Charge-transfer phenomena in novel, dual-component, sugar-based organogels

Article abstract of DOI:10.1021/ja012585i

The synthesis of a new saccharide-based gelator (2) containing a donor moiety has been described. Gelation experiments of a dual-component gel consisting of a saccharide-based gelator bearing an acceptor group (1) and of 2 have been performed in a variety

Full text of DOI:10.1021/ja012585i

Published on Web 08/17/2002
Charge-Transfer Phenomena in Novel, Dual-Component,
Sugar-Based Organogels
Arianna Friggeri,†,§ Oliver Gronwald, Kjeld J. C. van Bommel, Seiji Shinkai,*,† and
†
†,§
‡
David N. Reinhoudt
Contribution from the Chemotransfiguration Project, Japan Science and Technology
Corporation, Aikawa, Kurume, Fukuoka 839-0861, Japan, and the Chemotransfiguration
+
Project, MESA Research Institute, UniVersity of Twente, 7500 AE Enschede, The Netherlands
Received November 26, 2001. Revised Manuscript Received April 2, 2002
Abstract: The synthesis of a new saccharide-based gelator (2) containing a donor moiety has been
described. Gelation experiments of a dual-component gel consisting of a saccharide-based gelator bearing
an acceptor group (1) and of 2 have been performed in a variety of organic solvents and water. Moreover,
gelation tests at different molar ratios of 1 and 2 have been performed in water, octanol, and diphenyl
ether. In these last two solvents a gel color change was observed, from colorless to yellow, upon cooling
of the sample to room temperature. This phenomenon was further investigated by UV-visible spectroscopy,
which revealed the presence of charge-transfer interactions in the gel, in octanol. Temperature-dependence
UV spectroscopy confirmed that such interactions occur in the gel but not in the corresponding solution
sample. Furthermore, Tgel measurements show that dual-component gels of 1 and 2 present increased
thermal stability at a 50:50 ratio of the two gelators, in dependence of the solvent. Transmission electron
microscopy (TEM) images of the single-component gels in diphenyl ether revealed that they consist of a
fibrous network, while the dual-component gel presents a novel, helical, fibrous-bundle structure.
Introduction
Organogels are thermoreversible, viscoelastic materials con-
special attention has been paid to saccharide-based gelators with
the aim of synthesizing a large carbohydrate-based gelator
library. Gelation ability and strength has been found to correlate
5
sisting of low molecular weight compounds self-assembled into
complex three-dimensional networks.1 The aggregation of
gelator molecules into fibrous networks is driven by multiple,
weak interactions such as dipole-dipole, van der Waals, and
significantly with the absolute configuration of the sugar, as
well as with the substituents present on the sugar unit.
In recent years organogels have attracted considerable atten-
tion due to their numerous industrial applications as well as
1
hydrogen-bonding interactions. Gelators are thus generally
1
their interesting supramolecular architectures. In particular,
classified according to their driving force for molecular ag-
gregation, into two categories: non-hydrogen bond-based and
there has been an increasing interest in the development of dual-
component gels for the exploitation of new functions inherent
to these systems. For practical applications, however, improve-
ment of gel strength and stability are essential. Postpolymer-
2
hydrogen bond-based gelators. Cholesterol derivatives are
3
examples of the former, while amide-, urea-, and saccharide-
1
c,4
based compounds fall into the latter category. In our group
6
7
ization of gel fibers, and the addition of polymers or hydrogen-
8
bonding guests, have proven to be successful approaches toward
*
To whom correspondence should be addressed. E-mail: seijitcm@mbox.
gel stabilization. The introduction of metal ions, on the other
hand, has been carried out to investigate the sensing and catalysis
nc.kyushu-u.ac.jp.
†
‡
§
Japan Science and Technology Corporation.
University of Twente.
