A chromo-fluorogenic tetrazole-based CoBr2 coordination polymer gel as a highly sensitive and selective chemosensor for volatile gases containing chloride

Article abstract of DOI:10.1002/chem.201003279

Catching chloride: A tetrazole-based ligand without long alkyl chains is used to form a coordination polymer gel with CoBr² (1). Gel 1 has a spherical structure with a 20-30 nm diameter. More interestingly, gel 1 selectively recognizes toxic ga

Full text of DOI:10.1002/chem.201003279

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DOI: 10.1002/chem.201003279
A Chromo-Fluorogenic Tetrazole-Based CoBr2 Coordination Polymer Gel as
a Highly Sensitive and Selective Chemosensor for Volatile Gases Containing
Chloride
Hyejin Lee,[a] Sung Ho Jung,[a] Won Seok Han,[a] Jong Hun Moon,[b] Sunwoo Kang,[b]
Jin Yong Lee,*[b] Jong Hwa Jung,*[a] and Seiji Shinkai*[c]
Crystalline coordination polymers are a newer class of or-
ganic–inorganic hybrid nanomaterials created by infinitely
extending metal–ligand coordination interactions. These
polymers show promise in a broad range of applications, in-
cluding gas storage, molecular sieves, ion exchange, sensing,
magnetism, and catalysis.[1,2] Recently, a rational-design
strategy for supramolecular gels based on the concept of co-
ordination polymers is attracting interest.[3–6] In particular,
recent studies have demonstrated that simple bridging or-
ganic units can facilitate the formation of coordination poly-
mer gels in the absence of auxiliary moieties (e.g., urea,
sugar, cholesterol, long alkyl chains), offering new possibili-
ties to produce functional soft materials from structurally
simple building blocks.[7] Although the literature includes
several reports on the synthesis and characterization of bulk
coordination polymer gels, there are only a few examples of
such gels used in practical catalysis applications.[8] Also, to
best of our knowledge, there is no report on the usage of co-
ordination polymer gels as chemosensors or adsorbents for
toxic gases. Coordination polymer gels feature easy prepara-
tion and handling, good stability, and recyclability. As such,
they will act as very useful chemosensors if they can bind to
specific toxic gases.
well-defined and extensively developed metal–organic
framework (MOF).[9] A tetrazole derivative, with multibind-
ing sites, promotes cross-linking to form a polymer network,
which is a critical condition for gelation. Herein, we describe
the preparation, morphology, fluorescence, and lifetime of a
tetrazole-appended coordination polymer gel incorporating
Co2+ ions. In addition, we tested the CoBr2 coordination
polymer gel (1) as a selective chemosensor for chloride gas,
such as HCl, SOCl2, (COCl)2, and COCl2. Upon the addition
of several gases containing chloride atoms, gel 1 changed
from red to blue in the gel state through interconversion.
This is a new phenomenon that has never before been re-
ported for coordination polymer gels.
To design a chemosensor, we chose tetrazole-appended
benzene as the framework to attach ligands because of its
In a typical experiment, 1,2,4,5-tetra(2H-tetrazole-5-yl)-
benzene (TTB) was dissolved in organic solvent and the
Co2+ salt was dissolved in the same solvent. The Co2+ solu-
tion was then added to the TTB solution without heating.
The molar ratios of metal ions to TTB was in the range of
1–5:1, and the amount of the coordination polymer in the
gel ranged from 1 to 5 wt%, corresponding to a molar ratio
of gelator/solvent of ꢀ1:104. TTB immediately formed a gel
[a] H. Lee, S. H. Jung, Dr. W. S. Han, Prof. Dr. J. H. Jung
Department of Chemistry and Research Institute of Natural Sciences
Gyeongsang National University, Jinju 660-701 (Korea)
Fax : (+82)55-758-6027
E-mail: jonghwa@gnu.ac.kr
[b] J. H. Moon, S. Kang, Prof. Dr. J. Y. Lee
Department of Chemistry and Institute of Basic Science
Sungkyunkwan University, Suwon 440-746 (Korea)
Fax : (+82)31-290-7075
ꢁ
upon addition of Co2+ salts of ClO4 , OAcꢁ, Clꢁ, Brꢁ, Iꢁ, or
E-mail: jinylee@skku.edu
ꢁ
NO3 in polar solvents, such as DMF, DMA, and DMF/
[c] Prof. Dr. S. Shinkai
methanol (1:1 v/v, Figure 1A, as well as Figure S1 and
Table S1 in the Supporting Information). Except for the so-
lution containing Clꢁ, these gels exhibited a red color, as
will be discussed in detail shortly. Figure 1B shows a field-
emission scanning electron microscope (FE-SEM) image of
1. A spherical structure is seen with 20–30 nm diameter par-
ticles. The sample uniformity and narrow diameter were
Institute for Advanced Study, Kyushu University
744 Moto-aka, Nishi-ku, Fukuoka 819-0395 (Japan)
and Department of Nanoscience, Faculty of Engineering
Sojo University, 4-22-1 Ikeda, Komamoto 860-0082 (Japan)
Fax : (+81)92-805-3814
E-mail: shinkai_center@mail.cstm.kyushu-u.ac.jp
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201003279.
