Synthesis and luminescence properties of a lollipop-shaped molecule combined with rod and disc-like mesogens

Article abstract of DOI:10.1080/15421406.2011.600141

A novel lollipop-shaped molecule containing 1,3,4-oxadiazole moieties was designed and synthesized. The molecular structure of this 1,3,4-oxadiazole derivative was confirmed by FT-IR, ¹H-NMR spectroscopy and elemental analyzer. Its phase behavi

Full text of DOI:10.1080/15421406.2011.600141

Mol. Cryst. Liq. Cryst., Vol. 551: pp. 60–68, 2011
Copyright © Taylor & Francis Group, LLC
ISSN: 1542-1406 print/1563-5287 online
DOI: 10.1080/15421406.2011.600141
Synthesis and Luminescence Properties
of a Lollipop-Shaped Molecule Combined with Rod
and Disc-Like Mesogens
E-JOON CHOI,1,∗ FEI XU,1 JONG-HO SON,2
AND WANG-CHEOL ZIN2
1Department of Polymer Science and Engineering, Kumoh National Institute of
Technology, Gumi, Gyeongbuk, 730-701, Korea
2Department of Material Science and Engineering, Pohang University of Science
and Technology, Pohang, Gyeongbuk, 790-784, Korea
A novel lollipop-shaped molecule containing 1,3,4-oxadiazole moieties was designed
and synthesized. The molecular structure of this 1,3,4-oxadiazole derivative was con-
firmed by FT-IR, 1H-NMR spectroscopy and elemental analyzer. Its phase behavior and
mesomorphic property were investigated by DSC, POM and X-ray diffraction measure-
ments. We found that this compound showed a typical mosaic texture of columnar phase
during cooling run. The electron excitation and luminescence properties of this com-
pound were investigated by UV-vis absorption spectroscopy and photo-luminescence
spectroscopy both in CHCl3 solution and in solid state.
Keywords: luminescence properties; 1, 3, 4-oxadiazole; discotic columnar mesophase;
lollipop-shaped mesogen; blue light emitting; liquid crystalline OLED
Introduction
Organic light-emitting diodes have been intensively studied due to their potential in next
generation full-color displays1–5 and solid-state lighting.6,7 To date, a considerable number
of organic conjugated low molar mass molecules have been synthesized and reported for
applications in the field of OLEDs. In order to fabricate full-color OLED displays, we need
high performance red,8–10 green,2,11 and blue12,13 materials with high EL efficiencies, good
thermal properties and long lifetimes as well as pure color coordinates. However, the most
widely studied emissive molecules are predominantly hole-transporting materials which
possess low electron affinities,14,15 and consequently, the device exhibits relatively low EL
efficiency. Its current focus and challenge lie in the optimization of the molecular structure
with high electron-transporting property. Typical electron-transporting materials usually
contain a π-electron deficient heterocyclic moiety. In addition, the use of heterocycles to
tune emission behavior is widely practice since these materials exhibit both interesting
electrical and optical properties and have excellent thermal and chemical stability.16
Among these heterocyclics, oxadiazoles is one of the most widely used moieties
for OLED.17–19 Various kinds of oxadiazole molecules have been used to obtain high
electroluminescence, such as bisoxadiazoles,20,21 branched or dendrimeric oxadiazoles,22
∗Corresponding author. E-mail: ejchoi@kumoh.ac.kr
[380]/60

