Cu(i)-MOF: Naked-eye colorimetric sensor for humidity and formaldehyde in single-crystal-to-single-crystal fashion

Article abstract of DOI:10.1039/c3cc47723a

A porous Cu(i)-MOF was constructed from CuI and 1-benzimidazolyl-3,5-bis(4- pyridyl)benzene. This Cu(i)-MOF can be a highly sensitive naked-eye colorimetric sensor to successively detect water and formaldehyde species in a single-crystal-to-single-crystal fashion. Solid-state guest-responsive luminescence is also used to monitor the sensing process. This journal is The Royal Society of Chemistry.

Full text of DOI:10.1039/c3cc47723a

View Article Online
View Journal
ChemComm
Accepted Manuscript
This article can be cited before page numbers have been issued, to do this please use: Y. Dong, Y. Yu, X. Zhang, J. Ma, Q. Liu
and P. Wang, Chem. Commun., 2013, DOI: 10.1039/C3CC47723A.
This is an Accepted Manuscript, which has been through the RSC Publishing peer
review process and has been accepted for publication.
Accepted Manuscripts are published online shortly after acceptance, which is prior
Chemical Communications
www.rsc.org/chemcomm
Volume 46 | Number 32 | 28 August 2010 | Pages 5813–5976
to technical editing, formatting and proof reading. This free service from RSC
Publishing allows authors to make their results available to the community, in
citable form, before publication of the edited article. This Accepted Manuscript will
be replaced by the edited and formatted Advance Article as soon as this is available.
To cite this manuscript please use its permanent Digital Object Identifier (DOI®),
which is identical for all formats of publication.
More information about Accepted Manuscripts can be found in the
Information for Authors.
Please note that technical editing may introduce minor changes to the text and/or
graphics contained in the manuscript submitted by the author(s) which may alter
content, and that the standard Terms & Conditions and the ethical guidelines
that apply to the journal are still applicable. In no event shall the RSC be held
responsible for any errors or omissions in these Accepted Manuscript manuscripts or
any consequences arising from the use of any information contained in them.
ISSN 1359-7345
COMMUNICATION
FEATURE ARTICLE
J. Fraser Stoddart et al.
Directed self-assembly of
ring-in-ring complex
Wenbin Lin et al.
a
Hybrid nanomaterials for biomedical
applications
1359-7345(2010)46:32;1-H
www.rsc.org/chemcomm
Registered Charity Number 207890

ChemComm
Page 1 of 4Journal Name
Dynamic Article Links ►
View Article Online
DOI: 10.1039/C3CC47723A
Cite this: DOI: 10.1039/c0xx00000x
www.rsc.org/xxxxxx
ARTICLE TYPE
Cu(I)-MOF: naked-eye colorimetric sensor for humidity and
formaldehyde in single-crystal-to-single-crystal fashion
Yang Yu, Xiao-Meng Zhang, Jian-Ping Ma, Qi-Kui Liu, Peng Wang, Yu-Bin Dong*
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
5
DOI: 10.1039/b000000x
A porous Cu(I)-MOF is constructed from CuI and 1-
benzimidazolyl-3,5-bis(4-pyridyl)benzene (L). This Cu(I)-
MOF can be a highly sensitive naked-eye colorimetric sensor
to successively detect water and formaldehyde species in a
10 single-crystal-to-single-crystal fashion. The solid-state guest-
responsive luminescence is also used to monitor the sensing
process.
50
Scheme 1. Synthesis of 1.
Porous metalꢀorganic frameworks (MOFs),1 consisting of both
organic and inorganic components, are potential hybrid materials
15 for sensing. The functionlization of the framework ligand with
nonꢀcoordinated heteroatoms would enhances the interior surface
interaction of host frameworks toward specific guest analytes and
causes the changes in photoꢀ, electrochemical or other properties.
So far, only handful porous MOFs with guest sensing properties,
20 generally luminescenceꢀbased (including emission wavelength
and /or intensity) MOF sensors, have been described. Recent
progress of luminescenceꢀbased MOF sensors is summarized
recently by Hupp and Chen2 et al in two review papers. As we
know, the most practical and convenient sensor candidates are
25 nakedꢀeye colorimetric ones. Compared to the luminescent
colorimetric MOF sensors, MOFs3 exhibit guestꢀresponsive
nakedꢀeye colorimetric property after incorporating certain guest
species (including cationic, anionic and neutral species) are
extremely rare.
