Synthesis and characterization of halogen-containing ferroelectric liquid crystals and side chain liquid crystalline polymers

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Liquid Crystals

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Synthesis and characterization of

halogen-containing ferroelectric liquid crystals and side

chain liquid crystalline polymers

Ging-Ho Hsiue a , Yi-An Sha a , Shih-Jung Hsieh ac , Ru-Jong Jeng b & Wen-Jang Kuo a

a

Department of Chemical Engineering, National Tsing Hua University, Hsinchu, Taiwan 300, ROC

b

Institute of Applied Chemistry, National Chiao Tung University, Hsinchu, Taiwan 300, ROC

c

Department of Chemical Engineering, National Chung Hsing University, Taichung, Taiwan 402, ROC

Published online: 06 Aug 2010.

To cite this article: Ging-Ho Hsiue , Yi-An Sha , Shih-Jung Hsieh , Ru-Jong Jeng & Wen-Jang Kuo (2001) Synthesis and characterization of halogen-containing ferroelectric liquid crystals and side chain liquid crystalline polymers, Liquid Crystals, 28:3, 365-374, DOI: 10.1080/02678290010015324

To link to this article: http://dx.doi.org/10.1080/02678290010015324

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Liquid Crystals, 2001, Vol. 28, No. 3, 365± 374

Synthesis and characterization of halogen-containing ferroelectric

liquid crystals and side chain liquid crystalline polymers

GING-HO HSIUE*, YI-AN SHA, SHIH-JUNG HSIEH†, RU-JONG JENG‡ and WEN-JANG KUO

Department of Chemical Engineering, National Tsing Hua University, Hsinchu, Taiwan 300, ROC

† Institute of Applied Chemistry, National Chiao Tung University, Hsinchu, Taiwan 300, ROC

‡Department of Chemical Engineering, National Chung Hsing University, Taichung, Taiwan 402, ROC

(Received 6 June 2000; accepted 30 August 2000)

A new series of ferroelectric liquid crystals and side chain liquid crystalline polymers based on halogen-containing chiral centres has been synthesized. Chemical structures were analysed by NMR. Liquid crystal phases were characterized by diŒerential scanning calorimetry, optical polarizing microscopy, and X-ray diŒractometry. The behaviour of the liquid crystalline phases was investigated as a function of spacer units and diŒering terminal asymmetric moieties. It was found that phase transition temperatures decreased with increasing length of the oligooxyethylene spacer unit. DiŒering terminal asymmetric moieties led to diŒering mesophase phenomena. Furthermore, a wide temperature range (including room temperature) of a chiral smectic C phase was achieved.

1. Introduction unit favours a reduction in the phase transition temper-atures. As the number of oxyethylene units increases, During the past decade, ferroelectric liquid crystals

the transition temperature decreases [11]. The chiral (FLCs) have been extensively studied because of their

centre is usually the major part of the terminal chain fast response time and memory eŒect toward an applied

because the P_svalue is aŒected by the asymmetric atom electric eld [1–6]. These characteristics make them

and its position, and the length of the terminal chain suitable for electro-optical applications. Prompted by

unit. In addition, incorporation of a polar group (F, Cl, the development of surface stabilized ferroelectric liquid

Br, CN, CF₃, etc.) onto the chiral centre close to the crystal (SSFLC) display technology [7], FLC materials

core can maximize the P_svalue [12, 13]. and SSFLC cells have been vigorously investigated [8].

In our previous work, a series of chlorine-containing For display use, FLC materials have mostly been

FLCs was synthesized and characterized [14]. These designed to provide a wide temperature range of the

monomers have a wide temperature range of the chiral FLC phase, including room temperature [9], and a

smectic C phase—including room temperature; their large value of the spontaneous polarization (P_s). These

chemical structures are shown below: properties are in uenced by the design of the molecular

structures of FLC mesogens. The fundamental molecular structure of an FLC includes a mesogenic group, a spacer chain unit, and a chiral centre unit (terminal chain

In this paper, a new series of ferroelectric liquid units). The mesogenic group often consists of at least

crystals and side chain liquid crystalline polymers is two linked rigid groups with lateral substituents [10].

reported. These liquid crystalline materials consist of The length and chemical structure of the spacer unit

various halogen-containin g chiral moieties (F, Cl, and appear to be important factors in LFC phase formation

Br), oligooxyethylen e spacers, and ester core units. The and temperature range. The oxyethylene group as spacer _{in uences of the varied halogen-containin g chiral tails} and spacer units on mesophase formation are also discussed.

