L-Tartaric acid assisted binary organogel system: Strongly enhanced fluorescence induced by supramolecular assembly

Article abstract of DOI:10.1039/b504945h

A new series of binary organogels, which showed good gelated capability and strongly enhanced fluorescence emission, were designed and prepared by simply mixing two components of L-tartaric acid and alkoxyl substituted stilbazoles. It was found that L-tartaric acid could not only introduce stilbazole with fluorescent property into the gel system, but also provided a main motif for the formation of the gel-phase via multiple hydrogen bonds. Meanwhile, π-π interactions between the complexes also played a key role in the gel formation. The decreased nonradiative process, suggested by the longer fluorescent lifetime of aggregates than that of monomers and the presence of the J-aggregation for the aggregated state, favored the enhanced fluorescence emission The Royal Society of Chemistry 2005.

Full text of DOI:10.1039/b504945h

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A R T I C L E
L-Tartaric acid assisted binary organogel system: strongly enhanced
fluorescence induced by supramolecular assembly
Chunyan Bao, Ran Lu,* Ming Jin, Pengchong Xue, Changhui Tan, Guofa Liu and
Yingying Zhao
College of Chemistry, Jilin University, 2519 Jiefang Road, Changchun, 130023, P. R. China.
E-mail: luran@mail.jlu.edu.cn
Received 11th April 2005, Accepted 27th April 2005
First published as an Advance Article on the web 11th May 2005
A new series of binary organogels, which showed good gelated capability and strongly enhanced fluorescence
emission, were designed and prepared by simply mixing two components of L-tartaric acid and alkoxyl substituted
stilbazoles. It was found that L-tartaric acid could not only introduce stilbazole with fluorescent property into the gel
system, but also provided a main motif for the formation of the gel-phase via multiple hydrogen bonds. Meanwhile,
p–p interactions between the complexes also played a key role in the gel formation. The decreased nonradiative
process, suggested by the longer fluorescent lifetime of aggregates than that of monomers and the presence of the
J-aggregation for the aggregated state, favored the enhanced fluorescence emission.
the corresponding 4-(4-alkoxybenzoyloxy)-4ꢀ-stilbazoles (nSZ)
Introduction
in THF. The formation of the intermolecular hydrogen bonds
Gels derived from low molecular mass compounds have at-
between nSZ and L-tartaric acid in the complexes can be
tracted considerable interest in recent years on account of their
confirmed by FT–IR spectroscopy due to the appearance of
unique features and potential applications for new soft organic
two broad peaks at ca. 2450 cm−1 and 1930 cm−1.23 Neither
materials,1 template synthesis,2 drug delivery,3 separations and
nSZ nor L-tartaric acid alone is capable of gelating the solvents,
biomimetics.4 Compounds with various structural diversities
while the complexes can gel most chlorinated solvents including
have been reported to gelatinize organic liquids efficiently, such
chloroform, dichloromethane, dichloroethane, chlorobenzene
as the derivatives of carbohydrates,5–7 amino acids,8–9 urea10–11
and 1,3-dichloropropane. In dichloromethane, for example, the
and cholesterol.12–15 Recently, there has been increasing interest
complexes are insoluble at rt, but turn into a clear solution
in the development of the functional gel systems with p-
by heating to 40 ◦C in a capped vial and, upon cooling to
conjugated molecular structures due to their potential applica-
rt, an immobilized transparent gel is readily formed within
tions in various optoelectronic fields, including enhanced charge
several hours at contents as low as 0.3 wt% (w/v). During the
preparation of sols, gels and xerogels of complexes (I–III), it was
transport, fluorescence and sensing abilities.16 However, it was
often difficult to introduce the desired functional groups into the
noticed that their colour changes sensitively. They are almost
individual building blocks for the gel systems. It is believed that
colorless in the solid-phase or in nonchlorinated solvents, but
binary gel systems may assist to introduce the functional groups
when the complexes were added into the chlorinated solvents
and obtain organogelators with functionalities in a facile way,
the mixture turned light yellowish–green and the gels obtained
in which the two components may interact with each other to
showed a yellowish–green colour. To reveal the origin of the
obtain a complex by electrostatic action,17 hydrogen bonding,18
colour changes and the formation of the gel, the optical and
donor–acceptor interactions19 and metal coordination,20 then
further self-assemble via intercomplex interactions, e.g. hydrogen
bonding, p-p interactions or Van der Waal’s forces, to form
the fully gelated network. Until now, limited two-component
gelators had been reported.
