Smectic C Mesogens with Terminal Perfluoroalkyl Chains

Article abstract of DOI:10.1080/15421406.2014.915634

The fluorocarbon derivatives were synthesized. Liquid crystalline states were studied by DSC and microscopic observation. TNI of those compounds were from 161.2°C to 191.7°C, which represented those compounds had very good liquid crystalline stability. Sm

Full text of DOI:10.1080/15421406.2014.915634

Mol. Cryst. Liq. Cryst., Vol. 606: pp. 90–100, 2015
Copyright © Taylor & Francis Group, LLC
ISSN: 1542-1406 print/1563-5287 online
DOI: 10.1080/15421406.2014.915634
Smectic C Mesogens with Terminal
Perfluoroalkyl Chains
JUN YEOL JANG AND YOUNG WOOK PARK∗
Department of Polymer Science and Engineering, Engineering Research
Institute, Gyeongsang National University, Jinju, Republic of Korea
The fluorocarbon derivatives were synthesized. Liquid crystalline states were studied
by DSC and microscopic observation. TNI of those compounds were from 161.2◦C to
191.7◦C, which represented those compounds had very good liquid crystalline stability.
Smectic C temperature range was 91.7◦C for F8H2OPMB, 54.9◦C for F8H2OPCB,
41.1◦C for F8H2OPMB-F, 36.2◦C for F8H2OMB, 27.4◦C for F8H2OPCB-F, and 10.8◦C
for F6H2OMB. The structures of F8H2OPCB and F8H2OPCB-F at room temperature
were tilted herringbone packing of two aromatic ring ester cores (smectic K phase) and
perfluoroalkyl chains were located up and down alternatively. Smectic K was proved by
high intensity of 003 and 004 reflections in X-ray diffraction.
Keywords DSC; fluorocarbon; microscopic texture; smectic C; smectic K; XRD
Introduction
Fluorocarbons and hydrocarbons possess different properties and are incompatible with one
another. Those are due to the different chemical structures of H and F atoms leading to dif-
ferent chain conformations; (CH2)n segments arrange in the usual zigzag conformation with
a cross-sectional area of 18.5 Å2/molecule, whereas a (CF2)m chain possesses a typical 15/7
helix conformation and cross-sectional area of 28 Å2/molecule [1, 2]. When fluorocarbons
are substituted for alkanes in calamitic liquid crystals (LCs), both the transition temper-
ature and the temperature range of mesophase stability are increased. This is due to the
increased rigidity of the fluorocarbon moieties, which enhances the linearity of molecules
relative to their alkane counterparts [3]. However, the intrinsic incompatibility between the
fluorocarbon tails and the hydrocarbon mesogenic core leads to a nanophase separation of
the chemically distinct components which results in layered (smectic) structure composed
of fluorocarbon-rich and hydrocarbon-rich strata [3–6].
To study this phenomenon, we have made F6H2OCB and F8H2OCB, which both
compounds exhibit smectic C phase with wide temperature range, and showed smectic K
phase, proved by X-ray diffraction (XRD) spectra at room temperature [7].
In this paper, we made many derivatives of F8H2OCB, which are compounds with
methoxy terminal group for high smectic C phase stability, ester linkage for extending rigid
group length and fluoro substitution in aromatic ring for reducing transition temperature
∗Address correspondence to Young Wook Park, Department of Polymer Science and Engi-
neering, Engineering Research Institute, Gyeongsang National University, Jinju, Republic of Korea.
E-mail: ywpark@gnu.ac.kr
Color versions of one or more of the figures in the article can be found online at
www.tandfonline.com/gmcl.
90