Present address: BiOMaDe Technology Foundation, Nijenborgh 4,
9
potentials of gels. Furthermore, two-component gel systems
9
747 AG Groningen, The Netherlands.
(4) (a) Yoza, K.; Ono, Y.; Yoshihara, K.; Akao, T.; Shinmori, H.; Takeuchi,
M.; Shinkai, S.; Reinhoudt, D. N. Chem. Commun. 1998, 907. (b) Yoza,
K.; Amanokura, N.; Ono, Y.; Akao, T.; Shinmori, H.; Takeuchi, M.;
Shinkai, S.; Reinhoudt, D. N. Chem. Eur. J. 1999, 5, 2722. (c) Luboradzki,
R.; Gronwald, O.; Ikeda, M.; Shinkai, S.; Reinhoudt, D. N. Chem. Lett.
2000, 1148. (d) Luboradzki, R.; Gronwald, O.; Ikeda, M.; Shinkai, S.;
Reinhoudt, D. N. Tetrahedron 2000, 56, 8697.
(
1) For recent reviews see (a) Terech, P.; Weiss, R. G. Chem. ReV. 1997, 97,
3
133. (b) van Esch, J. H.; Feringa, B. L. Angew. Chem., Int. Ed. 2000, 39,
263. (c) Gronwald, O.; Shinkai, S. Chem. Eur. J. 2001, 7, 4328 and
2
references therein.
(
2) (a) Terech, P.; Furman, I.; Weiss, R. G. J. Phys. Chem. 1995, 99, 9558.
(
b) Murata, K.; Aoki, M.; Suzuki, T.; Hanada, T.; Kawabata, H.; Komori,
T.; Oseto, F.; Ueda, K.; Shinkai, S. J. Am. Chem. Soc. 1994, 116, 6664.
(5) Gronwald, O.; Sakurai, K.; Luboradzki, R.; Kimura, T.; Shinkai, S.
Carbohydr. Res. 2001, 331, 307.
(c) James, T. D.; Murata, K.; Harada, T.; Ueda, K.; Shinkai, S. Chem.
Lett. 1994, 273.
(6) (a) de Loos, M.; van Esch, J.; Stokroos, I.; Kellogg, R. M.; Feringa, B. L.
J. Am. Chem. Soc. 1997, 119, 12675. (b) Tamaoki, N.; Shimada, S.; Okada,
Y.; Belaissaoui, A.; Kruk, G.; Yase, K.; Matsuda, H. Langmuir 2000, 16,
7545.
(7) (a) Kobayashi, H.; Amaike, M.; Jung, J. H.; Friggeri, A.; Shinkai, S.;
Reinhoudt, D. N. Chem. Commun. 2001, 1038.
(8) (a) Jeong, S. W.; Murata, K.; Shinkai, S. Supermol. Sci. 1996, 3, 83. (b)
Inoue, K.; Ono, Y.; Kanekiyo, Y.; Ishi-i, T.; Yoshihara, K.; Shinkai, S. J.
Org. Chem. 1999, 64, 2933.
(
3) (a) Hanabusa, K.; Okui, K.; Karaki, K.; Shirai, H. J. Chem. Soc., Chem.
Commun. 1992, 1371. (b) Hanabusa, K.; Yamada, Y.; Kimura, M.; Shirai,
H. Angew. Chem., Int. Ed. Engl. 1996, 35, 1949. (c) Hanabusa, K.; Shimura,
K.; Hirose, K.; Kimura, M.; Shirai, H. Chem. Lett. 1996, 885. (d) Hanabusa,
K.; Kawakami, A.; Kimura, M.; Shirai, H. Chem. Lett. 1997, 191. (e) de
Vries, E. J.; Kellogg, R. M. J. Chem. Soc., Chem. Commun. 1993, 238. (f)
Takafujii, M.; Ihara, H.; Hirayama, C.; Hachisoko, H.; Yamada, K. Liquid
Cryst. 1995, 18, 97.