In this paper, we report the design of a series of binary gel sys-
structural properties of the complexes in the monomer state and
in the gel-phase were examined.
To obtain a visual image for the assemblies of the com-
plexes, the scanning electron microscope (SEM) pictures of the
dichloromethane gels were measured, as shown in Fig. 1. The
SEM picture of complex II showed a three-dimensional network
consisting of ribbons with largely knotted nodes (connected
tems comprised of L-tartaric acid and 4-(4-alkoxybenzoyloxy)-
4ꢀ-stilbazole (nSZ) derivatives, in which the alkyl chain sub-
spots), which suggested that the gel constituted by a robust
stituted stilbazoles have been shown to be versatile materials
gel system with permanent solid-like networks. Furthermore,
for the construction of molecular materials, including optically
the SEM picture for complex I, with a shorter alkyl chain in
the stilbazole unit, was filled with more or less straight ribbons,
whereas that of complex II was filled with frizzy ribbons. On the
other hand, the xerogel of complex III resulted in a sheet-like
nonlinear systems, pyroelectric Langmuir–Blodgett fabrications
and metallomesogens,21 but they generally cannot self-assemble
into supramolecular gels by themselves. As known, L-tartaric
acid is a good proton donor which can interact with stilbazoles
structure. It was assumed, therefore, that the complexes with an
via multiple hydrogen bonds to access the self-assembly.22
alkyl chain shorter or longer than that of complex II (n = 10)
Herein, excellent transparent gels can be formed based on the
tended to aggregate into two-dimensional morphologies.
hydrogen bonded complex derived from L-tartaric acid and
In order to ascertain how the complex aggregated into a
stilbazoles. Interestingly, such binary gels showed good gel
supramolecular structure, both FT–IR and UV–vis investi-
capability and unusually enhanced fluorescence, which is novel
gations were performed. From FT–IR absorption data, as
and very significant for functional gel materials.
shown in Table 1, it was found that the OH absorption in
the xerogel state shifted to shorter wave-numbers and split
=
into two peaks. The absorption of C O also shifted to lower
wave-numbers compared with the complexes before gelation,
Results and discussion
which showed that the hydrogen bonds did indeed take place
The complexes (I–III) of the two-component gelators as shown
in Scheme 1 were prepared from L-tartaric acid and two eq. of
=
between the OH and C O in the intermolecular L-tartaric
2 5 0 8
O r g . B i o m o l . C h e m . , 2 0 0 5 , 3 , 2 5 0 8 – 2 5 1 2
T h i s j o u r n a l i s
T h e R o y a l S o c i e t y o f C h e m i s t r y 2 0 0 5
©

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Scheme 1 The synthetic routes for complexes I–III.
Fig. 1 The SEM pictures of the xerogels obtained from 0.3 wt% dichloromethane gels: (a) n = 8; (b) n = 10; (c) n = 12.
