Mutual responsive hydrazide-based low-molecular-mass organic gelators: probing gelation on the molecular level

Article abstract of DOI:10.1002/chem.200800540

A study was conducted to demonstrate two gel-responsive and solvent polarity sensitive hydrazide based low molecular-mass organic gelators (LMOG) through specific interactions and a large volume of immobilized organic liquid. The study demonstrated that gel-solution phase of these LMOGs can be achieved in a manner, which will find wide applications in designing functional materials. The structures of hydrazide-based oligomers were also modified, during continuous hydrogen bonding mediated self-assembly. It was found that hydrogen bonding donor/acceptor sites that reside at the termini of each duplex interacts intermolecularly, to form a polymeric hydrogen bonding mediated supramolecular zipper structure, in addition to forming molecular duplex strands. It was also observed that addition of small amount of hydrogen bonding competitive solvents, such as methanol, led to sol-gel transformation.

Full text of DOI:10.1002/chem.200800540

DOI: 10.1002/chem.200800540
Mutual Responsive Hydrazide-Based Low-Molecular-Mass Organic Gelators:
Probing Gelation on the Molecular Level
Yong Yang, Ting Chen, Jun-Feng Xiang, Hui-Juan Yan, Chuan-Feng Chen,* and
Li-Jun Wan*[a]
Organogels are a new class of soft materials, which are
composed of a self-assembled suprastructure of low molecu-
lar-mass organic gelators (LMOGs) through specific interac-
tions and a large volume of organic liquid immobilized
therein.[1] Stimulated by their wide potential applications in
nanomaterials and delivery or modification agents for
paints, inks, cleaning agents, cosmetics, and drugs, great ef-
forts have been paid to this kind of new materials during
the past decade. Different types of LMOGs have been de-
veloped, including steroid,[2] amino acid,[3] anthryl deriva-
tives,[4] urea and thiourea derivatives,[5] sugar compounds,[6]
linear p-systems and chromophores.[7] However, most of
these LMOGs were discovered largely serendipitously or
via structural modification on the known gelators, and the
relationship between the structures of LMOGs and the
structural and rheological properties of their self-assembled
fibrillar networks (SAFINs) is not well understood. Special-
ly, stimuli sensitive or “smart” or “responsive” materials,[8]
such as photoactive gelators[9] featuring azo and stilbene
groups, pH-sensitive gelators,[9c,10] and electroactive gela-
tors[11] are very appealing for potential applications. In this
paper, we report, to the best of our knowledge, for the first
time two mutual “gel responsive” and solvent polarity sensi-
tive hydrazide based LMOGs.[12] Their gel–solution phase
transition can be achieved in a predictable fashion, which,
we believe, will find wide applications in the design of func-
tional materials.
thesize diacetyl-terminated oligomer 1a from N1’-acetyl-4,6-
bis(octyloxy)-benzene-1,3-dihydrazide and malonic acid
AHCTREUNG
using EDC·HCl as coupling agent in CH2Cl2 (Scheme 1),
stirring at room temperature for several hours resulted in a
viscous solution. With purified 1a at hand, extensive heating
also led to a solution in CHCl3 at a concentration of about
2% (w/w) viscous enough to perform top-down experiment.
This viscous mixture demonstrated gravity-induced flow
after a few minutes upon being inverted (Figure 1a). Addi-
tion of a small amount of methanol into the viscous mixture
resulted in a clear solution. A precipitate then formed when
more methanol was added. Similar results were also ob-
tained with the dibutyryl-terminated oligomer 1b. The only
difference is that the concentration competent for top-down
experiment dropped to as low as 1% (w/w). With isobuty-
loxy-derived oligomer 1c, only a precipitate in chloroform
was found. Atomic force microscope (AFM) images (Fig-
ure 1a) of xerogels of 1a or 1b showed needle-like nanofib-
er structures. Scanning electronic microscope (SEM) images
also provided similar results.[16] We further investigated self-
assembly of 1a on HOPG (High Oriented Pyrolytic Graph-
ite) by scanning tunneling microscope (STM) technology.
As provided in Figure 1b, STM image revealed an offset di-
meric structure for 1a. We propose that in contrast to oligo-
mers with complementary hydrogen boding sites,[15a] in addi-
tion to forming molecular duplex strands, abundant hydro-
gen bonding donor/acceptor sites that reside at the termini
of each duplex may further interact intermolecularly to
form a polymeric hydrogen bonding mediated supramolec-
ular zipper structure (Scheme 1). Further entanglement of
the polymeric zippers resulted in entrapment of solvent mol-
ecules. Van der Waals interactions among the interdigitated
octyl side chains as found in the X-ray structure of diacetyl-
terminated monomer[13a] may also play an important role in
the formation of the viscous mixture. Because the main in-
termolecular interactions stabilizing the self-assembled net-
works are hydrogen bonding, addition of competitive sol-
vents such as methanol can substantially affect the self-as-
sembly process, and ultimately destroy the networks which
During our ongoing project of hydrogen bonding mediat-
ed self-assembly,[13–15] we systematically modified the struc-
tures of hydrazide-based oligomers. When we tried to syn-
[a] Dr. Y. Yang, T. Chen, Dr. J.-F. Xiang, Dr. H.-J. Yan, Prof. C.-F. Chen,
Prof. L.-J. Wan
Beijing National Laboratory for Molecular Sciences
Institute of Chemistry, Chinese Academy of Sciences
Beijing 100190 (China)
Fax : (+86)10-62554449
E-mail: cchen@iccas.ac.cn
Supporting information for this article is available on the WWW
under http://www.chemistry.org or from the author.
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COMMUNICATION
Scheme 1. Synthesis and representation of self-assembly of 1 in apolar solvent, with hydrogen bonding highlighted.
entrapped solvent molecules to form organogels. Compound
1a in chloroform also displayed a concentration dependency
in the 1H NMR spectra. Because of the viscous mixtures,
only a solution of 1a in CDCl3 as low concentration as 1 mm
displayed a resolvable spectrum. Addition of 2% CD3OD
into a viscous solution of 5 mm of 1a in CDCl3 also rendered
a resolvable spectrum. These findings also confirmed the
above hypothesis.
0.8%, w/w) 2 in CHCl3 could form a gel. Addition of a
small amount of hydrogen bonding competitive solvents
such as methanol also led to gel–sol transformation. AFM
image (Figure 2a) on xerogel revealed entangled fibrous
structure. The longest fibers reached micrometer scale. We
propose that as in the case of 1, abundant hydrogen bonding
sites also led to a polymeric zipper structure as shown in
Scheme 2. Gelation was not observed with other similar
structures as diphenyl- or monophenyl-mononaphthyl termi-
nated oligomers.[17] In addition to hydrogen bonding and van
der Waals interactions, p–p
Gelation was also observed with dinaphthyl terminated
oligomer 2 (Scheme 2). As low concentration as 10 mm (ca.
stacking interactions between
naphthalene rings as depicted
in Scheme 2 may also play an
important role in stabilizing the
junction zone and further the
gel formation. Interestingly, the
STM image revealed a polymer-
ic double helical structure for 2
on HOPG (Figure 2b).
As stated above, intermolec-
ular hydrogen bonding interac-
tions and/or p–p stacking inter-
actions promoted aggregation
Figure 1. a) AFM image of 1a on silica plate, showing the needle-like nanofiber structure, with digital photo-
graphs of (from left to right) gels of 1a in CHCl3, 2% (w/w); 1b in CHCl3, 1% (w/w); 1a in CHCl3, 2% (w/w)
demonstrating gravity-induced flow inserted. Scan size=5.00 um, Scan rate=1.001 Hz, Data scale=100 nm. b)
STM image of 1a on HOPG, showing the offset dimeric structure. The imaging conditions: Ebias =À500 mV,
along one dimension to form
nanofiber structures, and fur-
ther entanglement of these
nanofibers into networks ach-
I
tip =612.4 pA.
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C.-F. Chen, L.-J. Wan et al.
Scheme 2. Representation of self-assembly of 2 in apolar solvent, with hydrogen bonding and p–p stacking interactions highlighted.
solution. Signals for 2-Ha and 2-
He displayed up-field shifts.
This may result from their spe-
cific positions: they were at the
frontier of complexation. Con-
tacts between 1-Hf and 2-He, g
zone and h zone were observed
in the 2D NOESY spectrum
(Figure 4), which provided a di-
agnostic evidence for the hete-
rodimer structure. Variable
temperature 1H NMR experi-
ments revealed unique dynamic
behavior for complex 1·2: mul-
tiple minor signals for NHs ap-
peared in the area between d 8
and 9 ppm and there displayed
a complicated picture in the
Figure 2. a) AFM image of 2 on silica plate, showing the entangled fibrillar networks structure, with digital
photograph of gel from 2 in CHCl3, 0.8% (w/w) inserted. Scan size=8.00 um, Scan rate=0.5003 Hz, Data
scale=20 nm. b) STM image of 2 on HOPG, showing the polymeric double helical structure. The imaging con-
ditions: Ebias =793.9 mV, Itip =570.6 pA.
