Transcription of chirality in the organogel systems dictates the enantiodifferentiating photodimerization of substituted anthracene

Article abstract of DOI:10.1002/chem.200902936

An organogelator (G) that contains 2-anthracenecarboxylic acid (2Ac) attached covalently to a gelator counterpart that consists of 3,4,5-tris(ndodecan-l-yloxy)benzoic acid by means of a chiral amino alcohol linkage has been synthesized. G acts as an efficient gelator of organic solvents, including mixed solvents and chiral solvents. Photodimers isolated after the photoreaction of the gel samples display different degrees of stereoselectivity. In the gel state, the formation of head-to-head (h-h) photodimers is always favored over head-to-tail (h-t) photodimers. Enantiomeric excess (ee) values of the major h-h photodimers reached as high as -56% in the case of the gels with enantiomeric glycidyl methyl ethers. Here, the solvent chirality is outweighed by the intrinsic chirality of the gelator molecule. The packing of the chromophore in the gel state has been characterized by the absorption and the emission behaviors and their variations during the course of gel-tosol phase transition. Whereas for the hexane gel, emission intensity increases with an increase in temperature, other systems show a decrease in emission intensity. Redshift of the λ_max in the gel spectra indicates the J-aggregate arrangement of the chromophores. Chiral transcription in the gel state has been investigated by CD spectroscopy, which shows a decrease in CD intensity during the gel-to-sol phase transition. The X-ray diffraction study clearly differentiates among the gels in terms of the order of molecular arrangements. The gel systems are categorized as strong, moderately strong, and weak, that originate from the cooperative or individual participations of intermolecular hydrogen-bonding and π-π interactions, fine-tuned by the solvent polarity and the gelation temperature. A simple model based on the experimental findings and the molecular preorientation as evidenced by the stereochemistry of the photodimers has been proposed.

Full text of DOI:10.1002/chem.200902936

DOI: 10.1002/chem.200902936
Transcription of Chirality in the Organogel Systems Dictates the
Enantiodifferentiating Photodimerization of Substituted Anthracene
Arnab Dawn,[a] Tomohiro Shiraki,[b] Shuichi Haraguchi,[c] Hiroki Sato,[c]
Kazuki Sada,[c] and Seiji Shinkai*[a, b]
Abstract: An organogelator (G) that
contains 2-anthracenecarboxylic acid
(2Ac) attached covalently to a gelator
counterpart that consists of 3,4,5-tris(n-
ty is outweighed by the intrinsic chirali-
ty of the gelator molecule. The packing
of the chromophore in the gel state has
been characterized by the absorption
and the emission behaviors and their
variations during the course of gel-to-
sol phase transition. Whereas for the
hexane gel, emission intensity increases
with an increase in temperature, other
systems show a decrease in emission in-
tensity. Redshift of the lmax in the gel
spectra indicates the J-aggregate ar-
rangement of the chromophores. Chiral
transcription in the gel state has been
investigated by CD spectroscopy, which
shows decrease in CD intensity
a
during the gel-to-sol phase transition.
The X-ray diffraction study clearly dif-
ferentiates among the gels in terms of
the order of molecular arrangements.
The gel systems are categorized as
strong, moderately strong, and weak,
that originate from the cooperative or
individual participations of intermolec-
ular hydrogen-bonding and p–p inter-
actions, fine-tuned by the solvent polar-
ity and the gelation temperature. A
simple model based on the experimen-
tal findings and the molecular preor-
ientation as evidenced by the stereo-
chemistry of the photodimers has been
proposed.
dodecan-1-yloxy)benzoic
acid
by
means of a chiral amino alcohol link-
age has been synthesized. G acts as an
efficient gelator of organic solvents, in-
cluding mixed solvents and chiral sol-
vents. Photodimers isolated after the
photoreaction of the gel samples dis-
play different degrees of stereoselectiv-
ity. In the gel state, the formation of
head-to-head (h–h) photodimers is
always favored over head-to-tail (h–t)
photodimers. Enantiomeric excess (ee)
values of the major h–h photodimers
reached as high as À56% in the case of
the gels with enantiomeric glycidyl
methyl ethers. Here, the solvent chirali-
Keywords: chiral induction · enan-
tioselectivity · organogels · photodi-
merization
chemistry
·
supramolecular
Introduction
synthesis, mostly because of the nature of the short-lived
weak interactions involved in the excited state. In this con-
text, supramolecular photochirogenesis has become a prom-
ising interdisciplinary field of research, in which chiral infor-
mation is induced, monitored, or controlled through the
principles of a supramolecular approach that involves differ-
ent types of noncovalent interactions.[1] Among the different
stereochemical processes, photodimerization of anthracene
is one of the oldest known photochemical reactions.[2] The
stereochemistry of unsymmetrically substituted anthracenes
such as 2-anthracenecarboxylic acid (2Ac) has been less ex-
plored until recently, probably due to the complex nature of
their photoproducts, which include four [4+4] photocyclo-
dimers: anti and syn head-to-tail (h–t, A and B) and anti
and syn head-to-head (h–h, C and D), among which B and
C are chiral (Scheme 1). In a more positive light, however,
one can consider this complexity to be a probe to explore
and monitor the molecular phenomena involved in the ste-
Photochirogenesis still remains a big challenge for chemists
in spite of recent advancements in ground-state asymmetric
[a] Dr. A. Dawn, Prof. S. Shinkai
Department of Nanoscience, Faculty of Engineering
Sojo University, 4-22-1 Ikeda, Kumamoto 860-0082 (Japan)
Fax : (+81)92 805 3814
E-mail: shinkai_center@mail.cstm.kyushu-u.ac.jp
[b] Dr. T. Shiraki, Prof. S. Shinkai
Institute of Systems
Information Technologies and Nanotechnology (ISIT)
203-1 Moto-oka, Nishi-ku Fukuoka 819-0385 (Japan)
[c] S. Haraguchi, H. Sato, Prof. K. Sada
Department of Chemistry and Biochemistry
Graduate School of Engineering, Kyushu University
744 Moto-oka, Nishi-ku, Fukuoka 819-0395 (Japan)
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.200902936.
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Chem. Eur. J. 2010, 16, 3676 – 3689

FULL PAPER
valent interactions, which in-
clude electrostatic, hydrogen-
bonding, p–p stacking, and van
der Waals interactions. Such a
highly ordered molecular envi-
ronment is expected to provide
an ideal platform for perform-
ing a stereoselective process.
On the other hand, quantifying
the transcription of chiral mes-
sage in a supramolecular gel
system by means of analyzing
the photoactive chromophore
as a functional probe might
give us real scope to “esti-
mate” the efficiency of such a
hierarchical process. In spite of
the presence of several chirally
well-defined gel systems, such
Scheme 1. Schematic presentation of [4+4] photocyclodimerization of 2-anthracenecarboxylic acid in the free
state, and gelator G used in this study.
a
quantitative result is still
missing. Motivated by the
reochemical course of a process, especially in terms of mo-
lecular arrangement, fine-tuned by noncovalent interactions
that participate in supramolecular chirogenesis. Substrate
preorientation prior to photoreaction, governed by means of
the supramolecular interactions, has recently been exploited
to achieve highly regio- and enantioselective photodimeriza-
tion of 2Ac in solution,[3] in the solid state[4] by using the in-
herently chiral cavity of g-cyclodextrin derivatives, and in
two-component liquid crystalline medium as well.[5] Howev-
er, all are sensitive to the design or fine-tuning of the appro-
priate host molecules.
