Optically active polymer film tuned by a chirally self-assembled molecular organogel

Article abstract of DOI:10.1016/j.tet.2007.02.036

Chirally-oriented lipid aggregates were doped into a solid polymer film, which was readily prepared by casting and removing a solvent from anionic l-glutamide-derived organogels and polystyrene. When a cationic achiral dye was mixed into this system, the

Full text of DOI:10.1016/j.tet.2007.02.036

Tetrahedron 63 (2007) 7489–7494
Optically active polymer film tuned by a chirally self-assembled
molecular organogel
Makoto Takafuji,a Yoshiko Kira,a Hideaki Tsuji,a Shiro Sawada,a Hiroshi Hachisakob and
Hirotaka Iharaa,
*
aDepartment of Applied Chemistry and Biochemistry, Kumamoto University, 2-39-1 Kurokami, Kumamoto 860-8555, Japan
bDepartment of Applied Chemistry, Sojo University, 4-22-1 Ikeda, Kumamoto 860-8691, Japan
Received 8 November 2006; revised 22 January 2007; accepted 1 February 2007
Available online 14 February 2007
Abstract—Chirally-oriented lipid aggregates were doped into a solid polymer film, which was readily prepared by casting and removing
a solvent from anionic L-glutamide-derived organogels and polystyrene. When a cationic achiral dye was mixed into this system, the resultant
polymer film showed tunable circular dichroism (CD) due to chiral complex formation with the achiral dye on the highly-ordered structures
based on the L-glutamide lipid.
Ó 2007 Published by Elsevier Ltd.
1. Introduction
molecular chirality because they are usually induced by
long-range chiral orientation among the chromophoric
groups. Therefore, observed CD spectra are useful as an in-
dicator to estimate their oriented structures. The enhanced
chirality is one of the attractive features of self-assembling
molecules and useful for sensing and switching because
the chiral orientation of the molecules is strongly affected
by environmental factors such as temperature, pH, and addi-
tives. We have reported that L-glutamide-derived lipids
(Scheme 1), as chiral self-assembling compounds, can
form gels in organic solvents. This is brought about through
nanofibrous network formation and entrapment of solvent.
Organogel systems with self-assembling low molecular
compounds have been attractive as a construction tool for
bottom-up nanotechnology because they are essentially gen-
erated through three-dimensional network formation from
nano-sized one-dimensional fibrillar aggregates, constructed
of highly-ordered assemblies like aqueous lipid bilayer
membranes. It is also known that a number of chiral com-
pounds form nano-sized one-dimensional unique aggregates
with large curvatures such as nanotubes, nanohelices, and
nanoribbons.1 These aggregates are morphologically in-
teresting and applicable as templates for the creation of
nanomaterials. Gu et al.2 and Hafkamp et al.3 reported
mesoporous polymer materials by using organogel systems.
They were prepared by polymerization of a monomer, which
was used as a solvent to produce an organogel and sub-
sequent extraction of the nanofibrillar aggregates from the
polymer. The other application has been the construction
of inorganic nanomaterials by condensation of alkoxysilane
or chlorosilane with self-assembled fibrous/helical aggre-
gates as templates.4 This template method has been also de-
veloped to create various inorganic,5 metal,6 and polymer7
nanomaterials.
A specific circular dichroism (CD) signal is often observed
in organogel systems8–10 because of the chirality of organo-
gelators. These CD patterns are rather unique compared with
Keywords: L-Glutamic acid; Induced circular dichroism; Self-organization;
Nanofibrils; Polymer composite.
Scheme 1. Chemical structures of L-glutamide-derived lipids and cationic
dyes.
* Corresponding author. Tel.: +81 96 342 3661; fax: +81 96 342 3662;
e-mail: ihara@kumamoto-u.ac.jp
0040–4020/$ - see front matter Ó 2007 Published by Elsevier Ltd.
doi:10.1016/j.tet.2007.02.036

