Coordination polymeric fluorescent gel: effect of removal of branch substituents of the central core over properties

Article abstract of DOI:10.1080/00958972.2019.1606908

A fluorescent gel has been prepared by deprotonation of non-fluorescent adipic acid derived ligand (H₄AL) with LiOH followed by coordination with Cd(II) in DMF. The mechanism of gelation is examined by IR, ¹H NMR, UV-vis, Job’s plot, PXRD, SEM and TEM. Chelation enhanced fluorescence by Cd(II) has been established by fluorescence experiments. In addition to synthesis and characterization of this gel, we compared the gel properties by varying the core of the ligand using citric (H₆CL) acid derivatives.

Full text of DOI:10.1080/00958972.2019.1606908

Journal of Coordination Chemistry
ISSN: 0095-8972 (Print) 1029-0389 (Online) Journal homepage: https://www.tandfonline.com/loi/gcoo20
Coordination polymeric fluorescent gel: effect of
removal of branch substituents of the central core
over properties
Vinay Kumar Pandey, Vikas Kumar Singh, Subhash Chandra & Syed Hadi
Hasan
To cite this article: Vinay Kumar Pandey, Vikas Kumar Singh, Subhash Chandra & Syed
Hadi Hasan (2019): Coordination polymeric fluorescent gel: effect of removal of branch
substituents of the central core over properties, Journal of Coordination Chemistry, DOI:
10.1080/00958972.2019.1606908
To link to this article: https://doi.org/10.1080/00958972.2019.1606908
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JOURNAL OF COORDINATION CHEMISTRY
https://doi.org/10.1080/00958972.2019.1606908
Coordination polymeric fluorescent gel: effect of removal
of branch substituents of the central core over properties
Vinay Kumar Pandey, Vikas Kumar Singh, Subhash Chandra and Syed
Hadi Hasan
Department of Chemistry, Indian Institute of Technology (BHU) Varanasi, Uttar Pradesh, India
ABSTRACT
ARTICLE HISTORY
A fluorescent gel has been prepared by deprotonation of non-
Received 18 December 2018
Accepted 22 March 2019
fluorescent adipic acid derived ligand (H AL) with LiOH followed
4
by coordination with Cd(II) in DMF. The mechanism of gelation is
examined by IR, H NMR, UV-vis, Job’s plot, PXRD, SEM and TEM.
Chelation enhanced fluorescence by Cd(II) has been established
by fluorescence experiments. In addition to synthesis and charac-
terization of this gel, we compared the gel properties by varying
1
KEYWORDS
Coordination polymer; gel;
fluorescence; rheology
6
the core of the ligand using citric (H CL) acid derivatives.
1. Introduction
Supramolecular gels as a unique class of smart materials have grabbed interest due to
responsiveness to external perturbations, nanostructure, optical and physico-chemical
behavior [1]. Among them “metallogel” with incorporated metal center through coord-
inate or covalent bonds as well as weak interactions between molecules construct
functionalities beyond the constituent molecules and create extended networks with
voluminous entrapment of solvent [2].
CONTACT Syed Hadi Hasan
shhasan.apc@itbhu.ac.in
ß 2019 Informa UK Limited, trading as Taylor & Francis Group

2
V. KUMAR PANDEY ET AL.
Scheme 1. Schematic illustration of the strategy behind H AL designed from H CL. The synthesis
4
6
of metallogels along with morphological changes obtained from respective ligands.
Scheme 2. (A) Synthetic procedure for the synthesis of H AL and (B) stepwise color changes dur-
4
ing the course of metallogel synthesis from H AL. The plausible structure of Cd(II) coordination
4
compound involved in metallogel formation depicted at the bottom.
Metallogels have a large array of applications in catalysis, drug-delivery systems,
sensing devices, bio-imaging and photo-physical behavior [3]. The gelation behavior of
a metallogel depends on the diversity among the molecular structures with respect to
sequence, stereochemistry of organic linker and choice of metal centers as well as the
polarity of solvents. The metallogel properties can be tuned with small modifications
at the molecular level [4]. A number of reports selective to functional groups, stereo-
chemistry, relative position of donor/acceptor and active metal center with bisacylhy-
10
drazones with their d complexes are available. However, a metallogel involving
change in core with strategy toward tuning of morphological behavior has not been
reported. Change in flexibility of core could alter the mode of complexation [5]. We
examined the effect of core variation on structural and optical properties of the metal-
logel [6].
Deprotonation of H AL with 4 equivalents of LiOH in DMF followed by addition of
4
Cd(OAc) led to formation of a fluorescent gel (Scheme 1). To investigate variable core
2
dependent architectural and conformational limitations with comparison to citric [7]
(
H CL) derivatives, bisacylhydrazone Schiff base ligand H AL with two N,N,O-donor
6
4
chelating units separated by four –CH linker units have been synthesized (Scheme 2).
2

