Remarkable regioisomer control in the hydrogel formation from a two-component mixture of pyridine-end oligo(p-phenylenevinylene)s and N-decanoyl-l-alanine

Article abstract of DOI:10.1002/chem.201302100

N-Decanoyl-L-alanine (DA) was mixed with either colorless 4,4′-bipyridine (BP) or various derivatives such as chromogenic oligo(p-phenylenevinylene) (OPV) functionalized with isomeric pyridine termini in specific molar ratios. This mixtures form salt-type gels in a water/ethanol (2:1, v/v) mixture. The gelation properties of these two-component mixtures could be modulated by variation of the position of the "N" atom of the end pyridyl groups in OPVs. The presence of acid-base interactions in the self-assembly of these two-component systems leading to gelation was probed in detail by using stoichiometry-dependent UV/Vis and FTIR spectroscopy. Furthermore, temperature-dependent UV/Vis and fluorescence spectroscopy clearly demonstrated a J-type aggregation mode of these gelator molecules during the sol-to-gel transition process. Morphological features and the arrangement of the molecules in the gels were examined by using scanning electron microscopy (SEM), atomic force microscopy (AFM), and X-ray diffraction (XRD) techniques. Calculation of the length of each molecular system by energy minimization in its extended conformation and comparison with the XRD patterns revealed that this class of gelator molecules adopts lamellar organizations. Rheological properties of these two-component systems provided clear evidence that the flow behavior could be modulated by varying the acid/amine ratio. Polarized optical microscopy (POM), differential scanning calorimetry (DSC), and XRD results revealed that the solid-phase behavior of such two-component mixtures (acid/base=2:1) varied significantly upon changing the proton-acceptor part from BP to OPV. Interestingly, the XRD pattern of these acid/base mixtures after annealing at their associated isotropic temperature was significantly different from that of their xerogels.

Full text of DOI:10.1002/chem.201302100

DOI: 10.1002/chem.201302100
Remarkable Regioisomer Control in the Hydrogel Formation from a
Two-Component Mixture of Pyridine-End Oligo(p-phenylenevinylene)s and
N-Decanoyl-l-alanine
Subham Bhattacharjee,[a] Sougata Datta,[a] and Santanu Bhattacharya*[a, b]
Abstract: N-Decanoyl-l-alanine (DA)
was mixed with either colorless 4,4’-bi-
pyridine (BP) or various derivatives
such as chromogenic oligo(p-phenyl-
fluorescence spectroscopy clearly dem-
onstrated a J-type aggregation mode of
these gelator molecules during the sol-
to-gel transition process. Morphological
features and the arrangement of the
molecules in the gels were examined
by using scanning electron microscopy
(SEM), atomic force microscopy
(AFM), and X-ray diffraction (XRD)
techniques. Calculation of the length of
each molecular system by energy mini-
mization in its extended conformation
and comparison with the XRD patterns
revealed that this class of gelator mole-
cules adopts lamellar organizations.
Rheological properties of these two-
component systems provided clear evi-
dence that the flow behavior could be
modulated by varying the acid/amine
ratio. Polarized optical microscopy
(POM), differential scanning calorime-
try (DSC), and XRD results revealed
that the solid-phase behavior of such
two-component mixtures (acid/base=
2:1) varied significantly upon changing
the proton-acceptor part from BP to
OPV. Interestingly, the XRD pattern of
these acid/base mixtures after anneal-
ing at their associated isotropic temper-
ature was significantly different from
that of their xerogels.
ACHTUNGTRENNUNGenevinylene) (OPV) functionalized
with isomeric pyridine termini in spe-
cific molar ratios. This mixtures form
salt-type gels in a water/ethanol (2:1, v/
v) mixture. The gelation properties of
these two-component mixtures could
be modulated by variation of the posi-
tion of the ’’N’’ atom of the end pyridyl
groups in OPVs. The presence of acid–
base interactions in the self-assembly
of these two-component systems lead-
ing to gelation was probed in detail by
using stoichiometry-dependent UV/Vis
and FTIR spectroscopy. Furthermore,
temperature-dependent UV/Vis and
Keywords: gels · liquid crystals · re-
gioisomer control · self-assembly ·
X-ray diffraction
Introduction
considered to be the main reasons for physical gelation.[3]
LMOGs include a wide variety of materials that are derived
from sugar,[4] dendrimer,[5] oligo(p-phenylenevinylene),[6]
cholesterol,[7] urea,[8] porphyrin,[9] amino acid,[10] and so on,
which are capable of exhibiting efficient gelation in different
solvents. Among them, OPV-based gelators of well-defined
nanostructures have become an emerging class of gelators,
especially from the last decade.[6] These molecules form gels
as a result of cooperative hydrogen-bonding, p–p stacking
and van der Waals interactions.[6] Recently, the self-assembly
of OPV-based molecules has attracted considerable atten-
tion in the design of artificial light-harvesting (LH) assem-
blies,[6c,e] self-assembled organic molecular wires,[11] G-quad-
ruplex-like self-assembled organic nanoparticles,[12] molecu-
lar electronic devices,[6a] and so on. Furthermore, it has been
also used to prepare various functional nanocomposites with
different types of inorganic nanotubes and carbon allo-
tropes.[13]
Gels are solid-like viscoelastic materials despite the pres-
ence of a major amount of solvent molecules associated
with a minute quantity of the gelator (typically 0.1–10 wt%)
as a minor component.[1] According to Flory, a gel could be
defined as a material that has a continuous structure with
macroscopic dimensions and rheologically solid-like proper-
ties.[2] Low molecular-mass organic gelators (LMOGs) form
gels through multiple non-covalent forces, such as hydrogen-
bonding, p–p, dipole–dipole, electrostatic, van der Waals in-
teractions, and so on.[1a] The high available surface area of
the self-assembled-nanostructures (i.e., fibers, sheets, plates,
rods, tubules, vesicles, and so on) formed in the gel phases is
[a] S. Bhattacharjee, S. Datta, Prof. Dr. S. Bhattacharya
Department of Organic Chemistry, Indian Institute of Science
Bangalore, Karnataka 560012 (India)
Fax : (+91)80-23600529
E-mail: sb@orgchem.iisc.ernet.in
LMOGs comprising of a single molecular unit in the self-
assembly are known as single-component gelators.[6,8,9,14]
Sometimes, structurally two different components, which are
non-gelators or at least one of them is a non-gelator, form
efficient gels at certain molar ratios. These are known as
two-component gelators.[7b] When more than two compo-
nents are involved in gelation, they are known as multicom-
[b] Prof. Dr. S. Bhattacharya
Honorary Professor, Jawaharlal Nehru Centre for
Advanced Scientific Research
Bangalore, Jakkur 560064 (India)
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201302100.
