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Chemistry and Applications of Liquid Crystals.

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[I671 H . G . Frlihlich, Z. Gesamte Text.-Ind. 72, 793 (1970).
[168] “Specifications for Test Methods”, accepted by the Technical
Committee of the International Wool Textile Organization, IWTO-2-66;
IWTO-21-69.
[169] G . Tdup and R . Ehrlich, Anal. Chem. 30, 1146 (1958).
[I701 H.-D. Dinse and K.-H. Ewert, Faserforsch. Textiltech. 21, 541
( 1970).
[171] K . Edelmann and H. Wyden, Kaut. Gummi Kunstst. 23.96 (1970).
[172] .I. D. Kerber, At. Absorption Newslett. 10, 104 (1971).
Chemistry and Applications of Liquid Crystals
By Ralf Steinstrasser and Ludwig Pohl[*]
Approximately 5 % of all organic compounds are transformed at their melting point into
liquid crystals- thermodynamically stable, anisotropic liquids which in contrast to isotropic
melts appear turbid and are also known as mesophases. Such melts are classed as smectic,
nematic, and cholesteric liquid crystalline phases, depending upon the arrangement of the
constituent molecules. The discovery of numerous potential applications during the past
ten years has awakened the study of liquid crystals from its former slumber as a physical
curiosity and placed it in the limelight of the scientific stage. Uses in display systems
for measured values and for computer and process data, as well as for remote controlled
timetables, for windows of variable light-transmission, etc., appear particularly promising.
Not only black-and-white contrasts are now possible but also color production.
1. Introduction
2.1. Smectic Liquid-Crystalline Phases
Directionally dependent (anisotropic) properties are
observed independently of the state of aggregation only
for substances having a regular arrangement of constituent
molecules. If the order is three-dimensional in nature then
the substance is a crystalline solid, while two- or one-dimensional order is characteristic of crystalline liquids or liquid
crystals[’ - 51.
Smectic phases have a two-dimensional structure. As seen
in Figure 1 their molecules are arranged in layers. Since
hardly any interaction occurs between the ends of the
molecules the layers can readily slip over one another.
The high viscosity and surface tension of smectic phases
are a consequence of the high degree of order.
The main emphasis of the present progress report will
be placed on recent applications of liquid crystals and
on the connection between chemical structure and the
principal physical effects.
2. Structure and Properties of
Liquid-Crystalline Phases
Smectic, nematic, and cholesteric liquid-crystalline phases
can readily be distinguished on the basis of their optical,
rheological, and thermodynamic properties. The structures
characteristic of smectic phases were first observed with
soaps (Greek crpqypcl) under the polarizing microscope.
The nematic phases owe their name to their threadlike
(Greek vbpclros) appearance under the polarizing microscope. And finally the designation cholesteric derives from
cholesterol whose derivatives were the first compounds seen
to exhibit such phases.
[*] Dr. R. Steinstrasser and Dr. L. Pohl
Zentrallahoratorium fur Industriechemikalien
und Analytisches Zentrallaboratorium der E. Merck
61 Darmstadt, Frankfurter Strasse 250 (Germany)
Angew. Chem. internat. Edit.
Vol. 12 (1973)
1 No. 8
Fig. 1. Structural model of a smectic phase.
Depending on the molecular arrangement it is possible
to distinguish at least five different smectic states which
are classified by the symbols A to E according to H .
Sackmann and Demud21. Three additional smectic phases
designated F, GI6], and HI7] have also been proposed.
Pure compounds too, on warming or cooling, can pass
through several smectic phases bounded by first-order
617
transition points. The transition points of such polymorphous systems can be determined by differential thermal
analysis (DTA). Figtire 2 shows the DTA curves of /,-bis(p11-butylphenyliminomethyl)benzene(/I, a compound for
which EIJ~/OI.
of rd.I8l observed one nematic and three
smectic phases (S,+Sc, Se).The upper curve a ) was recorded
on warming of the crystalline solid and the lower one
b) on cooling of the isotropic melt. Phase transitions
between several liquid crystalline phases and the isotropic
phase cannot be supercooled.
MP
I
ai
Clearing point
t5JQqGt
2.1.3. X-Ray Analysis’
7.
I
Ih1
The layer arrangement of the molecules could be confirmed
for the A, B, and C phases by X-ray diffraction diagrams~”t.
Such a n arrangement, although of higher order, is highly
probable also for the smectic E phase, while a cubic structure is under discussion for the smectic D phase.
The smectic H modification was found by de Vrirs K f
r11.[’]
for p-ethyl-N-(p-n-butoxybenzylidene)aniline ( 2 )
between 40.4 and 51 C and classified on the basis of X-ray
diffraction and microscopic investigations. A three-dimensional layer structure has been proposed in which the
long molecular axes are inclined to the plane of the layer
and the molecular planes adopt a herringbone arrangement.
This example clearly demonstrates that it is often difficult
to distinguish between a solid and a liquid crystalline
state. While the high order suggests a second solid crystalline modification, the fact that the phase transition at
51 C cannot be supercooled indicates the existence of
a true liquid crystalline phase which shows the known
smectic textures under the polarizing microscope. The lines
I
I
of discontinuity merge and reorder themselves when a
3
50
100 150 200 250 300
shearing force is exerted on the objects by application
T[TI
of light pressure with a needle or by lateral displacement
Fig. 2. Differential thermal analysis of p-bis~/~-i~-butylphenyliminumethyl)- of the cover glass[’I.
-
benzene ( I j ; a ) heating curve, b) cooling curve.
The identification and assignment of smectic phases has
been pursued in recent years above all by ti. Sackmrrri,~
P? a/.[’.’- ”1.
Apart from thermal analysis, three main
methods of study are preferred.
The only significant application of smectic phases discovered so far is their use as solvents in ESR and Mossbauer
spectroscopy (cf. Sections 5.3 and 5.4)[*1.
2.2. Nematic Liquid-Crystalline Phases
2.1.1. Microscopic Studies
In most cases characteristic patterns can be recognized
under the microscope when liquid crystalline preparations
are viewed between crossed Nicols. These textures arise
from thediffering orientation of the optical axes of adjacent
regions that are in themselves optically homogeneous. The
boundary lines between these regions, the lines of discontinuity, are typical of the textures which permit a first,
albeit not always unequivocal, distinction of the various
smectic phases. The three smectic phases characteri7ed
thermoanalytically in Figure 2 were found to give a focal
conic (S,%),
a smectic schlieren (SC),and a mosaic texture
The uniform order within a molecular plane of smectic
systems is absent from nematic phases (Fig. 3). The latter
merely exhibit a time- and spatially averaged parallel orientation of the long molecular axes within relatively small
regions and are consequently much less viscous than smectic phases”’].
( SB)‘Hl.
