Luminescent Calix[4]arene-based metallogel formed at different solvent composition

Article abstract of DOI:10.1021/ic500266f

We have synthesized a calix[4]arene derivative (1) containing terpyridine and showed that gelation occurred in the presence of Pt²⁺ in DMSO/H₂O of varying compositions. Gelation was presumably mediated by the Pt-Pt and π-π stacking interactions. The scanning electron microscopy image of the xerogel showed a spherical structure with diameter of 1.8-2.1 μm. Interestingly, the metallogel showed strong luminescence enhancement, which was dependent on the DMSO/H₂O ratio of the solvent. We examined the effects of concentration, temperature, and time resolution on the luminescence emission of both the gel 1-Pt²⁺ and the sol 1-Pt ²⁺. The luminescence lifetimes of the metallogel were particularly long, on the order of several microseconds. The luminescence lifetimes were also strongly dependent on the solvent composition. We also determined the thermodynamic parameters for the self-assembly of the gel by the Birks kinetic scheme. Furthermore, the rheological properties of the metallogels in the presence of more than 4.0 equiv of Pt²⁺ were independent of the concentration of Pt²⁺ applied.

Full text of DOI:10.1021/ic500266f

Article
pubs.acs.org/IC
Luminescent Calix[4]arene-Based Metallogel Formed at Different
Solvent Composition
Jaehyeon Park,† Ji Ha Lee,† Justyn Jaworski,‡ Seiji Shinkai,∥ and Jong Hwa Jung*,†
†Department of Chemistry & Research Institute of Natural Science, Gyeongsang National University, Jinju 660-701, Republic of
Korea
‡Department of Chemical Engineering, Hanyang University, Seoul 133-791, Korea
∥Institute for Advanced Study, Kyushu University, Fukuoka 819-0395, Japan
S
* Supporting Information
ABSTRACT: We have synthesized a calix[4]arene derivative
(1) containing terpyridine and showed that gelation occurred
in the presence of Pt2+ in DMSO/H2O of varying
compositions. Gelation was presumably mediated by the Pt−
Pt and π−π stacking interactions. The scanning electron
microscopy image of the xerogel showed a spherical structure
with diameter of 1.8−2.1 μm. Interestingly, the metallogel
showed strong luminescence enhancement, which was
dependent on the DMSO/H2O ratio of the solvent. We
examined the effects of concentration, temperature, and time
resolution on the luminescence emission of both the gel 1-Pt2+
and the sol 1-Pt2+. The luminescence lifetimes of the metallogel were particularly long, on the order of several microseconds. The
luminescence lifetimes were also strongly dependent on the solvent composition. We also determined the thermodynamic
parameters for the self-assembly of the gel by the Birks kinetic scheme. Furthermore, the rheological properties of the metallogels
in the presence of more than 4.0 equiv of Pt2+ were independent of the concentration of Pt2+ applied.
striking changes in the color and luminescence enhancement of
the organometallic alkynylplatinum(II) terpyridyl complexes
are some of the most interesting from an applications
perspective, since changes in their intermolecular interactions
are easily caused by solvent, temperature, and other stimuli.
Recently, Yam and other groups have reported that the sol−gel
transition of alkynylplatinum(II) terpyridyl complexes afforded
very distinctive changes in color.21−23 Also, Yam’s group
demonstrated that the solvent composition could be used to
control the supramolecular self-assembly of amphiphilic anionic
platinum(II) pyridyl complexes24 via π−π stacking interactions
and aggregation of Pt−Pt.
Furthermore, their luminescence lifetimes are on the order of
nanoseconds. However, the effect of external stimuli on the
luminescent properties of metallogels induced by Pt2+ has been
less well studied, particularly stimuli such as variation of solvent
composition and the change of concentration of the ligand. In
the present study, we used the framework of calix[4]arene for
ligand attachment, since the calix[4]arene moiety has proven to
be an effective core species for self-assembly. In addition, the
multifunctional groups can easily allow the introduction of
carboxyl functionalities for hydrogen-bonding and specific
ligand groups as sites for coordination. In particular, the 1,3-
alternate calix[4]arene moiety can form a coordinated polymer
INTRODUCTION
■
Recently, the generation of supramolecular gels from
components ranging in size from small molecules to polymeric
species has been realized by employing noncovalent inter-
actions, which include hydrophobic interactions, π−π stacking,
hydrogen bonding, and even metal−ligand interactions.1−8 The
application of supramolecular gels to the field of material
science has afforded benefits including information recording
materials, drug releasing materials, and adaptive material.9−12
The utilization of metal−ligand coordination chemistry for
the directed self-assembly of small molecules has opened a new
branch of materials research. The diverse and unidirectional
properties of metal−ligand interactions allow the formation of
controllable and extended structures, thus establishing these
interactions as the primary driving force in the construction of
supermolecular structures. Depending on the metal centers, the
coordination geometry, and the coordinating ligand, these
metal complexes offer various functional properties, such as
intriguing spectroscopic, catalytic, and redox properties among
others.13−15 The incorporation of metal centers into supra-
molecular gels may yield multifunctional metallogels that could
not be achieved in organogels.
