Note: Descriptions are shown in the official language in which they were submitted.
3~:
Background of the Invention
This invention relates generally to the art of visual
displays and similar light-control devices and relates more par-
ticularly to liquid crystal displays which incorporate an azine
compound as part of the system for selective transmission of
visible light.
Organic substances which exhibit a mesophase have been
known for many years, but it has been only more recently that the
technology of liquid crystal materials has been deve~oped suffi-
10 ciently to achieve commercial application in such devices aswrist watches and digital readouts.
The substances which exhibit a liquid crystal phase, as
postulated by Gray and Harrison in U.S. Patent No. 3,947,374 for
example, comprise a molecule with a central linkage group and a
pair of distal chemical groups of varying character. These paten-
tees recognize the prior utility of Schiff Bases and ~hemselves
disclose commercially useful liquid crystal properties for cer-
tain biphenyl compounds. Gray and Harrison also teach that the
presence of an unsaturated group in the linkage unit is asso-
20 ciated with undesirable, chemical and/or photochemical instability.In addition, Kmetz and Willisen in "Nonemissive Electrooptic
Displays", Plenum Press, New York and London (1976), have
suggested possible display performance for azine compounds
with identical distal groups, although the authors admit
that almost nothing is known about the properties of liquid
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crystal azine compounds.
Summary of the Invention
Contrary to the teachings and sugges~ions of
the prior art, appl.icants have discovered that certain azine
compounds 9 despite the presence of two doubLe bonds in the
central linkage group, are ade~uately stahle and have other
useful proper~.ies for display devices. In particular, the ~
azine compounds of the present invention have dissimilar -
distal groups, i.e. the azine molecule is asymmetric.
Accordingly, a general object of the presen~
invention has been to provide a new class of liquid crystal
compounds which have unexpected utility in a variety of
light-control devices.
Another object of the present invention is to
provide a new class o~ liquid crystal compounds which are
stable and which are easily and economically synthesized. ~;~
Applicants have established that a liquid crys-
tal compound which is usëful for visual display purposes
possesses a melting point of no higher than about 75C,
20 has a comparatively low viscosity in order to exhibit a
short turn-off time, is chemically stable and non-toxic,
and shows a high transition temperature from the mesomorphic
phase to the isotropic liquid state of at least about 50 C. ;
In order to formulate a material that is acceptable for a
given display application, it is sometimes necessary to mix
a particular asymmetrical azine compound of the present in-
vention with another like compound oe with some other
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substance, especially to obtain ~ desired melting point,
it having been found that mixtures of some liquid crystal
materials have melting points which are depressed to a con-
siderably greater extent than is predictable from simple
thermodynamics. On the other hand, the temperatures of
~ransition to isotropic liquid ordinarily vary linearly
with molar average composition, between the respective
temperatures or the pure components of the mixture.
Description of the Preferred Embodiments
The particular azine compounds of the present
- invention have the general formula:
X y
Rl - ~ _ C = N _ N = C _ ~ _ R2
wherein Rl and R2 are different para-substituents whereby
to produce an asymmetrical molecular structure, wherein
Rl and R2 are selected from the group comprising cyano,
halogen, alkyl; and substituted alkyl radicals in which the
alkyl radical has from one to nine carbon atoms, and
wherein X and Y are selected from the group comprising hydro-
gen and methyl radicals. The transition temperatures for
specific compounds answering this definLtion and comprising
examples of asymmetrical azines which we have specifically
synthesized, are set forth in Table I below.
... . , , I
3~iiZ
:
TABLE I
TR~NSITION TEMPERATURES FOR
SELECTED, AS ~ETRICAL AZINE COMPOUNDS ~ -
Crystal
(or Smectic) Nematic to
to Nematic Isotropic Liquid ~;
Rl R2 X YTransition Transition
,
F n-C3H7 CH3 H (Sm) 69C . 75
CN n-C4Hg c~3 H (Sm) 56 117
10 CN n-C5Hll H CH3 71 131
CH3 n-C4Hg H H - 58 87
CH3 n-C5HLl H H 60 100
CH3 n~C7HlS H H~Sm) 56 89
CH3 8H17 H H 62 85
CH3 9 19 H H 64 88
C2H5 n-C4H9 H H 39 82
C~H5 n-C5Hll H 32 92
C2H5 n-C6H13 H H 37 82
C2H5 ~-C6Hl3 CH3 H 32 62
20 C2Hs n-C8H17 H H 33 75
n-C3H7 n-C4Hg H H 37 95
n-C3H7 n-C4H9 CH3 H 39 76
n-C3~7 n-C4H9 H CH3 44 77
n-C3H7 n-C5Hll H H (Sm) 34 105
n-C3H7 n-C5Hll H CH3 43 83
n-C3H7 n-C6H13 H H . 23 92
n-C3H7 n-C7H15 H H 37 95
3~
TABLE I
CONT'D.
