High tilted antiferroelectric liquid crystalline materials

R. Dabrowskia,*, J. Gasowskaa, J. Otonc, W. Piecekb, J. Przedmojskid, M. Tykarskaa

aInstitute of Chemistry, Military University of Technology, ul. Kaliskiego 2, 00-908 Warsaw, PolandbInstitute of Physics, Military University of Technology, 00-908 Warsaw, Poland

cUniversidad Politecnica de Madrid, ETSI Telecomunicacion, 28040 Madrid, SpaindFaculty of Physics, Warsaw University of Technology, 00-662 Warsaw, Poland

Available online 6 May 2004

Abstract

Phase transitions, smectic layer structure, helical pitch and electrooptical properties of recently prepared high tilted antiferroelectric

compounds are characterized. The compounds enable to prepare broad temperature range (220 to 100 8C) orthoconic antiferroelectric

mixtures, having 458 tilt independent of temperature in broad temperature range, and exhibiting excellent contrast and grey level scale.

q 2004 Elsevier B.V. All rights reserved.

Keywords: Orthoconic antiferroelectrics; Phase transition; Enthalpy transition; Layer spacing; Helical pitch; Hystereses loop; Electrooptical cell

characterisation

1. Introduction

Surface stabilized antiferroelectric liquid crystals

(SSAFLCs) can be used to build fast response electrooptic

devices and displays [1–3]. These materials have several

advantages in comparison to surface stabilized ferroelectric

liquid crystals (SSFLCs) such as inherent DC compen-

sation, grey scale capability, relatively wide viewing angle,

driving voltages acceptable for integrated drivers. Due to

their tristability, a simple passive matrix driving scheme can

be utilized. Using this technology even big flat panels with

video rate were demonstrated [4], but their commercial

production was not started.

Two major problems hamper the application of SSAFLC.

The first one is so called pre-transitional effect [5–7]

(see Fig. 1). It is of the dynamic nature and seems to be more

important. The second one is poor optical uniformity in off

state caused by existence of microscopic defects and

existence of two preferred orientations of normal to the

smectic layer which differ by few degrees from each other.

All AFLCs currently known exhibit Iso – SmA –

SmCantip –Cr or Iso–SmCp–SmCanti

p –Cr or Iso–SmCantip –

Cr or Iso–SmA–SmCp–SmCantip –Cr phase sequences.

Due to the lack of the nematic phase as well as

heterogeneities in rubbing direction on both surfaces of

the sample there are some difficulties in forming of

uniform layers (and layer normal direction) at Iso–SmA or

Iso–SmCantip phase transitions. Such a structure placed in

birefractive set-up exhibits a light transmission even if the

average optical axis is parallel with an analyzer or

polarizer.

Dynamic contribution to the poor dark state comes from

the field-induced turning of optical axis and local switching

from the antiferro- to the ferro state below the threshold

voltage. During the transition from the orthogonal SmA

phase to the anticlinic SmCantip phase directly or via SmCp

phase a shrinkage of smectic layers occurs and it leads to the

formation of chevrons. These chevrons are straightened out

in the field direction when an electric field is applied and the

layers are bent in the plain of the cell, what causes the

spatial fluctuations of optic axis [7].

Methods of obtaining a more uniform molecular order in

AFLC are being looked for, see for example Refs. [8,9].

Furue and Yokoyama [8] proposed to use a mixture of

AFLCs and a photocurable nematic monomer to obtain

Iso–N–Sm phase sequence leading to a uniform alignment

of molecules. After the photocure AFLCs should go back to

the origin phase sequence keeping the previous order. In the

case of tested LC’s: MHPOBC or Chisso 4001 mixture even

50 wt% part of acrylate monomer does not involve

appearance of nematic phase. An improved contrast was

obtained using a cell with polymer-stabilized template

network fabricated by removing ferroelectric mixture

0141-9382/$ - see front matter q 2004 Elsevier B.V. All rights reserved.

doi:10.1016/j.displa.2004.04.002

Displays 25 (2004) 9–19

www.elsevier.com/locate/displa

* Corresponding author. Tel.: þ48-22-6839-607; fax: þ48-22-6839-582.