*Author for correspondence; e-mail: [email protected]

http://www.tandf.co.uk/journals DOI: 10.1080/02678290010015324

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2. Experimental heating and second cooling scans. A Nikon Micro-photo-Fx polarizing optical microscope (POM) equipped 2.1. Materials

Allyl bromide, 2-chloroethanol , 2-(2-chloroethoxy) - with a Mettler FP82 hot stage was used to observe the thermal transitions and anisotropic textures. X-ray ethanol, 2-[2-(2-chloroethoxy )ethoxy]ethanol, hydrogen

uoride–pyridine, tri uoromethanesulfonic anhydride, diŒraction (XRD) measurements were performed with a Rigaku R-axis IIC powder diŒractometer. A mono-tetrabutylammoniu m uoride hydrate, and hydrogen

hexachloroplatinat e(IV) hydrate were purchased from chromatized X-ray beam from nickel- ltered CuK a radiation with a wavelength of 0.15406 nm was used. A Adrich Chemical Co. 4,4¾ -Dihydroxybiphenyl, N,N¾

-dicyclohexylcarbodiimide (DCC), 4,4¾ -biphenol, l-iso- thermal controller was added to the X-ray system for thermal measurement with a tolerance of Ô 0.5ßC. leucine, sodium nitrite, and pyridine were purchased from

Tokyo Chemical Industry Co. Ltd.

4-Dimethylamino-pyridine (DMAP) and benzophenone were purchased 2.3. Synthesis of monomers

The syntheses of the liquid crystalline monomers are from Lancaster Chemicals Ltd;

poly(methylhydrogen-siloxane) ([g]5 30) was purchased from United Chemical outlined in schemes 1–3. Technologies, Inc; hexane, ethyl acetate, dichloromethane ,

acetonitrile, and methanol were purchased from TEDIA. 2.3.1. 4-(2-Allyloxyethox y)benzoic acid (S-1);

4-[2-(2-ally loxyethoxy)ethoxy]benzoi c acid (S-2);

All these chemicals were used as received.

Tetrahydro-furan and toluene purchased from TEDIA were distilled 4-{2-[2-(2-allyloxyethox y)ethoxy]ethox y}benzoic acid (S-3)

over sodium using benzophenone as indicator under a

nitrogen atmosphere. The syntheses of these compounds via esteri cation

and etheri cation reactions have been reported previously [14].

2.2. Characterization methods

1H NMR and 19F NMR spectra were obtained on

Bruker AM-400 NMR or Jeol JNM-FX100 spectro- 2.3.2. (2S,3S)-2-Fluoro-3-methylpentanoi c acid (Fs-1)

In a 500 ml round-bottom ask, hydrogen uoride-meters. Transition temperatures were determined as the

maxima of endothermic or exothermic peaks using a pyridine (100 g) was added dropwise to pyridine (60 ml ) under N₂ at 0ßC; after 10 min, l-isoleucine (7.87 g, Seiko SSC 5200 diŒerential scanning calorimeter (DSC);

heating and cooling rates were 10ßC minÕ 1. The transition 60 mmol ) was added to the reaction mixture. Sodium nitrite powder (6.21 g, 90 mmol) was then added under temperatures were speci cally obtained from the rst

Scheme 1. Synthesis of monomers

MDn12Fs (n5 1, 2, 3).

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367

Halogen-containin g FL Cs

Scheme 2. Synthesis of monomer MD312Fr.

N₂ at 0ß C. After stirring the reaction mixture for 5 h, after evaporating the solvent, a dark yellow oily pro-cooling was removed, and the mixture held at room duct was obtained; yield 80%. 1_{H NMR (DMSO-d}

6): temperature for 24 h. The reaction mixture was then d5 0.84 (t, 3H, CH₂ CH₃), 0.87 (d, 3H, CH(CH₃) ), added to 500 ml of water and the mixture was extracted 1.02–1.29 and 1.29–1.53 (m, 2H, CH₂CH₃), 1.53–1.57 with ethyl ether. The organic phase solvent was evaporate d (m, 1H, CHCH₂), 3.79 (m, 1H, CH(OH) ).

and a colourless oily product was nally obtained after distillation (78–80ßC, 4 mm Hg); yield 27.6%.1H NMR 2.3.4. Ethyl (2S,3S)-2-hydroxy-3-methylpentanoat e (DMSO-d₆): d5 0.97 (t, 3H, CH₃ CH₂ ), 1.12 (d, 3H, (Fr-2) CH(CH₃) ), 1.44 and 1.63 (m, 2H, CH₃CH₂ ), 2.17 A mixture of 39.5 g (299 mmol ) of Fr-1, 7.5 ml of (m, 1H, CH(CH₃) ), 4.93 (d, 1H, CH(F) ).19_{F NMR}

H₂SO₄ and 300 ml of anhydrous ethanol was stirred (DMSO-d₆): d5 Õ 243.42 (s, CH(F)).

under re ux for 24 h; ethanol was then distilled oŒ. The reaction mixture was extracted with ethyl ether after 2.3.3. (2S,3S)-2-Hydroxy-3-methylpentanoi c acid (Fr-1)

saturated with an aqueous solution of NaHCO₃. The l-isoleucine (49.5 g, 377 mmol ) was added to 2.67M

organic phase solvent was evaporated oŒ, and a colour-H₂SO₄ aqueous solution (586 ml) at 0ßC under N₂.

less oily product obtained by distillation (62–65ß C, Sodium nitrite (39.4 g, 570 mmol) dissolved in water

6 mm Hg). 1_{H NMR (DMSO-d}

6): d5 0.91 (t, 3H, (150 ml ) was added dropwise under N₂ at 0ß C and the

CH₂CH₃), 0.99 (d, 3H, CHCH₃ C₂H₅), 1.13–1.53 mixture stirred at room temperature overnight. The

reaction mixture was then extracted with ethyl ether; (m, 2H, CH₂CH₃), 1.31 (t, 3H, COOCH₂CH₃),

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Scheme 3. Synthesis of monomers

MDn12Bs (n5 1, 2, 3).

