2-D columnar assemblies of diblock rod-coil molecules incorporating cholesteryl group

Article abstract of DOI:10.14233/ajchem.2014.16021

Rod-coil molecules have a strong tendency to self-organize into supra-molecular nanostructures in the bulk state. In this article, we report synthesis and self-assembling behaviour of rod-coil molecules 1-3, consisting of a biphenyl and cholesteryl group

Full text of DOI:10.14233/ajchem.2014.16021

Asian Journal of Chemistry; Vol. 26, No. 4 (2014), 1127-1132
ASIAN JOURNAL OF CHEMISTRY
http://dx.doi.org/10.14233/ajchem.2014.16021
2-D Columnar Assemblies of Diblock Rod-Coil Molecules Incorporating Cholesteryl Group
†
†
*
*
YANG LIU , YIRONG PEI , ZHUOSHI WANG, JUNJIE CUI, TIE CHEN , JINGZHE XU and LONG YI JIN
Key Laboratory for Organism Resources of the Changbai Mountain and Functional Molecules, Ministry of Education and Department of
Chemistry, College of Science, Yanbian University, No. 977 Gongyuan Road, Yanji 133002, P.R. China
*Corresponding authors: E-mail: tchen@ybu.edu.cn; lyjin@ybu.edu.cn
†These authors contributed equally to this work
Received: 9 July 2013;
Accepted: 10 September 2013;
Published online: 15 February 2014;
AJC-14714
Rod-coil molecules have a strong tendency to self-organize into supra-molecular nanostructures in the bulk state. In this article, we report
synthesis and self-assembling behaviour of rod-coil molecules 1-3, consisting of a biphenyl and cholesteryl group as a rod segment and
poly(ethylene oxide) (PEO) with a degree of polymerization of 7, 12 and 17 coil segments. Self-organization behaviour of these molecules
investigated by means of differential scanning calorimetry, thermal polarizing optical microscopy and small-angle X-ray scattering in the
bulk state reveal that volume fraction of PEO coil segments containing alkyl chain of cholesteryl group dramatically influence the self
assembling behaviour of rod building block in the liquid crystalline phase. Molecules 1-3 self-assemble into 1-D bilayered structures with
different d-spacing according to the lengths of coil domains in the solid state. While, self-organized molecular structures transfer into
oblique columnar structure for molecule 1 and 2 from the crystalline to the liquid crystalline phase, further increasing coil to rod volume
fraction, molecule 3 self assemble into hexagonal columnar aggregate through π-π stacking of rod building blocks.
Keywords: Self-assembly, Rod-coil, Small-angle X-ray scattering, Cholesteryl, 2-D Columnar.
With this in mind,we synthesized rod-coil molecules
containing cholesteryl group as a part of rod segment and poly-
(ethylene oxide) with a degree of polymerization of 7, 12 and
17 linked together with biphenyl group as coil segments
(Scheme-I). The self-assembling behaviours of these mole-
cules investigated by using differential scanning calorimetry
(DSC), thermal polarizing optical microscopy (POM) and
small-angle X-ray scatterings (SAXS) in the bulk state.
INTRODUCTION
A considerable amount of research has been done to
control the supramolecular structures with well-defined shapes
and sizes which have potential for fundamental and practical
implications in areas such as materials science, molecular elec-
tronics and biomimetric chemistry1-6. Recently, functionalized
rod-coil molecules have been studied by researchers for
construction of nano-scale materials with potential electro-
photo properties7-13. Self-assembly of rod-coil block molecules
provide various supramolecular nanostructures from 1-D
lamellar, 2-D columnar, 3-D bundles in the bulk state, through
non-covalent forces including hydrophobic and hydrophilic
effects, electrostatic interaction, hydrogen bonding and micro-
phase segregation14-23. It has been reported that molecular para-
meters such as volume fraction of rod to coil segment, the cross-
sectional area of coil segment, the shape of rigid rod segment
dramatically influence driving force of molecules to form
various nanostructure24-27. Cholesterol-substituted molecules
have strong tendency to self-assemble into twisted helical
EXPERIMENTAL
4,4'-Biphenol, poly(ethylene glycol) methyl ether (MW =
350, 550, 750), toluene-p-sulfonyl chloride (TsCl, 99 %),
cholesteryl chloroformate, pyridine, potassium carbonate, tri-
ethylamine (99.5 %) and conventional reagents were used as
received. Compounds 4-6 and 7-9 were prepared according to
the similar procedures described elsewhere12,13
.
Techniques: 1H NMR spectra was recorded from CDCl3
solution on a Bruker AM 300 spectrometer. A Perkin Elmer
Pyris Diamond differential scanning calorimeter was used to
determine the thermal transitions with the maxima and minima
of their endothermic or exothermic peaks, controlling the heating
and cooling rates to 10 °C min-1. X-Ray scattering measure-
ments were performed in transmission mode with synchrotron
radiation at the 1W2A X-ray beam line at BeijingAccelerator
Laboratory, China and at the 3C2 X-ray beam line at Pohang
aggregations with distinct optical, chiroptical properties28-33
.
Hence, it is growing interest in the design of synthetic rod-
coil molecules incorporating cholesteryl block that are able to
self-assemble into diverse supramolecular nanostructures for
application in nano-wire and materials science.

