Fluorescence enhancement of a tetrazole-based pyridine coordination polymer hydrogel

Article abstract of DOI:10.1039/c0nj00840k

Hydrogelation of a pyridine derivative (1) possessing tetrazole moieties as end groups, without long alkyl chain groups, results in the formation of a Mg(NO₃)₂ coordination polymer gel. The polymer exhibits a strong fluorescence enha

Full text of DOI:10.1039/c0nj00840k

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PAPER
Fluorescence enhancement of a tetrazole-based pyridine coordination
polymer hydrogelw
Ji Ha Lee,a Hyejin Lee,a Sungmin Seo,a Justyn Jaworski,a Moo Lyong Seo,a
Sunwoo Kang,b Jin Yong Lee*b and Jong Hwa Jung*a
Received (in Montpellier, France) 28th October 2010, Accepted 24th February 2011
DOI: 10.1039/c0nj00840k
Hydrogelation of a pyridine derivative (1) possessing tetrazole moieties as end groups, without
long alkyl chain groups, results in the formation of a Mg(NO3)2 coordination polymer gel. The
polymer exhibits a strong fluorescence enhancement upon gel formation. 1 can also be gelated
with a variety of magnesium anions such as SO42ꢀ, NO3ꢀ, Clꢀ, Brꢀ and Iꢀ, indicating that the
coordination polymer gel formation of 1 does not strongly depend on anions. The SEM and
AFM images of Mg2+ coordination polymer gel 1 display a fibrillar network several micrometres
long, with widths in the range of 60–70 nm and thicknesses of about 3 nm. In addition,
photophysical studies show that the hydrogel exhibits a typical p–p* transition and gives rise
to high fluorescence behavior. The coordination polymer hydrogel exhibits viscoelastic behavior
as evidenced from the rheological studies.
derivatives used as fluorescent dyes,31 it is expected that such
Introduction
compounds may exhibit interesting spectroscopic and lumines-
The self-assembly of small molecules to form functional
cence properties during the sol–gel transition reaction. In
particular, fluorescent Mg2+ coordination polymeric gels are
materials is a powerful tool for the development of new device
very rare28 in comparison to transition metal ion gels.8,16,17,23,24,26
technologies. In particular, supramolecular self-assembly of
In addition, Mg2+ can take place in various coordination
low-molecular-mass organic gelators leading to gel formation
has attracted increasing interest.1–17 Supramolecular gels have
numbers. With this in mind, herein, we report the formation of
been studied as soft materials for use in applications such as
a coordination polymer gel of tetrazole-appended pyridine
derivatives (1) containing a Mg2+ ion, without long alkyl
drug-delivery systems, tissue engineering, sensing devices, separa-
tion and optoelectronic devices.1–17 Many researchers have
chain groups. In addition, we prepared ligand 2 without a
successfully incorporated metal ions into organic gelators to
central nitrogen atom as a reference. Remarkably, the fluores-
form metallogels and coordination polymer gels.18–20 Metallogels
cence intensity is enhanced upon the formation of a coordina-
and coordination polymer gels have been reported to show
unusual functional properties such as redox responsiveness,21,22
tion polymer gel, relative to 1 in aqueous solution. Of the few
reports on the enhancement of photoluminescence intensity
catalytic action,23,24 absorption,25,26 emission,27–29 magnetism26
upon gelation, most utilize organic gels,32 two component
and electron emission.24 It has also been shown that the gelation
gels33 or emissive materials immobilized in a gel network.34
of coordination polymers makes the material more processable
for direct device applications.30
To date, there is relatively little information in the literature
Results and discussion
regarding the photoluminescence properties of coordination
When a basic aqueous solution comprising ligand 1 or 2 is
reacted with Mg2+ ions in solution, both 1 and 2 will form
polymer gels, including switching of the emission colour, field
emission and light emitting properties.27–29 In view of the
hydrogels instantly at pH = 12, as confirmed by the inverted
high photoluminescence quantum efficiencies of pyridine
test tube method (Fig. 1A, Fig. S1, ESIw, and Scheme 1). In
addition, 1 forms hydrogels with Cu2+, Co2+, Zn2+ and Ni2+
a Department of Chemistry and Research Institute of Natural Science,
Gyeongsang National University, Jinju 660-701, Korea.
ions at pH = 12 (Fig. S2, ESIw). No gelation was observed
when the same reaction was repeated with other solvents.