E-mail address: rdabrowski@wat.edu.pl (R. Dabrowski).

Felix 020 giving easier the nematic phase. They also

obtained better alignment and contrast in AFLCs by

covering surface with a thin layer of a ferroelectric material

with SmCp–SmA–N transition [9]. Usefulness of those

methods for applications seems to be rather limited.

It seems that the result of our common work with

Lagerwall’s group at Chalmers University on chain

fluorinated compounds [7,10–12] leads to the more

perspective and simple method in solving bad contrast of

AFLCs.

Partially fluorinated molecules may be tilted at 458 in

their smectic layers. The tilt direction in such antiferro-

electric alternates from layer to layer by 908, therefore it is

named orthoconic antiferroelectric liquid crystal (OAFLC).

After switching to the ferroelectric state the cone angle

between two electrically induced ferroelectric states (F(þ ))

and (F(2 )) is also 908. Conventional AFLC materials (also

called regular) are tilted between 20 and 358. In surface

stabilized cell they are usually optically biaxial, positive

liquid crystals, with effective optic axes along the smectic

layer normal (parallel to cell plates), while orthoconic

antiferroelectrics are uniaxial negative liquid crystals with

optic axis perpendicular to the cell glass plates at normal

incidence (Fig. 2). OAFLC placed between crossed

polarizers behaves as an isotropic medium at zero field

[10–12] for the incident light beam orthogonal to the

sample plane. Surface defects are not seen, what generates

the excellent true dark state even the sample is rotated under

crossed polarizers.

After switching to one of two ferroelectric states F(þ )

and F(2 ), the polarization plane of the incident light rotates

by 908 hence, the optimum transmission is obtained. The

bright state is excellent and even brighter and more

homogenous than observed in the regular antiferroelectric.

The obtained contrast is limited theoretically by the quality

of polarizers. Moreover, experimental measurements (see

Fig. 1) show that also the pre-transitional effect in

orthoconic materials is not present or it is extremely

small [13]. Recently Abdulhalim [14] calculated that having

two condition satisfied: u ¼ 458 and dielectric permittivity

11 < ð12 þ 13Þ=2; then medium behaves nearly as an

isotropic medium for all incidence angle of light, hence

the contrast ratio does not depend strongly on viewing

angle. Above implies a search for a OAFLC having properly

tuned dielectric tensor.

The antiferroelectric phase (chiral anticlinic phase— also

marked as SmCantip ) is observed in limited number of

chemical structures [15,16]. Majority of currently known

compounds are of ester type and may be expressed by the

general formula 1.

ð1Þ

The rigid core of the esters 1 consists of unsubstituted or

different laterally substituted benzene rings joined with

bridge groups X1 and X2. They are usually carboxylic group

(COO) or sometimes methylenoxy group (CH2O) or a single

bond. A limited number of structures with a heteroaromatic

ring instead of benzene are also found. All those esters

should have branched chain CH(R2R3), introducing chir-

ality for R2 – R3. For the creation of chirality chiral

alcohols usually (R) or (S)-2-methylalkanol and (R) or (S)-

2-(trifluoromethyl)alkanol are often used. Molecules of

ester 1 are tilted in smectic layer at moderate angle, between

20 and 308.

Inukai et al. [17,18] found high tilted ferroelectrics

(chiral synclinic smectic C, SmCp) among the

compounds 2 with the direct SmCp – Ch transition

Fig. 1. Comparison of low frequency typical electrooptic response for

regular (dashed line) and orthoconic (solid line) antiferroelectric liquid

crystals.

Fig. 2. Electrooptical properties of orthoconic AFLCs in antiferroelectric

state (E ¼ 0; P ¼ 0) and after transition to ferroelectric state (E . Eth;

P ¼ 0).

R. Dabrowski et al. / Displays 25 (2004) 9–1910

(which is always first order transition).

ð2Þ

Unsubstituted esters 2 (Y2 ¼ H) for members m ¼ 7; 8, 9

and 10 are tilted at 458 in both (S) and (R) enantiomeric

forms and the tilt stays independent throughout the

temperature range at T 2 TC–Ch ¼ 25 to 230 8C.

