Matrix-molecule induced chiral enhancement effect of binary supramolecular liquid crystals

Article abstract of DOI:10.1039/b710701c

Chiral enhancement effects associated with supramolecular liquid crystalline structures are studied, using a complex of achiral molecules (4,4′-bipyridine, 4Bpy) and chiral cholesteric liquid crystalline molecules (3-cholesteryloxycarbonylpropanoic acid, C4) as the model system. Non-mesogenic achiral molecule 4Bpy is used as the matrix element. The chiral enhancement of the supramolecular liquid crystalline structures can be revealed by circular dichroism (CD) and helical twisting power measurements. It is interesting that CD spectra of the complex exhibited an appreciably enhanced chiral property in comparison with that of the pure chiral cholesteric C4 molecules at room temperature. The helical pitch measurements by the Grandjean-Cano method also confirm the chiral enhancement effect. In addition, polarizing optical microscopy (POM) measurements indicate that the addition of matrix molecules leads to the explicit expression of the chiral liquid crystalline texture, i.e. twisted grain boundary (TGBA*) phase, at the liquid crystal temperature. Differential scanning calorimetry (DSC) and variable-temperature X-ray diffraction measurements further confirm the existence of the TGBA* phase. The binary supramolecular assembly structures are investigated by scanning tunneling microscope (STM) and the formation of hydrogen bonds between the chiral mesogens and the achiral matrix molecules can be directly observed. The variable-temperature Fourier transform infrared (FTIR) results further demonstrate that the hydrogen bonds persist until 168 °C. These results indicate that the introduction of a bifunctional aromatic base such as bipyridine could lead to self-assembled supramolecular architectures with discernible enhancements of chiral properties. The Royal Society of Chemistry.

Full text of DOI:10.1039/b710701c

View Article Online / Journal Homepage / Table of Contents for this issue
PAPER
www.rsc.org/materials | Journal of Materials Chemistry
Matrix-molecule induced chiral enhancement effect of binary
supramolecular liquid crystals{
Xiao-Jing Ma,a Yong-Tao Shen,b Ke Deng,a Hong Tang,c Sheng-Bin Lei,a Chen Wang,*a Yan-Lian Yang*a
and Xi-Zeng Feng*b
Received 12th July 2007, Accepted 14th September 2007
First published as an Advance Article on the web 24th September 2007
DOI: 10.1039/b710701c
Chiral enhancement effects associated with supramolecular liquid crystalline structures are
studied, using a complex of achiral molecules (4,49-bipyridine, 4Bpy) and chiral cholesteric liquid
crystalline molecules (3-cholesteryloxycarbonylpropanoic acid, C4) as the model system. Non-
mesogenic achiral molecule 4Bpy is used as the matrix element. The chiral enhancement of the
supramolecular liquid crystalline structures can be revealed by circular dichroism (CD) and helical
twisting power measurements. It is interesting that CD spectra of the complex exhibited an
appreciably enhanced chiral property in comparison with that of the pure chiral cholesteric C4
molecules at room temperature. The helical pitch measurements by the Grandjean–Cano method
also confirm the chiral enhancement effect. In addition, polarizing optical microscopy (POM)
measurements indicate that the addition of matrix molecules leads to the explicit expression of the
chiral liquid crystalline texture, i.e. twisted grain boundary (TGBA*) phase, at the liquid crystal
temperature. Differential scanning calorimetry (DSC) and variable-temperature X-ray diffraction
measurements further confirm the existence of the TGBA* phase. The binary supramolecular
assembly structures are investigated by scanning tunneling microscope (STM) and the formation
of hydrogen bonds between the chiral mesogens and the achiral matrix molecules can be directly
observed. The variable-temperature Fourier transform infrared (FTIR) results further
demonstrate that the hydrogen bonds persist until 168 uC. These results indicate that the
introduction of a bifunctional aromatic base such as bipyridine could lead to self-assembled
supramolecular architectures with discernible enhancements of chiral properties.
of chirality has been investigated mostly in polymer and
Introduction
macromolecular aggregates formed by noncovalent interac-
tions, such as p–p stacking or hydrogen bonds.7–9 The
currently reported magnitude of enhancement or amplification
Amplification of chirality systems has been extensively
pursued for their relevance to enantioselective synthesis and
explanation of the origin of homochirality in nature.1 The
of chirality in numerous publications covers a wide range,
chiral amplification effect has been extensively pursued in
from simple enhancement to amplification by orders of
the studies for chiral recognition,2 asymmetric reactions3 and
magnitude.
