Coordination polymer gel derived from a tetrazole ligand and Zn 2+: Spectroscopic and mechanical properties of an amorphous coordination polymer gel

Article abstract of DOI:10.1039/c2sm07231a

A tetrazole-based ligand forms a supramolecular gel in the presence of transition metal ions, particularly Zn²⁺, Cu²⁺ and Co ²⁺. The gels have been characterized by SEM, TEM, fluorescence spectroscopy, rheometry and fluore

Full text of DOI:10.1039/c2sm07231a

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PAPER
Coordination polymer gel derived from a tetrazole ligand and Zn2+:
spectroscopic and mechanical properties of an amorphous coordination
polymer gel†
a
b
b
Hyejin Lee, Sunwoo Kang, Jin Yong Lee and Jong Hwa Jung
a
*
*
Received 22nd November 2011, Accepted 6th January 2012
DOI: 10.1039/c2sm07231a
A tetrazole-based ligand forms a supramolecular gel in the presence of transition metal ions,
particularly Zn2+, Cu2+ and Co2+. The gels have been characterized by SEM, TEM, fluorescence
spectroscopy, rheometry and fluorescence microscopy. The polymer exhibits a strong fluorescence
enhancement upon gel formation. According to DFT calculations, one of Zn2+ is bound to 4 tetrazole
moieties and 2 solvent molecules by octahedral structure. The Zn2+-induced gelation does not strongly
depend on anions. The Zn2+-tetrazole ligand gel shows a spherical structure with 20–50 nm diameter.
The rheological properties of the Zn2+-tetrazole ligand gel were strongly dependent on the
concentration of Zn2+.
process of the coordination polymer structure. Thus, the study
Introduction
for coordination polymer gels in the presence of various
concentrations of metal ions is important to our understanding
of the formation process of gelation and the resulting physical
properties.
Organogels and hydrogels composed from small molecules or
‘low molecular weight gelators’ (LMWGs), which are linker into
fibers by supramolecular interactions is a current topic of great
interest. Supramolecular gels have been studied as soft materials
for use in applications such as drug-delivery systems, tissue
engineering, sensing devices, separation and optoelectronic
devices.1–17 The incorporation of labile metals or anions within
these supramolecular gels is of particular interest because met-
allogels and coordination polymer gels with metal–organic
framework structures are applicable in various fields such as
redox responsiveness,18,19 catalytic action,20,21 absorption,22,23
emission,24–27 magnetism23 and electron emission.21 In addition,
the gels composed of a metal–organic framework (MOF) struc-
ture can be rapidly, efficiently, and easily prepared under mild
conditions in comparison to a typical crystalline MOF, which
would display similar properties.28
With this in mind, we chose tetrazole-appended benzene as the
framework for the attachment of ligands because of its well-
defined and extensively developed MOF chemistry.29 A tetrazole
derivative, with multiple binding sites, promotes cross-linking to
form a polymer network that is a critical condition for gelation.
Herein, we report the formation of a coordination polymer gel of
the tetrazole-based ligand (1) containing Zn2+ ion and its influ-
ence on the rheological properties of the coordination polymer
gels at different concentrations of Zn2+. We also prepared ligand
(2), which contains two tetrazole moieties as a reference. Inter-
estingly, its fluorescence intensity is enhanced upon the forma-
tion of a coordination polymer gel 1 with Zn2+, relative to sol 1 in
organic solvents. The rheology of Zn2+ coordination polymer gel
1 strongly depends on the concentration of Zn2+.