' 1
.. _ . . .. _ .
Crystal
(o~ Smectic) ~ematic to
to Nematic Isotropis Liquid
Rl R2 X Y Transition Transition
n-C3H7 n-C8H17 H H (Sm) 28 89 `
n-C3H7 n-CgHl9 H H ~Sm) 42 93
n-C4Hgn~C6HI3 H H (Sm) 38 86 ;~
~ 9 7 15 H 34 91 ~:
n-C4Hg n-C8H17 H H (Sm) 33 84
n-CsHlln-C7EI15 H H ~ 48 98
n-C5Hlln-C8H17 H H 43 9~ :~
CH~CH~CN n-C4Hg H H 71 105
n-C5Hll n-C9Hl9 H H 54 94
n C6H13 n-C7H15 H H 47 86 ~-
n-C6~13 n-C8H17 H H 37 84
n-C6H13 ~-C9Hl9 H H 59 87
It will be noted tha~ all of the compounds in
Table I have melting points (crystalline to smetic or
to nematic transition point~ below the desired temperature
of 75C; and in fact, many of them are below 30 C. More-
over the clearing points (nematic to isotropic liquid
transition points) are in excess of 5Q C and, ln many
cases, above 80 C. By comparison; a symmetrical azine .:
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35~
composition in which both Rl and R2 are alkyl radicals
containing eight carbon atoms, the smectic to nematic -~
transition temperature has been observed to be 68 C
with a nematic range of only 20 C.
The as~mmetrical azine compounds of the present
invention are also characterized by desirably low viscosity,
especially when formulated in mixtures for optical display
purposes. Fast response or turn-off times result, as
much as twice as fast as mixtures of ester liquid crystal
10 compounds. For example, a display with an ester liquid
crystal mixture showed a turn-off time of about 140 micro-
seconds, whereas a corresponding display employing a mixture
of asymmetrical azine compounds according to the present
invention exhibited a turn-off time of about 60 microseconds.
Thus, the asymmetrical azine compounds of the present invention
exhibit viscosities and turn-off times equivalent to commercial
biphenyl liquid crystal compounds while, at the same time,
possessing higher and more desirable isotropic transition
temperatures. In addition5 the asymmetrical azine compounds
20 o~ the present invention exhibit improved chemical and
photochemcial stability over Schi~f Base liquid crystal
compounds, although not as great as biphenyl or ester liquid
crystal compound 8 ~
The asymmetrical azine compounds of the present ~r
invention can be combined in mixtures with other liquid crystal
compounds as well as with other asymmetrical azine compounds
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' 1
using conventional methods such that the resulting oomposition
exhibits a greater mesomorphic range than that of the indivi-
dual compounds~ In addition, asymmetrical ~zine compounds
of the present invention, especially those having melting polnts -
of 60 C or higher, can be usefully combined with biphenyl
liquid crystal compounds or with Schiff Base liquid crystal
compounds or with ester liquid crystal compounds to elevate
the ~ransition temperature from thenematic phase to the
isotropic liquid phase of the resultant mi~ture. The compo- ;
10 sition of one eminently useful mixture, according to the
the present invention is set forth in Table II below.
TABLE II
M~XTURE #1
.. . ... _ .... . .. _
Compound % by Weight
R Rl X
C3H7 C6H13 H . H 14.3 :~:
C3H7 C5Hll H H 19
C~Hg -C6H13 H H 14.3
ClH3 C4H9 H H 9.5
20 ClH3 C8H17 H H 9.5
C3H7 C8H17 H H 14.3
F C3~7 CH3 H 14.3
CN C4H9 CH3 H 4.8
The nem~tic range of Mixture #1 is from -10 C to 85 C.
~ 8
, ~, . . .
35~
A twisted nematic display requires the use of
a liquid crystal mixture having a positive dielectric -
anisotropy; and for prior art liquid crystal compounds, this
is ordinarily accomplished by utili~ing a cyano (CN) group
as one of the substituents, thus introducing a dipole along
the long axis of the molecuIe. However, the asymmetrical
azine compounds of the present invention inherently possess
a mildly positive dielectric anisotropy without need to resort
to synthesis so as to incorporate a cyano group. However,
10 when a display device with a low voltage threshold is to be ~
fabricated, a cyano group is advantageously employed as one of ;;
the para-substituents in the azine compounds of the present
Lnvention; and as will be noted, Mixture`#l contains such a `~
compound. A twisted nematic display using Mixture ~1 has dis-
played a threshold voltage of 2.8 volts. In addition to com-
positions employing two or more asymmetrical azine compounds, ;
~.',;
:~
:
:
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~93 3~
other compositions can be formulated using other types of
liquid ~rystal material. For example, ten percent by weight
of a cyano ester, specifically cyano-phenly-pentyl benzoate,
may be added to Mixture #l; and this latter composition has
exhibited a nematic range of from -22~C to 83 C. In a
twisted nematic display, it exhibits a threshold voltage of
1.60.