Research interest in square planar platinum(II) terpyridyl
complexes has grown in recent years, as these coordination
compounds have exciting luminescence and spectroscopic
properties, in addition to their propensity for π−π as well as
metal−metal interactions.16−22 Among these compounds, the
Received: February 3, 2014
© XXXX American Chemical Society
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structure.25−27 Herein, we describe the preparation of a
terpyridine-appended calix[4]arene metallogel incorporating
Pt2+. We investigated its luminescence properties in various
compositions of solvent, and we observed its luminescence
emission as affected by concentration dependence, variation in
temperature, and time-resolved luminescence emission.
the crude product was recrystallized from ethyl acetate (300 mL) and
was washed with acetic acid (100 mL) to give white crystalline solid 2
in 56.79% yield. mp 343 °C; IR (KBr pellet) 3176, 2958, 2866, 1603,
1
1482, 1361, 1242, 1200, 1042, 871, 816, 783; H NMR (300 MHz,
DMSO-d6) δ ppm 10.36 (s, 4H), 7.07 (s, 8H), 3.52 (s, 4H), 1.23 (s,
36H); 13C NMR (75 MHz DMSO-d6) δ ppm 147.62, 143.63, 126.96,
125.74, 34.52, 33.1, 31.2; ESI-MS: calculated for C44H56O4 [M + H]+
649.42, found 649.35; anal. calcd for C44H56O4: C, 81.44; H, 8.70.
Found: C, 81.75; H, 8.59.
EXPERIMENTAL SECTION
■
Synthesis of Compound 3. A suspension of AlCl3 (24 g, 180
mmol) and toluene (150 mL) was stirred in a 1 L three-necked round-
bottom flask. The contents of the flask were poured into a suspension
of compound 2 (20 g, 30.8 mmol), CH2Cl2 (200 mL), and toluene
(50 mL). After the reaction mixture was stirred for 0.5 h, CH2Cl2 (100
mL) and 10% aqueous HCl (400 mL) solution were added to the
reaction mixture in an ice bath. Finally, the reaction mixture was
extracted with CH2Cl2 (3 × 200 mL), washed twice with water, and
dried over anhydrous MgSO4, and the solvent was removed in vacuo.
The crude product was recrystallized from CH2Cl2/ethyl ether (1:30,
v/v) to give a beige crystalline solid 3 in 66.5% yield (8.7 g). mp 315
°C; IR (KBr pellet) 3160, 2935, 2870, 1594, 1465, 1448, 1244, 752;
1H NMR (300 MHz, DMSO-d6) δ ppm 9.76 (br, 4H), 7.16 (d, 8H),
6.66 (t, 4H), 3.89 (s, 4H); 13C NMR (75 MHz DMSO-d6) δ ppm
149.8, 129.2, 129.0, 121.7, 31.1; ESI-MS: calculated for C28H24O4 [M
+ H]+ 425.17, found 425.28; anal. calcd for C28H24O4: C, 79.22; H,
5.70. Found: C, 79.31; H, 5.65.
Synthesis of Compound 4. Compound 3 (5.00 g, 11.78 mmol)
and Cs2CO3 (38.4 g, 11.78 mmol) were suspended in dry acetone
(250 mL) and added to the solution of ethyl 2-bromoacetate (11.81 g,
70.68 mmol) in dry acetone (25 mL). The reaction mixture was
refluxed for an additional 4 h. After cooling to room temperature, the
salt was filtered, and the solvent (acetone) was removed in vacuo. To
the resulting pale yellow oil, 10% aqueous HCl (100 mL) solution and
CH2Cl2 (100 mL) were added, and the organic layer was separated,
washed twice with water, and dried over anhydrous MgSO4, and the
solvent was removed in vacuo. The crude product was recrystallized
from CH2Cl2/n-hexane (1:30, v/v) to give a white crystalline solid 4 in
45.5% yield (4.12 g). mp 118 °C; IR (KBr pellet) 3062, 2980, 2938,
1758, 1453, 1180, 1095, 1060, 769; 1H NMR (300 MHz, DMSO-d6) δ
ppm 7.07 (d, 8H), 6.65 (t, 4H), 4.17 (q, 8H), 3.96 (s, 8H), 3.79 (s,
8H), 1.23 (t, 12H); 13C NMR (75 MHz DMSO-d6) δ ppm 169.9,
158.2, 133.8, 130.6, 122.7, 69.8, 60.7, 35.7, 14.5; ESI-MS: calculated
for C44H48O12 [M + Na]+ 791.30, found 791.25; anal. calcd for
C44H48O12: C, 68.74; H, 6.29. Found: C, 68.68; H, 6.34.
