New series of chiral smectic chlorinated liquid crystals

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New series of chiral smectic chlorinated liquid crystalsAlexej Bubnov a; Věra Hamplová a; Miroslav Kašpar a; Milada Glogarová a;Přemysl Vaněk aa Institute of Physics, Academy of Sciences of the Czech Republic, Prague 8, CzechRepublic

First Published on: 01 May 2000To cite this Article: Bubnov, Alexej, Hamplová, Věra, Kašpar, Miroslav, Glogarová,Milada and Vaněk, Přemysl (2000) 'New series of chiral smectic chlorinated liquidcrystals', Ferroelectrics, 243:1, 27 - 35To link to this article: DOI: 10.1080/00150190008008004URL: http://dx.doi.org/10.1080/00150190008008004

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Fermlccrrics. 2000. Vol. 243. pp. 27-35 Reprints available directly from the publisher Photocopying permitted by license only

0 UWH) OPA (Overseas Publishers Assoc'ration) N.V. Published by license under the

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New Series of Chiral Smectic Chlorinated Liquid Crystals

ALEXEJ BUBNOV, VERA HAMPLOVA, MIROSLAV KABPAR, MLADA GLOGAROVA and P k M Y S L VANEK

Institute of Physics, Academy of Sciences of the Czech Republic, Nu Slovance 2, 182 21 Prague 8, Czech Republic

(Received August 30, 1999)

Two series of ferroelectric liquid crystalline materials, containing a lateral chlorine group on the aromatic ring of the molecular core, have been synthesised and investigated. Sequences of mesophases and phase transition temperatures have been determined for all substances. The temperature dependences of the complex permittivity and the spontaneous polarisation were determined in the whole range of the ferroelectric SmC* phase. The substitution of chlo- rine increases the values of spontaneous polarisation more than 5 times in comparison with non-chlorinated compounds.

Keywords: ferroelectric liquid crystals; chlorinated liquid crystals; spontaneous polarisation; dielectric properties

INTRODUCTION

In works [ 1,2], a positive effect of lateral substitution by chlorine in the

series of biphenyl benzoate FLCs with Zalkoxypropionate c h i d unit

has been found. Strictly speaking, the range of the SmC* phase shifts by

approximately 20 K to lower temperatures and the saturated values of

the spontaneous polarisation increase from 80 nC/cm2 to the values

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28 ALEXEl BUBNOV et al.

about 120 nClcm2 for chlorinated materials compared to the non

substituted compounds.

A similar effect can be expected also with liquid crystals derived

from 4-hydroxybiphenyl-4'-carboxylic acid. To study that, two new

series of liquid crystals, containing 3chloro-4-hydroxybiphenyl-4'-

carboxylic acid in the core, have been prepared and investigated. General

formulae of the studied materials are:

abbreviated as BCI dm and BCI n/** with one or two c h i d centres,

respectively. Indexes I) and m denote the numbers of carbon atoms in

the hydrocarbon chain of the alkylbenzoate and alkoxypropionate units,

respectively.

SYNTHESIS

The synthesis was carried out according to the Scheme 1. Structure of

intermediates and final products was confirmed by 'H-NMR

spectroscopy (200 MHz, Varian Gemini 2000, CDCl3).

4-alkoxybiphenyl 1 was synthesised by alkylation with appropriate

alkylbromide in natriumethoxide/ethanol solution.

3-chloro-4-alkoxybiphenyl2: 0.2 mole of 1 was mixed with 54.0g

of sulphuryl chloride and refluxed for two hours. The cold solution was

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CHLORINATED FLC MATERIALS 29

poured into 1 litre of water, extracted with diethylether, washed with

water, dried over CaClz and evaporated to dryness. Crystallisation of

500 ml of ethanol yielded about 40 g of white crystals.