The asymmetrical azine liquid crystal compo~mds
of the present invention also show a very low order of bire-
10 fringence; and thus, digital and other displays can be fabri- ;
cated which are thin and possessed of fast response times
without undesirable color variations such as mottling.
Although the classical preparation o~ a sym-
metrical azine compound typically involves reaction of a
hydrazine with an excess of an aldehyde, the asymmetrical
azines of the present in~ention are advantageously synthesized
by first preparing the hydrazone of one of the components by
use of the procedure of G.R. Newkome and D.L Fishel (J. Org.
Chem., 31? 677 [1966]). The reaction then proceeds as i~ the
20 following typical equation:
tl H MeoH H ,H r~
7 3 ~ C_N- NH2 ~ O=C ~ CsHll~ H7C3 ~ C--N-N-C ~ CSHll
( ) ; ..
In the foregoing reaction, the hyd~azone (A~
which has been s~eci~ically prepared from n-propylbenzaldehyde,
is mechanically mixed with the molecular equivalent weight or
(B), n-pentylbenzaldehyde, in a suitable solvent such as
_ g _ :
3~:
,~ .
methanol, for a period of about 15-30 minutes. The resulting
azine product is then separated from the reaction mixture and
purified by recrystallization.
In order that the principles of the present
invention may be more readily understood5 a physical em-
bodiment ther~o~applied to a light-control device or display,
is shown in the accompanying drawing wherein Fig. 1 is a
schematic, cross-sectional view showing an electrooptical
display incorporating an asymmetrical azine liquid crystal
composition according to the present invention.
Referring in detail to the drawing, a light- ~
control device or shutter indicated generally by the refer- ~ -
ence numeral 10 is shown to comprise a first and a second
planar light-transmitting member, 12 and 14. A thin film
of liquid crystal material 16 is ~onfined between the planar
members 12 and 14; and in order to retain the liquid cr~stal ;
material 16 in place3 a continuous liquid-stop ring 18 en-
compasses the liquid crystal material between the planar
members 12 and 1~. As will~be appreciated, the ring 18 may
20 be configurated to define the perimeter of a numerical dig;t
or other symbol which it is desired to illuminate selectively.
In order to provide external electrical con-
nection to the liquid crystal material 16, the planar members
12 and 14, which are advantageously fabricated from a suitable
glass, are coated on their confronting surfa~es with thin
conductive films 20 and 22 respectively. These films are
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35~
advantageously fabricated of tin oxide and form respective
electrsdes for suitable attachment to an external electrical
circuit 24. Circuit 24 comprises a direct current source 26,
such as a battery, and a manual or other switching device 28.
As will be appreciated, the planar members 12 and 14 are out-
wardly offset from the liquid stop ring 18 in order to facili-
tate connections with the circuit 24.
According to conventional practice, a polariz-
ing filter 30 is applied to the outer planar surface o~
member 14. Similarly, a coating 32 is applied to the outer
planar surface of member 12; and the coating 32 may be a
cooperating polarizing filter with a reflective material
such as me~allic silver depending on whether it is desired
that ~he device be transmissive or reflective. In addition,
a light source 34 is focused on the polarizing filter 30; and
in the case where the dQvice 10 is reflective in nature, a
viewing element 36, such~as a photocell or the human eye, is
disposed to observe the device 10 from the same aspect as~'
light source 34.
In accordance with the present invention, the
liquid crystal material 16 includes a compound which has a
transition temperature of at least about 70 C, which has a
melting point of no higher that about 60 C, and which
comprises an asymmetrical azine liquid crystal compound as -~
described hereinabove. Advantageously, the liquid crystal
material 16 may take the ~orm of Mixture ~1 which has been
described hereinabove.
In operation, when the switching device 28 is
directed into a circuit-opening condition, the viewing
element 36 will contemplate a bright background surrounding
an opaque image of the indicia or other pattern created by
the liquid crystal material 16. Contrariwise, when the
switching device 28 is directed into a condition completing
the circuit 24, the liquid crystal material 16 becomes
light-transmissive, and the entire format of the device 10
10 appears reflective.
Various arrangements of light and dark areas
may, of course, be created as circumstances make desirable.
The term "light-control device" as used herein is intended
to mean an optical gate for permitting or obstructing the
passage of light.
The drawing and the foregoing disclosures are
not intended to represent the only forms of our invention
in regard to the details of fabrication and manner of ap-
plication. Changes in the construction of display devices
and in the formulation of liquid crystal compositions, as
weIl as the substitution of equivalents, are contemplated
as circumstances may suggest or render expediènt; and al-
though specific terms have been employed, they are intended
in a generic and descrip~ive sense only and not for the
purposes of limitation, the scope of the invention being
delineated in the following claims.
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