~~~~~ Ferroelectric and antiferroelectric liquid crystalline phases in some pyridine carboxylic acid derivatives N. Kasthuraiah,"B. K. Sadashiva,""S. Krishnaprasadb and Geetha G. Nairb "Raman Research Institute, C.V Raman Avenue, Bangalore-560 080, India bCentrefor Liquid Crystal Research, P.O. Box 1329, Bangalore-560 013, India The synthesis and mesomorphic properties of two series of compounds uiz. (S)-(+)-4-( l-methylheptyloxy)phenyl4'-(6"-alkoxypyridine-3-carbonyloxy) benzoates and (S)-(+)-1-methylheptyl4-[ 4'-( 6"-alkoxypyridine-3-carbonyloxy)benzoyloxyl-benzoates are reported; the homologues of the former series exhibit smectic A and smectic C* phases while the derivatives of the latter series show rich polymesomorphism including the antiferroelectric phase.The mesophases have been characterised by using optical polarising microscopy and differential scanning calorimetric methods. Some physical properties such as the spontaneous polarisation, helical pitch, tilt angle and relative permittivity of two derivatives have also been investigated. Since the discovery of the antiferroelectric chiral smectic phase (S,,) by Chandani et ul.,' and subsequent reports by Takezoe et uL,~a number of new materials exhibiting this phase have been ~ynthesised.~-' The compounds exhibiting the S,, phase generally show subphases such as smectic C,* (S,*a) and ferrielectric (SCsIand ScsII)phases which are being investigated by various groups. Although some molecular structural fea- tures have been identified for compounds which favour the formation of Sc, and other subphases, these have to be examined in a number different systems for a clear understand- ing of such behaviour.Subtle changes in the molecular struc- ture, including the nature and location of the chiral group, seems to play an important role in the appearance of these phases. Recently, the effect of the size of the lateral substituent has been investigated for some phenyl propiolates and benzo- atesg and a few other analogous compounds.lO.ll In this paper, we examine the influence of a substituted pyridine carboxylic acid moiety on the formation of these chiral smectic phases. Two series of compounds I and 11, which differ from one another in the way in which the chiral moiety is linked to the core unit, have been synthesised.I Some of the properties of the smectic C* phase, such as the spontaneous polarisation as a function of temperature, the helical pitch, tilt angle and the relative permittivity have been measured for one homologue of each series. The influence of the hetero nitrogen atom on the mesophases has been evaluated by comparing the mesomorphic behaviour of the correspond- ing carbon analogues. Experimental Synthesis The two series of compounds I and I1 were synthesised following the pathway shown in Scheme 1.4-Benzyloxybenzoic acid was prepared by refluxing ethyl 4-hydroxybenzoate with benzyl chloride in the presence of anhydrous potassium carbon- ate in butan-2-one and hydrolysing the resulting ethyl 4- benzyloxybenzoate with alkali.6-Alkoxypyridine-3-carboxylic acids were prepared following a procedure already described.I2 The final compounds were purified by column chromatography on silica gel using chloroform as eluent and repeated crystallis- ations using suitable solvents. The purities of all the compounds synthesised were checked by thin layer chromatography (Merck Kieselgel 6OFZ5, pre-coated plates) and by normal phase high performance liquid chromatography using p porasil column (3.9 mm x 300 mm, Waters Associates Inc.) and 2% ethyl acetate in heptane as the eluent. The purities were found to be greater than 99.5%. The yields of these compounds were in the range 60-70%. The chemical structures of all the compounds were confirmed by using a combination of nuclear magnetic resonance spectroscopy (Bruker WP8OSY spec-trometer), infrared