J. MATER. CHEM., 1994, 4( ll), 1673-1688 Synthesis, Transition Temperatures, some Physical Properties and the Influence of Linkages, Outboard Dipoles and Double Bonds on Smectic C Formation in Cyclohexylphenylpyrimidines Stephen M. Kelly" and Jurg Funfschilling F. Hoffmann-La Roche Ltd., Dept. RLCR, CH-4002 Basle, Switzerland A trans-l,4-disubstituted cyclohexane ring has been introduced into known two-ring phenylpyrimidines to produce a wide variety of new three-ring cyclohexylphenylpyrimidines. The length and type of the terminal chains and linking units have been varied systematically. The effect of introducing a carbon-carbon double bond of defined configuration into various positions of both terminal chains has also been investigated. The influence of lateral dipoles (i.e oxygen and carboxyl groups) in different positions (central and terminal) in the molecular core of a model system on the smectic C (S,) transition temperature has been studied and related in a simple empirical way to standard theories for S, phase formation.Isolated, non-conjugated outboard dipoles (i.e. in cyclohexyl ethers and esters) have been found to destabilise the Sc and nematic (N) phases. Conjugated outboard dipoles (i.e. in phenyl ethers and esters) lead to substantial increases in the Sc transition temperature and usually to a widening of the Sc temperature range. Most of the new cyclohexylphenylpyrimidines exhibit a variety of smectic phases as well as Sc and N phases. Several homologous series of the most interesting cyclohexylphenylpyrimidines incorporating oxygen atoms or carboxy groups and/or a carbon-carbon double bond were synthesized and found to exhibit a relatively wide-range Sc phase at elevated temperatures.In admixture with a chiral smectic C (S,. base mixture, some of the new three-ring cyclohexyl- phenylpyrimidines can induce a substantial increase in the Sc-and N transition temperatures without increasing the viscosity (and thus response times) excessively. Electro-optic display devices utilising ferroelectric liquid crys- tals (FLCDs) are being developed for commercial applications with a high information Content.'-'' These display devices are characterised by exceptionally fast response times (ps), high contrast and good viewing angle dependence. The successful commercialisation of these displays depends not only the resolution of problems concerned with display device con- struction, stability and addressing, but also on the optimis- ation of the ferroelectric mixtures used, which must exhibit a broad spectrum of narrowly defined physical A chiral smectic C (S,*) phase in conjunction with a narrow smectic A (S,) and nematic (N) phases are usually regarded as in order to obtain good orientation for sur- face-stabilised ferroelectric display devices ( SSFLCDS).',~A smectic A phase is not required for those effects based on orientation by electric fields (e.g.short pitch bistable and dis- torted helix ferroelectric display devices SBFLCDs and DHFLCDs).'-'* Although SSFLCDs require only a moder- ately high value of spontaneous polarisation (P,) and long pitch values (p) and SBFLCDs and DHFLCDs require the opposite (i.e.very high P, values and a short pitch), the non- optically active base mixture for each display type can be devised using different amounts of the same components and then doped with the appropriate optically active (chiral) dopants to achieve the desired values for the pitch and spontaneous polari~ation.l~-~~ Each base mixture must be chemically, thermally, photo- and electro-chemically stable, exhibit a small birefringence (An), a low rotational viscosity (y), low melting point (Tm),large tilt angle (0) and high smectic C (S,) transition temperatures (TSc*).Thus, there is a techno- logical requirement for compounds with high S, transition temperatures that can be used to increase the S,* transition temperature (Ts,*) without being seriously detrimental to the other essential physical properties [e.g.melting point, response times (T)~tilt angle].Three-ring systems offer the possibility of introducing the trans-1,4-disubstituted cyclohexane ring into the core in order to reduce the viscosity and birefringence. Almost no two-ring compounds incorporating the trans-1,4-disubstituted cyclo- hexane ring have been found to exhibit an S, phase. There are numerous reports concerning alkyl/alkoxy-su bstituted two- and three-ring phenylpyrimidine~,l~-~~ some ()f which incorporated a trans-1,4-disubstituted cyclohexane ring.29-34 However, preliminary investigations of a small number of 5-(trans-4-alkylcyclohexyl)-2-(4-alkylphenyl)pyrimidinr~s34with various linking units (e.g.CH,CH2, CH,O, CO,) between the cyclohexane and benzene rings revealed that these materials could be used to increase the S, transition temperature of a host mixture without increasing the viscosity (and .bus, the response time) excessively, while also decreasing the birefrin- gence. These investigations have now been extended to include variations in the length and type of the terminal chains and linking units. As it has recently been sho~n~~-~~ that the introduction of a carbon-carbon double bond in certain positions and configurations into various positions of the terminal chains of two-ring phenylpyrimidine~~~-~~ can lead to improvements in the transition temperatures, response times, tilt angles etc.of the base mixtures containing them, this effect has also been investigated for the new 5-(trans-4- alkylcyclohexyl)-2-(4-alkylphenyl)pyrimidines with various linking units [ie. -(direct bond), CH2CH2, CH20, COz] between the cyclohexane and benzene rings under study. Only substances containing a trans-l,4-disubstituted cyclo- hexane ring have been prepared as it is most probiible that the corresponding materials incorporating a cis-1,4-disubsti- tuted cyclohexane ring would not be liquid crystalline. Even small amounts of the latter lead to substantial reductions in the liquid-crystal transition temperatures of the former and must be removed during synthesis. This is due to the angled, non-collinear configuration of the cis-1,4-disubstitutcd cyclo- hexane This is well documented and in order to form calamitic liquid crystals a semi-rigid, lath-like structure is required (i.e.the ring-substituent bonds should be co-axial or at least parallel to each ~ther).l~-~l J. MATER. CHEM., 1994, VOL. 4 Most statistical theories for the Sc phase postulate that lateral dipole moments at an angle to the molecular axis promote S, mesophase behavio~r.~’~~~ However, it has been shown that such dipole moments are not essential for Sc formation, but often still lead to higher Sc transition tempera- ture~.~~,~’However, only fully aromatic systems were investi- gated.44,45 This is equally valid for models based on packing forces, where only steric forces are taken into account.Therefore, it was decided to investigate the influence of different lateral dipoles in various positions in the molecular core of a three-ring cyclohexylphenylpyrimidine model system on the S, transition temperature in order to determine the validity of these theories for the case where the outboard terminal dipoles are attached alternatively to aromatic and/or aliphatic rings. c5Hll~--Z+CN -HCVEtOWtoluene1 NHfitOHl 1 ‘N NaOCH,JCH30H CQ-’ “-/+ Experimental Synthesis The three-ring cyclohexylphenylpyrimidines 1-6 and 13-18 either directly linked or with an ethyl linkage [Z = -(direct bond) or C2H4] were synthesized as shown in Scheme 1 from the known two-ring cyclohexyl ben~onitriles.~~.