acid (hydrogen bonding b, as shown in Scheme 2) during the
formation of the organized superstructure.24 In addition, and as
mentioned above, the appearance of two broad peaks at ca. 2450
and 1930 cm−1 confirmed another kind of hydrogen bonding
(hydrogen bonding a, as shown in Scheme 2) formed between
nSZ and L-tartaric. Therefore, the hydrogen bonding not only
introduced the functional stilbazole groups into the gels, but also
helped the formation of supramolecular aggregations, which
could be further proved by the below experiment. It was found
that one small additional drop of methanol could completely
destroy the gel in solution, in which the hydrogen bonding
between the pyridine in the nSZ and the carboxyl in the L-tartaric
O r g . B i o m o l . C h e m . , 2 0 0 5 , 3 , 2 5 0 8 – 2 5 1 2
2 5 0 9

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Table 1 FT–IR absorption data for A, nSZ; B, hydrogen bonding complexes before the gelation; and C, xerogels of the complexes
−1
OH/cm−1
mas(CH2)/cm−1 ms(CH2)/cm−1 Hydrogen bonds of kind a in the complexes/cm−1
=
C O/cm
Number of alkyl chains
n = 8
A
B
C
A
B
C
A
B
C
2915.2
2923.8
3264.3, 3128.4 2922.8
2915.1
2921.0
3278.5, 3131.2 2919.2
2915.2
2850.6
2853.9
2852.7
2849.2
2852.3
2851.5
2848.8
2851.1
2850.6
1721.9
1730.9
1724.9
1722.1
1728.3
1721.7
1721.8
1729.8
1722.0
3422.9
2452.3, 1916.8
2552.2, 1921.6
n = 10
n = 12
3412.7
2431.5, 1913.9
2559.9, 1938.5
3435.3
2922.2
3277.1, 3135.6 2917.2
2431.5, 1933.4
2543.2, 1950.2
Scheme 2 Schematic illustration for the structure of the complexes and the possible self-assembled mode of the complexes in the gel-phase.
acid were still maintained, due to the FT–IR data being similar
to those of the samples without treatment with CH3OH. It
was suggested that the hydrogen bonding aggregations were
broken into the monomer state and intermolecular hydrogen
=
bonds between the OH and C O (hydrogen bonding b) in
the intermolecular L-tartaric acid were the key motif for gel
formation.
The UV–vis absorption spectra for the gel in dichloromethane
were measured and compared with the corresponding monomer
in cosolvent (40 lM in CH2Cl2–CH3OH = 9 : 1 by volume).
For example (as seen in Fig. 2), for the series of n = 10, the
peak assignable to stilbazole gave a broadened and red-shifted
band in the gel-phase, which was quite different from that of the
monomer state of the complex, which illustrated the formation
of J-aggregation within the gel. Therefore, it was suggested that
the p–p interaction also played a key role on the formation of
the binary gels. Based on the above results, the aggregated mode
could be suggested to be as is shown in Scheme 2.
Fig. 3 (a) Fluorescence spectra of complex II (1) in CH2Cl2–CH3OH =
9 : 1 solution at 0.3 wt% (w/v) and (2) in CH2Cl2 gel at 0.3 wt%.
(b) Fluorescence images of CH2Cl2–CH3OH = 9 : 1 solution (9/1) at
0.3 wt% (left), CH2Cl2 solution at 40 lM (center) and CH2Cl2 gel at
0.3 wt% (right) under 365 nm illumination.
Fig. 2 UV–vis absorption spectra for the series of n = 10: (a) the
monomer state of complex II in mixed solvents CH2Cl2–CH3OH = 9 :
1 by volume; (b) the gel state in dichloromethane.