2.1.2. Miscibility Relationships in Binary Systems12.10-t21
Evaluation of phase diagrams of two compounds having
liquid-crystalline properties assumes that perfect miscibility of two anisotropic phases is only possible if both
have the same kind of structure. Perfect miscibility of
an unknown smectic phase with a component known to
be, e . 8 . SH,would indicate the presence of a smectic B
phase. This method has been employed to establish the
identity of numerous phases formerly known merely as
“smect ic”.
618
Frg. 3. Strtrctural model of a nematic phase.
The ideal case-i. e. parallel orientation of all molecules
over large regions-can only be achieved by expenditure
.
.
. ... ..
[*] Note d l r d in p r o o f (July 23. 1973): I t has meanwhile been found
!hat sniectic C phases are suited for the producrlon of electrooptical
memoi-y devices [188].
of additional energy, c'.y. by electric or magnetic fields.
In real nematic liquids. however, the preferred orientation
is subject to continuous change, the angle by which the
preferi-ed direction changes froin place to place not being
defined as it is in cholesteric liquid crystals (Section 2.3).
The quality of the parallel orientation can be characterized
b) the degree o f order S which is unity for the ideal
nematic phase and zero for the isotropic phase. I t has
21 valtic between 0.3 and 0.8 for normal nematic liquid
crystals and decreases steadily M i t h rising temperatures.
suddenlq falling to 0 at the transition point to Ihe isoti-opic
mcltl-5.l X 1 .
Is" (S= I ) arc optically and dielectricall) uniaxial, the synimetrq axis running parallel to the
molecular axis. The values of the dielectric constant c.
the susceptibility x. the refractive index / I . or of other
anisotropic properties measured in this direction arc given
the subscript 1. and the values measured in a perpendicular
direction the subscript 2. I n nematic liquid crqstals the
molecular long axis is also the direction of maximum
polarizability of the molecule ; thus a positive birefringence
( I I ~> n z ) is always observedtly-'21.
The diamagnetic anisotropy of nematic phases ( A 1 =
- l r )is also always positive since the susceptibiliti is
always greater along the molecular long axis than in the
perpendicular direction.
I n contrast. the dielectric anisotropy ( A & = & -cz)
,
can be
positive 01-negative and depends upon the varying contributions of shift and orientation polarization in the different
axial directions which are detcrniincd bq the molecular
slructurel"'.
ZJI
a compensated cholesteric liquid crysta1l'"I. Such cliolesteric-nematic phase transitions can be induced not only
thermally but also by application of electric!". "I and
m
l.ig. 4. Striictural model of a c I i o I c \ ~ c ~phiisc.
. ~ ~ Ihc pi-cfei Fed oriciitii~ion
of llic long iiioleciiliir iixcs w i t h i n ii 1ii)i.r is rotated throiipli ii constant
angle irclativc t o that within the adjacent laqcr (helical s1i-iic1i1rcl
magnetic fields' 3').J-ol. Moreover. the cholesteric structure
can also be pi-oduced by addition of ii chiral. not nccessnrily
I iqu id-crystalline. compound to a nematic liqu id1A ' I or
b y resolution of nematic racematcs[4'l. thus clearly illustrating the close relationship between cholesteric and
nematic structure.
Most of the optical properties restili from the twisted
structure,<>.
61. thecxtrcmcl) high optical rotation, the circular dichi-oism,and tlie technicall) important selective rcflcction of the specti-a1 I-egion whose wavelength corresponds
to the pitch of the hclix~'3~.A change in pitch caused
bq thcrmall'3-'41, magneticl'71. or electrical energ) 1"' 1'
manifests itself in a change in optical properties.
~
IJniformlq oriented nematic preparations also exhibit anisotropic electric conductiviti 12' ''I1 . thermal conduct ivi t ) [30.3 1 I. ultrasonic velocityt3'?! diffusion'"I. and viscosity1"I. Values of anisotropic mcasurements must always
be accompanied by tlie measuring temperature since they
are all strongly temperature-dependent. The manifold
applications of nematic phases will be described in Section 4.
2.3. ('holesteric I,iquid-('rystalline Phased I . 'I
The cholesteric mesophase represents a special case of
the nematic structure in that it is also characterized by
;i par;illci orientation o f tlie long axes of the molecules
which howevei- varies rcgularl) firom place t o place. As
shown in Figure 4 there i s ;I uniform pi-eferrcd direction
14 i t h i n :I given layer which is rotated in ii uniform direction
through ;I constant angle rclntive to t h a t in tlic adjacent
l a ~ e rSeen
.
o\erse\eral layers thisleadstoa skewed arrangement of the moIecuIar Iayerst3'1. 7'he [Misted structure
and the sense of the rotation are induced b) the chiral
form o f tlic pal-ticipating molecules. (%olestei-ic phases
were first obscr\cd in cholesterol derivati\ es.
Mixinp of cholcstcryl esters ha\ ing right-handed and lefthanded hclical arrangements i n a particular ratio and at
21 dcfinite temperature. the nematic temperature 7,. at
which tlic rotator) powers of the components canccl each
olhcr. lcads to ii nematic liquid crystal u,liich is termed
Of particular importance in practice is the temperature
dependence of selective reflection i n the visible spectrum
because it enables cholcsteric liquid ci-ystals to be iised
21s teinpcraturc indicators in medical diagnoses1'"- "'I . in
nondestructive testing of materials". 521. or for infrared
image con\erters""]. Experimcnts have also be performed
t o exploit this effect for the production of color TV
tubcs15'.5.'i but h a w so f;i . been thwarted bj the difficultics
inherent in converting an electrical impulse into a local
warming effect in ;I manncr satisfq m y practicnl requirements. Other clectroopticol applications of cholcstcric
liquid crystals iirc bascd cithci on tlie :ihove mentioned
cliolesteric-nernatic phase transition b) electric fields'"''
01- on tlie conversion of the transparcnt p l a n x cliolcsteric
texture into tlie strongly dispersive focal conic tcxtui-c.
which opens up the possibilit) of producing optical storage
5 h l (cf. Section 4.4).
3. Relationship betveen Chemical Structure
and the Occurrence of Liquid-('rystalline I'liases
Recent studies on the irelationship between molecular structure and the appearance of anisotropic mcsophascs ha\ e
been performed mainly by B i . o i i d s " l . L)tri.rl""l. Grtr 1.1 "' '1,
t / i , Jiwl"". I<t~.si[".~I.
and II' i q ~ i i i i / ~ " K~ lt r. s / also compllcd
a list o f all compounds known u p to 1959 that posscss
liquid-ciq stalline properties["".
619
( 6 ~ ) .R=R’=n-C4H9 [69],clearing pt. 28-C (m.p. 14 C)
(661, R=n-C4H9, R’=OC4H9 [70], clearing pt. 91 ’C (m.p. 37°C)
( 6 ~ ) .R=R‘=OC4H9 [82], clearing pt. 137 C (m.p. 102-C)
3.1. Molecular Structure of Nematic
and Smectic Compounds
The thermodynamic stability of liquid crystalline phases
can be characterized in terms of the clearing point, i.e.
the temperature at which the energy content of the anisotropic and the isotropic liquid is equal and both modifications are present.