~~ The nitrile function was converted in the usual way with ethanolic hydrochloric acid into a benzimidoethyl ether hydrochloride and then into an benzimidamide hydrochloride using ethanolic Condensation with various straight chain (2-methoxymethylidene) aldehydes prepared in situ from the corresponding tetraacetals as ~s~a1~~,~~ yielded the desired three-ring cyclohexylphenylpyrimidines 1-6 and 13-18.A Williamson alkylation of the required 4-(5-n-alkylpyrimidin-1-6 and 13-18 59 and 69-73 \ 44,54and 65-68 Scheme 1 J. MATER. CHEM., 1994, VOL. 4 2-yl)phenols3' with the appropriate (trans-4-n-alkylcyclohex-y1)methyl bromides4' yielded ethers 7-9 and 19-21, see Scheme 2.Esters 10-12 and 22-32 were prepared in the usual way by esterification of the required 4-( 5-n-alkylpyrimidin-2- yl)phenols3' with the necessary trans-4-alkyl- and trans-4- alkenyl-cyclohexane-1-carboxylic a~ids~~,~'using N,N-dicyclohexylcarbodimide (DCC),sO see Scheme 2. Ether 33 was prepared by alkylation of 2-[4-trans-4-hydroxycyclohexyl)phenyl]-5-decylpyrimidine" using butyl tolulene-p-sulfonate,52 see Scheme 3. Ether 34 was prepared by alkylation of trans-4-propylcyclohexan-1-01~~with 4-( 5- decylpyrimidin-2-yl) benzyl toluene-p-sulfonate prepared from 4-( 5-decylpyrimidin-2-y1) benzyl alcohol after reduction of 4-( 5-decylpyrimidin-2-y1) benzoic acid3' with lithium aluminium hydride. see Scheme 4.Alkoxy-substituted ether 35 was synthesized from 2- [4-(trans-pentylcyclo hexyl) phenyl ]pyrmi-din-5-olS3 as shown in Scheme 1. The 4-(trans-4-pentylcyclo- hexy1)benzimidamide hydr~chloride~~.~~ was reacted with (2-ethoxy -3-dimethylaminopropenylidene) dimethylammon -ium perchlorateS4 to yield 2-[ 4-trans-pentylcyclohexyl)phenyl] 5-ethoxypyrimidine. Removal of the ethoxy group by base at high temperature^^^ resulted in 2-[ 4-(trans-pentylcyclo- hexyl)phenyl]pyrimidin-5-01,~~which was alkylated with bromononane in a Williamson ether synthesis to produce nonyloxy substituted ether 35. Ether 36 was prepared by alkylation of 2-[ 4-(trans-4-hydroxycyclohexyl)phenyl]-5-non-yloxypyrimidinesl using butyl toluene-p-sulfonate,54 see Scheme 3.Ether 37 was prepared by alkylation of 4-(5-de~ylpyrimidin-2-yl)phenol~~with (trans-hydroxycylohexy1)-methyl toluene-p-sulfonate,33~55and subsequent alkylation of the hydroxy group and ethyl toluene-p-sulfonate, see Scheme 2. Ether 38 was prepared by alkylation of 4-(5-benzyloxypyrimidin-2-yl)phenolS2with (trans-4-propylcyclo- 10-1 2 and 22-24 I 7-9 and 19-21 hexy1)methanol in a Mitsunobu reaction, followed by depro- tection with hydrogen and palladium on charcoal to produce 4-{ 5-[( trans-4-prop ylcyclohexyl ) methoxy] pyrimidin-2- y1)phenol and subsequent alkylation with bromononane, see Scheme 5. Ether 39 was synthesized by alkylation of 4-(5- benzyloxypyrimidin-2-yl)phenols2with (trans-hydroxycyclo- hexy1)methyl toluene-p-sulfonate,33.55 alkylation of the hydroxy group with ethyl toluene-p-sulfonate, removal of the benzyl group and subsequent alkylation with bromononane, see Scheme 5.Ester 40 was synthesized by esterification of 2-[4-(trans-4-hydroxycyclohexyl ) phenyl 1-5-de~ylpyrimidine~~with butanoic acid as usual using DCC," see Scheme 3. Ester 41 was prepared as usual by esterification of trans-4-propylcyclohexan-1-01~~with 4-( 5-decylpy rimidin-2-y1)benzoic acid using DCC, see Scheme 4.The 2-[ 4-trans- pent ylcyclohexyl )phenyl ]pyrimidine- 5-oIS3 was es terif ied with nonanoic acid to produce ester 42, see Scheme 1. Ester 43 was synthesized by esterification of 4-{5-[(trans-4-propylcyclo-hexyl)methoxy] pyrimidin-2-yl} phenol with nonanoic acid, see Scheme 5.Esterification of 2-[ 4-(trans-pentylcyclohexyl) phenyl] pyrimidin-5-olS3 with nonanoic acid yielded ester 45, see Scheme 1. The 2-[4-(trans-pentylcyclohexyl)phenyl]pyri-midin-5-olS3 was alkylated in a Mitsunobu reaction with the appropriate (E)-alk-2-en-l-ols and alken-1-01s with a terminal double bond to yield alkenyloxy-substituted ethers 44, 54 and 65-68 and 59 and 69-73, respectively, or alkylated in a Williamson ether synthesis with bromononane to produce nonyloxy-substituted ether 53 or esterified with alkanoic acids using DCC to produce esters 60-64, see Scheme 1. The methods of synthesis and structural analysis of the new three-ring cyclohexyl phenylpyrimidines are described in detail below. The configuration of the carbonsarbon double bond in the alkenyl chain of new ester 45 and ethers 44, 54 and n 37 Scheme 2 J.MATER. CHEM., 1994, VOL. 4 33 \ 1 40 -00 --OCQHIQ 36 Scheme 3 65-68 was confirmed by 'H nuclear magnetic resonance (NMR) spectroscopy (the trans-olefinic coupling constants, z12-18 Hz, are larger than those of the corresponding cis- olefinic coupling constants, ~7-11 Hz) and by infrared (IR) spectroscopy (the trans-absorption bands are narrow and exact, M 970-960 cm-', while the cis-absorption bands are observed at distinctly different wavelengths, M 730-675 cm- I). The structural and isomeric purity was determined by differential thermal analysis (DTA) and capillary gas chroma- tography (GC) as usual and, where necessary, on liquid-crystal-packed columns.56 The transition temperatures of the compounds prepared were determined by optical microscopy using a Leitz Ortholux I1 POL BK microscope in conjunction with a Mettler FP 82 heating stage and FP 80 control unit.All the monotropic liquid crystal phases could be observed using a microscope and no virtual values (extrapolated) were deter- mined. The transition temperatures were also determined using a Mettler DTA TA 2000. The purity of the compounds was determined by thin layer chromatography (TLC), GC and DTA. A Perkin-Elmer 83 10 capillary gas chromatograph and GP-100 graphics printer were used. Precoated TLC plates (4 cm x 8 cm), SiO, SIL G/IV2s4, layer thickness 0.25 mm (Machery-Nagel, Diiren, Germany) were utilised. Column chromatography was carried out using silica gel 60 (230-400 mesh ASTM).Reaction solvents and liquid reagents were purified by distillation or drying shortly before use. Reactions were carried out under N, unless water was present as a reagent or solvent. All temperatures were meas- ured externally unless otherwise states. The 'H NMR spectra were recorded at 60 MHz (Varian T-60), 80 MHz (Bruker QP-80) or 250 MHz (Bruker HX-270). Tetramethylsilane was used as the internal standard. Mass spectrometry (MS) was carried out by using an MS9 (AEZ Manchester) spectrometer. The S,* mixture SCO 1014 consists of 4-(trans-4-{[(R)- 2-fluorohexanoyl] oxy) cyclohexy1)phenyl 2,3-difluoro-4-(octy- 1oxy)benzoate( 16 wt.%), 2-[4-( hexyloxy)phenyl]-5-nonylpyr-imidine (24 wt.%), 2-[4-(nonyloxy)pheny1]-5-nonylpyri-midine (24 wt.%), 2-[ 4-(nonyloxy)phenyl]-5-heptylpyri-midine ( 12 wt.%), 2-[4-( hexyloxy)phenyl]-5-octylpyrimidine ( 12 wt.%) and 2-[ 4-(decyloxy)phenyl]-5-octylpyrimidine (12 wt.%).The determination of the physical properties of the chiral mixtures containing the new esters was carried out as pre- viously de~cribed.~~.~~ Synthesis of Ethoxy [4-(tvans-4-pentylcycIohexyl)phen ylme t hyl- amine Hydrochloride A solution of 4-(trans-4-pentylcyclohexyl)ben~onitrile~~ (51.1 g, 0.2 mol) in ethanol (35 cm3) and toluene (200 cm3) was saturated with hydrogen chloride at 0 "C and then stirred at room temperature for 2 days. The reaction mixture was evaporated down under reduced pressure, shaken with ether (500 cm3), filtered, washed with portions of ether and finally J.MATER. CHEM., 1994, VOL. 4 o~;>cloH21HO a\LIAIH,/ether H&12 HO/434>ClOH2' 0 41 34 Scheme 4 dried under vacuum to yield 59.6 g (88%) of the hydrochloride. IR (KBr) vmax/cm-': 2994,2922,2850, 1648, 1612,1443,1053 849. MS m/z: 301 (M'), 273, (C,,H,,NO+). 