Thinking in a different way, we have noticed that in the
supramolecularly assembled systems that are produced from
the low-molecular-weight gelators (LMGs), the chirality
within an individual molecule can be transcribed to the
nano- and mesoscale fibrous assembly by means of hierarch-
ical self-assembly process.[6] So far, such types of soft materi-
als have scarcely been employed in the stereochemical pro-
cess except in some isolated preliminary examples such as
photoinduced isomerization of azobenzenes,[6a,7] the devel-
opment of photoresponsive LMGs that contain stilbene[8] or
substituted anthracene,[9] or the study of the gelation behav-
ior of monomeric and dimeric derivatives of anthracene.[10]
There is also an earlier example in which a cholesterol-
based organogelator containing 2-substituted anthracene
was developed and the effect of photodimerization in the
gel, liquid crystalline, and isotropic phases were studied.[11]
In another example, the principle of capturing the translated
chiral information has been used to produce helical silica
fibers by the polymerization of tetraethoxysilane in the
chiral gel template.[12] Even in these cases, however, an at-
tempt has never been made to analyze the final stereochem-
istry precisely or to estimate the degree of chiral informa-
tion transcribed. The principles of organogel formation are
considered to be through the one-dimensional alignment of
the gelator molecules supported by different types of nonco-
above prospect, our group recently has developed a two-
component organogelator system containing a 2Ac molecule
attached noncovalently to a gelator counterpart that con-
tains a long alkyl chain substituted gallic acid backbone and
a chiral alanine moiety. The photodimerization of this
system in the gel phase provided an unprecedented stereose-
lectivity and produced only h–h photodimers.[13] However,
only a moderate enantiomeric excess (ee) could be achieved
during the process.
To fulfill the chirality factor, which is indispensable for
the application of such a soft template for chirogenesis, here
we have designed a 2Ac-bearing chiral organogelator G
(Scheme 1). Gel samples have been prepared by using dif-
ferent organic solvents. After photodimerization, the 2Ac
moiety could easily be cleaved from the gelator backbone
by hydrolysis. The photoproducts were analyzed and the
photodimer distributions were determined precisely by
using chiral HPLC. Remarkably, in the gel states, we ob-
tained variable ee values that ranged from low to high, de-
pending on the solvents. The assembly of the gelator mole-
cules prior to the photoreaction has been investigated from
absorption and emission behaviors with varying tempera-
tures. For the first time, we have observed here that the phe-
nomenon of emission enhancement occurs during two oppo-
site thermal events, gel-to-sol and sol-to-gel phase transi-
tions, with the same gelator systems differentiated by the
solvents only. The transmission of the chiral information
within the supramolecular assembly has been investigated
by CD spectroscopy. The structural elucidation is carried
out based on the X-ray diffraction, spectroscopic data, and
analysis of the photodimer distributions. Isotropic and mo-
lecularly aggregated systems were also investigated simulta-
neously to demonstrate the inherent selectivity and chirality
in the gel state. The mechanistic background of the chiral
selection process has been discussed and correlated with the
different aspects of the gel chemistry.
Chem. Eur. J. 2010, 16, 3676 – 3689
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3677

S. Shinkai et al.
Results and Discussion
Investigation of different gel systems and their selection to
perform the photodimerization process: Gelator G consists
of a 2Ac moiety attached covalently to a gallic acid back-
bone by means of a chiral amino alcohol linkage. In G, the
2Ac molecule is linked directly to the chiral center by an
ester bond so that pronounced chiral induction can be trans-
mitted to the ground-state orientation of the 2Ac moiety. G
is expected to be capable of forming a one-dimensional mo-
lecular arrangement by means of intermolecular hydrogen-
bonding through the amide linkage, p–p stacking interaction
of the anthracene and benzene ring (in the gallic acid part),
or even by the van der Waals interaction involved in long
alkyl chains. The gelation ability of G has been tested in dif-
ferent organic solvents of varying polarity, including mixed
solvents and chiral solvents (Table S1 in the Supporting In-
formation). Firstly, to differentiate the systems precisely in
terms of the solvent polarity, we have used mixed solvent
systems. Secondly, to investigate the probability of inducing
chirality directly by the solvent molecules, we have consid-
ered gel systems composed of enantiomeric and racemic va-
rieties of the solvent, glycidyl methyl ether.
Keeping the versatility of the solvents in mind, we have
selected the gel systems (Table 1) for performing the photo-
reaction. It can be seen that among the different systems,
only hexane can form a gel at room temperature. All other
gels have been prepared by quenching below room tempera-
ture from 5 to 08C, depending upon the solvents. However,
after formation, the gels G-Mc, G-Mc/Hx, G-Mc/Ch, and G-
Ch/Hx were found to be stable (to an inversion of the test
tube) even at room temperature. To elucidate the inherent
strength and stability of the gel systems, we have examined
their gel melting temperatures (Tgel). The results are pre-
sented in Figure 1. The phase above each curve is the sol,
whereas the phase below each curve is the gel. In general,
the Tgel values increase with an increase in the gelator con-
centration and finally tend to saturate. From these plots, one
can clearly notice three distinct zones along the Tgel axis.
Based on this observation, we have preliminarily catego-
rized different gel systems as strong, such as G-Hx; moder-
ately strong, such as G-Mc, G-Mc/Hx, G-Mc/Ch, and G-Ch/
Hx; and weak gels like G-Ge, G-RGe, and G-SGe. The
values of critical gelation concentration (CGC) and quench-
ing temperatures also justify such classifications.
Figure 1. Plots of Tgel versus concentration for the gel systems with gela-
tor G.
The enantiomeric solvent systems G-RGe and G-SGe
give almost identical plots that also closely resemble those
of the racemic variety. That means that the opposite chirali-
ties of the enantiomeric solvents cannot influence the gela-
tion ability of G. In the series of moderately strong gels, we
can arrange the different solvent systems in order of increas-
ing polarity: Ch/Hx<Mc/Hx<Mc/Ch<Mc, which is indeed
reflected in the plots. Throughout the text, we shall discuss
the different gel systems with their representative sample
names (Table 1) and also when necessary we shall introduce
new solvent systems for the discussion depending upon their
relevancy. All the forthcoming experiments including the
photodimerization process have been carried out using the
same gelator concentrations as indicated in Table 1 unless
otherwise noted, so that we can safely discuss and correlate
among different experimental results.
Morphology of the gel samples: The morphology of the gel
samples provides us with direct evidence about the existence
of a one-dimensional molecular arrangement. Before pro-
ceeding to the photoreaction step, therefore, we have per-
formed SEM studies of the xerogels as presented in Figure 2
(corresponding TEM images are presented in Figure S1 in
the Supporting Information). A one-dimensional fibrillar ar-
rangement is evident in all the images. The fibrillar diame-
ters for the different samples lie in the range of 85 to
102 nm on average. The minute variations in the morpholo-
gies are likely to originate from the different degrees of par-
ticipation of the noncovalent interactions responsible for
creating the one-dimensional
molecular arrangement. The
Table 1. Specifications of the investigated gel systems of G.
[c]
relative solvent polarities prob-
ably play the major role in this.