Lollipop-Shaped Liquid Crystalline OLED
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polymers containing oxadiazole units either in the main chain23,24 or side chain25–27 and
so on. Especially, 1,3,4-oxadiazole derivatives have been used as blue emitters as well as
electron transport and hole blocking materials due to the electron-withdrawing character
of the 1,3,4-oxadiazole rings.28,29
On the other hand, liquid crystalline materials display unique properties which can
be exploited in organic light-emitting diodes. Characteristic features of liquid crystals are
the anisotropy of electronic properties, a strong coupling to external fields as well as a
tendency to from spontaneously homogeneous monodomain films. The self-organization
caused likewise the anisotropy of transport properties including the anisotropy of the
mobility of charge carriers.30 It is found that liquid crystalline materials can be used in
light-emitting diodes to control the state of polarization of the emitted light, the magnitude
of the onset field for emission as well as the quantum efficiency. Both low molar mass
and polymeric liquid crystals have been introduced with great success in single as well as
in multilayer devices. Especially, discotic liquid crystals able to form columns have been
found to reveal unusually large mobility for holes and probably also for electrons along the
columns.31–33
Our present work was motivated by the above background and succeeded in designing
and synthesizing a the lollipop-shaped oxadiazol-phenylene derivative consisting of two dif-
ferent types of mesogens. In addition, the insertion of the flexible alkyl segments on the side
chain of the aromatic molecular frame could moderate the molecular flexibility and melting
temperature which could promote the solubility of materials in common organic solvents.
The thermal properties were investigated by a differential scanning calorimetry (DSC).
The mesogenic properties were investigated using crosspolarizing optical microscopy and
X-ray diffractometry. The physical properties were investigated by UV-vis absorption and
photoluminescence (PL) spectroscopy. Interestingly, found was a blue-light-emitting liquid
crystalline molecule which formed a discotic columnar mesophase.
Experimental
Synthesis
The synthesis of the lollipop-shaped compound was carried out by modification of proce-
dure previously reported by us.34
Synthesis of 4-(dodecyloxy)benzohydrazide (1). Ethyl 4-(dodecyloxy)benzoate (4.0
g, 12.0 mmol) and excess hydrazine monohydrate in ethanol was refluxed for 40 h. The
resulting solution was poured into water. The precipitate was collected and dried under
vacuum. The crude product was recrystallized from ethanol to give pure product 1 in 78.4%
yield. IR (KBr Pellet, cm−1): 3319 (NH2 stretch), 3297, 3243 (NH stretch), 3018 (sp2
C-H stretch), 2921, 2848 (sp3 C-H stretch), 1617 (Conj. C O stretch), 1346, 1124 (C-O
1
stretch); H NMR (CDCl3, δ in ppm): 7.69 (d, 2H, Ar-H), 7.32 (s, 1H, Ar-CONHNH2),
6.90 (d, 2H, Ar-H), 3.96 (t, 2H, Ar-OCH2CH2), 1.76 (m, 2H, Ar-OCH2CH2CH2), 1.33
(m, 2H, Ar-O(CH2)10CH2CH3), 1.24 (m, 16H, Ar-OC2H4(CH2)8C2H5), 0.85 (t, 3H, Ar-
O(CH2)11CH3).
Synthesis of methyl 4-(2-(4-(dodecyloxy)benzoyl)hydrazinecarbonyl)benzoate (2).
At the atmosphere of nitrogen, monomethyl terephthalate (2.0 g, 10.8 mmol) was added into
SOCl2 and then added pyridine into three neck flask. The reaction mixture was refluxed
at 80◦C for 6 h. The reaction mixture was cooled to 40◦C and SOCl2 was removed at
the vacuum state. DCM (30 mL) was added to dissolve the residue absolutely and then

