Rigid dendritic gelators based on oligocarbazoles

Article abstract of DOI:10.1039/b918986f

The first rigid dendritic gel with strong emission based on oligocarbazole was reported. It is interesting that the second generation of dendron exhibited gelation ability under ultrasound, and two-component gel could be formed from the first generation o

Full text of DOI:10.1039/b918986f

View Article Online / Journal Homepage / Table of Contents for this issue
COMMUNICATION
www.rsc.org/chemcomm | ChemComm
Rigid dendritic gelators based on oligocarbazolesw
Xinchun Yang, Ran Lu,* Fangyuan Gai, Pengchong Xue and Yong Zhan
Received (in Cambridge, UK) 14th September 2009, Accepted 26th November 2009
First published as an Advance Article on the web 16th December 2009
DOI: 10.1039/b918986f
The first rigid dendritic gel with strong emission based on
oligocarbazole was reported. It is interesting that the second
generation of dendron exhibited gelation ability under ultra-
sound, and two-component gel could be formed from the first
generation of dendron assisted by 1,6-diaminohexane.
chloroform–hexane under ultrasound. Although the first
generation of dendritic carbazole 1 can not form organogel
itself under heating–cooling and ultrasound treatment, robust
two-component gel assisted by 1,6-diaminohexane can be
obtained.
The synthetic routes and characterization data for the
carbazole-based dendrons 1, 2, and 3 were given in the ESI.w
After the hydrolyzation reaction in basic condition, the first
generation of carbazole dendron 1 could be obtained from
3,6-di-tert-butyl-9-(4-methoxycarbony l)-9H-carbazole, which
was synthesized from 3,6-di-tert-butyl-9H-carbazole9 and
1-iodo-4-methbenzoate via the Ullmann condensation
reaction. The second generation of carbazole dendron 2 could
also be synthesized via the hydrolyzation reaction of the
corresponding ester derivative. Due to the difficult hydrolyzation
for the third generation ester, we obtained the third generation
of carbazole dendron 3 from the aldehyde derivative under
oxidation by KMnO4.
Dendrons and dendrimers have attracted particular attention
in the construction of gel-phase materials since Newkome and
co-workers reported the first dendritic gelator based on
bola-amphiphiles.1 Dendritic gelators, as the bridge between
the polymeric and low-molecular-weight gelators, have lasted
in the extension of the gels due to their well-defined,
monodisperse, branched structures with a huge array of
different chemical functionalities as well as favorableness for
exploring structure-property relationships in gels.2 Especially,
the ‘dendritic function’ can be easily achieved from individual
dendritic gelators driven by multiple non-covalent weak inter-
actions avoiding time-consuming synthetic chemistry.3 Till
now, the reported dendritic gelators are focused on flexible
dendritic compounds.4 For instance, Aida’s group has
The gelation properties of the rigid carbazole-based
dendrons 1, 2 and 3 were tested in various solvents by means
of the ‘‘stable to inversion of a test tube’’ method (Table S1w).
It was found that 1–3 could not gel the tested solvents during
the typical heating–cooling process. Significantly, we found
that ultrasound irradiation could make the second generation
of dendron 2 form gels in chloroform–hexane (v/v = 1/10,
cgc = 3.5 Â 10À4 M) and chloroform/cyclohexane (v/v = 1/12),
while no organogel based on compounds 1 and 3 could be
obtained under similar conditions. In addition, considering the
carboxyl groups in dendrons 1–3 might interact with amines, a
series of aliphatic and aromatic amines, including dodecylamine,
hexylamine, 1,6-diaminohexane, ethylenediamine, aniline and
1,4-phenylenediamine, were employed to assist the gel formation
of the rigid dendrons 1–3. Notably, only two-component
organogel containing 1 and 1,6-diaminohexane (the molar
ratio is 2 : 1) could be gained in toluene after the hot solution
was cooled to room temperature within 1 min (cgc = 5.5 Â
10À4 M). However, the tested amines were unable to form gel
for 2 and 3. Therefore, the generation of dendron played a key
role in the gelation behaviors of the dendritic molecules.
designed the organogelators based on Frechet-type dendrons
´
functionalized at the focal point with different dipeptide
units.5 Smith and coworkers have investigated the unique
self-assembling behaviors of L-lysine dendrons in one-component
and two-component gel phases.6 However, no rigid dendritic
gelators have been reported so far. It is well-known that the
rigid dendrimers usually contain extended p-conjugated
structures, which possess important applications in organic
light emitting diodes, field effect transistors, solar cells,
molecular wires, and a light-harvesting molecular antenna
system.7 Therefore, the strategy for the construction of an
organogel system from a rigid dendritic organogletor would be
significant not only in the development of dendritic supra-
molecular chemistry but also in the extending gel-phase
material with unique photonic and electronic functionality.