Instruments. The NMR spectra for 1H and 13C NMR were
measured using a Bruker ARX 300 apparatus. For the IR spectra, KBr
pellets were formed, and the IR spectra were observed over the range
400−4000 cm−1, with a Shimadzu FT-IR 8400S instrument. In
addition, the mass spectra were observed using a JEOL JMS-700 mass
spectrometer. A UV−visible (UV−vis) spectrophotometer (Thermo
Evolution 600) was used to obtain the absorption spectra, and the
fluorescence spectra were obtained using a RF-5301PC spectropho-
tometer, which was also used for the variable-temperature emission
measurements. These measurements were obtained from a single-cell
to control the working temperature in the range 25−90 °C. Gels were
measured using a 0.50 mm path length quartz cuvette as well as a 4
mm path length quartz cuvette for emission measurements. The
thermodynamic parameters were determined by a literature method
reported previously.28
Electron Microscopy. A JEOL JEM-2010 transmission electron
microscope operating at 200 kV was used for examining the samples
using an accelerating voltage of 100 kV and operating at a 16 mm
working distance. A field emission scanning electron microscope
(Philips XL30 S FEG) was also used to observe the samples in which
an accelerating voltage of 5−15 kV with an emission current of 10 μA
was used. Prior to scanning electron microscopy (SEM) visualization,
the metallogel was also freeze-dried under vacuum at −5 °C.
Rheological Measurements. Fresh Pt-terpyridine gels were
analyzed for their rheological properties using a controlled stress
rheometer (AR-2000ex, TA Instruments Ltd., New Castle, DE, USA).
Throughout the experiments, a cone-type geometry of 40 mm
diameter was used. By using a frequency of 0.1 rad s−1, the dynamic
oscillatory behavior was examined. In addition, the response was
observed upon increased amplitude oscillations with up to 100%
apparent strain on shear and having frequency sweeps at 25 °C (from
0.1 to 100 rad s−1, respectively). Finally, the unidirectional shear was
examined at 25 °C with a shear-rate regime between 10−1 and 103 s−1
using transient measurements in the mechanical spectroscopy routines.
Fluorescence Lifetime Measurements. By using a conventional
laser system, the emission lifetimes were measured upon generation
with an excitation source (420 nm output of a Spectra-Physics Quanta-
Ray Q-switched GCR-150−10 pulsed Nd/YAG laser). The signals for
the luminescence decay were obtained using a Hamamatsu R928
PMT, and the data was recorded on a Tektronix model TDS-620A
(500 MHz, 2 GS/s) digital oscilloscope from which it was
exponentially fit for analysis.
Fluorescence Microscopy. A Nikon microscope ECLIPSE 80i
was used to record images with a 420 nm UV light excitation source.
The emission spectra between 465 and 495 nm were obtained with
100× magnification. The samples for microscopy were formed by the
drop-casting method on to a glass slide and allowing for slow
evaporation.
Typical Experimental Procedure for the Formation of Pt-
Calix[4]arene-Based Gel 1. Compound 1 (4 mg, 1.0 wt %) was
added to DMSO (0.28 mL). PtCl2 (0−6.0 equiv) was dissolved in a
small volume of H2O (0.12 mL). Compound 1 dissolved in DMSO
was mixed in the Pt2+ solution. The sample was maintained at room
temperature to form the gel.
Synthesis of Compound 5. A solution of compound 4 (2 g, 2.6
mmol) in the mixture of THF (40 mL) and EtOH (40 mL) was
heated to reflux temperature. The reaction mixture was then added to
aqueous KOH (1 mL, 26 mmol). After refluxing for 4 h, the organic
solvents were removed in vacuo, and water (10 mL) was added. The
remaining aqueous solution was acidified to pH 1 by addition of 6 N
HCl. The resulting precipitate was filtered and washed with water. The
precipitation was dried under vacuum to give compound 5 as a white
solid in 79.7% yield (1.36 g,). mp 303 °C; IR (KBr pellet) 3448, 3015,
1
2925, 1731, 1460, 1356, 1322, 1195, 1058, 767; H NMR (300 MHz,
DMSO-d6) δ ppm 12.48 (br, 4H), 7.12 (d, 8H), 6.69 (t, 4H), 4.12 (s,
8H), 3.83 (s, 8H); 13C NMR (75 MHz DMSO-d6) δ ppm 169.5,
154.4, 134.7, 124.1, 122.7, 72.5, 34.1; ESI-MS: calculated for
C36H32O12 [M + K]+ 695.15, C36H32O12 [M + Na]+ 679.17, found
695.25, 679.50; anal. calcd for C36H32O12: C, 65.85; H, 4.91. Found: C,
66.21; H, 4.97.
Synthesis of Compound 8. 4′-Chloro-[2,2′,6′,2″]terpyridine (1
g, 3.73 mmol) (7) was suspended in 1,3-propane diamine (8.305 g,
111.9 mmol). Upon heating, the solution became yellow. The reaction
mixture was then heated under reflux conditions (120 °C) overnight.
