J. MATER. CHEM., 1994, 4( l), 71-79 Helix Inversion in the Chiral Nematic Phase of a Ferroelectric Liquid Crystal Containing a Single Chiral Centre Christa Loubser: P. L. Wessels: Peter Styringb and John W. Goodby*b a Department of Chemistry, University of Pretoria, 0002 Pretoria, South Africa School of Chemistry, The University of Hull, Hull, UK HU6 7RX An inversion in the cholesteric phase has been found to occur with change in temperature. Additionally, the material under investigation was found to exhibit unusual ferroelectric properties on cooling from the chiral nematic phase. We report the synthesis and physical properties, including pitch and polarization data, for (S)-4-n-octyloxy-2,3-difluorobiphenyl-4’-yl 3-difluorobiphenyl-4’-yl 3-fluoro-4-(2-fluorooctanoyloxy)benzoate. Over the past few years there has been considerable interest it has helical inversions in both the cholesteric and smectic in the synthesis and investigation of chiral liquid crystals, and in particular materials that exhibit ferroelectric smectic C* phases.’ In this context we have investigated several chiral systems where an inversion in chiral properties with respect to temperature has been observed.For example, consider the materials shown below. The first compound,2 (S)-2-methylbutyl4-n-nonanoyloxybiphenyl-4-carboxylate,1, exhibits a smectic A and a ferroelectric smectic C* phase. Upon cooling into the ferroelectric phase a spontaneous polarization develops along the C, axis of the phase, with its value reaching a maximum before falling again.3 The magni- tude of the polarization reaches zero before surprisingly increasing again.Initially, at higher temperatures in the ferro- electric phase, the spontaneous polarization is defined as being negative,’ Ps(-),but as the temperature falls the polarization direction inverts and has the opposite sign, P,(+). CeH174m1 CBHl9O 1 The second compound (S)-2-chloropropyl 4-(4-n-nonyloxyphenylpropioloyloxy)biphenyl-4-carboxylate~ 2, exhibits cholesteric and smectic A phases, and on cooling from the isotropic liquid the cholesteric helix unwinds and then rewinds. Thus, at higher temperatures in the cholesteric phase this material possesses a left-handed helix, but as the temperature is lowered the helix unwinds through a cholesteric or chiral nematic phase which has an infinite pitch to give a cholesteric phase that has a right-handed helix.Compound 3, (S)-2-chloro-4-methylpentyl 4‘-(4-n-hexadecyloxyphenyl-propiolo ylox y) biphenyl-4-carboxylate, behaves somewhat similarly to compound 2, except that the helical inversion takes place in the helical ferroelectric smectic C* phase.5 However, in this case the direction of the spontaneous polariz- ation does not invert with temperature. Finally in this series of chiral substances, compound 4 is quite remarkable in that C* phases, and accompanying the helix transposition in the C* phase the direction of the spontaneous polarization also crosses over.6 We have attempted to interpret these effects in terms of a model of interconverting, but competing, species whose con- centrations are temperature dependent.From this hypothesis we have related the interconverting species to changes in the conformational structures of the molecules.3-6 In this current investigation we have examined the liquid-crystalline properties of compound 5, (S)-4-n-octyloxy-2,3-difluorobiphenyl-4-y1 3-fluoro-4-(2-fluorooctanoyloxy)-benzoate and have found that it too undergoes a helix inversion in the cholesteric phase. However, it is clear that an accompanying helix inversion in structure or a flip-flop in the direction of the spontaneous polarization does not take place in the ensuing ferroelectric C* phase. Hence, this mate- 2 F 4 rial provides yet another example of the first class of inversion phenomena (i) listed below.Thus, so far we have observed the following inversions in the chiral properties/structures of optically active liquid crystals: (i) helix inversion in the cholesteric phase; (ii) helix inversion in the smectic C* phase but no polarization flip-flop; (iii) spontaneous polarization crossover in smectic C* but no helix inversion; (iv) inversions in the helices in both cholesteric and smectic C* phases, and polarization reversal in the ferroelectric phase. The only combinations of inversion effects not yet observed are (v) a helix reversal in the cholesteric phase but not in the smectic C*,accompanied with a polarization flip-flop in the ferroelec- tric phase, and (vi) helix inversions in both smectic C* and cholesteric phases but no crossover in the spontaneous polarization.In the following sections of this article we report on the J. MATER. CHEM., 1994, VOL. 4 EF F 5 synthesis and physical properties of (S)-4’-n-octyloxy-2,3-difluorobiphenyl-4’-yl 3-fluoro-4-(2-fluoro-octanoyloxy) benzoate, and describe results which show clearly that this material undergoes an inversion in its chiral nematic phase. Experimental General Synthetic Procedures The compound, (S)-4-n-octyloxy-2,3-difluorobiphenyl-4’-yl 