Synthesis and self-assembly of dichalcone substituted carbazole-based low-molecular mass organogel

Article abstract of DOI:10.1039/b602520j

We report the synthesis and self-assembly of a new π-conjugated dichalcone substituted carbazole-based low molecular mass organogelator. It could form stable gels in most halogen-aromatic solvents. The transmission electron microscopy (TEM) images revealed that the gel formed fibrous structures with diameter of 50-100 nm, which consisted of several thinner fibers. The FT-IR, UV-vis and XRD results suggested that the H-bonds and π-π interactions were the main driving forces for the formation of the self-assembled gel, in which the U-shaped molecules were stacked into lamellar structures. The fluorescent spectra showed that the emission of the xerogel red-shifted markedly compared with the sol state, which resulted from the aggregation of the molecules. The Royal Society of Chemistry 2006.

Full text of DOI:10.1039/b602520j

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PAPER
www.rsc.org/obc | Organic & Biomolecular Chemistry
Synthesis and self-assembly of dichalcone substituted carbazole-based
low-molecular mass organogel
Lihong Su, Chunyan Bao, Ran Lu,* Yulan Chen, Tinghua Xu, Dongpo Song, Changhui Tan, Tongshun Shi and
Yingying Zhao
Received 20th February 2006, Accepted 16th May 2006
First published as an Advance Article on the web 23rd May 2006
DOI: 10.1039/b602520j
We report the synthesis and self-assembly of a new p-conjugated dichalcone substituted
carbazole-based low molecular mass organogelator. It could form stable gels in most halogen-aromatic
solvents. The transmission electron microscopy (TEM) images revealed that the gel formed fibrous
structures with diameter of 50–100 nm, which consisted of several thinner fibers. The FT-IR, UV-vis
and XRD results suggested that the H-bonds and p–p interactions were the main driving forces for the
formation of the self-assembled gel, in which the U-shaped molecules were stacked into lamellar
structures. The fluorescent spectra showed that the emission of the xerogel red-shifted markedly
compared with the sol state, which resulted from the aggregation of the molecules.
Introduction
account of their unique features and potential applications for
2
3
4
new soft organic materials, template synthesis, drug delivery,
Gels are always thought to be generated through the initial
assembly of the gelator molecules into fibrous nanostructures
which then further organize into a three dimensional lattice,
trapping the solvent within the free space of the network. The
gels formed from small organic molecules in organic solvents are
5
separations and biomimetics.
Well-known organogelators including, for instance, certain
6,7
8,9
10,11
derivatives of carbohydrates,
cholesterol
amino acids,
urea
and
12–15
have been prepared and their self-assembling prop-
erties have been well studied. Nowadays, there has been increasing
interest in the development of functional gels with p-conjugated
systems due to their potential applications in various optoelec-
tronic fields, such as enhanced charge transport, fluorescence and
1
often called physical gels or supramolecular gels. The following
general guidelines for the design principles are accepted: i) the
presence of strong self-complementary and unidirectional interac-
tions to enforce one-dimensional self-assembly; ii) control of the
interfacial energy between the fibers and solvent to counterpoise
the solubility and crystallisation; and iii) some factor to induce
fibers to form cross-linking network. In these low molecular mass
organogel systems, the three-dimensional network is held together
by noncovalent forces, such as hydrogen bonding, p–p stacking,
electrostatic forces, and van der Waals interactions. Low molecular
mass organogels have received considerable attention recently on
16
sensing abilities. As an attempt to obtain a new functional
organogelator with potential photonics applications, we have
designed and synthesized a p-conjugated dichalcone substituted
carbazole derivative 1, which can form stable gels in halogen-
aromatic solvents, and cannot be destroyed for several months. To
the best of our knowledge such a carbazole-based organogel is the
firs reported among the limited previous functional organogels.
Compound 1 was synthesized from dialdehyde carbazole 2 and
4-N-lauroylamino-acetophenone 4 (Scheme 1). It was found that
1
could form a stable gel in halogen-aromatic solvents via the
Key Laboratory of Supramolecular Structure and Materials, College of
Chemistry, Jilin University, Changchun, 130012, P. R. China. E-mail:
luran@mail.jlu.edu.cn; Fax: +86 431 8923907
lamellar stacking of the molecules with U-shaped configuration.
The fluorescence emission of the xerogel 1 red-shifted markedly
Scheme 1 The synthetic route for the dichalcone substituted carbazole-based organogel.
