Mesomorphic properties of a homologous series of liquid crystals with short fluorocarbon Chain

Article abstract of DOI:10.1080/10587250210547

A homologous series of short fluorocarbon chain-containing liquid crystals has been synthesized. Several of them show the phase sequence N → SmA → SmC in the cooling process from the isotropic liquid. The mesomorphic properties have been studied by polari

Full text of DOI:10.1080/10587250210547

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Mesomorphic Properties
of a Homologous Series of
Liquid Crystals with Short
Fluorocarbon Chain
Y. G. Yang a , G. Tang a & J. X. Wen a
a Shanghai Institute of Organic Chemistry , Chinese
Academy of Sciences , Shanghai, China
Published online: 18 Oct 2010.
To cite this article: Y. G. Yang , G. Tang & J. X. Wen (2002) Mesomorphic Properties
of a Homologous Series of Liquid Crystals with Short Fluorocarbon Chain, Molecular
Crystals and Liquid Crystals, 373:1, 25-32, DOI: 10.1080/10587250210547
To link to this article: http://dx.doi.org/10.1080/10587250210547
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Mol. Cryst. and Liq. Cryst., 2002, Vol. 373, pp. 25-32
# 2002 Taylor and Francis, Inc.
Mesomorphic Properties of a
Homologous Series of Liquid Crystals
with Short Fluorocarbon Chain
Y. G. YANG, G. TANG and J. X. WEN
Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences,
Shanghai, China
A homologous series of short fluorocarbon chain-containing liquid crystals has been synthe-
sized. Several of them show the phase sequence N ! SmA ! SmC in the cooling process from
the isotropic liquid. The mesomorphic properties have been studied by polarizing microscopy
and DSC measurements. The thermostability of SmC phases was enhanced along with the
lengthening of fluorocarbon chains.
Keywords: Fluorocarbon, liquid crystal, mesomorphic
Earlier work on the materials with either long perfluorocarbon or semi-
perfluorocarbon chains showed that smectic mesophases were enhanced [1–
10]. Compounds with semiperfluorocarbon chains have a tendency to form
tilted smectic phases. Due to the existence of strong fluorophobic/
fluorophilic self-assembly, only a few of them show the phase sequence
N ! SmA ! SmC [11]. To weaken polyphilic self-assembly and promote
nematic phase formation, a homologous series of liquid crystals (F4–9)
comprising a short fluorinated terminal chain were synthesized. The lateral
fluorosubstitution is expected to reduce the melting point of the parent
system and to suppress the formation of high-order smectic phases. If the
compounds (F4–9) exhibit the desired phase types—N; SmA, and SmC—
they may be applied as possible hosts for commercial SmC* mixtures to
FLC devices.
Received 6 March 2001; accepted 31 May 2001.
The authors gratefully acknowledge the National Natural Science Foundation of China for
partial financial support.
Address correspondence to Y. G. Yang, Shanghai Institute of Organic Chemistry, Chinese
Academy of Sciences, 354 Fenglin Lu, Shanghai 200032, China. E-mail: ygyang@pub.sioc.ac.ch
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Y.G. YANG et al.
The compounds are synthesized in four steps following the chemical
pathway depicted in Scheme 1. The protected acid was esterified with the
appropriate alcohol, using dicyclohexylcarbodiimide (DCC) as a dehy-
drating agent and N, N-dimethylaminopyridine (DMAP) as a catalyst, in
dry tetrahydrofuran solution. Benzyl ether was then converted to the cor-
responding phenol by the removal of the benzyl protecting group, which was
achieved by hydrogenation at room temperature with H2 atmosphere in the
presence of palladium on active carbon in ethyl acetate. After that the
phenol was esterified with p-iodobenzoic acid using DCC as a dehydrating
agent and DMAP as a catalyst, in tetrahydrofuran solution. Finally the
coupling reaction between the ester and 4-alkoxytetrafluorophenylacetylene
under the catalysis of dichlorobis(triphenylphosphine)palladium(II), tri-
phenylphosphine, and copper(I) iodide in dried triethylamine gave the
desired polyfluorinated substituted homologous compound. Yields of 60–
70% were achieved and characterization was by MS, elemental analysis, IR,
1H NMR, and 19F NMR spectroscopy.
The phase transition temperatures of the target compounds were
measured by optical microscopy using a polarizing microscope (Olympus
PM-6) fitted with a heating stage (Mettler FP-80) and a controlling unit
(FP-82), and by differential scanning calorimetry (DSC, Shimadzu DSC-
50 calorimeter with a data system and heating and cooling rate of 5ꢀC
min7 1). The transition temperatures reported in this paper were the
peak values of the transition on DSC traces and are summarized in
Table 1.
All the homologues (F4–9) exhibit enantiotropic nematic and smectic A
phases. The monotropic smectic C phase was observed for homologues (F6–
F9). The compound F9 shows a very narrow nematic phase, which can be
found visually by polarized optical microscopy (the temperature range
cannot be determined even by polarized optical microscopy). The transition
temperatures are plotted against the number of carbon atoms in the alkoxy
chain, n, in Figure 1. Generally speaking, nematic stability (TN-I) decreases
with the increasing of chain length.
To study the effect of lateral fluorosubstitution, homologous series F4–9 is
compared with series H [12]. The transition temperatures are plotted against
the number of carbon atoms in the alkoxy chain, n, in Figure 2. All the
homologues of series H exhibit enantiotropic SmA and SmC phases, except
the cases when n ¼ 4 and n ¼ 16.
Generally, the melting point and clearing point of the compounds of
series H are higher than those of the corresponding compounds of series F.
The nematic phase is more readily formed in series F than in series H and

