Design of Smectic Liquid Crystal Phases Using Layer Interface Clinicity

Chapter 20

Design of Smectic Liquid Crystal Phases Using Layer Interface Clinicity 1

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David M. Walba , Eva Körblova , Renfan Shao , Joseph E. Maclennan , Darren R. Link , Matthew A. Glaser , and Noel A. Clark Downloaded by NORTH CAROLINA STATE UNIV on September 17, 2012 | http://pubs.acs.org Publication Date: November 2, 2001 | doi: 10.1021/bk-2001-0798.ch020

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Departments of Chemistry and Biochemistry and Physics, Ferroelectric Liquid Crystal Materials Research Center, University of Colorado, Boulder, C O 80309

The design and synthesis o f a ferroelectric bow-phase

(banana

-phase) composed of racemic molecules is described. T o date many hundreds o f bent-core molecules have been screened for l i q u i d crystallinity, motivated by the recent discovery of very unusual and interesting chiral, antiferroelectric smectic phases from achiral mole cules. Based upon a model for bow-phase antiferroelectric structure driven by layer interface c l i n i c i t y , incorporation o f one racemic methylheptyloxycarbonyl

t a i l into the

prototype

bow-shaped

mesogen structure was accomplished. This led to a material show ing the unmistakable B 7 texture by polarized light microscopy. This B 7 phase was shown to be ferroelectric by a combination o f elec trooptic properties and current response in 4 μ m transparent capacitor liquid crystal cells. T o our knowledge, this is the first ferroelectric smectic LC composed of racemic molecules.

Since M e y e r ' s revolutionary w o r k o f 1974, the paradigm for ferroelectric smectic 1

2

liquid crystals has required enantiomerically enriched molecules * .

The recent ob

servation o f antiferroelectric switching i n smectic liquid crystals ( L C s ) composed o f achiral molecules has changed the polar smectic L C landscape, however. First, SotoBustamante and B l i n o v et al., working i n the Haase labs i n Darmstadt, reported antif erroelectric behavior i n an achiral polymer/monomer blend.

3

Shortly thereafter,

Takezoe and Watanabe et a l . reported a smectic l i q u i d crystal phase composed o f achiral bent-core, or bow-shaped (also k n o w n as banana-shaped) molecules exhibiting high-susceptibility electrooptic (EO) behavior characteristic of ferroelectric or antifer-

268

© 2002 American Chemical Society

In Anisotropic Organic Materials; Glaser, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2001.

269 4

roelectric liquid crystals ( F L C s or A F L C s ) . The observed E O behavior was ascribed to a ferroelectric supermolecular structure with C 2 V symmetry. This phase, k n o w n as B 2 or S m C P (see discussion below for a detailed descrip tion of the S m C P nomenclature), indeed possesses a polar layer structure, wherein the molecular bows are oriented with their imaginary "arrows" along a polar axis within each layer, as suggested by Takezoe and Watanabe. roelectric,

The phase, however, is antifer

with antiparallel orientation of the polar axis in adjacent l a y e r s

5,6

7

' .

Add

ing to the novelty of the B 2 system, a tilt o f the molecular bows about the polar axis Downloaded by NORTH CAROLINA STATE UNIV on September 17, 2012 | http://pubs.acs.org Publication Date: November 2, 2001 | doi: 10.1021/bk-2001-0798.ch020

7

leading to a chiral layer structure, was discovered by C l a r k et a l . . The global free energy m i n i m u m , the S m C g P ^ structure, is a macroscopic racemate, where adjacent layers are heterochiral.

M o s t interestingly, however, an apparently

metastable,

though easily observed, second antiferroelectric phase, the S m C ^ P ^ , composed o f chiral macroscopic domains, was also described i n the " B 2 phase" temperature range 7

8

of the classic materials . This system represents the first known liquid conglomerate . Since the initial report, many hundreds o f mesogens i n the bent-core family have 9

been characterized, and seven "banana phases ," B 1 - B 7 , have been identified based mainly upon observed textures i n the polarized light microscope and by X - r a y dif 5

fraction .

