J O U R N A L O F C H E M I S T R Y Materials EVects of nitro substituents on the properties of a ferroelectric liquid crystalline side chain polysiloxane Magnus Svensson,a Bertil Helgee,*a Kent Skarpb and Gunnar Anderssonb aDepartment of Polymer T echnology, Chalmers University of T echnology, S-412 96 Go�teborg, Sweden bDepartment of Physics, Chalmers University of T echnology, S-412 96 Go�teborg, Sweden The syntheses of chiral liquid crystalline side chain polysiloxanes with lateral nitro substituents in the mesogenic core are described. The influence of the substituents and substituent positions on phase behaviour and electro-optical properties are investigated and compared.The lateral nitro groups strongly aVect the phase behaviour of the side chain precursors as well as the liquid crystalline polymers.Properties in smectic A and C* phases are discussed with respect to substituent position. One side chain precursor exhibits very large spontaneous polarization of ~700 nC cm-2. The possibility of designing and using chiral liquid crystalline acid moiety. The nitro group is introduced at the 2- and 3- side chain polymers for technical applications in the future position in the phenyl ester and at the 3¾-position in the depends on the understanding of structure–property relation- biphenyl part.The side-chain precursors are attached to a ships. A vast number of new molecules, architectures and poly(dimethyl-co-methylhydrogen)siloxane backbone. It was concepts of chiral liquid crystalline side chain polymers have found that the introduction of substituents changes the phase been synthesized, examined and reported1,2 since the first behaviour dramatically depending on position.One of the ferroelectric liquid crystalline polymer was prepared by Shibaev side-chain precursors only exhibits a 10 °C temperature interval et al.3 in 1984. Research on the eVects of structural changes in of the N*-phase but in the corresponding polymer smectic the diVerent parts of the mesogens like the chiral centre,4,5 phases over a temperature range of 70 °C are again obtained.mesogenic core6–8 and alkyl chain9–11 has been carried out. Another side-chain precursor shows very large spontaneous The influence of the polymer main-chain on properties has polarization (nearly 700 nC cm-2).25,26 The same system shows also been investigated.12–17 Nevertheless, a large part of today’s interesting NLO-properties reported earlier,27 where a controlinformation on structure–property relationships in chiral liquid lable SHG-intensity in the smectic A phase was described for crystalline polymers relies on extrapolations from research on the first time.For the reference system (without nitro substitulow molar mass liquid crystals performed during the 1970s.ent), both the low molar mass mesogen and the polymeric Concerning lateral substituents Osman18 has shown that liquid crystal have been described in detail previously.24,28 steric eVects and thereby the van der Waals volume of the substituent are usually more important with respect to phase Experimental behaviour than dipolar interactions.The more detailed influences of lateral substituents depend on the structure of the Techniques rigid core and the position of the substituent. Generalizations 300 MHz 1H NMR spectra were obtained using a Varian are diYcult, since the eVects diVer between polar and non- VXR300 spectrometer. All spectra were run in CDCl3 or [2H6] polar mesogens and probably also change from low molar mass to polymer liquid crystals.Using electron attracting DMSO solutions. Infrared spectra were recorded on a Perkinsubstituents Masuda et al.19 showed that substituents in the Elmer 2000 FT-IR spectrophotometer using KBr pellets. The centre of the mesogenic core not only increased the intermol- phase behaviour of the diVerent materials was identified by ecular distance, thus destabilizing the liquid crystal behaviour, combining optical microscopy, diVerential scanning calorbut also hindered the formation of smectic phases.With imetry and electro-optical measurements. Optical microscopy multiple substituents where the second and/or third does not was performed using a Mettler FP82HT hot stage, Mettler increase the steric eVect but adds possibilities for polar inter- FP80HT central processor and an Olympus BH-2 polarizing actions the smectic phases can be regained.20 A study by Hird microscope.DSC measurements were recorded on a Perkinet al.21 shows the same tendencies. Elmer DSC 7 diVerential