Robinson et al. [19,20] joined two molecules 2 (unit A)

by dimethyl siloxane spacer creating bimesogenic twin

molecule 3.

ð3Þ

The antiferroelectric phase in the organosiloxane liquid

crystal 3 with m ¼ 11 and Y2 ¼ F or Cl or Br was evidenced

for the even value r þ 1 (3 or 5). Those materials showed

direct transition SmCantip –Iso and the tilt above 408

independent of temperature.

Switching from the antiferroelectric state to ferroelectric

one showed a large pre-transitional effect and slow response

time, about 1 ms at reduced temperature T 2 TC–Ch ¼

240 8C was observed.

We have found that high tilted materials without pre-

transitional effects are possible to create within the

structures 1, if R1 ¼ CnH2nþ1 in the vicinity of the

carboxylic group (X ¼ COO) is exchanged by fluorinated

unit (R1 ¼ CnF2nþ1). We have started to investigate,

systematically, an influence of the molecular building

units (R1, R2, R3, X, X1, X2, Y, Y1, Y2) on the phase

sequence, the optical tilt angle, polarizations, helical pitch,

driving voltages and electrooptical responses times [21–30].

Low melting and broad temperature range SmCantip com-

pounds are currently being searched. Recently prepared

homologous series of phenyl biphenylates (formulas 4–6)

and biphenyl benzoates (formula 7) are presented below:

ð4Þ

ð5Þ

ð6Þ

ð7Þ

2. Methods

The compounds were prepared in the same way as it was

described in Ref. [26]. Phase transitions were investigated

by polarizing optical microscope (BIOLAR-PZO) con-

nected with LINKAM-THMS-600 heating stage and by

DSC-SETARAM 141. The measurements of the helical

pitch are based on the phenomenon of selective reflection of

the light. The spectrophotometer UV–vis Varian Cary 3E

was used for measurements. The temperature was controlled

by Peltier element with the accuracy of 0.18. The sample

was put on the glass plate with homeotropic aligning layer

without covering with another glass plate. A temperature

characteristics of the smectic layer thickness has been

studied using XRD method. An X’Pert (by Philips) powder

diffractometer system (with Cu lamp, Ni filter, proportional

counter) with temperature controller (UNIPAN 660) driven

hot-stage has been utilized. A movement of the optical axis

of the AFLC slab affected by an electric field has been

studied in birefractive set-up. A standard measuring cell has

been used. The cell consists of two flat glasses covered

subsequently by a conducting (ITO), isolating (SiO2) and

orienting polyimide layers. The cell is assembled using

2.2 mm glass spacers. The measuring set-up consists of a

stabilized light source (a halogen lamp), polarizing

microscope (BIOLAR PI, PZO) with a hot-stage

(THMS-600, Linkam), silicon photo diode (PIN 20, FLC

Electronics), arbitrary pulse generator (HP 33120A, Hewlett

Packard), a digital oscilloscope (HP 54501B, Hewlett

Packard) and rotating stage. An optical axis orientation

has been identified as an angular position of the measuring

cell placed in the hot-stage between crossed polarizers

yielding a full extinction of incident light. Electrooptical

measurements have been done in 1.5 mm cells with nylon–

nylon orienting layers. Both layers were rubbed parallely.

A measuring set-up has been described above.

R. Dabrowski et al. / Displays 25 (2004) 9–19 11

3. Results and discussion

3.1. Characterization of phase transitions

Temperatures and enthalpies of phase transitions for

biphenylates (series n Fm Bi) with fixed fluorinated terminal

unit (n ¼ 3; C3F7COO) and variable spacing group ðmÞ are

compared in Table 1.

The compound without spacing group (m ¼ 0; 3F0Bi)

has very high melting point and melting enthalpy. The

melting temperature of compounds rapidly decreases as m is

growing. Tilted anticlinic phase (SmCantip ) appears for

compounds with m ¼ 3; 4, 5 and 6 in broad temperature

range. The compound 3F6Bi has melting point near room

temperature (29.4 8C), very low melting enthalpy

(3.77 kcal/mol) and the antiferroelectric phase in the

temperature range broader than 808. Phase sequence

Cr–SmCantip –SmCp–SmA–Iso is characteristic for all four

compounds and the temperatures of SmCantip –SmCp,

SmCp–SmA and SmA–Iso transition depend only a little

upon the value m: The transitions SmA – Iso and

SmA–SmCp are average at 135 and 125 8C, respectively.

