Chiral compounds

Details Chiral compounds Chiral compounds

1998-08-13

2003-01-28

Wu, Shean C. (Department: 1756)

Stock material or miscellaneous articles

Liquid crystal optical display having layer of specified...

C252S299610, C252S299630, C252S299660, C560S065000, C560S083000, C560S128000

Reexamination Certificate

active

06511719

ABSTRACT:

The invention relates to chiral compounds of formula I
wherein
Sp in each case independently denotes a spacer group with up to 20 C atoms,
X in each case independently denotes —O—, —S—, —CO—, —COO—, —OCO—, —OCO—O—, —CO—NH—, —NH—CO—, —OCH
2
—, —CH
2
O—, —SCH
2
—, —CH
2
S— or a single bond,
Y in each case independently denotes —O—, —S—, —CO—, —COO—, —OCO—, —CO—NH—, —NH—CO—, —CH
2
CH
2
—, —OCH
2
—, —CH
2
O—, —SCH
2
—, —CH
2
S—, —CH═CH—, —CH═CH—COO—, —OCO—CH═CH—, —C≡C— or a single bond,
a, b, c and d are in each case independently 0 or 1, provided that each a and d are not at the same time 0, (i.e., there must be at least one MG
1
or MG
2
group)
R
1
and R
2
are independently of each other H, CN, halogen or a straight-chain or branched alkyl radical with up to 25 C atoms which may be unsubstituted, mono- or polysubstituted by halogen or CN, it being also possible for one or more non-adjacent CH
2
groups to be replaced, in each case independently from one another, by —O—, —S—, —NH—, —N(CH
3
)—, —CO—, —COO—, —OCO—, —OCO—O—, —S—CO—, —CO—S— or —C≡C— in such a manner that oxygen atoms are not linked directly to one another, or alternatively are denoting P-(Sp-X)
n
-, with Sp and X having the meanings given above, n being 0 or 1 and P being a polymerizable group, and
MG
1
and MG
2
are each independently a mesogenic or mesogenity supporting group, preferably selected of formula II
-(A
1
-Z)
m
-A
2
-  II
with
Z denoting —O—, —S—, —CO—, —COO—, —OCO—, —CO—NH—, —NH—CO—, —CH
2
CH
2
—, —OCH
2
—, —CH
2
O—, —SCH
2
—, —CH
2
S—, —CH—═CH—, —CH═CH—COO—, —OCO—CH═CH—, —C≡C— or a single bond,
A
1
and A
2
being in each case independently 1,4-phenylene in which, in addition, one or more CH groups may be replaced by N, 1,4-cyclohexylene in which, in addition, one or two non-adjacent CH
2
groups may be replaced by O and/or S, 1,4-cyclohexenylene, 1,4-bicyclo(2,2,2)octylene, piperidine-1,4-diyl, naphthalene-2,6-diyl, decahydronaphthalene-2,6-diyl, or 1,2,3,4-tetrahydronaphthalene-2,6-diyl, it being possible for all these groups to be unsubstituted, mono- or polysubstituted with halogen, cyano or nitro groups or alkyl, alkoxy or alkanoyl groups having 1 to 7 C atoms wherein one or more H atoms may be substituted by F or Cl, and
m being 0, 1, 2 or 3.
The invention also relates to a liquid crystalline mixture containing at least one chiral compound of formula I.
The invention also relates to a polymerizable liquid crystalline mixture comprising at least one chiral compound of formula I and at least one polymerizable mesogenic compound.
The invention furthermore relates to the use of such liquid crystalline mixtures for the preparation of anisotropic polymer films with a chiral liquid crystalline phase, for active and passive optical elements like polarizers, compensators or color filters and for liquid crystal displays, for example STN, TN, AMD-TN, temperature compensation, guest-host or phase change displays, or surface stabilized or polymer stabilized cholesteric texture (SSCT, PSCT) displays.
The invention also relates to liquid crystal displays comprising a liquid crystalline mixture or a polymerized liquid crystalline mixture and at least one chiral compound of formula I, and to polymer films with a chiral liquid crystalline phase obtainable by (co)polymerizing a liquid crystalline mixture comprising at least one chiral compound of formula I and at least one polymerizable mesogenic compound.
Upon further study of the specification and appended claims, further objects and advantages of this invention will become apparent to those skilled in the art.
BACKGROUND OF THE INVENTION
Chiral compounds can be used as dopants to induce or enhance a helical twist of the molecules of a liquid crystalline mixture that can be used for example in liquid crystal displays. The pitch p of the molecular helix in the first approximation, which is sufficient for most practical applications, is inversely proportional to the concentration c of the chiral dopant in the liquid crystal host mixture according to equation (1):