1.73–1.93 (m, 1H, CH(CH₃) ), 2.78 (br, s, 1H, graphy on silica gel with ethyl acetate and hexane as eluant. A colourless product was obtained by distil-CH(OH) ), 4.08 (d, 1H, CH(OH) ), 4.26 (q, 2H,

COOCH₂CH₃). lation (59–63ßC, 9 mm Hg); yield 81.9%. 1H NMR

(DMSO-d₆): d5 0.95 (t, 3H, CH₂CH₃), 0.96 (d, 3H, CHCH₃ C₂H₅), 1.19–1.65 (m, 2H, CH₂CH₃), 1.32 2.3.5. Ethyl (2S,3S)-2-(tri

uoromethyl)sulfonyloxy-3-methylpentanoat e (Fr-3) (t, 3H, COOCH₂CH₃), 1.75–2.12 (m, 1H, CHCH₃ ),

4.28 (q, 2H, COOCH₂CH₃), 5.01 (d, 1H, CHF ). In a 500 ml polyproplene reactor, 34.5 g (325 mmol)

of Fr-2, 200 ml of anhydrous dichloromethane and 19_{F NMR (DMSO-d}

6): d5 Õ 251.26 ( CHF ). 18.6 ml of anhydrous pyridine was stirred under N₂ at

0ß C. Tri uoromethylsulfonic anhydride (50 g, 177 mmol)

2.3.7. (2R,3S)-2-Fluoro-3-methylpentanoi c acid (Fr-5)

was added dropwise. After 2 h the reaction mixture was

A mixture of 16.7 g (103 mmol ) of Fr-4, 6 ml of 50 wt % added to 500 ml of water and the mixture extracted

aqueous sodium hydroxide, 20 ml of ethanol, and 200 ml twice with dichloromethane. The organic phase was

of water was stirred under re ux for 24 h; ethanol was extracted with a 10 wt % aqueous solution of HCl, and

then distilled oŒ. After cooling at 0ßC, the reaction the solvent evaporated to yield a colourless product

mixture was acidi ed with 5N hydrochloric acid and after distillation (55–61ßC, 0.4 mm Hg); yield 85.4%.

extracted with ethyl acetate. The organic phase solvent 1H NMR (DMSO-d₆): d5 0.95 (t, 3H, CH₂CH₃), 1.07

was evaporated oŒ, and a colourless oily product (d, 3H, CHCH₃ C₂H₅), 1.19–1.64 (m, 2H, CH₂CH₃),

obtained by distillation (78–80ßC, 4 mm Hg); yield 76%. 1.33 (t, 3H, COOCH₂CH₃), 2.05–2.27 (m, 1H, ₁ H NMR (DMSO-d₆): d5 0.89 (t, 3H, CH₃CH₂ ), CHCH₃ ), 4.31 (q, 2H, COOCH₂CH₃), 5.01 (d, 1H, 0.97 (d, 3H, CH(CH₃) ), 1.06–1.32, 1.32–1.56 (m, 2H, CH(OSO₂CF₃) ). CH₃CH₂ ), 1.71–2.13 (m, 1H, CH(CH₃) ), 5.01 (d, 1H, CHF ). 19_{F NMR (DMSO-d} 6): d5 Õ 251.26 2.3.6. Ethyl (2R,3S)-2- uoro-3-methylpentanoat e (Fr-4) ( CH(F) ). A solution of tetrabutylammoniu m uoride hydrate

(40 g) in 150 ml acetonitrile was added dropwise to a

solution of Fr-3 (36.7 g, 133 mmol) in 150 ml aceto- 2.3.8. (2S,3S)-2-Bromo-3-methylpentanoi c acid (Bs-1)

l-isoleucine (7.9 g, 60 mmol ) and 75 ml 6N HBr were nitrile at 80ßC under N₂. After stirring the reaction

mixture under re ux for 3 h, the acetonitrile was distilled placed in a 500 ml round-bottom ask equipped with a magnetic stirrer at 0ßC. Sodium nitrite (6.21 g, 90 mmol ) oŒ. The crude product was puri ed by ash