After cooling to room temperature, H2O (50 mL) was added, and a
white precipitate was formed. After filtration, the solid was washed
with H2O. The solid was dissolved in CH2Cl2 and extracted twice with
H2O. The organic layers were combined, dried over anhydrous
Na2SO4, and filtered, and the solvent was removed under reduced
pressure to yield a white solid in 88.67% yield (1.01 g). mp 148 °C; IR
Synthesis of Compound 2. p-tert-Butyl phenol (150 g, 1 mol)
and NaOH (1.8 g, 45 mmol) was dissolved in 37% formaldehyde
(100.7 g, 1.24 mol). The reaction mixture was refluxed at 120 °C for
12 h. After the solution was cooled to room temperature, H2O was
removed in vacuo, and then, diphenyl ether (450 mL) and toluene
(150 mL) were added. The reaction mixture was refluxed at 250 °C,
again. The color of the reaction mixture changed to dark brown. Then,
B
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Scheme 1. Schematic Synthetic Route of Ligand 1
(KBr pellet) 3410, 3380, 3230, 3056, 3010, 2919, 2860, 1605, 1582,
1566, 1511, 1465, 1457, 1446, 1237, 1111, 1090, 1043, 990, 858, 835,
793, 743, 656; H NMR (300 MHz, MeOD) δ ppm 8.62 (m, 2H),
8.50 (d, 2H), 7.95 (m, 2H), 7.52 (s, 2H), 7.44 (m, 2H), 3.38 (t, 2H),
2.82 (m, 2H), 1.88 (q, 2H); 13C NMR (75 MHz MeOD) δ ppm
156.5, 155.7, 155.1, 149.2 135.8, 124.2, 121.3, 104.5, 41.6, 40.8, 32.3;
ESI-MS: calculated for C18H19N5 [M + H]+ 306.17, found 306.25;
anal. calcd for C18H19N5: C, 70.80; H, 6.27; N, 22.93. Found: C, 70.74;
H, 6.23; N, 22.88.
the yellowish solution was concentrated under vacuum. Acetone (10
mL) was added to the mixture. The orange precipitate was collected,
washed with acetone, and dried under vacuum (0.21 g; yield 73%). mp
220 °C; IR (KBr pellet) 3393, 3247, 3080, 2932, 1625, 1541, 1508,
1
1
1457, 1363, 1245, 1193, 1094, 1034, 782, 623; H NMR (300 MHz,
DMSO-d6, T = 373 K) δ ppm 8.70 (d, J = 5.58 Hz, 8H), 8.47 (d, J =
4.98 Hz, 8H), 8.30 (m, 8H), 7.80 (m, 8H), 7.42 (s, 8H), 7.14 (d, J =
7.60 Hz, 8H), 6.82 (m, 4H), 6.74 (m, 4H), 4.05 (s, 8H), 4.00 (m, 4H),
3.95 (s, 8H), 3.47 (m, 8H), 3.27 (m, 8H), 1.88 (m, 8H); 13C NMR
(125 MHz, DMSO-d6, T = 373 K) δ ppm 168.22, 159.23, 156.88,
154.28, 143.83, 142.26, 130.71, 126.24, 125.58, 125.22, 124.52, 106.21,
67.7, 40.78, 38.25, 32.45, 28.12; ESI-MS: calculated for
[C108Cl4H100N20O8Pt4Cl4]4+ 681.88, found 681.81; anal. calcd for
C108Cl8H100N20O8Pt4: C, 47.55; H, 3.69; N, 10.27. Found C, 47.47; H,
3.70; N, 10.28.
Synthesis of Compound 1. To a suspension of compound 5 (0.2
g, 0.30 mmol) in toluene (20 mL) was added SOCl2 (0.265 mL, 3.65
mmol). The reaction mixture was refluxed for 4 h and was cooled to
room temperature. The solvent and unreacted SOCl2 were removed to
give compound 6. Compound 6 without purification was dissolved in
CH2Cl2 (10 mL). Then, the solution of compound 8 (0.465 g, 15.2
mmol) in CH2Cl2 (10 mL) and TEA (0.5 mL) was added and stirred
for 12 h. After filtration of the reaction mixture, the solvent was
removed in vacuo. The crude product was dissolved in CH2Cl2,
washed twice with water, and dried over anhydrous MgSO4, and the
solvent was removed in vacuo. The crude product was recrystallized in
CH2Cl2/ethyl ether (1:30, v/v) to give a withe solid 1 as a final
product in 70.0% yield (0.38 g). mp 207 °C; IR (KBr pellet) 3397,
3245, 3055, 3010, 2928, 1647, 1605, 1585, 1565, 1461, 1445, 1405,
1355, 1246, 1225, 1199, 1092, 1042, 996, 984, 858, 792, 745, 622; 1H
NMR (300 MHz, DMSO-d6) δ ppm 8.64 (d, J = 4.68 Hz, 8H), 8.53
(d, J = 7.61 Hz, 8H), 7.91 (m, 8H), 7.67 (s, 8H), 7.40 (m, 8H), 7.03
(d, J = 7.68 Hz, 8H), 6.71 (t, J = 7.40 Hz, 4H), 6.59 (t, J = 5.43 Hz,
4H), 3.83 (s, 16H), 3.32 (br, 4H), 3.17 (m, 16H), 1.74 (m, 8H); 13C
NMR (125 MHz, DMSO-d6) δ ppm 167.58, 158.48, 157.22, 155.48,
148.55, 136.33, 134.85, 123.64, 122.52, 120.23, 106.54, 67.58, 40.58,
38.12, 33.54, 28.15; ESI-MS: calculated for C108H100N20O8 [M + H]+
1805.81, [M + 2H]2+ 903.91, [M + 3H]3+ 602.94, [M + 4H]4+ 452.46,
found 1806.33, 903.75, 602.92, 452.58; anal. calcd for C108H100N20O8:
C, 71.82; H, 5.58; N, 15.51. Found: C, 71.72; H, 5.54; N, 15.56.
