Synthesis and mesomorphic properties of tolane-based fluorinated liquid crystals with an acrylate linkage

Article abstract of DOI:10.1080/15421406.2010.504638

Six series of tolane-based fluorinated liquid crystals containing an acrylate linker were synthesized and their mesomorphic properties were characterized. The compounds generally exhibit SmA and N phases in wide temperature ranges. The introduction of acrylate linkers increases the clearing points by 20°C-30°C and suppresses the formation of higher ordered monotropic SmB phases. para-Fluorination is favorable for the mesophase stability, whereas meta-fluorination is not. The introduction of a 2,3,5,6-tetrafluorophenylene group decreases the clearing points and also suppresses the formation of the monotropic SmB phase. Copyright Taylor & Francis Group, LLC.

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Synthesis and Mesomorphic Properties of
Tolane-Based Fluorinated Liquid Crystals
with an Acrylate Linkage
Ximin Chen a , Yuehai Shen b & Jianxun Wen a
a Shanghai Institute of Organic Chemistry, Chinese Academy of
Sciences , Shanghai, China
b Faculty of Life Sciences and Technology, Kunming University of
Science and Technology , Kunming, China
Published online: 20 Oct 2010.
To cite this article: Ximin Chen , Yuehai Shen & Jianxun Wen (2010) Synthesis and Mesomorphic
Properties of Tolane-Based Fluorinated Liquid Crystals with an Acrylate Linkage, Molecular Crystals
and Liquid Crystals, 528:1, 138-146, DOI: 10.1080/15421406.2010.504638
To link to this article: http://dx.doi.org/10.1080/15421406.2010.504638
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Mol. Cryst. Liq. Cryst., Vol. 528: pp. 138–146, 2010
Copyright # Taylor & Francis Group, LLC
ISSN: 1542-1406 print=1563-5287 online
DOI: 10.1080/15421406.2010.504638
Synthesis and Mesomorphic Properties of
Tolane-Based Fluorinated Liquid Crystals
with an Acrylate Linkage
XIMIN CHEN,1 YUEHAI SHEN,2
AND JIANXUN WEN1
1Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences,
Shanghai, China
2Faculty of Life Sciences and Technology, Kunming University of
Science and Technology, Kunming, China
Six series of tolane-based fluorinated liquid crystals containing an acrylate linker
were synthesized and their mesomorphic properties were characterized. The
compounds generally exhibit SmA and N phases in wide temperature ranges.
The introduction of acrylate linkers increases the clearing points by 20ꢀC–30ꢀC
and suppresses the formation of higher ordered monotropic SmB phases. para-
Fluorination is favorable for the mesophase stability, whereas meta-fluorination is
not. The introduction of a 2,3,5,6-tetrafluorophenylene group decreases the clearing
points and also suppresses the formation of the monotropic SmB phase.
Keywords Cinnamate; fluorine; liquid crystals; tolane
Introduction
Because of their unique polar and steric effects, fluorine substituents in liquid
crystals (LCs) often bring desirable changes in respect to melting point, mesophase
morphology, transition temperatures, thermal stability, and essential physical
properties such as optical and dielectric anisotropy [1–5]. Tolane-based fluorinated
LCs are useful electro-optic media with low viscosity and high birefringence and
thus have attracted considerable attention recently [6–8]. Study along this line
has covered many three- and four-ring systems; for example, bistolane [9],
biphenyltolane [10,11], biphenyl bistolane [12], tolane benzoate [13,14], and tolane
cyclohexanylbenzoate [15].
Compared with an acetylene or an ester linker, an acrylic ester bridge
=
[(E)ꢁCH CHꢁCOOꢁ] in a rod-like LC molecule extends the conjugated system
and is likely to result in a higher clearing point. Liquid-crystalline cinnamate photo-
polymers are also used in making orientation films of liquid-crystal displays (LCDs)
[16]. Therefore, we designed a three-ring system containing fluorinated cinnamate
Address correspondence to Yuehai Shen, Faculty of Life Sciences and Technology,
Kunming University of Science and Technology, 296 Bailong Si, Kunming 650216, China.
E-mail: yuehaishen@gmail.com
138