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Table 1 Gelation test of 1 in different solvents
a
a
Entry Solvent
Gelation
Entry Solvent
Gelation
1
2
3
4
Toluene
THF
Benzene
Xylene
I
S
I
5
6
7
8
Chlorobenzene
Bromobenzene
Acetonitrile
G
G
P
I
G
Ethyl acetate
a
Gelator = 2.0% (wt/vol); G, stable gel formed at room temperature; S,
soluble; I, insoluble; P, precipitate.
compared with the sol state, which resulted from the aggregation
of the molecules.
Fig. 2 The FT-IR spectra of xerogel 1 from chlorobenzene.
Results and discussion
The gelation ability of compound 1 was estimated in different sol-
vents. As summarized in Table 1, we found that 1 could form stable
gels in xylene and halogen-benzene, and the self-assembled gels
were thermoreversible in these solvents, for example, the gelator
was insoluble at room temperature, and then turned into a clear so-
lution by heating to reflux, upon cooling to room temperature, the
immobilized gels appeared. The morphologies of the gels were ex-
amined by TEM, whichshowed fibers with diameters of50–100nm
and the fibers consisted of several thinner fibers with diameters of
1
5–20 nm (as shown in Fig. 1). In the previous gel system, hydrogen
bonding-directed self-assembly is a well studied mechanism for the
formation of the superstructure in an arranged system, whereas
for compound 1, p–p interactions may also play a key role on
the formation of the organogel due to the extended p-conjugated
moiety. To study the driving forces for the self-organization of
organogelator 1, FT-IR and UV-vis spectra were measured.
−
6
Fig. 3 The UV-vis spectra for the chlorobenzene solution at 10 M (the
−
5
solid line), and at 10 M (the dashed line), and the chlorobenzene gel at
.0 (wt/vol)% (the dotted line).
2
to 420 nm with a shoulder at 449 nm in the gel phase compared
with that at 395 nm (with a shoulder at 422 nm) for the sol state.
It illustrated that p–p interactions happened and J-aggregation
18
was formed in the gel state. Therefore, it indicated that the H-
bonds and p–p interactions were the main driving forces for the
formation of self-assembled gel.
To reveal how the molecules packed into the fibrous self-assem-
blies, XRD was investigated for the xerogel 1 obtained from chloro-
benzene. In Fig. 4, the small-angle diffraction pattern of the xerogel
showed two peaks at a d-spacing of 3.35 nm and 1.64 nm, which is
1
2
Fig. 1 TEM images of 1 in chlorobenzene gel at 2.0 (wt/vol)%. (a), the
close to 1 : , indicating the layered stacking of the molecules with
whole image; (b), the magnified image of a single fiber.
a period of 3.35 nm. The optimized structure for the molecule by
CPK molecular modeling showed that the molecule was present
as a U-shaped configuration with a length of 3.4 nm, which was
similar to the result from XRD. Therefore, we could conclude that
the U-shaped molecules formed lamellar packing of J-aggregates
in favor of H-bonding and p–p interactions. The stacking model
is shown in Scheme 2 (edge-on view and top-on view) based on
the above results and the optimized molecular structure.
The FT-IR spectra (as shown in Fig. 2) for the xerogel 1 from
−
1
chlorobenzene showed an absorption at 3355 cm for the N–H
−
1
−1
stretching vibration, 1650 cm for amide I and 1564 cm for
amide II, which suggested that weak hydrogen bonding between
17
amide units was formed in the gel state. Fig. 3 shows the UV-
vis absorption of the gel and the corresponding dilute solution at
−
6
the concentration of 1 × 10 M, in which the molecules may be
considered as monomers. The absorption band of 1 red-shifted
Fluorescence spectra also provide important information on
19
the molecular organization of fluorophores. Fig. 5 shows the
2
592 | Org. Biomol. Chem., 2006, 4, 2591–2594
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−
5
and 10 M, and gave a red-shifted emission peak (around 500 nm)
−
4
at 10 M and a more red-shifted emission peak (around 520 nm) in
the gel state. The UV-vis spectra depending on the concentration
in the Fig. 3 also showed a red-shift, which suggested that the
molecules would aggregate when the concentration increased, and
the large degree of p-aggregations induced the great red-shift of
the fluorescence emission for the gel. Therefore, the fluorescence
could be modulated by controlling the aggregated degree of the
molecules, which may be useful for some sensors and some other
optical devices.
