Pyridyl-Amides as a Multimode Self-Assembly Driver for the Design of a Stimuli-Responsive π-Gelator

Article abstract of DOI:10.1002/asia.201500331

An oligo(p-phenylenevinylene) (OPV) derivative connected to pyridyl end groups through an amide linkage (OPV-Py) resulted in a multistimuli-responsive π-gelator. When compared to the corresponding OPV π-gelator terminated by a phenyl-amide (OPV-Ph), the aggregation properties of OPV-Py were found to be significantly different, leading to multistimuli gelation and other morphological properties. The pyridyl moiety in OPV-Py initially interferes with the amide H-bonded assembly and gelation, however, protonation of the pyridyl moiety with trifluoroacetic acid (TFA) facilitated the formation of amide H-bonded assembly leading to gelation, which is reversible by the addition of N,N-diisopropyethylamine (DiPEA). Interestingly, addition of Ag⁺ ions to a solution of OPV-Py facilitated the formation of a metallo-supramolecular assembly leading to gelation. Surprisingly, ultrasound-induced gelation was observed when OPV-Py was mixed with a dicarboxylic acid (A₁). A detailed study using different spectroscopic and microscopic experimental techniques revealed the difference in the mode of assembly in the two molecules and the multistimuli-responsive nature of the OPV-Py gelation.

Full text of DOI:10.1002/asia.201500331

DOI: 10.1002/asia.201500331
Full Paper
Self-Assembly
Pyridyl-Amides as a Multimode Self-Assembly Driver for the
Design of a Stimuli-Responsive p-Gelator
[a]
[a]
[a]
Kalathil K. Kartha, Vakayil K. Praveen, Sukumaran Santhosh Babu,
[a, b]
[a, b]
Sandeep Cherumukkil,
and Ayyappanpillai Ajayaghosh*
Abstract: An oligo(p-phenylenevinylene) (OPV) derivative
connected to pyridyl end groups through an amide linkage
to gelation, which is reversible by the addition of N,N-diiso-
+
propyethylamine (DiPEA). Interestingly, addition of Ag ions
(
OPV-Py) resulted in a multistimuli-responsive p-gelator.
to a solution of OPV-Py facilitated the formation of a metal-
lo-supramolecular assembly leading to gelation. Surprisingly,
ultrasound-induced gelation was observed when OPV-Py
When compared to the corresponding OPV p-gelator termi-
nated by a phenyl-amide (OPV-Ph), the aggregation proper-
ties of OPV-Py were found to be significantly different, lead-
ing to multistimuli gelation and other morphological proper-
ties. The pyridyl moiety in OPV-Py initially interferes with the
amide H-bonded assembly and gelation, however, protona-
tion of the pyridyl moiety with trifluoroacetic acid (TFA) fa-
cilitated the formation of amide H-bonded assembly leading
was mixed with a dicarboxylic acid (A ). A detailed study
1
using different spectroscopic and microscopic experimental
techniques revealed the difference in the mode of assembly
in the two molecules and the multistimuli-responsive nature
of the OPV-Py gelation.
Introduction
in the formation of one-dimensional (1-D) fibers is a good way
[
6,7]
of controlling the molecular assembly and gel formation.
p-Gels are an intriguing class of soft materials created through
Several amide-functionalized self-assembling systems and
gelators containing ferrocene, tetrathiafulvalene, thiophene,
pyridine, etc. are known to respond to various external stimuli
[1]
the self-assembly of p-systems. Research on these materials
has been intensively pursued by virtue of their interesting op-
toelectronic properties useful for applications in organic elec-
[
7,8]
such as redox reagents, metal ions, acids, and ultrasound. In
such cases, favorable and/or preferred aggregation pathways
[1–3]
tronics and photonics.
In many of the cases, self-assembly
[
8]
of p-systems results in the formation of extended superstruc-
tures that leads to the gelation of solvents depending on fac-
tors such as structure of molecules, solvent polarity, tempera-
can be triggered by the introduction of a second component.
For this purpose, pyridine is a preferred moiety, which is
[
9–11]
known to interact with different metal ions and acids.
Even
[1–4]
ture, and concentration.