10754
9
J. AM. CHEM. SOC. 2002, 124, 10754-10758
10.1021/ja012585i CCC: $22.00 © 2002 American Chemical Society

Charge-Transfer Phenomena in Sugar-Based Organogels
A R T I C L E S
Scheme 1. Synthesis of Gelators 1 and 2a
Table 1. Organic Solvents Tested for Gelation of 1, 2, and 1/2
Mixtures (50:50)a
solvent
1
2
1/2 mix
Group I
1
2
3
4
5
6
7
8
9
0
1
n-hexaneb
Psc
I
I
I
I
I
G
P
P
P
P
b
c
n-heptane
Ps
Ps
Ps
Ps
G
G
G
S
G
G
b
c
n-octane
cyclohexaneb
c
a
(
a) 2 equiv of p-NO2C6H4CHO, H2SO4, DMSO, 10 days, rt, 44%; (b)
b
c
methylcyclohexane
H2, 1 wt % Pd/C, ethanol, 30 min, 98%.
b
c
c
c
c
Pc
benzene
toluene
b
c
c
Ps
Ps
p-xyleneb
c
c
are also being used to investigate molecular properties/interac-
tions such as chirality and donor-acceptor interactions within
gel fibers. To the best of our knowledge, Maitra et al.
presented the first and only report on donor-acceptor interac-
tions in organogels, only a few years ago, by doping gels of
donor-substituted derivatives with trinitrofluorenone.
In this paper, we present a novel donor-acceptor, sugar-based
gelator system in which the simultaneous gelation of two
gelators, one containing an acceptor group (1) and the other a
donor group (2), gives rise to charge-transfer phenomena within
the gel (Scheme 1). Moreover, these dual-component gels
present an increased thermal stability which was found to be
strongly dependent on the solvent and on the ratio of the two
gelators. Furthermore, transmission electron microscopy (TEM)
images of the two-component gel network in diphenyl ether
have revealed a novel organogel structure, with a helical motif,
that differs greatly from that of the single-component gels in
the same solvent.
nitrobenzeneb
10
11
b
c
c
c
Pc, Gd
G
1
1
carbon tetrachloride
1
2
11
e
c
carbon disulfide
G
I
diethyl etherb
I
c
c
12
b
c
c
13
14
15
diphenyl ether
G
S
S
S
Ps
P
G
P
S
P
P
P
G
b
ethyl formate
methyl acetateb
c
c
16
7
8
triethylamine
triethylsilane
tetraethoxysilane
c
c
1
1
c
Group II
1
2
9
0
methanol
ethanol
n-propanol
n-butanol
n-hexanol
octanolb
n-decanol
glycerol
hexanoic acid
water
P
P
S
S
S
G
G
Ps
G
G
G
G
G
G
G
G
P
P
S
21
c
22
23
24
25
P
S
c
26
2
2
7
8
reaction
S
c
Sc
a
All 1/2 mixtures are 3.0% (w/v) unless specified. G ) gel, PS ) self-
Results and Discussion
b
supporting precipitate, P ) precipitation, S ) solution, I ) insoluble. Dried
over molecular sieves 4 Å. c 1.0% (w/v). d 0.5%, 0.25% (w/v). Dried over
anhydrous magnesium sulfate.
e
In analogy to the preparation of nonsubstituted methyl 4,6-
5
O-benzylidene derivatives, the condensation of the C-4 and
C-6 hydroxyl groups with p-nitrobenzaldehyde in the presence
Table 2. Gelation Tests for Different Molar Ratios of 1 and 2 in
Water, Octanol, and Diphenyl Ether, with Corresponding Gel
of sulfuric acid in dimethyl sulfoxide resulted in methyl 4,6-
Colorsa
13
O-(p-nitrobenzylidene)-R-D-glucopyranoside (1) (Scheme 1).