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FULL PAPER
ponent gels.[15] Two-component or multi-component gelators
have distinct advantages over single-component gelators be-
cause their mechanical, optical, and other properties can be
conveniently tuned by changing the molar ratio of different
components in the mixture.[15b,16] On the basis of molecular-
level interactions among different components, the two-
component gels may be classified into four categories:[10e]
1) metal-ion-coordinated gels,[17] 2) hydrogen-bonding- and
halogen-bonding-induced gels,[18] 3) donor–acceptor-type
gels[19] and 4) salt-type gels.[20]
Previously, we reported supramolecular hydrogelation by
mixing lithocholic acid and oligomeric amines and investi-
gated them by various physical methods to deduce the struc-
ture–property relationship with this class of soft materi-
als.[20e] The cavities inside such gel networks could encapsu-
late different kinds of nanoparticles to give rise to new
nanocomposites with new properties.[21] Hydrogelators de-
rived from mixture of fatty acids and oligomeric amines
may be also used to template the synthesis of Ag nanoparti-
cles.[21] Generally, to form a salt, the pKa value of the acid
must be low compared with that of the conjugate acid of the
base. Accordingly, an OPV derivative with terminal pyridyl
groups could be used to verify this fact. Up to now, there
are several reports in literature comprising of OPV-based
single-component gelator.[6,13] Ajayaghosh et al. reported
a series of single-component organogelators based on struc-
turally divergent OPV-based systems.[6a–c,13a–b] Recently, we
have also demonstrated the hydrogelation by a single-com-
ponent biscationic phenylenedivinyline-based p-molecule.[6f]
However, reports pertaining to OPV-based two-component
salt-type gel are rare.[22]
Results and Discussion
Synthesis: Different isomeric pyridyl-appended OPV deriva-
tives NP, NM, and NO were synthesized by using the
Horner–Wadsworth–Emmons reaction (Figure 1a). Each of
the isomeric pyridine aldehydes was treated with the corre-
sponding 1,4-di-n-alkoxy-2,5-biphosphonate in the presence
of tBuOK under an Ar atmosphere (the Supporting Infor-
mation). The residue obtained after the work up was leach-
ed with n-hexane several times to furnish the pure product
(the Supporting Information). DA was synthesized according
to a previously reported procedure.[10d] The final products
1
were characterized by using H/13C NMR spectroscopy, ESI-
MS, FTIR, and elemental analysis (the Supporting Informa-
tion, Figure S1–S3).
Fatty acid amides of natural amino acids are also biocom-
patible; for example, lauric acid amide of l-alanine is an ex-
cellent phase-selective gelator, which has been shown to
have potential in oil-spill remediation.[10b] N-Decanoyl-l-ala-
nine (DA) is capable of exhibiting gelation in hydrocarbon
solvents (i.e., n-hexane and n-heptane).[10d] Herein, we
report the salt formation of DA with oligo(p-
phenylenevinylACHTUNGTRENNUNGene) functionalized with various isomeric
pyridyl termini (OPV) at specific molar ratios. Various iso-
meric bipyridines have been chosen as a non-fluorescent an-
alogue to compare the results with OPVs. Interestingly, the
salt formation of DA modulates its aggregation properties
in such a way that these acid/base mixtures are capable of
forming gel in 2:1 water/ethanol system. The incumbent
self-assembly phenomenon involved in the sol-to-gel transi-
tion process has been investigated by UV/Vis absorption,
fluorescence, and FTIR spectroscopy. The morphological be-
havior and structural information of these self-assemblies
have been examined by using SEM, AFM, optical profilo-
metric experiments, and XRD techniques. Liquid-crystalline
properties of the 2:1 acid/base mixtures have been explored
by POM and DSC. Additionally, we have demonstrated
a notable regioisomer control in gelation. Furthermore, to
the best of our knowledge, this is the first time that the syn-
thesis and characterization of two-component OPV-based
salt-type hydrogel has been reported.
