A roomerature liquid crystalline polymer based on discotic 1,3,4-oxadizole

Article abstract of DOI:10.1039/c5ra05983f

A side-chain polymer POXD and the corresponding monomer MOXD based on discotic 1,3,4-oxadiazole have been synthesized and characterized. Both POXD and MOXD exhibit liquid crystalline behaviors at room temperature with excellent thermal stability, while the mesomorphic temperature range of POXD is much wider than that of MOXD. In addition, they are photoluminescent in solution, in the solid state and in liquid crystalline phases. Notably, the fluorescent intensity of POXD is almost 12 times that of MOXD in both the solid state and liquid crystalline phases.

Full text of DOI:10.1039/c5ra05983f

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A Room-Temperature Liquid Crystalline Polymer Based on
Discotic 1,3,4-Oxadizole#
Jie Han*, Zhuo-Zhi Wang, Jing Wu, and Li-Rong Zhu
College of Chemistry and State Key Laboratory of Elemento-organic Chemistry, Nankai University,
Tianjin 300071, China. E-mail hanjie@nankai.edu.cn; Fax: +86-22-23509933; Tel:
+86-22-23509933
Abstract: A side-chain polymer POXD and the corresponding monomer MOXD based on discotic
1,3,4-oxadiazole have been synthesized and characterized. Both POXD and MOXD exhibit liquid
crystalline behaviors at room temperature with excellent thermal stability, while the
mesomorphic temperature range of POXD is much wider than that of MOXD. In addition, they
are photoluminscent in solution, in solid state and in liquid crystalline phases. Notably, the
fluorescent intensity of POXD is almost 12 times of that of MOXD in both solid state and liquid
crystalline phases.
1,3,4ꢀOxadiazoleꢀbased liquid crystals may exhibit various mesophases such as nematic,
smectic, blue phases, and columnar mesophases, and have been attracting continuous
interest during the past decade.1,2 Furthermore, 1,3,4ꢀoxadiazole derivatives can be used as
emissive and/or electroꢀtransporting materials in organic lightꢀemitting devices due to the
electron deficiency, high photoluminescence quantum yield, and excellent thermal and
chemical stability.3 Among the reported liquid crystalline 1,3,4ꢀoxadiazoles, much
attention has been paid on the rodꢀlike or bentꢀshaped molecules, and the relationship
# Electronic Supplementary Information (ESI) available: Synthetic details and characterization data
for all new compounds, TGA and DSC thermographs of MOXD and POXD . See
DOI: 10.1039/x0xx00000x

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between the molecular structures and liquid crystalline properties has been investigated
intensively. In contrast, the examples of discotic mesogenic 1,3,4ꢀoxadiazoles are
relatively limited due to lack of enough structural skeletons to construct this type of
compounds.4
Liquid crystalline polymers have many advantages such as selfꢀorganizing
characteristics, enhanced mechanical and thermal stabilities, good process ability and
simple fabrication over the conventional polymeric materials.5 In particular, the
1,3,4ꢀoxadiazole based liquid crystalline polymers may display fluorescent property with
good electron accepting ability. To date, a variety of 1,3,4ꢀoxadiazole based liquid
crystalline polymers, including mainꢀchain,6 sideꢀchain7 and mesogenꢀjacketed types,8
have been developed for different purpose. Generally, most of these polymers exhibit
relatively narrow mesomorphic temperature ranges with high melting points due to the
rodꢀlike 1,3,4ꢀoxadiazole mesogens. This has greatly hindered the development in practical
application, and also made it difficult to explore the structureꢀproperty relationship of this
kind of materials.
Herein, we describe the synthesis and liquid crystalline behaviors of a side polymer
POXD and the corresponding monomer MOXD derived from a discotic 1,3,4ꢀoxadiazole
unit. The synthetic route is shown in Scheme 1. Both POXD and MOXD exhibit liquid
crystalline behaviors at room temperature region. The photoluminescence behaviors of
POXD and MOXD in solution, in solid state, and in liquid crystalline phases have also
been studied in this work. To our best knowledge, this is the first example of liquid
crystalline polymer containing discotic 1,3,4ꢀoxadiazole units.

