Impact of “half-crown/two carbonyl”-Ca2+ metal ion interactions of a low molecular weight gelator (LMWG) on its fiber to nanosphere morphology transformation with a gel-to-sol phase transition

Article abstract of DOI:10.1039/c8sm01071d

We report here a smart functional low molecular weight gelator (LMWG) L, containing an unusual metal ion coordination site, i.e. “half-crown/two carbonyl”. The gelator L shows excellent gelation behavior with typical fibrillar morphology in acetonitrile, methanol and ethanol media. Upon Ca²⁺ ion binding with its “half-crown/two carbonyl” coordination site, the acetonitrile gel of L exhibits a fiber to nanosphere morphology transformation along with a gel-to-sol phase transition as confirmed by microscopic investigation and by direct naked eye visualization, respectively. The mechanism involved in this morphology transformation and gel-to-sol phase transition process was studied thoroughly with the help of computational calculations and various spectroscopic experiments and discussed.

Full text of DOI:10.1039/c8sm01071d

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ISSN 1744-683X
PAPER
Jure Dobnikar et al.
Rational design of molecularly imprinted polymers
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Impact of “half-crown/two carbonyl” - Ca2+ Metal Ion Interactions
of a Low Molecular Weight Gelator (LMWG) on its Fiber to
Nanosphere Morphology Transformation with Gel-to-Sol Phase
Transition
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
Arunava Maity,*ab Ananta Dey,bc Mrinal Kanti Si,bc Bishwajit Ganguly*bc and Amitava Das*bc
www.rsc.org/
We report here a smart functional low molecular weight gelator (LMWG) L, containing unusual metal ion coordination site,
i.e. “half-crown/two carbonyl”. The gelator L shows excellent gelation behavior with typical fibrillar morphology in
acetonitrile, methanol and ethanol medium. Upon Ca2+ ion binding with its “half-crown/two carbonyl” coordination site,
the acetonitrile gel of L exhibit fiber to nanosphere morphology transformation along with a gel-to-sol phase transition as
confirmed by microscopic investigation and by direct nacked eye visualization respectively. The mechanism involved in this
morphology transformation and gel-to-sol phase transition process was studied thoroughly with the help of computational
calculation and various spectroscopic experiments and discussed.
visual effect of the external stimulus on supramolecular gels,
the design of gelators molecules also become an important
issue. In fact, with a wide number of gelator molecules being
Introduction
Of the options available to produce and modulate well-
defined nanostructures from suitable molecular building
blocks, none is more versatile than molecular self-assembly.1
Among the various kinds of molecular assemblies,
supramolecular gels based on low molecular weight gelators
(LMWGs), which prevent the free movement of huge amount
of solvent opposite to the gravitational force inside the
entangled 3D molecular network are considered a very useful
class of components to generate distinct nanostructures and
for many proposed applications in a wide range of fields.2 The
driving forces for gelator molecules to self-assemble into
entangled nanostructures are the multiple weak non-covalent
interactions, such as intermolecular hydrogen (H) bonding, π-π
stacking, solvophobic, Van der Waals, electrostatic, and
charge-transfer (CT) interactions, metal-ligand coordination
etc.3 The weak nature of these non-covalent interactions
usually makes the supramolecular gels sensitive to various
external stimulus (e.g. chemical agents, protons, oxidation or
reduction reaction, temperature, light irradiation, sound,
etc.),4-7 and such system provides a good platform to modulate
wide variety of self-assembled structures and functions at
molecular level.2,8 Recently to realize the straightforward
developed and a deep understanding of their intermolecular
interactions, the design of the gelator molecules shifted from
serendipity to purposeful design. For instance, supramolecular
gels which respond to light irradiation have been achieved by
incorporating appropriate photo-responsive moieties (e.g.
azobenzene, stilbene, imine, bisthienylethene and spiropyran)
into the corresponding LMWGs derived from conventional gel
forming functionalities.9-13
In the midst of various chemical stimuli applied to modify
the gel behaviour, metal ions have been the most common
regulators, as because of the availability and the diversity of
metal-ligand coordination that could readily induce or control
the self-assembly process of the gel formations.14 Sometimes
this metal-ligand coordination also can induce the entrapped
solvent molecules to be released, resulting in shrinking or even
a gel-to-solution (sol) phase transition, and thereby influences
its self-assemble nanostructures. For example, Edwards and
co-workers reported the first example about gel-to-sol
disassembly through Ag+-alkene weak interaction as the
driving forces.15 Liu et al. have reported amphiphilic Schiff base
organogels which exhibited different behaviors with different
metal ions.16 The gel structure was disrupted when Zn2+ and
Ni2+ were added. However, the gel structure was maintained in
presence of Cu2+ and Mg2+ and shows twisted tape and fibrillar
morphologies respectively.16 Sobczuk and co-workers
a.Organic Chemistry Division, CSIR-National Chemical Laboratory, Pune 411008,
Maharashtra, India. E-mail: a.maity@ncl.res.in
b.Academy of Scientific and Innovative Research (AcSIR), New Delhi, India.
c. CSIR-Central Salt & Marine Chemicals Research Institute, Bhavnagar, 364002,
Gujarat, India. E-mail: ganguly@csmcri.res.in; a.das@csmcri.res.in
† Footnotes relating to the title and/or authors should appear here.