'H NMR aH (CDCI,; TMS standard; 250 MHz): 0.84-0.89 (3 H, t), 1.05-1.80 (16 H, overlapping peaks), 2.50-2.60 (1 H, overlap- ping peaks), 4.58-4.67 (2 H, q), 7.48-7.51 (2 H, d), 8.05-8.08 (2 H, d), 8.44 (2 H, s). Synthesis of 4-(tvans-4-Pentylcyclohexyl )benzimidamide Hydrochloride A saturated ethanolic ammonia solution (350 cm3) was added to a solution of 4-(trans-4-pentylcyclohexyl)phenylimido-ethyl ether hydrochloride (59.6 g, 176 mmol) and ethanol (350 cm3).The reaction mixture was stirred at room tempera- ture for 2 days and then evaporated down. The solid residue was shaken with ether (500cm3), filtered, washed with por- tions of ether and finally dried under vacuum to yield 53.6 g (98%) of the benzamidine. IR (KBr) vma,/cm-l: 3252, 3073, 2952, 2923, 2850, 1666, 1610, 1539, 1498, 1448, 849. MS m/z: (M'), 256, (C18H26N'). 'H NMR JH(CDCl,; TMS standard; 250 MHz): 0.84-1.50 (16 H, overlapping peaks), 1.81 (4 H, t), 2.50-2.58 (1 H, overlapping peaks), 2.50-2.58 (1 H, s), 7.45-7.48 (2 H, d), 7.76-7.80 (2 H, d), 9.19 (4 H, s). Synthesis of 2-[ 4-(trans-4-Propylcyclohexyl)phenyll-5-heptylpyrimidine, 1 A 30mol% solution of sodium methoxide in methanol (10cm3) was added dropwise to a mixture of 2-(methoxy-methylidene)n~nenal~~*~~(4.6 mmol), 4-(trans-4-pentylcyclo- hexyl) benzimidamide hydrochloride ( 1.O g, 3.6 mm ol), and methanol (15 cm3) at room temperature and stirred overnight.Concentrated hydrochloric acid was added (pH 3-41 and the inorganic material filtered off. The filtrate was conc;entrated under reduced pressure, dichloromethane (50cm,) was added and the resultant solution washed with water (2 x 100 cm3), then dried (MgSO,), filtered and evaporated down. The residue was purified by column chromatography on silica gel using a 9: 1 hexane-ethyl acetate mixture as eluent and recrystallised from tert-butyl methyl ether to yield 6.8 g (33%) of the desired product (1).IR (KBr) vma,/cm-': 2855, 2824, 2852, 1610, 1584, 1539, 1429, 797. MS m/z: 378 (hil'). The transition temperatures of ether 1 and of the similar ethers 2-6 and 13-18, prepared using this general method, are collated in Tables 1 and 2. Synthesis of 2-(4-[(trans-4-Propylcyclohexyl) methoxj 1-phenyl)-5-heptylpyrimidine, 7 A mixture of (trans-4-propylcyclohexyl)methyl bromide4' (0.24 g, 1.1 mmol), 4-( 5-heptylpyrimidin-2-yl)phen01.'~(0.30 g, 1.1mmol), potassium carbonate (0.43 g, 1.4 mmol) and butan- 2-one (50cm3) was heated under reflux overnight, filtered to remove any inorganic material, diluted with water ( 500 cm3) and then extracted into diethyl ether (3 x 100 cm3).'The com- bined organic extracts were washed with water (2 x 500 cm3), dried (MgSO,), filtered and then evaporated dciwn. The residue was purified by column chromatography on silica gel using a 9: 1 hexanHthy1 acetate mixture as eluent and recrystallised from ethanol to yield 0.35 g (77%) of the pure J. MATER. CHEM., 1994, VOL. 4 43 39 Scheme 5 Table 1 Transition temperatures for 1-12" ether. IR (KBr) v,,,/cm-l: 2922,2859,1585, 1547, 1519, 1427, 1255, 1167, 1028, 844, 796. MS m/z: 408 (M'), 270 (Cl7Hz2N2O+),185, (CllH9N20+). The transition tempera- tures of ether 7 and of similar ethers 8,9 and 19-21, prepared using this general method, are collated in Tables 1 and 2. Synthesis of 4-(5-Heptylpyrimidin-2-yl )phenyl trans-4-1 3 106 2 5 75 Propylcyclohexane-1-carboxylate,10 3 7 64 A solution of DCC (0.23 g, 1.1 mmol) in dichloromethane 4 3 101 (10 cm3) was added slowly to a solution of 4-(5-heptylpyrimi-5 5 92 6 7 86 din-2-yl)~henol~~(0.25 g, 0.9 mmol), trans-4-propylcyclohex- 7 3 113 ane-1-carboxylic acid48 (0.13 g, 0.9 mmol, 4-(dimethylamino)- 8 5 104 pyridine (0.04 g) and dichloromethane (25 cm3) at 0 "C.The 9 7 102 mixture was stirred at room temperature overnight, filtered 10 3 99 to remove precipitated material and then the filtrate was11 5 108 12 7 91 evaporated down under reduced pressure. The residue was purified by column chromatography on silica gel using a 9 : 1 a Values given in parentheses represent a monotropic transition hexane+thyl acetate mixture as eluent and then recrystallised temperature.from ethanol. The transition temperatures of ester 10 and J. MATER. CHEM., 1994, VOL. 4 Table 2 Transition temperatures for 13-24 compound Z n (C-S)/OC 13 3 72 77 116 151 14 5 60 83 93 131 152 -15 7 50 101 136 149 C2H4 3 88 92 105 131 C2H4 5 84 98 118 132 C2H4 7 70 101 124 130 19 CH20 3 93 101 -139 20 CH20 5 87 118 -142 21 CH20 7 86 120 -136 22 co2 3 70 89 -161 23 co2 5 64 85 104 -161 24 co2 7 78 89 116 -158 Table 3 Transition temperatures for esters 22-29 25 1 76 77 124 26 2 82 82 142 22 3 76 89 161 27 4 40 82 99 160 23 5 64 85 I04 161 6 24 7 78 89 116 158 8 28 9 77 85 123 153 29 10 77 87 125 150 Table 4 Transition temperatures for esters 23, 27 and 30-32 ~~ compound R (C-S,/S,/Sc)/°C (S,-S,)/OC (S4-S3)/”C (S,-S,)/”C (S,-S,-)/OC (S,-N)/”C (N-I)/”C 27 40 ---82 99 160W W30 71 ----92 162 W23 64 ---85 104 161 31 v 71 ----97 150 32 --If--52 56 64 69 -92 170 esters 11, 12, 22-32, prepared using this general procedure, (0.25 g, 0.8 mmol), potassium tert-butoxide (0.43 g, 3.1 mmol) are given in Tables 1-4.‘H NMR S, (CDC1,; TMS standard; and 1,2-dimethoxyethane (50 cm3) was stirred at room tem- 250 MHz): 0.88-0.90 (6 H, overlapping peaks), 1.27-1.32 perature overnight. It was then filtered to remove inorganic (10 H, overlapping peaks), 1.52-1.66 (4 H, overlapping peaks), material, diluted with water (500 cm3) and then extracted into 2.54-2.61 (4 H, overlapping peaks), 7.16-7.26 (2 H, overlap-diethyl ether (3 x 100cm3).The combined organic extracts ping peaks), 8.42-8.46 (2 H, d), 8.61 (2 H, s). IR (KBr) were washed with water (2 x 500 cm3), dried (MgSO,), filtered v,a,/cm-l: 2956,2925, 2853, 1731, 1598, 1476, 1205, 843. MS and then evaporated down. The residue was purified by 125 (C,H130). column chromatography on silica gel using a 9: 1 hexane-m/z: 396 (M’), 272 (C17H24N20), ethyl acetate mixture as eluent and recrystallised from ethanol to yield 0.25 g (66%) of the desired pyrimidine. IR (KBr)Synthesis of 2-[ 4-(truns-Butoxycyclohexyl)phenyl]-5-v,,,/cm-’: 2923, 2851, 1582, 1543, 1513, 1426, 1254, 1025, decylpyrimidine, 33 847, 790. MS m/z: 490 (M’), 270 (CI7Hz2N20+),185, A mixture of butyl toluene-4-sulfonate (TCI; 0.19 g, 0.9 mmol), (Cl,H,N20+).NMR 6, (CDCI,; TMS standard; 2SO MHz): 2-[4-( trans-4- h ydr ox yc yclo hex yl )p henyl ]-5 -de~ylpyrimidine~~ 0.88-0.89 (6 H, overlapping peaks), 1.29 (18 H, overlapping peaks), 1.62 (2 H, overlapping peaks), 2.60 (2 H, t), 4.02 (2 H, t) 6.96-7.00 (2 H, d), 8.32-8.36 (2 H, d), 8.57 (2 H, s). The transition temperatures of ether 33 are collated in Table 5. 4-(5-Decylpyrimidin-2-y1) benzyl Alcohol A solution of 4-( 5-decylpyrimidin-2-y1) benzoic acid3' (1.1 g, 3.9 mmol) in diethyl ether (25 cm3) was added dropwise to a solution of lithium aluminium hydride (0.2 g, 5.1 mmol) and diethyl ether (50cm3), which was cooled in an ice-bath. The reaction mixture was heated under gentle reflux for a further 2 h and then cooled to 0 "C in an ice-bath.Water (25 cm3) and 25% hydrochloric acid (50cm3) were added dropwise to the cooled reaction mixture. The organic layer was separated off and the aqueous layer extracted with ether (3 x 50 cm'). The combined organic layers were washed with water (500cm3) and saturated potassium carbonate (3 x 50 cm3), then dried (MgSO,), filtered and evaporated down to yield 1.Og (95%) of the desired pyrimidine. IR (KBr) v,,,/cm-': 3404, 3108, 3064, 2918, 2850, 1702, 1634, 1591, 1565, 1510, 1426,1299,1054,830. MS m/z:326 (M+),297 (C19H25N20'), 213 (CI3Hl3N2O+). 