J. MATER. CHEM., 1994, 4(11), 1715-1717 Synthesis and Mesomorphic Properties of 4-[(4-Cyanophenyl)acetylenyl]-2,3,5,6-tetrafluorophenyl 4-n-AIkoxybenzoates Jianxun Wen,* Hongbin Yu and Qi Chen Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences, Shanghai, 200032, China 4-[(4-Cyanophenyl)acetylenyl]-2,3,5,6-tetrafluorophenyl4-n-alkoxybenzoates, which contain a perfluorinated phenyl ring in their aromatic core system, exhibit nematic phases with a wide thermal range. In comparison with the homologues without fluorine, the introduction of the perfluorinated phenyl ring leads to the formation of a nematic phase and reduces the melting and clearing points. In recent years, laterally fluorinated aromatic liquid crystals have attracted much attention.lP5 The fluorine atom combines large electronegativity with small size, so that it significantly affects the physical properties of molecules without eliminating the possibility of mesophase formation.When one or two fluorine atoms are introduced into the aromatic core unit of liquid crystals, some valuable effects, such as a reduction in the melting point, an increase in cL6 and an increase in the width-to-length ratio,’ can be achieved. There have been some reports on liquid crystals which contain a perfluorinated phenyl These materials have high P, value^'^-'^ and low phase-transition temperature^'^ or else they favour the smectic C phasel4+l6 and thus have potential applications in message transportation and image displays.In order to pro- duce new materials with better physical properties, our group designed and synthesized new fluorinated liquid crystals. We reported some new perfluorophenyl-containing liquid crystals by coupling 4-alkoxy-2,3,5,6-tetrafluorophenylacetylenewith other materials.20P24 The liquid crystals obtained were ident- ified as having a chiral smectic C phase,24 a smectic A phase24 or an enantiotropic nematic However, for only a few of these perfluorophenyl-containing liquid crystals were their mesomorphic properties compared with their non-flu- orinated parent molecules. Most of them have the perfluoro- phenyl ring on the side of the molecular backbone. We report here a new family of compounds with a perfluorophenyl ring located at the centre of the core unit (1).Their non-fluorinated homologues (2) were reported previou~ly.~~ By comparing the mesophic properties of these two series, we can gain a deeper knowledge of perfluorophenyl-containing compounds. F\ IF F‘ ‘F 1 2 The starting material for producing 1 is 4-iodotetrafluoro- phenol. The preparation of these compounds affords a method of obtaining compounds with the perfluorophenyl ring at the centre of the core unit. The mesomorphic properties were studied by polarizing microscopy and differential scanning calorimetry (DSC). Experimental General Synthetic Procedures 4-[( 4-Cyanophenyl )acet ylen yl ]-2,3,5,6-tetrafluorophen yl 4-n- alkoxybenzoates (1) were prepared according to Scheme 1 Initially, 3 was treated with KOH in tert-butyl alcohol under reflux to yield 4.The main intermediate, 5,was obtained from a one-pot mild esterification between 4 and 4-alkoxybenzoic acid in the presence of both dicyclohexylcarbodiimide (DCC) F\ IF FeI FF 3 HO*I cop FF FF 4 5 a m=4 b m=5 c m=6 d m=7 e m=8 6 F ‘F 1 a m=4 b m=5 c m=6 d m=7 e m=8 Scheme 1 Preparation of 1. Treatments: (a) KOH, ButOH, reflux; (b) H(CH2)mo~CozH-DCC, PPY, CH,Cl,, room tempera- ture; (c) (Ph,P),PdCl,, CuI, Et,N, 60 “C. 1716 and 4-pyrrolidin-1-ylpyridine (PPY) catalyst in dried dichloromethane. 6 was synthesized following the method of ref. 26. The final products, 1 were prepared from 5 and 6 using dichlorobis( tripheny1phosphine)palladium as catalyst in triethylamine with copper(1) iodide.Characterization of Materials The final products were rigorously purified by flash chroma- tography over silica gel (200-400 mesh) using light petroleum (bp 60-90 "C)-ethyl acetate (60 : 1) as the eluent to give pale yellow solids which showed a single spot in thin-layer chroma- tography. The products were recrystallized from acetone-methanol to yield white crystals. The chemical structure of the intermediates and the final products were elucidated by 1R spectroscopy (Shimadzu IR- 440 spectrophotometer), 'H NMR spectroscopy (Varian EM390 and Varian EM360 spectrometer) with TMS as internal standard, 19F NMR spectroscopy (Varian EM3601 spectrometer) with trifluoroacetic acid as external standard and mass spectrometry (HP5989A spectrometer).The spectro- scopic data were consistent with the predicted structures. The melting points of the intermediates and the transition tempera- tures and phase assignments for the final products were determined by using an Olympus BH2 polarizing microscope in conjunction with a Mettler FP52 hot stage and FP5 control unit, while the observed textures were compared with those in the literat~re,~ to identify the mesomorphic phases. The enthalpies of transitions were investigated by DSC using a Shimadzu DSC5O calorimeter with a heating rate of 10°C min-'. Synthesis of Materials The preparation of 3,' and 626followed literature procedures. 2,3,5,6-Tetrujluoro-4-iodophenol(4) Pentafluoroiodobenzene (3) (14.7 g, 50 mmol) KOH (8.4 g, 150mmol) and tert-butyl alcohol (30ml) were refluxed at 90 "C for 6.5 h.19F NMR analysis of the reaction mixture showed that the reaction was complete. Then aqueous hydro- chloric acid (5%, 20 ml) was added and the aqueous tert- butyl alcohol (cu. 22 ml) was distilled off. The residue was acidified with aqueous hydrochloric acid (5%) and cooled. The mixture was allowed to stand for 30min and white crystals formed. The product was filtered off and washed with cold methanol. The filtrate was extracted with ether and dried over anhydrous sodium sulfate. The solvent was removed and the residue was combined with the white crystals which had been filtered off and dried under reduced pressure to give a white solid.Yield 12.8 g (87.7%), mp 46.0-46.5 "C. 'H NMR 6, (60 MHz; CCl,; TMS): 6.0 (s, OH). 19FNMR hF (60 MHz; CC1,; TFA): 48.35 (d, 2 F, J 18.8 Hz, F arom.), 83.6 (d, 2 F, J 18.8 Hz, F arom.). 2,3,5,6-Tetra~uoro-4-iodopheny~4-n-Butoxybenzoate(5a) 4-Alkoxybenzoic acids were prepared from 4-hydroxybenzoic acid and the corresponding alkyl bromide following the method of Gray and Jones.29 The general method of preparing 5 is as follows for 5a: a mixture of n-butoxybenzoic acid (1.65g, 8.5mmol), 4 (2.48g, 8.5mmol), DCC (1.93 g, 9.35 mmol) and PPY (60 mg, 0.4 mmol j in dry dichloro- methane (50 ml) was stirred at room temperature for 40 h. 19F NMR analysis revealed that complete reaction had occurred. The white precipitate was filtered off and the filtrate was washed with aqueous acetic acid (5%).The solvent was removed and the crude product was purified by flash column chromatography over silica gel (200-400 mesh) using a mix- J. MATER. CHEM., 1994, VOL. 4 ture of light petroleum (bp 60-90 "C) and ethyl acetate (30 :1) as the eluent. The solvent was removed from the collected fractions to give a white solid, 5a. Yield 1.82 g (77.8%), mp 62.9-64.4 "C. 'H NMR 6, (90 MHz; CDCl,; TMS): 8.15 (d, 2 H)/7.00 (d, 2 H) (AA'BB', J 9 Hz, H arom.), 4.09 (t, 2 H, J 6 Hz, OCH,), 0.8-2.0 [m, 7 H, (CH,),CH,:]. 19F NMR 6, (60 MHz, CDCl,, TFA): 42.7 (d, 2 F, J 18.8 Hz, F arom.), 73.0 (d, 2 F, J 18.8 Hz, F arom.). 5b-5e were prepared by the same general method and they had satisfactory 'H and 19FNMR spectra.4-(4-Cyanopheny2acetylenyl)-2,3,5,6-tetrujluorophen~l4-n-butoxybenzoate (la) The typical method of synthesis of 1 is as follows for la: a mixture of 5a (234 mg, 0.5 mmol), dichlorobis( triphenylphos- phine)palladium (20 mg, 0.03 mmol), copper(1) iodide (11.5 mg, 0.06 mmol) and 4-cyanophenylacetylene (63.5 mg, 0.5 mmol) in anhydrous triethylamine (15 ml) was stirred under an atmosphere of dry nitrogen at 60 C for 6 h. The precipitate was filtered off and washed with ether. The filtrate was washed with water and the solvent was removed. The residue was purified by flash column chromatography over silica gel (200-400 mesh) using a mixture of light petroleum (bp 69-90 "C)-ethyl acetate (60: 1) as the eluent to give a yellow solid which was recrystallized from acetone-methanol to yield white crystals of la.Yield 200 mg (85.6%). 'H NMR 6, (90 MHz; CDC1,; TMS): 8.15 (d, 2 H1'7.00 (d, 2 H) (AA'BB', J 9 Hz, ROC6H,), 7.70 (s, 4 H, CNC6H,), 4.09 (t, 2 H, J 6 Hz, OCH,), 0.8-2.0 [m, 7 H, (CH,),('H,]. 