Figure 1. a) Molecular structures of the different OPVs, BP, and DA used
in the present study. Photographs of solutions of b) BP and c) DA; pre-
cipitate of d) NP; e) sol of NO–DA (1:2), and f) NM–DA (1:2); typical
gels of g) NP–DA (1:2) and h) BP–DA (1:2) in 2:1 water/ethanol mixture;
[NP], [NM], [NO]=4 mm, [BP]=20 mm in each case, [DA]=8 mm in the
case of c), e)–g) and 40 mm in case of h).
Gelation studies: The gelation ability of N-decanoyl-l-ala-
nine (DA) was checked in aqueous/ethanol (2:1) mixture in
presence of various isomeric bipyridine and OPV deriva-
tives. Each of these individual components was soluble in
ethanol and gave a transparent, clear solution that never
formed gel, even upon prolonged ageing. However, they
were insoluble in water. Interestingly, we found that mixing
of BP with two equivalents of DA in 2:1 water/ethanol mix-
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S. Bhattacharya et al.
ture (v/v) followed by heating at 70–758C gave a transparent
solution, which upon subsequent cooling and sonication led
to the formation of a white colored, thermoreversible,
opaque gel (minimum gelator concentration [mgc]=
16.4 mm=[BP]) (Figure 1 h). It is important to note that the
mixture of 2,2’-bipyridine or 3,3’-bipyridine and DA did not
lead to gelation under the same conditions. This observation
prompted us to synthesize OPV derivatives functionalized
with various isomeric pyridyl termini (NP, NM, NO) to
obtain salt-type OPV-based gels. Among them, notably only
mixture of NP and DA (1:2) was able to form an orange-col-
ored, fluorescent, thermoreversible, opaque gel in 2:1 water/
ethanol (v/v) ([mgc]=3.31 mm=[NP]) (Figure 1g), whereas
other members did not lead to gelation with DA in that sol-
vent system. Interestingly, a 1:2 mixture of NP and DA
formed a gel at much lower mgc compared with that of the
BP–DA gel. Accordingly, 1 unit of NP–DA (1:2) system can
hold almost 8000 water molecules and 1300 ethanol mole-
cules inside the three-dimensional gel network. The chain
length of the OPV backbone is also important in determin-
ing the effect on gelation. Accordingly a control compound,
8-NP was synthesized (Figure 1a). Interestingly, 8-NP
formed a precipitate with DA in a 2:1 water/ethanol mix-
ture. This result clearly elucidates that a hydrophilic/hydro-
phobic balance is crucial for the gelation of this system.
Salt formation of each of NP, NM, and NO was also
checked in presence of different fatty acids of variable chain
lengths as well as aromatic acids, such as benzoic acid.
Among them, only NP resulted in the salt formation with
these acids, whereas NM and NO failed. Interestingly, the
salt of NP with fatty acids of variable chain lengths (no. of
carbon atoms (n)=8, 10, 12, 14, 16) led to the formation of
gelatinous precipitates in the 2:1 water/ethanol mixture.
This signifies that the hydrogen bonding through amide
units of DA is very important and plays a crucial role in the
aggregation as well as hydrogelation. Benzoic acid was
chosen as the simplest among the aromatic acids, which also
resulted in the formation of precipitate with NP in that sol-
vent system, because of the lack of both flexible aliphatic
hydrocarbon chain and hydrogen-bonding motif. Thus apart
from the acidity, the lipophilic/hydrophilic balance as well as
the flexibility and hydrogen bonding all contribute to the ge-
lation.
the pKa value of each of these series of compounds was ob-
tained from the plots of the absorbance versus pH. The ex-
perimentally determined pKa values of NP in 0.1m sodium
formate/formic acid and phosphate/citrate buffer solutions
are 3.77 and 3.72, respectively (Figures S4 and S5, the Sup-
porting Information). So, the average pKa value of NP is
3.75Æ0.04. Similarly, the experimentally determined pKa
value of NM is 3Æ0.11 (Figures S6 and S7, the Supporting
Information), whereas NO remains unprotonated even at
pH 2.8. This is clearly evident from the broad absorption
spectra of NO in sodium formate/formic acid buffer solution
(Figure S8, the Supporting Information). According to the
experimentally determined pKa values, the order of basicity
of these OPVs is NP>NM>NO.
Proton transfer during an acid/base reaction depends on
the DpKa value (pKa of the conjugate acid of pyridine
baseÀpKa of the carboxylic acid).[24] When DpKa <0, it re-
À
À
sults in a neutral O H···N H bond. On the other hand, 0<
+
À
À
À
DpKa <3 gave an intermediate O H···N/N H···O hydro-
gen-bond character. However, DpKa >3 results in complete
proton transfer to induce an ionic interaction N+ H···O .
À
À
Therefore, NP with relatively higher basicity should be able
to form a salt with DA with an intermediate O H···N/N
+
À
À
H···OÀ hydrogen-bond character, which accounts for the
reason of selective gel formation by NP. Whereas, NM and
NO, being relatively less basic compared with NP, most
likely fail to form a salt with DA. Furthermore, in case of
NO, the pyridyl ’’N’’ atom is probably buried under the p-
cloud of the OPV backbone. This results in a crowding
effect, which may be an alternative reason of the inability of
NO towards salt formation. It is important to note that pKa
was measured in pure buffered aqueous media. However,
the gelation was performed in 2:1 water/ethanol mixture,
and there will be changes in the pKa values in ethanolic
aqueous media. Moreover, it is known that pKa values of
different pyridine derivatives show changes at interfaces.[25]
Thus, the pKa of various isomeric OPV derivatives may also
change upon self-assembly. However, the extent of change
in pKa would be probably similar for the three isomers. So,
the trend in these values may explain the selective salt for-
mation and gel formation phenomena as observed here.