nanoscale functional materials.4 Supramolecular systems
As an important category of materials that could provide a
are regarded as suitable media for chiral amplification by
rich variety of structural ordering of molecules, supramole-
adjusting the entire supramolecular chirality via noncovalent
cular liquid crystalline structures may lead to new avenues for
interactions. The origin of such chiral amplification behavior
designing chiral architectures. Supramolecular liquid crystals
has been proposed as achiral components following the helicity
induced by the chiral components, as illustrated by the
‘‘Sergeants and Soldiers’’ principle.5 Dynamic studies revealed
that chiral amplification is dependent on time, as well as on the
stoichiometry of the chiral components.6 Common to these
studies is that orientational ordering of chiral groups is
essential for chiral expression. It is noted that amplification
have attracted much attention for a variety of well-defined,
easily formed and tunable structures derived from small mole-
cular weight components.10–14 Furthermore, supramolecular
liquid crystals can be arranged by specific molecular interac-
tions, such as hydrogen bonds, ionic bonds, charge-transfer
interactions, etc. Among these interactions, the hydrogen bond
is one of the most important interactions for constructing a
wide variety of supramolecular liquid crystal materials15,16 and
has been the subject of extensive studies.12,17 As an example,
Celebre et al.18 studied the chiral induction effect in liquid
crystals and found that a flexible, chiral molecule can produce
cholesteric phases of opposite handedness when dissolved in
different nematics.
aNational Center for Nanoscience and Technology, Beijing, 100080,
P. R. China. E-mail: wangch@nanoctr.cn; yangyl@nanoctr.cn;
Fax: +86 10 62650450; Tel: +86 10 62652700
bCollege of Life Science, Nankai University, Tianjin, 300071,
P. R. China. E-mail: xzfeng@nankai.edu.cn; Fax: +86 22 23507022;
Tel: +86 22 23507022
cDepartment of Chemistry, Tsinghua University, Beijing, 100084,
P. R. China
In this article, the chiral enhancement effects associated with
supramolecular liquid crystalline structures are studied, using
a complex of achiral molecules of 4,49-bipyridine (4Bpy) and
{ Electronic supplementary information (ESI) available: Structural
characterization of C4 molecules. See DOI: 10.1039/b710701c
This journal is ß The Royal Society of Chemistry 2007
J. Mater. Chem., 2007, 17, 4699–4704 | 4699

View Article Online
Scheme 1 Molecular structures of (a) C4, (b) 4Bpy and (c) C4BP. The chiral centers of C4 are marked in the molecular structure by *.
chiral molecules of 3-cholesteryloxycarbonylpropanoic acid
(C4) as the model system. Different from the conventional
chiral amplification method, here the enhancing chiral
phenomenon is observed by adding achiral non-mesogenic
molecules to pure chiral molecules. Shown in Scheme 1 are the
chemical structures of C4, 4Bpy and the proposed supra-
molecular structure of C4BP (2 : 1 mixture of C4 and 4Bpy).
Non-mesogenic achiral molecule of 4Bpy is added to used as
the matrix element. C4, with chiral centers marked in Scheme 1,
is a kind of cholesteric liquid crystalline molecule, which has
been extensively pursued as a novel display material suitable
for e-paper, commercial billboards and power-saving displays,
etc.19 Interestingly, the circular dichroism (CD) spectrum of
C4BP exhibits an enhanced chiral property in comparison with
that of the pure chiral cholesteric C4 at room temperature. The
helical pitch of C4 measured by the Grandjean–Cano method
is also larger than that of the C4BP complex, which indicates
that the helical twisting power (HTP) of C4BP is larger. In
addition, polarizing optical microscope (POM) measurements
indicate that the addition of matrix molecules 4Bpy leads to
the explicit expression of the chiral liquid crystalline texture,
i.e. twisted grain boundary (TGBA*) phase, which is not
observable for the pure chiral cholesteric C4. Differential
scanning calorimetry (DSC) and variable-temperature X-ray
diffraction (XRD) further support the existence of the TGBA*
phase. The molecular assembly structures of the binary
supramolecular liquid crystals consisting of C4 and 4Bpy are
investigated by scanning tunneling microscopy (STM), directly
visualizing the formation of the C4BP complex. The variable-
temperature Fourier transform infrared (FTIR) results indi-
cate that the hydrogen bond between C4 and 4Bpy persists
until 168 uC. These results demonstrate that the introduction
of a bifunctional aromatic base such as bipyridine could lead
to self-assembled supramolecular architectures with discernible
enhancement of chiral properties.
in tetrahydrofuran (THF) and then the complex was prepared
by evaporating the solvent.