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369

aqueous solution was added dropwise below 5ßC under (Bs-2) 75.5. (Fs-2) 1_{H NMR (CDCl3): d}₅ _{0.97 (t, 3H,} CH₃CH₂ ), 1.12 (d, 3H, CH(CH₃) ), 1.44 and 1.63 N₂. After stirring the reaction mixture for 5 h, cooling

was removed, and the mixture held at room temperature (m, 2H, CH₃CH₂ ), 2.17 (m, 1H, CH(CH₃) ), 4.93 (d, 1H, CHF ), 6.89, 7.15, 7.42 and 7.52 (4d, 8H, for 24 h; it was then extracted with ethyl ether. The

organic phase solvent was evaporated, and a red oily aromatic protons) ; 19F NMR (CDCl3): d5 Õ 243.42 ( CHF ). (Fr-6) 1H NMR (CDCl3): d5 0.97 (t, 3H, product obtained by distillation (140ßC, 20 mm Hg);

yield 73%. 1H NMR (DMSO-d₆): d5 0.96 (t, 3H, CH₃CH₂ ), 1.12 (d, 3H, CH(CH₃) ), 1.44 and 1.63 (m, 2H, CH₃CH₂ ), 2.17 (m, 1H, CH(CH₃) ), 5.05 CH₂CH₃), 1.13 (d, 3H, CHCH₃ ), 1.39 and 1.83 (m, 2H, CH₂CH₃), 2.18 (m, 1H, CHCH₃), 4.09 (d, 1H, (d, 1H, CHF ), 6.87, 7.12, 7.39 and 7.52 (4d, 8H, aromatic protons) ; 19F NMR (CDCl3): d5 Õ 251.26 CHBr ), 11.8 ( br, COOH). ( CHF ). (Bs-2) 1H NMR (CDCl3): d5 0.96 (t, 3H, CH₂CH₃), 1.13 (d, 3H, CHCH₃ ), 1.39 and 1.83 2.3.9. 4,4¾ -Dihydroxybipheny l (2S,3S)-2- uoro-3-methyl-pentanoate (Fs-2); 4,4¾ -dihydroxybiphenyl (m, 2H, CH₂CH₃), 2.18 (m, 1H, CHCH₃), 4.09 (d, 1H, CHBr ), 6.89, 7.15, 7.42 and 7.52 (4d, 8H, aromatic (2R,3S)-2- uoro-3-methylpentanoat e (Fr-6);

4,4¾ -dihydroxybipheny l (2S,3S)-2-bromo- protons).

3-methylpentanoat e (Bs-2)

These compounds were synthesized by similar 2.3.10. Synthesis of FL C monomers MDn12Fs, MDn12Fr, and MDn12Bs

methods. As an example, the synthesis of 4,4¾

-dihydroxy-biphenyl (2S,3S)-3- uoro-3-methylpentanoat e (Fs-2) is These products were synthesized, respectively, in the same manner as compounds Fs-2, Fr-6, or Bs-2;1H NMR described as follows. In a 100 ml round-bottom ask,

(2S,3S)-2- uoro-3-methyl pentanoic acid (3 g, 22.3 mmol), spectra are listed in table 1.

MDn12Fs stands for the following series: 4¾

-[(2S,3S)-4,4¾ -dihydroxybipheny l (11 g, 59 mmol ),

dimethylamino-pyridine (0.5 g, 4 mmol),N,N¾ -dicyclohexylcarbodiimid e 2- uoro-3-methylpe ntanoyloxy]-4-biphenyl 4-(2-allyloxy-ethoxy)benzoate (MD112Fs); 4¾ -[(2S,3S)-2-

uoro-3-(4.1 g, 20 mmol ), and dried THF (50 ml ) were stirred

under N₂ at 0ß C overnight. The solution was ltered methylpentanoyl oxy]-4-biphenyl 4-[2-(2-allyl-oxyetho xy)-ethoxy]benzoate (MD212Fs); 4¾ - [( 2S,3S)-2-

uoro-and the ltrate evaporated. The product was puri ed

by chromatograph y on silica gel with ethyl acetate 3-methylpentanoylox y]-4-biphenyl 4-{2-[2-(2-allyloxy-ethoxy)ethoxy]ethoxy}benzoat e (MD312Fs).

and hexane as eluant. Yield (Fs-2) 70.5%, (Fr-6) 69.7,

Table 1. Chemical shift and optical rotation values of compounds MDn12Fs, MD312Fr and MDn12Bs.

Compound [a]25

d a NMR spectrab

MD112Fs Õ _9.789 1_{H NMR: 0.97 (t, 3H, CH}

3CH2 ), 1.12 (d, 3H, CH(CH3) ), 1.44 and 1.63 (m, 2H, CH3CH2 ), 2.17 (m, 1H, CH(CH₃) ), 4.93 (dd, 1H, CHF ), 3.81–4.20 (m, 6H, CH₂ (OCH₂CH₂) ), 5.23 and 5.9 (m, 3H, CH₂ CH ), 6.9–8.15 (6d, 12 aromatic protons).19F NMR:Õ _{243.42 ( CHF ).} MD212Fs Õ _8.366 1_{H NMR: 0.98 (t, 3H, CH}