Although there are many reports on the formation of coor-
dination polymer gels, there is relatively little information in the
literature regarding the photoluminescence and rheological
properties of coordination polymer gels at different concentra-
tions of metal ions.24–27 In particular, the rheology of coordina-
tion polymer gels might be dependent on the concentration of
metal ions and is significantly influenced by the formation
Experimental
General
All reagents were purchased from Aldrich and Tokyo Kasei
Chemicals and used without further purification.
aDepartment of Chemistry, Research Institute of Natural Science,
Gyeongsang National University, Jinju, 660-701, Korea. E-mail:
jonghwa@gnu.ac.kr
bDepartment of Chemistry and Institute of Basic Science Sungkyunkwan
University, Suwon, 440-746, Korea. E-mail: jinylee@skku.edu
† Electronic Supplementary Information (ESI) available: Measurement
data. See DOI: 10.1039/c2sm07231a/
Characterization
1H and 13C NMR spectra were measured on a Bruker ARX 300
apparatus. IR spectra were obtained for KBr pellets, in the range
400–4000cmꢀ1, with a Shimadzu FT-IR 8400S instrument, and
mass spectra were obtained by
a JEOL JMS-700 mass
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spectrometer. The optical absorption spectra of the samples were
obtained at 378–278 K using a UV-vis spectrophotometer
(Hitachi U-2900). All fluorescence spectra were recorded with
a 378–278 K in RF-5301PC spectrophotometer.
The formation of the gel states was found to depend on the
concentration of metal salt added.
Preparation of ligand 1
A mixture of 1,2,4,5-tetracyanobenzene (3) (1 g, 5.61mmol),
NaN3(4.38 g, 67.38 mmol), and triethylamine hydrochloride
(9.27 g, 67.38 mmol) in 100 mL of toluene and 10 mL of meth-
anol was refluxed for 4 days. The precipitate was then collected
by hot filtration and dissolved in aqueous NaOH (1 M). The
resulting clear, colorless solution was titrated with HCl (1 M)
until the pH of the solution was 4. The ensuing white precipitate
was washed with successive aliquots of distilled water and diethyl
Preparation, TEM and SEM observations
For transmission electron microscopy (TEM), a piece of the gel
was placed on a carbon-coated copper grid (400 mesh) and
removed after one min, leaving some small patches of the sample
on the grid. The specimens were examined with a JEOL JEM-
2010 transmission electron microscope operating at 200 kV using
an accelerating voltage of 100 kV and a 16 mm working distance.
Scanning electron micrographs of the samples were taken with
a field emission scanning electron microscope (FE-SEM, Philips
XL30 S FEG). The accelerating voltage of the SEM was 5–15 kV
and the emission current was 10 mA.
1
ether to afford 1.8 g (91.8%) of product. H NMR (300 MHz,
DMSO-d6): d 9.20 (s, 2H, CH), 5.89 (s, 4H, NH); 13C
NMR(75 MHz, DMSO-d6): 156.4, 132.4, 126.2 ppm; IR
(KBr, cmꢀ1): 3431, 3050, 3016, 2957, 2923, 2885, 2839, 2796,
2771, 2717, 1854, 1637, 1567, 1486, 1438, 1392, 1359, 1292, 1269,
1235, 1228, 1173, 1147, 1087, 1045, 1022, 999, 934, 841, 786, 711,
653, 575, 508; MS (FAB): 352.15 (M + H)+ (calcd MW ¼ 351.11);
elemental analysis: calcd (%) for C10H6N16: C 34.29, H 1.73, N
63.98; found: C 34.36, H 1.75, N 64.32.
Fluorescence lifetime microscopy (FLM) measurements
Fluorescence lifetime images were acquired by an inverse time-
resolved fluorescence microscope, MicroTime-200 (PicoQuant
GmBH). The excitation wavelength, the spatial resolution, and
the time resolution were 375 nm, 0.3 mm and 60–70 ps, respec-
tively. The samples were prepared on one side of microscope
cover glasses. The manufacturer’s software was used to analyze
the data and calculate the lifetime maps.