Recent studies have revealed that
though a number of assemblies and gels based on pyridine-
functionalized systems are known, a clear understanding of
the role of pyridyl amide on the aggregation and gelation
properties of p-systems is lacking.
to attain a desired molecular organization and gel formation,
control over self-assembly and its nucleation-elongation path-
[5,6]
ways are important.
Nucleation followed by elongation
through extended amide H-bonding and p-stacking resulting
We have reported a number of functionalized OPVs and
their gelation from nonpolar hydrocarbon solvents at extreme-
[
12–14]
ly low concentrations.
Through a series of experiments,
the role of functional groups in OPVs and the role of solvents
[
a] Dr. K. K. Kartha, Dr. V. K. Praveen, Dr. S. S. Babu, S. Cherumukkil,
Prof. Dr. A. Ajayaghosh
[
12–14]
in the gelation process have been revealed.
Our success
Photosciences and Photonics Group, Chemical Sciences and Technology Di-
with OPV-based p-gelators have encouraged several research
vision
groups to modify OPV backbones with different functionali-
CSIR-National Institute for Interdisciplinary Science and Technology (CSIR-
NIIST)
[15]
ties. Herein, we demonstrate that the integration of a pyridyl
nitrogen as an additional noncovalent interacting position in
the OPV gelator (OPV-Ph) results in a dramatic change in the
self-assembly and gelation behavior, leading to a new multisti-
muli-responsive p-gelator (OPV-Py).
Trivandrum 695 019, Kerala (India)
E-mail: ajayaghosh@niist.res.in
[
b] S. Cherumukkil, Prof. Dr. A. Ajayaghosh
Academy of Scientific and Innovative Research (AcSIR)
CSIR-National Institute for Interdisciplinary Science and Technology (CSIR-
NIIST)
Trivandrum 695 019, Kerala (India)
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/asia.201500331.
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band at 518 nm was observed (Figure 2a). Similarly, a decrease
in temperature resulted in quenching of the fluorescence in-
tensity due to the formation of OPV-Ph aggregates (Figure S1a
in the Supporting Information). Contrary to the behavior of
OPV-Ph, no considerable shift in the absorption maximum was
noted for OPV-Py with an increase in temperature in toluene
(Figure 2b). However, a strong red-shifted vibronic shoulder
band at 520 nm was observed with relatively less decrease in
the intensity of the p–p* transition band at 448 nm. The emis-
sion of OPV-Py aggregates in toluene at 208C exhibited a shift
to lower wavelengths with an increase in intensity at higher
temperatures, indicating the breaking of the self-assembly (Fig-
Results and Discussion
The phenyl-amide functionalized gelator, OPV-Ph and the pyr-
idyl-amide functionalized OPV derivative, OPV-Py (Figure 1),
were synthesized as depicted in Scheme S1 in the Supporting
Information. For this purpose, the OPV-bisester (OPV2) was
synthesized using a Wittig–Horner reaction of the phospho-
ure S1b in the Supporting Information). In addition, we found
À5
that OPV-Py in CHCl (5”10 m; Figure S2 in the Supporting
3
Information) has the tendency to form aggregates, whereas
OPV-Ph was found to be highly soluble and exists as the mon-
omeric species (Figure S3 in the Supporting Information).
The stability of the aggregates was determined from the
plot of the fraction of aggregate (aagg) versus temperature (Fig-
ure 2insets). A melting transition temperature (T , temperature
m
at which aagg = 0.50) of 598C was obtained for OPV-Ph in tolu-
ene, whereas OPV-Py aggregates showed a Tm of 54 and 338C
in toluene and chloroform, respectively. These observations
suggest that the aggregation modes of OPV-Ph and OPV-Py
are significantly different from each other, which emphasize
the importance of the terminal functional moiety in the amide
linkage in controlling the aggregation properties. Having seen
a difference in the aggregation modes of the two molecules,
we further examined their gel formation in toluene. As expect-
ed, OPV-Ph formed a gel in toluene at a critical gelator con-
centration (CGC) of 0.45 mm (Figure S4 in the Supporting Infor-
mation). Surprisingly, OPV-Py did not form a gel, even at
higher concentrations, underlining that the difference in the
mode of aggregation has a significant influence on the gela-
tion behavior.