62/[382]
E.-J. Choi et al.
add 4-(dodecyloxy) benzohydrazide (3.5 g, 10.8 mmol) and a little pyridine. The mixture
was refluxed at 40◦C for 12 h. After cooling at room temperature, the resulting solution
was poured into distilled water (500 mL). The precipitate was collected and dried under
vacuum. The crude product was recrystallized from ethanol to give pure product 2 in 76.8%
yield. IR (KBr Pellet, cm−1): 3222 (NH stretch), 3019 (sp2 C-H stretch), 2917, 2850 (sp3
C-H stretch), 1724 (Conj. C O stretch), 1276, 1105 (C-O stretch); 1H NMR (CDCl3, δ in
ppm): 8.09 (d, 2H, Ar-H), 7.91 (d, 2H, Ar-H), 7.78 (d, 2H, Ar-H), 6.87 (d, 2H, Ar-H),
4.05 (t, 2H, Ar-OCH2CH2), 3.95 (s, 3H, Ar-COOCH3), 1.74 (m, 2H, Ar-OCH2CH2CH2),
1.33 (m, 2H, Ar-O(CH2)10CH2CH3), 1.25 (m, 16H, Ar-OC2H4(CH2)8C2H5), 0.85 (t, 3H,
Ar-O(CH2)11CH3).
Synthesis of methyl 4-(5-(4-(dodecyloxy)phenyl)-1,3,4-oxadiazol-2-yl)benzoate
(3). The purified methyl 4-(2-(4-(dodecyloxy)benzoyl)hydrazine carbonyl)benzoate (2.6
g, 5.3 mmol) was dissolved in phosphorous oxychloride (POCl3) and refluxed for about 40
h. Excess POCl3 was removed through distillation under reduced pressure and the residue
was slowly added into ice water. The precipitate was collected and dried under vacuum. The
crude product was further purified through a column of silica gel using 2% ethyl acetate in
chloroform as eluent to afford product 3 in 34.1% yield. IR (KBr Pellet, cm−1): 3070 (sp2
C-H stretch), 2925, 2846 (sp3 C-H stretch), 1695 (C N stretch), 1380, 1120 (C-O stretch);
1H NMR (CDCl3, δ in ppm): 8.18 (s, 4H, Ar-H), 8.04 (d, 2H, Ar-H), 7.03 (d, 2H, Ar-H),
4.05 (t, 2H, Ar-OCH2CH2), 3.92 (s, 3H, Ar-COOCH3), 1.74 (m, 2H, Ar-OCH2CH2CH2),
1.33 (m, 2H, Ar-O(CH2)10CH2CH3), 1.24 (m, 16H, Ar-OC2H4(CH2)8C2H5), 0.86 (t, 3H,
Ar-O(CH2)11CH3).
Synthesis of 4-(5-(4-(dodecyloxy)phenyl)-1,3,4-oxadiazol-2-yl)benzoic acid (4).
Methyl 4-(5-(4-(dodecyloxy)phenyl)-1,3,4-oxadiazol-2-yl)benzoate (7.7 g, 23.0 mmol)
and sodium hydroxide (4.2 g, 105.0 mmol) were added to the mixture of EtOH/H2O (500
mL, v/v = 1:1) under nitrogen. The mixture was stirred at 100◦C overnight. After reaction,
the solution was acidified with dilute hydrochloric acid to pH = 1 and then the reaction
mixture were cooled to room temperature. The resulting solid was collected by filtration and
washed with 1000 mL water for three times. The precipitate was collected by filtration and
dried under vacuum to afford product 4 in 79.6% yield. IR (KBr Pellet, cm−1): 3295–2443
(COOH stretch), 3068 (sp2 C-H stretch), 2921, 2844 (sp3 C-H stretch), 1687 (Conj. C O
stretch), 1606 (C N stretch), 1292, 1172 (C-O stretch); 1H NMR (DMSO-d6, δ in ppm):
8.26 (d, 2H, Ar-H), 8.15 (d, 2H, Ar-H), 8.09 (d, 2H, Ar-H), 7.18 (d, 2H, Ar-H), 4.07 (t, 2H,
Ar-OCH2CH2), 1.73 (m, 2H, Ar-OCH2CH2CH2), 1.33 (m, 2H, Ar-O(CH2)10CH2CH3),
1.24 (m, 16H, Ar-OC2H4(CH2)8C2H5), 0.85 (t, 3H, Ar-OCH2CH2(CH2)8
CH2CH3).
Synthesis of 2-(4-(dodecyloxy)phenyl)-5-(4-(5-(3,4,5-tris(dodecyloxy)phenyl)-
1,3,4-oxadiazol-2-yl)phenyl)-1,3,4-oxadiazole (5). At an atmosphere of nitrogen, 4-(5-
(4-(dodecyloxy)phenyl)-1,3,4-oxadiazol-2-yl)benzoic acid (2.8 g, 6.2 mmol) was added
into SOCl2 and then added pyridine into a three-neck flask. The reaction mixture was re-
fluxed at 80◦C for 6 h. The reaction mixture was cooled to 40◦C and SOCl2 was removed at
the vacuum state. DCM (30 mL) was added to dissolve the residue absolutely and then add
3,4,5-tris(dodecyloxy)-benzohydrazide (4.3 g, 6.2 mmol) and a little pyridine. The mixture
was refluxed at 40◦C for 12 h. After cooling at room temperature, the resulting solution
was poured into distilled water (500 mL). The precipitate was collected and dried under
vacuum. The crude product which was recrystallized from ethanol (2.5 g, 2.2 mmol) was
dissolved in phosphorous oxychloride (POCl3) and refluxed for about 40 h. Excess POCl3
was removed through distillation under reduced pressure and the residue was slowly added
into ice water. The precipitate was collected and dried under vacuum. The crude product