Synthesis of 1-[PtCl]4Cl4. Compound 1 (0.2 g, 0.11 mmol) was
allowed to react with PtCl2 (0.118 g, 0.44 mmol) in 5 mL of DMSO
with stirring at 100 °C for 2 days. After cooling to room temperature,
RESULTS AND DISCUSSION
■
Calix[4]arene-based ligand 1 was synthesized in six steps
(Scheme 1). The starting compound 2 was prepared by a
modified literature method. Compound 2 was treated with
AlCl3 in DCM/toluene to remove the tert-butyl group.
Compound 3 was reacted with ethyl 2-bromoacetate in the
presence of Cs2CO3 to obtain compound 4 in acetone. After
chlorination of compound 5, the coupling of compounds 6 and
8 synthesized from 4′-chloro-2,2′:6′,2″-terpyridine was per-
formed in methylene chloride in the presence of triethylamine
1
to produce desired ligand 1 in 70% yield, as confirmed by H
and 13C NMR, mass spectrometry (MS), and elemental
analysis.
A metallogel based on 1 was prepared by dissolving 1.0 wt %
of 1 in a mixture of DMSO (1:1 v/v) and with one of the
following solvents: chloroform, methylene chloride, toluene,
methanol, ethanol, ethyl acetate, acetonitrile, tetrahydrofuran,
acetone, and water. To this solution was added 4.0 equiv of Pt2+
C
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with 4.0. The following samples were then set, undisturbed, for
1 week. As shown in Figure S1 and Table S1 (Supporting
Information), ligand 1 after exposure to Pt2+ gelated in a
mixture of water and DMSO (1:1 v/v). In addition, the gelation
ability of ligand 1 was tested in a mixture of H2O/DMSO (7:3
v/v) in concentrations varying from 0 to 4.0 equiv with respect
to the ligand concentration. In a gelation test of 1 from 0 to 4.0
equiv of Pt2+ with H2O/DMSO of composition (7:3 v/v), we
observed that the critical gel concentration (CGC) of
metallogel 1 with Pt2+ (4.0 equiv) was 2.1 0.2 mM (Figure
1). Opaque gel 1 was obtained with 4.0 equiv of Pt2+ (Figure
(TEM), and fluorescence microscopy. The SEM image of Pt2+
metallogel 1 clearly displayed a spherical structure with
diameters of 1.8−2.1 μm (Figure 2). Fluorescence images of
metallogel 1 emitted a strong red color upon excitation at 420
nm. In particular, we performed elemental mapping by energy
dispersive X-ray spectroscopy (EDX) coupled with TEM
observation. The metallogel contained carbon, oxygen, nitro-
gen, and platinum components (Figure 2C). These findings
strongly support the view that 1 formed a complex with Pt2+ to
induce gelation.
Since ligand 1 could gelate a mixture of H2O/DMSO in the
presence of 4.0 equiv of Pt2+, quantitative luminescence
4+
properties of metallogel 1-[PtCl]4 were measured. In various
4+
solutions of H2O/DMSO (1:9, 2:8, 8:2, 9:1 v/v), 1-[PtCl]4
was weakly fluorescent (Figure 3A; a, b), but upon gelation
with H2O/DMSO (3:7−7:3 v/v), it emitted strong red
fluorescence under UV light (Figure 3A; c−g). The
fluorescence emission properties as well as the absorption
4+
4+
spectra of sol 1-[PtCl]4 and metallogel 1-[PtCl]4 were
observed. The absorption band of metallogel 1 appeared at 420
nm (Figure S3, Supporting Information), which was assigned as
dπ(Pt) → π*(tpy) metal-to-ligand charge transfer
(MLCT).29−33 The emission spectra of metallogel 1 with
different contents of H2O/DMSO were recorded upon
excitation at 420 nm (Figure 3B). In the absence of H2O, an
emission band centered at 570 nm was observed. The band
intensity dropped dramatically for higher proportions of H2O.
Simultaneously, a new emission band appeared at 660 nm,
which originated from an excimer emission that is believed to
be attributed to the formation of aggregate species. This
emission is attributed to a MLCT excited state that involves the
terpyridine π* as the lowest unoccupied molecular orbital. Pt−
Pt interactions are expected to affect the metal-based highest
occupied molecular orbital of metallogel 1-[Pt2+]4, thereby
corresponding to a dσ* function formed by overlapping dz2
orbitals that are formally antibonding in respect to the Pt−Pt
interaction. As a result of the metal−metal interaction, the
Figure 1. Photograph of metallogel 1 with (a) 1.0 equiv, (b) 2.0 equiv,
(c) 3.0 equiv, (d) 4.0 equiv, (e) 5.0 equiv, and (f) 6.0 equiv of Pt2+ in
H2O/DMSO (7:3 v/v).