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Here, when a pyrenyl group was introduced onto the head
part (1) as a chromophoric group, an enhanced CD signal
was observed around the absorption band of the pyrenyl
group.11 This indicates that the process is driven by chiral
stacking of the pyrenyl groups into highly-ordered struc-
tures. However, unfortunately the assembly is not stable
because a temperature-induced gel-to-sol transition is com-
monly accompanied by disappearance of chiral stacking.
To increase mechanical strength and heat stability of en-
hanced chirality, we have investigated the immobilization
and stabilization of specific chiral microenvironments from
an L-glutamide lipid by using a polymerizable solvent and
following photo-induced (UV-light) polymerization.10 By
this method, both the CD pattern and strength could be main-
tained in the resultant polymer film and was stable even at
temperature above the original gel-to-sol phase transition
temperature.
xerogel12 by TEM and SEM, respectively. No similar aggre-
gates were found in the absence of 1. The TEM image
(Fig. 1a) indicates that the minimum diameter of the fibrous
aggregates is approximately 30 nm, which corresponds to
almost 10 lipid molecules. Therefore, it is estimated that
the aggregates entangle each other, which leads to three-
dimensional network structures.
The critical aggregation concentration (cac) and critical ge-
lation concentration (cgc) of 1 in a benzene solution exist at
0.2–0.5 mM and 2.0–3.0 mM at room temperature, respec-
tively.11 No significant change in these values was found
in the mixed system with PMMA. Figure 2 (left) shows
UV and CD spectra of the mixed system. It is found that
a strong CD signal was detected at 10 ꢀC but not at 70 ꢀC.
This is due to the fact that the mixed system underwent
a gel-to-sol phase transition at temperature around 60 ꢀC.
To support this, DSC measurement showed distinct endo-
thermic peaks at 59 ꢀC and 67 ꢀC. These phenomena are
very close to those in the absence of PMMA. This indicates
that PMMA does not disturb the aggregation of 1 and that the
gelation due to 1 is brought about through the formation of
chirally-ordered aggregates at temperatures below their
phase transition temperature.
The present work is aimed to develop this method for the
preparation of optically active polymer composites because
there are still some disadvantages: for instance, the chromo-
phoric group of an organogelator is decomposed by photo
(UV-light) irradiation. Also a mechanical property of the
resultant polymer film is hardly controllable because of
difficulty in controlling the polymerization degree. In this
article, we introduce a facile method to create an optically
active polymer composite using an L-glutamide lipid-
derived organogel and also demonstrate that the resultant
optical activity is quite tunable by induced chirality based
on a doped achiral dye.
The solid film was prepared by casting the mixture on a glass
plate and removal of solvent. The resultant film was almost
transparent and colorless. Figure 2 includes UV and CD
spectra of the film. The CD pattern was similar to that in
the gel at 10 ꢀC but no change was observed, even at
70 ꢀC. These results indicate that the specific chirality
from the aggregates of 1 remains, even after casting, and is
stable at elevated temperature. Here, we have established
a facile method to immobilize specific chirality based on
a 1-organogel into a polymer solid film.
2. Results and discussion
2.1. Pyrenyl group-induced chirality
The lipid 1 (0.5 mM) and poly(methylmethacrylate)
(PMMA, 500 unit-mM) were dissolved in benzene at
70 ꢀC and then cooled to 10 ꢀC to form a gel. Fibrous net-
work formation was observed both in the cast film and
2.2. Chirality tune with induced chirality
It would be supposed that the CD pattern would be tunable
by changing the head group of the L-glutamide-derived lipid.
Figure 1. TEM images of 1 (a) and 2 (b) aggregates prepared from benzene solutions. Scale bars indicate 200 nm.