E-mail: jonghwa@gnu.ac.kr
b Department of Chemistry and Institute of Basic Science
Sungkyunkwan University, Suwon 440-746, Korea.
E-mail: jinylee@skks.edu
Hydrogel 1 is pH responsive and precipitates upon adjustment
to acidic pH values. On adjusting the pH to a strong basic
condition (pH = 12–14), the formation of a complex allows
the recovery of the gel structure. 1 can also be gelated with a
variety of magnesium anions such as SO42ꢀ, NO3ꢀ, Clꢀ, Brꢀ
w Electronic supplementary information (ESI) available: Details of
experimental section, SEM and TEM images, pictures of coordination
polymer gel 1 and spectroscopic data. See DOI: 10.1039/c0nj00840k
c
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Fig. 2 The B3LYP/3-21G* optimized structure for 1 complexed
with Mg2+
.
Lee–Yang–Parr exchange functional (B3LYP) employing 3-21G*
basis sets using a suite of Gaussian 03 programs.37 The
optimized structure for 1–Mg2+ complex is shown in Fig. 2.
The two ligand 1 and two nitrate anions octahedrally coordinated
to Mg2+ cation with trans conformation, where the nitrate
anions function as bidentate ligands. The distances between
the nitrogen of 1 and Mg2+ were calculated to be in the range
of 2.19–2.36 A, while the distances between the oxygen atoms
of nitrate and Mg2+ were in the range of 2.02–2.06 A. Using
the above optimized structure, we generated an expanded 2D
framework through the proper interactions and coordination
as shown in Fig. S6, ESI.w In ligand 1, only two nitrogen
atoms can coordinate to Mg2+ ion. In contrast, the other two
Fig. 1 (A) Photograph of sol 1 (left) and gel 1 (20 mM, right) with
Mg2+ (80 mM, 4 equiv.). (B) SEM and (C) AFM images of gel 1.
(D) Photograph of sol 1 (left) and gel 1 (20 mM, right) with Mg2+
(80 mM, 4 equiv.) by irradiation with UV light.
nitrogen atoms of the tetrazole unit of 1 did not bind to Mg2+
,
due to a steric hindrance effect. This result is in agreement with
previous results reported by Prof. J. R. Long. In addition, as
seen in the calculated structures, each tetrazole ligand 1 can
bind to 6 Mg2+ ions, of which 4 Mg2+ can be shared with
another tetrazole ligand. In addition, the oxygen atoms of the
amide groups of the tetrazole ligand 1 can be coordinated with
2 Mg2+ ions. Thus, stoichiometrically each tetrazole ligand
can bind to 4 Mg2+ ions on average, which is consistent with
experiment.
Scheme 1 Synthetic route of ligands 1 and 2.
and Iꢀ (Fig. S3, ESIw), indicating that the coordination
polymer gel formation of 1 does not strongly depend on
anions.
The absorption and emission properties of sol 1 and coordi-
nation polymer gel 1 with metal ions were extensively studied.
The UV-vis absorption band of Mg2+ coordination polymer
gel 1 appears at 302 nm (Fig. 3A), a typical p–p* transi-
tion.12,13 On the other hand, the p–p* absorption band of
sol 1 exhibits a 70 nm blue shift. The fluorescence spectra of
the coordination polymer gel 1 (lex = 302 nm) were also
obtained. As shown in Fig. 3B, Mg2+ coordination polymer
gel 1 exhibits a strong blue emission with a maximum at
l = 467 nm. The fluorescence intensity is enhanced drastically
Fig. 1B and C show the SEM and AFM images of Mg2+
coordination polymer gel 1, which clearly display a fibrillar
network. The fibers are several micrometres long with widths
in the range of 60–70 nm and thicknesses of about 3 nm. SEM
images of Mg2+ coordination polymer gel 1 with different
anions showed a similar fiber structure with diameters of
50–100 nm (Fig. S4, ESIw). These findings suggest that the
morphology of the coordination polymer gel 1 did not
strongly depend on the nature of the anion.
