Novel carbazole-based organogels modulated by tert-butyl moieties

Article abstract of DOI:10.1039/b713734f

tert-Butyl groups can modulate the self-assembling properties of carbazole derivatives; organogel fibers with a bright blue emission are generated, directed by the cooperation of hydrogen bonding as well as π-π interactions. The Royal Society of Chemistry.

Full text of DOI:10.1039/b713734f

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COMMUNICATION
www.rsc.org/chemcomm | ChemComm
Novel carbazole-based organogels modulated by tert-butyl moieties{
Xinchun Yang, Ran Lu,* Tinghua Xu, Pengchong Xue, Xingliang Liu and Yingying Zhao
Received (in Cambridge, UK) 6th September 2007, Accepted 19th November 2007
First published as an Advance Article on the web 28th November 2007
DOI: 10.1039/b713734f
tert-butyl groups instead of long alkyl or steroidal chains, that
can self-assemble into fibers with a strong fluorescent emission.
Compound 1 was synthesized via the hydrolyzing of 4-(3,6-di-
tert-butyl-9H-carbazol-9-yl)benzonitrile in a yield of 62%, which
itself was obtained from 4-(3,6-di-tert-butyl-9H-carbazol-9-yl)-
benzaldehyde.7 Under similar conditions, compound 2 was
tert-Butyl groups can modulate the self-assembling properties
of carbazole derivatives; organogel fibers with a bright blue
emission are generated, directed by the cooperation of
hydrogen bonding as well as p–p interactions.
Recently, low molecular mass organogels have received much
attention due to their unique supramolecular architectures and
potential applications in optoelectronic devices, template syntheses,
drug delivery and biomimetics systems.1 A good number of
organogelators have been designed, including derivatives of
perylene, phenylene, porphyrin, tetrathiafulvalene, carbohydrates,
amino acids, etc.,2 and most of them bear long carbon chains or
steroidal groups, favoring a balance between their solubility and
crystallization.3,4 Organogelators without steroidal units or long
alkyl chains, in which p–p interactions have important effects on
the self-assembling process, are rarely reported.5 It is well known
that carbazole is a typical p-conjugated system6 and a promising
candidate for optoelectronic materials due to its intense lumines-
cence and electron efficiency.7 In this Communication, we
present two novel carbazole-based organogelators, 4-(3,6-di-tert-
butyl-9H-carbazol-9-yl)benzamide (1) and 4-(3-(3,6-di-tert-butyl-
9H-carbazol-9-yl)benzamide (2) (Scheme 1), containing two
obtained
from
4-(3-(3,6-di-tert-butyl-9H-carbazol-9-yl)-9H-
carbazol-9-yl)benzaldehyde, which could be prepared via a two-
step Ullmann condensation reaction. In order to evaluate the effect
of tert-butyl units on gelation behaviour, compounds 3 and 4,
whose molecular structures are similar to compounds 1 and 2,
respectively, were synthesized.
The gelation ability of 1–4 in various organic solvents was
investigated by the ‘‘stable to inversion in a test tube’’ method.8 As
summarized in Table 1, we found that 1 showed robust gelation
capabilities in some apolar alkane liquids, such as cyclohexane,
hexane and petroleum ether. For example, a semi-transparent
cyclohexane gel was formed less than two minutes after a hot
solution was placed in room temperature surroundings with a
concentration as low as 0.2 wt% (Fig. 1(a)). In benzene, toluene,
chloroform, ethyl acetate, ethanol, THF and DMSO, compound 1
had a good solubility, while it precipitated from acetonitrile, so no
organogel could be obtained in these solvents. On the other hand,
compound 2 only formed an organogel in cyclohexane, and it
required more than 5 h for gel formation under the same
concentration as 1 (Fig. S2{). This indicates that compound 1
exhibits a better gelation ability than 2. However, compounds 3
and 4, with similar molecular structures to 1 and 2, respectively,
except for the absence of tert-butyl moieties, could not self-
assemble into gels in organic liquids, indicating that the tert-butyl
units play a key role in the formation of the organogels of 1 and 2.
Table 1 Gelation test of 1, 2, 3 and 4 in organic solvents
Solvent
1a
2a
3a
4a
Cyclohexane
n-Hexane
Petroleum ether
Benzene
Toluene
THF
Chloroform
Dichloromethane
Acetonitrile
Ethyl acetate
Ethanol
Methanol
1-Octanol
DMF
G
G
G
S
S
S
S
S
P
S
S
G
P
I
S
S
S
S
S
I
S
S
S
S
S
S
I
I
I
I
I
I
S
S
S
S
S
I
S
S
S
S
S
S
S
S
S
S
S
I
S
S
S
S
S
S
Scheme 1 The molecular structures of carbazole derivatives.
State 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: +86 431-88923907;
Tel: +86 431-88499179
{ Electronic supplementary information (ESI) available: Synthetic meth-
ods and characterization data for relevant compounds, SEM images, and
FT-IR and XRD spectra. See DOI: 10.1039/b713734f
S
S
S
S
DMSO
a
Gelator
P: precipitate.
=
0.2 wt%; G: stable gel; S: soluble; I: insoluble;
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Chem. Commun., 2008, 453–455 | 453