scanning calorimeter. The nitro group with its large dipole moment and electron attracting power oVers a way to drastically change the electron Materials distribution in the aromatic core.This has been used to Materials and reagents were of commercial grade quality and enhance NLO-eVects such as second harmonic generation used without further purification unless otherwise noted. Dry (SHG) in liquid crystals and in liquid crystalline polymers.22,23 toluene and methylene chloride were obtained by passing the In the present paper we have studied the eVect of introducing solvents through a bed of aluminium oxide (ICN Alumina a nitro substituent at various aromatic positions in a 4¾- N-Super I).(+)-2-(4-Hydroxyphenoxy)propionic acid was gen- alkoxybiphenyl-4-carboxylic acid phenyl ester mesogen. The erously provided by BASF. Poly(dimethylsiloxane-co-methyl- nitro group was chosen to increase the spontaneous polarizhydrosiloxane) with a copolymer ratio 2.7/1 and a degree of ation in the chiral smectic C phase.We were particularly polymerization of ca. 30 (according to manufacturer) and interested in how a strong dipole at diVerent locations in the the hydrosilylation catalyst dicyclopentadienylplatinum(II) polarizable aromatic core would change phase behaviour and dichloride were obtained from Wacker Chemie.Size-exclusion electro-optical properties of a ferroelectric liquid crystalline chromatography (SEC) analysis in chloroform of the copoly- polymer24 having a very broad smectic C* phase and good alignment properties. The chiral group is a substituted lactic mer performed at 30 °C on a Waters WISP712 instrument J.Mater. Chem., 1998, 8(2), 353–362 353HO CH3O CH3 O O CH3O O OH CH3O O OH NO2 HO O OH NO2 HO O OH O O OH O O OH NO2 i ii iii iv v vi v vi 1 2 3 4 5b 5a 6a 6b Scheme 1 Reagents: i, MeI, KOH, DMF; ii, AcCl, AlCl3, CH2 Cl2; iii, Br2, KOH, 1,4-dioxane; iv, HNO3, AcOH; v, HBr, AcOH; vi, undecenyl bromide, KI, EtOH equipped with three commercial Styragel columns and Waters boxylic acid 3 following closely the procedure reported by Percec et al.29 410 refractive index detector gave M : n=2.6·103 g mol-1 and M : w=5.4·103 g mol-1 with reference to polystyrene standards. 4¾-Methoxy-3¾-nitrobiphenyl-4-carboxylic acid 4. 4¾- Methoxybiphenyl-4-carboxylic acid 3 (0.01 mol, 2.28 g) in Synthesis acetic acid (50 ml ) was refluxed with concentrated nitric acid The synthesis of polymers 14a–d was carried out according to (6 ml) for 15 min.The reaction mixture was poured into water the reactions in Schemes 1–4. All structures were verified by and the precipitate was filtered oV. The product recrystallized 1H NMR spectroscopy and NMR data were in accordance from ethanol to yield 2.2 g (80%). dH ([2H6] DMSO) 3.98 (s, with the structures in all cases. 3 H), 7.48 (d, 1 H), 7.85 (d, 2 H), 8.02 (d, 2 H), 8.07 (dd, 1 H), 8.26 (d, 1 H). 4¾-Hydroxybiphenyl-4-carboxylic acid 5a. The acid was synthesized from 4-hydroxybiphenyl via 4-methoxybiphenyl 1, 4- 4¾-Hydroxy-3¾-nitrobiphenyl-4-carboxylic acid 5b. The acid 5b was synthesized as for 5a starting from 4. dH ([2H6] DMSO) acetyl-4¾-methoxybiphenyl 2 and 4¾-methoxybiphenyl-4-car- O Cl OH HO O O OH O O OH NO2 + HO O OH O * O O O NO2 O O * HO O OH O * NO2 HO O O O * NO2 HO O O O * HO O O O * HO O OH O * NO2 NO2 12a 11 12c 9 10 12b 7 8 i v v ii ii iii iv v Scheme 2 Reagents: i, pyridine, CH2Cl2, N2; ii, HNO3, AcOH, <35 °C; iii, (S)-ethyl lactate, PPh3, diethyl azodicarboxylate, THF, N2; iv, aq.KOH, EtOH; v, BuOH, HCl(g) 354 J. Mater. Chem., 19982), 353–3626a + 12a i (CH2)9 O O O O O H3C O * 13a 6a + 12b i (CH2)9 O O O O O H3C O * 13b NO2 6a + 12c i (CH2)9 O O O O O H3C O * 13c NO2 6b + 12a i (CH2)9 O O O O O H3C O * 13d NO2 Scheme 3 Reagents: i, DCC, DMAP, CH2Cl2 7.25 (d, 1 H), 7.81 (d, 2 H), 7.95 (dd, 1 H), 8.00 (d, 2 H), 8.22 (d, 1 H), 11.3 (s, 1 H). 4¾-( Undec-10-enyloxy)biphenyl-4-carboxylic acid 6a. The hydroxy-acid 5a (0.090 mol, 19.3 g) was dissolved in hot ethanol (1500 ml) and water (75 ml ) together with potassium hydroxide 86% (11.5 g) and a few crystals of potassium iodide.Undec-10-enyl bromide (0.18 mol) was added and the mixture was refluxed for 24 h. To hydrolyse any ester formed a 10% solution of potassium hydroxide in 70% ethanol was added and refluxing was continued for an additional 3 h.The reaction mixture was allowed to cool and was acidified with concentrated hydrochloric acid. The solid formed was recrystallized from acetic acid and ethanol to yield 22.2 g of product (69%). dH ([2H6] DMSO) 1.2–1.5 (m, 12 H), 1.72 (quintet, 2 H), 2.00 (q, 2 H), 4.00 (t, 2 H), 4.96 (m, 2 H), 5.79 (m, 1 H), 7.03 (d, 2 H), 7.67 (d, 2 H), 7.75 (d, 2 H), 7.98 (d, 2 H). 