3CH2 ), 1.12 (d, 3H, CH(CH3) ), 1.44 and 1.65 (m, 2H, CH3CH2 ), 2.17 (m, 1H, CH(CH₃) ), 4.93 (dd, 1H, CHF ), 3.61–4.22 (m, 10H, CH₂ (OCH₂CH₂)₂ ), 5.23 and 5.9 (m, 3H, CH₂ CH ), 6.9–8.15 (6d, 12 aromatic protons).19_{F NMR:}Õ _{243.42 ( CHF ).} MD312Fs Õ _8.647 1_{H NMR: 0.99 (t, 3H, CH}

3CH2 ), 1.12 (d, 3H, CH(CH3) ), 1.44 and 1.67 (m, 2H, CH3CH2 ), 2.19 (m, 1H, CH(CH₃) ), 4.93 (dd, 1H, CHF ), 3.58–4.22 (m, 14H, CH₂ (OCH₂CH₂)₃ ), 5.23 and 5.9 (m, 3H, CH₂ CH ), 6.9–8.15 (6d, 12 aromatic protons).19_{F NMR:}Õ _{243.42 ( CHF ).} MD312Fr Õ _6.17 1_{H NMR: 0.99 (t, 3H, CH}

3CH2 ), 1.09 (d, 3H, CH(CH3) ), 1.44 and 1.63 (m, 2H, CH3CH2 ), 2.20 (m, 1H, CH(CH₃) ), 5.05 (dd, 1H, CHF ), 3.58–4.22 (m, 14H, CH₂ (OCH₂CH₂)₃ ), 5.23 and 5.9 (m, 3H, CH₂ CH ), 6.98–8.15 (6d, 12 aromatic protons).19F NMR:Õ _{251.26 ( CHF ).} MD112Bs Õ _10.042 1_{H NMR: 0.95 (t, 3H, CH}

2CH3), 1.12 (d, 3H, CHCH3 ), 1.38 and 1.68 (m, 2H, CH2CH3), 2.18 (m, 1H, CHCH₃), 4.21 (dd, 1H, CHBr ), 3.81–4.20 (m, 6H, CH₂ (OCH₂CH₂) ), 5.23 and 5.9 (m, 3H, CH₂ CH ), 6.9–8.16 (6d, 14 aromatic protons).

MD212Bs Õ _15.187 1H NMR: 0.95 (t, 3H, CH₂CH₃), 1.12 (d, 3H, CHCH₃ ), 1.38 and 1.68 (m, 2H, CH₂CH₃), 2.18 (m, 1H, CHCH₃), 4.21 (dd, 1H, CHBr ), 3.81–4.32 (m, 10H, CH₂ (OCH₂CH₂)₂ ), 5.23 and 5.9 (m, 3H, CH₂ CH ), 6.9–8.16 (6d, 12 aromatic protons).

MD312Bs Õ _7.338 1H NMR: 0.95 (t, 3H, CH₂CH₃), 1.12 (d, 3H, CHCH₃ ), 1.38 and 1.68 (m, 2H, CH₂CH₃), 2.18 (m, 1H, CHCH₃), 4.21 (dd, 1H, CHBr ), 3.81–4.31 (m, 14H, CH₂ (OCH₂CH₂)₃ ), 5.23 and 5.9 (m, 3H, CH₂ CH ), 6.9–8.16 (6d, 12 aromatic protons).

a_{These values were measured in CHCl}

3at 25ßC. b_{These values were measured in CDCl}

3, using 400 MHz NMR spectroscopy (internal standard tetramethylsilane).

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MD312Fr stands for the following compound: chiral centre of MDn12Bs. The chemical structures of

compounds MDn12Fs and MD312Fr were also

identi-4¾ -[(2R,3S)-2- uoro-3-methylpentanoylox y]-4-biphenyl

4 - {2 - [2 - ( 2 - allyloxyethoxy)ethoxy]ethoxy}benzoate ed by 19F NMR. In the19F NMR spectra, the uorine chemical shifts appeared at Õ 243.49 and Õ 251.24 ppm (MD312Fr).

MDn12Bs stands for the following series: 4¾ -[(2S,3S)- for MDn12Fs and MD312Fr, respectively. Optical

rotations, [a]5

D, of these monomers are summarized 2-bromo-3-methylpentanoyloxy ]-4-biphenyl

4-(2-allyl-oxy-ethoxy)benzoate (MD112Bs); 4¾ -[(2S,3S)-2-bromo- in table 1.

The FLCPs were synthesized by hydrosilation reactions 3-methylpentanoylox y]-4-biphenyl 4-[2-(2- allyl-

oxy-ethoxy)ethoxy]benzoate (MD212Bs); 4¾ -[(2S,3S)-2-bromo- in which 10 mol % excess of the LFC monomers was employed to react with the Si–H groups on the poly-3-methylpentanoyloxy [-4-biphenyl

4-{2-[2-(2-allyl-oxy-ethoxy)ethoxy]ethoxy}benzoat e (MD312Bs). (methylhydrogens iloxane) backbone. Chemical structures of the FLCPs were characterized by 1H NMR spectra in which the Si–H peak (4.7 ppm) and vinyl protons

2.4. Synthesis of FL CPs, PS312Fr, PS312Fs, PS312C

and PS312Bs of the CH₂ CH group appearing between 5.23 and

5.90 ppm vanished after reaction. This con rmed that The structures of the synthesized liquid crystalline

poly-siloxanes are shown in scheme 4. They were synthesized complete reaction between FLC monomers and Si–H groups had taken place.

by similar methods; as an example, the procedure for PS312Bs is described below.