Figure 1. Molecular structures of phenyl-amide and pyridyl-amide functional-
ized OPV derivatives.
nate ester of ethyl 2-bromoacetate with the OPV-bisaldehyde
(
OPV1), which was prepared according to a known procedur-
[12b,16]
e.
Hydrolysis of OPV2 using KOH in methanol yielded
OPV3. The final reaction between OPV3 and the amine deriva-
tive was conducted using 1[bis(dimethylamino)methylene]-1H-
1
,2,3-triazolo[4,5-b]pyridinium 3-oxid hexafluorophosphate
(
HATU) in the presence of DiPEA. All these compounds were
1
13
characterized using FT-IR, H NMR, and C NMR spectroscopies
and MALDI-TOF mass spectrometry.
Variable-temperature absorption spectral changes of OPV-Ph
and OPV-Py in toluene (5”10 m) exhibited a distinct behav-
The above observations indicate that OPV-Ph preferably
forms an H-type assembly through the amide H-bonding (Fig-
ure 3a), whereas OPV-Py likely forms a random assembly (Fig-
ure 3b). Formation of such non-gelling random assembly can
be explained by the interference of the terminal pyridyl moiety
with the amide H-bonding in OPV-Py. If so, protonation of the
pyridyl moiety should facilitate the aggregation of
À5
ior as shown in Figure 2. OPV-Ph in toluene at 708C showed
an absorption maximum at 445 nm, corresponding to the
monomeric species. At 208C, a blue-shifted absorption band at
À1
lmax =420 nm (Dl=1338 cm ) with a small vibronic shoulder
the OPV-Py through amide H-bonding, leading to
the gelation of the solvent. As hypothesized, addition
of 1 equivalent of TFA to the solution of OPV-Py in
À5
CHCl (5”10 m) resulted in a red-shift in the ab-
3
sorption maximum from 448 to 474 nm with a slight
decrease in the intensity (Figure 4a). In addition, the
emission was significantly quenched and became
broad with loss of vibronic features (Figure 4ainset).
Addition of 2 equivalents of TFA induced a further
shift of the absorption maximum to 492 nm and the
emission maximum to 610 nm (Figure 4a). These
Figure 2. Variable-temperature absorption changes of (a) OPV-Ph and (b) OPV-Py in tolu-
ene (5”10 m). Arrows indicates relative changes in absorption with increase in temper-
ature from 20 to 708C. Insets show plots of fraction of aggregate (aagg) vs. temperature.
Figure 2a inset: OPV-Ph in toluene (*). Figure 2b inset: OPV-Py in toluene (~) and
changes in absorption and emission wavelengths are
also visible in the photographs (Figure S5a and S5b
À5
in the Supporting Information) of the CHCl solutions
3
chloroform (
&
).
of OPV-Py before (greenish-yellow color) and after
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Figure 3. Schematic representation of the self-assembling mode of (a) OPV-Ph and (b) OPV-Py, and the stimuli-responsive self-assembly of OPV-Py in the
+
presence of (c) TFA, (d) Ag , and (e) the aromatic dicarboxylic acid A
1
.
À5
the addition of TFA (orange-red color). Obviously, these spec-
tral shifts of OPV-Py with TFA are different from the blue-shift
in the absorption spectrum of OPV-Ph, indicating that the
mode of assembly in both molecules is different. While in the
latter case an H-type assembly (Figure 3a) is evident, the
former case points towards a J-type assembly (Figure 3c). Fur-
ther evidence for aggregation of OPV-Py in the presence of
OPV-Ph in CHCl (5”10 m) did not show any considerable
3
changes in the absorption and emission spectra (Figure S6 in
the Supporting Information), which point towards the impor-
tance of the pyridine heteroatom in the OPV-Py for inducing
a stimuli-responsive character.