3¾-Nitro-4¾-(undec-10-enyloxy)biphenyl-4-carboxylic acid 6b.The acid 6b was synthesized as for 6a starting from 5b. dH ([2H6] DMSO) 1.2–1.5 (m, 12 H), 1.73 (quintet, 2 H), 2.00 (q, 2 H), 4.21 (t, 2 H), 4.96 (m, 2 H), 5.78 (m, 1 H), 7.47 (d, 1 H), O Si O Si CH3 H CH3 CH3 n m O Si O Si CH3 CH3 CH3 n m + 13a–d m/ n = 2.7 O O O O O O H3C * NO2 14a–d Scheme 4 7.85 (d, 2 H), 7.98–8.06 (m, 3 H), 8.24 (d, 1 H). 4-Hydroxyphenyl benzoate 7. A mixture of hydroquinone (0.15 mol, 16.5 g) and pyridine (0.15 mol) in dry methylene concentrated nitric acid (11 ml ) was added dropwise. The temperature of the reaction mixture was kept below 35 °C. chloride was stirred at room temp. Nitrogen was bubbled through and benzoyl chloride (0.14 mol, 19.6 g) was added After 50 min the reaction mixture was poured into 700 ml of water.The precipitate was collected and purified on a silica slowly. After 2 h additional stirring the solvent was evaporated. The solid was washed with water to remove unreacted hydro- gel column using light petroleum–ethyl acetate (251) as eluent. Yield 5.28 g (51%). dH (CDCl3) 7.24 (d, 1 H), 7.49 (dd, 1 H), quinone and then treated with diethyl ether to dissolve the product; the diester remained as it is less soluble in diethyl 7.55 (t, 2 H), 7.67 (m, 1 H), 8.01 (d, 1 H), 8.19 (dd, 2 H), 10.5 (s, 1 H).ether. After evaporation of solvent the product was recrystallized from ethanol with increasing amounts of water. Yield 18.0 g (60%). dH ([2H6] DMSO–CDCl3) 6.87 (d, 2 H), 7.00 (d, 3-Nitro-4-[(1R)-1-ethoxycarbonylethoxy]phenyl benzoate 9 [2-(4-benzoyloxy-2-nitrophenoxy)propanoic acid ethyl ester]. 3- 2 H), 7.52 (t, 2 H), 7.65 (t, 1 H), 8.16 (d, 2 H), 9.1 (s, 1 H). Nitro-4-hydroxyphenyl benzoate 8 (0.016 mol, 4.15 g), S-ethyl lactate (0.016 mol, 1.89 g) and triphenylphosphine (0.020 mol) 3-Nitro-4-hydroxyphenyl benzoate 8. To hydroquinone monobenzoate 7 (0.04 mol, 8.6 g) in 100 ml of acetic acid, were placed in dry glass equipment with dry THF (100 ml) as J.Mater. Chem., 1998, 8(2), 353–362 355solvent. N2 gas was bubbled through the mixture at room (DMAP) in dry methylene chloride was treated with 4 mmol (0.83 g) of dicyclohexylcarbodiimide (DCC) at 0 °C. The tem- temp. and diethyl azodicarboxylate (0.020 mol) was added during 30 min. After 3 h of additional stirring at room temp.perature was allowed to rise to room temp. and stirring was continued for 3 h. Urea was filtered oV and the filtrate was the solvent was evaporated from the reaction mixture. The resulting solid was dissolved in ethyl acetate and on adding evaporated to dryness. Column chromatography on silica gel with light petroleum–ethyl acetate (951) as eluent gave 1.0 g four times the volume of light petroleum, triphenylphosphine oxide was precipitated.The solution was evaporated to dryness of pure product (80%). [a]D22-28.7 (CHCl3). dH (CDCl3) 0.89 (t, 3 H), 1.2–1.5 (m, 14 H), 1.6 (m, 2H), 1.69 (d, 3 H), 1.80 and the product recrystallized from light petroleum, 4.76 g (83%). dH (CDCl3) 1.26 (t, 3 H), 1.71 (d, 3 H), 4.23 (m, 2 H), (quintet, 2 H), 2.02 (q, 2 H), 4.00 (t, 2 H), 4.15 (m, 2 H), 4.82 (q, 1 H), 4.95 (m, 2 H), 5.80 (m, 1 H), 6.98 (d, 2 H), 7.03 (d, 1 4.84 (q, 1 H), 7.05 (d, 1 H), 7.40 (dd, 1 H), 7.53 (t, 2 H), 7.67 (t, 1 H), 7.79 (d, 1 H), 8.18 (d, 2 H).H), 7.38 (dd, 1 H), 7.58 (d, 2 H), 7.68 (d, 2 H), 7.78 (d, 1 H), 8.18 (d, 2 H). 2-(2R)-(4-Hydroxy-2-nitrophenoxy)propanoic acid 10. To 0.008 mol (2.87 g) of 9 in ethanol (120 ml), 0.016 mol of 4-[(1R)-1-Butoxycarbonylethoxy]-2-nitrophenyl 4¾-(undec- 10-enyloxy)biphenyl-4-carboxylate 13c.From 6a and 12c as for potassium hydroxide in a few millilitres of water was added. The mixture was stirred overnight at room temp. Ethanol was 13b. [a]D22+21.7 (CHCl3). dH (CDCl3) 0.93 (t, 3 H), 1.24–1.43 (m, 12 H), 1.48 (m, 2 H), 1.64 (m, 2 H), 1.69 (d, 3 H), 1.82 evaporated and the product was extracted from methylene chloride with water.Water was evaporated and the solid was (quintet, 2 H), 2.05 (q, 2 H), 4.02 (t, 2 H), 4.21 (t, 2 H), 4.82 (q, 1 H), 4.97 (m, 2 H), 5.82 (m, 1 H), 7.01 (d, 2 H), 7.23 (dd, dissolved in ethyl acetate. The insoluble part was filtered oV and the filtrate was evaporated to dryness and used without 1 H), 7.30 (d, 1 H), 7.6 (d+d, 3 H), 7.70 (d, 2 H), 8.22 (d, 2 H).further purification. dH ([2H6] DMSO) 1.47 (d, 3 H), 4.89 (q, 1 H), 7.01 (dd, 1 H), 7.07 (d, 1 H), 7.19 (d, 1 H), 9.9 (s, 1 H). 