p
=
1
HTP
·
1
c
(
1
)
The proportionality factor is the helical twisting power (HTP) of the chiral dopant.
For many applications it is desirable to have LC mixtures that exhibit a twist. Among these are e.g. phase-change displays, guest-host displays, passive and active matrix TN and STN displays like AMD-TN, including such displays with temperature compensated characteristics, e.g. by appropriate selection of the cholesteric compounds according to the invention either alone or in combination with further chiral dopants. For these applications it is advantageous to have available a chiral dopant with a high HTP in order to reduce the amount of dopant needed to induce the desired pitch.
For some applications it is desired to have LC mixtures that exhibit a strong helical twist and thereby a short pitch length. For example in liquid crystalline mixtures that are used in selectively reflecting cholesteric displays, the pitch has to be selected such that the maximum of the wavelength reflected by the cholesteric helix is in the range of visible light. Another possible application are polymer films with a chiral liquid crystalline phase for optical elements, such as cholesteric broadband polarizers or chiral liquid crystalline retardation films.
As can be seen from equation (1), a short pitch can be achieved by using high amounts of dopant or by using a dopant with a high HTP.
However, the chiral dopants of the prior art often exhibit low values of the HTP, so that high amounts of dopant are needed. This is a disadvantage because, as chiral dopants can be used only as pure enantiomers, they are expensive and difficult to synthesize.
Furthermore, and in many cases even more important, when using chiral dopants of prior art in high amounts, they often negatively affect the properties of the liquid crystalline host mixture, such as e.g. the dielectric anisotropy &Dgr;&egr;, the viscosity, the driving voltage or the switching times.
Thus, there is a considerable demand for chiral compounds with a high HTP which are easy to synthesize, which can be used in low amounts, show improved temperature stability of the cholesteric pitch, e.g., for utilizing a constant reflection wavelength, and do not affect the properties of the liquid crystalline host mixture.
SUMMARY OF THE INVENTION
The invention has an aim of providing chiral compounds having these properties, but which do not have the disadvantages of the chiral dopants of the state of the art as discussed above.
Another aim of the invention is to extend the pool of chiral compounds that can be used as dopants available to the expert.
It has been found that these aims can be achieved by the provision of chiral compounds according to formula I.
The inventive chiral compounds contain a chiral structure element based on hydrobenzoin, which bears several advantages. For instance, the hydrobenzoin group exhibits two centers of chirality and thus leads to chiral compounds with a high twisting power. Also, enantiomerically pure hydrobenzoins are easy to prepare from cheap, readily available starting materials. The preparation methods are also suitable for large scale production. Furthermore, it is possible to prepare both the R,R and S,S enantiomers. This allows the preparation of chiral compounds that can induce a cholesteric phase with either a right or a left handed helix. The availability of both helices can be a considerable advantage, e.g., for the use in security film applications.
Chiral compounds comprising a hydrobenzoin structure unit and a mesogenic monocarboxylic acid rest with directly linked rings are disclosed in the DE 3534777 A1. The DE 3617826 A1 discloses chiral compounds of a broad generic formula that, among many other possible groups, may also comprise a hydrobenzoin group, but gives no hint to specific compounds except those mentioned in the DE 3534777 A1. The EP 415 220 B1 discloses chiral compounds that among others may comprise a hydrobenzoin structure unit, but is limited t

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