Computational studies: The main factor that governs the se-
lective gel formation is the protonation of NP by DA, and
thereby self-aggregation of NP–DA (1:2) using several weak
non-covalent interactions leads to robust gel formation. To
obtain a reasonable explanation of these observations, each
of these molecules in the form of the HCl salt were opti-
mized by using the B3LYP G/6-31G* level of theory
(Figure 2). HCl was chosen as a proton source to mimic the
real situation. For these purposes, the two most relevant
conformations were considered; one in which the attached
N atom of the end pyridyl ring was placed facing towards
the vinylic bond and one in which the attached N atom was
placed away from the vinylic bond (in case of NM-HCl and
NO-HCl; Figure 2b and c). In striking contrast, only one
conformation was possible for NP-HCl, in which the at-
The role of basicity in selective gelation: The gelation prop-
erties of the OPV derivatives in presence of DA was found
to depend on the basicity of the ’’N’’ atom of the pyridyl
ring. We thought that it is important to determine the pKa
values of these compounds to find out the reason of selec-
tive gelation by NP. Accordingly, we obtained the pKa
values of these pyridine derivatives experimentally.
Since the protonated and non-protonated forms of these
OPV derivatives absorb differently, UV/Vis spectroscopy
was used as an efficient tool to determine their pKa
values.[23] We recorded the UV/Vis absorption spectra of
these compounds in 0.1m sodium formate/formic acid and
phosphate/citrate buffer solutions of various pH values and
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Regioisomer Control in Hydrogel Formation
FULL PAPER
To investigate the actual situation, computational studies
were further performed on NP–DA, NM—DA, and NO–
DA systems (the Supporting Information, Figure S9). NP ex-
À
periences negligible distortion from planarity as well as N
À
H O angle (179.978) in the case of NP-DA system. Howev-
er, the distortion of p-aromatic backbone of NM in case of
NM-DA (for the two conformers) and the deviation of the
À À
N H O angle (179.31 and 179.368, respectively) from lin-
AHCTUNGTREGeNNNU arity was moderate. In stark contrast, NO–DA (for the two
conformers) experiences a maximum distortion of p-aromat-
À À
ic backbone and deviation of the N H O angle (177.71 and
178.508, respectively) from linearity with the lowest energy
of E=À2928.054 and À2928.044 a.u., respectively, both of
which are considerably larger than the lowest energy of
NM–DA system (E=À2928.066 and À2928.065 a.u., respec-
tively, for the two conformers). Thus, the computational
studies provide an explanation of the selectivity in terms of
gel formation.
FTIR Spectroscopy: FTIR spectroscopy provides a useful
way to investigate the primary interactions involved in the
gelation of two-component systems, for example, acid and
À
base. DA in its solid state has a OH stretching frequency
Figure 2. Energy-minimized structures of a) NP-HCl, b) NM-HCl, and
c) NO-HCl; d) Distortion of p-aromatic backbone of NO-HCl.
À1
À
of the COOH group at 3348 cm . However, when mixed
with NP at a 1:2 molar ratio, this band shifted to a lower fre-
quency of 3300 cmÀ1 and a shoulder appeared at 3340 cmÀ1
indicating the formation of a salt (the Supporting Informa-
tached N atom was placed in the para position to the end
pyridyl ring. Energy minimization of NP in the form of the
HCl salt shows almost negligible distortions in the planar p-
À
tion, Figure S10). Similarly, the OH stretching of DA shift-
ed to 3290 cmÀ1 in 1:2 stoichiometric mixture of BP and
DA.
À À
aromatic backbone (Figure 2a). Moreover, the N H Cl
angle in the energy minimized structure was linear
(179.988), indicating that HCl forms a salt with NP without
any significant distortion of the OPV backbone.
The appearance of the amide I band at 1644 cmÀ1 for the
BP–DA xerogel and at 1645 cmÀ1 for NP–DA xerogel indi-
cates the presence of strong intermolecular hydrogen-bond-
À
On the other hand, energy minimization of NM (for the
two conformations) in the form of HCl salt gave the lowest
energy as E=À2267.726 and À2267.808 a.u., respectively,
both of the values are a little bit larger than the lowest
energy of the NP-HCl salt (E=À2267.810 a.u.). Additional-
ing between CONH groups (the Supporting Information,
Figure S10).[26a] The FTIR spectrum of DA showed a broad
C=O absorption associated with the carboxylic acid group in
the range of 1726–1707 cmÀ1, which was almost silent in case
of NP–DA (1:2) and BP–DA (1:2) systems. This observation
À À
À
ly, the N H Cl angle in the energy-minimized structure of
clearly suggests the transfer of the COOH proton to the
NM-HCl was distorted to 177.335 and 177.6118, respectively,
in the most probable two conformations, indicating that pro-
tonation of NM could be also achieved, although with addi-
tional difficulty compared to that of NP.
basic pyridine N-atom of NP or BP.[26b] In addition, the ap-
pearance of new peaks at 1615 and 1604 cmÀ1 for NP–DA
and BP–DA xerogels, respectively, in the low-frequency
region further proves the dissociation of the carboxylic acid
À
[26a]
À
In case of NO-HCl in which the N is away from the vinyl-
and the formation of anionic COO species.
