Investigation of the Mechanism Behind Conductive Fluorescent and Multistimuli-responsive Li+-enriched Metallogel Formation

Article abstract of DOI:10.1002/asia.202000630

A fluorescent metallogel (2.6 % w/v) has been obtained from two non-fluorescent components viz. phenyl-succinic acid derived pro-ligand H₂PSL and LiOH (2 equiv.) in DMF. Li⁺ ion not only plays a crucial role in gelation through aggre

Full text of DOI:10.1002/asia.202000630

CHEMISTRY
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Accepted Article
Title: Investigation of mechanism behind conductive fluorescent and
multi-stimuli responsive Li+-enriched metallogel formation
Authors: Jay Shukla, Yeeshu Kumar, Manish Kumar Dixit, Mahendar
Chinthakuntla, Vinay Kumar Sharma, Abul Kalam, and
Mrigendra Dubey
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10.1002/asia.202000630
Chemistry - An Asian Journal
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Investigation of mechanism behind conductive fluorescent and
multi-stimuli responsive Li+-enriched metallogel formation
Jay Shukla,[a] Yeeshu Kumar,[a] Manish K. Dixit,[a] Chinthakuntla Mahendar,[a] Vinay K. Sharma,[b] Abul
Kalam[c] and Mrigendra Dubey*[a]
[a]
J. Shukla, Y. Kumar, M. K. Dixit, C. Mahendar, Dr. M. Dubey
Soft Materials Research Laboratory, Department of Metallurgy Engineering and Materials Science
Indian Institute of Technology Indore, Simrol, Indore 453552, India
E-mail: mdubey@iiti.ac.in
[b]
[c]
V. K. Sharma
Indian Institute of Technology (BHU) Varanasi- 221005
Dr. A. Kalam
Department of Chemistry,
College of Science, King Khalid University, Abha 61413, KSA
Supporting information for this article is given via a link at the end of the document.
Abstract: A fluorescent metallogel (2.6% w/v) has been obtained
from the two non-fluorescent components viz. phenyl-succinic acid
derived pro-ligand H2PSL and LiOH (2 equiv.) in DMF. Li+ ion not only
plays a crucial role in gelation through aggregation, but it also
contributed towards enhancement of fluorescence by imposing
restriction over excited state intramolecular proton transfer (ESIPT)
followed by origin of chelation enhanced fluorescence (CHEF)
phenomenon. Further, the participation of CHEF followed by
aggregation caused quenching (ACQ) and aggregation induced
emission (AIE) in the gelation process have been well established by
fluorescence experiments. Transmission electron microscopy (TEM)
analysis disclosed the sequential creation of nanonuclei followed by
nanoballs and its alignment towards the fibre creation of about 3, 31
and ~40 nm diameter, respectively. The presence of long-range
fibrous morphology inside the metallogel was further attested by
scanning electron microscopy (SEM). The rheological studies over
metallogel proves its true gel phase material nature. Nyquist
impedance study shows the resistance value 7.4 kW for metallogel
which upon applying ultrasound increased to 8.5 kW, while at elevated
temperature of 70 oC caused reduction in resistance value to 4.8 kW.
The mechanism behind metallogel formation has been well
established by using FTIR, UV-vis, SEM, TEM, PXRD, 1H NMR,
fluorescence and ESI-MS.
metal-metal
interactions,
coordination
complexation/
polymerization, etc.[16-19]
Coordination of alkali/ transition/ lanthanide metal ions to
appropriately designed ligands produces metallogels which may
demonstrate the unique optical/fluorescent as well as other metal
associated properties.[20-26] Due to appearance of unique
fluorescence property in metallogels, these materials could be
utilized for sensing, cell imaging, security writing, etc.[27-30]
Involvement of CHEF, AIE and ACQ phenomenon towards
generation of fluorescence in metallogels are well established in
the recent past, but simultaneous occurrence and explanation of
all the three phenomena in a single metallogel is hardly
accomplished so far except our few reports.[23, 25, 26]
Metallogels, by virtue of its electrically conducting behaviour,
have gained substantial attention to be realized in the various
electronic and electrochemical device applications.[31, 32] Recently,
Dey et al. developed a metallogel based semiconductive device,
the conductivity of the metallogel was achieved to be 5.35 × 10−2
S/cm.[31] Further, Bhattacharjee et al. synthesised zinc containing
coordination driven metallogel and investigated its conducting
property in order to develop gel electrolyte for supercapacitor
application. The prepared metallogel exhibited very low
resistance value of 26.3 W, hence claimed its suitability as an
electrolyte for the targeted application.[32] Thus, these reports
suggest that the conductive properties of metallogels can be
utilized for fabrication of gel phase-based devices.