We attempted to determine the crystal structures of the
related complexes to understand the coordination behaviors
between ligand 1 and Mg2+, but repeated efforts were not
successful. As an alternative, we obtained the X-ray powder
diffraction pattern of freeze dried Mg2+ coordination polymer
gel 1. The tetrazole groups serve as multi-binding sites to form
a 2D or 3D coordination polymer in the presence of an
excess of metal ions.35,36 As shown in Fig. S5, ESIw, the X-ray
powder diffraction pattern of Mg2+ coordination polymer gel 1
shows a crystalline nature. The Mg2+ coordination polymer
gel 1 may correspond to an ordered 2D coordination polymer
network structure.
Fig. 3 (A) UV-vis spectra of (a) gel 1 (20 mM) with Mg2+ (80 mM,
4 equiv.) and (b) sol 1 (20 mM) without Mg2+. (B) Fluorescence
spectra of (a) gel 1 (20 mM, lex = 302 nm) with Mg2+ (80 mM,
4 equiv.), (b) sol 1 (20 mM, lex = 220 nm) without Mg2+ and (c) solid
1 (20 mM) with Mg2+ (80 mM, 4 equiv.) obtained at pH = 3.
To understand the coordinated molecular structure for 1
complexed with Mg2+, we carried out density functional
theory (DFT) calculations using Becke’s three parameterized
c
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compared to sol 1. On the other hand, the fluores-
cence intensity of the precipitated solid 1–Mg2+ obtained at
pH = 3 is much weaker than that of Mg2+ coordination
polymeric gel 1 (Fig. 3B(c)), because the solid 1 complexed
with Mg2+ might form H-type aggregation due to strong p–p
stacking.38 The photoluminescence of Mg2+ coordination
polymer gel 1 can be seen by the naked eye under UV light
(Fig. 1D). The planar conformation of 1 may be induced in the
gel state due to the strong intermolecular forces, which tend to
optimize close packing between molecules. This aggregation-
induced planarization extends the effective aromatic groups in
ligand 1 molecule. Furthermore, in the gel state, the coordina-
tion polymer structure of 1 with Mg2+ plays an important role
of favoring J-type aggregation, which restricts the formation
of the excimer complex.39 Consequently, molecule 1 in the
aggregated gel state is likely to show drastically enhanced
fluorescence emission compared to that of the isolated state
due to the synergetic effect of intramolecular planarization
and restricted excimer formation. In contrast, the fluores-
cence emission intensity of the Co2+, Zn2+, Ni2+ and Cu2+
coordination polymeric gel 1 decreased drastically compared
to Mg2+ coordination polymer gel 1, due to the photoinduced
electron transfer (PET) effect, which occurred upon complex
formation between the nitrogen atoms of both end tetrazole
groups of ligand 1 and Co2+, Zn2+, Ni2+ and Cu2+
(Fig. S7, ESIw).40
two possibilities for non-emission of 1 with Mn2+. The first is
due to the photoinduced electron transfer (PET) effect, which
occurred upon complex formation between the nitrogen
atoms of both end tetrazole groups of ligand 1 and Mn2+
.
The second is related to the complex structure between 1
and Mn2+. In this study, the non-emission of 1 with Mn2+
is primarily related to the complex structure. This finding
indicates that the strong fluorescence observed for gel 1 with
Mg2+ originates from the coordination polymer structure.