JOURNAL OF COORDINATION CHEMISTRY
3
2. Results and discussion
2.1. Materials and methods
Common reagents and solvents were purchased from Merck, Qualigens or S.D. Fine
Chem. Ltd., Mumbai, India and used as received. The solvents were thoroughly dis-
tilled and dried by standard literature methods prior to their use. Adipic acid, hydra-
zine hydrate and 2-hydroxybenzaldehyde were purchased from S.D. Fine synthesis Pvt.
Ltd. Hyderabad, and Spectrochem Pvt. Ltd. Mumbai, India and used as received.
FT-IR and electronic absorption spectra were obtained on a Perkin Elmer Spectrum
100 and Thermo scientific EVOLUTION 201 spectrophotometers, respectively.
Photoluminescence spectra were acquired on a Perkin Elmer LS 55 spectrophotometer.
1
H NMR spectra were obtained on a Bruker AVANCE III HD 500 spectrometer.
Electrospray ionization mass (ESI-MS) spectra were recorded on a Waters (Micromass
MS Technologies) QToF Premier. TEM and SEM images were captured using a JEOL
JEM 2100 and Carl Zeiss EVO/18 Research 2045, respectively. Rheology of metallogel
was performed on an Anton Paar MCR 702 Twin Drive Rheometer. Powder XRD data
were collected on a Rigaku MiniFlex 600 Detector D-tex ultra between 2h ¼ 5–80ꢀ.
2.2. Synthesis and characterization
2
.2.1. Synthesis of bis(acylhydrazones) and H AL
4
(a) To methanolic solution of adipic acid (1.00 g, 6.66 mmol) a few drops of conc. sul-
phuric acid were added and the reaction mixture was refluxed for 24 h. In vacuo
reduction of volume with mild heating was done to afford dimethyl adipate as viscous
transparent oil. Further, dimethyl adipate reacted with hydrazine hydrate (0.68 g,
1
3.65 mmol) in methanolic solution and was refluxed for 8 h. White crystals were
obtained on cooling to room temperature that were isolated by filtration, washed
1
with diethyl ether and dried in a desiccator. H NMR (500 MHz, DMSO, ppm): d ¼ 8.90
(
s, 2H, –CONH), 4.16 (s, 4H, –NHNH ), 1.94 (s, 4H, CH ), 1.77 (d, 2H, CH ).
2 2 2
(
b) To a methanolic suspension of hydrazone (1.00 g, 6.8 mmol), 2 mL water was
added to obtain a clear solution. Methanolic solution of 2-hydroxybenzaldehyde
1.67 g, 13.6 mmol) was added and refluxed for 1 h. The precipitate obtained after cool-
(
ing to room temperature was filtered and thoroughly washed with chloroform, metha-
1
nol and hexane to afford a white solid (H AL), yield 1.74 g (81%). H NMR (500 MHz,
4
DMSO, ppm): 11.58 þ 11.22 (2s, 2H, –NH), 10.19 (s, 2H, –OH), 8.28 þ 8.29 (2s, 2H,
ꢁ
1
CH ¼ N), 7.40-6.98 (m, 8H, Ar-H), 2.54-1.74 (m, 8H, –CH ). IR (KBr, cm ) m(OH) 3438,
2
–
m(NH) 3211, m(¼CH) 3073, m(C ¼ O) 1661, m(C ¼ N) 1484. ESI-MS m/z: [(C H N O )-H] ,
20 22 4 4
ꢁ
1
ꢁ1
3
81.11 (calc. 381.16). UV-Visible spectrum (DMF) [k , nm e, M cm )]: 321 (27030),
max
291 (40072), 279 (43200).
2.2.2. Synthesis of coordination polymer gel (CPG)
A clear solution was prepared by mixing H AL (10 mg, 0.027 mmol) in 0.6 mL DMF and
4
4
equivalents of LiOH (4.38 mg, 0.10 mmol). For gelation a solution of Cd(OAc) ꢂ2H O
2
2
(6.93 mg, 0.026 mmol) was added to a freshly prepared mixture of deprotonated AL in
DMF. A translucent and fluorescent gel (1% w/v) was obtained within a minute.