TEM images of G-RGe and
G-SGe (Figure S1) give some
hints of short-range twisted
fibers. Efforts to correlate
these findings with the other
results will be made in the
forthcoming sections. Howev-
er, it is clear that the above
System
Sample
Solvent
CGC[a]
[% w/v]
[G][b]
[% w/v]
Tq
G
R
[8C]
1
2
3
4
5
6
7
8
G-Hx
G-Mc
n-hexane
methylcyclohexane
0.2
1
1
1
1
25
5
5
5
5
0
0
0
0.25
0.25
0.25
0.25
0.8
G-Mc/Hx
G-Mc/Ch
G-Ch/Hx
G-Ge
G-RGe
G-SGe
methylcyclohexane/n-hexane (1:1 v/v)
methylcyclohexane/cyclohexane (1:1 v/v)
cyclohexane/n-hexane (1:1 v/v)
glycidyl methyl ether
(R)-glycidyl methyl ether
(S)-glycidyl methyl ether
1
1.5
1.5
1.5
1.0
1.0
[a] CGC=critical gelation concentration. [b] [G]=Concentration of G used. [c] Tq =quenching temperature.
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Chirality in Organogel Systems
FULL PAPER
Figure 2. SEM pictures of the xerogels prepared from gelator G with varying solvents: a) G-Hx, b) G-Mc, c) G-Mc/Ch, d) G-Mc/Hx, e) G-Ch/Hx, f) G-
Ge, g) G-RGe, and h) G-SGe.
morphological investigation proves not only the presence of
a perfect molecular platform to initiate a stereochemical
process, but also the scope to fine-tune the selectivity in the
photochemical process by the solvent-induced molecularly
different gelator arrangements.
tor backbone. In the next steps, the photoproducts were iso-
lated and finally subjected to the HPLC analysis (the details
of the process are described in the Experimental Section).
The relative yields and ee values for the different photodim-
ers were calculated from the HPLC chromatogram (Fig-
ure S3 in the Supporting Information).
Stereochemistry of 2Ac photodimers produced from the
photodimerization of G in the gel matrices: The gel samples
prepared under inert conditions were photoirradiated at a
wavelength of 366 nm at constant temperature. Photoreac-
tion is not reversible at this wavelength. As the samples
were photoirradiated, the intensity of the 1La absorption
band of monomeric 2Ac gradually decreased (Figure S2 in
the Supporting Information), thus signifying their conver-
sion to the dimer. Simultaneously, the physical state changed
from gel to sol. Since the photoreaction temperature re-
mained below the gel melting temperature, the observed
phase transition should be ascribed to a photoinduced pro-
cess. After the reaction the solvents were first removed and
then the samples were hydrolyzed in alkaline medium to
separate 2Ac dimers and unreacted monomer from the gela-
The distribution of the photodimers and corresponding ee
values obtained from different gel systems are summarized
in Table 2. In general, all the gel samples produce h–h pho-
todimers as the major products. It is worth mentioning here
that under the isotropic conditions, h–t photodimers are
always the major products due to their thermodynamic sta-
bility. The abundance of h–h photodimers is almost exclu-
sive to all the gel samples except for the systems with the
chiral solvent (racemic and enantiomeric) glycidyl methyl
ether, in which h–t photodimers are also present in a sub-
stantial fraction. The solvent polarity is probably responsible
for such a differentiation. Unlike nonpolar hexane, cyclo-
hexane, and methylcyclohexane, glycidyl methyl ether is rel-
atively polar in nature. The priority of the formation of h–h
photodimers in the gel state can be explained well in terms
Table 2. Distribution and enantiomeric excess (ee) values of the 2Ac photodimers obtained from [4+4] photocyclodimerization in the gel samples of G.[a]
Sample–state
Solvent
T [8C]
Conv.[b]
[%]
Relative yield [%][c]
ee [%][c]
B
(h–t)
A
B
C
D
A+B
(h–t)
C+D
(h–h)
C
ACHTUNGTRENNUNG(h–h)
G
E
G-Hx–gel
G-Mc–gel
G-Mc–sol
G-Mc/Hx–gel
G-Mc/Ch–gel
G-Ch/Hx–gel
G-Ch–aggregate[d]
G-Ge–gel
n-hexane
methylcyclohexane
5
5
25
5
5
5
10
5
25
5
5
25
25
60
88
47
70
43
81
71
82
75
73
88
9
7
47
50
44
49
50
50
44
30
24
31
31
26
37
43
36
43
43
43
38
24
18
26
25
20
16
7
20
8
7
7
18
46
58
43
44
54
84
93
80
92
93
93
82
54
42
57
56
46
30
36
16
41
30
41
20
12
7
À10
À30
À9
4
10
5
4
4
3
10
3
3
3
methylcyclohexane/n-hexane (1:1 v/v)
methylcyclohexane/cyclohexane (1:1 v/v)
cyclohexane/n-hexane (1:1 v/v)
cyclohexane
À36
À30
À33
À5
9
9
glycidyl methyl ether
28
32
26
27
29
18
26
17
17
25
À44
À1
G-Ge–sol
G-RGe–gel
G-SGe–gel
G-THF[d]
(R)-glycidyl methyl ether
(S)-glycidyl methyl ether
THF
11
12
10
À56
À56
4
[a] Gel samples were photoirradiated for 120 min and all other samples were photoirradiated for 30 min. [b] Conv.=conversion. [c] The absolute configu-
rations of B and C were not determined. The first eluted enantiomer is given a positive sign. Errors in relative yields are Æ0.5% and in ee values are
Æ1% for major products and Æ2% for minor products. [d] Gelator concentration: 1% w/v.
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S. Shinkai et al.
of the ground-state preorientation of the gelator molecules.
Intermolecular hydrogen-bonding and p–p interactions
should be the key factors that construct and control the one-
dimensional arrangement of the gelator molecules in the gel
state. Whereas the former is favored in h–h molecular orien-
tation, the latter can be operative in both kind of orienta-
tions, h–h and h–t. In nonpolar solvents, these two factors
operate cooperatively, thereby allowing h–h orientation
almost exclusively. On the other hand, with a relatively
polar solvent like glycidyl methyl ether, intermolecular hy-
drogen bonding is less favored, thereby resulting in some
h–t photodimers as well. These results also resemble our
previous studies.[13,14] That means that the preferential for-
mation of h–h photodimers in the gel state is now a well-
documented phenomenon.
discussion, we can explain this by the compactness of the ge-
lator arrangement in terms of the Tgel values. The variation
in ee values versus the corresponding Tgel values is depicted
in Figure 3. Here again, one can see three distinct domains
The next and most important issue for the “chirogenesis”
would be the chirality factor expressed in terms of ee values.
It is expected that the presence of excess chirality (in other
words, ee) in the photodimers is governed by the inherent
chirality of G, which is capable of differentiating preferen-
tially among the preorientations of the gelator molecules in
the ground state. However, it is remarkable that, depending
upon the solvent, the % ee values displayed by the gel sys-
tems vary from À10 (minimum) to À56 (maximum) for the
major chiral dimer C (h–h), and from +11 (minimum) to
+41 (maximum) for the minor chiral dimer B (h–t). Such a
high chiral excess value has never been achieved when using
a soft template like an organogel. This unprecedented find-
ing means that the present gel systems are truly capable of
leading an enantiodifferentiating process. Careful observa-
tion of the data reveals that the binary solvent system (Ch/
Hx) gives higher ee values relative to that of the correspond-
ing single-solvent system (Mc). In addition, considering ee
values for both the photodimers B and C, the chiral induc-
tion is most prominent for the binary solvent system Mc/Hx.