3-fluoro-4-( 2-fluorooctanoyloxy) benzoate, 5, was prepared according to the synthetic scheme shown in Scheme 1. Initially, 2-fluoroanisole (Aldrich), 6, was brominated using bromine in chloroform to give 4-bromo-2-fluoroanisole, 7 (structure con- (300 MHz, CDCl,, TMS): 7.17 (2 H, m), 6.81 [l H, apparent triplet, (H,H,)= 8.4 Hzz‘J (H,F)], 3.84 (3 H, s), 19F: 139.98 (rel.CFCl,)}. The bromo-substituent was replaced with a nitrile group via 6, firmed by NMR spectroscopy, 6 7 C8Hl7O 11 a cyanation reaction utilising dry copper(1) cyanide in N,N-dimethylformamide (DMF) to yield 4-cyano-2-fluoroanisole, 8. This product was demethylated and the nitrile group hydrolysed in one step by the action of aqueous hydrobromic acid (48% wt./vol.) in glacial acetic acid.7 The hydroxy func- tion of the resulting 3-fluoro-4-hydroxybenzoic acid, 9, was then protected with the use of methyl chloroformate to give the derivative 3-fluoro-4-methoxycarbonyloxybenzoic acid,’ 10.This acid was esterified,’ in the presence of diethylazodicar- boxylate (Merck) and triphenylphosphine (Merck), with 4-n- octyloxy-3,2-difluorobiphenol, 11, to produce the ester 4-n-oct yloxy- 3,2-difluoro biphenyl-4’-yl 3-fluoro-4-methoxycarbonyloxybenzoate, 12. The biphenol, 11, was prepared uia a boronic acid coupling reaction between 2,3-difluoro-4-octyloxyphenyl boronic acid 4-bromoiodobenzene in 1,2-dimethoxyethane (DME)” to give the intermediate 4‘-bromo-2,3-difluoro-4-octyloxybiphenyl which was then oxidised via the 4-boronic acid to the 4’-hydroxy derivative. The Mitsunobu reaction’ was used in the esterification of 10 and 11 because the protecting group was to be retained.Normal esterification conditions involving a base such as pyridine or N,N-dimethylaminopyridinehave a strong tendency to remove the methoxycarbonyloxy group. Subsequently, the protecting group of compound 12 was removed by stirring in a concentrated solution of ammonia in aqueous ethanol to yield the free hydroxy group (compound 13).’ 4-n-Octyloxy-3,2-difluorobiphenyl-4’-yl3-fluoro-4-hydroxybenzoate, 13, was then esterified with optically active 6 91;CH30CCI ~0H 10 + EF 12 5 Scheme 1 Synthesis of 5 J. MATER. CHEM.. 1994, VOL. 4 (S)-2-fluorooctanoic acid, 14, in the presence of dicyclohexyl- carbodiimide (DCC) (Merck) and N,N-dimethylaminopyrid- ine (DMAP) (Merck) to give the final product 5. The chiral fluoro-acid, 14, was prepared using the method reported by Nohira et al.," in which (R)-(+)-1,2-epoxyoctane was initially treated with pyridinium poly( hydrogen fluoride) in ether to give (S)-(-)-2-fluorooctan-l-ol. This alcohol was then esterified with acetic acid and the resulting ester oxidised in the presence of nitric acid to give (S)-2-fluorooctanoic acid, 14.The structure of 14 was confirmed by NMR spectroscopy [SH (300 MHz, CDCl,, TMS): 11.64 (1H, br), 4.98 [l H, dt, 'J (HF)=48.8 Hz, (HH)=5.9 Hz], 2.05 (2 H, m, (HF)= 24.4 Hz), 1.64 (2 H, m), 1.51 (6 H, m), 1.09 (3 H, t)]. Purity and Characterisation of Materials The final product was rigorously purified by flash chromatog- raphy over silica gel (200-400 mesh) using dichloromethane as the eluent.The combined fractions were found to show a single spot by thin-layer chromatography (TLC). After removal of the solvent, the final product was recrystallized sequentially from acetonitrile and light petrol (bp 40-60 "C) until constant transition temperatures were obtained. The chemical purity of the product was investigated by both normal and reversed-phase high-performance liquid chroma- tography; the purity was found to exceed 99% by both methods. Normal phase chromatography was performed over silica gel (5 pm pore size, 25 cm x 0.46 cm, Dynamax Scout Column), reversed-phase chromatography was performed over octadecylsiloxane (5 pm pore size, 25 cm x 0.46 cm, ODS Microsorb Dynamax 18 Column). Acetonitrile was the eluent used in both cases.Detection of eluting materials was achieved spectroscopically using a Spectroflow 757 UV-VIS detector (A=254 nm). The chemical structures of the intermediates in the synthetic route and the final product were determined by a combination of IR spectroscopy (Bomem Michelson 100 FTIR spectropho- tometer), NMR spectroscopy (Bruker AC300 NMR Spectrometer used at 25 "C) and mass spectral analysis (VG 7070H spectrometer operating at 70 eV). The optical rotation (Atago Polax-D Polarimeter) of the final product was deter- mined in solution (using chloroform as the solvent and a concentration of ca. 30 mg ml-l), and monitored carefully to ensure that no racemization in the synthetic procedures had occurred. The optical purity (e.e.) of the final product was determined from NMR spectroscopy using europium D-3-heptafluorobutyrylcamphorate (Lancaster Synthesis) as the chiral shift reagent.The melting points of the intermediates were determined using a Gallenkamp melting point apparatus (UK). The transition temperatures and phase assignments for the final products were determined to an accuracy ofk0.1 "C by ther- mal optical microscopy using a Zeiss