4-( 5-Decylpyrimidin-2-y1) benzyl Toluene-p-sulfonate A solution of toluene-4-sulfonyl chloride (0.60 g, 3.2 mmol) in dichloromethane (10 cm3) was added slowly to a solution of 44 5-decylpyrimidin-2-y1)benzyl alcohol ( 1.0 g, 3.1 mmol), pyridine (1.2 g, 15.3 mmol) and dichloromethane (50 cm3) at 0 "C.The reaction mixture was stirred at 0 "C for 6 h, washed with dilute hydrochloric acid (2 x 50 cm3), water (2 x 50 cm3) and dilute sodium carbonate solution (2 x 50 cm3), then dried (MgSO,), filtered and evaporated down to yield 0.9 g (62%) of the desired toluene-p-sulfonate. Synthesis of trans-l-[4-( 5-Decylpyrimidin-2-y1) benzyloxyll-4- propylcyclohexane, 34 A mixture of 4-( 5-decylpyrimidin-2-y1) benzyl toluene-p-sul- fonate (0.9 g, 0.6 mmol), trans-4-propylcyclohexan-1-0148 (0.3 g, 0.6 mmol), potassium tert-butoxide (0.23 g, 2.1 mmol) and 1,2-dimethoxyethane (50cm3) was stirred at room tem- perature overnight, then worked up and purified as above for 33 to yield 0.25 g (66%) of the desired pyrimidine.IR (KBr) vm,,/cm-': 2955, 2921, 2852, 1767, 1606, 1582, 1430, 1234, 1114, 1027, 851, 794. MS m/z: 466 (M'), 326 (C21H30N20f), J. MATER. CHEM., 1994, VOL. 4 188 (C, ,Hl2N20+). The liquid crystal transition temperatures of ether 34 are collated in Table 5. Synthesis of 2-[ 4-(tvans-4-Pentylcyclohexyl)phenyll-5-nonyloxypyrimidine, 35 A mixture of l-bromononane (Fluka; 0.19 g, 0.9 mmol), 2-[4- trans-4-pentylcyclohexyl) phenyl] pyrimidin-5-01 53 (0.25 g, 0.8 mmol), potassium carbonate (0.43 g, 3.1 mmol) and butan-2- one (50cm3) was heated under reflux overnight and then worked up and purified as described above for 7 to yield 0.25 g (66%) of the desired ether. IR (KBr) vmax/cm-': 2921, 2850, 1582, 1548, 1436, 1278, 1015, 851, 796. MS m/z: 450 (M').'H NMR bH (CDC1,; TMS standard; 00 MHz): 0.89-1.60 (34 H, overlapping peaks), 1.85-1.90 (6 H, overlap-ping peaks), 2.50 (1 H, t), 4.05-4.11 (2 H, t), 7.23-7.32 (2 H, t), 8.23-8.26 (2 H, d),8.43 (2 H, s).The transition temperatures of ether 35 and ether 53, prepared using this general method, are collated in Tables 5 and 6. Synthesis of 2-[ 4-(trans-4-Butyloxycyclohexyl)phenyll-5-nonyloxypyrimidine, 36 A mixture of butyl toluene-4-sulfonate (TCI; 0.17 g, 0.76 mmol), 2-[ 4-(trans-4-hydroxycyclohexyl)phenyl]-5-non-yl~xypyrimidine~' (0.10 g, 0.25 mmol), potassium tert-butox- ide (0.09 g, 0.83 mmol) and 1,2-dimethoxymethane (20 cm3) was heated under reflux overnight and then worked up and purified as described as above for 7 to yield 0.05 g (44%) of the desired ether.IR (KBr) v,,,/cm-': 3436, 2928, 2854, 1610, 1576, 1544, 1513, 1437, 1279, 1107, 847, 794. MS m/z: 452 (M'), 378 (C25H34N20+). 'H NMR 6, (CDCI,; TMS stan- dard; 250 MHz): 0.88-0.96 (6 H, overlapping peaks), 1.28-1.58 (22 H, overlapping peaks), 1.62 (2 H, overlapping peaks), 2.00 (2 H, d), 2.18 (2 H, d), 2.60 (1 H, t), 3.25 (1 H, overlapping peaks), 3.47-3.52 (1 H, t), 4.06-4.11 (1 H, t), 7.26-7.32 (2 H, t), 8.23-8.27 (2 H, d), 8.43 (2 H, s). The transition temperatures of ether 36 are collated in Table 5. Synthesis of 2-( [4-(trans-4-E thox yc yclohex y 1 )met hox y ]-phenyl}-5-decylypyrimidine,37 A mixture of ethyl toluene-4-sulfonate (TCI; 0.3 I g, 1.6 mmol), 2- (4- [(trans-4-hydroxycyclohexyl)methoxyJ phenyl- 5-decyl- pyrimidine33 (0.20 g, 0.5 mmol), potassium tert-butoxide (0.15 g, 1.4 mmol) and 1,2-dimethoxyethane (50 cm3) was Table 5 Transition temperatures for 14, 19, 22 and 33-43" Rl-0 R2.*I> 14 - 60 33 - 59 34 OCH2 79 19 CH,O 93 35 - 73 36 - 88 37 CH,O 95 38 CH,O 100 39 CH20 88 40 - 82 41 02c 90 22 COZ 76 42 - 122 43 CH,O 125 a Values given in parentheses represent a monotropic transition temperature.131 152 137 143 ~~ 91 139 139 176 153 167 111 130 -158 -154 150 158 96 142 -161 -178 -169 J. MATER.CHEM., 1994, VOL. 4 168 1 Table 6 Transition temperatures for two-ring ethers 46-52 and three-ring ethers 53-59" compound X R (C-S,/S,/N/I)/"C (S,-S,/N)/"C (S,-S,/N)/"C (SA-N/I)/"C (N-I)/"C ref. 46 36 - 53 85 - 39 47 56 - 65 82 - 39 48 52 (44) - 39 49 47 82 - 39 50 38 58 - 39 51 51 86 - 39 52 34 - 38 77 - 39 53 65 83 119 - 181 - 54 97 - 115 - 176 - 55 93 - 161 - 56 87 - 184 - 57 86 - 168 - 58 81 130 185 - 59 67 121 176 - a Values given in parentheses represent a monotropic transition temperature. heated under reflux overnight and then worked up and purified as described above for ether 7 to yield 0.03 g (14%) of the desired ether. IR (KBr) v,,,/cm-': 2923, 2851, 1607, 1583, 1541, 1513, 1430, 1252, 1165, 1110,1035, 847, 800.MS m/~:452 (M'), 312 (C19H24N202).'H NMR 6, (CDCl,; TMS standard; 250 MHz): 0.87 (3 H, d), 1.18-1.26 (21 H, overlapping peaks), 1.59 (2 H, overlapping peaks), 1.80 (2H, overlapping peaks), 2.00 (2 H, d), 2.18 (2 H, d), 2.60 (2 H, t), 3.25 (1 H, overlapping peaks), 3.52-3.56 (2 H, q), 3.81-3.84 (2H, d), 6.94-6.98(2 H, d), 7.26 (2 H, s), 8.32-8.36(2H,d), 8.56(2 H, s). The transition temperatures of this ether are collated in Table 5. Synthesis of 2-(4-[(trans-4-Propylcyclohexyl)methoxy]-phenyl)-5-benzylox ypyrimidine A mixture of (trans-4-propylcyclohexyl)methyliodide47(4.0g, 15.0mmol), 4-(5-benzyloxypyrimidin-2-yl)pheno158( 1.O g, 3.6mmol), potassium carbonate (2.0g, 14.3mmol) and butan- 2-one (50 cm3) was heated under reflux overnight.The reac- tion mixture was worked up and purified as described above for 7 to yield 0.6 g (40%) of the desired ether. IR (KBr) ~,,~/cm-~:2956, 2920, 2852, 1606, 1582, 1550, 1515, 1445, 1248, 1179, 1101, 846, 789, 747, 711. MS m/z: 416 (M'). Synthesis of 2-{4-[(trans-4-Propylcyclohexyl )methoxy]-phenyl)pyrimidin-5-01 A solution of 2-(4-[(trans-4-propylcyclohexyl)methoxy]-phenyl) -5-benzyloxypyrimidine (0.6g, 1.4mmol), ethyl acet- ate (50cm3), ethanol (50cm3), acetic acid (2cm3) and 10% palladium on active charcoal (0.2g) were hydrogenated until no more hydrogen was taken up. The catalyst was filtered off and the filtrate evaporated down and purified by recrystallis- ation from ethyl acetate to yield 1.Og (68%) of thr desired pyrimidine; mp 209-210 "C.IR (KBr) v,,,/cm-l: 3431, 2920, 2847, 1611, 1581, 1558, 1431, 1288, 844, 790. MS m/z: 326 (M + ), 188 (C,,H*N,O,). Synthesis of 2-{4-[(trans-4-Propylcyclohexyl) methoxyJ-pheny1)-5-nonyloxypyrimidine,38 A mixture of 1-bromononane (Fluka; 0.23g, 1.1 mmol), 2-(4-[(trans-4-propylcyclohexyl)methoxy]phenyl} pyrimi din-5-01 (0.25 g, 0.7mmol), potassium carbonate (0.43g, 2 9 mmol) and butan-2-one (50 cm3) was heated under reflux overnight. The reaction mixture was worked up and purified as described above for 7 to yield 0.10 g (32%) of the desired tbther. IR (KBr) vmax/cm-l: 2922, 2852, 1605, 1544, 1515, 1411, 1278, 1250, 1171, 1003, 845, 786.MS m/z: 452 (M'), 314 (CI9H2,N2O2),188 (C1,H8N202). 'H NMR dH (CD<'13; TMS standard; 250 MHz): 0.89-1.60 (29 H, overlapping peaks), 3.80-3.83 (2 H, d), 4.05-4.11 (2 H, t), 6.94-6.98 (2 H, d), 8.24-8.28 (2 H, d), 8.41 (2 H, s). The transition temperatures of this ether are collated in Table 5. Synthesis of 2-{[4-(trans-4-Hydroxycyclohexyl) methouy 1-phenyl)-5-benzyloxypyrimidine A mixture of (trans-4-hydroxycyclohexyl)methyltr duene-4-s~lfonate~~(2.2g, 7.7mmol), 4-(5-benzyloxypynmidin-2- J. MATER. CHEM., 1994, VOL. 4 (2.0 g, 7.0 mmol), potassium carbonate (3.9 g, Synthesisof 2-(4-[ trans-4-(Butanoyloxy)cyclohexyl] pheny1)- yl)phen01~~ 28.0 mmol) and butan-2-one (50cm3) was heated under reflux 5-decylpyrimidine, 40 overnight and then worked up and purified as described above for ether 7 to yield 0.5 g (18%) of the desired ether.IR (KBR) vmax/cm-': 3433, 2927, 2858, 1740, 1607, 1547, 1515, 1440, 1278, 1249, 1171, 844, 790. MS m/z: 390 (M'). Synthesis of 2-{[4-(trans-4-Ethoxycyclohexyl)methoxyl-phenyl)-5-benzyloxypyrimidine Amixture of ethyl toluene-4-sulfonate (TCI; 0.85 g, 4.2 mmol), 2-{4-[(trans-4-hydroxycyclohexyl)methoxy]phen yl} -5- benzyl- oxypyrimidine (0.50 g, 1.3 mmol), potassium tert-butoxide (0.43 g, 3.8 mmol) and 1,2-dimethoxyethane (50 cm3) was heated under reflux overnight and then worked up and purified as described above for ether 7 to yield 0.2 g (37%) of the desired ether. MS m/z: 418 (M+), 327 (Cl9H2,N2O3'), 278 (C17H16N202+ 1. Synthesis of 2-{4-[(trans-4-Ethoxycyclohexyl) methoxy 1- phenyl }pyrimidin-5-01 A solution of 2-(4-[( trans-4-ethoxycyclohexyl)methoxy]-phenyl} -5-benzyloxypyrimidine (0.15 g), ethyl acetate (50 cm3), ethanol (50cm3), acetic acid (1cm3) and 10% palladium on active charcoal (0.2 g) were hydrogenated until no more hydrogen was taken up and then worked up and purified as described above to yield 0.1 g (88%) of the desired pyrimidine. MS m/z: 328 (M').Synthesis of 2-{4[(~rans-4-Ethoxycylohexyl)methoxyl-phenyl}-5-nonyloxypyrimidine,39 A mixture of 1-bromononane (Fluka; 0.06 g, 0.3 mmol), 2-(4- [(trans-4-ethoxycyclohexyl)methoxy]phenyl) pyrimidin-5-01 (0.08 g, 0.25 mmol), potassium carbonate (0.14 g, 1.0 mmol) and butan-2-one (25 cm3) was heated under reflux overnight and then worked up and purified as described above for 7 to yield 0.06 g (53%) of the desired ether. IR (KBr) v,,,/cm-': 2926, 2854, 1606, 1544, 1514, 1434, 1273, 1250, 1168, 1109, 1031, 850, 791.MS m/z: 454 (M'), 314 (Cl9HZ6N2o2), 188 (CloH8N2O2). 