Bimodal self-assembly of an amphiphilic gelator into a hydrogel-nanocatalyst and an organogel with different morphologies and photophysical properties

Article abstract of DOI:10.1039/c6cc06971a

We design a flexible, amphiphilic LMWG consisting of donor and acceptor π-chromophores which self-assembles into a hydrogel and an organogel with different nano-morphologies. Different mechanisms of self-assembly evolve charge transfer (CT) emission in th

Full text of DOI:10.1039/c6cc06971a

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Table of Contents (TOC)
Solvent-dependent, bimodal self-assembly of a flexible, amphiphilic LMWG results in a charge-
transfer hydrogel and organogel with different nano-morphologies and the hydrogel is used as
nanocatalyst for Knoevenagel condensation reaction.

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Bimodal self-assembly of an amphiphilic gelator to hydrogel-
nanocatalyst and organogel of different morphologies and
photophysical properties
Received 00th January 20xx,
Accepted 00th January 20xx
Papri Sutar and Tapas Kumar Maji*
DOI: 10.1039/x0xx00000x
www.rsc.org/
we design a flexible, amphiphilic LMWG consists of donor and explored base-type catalytic site.7 However, inaccessibility of
acceptor π-chromophores which self-assembles to hydrogel and catalytic sites due to small pore size or pore blocking often
organogel with different nano-morphologies. Different limit their activity. In this respect hydrogels of amide-
mechanisms of self-assembly evolve charge transfer (CT) emission functionalized LMGWs are superior as wide pores of gel-
in hydrogel and LMWG-based emission in organogel. Moreover, network facilitate easy diffusion of reactive species towards
the hydrogel-nanostructure with surface exposed amide groups is the catalytic sites.8 Since hydrogels are metal-free and contain
explored for catalyzing Knoevenagel condensation reactions.
huge amount of water within, they are environmentally more
benign compared to MOFs. Moreover, hydrogel-nanocatalysts
could host different hydrophilic substrates on their outer
surface and their high surface-to-volume ratio could facilitate
contact with catalytic sites. However, use of LMWG-derived
hydrogel as heterogeneous catalyst for Knoevenagel
condensation is yet to be accounted. Herein, we design a new
Over a past few decades, there has been a growing interest in
the development of hydrogels or organogels from the self-
assembly of low molecular weight gelators (LMWGs).1 Owing
to the dynamic behaviour and high processability, such
supramolecular gels exhibit impending applications in diverse
fields, including tissue engineering, bio-medicine, opto-
electronics.2 Although many elegant design strategies have
been implemented to construct a number of LMWGs, it is still
a daunting task to design a prior a LMWG which can act as
both organogelator and hydrogelator.3 This is because a critical
balance between hydrophobicity and hydrophilicity is required
to design such LMWG. In this regard, a novel strategy would be
to design an amphiphilic LMWG with donor and acceptor π-
chromophores, connected by flexible alkylamide chain. Self-
assembly of such LMWG not only advances the possibility to
form CT gel, but also could result in different nanostructures of
hydrogel and organogel depending upon polarity of the
solvent medium.4 By considering the extensive opto-electronic
applications of supramolecular CT nanostructures, it is worth
to design such LMWGs which can pave the way towards
adaptive soft-material.5
C3-symmetrical LMWG
tri(methoxy)]-tris-benzene (tmb) core, connected to three
2,2 :6 ,2 -terpyridyl (tpy) moieties through flexible alkylamide
chain. The self-assembly of in aqueous and organic medium
(L) with a 4,4′,4″-[1,3,5-phenyl-
′
′ ″
L
result in hydrogel (HG) and organogel (OG) with nanosphere
and nanofiber morphologies, respectively. The HG exhibits CT
emission, whereas the OG shows normal LMWG-based
emission. Due to the presence of surface-exposed amide
Knoevenagel condensation is an important base-catalyzed
reaction in research and industry.6 Till date, many amide
functionalized metal organic frameworks (MOFs) or
microporous organic polymers (MOPs) are utilized as
heterogeneous Knoevenagel catalysts because amide is a well-
Scheme 1 Schematic showing the self-assembly of L to hydrogel and organogel and
their catalytic activity.
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Fig.1 (a) Photograph of HG hydrogel under day light and UV-light. (b) FESEM image of HG xerogel, inset showing FESEM image of single nanosphere. (c) TEM image of HG xerogel.
(d) High resolution TEM image of HG xerogel. (d) PXRD of HG xerogel. (f) Mechanism of self-assembly of L in HG. (g) UV-Vis spectra of methanolic solution of L (10-5, Black), solid L
(Red) and HG xerogel (Blue). (h) Emission spectra of methanolic solution of L (10-5, Red), solid L (Blue) and HG xerogel (Black).