spectroscopy (Shimadzu IR-435) and elemental analysis (Carlo-Erba 1106 analyser).The optical rotations of all the chiral compounds were determined in dichloromethane (Optical Activity AA 1000 polarimeter). (S)-(+)-4-( 1-Methylhepty1oxy)phenylbenzyl ether A This was prepared following a procedure described by Mitsunobu and Eguchi.', Thus, diethylazodicarboxylate (DEAD, 4.76g, 27.3 mmol) was added dropwise to a cold stirred solution of 4-benzyloxyphenol (5.0 g, 25 mmol), (R)-(-)-octan-2-01 (3.25 g, 24.9 mmol), triphenylphosphine (7.0 g, 26.3 mmol) and dichloromethane (56 ml) for 1 h. The reaction mixture was then stirred at room temperature for 4 h and the solid that formed was filtered off.The material obtained on removal of solvent from the filtrate was chromatographed on silica gel and eluted with a 3: 1 mixture of chloroform and light petroleum (bp 60-80 "C). The required compound was obtained as a viscous liquid (5.3 g, 76%); [a]L5 6.6; 'H NMR (CDC13) 6 0.7-2.0 (16H, m, 2 x CH,, 5 x CH2), 4.2 (lH, m, ArOCH), 4.1 (2H, s, ArCH,O), 6.83 (5H, s, ArH) and 7.29 (4H, s, ArH). (S)-(+)-4-( 1-Methylhepty1oxy)phenolB A mixture of (S)-(+)-4-( 1-methylhepty1oxy)phenylbenzyl ether (7.0 g, 22.43 mmol), ethanol (40 ml) and 5% Pd/C catalyst (1.Og) was stirred in an atmosphere of hydrogen until the calculated quantity of hydrogen was absorbed. The reaction mixture was then filtered and ethanol removed by distillation under reduced pressure.The viscous liquid thus obtained was dissolved in chloroform and filtered through silica gel. Removal J. Muter. Chern., 1996, 6(lo), 1619-1625 1619 I A I It B D 1Ill 1111 I II Scheme 1 Synthetic pathway used for the two series of compounds. Reagents and conditions: i, PPh,, DEAD, CH,Cl,; ii, H,, 5% Pd/C; iii, DCC, DMAP, CH,Cl,. of solvent afforded the required phenol (4.2 g, 84.3%); Calk5 7.6; 'H NMR (CDCI,) 6 0.7-2.0 (16H, m, 2 x CH,, 5 x CH,) 4.2 (lH, m, ArOCH), 6.78 (4H, s, ArH) and 7.2 (lH, s, ArOH). (S)-(+)-4-( l-Methylheptyloxy)phenyl4-benzyloxybenzoateC This was prepared following a procedure described by Hassner and A1e~anian.I~ Thus, a mixture of 4-benzyloxybenzoic acid (2.64 g, 11.6 mmol), (S)-(+)-4-(1-methylhepty1oxy)phenol (2.57 g, 11.6 mmol), N,N-dicyclohexylcarbodiimide (2.36 g, 11.6 mmol), 4-N,N-dimethylaminopyridine(0.14 g, 1.16 mmol) and anhydrous dichloromethane (20 ml) was stirred for 2 h at room temperature.The N,N-dicyclohexylurea formed was filtered off and the filtrate was washed successively with water (2 x 30 ml), 5% aqueous acetic acid (3 x 50 ml) and water (3x50ml), and dried (Na,SO,). The residue obtained on removal of solvent was chromatographed on silica gel using chloroform as eluent. Removal of solvent from the eluate afforded a white solid which was crystallised from ethanol (4.35 g, 87%); mp 118.5 "C; [a]h5 6; 'H NMR (CDCI,) 6 0.94-2.28 (16H, m, 2 x CH,, 5 x CH,), 4.35 (lH, m, ArOCH), 5.18 (2H, s, ArCH,OAr), 6.87 and 8.06 (4H, ABq, J 7.5 Hz, ArH), 6.83 (5H, s, ArH), 7.29 (4H, s, ArH).(27)-( +)-4-( 1-Methylheptyloxy) phenyl4-hydroxybenzoate D This was prepared as described above for compound B using the following quantities of reagents; (S)-(+)-4-( 1-Methylheptyl- oxy)phenyl 4-benzyloxybenzoate (4.7 g, 10.87 mmol), ethyl acetate (40ml) and 5% Pd/C catalyst (3.0g, 81.0%); mp 107°C; Cali5 7.1; 'H NMR (CDCI,) 6 0.75-2.0 (16H, m, 2 x CH3, 5 x CH2), 4.1 ( lH, m, ArOCH), 6.5-8.1 (7H, m, ArH), 7.2 (lH, s, ArOH). (S)-(+ )-4-(l-Methylheptyloxy)phenyl4-( 6"-heptyloxypyridine-3-carbonyloxy)benzoa te I (n=7) This was prepared following the same procedure described above for compound C using the following quantities of the reagents. 