Fig. 1 a) Singleꢀcrystal structure of 1 (two sets of frameworks are shown
in different colors). b) Singleꢀcrystal structure of 2. c) XRPD patterns of 1
and 2. d) 1H NMR (DMSOꢀd6) spectra of 1 and 2. The sample pictures of
30
Our research group has been exploring the reversible selective
adsorption, separation and sensing properties of organic and
inorganic guest species based on porous MOFs and discrete
molecular containers.4 Some of them do exhibit nakedꢀeye and
luminescent colorimetric sensing properties.5 Motivated by our
55 1-2 are inserted. The solidꢀstate UVꢀvis spectra of 1 and 2 further
confirmed this color change (Fig. S5†).
bimetallic {Cu2I2} cluster core (dCu⋅⋅⋅Cu = 2.7390(19 Å which is
shorter than twice of van der Waals radius of Cu(I), i.e., less than
2.8 Å) which is further ligated by four terminal pyridyl groups on
60 L into a porous 2D net (Fig. 1, Fig. S1†). These 2D nets interlock
together to form a rhombusꢀlike open channels (dimensions ~10 ×
11 Å) running along the crystallographic c axis (Fig. S1†). As
shown in Fig. 1, the channels contain disordered solvent
molecules of CH3CN, MeOH and H2O, which were further
65 confirmed by the 1H NMR spectra and thermogravimetric
analysis (TGA, Fig. S2†). Singleꢀcrystal analysis indicates that
the organic molecules are fixed inside by hydrogen bonding
interactions (Fig. S1†).
35 interest in this field, in this contribution, we report a new porous
Cu(I)ꢀMOF generated from a benzimidazolylꢀattached bent
organic ligand (L) (Scheme 1) with Cu(I) ion in solution.
Furthermore, it can serves as a sensitive visual colorimetric
sensor to successively apperceive humidity and formaldehyde in
40 a singleꢀcrystalꢀtoꢀsingleꢀcrystal fashion.
The combination of L (Experimental section †) with CuI in a
CH3CN/CH2Cl2/MeOH mixedꢀsolvent system to afford
1
(CH3CN⋅MeOH⋅1.5H2O⊂Cu2(L)2I2, experimental section, ESI†)
as bright yellow crystals (Scheme 1). Singleꢀcrystal Xꢀray
45 diffraction studies (ESI†) revealed that 1 crystallizes in the
monoclinic C2/c space group. As shown in Scheme 1, each Cu(I)
center in 1 lies in a tetrahedral {CuI2N2} coordination sphere.
Two Cu(I) nodes are bound together by two Iꢀ anions to afford a
Interestingly, when the yellow crystals of 1 were allowed to
70 stand at room temperature for a while, the crystal color gradually
changed from bright yellow to deep red brown with retention of
their singleꢀcrystal nature (Fig. 1b). The singleꢀcrystal analysis
This journal is © The Royal Society of Chemistry [year]
[journal], [year], [vol], 00–00 | 1

ChemComm
Page 2 of 4
View Article Online
DOI: 10.1039/C3CC47723A
revealed that the compounds 1-2 are identical and no phase
transition occurred. In 2, the encapsulated organic molecules
CH3CN and MeOH escaped and the water molecules in air came
into the cavities to generate a new hostꢀguest system of
4H2O⊂Cu2(L)2I2 (2). Such guestꢀexchange is further supported
by 1H NMR spectrum (Fig. 1d) and TGA (Fig. S2 †). The trapped
water molecules are bound together through hydrogen bonding
interactions into a H2Oꢀcluster (Fig. 1b), and no obvious
interactions between water cluster and framework are found (Fig.
5
10 S3†). Closer examination of 1 and 2, some structural parameter
changes in Cu2I2 cluster were observed. Compared to 1, the
Cu⋅⋅⋅Cu distance (2.7601(19) Å) in 2 increased by 0.02 Å, while
the CuꢀI bond lengths in 2 increased by 0.02 and 0.018 Å,
respectively. The XRPD pattern indicates that the bulk sample of
15 1 remains intact upon removal of organic solvent molecules (Fig.
1c). Therefore, the color change from 1 to 2 is certainly caused by
the different guest species encapsulation. Inspired by this, we
wondered if 1 could be a visual colorimetric humidity sensor.
To explore the possibility, the bright yellow crystals of 1 were
20 put in closed glass chambers (1L) which can respectively provide
different constant relative humidity (RH) of 5%, 33%, 43%, 57%,
and 75.8% in 24 h (ESI†).6 As indicated in Fig. 3, no obvious
color change of 1 was observed within 5 h in dry air (RH 5%).
