Ferroelectric Properties of Chiral Thiobenzoates from New Homologous Series

Ferroelectrics, 343:215–224, 2006Copyright © Taylor & Francis Group, LLCISSN: 0015-0193 print / 1563-5112 onlineDOI: 10.1080/00150190600962499

Ferroelectric Properties of Chiral Thiobenzoatesfrom New Homologous Series

M. D. OSSOWSKA-CHRUSCIEL,1 P. PERKOWSKI,2

W. PIECEK,2 S. ZALEWSKI,1 J. CHRUSCIEL,1

AND W. HAASE3

1Institute of Chemistry, University of Podlasie, 3-go Maja 54,08-110 Siedlce, Poland2Institute of Applied Physics, Military University of Technology, Kaliskiego 2,00-908 Warszawa, Poland3Institute of Physical Chemistry, Technical University, Petersenstr. 20,Darmstadt, D-64287, Germany

Chiral liquid crystalline compounds (S)-MHOBS5 and (S)-MHOBS6 from a new homol-ogous series of the (S)-(+)-4-(1-methylheptyloxy)biphenyl 4-alkylthiobenzoates, abbre-viated (S)-MHOBSn (where n denotes the number of carbon atoms in the alkyl chain) inwhich the chiral branched chain is attached to the biphenyl have been studied by DSCcalorymetry, dielectric spectroscopy and electro-optic methods. (S)-MHOBS5 and (S)-MHOBS6 possesses a rich phase polymorphism, including enantiotropic ferroelectricsmectic C phase (SmC∗).

Keywords Ferroelectric; liquid crystal; spontaneous polarization; relaxationfrequency

1. Introduction

The first information about synthesis of the chiral thiobenzoates with branched chain waspublished in 1989 [1]. The effect of the methyl group position in branched alkyl chainson the thiolic [1] and phenolic end [2] of 4,4′-disubstituted phenylthiobenzoates and someoptically active analogous as ferroelectric liquid crystals were discussed. The ferroelectricSmC∗ phase with large spontaneous polarization PSwas obtained when required branchedchain was patterned by 2-ethoxy-1-propoxy esters [3].

The first thioester with biphenyl group C8H17O-C10H8COSC6H4-O(CH2)2CH(CH3)Etshowed an enantiotropic sequence of SmC∗, SmA and N∗ phases with monotropic SmG∗

phase which supercooled considerably [4]. The chiral group of the branched terminal chainhas been connected to benzene ring. It was of interest to determine how the addition of thechiral branched chain, 1-methylheptyloxy, directly to the biphenyl group would influencethe ferroelectric behaviour of the thioester. We reported properties of two ferroelectricliquid crystals, (S)-MHOBS5 and (S)-MHOBS6, belonging to a new homologous seriesof chiral thiobenzoates, the (S)-(+)-4-(1-methylheptyloxy)biphenyl 4-alkylthiobenzoates,

Received September 12, 2005.∗Corresponding author. E-mail: [email protected]

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216 M. D. Ossowska-Chrusciel et al.

abbreviated (S)-MHOBSn, in which the chiral branched chain (S)-(1-methylheptyloxy) isattached directly to the biphenyl group.

2. Results and Discussion

2.1. Mesomorphic Properties

Chiral liquid crystalline compounds (S)-MHOBS5 and (S)-MHOBS6 from a new homol-ogous series of the (S)-(+)-4-(1-methylheptyloxy)biphenyl 4-alkylthiobenzoates, abbrevi-ated (S)-MHOBSn, having the following chemical formula:

C6H13C∗H(CH3)-O-C10H8-COS-C6H5-CnH2n+1

(for (S)-MHOBS5 n = 5 and (S)-MHOBS6 n = 6) were synthesized. The final compounds(S)-MHOBSn were obtained by condensation of the acid chloride, C6H13C∗H(CH3)-O-C10H8-COCl, and 4-n-alkylthiophenol, HS-C6H4-CnH2n+1, in toluene in the presence ofpyridine or triethylamine is presented on scheme1 (detailed discussion was presented in[5]).

Scheme 1. Final stage of (S)-MHOBSn synthesis.

The length of the terminal alkyl chain was the only variable parameter of the chemicalstructure. The chemical purity of final compound turned out to be the important point ofsynthesis. The phase situation depends on purity of the final product in a essential way(detailed discussion is presented in [6]). The transition temperatures were determined bymeans of the polarization microscope (PM) with a Linkam programmable heating stageTHMSE 600. The DSC measurements were performed using the differential scanningcalorimeter (DSC822e, Mettler-Toledo) in the temperature range from −30◦C to + 130◦C.The mass of the sample was 9–10 mg and the purity carried out 99.2%. The phase behaviourwas examined during heating and cooling at 2 K min−1. The phase situation receivedduring heating and cooling for (S)-MHOBS5 and (S)-MHOBS6 are shown in Figs. 1 and

New Ferroelectric Liquid Crystals 217

Figure 1. Exemplary DSC data obtained for (S)-MHOBS5 with the rates of ± 2K min−1 and 0.2 ±2 K min−1

2. Transition temperatures and associated enthalpy and entropy changes for (S)-MHOBS5and (S)-MHOBS6 are presented in Table 1.

