Non-Symmetric liquid crystal dimers containing azobenzene and 1,3,4-oxadiazole group: Synthesis and mesomorphic behaviour

Article abstract of DOI:10.2174/157017812799303962

We report on the synthesis and mesomorphic behaviour of a new series of non-symmetric liquid crystal dimers with long terminal chains containing 1,3,4-oxadiazole group and azobenzene group as the mesogenic units. These nonsymmetric liquid crystal dimers are evidenced to display monolayer smectic A phase. Microphase segregation and specific intermolecular interactions between the two different mesogenic units are the major driving forces for the formation of the monolayer smectic A structure.

Full text of DOI:10.2174/157017812799303962

76
Letters in Organic Chemistry, 2012, 9, 76-80
Non-Symmetric Liquid Crystal Dimers Containing Azobenzene and 1,3,4-
Oxadiazole Group: Synthesis and Mesomorphic Behaviour
Binglian Bai*,1,2, Haitao Wang1, Xiaolong Lin2, Xia Ran1, Chengxiao Zhao1 and Min Li*,1
1Key Laboratory for Automobile Materials (JLU), Ministry of Education, Institute of Materials Science and
Engineering, Jilin University, Changchun 130012, P. R. China
2College of Physics, Jilin University, Changchun 130012, P.R. China
Received July 12, 2011: Revised November 04, 2011: Accepted November 04, 2011
Abstract: We report on the synthesis and mesomorphic behaviour of a new series of non-symmetric liquid crystal dimers
with long terminal chains containing 1,3,4-oxadiazole group and azobenzene group as the mesogenic units. These non-
symmetric liquid crystal dimers are evidenced to display monolayer smectic A phase. Microphase segregation and
specific intermolecular interactions between the two different mesogenic units are the major driving forces for the
formation of the monolayer smectic A structure.
Keywords: Azobenzene, intercalated structure, interdigitated structure, liquid crystal dimers, 1,3,4-oxadiazole, non-symmetric
dimers, symmetric dimers.
1. INTRODUCTION
signifies the number of methylene units in the spacer and n
indicates the length of terminal alkoxy tail) with long
terminal chains exhibit a monolayer smectic C phase, for
which intermolecular hydrogen bonding between hydrazide
groups is considered to be the main driving forces [7]. So, in
order to validate the importance of intermolecular hydrogen
bonding between hydrazide groups in formation of
monolayer smectic phase, we have prepared non-symmetric
dimers containing azobenzene and 1,3,4-oxadiazole group
with long terminal alkoxy chains, i.e. 1-[4-(4’-
methoxyphenylazo)phenoxyl]-m- [(N-(4- cetyloxybenzoyl) -
N’-(benzoyl-4’-oxy) 1,3,4-oxadiazole] alkyl (EmC16-OXD,
Scheme 1). Interestingly, the compound E6C16-OXD, non-
symmetric liquid crystal dimer with long terminal chains,
exhibits a monolayer smectic A phase.
Liquid crystal dimers, which contain either two identical
(symmetric) or non-identical (non-symmetric) mesogenic
units connected via a flexible central spacer [1], have
attracted increasing attention because not only they are
regarded as model compounds for polymeric liquid crystals,
but also they are of significant interest in their own right and
exhibit quite different behaviour to conventional low molar
mass mesogens, such as their novel molecular architecture,
unique phase behaviours and mesophase polymorphism etc.
[2-4].
There are remarkable differences in the behaviour of
non-symmetric and symmetric dimers. As has been reported,
symmetric dimers containing terminal alkyl chains appear to
have a strong tendency to exhibit monolayer smectic phases,
the driving force for which is thought to be the
incompatibility between the terminal alkyl chains and the
spacers, leading to a microphase separation into three
regions: terminal chains, mesogenic groups and flexible
alkyl spacers [1]. In contrast, the example of non-symmetric
liquid crystal dimers exhibiting monolayer smectic phases is
rarely [5]. Usually, non-symmetric liquid crystal dimers
exhibit intercalated smectic phases, in which specific
intermolecular interactions between the two different
mesogenic units are responsible for this specific phase
behavior [1, 6].
2. EXPERIMENTAL
2.1. Synthesis
The target non-symmetric dimers, abbreviated as
EmC16-OXD (where m <m=5, 6> signifies the number of
methylene units in the spacer), were synthesized through the
route shown in Scheme 1. The chemical structures of the
1
resulting compounds were confirmed by FT-IR, H NMR
spectroscopy and elemental analysis (Because of the poor
1
solubility, H NMR experiment of E6C16-OXD was not
performed.).
In our previous work, it has been demonstrated that non-
symmetric liquid crystal dimers, 1-[4-(4’-methoxyphenyl-
azo)phenoxyl]-m- [(N-(4- alkoxybenzoyl) -N’-(benzoyl-4’-
oxy)hydrazine)] alkane (EmCn) (see Scheme 1, where m
Synthesis of 1-[4-(4’-methoxyphenylazo)phenoxyl]-5-
[(N-(4- cetyloxybenzoyl) -N’-(benzoyl-4’-oxy)-1,3,4-oxadia-
zolyl)] pentane (E5C16-OXD) (see Scheme 1).
1-[4-(4’-methoxyphenylazo)phenoxyl]-5- [(N-(4- cetyl-
oxybenzoyl)
-N’-(benzoyl-4’-oxy)hydrazine)]
pentane
*Address correspondence to this author at the Key Laboratory for
Automobile Materials (JLU), Ministry of Education, Institute of Materials
Science and Engineering, Jilin University, Changchun 130012, P. R. China;
Tel: +86-431-85168254;
(E5C16) (1.50 g, 1.89 mmol) was added into 70 mL
phosphorous oxychloride (POCl3) and refluxed for about 24
h. After cooling to room temperature, the mixture was
poured into water. And the precipitate was filtered, washed
E-mails: minli@mail.jlu.edu.cn; baibinglian@jlu.edu.cn
© 2012 Bentham Science Publishers
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Non-Symmetric Liquid Crystal Dimers Containing Azobenzene
Letters in Organic Chemistry, 2012, Vol. 9, No. 1
77
O(CH2)mO
COOH
H3CO
N
N
Em
THF
r.t
C16
H2NHNOC
O(CH2)16H
O
C
H
O(CH2)mO
H3CO
N
N
N
O(CH2)16
EmC16
H
N
H
C
O
O
O(CH2)16
H
O(CH2)mO
N
N
H3CO
N
N
EmC16-OXD
m=5, 6
Scheme 1. The synthetic routes for EmC16-OXD.
thoroughly with water, dried completely, and recrystallized
from DMF.
Elem. Anal: Found: C74.39, N7.12, H8.40; Calcd for
C49H64N4O5: C74.59, N7.10, H8.18.
1H NMR (300 MHz, CDCl3), (ppm, from TMS): 8.05
(dd, J = 8.97, 2.45 Hz, 2H), 7.92 (dd, J = 9.05, 1.46 Hz,
1.8H), 7.60 (s, 0.2H), 7.04-6.99 (m, 3.7H), 6.91-6.86 (m,
0.3H), 4.12-4.01 (m, 6H), 3.88 (d, J = 6.50 Hz, 3H), 1.95-
1.26 (m, 30H), 0.88 (t, J = 6.75 Hz, 3H). FTIR (KBr, pellet,
cm-1): 2921, 2851, 1612, 1603, 1582, 1497, 1472, 1396,
1251, 1176, 1145, 1104, 1035, 835, 743, 676. Elem. Anal:
Found: C74.31, N7.21, H8.06; Calcd for C48H62N4O5:
C74.39, N7.23, H8.06.
2.2. Characterization
1H NMR spectra were recorded with a Mercury-300BB
300 MHz spectrometer, using CDCl3 as solvent and
tetramethylsilane (TMS) as an internal standard. FT-IR
spectra were recorded with a Perkin–Elmer spectrometer
(Spectrum One B) using KBr pellets. The thermal properties
of the compounds were investigated with a Mettler-Toledo
DSC821e instrument. The rate of heating and cooling was
10ꢀmin-1; the weight of the sample was about 2 mg, and
indium and zinc were used for calibration. The peak
maximum was taken as the phase transition temperature.
Optical textures were observed by polarizing optical
Using the same method, compound E6C16-OXD was
successfully synthesized and characterized. FTIR (KBr,
pellet, cm-1): 2918, 2849, 1616, 1603, 1583, 1497, 1473,
1465, 1396, 1249, 1176, 1152, 1103, 1026, 843, 744, 674.
(a)
(b)
Fig. (1). DSC curves of E5C16-OXD and E6C16-OXD (a) on the first cooling run and (b) on the second heating run.