Preparation of ligand 2
Compound 2 was prepared as described previously.27 1H NMR
(300 MHz, DMSO-d6): d 8.12 (s, 4H, CH), 4.58 (s, 4H, NH); 13
C
NMR (75 MHz, DMSO-d6): 156.4, 132.4, 126.2 ppm; IR (KBr,
cmꢀ1): 3534, 3373, 3179, 3116, 3073, 3055, 3037, 3002, 2946,
2913, 2882, 2844, 2829, 2798, 2764, 1666, 1636, 1618, 1581, 1570,
1560, 1526, 1449, 1407, 1380, 1274, 1251, 1213, 1162, 1143, 1127,
1105, 1083, 1028, 1012, 998, 911, 853, 781, 742, 630, 530, 484,
419, 409; MS (FAB): 214.07 (M ꢀ H)ꢀ (calcd MW ¼ 213.08);
elemental analysis: calcd (%) for C8H6N8: C 44.86, H 2.82, N
52.32; found: C 44.92, H 3.12, N 52.46.
Rheological measurements
The rheological properties were observed on freshly prepared
gels using a controlled stress rheometer (AR-2000ex, TA
Instruments Ltd, New Castle, DE, USA). Cone type geometry of
40 mm diameter was employed throughout. Dynamic oscillatory
work kept a frequency of 1.0 rad sꢀ1. The following tests were
performed: increasing the amplitude of oscillation up to 100%
apparent strain on shear, time and frequency sweeps at 25 ꢁC
(60 min and from 0.1–100 rad sꢀ1,ꢁrespectively), and a heating run
Results and discussion
up to 90 C at a scan rate of 1.0 C minꢀ1. Unidirectional shear
ꢁ
Ligands 1 and 2 were conveniently prepared in a one pot
synthesis according to methods described previously (Scheme 1).
Tetracyanobenzene or dicyanobenzene was reacted with sodium
azide in the presence of triethylammonium chloride in toluene
ꢁ
routines were performed at 25 C covering a shear-rate regime
between 10ꢀ1 and 103 sꢀ1. Mechanical spectroscopy routines were
completed with transient measurements. In doing so, the desired
stress was applied instantaneously to the sample and the angular
displacement was monitored for 60 min (retardation curve).
and ethanol to yield 1 or 2, as confirmed by H and 13C NMR
1
spectroscopy.
Photophysical studies
UV-Vis absorption and emission spectra of the gel were observed
at room temperature and in the range 200–800nm. The absorp-
tion properties of gel 1 were studied extensively. UV-vis
absorption spectra of gel 1 (20 mM) were observed in the pres-
ence of Zn2+ (0–2.0 equiv).
Gelation test
Gels were prepared by dissolving 3.0 percent by weight of 1–2 in
organic solvents. To this solution 2.0 equivalents of metal ion in
organic solvents were added with respect to the ligand concen-
tration. The samples were then left to stand, typically for a week.
Scheme 1 Synthetic routes of ligands 1 and 2.
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A coordination polymer gel based on 1 was prepared by dis-
solving one percent by weight of 1 in organic solvents. To this
solution a small volume of metal ions in organic solvents was
added in concentrations varying from 0.1–2.0 equivalents with
respect to the ligand concentration. The samples were then left to
stand for a week, since some gels can change into solution or
precipitate. Table 1 and S1 (ESI†) summarize the results of
a gelation test of 1 and 2 in the presence of transition metal ions
(2.0 equivalents) with selected solvents. Translucent gel 1 was
obtained with Zn2+, Cu2+ and Co2+ ions in DMF, DMA, or
DMF/MeOH (Fig. 1). On the other hand, ligand 1 in the absence
of metal ions did not form a gel in organic solvents or water. In
addition, 1 can also be gelated with a variety of zinc counter ions
such as ClO4ꢀ, OACꢀ, Clꢀ, Brꢀ, and Iꢀ (Fig. S1, ESI†), indicating
that the coordination polymer gel formation of 1 does not
strongly depend on the nature of the anion. On the other hand, 2,
possessing two tetrazole moieties did not form a gel in the
presence of Zn2+, Cu2+ or Co2+ ions in organic solvents (Fig. S2,
ESI†). These findings suggest that the numbers of the tetrazole
moieties are important to form the coordination polymer
structure.