Recently, we reported chiral tartaric acid/ citric acid derived
ligands/ a series of bolamphiphiles including a succinic acid
derived ligand H2SL lead to formation of highly fluorescent
metallogels either upon addition of requisite amount of LiOH
or/and Zn(OAc)2/Cd(OAc)2.[25,33] We also tuned the morphology
from nanoball to twisted fibers, optical properties from CHEF to
intramolecular charge transfer (ICT) upon incorporation of minor
changes in molecular functionality in tartaric acid-based
ligands.[23] On the other hand, metallogels obtained from citric
acid derived ligand having additional arm compared to tartaric
acid one ligands showed morphological transformation from
random to lucid nanofibrous morphology, 10 fold enhancement in
conductance under influence of ultrasound.[24] Inspired from all
these findings, herein, we modified H2SL ligand by incorporating
additional benzene ring to obtain the chiral H2PSL ligand which
Introduction
Gels are well known smart materials, gathered alluring
importance in the recent past due to their divergent and
interdisciplinary applications.[1-6] Such smart materials, when
subjected to external stimuli e.g. pH, temperature, light, sound
and chemicals, usually undergo reversible gel-sol transition.[4-6]
As a result of their responsiveness towards a variety of external
stimuli, supramolecular gels find important applications in the field
of catalysis, drug delivery, tissue engineering, sensing and light
harvesting.[7-9] Metallogels as an emerging subclass of the
supramolecular gels find an increasing interest due to some
additional enhanced properties such as conductance, optics,
catalysis,
rheology,
magnetism,
etc.[10-15]
Similar
to
supramolecular gels, metallogel formation also takes place with
the help of H-bonding/ Van der Waals/ p-p stacking weak
interactions in addition to characteristic driving forces such as
1
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10.1002/asia.202000630
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lead to formation of a multi-stimuli responsive, conductive and
fluorescent metallogel upon treatment with 2 equivalents of LiOH.
Sections and Scheme S1, Supporting Information). The phenyl
group present in H2PSL is expected to provide a platform for
additional p-p stacking for aggregation and ONO’ donor sites
would accommodate Li+ ion to produce CHEF assisted
fluorescent metallogel (Scheme 1). Further, it is also expected
that metallogel enriched with Li+ ions would demonstrate the
conductance property.
2 eq.
LiOH
1H NMR of synthesized H2PSL ligand in d6-DMSO displayed
the existence of three conformers (syn/anti, anti/anti and syn/syn)
which upon treatment with LiOH converted into a single conformer
(syn/syn) as demonstrated in figure S1 (Supporting
H2PSL + LiOH
H2PSL
Ultrasound
5 min
Information).[25,25,33] The deprotonation of H2PSL with
2
Fluorescent
metallogel
equivalents of LiOH in DMF followed by sonication for about 5
minutes led to the formation of blue fluorescent metallogel at 2.6%
w/v, (Scheme 1 and Figure 1). The gelation was further confirmed
by the conventional falling ball and inverted vial tests. The
metallogel obtained was found to be stimuli-responsive towards
heat, mechanical shaking, whereas, resistant against brief
ultrasonication (Figure 1). Furthermore, H2PSL alone or in
presence of other alkali bases like NaOH, KOH, CsOH except
LiOH failed to produce metallogel under aforementioned similar
conditions implies the selectivity towards Li+ for gelation (Figure
S3-S5, and Table S1, Supporting Information). Gelation test of
H2PSL with LiOH was also tested in presence of other common
laboratory solvents such as CH3OH/ DMSO/ CHCl3, but except
DMF, the compound failed to form a gel in other solvents tried.