Figures 3 and 4 presents phase diagrams of (S)-MHOBS5 and (S)-MHOBS6mesophases determined from transmitted light intensity (TLI) studies. The phase transi-tion temperatures are comparable to those obtained from DSC and PM methods.

Figure 2. Exemplary DSC data obtained for (S)-MHOBS6 with the rates of ± 2 K min−1.


Table 1Phase transition temperatures (◦C), enthalpies (kcal mol−1) and entropie (cal mol−1K−1) oftransition for the (S)-MHOBS5 and (S)-MHOBS6 determined using DSC at 2 K min−1: GS,Cr, SmG∗, SmI∗, SmC∗, N∗, BP and I are glassy state, smectic G∗, smectic I∗, ferroelectric

smectic C∗, paraelectric N∗, blue and isotropic phases

CompoundMole

weightPhase

transitionPhase transitiontemperature [◦C]

Enthalpy[kcal/mol]

Entropy[cal/mol·K]

(S)-MHOBS5 488 g Cr →SmG∗ 64,2 4,36 12,92SmG∗ → SmI∗ 68,0 0,53 1,54SmI∗ → SmC∗ 73,0 0,31 0,91SmC∗ → N∗ 94,4 0,22 0,60N∗ → BP 126,5 0,03 0,07BP → I 126,7 0,10 0,25

(S)-MHOBS6 502 g Cr → SmG∗ 62,3 2,62 7,81SmG∗ → SmI∗ 70,9 0,01 0,04SmI∗ → SmC∗ 77,0 0,23 0,65SmC∗ → N∗ 99,2 0,19 0,51N∗ → BP 121,2 0,02 0,05BP → I 121,4 0,09 0,22

The mesomorphic phases were identified using miscibility and XRD [5]. They arecaused by the development of a glassy phase, and this effect is also seen by the optic method.The blue phase (BP) was observed if the heating rate was lower then 0.5◦C/min. Texture ofthe blue phase (BP, 123◦C), the phase transition between blue phase and cholesteric (N∗,paraelectric) phase (BP-N∗, 122.2◦C), the N∗ and tilted smectic C∗ (SmC∗, ferroelectric,94◦C) phases obtained during cooling (S)-MHOBS6 are presented in Fig. 5.

The compounds (S)-MHOBS5 and (S)-MHOBS6 possess a rich polymorphism in therange from −30◦C to +130◦C (I, BP, N∗, SmC∗, SmI∗, SmG∗, GS, Cr).

2.2. Spontaneous Polarization and Electro-Optic Studies

Preliminary results have been obtained using cell 5 µm thick with electrodes of 0.725 cm2

each. Both glass plates of the cell have been covered with the evaporated ITO conductive lay-ers and spin-coated polyimide orienting layers. The cell has been inspected optically usingpolarising microscope PZO Biolar PI equipped with Linkam TMS 93 hot stage for temper-ature stabilisation (temperature stabilisation better than 0.1◦C), and dielectrically inspectedbefore filling (using HP4192A impedance analyser). A static conductivity (S < 1 µS) aswell as empty cell capacitance (C0 =129 pF) and parasitic capacitance (Cm = 6 pF) havebeen evaluated (at frequency f = 100 Hz, and U = 1 Vpp). The cell has been filled in theisotropic state by the capillary action at the temperature of 130◦C. Subsequently, the cellhas been conditioned during several melting, cooling processes. Final texture of the SmC∗

phase has been obtained during slow cooling (0.01◦C/min) at the vicinity of the electric field(frequency f = 100 Hz, electric field strenght E = 10 Vpp/5 µm). The optical observationsand electro-optical studies have been carried out during slow cooling (0.01◦C/min) fromisotropic state to the room temperature.


Figure 3. TLI curves obtained during heating and cooling for (S)-MHOBS5 with the rates of ±2 K min−1.

Figure 4. TLI curves obtained during heating and cooling for (S)-MHOBS6 with the rates of ±2 Kmin−1.


Figure 5. Textures of liquid crystalline phases for (S)-MHOBS6 obtained during cooling: a) BP(123◦C), b) BP-N∗ (122.2◦C), c) N∗ (120◦C), d) SmC∗ (94◦C) (See Color Plate XV)

Spontaneous polarisation studies have been carried out using two methods: so calledreversal current method (described in detail elsewhere [7]) and using Diamant bridge method(see ref. [8]). The reversal current method has been observed using triangular voltagedriving pulses (generated by HP 33120A pulse generator). A repolarisation current has beenregistered using HP54601 digital oscilloscope as a voltage drop on a resistance. Parasiticcurrents have been eliminated before the numerical integration of the repolarisation currentof the FLC in the measuring cell.

Temperature dependence of the spontaneous polarisation value for (S)-MHOBS5 and(S)-MHOBS6 has been presented in the Fig. 6. Spontaneous polarisation behaviour above85◦C seems to be regular with relatively high value of the spontaneous polarisation closeto SmC∗-N∗ phase transition. This thermal characteristics consists of a low temperatureregion which corresponds to the more orderly smectic phase (SmI∗ phase).