Fig. 1 Photographs of the coordination polymer gel 1 with Zn2+ in; (a)
MeOH, (b) DMF, (c) DMSO, (d) DCM, (e) THF, (f) toluene, (g)
acetonitrile, (h) DMA, (i) EA, (j) CHCl3, (k) dioxane, (l) H2O, (m)
acetone, (n) DMF:MeOH (1 : 1 v/v) and (o) DMF:H2O (1 : 1 v/v).
sol 1 exhibited a 90 nm blue shift. The fluorescence spectra of the
coordination polymer gel 1 (lex ¼ 410 nm) were also obtained.
As shown in Fig. 3B, Zn2+ coordination polymer gel 1 exhibited
a strong blue emission with a maximum at l ¼ 475 nm. The
fluorescence intensity is enhanced markedly as compared to sol 1
(lex ¼ 320 nm). The photoluminescence of Zn2+ coordination
polymer gel 1 can be seen by the naked eye under UV light
(Fig. 3C). Conversely, solid ligand 1 without Zn2+ did not show
emission at any wavelength (Fig. S5). Therefore, the strong
emission of gel 1 with Zn2+ originates from the self-assembled
supramolecular structure formed through strong intermolecular
forces and not from the monomeric ligand 1. Furthermore, in the
gel state, the coordination polymer structure of 1 with Zn2+ plays
an important role in favoring J-type aggregation, which restricts
the formation of the excimer complex.31 We also observed the
absorption and emission spectra of gel 1 with Zn2+ as obtained
from DMF and DMA as shown in Fig. S6 (ESI†). The absorp-
tion and emission properties of gel 1 with Zn2+ from DMF and
DMA were similar to that obtained from DMF:MeOH.
The morphology of the xerogels obtained with different
cations and anions was investigated by scanning electron
microscopy (SEM), and transmission electron microscopy
(TEM). The SEM images of Zn2+ coordination polymer gel 1
clearly displayed a nanospherical structure with 30–50 nm in
diameters (Fig. 2 and S3, ESI†). The SEM image of the Zn2+
coordination polymer gel 1 with different anions showed
a similar nanospherical structure with 30–50 nm in diameters. In
addition, the coordination polymer gel 1 formed with other
cations such as Co2+, and Cu2+ showed a similar nanospherical
structure (Fig. S4, ESI†). These findings suggest that the
morphology of the coordination polymer gel 1 was not strongly
influenced by the nature of the cation and anion.
While we tried for a long time to obtain single crystals suitable
for X-ray analysis in CH3CN, the single crystal of 1 in complex
with Zn2+ could not be attained. Therefore, molecule 1 in the
aggregated gel state is likely to show dramatically enhanced
fluorescence emission compared to that of the isolated state due
to the synergistic effect of intramolecular planarization and
restricted excimer formation. The fluorescence emission of Zn2+
coordination polymer gel 1 with different anions was also
observed. The fluorescence intensities of Zn2+ coordination
polymer gel 1 with ClO4ꢀ, Clꢀ and Iꢀ were almost the same
(Fig. S7, ESI†). In contrast, the fluorescence emission intensity of
Zn2+ coordination polymer gel 1 with Iꢀ decreased considerably
as compared to the emission of coordination polymer gel 1
induced by other anions, due to the intersystem crossing effect.
To better understand the molecular structure of
[Zn(1)(MeOH)(DMF)]2+, we have carried out density functional
theory (DFT) calculations using the nonlocal density functional
of Becke’s three parameters employing Lee–Yang–Parr exchange
We examined the absorption and fluorescence emission
properties of sol 1 and coordination polymer gel 1 with metal
ions. The UV-vis absorption band of Zn2+ coordination polymer
gel 1 appeared at 410 nm (Fig. 3A), indicating a typical p–p*
transition.13,30 On the other hand, the p–p* absorption band of
Table 1 Gelation test of ligand 1 with metal ions (2.0 equiv) in organic
solventsa
Entry
Solvent
Zn(ClO4)2
Cu(ClO4)2
Co(ClO4)2
1
2
3
4
5
6
7
8
MeOH
DMF
DMSO
DCM
THF
Toluene
ACN
DMA
PG
G
S
P
PG
P
P
G
P
P
P
P
G
G
P
P
P
P
G
P
P
P
P
P
P
P
G
G
S
P
P
P
P
G
P
P
P
P
P
G
P
9
EA
10
11
12
13
14
15
CHCl3
Dioxane
H2O
Acetone
DMF:MeOH
DMF:H2O
P
P
G
PG
Fig. 2 SEM images of Zn2+ coordination polymer gels 1 with different
anions (2.0 equiv) such as (A) ClO4ꢀ, (B) Iꢀ and (C) Clꢀ in DMF:MeOH
(1 : 1 v/v).