Electronic Supplementary Information (ESI) available: [details of any
supplementary information available should be included here]. See
DOI: 10.1039/x0xx00000x
displayed
a
gel based on crown ether appended
quaterthiophene; which shows well-defined fibrillar
nanostructure in alcoholic solvents. However, as in response to
alkali metal cation, the fibrillar nanostructure of the
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corresponding gelator completely vanishes upon gel-to-sol
phase transition with enhanced fluorescence emission.17
Terech and co-workers reported a metallosupramolecular gel
based on a multitopic cyclam and bis-terpyridine platform that
showed a redox-responsive gel-to-sol phase transition and
electrochromic properties.18 In addition, Deng and co-workers
displayed gels of amine-modified cyclodextrins and such gels
exhibited a response towards Co3+, Ni2+, Cu2+ and Ag+.19 In all
these instances, the gelators have been deliberately designed
to have various metal-binding motifs and shown the profound
effects of metal-ligand coordination on the self-assembly
process, self-assembled nanostructures with number of smart
properties, including luminescence, redox activity etc.
However, to further advance towards precise control of the
self-assembly process and hence towards more complex
applications, novel molecular architectures are still required.
Polyoxyethylenes, are a class of highly flexible, non-cyclic
crown ether type functional group, which is known to form the
complexes with metal ions (mostly alkali and alkaline earth
metal ions) in the same manner as the cyclic crown ether.20
Typically, these non-cyclic ether derivatives do not show a
strong metal complexation ability compared to that of cyclic
crown ethers.21 However, the flexible nature of this acyclic
crown ethers allows an appreciable change in conformation of
its derivatives from a linear structure to a pseudocyclic one
upon the coordination with metal ions. Such conformational
changes help in improving the eventual binding affinity
towards metal ion.20,21 By taking advantage of this metal ion
induced structural modulation behaviour, Nakamura and co-
workers had developed a series of molecules that consisted of
mono- or bis(chromophores) linked with this polyoxyethylene
moiety and explored their metal ion sensing property, photo-
dimerization of suitable chromophore etc.22 Usually, in this
kind of molecular system, the chromophoric moiety and the
complexing part, transduced the chemical information
produced by metal ion binding event into an optical signal,
such as colorimetric and/or fluorometric changes. Literature
reports also suggest that polyoxyethylene unit is utilized as a
spacer in the modulating the properties of various
supramolecular and biologically active systems.23 However, the
influence of weak metal ion binding of such non-cyclic crown
ether type functional moiety for transforming the self-
assembled nanostructure upon gel-to-sol phase transition has
seldom been reported.
View Article Online
DOI: 10.1039/C8SM01071D
Fig. 1 (a) The molecular structure of the Gelator
L
; (b) Pictorial presentation of
metal ion coordination site, which termed here as “half-crown/two carbonyl”.
The morphological transformation was confirmed by capturing
the scanning electron microscopy (SEM) and transmission
electron microscopy (TEM) images. The metal ion (Ca2+)
coordination event with the gelator L was investigated in detail
with the help of the computational as well as with various
spectroscopic studies. The computational and the
experimental results reveal that the gelator molecule
L binds
with Ca2+ ion using its “half-crown/two carbonyl” functional
motif (Fig. 1b). Apart from the Ca2+ ion, in response to few
other alkali and alkaline earth metal ions such as Li+, Na+, K+,
Mg2+, Sr2+, Ba2+ etc. the organogel of
L also exhibits a “naked
eye” gel-to-sol phase transition. So overall, the impact of the
metal ion binding event with the “half-crown/two carbonyl”
motif on the mechanism of fiber to nanosphere morphology
transformation along with gel-to-sol phase transitions of the
gelator L was presented.
Experimental section
Materials
Unless otherwise stated, all reagents and organic solvents used
for the synthesis of
L were purchased from commercial
suppliers and were used as such without further purification.
Solvents were dried when required according to the standard
procedure.25 Silica gel 100-200 mesh was used for column
L and two of its intermediate 1
and (Scheme 1). HPLC grade solvents were used for gelation
chromatography for purifying
2
test and for recording the spectrometric data of the L.
Instruments
1H and 13C NMR spectra for the synthesized compounds were
recorded on Bruker 400/500 MHz FT NMR (Model: Avance-
DPX 400/500) using tetramethylsilane (TMS) as an internal
standard. High-resolution mass spectra (HRMS) were recorded
on JEOL JM AX 505 HA mass spectrometer. SEM images were
obtained using Nova Nano SEM 450 and Quanta™ Scanning
Electron Microscope. TEM images were recorded using a FEI
Tecnai G2 F20 X-TWIN TEM at an accelerating voltage of 200
kV. Rheological measurements were carried out on a Rheoplus
MCR302 (Anton Paar) rheometer with parallel plate geometry
and obtained data were processed with start rheometer
software. The gap distance between the plates was fixed at 0.5
mm. The powder X-ray diffraction (PXRD) pattern of the air-
Bearing in mind to expand the scope of responsiveness of
the metal ion interactions with the polyoxyethylene units,
herein, we design and synthesized a non-cyclic half-crown
ether like gelator
L
(Fig. 1),23a in which 4,7,10-trioxa-1,13-
tridecanediamine is symmetrically derivatises through amide
bond formation with an amide conjugate of 1-naphthalene
acetic acid and L-phenylalanine (Compound 2; Scheme 1).24
The gelator
L showed translucent, homogeneous and stable
organogel behavior and adopt fibrillar type morphology. It is
imperative that in response to alkaline earth metal ion (Ca2+),
the fibrillar morphology of the
L transform to nanospheres,
dried acetonitrile gel of
L was recorded on a Phillips
PANalytical diffractometer using Cu Kα radiation (λ = 1.5406 Å).
which is associated with the instant gel-to-sol phase transition.