STM experiments were performed in constant-current mode
under ambient conditions (Nanoscope IIIA SPM system,
Veeco, USA). The tips used were mechanically formed Pt–Ir
wire (80 : 20). The STM samples were prepared by depositing a
drop (4.5 ml) of solution containing C4BP at a concentration of
approximately 1026 mol l21 on a freshly cleaved atomically
flat surface of highly oriented pyrolytic graphite (HOPG)
(quality ZYB). The solvent used was THF of HPLC grade.
The STM tunneling conditions are described in the corre-
sponding figure captions.
We performed theoretical calculation using density func-
tional theory (DFT) provided by the DMol3 code.20 The
Perdew and Wang parameterization21 of the local exchange–
correlation energy were applied in the local spin density
approximation (LSDA) to describe exchange and correlation.
We expanded the all-electron spin-unrestricted Kohn–Sham
wave functions in a local atomic orbital basis. In such a
double-numerical basis set polarization was described. All
calculations were all-electron ones, and performed with the
Extra-Fine mesh. The self-consistent field procedure was done
with a convergence criterion of 1025 a.u. on the energy
and electron density. Infrared spectra were recorded on a
PerkinElmer Fourier transform infrared spectrometer at a
resolution of 4 cm21. Spectra recorded at elevated tempera-
tures were obtained by using a SHIMADEN SR91 heating
controller. CD measurements were conducted on a JASCO
J-810 spectrometer at room temperature. The films for CD
measurements were prepared following the procedures below.
Samples of 100 ml solutions containing C4 or C4BP were
dropped onto quartz at 100 uC to evaporate the solvent
quickly to decrease the possible uneven effect caused by
interface tension. The films were annealed at 135 uC (for C4)
and 165 uC (for C4BP) for 1 min and then cooled quickly. The
possible effect of linear dichroism of the film was removed by
rotating the film plate around the incident light.22 To avoid the
uncertainty induced by different thicknesses of C4 and C4BP
films, 30 slices of C4 and C4BP films were obtained by such a
preparation method and the thickness of each slice was mea-
sured by using a stylus profiler (DEKTAK 6 M, Veeco, USA).
Cholesteric pitches and HTP were obtained at room
temperature using the Grandjean–Cano method.23
Experimental
Cholesterol (3.867 g, 10 mmol), succinic anhydride (1.001 g,
10 mmol) and 4-dimethylaminopyridine (DMAP) (0.122 g,
1 mmol) were dissolved in 100 ml dichloromethane. The
mixture was refluxed for 24 h. The solvent was removed by
rotary evaporation and the residual solid was recrystallized
twice in glacial acetic acid to get C4 as a white powder.
Detailed structural characterization can be obtained from
the electronic supplementary information (ESI{). 4Bpy (98%,
Alfa-Aesar Co.) was used without further purification. The
mixture of C4 and 4Bpy (molar ratio = 2 : 1) was first dissolved
POM and DSC measurements were performed using a Leica
polarized optical microscope with a Linkam LTS350 hot stage
and a DSC-10 calorimeter (PerkinElmer Instrument) with
nitrogen purging. Variable-temperature XRD measurements
were performed on a Philips X9 pert pro, with a 3 kW ceramic
4700 | J. Mater. Chem., 2007, 17, 4699–4704
This journal is ß The Royal Society of Chemistry 2007

View Article Online
tube as the X-ray source (CuKa) and an X9 celebrator
connected by a non-mesogenic fragment.25 The periodicity
between two adjacent C4 dimers is measured to be 3.80 ¡
0.05 nm along the a direction and 1.70 ¡ 0.05 nm along the b
direction. The angle a in the unit cell is about 97 ¡ 2u. A
theoretically simulated packing model for C4BP system is
proposed in Fig. 1(b). Based on the theoretical analysis, the
parameters of the unit cell can be obtained (a = 3.72 nm, b =
1.67 nm, a = 96.4u), which are consistent with the measured
data from the STM images.
detector. The cooling and heating rates of POM, DSC and
XRD were 0.2 uC min21, 5 uC min21 and 5 uC min21
respectively.