Universal Polarizing Light Microscope equipped with a Mettler FP52 microfurnace in conjunction with a FP5 control unit, or by using a Leitz Laborlux Polarized light microscope fitted with a hot stage. Photomicrographs were taken using a Zeiss Universal polariz- ing microscope fitted with an RCA Newvicon video camera in conjunction with an Hitachi VY-200A videoprinter.Temperatures and enthalpies of transitions were investi- gated by differential scanning calorimetry (DSC) using a Perkin-Elmer PC Series DSC7 calorimeter. As a check on instrumental accuracy an indium standard was run at a scanning rate of 10.0"C min-'. The measured latent heat, 28.53 J g-l, compared well with the standard value for indium of 28.45 J g-'. The material was studied at various scanning rates (2, 5 or lO"Cmin-'), for both heating and cooling cycles, after being encapsulated in aluminium pans. Similarly, the measured melting temperature of 156.7 "C compared well with the literature value (156.6 "C). The pitch in the cholesteric phase was measured by determining the distance between the dechiralization lines in the fingerprint texture of the phase using a calibrated Filar eyepiece attached to the Zeiss polarizing microscope.The Filar eyepiece was calibrated against a graduated 1 mm microscope scale (10 pm spacing). The magnitude of the spontaneous polarization of com- pound 5 was measured in 0.25 cm2 indium tin oxide ([TO)-coated test cells that were obtained from Electronics Chemicals High Technology Group, Japan. The internal sur- faces of the cells were coated with polyimide and unidirection- ally buffed. Ac fields were applied using an Advance Electronics AF signal generator J2C in a sine-wave mode. The hysteresis loop was observed on a Dartron Instruments dual trace oscilloscope D17 and the spontaneous polarization (P,) was determined using a Diamant bridge.12 P, of com-pound 5 was evaluated using an applied ac voltage of 1OV p.p.at 60 Hz. The data reported were derived from three different runs and are plotted on a single graph so that the average value could be taken as a curve through the data points. Synthesis of Materials Preparation of 3-Fluoro-4-methoxycarbonyloxybenzoic Acid, 10 A solution of sodium hydroxide (0.25 g) in water (10 ml) was chilled to 0 "C in ice. To this, 3-fluoro-4-hydroxybenzoic acid, 9, (0.5 g, 3.2 mmol) was added. Methylchloroformate (0.606 g, 6.4 mmol) was added slowly to prevent the temperature from rising above 5°C. The reaction mixture was stirred at 0.5"C (3 h) during which time a white suspension formed gradually.The pH was adjusted to 4.5 with addition of hydrochloric acid-water (1:1). The resulting precipitate was filtered off, washed with water and recrystallised from ethanol to give the protected acid 10 as a white solid. Yield, 0.918 g, 67%; m.p. 137-139 "C;aH(300 MHz, solvent CDCl,, standard TMS) 7.90 (2 H, m), 7.34 [1H, dd, 4J (FH,) 8.6 Hz, (H,H,) 7.4Hz1, 3.94 (3 H, s) -COOH not observed, 19F: 131.53 (rel. CFC1,); v/cm-l (KBr Disc): 3200-2600 (br), 1776 (vs), 1694 (s), 1598 (m), 1513 (m),1448 (s), 1285 (br, s), 1197 (s) 930 (s), 769 (s). Preparation of 4-n-Octyloxy-2,3-d~uorobiphenyl-4'-yl 3-fluoro-4-methoxycarbonyloxybenzoate, 12 A solution of 4-n-octyloxy-2,3-difluorobiphenol,11, (0.78 g, 2.33 mmol), 3-fluoro-4-methoxycarbonyloxybenzoicacid, 10, (0.5 g, 2.33 mmol) and diethylazodicarboxylate (0.488 g, 2.8 mmol) in dry THF (50 ml) was prepared under an atmos- phere of dry nitrogen.A solution of triphenylphosphine (0.733 g, 2.8 mmol) in dry THF (15 ml) was then added slowly and the solution stirred overnight at room temperature. The solvent was removed in uacuo and the crude product was purified by flash-column chromatography over silica gel (200-400 mesh) using a mixture of light petroleum (bp 40-60 "C) and dichloromethane (1:1) as the eluent. The solvent was removed from the collected fractions and the residue was purified by repeated recrystallizations from pen- tane to give compound 12 as a white solid. Yield, 0.526 g, 43%; K, 84.0 "C K, 103.8 "C N 177.5 "C I; SH(300 MHz, solvent CDCl,, standard TMS): 8.02 (2 H, m), 7.54 (2 H, m), 7.39 (1H, dd), 7.26 (2 H, m), 7.08 (1 H, td), 6.79 (1 H, td), 4.07 (2 H, t) 3.96 (3 H, s), 1.83 (2 H, pentet), 1.47 (2 H, pentet), 1.32 (8 H, m), 0.88 (3 H, t); v/cm--' (KBr disc): 2949 (s), 2922 (s), 2859 (s), 1775 (vs), 1743 (vs), 1508 (vs), J.MATER. CHEM., 1994, VOL. 4 Plate 1 Microscopic defect texture of (S)-4-n-octyloxy-2,3-difluoro-biphenyl-4'-yl 3-fluoro-4-( 2-fluorooctanoyl-0xy)benzoate at the point where an inversion occurs in the twist direction of the helical structure of the cholesteric phase. In the dark areas of the preparation the homeotropic texture of the nematic phase predominates, whereas in the birefringent fingerprint region remnants of the cholesteric phase remain (x 100) Plate 2 Microscopic defect texture of the contact region between (S)-4-n-octyloxy-2,3-difluorobiphenyl-4'-yl 3-fluoro-4-( 2-fluoro- octanoyloxy) benzoate, 5 (bottom left) and (S)-4-n-hexyloxyphenyl 4-[ 4-(4"-methylhexyloxy)benzoyloxy]benzoate, 15 (top right).