Smectic C Mesogens
91
were synthesized. These compounds are shown in Fig. 1. Ferroelectric smectic C∗ could
be formed as a result of adding a chiral component as a dopant to smectic C matrix.
Experimental
4-(2-perfluorohexyl)-ethoxy-4ꢀ-methoxybiphenyl (F6H2OMB)
1H, 1H, 2H, 2H-perfluoro-1-octanol (3.64 g, 10 mmol), 4-hydroxy-4ꢀ-methoxybiphenyl
(2.01 g, 10 mmol) and triphenylphosphine (2.62 g, 10 mmol) were dissolved in dry tetrahy-
drofuran (THF) (30 mL). Diisopropyl azodicarboxylate (2 cm3, 10 mmol) was added
dropwise to the solution for 30 min under N2 at 0◦C. The solution was stirred at 0◦C
overnight under N2 atmosphere. After quenching with cold water (1 mL), the solvent was
removed under reduced pressure. The residue was purified by column chromatography
on silica gel (eluent 2.5:1 hexane and ethyl acetate), and recrystallized in 95% ethanol to
produce 4-(2-perfluorohexyl)-ethoxy-4ꢀ-methoxybiphenyl (F6H2OMB) as a white crystal
(0.27g) in 5% yield. 1H NMR and IR confirmed the structure as (δ(ppm)) 7.50 (doublet, J =
8.9 Hz, 4H), 6.98 (doublet, J = 8.9 Hz, 4H), 4.33 (triplet, J = 6.8 Hz, 2H), 3.87 (singlet,
3H), 2.67 (multiplet, J = 6.7 and 11.7 Hz, 2H), and IR(vCF and vCO = 1257, 1182, 1141,
1087 cm−1).
4-(2-perfluorooctyl)-ethoxy-4ꢀ-methoxybiphenyl (F8H2OMB)
F8H2OMB was synthesized by the same method as F6H2OMB synthesis.
1-(2-perfluorooctyl)-ethoxy-4-methoxybenzene (F8H2OMP)
1H, 1H, 2H, 2H-perfluoro-1-decanol (11.6 g, 25 mmol), 4-methoxyphenol (3.1 g, 25 mmol)
and triphenylphosphine (6.55 g, 25 mmol) were dissolved in dry tetrahydrofuran (THF)
(35 ml). Diisopropyl azodicarboxylate (5 cm3, 25 mmol) was added dropwise to the solution
for 30 min under N2 at 0◦C. The solution was stirred at 0◦C overnight under N2 atmosphere.
After quenching with cold water (1 mL), the solvent was removed under reduced pressure.
The residue was purified by column chromatography on silica gel (eluent 2.5:1 hexane and
ethyl acetate), and recrystallized in 95% ethanol to produce 1-(2-perfluorooctyl)-ethoxy-4-
methoxybenzene as a white crystal (0.71g) in 5% yield. 1H NMR confirmed the structure
as (δ(ppm)) 6.87 (singlet, 4H), 4.25 (triplet, J = 6.8 Hz, 2H), 3.80 (singlet, 3H), 2.62
(multiplet, J = 6.8, and 11.7 Hz, 2H).
1-(2-perfluorooctyl)-ethoxy-4-hydroxybenzene
1-(2-Perfluorooctyl)-ethoxy-4-methoxybenzene (12.5 g, 22 mmol) was dissolved in
dichloromethane (50 mL). Boron tribromide (5.1 cm3, 53 mmol) in dichloromethane (50)
was added dropwise to the solution at 0◦C. The solution was stirred at room temperature
overnight. After quenching with cold water (5 mL), the solvent was removed under re-
duced pressure. The residue was dissolved by the addition of 100 mL of ether. The organic
layer was separated and extracted with water, 10% NaHCO3 and water. The ether extract
was dried over anhydrous magnesium sulfate and recrystallized in toluene to produce
1-(2-perfluorooctyl)-ethoxy-4-hydroxybenzene as a white crystal (10.6 g) in 87%
yield. 1H NMR confirmed the structure as (δ(ppm)) 6.83 (doublet, J = 8.7 Hz,
2H), 6.79 (doublet, J = 8.7 Hz, 2H), 4.78 (singlet, 1H), 4.24 (triplet, J = 6.8 Hz, 2H), and
2.62 (multiplet, J = 6.8 and 11.6 Hz, 2H).

92
J. Y. Jang and Y. W. Park
Figure 1. The structure of the liquid crystalline compounds with terminal perfluoroalkyl chains.

Smectic C Mesogens
93
Figure 2. Synthesis of F-6H2OMB, F8H2OPCB and F8H2OPCB, F8H2OPMB and F8H2OPCB-F,
F8H2OPMB-F.
4-(2-perfluorooctyl)-ethoxyphenyl-4ꢀ-cyanobenzoate (F8H2OPCB)
1-(2-Perfluorooctyl)-ethoxy-4-hydroxybenzene (0.56 g, 1 mmol), 4-cyanobenzoyl chloride
(0.26 g, 1 mmol) and pyridine (0.16 cm3, 2 mmol) were dissolved in chloroform (25 mL).
The solution was stirred for 3 hr at room temperature. After quenching with cold water
(1 mL), the solvent was removed under reduced pressure. The residue was purified by

