Chinese Journal of Chemistry, 2006, 24, 99—102 Full Paper

* E-mail: ronggb@ecust.edu.cn Received April 1, 2005; revised September 13, 2005; accepted October 30, 2005.

© 2006 SIOC, CAS, Shanghai, & WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Synthesis and Mesomorphic Properties of Chiral Mesogens Bearing Fluorinated Alkyl Terminal Tails

QIN, Chuana(秦川) RONG, Guo-Bin*,a(荣国斌) CHEN, Bao-Quanb(陈宝铨) WEN, Jian-Xunb(闻建勋)

a Department of Chemistry, East China University of Science and Technology, Shanghai 200237, China b Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences, Shanghai 200032, China

Ten chiral mesogenic compounds containing fluorinated alkyl terminal tails were prepared and characterized by IR, NMR, MS. The phase transition behavior was investigated by DSC and polarized optical microscope. Two-ring liquid crystal compounds exhibited no mesogenic phase or only a monotropic smectic A phase. The longer fluori-natd alkyl chain on the three-ring was advantageous to the formation of smectic phase. The mesogens with chiral center located between fluorinated tail and mesogenic core exhibited chiral smectic C phase and other smectic phases when the alkoxyl adhering to the other side of the mesogenic core was of intermediate length.

Keywords chiral mesogenic compound, fluorinated alkyl chain, mesogenic phasee

Introduction

In the past decade, the modifications of the structural core of mesogens, such as the substitution of one or more protons of the aromatic core by fluoroatoms, the substitution positions, and central linkage, were actively studied.1-11 Recently, the research on fluorinated liquid crystals (LC) were mainly focused on the role of fluori-nated chain on the liquid crystalline properties. Because of the imcompatibility between fluorocarbons and hy-drocarbons, the fluorocarbon chains in LC molecules prefer to self-organize. The results of such aggregation in fluorinated LC with fluorocarbons to have mesogens would favor smectic formation.12-19 However, only rare studies have been carried out on the synthesis of LC with fluorocarbon chains, partly because the fluorinated reagents were rarely available and chemical modifica-tion of hydrocarbons into fluorocarbons was far difficult. In our laboratory, we have synthesized and investigated several series of fluorinated LC with highly fluorinated chains, and showed that smectic nature was enhanced to a great extent in such highly fluorinated LC and the mesogens with fluorinated tails could hold broader smectic C phase than the corresponding nonfluorinated mesogens.20,21 Hence, it is possible that mesogens with highly-fluorinated tails would be a potential ferroelec-tric LC by introduction of chiral center into terminal chain. Here, we present ten compounds with fluorinated alkyl chains and different chiral ones 1—10 as shown in Scheme 1.

Scheme 1 Ten compounds with fluorinated alkyl chains and different chiral ones

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© 2006 SIOC, CAS, Shanghai, & WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Results and discussion

The phase transition temperatures of chiral fluori-nated liquid crystals 1—10 are listed in Table 1. The ten compounds, all containing fluorinated alkyl chain as terminal groups, exhibit no nematic phase, while 1 and 3 are not mesogenic. With reversion and relocation of two ester linkages, the 2 and 4 only show a monotropic smectic A phase. The reason may be that the core ester linkage destroys the conjugation of the two aromatic rings, and the polarity of the molecules 2 and 4 is larger than that of 1 and 3.

Table 1 The phase transition temperatures of the ten chiral fluorinated liquid crystals synthesized

Mesogens

Cr SmC* SmA I

1 • 89.4 — — •

2 • 73.1 — • 49.6 •

3 • 66.9 — — •

4 • 40.8 — • 23.7 •

5 • 107.3 — • 199.8 •

6 • 116.1 • 135 • 199.2 •

7 • 88.1 • 107.2 • 167.3 •

8 • 81.0 • 100.4 • 157.8 •

9 • 84.8 (SmB 95.0)a • 118.4 • 151.0 •

10 • 91.3 (SmB 122.8)a — • 138.4 • a SmB: smectic B phase.

By insertion of a 1,4-phenylene group into the core structure of 2, the three-ring compound 5 displays a wide smectic A phase from 107.3 to 199.8 ℃, but no chiral smectic C phase. With additional two CF2 units into the fluorinated tail, the resulting comound 6 shows a very stable chiral smectic C phase as well as smectic A phase. It seems that a longer fluorinated alkyl chain benefits the formation of smectic phase. 21

Further study was considered to introduce a chiral center into the side of fluorinated chain with expectation of finding LC with broad chiral smectic C phase. In this work, lactyl block was selected and introduced into mesogens between mesogenic core and fluorinated chain. Four compounds were synthesized and their mesogenic properties were characterized. Both 7 and 8 show a chiral smectic C phase, accompanied by a wide smectic A phase. 9 shows three smectic phase, a smectic B from 84.8 to 95 ℃ connected with a chiral smectic C phase until 118.4 ℃ and a smectic A phase ended at 151 ℃. 10 exhibits no smectic C phase but smectic B and smectic A phases. As Figure 1 shows, the mesophase range shrinks with increasing length of end alkyl chain, and the intermediate length alkyl chain may form broader smectic C phase.