Comparison of the clearing points of compounds of analogous structure differing only in one particular feature permits’assignment of the contribution of such partial structures to liquid-crystalline character. The most important
knowledge gained in this way will now be summarized
and illustrated with the aid of a characteristic example:
The greater the anisotropy of the polarizability of a
molecule, the higher the clearing point and the stability
of mesophases. Accordingly, the compounds forming liquid
crystals generally are composed of long, planar rigid
molecules usually consisting of a conjugated aromatic
system.
x
Name
(3)
(4)
-CH=CH-
(5)
-N=N-N=N-
rrans-stilbenes
tolanes
azobenzenes
azoxybenzenes
(6)
(7)
(8)
(9)
-c=c-
-
0
-CH=N(O)-CU=N-
-0-co-
Ref
nitrones
Schiff bases
phenyl benzoates
Compounds of types ( 3 ) to ( 9 ) (Table 1) have been intensively s t ~ d i e d [ ’ ~ - In
~ ’ connection
~
with the applications
considered in Section 4. Of the stilbenes ( 3 ) , only the
trans isomers are able to form liquid-crystalline phases.
Although the correspondingly substituted cis-stilbenes are
also planar they d o not have a linear molecular structure.
Owing to their low degree of conjugation the esters ( 9 )
have the lowest clearing points. Methyl substituents ortho
to the bridge or on the bridge itself in the case of stilbene
effect mutual twisting of the phenyl rings for steric reasons,
thus reducing the polarizability along the long molecular
axis and thereby the stability of the mesophaser66.791.Replacement of aromatic rings by bi- or mono-cyclic aliphatic
systems has a similar
‘1. Additional permanent
dipoles shift the clearing points to higher temperatures.
Whereas the central bridge of the molecule induces anisotropic melting properties, the nature and chain length
of the wing groups determine which of the mesophases
occurs.
If the wing groups are ether or ester groups linked to
the molecular skeleton via oxygen, the lone pairs also
contribute to the n-electron system, thus enhancing the
thermal stability of the mesophases:
620
The acetic ester ( l o ) , n = 1, goes directly from the solid
to the isotropic liquid state at 114.5 “C; only on careful
supercooling to 105“C does the latter phase yield a metastable nematic phase. Such systems are termed monotropic.
An enantiotropic, i. e. thermodynamically stable, nematic
phase is observed for the propionic ester (ZO), n = 2. The
compounds with n = 3 and 4 also give monotropic smectic
phases in addition to enantiotropic nematic phases, while
both the nematic and the smectic phases of the higher
homologs (n= 5-7) are enantiotropic.
As the chain length increases, the tendency to form smectic
phases increases at the expense of the nematic phase until
eventually only smectic phases are observed at a chain
length of, in the present case, more than seven carbon
atoms.
This behavior has been observed in all homologous series
of ethers, esters, ketones, or directly linked alkyl chains
in various classes of compounds and can therefore be
generalized.
Table I . Compounds of types (3)-(9)
Type
Table 2 illustrates the effect of varying the wing groups
on the liquid-crystalline properties of a homologous
Table 2. Transition points [ T I of a homologous series of type ( 1 0 )
[83]. Monotropic transitions are indicated in parentheses.
n
solid/
isotropic
smectic [a]
I
2
3
4
5
6
7
8
9
114.5
SA:
SA:
Sc:
sc:
Sc:
Sc:
Sc:
(73.5)
(84.0)
86.5
94.5
89.0
83.0
79.0
nematic
106.0
87.5
101.0
92.0
100.0
107.0
-
anisotropic/
isotropic
(clearing pt.)
(105.0)
113.0
115.0
105.5
110.5
107.0
1 1 1.5
1 11.5
1 16.0
[a] SA and Sc signify smectic A and C phases. Further transitions to
monotropic smectic phases were observed.
Table 2 also reveals another generally valid principle:
the clearing points alternate within a homologous series,
usually diverging towards lower values but sometimes also
to higher values[62].
Chain branching in the substituent reduces the tendency
to form anisotropic melts because steric hindrance precludes an optimum parallel arrangement of the molecular
skeletons. The intramolecular steric hindrance increases
the nearer the branching point approaches the rigid central
part of the molecule[841.
Young et aZ.[”l found that incorporation of a particularly
readily polarizable atom can compensate the unfavorable
steric effect of chain branching. While the tert-butyl comAngew. Chem. internat. Edit. / Vol. 12 (1973) / No. 8
pound ( I 1 a ) is not liquid crystalline, the trimethylsilyl
( I 1b ) ,trimethylgermyl(1 I c), and trimethyltin (1 1d ) analogs form smectic mesophases (Table 3).
Table 3. Melting and clearing points [ C] of compounds ( I I a)-(
CPd.
X
m. p.
IId).
clearing point
The central part of the molecule can be modified in
a variety of ways. Thus when bearing suitable substituents,
compounds having the structural features (12)-(18)
exhibit liquid crystalline
[X and Y in (15)
have the same significance as in compound (1) and the
compounds of types (3)-(9)]. Free carboxylic acids,
such as para-substituted benzoic or cinnamic acids, occur
as double molecules in the melt and thus form a planar
system of sufficient length.
scribed in 1969 by Kelker et al.[731.Moreover, binary
eutectic mixtures of Schiff bases have been prepared, some
of which melt far below 0°C[73,741.
However, since the
sensitivity of Schiff bases to hydrolysis severely impairs
their technical utilization, attempts are made to stabilize
them by introduction of an intramolecular hydrogen
bond as in compound (20), m.p. 44.7"C,clearing point
64.50C1s91.A significant but insufficient stabilization is
noted. The increase in the clearing point is accompanied
by an increase in melting point compared to compound
(19).
Studies on more stable structural types showed that the
substituent combination p-n-butyl/p-methoxy also leads
to the lowest melting representatives (21) and (22), respectively, of the azo- and azoxybenzenes[68,701[(21), m. p.
32"C,clearing point 47"C;(22), m. p. 16"C,clearing point
76"CI.The azoxy product (22) consists of a mixture of
the t'wo N-0 isomers. Admixture of 35 YOof the isomeric
pair p-ethyl-p-methoxyazoxybenzene gave a eutectic (clearing point 75 "C)melting at - 5 0C[701.
(12) 186, 871
n
($4)
15)
".owim
0 H-O'
(16)
(17) [881
(0)
3.2. Nematic Liquid Crystals Having Special Properties
Subsequent to the discovery of their possible electrooptical
applications (Section 4) an intense search was instituted
for suitable nematic substances having low melting points
and high clearing points, favorable anisotropic properties,
and adequate stability towards chemical and physical
effects.
