Two-dimensional copper(I) coordination polymer materials as photocatalysts for the degradation of organic dyes

Article abstract of DOI:10.1021/ic302273h

Two isomeric two-dimensional copper(I) coordination polymer materials based on an in situ generated 5-(3-pyridyl)tetrazole ligand show similar layer structures but distinct photoluminescent and photocatalytic properties, which present an interesting compa

Full text of DOI:10.1021/ic302273h

Communication
pubs.acs.org/IC
Two-Dimensional Copper(I) Coordination Polymer Materials as
Photocatalysts for the Degradation of Organic Dyes
Tian Wen, De-Xiang Zhang, and Jian Zhang*
State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences
(CAS), Fuzhou, Fujian 350002, People's Republic of China
S
* Supporting Information
compounds are stable in air and insoluble in common solvents.
ABSTRACT: Two isomeric two-dimensional copper(I)
coordination polymer materials based on an in situ
generated 5-(3-pyridyl)tetrazole ligand show similar layer
structures but distinct photoluminescent and photo-
catalytic properties, which present an interesting com-
parative study on the structure−property correlation
between isomeric materials.
Because they are structural isomers, compounds 1 and 2 have
very similar thermal behaviors. Thermogravimetric analyses
(TGA) indicate that there is no obvious weight loss before 250
°C for both isomers (Figure S6 in the Supporting Information,
SI). It is notable that no solvent molecules have been trapped in
the structures of 1 and 2. However, the formation of these two
isomers with slight structual differences must be closely
associated with the solvent environment during synthesis.
The intermolecular interactions between solvent molecules
(ethanol for 1 and toluene for 2) and the coordination
framework might induce two isomeric frameworks.
oordination polymers (CPs) or metal−organic frame-
Cworks have attracted great interest in recent years because
of their diverse structures and potential applications in gas
sorption, separation, and catalysis.1−3 A new emerging
application of CPs is photocatalysis, and some results have
demonstrated that CPs are efficient photocatalysts on the
degradation of organic dyes, water splitting, or photoreduction
of CO2.4,5 However, such kinds of applications of CPs on
visible-light-driven photocatalysis are just beginning to emerge.
How to achieve inexpensive, stable, efficient, and band-gap-
tunable photocatalysts based on CPs is still a big challenge.
In this work, we report two isomeric two-dimensional
copper(I) CPs with interesting photocatalytic properties on the
degradation of organic dyes. Remarkably, two isomers show
similar layer structures and have the same formula Cu(ptz) [1
and 2, ptz = 5-(3-pyridyl)tetrazole], but they exhibit totally
different appearances and physical properties including
luminescence and photocatalytic efficiency. Compound 1 with
a band-gap size of 1.65 eV shows much higher photocatalytic
efficiency on the degradation of methylene blue (MB),
rhodamine B (RhB), and methyl orange (MO) than compound
2 does with a band-gap size of 2.24 eV. It is notable that both
materials can be used several times without decreasing the
photocatalytic efficiency.
In the structure of 1, each CuI center is coordinated by three
N atoms from three tetrazole groups and one N donor from a
pyridyl group, showing a distorted tetrahedral coordination
geometry (Figure 1a). Each ptz ligand links four CuI centers
Figure 1. (a) Layer structure in 1. (b) Two stacking layers in 1. (c)
Layer structure in 2. (d) Two stacking layers in 2.
Both isomeric materials 1 and 2 were solvothermally
synthesized through “click” reactions of Cu2O, 3-cyanopyridine,
and NaN3 in different mixed solvents (e.g., 1 in N,N-
dimethylformamide (DMF)/ethanol and 2 in DMF/toluene.6
The ptz ligand is in situ generated from a “click” reaction
between 3-cyanopyridine and NaN3. Such in situ “click”
solvothermal synthesis of a tetrazolate ligand has been well
introduced in a number of tetrazolate-based CPs.7 Single-crystal
X-ray diffraction measurements reveal that both isomers
through three tetrazole N donors and one pyridyl N donor.