In the literature the S m C § P ^ and S m C ^ A phases are typically, though

incorrectly, lumped together under the name " B 2 phase".

B o t h B 2 and B 7 phases

show electrooptic switching. Τ date these materials have been shown to exhibit be havior characteristic of antiferroelectric

supermolecular structures.

That is, a ferroe

lectric state is accessible by application of an electric field, but this state rapidly re verts to an antiferroelectric structure upon removal of the field. Herein we report the directed design, synthesis, and characterization o f a ferroelectric bow-phase l i q u i d crystal system. The new phase is composed of racemic molecules, and, being m i c r o scopically chiral, forms a ferroelectric liquid crystal conglomerate. This ferroelectric smectic L C , composed o f racemic molecules, breaks the M e y e r paradigm for forma tion o f F L C s .

N o new paradigm is involved, however.

The mesogen showing the

target phase was designed using phenomena well known i n the F L C literature.

The First Liquid Conglomerate The "classic" bow-phase mesogenic compound, which we term N O B O W ( N o n y l O x y B O W - p h a s e mesogen) has the bis-Schiff-base diester structure shown i n Figure 1. The C2-symmetrical series o f alkoxy homologues containing this material was first 10

prepared and characterize by Matsunaga .

Based upon differential scanning calo-

rimetry and X - r a y studies (experimental measures o f the layer spacings), i n combina tion w i t h molecular lengths o f the series o f homologues measured from molecular models, Matsunaga proposed that materials of this type show mesophases o f the "smectic C type." H i s analysis and conclusions were elegant and substantially cor rect.


Downloaded by NORTH CAROLINA STATE UNIV on September 17, 2012 | http://pubs.acs.org Publication Date: November 2, 2001 | doi: 10.1021/bk-2001-0798.ch020

270

Figure I.

Structure and phase sequence of NOBOW, mesogen.

a prototypical

bow-phase

A s shown i n Figure 1, N O B O W exhibits the " B 4 " phase at temperatures below the smectic L C temperature range. This phase, also termed "blue crystal," is actually a crystal modification showing a strong circular dichroism, and clear indications o f 11

helical layer deformations by atomic force microscopy .

T h i s crystalline phase is

composed o f macroscopic chiral domains of "random" handedness.

The opposite

sign of the C D exhibited by enantiomeric domains is easily observed. W h i l e the blue crystal bow-phase is certainly extremely interesting, there is nothing new i n finding chiral crystals growing from achiral or racemic molecules. Pasteur first documented this type of spontaneous breaking of achiral symmetry in his classic 1848 study o f the "racemic a c i d " sodium a m m o n i u m salt

12

conglomerate .

Since then a very large number of examples of spontaneous chiral symmetry breaking in the formation of crystals have been reported, including good evidence for the for 13

mation o f 2-dimensional crystal conglomerates on graphite , and at the air-water i n 14

terface . 7,8

The discovery o f the N O B O W liquid c o n g l o m e r a t e , however, represented a paradigm shift. U n t i l then, a central dogma in L C design held that chiral L C phases only occurred with enantiomerically enriched molecules. For example, when Feringa observed a chiral nematic phase after irradiation with circularly polarized light ( C P L ) of an achiral nematic doped with specially designed photoactive molecules, he confi 1 5

dently concluded that the dopant molecules must be partially resolved by the C P L . Meyer*s prediction o f the ferroelectric nature o f the chiral S m C * phase used the same argument by relying upon compounds composed of enantiomerically enriched mole cules to break the macroscopic achiral (and nonpolar) symmetry o f the S m C phase.

The Metastable SmC P A

A

Phase of NOBOW

C l e a r l y molecules o f N O B O W i n the isotropic liquid or gas phase are "achiral" by the conventional chemical definition. That is, the compound N O B O W is not re solvable at r o o m temperature. But, this material does form chiral liquid crystal su 7

permolecular structures, as we have reported in detail .

The bow-phases are very


271 similar to the well-known conventional S m C * system of M e y e r . In order to aid in the f o l l o w i n g discussion, the tilt plane, layer plane, and polar plane of a small cube of S m C * / S m C P sample are illustrated in Figure 2. First, for all k n o w n smectic ferroe lectric L C s there exists one two-fold axis of symmetry (singular points at the layer interfaces and the " m i d d l e " of the layers)—the polar axis.