FLC monomer, MD312Bs (0.5 g, 10 mol % excess 3.2. T hermal properties

Phase sequences and their corresponding transition versus the Si–H groups present in the polysiloxane), was

dissolved in 50 ml of freshly distilled toluene together temperatures for compounds MDn12Fs, MD312Fr,

MDn12C, and MDn12Bs are shown in table 2.

with the appropriate amount of poly(methylhydrogen-siloxane). This solution was heated to re ux under N₂.

Hydrogen hexachloroplatinat e(IV) hydrate (100 mg) in 3.2.1. MDn12Fs series

There are three compounds in this series; they con-dry THF was then injected via a syringe, and the

solution heated at re ux under N₂ for 24 h. After this sist of the (2S,3S)-2- uoro-3-methylpentanoylox y chiral

moiety and an oxyethylene spacer unit. MD112Fs and reaction time, 1H NMR analysis indicated that the

hydrosilation reaction was completed. The solution was MD212Fs exhibited similar liquid crystal behaviours with a cholesteric–chiral smectic C–chiral smectic F sequence. evaporated under reduced pressure to give crude yellow

powder; this product was further puri ed by precipitation No chiral smectic F phase was observed in MD312Fs. The phase assignment was made by POM and XRD. with methanol, and dried under vacuum.

An optical polarized micrograph reveals an oily streak texture at 80ßC which corresponds to a cholesteric

3. Results and discussion

3.1. Synthesis structure. Figure 1 presents the temperature-dependen t

XRD diagrams obtained from a powder sample of The synthetic routes for FLCs and FLCPs are

out-lined in schemes 1–4. Chemical structures of the com- MD312Fs at 30, 40, 50, 60, and 70ßC. Upon further cooling from the cholesteric phase, a sharp low angle pounds were identi ed by 1H NMR and 19F NMR. In

the 1H NMR spectra, the chemical shift appearing at re ection (associated with the smectic layer) and a broad wide angle re ection (associated with lateral packings) 4.9 ppm can be associated with the CHF proton of

the chiral centre of MDn12Fs. The peak appearing were observed. Curve A exhibits a diŒused re ection at 4.81 AÃ and a very weak re ection at 28.26 AÃ , which at 5.1 ppm can be assigned to the CHF proton of

the chiral centre of MD312Fr. The peak appearing at correspond to smectic layers. Moreover, the d-spacing

of the rst order re ection reduces from 28.36 to 27.26 AÃ 4.21 ppm can be assigned to the CHBr proton of the

Scheme 4. The polysiloxane series PS312Fr, PS312Fs, PS312C and PS312Bs.

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371

Table 2. Phase transitions and phase transition enthalpies for compounds MDn12Fs, MD312Fr, MDn12C and MDn12Bs:

Cr5 crystalline phase, SmX5 high order smectic phase, SmF*5 chiral smectic F phase, SmC*5 chiral smectic C phase, Ch5 cholesteric phase, I5 isotropic phase.

Heating

CoolingPhase transition temperature/ßC; (corresponding enthalpy changes/mJ mgÕ 1)