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UV−Vis absorption spectra measurements were documented 133.14, 132.56, 131.82, 129.07, 128.14, 127.9, 12V7ie.5w8A,rti1cl2e6O.n8li2ne,
DOI: 10.1039/C8SM01071D
using Perkin Elmer Lambda 950 UV-Vis spectrometer. All 126.11, 125.75, 125.41, 125.29, 124.14, 69.63, 69.42, 67.80,
emission spectral measurements were performed using PTI 53.99, 37.93, 35.70, 29.03; HRMS (ESI): m/z calculated for
Quanta Master™ steady state spectrofluorometer. Fourier C52H59N4O7 [M + H]+: 851.436, found 851.437; Elemental
transform infrared (FT-IR) transmittance spectra were analysis: C52H58N4O7: C, 73.39; H, 6.87, N, 6.58., found: C,
recorded on a FTIR-8300 (Shimadzu) spectrometer with 2 cm-1 73.26, H, 6.9, N, 6.6.
resolution at room temperature.
Computational methods
Synthesis of gelator L
The conformational search of the gelator
L and the metal ion
(Ca2+) binding event with its ‘half-crown/two carbonyl’ binding
motif was examined with MMFF94 (Merck Molecular Force
Field) force field.26-28 The conformation search yields several
conformers of gelator
L and its corresponding complex with
the Ca2+ ion. We have chosen the most stable conformer
among them for further calculations. The conformational
search has been performed using the Spartan 08 program.29
Further, we considered the most stable structure from the
conformational analysis and optimized the selected structure
with DFT B3LYP functional using the 6-31G* basis set.30-32
Water was employed as a solvent using SMD solvent model.33
The vibrational frequency calculations suggest that the
obtained geometry is
a minimum. All calculations are
Scheme 1. Schematic presentation of the methodology that was adopted for the
synthesis of gelator
performed in Gaussian 09 suite of programs.34
L.
General description of different experimental techniques
Gelation test
Initially, the amide conjugate of 1-naphthalene acetic acid and
L-phenylalanine (compound , Scheme 1), an intermediate
that was used for the synthesis of gelator , was prepared by
following a previously reported procedure.24 This synthesized
intermediate; compound (1.0 mM) was taken in 25 mL of dry
tetrahydrofuran (THF) and was cooled to 0 °C for 10 minutes
under N2 atmosphere. To this cold solution, 4,7,10-trioxa-1,13-
tridecanediamine (0.5 mM) was added, followed by N,N'-
2
L
A weighted amount of gelator
L was taken in a glass vial and
suspended in a series of selected organic solvents (1.0 mL) and
tried to dissolve by heating in closed vial condition. The hot
clear solution was cooled to room temperature and then
sonicated for 2-3 minutes. The system suddenly turned to a
semisolid like soft mass. The “stable-to-inversion protocol of a
glass vial” was adopted to ensure the gel formation. Gelation
was observed in acetonitrile and alcoholic (methanol, ethanol)
solvent.
2
dicyclohexylcarbodiimide
(DCC,
1.0
mM)
and
1-
hydroxybenzotriazole (HOBT, 1.0 mM). The reaction mixture
was stirred for about 6 hrs under the inert condition at room
temperature. The progress of the reaction was monitored by
TLC (eluent phase was 4 : 96; MeOH : CHCl3). After completion
of the reaction, THF was removed from the reaction mixture
under vacuum and 100 ml of dichloromethane (DCM) was
added to it. The DCM layer was washed with saturated
NaHCO3 (3 × 30 mL), dilute HCl (3 × 30 mL), water (3 × 30 mL)
and brine solution (2 × 30 mL). This organic layer was dried
over anhydrous sodium sulfate and subsequently, DCM was
Scanning electron microscopy (SEM)
A small portion of the freshly prepared acetonitrile, methanol
and ethanol gel of
L were scooped out and diluted with the
respective solvents. The Resulting solutions were drop-casted
on a silicon wafer and allowed to air dry for 6 hours in a dust
free place. Finally, it was dried in a desiccator for overnight.
Before taking images, samples were coated with gold vapor.
To check the effect of Ca2+ metal ion coordination with
morphology, a required amount of Ca2+ (in acetonitrile; 2.0
equivalent) was added to the preformed acetonitrile gel of
On addition of Ca2+, the gel turns into a solution. After 1 hour,
a small portion of this subsequent solution was pipet out and
diluted with acetonitrile and used for preparing the SEM
sample by following the above-mentioned procedure.
removed under reduced pressure to get crude
purified by column chromatography using the 2 % MeOH in
CHCl3 gradient solvent system to give pure gelator as white
L, which was
L on its
L
L.
solid. Yield = 56 %. 1H NMR (500 MHz, DMSO-d6): δ (ppm) 8.43-
8.42 (2H, d, J = 8, NH), 7.98-7.96 (2H, t, NH), 7.90-7.87 (4H, t,
Nap-H), 7.78-7.76 (2H, d, J = 8, Nap-H), 7.49-7.46 (2H, t, Nap-
H), 7.44-7.41 (2H, t, Nap-H), 7.39-7.36 (2H, t, Nap-H), 7.28-7.27
(2H, d, J = 7, Nap-H), 7.22-7.20 (10H, broad, Ar-H), 4.50-4.46
(2H, m, -CHCO), 3.90-3.88 (4H, broad, Nap-CH2), 3.50-3.48 (4H,
m, -C-CH2-O), 3.43-3.42 (4H, m, -C-CH2-O), 3.31-3.29 (4H, t, -N-
CH2-C), 3.11-3.05 (4H, m, -C-CH2-O), 2.98-2.94 (2H, m, Ar-CH2),
2.83-2.78 (2H, m, Ar-CH2), 1.56-1.55 (4H, broad, -C-CH2-C); 13C
NMR (125 MHz, DMSO-d6): δ (ppm) 170.79, 169.68, 137.71,
Transmission electron microscopy (TEM)
The prepared samples (with and without Ca2+ treated); which
were used for (SEM analysis), were drop-casted on carbon-
coated copper grids (200 mesh) and initially allowed to air dry
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for 6 hours in a dust-free place followed by under desiccator isolated in pure form after necessary workup and was
View Article Online
DOI: 10.1039/C8SM01071D
for overnight. The dried samples were employed for taking characterized by various analytical/spectroscopic techniques.