6-Heptyloxypyridine-3-carboxylic acid (138 mg, 0.46 mmol), (S)-(+)-4-( 1-methylhepty1oxy)phenyl4-hydroxy-benzoate ( 157 mg, 0.46 mmol), N,N-dicyclohexylcarbodiimide (94 mg, 0.46 mmol), 4-N,N-dimethylaminopyridine (5 mg, 0.046 mmol) and dry dichloromethane (10 ml).The product was crystallised from ethanol (283 mg, 87%); mp 89.3 "C; [a];' 4.08; vmax/cm-' 2950, 1740, 1720, 1605, 1490, 1270 and 1050; 'H NMR (CDC1,) 6 0.7-2.0 (29H, m, 3 x CH,, 10 x CH,), 5.0-5.34 (3H, m, ArOCH and ArOCH,), 6.82 (2H, d, J 9.7 Hz, ArH), 7.32 and 8.15 (4H, AB q, J 8.4Hz, ArH), 7.4 and 8.3 (4H, J 8.5 Hz, ArH), 9.0 (lH, d, J 2.0 Hz, ArH) (Found: C, 72.53; H, 7.85; N, 4.34. C34 H4, NO6 requires C, 72.85; H, 7.67; N, 4.49%).Measurements The transition temperatures were determined using a polarising microscope (Leitz Laborlux 12 POL) equipped with a heating stage and a controller (Mettler FP52 and FP5 respectively), and also from thermograms recorded on a differential scanning calorimeter (Perkin Elmer Model DSC-4 or Model DSC-7). The physical measurements were performed using samples sandwiched between ITO-coated glass plates. Mylar spacers were used to define the thickness of the cell (typically -10 pm for the polarisation and tilt angle measurements and 50 pm for pitch determination). For spontaneous polarisation (P) measurements the triangular wave meth~d'~,'~ was employed. To identify/confirm the presence of antiferroelectric and/or the 'sub phases' a low frequency (0.97 Hz) probing field had to be used.However, the measurements as a function of temperature were carried out at a higher frequency (9.7 Hz) to avoid conductivity problems associated with low frequency large magnitude fields. The tilt angle (8) data were obtained by applying a near DC (0.1 Hz) switching field. The pitch values were determined by the optical diffraction method.17 Dielectric measurements were carried out by using an impedance ana- lyser, the details of which are described elsewhere.'* Results and Discussion The transition temperatures together with the transition enthalpies for the two series of compounds I and I1 are summarised in Tables 1 and 2, respectively. All the compounds in both series are mesomorphic in nature.In series I, where the chiral tail is attached to the core through an ether linkage, only smectic A (S,) and chiral smectic C (Sp) phases were 1620 J. Mater. Chern., 1996, 6(lo), 1619-1625 Table 1 Phase sequences, transition temperaturesrc and enthalpies/kJ mol-' for the compounds of series I compound n C SC* SA I 1 7 0 89.3 0 0 100.0 0 25.42 3.75 2 8 0 76.2 0 0 108.0 0 33.98 4.24 3 9 0 52.7 0 80.5 0 100.5 0 33.16 0.023 4.1 7 4 10 0 56.7 0 86.1 0 101.3 0 37.38 0.048 4.75 5 11 0 65.7 0 91.6 0 100.6 0 46.0 0.098 4.92 6 12 0 65.8 0 94.0 0 100.5 0 43.65 0.15 5.1 observed. These two phases were identified from the character- istic optical textures exhibited by these compounds.On cooling the isotropic liquid, the transition from the focal-conic SA phase to the striated focal-conic texture of the S,* phase was quite clearly observable. Thermodynamically, although this is a weak transition, it could still be seen on a DSC thermogram with a low enthalpy value. A plot of the transition temperatures as a function of terminal alkyl chain length for this series is shown in Fig. 1. It is seen that there is a gradual decrease in the SA mesophase range while the Sc* mesophase range increases on ascending the series. As can be seen in Table 2, the compounds of series I1 exhibit a fairly rich polymesomorphism. The difference between series I and I1 is that there is an ester linkage between the core and the chiral group in the latter.Hence it is reasonable to assume that this is responsible for the appearance of S,z and other sub-phases in the latter series of compounds. The mesophases could be identified under a microscope in thin films by sandwiching a sample between a glass slide and cover slip. For example, on cooling the isotropic liquid of compound 11, the homeotropic and focal-conic texture of a S, phase was observed. The SC*.phase was hardly detectable by the above method because it's texture