than that of the nonfluorescent monomer (the left side of
Fig. 3b) under the same concentration of 0.3 wt %. To the best
Significantly, a remarkable fluorescence enhancement induced
by self-assembly of the complexes was found (see Fig. 3a). When
excited at 365 nm (using the series of n = 10 as the example),
the fluorescence intensity for the complex in the CH2Cl2 gel-
phase (the right side of Fig. 3b) was almost 1000 times stronger
of our knowledge, this is the highest fluorescence modulation
among the previous organogel systems for showing fluorescent
enhancement.25 Furthermore, the red-shift of the fluorescence
maxima of the gel compared with the monomer state was
based on the stabilization effect of aggregation in the excited
2 5 1 0
O r g . B i o m o l . C h e m . , 2 0 0 5 , 3 , 2 5 0 8 – 2 5 1 2

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state, which matched the UV–vis absorption results. The dilute
solution in 40 lM CH2Cl2 also showed the yellowish–green
luminescence similar to the luminescence of the gel in CH2Cl2,
which suggested that the complex had already self-assembled
into H-bonded aggregates even in the dilute solution. This
was also confirmed by the IR absorption data being similar
to those of the gel. Generally, the enhanced emission is always
attributed to the combined effects of molecular planarization,
restricted molecules motions and J-aggregation formation.26
Here, considering the maximum absorption of the aggregated
complexes gives only one bathochromic shift, the molecular
planarization does not happen. Based on the UV–vis spectra,
the J-aggregations were formed in the self-assemblies and the
molecules arranged into a slanted stack and the transition to
the lower couple excited state of the molecules was allowed,
which favored the enhancement of the emission.27 When the hy-
drogen bonded aggregates formed, the interactions between the
molecules confined the rotation of the groups in the molecules.
Therefore, the synergetic effect of the limited molecular motions
and the formation of the J-aggregation induced the enhancement
of the fluorescence emission. This significant enhanced emission
may be useful for some sensors, such as an on/off fluorescent
switch which is sensitive to temperature.
lar self-assembling. It is hoped that such fluorescence modulated
organogels may be useful for some sensors. In addition, it
provides an extensive way to prepare functional gels via non-
covalent interactions.
Experimental
Measurements
1H NMR spectra were determined with JEOL JNM-
500EX. UV–visible absorption spectra were recorded with
a Shimadzu UV-2201 UV–visible spectrophotometer. Fourier
transform infrared (FT–IR) spectra were measured at rt on
a Nicolet Impact 410 FT-IR spectrometer. Scanning electron
microscopy (SEM) was taken with a Japan Hitachi model X-650
San electron microscope. Fluorescence spectra were recorded on
a Shimadzu RF-5301 Luminescence Spectrometer. Fluorescence
lifetimes were measured using the time correlated single photon
counting technique with FL920-fluorescence lifetime spectro-
meter. The excitation source was a nF900 nanosecond flashlamp.
Lifetimes were obtained by deconvolution of the decay curves.
Materials
To obtain further information on the nature of the excited
state, a nanosecond time-resolved fluorescence was examined.
The emission decay profiles were monitored in 450 nm for the
monomer state solution (40 lM in CH2Cl2–CH3OH system)
and 500 nm for aggregated state solution in 40 lM CH2Cl2. As
shown in Fig. 4, the monomer solution gave a short lifetime
of 1.2 ns and the aggregated state solution under the same
concentration showed a long lifetime of 3.5 ns. It implies that
the formation of aggregation restricts the rotation and vibration
of the groups in the molecules, so that longer emission lifetimes
were dectected.16a,26 The limited molecular motions led to the a
decreasing of the nonradiative relaxation process, which led to
the enhanced fluorescence emission effectively.
All the materials were used as received and the solvents were
dried and distilled prior to use. The alkoxybenzoyl chlorides
were prepared according to a general procedure and were used
without any further distillation.
4-(4-Octyloxybenzoyloxy)-4ꢀ-stilbazole (8SZ). 4-Hydroxy-
4ꢀ-stilbazole (3.0 g, 15.2 mmol) was dissolved in dry pyri-
dine (100 ml) and a solution of 4-octyloxybenzoyl chrolide
(17.3 mmol) in dry THF (20 ml) was added dropwise to the
pyridine solution within 30 min at rt. After the mixture had been
stirred overninght, most of the THF and pyridine was distilled
off. The residue was poured into a large amount of water, the
precipitate was filtered and recrystallized twice from ethanol
and twice from cyclohexane. Yield 80%. Elemental analysis
calculated for C28H31NO3: C, 78.30; H, 7.22; N, 3.26%. Found:
C, 78.04, H, 7.35; N, 3.23%. FT–IR (cm−1): 2915.3, 2850.7,
1
1721.9, 1609.4, 1592.3, 1511.8. H NMR (CDCl3, 300 MHz):
8.597 (2H, s, PyH), 8.145 (2H, d, ArH), 7.600 (2H, d, ArH),
=
7.360 (2H, d, PyH), 7.304 (1H, s, CH CH), 7.234 (2H, d, ArH),
=
7.027 (1H, s, CH CH), 6.968 (2H, d, ArH), 4.048 (2H, t, CH2),
1.300–1.851 (12H, m, CH2), 0.896 (3H, t, CH3).