H
2
O
octanol
(3 wt %)
diphenyl ether
(3 wt %)
Hydrogenation in ethanol with palladium on activated carbon
induces the formation of the corresponding amino compound 2
in nearly quantitative yield (98%). As expected, the exchange
of the electron-withdrawing nitro substituent with the electron-
donating amino substituent induces a high-field shift of the
1
:2
(1 wt %)
color
color
color
none
1
00:0
0:10
80:20
0:30
60:40
0:50
0:60
30:70
0:80
0:90
G
G
G
G
Gp
S
S
S
S
S
none
none
none
none
none
none
none
none
none
none
none
G
G
WG
WG
WG
G
G
P
P
P
whiteb
G
G
Gp
G
G
G
G
G
G
G
whiteb
9
yellow 1
yellow 1
yellow 1
yellow 1
yellow 1
yellow 2
yellow 2
yellow 2
yellow 2
none
yellow 1
yellow 1
yellow 1
yellow 2
yellow 2
yellow 2
yellow 1
yellow 1
white
7
1
aromatic protons in the H NMR (2, δH ) 6.49 and 7.04 ppm;
5
4
1
, δH ) 7.68 and 8.23 ppm). In addition, the UV absorption
spectrum in water confirms with a significant blue shift in the
absorption maximum from λmax ) 266.0 nm (1) to λmax ) 239.4
nm (2) the successful transformation. The coupling constants
of 1-H (δH ) 4.60 ppm, d, J1-H,2-H ) 4.2 Hz) and 2-H (δH )
2
1
c
0:100
S
P
G
a
4
.08 ppm, dd, J1-H,2-H ) 4.2 Hz, J2-H,3-H ) 9.3 Hz) with the
neighboring protons prove their axial (1-H) and equatorial
2-H, 3-H) orientation and thus that the gluco configuration of
G ) gel; Gp ) partial gelation; WG ) weak gel. Yellow 1 is a paler
yellow than yellow 2. b Turbid. c 1.0% (w/v).
(
solvents, where it has a tendency to form self-supporting
the sugar moiety remained unchanged.
1
3,14
precipitates (Table 1).
Gelator 2, on the other hand, has a
Gelator 1 can gelate both aromatic solvents, as well as
nonaromatic polar solvents, and is also a very good gelator for
water, while it performs rather poorly in nonaromatic nonpolar
much more limited solvent gelation range (Table 1). However,
mixtures of 1 and 2 can gelate several solvents, in particular
water, octanol, and diphenyl ether (Table 2). Evidence showing
that both gelators are present in the gel fibers was obtained from
(
9) (a) Ishi-I, T.; Ono, Y.; Shinkai, S. Chem. Lett. 2000, 808. (b) Sohna, J.-E.;
Fages, F. Chem. Commun. 1997, 327.
1
H NMR spectroscopy of a solution of 1 and 2 (2 wt % total,
(
(
(
10) de Loos, M.; van Esch, J.; Kellog, R. M.; Feringa, B. L. Angew. Chem.,
Int. Ed. 2001, 40, 613.
1:1 molar ratio, in octanol), in which the sharp aromatic proton
signals of both 1 and 2 all broadened to the same extent upon
11) Maitra, U.; Kumar, P. V.; Chandra, N.; D’Souza, L. J.; Prasanna, M. D.;
Raju, A. R. Chem. Commun. 1999, 595.
12) (a) K o¨ lbel, M.; Menger, F. M. Langmuir 2001, 17, 4490. (b) Partridge, K.
S.; Smith, D. K.; Dykes, G. M.; McGrail, P. T. Chem. Commun. 2001,
S
(14) Self-supporting precipitate (P ): gellike precipitate. The transition from
3
19. (c) Xu, X.; Ayyagari, M.; Tata, M.; John, V. T.; McPherson, G. L. J.
gel phase to solution is not reversible and mechanical disruption (i.e.,
shaking) causes a visible separation into a solid and a liquid phase. See
ref 5.
Phys. Chem. 1993, 97, 11350.
(
13) Gronwald, O.; Shinkai, S. J. Chem. Soc., Perkin Trans. 2 2001, 1933.
J. AM. CHEM. SOC.
9
VOL. 124, NO. 36, 2002 10755

A R T I C L E S
Friggeri et al.
gelation of the sample.15 In octanol and dipehenyl ether, an
interesting phenomenon was observed: upon heating of a
mixture of 1 and 2 in a 1:1 molar ratio, in octanol or diphenyl
ether, the two white powders dissolved, yielding a colorless
solution that turned into a yellow gel upon cooling to room
temperature. Several cycles of heating (gel melting) and cooling
(gel formation) of the same sample proved the reversibility of
this process and ruled out the possibility of a change in color
1
due to degradation of the gelators. Moreover, H NMR spectra
of 1 and 2, after such heating and cooling cycles, also confirmed
that the molecular structure of the two gelators had not changed.