4-[(1R)-Butoxycarbonylethoxy]phenyl-3¾-nitro-4¾-(undec-10- enyloxy)biphenyl-4-carboxylate 13d. From 6b and 12a as for 13b. [a]D22+16.0 (CHCl3). dH (CDCl3) 0.92 (t, 3 H), 1.2–1.55 2-(2R)-(4-Hydroxy-3-nitrophenoxy)propanoic acid 11.To a solution of 2-(2R)-(4-hydroxyphenoxy)propanoic acid (m, 16 H), 1.64 (d, 3 H), 1.88 (quintet, 2 H), 2.05 (q, 2 H), 4.17 (m, 4 H), 4.75 (q, 1 H), 4.97 (m, 2 H), 5.82 (m, 1 H), 6.94 (d, 2 (0.027 mol, 5.0 g) in acetic acid (100 ml) concentrated nitric acid (1.5 ml) was added dropwise. The temperature of the H), 7.14 (d, 2 H), 7.18 (d, 1 H), 7.69 (d, 2 H), 7.80 (dd, 1 H), 8.13 (d, 1 H), 8.26 (d, 2 H).reaction mixture was kept below 35 °C. After 30 min the reaction mixture was poured into water. The product was Polymers 14a–d. Dry equipment was used. 0.26 g of poly(di- extracted from the aqueous phase using diethyl ether. No methyl-co-methylhydrogen)siloxane (2.751) (corresponding to further purification was done. Yield 5.1 g (80%).dH ([2H6] 1.0 mmol Si-H) and 1.1 mmol of 13a-d were dissolved in dry DMSO) 1.53 (d, 3 H), 4.89 (q, 1 H), 7.11 (d, 1 H), 7.24 (dd, 1 toluene (2 ml). Catalyst solution [0.8 ml; dicyclopentadienyl- H), 7.39 (d, 1 H), 10.6 (s, 1 H). platinum(II) chloride in dry toluene; 0.1 mg ml-1] was added and the reaction flask was sealed with a septum and heated to Butyl 2-(2R)-(4-hydroxyphenoxy)propanoate 12a.A solution 110 °C. After 2 d an additional 0.8 ml of catalyst solution was of 2-(2R)-(4-hydroxyphenoxy)propanoic acid (0.01 mol, 1.9 g) added and heating was continued. The reaction was monitored in butanol (150 ml) was treated with hydrogen chloride gas by IR spectroscopy and further addition of catalyst continued until no further heat was evolved. The reaction mixture was until the SiMH signal at ca. 2155 cm-1 was constant relative evaporated to dryness. The product was separated by column to the CNO signal at ca. 1740 cm-1. The reaction mixture chromatography on silica gel with light petroleum–ethyl acetwas added dropwise to methanol (400–800 ml) during vigorous ate (251) as eluent. Yield 1.6 g (64%). [a]D22+33.9 (CHCl3). stirring. The product was collected by centrifugation.dH (CDCl3) 0.88 (t, 3 H), 1.30 (m, 2 H), 1.6 (d+m, 5 H), 4.14 Reprecipitation from chloroform in methanol was continued (m, 2 H), 4.65 (q, 1 H), 6.72 (m, 4 H). until no free side-chain could be detected by thin layer chroma- Esters 12b and c were synthesized as for 12a starting from tography. Treatment of the product by dissolving it in ethyl 10 and 11 respectively.acetate and passing the solution through a 0.2 mm Teflon filter often reduced the amount of remaining SiMH drastically. Yield Butyl 2-(2R)-(4-hydroxy-2-nitrophenoxy)propanoate 12b. 0.5–0.8 g. 1H NMR spectra of the polymers show broader [a]D22-84 (CHCl3). dH (CDCl3) 0.90 (t, 3 H), 1.31 (m, 2 H), signals than those of the side chain precursors, but are other- 1.60 (m, 2 H), 1.66 (d, 3 H), 4.16 (m, 2 H), 4.74 (q, 1 H), 6.94 wise very similar except that the signals from methyl groups (d, 1 H), 6.97 (dd, 1 H), 7.32 (d, 1 H).attached to silicon at d 0–0.16 and a methylene group attached to silicon at d 0.5 are present, while the olefinic protons at d Butyl 2-(2R)-(4-hydroxy-3-nitrophenoxy)propanoate 12c. 4.95–4.97 and d 5.80–5.82 are absent.A SiMH signal appeared [a]D22+77.1 (CHCl3). dH (CDCl3) 0.92 (t, 3 H), 1.35 (m, 2 H), at d 4.7, equivalent to 10–15% unreacted hydrogens. 1.64 (d+m, 5 H), 4.18 (m, 2 H), 4.73 (q, 1 H), 7.10 (d, 1 H), 7.27 (dd, 1 H), 7.48 (d, 1 H), 10.3 (s, 1 H). 14a [a]D22+13.8 (CHCl3). The last reaction step to the mesogenic side-chain precursors 14b [a]D22-10.0 (CHCl3). 13a–d was similar for all and is described for 13b. 14c [a]D22+15.3 (CHCl3). 4-[(1R)-Butoxycarbonylethoxy]phenyl 4¾-(undec-10- 14d [a]D22+10.0 (CHCl3). enyloxy)biphenyl-4-carboxylate 13a. From 6a and 12a as for 13b. [a]D22+18.0 (CHCl3). dH (CDCl3) 0.90 (t, 3 H), 1.2–1.5 (m, 14 H), 1.6 (d+m, 5 H), 1.81 (quintet, 2 H), 2.04 (q, 2 H), 4.00 (t, 2 H), 4.17 (m, 2 H), 4.74 (q, 1 H), 4.97 (m, 2 H), 5.81 Sample preparation and electric measurements (m, 1 H), 6.92 (d, 2 H), 7.00 (d, 2 H), 7.13 (d, 2 H), 7.58 (d, 2 H), 7.68 (d, 2 H), 8.21 (d, 2 H).For the study of physical properties such as electro-optic eVects in smectic liquid crystals, the achievement of well aligned samples is essential. Our standard shear cell,30 built for 4-[(1R)-Butoxycarbonylethoxy]-3-nitrophenyl 4¾-(undec-10- enyloxy)biphenyl-4-carboxylate 