K 1906, C09K 1934, C09K 1912, C09K 1920

Patent

active

053666577

DESCRIPTION:

BRIEF SUMMARY
DESCRIPTION

The unusual combination of anisotropic and fluid behavior of liquid crystals has resulted in their use in a wide range of electrooptical switching and display devices. In these, their electrical, magnetic, elastic and/or thermal properties can be utilized to give changes in alignment. Optical effects can be achieved, for example, with the aid of birefringence, the inclusion of dye molecules which absorb dichroically ("guest-host mode") or light scattering.
In order to satisfy the ever increasing demands of practice in the various fields of application, there is a constant demand for novel improved liquid-crystal mixtures and thus also for a large number of mesogenic compounds of various structures. This applies both in the areas where nematic LC phases (for example TN="twisted nematic" STN="supertwisted nematic", SBE="super-twisted birefringence effect", ECB="electrically controlled birefringence") are used and also in those having smectic LC phases (for example ferroelectric or electroclinic).
Many of the compounds which are suitable for LC mixtures may be described by a structure principle [see, for example, J. Am. Chem. Soc., Vol. 108, 4736 (1986), Structure I; Science, Vol. 231, 350 (1986), FIG. 1 A; J. Am. Chem. Soc., Vol. 108, 5210 (1986), FIG. 3] in which rings from cyclic compounds--aromatics, heteroaromatics or alternatively saturated ring systems--are linked to alkyl side chains which are straight-chain or substituted in the chain by small groups (for example methyl or chlorine) and are thus monobranched.
Recently, there has been increasing interest in ferroelectric liquid-crystal systems as display elements in electrooptical components. A prerequisite for such systems to be usable in practice is frequently the formation of a smectic C phase or a chiral smectic C phase (S.sub.c or S.sub.c * phase) in a broad temperature range, in particular also at temperatures around and below room temperature. In most known compounds which form an S.sub.c phase, this is formed at a temperature considerably above room temperature. The object is therefore to reduce the melting point and in particular the lower phase-transition point of the S.sub.c phase.
It is known that the melting point depression in mixtures is the more pronounced the greater the structural difference between the components of the mixture (see, for example, J. Chem. Soc. 1955, 4305). This also applies to the melting point depression in systems which have a phase sequence C.revreaction.S.sub.c .revreaction.S.sub.A .revreaction.N.revreaction.I which is ideal for the production of electrooptical components. However, other essential characteristic values tend to be retained here if the components of the mixture are structurally similar. The two objects--melting point depression and shift of the lower temperature limit of the S.sub.c phase to lower temperatures on the one hand and as far as possible production of the other characteristic values on the other hand--are thus contradictory.
It has now been found that geminal dimethylalkyl compounds of the formula (I) satisfy said demands, ##STR2## in which: R.sup.1 is CH.sub.3, straight-chain or branched (with or without an asymmetric carbon atom) alkyl or alkenyl having 2 to 16 carbon atoms, it also being possible for one or two non-adjacent --CH.sub.2 -- groups to be replaced by --O--, --S--, --CO--, --CO--O--, --O--CO--, --Si(CH.sub.3).sub.2 -- or --O--O--, and it also being possible for H of the alkyl radical to be replaced by F, or is one of the following radicals ##STR3## A.sup.1, A.sup.2 and A.sup.3 are identical or different 1,4-phenylene, in which 2H may be replaced by F, 1,4-cyclohexylene, pyrazine-2,5-diyl, pyridazine-3,6-diyl, pyridine-2,5-diyl, pyrimidine-2,5-diyl or (1,3,4)-thiadiazole-2,5-diyl, --CO--S--, --S--CO--, --CH.sub.2 --O--, --O--CH.sub.2 --, --C.tbd.C-- or alkylene having 1 to 16 carbon atoms in which, in addition, one or two non-adjacent --CH.sub.2 -- groups may be replaced by --O--, --S--, --O--CO--, --CO-- O--, --CO-- or --O--CO--O--,
R.sup.2, R.sup.3, R.sup.4 a

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