We


define the tilt plane as the plane nor mal to the polar axis i n an F L C .

For

antiferroelectrics, the tilt plane is de fined as the plane normal to the polar axis i n the ferroelectric state.

The

polar plane is defined as the plane containing the layer normal and the polar

axis.

For

the

bow-phase

mesogens, we define the director as Figure 2.

The SmCP geometry.

being along the imaginary bow-string of the molecular bows.

U s i n g these

conventions, the geometry i n the S m C * and S m C P systems is identical. The smectic mesogenicity of N O B O W is actually quite complex and interesting, as follows. U p o n melting from the chiral blue crystal phase, a liquid crystalline con glomerate is seen i n 4 p m transparent capacitor L C cells with l o w pre-tilt p o l y i m i d e 16

alignment layers, parallel-rubbed .

The alignment seen i n the sample, seemingly

directly upon melting, has the layers normal to the substrates (bookshelf alignment), w i t h the tilt plane parallel to the substrates and the polar plane normal to the sub strates. This is the same geometry observed i n surface stabilized F L C ( S S F L C ) ceils in the well-known bookshelf or quasi-bookshelf alignment. Note, however, that while the zenithal anchoring appears strong (the director is oriented parallel to the sub strates), no azimuthal anchoring o f the director is observed, resulting i n a random focal conic texture by polarized light microscopy ( P L M ) . The macroscopically chiral domains of this conglomerate behave i n a character istically antiferroelectric and definitively chiral manner as evidenced by E O behavior upon application o f a triangular d r i v i n g waveform.

Specifically, a smooth purple

focal conic is observed i n the absence of an applied field. This texture is "smectic A i i k e " (i.e. extinction brushes i n cylindrical focal conic domains are oriented parallel and perpendicular to the crossed polarizer/analyzer, showing that the optic axis i n the phase at zero field is along the layer normal). W h e n a field is applied above a thresh o l d o f about 5 V/μτη dramatic E O switching is observed, g i v i n g a green focal conic structure where the optic axis is rotated from the layer normal by about ± 3 0 ° , de pending upon the sign o f the applied field. The observation o f optic axis rotation ei ther clockwise or counterclockwise depending upon the sign o f an applied field nor-


272 mal to the "clock face," with heterochiral domains showing enantiomeric behavior, proves a chiral structure. The structure o f one o f the enantiomers o f this phase is illustrated i n Figure 3, along with those o f the two degenerate ferroelectric states. S y m b o l i c representations of the bow-shaped structure indicate the molecular bows oriented with the polar plane as the plane o f the page i n the drawings at the top of the Figure, and with the tilt plane in the plane o f the page i n the drawings below. In both orientations, the layer normal


is vertical; the layers are normal to the plane o f the page and horizontal. Projected onto the polar plane, the director is tilted out o f the plane, as is suggested by the heavier and lighter lines making up the molecular bows. In the projection i n the tilt plane, the molecular bow plane is normal to the page. Experimentally, the substrates

Figure 3.

Structure of the SmC P A

A

antiferroelectric phase and two

SmC P s

F

ferroelectric states accessed by application of a field. The typical antiferroelectric current response is indicated at the bottom of the Figure.


273 in L C cells are parallel to the tilt plane. Thus, in the P L M experiments the system is being viewed as projected onto the tilt plane. The drawings i n Figure 3 represent a domain of negative chirality according to our convention (positive i f ζ χ η is pointing parallel to an imaginary arrow fitted to the bow). A t zero field, the structure observed is antiferroelectric, denoted S m C ^ P ^ . S m C descriptor indicates a smectic structure with coherent, long-range tilt of the d i rector relative to the layer normal. The S m C P notation conveys the unique identify


ing spontaneous breaking o f nonpolar symmetry which occurs i n the switching smec tic bow-phases; polar orientation of the molecular bows within each layer. The sub scripts refer to the relative structure of adjacent pairs of layers: C ^ denotes layer interfaces i n the tilt plane, while P ^ denotes antiferroelectric layer pairs i n the polar plane.

anticlinic

order for adjacent

Note that i n the S m C ^ A ground state, the average

optic axis is along the layer normal.