ieved entrapment of apolar solvents. Examination of the
chemical structures of 1 and 2 revealed that the hydrogen-
bonding sites in 1 and 2 were complementary. We wonder if
there exists a multipoint recognition directed supramolec-
ular substitution reaction to form a discrete heterodimer via
eight hydrogen bonds as depicted in Scheme 3, which blocks
abundant hydrogen-bonding sites, renders polymeric zipper
structures impossible and thus leads to mutual responsive
organogels? Our hypothesis was first evidenced by 1:1 mix-
ture of 1 and 2 in CDCl3. A clear solution appeared
(Scheme 3) and 1H NMR experiment revealed a well re-
solved spectrum (Figure 3). However, 1 alone in CDCl3 at
10 mm can not give a resolvable spectrum due to the viscous
area between d 10.5 and 12.5 ppm at low temperatures
(Figure 3). A contact between 1-Hf and h zone was also ob-
served in 2D NOESY spectrum. These findings might sug-
gest intermolecular slide perpendicular to hydrogen bonds
between the two constituent molecules of the heterodimer.
In conclusion, we reported in this paper two mutual re-
sponsive hydrazide based oligomers 1 and 2 as LMOGs. In
addition to forming hydrogen bonded molecular duplex
strands, abundant hydrogen-bonding sites and/or p–p stack-
ing interactions further promoted aggregation along one-di-
mension to form fibrous structures. Because the main non-
covalent forces stabilizing the self-assembled networks are
hydrogen bonding, the gels are solvent polarity sensitive.
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Organic Gelators
COMMUNICATION
and 2 are complementary and a
multipoint recognition directed
supramolecular substitution re-
action leads to gel–sol phase
transition. Thus the gel–sol
transition can be controlled in a
predictable fashion. We believe
the new gel systems we present-
ed here will find wide applica-
tions in the future.
Experimental Section
Chemical preparation: Gelators 1 and
2 were prepared from coupling reac-
tions of corresponding hydrazide and
acid derivatives using EDC·HCl as
coupling agent. The full synthetic
routes and characterization data for
new compounds were provided in the
supporting information.
Gel preparation: Gelators 1 or 2 in a
sealed tube were heated extensively in
methanol free chloroform (typically
2 mL) until all solids dissolved. Upon
cooling at room temperature, gels
were obtained.
STM measurement: A Nanoscope IIIa
SPM (Digital Instruments, Santa Bar-
bara, CA) was employed to carry out
the STM experiments using a standard
constant-current mode under ambient
condition. The samples for the STM
measurements were prepared by plac-
ing a drop of octylbenzene solution
containing 1 mm corresponding com-
pounds on a freshly cleaved atomically
Scheme 3. Representation of supramolecular substitution reaction between 1 and 2, which leads to gel–solution
phase transition, with a photograph of a solution of 1 and 2 1:1 in CHCl3, each 2% (w/w) inserted and proton-
labeling Scheme indicated.
For a 1:1 ratio of 1 and 2 in apolar solvents, gelation was no
longer observed, which could be explained on the basis of
the chemical structures that the hydrogen-bonding sites in 1
Figure 3. Stacked partial 1H NMR spectra of a) 2 in CDCl3 at 298 K
(10 mm, gel state); 1 and 2 1:1 in CDCl3 at b) 298 K, c) 253 K, d) 223 K,
each 10 mm, 600 MHz.
Figure 4. Partial 2D NOESY spectrum of 1 and 2 1:1 in CDCl3, with con-
tacts between g zone and h zone, 1-Hf and 2-He, 1-Hf and h zone shown,
each 10 mm, 300 MHz, 298 K.
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C.-F. Chen, L.-J. Wan et al.
flat surface of HOPG (ZYB quality). STM tips were mechanically cut Pt/
Ir wire 90:10. All STM images are presented in the paper without further
processing. The tunneling conditions used are given in the corresponding
Figure captions.
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Acknowledgements
We thank the National Natural Science Foundation of China (20625206),
CMS-Y 200708 and National Basic Research Program of China
(2007CB808004, 2008CB617501) for financial support.
Keywords: hydrazides
·
hydrogen bonds
·
gels
·
self-assembly · supramolecular chemistry
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Received: March 25, 2008
Published online: May 13, 2008
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