TEM images without staining.
The pure gelator
First, the gelation ability of
organic solvents (Table 1). Among various solvents,
L
was utilized for further studies.
was examined in different
was
L
Rheology
L
found to be sparingly soluble in acetonitrile, methanol and
ethanol under ambient condition. However, it was found to
dissolve completely on heating in these solvents. The hot clear
solution upon cooling and subsequent sonication for 2-3
minutes, the system suddenly turned into a semisolid like soft
mass, which we identified as a gel by the “stable-to-inversion
protocol of a glass vial”. Fig. 2a shows the photograph of the
A freshly prepared acetonitrile gel (0.9 mg/mL) of
L was
carefully scooped out and quickly placed on the parallel plate
of the rheometer to ensure minimum solvent loss through
evaporation. Dynamic strain sweep tests were carried out to
increase the amplitude of oscillation from 0.1% up to 100%
apparent strain shear (with a frequency ω = 10 rad·s−1) at 25
°C. Frequency sweep experiment was performed from 0.1 to
100 rad/s at constant strain (γ) of 1 % at 25 °C.
corresponding acetonitrile gel (CGC = 0.9 wt %) of L, which is
translucent in appearance and remained stable for a long
period of time under the closed-vial condition. The resulting
gel was found to be highly thermo reversible; upon heating, it
was transformed into the solution and reverted to gel state
after cooling to room temperature with little sonication (Fig.
Powder X-ray diffraction (PXRD)
A sufficient amount of acetonitrile gel of
L was prepared in a
capped glass vial. The Glass vial, containing gel was kept in
uncapped condition for few days to evaporate the entrapped
solvent within the gel matrix. After complete evaporation of
the solvent, the air-dried solid mass was used for the PXRD
study.
2b). The gelator
L turn into a partial gel in 1,4-dioxane and
toluene, whereas it was insoluble in ethyl acetate and hexane
and was found to be soluble in each of the following solvents
like CH2Cl2, CHCl3, THF, DMF and DMSO (Table 1).
Table 1 Gelation properties of L
in different solvents.a
UV-vis and fluorescence studies
Solvent
Hexane
Toluene
Ethyl acetate
1,4-Dioxane
DCM
State
I
PG
I
PG
S
CGCs (wt %)
-
-
-
-
-
-
0.9 w/v
1.0 w/v
1.0 w/v
-
-
-
A stock solution of
L
(1.0 × 10-3 M) was prepared in
acetonitrile, and this solution was used for all electronic and
fluorescence spectral studies after appropriate dilution with
acetonitrile. Except for the K+, the perchlorate slats of each
alkali and alkaline earth metal ions (Li+, Na+, Mg2+, Ca2+, Sr2+
and Ba2+) were used to prepare initially 1.0 × 10-3 M of stock
solution in acetonitrile and used these solutions after
appropriate dilution. For K+ ion, we had used its
hexafluorophosphate salt.
CHCl3
S
Acetonitrile
Methanol
Ethanol
THF
DMF
DMSO
G
G
G
S
S
S
Fourier transform infrared (FT-IR) study
2.0 mM CD3CN solutions of
L (in presence and absence of 5.0
equivalent of Ca2+ metal ion) were used for FT-IR
measurements. Approximately 60 microliters of freshly
prepared appropriate sample solutions were loaded into a
demountable cell consisting of two windows (CaF2, 3 mm
thickness, Shenzen Laser), separated by a mylar spacer of 50
micrometers thickness, and FT-IR spectra were recorded.
Before plotting both the FT-IR spectrum, the solvent (CD3CN)
alone spectrum was subtracted as background.
aI = insoluble, S = solution, G = gel, PG = partial gel; gel formed by heating cooling
followed by slight sonication.
Following standard method “stable-to-inversion protocol
of a glass vial” (Fig. 2a), it was ensured that the translucent
semisolid mass was a gel. We further examined its viscoelastic
behavior by rheology experiment. The viscoelastic behavior of
gel is generally determined by two key parameters, elastic
modulus (G': solid-like behavior) and viscous modulus (G'':
liquid-like behavior). Fig. 2c and 2d, respectively, show the
dynamic oscillatory stress sweep and a frequency sweep of the
soft mass (Fig. 2a), obtained from acetonitrile. It is evident
from the Fig. 2c, initially G' is higher than G'' and both
accomplish a linear signature. The linear viscoelastic region
(Fig. 2c), where G' > G'', reveals the solid character of this soft
mass. Beyond the linear viscosity region at certain stress
(25.02 Pa), G' falls lower than G'', and the soft mass transforms
into a liquid-like appearance (Fig. 2c). Therefore, beyond the
25.02 Pa of oscillatory stress, it begins to flow. The frequency
sweep experiment, where the G' and G'' were measured as a
Results and discussion
The methodology that was adopted for the synthesis of gelator
L
is shown in Scheme 1. Our design strategy helped us to have
the central oxyethylene spacer (half-crown) along with two
carbonyl units as metal ion binding scaffold, and the two
terminal amide conjugate of 1-naphthalene acetic acid and L-
phenylalanine moiety (Compound 2; Scheme 1) for favouring
the self-assembly process through multiple noncovalent
interactions (such as H-bonds, π-π stacking etc.) to achieve an
improved gelation property.24 Desired gelator molecule
L was
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Fig. 2 Photographs showing (a) freshly prepared translucent acetonitrile gel of
L and (b) its temperature induced reversible gel-to-sol transition. Dynamic oscillatory
stress sweep (c) and frequency sweep (d) studies of the acetonitrile gel of L.