Cinnamate Tolane-Type Liquid Crystals
139
Scheme 1. Chemical structures of the target compounds.
ester moieties to further explore the structure-property relationships of tolane-based
LCs (Scheme 1). The role of the trans-double bond can be revealed by comparing
with reported tolane benzoates. In addition, the effect of lateral fluorination can
be investigated by introducing a 2,3,5,6-tetrafluorophenylene group.
Results and Discussion
The synthesis of the target fluorinated liquid-crystal series are summarized in
Scheme 2. Fluorinated benzaldehydes 1a-c were prepared by formylation of poly-
fluorophenyl Grignard reagents, and 4-trifluoromethylbenzaldehyde (1d) was
Scheme 2. Synthesis of target compounds: (a) 1, Mg, THF; 2, DMF; (b) FSO2CF2CO2CH3,
CuI, DMF, 75ꢀC; (c) malonic acid, pyridine, piperidine, reflux; (d) 4-iodophenol, DCC, cat.
DMAP, CH2Cl2; (e) cat. Pd(PPh3)2Cl2, CuI, Et3N, reflux.

140
X. Chen et al.
obtained from 4-iodobenzaldehyde by a copper-catalyzed trifluoromethylation using
methyl 2,2-difluoro-2-(fluorosulfonyl)acetate (FSO2CF2CO2CH3) [17]. Benzalde-
hydes 1a-d were then converted to cinnamic acids 2a-d through reactions with
malonic acid in pyridine and piperidine. N,N-Dicyclohexylcarbodiimide (DCC)-
promoted esterifications afforded aryl iodides 3a–d, which were then reacted with
phenyl acetylene 4 or 5 respectively under standard Sonogashira conditions to afford
the target compounds.
The phase transition temperatures of series A, A0, B, C, D, and D0 are summar-
ized in Table 1. All of the compounds exhibit enantiotropic mesophases in very
broad temperature ranges. The compounds show surprisingly high thermal stability.
There is no decomposition observed even at 250ꢀC.
The effect of the alkoxy chain length on the mesomorphic properties is visualized
with a plot of the transition temperatures as a function of methylene units (n) in the
alkyl chain (Figs 1–3). The compounds in series A display smectic A and nematic
phases on both heating and cooling (Fig. 1). With the length of the alkoxy chain
increasing, the clearing points and the temperature ranges of the nematic phase
Table 1. Phase transition temperature of target compounds
Compounds
n
Transition temperature (ꢀC)
A6
A7
A8
A9
A10
A12
A70
B6
B7
6
7
8
C 122.3 SmA 129.9 N 219.5 I 219.2 N 129.4 SmA 81.4 Recr
C 98.1 SmA 158.6 N 214.4 I 214.2 N 157.8 SmA 78.8 Recr
C 98.7 SmA 170.9 N 209.7 I 209.4 N 170.5 SmA 72.4 Recr
C 103.0 SmA 178.8 N 203.2 I 203.0 N 178.5 SmA 74.6 Recr
C 87.2 SmA 182.7 N 198.0 I 197.8 N 182.5 SmA 66.5 Recr
C 89.4 SmA 176.7 N 181.7 I 181.5 N 176.5 SmA 73.9 Recr
C 77.5 SmA 146.9 N 184.8 I 184.3 N 146.5 SmA 42.4 Recr
C 107.1 SmA 177.9 N 198.7 I 198.5 N 177.5 SmA 78.3 Recr
C 105.1 SmA 182.1 N 192.9 I 192.7 N 181.7 SmA 74.1 Recr
C 81.9 SmA 185.2 N 190.2 I 190.0 N 184.9 SmA 72.8 Recr
C 98.5 SmA 186.5 I 186.4 SmA 69.9 Recr
9
10
12
7
6
7
B8
B9
8
9
B10
B12
C10
10
12
10
C 81.4 SmA 177.6 I 77.4 SmA 57.2 Recr
C 84.6 SmA 183.8 I 183.6 SmA 76.1 Recr
C 83.3 SmA 106.3 N 133.0 I 132.6 N 105.6 SmA 82.6 SmC
66.1 Recr
D6
D7
D8
6
7
8
C 124.2 SmA 225.6 N 242.5 I 241.6 N 225.4 SmA 120.1 SmB
77.0 Recr
C 127.8 SmA 227.2 N 234.8 I 234.5 N 226.7 SmA 119.9 SmB
88.5 Recr
C 137.5 SmA 226.2 N 229.9 I 229.6 N 225.8 SmA 118.5 SmB
100.4 Recr
D9
9
10
12
6
C 117.2 SmA 224.8 I 224.5 SmA 116.0 SmB 82.9 Recr
C 112.4 SmA 222.0 I 221.5 SmA 118.2 SmB 84.3 Recr
C 116.0 SmA 214.5 I 213.9 SmA 115.5 SmB 77.2 Recr
C 76.3 SmA 196.1 N 221.7 I 221.4 N 195.7 SmA 37.3 Recr
D10
D12
D60
Cr ¼ crystal; SmA ¼ smectic A phase; SmB ¼ smectic B phase; SmC ¼ smectic C phase;
N ¼ nematic phase; I ¼ isotropic liquid; Recr ¼ recrystal.