78 Letters in Organic Chemistry, 2012, Vol. 9, No. 1
Bai et al.
Table 1. Transition Temperatures ꢁꢀ) and Enthalpies (kJ mol-1, in Parentheses) of EmC16-OXD
Compds
First cooling
Second heating
E5C16-OXD
E6C16-OXD
E5C16
I 164 (56.3) Cr3 105 (2.3) Cr2 47 (4.4) Cr1
I 168 (6.8) SmA 127 (52.1) Cr 118 (2.8) Cr
I 169 (8.4) SmC 147 (36.9) Cr 116 (1.2) Cr
I 199 (14.6) SmC 177 (42.7) Cr 156 (16.7) Cr
Cr1 53 (4.3) Cr2 120 (2.9) Cr3 170 (57.6) I
Cr 132 (8.8) Cr 140 (37.1)Cr 145 (11.8) SmA 170 (7.3) I
Cr 119 (1.0) Cr 141 (20.0) Cr 170 (47.7) I
Cr 188 (74.6) SmC 201(14.1) I
E6C16
Cr, SmA, SmC and I indicate crystalline state, smectic A, smectic C phase and isotropic liquid, respectively. The rate of heating and cooling was 10ꢀmin-1.
microscopy (POM) using a Leica DMLP microscope
equipped with a Leitz 350 heating stage. X-ray diffraction
region implying the formation of a layered structure, and a
broad diffuse peak in the wide angle region centered at a
spacing of 4.6Å, indicating liquid-like arrangement of the
molecules within the layers, as shown in Fig. (3). The layer
spacing (d) of E6C16-OXD is 59.49 Å, which is a little bit
longer than the estimated all-trans molecular length (l) of the
most extended conformation of 56.07 Å, considering its
texture in mesophase, indicating that the molecules of
E6C16-OXD are arranged in a monolayer arrangement with
the molecular long axis perpendicular with respect to the
layer normal (SmA).
was carried out with
diffractometer.
a Bruker Avance D8 X-ray
3. RESULTS AND DISCUSSION
The phase behavior of E5C16-OXD and E6C16-OXD
were studied by polarized optical microscopy (POM),
differential scanning calorimetry (DSC), and powder XRD.
Fig (1) shows the DSC thermograms of E5C16-OXD and
E6C16-OXD on the first cooling and on the second heating
run at a scanning rate of 10 ꢀ/min. The phase transitional
temperatures and the associated enthalpic changes for
E5C16-OXD and E6C16-OXD are summarized in Table 1.
It can be seen that spacer plays an important role in the
formation of the mesophase. E6C16-OXD shows
enantiotropic liquid crystalline behaviours, whereas E5C16-
OXD is non-mesomorphic. And because of the higher
melting point suppressing the LC phase for E5C16-OXD,
the characteristic odd–even effect, as usually observed in
liquid crystal dimers, is not observed. This phenomenon is in
accordance with the results of the Ep-m-NO2 systems
reported previously by us [8]. The polarizing optical
photomicrograph of compound E6C16-OXD is shown in
Fig. (2), and no schlieren texture is observed on shearing.
Fig. (3). XRD pattern of E6C16-OXD at 160ꢀ on first cooling
run.
We have reported previously that non-symmetric liquid
crystal dimers (EmC16, Scheme 1) with long terminal
chains exhibit a monolayer smectic C phase, for which the
intermolecular hydrogen bonding between hydrazide groups
is considered to be the main driving forces. In contrast, for
E6C16-OXD, 1,3,4-oxadiazole group replacing the
dihydrazide group of E6C16, no hydrogen bonding
interactions existed in mesophase. We found that both the
melting and clearing points of E6C16-OXD are much lower
than those of E6C16, and the enthalpy change of clearing
point transition of E6C16-OXD was two times lower than
that of the E6C16, and E6C16-OXD exhibits a monolayer
smectic A phase. It is confirmed that the lateral hydrogen
bond between the dihydrazide groups indeed plays an
important role for the formation of mesophases for E6C16,
but it is not the exclusively factor for the formation of
monolayer smectic structure. And the terminal groups and
spacers must also be important.
Fig. (2). Polarizing optical photomicrograph of E6C16-OXD at 152
ꢀ (ꢀ200).
In order to obtain further information on molecular
arrangements in their mesophase, variable temperature XRD
was performed. The XRD pattern of E6C16-OXD in the
smectic phase contains two sharp peaks in the low angle
For E6C16-OXD, the terminal chain length is farther
greater than the spacer length, the incompatibility between