Notably, the color of 1, however, immediately changed in the
25 atmospheres with RH above 33%. At moderate RH of 33ꢀ57 %,
the color of 1 changed to redꢀbrown (2) after 2ꢀ4 h, respectively.
At high RH of 78.5 %, the color change from 1 to 2 took only 1.5
h (Fig. 2).
60 Fig. 2 Left: Photographs showing the color change of the bulk crystal
samples of 1 in different atmospheres with different RH. The single
crystal nature was kept during the color change process. Right: The
corresponding solidꢀstate emission spectra of 1 in different atmospheres
with different RH (33ꢀ78.5%).
In addition, the sensing process was monitored by solidꢀstate
30 emission spectra. The emission spectra on different waterꢀloaded
samples obtained from different RH atmospheres also reflect the
humidity change at ambient temperature. As indicated in Fig. 2,
the maximum emission of 1 is 607 nm upon excitation at 418 nm.
The emission might be assigned to be clusterꢀcentered (CC∗)
35 transition having mixed halideꢀtoꢀmetal charge transfer (XMCT),
d→(s, p) character.7 The emission of 1, however, is strongly
quenched by the encapsulated water molecules. Fig. 2 shows that
the emission intensity of 1 decreased by degrees as time
progressed. At RH 33, 43, 57, 78.5%, the emission intensity of 1
40 was strongly quenched by water after 4, 3, 2 and 1.5h,
respectively. As mention above, the water guests are suspensed in
the cavities and not bound to the framework. So the observed
emission quenching could be attributed to nonꢀrediative energy
transition.8 In addition, the emission maximum is slightly redꢀ
45 shifted to 613 nm from 1 to 2, which might be caused by the
slight variation of the bond distances within Cu2I2 cluster core.
Interestingly, the encapsulated water molecules in 2 can be
further replaced by formaldehyde molecules under ambient
conditions. Again, compound 2 retains its single crystallinity after
50 exchange of formaldehyde. When the crystals of 2 were exposed
to formaldehyde vapour for around 24 h at room temperature
(Experimental section†), the Xꢀray singleꢀcrystal analysis
indicated that the three of four encapsulated water molecules per
formula had been replaced by two formaldehyde molecules to
55 generate Cu2(L)2I2⋅2(HCHO)(H2O) (3) (Fig. 3a). The incursive
formaldehyde molecules are hydrogenꢀbonded to the framework
via O⋅⋅⋅HꢀC and O⋅⋅⋅HꢀN bonds (Fig. S3†). Again, no phaseꢀ
transition was found during the color change process. Compared
65
Fig. 3 a) Crystal structure of 3. b) XRPD pattern of 3 and its sample
picture. c) TGA trace of 3. The observed solvent mass loss is 6.8%
(calculated 6.8%). d) H NMR spectrum of the CDCl3 extract of 3. Tiny
1
amount of paraformaldehyde species, which might be formed during the
70 solidꢀliquid extraction process, was detected (Fig. S6†).
Fig. 4 Photographs showing the color change of the bulk samples of 2 in
formaldehyde vapour with different vapor concentrations. The single
crystal nature was kept during the color change process.
75 to 2, the Cu⋅⋅⋅Cu contact in 3 decreased only by 0.004 Å, and the
corresponding CuꢀI bond lengths decreased by 0.02 and 0.03 Å,
respectively (Table S5ꢀ7†). Meanwhile, the crystal color changed
from redꢀbrown to orange (Fig. 3b). The XRPD pattern shows
that the solventꢀexchange reaction is clean (Fig. 3b). The
80 included solvent amount and type are well supported by the TGA
and 1H NMR measurements (Fig. 3c and 3d). The emission
2
| Journal Name, [year], [vol], 00–00
This journal is © The Royal Society of Chemistry [year]

Page 3 of 4
ChemComm
View Article Online
DOI: 10.1039/C3CC47723A
energy mainly affected by the Cu(I)ꢀhalide core structural feature
concentrations. In addition, the guestꢀexchange processes are
in the ground state molecular structure has been described by 55 reversible (Reversible guestꢀexchange, ESI†) and singleꢀ
many previous reports.9 Singleꢀcrystal analysis and XRPD
patterns revealed that the structures of quenched samples
obtained from different RH atmospheres are the same as 2, and
the TGA on the quenched samples demonstrated that the water
loss is comparable to that of 2.
crystallinity of the Cu(I)ꢀMOF is maintained during the sensing
process. Such a property is definitely important for certain
devices. Work is in progress to obtain new nakedꢀeye or
luminescent colorimetric MOFꢀsensors based on other bent
60 ligands and transition metals.