Upon increasing the temperature from 25 to 508C, OPV-Py
in CHCl in the presence of 2 equivalents of TFA (Figure 4b)
3
1
À1
TFA is obtained from the H NMR spectrum, which exhibited
displayed a blue-shift (Dl=963 cm ) in the absorption maxi-
a broadening of peaks upon addition of TFA (Figure S5c in the
Supporting Information). The red-shift in the absorption and
mum without much change in the intensity, and the fluores-
cence profile showed a slight increase in the intensity. The de-
crease in the absorbance at 560 nm with increasing tempera-
ture must be due to the breaking of the protonated OPV-Py
aggregates. Addition of 2.2 equivalents of organic base (DiPEA)
to OPV-Py-TFA aggregates resulted in the formation of mono-
1
emission spectra as well as broadening of H NMR signals indi-
cate the formation of extended hierarchical assemblies of OPV-
Py in the presence of TFA and are further revealed from mor-
phological studies (see below). Addition of TFA to a solution of
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+
concentration of Ag ions (Figure 4d). At 1:1 molar
ratio, the absorption and emission maxima were red-
shifted with a further decrease in the intensity. Upon
increasing the concentration of OPV-Py, a stable gel
+
in CHCl was obtained in the presence of Ag ions.
3
This observation can be attributed to the formation
of an extended linear metallosupramolecular polymer
in solution (Figure 3d), the growth of which may be
assisted by amide H-bonding and p-stacking.
The overall multistimuli-responsive behavior of
OPV-Py is summarized in Figure 5. Upon addition of
+
different external agents, like TFA, A , and Ag , to
1
a required concentration of OPV-Py in CHCl , gelation
3
was observed after heating followed by cooling. Sub-
[
18]
sequently, a two-component gel was derived from
a hot toluene solution of OPV-Py-A upon sonication
1
for 2 min. The critical gelator concentration (CGC)
and gel melting temperature (T ) were determined
gel
by the ‘inversion tube method’ and the ‘dropping
[
12]
ball method’, respectively. The results are summar-
ized in Tables S1 and S2 in the Supporting Informa-
tion. The CGC of the OPV-Ph toluene gel is 0.45 mm,
+
a value lower than those of the OPV-Py-Ag gel in
CHCl (2.83 mm), OPV-Py-TFA gel in CHCl (2.92 mm),
Figure 4. (a) Absorption spectra of OPV-Py in CHCl
a) equiv. of TFA. (b) Variable-temperature absorption spectra of OPV-Py in CHCl
taining 2 equiv. of TFA. (c) Absorption spectra of OPV-Py-TFA before (a) and after
3
containing 0 (c), 1 (d), and 2
3
3
(
3
con-
OPV-Py-A1 gel in CHCl (3.27 mm), and the sonica-
3
tion-induced gel in toluene (3.15 mm). Gel melting
temperature studies showed a regular increase in gel
stability with increasing concentration (Figure S7 in
the Supporting Information). At a particular concen-
(
c) addition of 2.2 equiv. of DiPEA. (d) Absorption spectra of OPV-Py in chloroform
(
containing 0 (c), 0.5 (d) and 1 (a) equiv. of AgOTf. Insets show the correspond-
À5
ing emission spectra. The concentration of OPV-Py was 5”10 m in all experiments.
(
1
e) Molecular structure of the dicarboxylic acid derivative A .
mers as evident from the absorption and emission spectral
shift owing to deprotonation followed by the complete disas-
sembly of the OPV-Py (Figure 4c).
With these results in hand, we attempted the preparation of
H-bonded supramolecular polymers of OPV-Py with an appro-
priate carboxylic acid. We have chosen a few organic acids for
this purpose as shown in Chart S1 in the Supporting Informa-
tion. Refluxing OPV-Py with the appropriate acid in a 1:1 stoi-
chiometry in THF for 2 h at 608C followed by the removal of
the solvent afforded the H-bonded complexes. Among the dif-
ferent acids tested, 4,4’-(nonane-1,9-diylbis(oxy))bis(3,5-dime-
thoxybenzoic acid), A1 (Figure 4e), gave the best results. At
higher concentrations, the OPV-Py-A complex in CHCl result-
Figure 5. Multimode gelation of two-component systems based on OPV-Py.
1
3
+
ed in a stable gel upon heating and cooling, whereas precipi-
tation occurred in toluene. Surprisingly, the precipitates of the
tration, OPV-Py-TFA and OPV-Py-Ag gels in CHCl exhibited
3
a maximum stability in comparison to OPV-Py-A CHCl and
1
3
OPV-Py-A complex formed a weak gel in toluene upon appli-
cation of ultrasound (0.23 Wcm , 37 kHz) for 2 min. The soni-
toluene gels.