For the highly stable and strong gel system G-Hx, the ee
value for h–h photodimers is remarkably small, even though
it possesses moderately high ee values for h–t photodimers.
On the other hand, G-RGe produces the highest ee values
for h–h photodimers, even though it gives the lowest ee
values for h–t photodimers. The origin of such discrimina-
tions is probably the presence and the absence of intermo-
lecular hydrogen bonding for h–h and h–t molecular orienta-
tions, respectively. Unlike the p–p stacking interactions, the
hydrogen bonding is really sensitive to the solvent polarity.
Thus, it is understandable that in the series of nonpolar sol-
vents with little variation, ee values for the h–t photodimers
do not change too much. However, in relatively polar sol-
vent systems with glycidyl methyl ether (categorized as
weak gels), the lack of favorable hydrogen-bonding interac-
tions weakens the systems, especially the domain that con-
sists of h–t molecular orientation, which is dependent solely
on p–p stacking interactions and the compactness of the
neighborhood. Here a big question comes to mind. Why
does the strong gel (G-Hx) produce the lowest ee values,
whereas the weak ones (G-Ge/G-RGe/G-SGe) provide the
highest values for the major photodimers? At this stage of
Figure 3. Plots of the ee values obtained from the photodimers produced
in the gel state against the corresponding gel melting temperature
*
(dotted rectangular boxes indicate different zones in the plot): , h–h
photodimer C; &, h–t photodimer B.
(as indicated by the dotted rectangular boxes) that belong
to strong, moderately strong, and weak gel systems based on
the corresponding Tgel values. At this point, we should con-
sider the gelation temperature (to initiate the gelation) too,
because in principle it should follow a similar trend as that
of the corresponding Tgel. Whereas the strong gel is formed
at room temperature, others are formed only at low temper-
atures (Table 1). The energy difference between the relative
orientations of 2Ac moieties that give enantiomeric photo-
dimers should be very small. Such a differentiation is ex-
pected to become more prominent at low temperature. This
is the reason why the ee values for h–h photodimers follow
such an unexpected trend. This phenomenon is nicely sup-
ported by the similar ee values displayed by the gels formed
under the similar quenching conditions. For h–t photodim-
ers, the scenario is completely different, especially due to
the lack of intermolecular hydrogen-bonding sites. Here, the
environmental rigidity plays the key role in facilitating the
excess chirality in the photodimers. Significantly, chiral h–h
photodimers (i.e., C) have the highest relative yield in the
gel state, thereby implying the environmental preference for
chiral moieties. More detailed and informative discussions
about such a chiral selection process will be made in the
later sections.
To compare the results obtained from the gel state with
the other isotropic states, we have performed similar photo-
reactions in two additional environments. The sol states of
the corresponding gel states for the systems G-Mc and G-
Ge (as representatives) give an increased amount of h–t
photodimers for the former system, and for the latter
system, the selectivity is reversed to give h–t photodimers as
major product. However, the presence of a high percentage
of h–h photodimers even in the sol state for the system G-
Mc indicates that some sort of molecular association is still
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Chem. Eur. J. 2010, 16, 3676 – 3689

Chirality in Organogel Systems
FULL PAPER
taking place therein. Actually, it may not be all that unusual
to consider the possibility of hydrogen-bonded molecular as-
sociation in such a nonpolar solvent. To support our predic-
tion, we have performed similar experiment with cyclohex-
ane (we shall denote the system as G-Ch), which forms only
the molecular aggregate (Figure S4 in the Supporting Infor-
mation) rather than the macroscopic gel. This system gives a
similar result to those of the sol systems. That means the
molecular association is the key factor for the high degree
of chiral selection in the gel phase. The decrease in ee
values in the sol and the aggregate systems justifies the
same. The lowest ee value for G-Ge–sol system is evident
due to the absence of any significant fraction of molecular
association by means of the intermolecular hydrogen bond-
ing in a relatively polar solvent environment, particularly at
elevated temperatures. Similar photodimer distribution, in-
cluding the ee values, is obtained from using THF, a nonge-
lating solvent. From the above discussion, there is no doubt
that the stereochemical and chiral selectivity is inherent in
the gel system in which the small difference in molecular
orientations is amplified through the tight molecular packing
mode.
In spite of the fact that the gelator chirality is likely to be
the origin of excess chirality in the photodimers, we still ex-
pected some degree of perturbation from the enantiomeric
solvent pair based on the preferential interaction of G with
a particular enantiomer. To our surprise, both of the enan-
tiomeric solvent systems provide almost identical photodi-
mer distributions, including ee values with the same “sign.”
This result means that solvent chirality has no influence on
the preorientation of the gelator molecules. In other words,
the gelator chirality apparently outweighed the effect of sol-
vent chirality.
To examine the sensitivity of photodimer distribution to-
wards the different physical parameters, we have performed
a series of photoreactions by tuning the external factors. For
a given range of concentrations, the product distribution re-
mains almost unaffected for a particular solvent system
(Table S2 in the Supporting Information). We have also ex-
amined the effect of quenching conditions during the forma-
tion of gel on the photodimer distribution (Table S3 in the
Supporting Information). Here again, the photodimer distri-
butions show minimal dependency on the quenching rate
and even on the quenching temperature. We also have
varied the relative proportions of the solvents in a given
binary solvent system (Table S4 in the Supporting Informa-
tion). The results show little variation in stereo- and enan-
tioselectivities, probably due to the minute differences in
the resultant solvent polarities. From the above findings, it is
worth noting that the current gel systems are less sensitive
to the environmental perturbations. In other words, the final
stereochemistry is predetermined and maintained in a par-
ticular solvent system by immobilizing the orientations of
the gelator molecules. This conclusion is a remarkable ach-
ievement, especially for a delicate process like chiral photo-
reaction.
In the forthcoming sections, we shall try to shed some
light on the different mechanistic aspects that operate indi-
vidually or cooperatively to determine the final stereochem-
istry of the photodimers. Of course, the final molecular ori-
entations analyzed from the stereochemistry of the photo-
dimers will be the key issue, based on which the correlations
among the different experimental findings have to be made.
The absorption and the emission behaviors of the gel sam-
ples and their temperature dependency during the course of
gel-to-sol phase transition: It is a well-established and easily
understandable phenomenon that the physical organogels
are responsive to external stimuli such as temperature,
through which the aggregate structure is stabilized or desta-
bilized. During the stabilization process, the organogel struc-
ture is transformed to a more ordered, self-assembled mode.
On the other hand, in the destabilization process such an ag-
gregate structure is dissociated to reversibly form the molec-
ularly free state, which results in a gel-to-sol phase transi-
tion. Thus, it provides us with a great opportunity to use a
chromophore-bearing organogelator to follow the optical
properties of the resulting assemblies in which the modula-
tion of the chromophore packing involved in a hierarchical
process directly influences the absorption and emission be-
haviors. Keeping in mind the above points, we have investi-
gated the absorption and emission phenomena of the differ-
ent solvent systems in the gel state (maintaining the condi-
tions close to those of the photoreaction experiments) and
also during the gel-to-sol phase transition with varying tem-
perature. Among eight gel systems we had initially selected,
the absorption and emission spectra (samples were excited
at 400 nm) of the four representative solvent systems are
presented in Figure 4. For the purposes of discussion, and
where relevant, the rest of the spectra are presented in the
Supporting Information. To study the absorption spectra,
optically diluted samples (0.75 and 0.5% w/v for G-Ge and
other gels, respectively) have been used. Here, we have
1
monitored the La band of the anthracene moiety. The ab-
sorption spectra follow a similar response to the tempera-
ture enhancement for all the samples. The gel spectra are
always less structured, and the absorption maxima are also
redshifted relative to the sol phase attained at higher tem-
peratures. One can consider this trend as aggregation-in-
duced broadening, whereas the fine structure remains essen-
tially the same. However, the G-Ge gel spectrum shows
only a little deviation relative to that of the free state. The
polar solvent probably limits the participation of hydrogen
bonding, which is expected to be more sensitive to the tem-
perature variation. In spite of that, the appearance of a
small shoulder in the high-wavelength region is quite clear,
and it also tails over 450 nm. Similar bands are visible for all
other systems. Though the exact nature of the self-assem-
bled aggregates formed in a gel structure is difficult to es-
tablish unambiguously, the redshifted spectral features sug-
gest that the J-type aggregate structure is likely to be
formed in the assembled state.[15]
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S. Shinkai et al.