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ferring to the previously reported crystal structure in a simi-
lar system,[9,10] we generated the expanded framework by as-
sembling the optimized structure. As shown in Figure S4 in
the Supporting Information, the intercobalt(II) distance was
approximated to 14.43 ꢂ. This cavity size gives at least a
reasonable assembled structure in the 2D space.
The absorption and emission properties of TTB and 1 in
solution were studied by UV/Vis spectroscopy and fluorom-
etry. The UV/Vis absorption band of 1 appears at 475 nm
(Figure 3A), which is a typical p–p* absorption band.[4] In
Figure 1. A) Photograph of TTB in solution (a, 20 mm) and 1 (b, 20 mm,
3.0 equiv). B) SEM and C) TEM images of 1.
confirmed by TEM as shown in Figure 1C. In addition,
TEM images of the coordination polymer gels obtained with
different anions than Brꢁ show a spherical structure with
20–30 nm diameter particles (Figure S2 in the Supporting
Information). These findings suggest that the morphology of
1 does not strongly depend on the nature of the anion.
To understand the coordination behavior between TTB
and Co2+, we attempted to obtain crystal structures of the
related complexes, but were not successful. As an alterna-
tive, we obtained the X-ray powder diffraction (XRD) pat-
terns of freeze-dried 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.[9,10] As shown in Fig-
ure S3 in the Supporting Information, the XRD pattern of 1
reveals a crystalline structure. The crystallinity of TTB may
be due to the formation of a highly ordered 2D-coordination
polymer structure.[9,10]
To obtain the coordination structure for Co2+ with Brꢁ,
MeOH, and DMF, we carried out DFT calculations with
Beckeꢁs three parameterized Lee–Yang–Parr exchange func-
tional (B3LYP) implemented with the 6-31G* basis sets
with a suite of Gaussian 03 programs.[11] We used the octahe-
dral coordination structure as an initial geometry in which
Co2+ was coordinated by two trans oriented TTB ligands,
two trans oriented Brꢁ ions, one MeOH, and one DMF. The
optimized structure is shown in Figure 2. Even though the
Figure 3. A) UV/Vis spectra of TTB (a, 20 mm) and
1 (b, 20 mm,
3.0 equiv) in DMF/MeOH (1:1 v/v). B) Fluorescence spectra of TTB (a,
20 mm) in DMF/MeOH (1:1 v/v) and 1 (b, 20 mm, 3.0 equiv) upon excita-
tion at l=300 nm. C) Fluorescence spectra of 1 at different temperatures
upon excitation at l=300 nm. D) Plot of emission intensities of 1 at l=
375 nm versus temperature.
contrast, the p–p* absorption band of TTB shows a large
blueshift of l=300 nm. The fluorescence spectrum of 1
(20 mm) upon excitation at l=300 nm is shown in Fig-
ure 3B. Gel 1 exhibits strong blue emission with a maximum
at 375 nm. The fluorescence intensity is greatly enhanced
compared to TTB. The photoluminescence of 1 can be seen
by the naked eye under UV light. Interestingly, the fluores-
cence intensity of 1 gradually increased up to the addition
of three equivalents of Co2+ ions (Figure S5 in the Support-
ing Information) and then remained constant above this
concentration.
Controlled experiments were conducted to investigate
whether the fluorescence enhancement is related to the gel
formation or to the complexation with Co2+. The emission
properties of a solution of TTB (20 mm) upon addition of
Co2+ were studied. No significant change in fluorescence in-
tensity was observed upon excitation at l=300 nm. This
finding indicates that complexation of Co2+ ion with TTB
did not result in a fluorescence enhancement compared with
the free ligand.
Figure 2. Optimized structure for [(TTB)2CoBr
ed with B3LYP/6-31G.