(Table S2, supporting information).
Li
O
Li+
Ionic
O
O
N
H
N
N
H
N
O
Li
O
interaction
HN
N
O
O
Li
p-p
stacking
O
Li
N
NH
Scheme 1. Representation of the steps involved in metallogel formation along
with the changes in fluorescence.
Results and Discussion
With an objective to explore the effect of inclusion of an additional
benzene ring to H2SL ligand over metallogelation, we synthesized
a phenyl-succinic acid derived ligand H2PSL (Experimental
Morphological Characterization
(B)
To acquire the morphological insights, the metallogel was
examined under field emission scanning electron microscopy
(FESEM) and transmission electron microscopy (TEM). TEM
disclosed an interesting mechanism behind fiber formation within
the metallogel matrix. It revealed the formation of unique
nanonuclei with average diameter of about ~3 nm which perhaps
(A)
(B)
Heat
Cool
Ultrasound
Rest
UV light
Vis light
(E)
(A)
(C)
(C)
(D)
Rest
Shake
(D)
Figure 2. TEM images of diluted metallogel (1x10-4 M) at four different
magnifications.
Figure 1. Stimuli-responsive behaviour of (A) metallogel towards (B)
temperature, (C) UV light, (D) mechanical shaking and (E) resistant for
ultrasound.
2
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10.1002/asia.202000630
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aggregate into nanoballs of about ~31 nm average diameter
(Figure 2).[33] Moreover, these nanoball aggregates lead to
fabrication of fibrous network (average fiber diameter ~40 nm)
within metallogel matrix which was evident through FESEM
analysis where we observed long range intermingled fibrous
morphology (Figure 3).
4 and 8).[24,25] A plausible molecular arrangement has been
depicted in figure 8. However, the crystallization of an insignificant
amount of Li+ salts during the process of drying of metallogel
cannot be ruled out.[26]
H-bonding
Figure 4. PXRD pattern of the xerogel (H2PSL/Li+, black lines) and the ligand
(H2PSL, red lines).
7000
437.2
Experimental
Simulated
5000
3000
438.2
1000
436.2
436
439.2
439
437
440
438
m/z
Figure 5. ESI-Mass spectra show isotopic abundance pattern represented the
simulated (red dotted line) and experimental (black line) for diluted metallogel.
Figure 3. FESEM images of the xerogel at different magnifications (A-B).
To investigate the packing of molecular aggregates and
presence of weak interactions powder X-ray diffraction (PXRD)
study performed on xerogel. High intensity peaks in the PXRD
patterns of H2PSL in the range 2θ = 5o-50o indicative towards its
crystalline nature while that of the xerogel showed broad peaks at
Structural Analysis
To find out the chemical structure involved in formation of
metallogel, we performed the FTIR, UV-vis and ESI-Mass studies.
We could not observe the significant changes in FTIR spectra of
ligand H2PSL and xerogel H2PSL/Li+ due to the only replacement
of H+ with Li+ as demonstrated in scheme 1 and figure S6
(Supporting Information). Further, UV-vis spectra also did not
display any kind of significant changes in two intense coupled p-
p* transition and n-p* transition bands at 282, 291 and 323 nm,
respectively, upon aliquot addition of LiOH (Figure S7, supporting
information). FTIR and UV- vis spectra suggest the existence of
weak interaction between H2PSL and Li+.[24,36] Moreover, to
ascertain the adduct formation between H2PSL and Li+, we
recorded the ESI-MS spectra over diluted metallogel (Figure 5
and S8, Supporting Information). A molecular ion peak found to
be at m/z 437.2 corresponding to the molecular formula
2θ
=
7.84o, 14.24o, 17.04o, 22.48o and 30.32o suggests
amorphous nature of fibers (Figure 4). Three periodic diffraction
peaks were obtained in the PXRD pattern of the xerogel at 2θ =
7.82o, 14.24o and 22.48o corresponding to d-values 11.29Å, 6.21Å
and 3.95Å, respectively, with the sequential ratio close to d :d/2 :
d/3. The characteristic ratio of d-values advocates the presence
of layered arrangement between molecular aggregates with the
distance between layers equalling to 11.29Å.[25] Also, the
existence of the diffraction peaks at 2θ = 17.04o and 22.48o
corresponding to the d-values 5.19Å and 3.95Å, respectively,
indicates the presence of p-p stacking (Figure 4 and 8).[24,25]
Appearence of small peaks in the range of 2θ = 30o to 40o signifies
the presence of H-bonding interaction in the gel network (Figure
3
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[C24H22N4O4+Li]+, whereas the isotopic abundance pattern
matches nicely with the simulated peak pattern which confirms
the Li+ interaction with H2PSL.