Figure 4. Representative absorption and emission spectra of the gel samples: a,b) G-Hx; c,d) G-Mc; e,f) G-Ch/Hx; g,h) G-Ge with varying temperatures.
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Chirality in Organogel Systems
FULL PAPER
The response of the emission spectra to thermal stimula-
tion is even more drastic. In general, the gel-state emission
spectra are broad and almost structureless. Similar to the ab-
sorption spectra, here, too, the lmax is redshifted in the ag-
gregate state relative to that of the free state, thereby sup-
porting the presence of a J-aggregate structure of the chro-
mophore. The driving forces for the chromophore organiza-
tion are expected to be the hydrogen-bonding and p–p in-
teractions. Here it should be kept in mind that unlike p–p
stacking, the direct hydrogen bonding between two 2Ac
molecules is not possible, as they are coupled with the gela-
tor counterpart by means of the ester linkage. However, the
amide group present in the gelator backbone can associate
two units by intermolecular hydrogen bonding. The pres-
ence of the hydrogen-bonding motif away from the 2Ac
moiety provides some sort of flexible microenvironment
near the chromophore. Maybe this is the reason for the
broad nature of the emission spectra. In support of this, G-
Hx–gel (categorized as a strong gel), which bears the most
favorable hydrogen-bonded molecular association produced
from the least polar hexane, displays relatively well-defined
emission spectra relative to the other systems. At the same
time, the presence of two bands that correspond to the ag-
gregate species (at a higher wavelength) and molecularly
isolated species (at a lower wavelength) for the G-Hx
system, even at 608C, support the view that still not all the
chromophores are in the molecularly free state. This indeed
reflects the rigidity of the gel structure. To generalize the
idea, we have investigated the absorption and the emission
behavior of gelator G dissolved in cyclohexane (G-Ch),
which does not show any gel formation (Figure S4 in the
Supporting Information). However, due to the nonpolar
nature (similar to hexane and cyclohexane) of the solvent,
we still expected the formation of molecular aggregates, par-
ticularly at lower temperatures. Modification of the absorp-
tion and the emission spectra with varying temperature
points to our prediction. The presence of an emission peak
that corresponds to the free chromophore, even at low tem-
peratures, and the smaller degree of deviation relative to
the other gel samples is justified when one considers the
system G-Ch as an intermediate between highly organized
gel and molecularly free solution state. As expected from
the previous investigations, the enantiomeric solvent systems
behave similarly (Figure S5 in the Supporting Information).
Also, the binary solvent systems show little difference in the
emission spectra (Figure S6 in the Supporting Information)
because of the similar resultant solvent polarities.
the aggregate state is explained by the “aggregation-induced
enhanced emission” in which the restriction of intramolecu-
lar rotation is identified as the main cause.[17] In another ex-
ample, the luminescence enhancement of metallogels at ele-
vated temperatures has been explained in terms of the
higher degrees of freedom and motion of the molecules at
higher temperatures, thereby enhancing the chance of inter-
molecular aggregate formation to populate excimer forma-
tion.[18] However, neither of these phenomena may be fully
applicable to our systems, as we found opposite behaviors in
the systems that have the same gelator molecule but differ-
ent solvents. Therefore, a more meaningful choice would be
to explain things in terms of the differentiating microenvir-
onments created in the vicinity of the chromophore moiet-
ies.
To distinguish among the nature of molecular assemblies
involved in the gel systems, we have plotted the emission in-
tensities against the temperature (Figure 5). It is worth
Figure 5. Plots of the emission intensity against the temperature for the
gel samples with gelator G.
noting that even in this plot there are three distinct zones
that consist of the three types of gel samples (strong, moder-
ately strong, and weak). All the samples show a single-step
sigmoidal transition, thus indicating the melting of the ag-
gregates to the corresponding isotropic states.[19] The influ-
ence of temperature is mostly prominent for the moderately
strong gel systems for which the emission enhancements are
six- to eightfold higher than the corresponding sol states.
Enhancements are only threefold higher only for the glycid-
yl methyl ether solvent systems or the weak gels. As expect-
ed, G-RGe and G-SGe behave almost identically and also
similar to their racemic counterpart G-Ge. In the sol state
that usually occurs at elevated temperatures, the molecularly
free species of the gelator is conformationally flexible,
which typically allows the faster and more efficient nonra-
diative decay from the excited state. Formation of the gel
upon molecular association reduces the conformational flex-
ibility and consequently slows down the nonradiative decay
process, thereby resulting in enhanced emission.[15f] Howev-
er, the above interpretation may be applicable to all systems
except G-Hx. For G-Hx, the packing of the chromophore is
strongest where we can apply the classical theory of concen-
During the investigation of the optical properties, the
most remarkable finding was the completely opposite re-
sponses of the emission property of the gel samples during
the gel-to-sol phase transition. In the G-Hx system, the
emission intensity increases with the increase in tempera-
ture, whereas for all other systems it decreases as a result of
temperature enhancement. This unusual behavior is rarely
encountered, as it represents a sharp contrast to that exhib-
ited by other typical thermotropic organogels.[16] For a class
of silole fluorogens, the enhancement of the fluorescence in
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S. Shinkai et al.
tration quenching caused by the formation of excimers and
exciplexes aided by the interactions between the aromatic
molecules in the excited and ground states.[20] This situation
is very close to the solid-state fluorescence quenching. In
moderately strong gel systems, the chromophores are placed
in a comparatively flexible microenvironment that does not
allow the concentration quenching due to the lack of chro-
mophore proximity and at the same time reduces the proba-
bility of collisional quenching that is likely to occur at
higher temperatures in the sol state. We can assume, there-
fore, that with an increase in temperature, G-Hx undergoes
a transition from a tightly packed state to a conformational-
ly flexible (however, still assembled) state causing the en-
hancement of the emission intensity. Here, the strength of
the intermolecular association overcomes the effect of ther-
mal agitation, which generally favors the nonradiative decay
at elevated temperatures. It is remarkable that even at 608C
the emission enhancement of G-Hx does not plateau (we
did not increase the temperature further due the possibility
of concentration modification over the boiling point of the
solvent). It clearly demonstrates a very high degree of mo-
lecular association. As expected, the weak gel systems
behave similarly to the moderately strong gels with a lesser
extent of emission modification. To the best of our knowl-
edge, this is the first example in which a single gelator mole-
cule executes completely different emission behavior based
on the differentiating microenvironments provided by the
solvent polarity that fine-tunes the supramolecular interac-
tions.