M. Takafuji et al. / Tetrahedron 63 (2007) 7489–7494
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Figure 2. UVand CD spectra of 1 aggregates–PMMA mixed system in a benzene solution (left) and in a solid film (right) at 10 ꢀC and 70 ꢀC. Benzene solution:
[1]¼0.5 mM, [PMMA]¼500 unit-mM.
For instance, azobenzene8 and spiropyran13 derivatives
formed highly-ordered structures with specific chirality in
organic solvents. However, a bulky chromophoric group of-
ten prevents adequate molecular packing for highly-ordered
aggregation. On the other hand, it is well known that a chiral
stacking phenomenon can be induced in achiral molecules
bound to chirally-oriented aggregates.9,14,15 Therefore, a
dye-on-lipid aggregate complex as a host–guest system
would be attractive and useful to tune the optical activity
of a polymer composite film because the combination of
self-assembled aggregates and a chromophoric guest mole-
cule can be easily varied.
An ethanol solution of MB as an achiral guest molecule was
added to a benzene solution of a mixture of 2 and poly-
styrene as a supporting polymer. The final concentrations
of 2, MB, and polystyrene were adjusted to 5 mM,
0.25 mM, and 400 unit-mM, respectively. The solvent ratio
of benzene and ethanol was 100:1. The mixed solution
was heated to 70 ꢀC and cooled rapidly to 10 ꢀC and then
aged for 15 min. The solution formed a transparent blue or-
ganogel. The organogel was shaken well in a vial to become
a liquid and cast on a glass plate to obtain a blue solid film
(Fig. 3c). Figure 4 shows UVand CD spectra of a 2–MB mix-
ture in a benzene solution and a solid film. The absorption
band appeared at 655 nm as lmax in a sol state (at 70 ꢀC),
which was assigned to a monomeric dispersion state of
MB. In a gel state (at 10 ꢀC), the lmax shifted to 607 nm
accompanied with induction of CD with a positive signal
at 667 nm and a negative signal at 594 nm. These results
indicate that MB undergoes a monomer-to-aggregate transi-
tion16 according to a sol-to-gel transition of the solution of
2 and the observed CD signals are induced for both mono-
meric and H-like aggregated MBs. It should be also noted
that CD spectra showed almost no change in the absence
and presence of polystyrene, and thus the chiral environment
experienced by MB due to 2-aggregates is therefore insensi-
tive to polystyrene.
On the basis of this idea, we applied an induced CD (ICD) ef-
fect for preparing optically active polymer composites. For
this purpose, we selected lipid 2 as an organogelator because
we have already found that 2 can form fibrous aggregates in
organic solvents such as benzene, toluene, and cyclohexane,
and when a cationic achiral cyanine dye was added to the
2-benzene gel, distinct ICD appeared around the absorption
band of a cyanine dye.9 In the present work, we used several
cationic dyes, such as methylene blue (MB), acriflavin (AF),
and neutral red (NR) instead of a cyanine dye because a
cyanine dye is relatively fragile in light and also very sensi-
tive when subjected to environmental factors.

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M. Takafuji et al. / Tetrahedron 63 (2007) 7489–7494
Figure 3. Photos of polymer composite films prepared from the 2–cationic dye mixture in benzene.
UV and CD spectra of the MB-containing solid film are
shown in Figure 4. There are several similarities with the
case of the pyrenyl lipid 1 system: (1) aggregation of
MB was promoted while the monomer-to-aggregate com-
position was slightly different, (2) ICD was detected, and
(3) there was no significant change in the CD pattern and
its intensity was independent of temperature.
A similar procedure was applied for the other cationic dyes,
AF and NR, and transparent yellow (Fig. 3a) and red (Fig. 3b)
solid films were obtained, respectively. Typical CD spectra
are shown in Figure 5. The CD patterns, which appeared at
each absorptionbandof thedyes in benzenesolution, were re-
tained in the solid films. Therefore, the optical activity of the
polymer films is readily tunable by proper selection of dyes.
Figure 4. UVand CD spectra of methylene blue (MB) with 2 and polystyrene in a benzene solution (left) at 10 ꢀC and 70 ꢀC, and in a solid film (right) at 10 ꢀC
before and after heat treatment at 70 ꢀC for 10 min. Benzene solution: [MB]¼0.25 mM, [2]¼5 mM, [polystyrene]¼400 unit-mM.