SCHEME 1

3-chloro-4-alkoxybiphenyl-4’-carboxylic acid 4: 0.5 litre of

dichloroethane and 0.3 mole of anhydrous aluminium chloride were

cooled into iceiwater bath at 0-5’C and 0.2 mole of acetic anhydride was

added dropwise under stirring and cooling. Then, a solution of 0.2 mole

of (2) in dichloroethane was added dropwise. The reaction mixture was

stirred under cooling for 1 hour and then at room temperature for 2

hours. The resulting green mixture was poured into 500 g of ice/lOO ml

of conc. HCI mixture, extracted by chloroform and evaporated to

dryness. The product 3 was oxidised by a cool NaBrO solution (80g

NaOH, 30 ml Brz, 400 ml H20) in 300 mi of dioxane. During the

addition, the temperature was allowed to arise to 35-40°C. After stirring

for an additional 30 min the suspension of the sparingly soluble Na salt

was treated with sufficient amount of sodium bisulphate to destroy the

excess of hypobromite. One litre of water was added and about 0.5 litre

of the liquid was boiled off to remove the dioxane and bromoform.

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30 ALEXEJ BUBNOV et a6.

Acidification of the hot solution gave yellow product after cooling.

After two recrystallisations from ethanol 0.05-0.07 mole (25-35%) light

yellow crystals 4 yielded.

n-Alkyl-2(S)-(4-hydroxybenzoyloxy)propanoates 5 was

synthesised according the previous work [3]. Synthesis of the final

products were carried by usual esterification of 4 and 5 with

dicyclohexylcarbodiimide as a condensation agent in CH2Cl2 and 4-

pyrrolidinopyridine as a catalyst at a room temperature.

The crude products were chromatographed on silica gel and

crystallised from the ethanol-acetone mixture. The purity of substances

was checked by high pressure liquid chromatography on silica gel

(Biospher SI 100, Watrex) using mixture of 99.9% toluene and 0.1%

methanol as the eluent.

'H NMR BCllW12: 8,25d J= 8,4 2 HAr ortho to -COOAr; 8,19d J=8,8 2 HAr otho to -COOC*; 7,68m 3 HAr ortho to CI, meta to -COOAr; 7,5dd J I= 8,5 J2= 2,2 1 HAr para to C1; 7,35d J=8,6 2 HAr ortho to W O O - ; 7,02d J=8,6 1 HAr ortho to RO-; 5,34q J=7,1 1 HC*H; 4,18m 2H C O O B ; 4,08t J=6,5 CH20Ar; 1,87quint J=7,3 2 H B C H 2 0 A r ; 1,65d J=7,0 3 H CH3-C*; 1,5 quint J=7,0 2 H m C H 2 0 C O ; 1,2-1,4m 36 H CH2; 0,9m 6 H CH3.

EXPERIMENTAL RESULTS

With all substances sequences of mesophases and phase transition

temperatures have been determined from characteristic textures and

their changes observed in a polarising microscope (see Table 1). For

several compounds, the phase transition temperatures were checked by

DSC measurements. All compounds exhibit the chiral SmC* phase. The

temperature dependences of the spontaneous polarisation, P,, (see

Figure 1 and Figure 2) were determined from the hysteresis loop

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CHLORINATED FLC MATERIALS 31

detected during P, switching in the a.c. electric field 40 kVcm-' of the

frequency 60 Hz on 25 pn thick samples in book-shelf geometry. The

switching observed at a quasistatic electric fields shows a typical

ferroelectric behaviour [4].

TABLE 1 Phase sequences and transition temperatures TI ("C) for the homologous series BCl UP* and BCl dm.

Substance Cr. Tt %C* T, SmA TI N' TI Is0 BCI &"* 17 29 89 - BCI 7 P BCI W* BCI 6/6 BCI 7/6 BCI &!3 BCI 8/6 BCI iW BCI 8/12 BCI 10/3 BCI 1016 BCI I0112

10 a 48 72 80 0 6 0 5 4 - 81 0 . 5 2 0 . 9 1 -

19 38 83 86 . 20 . 53 . 95 . 97 20 47 84 90 22 40 89 - . 12 24 89 - . 13 59 87 93 18 62 77 88

0 9 4 9 . 8 5 - . BC112/12 12 60 89 - .