Cinnamate Tolane-Type Liquid Crystals
141
Figure 1. Transition temperatures versus number of methylene units (n) of series A.
decrease, whereas the ranges of the smectic A phase have an increasing trend because
of the increasing lateral interactions; the melting points tend to decrease, but the
melting point of compound A9 is higher than those of compounds A8 and A10.
Similarly, the clearing points and nematic ranges of series B decrease as the chain
length increases (Fig. 2). When the alkoxy chain is longer than nine carbons, only a
smectic A phase exists. Series D, with a trifluoromethyl as the end group, possesses
monotropic higher-order smectic B phases besides enantiotropic smectic A and
nematic phases (Fig. 3). When n > 9, only smectic A and B phases exist. The clearing
points also decrease as the length of the alkoxy chain increases.
Comparison between series A and reported benzoate analogues 34Fn [13] shows
the effect of the trans-double bond on the liquid-crystalline properties. The clearing
points of series A and 34Fn were plotted (Fig. 4) as a function of the length of the
alkoxy chain. The clearing points of series A are about 20ꢀC–30ꢀC higher than those
of the corresponding benzoate analogues. The enhanced mesophase stability of series
A is attributed to the longer conjugated system when a trans-double bond is added
into the rigid liquid-crystalline core. It is worth noting that the introduction of
the acrylic ester bridge simplifies the mesomorphic behavior by suppressing the
formation of the higher ordered smectic B phase in cooling processes.
Figure 2. Transition temperatures versus number of methylene units (n) of series B.

142
X. Chen et al.
Figure 3. Transition temperatures versus number of methylene units (n) of series D (cooling).
The clearing points of series A, B, D, and compound C10 as a function of the
length of the alkoxy chain are plotted (Fig. 5) in order to study the influence of dif-
ferent fluorination patterns of the end phenyl group on mesomorphic properties. It is
found that if the length of alkoxy chains remains the same, the compounds with a
4-trifluoromethylphenyl terminal group (series D) have the highest clearing points,
and the compounds in series C, with a 3,5-difluorophenyl, exhibit the lowest clearing
points. The clearing points of 3,4-difluorophenyl-containing series A are generally
higher than those of 3,4,5-trifluorophenyl-containing series B. It is concluded that
for different numbers and positions of fluorination, the order of mesophase stability
is 4-CF3- > 3,4-difluoro- > 3,4,5-trifluoro- > 3,5-difluorophenyl. Apparently, a para-
F or CF3 group increases both molecular polarity and molecular length and thus
raises the clearing points. On the contrary, meta-fluorination broadens the mole-
cules, which is unfavorable for formation of liquid-crystalline phases [1].
Figure 4. Clearing points versus number of methylene units (n) of series A and 34Fn.