À À
ic bond, the N H Cl angle was found to be 175.28. Where-
as, in NO-HCl in which the N is buried and/or close to the
SEM, AFM, and optical profilometry experiments: To dis-
cern the microstructures present in the three-dimensional
(3D) networks of the gelators, we examined these gel sam-
ples by SEM and AFM. Representative SEM images of the
BP–DA mixtures showed fiber-like nanostructures at four
different molar ratios. The fibers are of very high aspect
ratios with several hundreds of micrometers length and di-
ameters ranging from about 0.7 to 7.7 mm (Figure 3a–d). On
the other hand, the SEM image of NP–DA (1:0.5 to 1:2), ex-
hibited the presence of rectangular plate-like morphologies
(Figure 3e–h). Generally, the fibers or sheet-like structures
are found in most of the gels reported. Hence, the present
À À
vinylic bond, the N H Cl angle experiences the greatest
degree of distortion of about 1648 (Ddistortion ca. 168).
Moreover, the p-aromatic core experiences a high distortion
from the planarity to accommodate HCl, and to minimize
the energy of NO-HCl salt, meaning that HCl forms a salt
with NO with great difficulty. It is important to note the dis-
À À
tortion of aromatic core and the N H C angle was greatest
in case of NO-HCl, followed by NM-HCl. Whereas NP-HCl
experiences the least distortion in both cases. The results ob-
tained from computational studies are in accord with the ba-
sicity trend of the OPVs.
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S. Bhattacharya et al.
Figure 4. AFM images of BP–DA at molar ratios of a) 1:0.5, b) 1:1,
c) 1:1.5, and d) 1:2, respectively; [BP]=7.18 mm in each case; [DA]=3.6,
7.2, 10.8, and 14.4 mm, respectively. Optical profilometric images of NP–
DA at molar ratios of e) 1:0.5, f) 1:1, g) 1:1.5, h) 1:2, and i) 1:3, respective-
ly; [NP]=5.8 mm in each case, [DA]=2.9, 5.8, 8.7, 11.7, and 17.5 mm, re-
spectively.
Figure 3. SEM images of BP–DA at different molar ratios of a) 1:0.5,
b) 1:1, c) 1:1.5, and d) 1:2, respectively; [BP]=7.18 mm in each case,
[DA]=3.59, 7.18, 10.77, and 14.37 mm, respectively. SEM images of NP–
DA at different molar ratios of e) 1:0.5, f) 1:1, g) 1:1.5, h) 1:2, and i) 1:3,
respectively; [NP]=5.8 mm in each case, [DA]=2.9, 5.8, 8.7, 11.7, and
17.5 mm, respectively.
UV/Vis and fluorescence spectroscopy: Thermoreversibility
and molecular interactions involved in the gel-to-sol transi-
tion process were followed by temperature-dependent UV/
Vis absorption spectroscopy. These systems show broad ab-
sorption spectra at their associated minimum gelator con-
centrations and the sigmoidal nature of the gel melting plot
was an indication of an isodesmic mechanism during self-as-
sembly (the Supporting Information, Figures S11 and
S12).[14b] Since aggregation was still present at low concen-
trations, we recorded their UV/Vis spectra in dilute solu-
tions.[14b] Two absorption maxima (lmax) at 352 and 439 nm,
respectively, were observed for the NP–DA (1:2) system in
2:1 water/ethanol mixture at 208C and shifted to 341 and
423 nm at 608C, indicating a gradual change in the aggregat-
ed absorption to monomeric absorption (Figure 5a). This
phenomenon clearly reveals a J-type aggregation of the
OPV-based gelator.[9b,10g] The intensity of absorption increas-
es with concomitant increase of temperature. This may be
explained on the basis of heat-induced ’’destacking’’ of the
p-aromatic backbone of the NP–DA (1:2) system and subse-
quent generation of monomeric unit, which has higher
molar absorptivity (e) compared with the self-assembled
molecular aggregates. The transition temperature (ca. 428C)
associated with the melting of the aggregates was calculated
from the plot of Abs.439 versus temperature (the Supporting
Information, Figure S13a). The sigmoidal nature of the plot
indicates that an isodesmic mechanism is probably followed
during the aggregation process.[29] In contrast, the BP–DA
(1:2) system exhibited only one absorption maximum at
239 nm at 208C and did not show any significant shift in the
absorption spectra upon increasing the temperature (the
Supporting Information, Figure S14).
work is a rare example in which rectangular gel particles
have been observed in the xerogel. Sada and co-workers
also reported the formation of uniform cubic gel particles
with well-defined edges and square faces using internal
cross-linking of the CD-MOF crystals followed by loss of co-
ordinating metal ions.[27] However, with increasing the stoi-
chiometry of DA, the rectangular plates gradually abolished
and self-assembled clusters were formed (Figure 3i).