13b.From 6a and 12b. Dry measurements on low molar mass ferroelectric liquid crystals, has proved very useful for obtaining aligned polymer samples glassware was used. A mixture of 2 mmol (0.73 g) of 12b, 2 mmol (0.57 g) of 6a and 15 mg of dimethylaminopyridine and was used for all polymeric liquid crystals and some low 356 J. Mater. Chem., 1998, 8(2), 353–362Fig. 1 Experimental set-up and cell geometry molar mass liquid crystal compounds in this study. The sample Fig. 2 Phase behaviour of side-chain precursors and polymers is applied to the lower glass plate and melted into the isotropic phase to cover the whole glass area. Then the upper glass plate is put on and the shear cell is assembled. Orientational shear third chiral group (compound 12b) was synthesized via a was applied in the smectic A phase close to the isotropic phase.Mitsunobu reaction of (S)-ethyl lactate with nitrohydroquinone A special electrode pattern ensures that the active area is monobenzoate. The reaction was performed under standard always 16.8 mm2, independent of shear position. Spacers con- Mitsunobu conditions and proceeds with inversion of consist of thermally evaporated 2 mm SiO2, and the orientation of figuration.31 In the final step of the side chain precursor the smectic layers is such that the layer normal (k� ) is perpen- synthesis, the biphenylcarboxylic acids were esterified with the dicular to the shear direction, cf.Fig. 1. A 1000 A° protective chiral propanoic ester derivatives using dicyclohexylcarbodiim- SiO2 layer covers the ITO electrodes.The shear alignment cell ide (Scheme 3). The side chain liquid crystalline polymers were is mounted in a Mettler FP52 hot stage for temperature finally obtained by a hydrosilylation reaction (Scheme 4). The control and put in a Zeiss Photomicroscope equipped with a extent of reaction was monitored by FT-IR. Attempts to fast electro-optic recording system.The sample temperature is quantify the optical purity of the mesogens by 1H NMR using independently measured by a Pt 100 resistor element placed a chiral shift reagent, (+)-praseodymium tris (3-heptafluorobuin the sample holder close to the sample. For some of the low tyrylcamphorate), have been made. Addition of the chiral shift molar mass samples good alignment could also be achieved in reagent caused no observable separation of peaks, although standard commercial surface-coated cells with 4×4 mm active this has been reported by others.32 Materials up to three years area and 4 mm thickness, obtained from EHC.old show no reduction in the specific optical rotations due to The measurement of the spontaneous polarization Ps was racemization.done with either the bridge method or the triangular wave method. The measured polarization is taken from a series of Liquid crystal properties hysteresis curves, where the reading of the amplitude of the The system used as reference24 (13a, 14a) is easy to handle loop directly gives the spontaneous polarization by eqn. (1), during physical measurements and shows typical ferroelectric behaviour.The introduction of a strong electron attracting Ps= 1 2A kDU G C1 (1) group at diVerent aromatic positions in the mesogenic core was thought to give indications of the importance of electric where A is the active sample area, k is a correction factor for dipoles in the mesogen. The influence on spontaneous polarizthe SiO2 protective layer (k=1.1 for the shear cell glasses), G ation in the chiral smectic C phase was of special interest.is the gain of the instrumental amplifier, DU is the amplitude of the loop and C1 is the reference capacitor in series with the Phase behaviour. The nitro substituent aVected the phase ferroelectric LC sample. In order to measure the smectic tilt behaviour markedly for the low molar mass materials as can angle H in the C* phase, we applied a low frequency square be seen in Table 1 and Fig. 2.The lateral substituent lowers wave field and determined the two extinction orientations by the clearing temperature in all cases, a well-known eVect. rotating the sample between crossed polarizers. To investigate Furthermore the nitro group alters the type of liquid crystalline the electroclinic eVect, measurements of induced tilt angle and phase that appears, depending on substituent position.Only optical response time were carried out as a function of applied in mesogen 13b is the smectic C* phase retained and a field and temperature. comparison of ferroelectric behaviour can be made with