The azomethines and azoxybenzene derivatives mentioned
so far exhibit a negative dielectric anisotropy. If there
is no permanent electric dipole perpendicular to the long
molecular axis (EZ) or the para position carries a strongly
polar group, then the dielectric constant E' in the direction
of the long axis will predominate, and the dielectric anisotropy (A&=&'- 6 2 ) will be positive. The dielectric anisotropy of the azoxybenzene derivative (22) amounts to
- 0.2,and that of the azobenzene analog (21) + 0.2.That
the ether oxygen in (22) also makes a crucial contribution
to the permanent transverse dipole moment is apparent
from a comparison of the dielectric anisotropy of (22)
with that of p,p'-di-n-butylazoxybenzene ( +o. 19)169.901.
Nitrile groups in one of the para positions increase the
dielectric anisotropy. Thus a dielectric anisotropy of 14
was recently reported~9 11 for the eutectic system comprising
2/3 ( 2 3 ) and 113 (24), m.p. -3O"C,clearing point 62°C.
+
~~
n - C , H , G C H = N G C N
The first nematic compound found to be a liquid at room
temperature, p-n-butyl-N-(p-methoxybenzy1idene)aniline
(MBBA) (19), m.p. 20"C,clearing point 47"C,was deAngew. Chem. internat. Edit. / Vol. I 2 ( 1 9 7 3 ) J N o . 8
(24)
The phase diagrams of binary nematic systems display
the known depression of freezing point which leads in
621
6c
I
some cases to very low-melting eutectics. Since the melting
temperature of such mixtures can be calculated bq the
classical van't I-ioff-Scllriider-Van Laar formula under certain conditions, it is also possible t o calculate the eutectic
composition o f binary systems""l.
bond in steroids and triterpencs on the formation and
type of mesophases revealed that mesophase formation
is not supported b) a <--I. C7-4. or ( - 3 double bond in
the steroid molecule, whereas a double bond between C*-9
and C-I 1 permits a cholesteric phase. The nature and
length of the 17p side chain also excrt ;I considerable
influence on tlie mesomorphic behavior of tlic moleculcl'"ll. Accordingly, extensive variations of the cholesterol molecule are possible without loss of liqt~id-crqst~~lli~ie
properties. I t is therefore hardly surprising t h a t cholesteric
mesophases have also been found for derivatives of other
naturallq occurring steroids such as sitostero11"51or 9,13cyclopropa nc t rit el-penes1I '1. ti OM ever. nonsteroidal COIIIp ( b i t n d \ i > l n t ~ i i i ; i t i y c i i i c\II IIL'I III-c' L.;iii ;il\o foi-ni c,liiilt~\lt*i.ic
mesophases provided that they contain optically active
s I1 k
hI1 I LICll 1
' ".(I i i 'Ip]>c'llb I l l a t ;ill 'ib) lllnlc~I-lcC C l l l C l
suffices to impart to the ncmatogenic molecules tlie helical
arrangement necessary for formation of a cholesteric mesophase.
\'I'
-
4. Use of Nematic Liquid Crystals in Electrooptic
Display Devices
Tlieclearing points lie practically on a straight line between
the clearing points of tlie t n o pure components. I-igure
5 depicts a phase diagram of tlic monotropic nematic
compounds f 2-51 and 2 6 ) M hich form an enantiotropic
nematic cutcctic. The 2 : 1 composition of the eutcctic.
also found in the ( 2 3 ~(241
. mixture, is typical ofnumerous
binary rieiiiatic systemsly31.
Application of electric fields of ;I few hundred t o sc\cral
tcn-thousand V c m can induce changes in the state of
ordcr of thin nematic liqiiid-cr),stnllincfilms. Thcsc changes
in ordcr are ohscr\ahlc as changes in optical properties
sucl: as transparcncj. light scattering. birefringence. o r
color. Such clcc~troopticaldfccts c m be ohscricd and titilizcd in ;I set-up conskting of t w o conducting glass plates
coated on the inside v, itli ;I tr:rnsparcnt S n 0 2 or l n 2 0 . 1
laqcr and hctu ccw ikliicli thcrc is ;i 5~ 50 pii uniformly
2
\
1
3.3. Molecular Structure of Cholesteric Compounds
('holesteric mesophases occur primarily in cholesterol dci-ivativesoftype(27) in which the Iiydroxyl group in position
3 is esterified with
or aroinaticl"."'
carboxylic acids. hydrogen carbonates'"'! or inorganic acids'"''.
Here. too. long-chain substituciits enhance the tendency
t o form enantiotropic smcctic phases which undci-go
transition to cholestcric mesophases at elevated temperntllrcs.
\I
3
A
1
I
I
I
oriented ncmatic film (Fig. 0). Thinnci- 1:i~ci-sai-c difficult
to pi-odiicc and thicLcr one5 cxhihit strong light scattering.
4.1. Orientation of Nematic Liquid <'r\stals at Interfaces
The only hydrogenation product of cholesterol able to
for-m esters ha\ ing cholesteric properties is 53-cholestan311-01. I n the Sp compound. rings A and B 31-c c.i.\ to
cach other. thus ruling out the planar conformation whic11
is also a11 inipoi-tant structural feature of the cholesteric
mesopliase'"'.'Jx'. Studies b) Polilriitrrirt of tr/.l""l and Aitr//rrh
('I
tr/.l'""I on the influence of the position of a double
622
In cells of the above t j p c the liquid crystill ~nolcculcs
can adopt oi-iciitations~ i t their
h
long axiseither perpendicular or parallel to the ualls (Figs. 7 a and 7b). The former
c;isc is dcscrihcd 21s a homeoti-opic phase 01-orientation.
and the latter :IS ;I homogeneous one. The orientation
o f the molecules ;it the interface is transmitted to tlic
internal molecules b) dispersive interaction. The methods
used for production of uniformly oriented layers ;ire still
1a rgcl j em pi I- ica I.
Nematic crystals exhibiting positive dielectric anisotropy
(cf. Section 3.2)-this applies to molecules having a strong
electric dipole in the direction of the long molecular axisalign their long axes parallel to the direction of the field
under the influence of a DC or AC potential. This homeotropic orientation is further promoted by a flow of charge
carriers between the electrodes.
Field effects are more complicated in the case of nematic
substances having negative dielectric anisotropy[". '*I. For
small AC potentials (5-10 V depending on conductivity
and thickness of sample) a static deformation of the layer
takes place, irrespective of whether it was originally homogeneous or homeotropic. A system of largely parallel striations is observed under a polarizing microscope. The line
textures are called Williams domains after their discovererfl16-1181 . The long axes of the molecuIes are arranged
substantially parallel t o the surface of the electrodes.
Fig. 7. Possible orientations of thin liquid crystalline layers; a, homeotropic; b, homogeneous; c. disordered, dynamic scattering; d, deformed
homeotropic; e, twisted homogeneous; f, cholesteric planar; g, cholesteric
focal conic structure.
The homeotropic orientation (Fig.. 7 a) can be achieved
by treatment of the glass plates with an ethanolic lecithin
solution['041 or by addition of polyamide resins"051 or
quaternary ammonium saltsf'061 to a nematic liquid crystal.