In our previous work, we have employed monomeric and
dimeric carbazoles bearing tert-butyl groups for the gelation of
organic solvents.8 With these results in mind, we went on to
extend such a p-conjugated system and synthesized dendritic
3,9-linked carbazoles in order to gain organogelators based
on rigid dendrons (Scheme 1). It is interesting that the
second generation of dendritic carbazole 2 can form gel in
State Key Laboratory of Supramolecular Structure and Materials,
College of Chemistry, Jilin University, Changchun 130012,
P. R. China. E-mail: luran@jlu.edu.cn; Fax: +86-431-88923907;
Tel: +86-431-88499179
w Electronic supplementary information (ESI) available: Synthetic
methods and characterization data for relevant compounds. Gelation
properties of dendrons 1, 2 and 3, FT-IR spectra and XRD patterns.
See DOI: 10.1039/b918986f
Scheme 1 Molecular structures of carbazole-based dendrons 1, 2 and 3.
ꢀc
This journal is The Royal Society of Chemistry 2010
1088 | Chem. Commun., 2010, 46, 1088–1090

View Article Online
Fig. 2 UV-vis absorption (A) and fluorescent emission spectra (B) of
1 (’) and 2 (m) in solutions (1 Â 10À6 M) and in dried two-component
gel (&) containing 1 (7.0 mM) and 1,6-diaminohexane (3.5 mM)
obtained from toluene as well as dried gel 2 (n, 3.6 mM) obtained
from chloroform–hexane (v/v = 1/10), lex = 297 nm.
Fig. 1 SEM (A and C), TEM (B and D) and FM (E and F) images:
A, B and E for the two-component gel based on 1 and 1,6-diamino-
hexane (the molar ratio is 2 : 1) obtained from toluene, and C, D, and
F for gel 2 obtained from chloroform–hexane (v/v = 1/10).
absorptions at 301, 336 and 351 nm, which red-shifted from
298, 335 and 347 nm in dilute solution. So, p–p stacking
interaction played a key role in the formation of gel 2, and
the gelators might self-assemble into J-aggregates in a
gel state.
SEM and TEM images of the two-component gel
containing 1 (7.0 mM) and 1,6-diaminohexane (3.5 mM) in
toluene as well as gel 2 (3.6 mM) in chloroform–hexane
(v/v = 1/10) were shown in Fig. 1. It was found that two-
component organogel showed entangled bundles of fibers,
which were built up from thinner fibrils. Gel 2 created three-
dimensional networks consisting of fibrous bundles which
were longer than those in the two-component gel.
XRD studies further revealed the generation of well-ordered
microstructures in gel phases. The small angle X-ray
diffraction pattern of the two-component xerogel containing
1 and 1,6-diaminohexane gave three peaks at d-spacings of
2.70, 1.35 and 0.90 nm (Fig. S3w). It was exactly in the ratio of
1/1 : 1/2 : 1/3, suggesting a lamellar organization with a long
period of 2.70 nm. To further reveal the molecular packing
model, the optimized configurations of the organic salt based
on 1 and 1,6-diaminohexane was evaluated by the semi-
empirical quantum mechanical method (AM1 force field). As
a result, the extended length of the organic salt was calculated
to be 3.84 nm, which was larger than the long period (2.70 nm)
based on the XRD result. So, the molecular packing model in
the two-component gel was proposed as shown in Fig. 3A. The
organic salt complexes were stacked into one-dimensional
aggregates via hydrogen-bonding,13 which were tilting to the
normal of the lamella afforded by the assembling of 1D
structures.14 Nevertheless, only one peak at the d-spacing of
1.65 nm was detected for xerogel 2 (Fig. S3w). As calculated by
the semi-empirical quantum mechanical method (AM1 force
field), the length and width of the hydrogen-bonded dimer of
molecule 2 are 3.75 and 2.05 nm, respectively. Because the
length of dimer 2 is too large to match the XRD result, it is
reasonable to deduce that molecule 2 forms a lamellar structure
with a molecular width as a long period, as shown in Fig. 3B.
The fluorescent microscopy (FM) images as shown in Fig. 1
illustrated that the obtained nanofibers could emit strong blue
and bluish green light in two-component gel comprising 1 and
1,6-diaminohexane as well as in gel 2, respectively, excited at
330–385 nm. We subsequently studied the fluorescent
properties of dendrons 1 and 2, whose emission spectra in
solution and gel phases were shown in Fig. 2. When excited at
297 nm, the dilute solution of 1 gave an emission band at
407 nm, which only red-shifted to 412 nm in the two-component
gel phase. On the other hand, the emission of gel 2
red-shifted significantly to 458 nm compared to its dilute
solution (407 nm, chloroform–hexane, v/v = 1/10). It
suggested that p–p stacking interaction between molecules 2
are stronger than that between molecules 1 in the gel phases,
according to the results from the UV-vis absorption.
The FT-IR was a useful technique to investigate the driving
forces for the formation of the gel-phase materials.10 In
absence of 1,6-diaminohexane, the peaks located at 2670,
2550 and 1690 cmÀ1 were detected in powder 1, suggesting
the formation of intermolecular hydrogen-bonding between
carboxyl groups in the solid phase. However, in the
two-component gel the vibration peaks for powder 1 and
1,6-diaminohexane (3340, 1640, 1570 cmÀ1) disappeared, and
a new band at 1600 cmÀ1 emerged (Fig. S1w). It indicated that
the organic salt comprising of an anion of 1 and hexane-1,6-
diammonium was yielded.11 In addition, the observation of
the peaks at 2654, 2553, 1698 and 1609 cmÀ1 for gel 2
illustrated the formation of a hydrogen-bonded dimer of
carboxylic acid (Fig. S2w), so hydrogen-bonding played an
important role in the formation of gel 2.12 However, in the IR
spectrum of powder 3 we could find the peaks at 1720 and
1630 cmÀ1, but the peaks at 2500–2700 cmÀ1 were absent,
indicating that compound 3 existed as a monomer in solid
state because the branched conformation might restrain the
formation of a hydrogen-bonded dimer of acid. This is one of
the reasons why dendron 3 can not form gel. Therefore, we
may conclude that the generation of the dendron could affect
the strength of the intermolecular interactions, resulting in
different gelation abilities of the rigid dendrons.
The UV-vis absorption spectra of 1 and 2 in dilute solutions
and in gel phases were shown in Fig. 2. We can find that p–p*
transition band of dendron 1 appeared at 297 nm in toluene,
and shifted to 301 nm in the two-component gel. Meanwhile,
the absorption bands ascribed to the n–p* transition emerged
at 336 and 346 nm in the solution, and shifted to 335 and
347 nm in the gel phase. Such a slight shift of the absorption
bands indicated that p–p stacking interaction had less effect on
the formation of the two-component gel. Nevertheless, a new
broad shoulder at 356–416 nm was detected in gel 2 obtained
from chloroform–hexane (v/v = 1/10) except for three
ꢀc
This journal is The Royal Society of Chemistry 2010
Chem. Commun., 2010, 46, 1088–1090 | 1089

View Article Online
Notes and references
1 (a) G. R. Newkome, G. R. Baker, S. Arai, M. J. Saunders, P. Russo,
V. K. Gupta, Z. Yao, J. E. Miller and K. Bouilion, J. Chem. Soc.,
Chem. Commun., 1986, 752; (b) G. R. Newkome, G. R. Baker, S. Arai,
M. J. Saunders, P. S. Russo, K. J. Theriot, C. N. Moorefield,
L. E. Rogers and J. E. Miller, J. Am. Chem. Soc., 1990, 112, 8458.
2 (a) G. H. Kuang, Y. Ji, X. R. Jia, E. Q. Chen, M. Gao, J. M. Yeh and
Y. Wei, Chem. Mater., 2009, 21, 456; (b) Y. L. Chen, Y. X. Lv, Y. Han,
B. Zhu, F. Zhang, Z. S. Bo and C. Y. Liu, Langmuir, 2009, 25, 8548.
3 (a) A. R. Hirst and D. K. Smith, Top. Curr. Chem., 2005, 256, 237;
(b) D. K. Smith, Chem. Commun., 2006, 34.
4 (a) E. R. Zubarev, E. D. Sone and S. I. Stupp, Chem.–Eur. J., 2006,
12, 7313; (b) C. Kim, K. T. Kim and Y. Chang, J. Am. Chem. Soc.,
2001, 123, 5586; (c) H. S. Ko, C. Park, S. M. Lee, H. H. Song and
C. Kim, Chem. Mater., 2004, 16, 3872.
5 (a) W. D. Jang, D. L. Dong and T. Aida, J. Am. Chem. Soc., 2000, 122,
3232; (b) W. D. Jang and T. Aida, Macromolecules, 2003, 36, 8461.
6 (a) J. G. Hardy, A. R. Hirst, D. K. Smith, C. Brennan and
I. Ashworth, Chem. Commun., 2005, 385.
7 (a) A. W. Bosman, H. M. Janssen and E. W. Meijer, Chem. Rev.,
1999, 99, 1665; (b) S. C. Lo and P. L. Burn, Chem. Rev., 2007, 107,
1097; (c) T. H. Xu, R. Lu, X. L. Liu, X. Q. Zheng, X. P. Qiu and
Y. Y. Zhao, Org. Lett., 2007, 9, 797; (d) K. Albrecht and
K. Yamamoto, J. Am. Chem. Soc., 2009, 131, 2244; (e) J. Q. Ding,
B. Wang, Z. Y. Yue, B. Yao, Z. Y. Xie, Y. X. Cheng, L. X. Wang,
X. B. Jing and F. S. Wang, Angew. Chem., Int. Ed., 2009, 48, 6664.
8 (a) X. C. Yang, R. Lu, T. H. Xu, P. C. Xue, X. L. Liu and Y. Y. Zhao,
Chem. Commun., 2008, 453; (b) X. C. Yang, R. Lu, P. C. Xue, B. Li,
D. F. Xu, T. H. Xu and Y. Y. Zhao, Langmuir, 2008, 24, 13730.
9 T. H. Xu, R. Lu, X. L. Liu, P. Chen, X. P. Qiu and Y. Y. Zhao,
J. Org. Chem., 2008, 73, 1809.
10 (a) C. Y. Bao, R. Lu, M. Jin, P. C. Xue, C. H. Tan, T. H. Xu,
G. F. Liu and Y. Y. Zhao, Chem.–Eur. J., 2006, 12, 3287;
(b) P. C. Xue, R. Lu, G. J. Chen, Y. Zhang, H. Nomoto,
M. Takafuji and H. Ihara, Chem.–Eur. J., 2007, 13, 8231;
(c) P. Chen, R. Lu, P. C. Xue, T. H. Xu, G. J. Chen and
Y. Y. Zhao, Langmuir, 2009, 25, 8395.
Fig. 3 Schematic illustration of the layered structures with a period
of (A) 2.7 nm in two-component gel based on 1 and 1,6-diamino-
hexane (the molar ratio is 2 : 1), and (B) 1.65 nm in gel 2.
In conclusion, for the first time, we have reported rigid
dendritic gelators based on oligocarbazoles. It is interesting
that the second generation of dendron 2 exhibited gelation
ability under ultrasound, and the robust organogel can be
formed from the first generation of dendron 1 assisted by
1,6-diaminohexane. In addition, the obtained gels gave strong
fluorescence emission. It should be noted that such rigid
dendritic gel systems with strong emission as well as ordered
microstructures may find applications in optical and opto-
electronic materials. Meanwhile, the design of rigid dendritic
gelators provides a new strategy for development of dendritic
supramolecular chemistry.
11 (a) M. George and R. G. Weiss, J. Am. Chem. Soc., 2001, 123, 10393;
(b) M. Suzuki, H. Saito and K. Hanabusa, Langmuir, 2009, 25, 8579.
12 X. Z. Luo, W. Xiao, Z. F. Li, Q. Wang and J. L. Zhong, J. Colloid
Interface Sci., 2009, 329, 372.
This work is financially supported by the National Natural
Science Foundation of China (NNSFC, No. 20874034),
973 Program (2009CB939701), and the Open Project of State
Key Laboratory of Supramolecular Structure and Materials
(SKLSSM200901).
13 (a) P. Sahoo, N. N. Adarsh, G. E. Chacko, S. R. Raghavan,
V. G. Puranik and P. Dastidar, Langmuir, 2009, 25, 8742;
(b) D. R. Trivedi, A. Ballabh, P. Dastidar and B. Ganguly,
Chem.–Eur. J., 2004, 10, 5311.
14 P. C. Xue, R. Lu, D. M. Li, M. Jin, C. Y. Bao, Y. Y. Zhao and
Z. M. Wang, Chem. Mater., 2004, 16, 3702.
ꢀc
This journal is The Royal Society of Chemistry 2010
1090 | Chem. Commun., 2010, 46, 1088–1090