the reference system, 13a. In mesogen 13c the smectic phases are Results and Discussion lost and only in a narrow temperature interval does a chiral nematic phase remain.The nitro group ortho to the central Synthesis ester linkage probably induces greater order and thereby causes crystallization. This can be explained by an increase in attract- The synthetic route to these side chain liquid crystalline polymers consists of four parts as already shown in Schemes ive forces by dipole–dipole interactions between mesogens or by a decrease in intramolecular mobility on introduction of 1–4.The synthesis of 4¾-hydroxybiphenyl-4-carboxylic acid9,29 and its etherification also including the 3¾-nitro derivatives the nitro group in this position. The observed drastic lowering of the clearing temperature would at first make the former were straightforward (see Scheme 1).Scheme 2 shows the route to the chiral groups of which two were made from (+)-2-(4- explanation less probable but dipole–dipole interactions are known to be strongly temperature dependent33 and rotation hydroxyphenoxy)propionic acid kindly supplied by BASF. The J. Mater. Chem., 1998, 8(2), 353–362 357about the molecular long axis could be restricted by steric or electronic interactions involving the nitro group or by an increased moment of inertia.The location of the substituent towards the centre of the mesogenic core hinders the formation of layered phases. With a diVerent chiral group Walba22 described similar behaviour although the smectic phases were not completely lost. With the nitro group in the biphenyl as in 13d a broad smectic A phase and aarrow nematic region are observed (Fig. 2). Compared to 13a the core is broadened by the nitro substituent which according to Goodby disfavours formation of a smectic C phase.34 It also reduces the anisotropy of molecular polarizability and increases the polarizability perpendicular to the long molecular axis. This is known to diminish liquid crystalline order,6 as is also seen in system 13d.The changes in phase behaviour when the side-chain precursors are attached to the polysiloxane backbone follows the general expectations. All clearing temperatures are raised by 25–45 °C and crystallization is prohibited (Fig. 2). The stability of the smectic phases is increased at the expense of the nematic phases which disappears.In the polysiloxane systems, microphase separation of main chain and side chains exerts an extra Fig. 3 Spontaneous polarization of side-chain precursors versus tem- driving force for this formation of layered structures. In the perature with reference to SA–SC* phase transition: (+) 13a and (#) 13b case of 13b the smectic C* phase cannot be detected when going to polymer 14b which is somewhat unexpected.Naciri et al.26 reported a smectic C* phase ranging from Tg to 37 °C which have the polar lateral substituents at the end of the stiV core. The formation of distinct smectic layers is more favour- for a polymer similar to 14b with one methylene unit less in the spacer and a diVerent co-polymerization ratio in the able than polar interactions between mesogens giving a small overlap of the stiV cores.The intermesogenic interactions are siloxane backbone. It is possible that polymer 14b possesses a smectic C* phase but that it has a very short helical pitch reduced and a nitro substituent at the end of the core in the polymeric liquid crystals would favour a smectic A phase over which we have not succeeded in unwinding. Therefore the sample behaves as a smectic A phase.Comparing polymer 14a a smectic C*. In the case of 14c the formation of distinct smectic layers arranges the mesogens to give large overlap of without a lateral substituent in the mesogenic core with 14b–d containing a nitro group, it can be seen that the clearing the stiV cores although a disturbing substituent is positioned in the central part of the core. Without the restricting polymer temperatures are lowered by the substituent eVect.In 14b and 14d the isotropic transition is moved by 15 and 20 °C, respect- sublayers this substituent causes a longitudinal shift of the mesogens to give only a nematic phase, 13c. Liquid crystalline ively. In 14c the substituent in the central part of the core gives a more drastic eVect and the lowering of the transition phase behaviour seems to depend on a very delicate balance between attractive and repulsive intermesogenic forces.The temperature is 60 °C. Another striking diVerence is that 14b and 14d only