with [12]-crown-4 and observed the complete quenching of
fluorescence intensity which is due to trapping of Li+ by [12]-
crown-4 or in other words removal of CHEF (Figure S9,
supporting Information).[26] Thermo-reversibility of the metallogel
was further investigated by the variable temperature fluorescence
experiment in the temperature range of 30-70oC. Upon increasing
the temperature from 30 to 70oC, fluorescence intensity of
metallogel decreased with red shift (Dl=6 nm) indicative towards
the slight segregation of gel network (Figure 6). Further, upon
cooling from 70 to 30oC, it regained 80% of its original
fluorescence intensity supports the observation made about
metallogel as stimuli-responsive towards heat (Figure 1 and 6).
In order to confirm the presence of CHEF, AIE and/or ACQ
phenomena, we performed fluorescence dilution experiment on
Aggregation Study
With an objective to explore the mechanistic steps involved in the
gelation process, we performed various fluorescence
experiments over H2PSL and the freshly prepared metallogel. The
weakly fluorescent gelator H2PSL (λex=320nm, λem=481nm,
Stokes shift= 62112 cm-1, 1x10-3 M, DMF, blue line) displayed a
noticeable enhancement in the emission intensity upon aliquot
addition of LiOH (1x10-1 M, DMF, red line) along with blue shift of
9 nm (Figure 6).[24-26] It might be because of the removal of ESIPT
and origin of CHEF phenomenon, endorsed by the deprotonation
followed by chelation of Li+ to the deprotonated species of gelator
PSL2- as well as the AIE phenomenon (vide infra, scheme 1).[35,
37] Further, to ascertain the role of Li+ towards enhancement of
fluorescence intensity of H2PSL via CHEF phenomenon, we
treated the diluted metallogel (H2PSL/Li+, 10-4 M, DMF)
freshly prepared metallogel within the range from 10-2 M to 10-4
(Figure 6). Upon dilution of metallogel from 10-2 M to 10-3
M
M
concentration, the fluorescence intensity was decreased with a
small blue shift of 5 nm suggests the presence of AIE
phenomenon during the gelation process. Surprisingly, further
dilution from 10-3 M to 10-4 M lead to the notable enhancement of
fluorescence intensity with blue shift of 9 nm. This continuous
enhancement in fluorescence intensity upon dilution 10-3 M to 10-
8
6
4
2
0
7
o
(A)
(B)
+
30 C
H PSL/Li
2
2
o
70 C
4
H PSL
M advocates the involvement of ACQ phenomenon in the
5
Cooled
to 30 C
metallogelation. The occurrence of both quenching and
enhancement of emission intensity with the dilution of the
metallogel refers to the presence of AIE and ACQ, respectively,
o
3
1
along with CHEF phenomena. Extreme dilution below 10-4
M
concentration lead to usual decrease in emission intensity due to
dilution effect.[26] The fluorescence lifetime measurements were
performed over the synthesized metallogel (H2PSL/LiOH; ~10-2
M) and its diluted conditions (10-3 and 10-4 M). A sequential
increase in t values 0.77 ns, 0.79 ns and 1.07 ns were observed
the upon dilution from ~10-2 M to 10-3 and 10-4 M (diluted
metallogel), respectively which supports the observation made
from fluorescence gel dilution experiment (Figure 7).