1
À2
Morphological features of OPV-Py assemblies prepared from
toluene and CHCl (5”10 m) were examined by atomic force
À5
cation-induced gelation is most likely due to re-organization of
the noncovalent interactions, as commonly observed in the
3
microscopy (AFM) and transmission electron microscopy (TEM).
Formation of short fiber bundles of 1–3 mm in length, 100–
400 nm in width, and 50–300 nm in height were observed for
the assemblies from toluene (Figure 6a and Figure S8a in the
Supporting Information). However, the morphological feature
of the assemblies obtained from CHCl3 was different from
those obtained from toluene. In this case, AFM images dis-
played short rods of 1–3 mm in length, 50–250 nm in width,
[
17]
case of ultrasound-induced gelation of H-bonded systems.
Finally, we tried to modulate the aggregation properties of
+
OPV-Py with Ag , which is known to coordinate with a pyri-
[
9]
dine moiety. We titrated a solution of OPV-Py in CHCl (5”
3
À5
1
0
m) with silver triflate (AgOTf). Electronic absorption and
fluorescence spectral studies revealed that absorption and
fluorescence intensities were decreased with an increase in the
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À5
Figure 7. AFM images of OPV-Py-A
1
aggregates in toluene (5”10 m)
(a) before and (b) after sonication for 2 min. Insets show the corresponding
cross-section analysis.
The packing of the molecules in the self-assembled state
was investigated using FT-IR spectroscopy and X-ray diffraction
(
XRD) techniques. The xerogel film of OPV-Ph in toluene
showed IR absorptions corresponding to H-bonded secondary
À1
amides such as n
at 3267 cm (amide A band) and nC
=
O
at
NÀH
À1
1
657 cm (amide I band) (Figure S11a in the Supporting Infor-
[
6b,19]
mation).
Similarly, in the case of OPV-Py-TFA and OPV-Py-
Ag , the amide A and amide I bands were observed at 3263
+
À1
and 1663 and at 3262 and 1666 cm , respectively (Fig-
ure S11b, c in the Supporting Information), whereas the film
state IR spectrum of OPV-Py aggregates in toluene displayed
À1
a broad n
band in the 3100–3600 cm region due to multi-
NÀH
À5
ple-type H-bonding (Figure S12 in the Supporting Information).
The XRD pattern of the OPV-Ph xerogel prepared from toluene
exhibited several well-defined peaks in the small- and wide-
angle region, which suggests long-range ordering of the mole-
cules (Figure S13a in the Supporting Information). The XRD
peaks with d-values of 4.67 and 3.76 Š found in the wide-angle
region can be ascribed to secondary amide H-bonding assisted
Figure 6. TEM images of OPV-amide aggregates (5”10 m). (a) OPV-Py in
toluene, (b) OPV-Py in CHCl , (c) OPV-Ph in toluene, (d) OPV-Py-TFA in
, (e) OPV-Py-Ag in CHCl , and (f) OPV-Py-A in CHCl
3
+
CHCl
3
3
1
3
.
and 2-10 nm in height (Figure S8b in the Supporting Informa-
tion), which was also confirmed by TEM studies (Figure 6b).
However, in the case of OPV-Ph, samples prepared from a tolu-
ene solution (5”10 m) showed the presence of an extended
fibrillar morphology having several micrometers in length,
[
6b,19,20]
1-D-stacking of the molecules.
Comparable observations
À5
were made in the XRD pattern of the OPV-Py-TFA and OPV-
+
Py-Ag xerogel films prepared from CHCl3, which showed
1
00–300 nm in width, and 6–12 nm in height, as shown in Fig-
peaks with d-spacings of 4.43 and 3.75 Š and of 4.46 and
3.79 Š, respectively (Figure S13b and S13c in the Supporting
Information). In all the cases, a prominent diffraction signal
with a d-spacing value of around 27 Š was observed in the
small-angle region, which is close to the calculated length be-
tween the two terminal amide units of OPV-amides (26.12 Š).
The effect of sonication on gelation was monitored with FT-
IR and XRD studies as shown in Figure S14 and S15, respective-
ly, in the Supporting Information. The IR spectrum of the OPV-
ure 6c and Figure S8c in the Supporting Information.