In addition, the birefringence is lowered as

viewed normal to the tilt plane by the anticlinicity—the conjugated aromatic system is substantially excited for light either parallel or perpendicular to the layer normal. A p p l i c a t i o n o f a field above the switching threshold causes precession o f the d i rector i n a set o f alternate layers, providing a ferroelectric state. T w o homomeric ferroelectric states, differing only i n orientation, are accessed by application of fields of opposite sign. In Figure 3, these are denoted S m C $ P p (1) and S m C g P p (2) ( C § refers to synclinic adjacent layer pairs i n the tilt plane, and P p refers to ferroelectric order i n adjacent layer pairs i n the polar plane). The chirality o f layers does change during this k i n d o f switching. A l l the layers shown i n the Figure are o f negative chirality. Clearly, the switching causes the optic axis o f the system to rotate off the layer normal by the S m C tilt angle of one sign or the other depending upon the sign o f the applied field. A s s u m i n g the dipole moment o f N O B O W molecules points antiparallel to the "arrow," (we use the physics convention that dipoles are pointing from negative to positive; the direction of the dipole moment was obtained with M O P A C using A M I ) , the domains with negative chirality have positive ferroelectric polariza tion (P along ζ χ η ) . The classic antiferroelectric current response is given at the bottom o f Figure 3. Note two current peaks during each "half-cycle" o f driving. Starting at the bottom o f the triangular d r i v i n g waveform, the sample is switched into S m C g P p (1).

A s the

field moves towards zero from its m a x i m u m value ( w h i c h is above the switching threshold), the system switches to the antiferroelectric "ground state", releasing half the ferroelectric polarization o f the ferroelectric state, which is seen as a positive cur rent peak. This is due to the re-orientation of dipoles i n half the layers as the sample switches to the S m C ^ P ^ phase. A s the field crosses zero and continues to towards a m i n i m u m (maximum negative field), the second half o f the polarization reversal cur rent is seen when the system switches to the S m C ^ P p (2) state.


274

Supermolecular Diastereomers Exhibited by NOBOW The S m C ^ P A phase o f N O B O W is metastable.

U p o n standing i n the S m C P tem

perature range, the sample w i l l spontaneously convert to a new phase e x h i b i t i n g a very distinctive green stripe texture. The sample never seems to fully convert to this "green stripe" phase, however.

E v e n upon c o o l i n g from the isotropic melt, about 7

10% o f the sample exhibits the S m C ^ A texture. A s described previously i n detail , this phase is also antiferroelectric, with a structure denoted S m C g P A -

The S m C g P ^

antiferroelectric phase switches to the S m C P p ferroelectric state upon application o f


A

a field. The S m C g P ^ phase and its corresponding ferroelectric state are macroscopic racemates, wherein adjacent layers possess heterochiral structures, these being macroscopically achiral. The preceding discussion details the observation o f six different supermolecular structures for the " B 2 phase" o f N O B O W , illustrated graphically i n Figure 4.

Bor-

Ferro/Antiferro SmC P Meso

Layer Chirality < ^

s

90

e

A

^ Enantiomers ^

90°

I

«««-

SmC PA dl A

ο Β τη

SÎÏICSPF

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t Enantiomers

dl

*f-

SmC P

1

+\ \ \ \ " / / / / Figure 4.

90°

^

A

M

E

S

F

°

Illustration of the diastereomeric supermolecular structures exhibited by NOBOW. The graphies on the right are a symbolic representation of the structures projected onto the polar plane, while those on the left represent the structure projected onto the tilt plane. The only diastereomeric structures observed in the absence of an appliedfield areantiferroelectric, one racemate (meso) and one conglomerate (dl). The corresponding ferroelectric states are accessed by application of a field above the switching threshold.


275 r o w i n g from the language o f molecular stereochemistry, we find it useful to consider these supermolecular stereoisomers.