4
V. KUMAR PANDEY ET AL.
ꢁ
1
Characterizations via IR and ESI-MS are explained in respective sections. IR (KBr, cm
)
ꢁ
1
ꢁ1
–
m(C ¼ O) 1603, 1548 cm , m(C ¼ N) 1461, 1433 cm . ESI ꢁ MS m/z: [C H N O Cd-H] ,
20 20 4 4
493.00 (calcd. 493.04).
2.3. FTIR study
FTIR spectral analysis of gelator (H AL) shows characteristic m(–OH) and m(–NH) at
4
ꢁ
1
ꢁ1
ꢁ1
3
438 cm
and 3211 cm , respectively, while strong band at 1661 cm
indicates
ꢁ
1
m(-C ¼ O); in addition to aforementioned peaks m(-C ¼ N) is observed at 1484 cm
which suggests the enolic form of ligand [7]. A comparative study of FTIR spectra of
gelator and xerogel H AL suggests the disappearance of –OH band, broadening of
4
ꢁ
1
ꢁ1
aforementioned N-H band and shifting of m(-C ¼ O) to 1603 cm (Dm ¼ 58 cm ) which
2
þ
may be attributed to coordination of Cd through O,N,O binding [8, 9]. Alkali base (4
equiv.) was required for deprotonation signifying the tetra basic hexadentate nature of
ligand as a necessary condition for coordination of a metal ion through deprotonation
of both phenolic and –NH protons adjacent to azo-methine group.
2.4. UV-vis study
2
þ
To prove the complexation between H AL and Cd as well as gelation behavior, we
4
ꢁ
5
examined electronic absorption spectra. Two coupled bands for H AL (1 ꢃ 10
M,
4
ꢄ
DMF, 298 K) at k
¼ 279 nm and 291 nm are assigned to intraligand p-p transition
max
ꢄ
while a less intense band at 321 nm to n-p transition. Splitting of peak at 279 and
2
91 nm suggests dissimilar planarity of both hydrazones, i.e. both free hydrazones
exist in anti-cis configuration. Insignificant change in spectra was observed upon
ꢁ
5
deprotonation of H AL (1 ꢃ 10 M, DMF) with 4 equiv. of LiOH, while upon addition
4
ꢁ
3
of Cd(OAc) (1 ꢃ 10 M, DMF) to this solution, disappearance of the band at 320 nm
2
was observed with appearance of a new peak at 383 nm (Dk ¼ 63 nm) (Figure 1). Two
isosbestic points at 304 and 343 nm indicate equilibrium between two species during
their coordination (Figure 2). The end point of UV titrations was found to be 1.0–1.2
equivalents, indicating formation of the coordination complex [10] between Cd(II) with
2
þ
H4AL at 1:1 ratio. Stoichiometry model for Cd complexation gelator demonstrated
through Job’s plot was in good agreement to above spectroscopic result where gelling
combination was close to 1:1 metal to ligand ratio (Figure 3).
2.5. Mass spectral analysis
In order to prove the coordination polymeric nature indicated by Job’s plot, we per-
formed detailed ESI-MS spectral analysis of diluted metallogel. From the neagtive
–
mode ESI-MS spectra, a base peak was observed for gelator [C H N O -H] at m/
2
0 22 4 4
z ¼ 381.11 (calcd. 381.15). Another significant peak was observed in diluted metallogel
–
for [C H N O Cd-H] at m/z ¼ 493.00 (calcd. 493.04). Negative mode ESI-MS spec-
2
0 20 4 4
trum for diluted metallogel matches nicely with simulated isotopic abundance pattern.
Thus, we have found noticeable intense peak for 1:1: : H AL:Cd in mass spectrum
2

JOURNAL OF COORDINATION CHEMISTRY
5
Figure 1. Comparative IR spectrum of gelator (black) and xerogel (red) showing significant change
in gelator moiety upon complexation with Cd(II).
–5
Figure 2. UV-vis titrations in DMF (A) ligand H4AL (black line; kmax 322 nm; 1 ꢃ 10 M), deproto-
nation with LiOH (2 equiv., red line) and addition of Cd(OAc)2 (blue lines).
which suggested the coordination polymeric nature of metallogel (Figure 4). The
coordination polymeric gels chemistry has already been reported [7a].
2
.6. Fluorescence study
Coordination polymer shows significant enhancement in fluorescence intensity (kmax
75 nm) as compared to non-fluorescent H AL (k ¼ 320 nm), but with respect to
¼
4
4
ex
deprotonated (kmax ¼ 471 nm) its fluorescence intensity was quenched as expected in