Some commercially available nematic phases are already
doped so as to automatically adopt a homeotropic orientationl'"'1.
The homogeneous orientation (Fig. 7 b) can be brought
about by unidirectional rubbing of the electrode surfaces
with polishing cloths['081or pastes containing diamond
dust['"']. Such treatment produces submicroscopic grooves
parallel to which the molecules adjacent to the interface
become aligned. As demonstrated by Bcrrcmun" I"], any
other orientation of these molecules would incur expenditure of additional elastic energy. Homeotropic or homogeneous orientations are reported to be inducible according
to choice by dipping the electrodes into solutions of N-hexadecyl-N,N,N-trimethylammonium bromide depending on
the concentration and rate of immersion'' '1. More reproducible homogeneous phases are obtained at electrode
surfaces whose conducting surface has been condensed
on at an angle of about 85
'
4.2. Interactions between Electric Fields
and Nematic Liquid Crysta1~~'~31
Three kinds of electrooptical effect are known to occur
in liquid crystals, G I Z . dielectric, piezoelectric, and electrohydrodynamic effects. The first two arise solely from the
influence of electric fields without simultaneo~isparticipation of charge transport and are known collectively as
field effects. They result from static, so-called dielectric
deformation of the existing orientation of the liquid crystals
and become operative above a certain threshold voltage
that, for dielectric effects, is independent of the layer
thickness but dependent upon the dielectric anisotropy
and the splaying, bending, or twisting of the liquid crystalline layer[ ' 1 4 . ' I S 1
Angen. Chem. rntrniut. Edit. I Vol. 12 ( l Y 7 3 )
No. H
The same phenomenon is observed at higher potentials
as long as their frequency is in excess of the cut-off frequency. This term denotes the frequency at which the
preferred orientation of the liquid crystalline molecules
due to the field is no longer subject to interference by
migration of charge carriers[90.' 191. It is dependent upon
the conductivity of the material and lies below 30 Hz for
liquid crystals having R > 10' ohm cm and above 30 Hz
for those with Rc 1012ohm cm.
With potentials having frequencies lying below the
cut-off frequency, fields in excess of about 10kVcm-'
lead to perturbation of the molecules oriented parallel
by the field owing to the flow of charge carriers between
the electrodes, thus displacing the molecules from their
stable equilibrium positions and destroying the overall
optical homogeneity except for very small residual regions
which act as scattering centers for light (Fig. 7c). Viewed
under a polarizing microscope these small regions are
seen to undergo vigorous turbulent motion. Macroscopically, this turbulence manifests itself in a sharp increase
in scattering, conveying the impression that a transparent
glass is transformed into a frosted glass on application
of the DC or AC potential. This phenomenon is known
as "dynamic scattering'"' 201.
4.3. Electrooptical Display Devices
Employing Nematic Liquid Crystals
Most of the commercially available displays utilizing
liquid crystals are currently based on the dynamic scattering effect. In addition, increasing use is being made of pure
field effects for electrooptical applications.
4.3.1. Displays Based on Dynamic Scattering"
21.
' '*I
Dynamic scattering cells can be constructed in a transniission or a reflection mode. In the former case both electrodes
are transparent, and in the second the rear electrode is
a mirror. All systems of this kind are characterized by
their flat construction (2-3 mm thick), high contrast ratios
in excess of 100: 1 (the best contrast being obtained with
homeotropic or homogeneous phases), low driving voltages
623
between about 20 and 30V, and minimum power consumption of at most 500 pW/cm2 of active display area.
case the molecules flip out of their homeotropic into a
homogeneous orientation. The pdtential required for displacement of the molecules lies in the range 5-10V and
is thus much lower than the corresponding driving voltage
of 20-30 V for dynamic scattering. The occurrence of
dynamic scattering during deformation is avoided by the
use of high resistance liquid crystals and AC potentials
having frequencies above the cut-off frequency.
Homeotropic liquid crystalline layers behave like optically
positive, uniaxial crystals. Light entering the crystals in
the direction of the optical axis emerges unchanged. If
such a liquid crystalline layer is situated between crossed
polarizers the overall system cannot transmit light and
thus appears black.
Fig. 8. Example of a liquid crystal display based on dynamic scattering.
The lifetime of such display systems depends upon whether
they work with AC or DC. For AC operation values
in excess of 10000 operating hours are reported. DC operation often leads to gradual deposition of the ions required
for dynamic scattering so that the conductivity often drops
below the threshold potential for dynamic scattering. This
effect can be suppressed by addition of less than 1 YO
of a charge-transfer acceptor, c. g. chloranil, which lowers
the threshold potential by a factor of 211231.
The switching times depend on the layer thickness of the
nematic phases, the driving voltage, the conductivity,
the dielectric anisotropy, and the viscosity. The following
values are typical for the standardized commercial nematic
material['071used for dynamic scattering applications (conductivity: 4 x lo-" to l o x lo-'' ohm-' cm-'):
T, (raise time).
1; (decay time):
18 to 28 ms
25 to 50 ms
[measuring conditions: d = IOpm, C=2SV/SO Hz. S n 0 2 electrodes)
Apart from digital and analog display of measured values,
time ofday, and computer and process data (Fig. S), display
systems with nematic liquid crystals are suitable for use
in, e. g. remotely controlled flight timetables, windows of
variable transmission, or advertising displays. Unlike conventional luminous display they become more and more
legible as the brightness of the environment increases.
4.3.2. Display Devices Based on Electroelastic Deformation of Vertically Aligned Nematic Layers (DAP Effect)
Homeotropically oriented layers of negative dielectric anisotropy readily undergo elastic deformation on application
of an electric field parallel to the principal optical axis
of the molecules, leading to a displacement from their
perpendicular orientation (Fig. 7d).This effect is designated
as deformation of vertically aligned phases or in abbreviated form as the DAP effect1' 24. ' 25J.The degree of deformation depends on the applied voltage. In the extreme
624
If the liquid crystalline layer is deformed by application
of an electric field, the angle between the optical axis
of the liquid crystal and that of the incident light, i.r.
the angle of incidence, differs from zero and birefringence
occurs. Under these conditions, and using parallel light
the intensity ( I ) of the emerging light depends on the
phase difference 6 between the ordinary and the extraordinary ray as shown in eq. (1).
The phase difference itself varies with the difference between
the refractive index of the extraordinary ray (n,) and the
ordinary ray (no)according to eq. (2).
&=--
Zxnxd
An.
An
=
n, - no
(2)
For a given layer thickness d and wavelength h (monochromatic light) the angle of incidence can be varied at will
between 0 and 90"via the deformation of the homeotropic
phase by the applied voltage. It is thus possible to switch
the system from opacity to transparency, i.e. from dark
to light. Contrast ratios in excess of 500: 1 are perfectly
feasible" 26J.