T w o of the structures are supermolecular race-

mates (analogous to meso compounds), one antiferroelectric and one ferroelectric. The other four structures, two antiferroelectric and two ferroelectric, are supermo lecular conglomerates (analogous to d l pairs), exhibiting macroscopically chiral do mains of opposite handedness (supermolecular enantiomers). W h i l e three distinct ferroelectric structures have been documented for N O B O W (one


racemate and two enantiomers), these are not ferroelectric phases, being rather fer roelectric states resulting from application o f a field to antiferroelectric distinction is clear and important).

phases (this

In fact, a l l of the electrooptically active b o w -

phases reported to date are apparently antiferroelectric. Quite obviously this state o f affairs suggests a ferroelectric bow phase (or ferroelectric banana) as an attractive target for supermolecular stereocontrolled synthesis. Such a material, with an indefi nitely stable net polar structure, could be useful as w e l l .

F o r example, the highest

second order nonlinear optical susceptibility yet reported for a liquid crystal was seen 1 7

i n a ferroelectric state o f N O B O W .

Capturing this structure i n a thermodynamic

phase i n the absence of applied fields would be interesting.

Design and Synthesis of a Ferroelectric Banana Intuition suggests that the antiferroelectric nature of the bow-phases is driven by a C o u l o m b i c free energy term. In fact, macroscopically, ignoring "edge effects," fer roelectric and antiferroelectric order i n adjacent layer pairs have the same free energy. W e suggest that the k n o w n bow-phases are antiferroelectric simply due to a free en ergy preference for synclinic layer interfaces

in the polar plane.

A s shown i n Figure

4, the antiferroelectric S m C g P ^ has synclinic layer interfaces projected i n both the tilt plane and polar plane, while the antiferroelectric S m C ^ A phase is synclinic i n the polar plane. Consider that of a l l the k n o w n tilted smectics, the vast majority are synclinic S m C and S m C * . F o l l o w i n g this reasoning, i f a bow-phase mesogen could be produced w h i c h causes a thermodynamic preference for anticlinic

layer interfaces i n the polar plane,

then a ferroelectric phase should result. In fact no new paradigm is required to design such a system. Thus, one o f the most important results i n the F L C field since the end 1 8

of the 1980s was the discovery o f antiferroelectric smectic L C s . In such materials, exemplified by the prototype chiral antiferroelectric methylheptyloxycarbonylphenyl octyloxybiphenylcarboxylate ( M H P O B C ) , a l l layer interfaces are anticlinic, as s h o w n i n Figure 5 for chiral M H P O B C with positive P.

A p p l i c a t i o n o f a field

switches the sample to a ferroelectric state with a l l synclinic layer interfaces.

Note

that by our definition these illustrations show the director structure projected i n the tilt plane.


276

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A * Antiferroelectric Phase Figure 5.

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Antiferroelectric phase andferroelectric state of chiral with positive P, projected in the tilt plane.

MHPOBC

Importantly, it is w e l l k n o w n that anticlinic layer interfaces are also obtained i n a phase o f racemic M H P O B C , proving that enantiomeric excess is not required to achieve the anticlinic structure, though racemic M H P O B C is neither ferroelectric nor antiferroelectric. In the bow-phase system, the relationship between layer interface ciinicity and ferro/antiferroelectricity is "reversed," as illustrated i n Figure 6. T h e key defining characteristic here is the spontaneous polar order along the "arrows" o f the molecular bows. G i v e n this polar layer structure, the tilt plane of M H P O B C becomes the "bow plane", tilted from the polar plane by the S m C tilt angle (about 3 0 ° i n the case o f NOBOW).

The tilt plane is now vertical, perpendicular to the plane o f the page. In

Figure 6, the structures o f the ferroelectric and antiferroelectric S m C P p / s

trated projected onto this macroscopic bow plane.

A

are illus

In these drawings the layers are

not perpendicular to the page, a non-standard view. Nevertheless, these illustrations make a comparison with M H P O B C very simple. Synclinic layer interfaces provide an antiferroelectric structure while anticlinic layer interfaces provide a ferroelectric structure.

SmC P Ferroelectric s

Figure 6.