Fluorescence of the coordination polymer gel 1 was also
measured as a function of temperature, as shown in Fig. S10,
ESI.w No significant spectral changes were observed until
55 1C. The fluorescence intensity of Mg2+ coordination
ꢀ
polymer gel 1 with NO3 slightly decreases at 57 1C. Further
increase in temperature resulted in a large reduced emission.
These results suggest that the emission of the coordination
polymer gel 1 decreases as it starts to melt at 55–59 1C. The
decreasing fluorescent intensity of gel 1 above 56 1C is due to
this transformation to a solution phase. This phenomenon is
the same to that observed with Mn2+
.
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 supra-
molecular interactions while still in the solution state.
Interestingly, the fluorescence intensity of Mg2+ coordina-
tion polymer gel 1 gradually increased until the addition of 4
equivalents of metal ion and remained constant upon further
addition (Fig. S8, ESIw). These results indicate that one
molecule of gelator 1 binds to 4 metal ions. Furthermore, we
observed the fluorescence properties of Mg2+ coordina-
tion polymer gel 1 with different anions such as Clꢀ, Brꢀ, Iꢀ
and SO42ꢀ. To probe the stoichiometry, we prepared the
precipitation of 1 upon the addition of an excess of Mg2+
(6.0 equivalents) at pH = 3. To remove the uncoordinated
Mg2+, the solid product was washed with aqueous solution.
The solid sample, containing ligand 1 and Mg(NO3)2, was
examined by elemental analysis and ICP analysis. Elemental
analysis and ICP of the precipitated sample demonstrated
a content profile of 23.99% for C, 1.43% for H, 25.37% for
N and 9.32% for Mg. Our DFT calculations also show the
following results, which are in excellent agreement with the
experimental data: calculated C(24.09%), H(1.44%),
O(39.74%), N(25.42%), Mg(9.29%). These findings indicate
To garner an insight into the thermally promoted stability
of Mg2+ coordination polymer gel 1, the transition tempera-
ture (Tsol–gel) of coordination polymer gel 1 was measured by
differential scanning calorimetry (DSC) (Fig. S11, ESIw).
Mg2+ coordination polymer gel 1 showed a sharp phase
transition at 56 1C, an endothermic reaction. This endothermic
thermogram is due to the transition of the Mg2+ coordination
polymer gel 1 into a solution phase as observed in Fig. S10,
ESI.w This sol–gel transition temperature of Mg2+ coordina-
tion polymer gel 1 is consistent with the results obtained by
fluorescence spectroscopy.
The luminescence properties of Mg2+ coordination polymer
gel 1 were studied by time-resolved fluorescence confocal
microscopy. The emission decay profiles were monitored at
l = 405–490 nm for Mg2+ coordination polymer gel 1 (Fig. 4).
The fluorescence decay of Mg2+ coordination polymer gel 1
was fitted with a single exponential component yielding a
that the stoichiometry of sample is consistent with
a
4 : 1 formulation. The Mg2+ coordination polymer gel 1 with
NO3ꢀ exhibits the strongest emission. In contrast, a quenching
effect was observed for gel 1 with MgBr2 and MgI2, induced by
this intersystem crossing.41 In particular, the binding strengths
for Brꢀ and Iꢀ coordinated to Mg2+ might be stronger than
that of NO3ꢀ, thereby inducing a large quenching effect.
Controlled experiments were conducted to investigate whether
the fluorescence enhancement arises from the coordination
polymer gel formation or from complexation with Mg2+. The
emission properties of the solution of 1 (20 mM) upon addi-
tion of Mn2+ (4 equiv.) were studied. Even though 1 might
form a simple complex with Mn2+ in aqueous solution, the
solution 1 with Mn2+ demonstrates non-emission. There were
Fig. 4 (a) Fluorescence image of Mg2+ coordination polymer gel 1
by time-resolved fluorescence confocal microscopy and (b) its fluores-
cence decay.
c
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lifetime of 3.78 ns, indicating that this emission is fluorescence.
