5-Arylindane derivatives and ferroelectric liquid crystal...

Details 5-Arylindane derivatives and ferroelectric liquid crystal... 5-Arylindane derivatives and ferroelectric liquid crystal...

2000-08-31

2002-11-26

Wu, Shean C. (Department: 1756)

Stock material or miscellaneous articles

Liquid crystal optical display having layer of specified...

C252S299620, C252S299610, C544S334000, C544S335000, C546S345000, C546S346000, C570S183000

Reexamination Certificate

active

06485797

ABSTRACT:

The present invention relates to novel 5-arylindane derivatives and ferroelectric liquid crystal mixtures. More particularly, it relates to a ferroelectric liquid crystal mixture, which shows a high switching speed when driven at a low voltage, and a liquid crystal display device with the use of this liquid crystal mixture.
Since Clark and Lagerwall found Surface Stabilized Ferroelectric Liquid Crystals (SSFLC) in 1980 [N. A. Clark and S. T. Lagerwall, Appl. Phys. Lett., 36, 899 (1980)], these liquid crystals have attracted attentions as display materials in the coming generation and a number of studies have been carried out thereon. The reasons therefore are as follows. (1) These ferroelectric liquid crystals have a high response speed. (2) They have memory properties which enable a display of a large information capacity and they can be produced at a relatively low cost, since no active device (thin film transistor, etc.) is needed. (3) They have a broad viewing angle. Thus, these liquid crystals are expected to be usable in a display device having a large screen size and a large display capacity.
To use a ferroelectric liquid crystal display device in practice, it is an important factor to achieve a highly defined contrast. It is very difficult to establish a highly defined contrast at the desired level by using ferroelectric liquid crystals. The reasons therefore reside in, for example, the zigzag defect in the smectic C phase, a decrease in the effective cone angle due to the chevron geometry, the insufficient memory properties, etc. There have been proposed various methods for achieving a highly defined contrast. Examples of these methods include the use of an oblique vapor-deposition film as an alignment layer, the C1 uniform method by using an alignment layer having a high pretilt, the utilization of a quasi-bookshelf geometry through an AC electric field processing or by using a naphthalene-based compound, and the use of a material having a negative dielectric anisotropy. Among the above-mentioned methods, the one with the use of a material having a negative dielectric anisotropy (&Dgr;∈) depends on a phenomenon that, when an electric field of a high frequency is applied perpendicularly to the electrode substrate, liquid crystal molecules having a negative As are aligned in parallel with the electrode substrate. This phenomenon is called the AC stabilization effect.
Multiplexed FLC devices can operate in two different ways: the so-called “normal mode” and the so-called “inverse mode”, the latter also sometimes being referred to as “&tgr; V
min
−mode” or “negative dielectric mode”. The distinction of both modes lies in the addressing schemes and in the different requirement with respect to the dielectric tensor of the FLC material, i.e. the FLC mixture. Surveys are given, for example, in “Fast High Contrast Ferroelectric Liquid Crystal Displays and the Role of Dielectric Biaxiality” by J. C. Jones, M. J. Towler, J. R. Hughes, Displays, Volume 14, No. 2 (1993) 86-93 (referred to as Jones hereafter); M. Koden, Ferroelectrics 179, 121 (1996) and references cited therein.
In general, the switching characteristics of FLC can be discussed in terms of a diagram having the driving voltage (V) on the horizontal axis and the driving pulse width (&tgr;, time) on the vertical axis as in Jones, FIG. 4, 8, 10 or 11.
A switching curve is determined experimentally and divides the V,&tgr; area into a switching and non-switching part. Usually, the higher the voltage, the smaller is the pulse width for switching. Such a behaviour is typically observed for the so-called “normal mode” FLC devices within the range of applied driving voltages.
For a suitable material, however, the V,&tgr; curve reaches a minimum (at voltage V
min
) as—for example—shown in Jones, FIG. 8, 10, 11 and then shows an upturn for higher voltages which is due to the superposition of dielectric and ferroelectric torques. FLC devices work in the inverse mode, if in the temperature range of operation, the sum of row and column driving voltage is higher than the voltage at the minimum of the V,&tgr; curve, i.e. V
row
+V
col
>V
min
.
Examples of this driving mode are given in P. W. H. Surguy et al., Ferroelectrics 1991, 122, 63 (referred to as Surguy hereafter) and P. W. Ross, Proc. SID, 1992, 217.
Surguy reported a driving system in which switching is effected under the voltage
|V
s
−V
d
| but not under
|V
s
+V
d
| or |V
d
|.
(V
s
: strobe pulse; V
d
: data pulse)
The driving voltage in this system is determined by (&tgr;−V
min
) characteristics for the materials. According to Surguy, the value V
min
is defined as follows;
V
min
=
E
min
*
d
=
P
s
*
d
3
*
ϵ
0
*
Δ
⁢
 