1). On the other hand, calix[4]arene-based ligand 1 did not
form a gel with less than 4.0 equiv of Pt2+. The tetra-terpyridine
group of 1 formed a complex with four Pt2+ ions to induce gel
formation, which had a square planar structure. The molecular
weight of the 1-Pt2+ complex was measured by electrospray
ionization (ESI) MS. As shown in Figure S2 (Supporting
Information), evidence for the 1:4 complex is found in the ESI
4+
mass spectrum of the 1-[PtCl]4 complex (681.84 m/e).
The morphology of xerogel 1 obtained with Pt2+ (4.0 equiv)
was investigated by SEM, transmission electron microscopy
Figure 2. (A) SEM and (B) fluorescence microscopy images of metallogel 1 (1.0 wt %) with Pt2+ (4.0 equiv) in H2O/DMSO (7:3 v/v). (C) EDX
mapping of metallogel 1 by TEM observation; (a) bright-field image, (b) platinum, (c) carbon, (d) oxygen, and (e) nitrogen components.
D
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well as Pt2+ at low temperature but dissociated at high
temperatures. Figure S5 (Supporting Information) shows the
proposed structure of metallogel 1 with Pt2+ based on
fluorescence, mass, and NMR observations. Metallogel 1 with
Pt2+ could be formed through a self-assembled network
structure by intermolecular interactions such as π−π stacking
and MLCT. In particular, the distance between the terpyridine
and the terpyridine groups involved in the intermolecular
interactions such as π−π stacking as well as the Pt−Pt distances
were in the range 3.3−3.8 Å.22,34
Figure 4A shows the changes in the emission spectra of
different concentrations of 1-[PtCl]44+ in H2O/DMSO (3:7 v/
v), which is the sol phase. At concentrations decreasing from
1.0 × 10−4 to 1.0 × 10−6 M at 298 K, the emission band at 660
nm decreased in intensity with an associated appearance of a
new emission band at 570 nm. The luminescence spectra of the
dilute samples ([1-[PtCl]44+] = 1.0 × 10−4 M) were also
observed by varying the temperature (Figure 4B). By elevating
4+
the temperature of the dilute solution of 1-[PtCl]4 from 313
to 353 K, we could also observe a decrease in the 660 nm
emission band, while the band at 570 nm increased in intensity
upon excitation at λ = 420 nm, with an isoemissive point at 620
nm (Figure 4B). Given that the emission band at 660 nm
depends on temperature as well as concentration, we propose
an excimer emission originating from the formation of an
aggregate species in H2O/DMSO (3:7 v/v),28 since such a
decrease in temperature or increase in concentration is known
to lead to aggregate formation.32 While excimer emissions at
high concentration are proposed attributable to the dependence
of the emission band with respect to concentration, Birks
kinetic scheme was used to describe temperature-dependent
changes in the ratio of aggregate to monomer.35−38 The
Stevens−Ban plot39 of ln(Iagg/Imono) versus 1/T in H2O/
DMSO (3:7 v/v) yielded a positive slope (Figure 4C), which
has been suggestive of a high-temperature system limit (Iagg and
Imono are the emission intensities at 660 and 570 nm,
respectively). Below this limit, an excimer binding enthalpy
(ΔH) of −26.14 1.20 kJ mol−1 was found from the positive
4+
Figure 3. (A) Photograph of gel 1-[PtCl]4 (1.0 wt %) under UV
light at different solvent compositions of H2O/DMSO; (a) 1:9, (b)
2:8, (c) 3:7, (d) 4:6, (e) 5:5, (f) 6:4, (g) 7:3, (h) 8:2, and (i) 9:1. (B)
4+
Emission spectra of 1-[PtCl]4 at different solvent compositions of
H2O/DMSO; (a) 1:9, (b) 2:8, (c) 3:7, (d) 4:6, (e) 5:5, (f) 6:4, (g)
7:3, (h) 8:2, and (i) 9:1 at 25 °C. The emission spectra were
normalized at 570 nm.
emissive state of metallogel 1-[Pt2+]4 is thus designated as
3MMLCT (triplet metal−metal-to-ligand charge transfer).29−33
The higher emission of metallogel 1 formed in the presence of
Pt2+ with increasing H2O content is due to stronger Pt−Pt and
π−π stacking interactions than those formed in DMSO.29−33
We also studied H NMR spectra of metallogel 1 with Pt2+
1
4+
slope of the Stevens−Ban plot (Figure 4C) of 1-[PtCl]4 (2.7
(4.0 equiv) by increases in temperature (Figure S4, Supporting
Information). The protons of the terpyridine moieties were
shifted to a lower magnetic field by increasing temperature. In
addition, the proton peaks of the terpyridine moieties became
sharper at increasing temperature. These results indicated that
ligand 1 with Pt2+ forms a self-assembled structure by
intermolecular interactions between the terpyridine groups as
× 10−6 M) in H2O/DMSO.25,26 A corresponding binding
entropy (ΔS) of −26.36 J mol−1 K−1 was observed from the y
intercept of the Stevens−Ban plot and the negative slope of the
plot of Iagg against Imono. The Gibbs free energy change (ΔG) of
the excimer formation was calculated to be −18.05 kJ
mol−1.28,40,41
Figure 4. (A) Emission spectra of different concentrations of 1-[PtCl]44+ (1.0 × 10−6 to 1.0 × 10−4) at H2O/DMSO (3:7 v/v). (B) Emission spectra
of 1-[PtCl]44+ at different temperatures (increasing from 313 to 353 K). (C) Corresponding Stevens−Ban plot of ln(Iagg/Imono) vs 1/T (Iagg and Imono
are the emission intensities at 660 and 570 nm at corresponding temperatures). Inset: corresponding plot of Iagg/Imono. The emission spectra in (A)
were normalized at 570 nm.