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Y.G. YANG et al.
TABLE 1 Phase transition temperatures of the compounds F
Phase transition temperatures=ꢀC
Compounds n
F4
F5
F6
F7
F8
F9
4 Cr 117.7 SmA 176.9 N 185.5 I 185.3 N 175.6 SmA 87.5 Recr
5 Cr 120.5 SmA 170.3 N 176.1 I 175.5 N 169.5 SmA 98.7 Recr
6 Cr 104.8 SmA 166.5 N 170.1 I 169.5 N 165.6 SmA 91.0 (SmC 86.5) Recr
7 Cr 104.2 SmA 158.7 N 160.3 I 159.8 N 155.8 SmA 96.6 (SmC 75.2) Recr
8 Cr 99.4 SmA 156.8 N 158.8 I 158.6 N 156.6 SmA 96.7 (SmC 74.5) Recr
9 Cr 104.8 SmA 150.9
I 150.6
SmA 91.0 (SmC 86.5) Recr*
Cr-crystal; N, nematic; SmA, smectic A; SmC, smectic C; I, isotropic; Recr, recrystallization
*The compound F9 shows very narrow enantiotropic nematic phase for which transition temperatures can
not be determined even on OPM.
SmC phases are more favorable in series H than in series F. The latter may
be due to steric effects caused by the protrusion of the fluorosubstituent,
which tend to disrupt the side-to-side intermolecular forces of attraction and
reduce the melting point and the tendency to form smectic mesophases.
Because the fluorine atoms broaden the molecules, the mesomorphic range
of the compound in F series is narrower than that of the corresponding
compound in H series.
FIGURE 1 Transition temperatures as a function of the number of carbon atoms in the al-
koxy chain of compounds F4–F9.*, MP; D, SmA-N; ^, N-I transitions.

SHORT FLUOROCARBON CHAIN COMPOUNDS
29
FIGURE 2 Transition temperatures as a function of the number of carbon atoms in the al-
koxy chain of compounds H. *, MP; D, SmC-SmA; ^, SmA-I; H, N-I transitions.
To study the relationship between the mesomorphic properties and the
length of the fluorocarbon tails, three other compounds, A1–A3, were also
synthesized (see Scheme 3). Comparison of A2 and A3 with F8 reveals
that as the terminal fluoro-chain is lengthened gradually from -(CF2)2- to
-(CF2)4- to -(CF2)6-, the fluorophobic=fluorophilic effect became so strong
that the compounds are more favorable to form tilt layer structure with
the lengthening of the fluorocarbon tails. But the clearing points of
these compounds are nearly the same. Although this may be ascribed to
the interaction between the fluorocarbon tails, the lateral-lateral interac-
tion does not change much along with the lengthening of fluorocarbon
chains.
The compounds with long terminal fluorocarbon chains often
show smectic mesomorphism and do not show nematics [1, 4, 7]. The series
in this paper with short fluorocarbon chains show a very broad temperature
range and phase sequence of N; SmA and SmC in the cooling process.
Unfortunately, SmC phases are monotropic. In the future, the compounds
with less fluorosubstituents on the core will be synthesized. Enantiotropic
SmC phase might be formed. This kind of liquid crystal might be useful as a
host material in ferroelectric LCDin the future.

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Y.G. YANG et al.
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