Both of them are strongly first order. Enthalpy of the latter is

between 0.3 and 0.47 kcal/mol.

The smectic A phase exists in small temperature range of

0.48 for compound 3F4Bi only and in larger temperature

range of 8.18 for 3F5Bi.

The high enthalpy of the SmA–SmCp transition seems to

be a characteristic feature of high tilted synclinic and

anticlinic compounds. The SmCantip –SmCp transition is

hardly of first order with the enthalpy about 80 times lower

than observed for the SmA–SmCp transition. The tempera-

ture of SmCp–SmCantip transition is lower for the compound

with m ¼ 6 than with m ¼ 3; 4 and 5. During cooling a more

ordered monotropic SmIp phase was observed in the case of

compound 3F3Bi only. For the others this phase was not

present up to temperature 220 8C.

Two and four-ring analogous compounds were also

prepared. Two-ring ester is not mesomorphic yet while

four-ring ester shows antiferroelectric phase in broad

temperature range but its melting point is not so convenient

as in three ring compounds.

Table 1

Phase transition temperatures (8C, upper line) and enthalpies (kcal/mol, lower line) for the fluorinated compounds of 3Fm Bi series

Acronym m Cr1 Cr Iantip Canti

p Cp A Iso

3F0Bi 0 * 137.8 * 157.3 – – – – *

1.78 11.49

3F3Bi 3 – * 83.8 (* 54.0) * 121.3 * 123.8 * 128.9 *

[Ref. 26] 5.58 0.10 0.03 0.35 0.90

3F4Bi 4 – p 68.4 – p 120.1 p 126.6 p 127.0 p

[Ref. 26] 3.86 0.02 0.47 0.79

3F5Bi 5 * 51.9 p 65.5 – p 121.7 p 124.6 p 132.7 p

1.92 4.70 0.02 0.30 0.76

3F6Bi 6 – p 29.4 – p 111.4 p 122.5 p 129.3 p

3.77 0.015 0.33 0.70

Table 2

Phase transition temperatures (8C upper line) and enthalpies (kcal/mol, lower line) for the protonated compounds of 3Hm Bi series

Acronym m Cr Iantip Canti

p Cp A Iso

3H0Bi 0 p 148.7 – – – ( p 143.3) p

10.22 0.81

3H3Bi 3 p 66.6 ( p 43.0) p 92.4 – p 117.3 p

[Ref. 26] 5.33 0.3 0.02 1.22

3H4Bi 4 p 71.0 – p 92.4 p 100.5 p 110.0 p

[Ref. 26] 9.74 0.01 0.08 1.04

3H5Bi 5 p 74.5 – p 95.3 – p 108.5 p

8.78 0.10 0.94

3H6Bi 6 p 61.0 – p 87.5 p 98.1 p 104.5 p

6.06 0.01 0.10 0.85

R. Dabrowski et al. / Displays 25 (2004) 9–1912

Phase situation and its thermodynamic characterization are

found quite different for analogous protonated compounds

(series 3Hm Bi) (Table 2).

The melting points of the compounds 3Hm Bi depend

on m a little and they are rather high (about 708). The

melting enthalpies are also higher than in series 3Fm Bi.

The clearing points (SmA–Iso transition) are much

lower and they are decreasing with the increase of m:

All compounds in Table 2 have SmCantip phase, but it is

directly below the SmA phase for m ¼ 3 and 5 and

below the SmCp phase for other m: Enthalpies of

transitions from the orthogonal smectic A phase to tilted

phases are small—about 0.1 kcal/mol only. This is

typical for low tilted compounds.