This result reflects the fact that coordination polymer gel 1 in
its aggregated state is more rigid, restricting the rotational and
vibrational movements of molecules. In contrast, the fluorescence
lifetime of solid 1 was less than 2.3 ns (Fig. S12, ESIw).The
limited molecular motions decrease the nonradiative relaxa-
tion process, which leads to the longer lifetime and fluores-
cence enhancement.42
which is a characteristic of a strong cohesive gel.43 A sharp
melting process then ensues, which results in ‘‘liquefaction’’ of
the sample between 54–57 1C, indicating a thermally labile
assembly (Fig. 5B). This observation is consistent with the
variable temperature fluorescence studies.
Conclusions
To understand the mechanical properties of hydrogel 1,
dynamic oscillation and steady shear measurements were
carried out. The linear viscoelastic region (LVR) of Mg2+
coordination polymer gel 1, as a function of the amplitude of
deformation on shear, was determined with a strain amplitude
ranging from 0.01% to 100% at 1 rad sꢀ1 (Fig. S13A, ESIw).
Both the in-phase storage modulus (G0) and the out-of-
phase loss modulus (G00) remain constant up to B1% strain
(G0 > G00), an outcome which defines the uppermost bound of
LVR. Beyond this level of deformation, a catastrophic disrup-
tion of the network occurs as indicated by the steep drop in the
values of both moduli and the reversal of the viscoelastic signal
(G00 > G0). Based on these data, we decided to perform
subsequent measurements on the gel at 0.1% strain, which
lies comfortably within LVR.
In conclusion, we have demonstrated that the formation of a
coordination polymer gel can be facilitated without long alkyl
chain groups. A tetrazole-appended ligand is shown to efficiently
produce a hydrogel by simple mixing with Mg2+. Photo-
physical studies show that the hydrogel exhibits a typical
p–p* transition and gives rise to high fluorescence 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. Together, measure-
ments of dynamic oscillation and steady shear indicate the
formation of a weak and thermally labile network whose
tenuous supramolecular structure is irreversibly disrupted by
mechanical and thermal treatments.
Experimental
Time-dependent oscillation measurements were used to
monitor the gelation process of Mg2+ coordination polymer
gel 1, which forms gradually upon mixing of ligand 1 and
Mg2+ (Fig. S13B, ESIw). The time sweep shows the rapid
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. Gel network
formation as evidenced by the time dependent oscillation is
consistent with the fluorescence studies, which showed that the
maximum fluorescence intensity is reached at about 2 min.
Once the aforementioned pseudo-equilibrium plateau was
reached, we were able to implement a frequency sweep between
0.1 and 100 rad sꢀ1 at the same temperature and within the
LVR (Fig. 5A). The results show that G0 > G00, confirming
that the Mg2+ coordination polymer 1 has predominantly an
elastic character. The elasticity of the gel is further evident
from the fact that G0 and G00 are minimally sensitive to O
and the loss tangent (tan d = G00/G0) approaches a value of
B0.1 in the tested frequency range. The double logarithmic
Equipments
1H and 13C NMR spectra were measured on a Bruker ARX
300 apparatus. IR spectra were obtained for KBr pellets, in the
range 400–4000 cmꢀ1, with a Shimadzu FT-IR 8400S, and
Mass spectra were obtained by a JEOL JMS-700 mass spectro-
meter. 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 in a
RF-5301PC spectrophotometer. The powder X-ray diffraction
(XRD) measurements of the samples were recorded on a
Bruker D8-Advance X-ray powder diffractometer using
Cu Ka radiation (l = 0.1542 nm) with scattering angles (2y)
of 3–701, operating at 40 keV, cathode current of 20 mA.
Differential scanning calorimetry (DSC) was performed on a
Seiko DSC6100 high-sensitivity differential scanning calorimeter
equipped with a liquid nitrogen cooling unit. Samples of the self-
assembled metal coordination polymeric gel were hermetically
sealed in a silver pan and measured against a pan containing
alumina as the reference. The thermograms were recorded at a
heating rate of 1 1C minꢀ1. Scanning electron micrographs of the
samples were taken with a field emission scanning electron
microscope (FE-SEM, FEI Sirion 400). The accelerating voltage
of SEM was 5–15 kV and the emission current was 10 mA.
