Effects of Polar Substituents on the Properties of
1,3,4-Oxadiazole-based Liquid Crystalline Materials Containing
Asymmetric Cores
C025
Hsiao-Hsien Sung (宋孝先) and Hong-Cheu Lin (林宏洲)*
Department of Materials Science and Engineering, National Chiao Tung University,
Hsinchu, Taiwan (ROC)
Tel: (03)5712121ext.55305, Fax: (03)5724727, E-mail: linhc@cc.nctu.edu.tw
[1]. The introduction of oxadiazole rings can provide not
ABSTRACT
A series of 1,3,4-oxadiazole (OXD)-containing liquid
crystals
are
synthesized
and
characterized.
only the lateral dipole from the oxygen and nitrogen
These
atoms, but also the bent shape of the rigid cores. It is due
molecular oxadiazole-based liquid crystals exhibit stable
to the polar substituents are highly variable, and its
mesogenic properties including the nematic and smectic A
attaching sites are also adjustable. Therefore, these
phases. With the analogous structural design, the
benefits allow us to access a wide variety of mesophases
transition temperatures, mesomorphic phases, optical
simply by changing the polar moieties or their attaching
properties, and internal quantum efficiencies show strong
positions. [2]
dependence on the terminal substitution. In general, by
Linearly polarized blue organic light-emitting diodes
increasing the terminal dipoles the temperature ranges of
have been demonstrated by taking advantage of the
the
Furthermore,
thermotropic mesomorphism to be aligned on a rubbing
poly(fluorene)-based copolymers with OXD pendants
polyimide (PI) substrate [3-5]. These liquid crystalline
exhibit glass-forming liquid crystalline properties and
materials possess a potential for spontaneous uniaxial
reveal much wider mesophasic temperature ranges than
alignment of nematic mesomorphism by spin-coating on a
that of PF. The thermal properties and mesomorphism of
rubbing substrate and therefore they are enabled to
these conjugated polymers are mainly affected by the
generate polarized light emission. PF and monodisperse
nature of these pendants. In addition, the tendencies of
oligofluorenes are among the most promising candidates
crystallization and aggregation of PF are also suppressed
for efficient polarized blue luminescence [6].
mesophases
are
enhanced.
by introducing the OXD pendants.
摘要
2. RESULTS AND DISCUSSION
一系列包含 OXD 雜環的液晶材料被合成與鑑
2-(6-alkoxy-naphthalen-2-yl)-5-phenyl-[1,3,4]oxadia
定，這些液晶分子有穩定的向列相和層列 A 相，其相
zoles containing various substituents at phenyl 4-position
轉移溫度、液晶相與光學性質皆受到末端基的影響。
(Fig. 1) exhibit stable mesogenic properties including the
大体而言，當末端基的極性增強液晶相的溫度範圍也
nematic
變廣。再者，當 OXD 基團懸接在 PF 側鏈上時，這些
incorporated with strong polar electron-withdrawing
高分子都展現了玻璃態的液晶性質，而且有較 PF 更寬
terminal groups (F, Cl, CN, and NO2) exhibit the SmA
廣的液晶相溫度範圍，這些高分子的熱性質與液晶相
phase and tend to form highly ordered smectic E (SmE)
主要受到 OXD 懸掛基團的影響。此外，把 OXD 基團
phase. All compounds with electron-donating terminal
導入 PF 高分子中也可有效的抑制結晶與聚集的現象。
groups (-Me and -OMe), i.e. n-NPO-Me and n-NPO-OMe,
1. INTRODUCTION
reveal the nematic (N) phase characterized by the
and
SmA
phases.
These
LC
materials
Dipole-dipole interactions and structural shapes are
schlieren textures. Whereas, when electron-donating
the fundamental elements in the design of liquid crystals
methyl and methoxy groups were served as terminal
43
moieties, the mesophase of these LC materials do not
show any SmE phase.
220
N
N
A
B
C
D
200
O
180
temperature ( C)
X
o
H2n+1Cn O
n-NPO-X
where
n = 6, 8, and 10
160
140
120
100
and X = Me, OMe, F, Cl, CN, and NO2
80
Fig. 1 Chemical structures of the synthesized molecules.
X=Me
OMe
Cl
F
CN
NO2
8-NPO-X
The angular mesogens carrying different terminal
groups indeed influence the mesogenic phases and the
Fig 2.