Thus, six compounds with chiral center on hydro-carbon chain and four compounds with chiral center between fluorocarbon chain and mesogenic core, which

are the first example of LC with chiral center located between fluorocarbon and core, have been synthesized, and their mesomorphic properties were investigated.

Figure 1 The relationship between the methylene units (m) of hydrocarbon and the mesophases of 7—10.

Experimental

The structures of all intermediates and final mesogens were determined by spectral methods. IR spectra were recorded on a PE-983G spectrometer with a KBr disc or neat liquid film. 1H NMR spectra with TMS as the internal standard and 19F NMR spectra with trifluoroacetic acid as external standard in CDCl3 as the solvent were recorded on an FX-90Q (90 MHz) or a Bruker 300AM instrument. For 19F NMR spectra, the high field was positive. Mass spectra were measured with a Finnigan 4021 spectroscope. The phase transition temperatures of all the target compounds were measured by optical microscopy via a polarizing optical micro-scope (Olympus PM-6) equipped with a heating stage (Mettler FP-80) and a control unit (FP-82), and by dif-ferential scanning calorimetry (DSC, Shimadzu DSC-50 calorimeter with a data system) with heating and cool-ing rate of 5 ℃/min. The transition temperatures shown in this paper were the peak values of the transitions on DSC trace. Phase identification was made by comparing the observed textures with those reported in the litera-ture.22

General procedure

The synthetic route was drawn in Scheme 2, where p-(2,2,3,3,4,4,5,5-octafluoropentoxy)benzoic acid II was prepared from methyl p-hydroxybenzoate and 2,2,3,3,4,4,5,5-octafluoropentyl trifluoromethylsulfo- nate, followed by hydrolyzation according to a literature method.23 The synthesis of chiral blocks including S-2-methylbutyl p-hydroxybenzoate, S-2-methylbutoxy- benzoic acid, p-[p-(S-2-methylbutoxy)phenyl]benzoic acid, S-isooctoxybenzoic acid and S-isooctyl p-hydroxy- benzoate was described in the previous papers.21,24 The acid II reacted with S-2-methylbutyl p-hydroxybenzoate

Chiral mesogenic compound Chin. J. Chem., 2006 Vol. 24 No. 1 101

© 2006 SIOC, CAS, Shanghai, & WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

and S-isooctyl p-hydroxybenzoate to yield 1 and 3 re-spectively. 2,2,3,3,4,4,5,5-Octafluoropentyl or 2,2,3,3,4, 4,5,5,6,6,7,7-dodecafluoroheptyl p-hydroxybenzoate IV and V were synthesized from corresponding fluoroal-cohols and benzyloxybenzoic acid, which was prepared by protection of hydroxyl group and release of acid function of methyl p-hydroxybenzoate I as described in the previous paper.25 Compound IV was reacted with p-S-2-methylbutoxybenzoic acid, p-[p-(S-2-methylbut-oxy)phenyl]benzoic acid or p-S-isooctoxybenzoic acid to form 2, 5 or 4 respectively. Compound V was reacted with p-[p-(S-2-methylbutoxy)phenyl]benzoic acid to generate 6. An important chiral acid VIII was prepared from p-benzyloxyphenol and methyl S-lactate by Mistunobu reaction, followed by hydrolyzation. The chiral acid VIII was reacted with 2,2,3,3,4,4,5,5-octa- fluoropentanol and, subsequently, removal of benzyl protection to produce compound 10, which was used to esterify p-(p-alkoxyphenyl)benzoic acids26 to afford chiral mesogens 7—10.