Compound na MD112Fs 1 Cr 17.6 (0.2) SmX 60.3 (0.1) SmF* 78.3 (1.2) SmC* 126.1 (5.6) Ch 168.5 (0.8) I I 166.9 (0.9) Ch 123.9 (5.8) SmC* 76.7 (1.5) SmF* 57.6 (0.1) SmX 17.6(Õ ₎bCr MD212Fs 2 Cr 44.6 (18) SmF* 48.2(Õ ) SmC* 100.5 (5.8) Ch 126 (0.6) I I 124.6 (0.7) Ch 98.7 (6.1) SmC* 20.8 (0.2) SmF* 8.7 (7.1) Cr MD312Fs 3 Cr 27.7 (14.6) SmC* 70.6 (5.1) Ch 83.9 (0.4) I I 81.3 (0.2) Ch 67.0 (4.9) SmC* 20.8 (0.3) SmXÕ _{6.8 (2.1) Cr} MD312Fr 3 Cr 29.1 (12.9) SmF* 42.9 (0.8) SmC* 82.7 (4.9) Ch 106.9 (5) I I 105.5 (0.5) Ch 80.9 (4.8) SmC* 40.2 (0.4) SmX 12.0 (10.5) Cr MD112C 1 Cr 29.9 (7.4 (SmX 57.1 (1.4) SmC* 113.2 (7.2) Ch 141.7 (0.9) I I 142.2 BPII 141.8 BPI 136 (1.1) Ch 112.1 (7.3) SmC* 53.9 (1.9) SmF* 23.9 (1.0) Cr MD212C 2 CrÕ 9.2 (4.2) SmC* 88.3 (7.3) Ch 103.5 (0.8) I I 102 BPII 96.6 BPI 90.2 (1.8) Ch 85.9 (7.8) SmC*Õ _{13.9 (5.8) Cr} MD312C 3 Cr 25.26 (Õ )bSmC* 57.59 (5.3) BP 63.5 (0.2) I I 63.77 (0.3) BP 58.6 (5.3) SmC*Õ _28.13(Õ ₎bCr MD112Bs 1 Cr 28.6 (8.9) SmF* 67.6 (33.3) SmC* 104.7 ( 5.8) Ch 138.2 (0.7) I I 136.9 (0.6) Ch 102.9 (5.9) SmC* 54.6 (0.6) SmF* 23.7 (3.2) Cr MD212Bs 2 CrÕ 10.0 (2.1) SmF* 25(Õ )bSmC* 81.3 (5.3) Ch 101.15 (0.8) I I 99.2 (0.6) Ch 77.7 (5.6) SmC* 23.1 (0.3) SmF*Õ _{13.6 (0.5) Cr} MD312Bs 3 CrÕ 25.4 (0.2) SmC* 53.85 (4.2) Ch 65.31 ( 0.4) I I 63.84 (0.4) Ch 51.31 (4.2) SmC*Õ _{28.6 Cr}

an corresponds to spacer length.

bEnthalpies were too small to be evaluated.

(curve A to curve E) as the temperature of measurement 3.2.2. MD312Fr

This compound contained the (2R,3S)-2- uoro-3-decreases from 70 to 30ßC. The temperature dependence

of the layer spacing for MDn12Fs is presented in gure 2. methylpentanoylox y chiral moiety and three oxyethylene spacer units. A cholesteric–chiral smectic C–chiral In one example, the optical polarizing micrograph of

MD312Fs reveals a striated fan texture ( gure 3 ) from smectic F liquid crystal sequence was found. Upon cooling from the cholesteric phase, the chiral smectic C 27.7 to 70.6ßC. These results imply the presence of a

tilted SmC* phase. In gure 4, the tilt angle is plotted phase, and chiral smectic F with striated fan textures were formed. The (2R,3S)-2- uoro-3-methy lpentanoylox y

as a function of temperature in the chiral smectic C

phase; it decreased with increasing temperature. The tilt chiral moiety showed a poorer mesomorphic behaviour as compared with MD312Fs.

angles were calculated from equation (1);d_SmC*_(Max) was chosen to be the maximum layer spacing of the chiral

smectic C phase. _3.2.3. _{MDn12C series}

The MDn12C series contained the

(2S,3S)-2-chloro-h5 cosÕ 1

A

dSmC*

d_SmC*_(Max)

B

. (1 ) _{3-methylpentanoylox y chiral moiety and oxyethylene} spacer units (n5 1–3) [14]. The three compounds in this series are all mesomorphic; all three compounds The temperature angle of the chiral smectic phase

was about 50ßC. From gure 5, it can be seen that the display the chiral smectic C phase. The mesophase ranges of MDn12C and MDn12Fs are shown in table 2. increasing number of oxyethylene units signi cantly

depresses the phase transition temperature [15]. This A wider chiral smectic C phase and lower clearing temperature for MDn12C were observed. MD112Fs depression has been attributed to the increased exibility

of the C–O bonds. and MD212Fs are more inclined to form the ordered

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Figure 1. X-ray diŒraction measurements for MD312Fs.

Figure 3. Optical polarizing micrograph of MD312Fs showing the chiral smectic C phase at 50ßC (400Ö ).

3.2.4. MDn12Bs series

The MDn12Bs series contained the

(2S,3S)-2-bromo-3-methylpentanoylox y chiral moiety and oxyethylene Figure 2. Layer spacing as a function of temperature in the _{spacer units (}_n₅ _{1–3). MD112Bs and MD212Bs exhibited}

chiral smectic C phase of compounds MDn12Fs.

similar liquid crystal behaviours with a cholesteric– chiral smectic C–chiral smectic F liquid crystal sequence. The chiral smectic F phase was not observed for the chiral smectic F phase as compared with MD112C

and MD212C. This implies that the decreasing dipole compound with longest spacer length, MD312Bs. XRD measurements and POM veri ed the assignment of moment and increasing atomic size of the halogen in

the chiral moiety can decrease the clearing temperature the mesophases for these compounds. For compounds MDn12Bs, a sharp low angle re ection and a broad wide

and widen the chiral smectic C phase.

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373

Figure 6. Optical polarizing micrograph of MD112Bs showing the chiral smectic C phase at 75ßC (400Ö ).

Figure 4. Tilt angles as a function of temperature in the chiral smectic C phase of compounds MDn12Fs.