The superstructures created in the gel network of 1:2 mix-
ture of BP–DA were also observed under AFM. AFM
images of the BP–DA system at different molar ratios also
showed fibrous networks, which is in accord with the SEM
finding (Figure 4a–d). To investigate the rectangular plate-
like morphology of the NP–DA in detail, an optical profilo-
metric experiment was carried out for NP–DA at different
molar ratios (Figure 4e–i). A rectangular plate-like morphol-
ogy was observed for NP–DA up to a stoichiometric ratio of
1:1.5. However, the plate-like morphology was almost abol-
ished and some cluster-type microscopic particles were ob-
served at amine/acid ratios of 1:2 and 1:3. Importantly, the
plates were of different heights and in some cases the plates
conserved their smoothness over large areas. Since the
height of these microplates was more than 5 mm, we could
not visualize them adequately by AFM. It is well-known
that molecular chirality can induce supramolecular chirality
efficiently in the self-assembled supramolecular fi-
ACHTUNGTRENNUNG
bers.[6b,22a,28] For example, Schenning and Meijer et al. have
described the fabrication of self-assembled 1D stacks by
a chiral supramolecular auxiliary approach.[28] However, in
the present case we have not observed any chiral amplifica-
tion to the supramolecular level.
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Regioisomer Control in Hydrogel Formation
FULL PAPER
ature-dependent fluorescence spectra. The fluorescence
spectra of a 1:2 mixture of NP and DA in 2:1 water/ethanol
(v/v) showed one emission maximum at 576 nm associated
with the aggregated species and a shoulder at 494 nm at
158C (Figure 5b). However, the emission intensity at
576 nm gradually diminished, which was followed by the ap-
pearance of a blueshifted hyperchromic peak at 494 nm (iso-
tropic emission) when increasing the temperature from 15
to 658C. This indicates an apparent breakdown of the supra-
molecular aggregates on heating. Conversely, the redshift of
the emission maximum as a result of self-aggregation clearly
ascribes to the presence of J-type aggregation mode of this
gelator system. The melting temperature (ca. 428C) of the
aggregates is obtained from the plot of F.I.576 (fluorescence
intensity at l=576 nm) versus temperature and the result is
in good agreement with the findings obtained on the basis
of the UV/Vis absorption spectroscopy (the Supporting In-
formation, Figure S13b).
Differential scanning calorimetry (DSC) and polarized opti-
cal microscopy (POM): Hydrogen bonding and salt forma-
tion have been used previously to congregate a carboxylic
acid derivative with compounds having a terminal-pyridyl
group to produce calamitic or linear rod-shaped structural
organizations, which in turn exhibit a mesogenic behavior.[30]
Keeping this in mind, DA was mixed with either BP or NP
at 1:2 molar ratio in THF. After evaporation of the solvent,
the solid residue was investigated using POM, DSC, and
XRD techniques. In the first heating cycle of the DSC ex-
periment, the solid sample was heated (25–1508C) to melt-
ing to remove the anisotropic history of the sample. Then
the resulting melt was cooled down at a rate of 58CminÀ1.
After the melt reached room temperature, new heating and
cooling cycles were performed under the same conditions as
before. Under these conditions, a 1:2 mixture of NP–DA
showed an endothermic transition at 104.58C (Figure 6a).
Interestingly, in the cooling cycle, it exhibited double exo-
thermic transitions at 121 and 79.58C, respectively (Fig-
ure 6a). A smaller peak appeared at higher temperature
and a larger exothermic transition appeared at lower tem-
perature during the cooling cycle. However, the DSC ther-
mograms of BP–DA (1:2) showed only a single and sharp
peak at 103.58C during the heating and at 72.58C in the
cooling cycle (Figure 6a).
Thermotropic properties of these acid/base mixtures were
further probed by using temperature-dependent polarized
optical microscopy. The samples were first heated to obtain
an isotropic melt (Figure 6b) and then slowly cooled down
to room temperature at the same rate of the DSC experi-
ment (58CminÀ1). Thermotropic transitions were observed
during cooling of the isotropic melt and different birefrin-
gent patterns appeared upon changing the temperature. For
the 1:2 stoichiometric NP–DA mixture, a straw-like texture
started appearing at about 1208C, which was associated with
a small exothermic transition in the DSC profile (Fig-
ure 6c).[31b] Interestingly, fan-shaped focal conic textures cor-
responding to the smectic-A (SmA) phase appeared at
Figure 5. Temperature-dependent changes in the a) UV/Vis absorption
([NP]=0.29 mm; [DA]=0.58 mm) at 25, 35, 45, 55, and 658C and b) fluo-
rescence spectra of NP–DA (1:2) in water/ethanol (2:1) mixture ([NP]=
0.23 mm; [DA]=0.46 mm) at 15, 20, 25, 30, 35, 40, 45, 50, 55, 60, and
658C.
A salt-type interaction-mediated selective gelation of NP
with DA has been further explored by following stoichiome-
try-dependent absorption spectroscopy in 2:1 water/ethanol.
Figure S15a (the Supporting Information) shows that two
peaks in the UV/Vis spectra appear at 334 and 405 nm, re-
spectively, at a 1:0.5 stoichiometric ratio of NP–DA. It shift-
ed to 350 and 433 nm, respectively, in the 1:2 mixture of
NP–DA, which was accompanied by a significant increase in
the intensity of absorption. This result is further supported
by the change in visible color of the solution from nearly
colorless to deep-yellow on increasing the molar ratio of
DA (the Supporting Information, Figure S15b). Schenning
and Meijer et al. have also reported similar large color shifts
as a result of formation of hydrogen-bonded ion pair.[28]
However, the UV/Vis peak at 380 nm in 1:0.5 mixture of
NM–DA exhibited a redshift of around 5 nm followed by
a little increase in the absorption intensity on gradual addi-
tion of DA starting from 0.5 to 2 equiv (the Supporting In-
formation, Figure S16). Interestingly, no significant shift and
change in the intensity were observed in case of NO with in-
creasing proportion of DA (the Supporting Information, Fig-
ure S17). This phenomenon clearly elucidates that NP is
readily able to form a salt with DA leading to gelation. In
contrast, NM and NO did not result any gel formation be-
cause of their inability to induce salt formation with DA.