94
J. Y. Jang and Y. W. Park
Table 1. Thermal transitions of the liquid crystalline compounds (Fig. 3)
Compound
Transition temperatures (◦C)
ꢀH (J/g)
(1) F6H2OCB[7] SK 89.4 SC 162.0 SA 162.7
(2) F8H2OCB[7] SK 103.7 SC 182.1 SA 182.9
I
I
29.35 & 8.28
37.07 & 10.54
12.41 & 12.53
& 12.90
4.57& 5.61 &
5.60
(3) F6H2OMB
Cr 124.5 S? 139.2 SC 150.0 SA 176.6
I
I
(4) F8H2OMB
Cr 104.3 S? 140.8 SC 177.0 SA 191.7
(5) F8H2OPCB
(6) F8H2OPMB
(7) F8H2OPCB-F SK 121.6 SC 149.0 SA 161.2
(8) F8H2OPMB-F Cr 104.9 SC 146.0 SA 162.3
SK 129.1 SC 184.0 SA 189.5
Cr 82.3 SC 174.0 SA 182.4
I
I
I
I
36.90 & 10.58
16.50 & 3.98
48.19 & 10.48
35.34 & 6.37
column chromatography on silica gel (eluent 2.5:1 hexane and ethyl acetate), and recrys-
tallized in 95% ethanol to produce 4-(2-perfluorooctyl)-ethoxyphenyl-4ꢀ-cyanobenzoate
1
(F8H2OPCB) as a white crystal (0.4 g) in 58% yield. H NMR and IR confirmed the
structure as (δ(ppm)) 8.32 (doublet, J = 8.4 Hz, 2H), 7.84 (doublet, J = 8.4 Hz, 2H), 7.18
(doublet, J = 9.0 Hz, 2H), 6.99 (doublet, J = 9.0 Hz, 2H), 4.32 (triplet, J = 6.8 Hz, 2H),
2.67 (multiplet, J = 6.7 and 11.7 Hz, 2H), IR(vCN = 2231 cm−1, vC = 1743 cm−1, vCF
O
and vCO = 1251, 1195, 1149, 1116 cm−1).
4-(2-perfluorooctyl)-ethoxyphenyl-4ꢀ-methoxybenzoate (F8H2OPMB)
F8H2OPMB was synthesized by the same method as F8H2OPCB synthesis. 1H NMR and
IR confirmed the structure as (δ(ppm)) 8.17 (doublet, J = 9.0, 2H), 7.16 (doublet, J = 9.0,
2H), 7.00 (doublet, J = 9.0, 2H), 6.96 (doublet, J = 9.1, 2H), 4.31 (triplet, J = 6.8, 2H),
3.92 (singlet, 3H), 2.67 (multiplet, J = 6.7 and 11.7 Hz, 2H), and IR(vC = 1735 cm−1
,
O
vCF and vCO = 1249, 1203, 1164, 1116 cm−1).
4-(2-perfluorooctyl)-ethoxyphenyl-3ꢀ-fluoro-4ꢀ-methoxybenzoate (F8H2OPMB-F)
1
F8H2OPMB-F was synthesized by the same method as F8H2OPCB synthesis. H NMR
and IR confirmed the structure as (δ(ppm)) 8.00 (doublet, J = 9.3, 1H), 7.92 (doublet, J =
9.6, 1H), 7.15 (doublet, J = 9.2, 2H), 7.07 (doublet, J = 8.4, 1H), 6.97 (doublet, J = 9.1,
2H), 4.31 (triplet, J = 6.8, 2H), 4.01 (singlet, 3H), 2.67 (multiplet, J = 6.6 and 12.0 Hz,
2H), and IR(vC = 1732 cm−1, vCF and vCO = 1261, 1203, 1145, 1101 cm−1).
O
4-(2-perfluorooctyl)-ethoxyphenyl-3ꢀ-fluoro-4ꢀ-cyanobenzoate (F8H2OPCB-F)
4-(Dimethylamino)pyridine(DMAP) (0.08 g, 0.7 mmol) was added to a solution of 1-
(2-perfluorooctyl)-ethoxy-4-hydroxybenzene (0.56 g, 1 mmol), 4-cyano-3-fluorobenzoic
acid (0.18 g, 1 mmol) and N,Nꢀ-dicyclohexylcarbodiimide(DCC) (0.28 g, 1.4 mmol) in
dichloromethane (25 mL). The solution was stirred for 26 hr at room temperature. After
quenching with cold water (1 mL), the solvent was removed under reduced pressure. The
residue was purified by column chromatography on silica gel (eluent 2.5:1 hexane and ethyl