Interestingly, the single-component gels in the same solvents
are colorless. Therefore, the change in color observed for the
dual-component system pointed to the occurrence of donor-
1
6,17
acceptor interactions between 1 and 2 in the gel fibers.
Figure 1. Tgel as a function of the molar ratio of 1 to 2, in dual-component
gels in diphenyl ether (9) and in octanol (2); the dotted line delineates the
passage from gel to precipitate for the mixtures in octanol only.
The gelation ability of mixtures of different molar ratios of
and 2, in water, octanol, and diphenyl ether, was studied (Table
). In the more apolar solvents, a definite change from colorless,
1
2
or white, to yellow could be observed upon going from the 100:0
or the 0:100 samples to the mixed samples of 1 and 2. In water,
on the other hand, although gelation did occur for mixtures up
to a ratio of 60:40 of 1 and 2, respectively, the samples remained
colorless. These results are in agreement with the behavior
shown by charge-transfer complexes, whereby solvent polarity
plays an important role; in fact, charge-transfer interactions
become visible only in nonpolar environments.16,17,18 In octanol
and in diphenyl ether, color intensification was observed for
mixed samples containing 50% or more of gelator 2 (Table 2).
This phenomenon has previously been observed in solution, by
Landauer and McConnell,17 for samples containing aniline and
various nitrobenzenes. Values of the formal extinction coef-
ficients of such samples were found to increase with increasing
aniline concentration. The authors attributed this behavior to
the existence of 1:1 and 2:1 donor-acceptor complexes,
respectively, and thus to light absorption by both of these types
of complexes.
To determine whether the donor-acceptor interactions be-
tween 1 and 2 induce stabilization of the dual-component gel,
Tgel measurements with different ratios of the two gelators were
1
9
carried out. In octanol the Tgel of 1 is 93 °C; upon increasing
amounts of 2, the Tgel at first decreases before increasing to
reach a local maximum (55 °C) at a 1:1 ratio of the two
gelators20 (Figure 1). This behavior is most probably the result
Figure 2. (a) UV-Vis spectra of a 2 wt % sample of 1 and 2, in a 1:1
molar ratio, in octanol. At t ) 0 min the sample is completely dissolved,
at t ) 10 min gelation occurs, and between t ) 10 and t ) 60 min the
yellow color of the gel intensifies considerably, (b) ꢀ at 420 nm as a function
of the temperature, for a 3 wt % sample (9) and for a 1 wt % solution
sample ([) of 1 and 2, in a 1:1 molar ratio, in octanol.
of two contrasting effects. Although 1 is a good gelator for
octanol, 2 precipitates in octanol; therefore, increasing amounts
of 2 in the gel of 1 cause a destabilization of the gel that is
reflected in decresing Tgel values. However, increasing amounts
of 2 also give rise to increased donor-acceptor interactions,
which seem to provide stabilization to the dual-component gel
system since the Tgel values increase to a local maximum. In
diphenyl ether, on the other hand, both 1 and 2 form gels and
a maximum Tgel value was measured for a 1:1 molar ratio
mixture of the gelators, showing a stabilization of about 30-40
°
C with respect to the gels of the single components (Figure
1). These results indicate that when donor and acceptor moieties
are covalently attached to gelators, their interaction can lead to
thermal stabilization of the gel, provided that the solvent
employed is one which the single components can also gelate.
To obtain spectroscopic evidence of the donor-acceptor
interactions taking place in the dual-component gels, UV-vis
spectroscopy was carried out on a sample containing both 1
and 2, 2 wt % total, in a 1:1 molar ratio, with octanol as the
(
15) Before gelation, the peak width at half-height of the aromatic proton signals
of 1 at 8.50 and 8.08 ppm and of 2 at 7.54 and 6.94 ppm was 0.025 ppm,
while after gelation this value increased to 0.056 ppm for all these signals.
16) Hunter, C. A.; Lawson, K. R.; Perkin, J.; Urch, C. J. J. Chem. Soc., Perkin
Trans. 2 2001, 651.
(
(
17) Landauer, J.; McConnell, H. J. Am. Chem. Soc. 1952, 74, 1221.