exhibit very broad smectic A phases while 14c distance between the mesogens, which is a key parameter, is aVected by geometry, electron distribution and polarizability has a broad smectic C* phase as well as a smectic A phase.In order to rationalize this let us consider the layer structures of of the mesogen. All these factors change when substituents are introduced or varied and this of course complicates the under- the polymers in relation to the low molar mass mesogens. In most cases of smectic phases of low molar mass mesogens, the standing of lateral substituent eVects in liquid crystals.layer structure is not especially well defined. This is seen in Xray studies which then only give the first order reflection. The Electro-optical properties, SmC* phase. The desired evaluation of the influence on ferroelectric behaviour exerted by the mesogens are thus able to make use of the most favourable intermesogenic interactions regardless of how much the mesog- nitro group in the mesogenic core becomes rather limited since the smectic C* phase is present only in 13b and 14c of the ens overlap each other.In a polymer liquid crystal with smectic phases there is a much more distinct layer structure. This is nitro-containing systems. Nevertheless interesting observations can be made.In Fig. 3 the spontaneous polarizations (Ps) of evident from X-ray diVraction studies where many polymeric smectic A phases show more than one order of reflection.35 In 13a and 13b are shown and one particularly interesting eVect of the nitro substituent can be seen. To our knowledge a Ps a polysiloxane system the backbone is confined to sublayers between the liquid crystalline layers because of the microphase value of ca. 700 nC cm-2 is the highest reported for a system with one chiral centre. One possible explanation could be the separation. This reduces the freedom of mesogen arrangement. The intermesogenic interactions that were favourable in the increased double bond character of the phenyl ether bond caused by the nitro group in an ortho position. This restricts low molar mass smectic phase may not give the lowest energy arrangement anymore.This can be the case in 14b and 14d rotational mobility around the ether linkage. Table 1 Phases and transition temperatures material phase sequence/°C 13a SX 30 SC* 74 SA 106 I 13b -5 SC* 52 SA 70 I 13c K 35 N* 45 I 13d -5 SA 59 N* 70 I 14a -5 SC* 105 SA 130 I 14b 0 SA 115 I 14c 0 SC* 61 SA 70 I 14d -10 SA 110 I 358 J.Mater. Chem., 1998, 8(2), 353–362Fig. 6 Tilt angle of polymers versus temperature with reference to Fig. 4 Spontaneous polarization of polymers versus temperature with SA–SC* phase transition: (6) 14a and (&) 14c reference to SA–SC* phase transition: (6) 14a and (&) 14c smectic A transition polymer 14c speeds up and response times In Fig. 4 the spontaneous polarization of the polymers are as low as 10 ms are recorded.The greater temperature depen- presented and 14c shows values up to twice as high as for 14a dence of dipole–dipole interactions is a possible explanation at the same reduced temperatures. Measured tilt angles are a for these latter observations. few degrees higher for the nitro containing systems as presented in Fig. 5 and 6. When comparing spontaneous polarization Electro-optical properties, SmA phase. Some details of the and tilt angle for a system, they usually exhibit the same electro-optical behaviour in the smectic A phase have been temperature behaviour, but as is seen in the figures this does examined for the side-chain precursors 13a, b, d. In Fig. 9 the not apply for 13a and 14a.The analysis of 13c and 14c was temperature dependence of the induced tilt shows the usual limited by the easily hydrolysed central ester linkage in the steep increase close to the SA–SC* transition for 13a and b, mesogen. while 13d has a smaller temperature dependence because of Optical response times (tr) are also of interest since they the diVerent phase sequence. The variations in induced tilt relate to collective side-chain dynamics, and from Fig. 7 and 8 with applied field are shown in Fig. 10 at 15 °C below the it is clear that for 13b and 14c with a nitro substituent in the Iso–SA transition temperature. Linear dependencies were mesogen, tr have a stronger temperature dependence than 13a obtained for all three systems. and 14a, respectively. 