Smectic C Mesogens
95
Figure 3. DSC thermogram of (a) F6H2OMB, (b) F8H2OMB, (c) F8H2OPCB, (d) F8H2OPMB,
(e) F8H2OPCB-F, and (f) F8H2OPMB-F.
acetate), and recrystallized in 95% ethanol to produce 4-(2-perfluorooctyl)-ethoxyphenyl-
1
3ꢀ-fluoro-4ꢀ-cyanobenzoate (F8H2OPCB-F) as a white crystal (0.42 g) in 60% yield. H
NMR and IR confirmed the structure as (δ(ppm)) 8.11 (doublet, J = 6.6 Hz, 1H), 8.05
(doublet, J = 7.8 Hz, 1H), 7.82 (doublet, J = 6.1 Hz, 1H), 7.17 (doublet, J = 9.1 Hz, 2H),
6.99 (doublet, J = 9.2 Hz, 2H), 4.31 (triplet, J = 6.5, 2H), 2.67 (multiplet, J = 6.9 and
11.4 Hz, 2H), IR(vCN = 2245 cm−1, vC O = 1732 cm−1, vCF and vCO = 1257, 1207, 1147,
1114 cm−1).
Differential scanning calorimetry (DSC) was performed on a TA DSC Q20 (TA In-
struments, New Castle, DE, USA) with ∼10 mg sample using a scanning rate of 10◦C
min−1
.

96
J. Y. Jang and Y. W. Park
Figure 4. (a) Broken fan-shaped texture of the SC phase of F8H2OPCB at 130◦C (200×). (b)
Fan-shaped texture of the SA phase of F8H2OPCB at 186◦C (200×).
Optical microscope evaluation was carried out using Olympus BX41 (Olympus Co.,
Shinjuku, Tokyo, Japan) polarizing microscope equipped with a Mettler FP82HT (Mettler
Toledo AG, Im Langacher, Greifensee, Switzerland) hot stage.
The XRD data were collected between 5◦ and 60◦ 2θ with a step increment of 0.02◦ 2θ
and a count time of 2 s/step. The well-known Bragg’s equation (λ = 2d sin θ) was used to
calculate the d spacing, wherein λ is a wavelength of 0.154 nm, d is the distance between
each adjacent crystal plane (d spacing), and θ is the Bragg angle.
Results and Discussion
Synthesis
F8H2OMB and F8H2OMP were synthesized by Mitsunobu reaction as shown in Fig. 2.
Boron tribromide was used for demethylation of methoxyphenyl, and fortunately 2-
perfluorooctyl-ethoxy group was stable in this condition. Esterification was carried by
conventional method, and the reaction scheme is shown in Fig. 2.
Mesomorphic Properties
The mesomophic properties of fluorocarbon compounds were investigated by DSC and
optical microscopy. The smectic C to smectic A transition is second-order transition.
The transition temperature was not appeared in DSC. The temperature was detected by

Smectic C Mesogens
97
Figure 5. Schematic representation of the crystalline structure of F8H2OPCB and F8H2OPCB-F at
room temperature.

98
J. Y. Jang and Y. W. Park
Table 2. Structural parameters of the compounds with cyano terminal group
Parameter
F6H2OCB[7]
F8H2OCB[7]
F8H2OPCB
F8H2OPCB-F
L ( 1Å)
d ( 1Å)
36.5
28.9
41.7
32.7
43.6
36.9
43.6
37.3
d(002) = 14.45
d(003) = 9.71
d(004) = 7.32
37.7
d(002) = 16.35
d(003) = 10.94
d(004) = 8.29
38.5
d(003) = 12.30
d(003) = 12.45
d(004) = 9.22
d(004) = 9.33
θ ( 1o)
32.2
31.2
Figure 6. X-ray diffraction patterns of (a) F8H2OPCB and (b) F8H2OPCB-F.
Figure 7. Tilted herringbone packing of phenyl benzoate cores (SK, tilted SE).