,
Results and discussion
Fig. 1 illustrates the STM images of the lamellae structures of
C4BP physisorbed on a HOPG surface from THF solution.
Fig. 1(a) shows a typical high-resolution STM image, which
can be obtained under both negative and positive bias
conditions. The two-row bright bands are ascribed to the
stereoscopic structure of the five-membered and six-membered
rings in the cholesteryl groups, and the bands with less contrast
in the middle represent the 4Bpy molecules linking the two
cholesteryl groups.24 Due to the stereoscopic structure, the
cholesteryl part of C4 seems to be much brighter than the end
alkyl chain and the carboxyl end group in STM images
[Fig. 1(a) inset image]. As shown in the high-resolution STM
image, each pair of two adjacent C4 molecules from the
neighboring rows appears as a dimer structure. The 4Bpy
molecules are located in the interspace between two neigh-
bouring stripes, forming the C4(4Bpy)C4 supramolecular
structures. This is consistent with the well-known optimized
2 : 1 trimer structure consisting of two mesogenic units
It should be noted that the chiral expression of the molecules
on the surface is a very important subject for understanding
the chiral enhancement properties in the bulk. However, the
stereoscopic structure of the cholesteryl part of C4 where the
chiral centers are located can not be clearly resolved by STM.
The fluctuations of the adsorption conformation for the
stereoscopic cholesteryl part and alkyl tails of the C4 could
cause inevitable uncertainties in the adsorption conformation
of the C4BP complex. From close examination of the STM
images, it can be recognized that the 4Bpy segments of the
C4BP complex display a range of orientations. The schematic
illustration in Fig. 1 is only a qualitative model showing the
bonding characteristics between C4 and 4Bpy, with
a
simplified adsorption conformation in which the molecular
plane of 4Bpy is parallel to the substrate surface. It is therefore
not sufficiently reliable to assign the adsorption conforma-
tions, as well as the assembling chirality, of the molecular
complex using the STM images.
C4 is known as a typical steroid and well miscible with other
liquid crystals.26 It is noticed in our study that the assembly
structure of C4 physisorbed on HOPG is difficult to observe
by using STM at room temperature, while the coadsorption of
4Bpy enables relatively high-resolution STM observations of
the assemblies of the C4BP complex. This phenomenon
indicates that the stability of the monolayer could be improved
by utilizing space-filling non-mesogenic elements as proton
acceptors. The simulated structural model in Fig. 1(b)
…
demonstrates that a O–H N hydrogen bond can be formed
between one 4Bpy molecule and two C4 molecules. The
…
calculated dissociation energy of the O–H N hydrogen bond
is about 39.1 kcal mol21. Furthermore, the side-by-side C4
dimer coadsorption can be ascribed to the van der Waals
interaction (5.7 kcal mol21) between the end alkyl chains of the
C4 molecules.
The infrared spectra of the C4BP complex and C4 in
…
Fig. 2(a) confirmed the formation of O–H N hydrogen bonds
between C4 and 4Bpy. The O–H bands at 2500 and 1920 cm21
indicate the strong hydrogen bond formation.25,27–28 The
carbonyl band centered at 1732 cm21 in the spectrum obtained
for the C4BP complex is ascribed to the free carbonyl groups,
while the carbonyl band centered at 1726 cm21 is observed in
the single component C4 system for the dimer formation of the
carboxyl acid. Variable-temperature FTIR spectra were
measured to investigate the stability of the hydrogen bond in
the C4BP complex. Fig. 2(b) presents the infrared spectra from
30 uC to 168 uC and the results illustrate that the hydrogen
bond could be stabilized up to 166 uC with O–H bands at 2500
and 1920 cm21, which is identical with the temperature ranges
of the mesophases observed by POM characterization. The
transmission intensities of the two characteristic O–H bands
Fig. 1 (a) STM image (15.5 nm 6 15.5 nm) of a self-assembled
monolayer of C4BP on a HOPG surface. The imaging conditions were
I = 305.2 pA and V = 722.7 mV. The inset at the upper right is a
magnified image (5 nm 6 5 nm) and a unit cell is superimposed on the
image. (b) Proposed packing model of the C4BP complex.