(x 100) 1472 (s), 1438 (s), 1305 (vs), 1217 (s), 1192 (s), 1077 (vs), 933 (s), 895 (s), 789 (m), 758 (m), 735 (m); m/z:530 [M'], 197, 153. Preparation of 4-n-Octyloxy-2,3-dijIuorobiphenyl-4'-yl 3-jluoro-4-hydroxybenzoate, 13 A solution of compound 12 (0.5 g, 0.94 mmol) dissolved in a mixture of dichloromethane (20 ml) and ethanol (20 ml) was added to an aqueous solution of ammonia (35%, 20ml) at room temperature. The mixture was stirred (4 h) until TLC showed the reaction to be complete.The solvents were removed in uucuu and the residue purified by column chroma- tography over silica gel (200-400 mesh) using a mixture of light petroleum (bp 40-60 "C) and ethyl acetate (2 :1) as the eluent. The solvent was removed from the collected fractions, and the product, compound 13, was purified by repeated recrystallizations from a mixture of ethyl acetate and hexane to give white crystals. Yield, 0.420 g, 94%; mp 135-137 "C; 6, (300 MHz, solvent CDCl, +10% [2H,]DMS0, standard TMS) 7.80 (2 H, m), 7.46 (2 H, m), 7.18 (2 H, m), 7.02 (1 H, td), 7.00 (1H, m), 6.73 Plate 3 Microscopic defect texture of the contact region between (S)-4-n-octyloxy-2,3-difluorobiphenyl-4'-yl 3-fluoro-4-( 2-fluoro- octanoyloxy)benzoate, 5, (bottom left) and (S)-J-n-decyloxyphenyl 4-[4'-(2"-methylbutyl)benzoyloxy]benzoate, 16, (top right) (x 100) Plate 4 Contact region between (S)-4-n-octyloxy-2,3-difluoro-biphenyL4'-yl 3-fluoro-4-(2-fluorooctanoyl-oxy)benzoate,5, and (S)-2-methylbutylphenyl4-n-octylbiphenyl-4-carboxylate, 17 (x 100) (1H, td), 4.00 (2 H, t), 2.17 (1H, s), 1.76 (2 H, pentet), 1.40 (2 H, pentet), 1.22 (8 H, m), 0.83 (3 H, t); v/cm-' (KBr disc) 3500-3200 (br), 2951 (s), 2921 (s), 2854 (s), 1730 (vs), 1619 (s), 1508 (vs), 1470 (s), 1406 (m), 1305 (vs), 1229 (s),1109 (s), 1087 (s) 894 (m), 869 (m), 799 (s), 751 (s); m/z 472 [M'], 335, 334, 222, 221, 140, 139.Preparation of (S)-4-n-0ctyloxy-2,3-d~uurobiphenyl-4'-yl 3-JEuoro-4-( 2-j7uorooctunoyloxy) benzoate, 5 A solution of dicyclohexylcarbodiimide, DCC, (0.12 g, 0.5.6 mmol) in dichloromethane (5 ml) was added to a solution of biphenol 13 (0.2 g 0.42 mmol), (S)-2-fluorooctanoic acid (0.068 g, 0.42 mmol) and dimethylaminopyridine, DMAP, (0.0154 g) in dry dichloromethane (20 ml) under an atmos- phere of dry nitrogen.The reaction mixture was stirred at room temperature (8 h) after which the dicyclohexyl urea that had formed was filtered off. The solvent was removed in uucuo from the resulting solution and the crude product was purified by flash chromatography over silica gel (200-400 mesh) using a mixture of light petroleum (b.p. 40-60 "C) and dichloro- methane (1:4) as the eluent. The solvent was removed from the fractions collected and the product, 5, was purified by repeated recrystallizations from hexane to give a white solid.Yield, 0.165 g, 64%; Found: C, 68.14 H, 6.47. Calc. for C,, J. MATER. CHEM., 1994, VOL. 4 H4, C, 68.17, H 6.54%, 6, (300 MHz, solvent CDCl,, standard TMS) 8.04 (2 H, m), 7.55 (2 H, m), 7.32 (1H, dd), 7.27 (2 H, m), 7.08 (1 H, td), 6.79 (1H, td), 5.20 (1H, dt, 2JHF48.6 Hz, 5.9 HZ), 4.07 (2 H, t), 2.06 (2 H, 2 X m, 3JHF 25.3 HZ), 1.83 (2 H, pentet), 1.59 (2 H, pentet), 1.45 (2 H, pentet), 1.32 (14 H, .m), 0.89 (6 H, 2 x t); v/cm-' (KBr disc) 2951 (s), 2918 (s), 2861 (s), 1769 (s), 1733 (s), 1508 (vs), 1469 (s), 1308 (s), 1202 (s), 1108 (s), 1079 (s), 892 (m), 867 (m), 793 (m), 749 (m); m/z 616 [M+], 503, 334, 283, 255, 222, 221, 140, 139; +5.1"; e.e.>90% (using europium D-3-heptafluorobutyryl-camphorate as the chiral shift reagent).The proton attached to the chiral centre (double of triplets) was found to shift downfield from 5.2 to 5.76, however, no detectable amount of the other enantiomer was found, therefore the optical purity was assigned a minimum value of 90%. Results In the following sections we detail results that unequivocally prove that compound 5 possesses a helix inversion in the cholesteric phase, and we also discuss the ferroelectric proper- ties of the smectic C* phase of the material. Thermal Polarized Light Microscopy Studies Studies on the Pure Material Thermal polarized microscopy of (S)-4-n-octyloxy-2,3-di-fluorobiphenyl-4'-yl3-fluoro-4-(2-fluorooctanoyloxy)-benzoate, sandwiched between an untreated glass slide and cover slip, showed that this material exhibited cholesteric and smectic C* phases.The following transition temperatures ("C) were determined at heating/cooling rates of less than 1"C min-'. K 89.7 Sp 139.3 N*R 140 N*, 140 N*L 149.6 I The cholesteric phase was easily identified from its Grandjean planar and fingerprint textures, and on cooling to just above the cholesteric to smectic C* transition (140-139 "C) an interesting phenomenon was observed. The helical structure of the cholesteric phase was found to unwind at 140°C so that the mesophase became totally untwisted, and then almost immediately upon further cooling a helical structure reformed, but with the opposite handedness.This is seen clearly for the