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As seen in Scheme 1, compounds
3,4,5ꢀtrihydroxy methylbenzoate with 1ꢀbromodecane. The yields of
48%, respectively. Compound is a key intermediate to synthesize the monomer MOXD
Similar compound to had ever been synthesized by a fourꢀstep reaction from
2
and
3
were synthesized in a single reaction of
2
and are 40% and
3
4
.
4
3,4,5ꢀtrihydroxy methylbenzoate in literature,9 which needs the protection of two of the
hydroxyl groups and cleavage the protective group. In this work, we have developed a
direct method to synthesize
4 by etherification of the hydroxyl group of 2 with
1ꢀbromodecanol in DMF at 60 °C using K2CO3 as base, which needs only two steps of
reaction from the same starting materials, and has a higher overall yield with simple
workup. The ester group of
hydroxyl acid in 94% yield. Esterification of
CH2Cl2 in presence of pyridine led to a mixed ester anhydride, which was then heated in a
solution of pyridine in water to afford in 72% yield according to the reported
procedures.9,10 3,4,5ꢀTridecyloxybenzohydrazide 711 was prepared in 94% yield by
reaction the ester with hydrazine in ethanol. Refluxing with 3,4,5ꢀtridecyloxybenzoic
acid chloride yielded the hydrazide , which was used without purification in the
4
was hydrolyzed with KOH in refluxing EtOH to yield the
5
5
with methacryloyl chloride at 0 °C in dry
6
3
7
8
cyclisation reaction to give the monomethacylate functionlized monomer MOXD with an
overall yield of 65%. Radical polymerization of MOXD initiated with AIBN was carried
out in THF, and the resulting polymer POXD was purified by silica column
chromatography to separate the unreacted monomer.

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OH
OH
OH
OC10H21
OC10H21
OC10H21
O
O
O
Br(CH2
)10H
OC10H21
+
OH
K2CO3
H3CO
H3CO
H3CO
OC10H21
2
1
3
NH2NH2 H2
O
K2CO3
Br(CH2)10OH
O(CH2 10OH
OC10H21
OC10H21
OC10H21
O(CH2
OC10H21
OC10H21
)
10OH
)
O
O
O
KOH
OC10H21
OC10H21
H2NHN
HO
H3CO
7
5
4
O
Cl
pyridine
2M HCl
pyridine
O
O
OC10H21
O(CH2)10O
O(CH2
OC10H21
OC10H21
)
10O
O
C10H21
O
O
OC10H21
1) SOCl2
POCl3
O O
2) 7, pyridine
HO
C10H21
OC10H21
HN NH
6
8
O
O
n
OC10H21
O(CH2)10O
OC10H21
O(CH2)10O
C10H21
O
O
OC10H21
C10H21
C10H21
O
O
OC10H21
AIBN
O
O
C10H21
OC10H21
OC10H21
N
N
N
N
POXD
MOXD
Scheme. 1 Synthetic route of the monomer MOXD and polymer POXD
1
The molecular structure of MOXD was characterized by H NMR, 13C NMR and high
1
resolution mass spectroscopy (HRMS). From the H NMR spectrum (Fig. 1a) of MOXD
,
the characteristic vinyl proton signals of the methacryl group at 5.50 and 6.10 ppm can be
seen clearly, and the peak integrations and chemical shifts of all the protons are in excellent
1
agreement with its expected structure. In contrast, the H NMR spectrum (Fig. 1b) of
POXD showed that the vinyl proton signals of the methacryloyl group from the monomer
have disappeared completely from the spectrum, indicating that the unreacted monomer
has been fully removed from the purified polymer.

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Fig. 1 1H NMR spectra of (a) MOXD and (b) POXD
The thermal stability of MOXD and POXD was investigated by thermogravimetric
analyses (TGA) under nitrogen atmosphere. The TGA curves of the solid samples of
MOXD and POXD are given in the Supporting Information (Fig. S1), and the onset
decomposition temperatures are listed in Table 1. Both of them showed no weight loss at
25 – 300 °C, indicative of the excellent thermal stability. The liquid crystalline behaviors
of MOXD and POXD were investigated by polarized optical microscopy (POM) and
differential scanning calorimetry (DSC). The transition temperatures and enthalpy values
derived from DSC results are listed in Table 1.
Table. 1 Phase transition temperature and corresponding enthalpies (kJ/mol) of MOXD and
POXD
Compd Phase transitionsa T[°C]b(ꢁH[J gꢀ1]) Tdec[°C]c
MOXD Cr1 → Cr2
Cr2 → ND
ꢀ21.0 (3.5)
14.7 (2.5)
27.9 (0.2)
24.6 (1.4)
ꢀ4.3 (ꢀ6.5)
ꢀ27.0 (ꢀ5.2)
ꢀ25.9 (0.1)
24.1 (17.9)
54.6 (2.8)
327
ND → Iso
Iso → ND
ND → Cr1
Cr1 → Cr2
POXD Cr1 → Cr2
Cr2 → Colh
366
Colh → Iso