1128 Liu et al.
Asian J. Chem.
Accelerator Laboratory, Korea. MALDI TOF-MS was perfor-
med on a Perceptive Biosystems Voyager-DE STR using a
2-cyano-3-(4-hydroxyphenyl) acrylic acid (CHCA) as matrix.
A Olympus optical polarized microscope equipped with a
Mettler FP 82 hot-stage and a Mettler FP 90 central processor
was used to observe the thermal transitions and to analyze the
anisotropic texture.
J = 4.8 Hz), 4.54-4.65 (m, 1H, OCH), 4.17 (t, 2H, phenyl
OCH2CH2O, J = 4.8 Hz), 3.88 (t, 2H, phenyl OCH2CH2O, J =
4.8Hz), 3.53-3.74 (m, 70H, -OCH2CH2O-), 3.38 (s, 3H, OCH3),
0.69-2.51 (cholesteryl part); MALDI-TOF-MS m/z [M]+ 1347.
RESULTS AND DISCUSSION
Synthesis of molecules 1-3: Rod-coil molecules containing
cholesteryl group as a part of rod segment and poly(ethylene
oxide) with a degree of polymerization of 7, 12 and 17 linked
together with biphenyl group as coil segments were synthesized
using 4,4'-biphenol and poly(ethylene glycol) methyl ethers
as starting materials. Molecules 1-3 were obtained by nucleo-
philic substitution reaction of cholesteryl chloroformate and
compound 7-9 in the presence of triethylamine as catalyst
(Scheme-I). The structures of these molecules were charac-
terized by 1H NMR spectroscopy (Fig. 1) and matrix-assisted
laser desorption ionization time-of-flight (MALDI-TOF) mass
spectroscopy (Fig. 2) were shown to be in full agreement with
the structures presented in Scheme-I.
Synthesis of molecules 1-3: Molecules 1-3 were synthe-
sized using the similar procedures. A representative example
is described for 1. A mixture of compound 7 (0.83 g, 1.55
mmol) and cholesteryl chloroformate (2.0 g, 7.7 mmol) were
dissolved in absolute toluene (40 mL) in a 100 mL two-neck
flask adding triethylamine (3 mL) as catalyst, then refluxed
for 24 h. The solvent was removed in a rotary evaporator and
the crude product was purified by column chromatography
on silica gel using CH2Cl2, ethyl acetate and ethyl acetate:
MeOH (20:1) as eluent to yield 0.9 g of yellow wax-like solid
1
(70 %). H NMR (300 MHz, CDCl3, δ, ppm) 7.46-7.55 (m,
4Ar-H, m to phenyl-OCO, m to CH2O phenyl), 7.22 (d, 2Ar-
H, o to phenyl-OCO, J = 9.0Hz), 6.97 (d, 2Ar-H, O to CH2O
phenyl, J = 9.0 Hz), 5.42 (t, 1H, C=CH, J = 5.1Hz), 4.54-4.67
(m, 1H, OCH), 4.17 (t, 2H, phenyl OCH2CH2O, J = 4.8 Hz),
3.88 (t, 2H, phenyl OCH2CH2O, J = 4.8 Hz), 3.54-3.74 (m, 24
H, -OCH2CH2O), 3.38 (s, 3H, OCH3), 0.68-2.51 (m, 46H,
cholesteryl part); MALDI-TOF-MS m/z [M]+ 995.
Molecule 2: Yield 73 %. 1H NMR (300 MHz, CDCl3, δ,
ppm) 7.46-7.55 (m, 4Ar-H, m to phenyl-OCO, m to CH2O
phenyl), 7.22 (d, 2Ar-H, o to phenyl-OCO, J = 9.0 Hz), 6.98
(d, 2Ar-H,o to CH2O phenyl, J = 9.0Hz), 5.42 (t, 1H, C=CH,
J = 4.8Hz), 4.54-4.65 (m, 1H, OCH), 4.17 (t, 2H, phenyl
OCH2CH2O, J = 4.8 Hz), 3.88 (t, 2H, phenyl OCH2CH2O, J =
4.8Hz), 3.64-3.73 (m, 46H, -OCH2CH2O-), 3.38 (s, 3H, OCH3),
0.69-2.51 (m, 46H, cholesteryl part); MALDI-TOF-MS m/z
[M]+ 1127.
Structure analysis in the bulk state: The self-assembling
behaviour of 1-3 was investigated by means of differential
scanning calorimetry (DSC), thermal polarizing optical micro-
scopy (POM) and X-ray diffraction (XRD) in the bulk state.
Fig. 3 shows the DSC heating and cooling traces of molecules
1-3 and exhibited their thermotropic liquid crystalline phase
at 64, 58 and 47 °C. On further increasing temperature, the
isotropic liquid phase was observed at 130, 113 and 76 °C,
respectively. Similar to conventional linear rod-coil molecules,
the melting transition temperatures of the rod-coil diblock
oligomers 1-3 increase as the poly(ethylene oxide) coil length
decrease3,5,12
.
On slow cooling of 1-3 from isotrophic liquid to liquid-
crystalline mesophase, focal conic textures were observed on
optical polarize microscope, indicating the presence of the 2-D
columnar liquid-crystalline mesophases (Fig. 4). Especially,
molecule 3 exhibited the pseudo focal conic texture in the
POM experiment which is indicative of existence of hexagonal
columnar structure (Fig. 4c).
Molecule 3: Yield 71 %. 1H NMR (300 MHz, CDCl3, δ,
ppm) 7.46-7.55 (m, 4Ar-H, m to phenyl-OCO, m to CH2O
phenyl), 7.22 (d, 2Ar-H, O to phenyl-OCO, J = 9.0Hz), 6.98
(d, 2Ar-H, O to CH2O phenyl, J = 9.0Hz), 5.42 (t, 1H, C=CH,
Scheme-I: Synthetic route of molecules 1, 2 and 3