Conclusion
A new organogelator with good gelated capability in halogen-
aromatic solvents based on dichalcone substituted carbazole
derivative was prepared. From the TEM images, FT-IR and UV-
vis results, it suggested that the molecules stacked into a fibrous
structure in favour of H-bonds and p–p interactions, and J-
aggregates formed. Combining the XRD result and the optimized
molecular configuration, the U-shaped molecules were stacked
into lamellar structures. The fluorescent spectra indicated that
the emission could red-shift obviously with increasing of the
aggregated degree of molecules, which provided a simple way to
modulate the emission band by controlling the aggregated degree
of the molecules.
Fig. 4 The XRD diffraction of the xerogel 1 from chlorobenzene.
Experimental
The dry tetrahydrofuran (THF) was distilled after refluxing in
Na–benzophenone for 3 h. The lauroyl chlorides were prepared
according to the general procedure and were used without any
further distillation. Other chemicals were used as received.
Measurements
1
1
H nuclear magnetic resonance ( H NMR) spectra were deter-
Scheme 2 The stacking model for 1 in the gel state based on the
CPK optimized configuration from the edge-on view and top-on view,
respectively.
mined with a Varian-300 EX spectrometer. UV-visible absorption
spectra were recorded with a Shimadzu UV-2201 UV-visible
spectrophotometer. Fourier transform infrared (FT-IR) spectra
were measured at room temperature on a Nicolet Impact 410 FT-
IR spectrometer. X-Ray diffraction (XRD) patterns were carried
out on a Japan Rigaku D/max-cA instrument. XRD was equipped
˚
with graphite monochromatized Cu-Ka radiation (k = 1.5418 A),
◦
−1
◦
employing a scanning rate of 0.02 s in the 2h range from 0.7
◦
to 10 . Transmission electron microscopy (TEM) was taken with
a Hitachi modes H600A-2 apparatus by wiping the samples onto
a 200-mesh copper grid followed by naturally evaporating the
solvent. Fluorescence spectra were measured on a Japan Hitachi
8
50 fluorescence spectrophotometer at room temperature.
Synthesis of compounds
-N-Lauroylamino-acetophenone (4). A mixture of 4-amino-
4
Fig. 5 The fluorescent spectra for1 at different concentrations (a), 10− M;
6
acetophenone (1.62 g, 0.012 mol) and triethylamine (4 mL,
.03 mol) in dry THF (10 mL) was added dropwise to a solution
−
5
−4
(
b), 10 M; (c), 10 M; and (d), gel state at 2.0 (wt/vol)% in chlorobenzene.
0
of lauroyl chloride (2.6 g, 0.012 mol) in THF (10 mL) at
0 C. After stirring overnight at room temperature the solution
◦
fluorescence spectra of compound 1 in chlorobenzene solutions
at different concentration and in the chlorobenzene gel state,
respectively. When excited at 325 nm, compound 1 showed an
was poured into water (100 mL), and the precipitate was filtered
and recrystallized in ethanol. Yield: 3.2 g, 85%. Mp 109.0–
−
6
◦
−1
emission peak at 460 nm under lower concentrations at 10
M
110.0 C. FT-IR (cm ): 3324, 2917, 2848, 1678, 1658, 1606, 1536;
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1
H NMR (CDCl
6H, CH ), 1.73 (m, 2H, CH
), 7.63 (d, 2H, ArH), 7.94 (d, 2H, ArH).
3
, 300 MHz, ppm): d 0.87 (t, 3H, CH
3
), 1.26 (m,
allowed to stand at room temperature for 5 h, the state of the
mixture was evaluated by the “stable to inversion of a test tube”
method.
1
CH
2
2
), 2.58 (s, 3H, COCH ), 2.90 (m, 2H,
3
20
2
9
-Butylcarbazole (3). Carbazole (6 g, 0.036 mol), benzyltri-
Acknowledgements
ethylammonium chloride (BTEAC, 0.2 g), and 1-bromobutane
were dissolved in benzene (20 mL), then NaOH aqueous (50%,
0 mL) was added, the obtained mixture was stirred and refluxed
for 3 h. Then the solvent benzene was removed under the reduced
pressure, and water (200 mL) was added to the flask and a white
solid formed. The white solid was collected by filtration and
washed with water. The solid was recrystallized from a mixture
of ethanol and water and gave white needle crystals. Yield: 6.4 g,
The work was financially by the National Natural Science
Foundation of China (NNSFC, No. 20574027).
1
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594 | Org. Biomol. Chem., 2006, 4, 2591–2594
This journal is © The Royal Society of Chemistry 2006