Lollipop-Shaped Liquid Crystalline OLED
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was further purified through a column of silica gel using 2% ethyl acetate in chloroform
as an eluent to afford product 5 in 29.8% yield. IR (KBr Pellet, cm−1): 3077 (sp2 C-H
stretch), 2919, 2854 (sp3 C-H stretch), 1606 (C N stretch), 1257, 1118 (C-O stretch); 1H
NMR (CDCl3, δ in ppm): 8.28 (s, 4H, Ar-H), 7.98 (d, 2H, Ar-H), 7.32 (s, 2H, Ar-H),
7.00 (d, 2H, Ar-H), 4.03 (t, 8H, Ar-OCH2CH2), 1.81 (m, 8H, Ar-OCH2CH2CH2), 1.33 (m,
8H, Ar-OCH2CH2(CH2)8CH2CH3), 1.25 (m, 64H, Ar-OCH2CH2(CH2)8CH2CH3), 0.85 (t,
12H, Ar-OCH2CH2 (CH2)8CH2CH3). Anal. Calcd. C: 76.18% H: 10.05% and N: 5.08%,
and found C: 76.19% H: 10.30% and N: 5.09%.
Measurements
FT-IR and NMR spectra were obtained by using Jasco 300E FT/IR and Bruker DPX 200
MHz NMR spectrometers, respectively. The chemical shifts were reported in ppm units
with tetramethylsilane (TMS) as internal standard. Elemental analysis was performed with
a Thermofinnigan EA1108. The transition behaviors were characterized by differential
scanning calorimetry (NETZSCH 200 F3). UV-vis absorption spectra were reported on
an OPTIZEN 3220UV spectrometer. Photoluminescence was measured by RF-5301PC
spectrometer. Optical texture observation was carried out using a polarizing microscope
(Carl Zeiss Axioskop 40) with a hot stage (Mettler FP82HT).
Results and Discussion
The synthetic route to the lollipop-shaped liquid crystalline molecule is shown in Scheme
1. This compound was prepared by cyclodehydration of the hydrazide precursors using
PCl3. The structures of all products were identified by using IR and NMR spectrometry as
already described in the experimental section. The all resultant data of the spectra were in
accordance with expected values. The purities of compounds were confirmed by using an
elemental analysis, and the resultant data was tolerable.
The differential scanning calorimetric (DSC) thermograms is presented in Figure 1.
The melting temperature (Tm), recrystallization temperature (Tc) and the related enthalpy
changes are listed in Table 1. The melting temperature and the enthalpy change for melt-
ing were 89.9◦C and 148.8 KJ/mol, respectively. During the cooling process, this liquid
crystalline molecule exhibited a discotic columnar phase. And the isotropic-to-mesophase
transition temperature and the enthalpy change were 72.5◦C and 1.5 KJ/mol, respectively.
As expected, introduction of trialkoxy groups to the rigid segment lowered the melting
temperature and increased the solubility remarkably. This indicates that flexible –CH2–
subunits can give an entropy benefit to the system and the bulky trialkoxy end group can
disturb the packing of molecules.
Table 1. Thermal properties of compound
Tm
aꢀHm
bTc
ꢀHc
Ti
ꢀHi
(◦C)
(kJ mol−1
)
(◦C)
(kJ mol−1
)
(◦C)
(kJ mol−1
)
89.9
148.8
66.5
27.1
72.5
1.5
aꢀHm: total enthalpy changes for multiple melting. b Tc: recrystallization temperature.