19FNMR 6, (60 MHz, CDCl,, TFA): 58.4 (d, 2 F, J 18.8 Hz, F arom.), 74.3 (d, 2 F, J 18.8 Hz, F arom.). IR (KBr) v/cm-': 2900, 2850, 2200, 1755, 1605, 1580, 1510, 1490, 1440, 1400, 1320, 1260, 1235, 1170, 1100, 1030, 990, 840. MS (willz) 468 (A4+ 1). Found: C, 66.58; H, 3.43; N, 2.90; F, 15.74%. Calc. for C2,H17F,N0,: C, 66.81; H, 3.64; N, 3.00; F, 16.27%. lb-le were prepared similarly. All of them had satisfactory elemental analyses and appropriate 'H and 19F-NMR, IR and MS spectra.Results and Discussion Optical Microscopy Studies The transition temperatures and enthalpies for compounds 1 are presented in Table 1. The reported transition tempera- ture~~~for 2a-2f are listed in Table 2. When m=4-8, com-pounds 1 had a wide thermal mesomorphic range and exhibited an enantiotropic nematic phase. The nematic ther- mal ranges were relatively stable around 114-1 18 "C. The melting points and the clearing points decreased with increas- ing the length of the alkoxy chain. The melting point decreased by 47.5 "C from the butoxy substituent (la) to the pentyloxy substituent (lb). The difference in melting points between lb and le were much smaller. A similar trend is apparent with the clearing points.Tinh et al. reported25 that compounds 2 were liquid crystal- line. The compounds with m =5 and 6 have an enantiotropic nematic phase and a monotropic S, phase. The enantiotropic S, phase appears from m=7. The nonyloxq and decyloxy derivatives have a stable re-entrant nematic phase. By compar- ing compounds 1 with compounds 2, it was found that the melting points and the clearing point reduced when the perfluorinated phenyl ring was introduced into the molecule. For m= 8, the melting point decreased from 86.0 "C to 77.3 'C and the clearing point decreased from 248'C to 193.1"C. Compounds 1 exhibited stable enantiotropic nematic phases without a smectic phase when rn =4-8. However, the smectic phase exists in compounds 2 from m=5 and re-entrant J.MATER. CHEM., 1994, VOL. 4 1717 Table 1 Transition temperatures, enthalpies and thermal ranges for compounds la-le T/”C (AH/J g-’) compound m K-N N-I recryst. temp./”C N thermal 1 ange/”C ~~ la lb lc Id le 4 5 6 7 8 146.54 74.05) 99.0( 61.93) 90.4( 81.69) 84.0( 59.09) 77.3( 85.63) 234.2( 2.74) 213.5( 2.05) 208.0( 1.44) 198.0( 2.3 1) 193.1( 1.83) 98.4 40.0 49.2 39.0 46.9 87.7 114.i 117.6 114.0 115.8 Table 2 Transition temperatures for compounds 2a-2f compound m K-S, SA-N N-S, SA-N N-I 2a 5 107 (105) --276 2b 6 113 (107) -~ 268 2c 7 102 108 --256 2d 8 86 96 --248 2e 9 90 (75.7) 141 183 239 2f 10 84 -102 208 233.5 nematic phases were observed when m=9 and 10. These two series of compounds contained the cyano group.In non-fluorinated compounds the highly polar cyano group can be attached to one end of the molecule, resulting in strong antiparrallel correlations between neighbouring molecules.30 That the fluorinated compounds exhibited only an enantio- tropic nematic phase indicated that the terminal attractive force between the molecules was dominant and the lateral attractive force was relatively weak so that the smectic phase could not form and the antiparrallel correlation between neighbouring molecules could not persist. Introduction of fluorine at the centre of the core unit disrupts the formation of the re-entrant phase when the highly polar cyano group is the terminal group. DSC Studies The phase transitions were also investigated by DSC, and the mesomorphic transition enthalpies for compounds 1 are shown in Table 1.The average value of the melting enthalpy of compounds 1 is 72.48 J 8-l. The nematic to isotropic transition enthalpies are relatively small: 1.44-2.74 J 8-l. Conclusion We synthesized a new series of liquid crystals which contain central perfluorinated phenyl ring in the aromatic core system and studied their mesomorphic properties. We have compared their mesomorphic properties with homologous non-fluori- nated compounds. Compounds 1 possessed wide nematic temperature ranges, lower melting points and lower clearing points. The authors gratefully acknowledge the Advanced Material R&D Program of China for financial support.References G. W. Gray, M. Hird, D. Lacey and K. J. Toyne, J. Chem. SOC., Perkin Trans. 2, 1989,2041. 2 G. W. Gray, M. Hird, D. Lacey and K. J. Toyne, Mol. Ctayst. Liq. Cryst., 1989, 172, 165. 3 G. W. Gray, M. Hird, D. Lacey and K. J. Toyne, Mol. Cryst. Liq. Cryst., 1991, 195, 221. 4 G. W. Gray, M. Hird, D. Lacey and K. J. Toyne, Mol. Cryst. Liq. Cryst., 1991,204,43. 5 M. Hird, G. W. Gray and K. J. Toyne, Liq. Cryst., 1992, 11, 531. 6 M. 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Paper 4/02944E; Received 17th hlay, 1994