18) Gilbert, A.; Baggott, J. Essentials of Molecular Photochemistry; Blackwell
Scientific Publications: Oxford, U.K., 1991.
(
(19) The Tgel was determined by placing an inverted, sealed tube in a
thermocontrolled oil bath and increasing the temperature at a rate of 1 °C
-1
min . Here, Tgel is defined at the temperature at which the gel disappears.
20) For samples containing more than 60% 2, Tgel values could not be
determined due to precipitate formation.
21
solvent. Under these conditions the sample is initially a
solution and slowly becomes a gel within approximately 1 h;
(
10756 J. AM. CHEM. SOC.
9
VOL. 124, NO. 36, 2002

Charge-Transfer Phenomena in Sugar-Based Organogels
A R T I C L E S
Figure 3. TEM images of a gel of 1 (a), a gel of 2 (b), and a gel of 1 and 2 in a 1:1 molar ratio (c and d), 3 wt %, in diphenyl ether. Scale bars ) 1.00 µm.
therefore, the gelation process can be monitored at room
temperature (Figure 2a). At first, a decrease in intensity of the
shoulder at 335 nm and a simultaneous increase in signal at
higher wavelengths (380-800 nm) were observed as the sample
changed from sol to gel (from t ) 0 to t ) 10 min), becoming
only slightly yellow in the process. Subsequently, while the
intensity of the shoulder at 335 nm continued to decrease, the
intensity of the signal at around 420 nm increased in cor-
respondence to a substantial intensification of the yellow color
of the gel. Such light absorption in the 350-600 nm range can
existence of not only 1:1 but also 2:1 donor-acceptor complexes
as proposed by Landauer and McConnell (vide supra), explain-
ing the intensification of the UV signal at ratios of 1 and 2
higher than 50:50. However, it should be noted that the Tgel
values of such mixtures show a maximum for the 50:50 sample
(Figure 1). Therefore, although charge-transfer interactions are
maximized in samples containing a higher ratio of 2, maximum
thermal stability of the dual-component gel is achieved at a 50:
50 ratio of the two gelators.
Transmission electron microscopy images of the dual-
component gels in both octanol and diphenyl ether (3 wt %,
1:1 molar ratio) provided further insight into the structure of
these organogel systems (Figure 3). In octanol no significant
differences were observed between the single-component and
the dual-component gels. All consist of networks of relatively
straight bundles of fibers approximately 60-200 nm in diameter.
In diphenyl ether, however, while TEM images of the single-
component gels showed networks of straight fibers (1, 30-150
nm; 2, 20-110 nm in diameter), images of the dual-component
gel revealed very large, helical bundles (250-500 nm in
diameter) of intertwined fibers (Figure 3c,d). The intertwined
fibrous structure of the dual-component gel in contrast to the
straight-fiber networks of the single-component gels seem to
reflect the increased thermal stability and intermolecular
cohesiveness of the former with respect to the latter, as
determined by the Tgel measurements (vide supra). Small-angle
X-ray scattering (SAXS) studies are currently being performed
to clarify the structures of the single- and dual-component gels.23
17
be interpreted as intermolecular charge-transfer spectra. These
+
spectra represent electronic transitions between (D, A) and (D -
-
A ), where D is the electron donor (2) and A is the electron
acceptor (1). The reason for intensification of the signal at 420
nm, instead of the appearance of a new shoulder as had
previously been observed in solution experiments,17 is as yet
not clear. The charge-transfer band for the complex of 1 and 2
might be broader in the gel than that of the aniline-nitrobenzene
complex in solution, due to extensive stacking of the donor and
acceptor molecules in the gel fibers. The temperature depen-
dence of the band at 420 nm, for a 3 wt %, 1:1 mixed gel of 1
and 2 in octanol was also examined by UV-vis spectroscopy.
The results presented in Figure 2b clearly show that the gelation
process is accompanied by a considerable increase in donor-
acceptor interaction, while in a true solution of 1 and 2 (1 wt
%, 1:1 molar ratio) only a very slight, regular increase of the
molar absorption coefficient (ꢀ) with decreasing temperature is
observed. Furthermore, the effect of the ratio of 1 and 2 on the
intensity of the charge-transfer signal in octanol was studied.