'H NMR dH (CDC1,; TMS standard; 250 MHz): 0.88 (2 H, t), 1.18-1.28 (19 H, overlapping peaks), 1.58 (3 H, overlapping peaks), 1.82 (2 H, d), 2.18 (2 H, d), 3.25 (1 H, overlapping peaks), 3.52-3.55 (2 H, q), 3.81-3.83 (2 H, d), 4.05-4.10 (2 H, t), 6.93-6.97 (2 H, d), 8.24-8.28 (2 H, d), 8.41 (2 H, s). The transition temperatures of this ether are collated in Table 5. A solution of DCC (0.40g, 0.9 mmol) in dichloromethane (10cm3) was added slowly to a solution of 2-[4-(trans-4-hydroxycyclohexyl)phenyl]-5-decylpyrimidine (0.50 g, 0.8 mmol), butanoic acid (Fluka; 0.17 g, 0.8 mmol), 4-(di- methy1amino)pyridine (0.04 g) and dichloromet hane (50 cm3) at 0°C and then worked up and purified as described above for ester 10 to yield 0.32 g (84%) of the desired ester.'H NMR 6, (CDCl,; TMS standard; 250 MHz): 0.88-0.99 (6 H, overlap- ping peaks), 1.26-1.68 (24 H, overlapping peaks), 2.00 (2 H, d), 2.18 (2 H, d), 2.60 (2 h, t), 2.65 (3 H, t), 3.47-3.52 (1 H, t), 4.06-4.11 (1 H, overlapping peaks), 7.26-7.33 (2 H, t), 8.31-8.34 (2 H, d), 8.60 (2 H, s). v,,,/cm-': 2924, 2859, 1729, 1611, 1586, 1546, 1431, 1181, 1013, 801. MS m/z: 464 (M+), 376 (C25H32N20+). The transition temperatures of ester 40 are collated in Table 5.Synthesis of tvans-4-Propylcyclohexyl4-(5-Decylpyrimidin-2-yl)benzoate, 41 A solution of DCC (0.43 g, 2.1 mmol) in dichloromethane (10cm3) was added slowly to a solution of rrans-4-propyl-cyclohexan-1-01 (0.25 g, 1.8 mmol), 4-( 5-decylpyrimidin-2- yl) benzoic acid (0.6 g, 1.8 mmol), 4-(dimethy1amino)pyridine (0.04 g) and dichloromethane (25 cm3) at 0 "C and then worked up and purified as described above for ester 10 to yield 0.32 g (84%) of the desired ester. 'H NMR 6, (CDCl,; TMS standard; 250 MHz): 0.87-0.90 (6 H, overlapping peaks), 1.26-1.56 (27 H, overlapping peaks), 1.91 (2 H, d), 2.22-2.35 (2 H, d), 2.60-2.66 (2 H, t), 4.88-5.02 (1 H, overlapping peaks), 8.12-8.15 (2 H, d), 8.46-8.49 (2 H, d), 8.65 (2 H, s). IR (KBr) v,,,/cm-': 2925,2852,1711, 1545, 1432, 1274, 1133, 762.MS m/z: 464 (M'), 323 (CZ1Hz7N20). The transition temperatures of benzoate 41 are collated in Table 5. Synthesis of 2-[ 4-(trans-4-Pentylcyclohexyl)phenyl] pyrimidin- 5-yl Nonanoate, 42 A solution of DCC (0.19g7 0.9mmol) in dichloromethane (10 cm3) was added slowly to a solution of 2-[4-(trans-4-pentylcyclohexyl)phenyl]pyrimidin-5-oI (0.25 g, 0.8 mmol), nonanoic acid (Fluka; 0.14 g, 0.8 mmol), 4-(dimethylamino)- pyridine (0.04 g) and dichloromethane (25 cm3) at 0 "C and then worked up and purified as described above for ester 10 to yield 0.32 g (84%) of the desired ester. 'H NMR 6, (CDCl,; TMS standard; 250 MHz): 0.89-0.90 (6 H, overlapping peaks), Table 7 Transition temperatures for 35,42,44and 45 c5Hl,---~~~z-c6H13 ~ _____~ 35 -O< 73 128 139 176 44 - O u 94 125 - 170 42 122 113 -_ 178 -OF0 110 --21645 -*y0 J.MATER. CHEM., 1994, VOL. 4 1.28 (28 H, overlapping peaks), 1.77 (2 H, q), 1.91 (4 H, t), 2.60-2.66 (3 H, t), 7.26-7.34 (2 H, t), 8.29-8.32 (2 H, d), 8.61 (2 H, s). TR (KBr) vmax/cm-l: 2920, 2850, 1768, 1546, 1429, 1232, 1133, 857. MS m/z:464 (M'). The transition tempera- tures of nonanoate 42, (E)-non-2-enoate 45 and esters 60-64 prepared using this general method are collated in Tables 5, 7 and 8. Synthesis of 2-{4-[(tvans-4-Propylcyclohexyl) methoxy1-phenyl )pyrimidind-yl Nonanoate, 43 A solution of DCC (0.13 g, 0.7 mmol) in dichloromethane (10cm3) was added slowly to a solution of 2-(4-[(trans-4-propylcyclohexyl)methoxy] phenyl} pyrimidin-5-01 (0.20 g, 0.6 mmol), nonanoic acid (Fluka; 0.10 g, 0.6 mmol), 4-(dimethy1amino)pyridine (0.04 g) and dichloromethane (25 cm3) at 0 "C and then worked up and purified as described above for ester 10 to yield 0.32 g (84%) of the desired ester.IR (KBr) v,,,/cm-': 2921,2852,1767,1606, 1508,1430,1234, 1027, 851. MS m/z: 466 (M'), 326 (C2,H2,N202), 188 (C,,H,N202). The transition temperatures of nonanoate 43 are listed in Table 5. Synthesis of 2-[ 4-(trans-4-Pentylcyclohexyl)phenyl]-5-{[(E)-non-2-en-l-yl] oxy) pyrimidine, 44 A solution of (E)-non-2-en-l-o1 (Johnson Matthey; 0.10 g, 8 mmol ), 2-[4-(trans-4-pentylcyclohexyl)phenyl]pyrimidin-5-01~~(0.25 g, 8 mmol), diethyl azodicarboxylate (0.13 g, 8 mmol), triphenylphosphine (0.20 g, 8 mmol) and tetrahydro- furan (25 cm3) was stirred at room temperature overnight and then evaporated down.The solid residue was taken up in warm hexane (25 cm3), filtered to remove the precipitate ( PPh30) and evaporated down once more. Purification of the residue by column chromatography on silica gel using a 9 :1 hexane-ethyl acetate mixture as eluent and then recrystallis- ation from ethanol yielded 0.16 g (48%) of the desired ether. IR (KBr) v,,,/cm-': 2956,2922,2850, 1574, 1543,1439,1390, 1277, 1000, 971, 789. MS m/z: 448 (M'), 324 (C21H2,N20). 'H NMR BH (CDC1,; TMS standard; 250MHz): 0.87-0.90 (6 H, q), 1.27 (21 H, overlapping peaks), 1.90 (4 H, t), 2.08-2.12 (2 H, q), 2.45-2.56 (1 H, t), 4.59-4.62 (2 H, d), 7.26-7.32 (2 H, t), 8.23-8.26 (2 H, d), 8.45 (2 H, s).The transition temperatures of ether 44 and similar ethers 53-59 and 65-73 prepared using this general method are recorded in Tables 6, 7, 9 and 10. Results and Discussion The transition temperatures of all the compounds synthesized were determined as single components. This allows several Table 8 Transition temperatures for esters 42 and 60-64" CmHzm+1 compound m (C-S,-/N)/"C ( S,-N)/"C (N-I)/"C 60 6 112 -188 61 7 118 -185 42 8 122 (113) 178 62 9 120 125 177 63 10 118 133 172 64 11 120 138 169 Values given in parentheses represent a monotropic transition temperature. 1683 Table 9 Transition temperatures for ethers 44, 54 and 65-68 compound m (C-S,-/N)/OC (S,-N)/"C ( N-I)/"C 65 3 99 -187 66 4 99 -177 54 5 97 115 176 44 6 94 125 170 67 7 86 135 167 68 8 93 140 163 Table 10 Transition temperatures for ethers 59 and 69 -73 C5H11---m:>O-(cHz)mk compound m (C-S,/N)/"C (S,-SA)/"C (S,N)/"C ~~~ (N-I)/"C 69 4 82 - - 184 70 5 55 65 112 185 59 6 67 96 121 176 71 7 59 91 142 176 72 8 55 103 145 169 73 9 57 97 151 168 important qualitative properties of the pure compounds to be determined, such as the tendency to form liquid-crystal phases, as reflected in the absolute value of the clearing point.Similarly, the tendency to form (tilted) smectic phases is shown by the upper temperature limit of the highest smectic (tilted) phase.However, the high melting point of a three-ring compound may obscure the existence of monotropic trans- itions. Additional information can be gained from mixtures of selected compounds in a base mixture, which exhibits all the phases (smectic and nematic) of interest. The addition of a fixed (relatively small) amount of the compound to be investigated results in relatively limited changes in the trans- ition temperatures of the base mixture. This allows compo- nents with greatly differing absolute transition temperatures and phase types to be compared. Furthermore, propcrties of interest of S,* mixtures designed for use in FLCDs such as the spontaneous polarization and switching time are only measurable in mixtures.Therefore, a fixed amount (1 5 wt.