a
pG: partially gel, P: precipitate, G: stable gel.
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Fig. 3 (A) UV-vis and (B) fluorescence spectra (excitation at l ¼ 320 nm
for sol 1 and 410 nm for gel 1) of (a) sol 1 (20 mM) and (b) gel 1 (20 mM)
with Zn(ClO4)2 (2.0 equiv) in DMF/MeOH (1 : 1 v/v). (C) The photo-
graph of coordination polymer gel 1 with Zn(ClO4)2 without (a) and with
(b) irradiation of UV light.
Fig. 5 The highest occupied molecular orbital (HOMO) and the lowest
unoccupied molecular orbital (LUMO) of [Zn(1)(MeOH)(DMF)]2+
.
functional (B3LYP) with 6-31G* basis sets using a suite of
Gaussian 09 programs.32 In the optimized structure of
[Zn(1)(MeOH)(DMF)]2+, Zn2+ is octahedrally coordinated by
four tetrazole moieties of the ligand 1, DMF, and MeOH
(Fig. 4). The distances between Zn2+ and coordinated nitrogen
atom of the ligand 1 were calculated to be in the range
ligand charge transfer (MLCT), which is consistent with the
experimentally observed transition property.
We also observed the changes in the fluorescence intensities of
the gel 1 at different concentrations of Zn2+. The fluorescence
intensity of Zn2+ coordination polymer gel 1 gradually increased
with the addition of Zn2+ until reaching the maximum fluores-
cence emission upon the addition of 1.0 equivalent of metal ion;
the maximum emission remained almost constant upon further
addition (Fig. S8, ESI†). Furthermore, we prepared a precipitate
of 1 by the addition of an excess of Zn2+ (2.0 equivalents) to
probe the stoichiometry in acetone or acetonitrile, since precip-
itated 1 complex with Zn2+ will form in acetone or acetonitrile as
shown in Table 1. To remove uncoordinated Zn2+, the solid
product was washed with aqueous solution. The solid sample,
containing ligand 1 and Zn(ClO4)2, was examined by elemental
analysis and ICP analysis. Elemental analysis and ICP of the
precipitated sample demonstrated a content profile of 15.57% for
C, 1.53% for H, 25.32% for N and 7.51% for Zn2+. Our DFT
calculations also showed the following results, which are in
excellent agreement with the experimental data: calculated
C(16.00%), H(1.56%), O(17.88%), N(25.05%), Zn2+(7.31%).
These findings indicate that the stoichiometry of the sample is
consistent with a 1 : 1 formulation.
2+
ꢀ
2.189ꢂ2.333 A, while those between Zn and the oxygen atom
ꢀ
of DMF and MeOH were calculated to be 1.985 and 2.106 A. In
contrast to a previous report,33 two Brꢀ are replaced with two
tetrazole moieties of ligand 1. As the ligand 1 is replaced, the
intermolecular H-bonds are generated between four tetrazole
moieties of the ligand 1. It is worth noting from the structure that
two intermolecular H-bonds exist between neighboring tetrazole
moieties of ligand 1, rendering a total of eight H-bonds (N/H–
N) in one unit structure. The H/N distances for the inner side
ꢀ
H-bonds were calculated be in the range 1.844ꢂ2.009 A, while
those in outer side were calculated to be slightly longer, in the
ꢀ
range 1.930ꢂ2.047 A.