2ACHUTGTNREN(UNG MeOH)CAHUTNGTREN(NUNG DMF)] calculat-
two bromides may have cis orientation, the expanded geom-
etry seems to be equivalent to the trans orientation as
shown in Figure 2. The coordinated distances between the
cobalt atom and the TTB ligand, bromide anions, MeOH,
and DMF were 1.98, 2.52, 2.25, and 2.09 ꢂ, respectively. Re-
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Coordination Gels as Sensors for Chloride Gases
COMMUNICATION
Variable-temperature fluorescence spectra of 1 are shown
in Figure 3C and 3D. No significant spectral changes were
observed as the temperature was increased from 25 to 708C.
A slight decrease in fluorescence intensity was seen when 1
was heated to 728C. Further increase in temperature result-
ed in decreasing emission. These results suggest that the
emission of 1 decreases as it starts to melt at 72–808C.
These findings indicate that the optimal emission of TTB
occurs when the gel is completely formed and that the emis-
sion of the coordination polymer gel decreases as it melts at
higher temperatures. This striking observation may be at-
tributed to the rigidification of the media upon gelation, a
process that slows down nonradiative decay mechanisms,
leading to fluorescence enhancement.
Additional experiments were carried out to gain insight
into the luminescence properties of 1 by time-resolved fluo-
rescence confocal microscopy. The emission decay profiles
were monitored at l=375 nm (Figure S6 in the Supporting
Information). The fluorescence decay of 1 was fitted with a
single-exponential component to yield a lifetime of 3.23 ns,
indicating that this emission is fluorescence. This result sug-
gests that 1, in the aggregate state, increases the rigidity and
restricts the rotational and vibrational movements of mole-
cules.[12]
octahedral structure of 1 changed to a tetrahedral coordina-
tion polymer gel with chloride ligands (2). This is the first
example of a chemosensor for detection of chloride and for
conversion in the gel state. Furthermore, ESI-MS data were
in agreement with the presence of octahedral and tetrahe-
dral structures, respectively (Figure S9 in the Supporting In-
formation).
The complex stability of the coordination polymers pre-
pared in this work would be determined mainly by the fol-
lowing factors: 1) the thermal stability of the coordination
structures, 2) interactions between solvent molecules and
anions, 3) electrostatic interactions between the polymer
complex and the counter anions, and 4) p–p stacking of aro-
matic rings. The configurational equilibrium between octa-
hedral and tetrahedral structures of the Co2+ ion is affected
by several factors, including crystal-field stabilization, prop-
erties of ligands (chemical structure, polarizability, p recep-
tor capacity), and crystal packing. Octahedral coordination
is favored in the solution phase, whereas tetrahedral coordi-
nation is favored in the gel-like state.[13,15]
The sol–gel transition temperature of 2 was slightly higher
than that of 1 (Figure S10 in the Supporting Information),
indicating that the tetrahedral structure of 2 is more stable.
On the other hand, no significant color changes were ob-
served for an excess of other anions, namely, HF, HBr, HI,
HNO3, and H2SO4 (Figure S11 in the Supporting Informa-
tion). These findings indicate that 1 is useful as a chemosen-
sor and adsorbent for chloride ions.
To extend the performance described above to a portable
chemosensor kit, gel 1 was coated in a capillary with 50 mm
inner diameter (Figure 5A). Then, both ends of the capillary
were closed with membrane filters of 1–2 mm pore size.
Figure 5 shows the 1-coated capillary before and after expo-
sure to phosgene gas. The color of the capillary coated with
1 was red before exposure(Figure 5B: a). As expected, gel 1
only changed from red to blue when exposed to chloride
We investigated the sensing ability of 1 as a selective che-
mosensor for toxic gases containing chloride, that is, HCl,
SOCl2, (COCl)2, and COCl2 (phosgene) by using UV/Vis
spectroscopy. The UV/Vis spectrum of 1 exhibits an absorp-
tion band at 475 nm with a red color, suggesting the forma-
tion of octahedral (Oh) complexes (Figure 4A).[13] Introduc-
Figure 4. A) UV/Vis spectra of 1 (10 mm) before (a) and after (b) diffu-
sion of phosgene gas (10 mm). B) Fluorescence spectra of 1 (lex =300 nm)
upon diffusion of phosgene gas; 0 (a), 5 (b), 10 (c), 15 (d), and 20 ppb
(e).
tion of a small amount of COCl2 (phosgene) gas causes a
redshift of 195 nm, resulting in a new band at 670 nm with
blue color (Figure 4A and Figure S7 in the Supporting In-
formation). It is likely that the bromide anions were re-
placed with chloride anions stemming from the phosgene
that decomposed in the gel matrix.[14] An absorption band at
670 nm is characteristic of the tetrahedral Co2+ complex
4
4
(Td, A2! T1(P)).[13] In addition, the fluorescence intensity
of 1 gradually decreases with exposure to phosgene gas (Fig-
ure 4B). Introduction of HCl, SOCl2, and (COCl)2 gases in
1 also induced a color change from red to blue (Figure S8 in
the Supporting Information). These results indicate that the
Figure 5. A) Representation of a 1-coated capillary as the portable che-
mosensor. B) Photographs of a 1-coated (20 mm) capillary before (a),
after 10 (b), and 30 s (c) of exposure to chloride gas (100 mm). C) Photo-
graphs of disktype pellets of dried 1 before (a) and after (b) exposure to
phosgene gas, and after rinsing the pellet with DMF and methanol con-
taining 100 mm HBr (c).