cholesteric phase (N*) at the point where the fingerprint texture gives way to the homeotropic texture of the nematic phase (N*,), as shown in Plate 1. This plate shows the crossover point in the helical-twist sense; in the black region of the photomicrograph the molecules are essentially ordered so that the direction of observation is along the optic axis of the phase, whereas in the fingerprint region a helical structure exists. As the preparation is cooled further, the fingerprints return over the whole of the specimen and eventually a Grandjean plane texture reforms. However, it should be noted that the lower temperature cholesteric phase does not exist over a very large temperature range as it quickly gives way to the formation of a ferroelectric C* phase.Rotation of the upper polarizer (either side of being crossed to the bottom polarizer) of the microscope, above and below the crossover point, reveals that the sign of the helix inverts with tempera- ture. A slight coloration is observed when the upper polarizer is rotated in the same direction as the helix. This experiment shows that the upper-temperature cholesteric phase is right- handed (1) whereas the lower phase is left-handed (d). This process of unwinding of the helix just before the transition to the smectic C* phase leads directly to the ferroelectric phase being formed with relatively good alignment, which is very unusual for a material exhibiting cholesteric to smectic C transition.For the smectic C* phase normal pseudo-homeotropic and schlieren defect textures are observed. Rotation of the upper polarizer confirms that the helical structure is left-handed (d), which is in agreement with the proposed rules linking twist sense and spontaneous polarization dire~ti0n.l~ Contact Studies Various contact preparations of materials of known helical- twist sense were studied in order to confirm the presence of a helix inversion in compound 5, and to determine the helical twist direction with respect to temperature. The first contact to be investigated was that between the test material 5 and the standard 15.13Compound 15, see structure 5 given below, has been reported to exhibit a laevo rotation of plane-polarized light in its cholesteric phase.Moreover, as this compound has an (S) absolute spatial configuration with respect to its chiral centre, which is itself removed from the rigid aromatic core by an odd number (0)of atoms, then this material is categorised as Sol (RH) by Gray and McDonnell rules.I4 Therefore, the material has a right-handed helical structure. The contact region between the two compounds shows no discontinuity between the two cholesteric phases. Plate 2 shows the texture of a typical region that includes the contact boundary at 147 "C. The green colour appears fairly uniform across the photomicrograph indicating that the two phases have roughly similar pitch lengths in their cholesteric phases, and furthermore the selective reflection of green light indicates that the pitch length is ca. 0.7-0.9 pm.This contact therefore confirms that for the upper temperature region of the cholesteric phase the helix has a right-handed twist. 15 17 A second contact preparation was made between compound 5 and ester 16.15 The standard material, 16, in this case is classified as Sed(LH) by the Gray and McDonnell rules. Therefore the two materials should form a nematic phase where the pitch diverges at the contact boundary as the two helical structures compensate. Plate 3 shows the contact boundary of the two materials at a temperature of 141.6 "C. It can be seen clearly from the rivulet of nematic phase (grey area) running across the preparation that there is a di\ ergence in the pitch.The strong variation in colour across the sample also indicates that the pitch length of the mesophase is changing sharply with concentration. The divergence in the pitch confirms that the upper temperature cholesteric phase does not possess a left-handed helix. When the preparation is cooled down to below the crossover point for the helix J. MATER. CHEM., 1994,VOL. 4 inversion in compound 5, the contact boundary shows no discontinuity confirming that the lower temperature choles- teric phase has a left-hand helical structure. A third contact study was performed with the commercially available material (S)-2-methylbutylphenyl 4'41-octylbiphenyl-4-carboxylate, CE8 (Merck), 17.16At high tem- peratures in the cholesteric phase, the test material, 5, and the standard, 17, were found to have helical structures that compensated, i.e.of opposite twist, and at lower temperatures no discontinuity was found at the contact hence the two cholesteric phase have the same twist sense. Thus, this again confirms that in the upper temperature regime of the choles- teric phase the test material possesses a right-handed helix, and at lower temperatures this inverts to give a left-handed helix. Plate4 gives the most comprehensive view of the inversion phenomenon. This figure shows the texture around the contact region at a