5
Further study demonstrated that compound 2 is very sensitive
to formaldehyde vapor even at very low concentration, which is
We are grateful for financial support from 973 Program (Grant
Nos. 2013CB933800 and 2012CB821705), NSFC (Grant Nos.
91027003, 21072118 and 21271120) and “PCSIRT”.
10 reflected by the clear color change in the solid state. As we know,
formaldehyde is widely used in the manufacture of construction
and decoration materials.10 Formaldehyde is very harmful to
human health as an indoor pollutant. OSHA (World Health
Organization) has set the immediately dangerous to life or health
15 limit at 20 ppm.11 Therefore, the development of formaldehyde
sensor, especially direct and convenient formaldehyde detector is
very significant.12 As shown in Fig. 4, the color of 2 changed to
orange, deepꢀorange and light brown in 1.6, 0.16 and 0.016 ppm
formaldehyde vapors, respectively. No obvious nakedꢀeye
20 detected color change was observed in 0.0016 ppm formaldehyde
vapor. Besides Xꢀray singleꢀcrystal analysis, the 1H NMR
measurement on these samples provided direct evidence for
formaldehyde enrichment (Fig. S7†).
Notes and references
65 College of Chemistry, Chemical Engineering and Materials Science, Key
Laboratory of Molecular and Nano Probes, Engineering Research Center
of Pesticide and Medicine Intermediate Clean Production, Ministry of
Education, Shandong Provincial Key Laboratory of Clean Production of
Fine Chemicals, Shandong Normal University, Jinan 250014, People’s
70 Republic of China. Email: yubindong@sdnu.edu.cn
† Electronic supplementary information (ESI) available: Synthesis and
characterization data for all compounds, including figures for ORTEP,
hydrogen bonding interaction and CIF files, UVꢀvis spectra and crystal
data. See DOI: 10.1039/b000000x/
75
80
1
2
H.ꢀC. Zhou , J. R. Long , O. M. Yaghi, Chem. Rev., 2012, 112, 673.
a) Y. Cui, Y. Yue, G. Qian, B. Chen, Chem. Rev., 2012, 112, 1126. b)
L. E. Kreno, K. Leong, O. K. Farha, M. Allendorf, R. P. V. Duyne, J.
T. Hupp, Chem. Rev., 2012, 112, 1105.
3
4
Recent visual colorimetric MOFs sensors, see: a) J. He, M. Zha, J.
Cui, M. Zeller, A. D. Hunter, S.ꢀM. Yiu, S.ꢀT. Lee, Z. Xu, J. Am.
Chem. Soc., 2013, 135, 7807. b) G.ꢀB. Li, L. Li, J.ꢀM. Liu, T. Yong,
C.ꢀY. Su, Cryst. Growth Des. 2013, 13, 1518. c) Y.ꢀQ. Chen, G.ꢀR.
Li, Z. Chang, Y.ꢀK. Qu, Y.ꢀH. Zhang, X.ꢀH. Bu, Chem. Sci., 2013, 4,
3678.
a) Y.ꢀB. Dong, P. Wang, J.ꢀP. Ma, X.ꢀX. Zhao, H.ꢀY. Wang, B.
Tang, R.ꢀQ. Huang, J. Am. Chem. Soc. 2007, 129, 4872. b) P. Wang,
J.ꢀP. Ma, Y.ꢀB. Dong, R.ꢀQ. Huang, J. Am. Chem. Soc. 2007, 129,
10620. c) Q.ꢀK. Liu, J.ꢀP. Ma, Y.ꢀB. Dong, J. Am. Chem. Soc. 2010,
132, 7005.
85
25
Fig. 5 The corresponding solidꢀstate emission spectra of 2 in different
atmospheres with different formaldehyde concentrations. The emission
intensity of the sample in 0.0016 ppm formaldehyde vapor was not
1
dramatically enhanced, which matches the nakedꢀeye observation and H
90
5
6
J.ꢀP. Ma, Y. Yu, Y.ꢀB. Dong, Chem. Commun., 2012, 48, 2946.
a) Z. Li, H. Zhang, W. Zheng, W. Wang, H. Huang, C. Wang, A. G.
MacDiarmid, Y. Wei, J. Am. Chem. Soc., 2008, 130, 5036. b) Q.