This journal is ß The Royal Society of Chemistry 2007
J. Mater. Chem., 2007, 17, 4699–4704 | 4701

View Article Online
Fig. 3 (a) CD spectra of C4 (solid line) and C4BP (broken line) at
room temperature. (b) Plot of the CD intensities vs. film thicknesses for
C4 (circles) and C4BP (squares) at room temperature.
…
strong O–H N hydrogen bond formed between C4 and 4Bpy.
Fig. 2 (a) Infrared spectra of C4 (solid line) and C4BP (broken line)
in the crystalline state at room temperature; (b) variable temperature
infrared spectra of C4BP from 30 uC to 168 uC.
To avoid the CD signal variations caused by different film
thicknesses, both the CD signals and the film thickness were
measured as shown in Fig. 3(b). It can be noted that the CD
signals were less than 10 mdeg at 282 nm with the film
thickness ranging from 4.7 to 7.9 mm without the addition of
4Bpy, while after the addition of 4Bpy, stronger CD signals
(from 55 to 80 mdeg) were observed in the same range of film
thickness from 4.7 to 6.7 mm. Even when the film thickness of
C4BP was lower, the CD intensity was still much stronger than
that of the pure chiral cholesteric C4.
decrease gradually with increasing temperature and disappear
above 168 uC [broken line in Fig. 2(b)], which indicates the
break-up of the strong hydrogen bonds between C4 and 4Bpy.
One should note that the stability of the hydrogen bond is
not simply a function of temperature12 and actually the ionic
hydrogen-bonded state is stabilized by proton transfer in the
mesophase.29 These results are supportive to the STM observa-
…
tions of the C4BP complex self-assembled by O–H N hydro-
To further examine the effects associated with the addition
of 4Bpy, the Grandjean–Cano method23 was used to measure
the helical pitches of C4 and C4BP at room temperature. The
helicity degree is given by the quantity of the pitches of the
helix. For C4, the HTP value is 3.16 and the helical pitch
is about 31.6 mm, while for C4BP, the HTP value is 4.56
and the corresponding helical pitch is decreased to 21.9 mm.
The reduced helical pitch and the enhanced HTP value are
supportive of the chiral enhancement effect observed by CD
measurements.
gen bonds. The experimental data also confirm the theoretically
proposed structural model, in which each 4Bpy molecule
…
interacts with two C4 molecules by forming O–H N hydrogen
bonds between the two end N atoms and carboxyl groups.
It is conceivable that the addition of non-mesogenic
elements as the hydrogen-bonded matrix molecules could lead
to varied orientational order of the liquid crystalline structure.
Such deviation of molecular ordering from the single com-
ponent architectures could subsequently affect the chirality
expression of the mesogens. Macroscopic chirality charac-
teristics were studied by CD measurements at room tempera-
ture. It can be observed that a relatively weak CD signal about
5.3 mdeg is located around 282 nm without the addition of
4Bpy, while a stronger CD signal about 64.1 mdeg was
observed at 280 nm after the addition of 4Bpy [Fig. 3(a)] when
the film thicknesses of both C4 and C4BP were 5.3 mm. The
blue shift of the maximum CD signal could be attributed to the
POM measurements on pure C4 and the C4BP complex
were carried out (Fig. 4 and 5) to determine the optical texture
of the supramolecular liquid crystals in liquid crystalline
phases. For the single component C4 (Fig. 4), the typical focal
conic texture was observed at 169 uC on cooling, which
indicates that cholesteric phases are formed, while no charac-
teristic chirality related textures are observable. In contrast,
the POM image of C4BP (Fig. 5) revealed an additional
4702 | J. Mater. Chem., 2007, 17, 4699–4704
This journal is ß The Royal Society of Chemistry 2007

View Article Online
Fig. 4 Polarized optical micrograph of C4. The focal conic texture of
cholesteric phase at 169 uC on cooling; magnification: 2006.
Fig. 6 DSC of C4 (a) and C4BP (b) in the second cooling process,
rate 5 uC min21. Cr: crystalline state, I: isotropic phase.