13b with a very high Ps has response The induced tilt as a function of temperature for the polymers times in the 100 ms region while 13a is ten times faster.It is (Fig. 11) does not give the usual behaviour for 14d. The curve evident that the stronger mesogenic interactions in 13b, which can be divided into three parts with diVerent slopes. As the are responsible for the high spontaneous polarization, are slowing the reorientation of the side-chains.The polymers are slower and show tr in the millisecond range but near the Fig. 7 Response times in the smectic C* phase of side-chain precursors versus temperature with reference to SA–SC* phase transition, at a field Fig. 5 Tilt angle of side-chain precursors versus temperature with of 8 V mm-1. For comparison the measured values of 13b were recalculated to this field (tr 3 1/E).(+) 13a and (#) 13b reference to SA–SC* phase transition: (+) 13a and (#) 13b J. Mater. Chem., 1998, 8(2), 353–362 359Fig. 10 Induced tilt as a function of applied field for side-chain Fig. 8 Response times in the smectic C* phase of polymers versus precursors at 15 °C below the I–SA phase transition (T-Ttr=-15 °C): temperature with reference to SA–SC* phase transition, at a field of (+) 13a, (#) 13b and (1) 13d 8 V mm-1.For comparison the measured values of 14c were recalculated to this field (tr 3 1/E). (6) 14a and (&) 14c. Other interesting properties. The side-chain precursor 13b phenomenon remains in a plot of the response time as a with its very large spontaneous polarization was of course of function of inverse temperature (Fig. 12) it cannot be regarded interest for NLO measurements. The results from these investias a viscosity eVect, if the viscosity is assumed to follow an gations have been reported earlier.27 From the SHG-intensity Arrhenius type of behaviour. Are there changes in the meso- in the smectic C* phase, a deff value of 0.055 pm V-1 was genic interactions through the temperature span of the smectic estimated.Furthermore, for the first time a field-controllable A phase? In Fig. 13 polymers 14b and d show large electroclinic SHG-intensity in the smectic A* phase dependent on the square coeYcients at low temperatures. This could indicate a smectic of the applied electric field was reported. C* phase at lower temperatures, but this has not been con- firmed. Further investigations of the polymers show response Conclusions times independent of applied field (Fig. 14). At the same reduced temperatures the response times were within the same We have studied the eVect of introducing nitro substituents order of magnitude for polymers 14a, b and d. When comparing in the mesogenic core, and have found it to be considerable. 14b and 14d at low temperatures (Fig. 13 and 14), polymer Drastic changes in phase behaviour with substituent position 14b shows larger electroclinic coeYcient and faster response. are observed. In one position the nitro group reduces the As the phase sequences are identical these observations can be transition temperatures for the low molar mass compound explained by the diVerence in position of the nitro group which and more than doubles the maximum spontaneous polarizmay cause a larger transverse dipole moment in 14b compared ation to a value of ca. 700 nC cm-2. Moving the nitro to 14d. substituent one position in the aromatic ring gives a mesogen Fig. 9 Induced tilt as a function of temperature with reference to I–SA Fig. 11 Induced tilt as a function of temperature with reference to I–SA phase transition, for polymers.Applied field 16.2, 53.5 and phase transition, for side-chain precursors. Applied field 7.5, 16.5 and 12.5 V mm-1 for (+) 13a, (#) 13b and (1) 13d, respectively. 23.8 V mm-1 for (6) 14a, ($) 14b and (2) 14d, respectively. 360 J. Mater. Chem., 1998, 8(2), 353–3622.4 2.6 2.8 3.0 3.2 3.4 3.6 10-3 10-2 10-1 100 101 102 103 104 response time/ms T–1/10–3 K–1 Fig. 12 Response times as a function of inverse temperature. Applied Fig. 14 Response times in the smectic A phase of polymers: (6) 14a, field 53.5 and 23.8 V mm-1 for ($) 14b and (2) 14d respectively. The ($) 14b and (2) 14d. Ttr refers to the I–SA transition. The lines are lines are linear regressions of the diVerent parts of the 14d plot.only a guide to the eye. Financial support from The Swedish Natural Science Research with no smectic phases. The position of the lateral nitro Council, The Swedish Research Council for Engineering substituent has a more pronounced eVect in the low molar Sciences and The Swedish Defence Research is gratefully mass compounds than in the polymer liquid crystals. 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