Vol. 26, No. 4 (2014)
2-D Columnar Assemblies of Diblock Rod-Coil Molecules Incorporating Cholesteryl Group 1129
Temperature (°C)
Fig. 3. DSC traces (10 °C min-1) recorded for heating and the cooling scans
of 1, 2 and 3
Fig. 1. . 1H NMR spectra of molecules 1, 2 and 3 (solvent: CDCl3)
Fig. 4. Representative POM micrograph (× 40) of the textures exhibited
by (a) an oblique columnar structure of 1; (b) an oblique columnar
structure of 2 and (c) a hexagonal columnar structure of 3 at the
transition from the isotropic liquid
To identify the detailed self-organizing structures in the
bulk state, X-ray scattering experiments of molecules 1-3 were
performed at various temperatures. Figs. 5a and 6a show X-
ray diffraction patterns for compound 1 recorded at 35 and
110 °C on cooling from the isotropic phase to the liquid-crysta-
lline phase. In the solid state, several peaks corresponding to
equidistant q-spacings in the small-angle region can be indexed
as the (001), (002) and (003) reflections for a 1-D lamellar
phase. A layer spacing (d-spacing) of 7.62 nm is calculated
from the (001) plane, which is larger than the corresponding
estimated molecular length 5.75 nm (by Corey-Pauling-Koltun
Fig. 2. MALDI-TOF-MS spectra of molecules 1, 2 and 3 (matrix: CHCA).