The dihedral angle between pyridyl and tetrazole rings in a ptz
ligand is 40.9° (Figure S1 in the SI). Every tetrazole group
bridges three CuI centers together, and the adjacent Cu···Cu
distances are 3.651 and 3.661 Å, respectively. A copper
tetrazole chain running along the a axis is thus formed. These
resulting chains further connect each other into a layer through
the pyridyl groups of the ptz ligands (Figure 1a). The layers
further stack into a three-dimensional structure. No obvious
crystallize in the same triclinic P1 space group and feature
̅
identical wavelike layer structures, but their unit cells are
distinct.8 The pure phase of each compound has been
demonstrated by powder X-ray diffraction (PXRD). Both
Received: October 17, 2012
Published: December 17, 2012
© 2012 American Chemical Society
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Inorganic Chemistry
Communication
interactions were observed between adjacent layers. In each
layer, all CuI centers almost fall in one plane. The distance
between such adjacent copper planes is 6.50 Å (Figure 1b).
The coordination environment in 2 is very similar to that in
1, where the μ4-ptz ligands link those tetrahedral CuI centers
into a wavelike layer (Figure 1c). However, there are three
crystallographically different CuI centers and three 3-ptz ligands
in the asymmetric unit of 2. The dihedral angles in three 3-ptz
ligands are 37.0°, 41.9° and 43.4°, respectively, which are totally
different from those in 1 (Figure S4 in the SI). Moreover, the
shortest Cu···Cu distance in the copper tetrazole chain is 3.684
Å. The distance between two adjacent copper planes in 2 is
6.57 Å (Figure 1d), which is a little longer than that in 1.
One breakthrough achieved in this work was to discover
distinct properties in these two isomers with such close
structural features. Actually, they have totally different perform-
ances on photoluminescent emissions and photocatalysis. The
solid-state photoluminescent properties of two isomers were
investigated at room temperature (Figure 2). Upon irradiation
The band-gap sizes of 1 and 2 were investigated by a UV/vis
diffuse-reflectance measurement method at room temperature.
The results give Eg (band-gap energy) values of 1.65 and 2.24
eV for 1 and 2, respectively (Figures S9 and S10 in the SI).
Different dihedral angles presented by the ptz ligands in two
isomers as well as different layer−layer distances in two
structures might lead to such distinct band-gap sizes for 1 and
2. Both 1 and 2 have absorption response to visible light
because of their narrow band-gap sizes. The photocatalytic
activities of both isomers were further studied. The degradation
of organic dyes was selected as the reference.
Two isomers showed high photocatalytic efficiency for the
degradation of MB, RhB, and MO in aqueous solution under
xenon arc lamp irradiation (Figure 3). First, for the degradation
Figure 2. Emission spectra of 1 (blue line, maximum 501 nm) and 2
(green line, maximum 533 nm) in the solid state at room temperature.
of ultraviolet light at 360 nm, the emission maximum
wavelengths for 1 and 2 are 501 and 533 nm, respectively.
The difference in the emission spectra of two isomers implies
that the properties of their excited states are mainly related to
the local coordination factors, such as the torsion of the ptz
ligand, the coordination between CuI and N atoms, or other
intramolecular interactions. The emission of the ptz ligand is
455 nm upon irradiation of ultraviolet light at 295 nm.
Compared to 1, the ptz ligands in 2 are closely packed and
show much stronger π···π interactions between adjacent layers.
Such strong π···π interactions probably lower the π* energy of
the ptz ligand. Thus, the emission energy level of 2 is lowered,
and longer wavelength emission is found. The emissions of 1
and 2 can be tentatively assigned to the Cu → π*(3-ptz) metal-
to-ligand charge transfer, which are similar to those observed in
some documented copper(I) compounds.9 Furthermore,
Cu···Cu interactions in the same layer, probably involving
with Cu···Cu [3d → 4s] cluster-centered excited states, might
also contribute to photoemission, as evidenced in other
copper(I) tetrazolate compounds.10
Figure 3. Photodecomposition of three dyes (a, MB; b, RhB; c, MO)
in solution over two isomers. Left: color change photograph image of
dye solutions. Right: time-dependent UV/vis spectra of three dyes
over two isomeric photocatalysts. Black line: 1. Red line: 2.
of MB, the characteristic absorption of MB at about 650 nm
was selected to monitor the adsorption and photocatalytic
degradation process. Figure 3a shows a comparison of the
photocatalytic activities of two isomers. The photocatalytic
activity of each sample was gradually enhanced with time
increasing from 0 to 25 min. It is obvious that 1 exhibits much
higher activity. When the light application time increases to 24
min, the degradation ratio of MB reaches 98% (Figure 3a).