440 480 520 560 600
Wavelength (nm)
400
450
500 550 600
Wavelength (nm)
-2
8
12.5
10.0
(C)
(D)
10
10
M
M
-3
-4
10
10
M
M
-3
6
4
2
0
7.5
5.0
2.5
Based on the observations drawn from the above
experiments, we summarized a plausible mechanism of gel
formation in figure 8.
0
440 480
560 600
520
520
440 480
560 600
Wavelength (nm)
Wavelength (nm)
Rheological Study
Figure 6. Fluorescence titration experiment of (A) H2PSL (10-3 M, DMF, blue
line) with LiOH (10-1 M, DMF, red lines), (B) Fluorescence variable temperature
experiment and (C and D) Fluorescence gel dilution experiment.
In order to establish the metallogel as true gel phase material and
to explore its viscoelastic properties, we performed detailed
rheological experiments over freshly prepared metallogel (2.6%
w/v). Strain sweep experiment shows that in response to applied
shear strain, the storage modulus (G’) was found to be ~1 order
magnitude higher than loss modulus (G’’) upto 4% strain value
and beyond this value, they crossed each other which indicates
the phase transition from gel to sol phase (Figure 9).[38, 40] When
the metallogel was subjected to dynamic shear stress, G’ was
found to be ~1 order magnitude higher than G’’ up to yield stress
of 2.4 Pa, supporting the metallogel as true gel phase material,
upon further application of shear stress beyond the yield stress
point both G’ and G” cross each other, indicating the deformation
of the gel network (Figure 9).[40] Dynamic frequency sweep
measurements show that the predominantly unchanged behavior
of the storage modulus (G’) and the loss modulus (G’’) within the
10000
Metallogel τ = 0.77 ns
-3
Diluted gel (10 M) τ = 0.79 ns
-4
Diluted gel (10 M) τ = 1.06 ns
1000
100
o
applied frequency range (0.1 to 100 rad s-1 at 25 C) advocates
400
1200
Time (ns)
0
800
2000
1600
the elastic nature of the metallogel (Figure 9). Moreover,
logarithmic plot of complex viscosity (h*) plotted against w
Figure 7. Fluorescence lifetime measurement of metallogel and its diluted
conditions.
4
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provides a negative slope of ~1 suggesting constant declination
was again observed at about 80oC (Figure 9). This result was
further attested by a curve of loss tangent (tanδ = G’’/G’) vs
of viscosity with respect to frequency. [38, 39] Effect of temperature
(3)
(4)
(1)
(5)
(2)
100 nm
Fluorescent
metallogel
Extensive Fibre
network
PSL2-/Li+
complex
100 nm
Single fibre
31 nm
Nanoball aggregates
(TEM Image)
H-bonding
p-p stacking
(1) Ultrasound (2) Aggregation (3) Nanoball formation (4) Aggregation of nanoballs (5) Extensive Fibre formation
Figure 8. A model representation for plausible mechanism of the gelation along with morphological changes.
on logarithmic plot of G’ and G’’ was recorded within the
temperature range 25oC-130oC (Figure 9). The plot shows
temperature plot (Figure 9). An abrupt change in the curve at
around 80oC confirms the phase transition temperature “Tgel” of
the metallogel. Thus, the overall rheological results show
that the present metallogel has characteristics of true gel phase
materials.[38, 40]
Figure 10. (A) Nyquist impedance plots, triangle points represent the best fit
model. Equivalent circuit model for (B) metallogel (C) after ultrasound and (D)
after heating the metallogel.
Conductance
The conductance property of the synthesised metallogel along
with effect of temperature and ultrasound were explored using
electrochemical impedance spectroscopic (EIS) measurements
within the frequency range 106 to 10 Hz at an amplitude voltage
of 10 mV. The Nyquist plots shown in figure 10, present a
semicircle in high frequency region followed by a linear evolution
in low frequency region for each plot. The semicircle can be fitted
Figure 9. (A) Dynamic oscillation strain sweep, (B) Dynamic shear stress of G’
and G’’ at frequency of 10 rad s-1 at 25oC, (C) Plot of Storage and loss modulus
vs Temperature, (D) Plot of Storage and loss modulus over dynamic frequency
ramp, (E) Plot of temperature vs complex viscosity and (F) Plot of temperature
vs tand.