The J-type aggregation mode of the OPV-based gelator
was further investigated by recording changes in the temper-
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S. Bhattacharya et al.
The oscillatory frequency-sweep experiments were carried
out for both BP–DA (1:2) and NP–DA (1:2) gels. The fre-
quency-sweep experiment revealed that the storage modulus
G’, was always greater than the respective loss modulus G’’
under an applied stress of 1 Pa in the frequency range of
0.1–10 Hz, thereby indicating the viscoelastic nature of the
gel over the entire angular frequency range (Figure 7a).
Figure 6. a) Solid-phase DSC profile of NP–DA (1:2) and BP–DA (1:2),
respectively; inset shows magnification of exothermic scan of NP–DA for
clarity. POM images of NP–DA (1:2) were captured at b) 130 (isotropic),
c) 120, d) 80, e) 75, f) 65, and g) 608C.
about 808C under a polarizing microscope, which gave
a sharp peak in the DSC experiment (Figure 6d–g).[31c] How-
ever, a fingerprint texture was found at about 728C during
the cooling of the isotropic melt of the 1:2 mixture of BP
and DA (the Supporting Information, Figure S18).[31a] The
associated transition in DSC appeared as a sharp exothermic
peak at 72.58C with a large enthalpy change of 75.36 JgÀ1.
However, a phase transition from the LC phase to a crystal-
line phase or glass transition was not observed in DSC,
probably due to the slow transition between the respective
phases.[32]
Figure 7. a) Frequency- and b) amplitude-sweep rheology data of NP–DA
(1:2) and BP–DA (1:2) in a 2:1 water/ethanol mixture; [BP] and [NP]=
18.7 mm, [DA]=37.4 mm.
To compare the mechanical properties of these two-com-
ponent gels, an oscillatory amplitude-sweep experiment was
performed. Each gel showed its own characteristic yield
stress (sy) according to its viscoelasticity to an oscillating
stress at the concentration of 18.7 mm. When these salt-type
gels succumbed to an applied stress, the gel composed of
BP–DA (1:2) started to flow at a shear stress of about 85 Pa.
However, the NP-DA (1:2) gel began to lose its solid-like
behavior at a critical stress of about 95 Pa (Figure 7b).
Therefore, the gel of NP–DA is more resistant to flow under
an applied mechanical force than that of the BP–DA gel.
Moreover, an oscillatory amplitude-sweep experiment was
also performed for NP–DA (1:2) gel at different concentra-
tions in 2:1 water/ethanol (v/v) mixture (the Supporting In-
formation, Figure S19). The yield stress progressively in-
creased with increasing gelator concentration. This observa-
tion clearly demonstrated the gradual increase in the viscoe-
lasticity of the NP–DA gel with increasing concentrations.
Rheology experiment: Gels are viscoelastic solid-like mate-
rials, and the viscoelastic properties of gels are best charac-
terized by rheological studies.[6d,f] Therefore, oscillatory fre-
quency-sweep and amplitude-sweep experiments were per-
formed to find out the elastic and loss moduli (G’ and G’’).
The elastic modulus G’ represents the solid-like character,
that is, the ability of the material to defy deformation,
whereas the loss modulus G’’ reflects the liquid-like behav-
ior, that is, the tendency of the material to flow. Generally,
gels exhibit a higher magnitude of the G’ value than the G’’
value, demonstrating viscoelastic character. In an oscillatory
amplitude-sweep experiment, the switchover point of G’ and
G’’ is termed as yield stress (sy), in which a transition of the
elastic solid-like character to a viscous liquid-like character
occurs.
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Regioisomer Control in Hydrogel Formation
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X-ray diffraction (XRD) plots of the xerogel of 1:2 mixtures
of BP and DA show the presence of two sets of lamellar
structures. The corresponding d-spacings of these two sets
are d1 =2.62, d2 =1.28 (1/2), d3 =0.9 (1/3), d4 =0.65 nm (1/4)
and d’1 =2.3, d’2 =1.12 (1/2), d’3 =0.78 nm (1/3), respectively.
This indicates the presence of a lamellar arrangement in the
three-dimensional (3D) gel network with interlayer spacing
of 2.62 and 2.3 nm, respectively. However, only a broad
peak at 1.3 nm has been observed in the XRD plot of the
annealed mixture of BP–DA (1:2). All of these results clear-
ly demonstrate the different modes of aggregation in the gel
phase and solid state. Moreover, the XRD pattern of these
two-component systems are completely different from that
of the individual component (the Supporting Information,
Figure S21).