Similar procedures were performed to check the photocatalytic
activities of two isomers upon degradation of RhB and MO. For
photocatalyst 1, the full degradation time for RhB is only 35
min (Figure 3b), and the degradation of MO in 45 min also
reaches 95% (Figure 3c). In contrast, photocatalyst 2 has to
take a little longer time for degradation of these dyes. It is clear
that 1 possessed higher activity than 2, but the detailed reason
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Inorganic Chemistry
Communication
(c) Wang, F.; Tan, Y.-X.; Yang, H.; Zhang, H.; Kang, Y.; Zhang, J.
Chem. Commun. 2011, 5828.
(4) (a) Wang, F.; Liu, Z.-S.; Yang, H.; Tan, Y.-X.; Zhang, J. Angew.
Chem., Int. Ed. 2011, 50, 450. (b) Das, M. C.; Xu, H.; Wang, Z.-Y.;
Srinivas, G.; Zhou, W.; Yue, Y.-F.; Nesterov, V. N.; Qian, G.-D.; Chen,
B.-L. Chem. Commun. 2011, 47, 11715.
is difficult to explain at the current stage. It is clear that the
slight structural difference between 1 and 2 leads to their
discrepancy in the band-gap sizes, which might also affect their
discrepancy in the photocatalytic activity.
In order to investigate the stability of two isomers as visible-
light photocatalysts, we repeated the photocatalytic degradation
of MB three times. It is interesting that they still keep similar
photocatalytic efficiencies and the PXRD patterns of two
isomers at the end of each repeated bleaching experiment are
almost identical with that of the as-prepared sample (Figures
S7, S8, and S17−S19 in the SI). We analyzed the total organic
carbon (TOC) of the organic residues after photocatalysis
experiments of isomer 1 (Table S1 in the SI). The results reveal
that the TOC decreases obviously, which infers that the dyes
should degrade into carbon dioxide. Compared to other
reported CPs with photocatalytic properties, both photo-
catalysts 1 and 2 show high visible-light-driven photocatalytic
efficiency.4a,5,11
In summary, through in situ “click” reactions in different
solvents, two isomeric two-dimensional copper(I) materials
based on an in situ generated ptz ligand were successfully
synthesized. The results reveal distinct photoluminescent and
photocatalytic properties presented by two such isomers with
very similar structural features. This work is an interesting
comparative study on the structure−property correlation
between isomeric materials. Considering their high efficiency
and high stability on the photocatalytic degradation of organic
dyes, both isomeric materials might be good candidates for
photocatalytic applications.
(5) (a) Fu, Y.-H.; Sun, D.-R.; Chen, Y.-J.; Huang, R.-K.; Ding, Z.-X.;
Fu, X.-Z.; Li, Z.-H. Angew. Chem., Int. Ed. 2012, 51, 3364. (b) Wen, L.-
L.; Zhao, J.-B.; Lv, K.-L.; Wu, Y.-H.; Deng, K.-J.; Leng, X.-K.; Li, D.-F.
Cryst. Growth Des. 2012, 12, 1603. (c) Mahata, P.; Madras, G.;
Natarajan, S. J. Phys. Chem. B 2006, 110, 13759.
(6) Synthesis of isomers 1 and 2: A mixture of Cu2O (0.0143 g, 0.1
mmol), 3-cyanopyrimidine (0.0105 g, 0.1 mmol), and NaN3 (0.0325 g,
0.5 mmol) in a mixed DMF/ethanol solvent (3/9 mL) along with a
few drops of NH3·H2O was stirred for 15 min. It was then transferred
and sealed in a 23 mL Teflon-lined stainless steel reactor, which was
heated in an oven to 160 °C for 72 h and then cooled to room
temperature. Pale-yellow sheet crystals of 1 were obtained as the main
product (86% yield). 2 was prepared by using a similar procedure
except with toluene replacing ethanol. Pale-blue sheet crystals of 2
were collected and dried in air (80% yield).