Transcription of molecular chirality in the gel systems: The
high ee values of the photodimers, which result from the gel-
state photoreaction, encouraged us to investigate the origin
of such chiral induction with the help of CD spectroscopy.
Representative CD spectra of different gel systems with
varying temperatures prior to the photoreaction are present-
ed in Figure 6. All the gels show an intense CD signal in the
wavelength range of 2Ac absorption. Though much weak-
ened, the CD signals still remain even in the sol state at ele-
vated temperatures. That signifies the molecular chirality of
G. However, the enhancement of the CD signals in the gel
state definitely points to the existence of supramolecular
chirality that originates from the molecular chirality, tran-
scribed through the molecular assembly by means of the
noncovalent interactions. The striking difference of G-Hx
CD spectra compared to other gel systems lies in, firstly, the
“sense” (or handedness) of chirality; secondly, very high el-
lipticity value; and thirdly, the well-defined “structure” of
the spectra. At present, it is not fully clear to us why the
sign of the CD signals inverts for the G-Hx system. Howev-
er, it may be feasible to explain this phenomenon based on
the final stereochemistry of the photodimers. Here, we have
considered that in the tightly packed gel phase, the ground-
state gelator arrangement does not alter in the short-lived
excited state during the photoreaction. From the analysis of
the photodimers, we could see that the signs of ee values
that correspond to h–h and h–t photodimers are opposite
(the absolute configurations are not determined). This
means that in the ground state, the directions of the resul-
Figure 6. Representative CD spectra of the gel samples: a) G-Hx, b) G-Mc, c) G-Ch/Hx, and d) G-Ge with varying temperatures.
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Chirality in Organogel Systems
FULL PAPER
tant operating dipoles are likely to be opposite for h–h and
h–t molecular orientations. Being major in the gel state, h–h
molecular orientation is expected to determine the resultant
CD signal. This is probably the reason why all the gel sys-
tems (except G-Hx) show “positive” CD signals. Our as-
sumption is supported by the fact that G-Ge gel comprised
of both h–h and h–t molecular orientations in competitive
fractions (from the HPLC study) produces a comparatively
weak, less-structured and an inconsistent variation of CD
signal with temperature. However, in strong G-Hx gel domi-
nated by the highly packed and conformationally rigid h–h
molecular orientation, the moderately flexible domain that
consists of h–t molecular orientation (lacking intermolecular
hydrogen bonding) catalyzes and transcribes the chirality
through the supramolecular interactions. The presence of
higher ee values for h–t photodimers than the h–h photo-
dimers justifies the situation. The high intensity of the CD
signal probably results from the long-range ordered struc-
ture of the G-Hx gel. The possibility of the CD artifact orig-
inating from the macroscopic anisotropy of the chromo-
phore was tested by measuring the linear dichroism (LD)
spectra of the gel samples under similar conditions. Alhough
a nonzero LD spectrum warns of possible CD artifacts, the
weak intensity compared to the total observed CD signal at
least ensures a confident discussion of the chirality.
meric solvent systems (Figure S7 in the Supporting Informa-
tion). This clearly demonstrates that the inherent molecular
chirality of G overcomes the effect of solvent chirality. How-
ever, with glycidyl methyl ether gel systems, the reason for
an unusual hike in the CD signal near 158C is not clear to
us. In none of the above samples could nanoscale chirality
be visualized by electron microscopy, probably due to the
absence of any helical twist in the fibrillar assemblies.
From the above discussions, it is clear that the molecular
chirality of the gelator molecule is indeed transmitted to the
nanoscale supramolecular chirality through the ordering of
the chromophore in the assembled phase. One may there-
fore expect that a simple correlation might exist between
the degree of supramolecular chirality and the ee values ob-
tained from the photodimers as a result of photodimeriza-
tion. To our surprise, however, the results are just the re-
verse. This means that the supramolecular chirality in the
ground state is not enough to control the final chirality ob-
tained from the photochemical process through the transi-
tion state. It is the molecular chirality that actually deter-
mines the preorientation of the gelator molecules depending
upon other physical factors that control the hierarchical pro-
cess. This might be the reason for the low ee values obtained
in our previous noncovalent approach.[13] The possible corre-
lations of the molecular chirality with other factors will be
discussed in the latter sections.
On account of the thermal responsiveness of self-assem-
bled fibers, variable-temperature CD spectroscopy provides
an ideal approach for demonstrating the formation of nano-
scale chiral aggregates. In Figure 7, the ellipticity values of
Structural investigation: To differentiate among the molecu-
lar arrangements in different gel samples, we have per-
formed X-ray diffraction experiments of the xerogels. The
results are presented in Figure 8. All four xerogel samples
show a diffuse halo in the wide-angle region that corre-
sponds to the distance of 4.0–4.5 ꢁ, which is usually attribut-
Figure 7. Plots of CD intensity against the temperature for the represen-
tative gel samples with gelator G (due to the difference in scale, the
G-Hx system is presented as an inset).
the CD spectra (at lmax ꢀ415 nm) for the gel samples are
plotted against the temperature. In all the cases, the CD sig-
nals diminish with heating, thus signifying that the self-as-
sembled nanostructure has nanoscale chirality.[6e,21] Again,
the sigmoidal pattern of the plots indicates a one-step se-
quential disassembly similar to what we observed from the
luminescence study. Here, too, we did not find any influence
of the solvent chirality on the supramolecular chirality, as
evident from the similar CD signals obtained with enantio-
Figure 8. X-ray diffractograms of the xerogels prepared from the gel sam-
ples with gelator G (diffraction peaks are indicated with arrows).
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S. Shinkai et al.
ed to the disordered alkyl chains.[15c] The sharp peak ob-
served in the low-wavelength region varies slightly between
17.1, 16.5, 17.3, and 16 ꢁ for the systems G-Hx, G-Mc, G-
Ch/Hx, and G-Ge, respectively. In addition, the G-Hx
system possesses diffraction peaks that correspond to the
distances of 3.2, 2.8, and 2.6 ꢁ, which are significantly
absent for other gel systems. To satisfy the cooperative con-
tribution of intermolecular hydrogen-bonding and p–p
stacking interactions, we have approximated that the ben-
zene ring in the gallic acid moiety and the anthracene
moiety are placed in more or less same molecular plane.
However, some offsets are likely to have taken place due to
the interaction among the similar groups. Now, we can con-
sider a typical columnar arrangement, as represented in
Figure 9, in which the gelator molecules are stacked in a
one-above-the-other fashion. The width of the column as
calculated from the energy minimized orientations is longer
(ꢀ25 ꢁ) than the distance we obtained from the low-angle
diffraction peaks present in all four samples. To rationally
explain this deviation, we considered some degree of tilt for
the stacking planes. This situation would also satisfy the J-
aggregate molecular assembly. The origin of the unique
wide-angle diffraction peak for the G-Hx system for a dis-
tance approximately 3.2 ꢁ can be attributed to the intraco-
lumnar spacing between two stacked molecular planes. This
distance is a little shorter than that of typical p-stacked sys-
tems.[15c,f,16d] However, in a very tightly packed gelator ar-
rangement, the stacking distance can be shortened. The ab-
sence of similar diffraction peaks for other gel systems signi-
fies the lack of such compactness. For G-Ge, even the low-
angle peak is less prominent. Also, the FTIR spectra (Fig-
ure S8 in the Supporting Information) of the xerogels clearly
À
demonstrate the lower N H stretching frequency for G-Hx
(3263 cmÀ1) relative to other systems (ꢀ3286 cmÀ1). That
means that the G-Hx system possesses the strongest hydro-
gen bonding.