Fig. 3 (a, b, c) SEM and (d, e, f) TEM images of the diluted acetonitrile, methanol and ethanol gel of L respectively
function of angular frequency at 0.1 % constant strain, confirmed that the identified semi-solid like soft mass (Fig. 2a)
displayed the G' value of the corresponding soft mass was had a certain degree of viscoelasticity and it was actually a
much higher than that of G'' for the entire of frequency range gelatinous material.
(Fig. 2d). Both G' and G'' values were found to be feebly
dependent on frequency and this signified that some weak the molecular self-assembled structures at the nanoscale level
matrixes existed in it.35 The higher magnitude of G' (Fig. 2d) of acetonitrile, methanol and ethanol gels of
. SEM image of
implied that a good mechanical strength was present in this the dilute solution of acetonitrile gel of showed thin plates,
soft mass. Thus, results of the rheological experiment which were connected to each other to form flat and flexible
Next SEM and TEM experiment were performed to reveal
L
L
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ribbon-like network morphology (Fig. 3a). However, SEM ions, a complete gel-to-sol phase transition was oVbieswerAvrteicdle,Oansliniet
images of the corresponding dilute solutions obtained from was observed for the Ca2+ ion.
DOI: 10.1039/C8SM01071D
methanol and ethanol gels of
structures of high aspect ratio (Fig. 3b,c). Although the SEM transition, thus one might expect that self-aggregated network
image of the acetonitrile gel of showed a thin plate-like structure can also be affected. To examine this, SEM and TEM
L
showed entangled fibrous
As the metal ion treatment influence the gel-to-sol phase
L
structure, the TEM result of the same solution appeared to characterization was performed of the dilute solution of the
show well-developed fibrous network structure (Fig. 3d). The corresponding transformed gel-to-sol solution (Fig. 4a)
observed fibrous network consisted of entangled thin fibrils of obtained after the addition of 2.0 equivalent of Ca2+ ion in
nanometer diameter and micrometer length (Fig. 3d). Similar acetonitrile gel of
findings were also observed in the TEM images of methanol of the transformed sol solution of
and ethanol gels of , respectively (Fig. 3e,f). Thus, results of nanosphere architecture (Fig. 4b, 4c) with complete
L. Interestingly both the SEM and TEM image
L
showed a spurted
L
the SEM and TEM experiments suggest the formation of the disappearance of their network structure, which was
entangled fibrillar structure as a key step for gel formation of otherwise observed in Fig. 3a and 3d. This observation again
L
.
confirms that the treatment of metal ion on the designed
is gelator has a serious effect on its self-assembly mode, which
expected to help in the binding this molecule to metal ion and eventually triggers its morphology transformation and gel-to-
influence the molecular self-assembly behavior of . To sol phase transition.
examine this, we gently pipetted various equivalent amounts After direct visualization of morphological transition from
of Ca2+ ion (as its perchlorate salt) as an acetonitrile solution fibrillar network to nanospheres structure microscopically and
onto the pre-formed acetonitrile gel (0.9 wt %) of . It was naked eye detection on gel-to-sol phase transition (Fig. 4), we
observed that the gel phase of became unstable and a gel-to- were curious to find out the involved non-covalent interactions
sol transformation was induced on immediate contact of the in gelator and the alteration of these interactions upon
Ca2+ ion. Till [Ca2+] was less than 1.0 mole equivalent of , the complexation with Ca2+ and also the mode of binding of Ca2+ in
acetonitrile gel of
existed in the partial gel state. However, its L•Ca2+ complex. As these are the crucial factors to
for [Ca2+]
1.0 mole equivalent a complete collapse of the gel determine the particular molecular arrangement in self-
(acetonitrile gel of
Presence of the “half-crown/two carbonyl” moiety23a in
L
L
L
L
L
L
L
L

L) phase from top to bottom was observed aggregated state and to establish the mechanism of self-
as the Ca2+ salts diffused through the gel and transmuted into assembly process. In this milieu, the solid-state single crystal
a homogeneous solution. Photograph of gel-to-sol phase structure is the ultimate proof to establish the precise
transition of acetonitrile gel (0.9 wt %) of
equivalent of Ca2+ ion is shown in Fig. 4a. Instability of the
L in response to 2.0 molecular arrangement and envisage the operating
noncovalent interactions of any assemblies in their self-
assemble state. Unfortunately, despite our repeated attempts
with various solvent composition and experimental condition
we failed to grow a single crystal of
L
and L•Ca2+. As an
and
alternative, we performed computational studies for
L
L•Ca2+ using density functional theory (DFT) calculations
employing the B3LYP functional and 6-31G* basis set.30-32
The optimized structure of the metal-free gelator L showed
that it attainted an interesting folded structure with the
assistance of several intramolecular non-bonded interactions
such as H-bonds between the carboxyamides and the ethereal
oxygen atoms and C-H···O interaction between aromatic
hydrogens and ethereal oxygens (Fig. 5a). A critical inspection
of the Fig. 5a reveals that the aromatic rings of the gelator
L
interact with each other via CH···π and π···π interactions. The
cumulative effect of these interactions, (namely intramolecular
H-bonds, C-H···O, CH···π and π···π interactions) helped the
Fig. 4 (a) Gel-to-Sol phase transition of acetonitrile gel (0.9 wt %) of
L with the
addition of 2.0 equivalents of the Ca2+ ion; (b and c) SEM and TEM image of the
corresponding transformed sol solution (after proper dilution) respectively, show
the nanosphere morphology formation.