1130 Liu et al.
Asian J. Chem.
q (nm–1)
Fig. 5. X-Ray diffraction patterns of lamellar structure for molecules 1, 2
and 3 measured at solid state
(CPK) molecular modeling). It can be explained that molecule
1 formed a bilayered lamellar structure in the solid state, in
which the hydrophobic and PEO segments are fully over-
lapped. In the liquid-crystalline phase, the small-angle X-ray
diffraction pattern of 1 measured at 110 °C shows four reflec-
tions. These reflections can be indexed as the (100), (110),
(010) and (200) planes (Fig. 6a) for a two-dimensional oblique
columnar structure with a characteristic angle γ = 60° and lattice
parameters a = 6.38 nm, b = 4.17 nm (Table-1). To further inves-
tigate inner packing arrangement of molecule 1, we performed
wide-angle X-ray scattering experiment. A broad single peak
centered at approximately 0.514 nm in the wide-angle region
indicative of liquid-crystalline ordering of the aromatic
segments within the rod domains. Based on the experimental
values of the unit cell parameters (a = 6.38 nm, b = 4.17 nm, c
= 0.514 nm and γ = 60°) and the density of 1 (ρ = 1.04), the
number of molecules in a single slice of the column is calcu-
lated about 8, according to the following equation, where M
is the molecular weight and NA is Avogadro's number.
q (nm–1)
Fig. 6. SAXS patterns of 1, 2 and 3 measured in the liquid-crystalline state,
plotted against q (= 4π sin θ/λ): (a) oblique columnar for 1 at 110
°C; (b) oblique columnar phase for 2 at 95 °C; (c) hexagonal
columnar for 3 at 57°C.
TABLE-1
SAXS DATA FOR AN OBLIQUE COLUMNAR STRUCTURE OF
1, IN THE LIQUID CRYSTALLINE PHASE
h
1
1
0
2
k
0
1
1
0
l
qobsd (nm-1)
1.1323
1.538
qcalcd (nm-1)
1.1323
0
0
0
0
1.5334
1.7386
2.2733
1.7386
2.2733
qobs and qcalc are the scattering vectors of the observed and calculated
reflections for the oblique columnar structure with lattice parameters
a = 6.38 nm, b = 4.173 nm and γ = 60°.
sional oblique columnar structure with a characteristic angle
γ = 61° and lattice parameters a = 6.86 nm, b = 4.59 nm. With
regard to molecule 2, the elongated chain magnifies the space
crowding thus leading to increasing the lattice parameters of
oblique columnar structure (Table-2). The number of mole-
cules of 2 in a single slice of the column is about 8 calculated
according the equation 1.
n = (abcsinγ)ρNA/M
(1)
Similar to molecule 1, in the solid state, XRD experiments
of molecule 2 and 3 display several peaks corresponding to
equidistant q-spacings in the small-angle region, that also can
be indexed as the (001), (002) and (003) or (004) reflections
for the 1-D lamellar phases. The layers spacing are about 8.05
nm and 10.07 nm calculated from the (001) plane, which are
also larger than the corresponding estimated molecular length
7.65 nm and 9.45 nm respectively, indicative of self-organizing
into bilayered lamellar structure in the solid state (Figs. 5b, 5c).
Compared with molecule 1, molecule 2 with a longer coil
segments give rise to slight change to the structure of liquid-
crystalline phase. Fig. 6b shows X-ray diffraction patterns for
molecule 2 recorded at 95 °C, several diffraction patterns
corresponding in the small-angle region also can be indexed
as the (100), (110), (010) and (200) planes for a two-dimen-
TABLE-2
SAXS DATA FOR AN OBLIQUE COLUMNAR STRUCTURE OF
2, IN THE LIQUID CRYSTALLINE PHASE
h
1
1
0
2
k
0
1
1
0
l
qobsd (nm-1) qcalcd (nm-1)
0
0
0
0
1.0431
1.3819
1.5343
2.0950
1.0431
1.3819
1.5334
2.0950
qobs and qcalc are the scattering vectors of the observed and calculated
reflections for the oblique columnar structure with lattice parameters
a = 6.86 nm, b = 4.585 nm and γ = 61°

Vol. 26, No. 4 (2014)
2-D Columnar Assemblies of Diblock Rod-Coil Molecules Incorporating Cholesteryl Group 1131
Fig. 7. Schematic representation of self-assembly of (a) an oblique columnar structure for 1, (b) an oblique columnar structure for 2, (c)
hexagonal columnar for 3
For molecule 3 with the longer poly(ethylene oxide) coil
segment, the SAXS patterns in the melt state measured at
57 °C displays three peaks with the d-spacing ratio of 1: 31/2:
2, which can be interpreted as a hexagonal columnar structure
with a lattice constant a = 9.04 nm. From characteristic of
pseudo-focal conic domains shown by the POM, together with
SAXS data of 3, we confirmed that molecule 3 self-assemble
into hexagonal columnar nanostructure in the liquid crystalline
mesophase. These results indicate that parameters of the coil
segments with different lengths of coil chains dramatically
influence the self-assembly of block molecules as well as the
cholesteryl alky unit, forming various supramolecular structure,
from the lamellar structure to diverse columnar liquid-crysta-
lline mesophases for molecules l-3 (Table-3). Based upon the
data presented so far, a schematic representation of the self-
assembled structures of 1-3 is illustrated in Fig. 7.
biphenyl and the other side with cholesteryl group were
successfully synthesized. XRD studies in the crystalline phase
reveal that these molecules self-organized into 1-D bilayered
structures with different d-spacings according to the lengths
of molecules. However, self-organized molecular structures
transfer to various columnar structures in the liquid-crystalline
phase. XRD experiments together with POM textures demons-
trate that molecule 1 and 2 convert into oblique columnar
structure. While, further increasing coil volume fraction, mole-
cule 3 self assemble into hexagonal columnar aggregate, which
have potentially application in materials science, molecular
electronics and biomimetric chemistry.
ACKNOWLEDGEMENTS
This work was supported by the National Natural Science
Foundation of China (grant number: 21164013) and the Jilin
Provincial Science and Technology Department (Grant number:
201115225). The authors are grateful to Institute of High Energy
Physics, ChineseAcademy of Sciences and PohangAccelarator
Laboratory, Korea for using Synchrotron Radiation Source.
TABLE-3
CHARACTERIZATION OF THE THERMOTROPIC
LIQUID-CRYSTALLINE BEHAVIOR OF
MOLECULES IN THE BULK STATE
Crystalline phase
(lamellar)
Liquid-crystalline
phase (columnar)
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