%) of a selection of the new compounds was added to a standard S,* base mixture SCO 1014, which exhibits the fcdlowing phases: C/Sx-Sc* = -7.6 "C, Sc*-SA =60.6 "C, S \-N* = 67.7 "C and N*-I =74.6 "C). The liquid-crystal transition tem- peratures (C-Sc*, SrSc*, Sc*-SA, SA-N* and N*-I) the spontaneous polarisation (P,) and the observed switching time (z) of the resulting mixtures were determined under standard conditions (z: 1OVpp p-' square wave, time to maximum current, at 25 "C; P,: 10 Hz, 10 Vpp p-' triangle). Influence of Linkages The transition temperatures of the heptyl homologues of the three-ring cyclohexylphenylpyrimidines (1-12) either directly linked or with ethyl, methoxy or ester linkages (denoted as -, CH2CH2, CH20 and C02, respectively) are collated in Table 1.The melting points (T',) of the directly linked com- pounds (1-3), the ethanes (4-6), the ethers (7-9) and the esters (10-12) are moderate for three-ring compounds (82, 93, 106 and 100"C, on average, respectively). The corresponding values for the clearing point (TNI)are relatively low (163, 140, 149 and 177"C, on average, respectively). Only three com- pounds (3, 6 and 9) exhibit an S, mesophase (monotropic). J. MATER. CHEM., 1994, VOL. 4 Compound 3 also possesses an enantiotropic SA phase. The transition temperatures for the corresponding decyl homol- ogues (13-24) are collated in Table 2. The extension of the terminal alkyl chain by three methylene units (CH,) results in lower melting and clearing points (ca.-10 to 26 "C) for all of the compounds studied. However, 11 of the 12 homol-ogues prepared now exhibit an S, phase at relatively elevated temperatures. In addition, the directly linked compounds (13-15) and the ethanes (16-18) possess and enantiotropic SA phase above the Sc phase. An ordered smectic phase (S3, not yet identified, and S,) is observed for the longest chain lengths for three of the four series studied. This is the usual behaviour observed for most mesogens of this type.17-22 Influence of Chain Length and Double Bonds The data collated in Table 3 for a homologous series of three- ring esters (22-29) reveal that even compounds with the shortest chains (n= 1)exhibit S, and N phases.This is unusual for cyclohexane compounds, where the plots of the Sc and N phases usually rise very sharply from the very low values of the short chains, they reach a maximum and then decrease s10wly.'~-~~Ordered smectic phases (S, and S,) are observed for most homologues. The effect of a carbon<arbon double bond on the transition temperatures of a number of esters (23, 27 and 30-32) is shown in Table 4. It is seen that T,, is lower for the alkenyl- substituted esters (30-32), while the ordered phase (S,) is completely suppressed. The TNIis sometimes higher but in one case it is lower. Although T, is higher for two alkenyl- substituted esters (30 and 31), these changes still result in an increase in the S, transition temperature range by the lowering of the ordered smectic (S, and S,) transition temperatures.The elimination or lowering of the ordered smectic phase is important for the low temperature behaviour of mixtures of these compounds (the double bond in a terminal position can also contribute to a low value for the birefringence). These observations are not completely consistent with previous results for related systems with an alkyl chain attached to a cyclohexane ring. Influence of Dipoles The effect of introducing an additional lateral dipole in the form of either an oxygen atom (0)or a carboxy group (0,C or CO,) into the core of an almost apolar dialkyl pyrimidine (14) is demonstrated by reference to Table 3. The model substance (14) exhibits three smectic modifications (S,, Sc and S,) as well as an N phase.The total lengths of the dialkyl pyrimidine (14), the ethers (19 and 33-39), the esters (22, 40-42) and the combined ester/ether (43) are kept constant (five units attached to cyclohexane ring and 10 units attached to the pyrimidine ring when all the rings are bonded directly and three units when separated by a two-unit linking group). The replacement of the apolar CH, unit between the cyclohexane ring and the alkyl chain of dialkyl pyrimidine (14) by an oxygen atom to yield the cyclohexyl ether (33) leads to the disappearance of the Sc phase. The transition temperatures of the orthogonal smectic phases (A and B) are increased (+9 and +8 "C, respectively), while TN, is lower (-9 "C).The introduction of a epoxymethano group (OCH,) between the cyclohexane ring and the pyrimidine ring with the oxygen atom attached to the cyclohexane ring to produce the ether (34) leads to the total elimination of the smectic phases. TNIis much lower (-61 "C) than that of the model substance (14).Thus, a single isolated (non-conjugated) dipole, either as an outboard or central dipole appears to destabilise the Sc and the N phases. A central dipole in the form of an oxygen atom attached to the benzene ring (CH,O) in the core of the aromatic ether (19) leads to an increase (+8 "C) in the S, transition temperature compared with the reference sub- stance (14). The orthogonal SA and SB phases are totally suppressed; TNIis also lower (-13"C).However, an oxygen atom attached to the pyrimidine ring in a terminal position in the aromatic ether (35) results in an even larger increase (+35 "C) in T,, and TNIand the broadest Sc temperature range (55 "C) of all the compounds collated in Table 5. Thus, an outboard or central dipole in conjugation with an aromatic ring stabilises the Sc and N phases. The presence of two outboard dipoles in the shape of oxygen atoms in terminal positions (one isolated next to the cyclohexane ring and one conjugated with the pyrimidine ring) in the ether (36) causes a decrease C-8°C) in Ts, compared with the reference substance (14). TNIis increased (+15"C), but not by as much as for the monoether (35). These transition temperatures could be regarded as the prod- uct of the two competing tendencies described above.The SA transition temperature is increased significantly (+ 22 "C).No ordered phase could be observed. The diether (37) with one isolated outboard dipole and one central conjugated oxygen atom possesses values for T, and T,, that do not differ greatly from those of the corresponding monoether (19). TNIis lower than that of either monoether (19 and 33) with oxygen atoms in the same positions. Thus, the transition temperatures are, in this case, clearly non-additive. Diether 38, with two conju- gated dipoles (one central, one outboard), exhibits an Sc transition temperature and a TNIat almost exactly intermedi- ate values between those of the corresponding monoethers (19 and 35) with the oxygen atoms in the same positions.However, the melting point for diether 38 is much higher than that of either monoether and thus, the Sc range is narrow (10 "C). Ether 39, with an oxygen atom in the three possible positions under consideration, exhibits a moderate T,, a relatively high Ts, and a relatively broad Sc temperature range (30°C). TNIis relatively high. The introduction of a third oxygen atom as an isolated, non-conjugated dipole next to the cyclohexane ring of diether 38 to produce triether 39 results in an increase in the S, and a decrease in TNI. The introduction of the larger dipole moment associated with the carboxy group (CO, and 0,C) of esters 22 and 40-42 results in similar trends in the temperatures as observed for the corresponding monoethers 19 and 33- 35, although at higher absolute temperatures, except for T,,.The replacement of the apolar CH, unit next to the cyclohexane ring of dialkyl pyrimidine 14 by a carboxy group to yield ester 40 leads to the