is similar to that of the SA phase. The ferroelectric Sc* phase appears with a striated fan-shaped or pseudo-homeotropic texture. Cooling further produces a transition to the ferrielectric phase the texture of which con- stantly moves, probably as a result of changes in helical pitch.As the temperature is decreased further, the antiferroelectric phase appears which looks like a ferroelectric phase. However, a homogeneous alignment of the sample can be used to distinguish the antiferroelectric phase from the ferroelectric I 90 9 80i= 70 60 50 1 I I I I 1 I 6 7 13 9 10 I1 12 1 number of carbon atoms in alkoxy chain Fig. 1 Plot of transition temperatures as a function of alkyl chain length for series I phase. A transition from S, to Spa phase could also be easily detected using this technique. In the SC*. phase the homo- geneous coloured regions of the SA phase becomes striated with thin, dark and clear bands parallel to the rubbing direction as observed by Cluzeau et aL7 At the transition to the S,* phase a texture reminiscent of ropes lying parallel to one another could be seen.The transition to the Scg phase was detected as a clear change of texture. The new texture appears with large stripes growing perpendicular to the rubbing direction. The phase behaviour of the compounds of series I1 as a function of alkoxy chain length is shown in Fig. 2. Here again, the clearing temperatures decrease gradually with no strong odd-even effect. The temperature range of existence of the SA phase decreases with increasing chain length, while that of the Sc* phase increases with increasing chain length. The S,-phase is injected into the series from compound 9 (n=8) as a monotropic phase.It is also seen that the Spa phase has a fairly wide range of temperature (2-3.5 "C). In order to confirm the existence of various mesophases observed in the compounds of series 11,miscibility studies were carried out between compound 11 and the standard material 4-(1-methylheptyloxycarbony1)phenyl 4'-octyloxybiphenyl-4-Table 2 Phase sequences, transition temperaturesrc and (in brackets) enthalpies/kJ mol- for the compounds of series 11" compound n C &** SC,* SC* SC. * SA I 7 6 0 76.5 0 0 0 0 0 112.5 0 36.56 4.7 8 7 0 80.3 40.1 6 0 0 (0 56.2) 0.015 (0 57.4) 0.013 0 107.5 4.23 0 9 8 0 72.2 (0 68.8) (0 71.0) 0 73.0 0 76.5 0 106.0 0 38.52 0.006 0.007 0.008 0.009 4.87 10 9 0 70.2 (0 66.7) 0 71.1 0 80.2 0 84.0 0 103.0 0 38.93 0.006 0.008 0.013 0.046 4.55 11 10 0 67.2 0 76.8 0 78.8 0 85.5 0 87.5 0 102.5 0 46.93 0.017 0.018 0.014 0.037 4.66 12 11 0 71.6 (0 60.1) (0 66.4) 0 89.4 0 90.1' 0 100.5 0 54.6 0.048 0.006 0.218 4.51 13 12 0 75.0 (0 64.6) (0 70.7) 0 91.0 0 0 99.0 0 55.66 0.01 4 0.009 0.335 4.54 " Key: C: Crystalline phase; Sc,*: Antiferroelectric phase; Sc,*: Ferrielectric phase; Sc*: Ferroelectric phase; Sc,*: Chiral smectic tl phase; S,: Smectic A phase; I: Isotropic phase.'Enthalpy could not be measured. J. Muter. Chem., 1996,6(lo), 1619-1625 1621 In the present series of compounds 11, a pyridine moiety has been chosen such that the nitrogen atom is ortho to the alkoxy lZO Ichain It is known that pyridine has a moment which is directed along a second order axis of symmetry in the direction of the unshared electron pair As a consequence any influence on the mesophase is due to dipolar effects and the steric effect 110' flis absolutely minimal A comparison of the effect on the 90 -80 -70-60-50 ! I I 1 I I I I 5 6 7 8 9 1011121 number of carbon atoms in alkoxy chain Fig.2 Plot of transition temperatures as a function of alkyl chain length for series I1 carboxylate (MHPOBC) The isobaric binary phase diagram thus obtained is shown in Fig 3 The mixtures were made as weight/weight ratio and mixed thoroughly in their isotropic states It can be clearly seen that there is continuous miscibility of all the phases over the whole composition