Fig. 5 shows the highest occupied molecular orbital (HOMO)
and the lowest unoccupied molecular orbital (LUMO) of
[Zn(1)(MeOH)(DMF)]2+. The UV-vis absorption of the assem-
bled structure of [Zn(1)(MeOH)(DMF)]2+ was experimentally
determined to have a lmax at 440 nm. The electrons in the
HOMO are degenerated over ligand 1 and Zn2+, while the elec-
trons in LUMO are distributed over ligand 1. From the HOMO
and LUMO, it is expected that the optical transition is contrib-
uted by the p–p* transition of ligand 1 as well as by the metal-to-
Fluorescence of the coordination polymer gel 1 was measured
as a function of temperature (Fig. S9, ESI†). No significant
spectral changes were observed until 55 ꢁC. The fluorescence
intensity of Zn2+ coordination polymer gel 1 with ClO4ꢀ slightly
ꢁ
decreased at 60 C. Further increases in temperature resulted in
a large reduction in emission. These results suggest that the
emission of the coordination polymer gel 1 decreased as it started
to melt at 60–65 ꢁC. The decreasing fluorescence intensity of gel 1
ꢁ
above 62 C is due to this transformation to a solution phase.
The optimal emission of 1 therefore occurs when the gel is
completely formed and decreases as the gel melts at higher
temperature. This striking observation may be attributed to the
rigidification of the media upon gelation, a process that slows
down nonradiative decay mechanisms, leading to luminescence
enhancement. Though the gel dissociates from the aggregated
state and shows a drastic drop in fluorescence intensity, the
complex still exhibits a considerable blue emission in the solution
state. This may be due to weak supramolecular interactions that
persist in the solution state.
To gain information about the thermally promoted stability of
Mg2+ coordination polymer gel 1, the transition temperature
(Tsol-gel) of coordination polymer gel 1 was measured by
Fig.
4
The
B3LYP/6-31G*
optimized
structure
of
[Zn(1)(MeOH)(DMF)]2+(H-bonds are denoted as a dotted green line).
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differential scanning calorimetry (DSC) (Fig. S10, ESI†). The
Zn2+ coordination polymer gel 1 reveals a sharp phase transition
at 62 ꢁC, indicative of an endothermic reaction. This endo-
thermic thermogram is due to Zn2+ coordination polymer gel 1
changing into a sol state as seen in Fig. S9 (ESI†). This sol–gel
transition temperature of Zn2+ coordination polymer gel 1 is
consistent with the results obtained by fluorescence spectroscopy.
The luminescence property of Zn2+ coordination polymer gel 1
was studied by time-resolved fluorescence confocal microscopy.
The emission decay profile was monitored over the range l ¼
405–490 nm for Zn2+ coordination polymer gel 1 (Fig. S11,
ESI†). The fluorescence decay of Zn2+ coordination polymer gel 1
was fitted with a single exponential component yielding a lifetime
of 3.36 ns (Fig. S11B-a), indicating that this emission is fluores-
cence. In addition, the fluorescence intensity of Zn2+ coordina-
tion polymer gel was more than 1000 times higher than that of
the ligand 1 alone. This result reflects the fact that coordination
polymer gel 1 in its aggregate state is more rigid, restricting the
rotational and vibrational movements of molecules. In contrast,
the fluorescence lifetime of solid 1 was less than 2.0 ns (Fig. S11B-
b). The limited molecular motions decrease the nonradiative
relaxation process, which leads to the longer lifetime and fluo-
rescence enhancement.34
Rheological information is an indicator of the behavior of the
gels when they are exposed to mechanical stress, especially the
‘‘storage’’ (or ‘‘elastic’’) modulus G0, which represents the ability
of the deformed material to ‘‘snap back’’ to its original geometry,
and the ‘‘loss’’ (or ‘‘viscous’’) modulus G00, which represents the
tendency of a material to flow under stress. Two rheological
criteria required for a gel are: (i) the independence of the dynamic
elastic modulus, G0, with respect to the oscillatory frequency, and
(ii) G0 must exceed the loss modulus G00 by about 1 order of
magnitude.