(7) (a) Zhao, H.; Qu, Z.-R.; Ye, H.-Y.; Xiong, R.-G. Chem. Soc. Rev.
2008, 37, 84. (b) Li, Z. M.; Li, D. Chem. Commun. 2008, 3390.
(c) Wen, T.; Li, M.; Zhou, X.-P.; Li, D. Dalton Trans. 2011, 40, 5684.
(d) Kolb, H. C.; Finn, M. G.; Sharpless, K. B. Angew. Chem., Int. Ed.
2001, 40, 2004.
(8) The diffraction data of 1 and 2 were collected on an Oxford
Xcalibur diffractometer equipped with graphite-monochromatized Mo
Kα radiation (λ = 0.71073 Å) at 293(2) K. Crystal data for 1:
C6H4CuN5, M = 209.68, triclinic, a = 5.8132(4) Å, b = 7.9619(12) Å, c
= 8.2074(9) Å, α = 67.801(12)°, β = 74.775(7)°, γ = 83.256(8)°, V =
339.29(7) Å3, T = 293(2) K, space group P1, Z = 2, 2025 reflections
̅
measured, 1189 independent reflections (Rint = 0.0531). The final R1
value was 0.0594 [I > 2σ(I)]. The final wR(F2) value was 0.1568 [I >
2σ(I)]. The final R1 value was 0.0638 (all data). The final wR(F2)
value was 0.1591 (all data). The goodness of fit on F2 was 1.196.
Crystal data for 2: C18H12Cu3N15, M = 629.08, triclinic, a = 9.022(3)
Å, b = 10.445(2) Å, c = 12.295(3) Å, α = 71.225(19)°, β = 75.08(2)°,
ASSOCIATED CONTENT
■
S
* Supporting Information
Experimental details, additional figures, TGA, PXRD patterns,
and a CIF file. This material is available free of charge via the
Internet at http://pubs.acs.org.
γ = 70.25(2)°, V = 1017.9(4) Å3, T = 293(2) K, space group P1, Z = 2,
̅
6030 reflections measured, 3311 independent reflections (Rint
=
0.1262). The final R1 value was 0.1041 [I > 2σ(I)]. The final wR(F2)
value was 0.2898 [I > 2σ(I)]. The final R1 value was 0.3096 (all data).
The final wR(F2) value was 0.3555 (all data). The goodness of fit on
F2 was 0.828. The structures were solved by direct methods and
refined by full-matrix least squares on F2 using the SHELXTL-97
program.
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: zhj@fjirsm.ac.cn.
Notes
The authors declare no competing financial interest.
(9) (a) Hu, J.-S.; Qin, L.; Zhang, D. M.; Yao, Q. X.; Li, Z. Y.; Guo, Z.-
J.; Zheng, H.-G.; Xue, Z.-L. Chem. Commun. 2012, 48, 681. (b) Han,
T.-Y.; Feng, X.; Tong, B.; Shi, J.-B.; Chen, L.; Zhi, J.-G.; Dong, Y.-P.
Chem. Commun. 2012, 48, 416. (c) Yam, V. W.-W.; Lo, K. K.-W. Chem.
Soc. Rev. 1999, 28, 323.
(10) (a) Kim, T. H.; Shin, Y. W.; Jung, J. H.; Kim, J. S.; Kim, J.
Angew. Chem., Int. Ed. 2008, 47, 685. (b) Chen, D.; Liu, Y.-J.; Lin, Y.-
Y.; Zhang, J.-P.; Chen, X.-M. CrystEngComm 2011, 13, 3827.
(11) Zhang, H.; Zong., R.; Zhu, Y. J. Phys. Chem. C 2009, 113, 4605.
ACKNOWLEDGMENTS
■
This work is supported by the 973 program (Grants
2011CB932504 and 2012CB821705), National Science Foun-
dation (NSF) of China (Grants 21073191, 21221001, and
91222105), NSF of Fujian Province (Grant 2011J06005), and
CAS.
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