Diagrams showing the phase transition temperatures and

the phase ranges upon the length of fluorinated unit CnF2nþ1

in the terminal chain (n changes between 1 and 7) are given

for phenyl biphenylates in Fig. 3 and for biphenyl benzoates

in Fig. 4, for both families the members are with fixed

methylene spacer m ¼ 6: In the investigated homologous

series of biphenylates and benzoates the phase transition

temperatures SmCantip –SmCp, SmCp–SmA and SmA–Iso

are growing with the increase of n (Figs. 3 and 4). For

members n ¼ 6 and 7 the stability of the smectic A phase

increases more than the stability of SmCp and SmCantip

phases.

In homologous series n F6Bi, the temperature range of

the SmA phase enhances to 13, 16 and 188 for n ¼ 5; 6

and 7, respectively. Melting point and melting enthalpy is

the smallest for n ¼ 3 member. The enthalpies of

the SmCp–SmA transitions are the highest for members

between 2 and 5.

The substitution of benzene ring by fluorine atom in

position 2 or 3 decreases all phase transition temperatures,

Fig. 3. Diagrams of the phase transition temperatures showing the range of

phases for phenyl biphenylates homologues series (numbers characterize

the transition enthalpies (kcal/mol); bottom for melting and between

mesophases and on the right side for isotropization).

Fig. 4. Diagrams of the phase transition temperatures showing the range of

phases for biphenyl benzoates homologues series (numbers characterize

the transition enthalpies (kcal/mol); bottom for melting and between

mesophases and on the right side for isotropization).

R. Dabrowski et al. / Displays 25 (2004) 9–19 13

except melting points, which are not changed. This result is

different from that observed in the compounds with shorter

spacer, where fluorination of ring decreases melting points

[28]. In homologous series n Fm Bi(3F), the temperature of

the SmCantip –SmCp transition decreases most strongly (10–

158) than the temperature of the SmCp–SmA and SmA–Iso

transitions (12 and 7 – 128, respectively). In series

n F6Bi(2F), the smectic A phase decreases more than tilted

SmCantip and SmCp phases. Members of series n F6Bi(2F)

have the smectic A phase in narrower range than in series

n F6Bi and n F6Bi(3F). In this case, the enthalpy of

transition SmCp–SmA depends only a little on the extension

of fluorination.

The homologous series n F6B with the opposite ring

sequences show similarities but also some differences, in

their phase situation, to n F6Bi. In this series, the

member n ¼ 3 has the lowest melting point (only

18.58) which is accompanying with very low melting

enthalpy and the broad range of the antiferroelectric

phase.

The temperatures of phase transitions SmCantip –SmCp,

SmCp–SmA and SmA–Iso are lower than in n F6Bi and

the stability of SmCpanti phase decreases more than SmCp,

therefore SmCp phase exists in a broader temperature

range than in n F6Bi (Fig. 4). The enthalpies of

SmCp–SmA transition are highest for member n ¼ 4

and 5. Analogous homologous series with shorter spacer

m ¼ 3 (n F3B) show similar phase behavior with the

exception that for longer fluorinated units (n ¼ 6 and 7)

the anticlinic phase is not observed and all members

have higher melting points. It is typical for compounds

with shorter spacer (smaller m). In the benzoates n F6B

the stability of the SmCp phase is similar as in

biphenylates n F6Bi but the SmCantip and SmA are

less stable.

The rigid core structure stabilizes the anticlinic phase as

follow:

3.2. Temperature dependence of smectic layer spacing

Temperature dependence of smectic layer spacing d was

measured and the ratio d=dA was used to compare how the

molecular structure influences molecules tilting (dA is the

maximum value of interlayer spacing in the smectic A phase).

The ratio d=dA gives direct information about layer tilt, if dA

is similar to molecular length l; the tilt should be proportional

to d=dA ratio. In case of fluorinated compounds, dA differs

much from l;dA=l may be lower than 0.9 [25]. It was suggested,

recently, that in fluorinated compounds similar to the

investigated ones the smectic A phase is of de Vries type

[31]. It means that the orthogonal layers are built from

disordered tilted domains. In this case, the ratio d=dA evidences

only the increase of the layer tilt and not its real value.

Dependence d=dA upon reduced temperature T 2 TC–A for

compounds with fixed C3F7 fluorinated unit and the different

length of spacer m is given in Fig. 5a and with fixed C3F7 unit

and with the different rigid core of molecules in Fig. 5b.