to
a parallel circuit of a charge-transfer resistance and
capacitance in each plot. The charge-transfer resistance (Rct1
,
Rct2 and Rct3) arises due to electrode-electrolyte interface,
whereas the capacitance (C1, C2, and C3) accounts for the
geometric capacitance of the cell.[24] The overall Nyquist plot has
been fitted with the equivalent circuit models in each case of the
metallogel (Heating and ultrasonication). At very high frequency
(106 Hz) there is appearance of a series resistance (Rs1, Rs2 and
that the metallogel can sustain temperature below 80oC, and
beyond 80oC, strong to weak gel transition has been observed.[38]
The phase transition temperature or strong to weak gel transition
was further ascertained by the plot of complex viscosity over
dynamic temperature ramp (25-130oC) where the gel transition
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Rs3) and the linear evolution could be fitted with a constant phase
elements for each case accounting for the non-ideal behaviour of
the electrode interface.[24,32] The resistance was measured to be
7.4, 8.5 and 4.8 kW for metallogel, after sonication and after
heating of the same metallogel, respectively, by extrapolating the
straight line of the Nyquist plot from very low frequency (10 Hz) to
an infinitely large frequency up to real axis (Z’).[41] Notably, the
resistance value of the metallogel obtained in various conditions
is the sum of series resistance and charge transfer resistance.
Initially, the metallogel shows resistance value of R1 = 7.4 kW,
however, upon sonication for 15 minutes resulted in the increased
resistance value R2 = 8.5 kW, this increment in the resistance, in
other words, decrement in the conductivity of the metallogel
perhaps attributed to the slight disorganisation caused in fibrous
network of the metallogel, since conductance property is more
dominant in highly organised system as compared to
disorganised system.[24,41] Furthermore, the metallogel was
heated up to 70oC, upon heating the resistance value significantly
decreased to R3 = 4.8 kW, this decrement in the resistance value
caused by reduced viscosity of the metallogel at elevated
temperature thereby increasing the mobility of ionic species inside
the gel matrix (Figure 10).[42] The effect of ultrasound and
temperature on the conductance property of metallogels are in
agreement with the observation made from rheology and stimuli-
responsive behaviour explored in earlier sections (Figure 1 and
10).
to their use. Phenylsuccinic acid, Hydrazine Hydrate, 2-
hydroxybenzaldehyde and LiOH.H2O were purchased from TCI
chemicals Pvt. Ltd, Sisco Research Laboratories Pvt. Ltd., and
Spectrochem Pvt. Ltd. Mumbai, India, respectively, and used as
received without any purification.
PerkinElmer Spectrum two and UV 2600 Schimazdu
spectrometers were used to obtain the FT-IR and electronic
absorption spectra, respectively. Photoluminescence spectra
were acquired on
a Perkin Elmer FL8500 Fluorescence
1
spectrophotometer. H NMR spectra were obtained on a Bruker
AVANCE III 400 Ascend Bruker BioSpin International AG
spectrometer. Electrospray ionization mass (ESI-MS) spectra
were recorded on a Waters (Micromass MS Technologies) QTof
Premier. TEM images were captured using a JEOL JEM 2100.
SEM images were recorded using JEOL 7610 F Plus. Rheology
of metallogel was performed on Anton Paar MCR 102 Rheometer.
Powder XRD data was collected on Rigaku Smart Lab between
angle 2q = 5o-50o. Melting point was obtained from MPA100
digital melting point apparatus.
UV-vis study
Stock solution of H2PSL (10-5 M) was prepared in DMF for
electronic absorption study. LiOH solution (10-3 M) was also
prepared in DMF. As performed in a typical titration, solution of
H2PSL was taken in a quartz cuvette (3.0 mL; path length, 1 cm)
and LiOH solution was added gradually with a micropipette. At
each addition, proper mixing and sonication was performed, and
spectra were recorded.