temperature of 136.8"C, which is close to the crossover point. On the right-hand side of the figure (high concentration of the test material) the texture appears black, this is due to a transition to the smectic C* phase which exhibits a pseudo-homeotropic texture.A grey-black line, which is a rivulet of nematic runs down the centre of the figure, this nematic region is caused by the inversion in the helix of the cholesteric phase of the test material. To the left there is another black area, this is the actual contact zone between the two materials. Although appearing black, this area in fact has a mosaic texture associated with it which is due to the formation of a blue phase (not resolved by the camera). In the blue-phase region, the pitch of the helical structure is ca. 0.5 pm, i.e. the contact zone shows continuity of the choIesteric phase.Thus, to the left-hand side of the rivulet of nematic in the centre of the figure, which includes the contact zone, the cholesteric phase has a left-hand helix, and on the right of the figure the cholesteric phase has a right-hand helix. 30.0-5.00 3.75-\2 22.5 --5 2.50-.c c. a,c 1.25-15.0-0.0 4 Differential Scanning Calorimetry The phase transitions of the material were also investigated by DSC as shown in Fig. 1. This shows the first heating cycle for compound 5, and it can be seen from the thermogram that the clearing point and cholesteric to smectic C* enthalpies are relatively small and that the peak shapes are broad, suggesting that the phase transitions are weakly first order in nature. It is also interesting to note from the enlargement of the region about the clearing point, shown in the insert, that there are accompanying shoulders on both the cholesteric to isotropic liquid and smectic C* to cholesteric transitions.The two shoulders are on opposite sides of the peaks to one another, indicating that these events are not artefacts of the experimental technique but are real. Further investigations of these phenomena were frustrated, however, because of decomposition of the sample in subsequent heating and cooling runs. Nevertheless, fresh specimens were found to give reproducible behaviour. In addition, no changes in the defect textures of the material, when observed in the polarizing light microscope, could be detected at the temperatures where the shoulders were found to occur.Therefore, the extra peaks/shoulders in the first-heating thermogram remain some- what of a mystery. The cooling cycle shows that these two events in the thermogram become better resolved, but the shoulder at the cholesteric to C* transition does not correspond in tempera- ture to the inversion point in the cholesteric phase. At the present time, therefore, we have no real explanation for the appearance of these shoulders; however, their presence is the subject of further investigations. Pitch The pitch of the helix in the cholesteric phase of compound 5 was measured as a function of temperature from a point I I I I 1 I 125 130 135 140 145 150 Fig. 1 DSC heating cycle for (S)-4-n-octyloxy-2,3-difluorobiphenyl-4'-yl3-fluoro-4-( 2-fluorooctanoyloxy)benzoate, 5.Heating rate 10"C min-'. The second trace shows a blow up of the smectic C* to cholesteric and cholesteric to isotropic liquid transitions. In this trace shoulders on both peaks are clearly visible. J. MATER. CHEM., 1994, VOL. 4 near to the clearing transition down to a temperature close to the phase change to the smectic C* phase. The pitch was found to diverge as the temperature was reduced; however, after the inversion in the sign of the helix had taken place the temperature range preceding the formation of the smectic C* phase was too short for accurate measurements to be made. Therefore, Table 1 gives the pitch (pm) measured up to the point of the inversion, and the values are plotted as a function of temperature in Fig.2. This figure demonstrates quite clearly that the pitch diverges with a reduction in temperature, thereby supporting the view that a change in handedness of the helix occurs. It should be noted in this figure that the temperature over which the cholesteric phase exists is shifted to lower temperature. This is the result of the experimental set-up where a thick cell and a different oven/microfurnace were used in the determination of the pitch. Spontaneous Polarization The spontaneous polarization was measured as a function of temperature in the smectic C* ferroelectric phase of (S)-4-n-octyloxy-2,3-difluorobiphenyl-4'-yl 3-fluoro-4-( 2-fluoro- octanoyloxy) benzoate, 5.The results obtained for three consecutive cooling runs are superimposed in Fig. 3. It can be seen from this graph that P, increases almost linearly with the decreasing temperature which is a little unexpected for a material that exhibits a direct cholesteric to ferroelectric C* transition. For a first-order phase change of this type it is to be expected that P, would show a sharp jump at the phase transition before levelling off rapidly as the temperature is reduced. This more gradual rise in the polarization might be due to the fact that the cholesteric to smectic C* transition is a relatively weak first-order transition as shown by DSC. The direction of the P, in the smectic C* phase was determined according to standard procedures' by poling the material which was held in an ITO-coated electrooptic cell.Throughout the whole temperature range of the phase only a positive spontaneous polarisation, P,(+), was observed, con- firming that no polarisation inversion occurred. Table 1 temperature/"C pitch/pm 136.4 8.5 137.6 6.2 138.1 5.6 140.2 4.4 141.5 3.6 143.2 1.8 + 8-6-Es-+I= .