The ratio d=dA of the compounds with the same length of

fluorinated terminal unit does not depend on spacer length

m: The ratio d=dA falls rapidly after the SmA–SmCp

transition and stays temperature independent at 408 below

the transition ðd=dA ¼ 0:91Þ:

Fig. 5. Dependence of d=dA ratio upon reduced temperatures T 2 TC–A: (a) for compounds 3Fm Bi with m ¼ 3; 4, 5, 6, (b) for compounds 3F6Bi, 3F6Bi(2F),

3F6Bi(3F), 3F6B and orthoconic mixture W-182.

R. Dabrowski et al. / Displays 25 (2004) 9–1914

The ratio d=dA depends on rigid core structure in the

following way:

Benzoate (3F6B) becomes more tilted than biphenylate

(3F6Bi) after the transition from the orthogonal SmA phase

to the synclinic and anticlinic phase. The decrease of the

d=dA ratio is observed as a result of ring fluorination but only

in the case of the fluorine atom placed in the vicinity of the

central carboxylic group (see Fig. 5b). The ratio d=dA for

3F6Bi(2F) is more similar to 3F6B than for 3F6Bi.

The influence of the length of fluorinated unit CnF2nþ1 on

the ratio d=dA in series n F6Bi, n F6Bi(3F), n F6Bi(2F) and

n F6B is shown in Fig. 6a–d. The d=dA ratio depends

strongly on the length of the fluorinated unit. It is smaller for

compound with short fluorinated unit than with long one,

see also data listed in Table 3.

3.3. Temperature dependence of optical tilt

The temperature dependence of optical tilt is given in

Fig. 7 for series n F6B. In Table 4 the values: molecular

length l; optical tilt u; d=dA; d=l; arccosðd=lÞ and arccosðd=dAÞ

(X-ray tilt) are compared at fixed reduced temperature

240 8C for different n in fluorinated CnF2nþ1 unit.

In series n F6B, members with fluorinated unit having

four or five carbon atoms show the highest optical tilt.

Fig. 6. Dependence of d=dA ratio upon reduced temperatures T 2 TC–A showing the influence of fluorinated unit CnF2nþ1 in series (a) n F6Bi, (b) n F6Bi(2F),

(c) n F6Bi(3F), (d) n F6B.

Table 3

The influence of the length of fluorinated unit CnF2nþ1 on the ratio d=dA in

series n F6Bi, n F6Bi(3F), n F6Bi(2F) and n F6B

Series Minimum

d=dA at

T 2 TC–A

¼ 2408C

n Maximum

d=dA at

T 2 TC–A

¼ 2408C

n F6Bi 0.910 2 , 3 < 4 , 5 , 1 , 6 , 7 0.941

n F6Bi

(2F)

0.896 2 , 3 , 4 , 1 , 6 , 7 0.938

n F6Bi

(3F)

0.918 3 , 2 , 4 , 1 , 5 , 6 , 7 0.938

n F6B 0.863 2 , 1 , 4 , 3 , 5 , 6 , 7 0.916

R. Dabrowski et al. / Displays 25 (2004) 9–19 15

It grows rapidly directly after the transition SmA–SmCp and

reaches maximum value of 458 at distance T 2 TC –A ¼ 2208

for pentyl derivative and at distance 2408 for butyl

derivative.

The relation that the highest optical tilt is observed for the

members with perfluorobutyl and perfluoropentyl group

while the smallest ratio d=dA is observed for methyl

fluorinated unit confirms the suggestion that the smectic A

phase must be of de Vries type. It is confirmed by the decrease

of dA=l ratio with the increase of n and small dependence of

d=l ratio and big dependence of dA=l ratio upon n:

3.4. Helical pitch length and its temperature dependence

In the antiferroelectric phase, the part of helice which

selectively reflects the light equals the half of the pitch,

p ¼ 2lmax=n:

Refractivity indices nk and n’ were measured for 3F3Bi

[30]. The average refractivity indexes n < 1:5; hence p <1:3lmax: Temperature dependence lmax of selective reflec-

tion band for compounds 3F6Bi and 3F6B is compared in

Fig. 8a and b.

Both the compounds reflect the light in near ultraviolet

and visible range of spectrum near room temperature.

Position of lmax for 3F6B is shifted to the red side in

comparison to 3F6Bi, although the observed difference is

smaller if the comparison is made at the same distance from

the SmC–SmA transition (Fig. 8b).

For 3F6Bi, maximum wavelengths of selectively

reflected light are lmax ¼ 0:42 and 0.6 mm at 20 and

50 8C, respectively, what gives helical pitch length p ¼ 0:55

and 0.78 mm, respectively. While for 3F6B there are lmax ¼

0:57 and 0.80 mm and pitches are equal p ¼ 0:74 and

1.04 mm, respectively. Such strong temperature increase of

helical pitch should promote creation of the surface

stabilized structure, because the cells are fulfilled at high

temperature and the surface stabilized structure is formed

during cooling.

Selective reflection strongly depends on the length of

fluorinated part of the chain. In series n F6Bi, lmax shifts to

the infrared region when n is increased from 1 to 7

(see Fig. 9), therefore pitch is increasing while n increases.

Similar behavior was observed in series n F6B [32],

although their lmax changed with n not quite regular.

Fig. 8. Comparison of maximum selective reflection for compounds 3F6Bi and 3F6B upon temperature (a) and reduced temperature T 2 TC–A (b).

Table 4

The comparison of parameters of the smectic layer upon the length of

fluorinated unit for series n F6B at reduced temperature T 2 TC–A ¼

240 8C

n 1 2 3 4 5 6 7

u (8) 35.0 43.7 37.5 44.0 43.0* 24.5 31.0

d=dA 0.895 0.863 0.918 0.909 0.892** 0.919 0.945

d=l 0.764 0.766 0.762 0.735 0.754 0.750 0.750

l (nm) 3.79 3.91 4.06 4.19 4.33 4.46 4.61

dA=l 0.878 0.897 0.855 0.825 0.845 0.833 0.818

DHC2A

(kcal/mol)

0.29 0.35 0.24 0.36 0.33 0.07 0.11

Arccos

d=l (8)

40.2 40.0 40.3 42.7 41.1 41.4 41.4

Arccos

d=dA (8)

26.5 30.3 23.4 24.6 26.9 23.2 19.1

* - (T 2 TC–A ¼ 220 8C); ** - value calculated from second order reflex.

Fig. 7. Temperature dependence of optical tilt u in homologues series n F6B

upon length of fluorinated chain.

R. Dabrowski et al. / Displays 25 (2004) 9–1916

The member 6F6Bi has shorter pitch than the members with

smaller n:

3.5. Electrooptical properties

3.5.1. Single compounds

If we want to achieve the maximum contrast in surface-

stabilized bookshelf geometry, the optical thickness of

smectic layer should be adjusted to the half wave condition

Dnsynd ¼ l=2 [11].

For investigated compounds an optical anisotropy Dn

was about 0.2 [30], hence the sample thickness was limited

to about 1.5 mm to fulfill the half wave condition. In Fig. 10,

electrooptical hystereses loops of two highly tilted com-

pounds 4F6Bi(2F) and 4FBi(3F) are compared at tempera-

ture 60 8C and their electrooptical parameters are listed in

Table 5.

Hysteresis loops in both compounds do not show pre-

transitional effects although their branches are not quite

symmetrical for opposite driving voltage polarizations.

Compound 4F6Bi(2F) tilted at 458 shows better static

contrast than 4F6(3F) tilted at 368, although it is much

slower. Higher dipole moment along long molecular axis in

4F6Bi(3F) is probably responsible for lower threshold and

saturation voltages and shorter response times.

3.5.2. Multicomponent mixtures

Multicomponent mixtures are utilized in displays and

devices because they exhibit the antiferroelectric phase at

low temperatures only. Their electrooptical properties may

be changed and optimized easier by the proper selection of

components also.

In the case of ferroelectric liquid crystal mixtures, they

are usually formulated from achiral synclinic components

having smaller viscosity than chiral ones and a small

amount of chiral twisting dopant with high spontaneous

polarization. Such a way of preparation enables to obtain the

composition with low viscosity and with desired spon-

taneous polarizations.