Atomic force microscopy (AFM) images were taken on an
AutoProbe CP/MT scanning probe microscope (XE-100(PSIA)).
Imaging was done in non-contact mode using a V-shaped
‘Ultralever’ probe B (Park Scientific Instruments, boron doped Si
plot of Z* (dynamic viscosity) versus (angular frequency)
2
shows a gradient close to ꢀ1 [note: Z* = (G0 + G00
)
2 1/2/o],
with frequency fc = 78.6 kHz, spring constants k = 2.0–3.8 N mꢀ1
,
and nominal tip radius 10 nm). All images were collected
under ambient conditions at 50% relative humidity and 23 1C
with a scanning raster rate of 1 Hz. Samples for AFM images
were prepared by depositing dispersions of hydrogel 1 in EtOH
on a freshly cleaved mica surface (Ted Pella Inc. Prod No. 50)
and allowing them to dry in air.
Fig. 5 (A) Frequency sweep of G0 and G00 for Mg2+ coordination
polymer gel 1 at a strain of 0.1%. (B) Temperature ramp G0 and G00 for
Mg2+ coordination polymer gel 1 at the heating rate of 1 1C minꢀ1
,
strain of 0.1% and frequency of 1 rad sꢀ1
.
c
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Rheological measurements
d = 164, 158, 139, 138, 132, 128, 119, 116, 99 ppm. IR
(KBr, cmꢀ1): 3364, 3335, 3093, 3063, 2229, 1678, 1662, 1533,
1522, 1490, 1447, 1300, 1238, 1159, 1114, 1085, 1017, 990, 856,
830, 796, 754, 617, 575, 537. HRMS (FAB+) m/z 368.1069
[(M + H)+ calcd for C2H13N5O2: 368.1574]. Anal. calcd for
C2H13N5O2: C, 68.71; H, 4.32; N, 19.43. Found: C, 68.66; H,
3.57; N, 19.06%.
These were carried out on freshly prepared gels using a controlled
stress rheometer (AR-1000N, TA Instruments Ltd, New Castle,
DE, USA). Parallel plate geometry of 40 mm diameter and
1.5 mm gap was employed throughout. Following loading, the
exposed edges of samples were covered with a silicone fluid from
BDH(100 cs) to prevent water loss. 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 1C
(60 min and from 0.1–100 rad sꢀ1, respectively), and a heating
run up to 90 1C at a scan rate of 1.0 1C minꢀ1. Unidirectional
shear routines were performed at 258 1C covering a shear-rate
Compound 1
A mixture of compound 5 (1 g, 27 mmol), NaN3 (2.123 g,
330 mmol), and triethylamine hydrochloride (4.496 g, 330 mmol)
in 100 mL of toluene and 10 mL of methanol was heated at
reflux 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
ether to afford 0.5 g (41.7%) of the product. 1H NMR
regime between 10ꢀ1 and 103 ꢀ1. Mechanical spectroscopy
s
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). After completion of the run, the imposed
stress was withdrawn and the extent of structure recovery was
recorded for another 60 min (relaxation curve). Dynamic and
steady shear measurements were conducted in triplicate and
creep (transient) measurements in duplicate.