Furthermore, the structural changes associated with the
birefringence textures were monitored by temperature-de-
pendent XRD. The relevant diffractograms of NP–DA at
variable temperatures were recorded and shown in Fig-
ure S22 (the Supporting Information). Interestingly the
XRD patterns of the NP–DA (1:2) system under these con-
ditions showed significant differences from that of the xero-
gel. At 1308C, no diffraction was observed, indicating an
isotropic liquid state of NP–DA. However, gradual cooling
to 1158C and subsequent data collection manifested only
a few diffraction peaks that correspond to the straw-like tex-
tures. Moreover, appearance of only few peaks also indi-
cates ’’loose’’ molecular packing (low enthalpic transition)
that, in turn, presumably causes very weak exothermic tran-
sition in the DSC experiment at around 1218C.[32] On fur-
ther cooling, the sample at 758C showed four major reflec-
tions at 2.6, 1.3, 0.95, and 0.67 nm, respectively, along with
some small peaks associated with the fan-shaped focal conic
liquid-crystalline phase. Interestingly, the solid material at
258C, exhibited only three reflections. The corresponding d-
values are d1 =2.6 nm, d2 =1.3 nm (1:2), and d3 =0.86 nm
(1:3) indicating a lamellar-type arrangement. Furthermore,
the longest d-spacing (2.6 nm) is less than the double of its
molecular length (1.7 nm) as calculated from an optimiza-
tion of geometry of the single molecule using B3LYP/6-
Figure 8. a) XRD plots of xerogels of NP–DA (1:2), BP–DA (1:2) from
a 2:1 water/ethanol mixture and annealed solid of NP–DA (1:2), BP–DA
(1:2). b) Proposed molecular packing of propagation of the aggregates of
NP–DA (1:2). c) A proposed model depicting the formation of rectangu-
lar gel particles by the NP–DA (1:2) system.
À
31G*. This analysis indicates that the OC4H9 chains inter-
digitate with each other to introduce effective van der Waals
X-ray diffraction and molecular modeling: X-ray diffraction
was performed to unveil the mechanism of packing of the
supramolecular organizations in the xerogels. Figure 8a
shows the diffraction pattern of the xerogel obtained from
a 1:2 mixture of NP and DA in 2:1 water/ethanol mixture. It
shows two sets of Bragg reflections. The corresponding d-
values of these two sets of peak are d1 =3.4 nm, d2 =1.14 nm
(1/3) and d’1 =2.8 nm, d’2 =1.44 nm (1/2), d’3 =0.93 nm (1/3),
respectively. These results clearly indicate the presence of
two sets of lamellar structures in the gel network with the
interlayer spacings of 3.4 and 2.8 nm, respectively. Interest-
ingly, the longest interlayer spacing (3.4 nm) is equal to the
double of its molecular length (1.7 nm) along the direction
of -OC4H9 backbone as calculated by an optimization of ge-
ometry of the single molecule using B3LYP/6-31G* level of
theory (Figure 8b).
À
interactions among the OC4H9 chains. Interestingly, the dif-
fraction pattern at 258C exactly matched with the diffraction
pattern of the annealed sample of 1:2 mixture of NP and
DA. It is important to note that the diffraction pattern of
the annealed sample of 1:2 mixture of NP and DA shows
significant differences from that of the xerogel.
As stated above, a rectangular gel particle was observed
instead of a fiber formation. It is possible that the gelation
observed here involves a frustrated crystallization pro-
AHCTUNGTRENNUNG
cess.[1a,33] In fact, there are several reports in which the crys-
tallization of the gelator molecule was achieved inside the
3D-gel network.[18g,33] Although in such cases, the gelator
molecules have substantial crystalline character (i.e., ab-
sence of long-chains, which usually decreases the crystallini-
ty of a compound). Indeed, gelation occurs when the pro-
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16679

S. Bhattacharya et al.
pensity of crystallization (i.e., the conversion of the solid
component into crystal from the respective solution) is miti-
gated.[1a] In the present case, NP is crystalline in nature due
Acknowledgements
This work was supported by J. C. Bose fellowship of DST to S.B. S.B. and
S.D. thank CSIR for senior research fellowship.
À
to the presence of short OC4H9 chains and DA is amor-
phous because it has a long n-decanoyl chain in its structure.
As a result, the combination of NP and DA in 1:2 molar
ratios by simple acid/base interactions led to a semi-crystal-
line composite, which, in turn, presumably yielded micro-
crystalline rectangular gel particles. Moreover, the formation
of fibrous (linear) aggregates is governed by the direction
and strength of the binding forces among the aggregates of
the gelators.[1a] In the present case, the interactions among
the aggregates of NP–DA (1:2) are probably not strong
enough to form such directional (fibrous) aggregates with
high aspect ratios, thus resulting in the formation of the rec-
tangular gel particle. Furthermore, when NP and DA was
mixed in 1:3 molar ratio, the overall crystallinity of these
composites was found to be even less compared with that of
the NP–DA (1:2) system. This resulted in the formation of
self-assembled clusters instead of forming rectangular gel
particle. Based on this rationale, a model has been proposed
for the formation of such rectangular gel particles (Fig-
ure 8c).
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We have demonstrated here the selective two-component
gelation by a 1:2 mixture of NP–DA system over the NM–
DA or NO–DA system in 2:1 water/ethanol (v/v). The basic-
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DA gelator, which showed the presence of fibrous morphol-
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experiment also supported the SEM results. The existence
of the two sets of lamellar structures in the three-dimension-
al gel network was confirmed by XRD experiments for both
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was investigated by POM and DSC experiments. During
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phase transition was observed for the NP–DA (1:2) system.
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the first time, by supramolecular polymerization with rare
gel morphology, but also evidenced excellent liquid crystal-
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Received: June 2, 2013
Revised: September 9, 2013
Published online: November 5, 2013
Chem. Eur. J. 2013, 19, 16672 – 16681
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
16681