groups, HG is exploited as heterogeneous nanocatalyst in corresponding to the d-spacing value of 3.6 Å (Fig. 1e and
Knoevenagel condensation reaction (Scheme 1).
The synthesis of is carried out via amide coupling between HG xerogel with
-[1,3,5-phenyl-tri(methoxy)]-tris-benzoic acid and and -C=O stretching frequency which indicates no H-bonding
,2 -terpyridin-4 and gelation is driven by only π-π
-yl-propane-1,3-diamine (Fig. S1-S3).2e interaction between
(10-5 M) shows a broad absorption stacking/hydrophobic interaction between tmb and tpy groups
S9).Comparison of fourier transform infrared (FTIR) spectra of
L
L
, does not exhibit significant change in -N-H
4,4
′
′
,4
″
′
2,2
:6
″
′
L
Methanolic solution of
L
band at 250-320 nm which could be assigned to the π-π* (Fig. S10). UV-Vis spectrum of HG xerogel exhibits maxima at
transition of the tmb core and terminal tpy moieties (Fig. S4- 290 nm and a broad shoulder at 410 nm which could be
S6). Upon excitation at 280 nm, the same solution exhibits attributed to the interligand CT transition between π-stacked
blue emission with maxima at 420 nm which could be tmb (
considered as combined emission from tmb and tpy groups of Ar tpy intraligand charge transfer (ILCT) transition is reported
(Fig. S4-S6). Gelation propensity of
is thoroughly checked in for many Zn2+ complexes of Ar-tpy derivatives.10 Upon exciting
various solvents. The solution of (9 mg/ml) in H2O/MeOH at 290 nm HG xerogel shows cyan emission with maxima at
mixture forms the translucent hydrogel HG at room 478 nm (Fig. 1h). Moreover, the intensity of the emission
π-donor) and tpy groups (π-acceptor) (Fig. 1g). Similar
→
L
L
L
λ=
(
)
temperature (Fig. 1a). Upon shaking, HG is transformed into a maxima increases upon excitation at 400 nm, further
sol, which again turned back to a gel on resting, thereby indicating CT based emission in HG (Fig. S11). Excitation
confirming its thixotropic nature. Insight into the morphology spectra show presence of ground state interaction, thus ruling
of HG is obtained by recording field emission scanning electron out the possibility of excited state exciplex formation (Fig.
microscopy (FE-SEM) and transmission electron microscopy S12).11 To further confirm the CT interaction, the absorption
(TEM) images of corresponding xerogel. FE-SEM and TEM and emission spectra of
L in MeOH/H2O mixture are recorded
images of HG xerogel reveal the presence of nanospheres with by gradually adding H2O in the MeOH solution of
L
. With
700±100 nm diameter (Fig. 1b, 1c, S7 and S8). High resolution incremental addition of H2O, the absorption spectra of
L show
TEM image shows that surface of the nanospheres consists of appearance of
a
new peak at 410 nm, suggesting
in mixed solvent
layered sheets (Fig. 1d). Formation of the nanosphere occurs intermolecular CT interaction of aggregated
L
through a two-step hierarchical self-assembly process as (Fig. S13). The corresponding emission spectra (λex=400 nm)
shown in Fig. 1f.9 First, in polar solvent the molecules are exhibit red shift of the emission maxima at 420 nm with
assembled through intermolecular π-π stacking between the increase in intensity, confirming CT emission of aggregated
tmb and tpy moieties and result in extended 2D sheets. As mixed solvent (Fig. S14).Comparison of absorption spectra of
time proceeds, the π-stacked tmb and tpy groups tend to HG xerogel with solid , shows that the CT band at 410 nm
minimize their contact with H2O/MeOH due to hydrophobic becomes more prominent in HG (Fig. 1g). This result indicates
effect, whereas amide groups present on the sheets tend to be that the CT interaction between the stacked tmb and tpy
exposed towards the polar medium due to H-bonding with groups becomes prominent in the ordered assembly of HG
L in
L
π-π
.
H2O/MeOH. As a result, sheets are folded to nanospheres Confocal fluorescence microscopy images of HG xerogel
whose exterior are decorated with amide groups. The confirm the formation of cyan emissive nanospheres with an
presence of π-π stacking in HG is supported by powder X-ray average diameter of 700±100 nm (Fig. S15).
diffraction (PXRD) which shows peak at 2θ= 24.5°,
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higher than the G
'
value of HG indicating greater mechanical
strength of OG compared to HG
.
Table
1 The catalytic reactions and the results with varying substituent on the
Fig. 2 (a) Photograph of OG organogel under day light and UV-light. (b) FESEM image of
OG xerogel. (c) TEM image of OG xerogel. (d) Mechanism of self-assembly of L in OG.
(e) PXRD of OG xerogel. (f) UV-Vis and emission spectra of OG xerogel.
aldehydes.
The presence of free amide groups on the exterior of
nanosphere of HG xerogel prompted us to study its catalytic
activity towards the Knoevenagel condensation of aldehydes
and active methylene compounds. In a typical condensation
reaction p-nitrobenzaldehyde (1 equiv), malononitrile (1