-4-P 4 2-4 136 138 140 142 144 TI"C Fig.2 Pitch of the helix in the cholesteric phase measured as a function of temperature for (S)-4-n-octyloxy-2,3-difluorobiphenyl-4'-yl 3-fluoro-4-(2-fluorooctanoyloxy)benzoate, 5 200-160-cu 'E 120-0 92 80-'Ol 80 90 100 110 120 130 TI"C Fig. 3 P, in the ferroelectric smectic C* phase measured as a function of temperature for (S)-4-n-octyloxy-2,3-difluorobiphenyl-4-yl 3-fluoro-4-( 2-fluorooctanoyloxy) benzoate, 5 The maximum value obtained for the polarization is of the order of 200 nC cm-' which is almost twice as large as that observed for the equivalent 2-methylalkyl-substituted systems, i.e.the fluoro-substituent at the chiral centre exchanged for a methyl group. This increase in size of the polarization can be therefore attributed to the increased effective dipole at the chiral centre. Tilt Angle In the process of measuring P,, the optical tilt angle was also determined as a function of temperature. The results obtained are shown in Fig. 4.The variation of the optical tilt angle with temperature was quite surprising.Initially, at the choles- teric to smectic C* transition, a jump in the value of the tilt angle was found as expected for a first-order phase transition. However, instead of the tilt angle levelling off with the reduction in temperature, it started to fall slowly, dmost halving its value over a 50"Ctemperature range. This behav- iour is not due to a falling value of the spontaneous polariz- ation as it increases steadily over the same temperature regime. Moreover, the tilt angle was found to be relatively small for a material that exhibits a cholesteric to smectic C* transition, for which tilt angles of the order to 45"are not uncommon. As the optical tilt angle is related closely to the positions that the transition moments of the molecules make with respect to the layers, these results appear to indicate that the angle which the aromatic core makes with the layer normal decreases with reduced temperature, i.e.the moleculeb stand up as the sample is cooled. This is the reverse of the normally expected behaviour. 251 ...V. 80 90 100 iio ' 120 ' 130 77°C Fig. 4 Optical tilt angle measured as a function of temperature for (S)-4-n-octyloxy-2,3-difluorobiphenyl-4'-yl3-fluoro-4-( 7-fluoro- octanoyloxy) benzoate, 5 J. MATER. CHEM., 1994,VOL. 4 ‘F L 19 Fig. 5 Zigzag shaped conformer structure of (S)-4-n-octyloxy-2,3-difluorobiphenyl-4’-yl3-fluoro-4-(2-fluorooctanoyloxy)benzoate, 5 The results suggest that the parallel core-core interactions (low tilt) in this particular system are relatively strong in comparison to the situation where they are only partially overlapping (high tilt). This hypothesis is somewhat supported by the fact that the phase transition from the cholesteric phase is only weakly first order, whereas typically for the cholesteric to smectic C* phase change it is strongly first order.Discussion The results obtained show that there is an inversion in the helical twist direction in the cholesteric phase of (S)-4-n-octyloxy-2,3-difluoro biphenyl4’-yl 3-fluoro-4-( 2-fluoro- octanoyloxy) benzoate, 5. The helical order, however, does not invert in the smectic C* phase, and polarization and tilt-angle studies show that the direction of P, remains constant with respect to temperature.In previous investigations of inversion phenomena we have ’1 22 23 Fig. 6 Structures of two conformers where the fluoro substituents have been brought into close proximity suggested that conformational structures play an important role in determining both the twist and polarization directions. In this material the structural architecture is very different from that of other compounds that show inversions. In particular, the chiral centre in compound 5 is closer to the core than in other ‘inversion’ materials which have chiral centres usually removed from the aromatic core by at least two atoms, thereby allowing some degree of free rotation about the chiral centre it~e1f.l~ Similarly, the peripheral ali- phatic chain is longer in compound 5 than in compounds 1 to 4 inclusive.We have suggested in the past that a longer peripheral chain has the effect of rotationally damping the motion of the chiral centre, consequently leading to higher spontaneous polarizations.’