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Iso → Colh
Colh → Cr2
Cr2 → Cr1
49.0 (ꢀ2.7)
4.1 (21.2)
ꢀ29.1 (ꢀ0.1)
aCr = crystalline phase; ND = discotic nematic phase; Colho = hexagonal columnar phase; Iso
= isotropic melt. bPeak transition temperature in the DSC thermograms obtained during the
second heating and first cooling cycles at 10
TGA.
°
C/min. cOnset of decomposition determined by
Fig. 2 The texture of (a) MOXD at 18 °C, ×100; (b) POXD at 29.6 °C, ×100, in the cooling
cycle.
At room temperature, the sample of the monomer MOXD placed between two glass
slides can be easily sheared to show birefringent texture, indicating that the compound is in
the liquid crystalline state at room temperature. The sample transforms into an isotropic
liquid upon heating to 28 °C. Upon cooling, little birefringent drops first appear from the
isotropic melt and then fully form a threaded Schlieren texture with two and four brush
defects and marbled textures (Fig. 2a) typical for a discotic nematic phase.12 The NDꢀI
transition enthalpy (0.2 J gꢀ1 (0.25 KJ molꢀ1)) may also support the assignment of discotic

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nematic phase, in which the NDꢀI transition enthalpy is usually < 1 KJ molꢀ1.13 Further
cooling to −4 °C results in the crystallization of the mesophase. The POM observations are
reasonably consistent with the results of DSC (Fig. S2a).
The polymer POXD is a sticky solid at room temperature. In the microscopic study,
typically pseudo focalꢀconic textures with a large area of homeotropic domains (Fig. 2b)
are observed both in the heating and cooling cycles, suggesting that the mesophase is an
enantiotropic hexagonal columnar phase.[4e,11,14] In addition, POXD exhibits the liquid
crystalline behaviors at room temperature region, and the mesomorphic temperature range
is much wider than that of MOXD. The DSC curves (Fig. S2b) of POXD exhibited clearly
two peaks and a glass transition in both the heating and cooling scans, which is well in line
with the POM observations.
The UVꢀvis absorption and photoluminscent spectra of MOXD, and POXM in
dichloromethane solution at room temperature are presented in Fig. 3. All compounds
exhibited a strong absorption λmax at ca. 310 nm, while the polymer POXD showed two
additional weak shoulders at 260 nm and 265 nm. They all emitted a strong blue emission
with a λmax at ca. 391 nm. The shapes of the photoluminscent spectra, the emission λmax
,
the intensity of emission peaks and the photoluminscent quantum yields (MOXD: 51%;
POXD: 64%) of these compounds are very similar, indicating that the emission was
mainly attributed to conjugated mesogenic core, the molecular weight and peripheral
alkoxy chains had negligible effect on the optical properties.

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0.7
0.6
0.5
0.4
0.3
0.2
0.1
0.0
700
600
500
400
300
200
100
0
MOXD
POXD
250
300
350
400
450
500
550
600
wavelength/nm
Fig. 3 Absorption and photoluminscent spectra of MOXD and POXM in CH2Cl2 (1×10ꢀ5 M)
at 25 °C. The excitation wavelength was 300 nm.
The emission spectra of MOXD and POXD in solid state and liquid crystalline phases
are presented in Fig. 4. For POXD, the fluorescent emission λmax in solid state at −78 °C is
blueꢀshifted (5 cmꢀ1) with respect to that in the columnar phase at room temperature.
Compared to the slight spectral shift, the fluorescent intensity increased dramatically from
columnar phase to solid state. Similar results are also observed in the case of MOXD. The
fact that the fluorescent intensity increases from liquid crystalline state to solid state may
be ascribed to the different extent of forming selfꢀquenching aggregates in different
states.15 This phenomenon often occurred in the compounds with an intramolecular
donorꢀacceptor structure,16 compounds like 1,3,4ꢀoxadiazoles without strong
electronꢀdonating groups were found to show the selfꢀquenching aggregates for the first
time. By comparison of the emission spectra of MOXD and POXD, it is found that the
fluorescent intensity increases significantly both in solid state and liquid crystalline phases.

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The fluorescent intensity of POXD is almost 12 times of that of the monomer MOXD in
solid state as well in liquid crystalline state.
800
600
400
200
0
POXD(ꢀ78°C)
POXD(r.t.)
MOXD(ꢀ78°C)
MOXD(r.t.)
350
400
450
500
wavelength/nm
Fig. 4 Fluorescence spectra of MOXD and POXD in solid state at –78 °C and liquid
crystalline phases at room temperature
Conclusion
A novel sideꢀchain polymer and the corresponding monomer based on discotic
1,3,4ꢀoxadiazole have been reported in this work. Both of them are room temperature
liquid crystals with excellent thermal stability. The polymer facilitated to form an ordered
hexagonal columnar phase a with wider temperature range than that of the monomer. Both
the polymer and the momomer are photoluminscent in solution, in solid state and in liquid
crystalline state, while the different states have a great effect on their optical behaviors. In
CH2Cl2 solution, they all display a fluorescent emission with similar quantum yields. In
contrast, the intensity of the polymer is almost 12 times of that of the monomer in both
solid state and liquid crystalline state.
Acknowledgments

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This work was financially supported by the National Natural Science Foundation of China
(No. 21272130) and the Open Project of State Key Laboratory of Elementoꢀorganic
Chemistry (No. 201211), which are gratefully acknowledged.
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