gelator
L to attain this folded structure (Fig. 5a). The gelator L
also have free amide (NHCO) functional groups that can be
involved in the formation of the extended intermolecular H-
acetonitrile gel of
L was further examined in the presence of
few other alkali and alkaline earth metal ions such as Li+, Na+,
Mg2+, Sr2+, Ba2+ (as their perchlorate salts) and K+ (as the
hexafluorophosphate salt). The KPF6 is preferred over KClO4 in
our study as the latter is known to be sparingly soluble in
acetonitrile; the solvent used in this study. The gel phase was
retained partially in the presence of Li+, Na+, K+, Mg2+, Sr2+, Ba2+
bond formation among the consecutive
L molecules and
eventually can result in a fibrillar network-like structure and
gel formation.
The energy optimized structure of the L•Ca2+ complex is
shown in Fig. 5b. In the metal-free gelator L, the central
oxyethylene moiety was poisoned in the anti-orientation of its
adjacent two carbonyl groups (Fig. 5a). However, the
metal ions having a concentration
With the addition of more than 1.0 equivalent of these metal
 1.0 mole equivalent of L.
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DOI: 10.1039/C8SM01071D
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ARTICLE
(a)
(b)
2.55
2.53
2.50
2.86
2.50
3.16 2.01
3.04
2.01
3.40
3.10
2.41
3.08
3.33
2.38
Fig 5 (a and b) The optimized structure of gelator
L
and L•Ca2+ complex at B3LYP/6-31G* method in aqueous phase. Non interacted H atoms are omitted for clarity.
(Color code: red = oxygen; blue = nitrogen; grey = carbon; green = chlorine; olive = calcium).
optimized metal complex structure (L•Ca2+) showed that in length (~ 12.607 Å, Fig. S1). This result validates the existence
favor of binding with Ca2+ both the carbonyl groups turned of the folded molecular structure (as shown in Fig. 5a) of
L
in
toward the same direction of the oxyethylene moiety (Fig. 5b). its self-assemble state. Furthermore, in the wide angle region
Due to this conformation change, gelator formed a crown of the PXRD of (Fig. 6) the appearance of a broad peak at 2θ
L
L
ether-like cavity and the Ca2+ ion was encapsulated within it = 16.76–24.50° (d = 5.2-3.6 Å) arises due to alkyl group packing
with the help of the coordination to the two central amide- and aromatic π-π stacking interaction of the gelator molecules
carbonyl oxygen and two ethereal oxygen atoms of the half- in its gel state.36 However the assignment of the peak at 2θ =
crown like triethylene oxide linkage of L (Fig. 5b). Interestingly, 10.43° (d = 8.5 Å) at present remains unsolved.
two counter anions (ClO4¯) also remained bonded to the Ca2+
metal ion effectively and hence the L•Ca(ClO4)2 complex
accomplished a 6-coordinated quasi-octahedral type geometry
with Ca2+···O distances within the range of 2.38-2.55 Å (Fig.
5b). The gelator
L, therefore, utilized its “half-crown/two
carbonyl” binding motif to coordinate the Ca2+ metal ion (Fig.
1b and 5b). Again, in comparison to the optimized structure of
L
, the L•Ca(ClO4)2 structure shows, upon coordination of Ca2+
metal ion, two naphthalene rings of
L were positioned far
away from each other and entire molecular orientation
changes in such a way that all the existed non-covalent
interactions (intramolecular multiple H-bonds, CH···π and π···π
Fig. 6 Powder X-ray diffraction (PXRD) pattern of the air-dried acetonitrile gel of
interactions etc.) in the metal-free gelator
L completely
L.
disappears. Further, the electrostatic repulsion is also a
common problem among the bound metal ions to a ligand,
which usually hampered the intermolecular interactions
among the consecutive molecules and results in their
disassembly process.15 Perhaps, in our molecular system, the
To investigate the mechanism of cation response on the
molecular scale, we record and analyze the FT-IR spectrum of
the gelator
with and without Ca2+ ion in CD3CN. The peak at
1669 cm-1 was attributed to the amide carbonyl stretch
(C=Oamidestr) of
in absence of metal ion (Fig. 7; blue line). The
position of this peak in the IR frequency range revealed an
unambiguous signature of a network of H-bonded amides of
the metal-free L
. When Ca2+ was added, the corresponding
L
gelator L also possesses this strong charge repulsion force after
L
the binding of Ca2+ ions, which profoundly affect to disrupt the
inter and intramolecular H-bonds, CH···π and π···π interactions
etc. and finally leads to a dissociation of the gel phase and its
fibrillar network to nanosphere morphology transition.
amide carbonyl (C=Oamidestr) peak shifted to 1645 cm-1 (Fig. 7;
red line). In presence of Ca2+, the shifting of this peak position
to lower wavenumber region indicates the weakening of the
C=Oamide bond strength (Fig. 7; red line). This is attributed to
To gain the additional insight into the modes of molecular
packing experimentally, powder X-ray diffraction (PXRD)
measurement were recorded of the air-dried acetonitrile gel of
L. In the small angle region of its PXRD pattern a peak at 2θ =
the Ca2+ binding to C=Oamide group of
the gel network of was mainly formed through the
L. It was presumed that
6.39° was observed (Fig. 6). The corresponding d-spacing value
13.7 Å for this peak calculated according to Brags equation, is
closely matched with its energy optimized folded molecular
L
intermolecular hydrogen bond interactions between the amide
(CONH) groups. Binding of Ca2+ to C=Oamide group was expected
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Journal Name
to destroy this corresponding intermolecular hydrogen bonds naphthalene moiety (Fig. 5), which results in their diamagnetic
View Article Online
shifts (up-field shifts).22c
DOI: 10.1039/C8SM01071D
between amide (CONH) groups within the gel networks of
and thus promoting its gel-to-sol transition.