The limited number of achiral anticlinic structure found

until now does not allow developing them in the same way.

Multicomponent low melting antiferroelectric mixtures may

be formulated mainly from chiral components at this

moment, what yields some problems with preparation of

mixtures with low viscosity and low spontaneous polariz-

ation. In the case of OAFLCMs it is still more difficult,

because the known structures with the high tilt are limited to

be prepared by us. The mixture W-107 was the first

orthoconic mixture achieving temperature independent

angle u ¼ 458 at temperature below 80 8C (408 below

transition SmCp–SmA). Its composition (wt%) is as

follows:

CF3CH2CH23Bi (6.31); 3F3Bi (20.72); 3F4Bi (32.45);

7F3Bi (40.47). The phase transitions and the properties at

40 8C [11] are given in Table 6.

In the component mixture, only 3F4Bi has the tilt higher

than 408 at the reduced temperature T 2 TC–A ¼ 220 8C:

The tilt is saturated at some distance from transition SmC–

SmA while polarization grows with decreasing temperature

all the time. In mixtures, the optimum tilt of 458 is easily

achieved than in a single compound. The compounds of

series n F6Bi and n F6B, described here, are characterized

by lower melting temperatures and lower melting enthalpies

in comparison to that prepared earlier [26]. Therefore, we

were able to formulate eutectic mixture with melting point

below 220 8C according to the well known equations

ln xk ¼DHk

m

R

1

T2

1

Tkm

� � Xn

k¼1

xk¼1

Recently, we prepared many such mixtures [33]. Mixture

W-193B can be given as an example of the best one at this

moment. Their phase sequence and electrooptical properties

are listed in Table 7.

The mixture W-193B exhibits high contrast in the static as

well dynamic transmission mode (70 and 183, respectively).

Fig. 10. Electrooptical hysteresis loops at temperature 60 8C for compounds (a) 4F6Bi(2F) and (b) 4F6Bi(3F).

Fig. 9. Comparison of maximum selective reflection upon reduced

temperature T 2 TC–A and the length of fluorinated unit for homologues

series n F6Bi.

R. Dabrowski et al. / Displays 25 (2004) 9–19 17

Significant higher contrast [34] can be obtained in reflective

mode. The thickness of reflective cells is only 0.8 mm, thus

improving helix unwinding of short pitch materials as

compared to transmissive cells.

The grey scale is excellently developed although some

asymmetry is observed between (þ ) and (2 ) cycles, what

can be attributed to short pitch of this material. Dynamic

responses are asymmetric, rise time ton is about 10 times

shorter than fall time toff : It is probably due to the used

aligning material (Nylon 6) being far from the optimum one.

Surface aligning layers with stronger anchoring energy

probably should decrease the asymmetry of the electro-

optical responses.

We are extensively working on improving properties of

orthoconic mixtures and we hope to present OAFLCMs

operating at lower voltages soon as well as having fully

symmetric branches of hysteresis loops as well as shorter

and symmetric response times.

4. Conclusion

The 458 tilted smectic compounds with antiferroelectric

phase (OAFLCs) existing in a broad temperature range were

found among esters family having partially fluorinated

terminal chain. The extension of fluorinated part influences

on tilting of molecules in smectic layer as well as on the

pitch length of the helical structure. LC mixtures operating

at room temperature and even at lower temperatures are

convenient for passive addressing schemes with high

multiplexing level at video rate, ensures excellent contrast

independent of viewing angle may be formulated.

Performance of present known materials is necessary to

improve by the increase of their pitch, what enable easier

depression of twisted structure in the cells. It is also

necessary to develop better aligning materials involving

strong anchoring of molecules with surface.

The orthoconic mixture showing V-shaped transmission

provided in certain frequency and temperature range

necessary for active matrix display is also possible to

formulate, and examples of such materials are described in

Ref. [35].

Acknowledgements

Financial support from Polish Ministry of Sciences and

Informatization PBS 701 and from EU projects IST

‘HEMIND’ and TRN ‘SAMPA’ is appreciated.

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