E
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Inorganic Chemistry
Article
4+
To better identify the excited-state behavior of metallogel 1
with Pt2+, we conducted a time-resolved emission study of
metallogel 1 with Pt2+ using various concentrations of 1 (Figure
5). In the sol state, the luminescence lifetimes were found to be
Table 2. Luminescence Lifetimes of 1-[PtCl]4 at Different
Solvent Compositions
H2O/DMSO (v/v)
τ1
τ2
phase
7:3
5:5
3:7
0:10
143.20 ns
109.90 ns
90.05 ns
25.16 ns
250.90 ns
210.12 ns
185.20 ns
95.25 ns
gel
gel
gel
sol
respectively. An increase of DMSO content led to shortening
the τ1 and τ2 values. At H2O/DMSO (3:7 v/v), we observed
4+
the shortest luminescence lifetime of metallogel 1-[PtCl]4
.
The largest fluorescence intensity from gels prepared from
H2O/DMSO (7:3 v/v) had longest lifetime, because the
strength of gel formation in H2O/DMSO (7:3 v/v) was
strongest due to intermolecular interactions such as MLCT and
π−π stacking. On the other hand, sol 1 with Pt2+ in DMSO was
weakly emissive. Thus, the lifetimes of solution 1 with Pt2+ were
25.16 ns for τ1 and 95.25 ns for τ2.
The storage and loss moduli (G′ and G″, respectively) were
obtained from rheology to offer a means for assessing the
behavior of the metallogel under mechanical stress. The criteria
of frequency independence of the dynamic storage modulus
and also that the storage modulus should be an order of
magnitude higher than the loss modulus must be met for a
metallogel. We examined the dynamic strain sweep in order to
find the appropriate conditions for the dynamic frequency
sweep of the metallogel at different concentrations of Pt2+.
Figure S7A (Supporting Information) illustrates that G′ and G″
were almost constant with the increase of frequency from 0.1 to
100 rad s−1. In addition, G′ is almost 5−7 times larger than G″
over the entire range (0.1−100 rad s−1), suggesting that the
metallogel is moderately tolerant to external force. As shown in
Figure S7B (Supporting Information), the storage modulus
(G′) and the loss modulus (G″) exhibited a weak dependence
from 0.1% to 1.0% of strain (with G′ dominating G″). The
observation of these met criteria indicates the sample as a
metallogel. Both the storage modulus and the loss modulus of
the metallogel in the presence of 4.0 equiv of Pt2+ were similar
to the metallogels in the presence of 5.0 and 6.0 equiv of Pt2+.
This suggests that the metallogel was almost completely formed
and stabilized in the presence of 4.0 equiv of Pt2+. The dynamic
frequency sweep was used to study the metallogel after the
strain amplitude was set to 0.01% (which is in the region of
linear response for the strain amplitude). In addition, time-
dependent oscillation measurements revealed the gelation
processes of metallogels 1 with Pt2+ (Figure S7C, Supporting
Information). A quick increase in the storage modulus and loss
modulus were both observed at the initial stage of the time
sweep. A long, slow approach to a final pseudoequilibrium
plateau was then found to occur. The final value of the storage
modulus as the end of the experiment was almost 1 order of
magnitude higher than that of the loss modulus.
Figure 5. Emission decay curves of (a) sol 1-[PtCl]44+ (0.3 wt %) and
metallogel 1-[PtCl]4 (b) 0.5 wt %, (c) 0.7 wt %, and (d) 1.0 wt %
obtained from H2O/DMSO (3:7 v/v).