Interestingly, upon going from a 1:2 ratio of 60:40 to 50:50
and then to 40:60, the intensity of the charge-transfer signal (at
Conclusions
In conclusion, we have presented the first example of donor-
acceptor interactions in dual-component systems, where both
20 nm) increases.22 This observation seems to support the
4
(
22) Lower ratios of 1 and 2 yielded erratic UV signals due to the weak gel
formation and to the turbidity of some of the samples, while higher ratios
could not be measured due to precipitate formation.
(
21) Measurements in diphenyl ether were also attempted, but overlap with the
diphenyl ether signals did not allow identification of the charge-transfer
band.
(23) Manuscript in preparation.
J. AM. CHEM. SOC.
9
VOL. 124, NO. 36, 2002 10757

A R T I C L E S
Friggeri et al.
donor and acceptor moieties are confined to separate gelators.
Furthermore, we have shown how such interactions can bring
about effective thermal stabilization of the gel in dependence
of the solvent employed and of the donor-acceptor ratio used.
Variations on this theme, i.e., different donor and acceptor
moieties on a library of sugars, are currently being investigated
in the hope of creating materials with novel electronic or optical
properties.
hydrogenated for 30 min. After thin-layer chromatography (TLC)
indicated the completion of the reaction, the suspension was filtered
and the solvent was evaporated in vacuo. Yield: 505 mg (1.69 mmol)
of 2 as colorless solid; 98% of theoretical value.
Experimental Section
All solvents used were of p.a. grade (Merck). Water was purified
1
R
t
(ethyl acetate/n-hexane 3/1) 0.15; mp 202.7-204.0 °C; H NMR
1
by Millipore membrane. H NMR spectroscopy was performed on a
(
5
)
300 MHz, DMSO-d
6
) δ
Η
) 3.24-3.41, 3.42-3.63 (m, 5 H, 3-H, 4-H,
-H, 6-H), 3.29 (s, 3 H, OCH ), 4.08 (dd, J1-H,2-H ) 4.2 Hz, J2-H,3-H
1
Bruker ARX 300 apparatus; H NMR of a solution and a gel of 1 and
3
2
D
(2 wt % in total, 1:1 molar ratio) was measured in octanol with a
2
O probe. UV spectroscopy was performed on a Jasco V-570 UV/
9.3 Hz, 1 H, 2-H), 4.60 (d, J ) 4.2 Hz, 1 H, 1-H), 4.96 (d, J ) 6.7
), 5.33
s, 1 H, benzylidene-H), 6.49 (d, 2 H, aryl-H), 7.04 (d, 2 H, aryl-H);
Hz, 1 H, OH), 5.10 (d, J ) 5.2 Hz, 1 H, OH), 5.12 (s, 2 H, NH
2
vis/NIR spectrophotometer with a 1 mm quartz cell; TEM was
performed on a Hitachi H-7100 at 100 kV, and TEM samples were
prepared by placing some gel onto a carbon film on a Cu grid and
then removing the excess gel after 1 min and allowing the samples to
dry overnight at room temperature before imaging. Compound 1 was
(
UV (H
2
O, lg ꢀ) λmax ) 239.4 nm (3.92); SIMS (glycerol, positive) m/z
98 (83%, MH ). Anal. found: C, 56.51; H, 6.53; N, 4.60. Calcd for
+
2
C
14
H
19
O
6
N: C, 56.54; H, 6.44; N, 4.71.
1
3
prepared according to literature procedure.
Acknowledgment. We thank Dr. R. Keim for help in the
preparation of the TEM images and Dr. Ir. F. C. J. M. van
Veggel for helpful discussion.
Methyl 4,6-O-(p-Aminobenzylidene)-r-D-glucopyranoside (2).
Methyl 4,6-O-(p-nitrobenzylidene-R-D-glucopyranoside (1) (570 mg,
.74 mmol) was dissolved in ethanol (50 mL), 57 mg of palladium on
1
activated carbon (10 wt %) was added, and the suspension was
JA012585I
10758 J. AM. CHEM. SOC.
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VOL. 124, NO. 36, 2002