All the esterifications were conducted in equal equivalent of acid to phenol or alcohol with 1.2 equiva-

Scheme 2 The synthetic route of 1—10

Reagents and conditions: a. 1) H(CF2) 4CH2OSO2CF3, Et3N, CH2Cl2,

2) NaOH, acetone, 3) +3H O. b. p-(S-C2H5CH(CH3)CH2OOC)C6H4OH,

DCC, DMAP, CH2Cl2. c. p-(S-C6H13CH(CH3)OOC)C6H4OH, DCC,

DMAP, CH2Cl2. d. 1) BuCl, NaOH, C2H5OH, 2) NaOH, H2O, 3) H3＋O.

e. 1) H(CF2)nCH2OH (n＝4, 6), DCC, DMAP, CH2Cl2, 2) Pd/C, H2,

CH3COOEt. f. p-[S-C2H5CH(CH3)CH2O]C6H4COOH, DCC, DMAP,

CH2Cl2. g. p-[S-C6H13CH(CH3)O]C6H4COOH, DCC, DMAP, CH2Cl2.

h. p-{p-[(S-C2H5CH(CH3)CH2O)]C6H4}C6H4COOH, DCC, DMAP, CH2Cl2.

i. S-HOCH(CH3)COOCH3, DEAD, Ph3P, CH2Cl2. j. 1) NaOH, CH3OH, 2) +3H O. k. H(CF2)4CH2OH, DCC, DMAP, CH2Cl2. l. Pd/C, H2, CH3COOEt.

m. p-[p-(CmH2m＋1O)C6H4]C6H4COOH (n＝6, 7, 8, 10), DCC, DMAP,

CH2Cl2-THF.

lent of dicyclohexylcarbodiimide (DCC) and catalytic N,N-dimethylaminopyridine (DMAP) in dried CH2Cl2 at room temperature, and monitored by TLC.

1: 24D[α] ＋0.13 (c 0.0241, CHCl3);

1H NMR (90 MHz, CDCl3) δ: 0.85—2.10 (m, 9H, aliphatic H), 4.18 (d, J＝6 Hz, 2H, OCH2), 4.58 (t, J＝13 Hz, 2H, OCH2CF2), 6.08 (tt, J1＝52 Hz, J2＝5.2 Hz, 1H, CF2H), 7.05 (d, J＝9 Hz, 2H, Ar), 7.30 (d, J＝9 Hz, 2H, Ar), 8.00—8.30 (m, 4H, Ar); IR (KBr) ν: 1736, 1703, 1606, 1511, 1273, 1210, 1170, 1067, 851, 760 cm－1; MS (70 eV) m/z (%): 543 (M＋), 335 (H(CF2)4CH2OC6H4CO＋, 100). Anal. calcd for C24H22F8O5 (542.4200): C 53.14, H 4.09, F 28.02; found C 53.00, H 3.98, F 28.42.

2: 24D[α] ＋0.47 (c 0.0238, CHCl3);

1H NMR (90 MHz, CDCl3) δ: 0.80—2.10 (m, 9H, aliphatic H), 3.89 (d, J＝6 Hz, 2H, OCH2), 4.85 (t, J＝13 Hz, 2H, OCH2CF2), 6.05 (tt, J1＝52 Hz, J2＝5.2 Hz, 1H, CF2H), 7.02 (d, J＝9 Hz, 2H, Ar), 7.20—7.48 (m, 2H, Ar), 8.05—8.40 (m, 4H, Ar); 19F NMR (60 MHz, CDCl3) δ: 42.3 (m, 2F), 47.9 (m, 2F), 52.6 (m, 2F), 59.5 (m, 2F); IR (KBr) ν: 1703, 1604, 1512, 1269, 1209, 1169, 1103, 855, 767 cm－1; MS (70 eV) m/z: 543 (M＋), 191, 121. Anal. calcd for C24H22F8O5 (542.4200): C 53.14, H 4.09, F 28.02; found C 53.89, H 4.21, F 27.97.

3: 24D[α] －0.36 (c 0.0232, CHCl3);

1H NMR (90 MHz, CDCl3) δ: 0.70—1.90 (m, 16H, aliphatic H), 4.16 (d, J＝6 Hz, 2H, OCH2), 4.58—5.18 (m, 3H, OCH2CF2

and C*H), 5.95 (tt, J1＝52 Hz, J2＝5.2 Hz, 1H, CF2H), 7.00—7.39 (m, 4H, Ar), 8.04—8.35 (m, 4H, Ar); 19F NMR (60 MHz, CDCl3) δ: 42.3 (m, 2F), 47.8 (m, 2F), 52.6 (m, 2F), 59.9 (m, 2F); IR (KBr) ν: 1730, 1695, 1604, 1513, 1272, 1178, 1149, 1061, 856, 769 cm－1; MS (70 eV) m/z (%): 585 (M＋), 335 (H(CF2)4CH2O- C6H4CO＋, 100).