4.3.3. Display Devices Based on an Electroelastic Deformation of Homogeneous Nematic Layers (FrCedericksz Effect)
Homogeneously oriented nematic liquid crystals of positive
dielectric anisotropy can be reoriented homeotropically
by application of a DC or AC field perpendicular to
the long molecular axis. This effect, the counterpart
of the deformation of vertically aligned phases, produces
the same observable effect as the latter and was described
by Fr6cdericksz as long ago as 1933["'J. Such a layer
between crossed polarizers can be switched from transparency to opacity, i. e. from light to dark.
4.3.4. Display Devices Based on Electroelastic Deformation of Twisted Nematic Layers (Schadt-Helfrich Effect)
Homogeneous phases can also be obtained in which the
liquid crystals exhibit a preferred orientation differing by
90" between the opposite electrode surfaces" 14] (Fig. 7e).
Polarized light passing through such a twisted layer
Angew. Chem. internat. Edit. J Vol. 12 (lY73) J
No. 8
is accordingly rotated through 90", and between crossed
polarizers the system is transparent. An electric field
can induce homeotropic reorientation of the phase if
the molecules possess a positive dielectric anisotropy
(Fig. 7 b). Polarized light is then unaffected and the system
is opaque, appearing black. This effect, discovered by
Schadt and Helfvich" 14], yields high contrasts, and exhibits
threshold potentials of less than 1 V1911,driving voltages
of 2-5 V"281, and power consumptions of 1-50pW/cmZ.
4.5. Production of Colors with Nematic Liquid Crystals
Apart from black-and-white contrasts, nematic liquid crystals can also be used for generating colors by virtue of
interference phenomena in homogeneous or homeotropic
layers or of reorientation of dichroic dyes embedded in
a liquid crystalline matrix.
4.5.1. Colors from Tunable Domains
4.4. Electrooptical Effects Based on
Textural Changes in Liquid Crystals
Addition of a chiral compound to a nematic liquid leads
to two further effects, depending upon the anisotropy,
designated as memory and transparency effects. Both arise
from light scattering by disordered phases. However, since
they represent stable textures and are not due to turbulence
within small liquid crystalline regions, they differ fundamentally from dynamic scattering.
4.4.1. Memory Effect in Liquid Crystals148.".
291
Mixtures of a nematic liquid crystal of negative dielectric
anisotropy and a cholestericl I 301 or non-liquid crystalline
chiral substance such as L-mentholI'3'] can be used to
prepare thin films whose order corresponds to the planar
cholesteric texture (Fig. 7 0 . Application of a D C potential
transforms this phase into a strongly deformed, light-scattering phase exhibiting a focal conic texture (Fig. 7g) and
having randomly oriented helical axes in the individual
subregions. After removal of the applied potential the disordered phase reverts to the original ordered state within
minutes or days, depending on the concentration of the
chiral component ( I - - I O Y O ) ~ ' ~An
~ ~AC
. potential of 50 to
100 V whose frequency must exceed the cut-off frequency
and is generally above 700 Hz, can reconvert the disordered
into the ordered phase within milliseconds[' 331. This
reversible change of texture permits the construction
of an optical memory device in which information can be
recorded with an DC potential and erased with an AC
potential.
4.4.2. Transparency Effect in Liquid Crystals"
341
For mixtures comprising 3&90% of a nematic liquid
crystal of positive dielectric anisotropy and 70--100% of
a cholesteric liquid crystal, a disordered molecular arrangement, or focal conic texture, is the most stable possibility.
Such a liquid crystalline film therefore exhibits pronounced
light scattering. An AC potential can transform the disordered phase into an unstable homeotropically oriented,
transparent layer which reverts to the disordered state
when the potential is removed. The switching times depend,
inter alia, on the thickness of the layer, the dielectric anisotropy, and the mixing ratio of nematic to cholesteric liquid
crystals, and lie below 10ms; the AC potentials required
are between 30 and 1OOV.
Anyew. Chem. internat. Edrr. J
Vof. 12 ( 1 9 7 3 ) No. 8
Homogeneously or homeotropically oriented layers (3 to
6 pm thickness) of high resistance ( > 10' ohm cm) nematic
liquid crystals with negative dielectric anisotropy exhibit
Williams domains (Section 4.2) when an applied potential
alternates at a frequency in excess of the cut-off frequency.
The width of the domains can be reduced by a factor
of 10 from e.y. 10 to I pni if the applied potential is
raised by the same factor, e.y. from 10 to 1OOV. Such
liquid crystalline layers therefore behave as a tunable optical diffraction gratings which, being phase gratings, are
highly efficient. If a directional filter which only transmits
light of a particular diffraction angle is placed behind
one of these gratings and white light shone onto the set-up,
any desired spectral color can be obtained by changing
the applied potential[I3'- I3'l.
4.5.2. Colors Produced by Reorientation of Aligned Phases
(DAP and Frtedericksz Effect)
If white light is used instead of monochromatic light with
the set-up described in Sections 4.3.2 and 4.3.3, then according to eq. (2) the phase difference between the ordinary
and the extraordinary ray occurring on deformation of
the homeotropic or homogeneous phase depends not only
upon the angle of incidence but also upon the wavelength.
Interference between these two rays thus leads to the
appearance of Newtonian colors. For a given direction
of observation any desired spectral color can be obtained
by variation of the applied potential.
4.5.3. Colors Produced by Dichroic Liquid-Crystalline
Color Switches
Dyes exhibiting differential absorption of polarized light
in the directions of the individual axes are called dichroic.
Representatives of this group include indophenol blue,
alizarin, and oil yellow. Since the guest molecules dissolved
in a nematic liquid crystal frequently adopt the same orientation as the liquid-crystalline host molecules and are reoriented with the latter by electric fields, dichroic dyes
can be utilized in the production of color switches['38- 14'1
that can be tripped from colored to colorless or vice versa
(Fig. 9).
A hornogmeottsly oriented film (Fig. 9a) of a nematic liquid
crystal of positive anisotropy doped with 1-2"/, of a dichroic dye absorbs only light polarized in the direction of
the transition moment. Such a system is therefore colored.
If a doped liquid crystalline layer is aligned homeotropically
by a DC or AC potential (Fig. 9b) interaction no longer
occurs between the chromophore and the incident light and
the system is colorless.
625
observed with oriented molecules. Corresponding studies
can be performed both in the solid state and in liquid
crystalline solvents[3,1 4 2 - 1451.
x-
i
'
Fig. 9. Inclusion ofdichroicdye molecules in a nematic layer: a. absorbing:
b, nonabsorbing: c, absorption curves.
Conversely, a corresponding horneotropic layer of a nematic
liquid crystal of nvgatice anisotropy is colorless in its quiescent state and becomes colored when it undergoes homogeneous reorientation. A color switch of this kind also
functions with unpolarized light. Since the driving voltages
required are of the same order as those required for
dynamic scattering the latter type of color switch, in
contrast to the former, requires AC potentials having
frequencies in excess of the cut-off frequency for dynamic
scattering.