Conclusion
Rheological study
In conclusion, we demonstrated the synthesis and properties of
metallogel, obtained from phenyl ring containing succinic acid-
based ligand and LiOH. The LiOH treatment with ligand H2PSL
not only contributed towards the metallogels formation, but also
originate the intense fluorescence through the CHEF and AIE. In
addition to the CHEF and AIE, another contradictory ACQ
phenomenon was explored to be present in metallogel. Further,
TEM and SEM analysis over diluted metallogel disclosed the
steps involved in fiber formation in sequence of nanonuclei to
nanoballs followed by nanofiber. Gelation mechanism is well
established by UV-vis, PXRD, fluorescence, SEM, TEM and ESI-
mass experiments. The true gel phase of the metallogel was
proved by a detailed rheology experiment. Furthermore, we
analyzed the external stimuli like ultrasound and thermal
response over conductance property of metallogel. Ultrasound
led to the disruption of metallogel network in turn increase in
resistance value (8.5 kW) was observed whereas heating simply
decreases the resistance value (4.8 kW) probably due to easy flow
of ion in gel matrix. This concept of metallogel synthesis would be
helpful to develop more stimuli responsive conductive fluorescent
materials for electrolytic applications.
Rheological measurements were carried out using an MCR 102,
Anton Paar rheometer equipped with stainless steel parallel
plates (20 mm diameter, 1.0 mm gap). The instrument facilitated
with a Peltier temperature control unit which was calibrated to give
a temperature in the sample within 0.1 oC of the preset value. All
the experiments were conducted over freshly prepared metallogel
(2.6% w/v). Dynamic amplitude sweep experiments were
conducted within stress and strain range of 1.2 to 2.8 Pa and
0.01% to 100%, respectively, at a constant frequency of 10 rad s-
1. Dynamic oscillatory frequency sweep was carried out at 25 oC
(20 min and from 0.1 to 100 rad s-1) at a constant shear strain of
1%. Dynamic temperature ramp experiment was performed within
the temperature range of 25 to 130 oC at a constant frequency of
10 rad s-1 and strain of 1%.
Conductance study
Electrochemical impedance spectroscopic (EIS) measurement
were performed using
workstation CHI604E model within the frequency range 106 to 10
Hz at a small perturbation voltage of 10 mV. To record Nyquist
impedance plots for metallogel at different conditions, the
a CH instruments electrochemical
Experimental Section
experiment was conducted in
a
cylindrical shaped
electrochemical cell with three electrodes system. Ag/AgCl
reference electrode, glassy carbon working electrode and Pt wire
counter electrode were used for the experiment. Further, Nyquist
plot was fitted with a suitable equivalent electrical circuit model
with the help of an inbuilt software provided by CH instruments.
General Information, Material and Physical Methods
Common reagents and solvents were purchased from Avra,
Merck, Qualigens or S.D. Fine Chem. Ltd., Mumbai, India and
used as received without further purifications. The solvents were
thoroughly dried and distilled by standard literature methods prior
6
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FESEM and TEM sample preparation
opaque blue fluorescent metallogel (2.6% w/v) at room
temperature. FTIR (KBr) n=3256, 3056, 1652, and 1532 cm-1.
Anal. Calc. for C24H21N4O4Li: C, 66.02; H, 4.85; N, 12.84. Found
C, 65.93; H, 4.79; N, 12.89. ESI-MS m/z: calcd for
[C24H22N4O4+Li]+: 437.18; found: 437.2.
A freshly prepared metallogel (1 mL) was dried inside vacuum
desiccator at ambient room temperature in order to obtain the
sample for FESEM analysis. The dried metallogel (xerogel)
sample was gently removed out from the vial and transferred to
the FESEM sample holder. Furthermore, gold coating was
applied on the samples for reduction of the charging under an
electron beam.
Acknowledgements
For TEM analysis, a sample was prepared by dilution of
metallogel (DMF, 1x10-4 M) followed by drop casting on a carbon-
coated copper grid (400 mesh) and dried in vacuum desiccator for
2 days.
M. Dubey acknowledges the Department of Science and
Technology, New Delhi, India, for financial support under a DST-
INSPIRE Faculty award (IFA-14/CH-156). CM and YK
acknowledge UGC New Delhi and IIT Indore for fellowship,
respectively. A. Kalam extend his appreciation to the Deanship of
Scientific Research at King Khalid University for funding this work
through research groups program under grant number RGP
1/181/41. We thank SIC IIT Indore for extending the instrumental
facilities.