2
(300 MHz, DMSO-d6) 10.79 (s, 2H, NH), 8.30 (d, J(H,H) =
2
8.7 Hz, 4H; Ar–H), 8.20 (d, J(H,H) = 8.4 Hz, 4H; Ar–H),
7.92 (t, 3J (H,H) = 3.9 Hz, 3H; Ar–H). 13C NMR (DMSO-d6)
d = 182, 165, 150, 140, 136, 129, 127, 112, 111 ppm. IR (KBr,
cmꢀ1): 3486, 3314, 3093, 3053, 3025, 2922, 2858, 2830, 2759,
2720, 2696, 2668, 2633, 2587, 2521, 1979, 1644, 1588, 1524,
1495, 1455, 1311, 1287, 1237, 1124, 1089, 998, 904, 854, 797,
731, 710, 644, 632, 585, 532, 501. HRMS (FAB+) m/z
454.1410 [(M + H)+ calcd for C22H15N11O2: 454.2013]. Anal.
calcd for C22H15N11O2: C, 56.02; H, 3.38; N, 34.03. Found: C,
55.63; H, 3.33; N, 33.98%.
Fluorescence lifetime microscopy (FLM) measurements
Fluorescence lifetime images were acquired by an inverse
time resolved fluorescence microscope, MicroTime-200
(PicoQuantGmBH). The excitation wavelength, the spatial
resolution, and the time resolution were 405 nm, 0.3 mm, and
60–70 ps, respectively. 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. The
color scales represent average lifetime and total number of
counts is indicated by color density at each point.
Compound 2
Compound 2 was prepared by a similar method as described
for compound 1. 1H NMR (300 MHz, DMSO-d6) 10.45
(s, 2H, NH), 8.40 (d, 2J(H,H) = 8.7 Hz, 4H; Ar–H), 8.30
(d, 2J(H,H) = 8.4 Hz, 4H; Ar–H), 7.30 (t, 3J (H,H) = 3.9 Hz,
3H; Ar–H). 13C NMR (DMSO-d6) d = 165, 155, 137, 136,
135, 131, 130, 129, 127, 113 ppm. IR (KBr, cmꢀ1): 3522, 3314,
2963, 2922, 2916, 2915, 2858, 2823, 1959, 1644, 1598, 1524,
1495, 1458, 1318, 1296, 1084, 1036, 1166, 977, 909, 906, 852,
773, 669, 577, 560. HRMS (FAB+) m/z 452.1458 [(M + H)+
calcd for C23H16N10O2: 452.4313]. Anal. calcd for C23H16N10O2:
C, 58.40; H, 3.26; N, 29.06. Found: C, 57.82; H, 3.23; N, 28.98%.
Preparation of the metal coordination polymer gel
In a vial, the solution of metal salt [0–4 equiv.] was added
to solution of gelator (3–5 wt%). The metal coordination
polymeric gel is formed immediately upon standing in ambient
temperature. The resulting reaction mixture was subjected to
sonication. The metal coordination polymeric gel was stable to
inversion of the vial.
Compound 5
Acknowledgements
Thionyl chloride (5.9 g, 50 mmol) was added dropwise to
compound 3 (7.4 g, 50 mmol) and triethylamine (5.1 g, 50 mmol)
in chloroform. The mixture was refluxed for 2 h and cooled
down to room temperature. Then an acetonitrile solution
of 2,6-diaminopyridine (2.7 g, 25 mmol) and triethylamine
(5.1 g, 50 mmol) were added dropwise to the resulting
compound 3 solution, cooled by salt and ice water. The solution
was stirred for 12 h, and then water was added. From the
resulting solution, yellow powder was filtered and washed with
a dilute Na2CO3 solution, distilled water, and then a small
amount of cold methanol. Compound 5 was obtained as
yellowish white powder (5.4 g, 59%). 1H NMR (300 MHz,
DMSO-d6) 10.88 (s, 2H, NH), 8.13 (d, 2J(H,H) = 8.1 Hz, 4 H;
Ar–H), 8.02 (d, 2J(H,H) = 8.1 Hz, 4 H; Ar–H), 7.899
(t, 3J(H,H) = 6.9 Hz, 3 H; Ar–H). 13C NMR (DMSO-d6)
This work was supported by a grant from World Class
University (WCU) Program (R32-2008-000-20003-0) and NRF
(2010-0018510) supported by Ministry of Education, Science,
S. Korea. The work at SKKU was supported by the NRF grant
(No. 20100001630) by MEST.
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