disappearance of the S, phase. The transition temperatures of the orthogonal smectic phases (A and B) are increased substantially (+ 33 and +19 "C, respectively), while TNIis somethwat higher (+6 "C). This is also the case for ester 41 with a carboxy group (0,C) between the cyclohexane and the benzene rings with the oxygen atom attached to the cyclohex- ane ring. Only an SA and an N phase could be observed. Thus, an isolated (non-conjugated) outboard dipole in the form of a carboxy (ester) function also destabilises the Sc phase.The presence of the carboxy group between the cyclo- hexane and the benzene rings in ester 22 with the oxygen atom of the carboxy group attached to the benzene ring results in an increase in T, and TNI(+ 14 and +9 "C, respect-ively). The SAand SBphases of the model dialkyl pyrimidine (14) have been totally suppressed and Ts, is a little lower (-4°C). The carboxy group with the oxygen atom bonded to the pyrimidine ring in ester 42 increases T, and TNIsubstan-tially (+62 and +26"C, respectively). T,, is increased to a lesser extent (+ 20 "C) and is, as a consequence, monotropic. SA or SB phases could not be observed. J. MATER. CHEM., 1994, VOL. 4 The combination of an expoxymethano group (CH,O) and an ester group (0,C) in 43 lead to the highest melting point observed for this series and, thus, a monotropic S, transition temperature.The clearing point (TN,)is high, but lower than that of the corresponding ester (13) with a direct linkage instead of the epoxymethano linkage. This is consistent with the other results collated in Table 5. The S,* transition temperatures (Tsc*)for mixtures 1-14 containing compounds 14, 19 and 33-43 are arranged in Table 11 in order of ascending value. It is clear from the data in the table that an outboard dipole [either in the form of an oxygen atom or a carboxy (ester)] group in position 1 or 2, when the dipole is attached to the cyclohexane ring, leads to a low Tsc,.The highest values are for compounds with a dipole attached to the polarisable pyrimidine ring. Intermediate values are obtained for compounds with a dipole in the middle of the molecule, when the dipole is bonded to the polarisable benzene ring. For compounds with two, or more conjugated dipoles the effects on the Ts,* are more or less additive. This is shown clearly in Fig. 1, where the transition temperatures of mixtures 1-14 are plotted in order of increasing Ts,*. The clearing point (N*-I) also increases generally in the same order. However, the SA-N* transition temperature shows no such dependency and is missing alto- gether for three mixtures. This indicates that the position and nature of the dipoles often has the same effect on the N and Sc phases, but not on the SA phase. This infers that previous the~ries~'-~' for the S, phase are primarily valid only for fully aromatic compounds without any isolated (i.e.non-conjugated) dipoles. Thus, thc expla- nation~~',~'proposed in order to explain the low nematic transition temperatures (or complete absence of an N phase) of mesogens incorporating isolated dipoles59p63 (e g. non-conjugated oxygen or nitrogen atoms) would also appear to be valid for the S, phase. These theories invoke intermolecular repulsive interactions between dipoles in adjacent molecules assuming certain packing arrangement^.^^,^' The z and P, values for the mixtures containing conipounds 14, 19 and 33-43 are shown in Fig. 2.There is a general dependence of the switching times and spontaneous polaris- ation on Ts,* suggesting that these values are primarily a result of the temperature dependence of the tilt angle. This infers surprisingly similar viscosity values. The major excep- tions are for mixtures 12 and 14 containing ether 35 and ester 42, respectively, where a high spontaneous polarization value and a relatively short switching time infer a low viscosity Table 11 Chiral smectic C transition temperatures (T,J for mixtures 1-14 consisting of 15 wt.% of selected members of compounds 14, 19, 22 and 34-43 and 85 wt.% of the mixture SCO 1014 1 2 3 mixture T,,,/"C position 1" position 2" position 3" compound 1 24.6 OCH, 34 2 53.7 OK 41 3 57.6 -33 4 61.0 -40 5 63.3 -36 6 63.9 -14 7 63.9 CH20 37 8 65.8 co2 22 9 67.0 CH,O 43 10 67.1 11 67.4 12 67.8 13 69.3 14 69.7 " 0=oxygen; CO, and 0,C =ester; CH,O and OCH, =epoxymethano; Fig.1 Chiral nematic-isotropic ( W, N*-I) smectic A-chiral nematic (0,SA-N*) and the chiral smectic C-smectic A (A,Sc*-SA) transition temperatures for mixtures 1-4 containing compounds 14, 19, 22 and 33-34 CH,O 19 CH,O 39 -35 CH,O 38 -42 -=direct bond. 20 200 180 18 cu 160 0516 140Q 120 14 100 135791113 n Fig. 2 Spontaneous polarisation (W, P,) and switching times for mixtures 1-14 containing compounds 14, 19, 22 and 33-43 J. MATER. CHEM., 1994, VOL. 4 associated with a high Ts,* value. Thus, ether 35 and ester lOOr 42 are the most interesting compounds of the 14 screened.Mixture 13 incorporating diether 38 exhibits an almost equal 90 ITsc*,but a lower spontaneous polarisation and a much higher switching time. A surprising characteristic of the data in Table6 and Fig. 1 and 2 is the fact that the presence or absence of an SAphase for the mixture seem to have no effect on the spontaneous polarisation or the switching time. Therefore, derivatives of monoether 35 and ester 42 were chosen for further study. Influence of Double Bonds and Dipoles It has recently been shown that the incorporation of carbon- 2 3 4 5 6 7 carbon double bonds of defined configuration in certain n positions of alkoxy and alkanoyloxy chains can beneficially Fig. 3 Chiral nematic-isotropic (m, N*-I), smectic '4-chiral nematic influence both the transition temperatures and other physical (m, SA-N*) and the chiral smectic C-smectic A( A,S,.*-SA)transition properties of direct relevance for commercial FLCDS.~~-~~temperatures of the mixtures, containing 15 wt.% of the two-ring Therefore, an additional trans-carbon-carbon double bond was introduced into nonyloxy ether 35 to yield nonenyloxy ether 44.The result of this manipulation is shown in Table 7. T, is increased (+21 "C) while both Tsc, and TNIare both marginally lower (-3 and -6 "C, respectively). The SA phase has been suppressed. The effect of a similar manipulation on nonyloxy ester 42 to yield (E)-non-2-enoyloxy ester 45 is to suppress the S, phase completely, while decreasing the T, (-10 "C) and increasing the clearing point (+39 "C) signifi-cantly. This kind of behaviour has also been observed for analogous two-ring phenylpyrirnidine~.~~ The data compiled in Table 6 allow a comparison of the transition temperatures of two-ring phenylpyrimidines (46-52)39with a nonyl chain attached to the pyrimidine ring (X =C,H,) and an octyloxy/octenyloxy chain on the benzene ring with those of the analogous three-ring ethers (53-59) with an additional trans-l,4-disubstituted cyclohexane ring (X =C,H,,).The most striking aspects of the results collated in Table 6 are the substitution of an N phase for the SA phase of two-ring ethers 46-52 for all but two isomers 58 and 59 of three-ring ethers 53-59.TNIof three-ring ethers 53-59 is high (176 "C,on average). The melting point is higher (+37 "C,on average) than that of two-ring ethers 46-52.58 However, Tsc is increased more (+54 "C, on average, comparing only those homologues with an S, phase for both series), which results in an enantiotropic S, phase (22 "C, on average). Thus, the increased rigidity of the trans-1,4-cyclohexane ring results, as expected, in higher transition temperatures. As the nonyl chain of the two-ring phenylpyrimidines can adopt many more non-linear conformations than the trans-4-pentylcyclo- hexyl moiety, the effective length: breadth ratio of the latter will be greater, thus leading to the higher transition tempera- tures observed. The transition temperatures of the mixtures containing 15 wt.