The effect of polar substituents on the transition
transition temperatures in 8-NPO-X analogues (see Fig.
temperatures (dipole increases from left to right). (A)
2). It shows that both of the melting temperature and
clearing point, (B)melting point, (C) liquid crystallization
liquid crystallization temperature (on cooling) increase
temperature (on cooling), (D)crystallization temperature.
with the terminal dipoles. The clearing temperature and
crystallization temperature also have a similar tendency,
70
except the methyl-substituted compound (8-NPO-Me).
heating
cooling
60
Another interesting trend is the mesogenic phase range
50
o
mesophase range ( C)
also increases with the terminal dipoles (shown in Fig. 3),
especially for the heating procedure, the mesomorphic
temperature ranges from 3.1 ℃ with methoxy-group
(8-NPO-OMe)
up
to
62.4 ℃
with
nitro-group
40
30
20
10
(8-NPO-NO2). From this result, it can be proved that the
0
mesogenic phase ranges of these oxadiazole-based
X=Me
structures can be enlarged by increasing the terminal
OMe
F
Cl
CN
NO2
8-NPO-X
dipoles, and both of the melting and clearing temperatures
can be raised as well.
Fig 3.
The effect of polar substituents on the mesophase
range during heating and cooling cycles. (dipole increases
from left to right).
44
temperature range. On the other hand, P4 (which is a
n
random copolymer comprising carbazole and Be-OXD
groups) is also an amorphous polymer with T g = 83 °C,
Ar
O
O
Ar
N
N O
N
O
and exhibits a nematic phase without any propensity to
N
P1 Ar=
crystallize on heating or cooling. From these results, we
P2 Ar=
can draw two conclusions from these facts. First, the
P3 Ar=
mesomorphism of these conjugated polymers are greatly
n
m
affected by the nature of the pendants. Second, the
N
mesophasic temperature range will be hugely promoted,
N
O
N
N
O
O
O
NN
P4
comparing with the mesophasic temperature range of
PFO (~100 °C), as well as the tendency of spontaneous
Fig 4. Chemical structures of polymers.
crystallization of PF can be suppressed by the
introduction of the OXD pendant groups.
Furthermore,
the
chemical
structures
of
poly(fluorene)-based copolymers with OXD pendants
Table 1. Molecular Weights and Thermal Properties of
are shown in Fig. 4. As revealed by DSC diagram
Polymers P1-P4
(shown in Fig. 5), P1 is an amorphous polymer with Tg
Polymer
Mn
Mw/Mn
= 60 °C (Table 1) and shows a stable glass-forming
Tda
Tg
Tc
(℃)
(℃)
(℃)b
nematic phase corroborated by polarizing optical
P1
10200
2.2
406
60
>300
microscopy. P2 contains Na-OXD (7b) pendants, which
P2
27000
2.1
414
80
>340
is a monotropic mesogenic material with nematic and
P3
55700
2.6
438
86
>340
SmA phases, shows Tg = 80 °C and only exhibits a
P4
45600
1.8
438
83
>340
nematic phase without any SmA phase. On the other
a
hand, P3 possesses Bi-OXD (7c) groups as pendants,
nitrogen.
Temperature of 5 % weight loss measured by TGA in
Bi-OXD has wider mesophasic temperature range (K
134 SmA 187 N 195 I) than that of Na-OXD. The
N
P1
N
P2
G
resulting polymer displays nematic and SmA phases
similar to their pendants with Tg = 86 °C. In comparison
Endotherm (up)
G
with P2, P3 shows two mesophases including the
nematic and SmA phases. The divergence may result
N
SmA
P3
G
from the wide SmA temperature range (53 °C) of
N
G
P4
Bi-OXD pendants, which induces the resulting polymer
to exhibit the SmA phase. In contrast to Bi-OXD,
0
50
100
150
200
250
300
o
Temperature ( C)
Na-OXD only has SmA temperature range of 8 °C and
consequently can not effectively induce P2 to generate
Fig 5. DSC thermograms of polymers during the second
the SmA phase. Compare P1 and P2-P3, we can find that
heating scan at 10 °C /min. Symbols: G, glass; N, nematic;
although Be-OXD (7a) does not exhibit mesogenic
K, crystalline.
property, the large steric groups can still suppress the
tendency of crystallization and enhance the mesophasic
45
3. CONCLUSION
The thermal properties and mesomorphism of these
OXD-containing liquid crystals are greatly affected by the
nature of terminal groups and pendants, these liquid
crystalline materials exhibit stable mesogenic properties
including the nematic and SmA phases.
4. REFERENCES
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(2004).
[3] U. Scherf, and E. List, Adv. Mater. 14, 477 (2002).
[4] M. Grel, D. D. C. Bradley, M. Inbasekaran, and E. P.
Woo, Adv. Mater. 9, 798 (1997).
[5] M. Hamaguchi, and K. Yoshino, Appl. Phys. Lett. 67,
3381 (1995).
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46