Non-Symmetric Liquid Crystal Dimers Containing Azobenzene
Letters in Organic Chemistry, 2012, Vol. 9, No. 1
79
the long terminal alkyl chains and the spacers does not
approve the intercalated structure. And the terminal nonpolar
methoxyl group does not approve the interdigitated structure
usually stabilized by the electrostatic interaction between the
polar groups such as cyanobiphenyl groups, nitryl groups
etc. [6a, 6b] in non-symmetric liquid crystal dimers with
long terminal alkyl chains. Thus the E6C16-OXD exhibits a
monolayer smectic A phase and we propose that the
structure of monolayer SmA phase of E6C16-OXD should
be as shown in Fig. (4) [1c, 5a], the driving force for which
is thought to be specific intermolecular interactions between
the two different mesogenic units and the incompatibility
between the long terminal alkyl chains and the spacers,
leading to a microphase separation into three regions: the
long terminal alkyl chains, mesogenic groups and flexible
alkyl spacers.
No.20090061120021),
Scientific
Forefront
and
Interdisciplinary Innovation Project, Jilin University (No.
200903321) and Project 985-Automotive Engineering of
Jilin University for their financial supports of this work.
DISCLOSURE
The part of information about compound EmCn included
in this article has been previously published in Journal of
Physical Organic Chemistry Volume 20, Issue 8, pages 589-
593, August 2007.
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Fig. (4). A sketch of the monolayer smectic A of E6C16-OXD
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ACKNOWLEDGEMENTS
The authors are grateful to the National Science
Foundation Committee of China (project No. 50873044,
21072076, 51073071), Special Foundation for PhD Program
in Universities of China Ministry of Education (Project

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