% of two-ring phenylpyrimidines 46-5239 or three-ring ethers 53-59 and 85 wt.% of the base mixture SCO 1014 are shown in Fig.3. The spontaneous polarisation and switching times of the same mxtures are shown in Fig. 4. Larger differences in the values determined for the spontaneous polarisation are observed than can be explained by differences in Ts,*. Therefore, it can be assumed that two-ring ethers 46-52 possess a smaller tilt angle. However, the switching times are lower than can be explained by the even lower tilt angle. Therefore, it must be concluded that the viscosity of the two-ring ethers (46-52) is lower (as could have been expected) than that of the three-ring ethers (53-59). However, the difference is not that large, thus, inferring that the viscosity of the three-ring ethers is low.Hence, the trans-1,4-cyclohexane ring has been shown to induce a low viscosity in the Sc phase as also observed for nematogens. phenylpyrimidines (46-52; solid lines) and the corresponding three- ring ethers (53-59; broken lines) and 85 wt.% of the chiral smectic C mixture SCO 1014 uersus the number of carbon atoms (n) from the core, where the carbon-carbon double bond in the alkenyloxy chain starts for each of the two series 2o '2oo 18 I 6 22 16 14 n Fig. 4 Spontaneous polarisation (m, P,) and switching times (e,z) of the mixtures containing 15 wt.% of same ethers as in Fig. 3, i.e. the two-ring phenylpyrimidines (46-52; solid lines) and the corre- sponding three-ring ethers (53-59 broken lines) and 85 wt.% of the chiral smectic C mixture SCO 1014 uersus the number of carbon atoms from the core, where the carbon-carbon double bond in the alkenyloxy chain starts for each of the two series The transition temperatures of a homologous series of esters (42 and 60-64) are collated in Table 8.Only S,-and N phases could be observed. T, is high (118 "C, on average) and, as a consequence, the range of the S, phase is narrow (13 "C, on average). TNIdecreases with increasing chain length. This leads to a significant narrowing of the nematic temperature range. The normal pattern of alternation for TNIis observed. Attempts to determine the relative merits of these esters in mixtures were not successful owing to solubility problems at room temperature for several of the homologues.The thermal data of a short homologous series of alkeny- loxy-substituted three-ring ethers (44, 54 and 65-68) are recorded in Table 9. The melting points are remarkably con- stant (95"C, on average). The S, phase is injected for octeny-loxy ether 54 and rises steeply as the chain lengthens. Thus, the temperature range of the S, phase increases strongly for longer chain lengths. Only an S, phase and an N phase could be determined. TNIdecreses steadily with increasing chain length and exhibits the normal alternation pattern. The transition temperatures of a short homologous series of alkenyloxy-substituted three-ring ethers (59 and 69-73) are J.MATER. CHEM., 1994, VOL. 4 listed in Table 10. S, and SAphases as well as an N phase are observed. T, and Tscare relatively low for three-ring systems (63 and 90 "C, on average, respectively), while the SA and N transition temperatures are high (134 and 176 "C, on average, respectively). As a consequence the temperature range of the S, and N phases is large (44 and 86 "C, on average, respect- ively). Both the s, and SA transition temperatures increase with increasing chain length, whereas TNI decreases pro- portionately. This is normal behaviour for such systems. The normal pattern of alternation for the clearing point is observed. The transition temperatures of the mixtures containing 15 wt.% of alkenyloxy-substituted three-ring ethers (44, 54 and 65-68; 59 and 69-73) and 85 wt.% of the base mixture SCO 1014 are shown in Fig.5. The clearing point (N*-I) is similar for both series. However, compounds 44,54 and 65-68 with a trans-carbonxarbon double bond exhibit a higher Tsc* and a broader N phase. Thus, the temperature range of the SA phase is much narrower than that observed for the corresponding series of alkenyloxy substituted three-ring ethers (59 and 69-73) with a carbon-carbon double bond in a terminal position. Fig. 6 shows the values of the spontaneous polarisation and switching ties for the same mixtures. They go together, i.e. their variation can be explained by differences 90 r Fig. 5 Chiral nematic-isotropic (N*-I), smectic A-chiral nematic (SA-N*) and the chiral smectic C-smectic A (Sc*-SA) transition temperatures of the mixtures containing 15 wt.% of ethers 44, 54 and 65-68 (solid lines), and 59 and 69-73 (broken lines) and 85 wt.% of the chiral smectic C mixture SCO 1014 versus the number of carbon atoms (n) in the alkenyloxy chain 6 7 8 9 10 11 n Fig.6 Spontaneous polarisation (m, P,) and switching times (0,z) of the mixtures containing 15 wt.% of the same ethers as in Fig. 5 [44,54 and 65-68 (solid lines) and 59 and 69-73 (broken lines)] and 85 wt.% of the chiral smectic C mixture SCO 1014 versus the number of carbon atoms (n) in the alkenyloxy chain [d =2, solid lines; d = (n -1)broken lines] 251 20-N'5 15-$?2 10-5-Ol I I10 20 30 40 50 60 70 TI'C Fig.7 Spontaneous polarisation (P,) and switching times (7) of mixture 11 versus temperature in the tilt angle, which is governed by the distance from Tsc,. This is shown clearly in Fig. 7, where the dependence of the spontaneous polarisation on the temperature is depicted. At about room temperature the increase in the spontaneous polarisation is ca. 0.15 nC cm-2 "C-'. Therefore, an average difference of 8°C in Tsc,corresponds to a difference in the spontaneous polarisation of 1.2 nC cmP2, which is equal to the differences observed. Conclusions Polar linkages between the cyclohexane and benzene rings in a cyclohexylphenylpyrimidine model system lead to higher S, transition temperatures than those observed for the analogous compounds with apolar linkages.Isolated dipoles such as oxygen atoms or carboxy groups attached to a non-polarisable ring (e.g. cyclohexane) leads to a lowering of Tsc and the clearing point (TNI)or to the total disappearance of the S, phase. The same dipoles attached to polarisable rings (e.g. benzene or pyrimidine rings) result in high Ts, and T,, values. Conjugated dipoles in the centre of the molecule usually exhibit intermediate effects. Combinations of dipole positions normally give rise to intermediate (additive or subtractive) effects. Conjugated outboard (terminal) dipoles have d greater effect on Ts, than central (conjugated) dipoles. 'The (E)-carbon-carbon double bond induces the highest T,,, and spontaneous polarisation of Sc* mixtures. The presence of the trans-l,4-disubstituted-cyclohexanering in the model system leads to compounds with moderately high Ts, and low viscosity values (i.e.short switching times) and are of interest for commercial mixture development for liquid-crj stal dis- plays based on ferroelectric effects. The authors express their gratitude to Mr. C. Haby and Mr. W. Janz for technical assistance in the preparation of the compounds and the determination of their physical data. Dr. W. Arnold (NMR), Mr. W. Meister (MS), Dr. M. Grosjean (IR), Mr. F. Wild and Mr. B. 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Cryst., 1988,3, 1173. 31 W. Hemmerling, I. Muller and R. Wingen, Ferroelectrics, 1988, 62 E. L. Steiger and H. J. Dietrich, Mol. Cryst. Liq. Cryst., 1972, 85,393. 16,279. 32 C. Escher, W. Hemmerling, G. Illian, I. Muller, P. Wegener and 63 M. A. Osman and L. Revesz, Mol. Cryst. Liq. Cryst., 1980,56, 133. R. Wingen, presented at the 13th International Liquid Crystal Conference, Vancouver, BC, Canada, 1990. Paper 4/01811G:Received 25th March, 1994