Synthesis and mesomorphism of novel star-shaped glassy liquid crystals containing pentaerythritol esters

Article abstract of DOI:10.1016/j.tetlet.2004.09.109

A new class of star-shaped glassy nematic liquid crystals based on pentaerythritol as a flexible core and ω-[4-(p-alkoxybenzoloxy) phenoxycarbonyl]valeric acid as side-chain mesogenic arms has been prepared. This is a new kind of glassy nematic liquid cry

Full text of DOI:10.1016/j.tetlet.2004.09.109

Tetrahedron
Letters
Tetrahedron Letters 45 (2004) 8953–8956
Synthesis and mesomorphism of novel star-shaped glassy
liquid crystals containing pentaerythritol esters
*
Dan-Shu Yao, Bao-Yan Zhang, Yuan-Hao Li and Wen-Qiang Xiao
The Research Centre for Molecular Science and Engineering, Northeastern University, Shenyang 110004, China
Received 4 May 2004; revised 31 August 2004; accepted 3 September 2004
Abstract—A new class of star-shaped glassy nematic liquid crystals based on pentaerythritol as a flexible core and x-[4-(p-
alkoxybenzoloxy)phenoxycarbonyl]valeric acid as the side-chain mesogenic arms has been prepared. Their structures and mesomor-
phism have been investigated in detail.
ꢀ
2004 Elsevier Ltd. All rights reserved.
Glassy liquid crystals (GLCs), which do not crystallize
upon cooling, but vitrify and form supercooled liquid
crystalline phases are useful materials for fabrication
of thermoelectrooptical storage displays and different
SGLCs, using pentaerythritol as a flexible core and x-
[4-(p-alkoxybenzoloxy)phenoxycarbonyl]valeric acid as
the side-chain mesogenic arms, which do not crystallize
upon cooling, but vitrify and form supercooled nematic
phase.
1
–3
optical components. As a rule, GLCs can be divided
4
into polymers and low molecularmass compounds.
,5
6,7
Compared with polymers, the main advantages of low
molecularmass compounds a re the lowerviscosity and
superior chemical purity, so low molecular mass GLCs
hold a fast and good orientation, which can be readily
The structure and synthetic route of SGLCs (3) were
0
shown in Scheme 1. 4-Hydroxy-4 -[(p-alkoxy)benzoyl-
oxy]phenyl (1) was synthesized according to a reported
1
1
procedure,
x-[4-(p-alkoxybenzoloxy)phenoxycarbon-
8
processed into macroscopically ordered solid films. As
yl]valeric acid (2) was synthesized from excess adipyl
chloride and compound 1. Purified compound 2 was
transformed into the corresponding acyl chloride and
reacted with the appropriate pentaerythritol (molar
ratio = 4.2:1) in pyridine for 16h stirring to give the
desired compound 3 in reasonable yields (48–56%).
Compound 3 showed good solubilities in common
we know, the unconventional liquid crystals, with
molecular structures deviating substantially from the
simple rod-like shape, are many which can form GLCs
such as branched compounds, derivatives of condensed
aromatic rings like naphthalene, phenanthrene, ortho-
or meta-substituted benzene derivatives. Among these
low molecularmass GLCs, sta -rs haped glassy liquid
crystals (SGLCs) are particularly attracted the atten-
tion, owing to their symmetrical molecular structures
and interesting properties. The star-shaped structure
usually has a small core and a few extended rigid meso-
genic units as the side-chain liquid crystal arms, which is
different from conventional rod-like or discotic mole-
cules. But most star-shaped molecules having high
molecular order usually crystallize, instead of vitrifying
into a glassy state, during cooling from the isotropic
1
organic solvents and it was carefully identified by H
1
NMR, IR and elemental analyses.
2
The thermal properties of 3 evaluated by DSC (differen-
tial scanning calorimetry) were summarized in Table 1
and the representative DSC thermograms were shown
in Figure 1 (Netzsch DSC 204, equipped with a liquid
nitrogen cooling system. Scan rate, 10ꢁC/min). As we
know, polymers such as the side-chain liquid crystalline
1
3
polymers are atactic, their disordered systems induce
vitrification rather than crystallization during cooling.
But the DSC curves of these low molecular mass 3
showed that they did not crystallize during cooling from
the isotropic melt, instead, vitrifying into a glassy state
just like most polymers. Furthermore, the glassy state
was quite stable, no sharp melting peak was observed
9,10
melt.
Here we report the synthesis of a new class of
Keywords: Liquid crystals; Star-shaped molecules; Glassy state;
Nematic phase.
*
Corresponding author. Fax: +86 24 8368 7446; e-mail:
baoyanzhang@hotmail.com
0
040-4039/$ - see front matter ꢀ 2004 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2004.09.109