A A A A A ~ ~ ~ t so

A BCI8/** O0\ AAAA

M . ==%I

4' 50

20 24 28 32 36 40 44 48 5 2 56 T I 'C

FIGURE 1 for indicated BCf n/+* hornologues (n = 6,7,8).

Temperature dependences of the spontaneous polarisation

Temperature dependences of the complex permittivity have been

measured in a whole temperature range of the SmC* phase where the

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32 ALEXEJ BUBNOV et al.

contribution of the Goldstone mode in BCI n/m homologues is detected

(see Figure 4). In BCI n/** homologues the real part of permittivity

exhibits a peak at the phase transition (see Figure 3), while the value of

permittivity in the whole range of the SmC* phase is very low. The

temperature dependences of E’ have been m a w s at 30 Hz and 100

Hz, both showing qualitatively the same behaviour.

The dielectric spectra have been also studied, which provide a

relaxation frequencies and dielectric strengths of mode contributing to

the permittivity. The results show that the relaxation frequencies in the

SmC* phase are low (-100 Hz), which is a consequence of rather low

temperature range of the observed mesophases.

0 BCI 7/6

%“so - I BCI10/6 . BCIIO/I2

v BCI 12/12

10 20 30 40 50 60 T I “c

FIGURE 2 for indicated BCI n/m homologues (n = 6,7,8, 10, 12; m = 3,6,9, 12).

Temperature dependences of the spontaneous polarisation

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CHLORINATED FLC MATERIALS 33

60

45

't,

30

1 5

0 0 10 20 30 40 50 60

T I "C

FIGURE 3 Temperature dependences of the real part of permittivity for BCl d** homologues measured on cooling at a frequency 30 Hz.

80

20

D BCI 6/6 0 BCI 7/6 a BCI 8/3 7 BCl8/6 - B C I W 1 BCI 8/12

BCI lO/3 BCI 10/6 BCI IO/l2 BCI 12/12

O ' ' ' . ' . ' . ' . ' . * > ' 0 10 20 30 40 50 60 70

T / " C

FIGURE 4 Temperature dependences of the real part of permittivity for BCl d m homologues measured on cooling at a frequency 30 Hz.

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34 ALEXEJ BUBNOV et a6.

DISCUSSION AND CONCLUSIONS

Comparing obtained values of the spontaneous polarisation to those for

similar non-chlorinated compounds [5] , one can conclude that the

substitution of chlorine results in the P, increase of more than 5 times.

As the increase of P, by the substitution of chlorine has been found also

with other compounds [2], it seems to be general property.

The temperature dependences of the real part of complex

permittivity behave in unusual way, which has not been observed so far. Namely, in BCIn/** homologues, the real part of permittivity, being

extremely low, exhibits a strong peak at the temperature of the phase

transition to the SmC* phase (see Figure 3). This could be viewed as a

behaviour of the antifemelectric structure, but a typical ferroelectric

switching observed in the whole temperature range of this phase

contradicts. The transition to the hexatic phase is also excluded in the

temperature range of the decrease of c’ in the BCf d m homologues as

no transition has been detected in this temperature range in DSC study.

Moreover, the X-ray study for both series proves a short range in-plane

molecular order (-5A) only in the whole temperature range assigned to

the SmC* phase (see Table 1).

ACKNOWLEDGMENTS

The authors are indebted to Dr. E. Gorecka and Dr. D. Pociecha for X- ray measurements. The work was supported by Grants No. 202/99/1120, No. 202/00/P044 and No. 106/97/0337 from the Grant Agency of the Czech Republic.

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CHLORINATED FLC MATERIALS 35

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