Smectic C Mesogens
99
Table 3. Experimental values of intensities of 00l reflections in the SK (tilted SE) and SC
phases and calculated values from the model of Fig. 5 [10]
hkl
001
002
003
004
Calculated[10]
Measured
SK
SC
100
100
?
?
?
17
13
20
21
?
30
13
63
39
40
40
1
0.2
71
12
52
F6H2OCB[7]
F8H2OCB[7]
F8H2OPCB
F8H2OPCB-F
?
?
116
polarized microscope. For the methoxy terminated LC, it was difficult to detect the transition
temperature due to high viscosity.
Figure 4(a) showed a broken fan-shaped texture characteristics of smectic C phase
of F8H2OPCB at 130◦C, Fig. 4(b) showed a fan-shaped texture characteristics of smectic
A phase of F8H2OPCB at 186◦C [8, 9]. All the compounds in this paper showed similar
texture as that of F8H2OPCB. The transition temperatures are reported in Table 1 and
Fig. 3. Methoxy terminated biphenyl compounds ((3) F6H2OMB and (4) F8H2OMB)
had higher isotropic temperature (13.9◦C and 8.8◦C respectively) compared with that of
cyano terminated biphenyl compounds ((1) F6H2OCB and (2) F8H2OCB), but smectic C
temperature range was greatly reduced from 72.8◦C to 10.8◦C and 78.2◦C to 36.2◦C in that
of (1) F6H2OCB to (3) F6H2OMB and (2) F8H2OCB to (4) F8H2OMB, respectively. The
addition of ester linking group ((5) F8H2OPCB) extends the core group length compare with
biphenyl core ((2) F8H2OCB). TNI temperature was slightly higher (6.6◦C), but smectic
C temperature range was slightly reduced from 78.4◦C in (2) F8H2OCB to 54.9◦C in (5)
F8H2OPCB. Compound ((6) F8H2OPMB) with methoxy terminal group and ester linking
group had most wide smectic C temperature range (91.7◦C).
The use of lateral fluoro substituents ((7) F8H2OPCB-F and (8) F8H2OPMB-F) is
helpful in the design of ferroelectric because they usually tend to reduce melting point
markedly, and the lateral polarity tends to favor the smectic C phase. Compounds (7) and
(8) reduced TNI 27.3◦C and 20.1◦C compared to that of compounds (5) and (6), respectively.
The smectic C temperature range was greatly reduced from 54.9◦C to 27.4◦C and 91.7◦C
to 41.1◦C in that of (5) F8H2OPCB to (7) F8H2OPCB-F and (6) F8H2OPMB to (8)
F8H2OPMB-F, respectively. The fluorocarbon LCs in this paper had great LC stability
based on TNI. The rate of smectic C temperature range of the LCs was in the following
order.
1. (6) F8H2OPMB (91.7◦C) 2. (5) F8H2OPCB (54.9◦C) 3. (8) F8H2OPMB-F (41.1◦C)
4. (4) F8H2OMB (36.2◦C) 5. (7) F8H2OPCB-F (27.4◦C) 6. (3) F6H2OMB (10.8◦C)
Structure of the Smectic K
The molecular lengths of F8H2OPCB and F8H2OPCB-F with fully extended conformation
were about 26.6Å, which were significantly smaller than the interlayer distance, 36.9Å and
37.3Å, respectively. A bimolecular structure should be considered since the cross-sectional
area of perfluoroalkyl and aromatic ring, the tilt model as shown in Fig. 5, was reasonable
for these compounds.

100
J. Y. Jang and Y. W. Park
The tilt angle (θ) was calculated by the comparison of d and L (Table 2): calculated
values were θ = 32.2o for F8H2OPCB and θ = 31.2o for F8H2OPCB-F. Herringbone
structure of two aromatic ring ester cores was identified by X-ray scattering studies (Fig. 6
and Table 3) [4, 10]. The characteristic peaks of herringbone structure in X-ray showed
strong intensity in 003 and 004 reflections (Fig. 6). Even though there is no data for 001 and
002 reflections, the strong intensity of 003 and 004 reflections proved that the structure of
F8H2OPCB and F8H2OPCB-F were tilted herringbone packing (smectic K phase) (Fig. 7)
[10].
Conclusion
LCs with fluorocarbon exhibited smectic C phase with wide temperature range. This
was proved by broken fan-shaped texture characteristics of smectic C phase in an opti-
cal microscope. Liquid crystal stability base on TNI from 161.2◦C to 191.2◦C was very
good. Smectic C temperature range was 91.7◦C for F8H2OPMB, 54.9◦C for F8H2OPCB,
41.1◦C for F8H2OPMB-F, 36.2◦C for F8H2OMB, 27.4◦C for F8H2OPCB-F, and 10.8◦C
for F6H2OMB. Smectic K phase was observed at room temperature for both F8H2OPCB
and F8H2OPCB-F up and down alternatively. Smectic K was proved by the comparison of
d (interlayer distance) and L (calculated dimer molecular length) and high intensity of 003
and 004 reflections in XRD.
Acknowledgments
Following are results of a study on the “Leaders in Industry-university Cooperation” Project,
supported by the Ministry of Education (MOE).
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