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spectrum of such a destroyed gel system, suggesting that some
hydrogen bonds were cleaved (Fig. S4{).
The UV-vis absorption spectra of compounds 1 and 2 in
cyclohexane solution and in their gel phase are shown in Fig. 2(a).
Compared with the cyclohexane solution of 1 (1 6 1026 M), in
which the molecules may be considered to be in a monomeric state,
the absorption bands of the cyclohexane gel of 1 are red-shifted
from 296, 331 and 342 nm to 300 and 335 nm, accompanied by a
broad shoulder at around 348 nm. A red shift of the absorption
bands of the cyclohexane gel of 2 also took place, compared to the
corresponding solution. These results indicate that p–p stacking
interactions between the aromatic moieties play an important role
in the formation of the gels, and might induce the formation of
J-aggregation.10
To reveal how the molecules packed into fibrous self-assemblies,
the XRD pattern of a dried gel of 1, obtained from cyclohexane,
was obtained (Fig. S5{). The small angle X-ray diffraction pattern
gave two peaks at d-spacings of 2.41 and 1.20 nm, and exactly in
the ratio 2 : 1, suggesting perfect lamellar organization with a
period of 2.41 nm in the aggregates of the gel of 1. The optimized
CPK model showed that the molecular length of 1 was 1.25 nm. It
is notable that the d value of 2.41 nm from the XRD pattern is
approximately twice the evaluated molecular length obtained from
the CPK model. We deduce that the molecules might form
bimolecular lamellar packing in the gel phase,11 which could be
supported by the formation of a hydrogen-bonded dimer, as
mentioned above. Therefore, we propose the molecular packing
model in the gel phase shown in Fig. 3. Bimolecular layers are
easily formed via hydrogen bonding, and the aromatic moieties are
enforced by p–p interactions to adopt the J-aggregation mode, and
further develop into 3-D superstructures.
Fig. 1 (a) Daylight and (b) 256 nm fluorescence emission-irradiated
photos of 1 in a cyclohexane solution at 70 uC (left vial) and gel at 25 uC
(right vial) at a concentration of 0.2 wt%. (c) TEM image, (d) AFM height
image (7.5 6 7.5 mm2) and (e) fluorescence microscopy image of a dried
gel of 1 obtained from cyclohexane.
The morphologies of the organogels were determined by SEM,
TEM and AFM. The SEM images (Fig. S1{) of the cyclohexane
gels of 1 and 2 show three-dimensional networks, consisting of
entangled bundles of fibres. From the TEM image (Fig. 1(c)) of
the cyclohexane gel of 1, we find that a great deal of the long fibres
consist of thin fibrils, which juxtapose or interwind with each
other, preventing the flow of solvent molecules. As shown in
Fig. 1(d), the AFM image of the cyclohexane gel of 1 shows that
the fibre bundles are built up from thin fibrils of 40–50 nm width
and several micrometers length. The AFM image of the
cyclohexane gel of 2 (Fig. S2{) shows many thicker and shorter
fibrils compared with those of the gel of 1, which also suggests that
compound 1 is a better organogelator than 2.
Fluorescence spectra may provide important information on the
molecular organization of fluorophores.12 Fig. 2(b) shows the
fluorescence emission spectra of 1 and 2 in cyclohexane solutions
and gel phases, respectively. When excited at 297 nm, 1 showed
two emission bands at 352 and 368 nm in dilute solution
(fluorescence quantum yield, WF = 0.37), and gave a broadened
band at around 402 nm in the gel state (WF = 0.75). Such red-
shifting of the emission bands results in a bright blue emission
from the gel phase, which could be confirmed by fluorescence
microscopy upon illumination with UV light (Fig. 1(e)), while the
emission of the solution was very weak (Fig. 1(b)). Compound 2
also showed a significant red-shift emission band at 456 nm in its
gel phase compared to those in solution (373 and 388 nm). Such
remarkable red-shifting of the emission could be ascribed to the
formation of p-aggregations in the gel phase. Therefore, the
In order to investigate the driving forces leading to the
formation of the organogels, FT-IR and UV-vis absorption
investigations were performed. Taking compound 1 as an example,
Fig. S3{ shows the IR spectrum of 1 in THF in a gel state. In THF
solution, the vibration absorption bands arising from the free
amide groups appear at 3448 (N–H bond stretching vibration
band) and 1682 cm21 (amide I band). For the cyclohexane gel of 1
(0.2 wt%), the band at 3448 cm21 disappears, while new bands at
3379 and 3209 cm21 are found, and the amide I band shifts from
1682 to 1651 cm21, suggesting the formation of hydrogen bonds
between the amide groups in the gel phase.8 Fortunately, in the
mass spectrum of compound 1, we found a strong peak at m/z =
797.4 (Fig. S13{), indicating the formation of a hydrogen-bonded
dimer. In addition, a small drop of methanol destroys the gel
completely into a transparent solution, which further confirms that
hydrogen bonding has an effect on the formation of the organogel
of 1.9 This is because a new peak at 1670 cm21 appears in the IR
Fig. 2 (a) UV-vis absorption and (b) PL emission (excited at 297 nm)
spectra of 1 and 2 in cyclohexane solution (1 6 1026 M) and as gels
(0.2 wt%).
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organogel.
fluorescence could be tuned by a reversible sol–gel transformation
(Fig. S6{).
In conclusion, we have found that tert-butyl groups can
modulate the gelation behavior of carbazole derivatives. The
fluorescence of compound 1 can be tuned by a reversible sol–gel
transformation, and an aggregation-induced bright blue emission
can be observed in the gel phase. Such unique organogelators may
have potential applications in sensor, switch and soft optical
materials.
This work was financially supported by the National Natural
Science Foundation of China (NNSFC, no. 20574027) and the
Program for New Century Excellent Talents in University
(NCET).
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