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J. H. Jung, J. Y. Lee, S. Shinkai et al.
glasses. The manufacturerꢁs software was used to analyze the data and
calculate the lifetime maps.
atoms, such as HCl, SOCl2, (COCl)2, and COCl2 (Figure 5B:
b and c). With the 1-coated capillary, it is hence possible to
detect low concentrations of chloride in gas in the gel by the
naked eye. The result implies that capillaries coated with 1
are applicable as portable colorimetric sensors for detection
of chloride atoms in the environment. Furthermore, a disk-
Photophysical studies: UV/Vis absorption and emission spectra in the
range of 200–800 nm of the gel dispersed in 1:1 DMF/MeOH were run at
room temperature. The absorption properties of 1 were studied exten-
sively. UV/Vis absorption of 1 ([1]=20 mm) were observed in the pres-
ence of Co2+ (0–5 equiv).
A
Differential scanning calorimetry: Differential scanning calorimetry was
performed on a Seiko DSC6100 high-sensitivity differential scanning cal-
orimeter equipped with a liquid-nitrogen cooling unit. Samples of Co2+
coordination polymer gels were hermetically sealed in a silver pan and
measured against a pan containing alumina as the reference. The thermo-
ure 5C). The red color of the disktype pellet of dried 1 was
changed to blue within 10 s when exposed by phosgene gas.
The color change was fully reversible when the pellet was
rinsed by DMF and methanol containing HBr.
grams were recorded at a heating rate of 0.58Cminꢁ1
.
The absorption and fluorescence changes of 1 were found
to correspond to the range of 1–20 ppb of phosgene (Fig-
ure 4B and Figure S12 in the Supporting Information). The
limit of detection of 1 for phosgene gas was determined to
ꢀ1.0 ppb, more than sufficient to sense the phosgene con-
centration in the atmosphere according to the American
conference of governmental industrial hygienistsꢁ (ACGIH)
limit (100 ppb). An evaluation of the time course for the ab-
sorption intensity of gel 1 at 670 nm indicated that immedi-
ately after the addition of phosgene gas, the absorption-
band intensity of 1 started to increase, and after 60 s the
band was almost saturated (Figure S13 in the Supporting In-
formation). Thus, the response time of this system is less
than 1 min, making it a rapid and convenient method for
quantitative analysis of phosgene.
Preparation of the gel: In a vial, a solution of the metal salt (150 mL,
3 equiv in MeOH) was added to solution of the gelator (150 mL, 1–
5 wt% in DMF). The metal-coordination polymeric gel was formed im-
mediately upon standing in ambient temperature. The resulting reaction
mixture was sonicated. The gelation state of the material was evaluated
by turning the test tube upside down.
Acknowledgements
This work was supported by a grant from World Class University (WCU)
Program (R32–2008–000–20003–0) and NRF (grant no. 2010–0003695)
supported from Ministry of Education, Science and Technology, S. Korea.
JYL acknowledges the NRF grant (no. 20100001630) from Ministry of
Education, Science and Technology, S. Korea.
In conclusion, we have demonstrated the first example of
a selective chemosensor for the detection of chloride by
conversion of a coordination gel. The gel is produced by
mixing a tetrazole-appended ligand with Co2+ ions. The
photophysical studies showed that the coordination polymer
gel exhibits a typical p–p* transition and gives rise to a
highly fluorescence behavior. Upon the formation of the co-
ordination polymer gel with a 2D structure, the complex
shows a pronounced fluorescence enhancement with a long
lifetime compared with the ligand. More interestingly, a
CoBr2 coordination polymer gel with octahedral structure
selectively recognizes toxic gases, such as HCl, SOCl2,
(COCl)2, and COCl2 containing chloride atoms. The results
suggest that coordination polymer gels may offer useful ap-
plications in chemical sensing.
Keywords: chlorine
tetrazole
· chromophores · gels · sensors ·
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Coordination Gels as Sensors for Chloride Gases
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Received: November 15, 2010
Published online: February 10, 2011
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