Figure 7. Plots of transition temperature versusn, the number of oxyethylene spacer units for compounds MDn12Bs. series is higher than for the MDn12Bs series. This may

result from the increase in dipole moment and decrease in molecular size of the asymmetric chiral center [16]. Figure 5. Plots of transition temperature versusn, the number

of oxyethylene spacer units for compounds MDn12Fs. _3.2.5. _{Polymer series PS312Fr, PS312Fs, PS312C and} PS312Bs

The phase sequences and corresponding transition angle re ection angle were observed in XRD diagrams;

these were respectively assigned to lateral packing and temperatures for the (n5 3) members of these series are shown in table 3. Polymers PS312Fs, PS312Fr, and the smectic layer below the cholesteric point. Moreover,

the tilt angle decreases as the temperature increases. In PS312C exhibited similar liquid crystal behaviour, with cholesteric and chiral smectic C phases on the heating one example, the optical polarizing micrograph reveals

a striated fan texture ( gure 6) from 67.6 to 104.7ß C for and cooling scans. XRD measurement for PS312Fs and PS312Fr show a smectic diŒraction pattern (a sharp low MD112Bs. This indicates the formation of the tilted

chiral smectic C phase. The temperature range of the angle re ection and a broad wide angle re ection). Layer spacings were plotted as a function of temperature in chiral smectic phase was about 80ßC for MD312Bs. The

clearing temperature and phase transition temperature the chiral smectic C phase ( gure 8); the layer spacing increases as the temperature increases, con rming the decreased as the number of oxyethylene units increased

( gure 7) . Moreover, the clearing point of the MDn12Fs presence of the chiral smectic C phase. On the other

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Table 3. Phase transitions and phase transition enthalpies _{more bulky molecular size of asymmetric chiral centre} for polymers PS312Fs, PS312Fr, PS312C, and PS312Bs: _{[16], which reduces the phase stability of ferroelectric} g5 glassy state, SmF*5 chiral smectic F phase, SmC*5 _{liquid crystal homopolymer.}

chiral smectic C phase, Ch5 cholesteric phase, I5 isotropic phase.

4. Conclusion

A new series of ferroelectric liquid crystal monomers Heating

CoolingPhase transition temperature/ßC;

Polymer na _{and polymers consisting of a halogenated chiral centre,}

(corresponding enthalpy changes/mJ mgÕ 1) oligooxyethylen e spacers, and an ester core unit con-taining three aromatic rings have been synthesized. All PS312Fr 3 g 25.9 SmC* 139.4 Ch 160.5 I _{of the compounds exhibit the chiral smectic C phase} I 150.5 Ch 131.4 SmC* 24.4 g _{except PS312Bs. Wide SmC* temperature ranges were} obtained in these monomers (~90ßC) and polymers PS312Fs 3 g 28.6 SmC* 143 Ch 163 I

I 153.34 Ch 137.4 SmC* 32.4 g (~100ßC). Several MDn12Fs, MDn12Fr, and MDn12Bs

compounds (n5 1, 2) exhibited chiral smectic F phases. PS312C 3 g 17 SmC* 131.9 Ch 149.8 I

I 145.5 Ch 128.1 SmC* 12 g When the number of oxyethylene spacer units increased, the clearing and phase transition temperatures decreased.

PS312Bs 3 —

I 105.4 (0.6) Ch 7.4 (0.7) g The lack of mesophases in PS312Bs is possibly due to the bulky substituted group at the chiral centre which

a_{n corresponds to spacer length.} _{disturbs the orientation of the side chain liquid crystal}

polymer.

The authors thank the National Science Council of Republic of China for nancial support of this work (NSC 89-2216-E-007-00 6).

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L iquid Crystals (London: Taylor & Francis Ltd), p. 51.

[11] Chen, J. H., Chang, R. C., Hsiue, G. H., Guu, F. W., and Wu, S. L., 1995,L iq. Cryst., 18, 291.

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tem-Zubia, J., 1992,J. Am. chem. Soc., 114, 7645. perature ranges of the SmC* phase as compared with

[13] Collings, P. J., and Hird, M., 1997, Introduction to MD312Fr, MD312Fs, and MD312C. For PS312C, the _{L iquid Crystals (London: Taylor & Francis Ltd), p. 119.} temperature range of the chiral smectic C phase was _{[14] Hsiue, G. H., Wu, J. L., and Chen, J. H., 1996, L iq.} about 120ß C. A exible polysiloxane backbone enhances Cryst., 21, 449.

[15] Kitamura, T., Fujii, T., and Mukoh, A., 1984, Mol. the decoupling between side chain and main chain,

Cryst. liq. Cryst., 108, 333. tending to give rise to a higher thermal stability of the

[16] Collings, P. J., and Hird, M., 1997, Introduction to mesophases, including the chiral smectic C phase [17]. _{L iquid Crystals (London: Taylor & Francis Ltd), p. 120.} Sample PS312Bs exhibited fewer mesomorphic phases _{[17] Hsiue, G. H., and Chen, J. H., 1995, Macromolecules,} and no chiral smectic C phase. This may result from the 28, 4366.