Kuang, C. Lao, Z. L. Wang, Z. Xie, L. Zheng, J. Am. Chem. Soc.,
2007, 129, 6070.
30 NMR measurement. The tiny amount of encapsulated formaldehyde
species, however, did cause the emission intensity enhancement as the
time going on. At this point, Cu(I)ꢀMOF is a really highly sensitive sensor
for formaldehyde analyte.
95
7
8
E. Cariati, X. Bu, P. C. Ford, Chem. Mater., 2000, 12, 3385.
J. J. Perry, C. A. Bauer, M. D. Allendorf, (2011) Luminescent Metal–
Organic Frameworks. In Metal-Organic Frameworks: Applications
from Catalysis to Gas Storage; D. Farrusseng, Ed.; WileyꢀVCH
Verlag GmbH & Co. KGaA: Weinheim, Germany, 2011; doi:
10.1002/9783527635856, Chapter12.
Besides nakedꢀeye detection, the HCHO encapsulation also
35 caused the dramatically change in their emission spectra. As
shown in Fig. 5, compared to 2, the emission intensities of the
HCHOꢀloaded samples are much enhanced with the increase of
the amount of formaldehyde, which might be contributed to the
structural rigidity enhancement imposed by the hostꢀguest
40 interactions. In addition, the structural variations of the Cu2I2 core
led to the emission slightly blueꢀshifted (from 613 to 598 nm).9
As we know, many prior formaldehyde sensors cannot exactly
detect the formaldehyde analyte because of the ineluctable
influence from water molecule in air,12 which largely limits the
45 sensor applications in realꢀworld. Fortunately, compound 2 herein
is able to detect formaldehyde in the presence of water molecules.
Moreover, the temperature changes (from r.t. to ~75°C) cannot
affect the color response. So Cu(I)ꢀMOF could be a potential
candidate for the portable sensing device toward to formaldehyde
50 pollutant in real world.
100
105
110
115
120
9
a) Z. Liu, P. I. Djurovich, M. T. Whited, M. E. Thompson, Inog.
Chem., 2012, 52, 230. b) M. Knorr, A. Pam, A. Khatyr, C.
Strohmann, M. M. Kubicki, Y. Rousselin, S. M. Aly, D. Fortin, P. D.
Harvey, Inorg. Chem., 2010, 49, 5834. c) H. Kitagawa, Y. Ozawa, K.
Toriumi, Chem. Commun., 2010, 46, 6302.
10 T. Salthammer, S. Mentese, R. Marutzky, Chem. Rev., 2010, 110,
2536.
11 S. J. Armour, International Task Force 40: Toxic Industrial
Chemicals (TICs)-Operational and Medical Concerns. U. S.
Government Printing Office: Washington, DC, 2001.
12 Several kinds of formaldehyde sensors based on amineꢀ
functionalized polymer film, silica material, nanoarray membrane
and enzyme test sheet have been reported: a) L. Feng, C. J. Musto, K.
S. Suslick, J. Am. Chem. Soc., 2010, 132, 4046. b) Y. Zhu, H. Li, Q.
Zheng, J. Xu, X. Li, Langmuir, 2012, 28, 7843. c) L. Zhuo, Y. Huang,
M. S. Cheng, H. K. Lee, C.ꢀS. Toh, Anal. Chem., 2010, 82, 4329. d)
N. Shinohara, T. Kajiwara, M. Ohnishi, K. Kodama, Y. Yanagisawa,
Environ. Sci. Technol., 2008, 42, 4472. To our knowledge, Cu(I)ꢀ
MOF herein is the first nakedꢀeye colorimetric MOF sensor for
formaldehyde.
In conclusion, we have prepared a new porous Cu(I)ꢀMOF
which can be
a keen nakedꢀeye colorimetric sensor to
successively detect water and formaldehyde at very low
This journal is © The Royal Society of Chemistry [year]
Journal Name, [year], [vol], 00–00 | 3

ChemComm
Page 4 of 4
Dynamic Article Links ►
Journal Name
View Article Online
DOI: 10.1039/C3CC47723A
Cite this: DOI: 10.1039/c0xx00000x
www.rsc.org/xxxxxx
ARTICLE TYPE
For table content
A highly sensitive visual Cu(I)ꢀMOF sensor which is able to successively detect water and
formaldehyde species in a singleꢀcrystalꢀtoꢀsingleꢀcrystal fashion is reported.
5
This journal is © The Royal Society of Chemistry [year]
[journal], [year], [vol], 00–00 | 4