equiv), and HG xerogel (1 mol %) are taken in dry THF and
stirred at 40°C under inert conditions. Formation of product is
Heating solution of L (8 mg/ml) in CHCl3/THF mixture followed
by cooling results in an opaque organogel (OG) (Fig. 2a).
FESEM and TEM images of OG xerogel reveal the typical
fibrous morphology (Fig. 2b, 2c and S16). Nanofibers are
several micrometer long with an average diameter of 80-100
nm. The presence of nanorings is also observed which are
formed by coiling of nanofibers. In CHCl3/THF mixture, self-
1
confirmed by H-NMR and GC-MS analysis (Fig. S23, S24). The
quantitative analysis of the product conversion is monitored
by a GC-MS analyser at regular intervals of times. The reaction
kinetics shows a sharp increase in conversion (69%) in 20
minutes, after 2 h 90% conversion of product is observed and
after 4 h almost all aldehyde is converted to product (99%)
(Fig. S25). However, no detectable amount of product is
assembly of
interaction between amide groups and π-π stacking between
tmb cores of neighbouring and results in 1D fibril (Fig. 2d).
L is driven by complementary H-bonding
L
These fibrils upon aggregation form fibers which further
entangle to form fibrous gel architecture. The presence of H-
bonding and π-π stacking in OG is supported by PXRD and FTIR.
obtained, when L is used as catalyst under the similar reaction
Comparison of FTIR spectra of
L
and OG xerogel exhibits
condition. This suggests that the direct contacts of the
substrate with the amide groups present on the surface of
nano-spheres are essential for catalytic activities. The catalytic
activity of HG xerogel towards different p-substituted
benzaldehyde (X = H, NO2, Cl, CH3 and OCH3) is studied and the
conversion results are summarized in Table 1. (Fig. S26-S29).
Although the conversion of product is significant for X= H, NO2
and Cl, the yield is reduced for X= CH3, OCH3. This suggests that
electron withdrawing group (NO2, Cl) at the para-position of
benzaldehyde enhances the electrophilicity of the carbonyl
carbon and thereby increases its reactivity.12 However, the
electron donating group (OCH3, CH3) reduces the
electrophilicity of the carbonyl carbon and therefore exhibits
lesser yield. The catalyst could be reused for at least 4 cycles.
TEM images of the catalyst after 4th cycle show retention of
morphology. The size distribution histogram plots of HG
xerogel before and after the catalytic reaction also indicate
decrease in − O and
C
=
−N−H stretching frequencies from 1693
cm-1 to 1688 cm-1 and from 3436 cm-1 to 3420 cm-1,
respectively, indicating the presence of intermolecular H-
bonding between the amide groups of
L (Fig. S17). PXRD
analysis of OG xerogel exhibits broad peck at 2θ= 21.7°,
corresponding to d = 4.0 Å which indicates the π-π stacking in
organogel (Fig. 2e and S18). Absorption spectrum of OG
xerogel shows maxima around 290 nm. The xerogel exhibits a
blue emission with maxima at 430 nm (λex=290 nm),
originating from both tmb and tpy, similar to the methanolic
solution of
L (Fig. 2f).This also suggests that self-assembly of L
is different in HG and OG. Oscillatory strain and frequency
sweep measurements on HG and OG show that storage
modulus (G
range of strain (%) indicating viscoelastic nature of the gels
(Fig. S19-S22). Interestingly, the G value of OG is two-fold
′) is higher than the loss modulus (G″) over a long
'
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S30, S31). The PXRD of HG xerogel do not change after
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amide groups are essential for the catalytic activity, we have
performed same catalytic reactions with nanofiber OG xerogel.
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12 hours. This suggests that OG xerogel cannot catalyze the
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The preceding results demonstrate design of a LMWG which
can self-assemble in both aqueous and organic medium
resulting into hydrogel and organogel, respectively. Hydrogel
shows unique spherical morphology and CT emission.
However, the organogel shows typical fibrous morphology.
The nanospheres of hydrogel are utilized for catalysing
Knoevenagel condensation reaction, whereas the nano-fibers
of organogel fail to show catalytic activity due to inaccessibility
of free amide groups. This work would open up a new
perspective to rationally design LMWG that can form both
organogel and hydrogel and explore their catalytic
applications. Furthermore, the effect of metal ion coordination
on self-assembly of this LMWG and their properties can be
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TKM is grateful to the Department of Science and Technology
(DST, Project No. MR-2015/001019), Government of India
(GOI) and JNCASR for financial support. PS is thankful to
Council of Scientific and Industrial Research (CSIR), GOI for
fellowship.
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