4: 24D[α] －0.26 (c 0.0182, CHCl3);

1H NMR (90 MHz, CDCl3) δ: 0.75—1.90 (m, 16H, aliphatic H), 3.86 (d, J＝6 Hz, 2H, OCH2), 4.35—5.10 (m, 3H, OCH2CF2

and C*H), 6.09 (tt, J1＝52 Hz, J2＝5.2 Hz, 1H, CF2H), 6.96 (d, J＝9 Hz, 2H, Ar), 7.10—7.39 (m, 4H, Ar), 8.15 (d, J＝9 Hz, 2H, Ar); 19F NMR (60 MHz, CDCl3) δ: 42.0 (m, 2F), 47.5 (m, 2F), 52.3 (m, 2F), 59.5 (m, 2F); IR (KBr) ν: 1741, 1604, 1508, 1211, 1168, 1149, 1063, 853, 764 cm－1; MS (70 eV) m/z (%): 585 (M＋), 233 (C8H17OC6H4CO＋, 100), 121.

5: 24D[α] －0.44 (c 0.1033, CHCl3);

1H NMR (90 MHz, CDCl3) δ: 0.82—2.05 (m, 9H, aliphatic H), 3.83 (d, J＝6 Hz, 2H, OCH2), 4.84 (t, J＝13 Hz, 2H, OCH2CF2), 6.05 (tt, J1＝52 Hz, J2＝5.2 Hz, 1H, CF2H), 7.00 (d, J＝9 Hz, 4H, Ar), 7.20—8.31 (m, 8H, Ar); IR (KBr) ν: 1731, 1603, 1496, 1276, 1168, 1125, 1080, 836, 770 cm－ 1; MS (70 eV) m/z (%): 618 (M＋ ), 267 (C5H11OC6H4C6H4CO＋, 100), 197 (27).

6: 24D[α] ＋0.67 (c 0.0673, CHCl3);

1H NMR (90 MHz, CDCl3) δ: 0.86—2.04 (m, 9H, aliphatic H), 3.86 (d, J＝6 Hz, 2H, OCH2), 4.86 (t, d, J＝13 Hz, 2H, OCH2CF2), 6.00 (tt, J1＝52 Hz, J2＝5.2 Hz, 1H, CF2H), 6.99 (d, J＝9 Hz, 2H, Ar), 7.19—7.83 (m, 6H, Ar), 8.06—8.36 (m, 4H, Ar); 19F NMR (60 MHz, CDCl3) δ: 42.3

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© 2006 SIOC, CAS, Shanghai, & WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

(m, 2F), 45.2 (m, 2F), 46.4 (m, 4F), 52.4 (m, 2F), 60.1 (d, J＝52 Hz, 2F); IR (KBr) ν: 1731, 1603, 1496, 1276, 1168, 1125, 1080, 836, 770 cm－1; MS (70 eV) m/z (%): 718 (M＋), 267 (C5H11OC6H4C6H4CO＋, 100), 197 (27).

Methyl S-α-(p-benzoxyl)phenoxylpropanoate (VII): 24D[α] －3.0 (c 0.0121, CHCl3);

1H NMR (90 MHz, CDCl3) δ: 1.60 (d, J＝6.8 Hz, 3H, CH3), 3.75 (s, 3H, OCH3), 4.68 (q, J＝5.1 Hz, 1H, C*H), 5.00 (s, 2H, OCH2), 6.88—6.92 (m, 4H, Ar), 7.29—7.44 (m, 5H, Ar); IR (neat) ν: 1731, 1508, 1288, 1233, 1136, 1095, 828, 776 cm－1.

S-α-(p-Benzyloxyl)propanoic acid (VIII): 24D[α]

－3.45 (c 0.0057, CHCl3); 1H NMR (90 MHz, CDCl3) δ:

1.62 (d, J＝7 Hz, 3H, CH3), 4.73 (q, J＝5 Hz, 1H, C*H), 5.03 (s, 2H, OCH2), 6.95 (m, 4H, Ar), 7.42 (m, 5H, Ar), 9.20 (s, 1H, COOH).

Compound IX: 24D[α] －3.14 (c 0.0049, CHCl3);

1H NMR (90 MHz, CDCl3) δ: 1.65 (d, J＝7 Hz, 3H, CH3), 4.52—4.92 (m, 3H, C*H and CH2CF2), 5.05 (s, 2H, OCH2), 6.06 (tt, J1＝60 Hz, J2＝6 Hz, 1H, CF2H), 6.94 (m, 4H, Ar), 7.31—7.54 (m, 5H, Ar); IR (KBr) ν: 1782, 1508, 1288, 1174, 1056, 829, 779 cm－1.