8954
D.-S. Yao et al. / Tetrahedron Letters 45 (2004) 8953–8956
RO
COOH
SOCl
2
HO
OH THF
1
RO
COO
OH
ClCOC(CH ) COCl THF
2
4
RO
COO
SOCl
OOC(CH ) COOH
2
2
4
2
C(CH OH)4 Pyridine
2
O
O
O
O
COO
COO
OR
OR
RO
RO
COO
COO
O
O
O
O
O
O
O
O
O
O
O
O
3
3
a: R=CH (n=1), 3b: R=C H (n=2), 3c: R=C H (n=3).
3 2 5 3 7
Scheme 1. The synthetic route of star-shaped glassy liquid crystals.
Table 1. Phase transition temperatures of star-shaped glassy liquid
crystals
a
b
c
T
d
Star-shaped Transition
glassy liquid temperature/ꢁC
DT
1
DT
2
crystals
(corresponding enthalpy
À1 heating
changes/Jg
)
cooling
d
g À 2:0ð0:3Þ N98:8ð2:1ÞI
3a
3b
3c
100.8 101.9 344
136.4 137.1 356
141.2 141.8 350
d
I95:6ðÀ2:2ÞN À 6:3ð0:3Þ g
d
g2:1ð0:4Þ N138:5ð2:4ÞI
d
I135:4ðÀ2:5ÞN À 1:7ð0:6Þ g
d
g À 5:5ð0:3Þ N135:7ð1:8ÞI
Figure 1. DSC thermograms of star-shaped glassy liquid crystal 3b.
d
I133:2ðÀ1:9ÞN À 8:6ð0:8Þ g
g = glass state, N = nematic, I = isotropic.
when we reheated the glass. This was due to the steric
effect, the crystallization of these star-shaped molecules
was hindered. As summarized in Table 1, with theirhigh
clearing points and low glass transition temperature, 3a–c
all showed the enantiotropic behaviour and a wide mes-
ogenic region. On heating, the mesogenic region of 3a
was extended to 100.8ꢁC compared with 136.4ꢁC for
a
Mesophase temperature ranges on heating cycle.
Mesophase temperature ranges on cooling cycle.
Temperature at which 5% weight loss occurred.
b
c
d
p
DC in J/(gK).
molecular maintain orient order through the acting
1
4
3
b and 141.2ꢁC for 3c. The reason was that the polarity
forces of molecular induction and polarization. As
the terminal alkoxy chain lengthened (from n = 1 to 3),
the molecularcoope ar tive packing was enhanced and
of the terminal group played a quite important role
during the liquid crystal phase formation. It made the