F

SmC P Antiferroelectric s

A

Illustrations offerroelectric and antiferroelectric SmC$P bow-phase structures with the bow-plane parallel to the plane of the page.


277 This idea was tested with the synthesis of a bow-phase analog of M H P O B C , similar to the classic N O B O W but possessing one racemic methylheptyloxycarbonyl tail. The structure of the new mesogen, termed M H O B O W (MethylHeptylOxycarbonyl19

B O W ) , is given i n Figure 7 . B y P L M the material indeed can be seen to possess an enantiotropic bow-phase. The texture, however, is not that o f one of the phases i n the B 2 motif, but rather the unmistakable, highly unusual " B 7 phase." The B 7 texture was first described by P e l z l et a l . i n bow-phase mesogens pos 20


sessing a nitro substituent at C l o f the resorcinol unit at the center o f the core . B y P L M the B 7 texture is characterized by the formation of twisted ribbons at the iso-B7 transition, and by the formation of beautiful focal conic domains containing unusual subtle stripes and other texture within the domains. Often the ribbons seem to anneal into focal conies with residual modulations from the ribbon structure. Double-helical intertwined twisted ribbon structures have been observed i n the a different B 7 bow21

phase mesogen .

O f course the ribbons have a chiral supermolecular structure, and

the samples are macroscopic conglomerates, with about half the ribbons being right handed, and half left handed, strongly suggesting a chiral structure for the phase, though the molecules are achiral. Recently, a bow-phase mesogen exhibiting the B 7 22

texture was shown to be antiferroelectric .

χ Figure 7.

^

^

B4

^

>·

B7

^

w

Iso

The structure and phase sequence of a bow-phase analog of MHPOBC: MHOBOW.

The electrooptic behavior o f the B 7 texture o f M H O B O W was studied i n 4 p m D i s 16

playtech c e l l s . The focal conic domains obtained by annealing the texture just be l o w the Iso-B7 transition had a gold birefringence color suggesting relatively l o w birefringence. In addition, extinction brushes i n cylindrical focal conic domains were parallel and perpendicular to the polarizer, showing that the optic axis was along the layer normal ( S m A - l i k e ) . A p p l i c a t i o n o f a triangular electric field waveform o f amplitude 10 V / p m p r o duced a striking analog rotation o f the optic axis in the focal conic domains of about ± 1 0 ° . The optic axis rotation is approximately linear with applied field (similar to an electroclinic effect), and the sample is monostable, always returning to the S m A - l i k e


278 brush orientation at zero field. having as a conglomerate.

This analog response is chiral, with the sample be

A b o u t half the domains rotate clockwise and half rotate

counterclockwise for increasing positive field, and vice-versa for increasing field o f opposite sign. Some domains seem to possess a decidedly smaller susceptibility, with optic axis rotation of about ± 5 ° in response to a 10 V/μτη triangular waveform. A p p l i c a t i o n o f a field above a threshold value o f about 12 V / p m causes a dra matic change i n the texture.

The gold focal conic domains, showing analog E O be


havior, are seen to change to smooth S m C * - l i k e bistable domains w i t h a blue bire fringence color indicating increased birefringence. Well-defined domain walls medi ate the change when the applied field is just above the threshold. This "bistable blue" texture shows typical ferroelectric hysteresis i n the E O switching, and behaves just as expected for a bistable S S F L C cell composed o f chiral S m C * material, except it is, o f course, a conglomerate. There is only one peak observed in each half-cycle o f driving (the classic S S F L C ferroelectric response) i n the bistable blue texture. The two bista ble states o f the conglomerate and the polarization current response o f the c e l l are shown i n Figure 8. T o our knowledge, no other bow-phase material i n the B 7 texture has been seen to exhibit this type of ferroelectric response.

Figure 8.

Left: Photomicrographs of a sample ofMHOBOW in 4pm transparent capacitor cells under drive. With a positivefieldapplied, a pair of heterochiral focal conic domains can be seen (A and B). The director is tilted off the layer normal in these domains by about (±)30 as can be seen by the orientation of the extinction brushes. Reversal of the ap pliedfield switches the director in these domains in opposite directions about the layer normal (A ' and B ). Right: The ferroelectric polariza tion response to an applied triangular driving field (1 kHz). c

f

A n a l o g electrooptics i n S m C * F L C s has been the subject o f considerable study and interest i n recent years.