Electrooptical performance has been studied to evaluate dynamic characteristics of thematerial using cells placed in birefractive set-up and driven by HP33210A pulse generator.The cell has been illuminated by 15 W halogen lamp powered by HP E 3136A DC stabilisedpower supply and the optical response has been registered by FLC PIN 20 silicone photodiode and HP54601B digital oscilloscope. At the zero field state the optical axis of thesample was collinear with the polarizer which corresponded with low luminance measured.The temperature characteristic of the transmitted light vs. voltage has been taken for thelow (1 Hz) and high frequency (100 Hz) using voltage of U = 10 Vpp which has saturatedthe electrooptical effect (see Fig. 7).


Figure 6. Temperature dependence for spontaneous polarization for (S)-MHOBS5 and (S)-MHOBS6.

As one can see the tilt angle decreases with the temperature which causes the decreasingof the contrast ratio (CR = bright state / dark state), and simultaneously decreases the ratiobetween the applied voltage and the optical response. This kind of behaviour is probablycaused by the flat temperature characteristic of the spontaneous polarisation.

2.3. Dielectric Measurements

For the dielectric measurements LF the impedance analyzer HP 4192A was used. We applied0.1 V electric measuring field as well as several different BIAS levels (from 0 V to 35 V).


Figure 7. An instance of temperature characteristic of transmitted light vs. voltage.

5 µm thick cells were used with ITO electrodes covered by the rubbed polyimide for theplanar orientation. The temperature was stabilized by the Linkam TMSE 600 temperaturecontroller. The temperature stabilization inside the Linkam TMS 92 hot stage was 0.1◦C.We investigated all liquid crystal phases: from isotropic to SmG∗ phases.

The results of the real part of the dielectric permittivity are presented on the figure 8.It can be seen that ε’ increases rapidly for the temperatures between 75 and 97◦C. This

Figure 8. Real part of dielectric permittivity ε′ as a function of temperature T for four frequencies(100 Hz, 1 kHz, 10 kHz and 100 kHz).


Figure 9. Imaginary part of dielectric permittivity ε′′ versus a real part ε′ in SmC∗ phase (85◦C).Plots are made for different BIAS electric fields (0 V, 4 V, 10 V, 20 V, 30 V).

fact is related to appearance of Goldstone mode. It means that for this temperature region,MHOBS5 shows ferroelectric SmC∗ phase. For Goldstone relaxation in SmC∗ phase wecalculate all parameters of Cole-Cole plots. Results are presented below (Fig. 9).

It is worth to underline that for the SmC∗ phase the increase of BIAS levelcauses the increase of the radius of the Cole-Cole plot. In many papers, it is said thatwe should observe the opposite effect: the suppression of the Cole-Cole plot radius[9].

For investigated compounds the radius of Cole-Cole plots depends on temperature quiteweakly. When temperature is decreasing, diameter of Cole-Cole plot is slightly decreasing(Fig. 10).

Figure 10. Diameter of Cole-Cole plots as a function of temperature (for BIAS = 0 V).


Figure 11. Relaxation frequencies f [kHz] as a function of temperature for ferroelectric SmC∗ phase.

On the Fig. 11 one can notice that the relaxation frequency increases a little (in average)when the temperature of SmC∗ phase is decreasing and frequencies are typical for Goldstonemode.

3. Conclusions

1. (S)-MHOBS5 and (S)-MHOBS6 compounds possess a rich polymorphism in the rangefrom−30◦C to 130◦C including blue phase, paraelectric (N∗), chiral ferroelectric smecticC (SmC∗) and two highly ordered tilted mesophases, SmG∗ and SmI∗.

2. The spontaneous polarization has a relatively high value in the SmC∗ phase.3. The relaxation frequency depends weakly on the temperature in the SmC∗ phase.4. High BIAS electric field does not suppress Cole-Cole diagram.

The work was partly supported by the Polish Ministry of Scientific Research andInformation Technology Grant 4 T09A 200 25.

References

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2. M. E. Neubert, D. Leonhardt, and S. Sabol-Keast, Mol. Cryst. Liq. Cryst. 172, 227 (1989).3. M. E. Neubert and S. Sabol-Keast, Mol. Cryst. Liq. Cryst. 188, 67 (1990).4. M. E. Neubert and I. G. Shenouda, Mol. Cryst. Liq. Cryst. 205, 29 (1991).5. M. D. Ossowska-Chrusciel, Liq. Cyst. 31, 1159 (2004).6. M. D. Ossowska-Chrusciel, J. Chruúciel, S. Zalewski, A. Rudzki, A. Feliks, and J. Przedmojski,

SPIE 5565, 92 (2003).7. K. Miysato, S. Abe, H. Takezoe, A. Fukuda, and E. Kuze, Jap. J. Appl. Phys. 27, L661 (1983).8. H. Diamant, K. Drenck, and R. Pepinsky, Rev. Sc. Instr. 28, 30 (1957).9. J. Rutkowska, Ferroelectrics 180, 83 (1996).

Ferroelectric Properties of Chiral Thiobenzoates from New Homologous Series

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