4-(4-Decyloxybenzoyloxy)-4ꢀ-stilbazole (10SZ). This com-
pound was prepared in a similar way to 8SZ. Yield 82%.
Elemental analysis calculated for C30H35NO3: C, 78.52; H, 7.50;
N, 3.16%. Found: C, 78.57, H, 7.78; N, 3.27%. FT–IR (cm−1):
2915.1, 2849.2, 1722.1, 1609.3, 1592.6, 1511.8. 1H NMR (CDCl3,
300 MHz): 8.599 (2H, s, PyH), 8.150 (2H, d, ArH), 7.590 (2H,
=
d, ArH), 7.391 (2H, d, PyH), 7.356 (1H, s, CH CH), 7.234 (2H,
=
d, ArH), 7.028 (1H, s, CH CH), 6.972 (2H, d, ArH), 4.049 (2H,
Fig. 4 The fluorescence decay profiles of complex II for monomer (ꢁ)
in 40 lM cosolvent CH2Cl2–CH3OH = 9/1 and H-bonded aggregate
(ଝ) state in 40 lM CH2Cl2 solution. The samples were excited at 365 nm
and monitored at 450 nm and 500 nm respectively.
t, CH2), 1.284–1.851 (16H, m, CH2), 0.889 (3H, t, CH3).
4-(4-Dodecyloxybenzoyloxy)-4ꢀ-stilbazole
(12SZ). This
compound was prepared in a similar way to 8SZ. Yield 75%.
Elemental analysis calculated for C32H39NO3: C, 79.14; H,
8.09; N, 2.88%. Found: C, 79.10, H, 8.11; N, 2.86%. FT–IR
(cm−1): 2915.2, 2848.8, 1721.8, 1609.3, 1592.9, 1511.8. 1H
NMR (CDCl3, 300 MHz): 8.599 (2H, s, PyH), 8.148 (2H, d,
ArH), 7.590 (2H, d, ArH), 7.372 (2H, d, PyH), 7.337 (1H, s,
Conclusions
A series of functional binary gels with good gelated capability
based on L-tartaric acid and stilbazoles were prepared and
showed good gelated capability in most chlorinated solvents.
Herein, L-tartaric acid not only introduced the functional groups
(fluorescent stilbazole group) into the gel system, but also
provided a main motif for the formation of the gel-phase via
multiple hydrogen bonds, which provide a simple and more
extensive way for the fabrication of functional gels through
non-covalent interactions. The obtained gels showed strongly
enhanced emission and a longer lifetime due to the supramolecu-
=
=
CH CH), 7.263 (2H, d, ArH), 7.013 (1H, s, CH CH), 6.972
(2H, d, ArH), 4.040 (2H, t, CH2), 1.270–1.845 (20H, m, CH2),
0.884 (3H, t, CH3).
Acknowledgements
The authors acknowledge the National Nature Science Founda-
tion of China (NNSFC) for financial support of this work.
O r g . B i o m o l . C h e m . , 2 0 0 5 , 3 , 2 5 0 8 – 2 5 1 2
2 5 1 1

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15 (a) K. Sugiyasu, N. Fujita, M. Takeuchi, S. Yamada and S. Shinkai,
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2 5 1 2
O r g . B i o m o l . C h e m . , 2 0 0 5 , 3 , 2 5 0 8 – 2 5 1 2