M. Takafuji et al. / Tetrahedron 63 (2007) 7489–7494
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s¼singlet, d¼doublet, t¼triplet, q¼quartet, m¼multiplet,
and b¼broad.
4.1.1. Preparation of N1,N5-didodecyl-N2-benzyloxycar-
bonyl-L-glutamide (4). N-Benzyloxycarbonyl-L-glutamic
acid (3, 7.0 g, 25 mmol), dodecylamine (12 ml, 53 mmol),
and triethylamine (8.9 ml, 63 mmol) were dissolved in
THF (350 ml). The solution was cooled to 0 ꢀC and diethyl-
phosphorocyanidate (DEPC) (10 ml, 64 mmol) was added to
the solution. The mixture was stirred for 1 h at this temper-
ature. After being stirred for 12 h at room temperature, the
solution was concentrated in vacuo, and the residue was dis-
solved in chloroform. The solution was washed with 0.2 M
NaOH (aq), 0.2 M HCl (aq), and water. The solution was
dried over Na2SO4, concentrated in vacuo, and the residue
was recrystallized from methanol to yield white solid pow-
der: yield 10.7 g (68%); mp 141–143 ꢀC; IR spectrum
(KBr) 3287 (N–H), 2917 (C–H), 2850 (C–H), 1686
Figure 5. CD spectra of cationic dyes with 2-aggregates in polystyrenefilms.
Solid films were prepared from benzene solutions ([AF]¼[MB]¼0.25 mM,
[NR]¼0.50 mM, [2]¼5 mM, [polystyrene]¼400 unit-mM).
1
(C]O, urethane), 1637 (C]O, amide), 1536 (N–H); H
NMR spectrum (CDCl3, 400 MHz) d 0.88 (t, J¼6.84 Hz,
6H, CH3), 1.20–1.37 (m, 36H, CH3 (CH2)9), 1.43–1.54 (m,
4H, CH2CH2NHC(]O)), 1.91–2.15 (m, 2H, CH2C*),
2.23–2.43 (m, 2H, NHC(]O)CH2), 3.10–3.35 (m, 4H,
CH2NHC(]O)), 4.09–4.20 (m, 1H, *CH), 5.10 (s, 2H,
CH2-Phe), 7.29–7.45 (m, 5H, aromatics). Elemental analy-
sis. Found: H, 10.9; C, 71.8; N, 6.75. Calcd for
C37H65N3O4: H, 10.6; C, 72.1; N, 6.82%.
3. Conclusions
Generation and enhancement of supramolecular functions
are often observed in self-assembling systems. Induction
of chirality is one of the typical features of self-assembling
systems. In this study, we have established a facile method
to embed a supramolecular complex composed of chirally-
ordered lipid aggregates and achiral dyes into a solid poly-
mer film. As a result, optical activity of the polymer film
is readily tunable using ICD around the absorption bands
of the dyes as schematically illustrated in Figure 6. This
method would be useful for the design of new soft materials
with interesting optical properties.
4.1.2. Preparation of N1,N5-didodecyl-L-glutamide (5).
Compound 4 (10.0 g, 16 mmol) was dissolved in 300 ml of
ethanol with heating and Pd–carbon (1 g) was added to the
solution. H2 gas was bubbled slowly into the solution for
8 h at 70 ꢀC. Pd–carbon was removed by filtration. The solu-
tion was concentrated in vacuo and the residue was recrystal-
lized from methanol to yield a white solid powder: yield
6.77 g (88%); mp 117–118.5 ꢀC; IR spectrum (KBr) 3324
(N–H), 2917 (C–H), 2810 (C–H), 1632 (C]O, amide),
1528 (N–H); 1H NMR (CDCl3, 400 MHz) d 0.88 (t,
J¼6.82 Hz, 6H, CH3), 1.15–1.38 (m, 36H, CH3(CH2)9),
1.41–1.70 (m, 4H, CH2CH2NHC(]O)), 1.83–2.03 (m, 2H,
CH2C*), 2.22–2.41 (m, 2H, NHC(]O)CH2), 3.14–3.30
(m, 4H, CH2NHC(]O)), 3.35–3.47 (m, 1H, *CH), Elemen-
tal analysis. Found: H, 12.2; C, 72.1; N, 8.74. Calcd for
C29H59N3O2: H, 12.3; C, 72.3; N, 8.72%.
Figure 6. Schematic illustration of optically active solid polymer film.
4.1.3. Preparation of N1,N5-didodecyl-N2-[4-carboxy-
butanoyl]-L-glutamide (1). Compound 1 was synthesized
according to the previous report;11 mp 190–195 ꢀC; Elemen-
tal analysis. Found: H, 9.6; C, 77.5; N, 5.4. Calcd for
C49H73N3O3: H, 9.8; C, 78.2; N, 5.6%.
4. Experimental
4.1. General information
All chemicals were reagent grade and purchased from chemi-
cal suppliers. The polymerization degrees of PMMA and
polystyrene were 13,500–14,000 and 2700, respectively.
The L-glutamide-derived lipids with pyrene or carboxylic
acid head group were synthesized as shown in Scheme 2.
The chemical structures of these compounds were identified
4.1.4. Preparation of N1,N5-didodecyl-N2-[4-carboxy-
butanoyl]-L-glutamide (2). Compound 5 (2.0 g, 4.2 mmol)
and triethylamine (1.75 ml, 12.6 mmol) were dissolved in
50 ml of THF and glutaric anhydride (0.96 g, 8.4 mmol)
was added to the solution. After being stirred for 1 day at
room temperature the solution was concentrated in vacuo.
The residue was recrystallized from methanol and dried in
vacuo, which gives a white solid powder: yield 0.76 g
(32%); mp 145–147 ꢀC; IR spectrum (KBr) 3294 (N–H),
2920 (C–H), 2851 (C–H), 1701 (C]O, carboxylic acid),
1635 (C]O, amide), 1546 (N–H); 1H NMR (CDCl3,
400 MHz) d 0.87 (t, J¼6.84 Hz, 6H, CH3), 1.15–1.38
1
by melting point, FTIR, H NMR, and elemental analysis.
Melting points were determined on a micro melting point ap-
paratus. FTIR spectra were performed on a JASCO FT/IR-
4000 spectrometer. H NMR spectra were recorded by a
1
JEOL JNM-EX400 spectrometer using tetramethylsilane as
an internal standard. The following abbreviations are used:

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M. Takafuji et al. / Tetrahedron 63 (2007) 7489–7494
Scheme 2. Synthetic procedure of L-glutamide-derived lipids.
3. Hafkamp, R. J. H.; Kokke, B. P. A.; Danke, I. M.; Geurts,
H. P. M.; Rowan, A. E.; Feiters, M. C.; Nolte, R. J. M. Chem.
Commun. 1997, 545–546.
4. (a) Ono, Y.; Nakashima, K.; Sano, M.; Kanekiyo, Y.; Inoue, K.;
Hojo, J.; Shinkai, S. Chem. Commun. 1998, 1477–1478; (b)
Jung, J. H.; Ono, Y.; Shinkai, S. Langmuir 2000, 16, 1643–
1649.
(m, 36H, (CH2)9), 1.90–2.17 (m, 4H, CH2C*,
CH2CH2C(]O)OH) 2.20–2.54 (m, 6H, NHC(]O)CH2,
NHC(]O)CH2CH2CH2C(]O)OH), 3.11–3.34 (m, 4H,
CH2NHC(]O)), 4.40–4.45 (m, 1H, *CH). Elemental analy-
sis. Found: H, 11.0; C, 67.1; N, 6.94. Calcd for C34H65N3O5:
H, 11.0; C, 68.5; N, 7.05%.
5. (a) Kobayashi, S.; Hamasaki, N.; Suzuki, M.; Kimura, M.;
Shirai, H.; Hanabusa, K. J. Am. Chem. Soc. 2002, 124, 6550–
6551; (b) Suzuki, M.; Nakajima, Y.; Sato, T.; Shirai, H.;
Hanabusa, K. Chem. Commun. 2006, 377–379.
6. (a) Chan, C. L.; Wang, J. B.; Yuan, J.; Gong, H.; Liuand, Y. H.;
Liu, M. H. Langmuir 2003, 19, 9440–9445; (b) Sone, E. D.;
Zubarev, E. R.; Stupp, S. I. Angew. Chem., Int. Ed. 2002, 41,
1705–1709.
7. (a) Hatano, T.; Bae, A.-H.; Takeuchi, M.; Fujita, N.; Kaneko,
K.; Ihara, H.; Takafuji, M.; Shinkai, S. Angew. Chem., Int.
Ed. 2004, 43, 465–469; (b) Hatano, T.; Bae, A.-H.; Takeuchi,
M.; Fujita, N.; Kaneko, K.; Ihara, H.; Takafuji, M.; Shinkai,
S. Chem.—Eur. J. 2004, 10, 5067–5075.
4.2. Measurements
Transmission electron micrographs were recorded by using
a JEOL JEM-2000FX. The samples were spotted on poly-
˚
vinylformal-coated copper grids (200 A). After excess of
the samples was removed by a tissue paper and air-dried,
they were stained with 2 wt % aqueous ammonium molyb-
date. UV–vis and CD spectra were recorded on a JASCO
V-560 and JASCO J-725 spectrometers, respectively, and
temperature was controlled by Peltier thermostatted cell
holder.
8. Ihara, H.; Hachisako, H.; Hirayama, C.; Yamada, K. J. Chem.
Soc., Chem. Commun. 1992, 1244–1245.
Acknowledgements
9. Takafuji, M.; Ihara, H.; Hirayama, C.; Hachisako, H.; Yamada,
K. Liq. Cryst. 1995, 18, 97–99.
10. Takafuji, M.; Ishiodori, A.; Yamada, T.; Sakurai, T.; Ihara, H.
Chem. Commun. 2004, 1122–1123.
This work was supported by the Grant-in-Aid for Scientific
Research from the Ministry of Education, Science, Sports
and Culture of Japan.
11. Sagawa, T.; Fukufawa, S.; Yamada, T.; Ihara, H. Langmuir
2002, 18, 7223–7228.
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