Cinnamate Tolane-Type Liquid Crystals
143
Figure 5. Clearing points versus the number of methylene units (n) of series A, B, C, and D.
Finally, comparing A7 with A07 and D6 with D06 reveals the effect of lateral fluor-
ination. The introduction of 2,3,5,6-tetrafluorophenylene moiety decreases both the
melting and clearing points. Suppressing formation of monotropic smectic B phase is
also observed (D6 versus D06). This is consistent with reported results on other sys-
tems [18].
Conclusion
In summary, two categories of cinnamate tolane-type LCs were synthesized and their
mesomorphic properties were characterized. The compounds generally exhibit
smectic A and nematic phases over wide temperature ranges. The effects of the
alkoxy chain length, the introduction of the trans-double bond and that of
2,3,5,6-tetrafluorophenylene, and the fluorination patterns of the end phenyl group
on mesomorphic properties were investigated. The introduction of acrylate linkers
increases the clearing points by 20ꢀC–30ꢀC and suppresses the formation of higher
ordered monotropic smectic B phases. para-Fluorination is favorable for the
mesophase stability, whereas meta-fluorination is not. The introduction of 2,3,5,6-
tetrafluorophenylene decreases the clearing points and also suppresses the formation
of the monotropic smectic B phase.
Experimental
Characterization
The structures of the intermediates and the target compounds were verified by spec-
trometric methods. Infrared (IR) spectra were recorded on a Perkin Elmer IR-983
spectrophotometer. Nuclear magnetic resonance (NMR) spectra were recorded on
a Bruker Advance-300 spectrometer (300 MHz), Varian EM 360 L spectrometer
(60 MHz), or a JEOL FX-90Q NMR (90 MHz) with tetramethylsilane (TMS) as
internal standard for1H-NMR and trifluoroacetic acid (TFA) as external standard
for 19F-NMR. The high field was set to be positive in 19F-NMR spectra. Mass
spectra were obtained with a Finnigan 4021 mass spectrometer using electrical
ionization. High-resolution mass spectra were recorded with Kratos Concept 1H
mass spectrometer.

144
X. Chen et al.
The transition temperatures of the final liquid-crystalline compounds were
measured by a polarizing optical microscope (Olympus PM-6) fitted with a heating
stage (Mettler FP-80) and a control unit (FP-82) and also by a differential scanning
calorimeter (Shimadzu DSC-50) with heating=cooling rate 5ꢀC=min. The transition
temperatures in this article are the peak values of the transitions on DSC traces.
Phase assignment was made by comparing the observed textures with those in the
literature [19–21].
Synthesis
Examples of the typical procedures for the preparation of the intermediates and the
target compounds are given as follows: All of the target compounds have satisfying
1
elemental analysis and appropriate H- and 19F-NMR, IR, and MS spectra data.
3,4-Difluorobenzaldehyde (1a). Into a 100 mL dried three-necked round-bottom
flask, equipped with a gas inlet and outlet and a dropping funnel, was added
magnesium filings (385 mg, 16 mmol), anhydrous tetrahydrofuran (THF; 10 mL),
and a small piece of iodine as the initiator. A solution of 3,4-difluorobromobenzene
(3.00 g, 15.5 mmol) in anhydrous THF (5 mL) was added dropwise into the flask
with stirring. The resulting reaction mixture was stirred at room temperature for
2 h until the magnesium filings disappeared. The obtained Grignard reagent solution
was ice-cooled, and a solution of DMF (1.13 g, 15.5 mmol) in anhydrous THF
(5 mL) was added. The reaction was stirred at 0ꢀC for 30 min, quenched by adding
saturated NH4Cl solution, and then extracted with diethyl ether. The etheral layer
was dried over anhydrous sodium sulfate and was concentrated. The crude product
was purified by column chromatography on silica gel (dichloromethane) to give
compound 1a as a colorless liquid (1.53 g, 63%). 19F-NMR (CDCl3=TFA) d 49.5
(m, 1F), 57.4 (m, 1F).
4-Trifluoromethylbenzaldehyde (1d). A solution of 4-iodobenzaldehyde (350 mg,
1.5 mmol) in Dimethylformamide (DMF) (5 mL) was added to a mixture of copper
(I) iodide (3.00 g, 15.7 mmol), methyl 2,2-difluoro-2-(fluorosulfonyl)acetate (2.59 g,
13.5 mmol), and DMF (20 mL) preheated to 75ꢀC. The reaction was stirred at
the same temperature for 10 h, and then cooled down to room temperature and
quenched by pouring into water. The mixture was filtered and the filtrate was
extracted with diethyl ether. The etheral extract was washed with water, dried over
anhydrous sodium sulfate, and concentrated to give a yellow oil (196 mg, yield
1
75%). H-NMR (CDCl3=TMS) d 7.65 (d, J ¼ 9.0 Hz, 2H), 7.90 (d, J ¼ 9.0 Hz, 2H),
9.95 (s, 1H). 19F-NMR (CDCl3=TFA) d ꢁ14.0 (s, 3F).
3-(3,4-Difluoropheny)acrylic acid (2a). 3,4-Difluorobromobenzaldehyde (1a)
(0.739 g, 5.2 mmol) and malonic acid (1.045 g, 10 mmol) were dissolved in pyridine
(10 ml) in a 50-ml dried 3-necked round bottom flask. Piperidine (0.5 ml) was added
in one portion. The resulting mixture was stirred at 85ꢀC for 25 hours, and then
refluxed for 3 hours. After cooling down to room temperature, the solution was
acidified by adding 10 M hydrochloric acid (7 ml), and extracted twice with diethyl
ether. The combined ethereal extracts were washed with water, dried over anhydrous
sodium sulfate, and concentrated to give the title compound as a light-pink
solid (710 mg, yield 74%). 1H-NMR (CDCl3=TMS) d 6.50 (d, J ¼ 16.0 Hz, 1H),