4+
4+
Table 1. Luminescence Lifetimes of 1-[PtCl]4 at Different
Concentrations in H2O/DMSO (7:3 v/v)
4+
conc of 1-[PtCl]4
τ1
τ2
phase
0.3 wt %
0.5 wt %
0.7 wt %
1.0 wt %
71.22 ns
82.39 ns
99.75 ns
143.20 ns
199.57 ns
201.25 ns
219.15 ns
250.90 ns
sol
gel
gel
gel
71.22 and 199.57 ns (Table 1), much longer times than those
reported in the literature.28,42 In general, the luminescence
lifetimes of nanomaterials induced with platinum ion are on the
order of nanoseconds.36,39 Therefore, the luminescence proper-
ties of 1 with Pt2+ in the sol phase are unique. The long
lifetimes of 1 with Pt2+ are due to the existence of aggregation
in the sol state. The biexponential luminescence decay may be
due to aggregate formation in the sol state. At 0.5 wt % of 1 in
the gel state, both the lifetimes, termed the τ1 and τ2 values,
were slightly longer than those observed with 0.3 wt % of 1 in
the sol state. An increase in the concentration of metallogel 1
(1.0 wt %) led to further increases of τ1 (143.20 ns) and τ2
(250.90 ns) values. On the other hand, the luminescence
lifetimes of the solid 1 in the presence of Pt2+ obtained with
H2O/DMSO (7:3 v/v) were found to be 79.82 and 205.32 ns
for τ1 and τ2, respectively. In the gel state, the π−π stacking
interaction between the terpyridine and tertpyridine moieties
was relatively weaker than those of the solid self-assemblies
because the solvent molecules were located between the gelator
molecules. Therefore, the emission in the gel state increased
with increasing concentration of 1.
CONCLUSIONS
■
In conclusion, calix[4]arene containing the terpyridine group in
the presence of Pt2+ formed a gel in solutions of various H2O/
DMSO compositions. Photophysical studies confirmed that the
metallogel exhibited typical Pt−Pt and π−π stacking inter-
actions that gave rise to strong luminescence behavior. The
metallogel showed a significant luminescence enhancement
having a long lifetime by comparison to the sol. From the Birks
kinetic scheme, we determined the values for the binding
We also investigated the time-resolved emission of metallogel
4+
1-[PtCl]4 with different ratios of H2O/DMSO (Table 2 and
Figure S6, Supporting Information). The luminescence life-
times of the metallogel obtained with H2O/DMSO (7:3 v/v)
were found to be 143.20 and 250.90 ns for τ1 and τ2,
F
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Inorganic Chemistry
Article
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change (ΔG) of the excimer. The dependence of concentration
and temperature on luminescence suggested that enhancement
was likely ascribed to the restricted molecular geometry being
in favor of aggregate formation in the gel state. The
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on solvent composition. The rheological properties of the
metallogel were independent of the concentration of Pt2+. The
results of this work provide an emphasis on the validity of
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realizing metallogel systems with versatile properties. We
anticipate that this may help open the way for a range of
responsive soft materials.
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ASSOCIATED CONTENT
* Supporting Information
■
(23) Tam, A. Y. Y.; Wong, K. M. C.; Wang, G.; Yam, V. W. W. Chem.
Commun. 2007, 2028−2030.
S
(24) Zhao, L.; Wong, K. M.; Li, B.; Li, W.; Zhu, N.; Wu, L.; Yam, V.
W. Chem.Eur. J. 2010, 16, 6797−6809.
Information on gelation test of ligand 1; photographs of ligand
1 with Pt2+ in different solvents; ESI mass spectra, UV−vis
absorption spectra, and 1H NMR spectra; proposed structure of
metallogel 1 with Pt2+; emission decay curves; and dynamic
oscillatory and steady-shear measurements. This material is
available free of charge via the Internet at http://pubs.acs.org.
(25) Cai, X.; Liu, K.; Yan, J.; Zhang, H.; Hou, X.; Liu, Z.; Fang, Y.
Soft Matter 2012, 8, 3756−3761.
(26) Zhang, J.; Guo, D. S.; Wang, L. H.; Wang, Z.; Liu, Y. Soft Matter
2011, 7, 1756−1762.
(27) Mecca, T.; Messina, G. M. L.; Marletta, G.; Cunsolo, F. Chem.
Commun. 2013, 49, 2530−2532.
AUTHOR INFORMATION
Corresponding Author
*E-mail: jonghwa@gnu.ac.kr; fax: +82-55-758-6027.
(28) Tam, A. Y.; Wong, K. M.; Yam, V. W. W. J. Am. Chem. Soc.
2009, 131, 6253−6260.
■
(29) Yam, V. W. W.; Wong, K. M.; Zhu, N. J. Am. Chem. Soc. 2002,
124, 6506−6507.
Notes
(30) Wong, K. M.; Yam, V. W. W. Acc. Chem. Res. 2011, 44, 424−
434.
The authors declare no competing financial interest.
(31) McGuire, R., Jr.; McGuire, M. C.; McMillin, D. R. Coord. Chem.
Rev. 2010, 254, 2574−2583.
ACKNOWLEDGMENTS
■
(32) Tam, A. Y.; Wong, K. M.; Zhu, N.; Wang, G.; Yam, V. W. W.
Langmuir 2009, 25, 8685−8695.
This work was supported by a grant from the NRF
( 2 0 1 2 0 0 2 5 4 7 , 2 0 1 2 R 1 A 4 A 1 0 2 7 7 5 0 , a n d
2014M2B2A9030338) supported from the Ministry of
Education, Science and Technology, Korea. In addition, this
work was partially supported by a grant from the Next-
Generation BioGreen 21 Program (SSAC, grant no.:
PJ009041022012), Rural Development Administration, Korea.
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dx.doi.org/10.1021/ic500266f | Inorg. Chem. XXXX, XXX, XXX−XXX