⁢
ϵ
*
sin
2
⁢
Θ
In the above formula, E
min
stands for the minimum strength of the electric field; d stands for the cell gap, P
s
stands for the spontaneous polarization; &Dgr;∈ stands for the dielectric anisotropy; and &THgr; stands for the tilt angle of the liquid crystal material.
By taking the biaxial anisotropy (&dgr;∈) into consideration, furthermore, M. J. Towler et al. (Liquid Crystals 1992, Bd. 11) obtained the values V
min
and &tgr;
min
as defined below.
&LeftBracketingBar;
V
min
&RightBracketingBar;
=
P
s
*
d
ϵ
0
*
3
*
(
sin
2
⁢
Θ
-
δϵ
)
τ
min
~
η
*
(
Δ
⁢
 
⁢
ϵ
*
sin
2
⁢
Θ
-
δϵ
)
P
s
2
(&eegr;: viscosity)
However, the ferroelectric liquid crystal material disclosed by Ross et al. still shows only a slow response speed and [V
s
+V
d
] exceeds 55 V, which makes it less usable in practice.
Further ferroelectric liquid crystal mixtures appropriate for driving systems with the use of the AC stabilization effect or driving systems with the use of the &tgr;−V
min
characteristics are disclosed, e.g. in JP A 168792/1989, 306493/1989 and 4290/1992, JP B 29990/1995 and JP A 503444/1990.
EP-B 0 546 338 discloses liquid crystal mixtures, especially ferroelectric (chiral smectic) liquid crystal mixtures containing specific indane-2,6-diyl-compounds. Apart from a 2,6-substitution the indane is not substituted.
DE-A43 03 634 discloses indane-2-yl-compounds which may be substituted in the 5- and/or 6-position by F, Cl, CF
3
, OCF
3
or OCF
2
H. The compounds may be used in liquid crystal mixtures.
JP-A-06263663 discloses 4,6-difluoro-indane-2-yl-compounds for liquid crystal mixtures.
However, since the development of ferroelectric liquid crystal mixtures can in no way be regarded as complete, the manufactures of displays are still interested in a very wide variety of mixtures. Another reason for this is that only the interaction of the liquid crystal mixtures with the individual components of the display device or of the cells (for example the alignment layer) allows conclusions to be drawn on the quality of the liquid crystal mixtures too.
The object of the present invention was therefore to provide ferroelectric liquid crystal mixtures and compounds thereof which are suitable for improving the property profile of ferroelectric liquid crystal displays, particularly of ferroelectric liquid crystal (FLC) displays operated in the inverse mode (using the (&tgr;−V
min
) characteristics).
The present invention provides a ferroelectric liquid crystal mixture comprising a compound of group A:
A. 5-Arylindane derivatives of the formula (I),
wherein the symbols and indices have the following meanings:
R
1
and R
2
, independently of one another, are
(a) a hydrogen atom, —F, —Cl, —CN, —CF
3
or —OCF
3
,
(b) a straight-chain or branched-chain alkyl group (with or without an asymmetric carbon atom) having from 1 to 20 carbon atoms, in which
b1) one or more non-adjacent and non-terminal —CH
2
— groups may be replaced by —O—, —S—, —CO—, —CO—O—, —O—CO—, —O—CO—O— or —Si(CH
3
)
2
— and/or
b2) one or more —CH
2
— groups may be replaced by —CH═CH—, —C≡C—, 1,4-cyclohexylene, 1,4-phenylene, cyclopr

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