Cinnamate Tolane-Type Liquid Crystals
145
7.65 (d, J ¼ 16.0 Hz, 1H), 7.50 (m, 3H). 19F-NMR (CDCl3=TFA) d 61.5 (m, 1F),
63.5 (m, 1F).
3-(3,4-Difluoropheny)acrylic acid 4-iodophenyl ester (3a). Compound 2a (580 mg,
3.15 mmol) and 4-iodophenol (763 mg, 3.46 mmol) were stirred with DCC (721 mg,
3.5 mmol) and 4-Dimethylaminopyridine (DMAP) (20 mg) in dichloromethane
(20 mL) in a single-necked round-bottom flask for 10 h. The resulting slurry was
filtered and the filtrate was concentrated. The solid residue was purified by
column chromatography on silica gel with petroleum ether=ethyl acetate (5:1) as
1
the eluent to give a white solid (1.12 g, yield 92%). H-NMR (CDCl3=TMS) d 6.45
(d, J ¼ 16.0 Hz, 1H), 7.65 (d, J ¼ 16.0 Hz, 1H), 6.90 (d, J ¼ 9.0 Hz, 2H), 7.65 (d,
J ¼ 9.0 Hz, 2H), 7.30 (m, 3H). 19F-NMR (CDCl3=TFA) d 55.0 (m, 2F), 58.0 (m,
1F). MS (m=z, %): 386 (Mþ, 8.58).
3-(3,4-Difluorophenyl)acrylic acid 4-(4-decyloxyphenylethynyl)phenyl ester (A10,
n ¼ 10). A mixture of compound 3a (120 mg, 0.31 mmol), compound 4 (n ¼ 10);
(80 mg, 0.31 mmol), bis(triphenylphosphine)palladium dichloride (15 mg), copper
(I) iodide (5 mg), and anhydrous triethylamine (15 mL) was stirred under reflux
for 3 h under nitrogen protection. After the reaction was completed, the mixture
was cooled to room temperature, diluted with diethyl ether, and filtered to remove
the precipitates. The filtrate was washed with water, dried over anhydrous sodium
sulfate, and concentrated. The pure product was obtained by column
chromatography on silica gel (petroleum ether=ethyl acetate 10:1) followed by
recrystallization from methanol-acetone to give compound A10 as white crystals
1
(72 mg, yield 45%). H-NMR (CDCl3=TMS) d 0.90 (t, J ¼ 6.0 Hz, 3H), 1.10–1.90
(m, 16H), 4.00 (t, J ¼ 6.0 Hz, 2H), 6.50 (d, J ¼ 16.0 Hz, 1H), 7.75 (d, J ¼ 16.0 Hz,
1H), 6.85 (d, J ¼ 8.0 Hz, 2H), 7.25 (d, J ¼ 8.0 Hz, 2H), 7.13 (d, J ¼ 8.0 Hz, 2H),
7.55 (d, J ¼ 8.0 Hz, 2H), 7.40 (m, 3H). 19F-NMR (CDCl3=TFA) d 55.30 (m, 1F),
58.30 (m, 1F). IR (KBr, cmꢁ1) 2955, 2920, 2851, 2219, 1735, 1725, 1641, 1605,
1513, 1281, 1204, 1169, 1020, 840, 776, 529. MS (m=z, %) 516 (Mþ, 23.06).
HRMS for C33H34F2O3: Calculated 516.2481; Found 516.2486.
Acknowledgment
This work is supported by the National Natural Science Foundation of China.
References
[1] Hird, M. (2007). Chem. Soc. Rev., 36, 2070.
[2] Guittard, F., Geribaldi, S., & Cambon, A. (1999). J. Fluorine Chem., 100, 85.
[3] Gray, G. W., Hird, M., Lacey, D., & Toyne, K. J. (1989). Mol. Cryst. Liq. Cryst., 172,
165.
[4] Gray, G. W., Hird, M., Lacey, D., & Toyne, K. J. (1991). Mol. Cryst. Liq. Cryst., 195,
221.
[5] Goulding, M. J., Greenfield, S., Coates, D., & Clemitson, R. (1993). Liq. Cryst., 14, 1397.
[6] Tsay, K. T., Hsu, C. S., & Wu, S. T. (1998). Proc. SPIE, 3421, 142.
[7] Ichinose, H., Nakajima, S., Sugiyama, Y., Numata, H., Naemura, S., & Manabe, A.
(2001). J. SID, 9, 287.
[8] Gauza, S., Wang, H., Wen, C. H., Wu, S. T., Seed, A. J., & Dabrowski, R. (2003). Jpn. J.
Appl. Phys. 1, 42, 3463.