D.-S. Yao et al. / Tetrahedron Letters 45 (2004) 8953–8956
8955
the structural anisotropy increased. The phase transition
temperatures displayed in Table 1 were reversible and
did not change on repeated heating and cooling cycles.
Meanwhile, TGA results showed that the temperatures
when 5% weight loss occurred (T ) were higher than
d
3
3
40ꢁC, which revealed that the synthesized compound
had a high thermal stability.
Figure 4. Diagram of star-shaped molecule anisotropic structure in the
nematic phase.
The textures of 3 studied with POM (polarizing optical
microscopy) were shown in Figure 2. The results showed
that 3 exhibited typical nematic thread-like textures in
small core with a longer flexible spacer derived from
hexanedioic acid, the arm-to-arm interaction between
neighbouring and within molecules existed. So the abil-
ity of such star-shaped molecules to fold into an aniso-
tropic shape gave rise to the appearance of a nematic
liquid crystalline order as shown in Figure 4.
1
5
theirliquid c yr stal state.
were also studied by wide angle X-ray diffraction
WAXD) and small angle X-ray scattering (SAXS).
The amorphous diffuse peaks of 2h was found at about
0ꢁ, and no sharp peak in the lower Bragg angle region
The quenched samples of 3
(
2
was observed as shown in Figure 3. This result suggested
that 3 exhibited only nematic mesophases, which was
consistent with the optical textures. In general, the ne-
matic state has been restricted just to the rod-like mole-
cules, which spontaneously ordered with their long axes
roughly parallel or to discotic molecules, which are sim-
ilar in structure to the rod-like nematic, although in the
later case the short axes of the molecules tend to lie par-
allel. The driving force for the nematic phase in this kind
of SGLCs was found that, in the symmetric star-shaped
molecules, the linearly extended arms were attached to a
In conclusion, we have synthesized a new kind of
star-shaped glassy liquid crystals, which show a wide
mesogenic region and high thermal stability. These com-
pounds do not crystallize on cooling, but vitrify and
form stable supercooled LC phase. Their optical texture
is threadlike, typical of nematic LC phase.
Acknowledgements
The authors are grateful to National Natural Science
Fundamental Committee of China and HI-Tech
Research and development program (863) of China for
financial support of this work.
References and notes
1
. Eichler, H. J.; Elschner, R.; Heppke, G. Appl. Phys. B
995, 61, 59–63.
. Elschner, R.; Macdonald, R. Mol. Cryst. Liq. Cryst. 1996,
82, 107–115.
1
2
2
3
4
. Tantrawomg, S.; Styring, P. Liq. Cryst. 1997, 22, 17–22.
. Brehmer, M.; Lub, J.; Witte, P. Adv. Mater. 1998, 10,
1
438–1442.
. Bobrovsky, A. Y.; Boiko, N. I.; Shibaev, V. P. Liq. Cryst.
998, 25, 393–398.
. Wedler, W.; Demus, D.; Zaschke, H.; Mohr, K. J. Mater.
Chem. 1991, 1, 347–356.
5
6
1
Figure 2. Polarized optical micrograph (200·) of 3b at heating to
5ꢁC.
7
8
. Witte, P.; Lub, J. Liq. Cryst. 1999, 26, 1039–1046.
. Chen, H. P.; Katsis, D.; Mastrangelo, J. C.; Chen, S. H.;
Jacobs, S. D. Adv. Mater. 2000, 12, 1283–1286.
. Bong, G. K.; Sehoon, K.; Soo, Y. P. Tetrahedron Lett.
9
9
2
001, 42, 2697–2699.
0. Lee, C. H.; Yamamoto, T. Bull. Chem. Soc. Jpn. 2002, 75,
15–618.
1. Hu, J. S.; Zhang, B. Y.; Feng, Z. L. J. Appl. Polym. Sci.
001, 80, 2335–2340.
1
1
200
1
1
1
6
000
2
8
6
4
2
00
00
00
00
2. All new compounds showed satisfactory spectroscopic
data. Typical procedure for compound 3b: 6.5g
(
16.8mmol) of compound 2b were stirred in 25mL of
thionyl chloride and 0.5mL of DMF as a catalyst was
added. The solution was heated at reflux for 5h whereby a
clearsolution was obtained. The excess of thionyl chlo ri de
were removed in vacuum and the acid chloride dried in
vacuum for 1h. For the esterification, the acid chloride
was dissolved in 10mL of dry THF and 25mL dry
pyridine and 0.54g (4.0mmol) of pentaerythriol were
added. The reaction mixture was stirred at 70ꢁC for16h in
5
10
15
20
θ(°)
25
30
35
2
Figure 3. X-ray diffraction pattern of 3b.

8956
D.-S. Yao et al. / Tetrahedron Letters 45 (2004) 8953–8956
a dry atmosphere. After cooling to room temperature, the
mixture was poured into 150mL cold water and acidified
with 6N hydrochloric acid. The precipitate was collected
by filtration and washed with distilled water. The crude
product was recrystallized from ethyl acetate/ethanol (1:1)
J = 8.9Hz, Harom); IR (KBr) 2923 (C–H); 1757, 1732,
(C@O); 1605, 1510 (Ar); 1254, 1187 (C–O–C) cm
À1
.
Anal.calcd forC 89H O : C, 66.41; H, 5.76. Found: C,
66.28; H, 5.43.
92
28
13. Ciferri, A. Liquid Crystallinity in Polymers: Principles and
Fundamental Properties; Wiley–VCH: NY, 1991.
14. Marcos, M.; Omenat, A.; Serrano, J. L. C. R. Chimie
2003, 6, 947–957.
15. Dierking, I. Textures of Liquid Crystals; Wiley–VCH:
Weinheim, 2003.
1
to afford a white solid of compound 3b in 52% yield. H
NMR (CDCl , 300MHz): d 1.48 (t, 12H, J = 6.3Hz, CH )
3
3
1.77–1.83 (m, 16H, CH
CH
2
), 2.60 (t, 16H, J = 6.3Hz,
COO), 4.02 (q, 8H, J = 6.3Hz, CH O), 4.17 (s, 8H,
2
2
COOCH ), 6.90–7.20 (m, 24H, Harom), 8.12 (d, 8H,
2