From the above observations, we can now safely conclude
G-Hx to be a strong gel, G-Mc, G-Ch/Hx, and similar sys-
tems to be moderately strong gels, and G-Ge and enantio-
meric solvent systems to be weak gels. To explain the abun-
dance of the photodimers in terms of relative orientations
(h–h or h–t), we have proposed two separate packing
models for h–h and h–t molecular orientations (Figure 9).
Similar arrangement modes are evident from the similar col-
umnar thicknesses, as indicated by the presence of low-angle
diffraction peaks irrespective of the nature of the solvents.
We can then consider a gelator arrangement that comprises
the domains for either h–h or h–t molecular orientations.
The gel systems other than the glycidyl methyl ether solvent
systems consist almost exclusively of a single domain that
has h–h molecular orientation. On the other hand, G-Ge
gels include both kinds of domains in competitive propor-
tions. The resultant chirality and relative compactness are
not demonstrated in this model because it would require de-
tailed molecular modeling based on precise energy consider-
ations, especially for the gels that have the same gelator
molecule differentiated only by the nature of solvents. Rela-
tive strengths of the gels are also taken into account in the
proposed model. Here, such categorization has been made
based on h–h molecular orientation only, because we consid-
er h–t orientation to be inherently weak due to the absence
of intermolecular hydrogen bonding. However, it is signifi-
cant to find the evidence for different degrees of compact-
ness from the X-ray data based solely on the nature of the
solvents.
Mechanistic aspects behind the stereo- and the enantioselec-
tivities: From all the above-mentioned discussions, we can
now interpret that the key factors behind the selective
nature of photodimerization in the gel state are, firstly, the
solvent polarity, which controls the participation of the in-
termolecular hydrogen bonding, and secondly, the gelation
temperature. Nonpolar solvents and low gelation tempera-
tures favor intermolecular hydrogen bonding, thereby lead-
ing to the cooperative participation with p–p interactions to
result in h–h molecular orientation; conversely, higher gela-
tion temperatures and a polar solvent environment favor h–
t molecular orientations based on p–p interactions and
Figure 9. Models for the gelator arrangements in the gel samples with ge-
lator G based on their orientations: a) molecular scale of G; b) h–h for
strong gel; c) h–t; d) h–h for moderately strong gel; and e) h–h for weak
gel.
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Chirality in Organogel Systems
FULL PAPER
steric factors due to the weak nature of intermolecular hy-
drogen bond. Thus, it is understandable why the nonpolar
solvents systems result in h–h photodimers almost exclusive-
ly. Similarly, the gels with the relatively polar glycidyl
methyl ether result in both h–h and h–t photodimers in com-
petitive proportions. Among the different gel systems stud-
ied, the rigid nature of the G-Hx system is also evident from
the least conversion during the photoreaction, thereby indi-
cating the unfavorable microenvironment for undergoing
such a process. Photoresponsive gel-to-sol phase transition
during the photodimerization definitely corresponds to the
disruption of molecular arrangement as a result of dimer
formation. This fact means that such a process demands
some degree of environmental flexibility (see Figure 9). In
the case of chiral induction, the situation is not so straight-
forward. Here, the gelation temperature plays a crucial role.
As the energy difference between the two molecular orien-
tations that give the enantiomeric photodimers is very small,
lower temperatures should facilitate such a differentiation.
At the same time, the connection of the anthracene moiety
to the chiral center demands conformational flexibility that
leads to some sort of deviation from ideal coplanar molecu-
lar arrangements. Such an orientational freedom is permissi-
ble in relatively flexible gelator arrangements, or, in other
words, for moderately strong and weak gel systems. These
factors are mostly prominent in G-RGe and G-SGe gels,
which possess the lowest gelation temperature and weaker
molecular arrangement, thereby giving rise to the highest ee
values for h–h photodimers. However, if temperature were
to be the sole factor, then even h–t photodimers would
result in high ee values. In fact, among the different gel sys-
tems, these systems possess the lowest ee values for the h–t
photodimers, thus signifying that in the absence of intermo-
lecular hydrogen bonding the h–t molecular arrangement is
too weak to induce the chirality. The high ee values of the
h–t photodimers for the G-Hx system confirm the same, be-
cause in the absence of intermolecular hydrogen bonding,
the h–t molecular arrangement gains the desired conforma-
tional flexibility. In the moderately strong gel systems, we
obtained the medium ee values for both h–h and h–t photo-
dimers. From the variation of ee values with Tgel (Figure 3),
it can be concluded that for the highly ordered h–h molecu-
lar orientation, gelation temperature dictates the resultant
chirality induced in the photodimers, whereas for the inher-
ently weak h–t molecular orientation, the effect of gelation
temperature is not reflected on account of the lack of mo-
lecular arrangement. As expected, the stereochemical selec-
tion is low in the sol state, lower in the molecular aggregate
state, and lowest in nongelating THF. This means that a
one-dimensional molecular arrangement created in the gel
state is the mandatory factor for a high degree of stereo-
and enantioselectivity. In this molecular platform, other
physical parameters operate individually or cooperatively to
control the gelator preorientation, which in turn determines
the final stereochemistry of the photodimers. In gel chemis-
try the stronger gel generally demands priority. However,
this is an extraordinary finding in which even the “weak”
gel demonstrates its superiority over the stronger member.
Here, for the first time, we have executed a highly stereose-
lective and enantiodifferentiating chiral photochemical pro-
cess using a soft template. Moreover, the stereochemistry of
the photoproducts can be tuned simply by the solvent selec-
tion. This study will certainly bring new prospects to the
wealth of gel chemistry.
Conclusion
In conclusion, we have demonstrated the highly stereoselec-
tive and enantiodifferentiating photodimerization process
performed in the gel state with an anthracene-bearing chiral
organogelator. Different degrees of stereo- and enantiose-
lectivities have been obtained from the gels prepared from
different organic solvents. Interestingly, the G-Hx system
behaves differently from the rest of the members due to
having the highest degree of molecular packing. A one-di-
mensional molecular arrangement mode and the gelation
temperature are found to be responsible for fine-tuning the
final stereochemistry of the photodimers. Photolumines-
cence measurements demonstrated unprecedented varia-
tions in which the emission intensity of the G-Hx system in-
creases during the gel-to-sol phase transition, unlike the
other systems, which show a decrease in the emission inten-
sity during this course. In the gel state, the ee values of h–h
(major) photodimers increase from strong to moderately
strong to weak gel systems, whereas h–t (minor) photodim-
ers maintain the reverse trend. On the basis of spectroscopic
findings, gel melting temperatures, and X-ray diffraction
patterns, the gel systems are categorized as strong, moder-
ately strong, and weak gels. This “model” can satisfactorily
explain the different experimental findings. This is the first
example in which a chiral photochemical process is per-
formed in the gel medium and, as a result, a high degree of
chiral induction has been achieved and controlled precisely
by the one-dimensional gelator assembly created by the
supramolecular interactions. This study is believed to add a
new dimension in the field of supramolecular chirogenesis.