6
V. KUMAR PANDEY ET AL.
Figure 3. (A) Job’s plot for LiOH deprotonated H4AL vs. Cd(OAc)2 showing 1:1 stoichiometry in
DMF, [CdII]/[CdII] þ [H4AL] vs. absorbance monitored at 383 nm. (B) The 1:1 stoichiometry suggests
that the possible complex may be coordination polymer as demonstrated through sketch diagram
along with asymmetric unit highlighted through dotted (red) circle.
Figure 4. From the negative mode ESI-MS (DMF) spectra of (A) diluted metallogel shows the
molecular ion peaks for H4AL and metallogel supporting the complex formation is at 1:1 ratio.
–
From the negative mode ESI-MS spectra, base peak was observed for gelator [C20H22N4O4-H] at
m/z ¼ 381.15 (calcd. 381.15). (B) The experimental isotopic abundance pattern of molecular ion
–
peak of [H2ALCd-H] matches nicely with simulated pattern which is in agreement with Job’s plot,
[H4AL]/[Cd2þ] ¼ 1:1.
ACQ phenomenon. In a similar experiment H CL derivative with an extra number of
6
binding arms show AIE phenomenon. This observation advocates the effect of central
core and number of binding sites over aggregation and photo-physical behavior. In
order to find out the assembly process dilution based experiments were performed.
Upon dilution emission peaks displayed a blue shift ꢅ26 nm with decrease in emission
intensity, which indicates disassembly of the self-assembled structure (Figure 5) [11].
2.7. Morphological study
Unambiguous morphological insights of obtained metallogel were examined under
Scanning Electron Microscopy (SEM) and Transmission Electron Microscopy (TEM).

JOURNAL OF COORDINATION CHEMISTRY
7
Figure 5. (A) Fluorescence spectra in DMF of H4AL (red line ¼ deprotonated ligand, blue line-
metal addition) and (B) upon dilution of metallogel.
¼
Figure 6. SEM images (a) and (b) showing intermingled fibers with knitted appearance while in
TEM images, (c) and (d) three-dimensional nanofibers were visualized.
Morphology revealed by H AL derived gel from SEM revealed extended and intercon-
4
nected fibers with close knit appearance. TEM study on diluted metallogel revealed
the three-dimensional (3-D) network of nano-fibers of about ꢅ30 nm in average diam-
eter with intermingled appearance [12]. Interesting observations were obtained upon
comparison to our previous work. In H CL derivative uniform fibers were obtained
6

8
V. KUMAR PANDEY ET AL.
Figure 7. Powder X-ray diffraction pattern of isomer AL (black line), H4AL/LiOH/Cd(II) (red line),
indicating the loss of crystalline behavior toward amorphous nature upon complexation and gel-
ation (xerogel) (red line).
only after sonication, which proved the effect of arm length and its presence over
morphology (Figure 6).
2.8. Powder XRD
During synthesis of the coordination polymer, metal center mostly binds to organic
anions; flexible ligands when linked to metal centers lack tendency to form periodic
ꢀ
networks [13]. PXRD patterns for H AL in 2h range 5-35 showed the crystalline nature
4
of ligand while amorphous nature of xerogel (Figure 7).
2.9. Rheological study
In order to examine mechanical properties of the metallogel various rheological stud-
ies were performed. Rheological experiments were conducted over freshly prepared
0
0
0
(
1.0% w/v) fixed concentration metallogel. Storage modulus (G ) and loss modulus (G )
ꢀ
ꢁ1
are obtained at 25 C and 1 rad s as a function of shear stress and strain. For metal-
0
00
logels when subjected to oscillatory shear, G was found to be higher than G by an
order of magnitude of shear stress. This is an absolute character for true gel phase
0
00
where no variation among G and G was observed with increasing applied stress. At a
yield stress of ꢅ1.6 Pa, they intersect each other establishing a mechanical breakup of
gel beyond which they deviate from linearity and follow gel-sol phase transition. On

JOURNAL OF COORDINATION CHEMISTRY
9
0
00
Figure 8. (A) Dynamic shear stress of G and G for gel and (B) dynamic oscillation strain sweep of
0
00
ꢁ1
ꢀ
G and G for gel at frequency of 1 rad s and 25 C.
0
the other hand both in-phase storage modulus (G ) and out-of-phase loss modulus
0
0
0
00
(
G ) remain constant up to 1.1% (G >G ), beyond which deformation of network
occurred (Figure 8).
3. Conclusion
This manuscript has examined effect of variation in core as change in number of arms
along with binding sites correlate with gelation behavior, photo physical properties
and morphology. Non-emissive solution of gelator became emissive upon deprotona-
tion and further upon metal addition ESIPT along with ACQ effect has been observed.
Central core length could affect the gelation behavior by restriction to rotation. Odd/
Even number of alkyl carbons as spacer could affect the intermolecular stacking
between terminal moieties and may distort the molecular conformation. Shorter
length of spacer may induce increased strain/repulsion between two binding moieties
while large spacer length will lead to high entropy and hence lesser probability
toward organized systems.
Acknowledgments
The authors VKP, VKS, and SC acknowledge IIT (BHU), Varanasi, and MHRD, New Delhi, India, for
the financial assistance. Further, Vinay Kumar Pandey acknowledges UGC, New Delhi, for SRF
award. The authors are thankful to the Central Instrument Facility Centre, IIT (BHU) Varanasi, for
providing the Instrumentation facilities.
Disclosure statement
The author declare that there is no conflict of interest.

1
0
V. KUMAR PANDEY ET AL.
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