Compound X: 24D[α] －4.17 (c 0.0287, CHCl3);

1H NMR (90 MHz, CDCl3) δ: 1.58 (d, J＝7 Hz, 3H, CH3), 4.43—4.87 (m, 3H, C*H and CH2CF2), 6.07 (tt, J1＝60 Hz, J2＝6 Hz, 1H, CF2H), 6.79 (s, 4H, Ar), 7.28 (s, 1H, OH); IR (KBr) ν: 1782, 1508, 1288, 1174, 1056, 829, 779 cm－1.

7: 1H NMR (90 MHz, CDCl3) δ: 0.91 (d, J＝7 Hz, 3H, CH3), 1.23—1.91 (m, 11H, aliphatic H), 4.07 (t, J＝6.5 Hz, 2H, OCH2), 4.52—4.96 (m, 3H, OCH2CF2 and C*H), 6.04 (tt, J1＝60 Hz, J2＝6 Hz, 1H, CF2H), 6.83—7.28 (m, 6H, Ar), 7.50—7.82 (m, 6H, Ar); 19F NMR (200 MHz, CDCl3) δ: 42.7 (m, 2F), 48.3 (m, 2F), 53.0 (m, 2F), 60.2 (m, 2F); IR (KBr) ν: 1731, 1604, 1504, 1131 cm－1; MS (70 eV) m/z (%): 676 (M＋), 281 (C6H13OC6H4C6H4CO＋, 100).

8: 24D[α] ＋2.45 (c 0.0047, CHCl3);

1H NMR (200 MHz, CDCl3) δ: 0.89 (d, J＝6.7 Hz, 3H, CH3), 1.29—1.88 (m, 13H, aliphatic H), 4.02 (t, J＝6.5 Hz, 2H, OCH2), 4.69 (t, J＝14.8 Hz, 2H, OCH2CF2), 4.87 (q, J＝14.8 Hz, 1H, C*H), 6.06 (tt, J1＝51.9 Hz, J2＝5.4 Hz, 1H, CF2H), 6.93 (d, J＝8.4 Hz, 2H, Ar), 7.00 (d, J＝8.8 Hz, 2H, Ar), 7.15 (d, J＝8.6 Hz, 2H, Ar), 7.59 (d, J＝8.6 Hz, 2H, Ar), 7.69 (d, J＝8.4 Hz, 2H, Ar), 8.21 (d, J＝8.4 Hz, 2H, Ar); 19F NMR (200 MHz, CDCl3) δ: 43.1 (m, 2F), 48.7 (m, 2F), 53.6 (m, 2F), 61.8 (m, 2F); IR (KBr) ν: 1732, 1604, 1507, 1251, 1187, 1131 cm－1; MS (70 eV) m/z (%): 690 (M＋), 295 (C7H15OC6H4- C6H4CO＋, 100).

9: 24D[α] ＋2.54 (c 0.0055, CHCl3);

1H NMR (90 MHz, CDCl3) δ: 0.80—1.95 (m, 18H, aliphatic H and CH3), 4.03 (t, J＝7 Hz, 2H, OCH2), 4.65—5.23 (m, 3H, CH2CF2 and C*H), 6.13 (tt, J1＝52 Hz, J2＝5.2 Hz, 1H, CF2H), 6.88—8.30 (m, 12H, Ar); 19F NMR (90 MHz, CDCl3) δ: 43.5 (m, 2F), 49.1 (m, 2F), 54.1 (m, 2F), 62.6 (m, 2F); IR (KBr) ν: 1732, 1604, 1507, 1251, 1187,

1131 cm－1; MS (70 eV) m/z (%): 704 (M＋), 309 (C8H17OC6H4C6H4CO＋, 100).

10: 24D[α] ＋2.08 (c 0.0078, CHCl3);

1H NMR (90 MHz, CDCl3) δ: 0.75—1.95 (m, 22H, aliphatic H and CH3), 4.01 (t, J＝7 Hz, 2H, OCH2), 4.55—4.95 (m, 3H, CH2CF2 and C*H), 6.04 (tt, J1＝52 Hz, J2＝5.2 Hz, 1H, CF2H), 6.80—8.28 (m, 12H, Ar); 19F NMR (90 MHz, CDCl3) δ: 42.5 (m, 2F), 48.3 (m, 2F), 53.9 (m, 2F), 62.1 (m, 2F); IR (KBr) ν: 1733, 1604, 1506, 1251, 1203, 1133 cm－1; MS (70 eV) m/z (%): 733 (M＋), 337 (C10H21OC6H4C6H4CO＋, 100).

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(E0504017 PAN, B. F.; LING, J.)