* The rotational damping, there- fore, should lead to one principal conformational structure being present, thereby negating the possibility of inversions occurring in chirality-dependent properties. This can be Seen to be clearly the case when the conformational structures of compound 5 are considered. The conformers related to struc- tural changes about the chiral centre are shown together in Fig. 5. In this figure, the structures of the conformers of compound 5 are shown in their most extended forms giving the molecule an overall gross zigzag shape which is conducive to the formation of smectic C* phases.” Structures 18 and 19 show a trans relationship of the carbonyl group and the aromatic fluoro substituent, whereas this arrangement is cis for structures 20 and 21.It is expected that the cis forms will be of much higher energy than the related trans structures because of the increased steric hindrance and strong repulsive polar effects. J. MATER. CHEM., 1994, VOL. 4 In all four structures, assuming that the peripheral aliphatic chain attached to the chiral centre is fully extended and, for reasons of packing constraints, the material retains an overall zigzag molecular shape, then the steric and polar properties about the chiral centre will be similar for each conformer, i.e.the steric bulk and dipole associated with the chiral centre will be on the same side of the molecule in all four conformers. If this is the case the four conformers will have the same associated twist and polarization directions, and therefore there will be no competition between conformers to drive the inversion phenomena. However, it is known2’ in a variety of difluoro-substituted systems that the fluoro substituents prefer to be located adjacent to one another with the fluorine atoms lying at the minimum van der Waals’ distance of approach. If we speculate that this is the case for compound 5, and there is some evidence from NMR studies on closely related materials which suggests this is in fact the then we can examine the conformers in slightly more detail with respect to their steric and dipolar properties about the chiral centre.Fig. 6 shows the structures of two possible conformers in the vicinity of the aromatic ring that carries the chiral group. The fluoro substituents of the chiral centre and the aromatic ring have been brought into close approximation so that the fluoro atom lies to the right of the ring in structure 22 and to the left in 23.It can be seen from the stereochemistry about the chiral centre in 22 that the peripheral chain lies closer to the long axis of the molecule than it does in structure 23.Thus, it might be expected that structure 22 is more conducive to forming liquid-crystal phases because of its lath-like shape, and therefore it may be preferred over structure 23.Nevertheless, the two structures will have different polar and steric properties, and more importantly when the two confor- mers are compared it can be seen that two effects will operate from opposing sides of the molecular structure. The opposing effects for the related fluoro-substituents, the carbonyl groups and the terminal aliphatic chains will put the two conformers into competition. Once we have generated a competition between the two conformer species we can suggest that their concentrations are temperature dependent. If the two species are intercon- vertable via a small energy barrier, then at a given temperature one species will dominate over the other, and when this dominance is reversed at another temperature an inversion in chiral properties will occur as suggested previou~ly.~~ Conclusions In conclusion we have demonstrated that (S)-4-n-octyloxy-2,3-difluorobiphenyl-4’-yl 3-fluoro-4-(2-fluoro-octanoyloxy)benzoate, 5, exhibits a twist inversion in helix of its cholesteric phase.We have shown that a conventional model of competing conformer species cannot be used to explain this inversion phenomenon, and therefore we speculate that the competing species can be created through the fluoro interactions of the fluoro substituent in the aromatic core and the fluoro substituent at the chiral centre. Compound 5 is also shown to possess unusual tilt-angle and thermodynamic properties.The authors would like to thank The University of Pretoria for an Overseas Research Grant for C. L., and Thorn EM1 CRL and Bell Northern Research (Europe) for support of a Lectureship to P. S. We are also grateful to Mr. A. Hassett for mass spectral analysis. References 1 J. W. Goodby, R. Blinc, N. A. Clark, S. T. Lagerwall, M. A. Osipov, S. A. Pikin, T. Sakura, K. Yoshino and H. Zeks, Ferroelectric Liquid Crystals, Gordon and Breach, Philadelphia, 1991. 2 J. W. Goodby, E. Chin, J. M. Geary, J. S. Patel and P. 1,. Finn, J. Chem. SOC., Faraday Trans. I, 1987,83,3429. 3 J. S. Patel and J. W. Goodby, Phil. Mag. Lett., 1987,55,283. 4 A. J. Slaney, I. Nishiyama, P. Styring and J. W. Goodby, J.Mater. Chem., 1992,2,805.5 A. J. Slaney, unpublished results, 1992. 6 P. Styring, J. D. Vuijk, I. Nishiyama, A. J. Slaney and J. W. Goodby, J. Mater. Chem., 1993,3, 399. 7 M. F. Nabor, J. T. Nguyen, C. Destrade, J. P. 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