146
X. Chen et al.
[9] Sekine, C., Konya, N., Minai, M., & Fujisawa, K. (2001). Liq. Cryst., 28, 1495.
[10] Liao, Y. M., Janarthanan, N., Hsu, C. S., Gauza, S., & Wu, S. T. (2006). Liq. Cryst.,
33, 1199.
[11] Xianyu, H. Q., Gauza, S., Song, Q., & Wu, S. T. (2007). Liq. Cryst., 34, 1473.
[12] Gauza, S., Wen, C. H., Wu, S. T., Janarthanan, N., & Hsu, C. S. (2004). Jpn. J. Appl.
Phys., 43, 7634.
[13] Yang, Y. G., Tang, G., Gong, Z., & Wen, J. X. (2000). Mol. Cryst. Liq. Cryst., 348, 153.
[14] Yao, L. H., Li, H. F., & Wen, J. X. (2005). Liq. Cryst., 32, 527.
[15] Chen, X. M., Wang, K., Li, H. F., & Wen, J. X. (2002). Liq. Cryst., 29, 1105.
[16] Park, J. G., Kim, D. Y., Choi, H. J., & Kim, J. Y. (2001). Cinnamate-containing photo-
polymer for orientation film of liquid crystal display (LCD) and method for using the
photopolymer to form an orientation film. U.S. Patent 6, 174,694.
[17] Chen, Q. Y. (1995). J. Fluorine Chem., 72, 241.
[18] Gray, G. W., Hird, M., Lacey, D., & Toyne, K. J. (1989). J. Chem. Soc., Perkin Trans. II,
2041.
[19] Demus, D., & Richter, L. (1979). Texture of Liquid Crystals, Verlag Chemie: Weinheim,
Germany.
[20] Gray, G. W., & Goodby, J. W. (1986). Smectic Liquid Crystals: Textures and Structures,
Leonard Hill: Glasgow, UK.
[21] Dierking, I. (2003). Textures of Liquid Crystals, Wiley-VCH: Weinheim, Germany.