range, confirming the optical observations From the available data it has been observed that the location of the transverse dipole on the phenyl ring containing the alkoxy chain has an influence on the mesophases Faye et ul have examined a number of fluoro substituted derivatives and concluded that a fluoro substituent ortho to the alkoxy chain does have an effect This steric effect is less pronounced on the cleanng temperatures They have inferred that the fluorine in the ortho position decreases the longitudinal moment without affecting the mesophase sequence Also, any substituent which increases the longitudinal moment also reduces the possibility of obtaining Sc* and Sc2 phases 9 120 i= 110 100 90 80 70 I I I I I I 1 I I 0 10 20 30 40 50 60 70 80 90 1 D % compound I1 clearing temperatures and the SA mesophase temperature range for three derivatives in each of the 4-( l-methylheptylcarbony1)-phenyl 4-(4-alkoxybenzoyloxy) benzoate (nHH8), 4-(l-methyl-heptylcarbony1)phenyl 4'-( 3-fluoro-4-alkoxybenzoyloxy) ben-zoate (nFH8) and series I1 has been made and are summarised in Tables 3 and 4 respectively X = Y = H nHHBBM7 (nHH series) X = F, Y = H nFHBBM7 (nFH series) For example, the clearing temperatures of the compounds of series JJ are about 30°C lower and those for the nFH series are about 8°C lower when compared with those for the corresponding nHH compounds However, as can be seen in Table 4, the thermal range of the SA phase of compounds of series I1 are between those for nHH and nFH compounds It is quite clear from the data shown in these two tables that the nitrogen atom of the pyridine moiety which is ortho to the Table 3 Clearing temperaturesrc for the three series nHH, nFH and I1 compounds n nHH8 nFH8 11 10 1355 127 1 101 5 11 131 4 124 6 1000 12 131 5 1230 99 8 Table4 Temperature rangePC of the S, phase for the three series nHH, nFH and I1 compounds n nHH8 nFH8 I1 10 20 2 13 8 14 8 11 140 10 3 110 12 12 3 75 93 60 40 N Eo C* s5 20 0 70 80 90 TI'C Fig. 3 Miscibility phase diagram (w/w ratio) between compound 11 and the standard compound MHPOBC Fig.4 Thermal variation of polarisation for compound 4 1622 J Muter Chem, 1996,6(10), 1619-1625 alkoxy chain does have an influence on the temperature without affecting the sequence and type of the mesophases. Polarisation Fig. 4 shows the temperature dependence of polarisation (P) for compound 4. The behaviour is typical for a compound exhibiting a second order SA-Sc* transition. The applied field was monitored and adjusted such that it was slightly higher than the field necessary to unwind the helix. Hence no induced polarisation is observed in the SAphase. Fig. 5 shows the current response to an applied triangular wave in different phases of compound 11.The switching profile in the Sc*. phase shows two peaks. As the sample is cooled to the Sc* phase one of the peaks vanishes. The profiles in the Sc-and SCea phases contain, in addition to a main peak, additional peaks, albeit of much smaller strengths. These oscilloscope traces are now accepted to be characteristic of the phases studied here.20 Notice that in the ferri- and antiferro- electric phases a strong buckJlow effect is also seen. The thermal variation of polarisation for compound 11 is shown in Fig. 6. As the frequency of the applied field was 9.7 Hz, the data represents saturation polarisation in all the phases. For this reason, one observes a smooth variation in P across the SC+-Sc*Y and Sc*rScX transitions.On the other half cycle I t Fig.5 Raw oscilloscope traces in the smectic Ca*, C*, C,* and C,* phases of compound 11. The arrows point to the subsidiary peaks. I20 cu ;E C*flo s 40 n-70 75 80 85 TI"C Fig. 6 Variation of spontaneous-induced polarisation as a function of temperature for compound 11. Notice the step in P at the smectic C,*-C* transition. hand, at the SA-SC*.transition there is a sharp but