4.6. Piezoelectric Effect in Nematic Liquid Crystals
The phenomenon of piezoelectricity as known for natural
electrets such as quartz or tourmaline and which nowadays
finds wide technical application in quartz crystal oscillators
was first detected in liquid crystals in 1972[14'1.
Fig 10. Distorted homeotropic layer exhibiting piezoelectric properties.
If an electric potential is applied perpendicular to the
molecular orientation of a homeotropic layer of nematic
liquid crystals of negative dielectric anisotropy, the orientation of the liquid crystals becomes distorted in the direction
of the field (Fig. 10). The liquid crystalline layer which
was originally isotropic to the incident light striking it
parallel to the preferred orientation becomes anisotropic
due to the distorted molecular arrangement and thus birefringent. This bending or piezoelectric effect leads, P. g. for
layer thicknesses of ca. 100pm and field strengths of
100 V/cm, to an optical phase difference between the ordinary and extraordinary refractive rays of 300 A.
5. Liquid Crystals as Anisotropic
Solvents for Molecular Spectroscopy
Anisotropic spectroscopic molecular properties, Y.g. polarization of an electronic or vibrational transition in the
UV or IR or the magnitude of a direct intramolecular
dipole-dipole interaction in NMR and ESR, can only be
626
Since the molecules dissolved in a liquid crystal are frequently aligned parallel to the preferred orientation of
the principal axis of the liquid crystal it becomes possible
to study not only the anisotropic molecular constants
of the liquid crystal itself but also those of the molecules
dissolved therein.
Planar or elongated molecules can be dissolved in nematic
liquid crystals in concentrations up to 30 mol-% without
destroying the liquid crystalline state. This limiting value
is frequently much lower for spherical molecules. The dissolved molecules usually exhibit a lower degree of order
than the solvent molecules. Since the clearing point is
invariably lowered more than the melting point the nematic
range is significantly narrower than that of the pure solvent.
Spectroscopic studies therefore require liquid crystals with
wide mesophase ranges, which should be liquid crystalline
at room temperature to facilitate experimental work.
5.1. Liquid Crystalline Solvents in
Optical Molecular Spectroscopy
Determination of the polarization of absorption or fluorescence bands is usually performed with single crystal sections. High intensity bands require single crystal sections
less than 1 pm thick whose preparation and handling is
not easy and sometimes even impossible. The alternative
procedure of working with non-absorbing single host crystals or with stretched plastic f i l m ~ ~ ' ~ into
~ - ~which
~*]
the sample under study has been previously incorporated
is complicated and again sometimes impossible. In contrast, nematic liquid crystalline solvents permit the recording of polarization spectra in the same simple manner
in which spectra are measured in isotropic solvents.
Investigation of the polarization of a transition with the
aid of homogeneously oriented liquid crystalline layers
requires use of linearly polarized light. Measurements in
homeotropic phases, however, also permit the direction
of the transition moment to be determined without polarized light, the spectrum of the nematic solution being
recorded first and then that of the isotropic solution above
the clearing point. The difference in intensity between the
individual bands allows unambiguous assignment of their
polarization'' 491.
5.1.1. Use of Liquid Crystals in Absorption
and Fluorescence Spectroscopy
The intrinsic absorption of nematic liquid crystals of the
Schiff base, azoxybenzene, or phenyl benzoate type preclude their use as solvents for recording polarized absorption of fluorescence spectra except for molecules whose
spectra lie above 350 nm[' 5 0 . 511. Compensated cholesteric
liquid crystals (see S e c t i a 2.3), on the other hand, represent
nematic solvents which transmit light above 250 nm and
which can readily be supercooled to give vitreous solutions
by quenching136.1 5 2 - 1541
While the degree of order S of dissolved molecules, and
its temperature dependence, can also be ascertained from
Angew. Chrm. infernu(. Edir.
1 Voi. 12 ( 1 9 7 3 ) i No. X
polarized absorption spectra['55these quantities
cannot generally be obtained from the corresponding fluorescence spectra since the polarized fluorescence of the
dissolved molecules is anisotropic in the various axial
directions, i. P. may be quenched to varying extents. Hence
the fluorescence intensity measured is no longer solely
dependent on the molecular orientation[ "I.
'
5.1.2. O R D and C D in Nematic Solvents
O n dissolution of chiral compounds in a nematic solvent
the solute molecules induce a helical ordering of the liquid
crystalline molecules and thus transform the nematic liquid
into a cholesteric phase which exhibits optical rotatory
dispersion (ORD) and circular dichroism (CD)I4'l; 1-2
mo1-Z of the chiral compound suffice to effect this transition. The effect is caused both by asymmetric molecules
such as r - m e n t h ~ l ~and
' ~ ~ by
' dissymmetric structures
such as diazacycloctatetraene derivatives[411,the sense of
rotation of the helix depending upon the chirality of the
solute molecules. Since the induced specific rotation of
the cholesteric liquid crystalline solution can far exceed
lo00 and is hence three orders of magnitude larger than
that of the molecular compounds or molecules dissolved
in isotropic solution, these effects, which were first described by Stegemeyer et a[.,are suitable for detecting small
amounts of optically active or chiral molecules having
very low optical activities.
5.1.3. Induction of Optical Activity
by Cholesteric Solvents
The complementary phenomenon, i. P. induction of a Cotton effect within the absorption bands of achiral molecules
with the aid of a cholesteric liquid crystalline solvent,
has meanwhile also been established['60- l h 4 ] . Here the
helical structure of the cholesteric solvent is impressed
upon the spatial arrangement of the dissolved molecules,
leading to molar amplitudes of the induced rotatory dispersion of the order of lo7 deg cm2. These values thus exceed
P. 9. those of the pronouncedly chiral compound hexahelicene (8.62 x lo5 deg cm2)by several orders of magnitude.
E . Sackmann['651and S a ~ r a [have
' ~ ~developed
~
a method
of establishing the polarization of the pertinent electronic
transition of the dissolved achiral molecule from the sign
of the induced Cotton effect. The method clearly supplements the techniques hitherto available for determining
the direction ofelectronic transition moments in molecules.
5.1.4. IR Spectroscopy in Nematic Solvents
Owing to their numerous absorption bands nematic liquid
crystals find only limited use as anisotropic solvents for
recording polarized IR spectra, which differ from the nonpolarized IR spectra only in the intensities of analogous
bands. This intensity difference which is known as infrared
dichroism permits determination of either the degree of
order[ 1 5 6 . 1 5 7 . 1 6 7 ] or the vibrational class of the individual
bands of the dissolved molecule[ ' '. l h 8 ] . The method has
been used primarily for investigation and assignment of
-C=N,
-C=O,
-C-C--,
and =C=O
stretching
vibrations.