Fig. 5 Polarized optical micrographs of C4BP. (a) The SmA* phase
(150.9 uC on heating); magnification: 5006. (b) The typical filament
texture of the TGBA* phase (152.1 uC on heating); magnification:
5006. (c) The N* phase (153.3 uC on heating); magnification: 5006.
phase—a TGBA* phase, which is generally located between
the cholesteric and the respective smectic phases and can only
be observed in chiral materials with large twisting power.30
The appearance of the TGBA* phase is consistent with the
relatively short pitch by HTP characterization.31 Fig. 5(a)
shows the SmA* phase at 150.9 uC on heating. The typical
filament-like texture characteristics of TGBA* phase can be
observed at 152.1 uC [Fig. 5(b)]. On further heating (153.3 uC),
the N* phase with focal conic pattern was observed [Fig. 5(c)].
The cooling rate was 0.2 uC min21 during the POM observa-
tion of the phase transitions. The I A N* A TGBA* A
SmA* A Cr transition in the second cooling process of
C4BP was confirmed by DSC (Fig. 6), which showed clear
exothermic peaks corresponding to the relative phase transi-
tions observed by POM. Fig. 7 shows XRD profiles of C4BP
at different temperatures. In Fig. 7, the X-ray scattering
intensity (I) is plotted vs. the scattering angle (2h) at 130 uC,
175 uC, 190 uC. In the crystalline state (130 uC), the I vs. 2h
curve shows several sharp peaks. When heated to 175 uC, the
ratio of diffraction peaks (1 : 2 : 3 : 4 : 5) indicates the typical
lamellar phase, which suggests the existence of the TGBA*
phase in agreement with the POM and DSC results. The
temperature difference between XRD, POM and DSC for
the TGBA* phase could be due to the difference between the
different hot stages. The expression of the chiral liquid
crystalline textures demonstrated the chiral enhancement effect
associated with the matrix element. Combined with the above
CD measurements, this chiral enhancement effect can be
identified in both the crystalline phase and the liquid crystal-
line phases.
Fig. 7 X-Ray diffraction patterns of C4BP at different temperatures.
The molecular interactions between cholesteric molecules
and matrix molecules are important in designing supra-
molecular assemblies. The combination of the stabilization
effects by molecular orientation, the geometry effect, and the
relatively strong carboxylic acid/pyridine interaction, leads to
the formation of supramolecular liquid crystalline materials
with stable mesomorphic behaviour. The stability and
molecular orientation for hydrogen-bonded systems have
also been discussed theoretically.3,32–34 Based on the above
studies, it can be seen that the addition of non-mesogenic
achiral 4Bpy molecules changes the micro-environment of the
liquid crystal molecules of C4, leading to the possibility of
chiral enhancement as revealed by CD spectra, POM and HTP
measurements.
This journal is ß The Royal Society of Chemistry 2007
J. Mater. Chem., 2007, 17, 4699–4704 | 4703

View Article Online
5 L. Brunsveld, A. P. H. J. Schenning, M. A. C. Broeren,
H. M. Janssen, J. A. J. M. Vekemans and E. W. Meijer, Chem.
Lett., 2000, 292.
6 M. M. A. J. Van der Schoot, L. Brunsveld, R. P. Sijbesma and
A. Ramzi, Langmuir, 2000, 16, 10076.
7 M. M. Green, M. P. Rediy, R. J. Johnson, G. Darling,
D. J. O’Leary and G. Willson, J. Am. Chem. Soc., 1989, 111, 6452.
8 M. M. Green, N. C. Peterson, T. Sato, A. Teramoto, R. Cook and
S. Lifson, Science, 1995, 268, 1860.
9 L. J. Prins, P. Timmerman and D. N. Reinhodt, J. Am. Chem. Soc.,
2001, 123, 10153.
10 T. Kato and N. Mizoshita, Curr. Opin. Solid State Mater. Sci.,
2002, 6, 579.
11 T. Kato, N. Mizoshita and K. Kanie, Macromol. Rapid Commun.,
2001, 22, 797.
12 T. Kato, Struct. Bonding, 2000, 96, 95.
13 W. Shu and S. Valiyaveettil, Chem. Commun., 2002, 1350.
14 N. Gimeno, M. B. Ros, J. L. Serrano and M. R. Fuente, Angew.
Chem., Int. Ed., 2004, 43, 5235.