Experimental Section
General: All starting materials and solvents were purchased from Tokyo
Kasei Organic Chemicals or Wako Organic Chemicals and used as re-
ceived. The 1H NMR spectra were recorded using
a Bruker 300
(300 MHz) spectrometer. Chemical shifts are reported in ppm downfield
from tetramethylsilane (TMS) as the internal standard. Mass spectra data
were obtained using a Perspective Biosystems Voyager DE RP MALDI-
TOF mass spectrometer. UV/Vis, circular dichroism (CD), luminescence,
and IR spectra were recorded using a Shimadzu UV-2500 PC spectropho-
tometer, a JASCO J-720WI spectropolarimeter, a Perkin–Elmer LS 55
luminescence spectrometer, and Perkin–Elmer Spectrum One FTIR
spectrometer, respectively.
Synthesis: Gelator G was prepared by the esterification of 2-anthracene-
carboxylic acid (2Ac) and alcohol 2 (Scheme 2). Compound 1 was syn-
thesized according to the method described earlier.[13]
Chem. Eur. J. 2010, 16, 3676 – 3689
ꢀ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
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S. Shinkai et al.
the mass started to flow. The error involved in the measurement of Tgel
was Æ18C.
SEM measurements: A thin layer of the gel sample was prepared over a
carbon-coated copper grid and dried under vacuum for 24 h to obtain the
xerogel. The sample was then shielded by Pt and examined using a Hita-
chi S-5000 scanning electron microscope.
TEM measurements: A thin layer of the gel sample was prepared over a
carbon-coated copper grid and dried under vacuum for 24 h to obtain the
xerogel. TEM studies were carried out using a JEOL JEM-2010.
XRD studies: X-ray data were recorded using a Rigaku R-axis instru-
ment. The gel sample was prepared in a sample tube and frozen in liquid
nitrogen. The frozen specimen was evaporated under vacuum for 24 h.
The obtained xerogel was put into a glass capillary (d=0.7 mm). The dif-
fractogram was recorded on an imaging plate using Cu radiation (l=
1.54178 ꢁ).
Scheme 2. Reagents and conditions: i) (R)-(À)-1-amino-2-propanol, BOP,
TEA, CH2Cl2, RT, 3 h; ii) 2-anthracenecarboxyl chloride, pyridine, dry
benzene, RT, 3 h.
General procedure for photochemical reaction, product isolation, and
product analysis: The different gel samples of gelator G were prepared in
a capped quartz cell of 1 mm path length under an argon atmosphere
and irradiated at a wavelength of 366 nm using a USHIO Optical Modu-
lex Deep UV 500 instrument through UV-35 and UV-D36C optical filters
at a constant temperature. After the photoreaction, the solvent was first
removed at 558C and then under high vacuum for 24 h. The dried prod-
uct was then hydrolyzed in a 1,4-dioxane/water mixture with 1m KOH
for 15 h and poured into a 25 mm borate buffer solution of pH 9. The re-
action mixture was filtered to isolate 2Ac photodimers and unreacted
monomer, and then they were neutralized partially (to a pHꢀ9). This so-
lution (50 mm, 10 mL) was subjected to HPLC analysis. The same proce-
dure was employed for the sol, aggregate, and THF systems. Analysis of
the photoproducts was performed using chiral HPLC with tandem col-
umns Inertsil ODS-2 (GL Sciences) and Chiralcel OJ-RH (Daicel). The
columns were kept at 358C. A mixture of 0.2m potassium dihydrogen
phosphate (adjusted to pH 2.5 by phosphoric acid) and acetonitrile
(62:38 by volume) was used as the eluent. Relative yield and ee values
were determined from the peak area on the HPLC chromatogram, de-
tected by the absorbance at 254 nm.
Synthesis of 2: Compound 1 (1 gm, 1.5 mmol) and benzotriazole-1-yloxy-
tris(dimethylamino)phosphonium hexafluorophosphate (BOP) reagent
(721 mg, 1.6 mmol) were dissolved in dry CH2Cl2. (R)-(À)-1-Amino-2-
propanol was added, followed by triethylamine (TEA), and the reaction
mixture was stirred for 3 h at RT under an Ar atmosphere. The progress
of the reaction was checked by TLC. The reaction mixture was first dilut-
ed by adding excess CH2Cl2 and then the organic layer was washed three
times with distilled water, dried over anhydrous Na2SO4, filtered, and the
filtrate was dried under reduced pressure. The dried solid was subjected
to column chromatography (silica gel, hexane/ethylacetate=1:1 (v/v)) to
give compound 2 (890 mg, 82%) as white solid. 1H NMR (300 MHz,
CDCl3, TMS): d=0.88 (t, J=6.8 Hz, 9H), 1.26–1.47 (m, 54H), 1.56 (m,
3H), 1.73–1.82 (m, 6H), 2.52 (s, 1H), 3.65 (m, 1H), 3.96–4.02 (m, 8H),
6.45 (m, 1H), 6.97 ppm (s, 2H); MALDI-TOF MS (dithranol): m/z: calcd
for [M+Na+]: 754.63; found: 754.56; elemental analysis calcd (%) for
C46H85NO5: C 75.46, H 11.7, N 1.91; found: C 75.45, H 11.64, N 1.93.
Synthesis of 2-anthracenecarboxyl chloride: 2Ac (289 mg, 1.3 mmol) was
dispersed in dry benzene (50 mL) at 508C and a catalytic amount of
DMF was added. Oxalyl chloride (1.7 g, 13 mmol) was added dropwise
and the reaction mixture was stirred at approximately 708C under reflux
conditions for 18 h. After cooling to RT, the solvent and excess oxalyl
chloride were distilled off, washed with dry benzene, and again evaporat-
ed to obtain a yellow solid that was used in the next step without further
purification.
Acknowledgements
Financial support was provided by Grants-in-Aid (nos. 19022031,
Synthesis of G: Compound 2 (800 mg, 1.1 mmol) was dissolved in dry
benzene (10 mL) that contained a trace of pyridine, and 2-anthracenecar-
boxyl chloride dissolved in dry benzene (20 mL) was added slowly with
constant stirring. The reaction mixture was allowed to stir for 4 h at RT.
The progress of the reaction was checked with TLC. Distilled water
(20 mL) was added and the mixture was extracted with chloroform sever-
al times. The organic layer was washed three times with distilled water,
dried over anhydrous Na2SO4, filtered, and the filtrate was dried under
reduced pressure. The dried solid was subjected to column chromatogra-
phy (silica gel, hexane/ethylacetate=1:1 (v/v)) to give compound G
(757 mg, 74%) as a yellow solid. 1H NMR (300 MHz, CDCl3, TMS): d=
0.88 (t, J=6.8 Hz, 9H), 1.26–1.47 (m, 54H), 1.56 (m, 3H), 1.69–1.77 (m,
6H), 3.79 (m, 2H), 3.93 (m, 6H), 5.45 (m, 1H), 6.63 (m, 1H), 6.91 (s,
2H), 7.52 (m, 2H), 7.98–8.02 (m, 4H), 8.44, (s, 1H), 8.57 (s, 1H),
8.82 ppm (s, 1H); MALDI-TOF MS (dithranol): m/z: calcd for [M+Na+
]: 958.69; found: 958.78; elemental analysis calcd (%) for C61H93NO6: C
78.24, H 10.01, N 1.50; found: C 78.10, H 9.98, N 1.49.
20045014, and 20685010),
(20245036) from the JSPS, and a JSPS fellowship for A.D.
a Grant-in-Aid for Scientific Research A
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Received: October 23, 2009
Published online: February 11, 2010
Chem. Eur. J. 2010, 16, 3676 – 3689
ꢀ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
3689