L
Fig. 7 FT-IR spectra of 2.0 mM CD3CN solution of gelator
L in the presence (red
line) and absence (blue line) of 5.0 equivalent of the Ca2+ ion.
In higher wavenumber region, two more peaks for the N-H
stretch (amide N-Hstr) of
L
appeared at 3350 cm-1 and 3391 cm-
Fig. 8 Partial 1H NMR spectra (500 MHz, 25°C) in DMSO-d6, of gelator
L (blue line;
20.0 mM) and after the addition of 5.0 equivalent of Ca2+ (red line).
1
also directs their H-bonded nature (Fig. 7; blue line).
However, due to the interference of water molecule with the
addition of Ca2+ salt, these two distinct N-H stretches of
The presence of chromophoric naphthalene unit in
further offered us an opportunity to probe the binding
phenomena of Ca2+ ion with
by using the UV-Vis and
fluorescence spectroscopy. The UV-Vis absorption spectrum of
(1.0 × 10-5 M) in acetonitrile solution at 298 K, showed band
L
gelator
(Fig. 7; red line). The peaks at 2869 cm-1 and 2936 cm-1 for
L
merged and a broad signal appeared at 3477 cm-1
L
L
were assigned to the aliphatic -C-H stretch (-C-Hstr, Fig. 7; blue
line), which also become broad and almost negligible with the
addition of Ca2+ (Fig. 7; red line).
L
maxima at 283 nm with two shoulders at 272 and 293 nm.
These were assigned to the π-π* transitions of naphthalene
moiety (Fig. 9a; black line). The fluorescence spectrum of the
corresponding solution showed two maxima at 328 and 337
nm (Fig. 9b; black line).
In order to further confirm the metal ion coordination
mode of
in the absence and presence of Ca2+ ion in DMSO-d6 at 298 K.
L in L L
Ca(ClO4)2, we performed the 1H NMR study of
Due to the low solubility and high gelatinous nature of
acetonitrile, it was not possible to investigate the 1H NMR
study in CD3CN. The partial H NMR spectra of
L in
1
L before and
after the addition of Ca2+ are shown in Fig. 8 as a typical result.
Peak assignments of the important protons were made by
using TOCSY NMR spectra (Fig. S2). The central oxyethylene
protons of L (protons a, b and c; Fig. 8) showed a marginal up-
field shift upon complexation to Ca2+. This suggests that Ca2+
ion was weakly coordinated to the ethereal oxygen atom. On
the other hand, the amide proton (identified as ‘f’; Fig. 8),
neighboring to the central oxyethylene moiety, showed
considerable up-field shifts (∆δ = - 0.09 ppm) when spectra
were recorded in the presence of Ca2+. This observation
suggested rather strong coordination of the Ca2+ ion to the two
carbonyl oxygen atom (OC=O). Another set of amide proton ‘g’
(1.0 × 10-5
L
(1.0 × 10-5 M) with
(Fig. 8) also showed
a significant up-field shift upon
Fig. 9 Uv-Vis absorption (a) and fluorescence (b) spectra of gelator
L
M) in absence (black line) and presence (orange line) of 20 mole equivalent of
complexation of
L
with Ca2+. The considerable shifts of amide
Ca2+ ion; (c) change in fluorescence spectral pattern for
varying the concentrations of Ca2+ (from 0.0 - 3.5 × 10-4); (d) Benesi-Hildebrand
(B-H) plot of 1/[I – I0] vs 1/[Ca2+]. For all studies were performed in pure
acetonitrile medium, for fluorescence spectra (λex = 283 nm).
protons and also the oxyethylene protons signals were
associated with the structural reorganization and
conformational changes of
L during the binding of its central
Upon addition of 20 mole equivalent of Ca2+ salts in
acetonitrile, failed to induce any significant change in the
‘half-crown/two carbonyl’ motif to Ca2+; an observation that
was also supported by the results of the computational studies
(Fig. 5). Due to the structural reorganization and
conformational change upon complexation, these mentioned
absorption spectrum of
the absorption spectrum, upon addition of 20 mole equivalent
Ca2+, a little enhancement in the fluorescence intensity of
L (Fig. 9a; orange line). In contrast to
L
protons of
L are probably covered with shielding area of
was obtained (Fig. 9b; orange line). The increased emission
8 | J. Name., 2012, 00, 1-3
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intensity of
rigid structure upon complexation with Ca2+ ion compared to study does not give any impression on thDeOinI:t1e0r.1a0c3t9io/Cn8oSMf 01071D
its metal-free flexible form. To understand the binding affinity the alkali and alkaline metal ions, the increase of emission
of
towards Ca2+, systematic fluorescence titration was also intensity upon addition of these metal ions to the dilute
carried out in acetonitrile medium with varying [Ca2+] (0.0 - 3.5 solution of
clearly indicates metal ion coordination role is
× 10-4 M), while the concentration of
was kept constant (1.0 operating here
× 10-5 M). With the increase of [Ca2+], a small but gradual
Together with the above experimental facts and theoretical
increase of emission intensity of
was observed (Fig. 9c) at calculations, it is now possible to summarize the impact of Ca2+
L
is must be due to the formation of the little bit referred metal ions (Fig. 10b). Although the Uv-VVisiewabArstioclrepOtniloinne
L
with
L
L
L
L
328 and 337 nm. Using parameters that were obtained from ion coordination with the “half-crown/two carbonyl” motif of
L
this titration plot (Fig. 9c), formation constant (Ka) for L•Ca2+ on the mechanism of its fiber to nanosphere morphology
was evaluated using Benesi-Hildebrand (B-H) plot (Fig. 9d), and transformation along with gel-to-sol phase transition. Metal-
it was found to be (9.7 ± 0.2) × 104 M−1. A good linear fit of the free gelator
L initially enriched with various noncovalent
B-H plot for 1/[I – I0] vs 1/[Ca2+] confirmed the 1:1 binding interactions such as intramolecular H-bonds, CH···π and π···π
stoichiometry.37 Likewise B-H plot, the Jobs plot analysis also interactions (Fig. 5a and 6). Along with these intramolecular
confirm a 1:1 binding stoichiometry between
(Fig. S3). The Calculated formation constant (from B-H plot, consecutive
Fig. 9d) for the L•Ca2+ complex in pure organic solvent ensured with the help of intermolecular H-bonding interactions among
L
and Ca2+ ion interactions, during the course of self-assembly, the
L
molecules build an extended network structure
that interaction between
L
and Ca2+ ion was moderate.