15 C. M. John and M. W. George, Chem. Rev., 1994, 94, 2383.
16 J. M. Lehn, Science, 2002, 295, 2400.
Conclusions
In summary, we have observed the chiral enhancement effects
in supramolecular architectures associated with non-mesogenic
matrix molecules by CD spectra, the HTP measurements and
POM textures. The STM and variable-temperature FTIR
measurements confirmed the strong hydrogen bond forma-
tion between C4 and 4Bpy. This work demonstrates a
complementary approach for enhancing chiral properties by
addition of achiral non-mesogenic molecules even higher than
that of the pure chiral molecules, which is different from the
conventional chiral amplification approaches by addition
of chiral dopants into achiral molecules with the chirality
no better than the pure chiral molecules. It could be
suggested that the approach in this study would strengthen
the capability of fine tuning of the chiral behaviour of
molecular architectures.
17 T. Kato, N. Mizoshita and K. Kishimoto, Angew. Chem., Int. Ed.,
2006, 45, 38.
18 G. Celebre, G. De Luca, M. Maiorino, F. Lemma, A. Ferrarini,
S. Pieraccini and G. P. Spada, J. Am. Chem. Soc., 2005, 127, 11736.
19 T. Ikeda, J. Mater. Chem., 2003, 13, 2037.
Acknowledgements
This work was supported by the National Natural Science
Foundation of China (Grant numbers: 90406019, 20473097,
90403140, 50573089) and the Tianjin Science Technology
Research Funds of China (No. TJ043801111). The Ministry of
Science and Technology (MOST) and Chinese Academy of
Sciences (CAS) are also appreciated for financial support. The
authors gratefully acknowledge Prof. Ming-Hua Liu and Dr
Peng-Lei Chen for helpful discussions on circular dichroism
and Dr Hong Tang for HTP measurements.
20 A. Becke, J. Chem. Phys., 1988, 88, 2547.
21 J. P. Perdew and Y. Wang, Phys. Rev. B, 1992, 45, 13244.
22 J. Yuan and M. Liu, J. Am. Chem. Soc., 2003, 125, 5051; L. Zhang,
Q. Lu and M. Liu, J. Phys. Chem. B, 2003, 107, 2565; P. Guo and
M. Liu, Langmuir, 2005, 21, 3410.
23 R. Cano, Bull. Soc. Fr. Mineral. Cristallogr., 1968, 91, 20.
24 Y. L. Yang, Q. L. Chan, X. J. Ma, Y. T. Shen, K. Deng, X. Z. Feng
and C. Wang, Angew. Chem., Int. Ed., 2006, 45, 6889.
25 T. Kato, M. J. J. Frechet, P. G. Wilson, T. Saito, T. Uryu,
A. Fujishima, C. Jin and F. Kaneuchi, Chem. Mater., 1993, 5,
1094.
26 H.-S. Kitzerow and C. Bahr, Chirality in Liquid Crystals, Springer,
Berlin, 2001.
27 S. L. Johnson and K. A. Rumon, J. Phys. Chem., 1965, 69, 74.
28 J. Y. Lee, P. C. Painter and M. M. Coleman, Macromolecules,
1988, 21, 954.
29 D. J. Price, K. Willis, T. Richardson, G. Ungar and D. Bruce,
J. Mater. Chem., 1997, 7, 883.
30 I. Dierking, Textures of Liquid Crystals, Wiley-VCH, Weinheim,
2003.
References
1 M. M. Green, J. W. Park, T. Sato, A. Teramoto, S. Lifson,
R. L. B. Selinger and J. V. Selinger, Angew. Chem., Int. Ed., 1999,
38, 3138.
2 T. Ishi-I, T. M. A. Mateos, P. Timmerman, M. C. Calama,
S. Shinkai and D. N. Reinhoudt, Angew. Chem., Int. Ed., 2003, 42,
2402.
3 G. Roelfes and B. L. Feringa, Angew. Chem., Int. Ed., 2005, 44,
3294.
4 T. Ishi-I, R. Kuwahara, A. Takata, Y. Jeong, K. Sakurai and
S. Mataka, Chem.–Eur. J., 2006, 12, 763.
31 A. J. Slaney and J. W. Goodby, J. Mater. Chem., 1995, 5, 663.
32 B. A. Veytsman, Liq. Cryst., 1995, 18, 595.
33 R. P. Sear and G. Jackson, Phys. Rev. Lett., 1995, 74, 4261.
34 P. Bladon and A. C. Griffin, Macromolecules, 1993, 26, 6604.
4704 | J. Mater. Chem., 2007, 17, 4699–4704
This journal is ß The Royal Society of Chemistry 2007