continuous increase in the value of P, perhaps suggesting a second order change. But remarkably, a step increase is also observed at the temperature corresponding to the SC*-Sc* transition. As we had ensured that the applied field was sufficient to unwind the helix, any effects due to the partial unwinding of the sample can be ruled out.To our knowledge, this is the first instance where a clear change in the P us. temperature plot has been seen at the SC*-Sc* transition. To understand the effect of finer variations of the molecular structure on the magnitude of P,let us compare the values obtained for compound 11, with those for compounds having a very similar structure. At 10°C away from the SA-Sc*= transition the value for compound 11 is cu. 120 nC cm-2 as against 60 nC cmd2 for 4-( 1-methylheptylcarbony1)phenyl4-(4-decyloxybenzoyloxy) benzoate ( 10HHBBM7), 70 nC cmU2 for 4-( 1 -methylhept ylcarbonyl )phenyl 4-(3-fluoro-4-decylox-Lh O 5 10 15 20 25rn 18 14 I 12 -0 2 4 6 5-T /"C Fig. 7 Plot of tilt angle us. reduced temperature for (a) compound 4 and (b) compound 11 J.Muter. Chem., 1996,6( lo), 1619-1625 1623 ybenzoyloxy)benzoate (lOFHBBM7), studied by Faye et ul (for details regarding the structural differences see an earlier section) The variation in the values of P indicates that the ring structure of the first phenyl ring, although very far from the chiral centre, plays a non-neghgible role 24 22 r 5 L I I I J E @ 5 10 15 20 0=4c1 3 2 0 2 4 6 c-T 1°C Fig. 8 Plot of helical pitch us reduced temperature for (a) compound 4and (b)compound 11 135 0 0 0 0 0 c; 0 0 0 045 0 0 0 0 0 n u~ 70 75 80 a5 90 85 86 87 88 T1"C Fig. 9 Temperature dependence of transverse static relative permit-tivity in the vanous phases of compound 11 The bottom panel shows the smectic A-C,* region on an enlarged scale 1624 J Muter Chern, 1996, 6(10), 1619-1625 Tilt angle and pitch Plots of the optical tilt angle 8 as a function of Tc-T (where Tc represents the temperature of transition from the Sc* phase) for compounds 4 and 11 are shown In Figs 7(u) and (b) respectively Although the value of the tilt angle far away from the transition is observed to be very similar in both the compounds, temperature variations are different Again com-paring with the studies carried out by Faye et u18 we notice that compound 11 shows a smaller tilt angle than 10HHBBM7 or 10FHBBM7 compounds mentioned above Combined with the fact that the polarisation values are high, this would mean that the presence of hetero nitrogen atom in the ring enhances the polarisation-tilt coupling The pitch values measured for a few temperatures in the Sc* phase of compounds 4 and 11 are shown in Figs 8(u) and (b)respectively The different types of temperature dependence may be associated with the fact that one compound has the SC*.phase intervening between the SA and S,* phases The pitch values observed are about 8 to 10 times higher than a thiobenzoate compound6 having a very similar structure Relative permittivity Fig 9 shows the temperature dependence of the real part of the relative permittivity measured at 105 Hz for compound 11 As the material cools from the S, to S,* phase, there is a large increase in the value, which is commonly observed In addition a small but clear step is seen at the SA-Sc*a transition Such a feature has been reported earlier by Gisse et ulZ1 In the S,* phase the main contribution to E~ comes from the Goldstone mode whose strength decreases as the material transforms from the Sc* to the Spa phase and completely vanishes in the Sc%phase This is reflected in the E values Application of a I 135 -(a) 00 0 oo 00 0 00 0 oo 90-0 001:: 0 0c; : 00 -0 0 0I 0 0 0 0 a0 I , I0 w+ 70 75 80 85 ! L I 85 86 87 88 T/"C Fig.10 (a)Static relative permittivity (E~)as a function of temperature for compound 11 Probing frequency f= 105 Hz with a DC bias of 125 V pm-l (b) Enlarged view, the arrow points to the smectic A-C,* transition I 1 Fig.11 Influence of the magnitude of the probing frequency on the thermal variation of cl for compound 11 DC bias used = 1 25 V pm The frequencies are (a) 0 105, (b) 10, (c) 5 0, (d) 100, (e) 20 0 and (f) 500 KHz DC bias field (1 25 V pm-') reduces the value in the SC*. transition (see Fig 10) Additionally, one clearly observes a small peak in the Scz phase, which exists for the zero bias case also but is very weak This peak vanishes as the frequency of the probing field is increased (Fig 11) Except for recalling that Gisse et a1 21 have also observed such a peak, we are not sure about the physical origin of this Finally, the change at the SA-Sc*n transition seems to be hardly affected by the application of a bias field Conclusions Two series of compounds incorporating 6-alkoxypyridine-3- carboxylic acids have been synthesised These two series differ from one another by the way in which the chiral group is linked to the core It is shown that the terminal ester group which has a strong transverse dipole is essential for obtaining the S,-and other sub-phases It is also shown that the dipole associated with the nitrogen of the pyridine moiety, which is ortho to the alkoxy chain, does have an influence on the clearing temperatures without affecting the sequence and type of mesophases We wish to thank Mr K Subramanya for recording the various spectra and for elemental analysis, and Dr A A Khan, Deputy Director, Indian Institute of Chemical Technology, Hyderabad for allowing us to use the Perkin-Elmer DSC-7 instrument References 1 A D L Chandani, E Gorecka, Y Ouchi, H Takezoe and A Fukuda, Jpn J Appl Phys ,1989,28, L1265 2 H Takezoe, J Lee, A D L Chandani, E Gorecka, Y Ouchi, A Fukuda, K Terashima and K Furukawa, Ferroelectrics, 1991, 114, 187, H Takezoe, A D L Chandani, J Lee, E Gorecka, Y Ouchi, A Fukuda, K Terashima, K Furukawa and A Kishi, Abstracts, 2nd International Symposium on Ferroelectric Liquid Crystals (Goteborg, 1989), p 26, H Takezoe, A D L Chandani, E Goreka, Y Ouchi and A Fukuda, Abstracts, 2nd International Symposium on Ferroelectric Liquid Crystals (Goteborg, 1989), p 108 3 J W Goodby, J S Pate1 and E Chin, J Muter Chem ,1992,2,197 4 S Inui, T Suzuki, N Iimura, H Iwane and H Nohira, Ferroelectrics, 1993, 148,79 5 S Inui, T Suzuki, N Oimura, H Iwane and H Nohira, Mol Cryst Liq Cryst, 1994,239,l 6 H T Nguyen, J C Rouillon, P Cluzeau, G Sigaud, C Destrade and N Isaert, Lq Cryst, 1994,17, 571 7 P Cluzeau, H T Nguyen, C Destrade, N Isaert, P Barois and A Babeau, Mol Cryst Liq Cryst, 1995,260,69 8 V Faye, J C Rouillon, C Destrade and H T Nguyen, Liq Cryst, 1995,19,47 9 I Nishiyama and J W Goodby, J Muter Chem ,1993,2,149 10 R P Tuffin,J W Goodby,D Bennemann,G Heppke,D Lotzsch and G Scherowsky, Mol Cryst Liq Cryst, 1995,260,51 11 Y Ouchi, Y Yoshioka, H Ishii, K Seki, M Kitamura, R Nayori, Y Takanishi and I Nishiyama, J Muter Chem ,1995,5 2297 12 A I Pavluchenko,N I Smirnova,V V Titov, E I Kovshevand K M Djumaev, Mol Cryst Liq Cryst, 1976,37,35 13 0 Mitsunobu and M Eguchi, Bull Chem SOC Jpn ,1971,44,3427 14 A Hassner and V Alexanian, Tetrahedron Lett, 1978,4475 15 K Miyasato, S Abe, H Takezoe, A Fukuda and E Kuze, Jpn J Appl Phys ,1983,22, L 661 16 S Krishnaprasad, Geetha G Nair and S Chandrasekhar, J Muter Chem ,1995,52253 17 S Krishnaprasad and Geetha G Nair, Mol Cryst Liq Cryst, 1991,202,91 18 S M Khened, S Knshnaprasad, B Shivkumar and B K Sadashiva J Phys II,1991,1, 171 19 V I Minkin, 0 A, Osipov and Y A Zhdanov, in Dipole Moments in Organic Chemistry, Plenum Press, New York, 1970,p 114 20 A Fukuda, Y Takanishi, J Isozaki, K Ishikawa and H Takezoe, J Muter Chem ,1994,4,997 21 P Gisse, J Pavel, H T Nguyen and V L Lorman, Ferroelectrics, 1993,147,27 Paper 6/02415G, Received 9th April 1996 J Mater Chem, 1996, 6(10), 1619-1625 1625