the amide (NHCO) groups and resulted into gel with entangled
fibrillar morphology (Scheme 2). This is quite sure that the
main driving force behind the fibrillar network structure is
these intermolecular H-bonding interactions among the amide
(NHCO) groups. Now with incorporation of Ca2+ ions,
L is
coordinated with the Ca2+ using its “half-crown/two carbonyl”
binding motif, as a result the strength of the H-bond becomes
weak; as the two carbonyl groups are no more available to
participate in the intermolecular H-bonds, and hence the gel
L
(1.0 × 10-5 M)
phase of
L
becomes unstable. Further, in favor of Ca2+ ion
also changes in a
Fig. 10 (a) Uv-Vis absorption, (b) fluorescence spectra of gelator
in absence and presence of 20 mol equivalent of different alkali and alkaline
coordination, the molecular orientation of
L
earth metal ions in acetonitrile medium. For fluorescence spectra (λex = 283 nm).
manner that the existed all intramolecular interactions
disappear (Fig. 5b); this also contributes to destabilizing the
self-assembled gel state of L. Finally, with the addition of the
excess equivalent of Ca2+ ion, the electrostatic repulsion
among the bound Ca2+ ions prevailed and this kept neighboring
We have also recorded both the UV-Vis absorption and
fluorescence spectra of the of
of 20 mole equivalent of other alkali and alkaline earth metal
ions (such as Li+, Na+, K+, Mg2+, Sr2+ and Ba2+) in acetonitrile. No
detectable change in absorption spectrum was observed (Fig.
L
(1.0 × 10-5 M) in the presence
L
molecules away from each other and disrupted the
aggregated structure for
L
(Scheme 2).15 As a result, fibrillar
10a); whereas a little increase in the emission intensity of the
L
network morphology disassembled into the basic spherical
structure and gel-to-sol phase transition took place.
was observed on the treatment with any one of the above-
Scheme 2 Schematic presentation of the mechanism of fiber to nanosphere morphology transformation along with gel-to-sol transition of
carbonyl”- Ca2+ coordination.
L upon “half-crown/two
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DOI: 10.1039/C8SM01071D
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ARTICLE
6385; (k) S. Kushwaha, A. Maity, M. Gangopadhyay, S.
Ravindranathan, P. R. Rajamohanan and A. Das, Langmuir.,
2017, 33, 10989; (l) H. Frisch, P. Besenius, Macromol. Rapid
Commun., 2015, 36, 346.
Conclusions
In conclusion, we have introduced a relatively new kind of
2
(a) P. Terech and R. G. Weiss, Chem. Rev., 1997, 97, 3133; (b)
metal ion binding motif (“Half-crown/two carbonyl”) based
L. Zhang, X. Wang, T. Wang and M. Liu, Small., 2015, 11
,
LMWG
morphological as well as gel-to-sol phase transition
mechanism. We suggested that, the gel network of is
L
, and established its metal ion (Ca2+) responsive
1025; (c) S. S. Babu, V. K. Praveen and A. Ajayaghosh, Chem.
Rev., 2014, 114, 1973; (d) L. A. Estroff and A. D. Hamilton,
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L
underpinned by intermolecular H-bonding interaction among
the amide (NHCO) groups, and the disruption of these H-bonds
upon binding of Ca2+ ion to the “Half-crown/two carbonyl”
binding motif of
the gel phase of
disassembled nanospheres morphology transition. In quick
summary, we have represented a unique example in which
“Half-crown/two carbonyl” - Ca2+ interactions play a vital role
in mediating a response in soft matter systems, providing
fundamental insight into the nature of this interaction and
acting as a step on the way to the development of metal-
responsive materials.
L
L
is the main driving force for the collapse of
and hence its assembled entangled fibers to
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Conflicts of interest
There are no conflicts to declare.
Huang, Soft Matter, 2011,
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9
,
8
Acknowledgments
4
A.M. and A.D. acknowledge CSIR for their research fellowship.
M.K.S is thankful to UGC, New Delhi, India for awarding a
fellowship. B.G. thanks (SIP, MSM, CSIR, New Delhi, DAE-BRNS,
Mumbai) and DST, New Delhi for financial support. A.D.
acknowledges SERB (India) (EMR/2016/001850 and
JCB/2017/000004/SSC) and DRDO (India) (ERIP/ER1502249/M/
01/1685) for funding. Supports from CSIR-NCL and CSIR-
CSMCRI) are also gratefully acknowledged.
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