Anqrw Chern. i n f r m a f . Edir. f
Vol. f 2 ( 1 9 7 3 ) / N o . 8
5.1.5. IR-ORD in Nematic Solution
The theory of rotatory dispersion predicts that Cotton
effects should also occur in the infrared. However, the
amplitudes of these anomalies are four orders of magnitude
smaller for vibrational than for electronic transitions.
Nevertheless, as Schruder and KortdLhy]
were able to show
for the first time, Cotton effects also occur in the IR
with amplitudinal values of several thousand deg per mm
when chiral molecules are dissolved in nematic solvents.
The frequencies at which these IR Cotton effects are
observed agree, as in the UV, with the absorption bands
of the liquid crystal but not with those of the dissolved
molecule.
5.2. NMR Spectroscopy in Nematic Solvents[3.
1431
Proton and fluorine nuclear magnetic resonance spectroscopy in nematic solvents is already a routine method for
determining bond angles and relative bonding distances
in proton deficient molecules and has been treated in
detailed reviews['70Advances in the field of Fourier
transform NMR spectroscopy have meanwhile facilitated
measurements on nuclei of low sensitivity and low natural
abundance, e.g. 3C-NMR spectra, without preliminary
isotopic enrichment['731. If the degree of order of the
solute molecules is known, the direct 13C-'H nuclear spin
coupling constants LA,,.-,t, obtained from these spectra.
which at 1 500 Hz are larger than the indirect I3C-'H coupling constants j I A ct,-by
, a factor of 10, can be utilized for
calculating not only the H-C-H
bond angles bur also
C-H bond lengths. In contrast, the anaiogous anisotropic
proton spectra yield only relative internuclear distances
via the direct 'H-IH coupling constants. Because of the
numerous possible dipole couplings in molecules containing many protons the resonances exhibit such extensive splitting, as in anisotropic proton spectra. that
analysis is no longer possible. The application of this
method is therefore limited to small molecules.
5.3. ESR Spectroscopy in Liquid Crvstals
ESR spectra in liquid crystalline solvents provide extremely
valuable information, also for the preparative chemiSt1174--176]. B0th liquid crystalline['75.1771 and glassi[177. 1781
nematic, supercooled compensated cholesterand also smectic liquid
are used in
these studies. The direct dipole-dipole interaction between
the unpaired electrons of a multiple radical which is averaged to zero by Brownian motion in isotropic solution is retained in liquid crystalline solution and appears-e.q. as a
triplet for a biradical-alongside the hyperfine splitting.
Thus an ESR spectrum in liquid crystalline solution. in
contrast to the results obtained in isotropic solution, provides a clear-cut answer to the question of how many unpaired electrons a radical contain^"'^.
It also permits
calculation of the concentration, and its temperature
dependence, of multiple radicals['"!
Highly complex ESR spectra can be considerably simplified
by electron-nucleus double resonance experiments
627
(ENDOR). Low-melting azoxybenzene derivatives have
proved particularly suitable for such studiesf'82!
[20] 0. Prller and P. Charelain, Bull. SOC. Fr. Mineral. 73, 154 (1950).
[ > I ] D. A. Balzarini. Phys. Rev. Lett 25, 914 (1970).
[22] 1. Hallrr, H . A. Huggins, and M J . Freiser, Mol. Cryst. Llq. Cryst.
16, 53 (1972).
5.4. Miissbauer Spectroscopy in Srnectie Phases
One of the few applications of smectic phases has recently
been found in Mossbauer spectroscopy1'83- 1871. The possibility of translational molecular motion in all three spatial
directions in nematic phases rules out their use as matrices
for Mossbauer measurements. Recoilless y-resonance is
feasible, however, in smectic solvents where translational
motion is permitted within a molecular layer in the direction of the long molecular axis but not in a perpendicular
direction. Measurement of Mossbauer spectra in a smectic
rather than a solid matrix has the advantage of simplified
sample preparation, provided the substance under study
is sufficiently soluble. The intensity of the spectra is albeit
lower than in crystalline matrices.
6 . Outlook
Although it is only in the last 10 years or so that the potential technical applications of nematic liquid crystals have
been studied in depth, these substances now occupy a
strong position in the field of electrooptics in such applications as digital indicators for electronic clocks, table-top
calculators, and measuring instruments ; in the near future
we shall see their application in remote controlled traffic
signs and information boards, vehicle instrument panels,
special window panes, in which the degree of light penetration may be controlled, and camera shutters. Looking
further into the future, we can also anticipate optical
memory devices and flat large area color television screens.
Received: May 2, 1973 [A 959 IE]
German version: Angew. Chem. 85,706 (1973)
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Reactive Monomeric Carbonic Acid Derivatives
By Engelbert Kuhle[*]
Monomeric carbonic acid derivatives that can react by addition or substitution are systematically discussed according to the three tabular surveys A, B, and c, from which a principle
for the classification of carbonic acid derivatives can be readily deduced. For each type
of compound, a brief description is given first of the principal syntheses and then of the
reactions, and the heterocycles obtainable by addition, disubstitution, or coupled addition-substitution are also considered. The carbonic acid derivatives discussed can be synthesized
from very simple and in most cases cheap base chemicals
Introduction
The derivatives of carbonic acid are usually discussed only
briefly in textbooks of organic chemistry, though carbonic
acid chemistry actually offers the synthetic worker a multitude of possibilities for the synthesis of new compounds,
and carbonic acid derivatives have a very wide range of
uses in industrial chemistry.
The industrial importance of carbonic acid chemistry is
clear from the following brief enumeration of its uses.
Urea-formaldehyde and melamine resins have been in use
for a long time; polycarbonates and polyurethanes are
modern plastics used e.g. as foams, paint components,
and products with rubbery properties. Cyanuryl chloride
is used as a reactive component for dyes and as a starting
material for herbicides. Arylalkylureas and N-arylcarbamic
esters also have herbicidal properties, while aryl N-methyl[*] Dr. E. Kuhle
Zentrale Forschung
Wissenschaftliches Hauptlaboratorium der Bayer AG
509 Leverkusen
630
carbamates, e.g. ( 1), and N-methylcarbamidoximes, c.g.
( 2 ) , are used as cholinesterase inhibitors for the control
of insects.
H3CN H - C - 0
ll
0
-
II,C S-C = N U C - N H C H3
I
I1
CH3
0
The fungicides used in agriculture include the dithiocarbamic acids ; N-trihalomethylthio compounds, which can also
be regarded as carbonic acid derivatives, are used for
the same purpose. Very many carbonic acid derivatives
are found in drugs. The range extends here from the oldestablished barbiturates to the modern antidiabetics based
on sulfonylureas. A number of sulfonamides used in pharmaceutical applications are also based on carbonic acid
derivatives. Finally, carbonic acid structures are found
in complicated natural products, e.g. in antibiotics (streptomycin) and vitamins (folic acid), and as essential components of the nucleic acids (guanine, cytosine, thymine).
Angew. Chem. internur Edit. 1 Vol. 12 f 1973)
NO. 8
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