64/[384]
E.-J. Choi et al.
C2H5OOC
H2NHNOC
OC12H25
N2H4
H3COOC
COOH
OC12H25
1
SOCl2
O
O
H3COOC
C NHNH C
OC12H25
2
POCl3
O
H3COOC
OC12H25
N N
3
NaOH/HCl
C12H25O
C12H25O
C12H25O
O
O
C NHNH2
HOOC
OC12H25
N N
4
1)SOCl2
2)POCl3
OC12H25
OC12H25
OC12H25
O
O
C12H25O
N N
N N
5
Scheme 1. Synthetic route to a lollipop-shaped compound.
Figure 2 showed the crosspolarizing optical micrographs of the liquid crystalline
compound obtained during cooling run. A typical mosaic texture of columnar phase was
found as shown in Figure 2 (a) and a transition from mesophase to crystal was found at
52.2◦C as Figure 2 (b). Figure 4 showed the X-ray diffraction data measured during the first
heating and fist cooling run. As cooled from the isotropic liquid, a strong peak appeared at
the small angle region, showing broad peaks in wide angle region, being matched with the
temperature range of mesophase formation (72.5–66.5◦C) defined by DSC.
The UV absorption and fluorescent properties of this liquid crystalline molecule have
been studied. The absorption and PL spectra in CHCl3 solution and in solid state are shown
in Figure 3. The data of λabs, max and λem, max are presented in Table 2. This compound
exhibited a distinctive absorption peak at 342–346 nm. As the UV-vis absorption spectra

Lollipop-Shaped Liquid Crystalline OLED
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cooling
1st heating
2nd heating
20
40
60
80
100
120
140
160
Temperature (oC)
Figure 1. DSC thermograms of compound (heating and cooling rate = 10◦C/min).
in CHCl3 solution and in solid state are compared, a shift to higher wavelength is evident
in the case of solid state. The PL spectra of this compound exhibit a well-defined vibronic
feature. The table 2 showed that the liquid crystalline compound could emit a blue light both
in CHCl3 solution and in solid state on photoexcitation. The PL maxima of the compound
occurred at 451–462 nm.
In order to investigate the OLED performances of this compound, we tried to make the
device through thermal evaporation. But it was failed to fabricate OLED device. The reason
was our compound’s melting temperature is so low that material shows bumping seriously
during evaporation, which leads very unstable thin film formation. Therefore, to con-
firm the OLED performance due to high electron-transporting property of this compound,
more studies are needed including structural modification for the fabrication of OLED
device.
(a)
(b)
Figure 2. Crosspolarizing optical micrographs of compound on cooling (Magnification 200X): (a)
T = 72.5◦C; (b) T = 52.2◦C.

66/[386]
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Table 2. Physical properties of compound
CHCl3 Solution
Film on Glass
λabs, max
(nm)
λem, max
(nm)
λabs, max
(nm)
λem, max
(nm)
342
451
346
462
1.0
UV spectrum in CHCl solution
3
UV spectrum in solid state
PL spectrum in CHCl solution
3
0.8
0.6
0.4
0.2
0.0
PL spectrum in solid state
300
400
500
600
Wavelength ( nm )
Figure 3. UV-Vis absorption and PL spectra of the liquid crystalline compound at room temperature.
Figure 4. X-ray diffraction patterns of compound at the given temperature from bottom to top: on
heating as prepared sample measured from room temperature to 90◦C; on cooling the isotropic liquid,
measured from 90◦C to room temperature.

Lollipop-Shaped Liquid Crystalline OLED
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Conclusions
We have successfully designed and synthesized a lollipop-shaped liquid crystalline com-
pound. Its optical and thermal properties were investigated. The solubility of this 1,3,4-
oxadiazole derivative promoted greatly as introducing flexible groups. In general, the UV-
vis absorption maxima peaks (342 nm-346 nm) and PL maxima peaks (451 nm-462 nm)
were observed at the blue-light-emitting region. Especially, we found that besides possess-
ing electron-transporting capability, a lollipop-shaped molecule based on 1,3,4-oxadiazole
combining rod mesogen with disk mesogen could form a discotic columnar mesophase
monotropically, which should improve the EL efficiencies.
Acknowledgment
This work was supported by the Korea Science and Engineering Foundation (KOSEF) grant
funded by the Korea government (MEST) (No. R01-2008-000-11521-0).
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