Contrary to the formation of needle-like short fibers in tolu-
ene, OPV-Py in the presence of TFA resulted in long flexible
fibers of micrometers in length, 100–300 nm in width, and 5-
3
0 nm in height (Figure 6d and Figure S9a in the Supporting
+
Information). Similarly, a 1:1 complex of OPV-Py and Ag ions
in CHCl (5”10 m) showed the presence of interconnected fi-
brous structures of a few micrometers in length, 100–300 nm
in width, and 6–24 nm in height that are densely packed in
large areas (Figure 6e and Figure S9b in the Supporting Infor-
mation). Similar observations were made for aggregates of
À5
3
À1
Py-A1 complex showed absorption peaks at 3314 cm for
À1
À1
amide A band, 1690 cm for nC
=
acid, and 1663 cm for
O
amide I band. In the IR spectrum of the aggregates of OPV-Py-
À5
OPV-Py-A in CHCl (5”10 m) as shown in Figure 6 f and Fig-
ure S9c in the Supporting Information. OPV-Py-A assemblies
A in toluene, these peaks showed a shift towards a higher
1
3
1
À1
wavenumber such as 3441 cm
(broad) for amide A,
1
À1
À1
from toluene are composed of ill-defined aggregates as well as
fibers (Figure 7a). Upon sonication for 2 min, entangled fibers
with a width of 50–250 nm, height of 10–50 nm, and several
micrometers in length were observed as shown in Figure 7b.
This is further confirmed through TEM analysis (Figure S10 in
the Supporting Information).
1722 cm for nC
=
O
acid, and 1693 cm for amide I. Upon soni-
cation, these peaks were shifted back to 3331, 1691, and
À1
1665 cm , respectively. As mentioned earlier, the aggregates
of the OPV-Py-A complex precipitate out from the solution
1
due to the formation of ill-defined aggregates. Upon sonica-
tion, the shift of the IR peaks to a lower wavenumber indicates
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the formation of an H-bonded assembly through acid–pyridine
interaction along with amide H-bonding. This is also supported
by XRD studies (Figure S15 in the Supporting Information). The
XRD profile of OPV-Py-A1 aggregates from toluene showed
a broad peak in the wide-angle region. By contrast, after soni-
cation, two distinct peaks were observed with a d-spacing of
Acknowledgements
A.A. is grateful to the Department of Atomic Energy, Govern-
ment of India, for a DAE-SRC Outstanding Researcher Award
and for financial support of this work. K.K.K. and S.C. thank the
Council of Scientific and Industrial Research (CSIR) and Univer-
sity Grants Commission (UGC), Government of India, respec-
tively for research fellowships. We thank Mr. Kiran Mohan of
CSIR-NIIST for TEM analyses.
4
.45 and 3.72 Š as in the case of the XRD patterns of the OPV-
Py-A xerogel in CHCl , which showed peaks with d-values of
1
3
4
.43 and 3.71 Š in the wide-angle region. This observation indi-
cates that the OPV-Py-A complex in CHCl self-organizes in
1
3
a well-defined manner when compared to that in toluene.
Under ultrasound energy, breaking of the ill-defined aggre-
gates by the reorganization of the noncovalent interactions
might have resulted in the formation of kinetically trapped ag-
gregates, leading to the nucleation of extended fibers that are
responsible for the gel formation.
Keywords: gels
phenylenevinylene)s · self-assembly
·
H-bonding
·
multistimuli
·
oligo(p-
[
[
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2
The observed difference in the self-assembly and gelation
properties of the two molecules can be rationalized as shown
in Figure 3. OPV-Ph forms the usual 1-D self-assembly through
the amide H-bonding and p-stacking, resulting in gel forma-
tion in toluene. Such an assembly is hindered by the pyridyl
moiety in OPV-Py through the involvement of the H-bonding
between the amide H-atom and the pyridyl N-atom, leading to
random supramolecular polymerization. This can be prevented
by the protonation of the pyridyl moiety with TFA, triggering
the interamide H-bonding self-assembly and gelation, which is
reversible by deprotonation with DiPEA. Upon complexation
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Manuscript received: April 1, 2015
Accepted Article published: April 30, 2015
Final Article published: June 12, 2015
Chem. Asian J. 2015, 10, 2250 – 2256
www.chemasianj.org
2256
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim