Fuorinated hydrogen bonding liquid crystals based on Schiff base

Article abstract of DOI:10.1016/j.jfluchem.2013.01.016

Two new Schiff base compounds, tailed by the fluoroalkoxy, were synthesized in a three-step process. Those supramolecular structures constructed by hydrogen bonding show good liquid crystal properties with higher clear points and wider mesomorphic phase ranges than other analogs containing a terminal alkoxy chain. Investigation of the optical textures by polarizing microscopy reveals that terminal fiuorinated substituents convert the nematic phase of supramolecular hydrogen-bonding complexes with terminal alkoxy chains into the smectic A phase of those with terminal fluoroalkoxy chains.

Full text of DOI:10.1016/j.jfluchem.2013.01.016

Journal of Fluorine Chemistry 147 (2013) 36–39
Contents lists available at SciVerse ScienceDirect
Journal of Fluorine Chemistry
journal homepage: www.elsevier.com/locate/fluor
Fuorinated hydrogen bonding liquid crystals based on Schiff base
a
a
a
a,b
Zhilian Liu a,b, , Jian Zhang , Tengfei Li , Zhenning Yu , Shuxiang Zhang
*
a School of Chemistry and Chemical Engineering, University of Jinan, Jinan 250022, China
b Shandong Key Laboratory of Fluorine Chemistry and Chemical Engineering Materials, Jinan 250022, China
A R T I C L E I N F O
A B S T R A C T
Article history:
Two new Schiff base compounds, tailed by the fluoroalkoxy, were synthesized in a three-step process.
Those supramolecular structures constructed by hydrogen bonding show good liquid crystal properties
with higher clear points and wider mesomorphic phase ranges than other analogs containing a terminal
alkoxy chain. Investigation of the optical textures by polarizing microscopy reveals that terminal
fluorinated substituents convert the nematic phase of supramolecular hydrogen-bonding complexes
with terminal alkoxy chains into the smectic A phase of those with terminal fluoroalkoxy chains.
ß 2013 Elsevier B.V. All rights reserved.
Received 13 December 2012
Received in revised form 15 January 2013
Accepted 18 January 2013
Available online 25 January 2013
Keywords:
Liquid crystal
Fluorous substituent
Schiff base
Hydrogen bonding
Supramolecule
1. Introduction
because these materials involving non-covalent interactions have
potential applications for dynamically functional molecular
Schiff base compounds are widely used in many fields, such as
catalytic chemistry [1], organometallic chemistry [2], biochemistry
[3], and photochemistry [4]. Schiff bases have attracted much
attention in the field liquid crystal researches since the first report
on the Schiff base liquid crystal compound 4-methoxybenzyli-
dene-40-butylaniline (MBBA) by Kelker et al. [5] in 1969. Over the
past few decades, lots of Schiff bases with low molecular weight
have been synthesized and investigated extensively toward
applications of liquid crystal [6–15]. It has been found that the
terminal or lateral groups, such as –F, –Cl, –CN, –CH3, and –OCH3,
have a significant influence on the liquid crystalline properties of
the Schiff bases especially in the cases of fluorine atoms. The small
size of the fluoro-substituent enables its incorporation into liquid
crystals’ structures without undue disruption, and hence liquid
crystalline phases can still be exhibited. However, it is the
combination of the small size and high polarity which serves to
modify melting point, mesophase morphology, transition tem-
peratures, and other physical properties including optical anisot-
ropy, dielectric anisotropy, and visco-elastic properties.
systems. Hydrogen bonding is one of the most important
interactions in chemical and biological processes. In nature,
hydrogen bonding plays a key role in molecular recognition and
molecular assembly because of its stability, dynamics, direction-
ality, and reversibility [21,22].
Herein we present synthesis and mesomorphic properties of
hydrogen bonding liquid crystals based on Schiff bases with
terminal fluoroalkoxy chains. For comparison, Schiff base liquid
crystals with terminal alkoxy chains were also synthesized.
2. Results and discussion
2.1. Synthesis
The Schiff bases A and B with terminal fluoroalkoxy chains were
synthesized in three steps (Scheme 1). 2,2,3,3-tetrafluoro-1-
propanol was reacted with 4-chloronitrobenzene (2) to form 4-
(2,2,3,3-tetrafluoropropoxy)nitrobenzene (3), which was further
treated with hydrazine hydrate, FeCl3 and activated carbon
affording 4-(2,2,3,3-tetrafluoropropoxy)aniline (4). Reaction of 4
with 3-pyridinecarboxaldehyde (5) in ethanol at 85 8C for 12 h
gave desired Schiff base B. The preparation of A is similar to that of
B. The Schiff base C with terminal alkoxy chains was synthesized in
one step with 4-propoxyaniline (6) and 3-pyridinecarboxaldehyde
(5) in ethanol at 85 8C for 12 h.
The use of molecular interactions caused by hydrogen bonding
for the design of liquid crystals [16–20] has attracted attention
* Corresponding author at: School of Chemistry and Chemical Engineering,
University of Jinan, Jinan 250022, China. Tel.: +86 531 82765475;
fax: +86 531 89736365.
4-n-Alkoxybenzoic acids (Dn) were synthesized as the litera-
ture [23]. The supramolecular hydrogen bonding liquid crystal
E-mail address: chm_liuzl@ujn.edu.cn (Z. Liu).
0022-1139/$ – see front matter ß 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.jfluchem.2013.01.016

Z. Liu et al. / Journal of Fluorine Chemistry 147 (2013) 36–39
37
N
CHO
Cl
NO2
FeCl3, C
5
2
RO
N
RO
NH2
NO2
RO
ROH
CH
N
CH3COOH
NH2NH2·H2O
KOH / NMP
1
4
3
A: R=CH2CF3
B: R=CH2CF2CF2H
N
CHO
5
CH3CH2CH2O
N
CH3CH2CH2O
NH2
CH3COOH
CH
N
6
C
O
OCnH2n+1
C
O
C
H O
HO
OCnH2n+1
RO
N
Dn
A, B or C
CH
N
Pyridine
LD-An: R=CH2CF3
LD-Bn: R=CH2CF2CF2H
LD-Cn: R=CH2CH2CH3
n = 4~8, 10, 12
Scheme 1. Synthesis of Schiff bases and supramolecular liquid crystal complexes.
complexes (LD–An, LD–Bn, LD–Cn) were prepared with equi-
molar Schiff bases (A, B or C) and 4-n-alkoxybenzoic acids in
pyridine. After evaporation of pyridine and drying under
vacuum, the supramolecular liquid crystal complexes were
obtained (Scheme 1).
2.2. Identification
The structures of Schiff bases A, B and C were identified by IR, 1H
NMR, 13C NMR, 19F NMR, mass spectra and elemental analyses. The
hydrogen bonds in the supramolecular complexes (LD–An, LD–Bn,
LD–Cn) were identified by infrared spectra for the two new peaks
centered at 2470 and 1874 cmꢀ1 compared with Schiff bases and
4-n-alkoxybenzoic acids (Fig. 1, LD–B8 for example) [24–26].
B8
LD-B8
140
130
D8
1874
2470
120
m.p. of LD-An
c.p. of LD-An
m.p. of LD-Bn
110
c.p. of LD-Bn
m.p. of LD-Cn
c.p. of LD-Cn
100
90
80
70
4000
3500
3000
2500
2000
1500
1000
500
3
4
5
6
7
8
9
10
11
12
13
wavenumber/cm-1
n
Fig. 2. Melting points (m.p.) and clear points (c.p.) of new supramolecular liquid
Fig. 1. Infrared spectra of Schiff base (B8) and 4-n-octoxybenzoic acid (D8) and their
supramolecular liquid crystal complex (LD–B8).
crystal complexes.
Table 1
Transition temperatures (8C) of new supramolecular liquid crystal complexes.
a
a
a
Compound
Transition temperature
Compound
Transition temperature
Compound
Transition temperature
LD–A4
LD–A5
LD–A6
LD–A7
LD–A8
LD–A10
LD–A12
Cr 108 SA136 I
Cr 103 SA137 I
Cr 95 SA136 I
Cr 87 SA140 I
Cr 90 SA141 I
Cr 95 SA144 I
Cr 91 SA141 I
LD–B4
LD–B5
LD–B6
LD–B7
LD–B8
LD–B10
LD–B12
Cr 107 SA119 I
Cr 102 SA125 I
Cr 91 SA123 I
Cr 85 SA126 I
Cr 87 SA132 I
Cr 91 SA135 I
Cr 99 SA131 I
LD–C4
LD–C5
LD–C6
LD–C7
LD–C8
LD–C10
LD–C12
Cr 103 N133 I
Cr 94 N130 I
Cr 83 N131I
Cr 75 N141 I
Cr 90 N129 I
Cr 71 N123 I
Cr 82 N123I
a
Cr: crystal state; I: isotropic liquid; SA: smectic A phase; N: nematic phase; the onset temperature of the phase transition from DSC of the second heating.

38
Z. Liu et al. / Journal of Fluorine Chemistry 147 (2013) 36–39
supramolecular liquid crystal complexes (LD–An and LD–Bn), with
the terminal fluoro-substituents, show good stability. Mesomor-
phic phases with fluorinated tails are thermally more stable
because fluorinated alkyl chains are stiffer than hydrocarbon alkyl
chains [27,28].
Investigation of the optical textures by polarizing microscopy
(POM) reveals that LD–An and LD–Bn show smectic A phase
instead of nematic phase of LD–Cn. The typical texture change is
shown in Fig. 3 for complex LD–A5, LD–B5 and LD–C5. The
substitution of the trifluoroethyoxyl and tetrafluoropropoxyl
resulted in the high degree of order in a liquid crystal phase.
The result was confirmed by DSC due to high melting points and
clear points. Mesomorphic phases with fluorinated tails are much
ordered because of strong affinity of fluoro-alkyl chains to
neighboring fluoro-alkyl chains (fluorophilic effect), and the
incompatibility with hydrocarbon chains and aromatic cores
(fluorophobic effect). The fluorophilic and fluorophobic effects of
fluorinated liquid crystals produce a micro-segregation and raise
smectic phase stability, and suppress nematic phase [29].
All the POM pictures were taken at different temperatures on
the second heating cycle according to DSC. Observations were
made under 20-fold magnification. The initial heating rate of
10 8C minꢀ1 was slowed to 2 8C minꢀ1 when the temperature was
close to the clearing point.
3. Conclusion
We have synthesized and characterized several novel supra-
molecular hydrogen bonding liquid crystals containing fluoroalk-
oxy as the terminal chains. The supramolecular liquid crystals
displayed good liquid crystal properties that can be adjusted by the
terminal substituent. They show a wide mesophase range and are
mesophase stable compared with alkoxyl analogs. It was also
found that terminal fluorinated substituents have a great impact
on liquid crystal textures.
4. Experimental
4.1. General considerations
All the reagents were analytical grade, purchased from
commercial sources and used as received. 1H, 13C and 19F NMR
were recorded on 400 MHz spectrometer (Avance III, Bruker) and
chemical shifts were reported using tetramethylsilane as an
internal reference. The solvent was CDCl3 unless otherwise
specified. The DSC was recorded on a differential scanning
calorimeter at a scan rate of 10 8C minꢀ1. Optical micrographs
were observed with a polarizing optical microscope (POM) (Nikon
LV100POL). Elemental analyses were performed on an EXETER CE-
440 Elemental Analyzer.
Fig. 3. POM textures of new supramolecular liquid crystal complexes: (a) LD–A5,
heating to 122 8C, SA phase; (b) LD–B5, heating to 135 8C, SA phase; (c) LD–C5,
heating to 124 8C, N phase (crossed polarizers, magnification 20ꢁ).
2.3. Liquid crystalline properties
4.2. General procedure for the preparation of A, B and C
The thermal behaviors of products Schiff bases, 4-n-alkox-
ybenzoic acids and supramolecular liquid crystal complexes were
investigated by differential scan calorimetry (DSC), and polarizing
optical microscopy (POM). The transition temperatures for all the
new supramolecular liquid crystal complexes are shown in Table 1.
The two series of new supramolecular liquid crystal complexes
(LD–An and LD–Bn) with the terminal fluoro-substituents, show
good liquid crystal properties. Long alkoxy substituents anchored
on the benzoic acids increased the transition temperature of the
clearing point (Fig. 2). The melting points of LD–An are close to
those of LD–Bn while the clear points of LD–An are higher than
that of LD–Bn. Compared with LD–Cn bearing the terminal alkyl-
substituent, all in all, the melting points and the clear points of LD–
An and LD–Bn are higher. It suggests that the two series of new
Potassium hydroxide (1.16 g, 0.02 mol) was added to a solution
of 2,2,2-trifluoroethanol or 2,2,3,3-tetrafluoro-1-propanol
(0.02 mol) in n-methyl-2-pyrrolidone (NMP, 50 mL). P-chloroni-
trobenzene (3.14 g, 0.02 mol) was added to the solution after
complete dissolution of potassium hydroxide at 60 8C. The
mixture was stirred at 60 8C for 12 h. The reaction mixture was
next poured into water and filtered to give solid. The solid was
purified by column chromatography on silica gel with petroleum
ether (60–90 8C)/acetic ether = 5/1 as the eluent. The second
fraction was concentrated in vacuo to afford the target product 3
as pale yellow solid.
Hydrazine hydrate (1.2 mL) was added dropwise to a stirred
mixture of product 3 (1.66 g, 0.0087 mol), FeCl3 (0.03 g), activated

Z. Liu et al. / Journal of Fluorine Chemistry 147 (2013) 36–39
39
carbon (0.31 g) and MeOH (29 mL) over the period of 1 h at 75 8C.
The resulting mixture was stirred 12 h at 70–80 8C. After having
been cooled, the mixture was filtered and the filtrate was
evaporated in vacuo. The residue was extracted with AcOEt-
hexane (1:1) for three times and then washed by water for three
times to give 4 as pale brown solid.
4-Pyridinecarboxaldehyde 5 (1.9 mL, 0.02 mol) and acetic acid
(2 mL) were added to a solution of 4 (or 6, 0.02 mol) in ethanol
(60 mL). The mixture was stirred at 85 8C for 12 h. After
evaporation and dry in vacuo, pale brown solid (A, B or C) was
obtained.
C15H16N2O: C, 74.97; H, 6.71; N, 11.66. Found C, 74.95; H, 6.72;
N, 11.65%.
Acknowledgment
The authors gratefully acknowledge the support of the
Shandong excellent Young Scientist Research Award Fund (Project
No. BS2011CL007).
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