T h i s desirable behavior has been achieved i n S S F L C cells

where the layers are uniformly tilted by the smectic C * tilt angle (Sony-mode V 23

shaped switching) . M o s t recently, the analog response observed i n "Thresholdless Antiferroelectric" ( T h A F L C ) c e l l s

24

has been interpreted to be resulting from an un-


279 usual alignment

mode i n the S m C * phase.

25

Indeed, such S m C * ceils are seen to

show analog and bistable response in the same c e i l .

25

W h i l e the details of the observed E O behavior of M H O B O W are still under i n vestigation, this material has never been seen to exhibit E O properties indicative of an antiferroelectric structure. A s suggested by the preceding discussion, the observation of an analog response i n ferroelectric L C s is w e l l known. The formation o f a very robust bistable texture showing the characteristic ferroelectric current response ap


pears to be unique to the B 7 phase of M H O B O W .

This E O evidence strongly sug

gests that a ferroelectric bow-phase, the target o f the design effort, has indeed been obtained by incorporation o f the M H P O B C tail into the bow-phase mesogen struc ture. Furthermore, we speculate that i n fact the M H O B O W " B 7 phase" has the SmCsjPp F L C conglomerate structure, precisely the target o f the design effort, while other materials s h o w i n g the antiferroelectric " B 7 phase" are

thermodynamic

S m C ^ A conglomerates. W e suggest that the l o w birefringence, S m A - l i k e gold fo cal conic texture occurs when the bow-plane of the sample is more or less parallel to the substrates (i.e. as shown i n Figure 6, with the glass plates parallel to the plane o f the page).

In this geometry substantial excitation of the aromatic rings occurs for

light polarized both parallel and perpendicular to the director, leading to two large indices of refraction and a small birefringence. A l s o , the director is oriented along the projection of the layer normal on the substrates, affording a S m A - l i k e orientation o f the extinction brushes.

O f course the ferroelectric polarization is parallel to the sub

strates for this type o f alignment—precisely the orientation leading to analog E O . Application of a field above the threshold of about 12 V / p m then causes an alignment change such that the bow plane, the ferroelectric polarization and smectic layers are now more or less normal to the substrates (bookshelf alignment), with the polar axis along the applied field. In this orientation the birefringence is much larger, since the aromatic rings are only excited for input light polarized parallel to the director.

Conclusions A design effort aimed at formation o f a ferroelectric bow-phase has led to the synthe sis o f the M H P O B C bow-phase analog M H O B O W .

T h i s structure was designed

based upon the proposal that induction o f anticlinic layer interfaces i n the bow-phase polar smectic m o t i f w o u l d stabilize a ferroelectric polar smectic structure.

In the

event, strong experimental evidence, based p r i m a r i l y upon electrooptic measure ments, and aided by the chiral response o f the system with bistable rotation o f extinc tion brushes i n c y l i n d r i c a l focal conic domains (the classic signature o f F L C E O ) suggests that indeed a ferroelectric bow-phase has been obtained.


280

Acknowledgements The authors gratefully acknowledge support o f this work by the Ferroelectric L i q u i d C r y s t a l Materials Research Center (National Science Foundation M R S E C Award N o . DMR-9809555).


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M e y e r , R. B.; Liebert, L.; Strzelecki, L.; K e l l e r , P. J. Phys., Lett. (Orsay,

Fr.)

1975, 36, L 6 9 - L 7 1 . 2

Putatively polar "polyphilic" S m A phases reported by Tournilhac et a l . [Tournil hac, F. G.; B o s i o , L.; S i m o n , J.; B l i n o v , L. M.; Y a b l o n s k y , S. V. Liq.

Cryst.

1993, 14, 405-414.] have since been shown to possess a nonpolar bilayer smectic A structure:

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Design of Smectic Liquid Crystal Phases Using Layer Interface Clinicity

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