Fluorescence lifetime measurements
TCSPC system from Horiba Yovin (Model: Fluorocube-01-NL)
were used to perform the lifetime measurements. Excitation
wavelength were fixed at 334 nm using a picoseconds diode laser
(Model: Pico Brite-375L) for as synthesized metallogel and diluted
metallogels at the concentration 10-3 and 10-4 M. Further, the data
analysis was performed using IBH DAS (version 6, HORIBA
Scientific, Edison, NJ) decay analysis software. The resulting
decay was best fitted by a bi-exponential decay.
Author contributions
JS contributed in the synthesis of ligand, metallogel and
preliminary characterization studies. YK helped in conductance
measurements, formal analysis and investigation of results. MKD
contributed to the fluorescence experiments and discussion. CM
performed the rheology, UV-vis experiments and analysed the
data. VKS and AK were involved in the TEM experiments,
characterization, analysis and correction in the manuscript. All the
authors analysed the experimental data, discussed the results,
wrote and commented on the manuscript. MD had responsibility
for funding acquisition, project administration, formal analysis,
writing of original draft, validation and the overall project concept.
Synthesis of H2PSL
To a methanolic solution of phenylsuccinic acid (1.00g, 5.1mmol)
2 or 3 drops of conc. sulphuric acid was added as a catalyst and
the reaction mixture was refluxed for 18hrs at 70ºC. In vacuo
reduction in volume with mild heating was done to obtain dimethyl
phenylsuccinate ester as an oily and sweet-smelling product. To
the obtained dimethyl phenylsuccinate (1.00g, 4.5mmol)
hydrazine hydrate (0.45g, 9mmol) was added in methanol and
reaction mixture was stirred for 6 hours at room temperature. A
white crystalline powder of phenylsuccinic hydrazide (PSAH) was
obtained from above solution which was filtered, washed with
methanol and diethyl ether and dried in desiccator. The
compound PSAH (0.300g, 0.32mmol) was dissolved in 2mL of
water and then mixed in 20mL methanol to obtain a clear solution.
To this solution, 2-hydroxybenzaldehyde (0.329g, 0.64mmol) in
methanol was added dropwise and solution was refluxed for 1
hours at 70ºC. The white precipitate afforded from the solution
was stirred for additional 2 hours at room temperature and then
filtered, washed thoroughly with methanol and diethyl ether to
afford a white solid (H2PSL). Yield 1.74g (81 %). Mp 280oC; Anal.
Calc. for C24H22N4O4: C, 66.95; H, 5.15; N, 13.02. Found C, 66.82;
H, 5.09; N, 13.10. 1H NMR [(CD3)2SO, 400 MHz]: d (ppm) =
12.00+ 11.80+11.37+11.14 (3d+1t, 2H, NH), 10.19+10.09 (2s, 2H,
OH), 8.43-8.27 (m, 2H, -CH=N), 7.55-7.32 (m, 8H), 6.94 (s, 5H, -
Ph), 5.03 (s, 1H, CH-Ph), 4.20 (s, 2H, CH2); 13C NMR [(CD3)2SO,
400 MHz]: d =168.79,163.25, 158.13, 132.48-128.15, 117.95
ppm; FTIR (KBr): n=3250, 3052, 1653, 1532 cm-1; UV-vis (DMF),
λmax (e:M-1 cm-1): 322 (26500), 291 (45000) and 282 (47500) nm.
Keywords: metallogels
•
fluorescence
•
conductance
•
nanofibers • rheology
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H2PSL/LiOH
(DMF)
Ultrasound
A Li+-induced multi-stimuli responsive florescent metallogel (2.6% w/v) has been synthesized and explored the presence of ESIPT,
CHEF, AIE and ACQ fluorescence phenomenon. The morphology, rheology and effect of ultrasound and heat was monitored on the
conductance property of metallogel.
Twitter user name: mrigendra dubey
@dubey_mrigendra
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