Fig. 6 Dynamic oscillatory and steady shear measurements of coordi-
nation polymer gel 1 at different concentration of Zn2+ at 25 ꢁC: (A)
strain sweep at a frequency of 1 rad sꢀ1; (B) frequency sweep at a strain
of 0.01%; 1:Zn(ClO4)2 ¼ 1 : 0.3 -: G0,C: G00; 1:Zn(ClO4)2 ¼ 1 : 0.6 O:
G0, >: G00; 1:Zn(ClO4)2 ¼ 1 : 1 G0:1, ;: G00; 1:Zn(ClO4)2 ¼ 1 : 2 *: G0,
B: G00.
growth of G0 and G00 in the initial stage of gelation, followed by
a slower long term approach to a final pseudo-equilibrium
plateau. At the end of the experiment, the value of G0 was about
an order of magnitude higher than G00. All values of G0 and G00 in
the coordination polymer gel are strongly influenced by the
concentration of the ligand 1 complex with metal ions.
We first used dynamic strain sweep to determine the proper
condition for the dynamic frequency sweep of the gel at different
concentration of Zn2+. As shown in Fig. 6A, the values of the
storage modulus (G0) and the loss modulus (G00) exhibited a weak
dependence from 0.1 to 1.0% of stain (with G0 dominating G00),
indicating that the sample is a gel. The values of both G0 and G00
of the gel in the presence of 1.0 equivalents of Zn2+ dramatically
increased in comparison to the gels in the presence of only 0.3 or
0.6 equivalents of Zn2+. These results reflect that the gel was
stabilized with almost complete coordination polymer structure
in the presence of 1.0 equivalents of Zn2+, because ligand 1 forms
a 1 : 1 complex with Zn2+.
Conclusions
We have reported the formation of a simple Zn2+-based coordi-
nation polymer gel. A tetrazole-appended ligand was shown to
efficiently produce an organogel by simple mixing with Zn2+.
Photophysical studies showed that the coordination polymer gel
exhibited a typical p–p* transition and gave rise to highly fluo-
rescent behavior. Upon the formation of the coordination
polymer gel, the complex shows a pronounced fluorescence
enhancement with a long lifetime, as compared to the ligand. The
rheological properties of the Zn2+-tetrazole coordination poly-
mer gel were strongly dependent on the concentration of Zn2+.
The tetrazole-based ligand 1 complex with Zn2+ was strongly
influenced to the concentration of metal ions to form the 3D
coordination polymer structure. The present results emphasize
the validity of ligand design that includes anion- and metal-
binding in realizing gel systems with very versatile and stimuli-
responsive properties. These concepts should open the door for
a wide range of responsive soft materials.
We used dynamic frequency sweep to study the gel after setting
the strain amplitude at 0.8% (within the linear response region of
the strain amplitude). Fig. 6B exhibits that G0 and G00 are almost
constant with the increase of frequency from 0.1 to 100 rad sꢀ1
.
The value of G0 is about 10 times larger than that of G00 over the
whole range (0.1–100 rad sꢀ1), suggesting that the gel is fairly
tolerant to external force. As observed by changes of dynamic
strain sweep, the values of both G0 and G00 of the gel at values
above 1.0 equivalent of Zn2+ concentration was 10 time larger
than that of the gel in the presence of 0.3 equivalent of Zn2+.
Furthermore, time-dependent oscillation measurements were
used to monitor the gelation process of Zn2+ coordination
polymer gel 1, which forms gradually upon mixing of ligand 1
and Zn2+ (Fig. S12, ESI†). The time sweep shows the rapid
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Acknowledgements
This work was supported by a grant from World Class
University (WCU) Program (R32-2008-000-20003-0) and NRF
(2011-0003903) supported from Ministry of Education, Science,
S. Korea. The work at SKKU was supported by the NRF
Grant (2011-0001211) funded by the Korean Government
(MEST).
€
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This journal is ª The Royal Society of Chemistry 2012
Soft Matter, 2012, 8, 2950–2955 | 2955