Electrically Conductive Coordination Polymer for Highly Selective Chemiresistive Sensing of Volatile Amines

Article abstract of DOI:10.1021/acs.inorgchem.7b02464

A new electrically conductive coordination polymer is synthesized using anthracene-incorporated organic ligand. The compound exhibits a band-like structure and long-range π-π stacking of ligand. Interestingly, the fabricated chemiresisitor using this coor

Full text of DOI:10.1021/acs.inorgchem.7b02464

Communication
pubs.acs.org/IC
Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
Electrically Conductive Coordination Polymer for Highly Selective
Chemiresistive Sensing of Volatile Amines
Siping Wang,†,§ Jie Liu, Hongmei Zhao, Zhifen Guo, Hongzhu Xing,* and Yuan Gao*,‡
‡,§
†
†
,†
†Provincial Key Laboratory of Advanced Energy Materials, College of Chemistry, Northeast Normal University, Renmin Street 5268,
Changchun 130024, China
‡
State Key Laboratory on Integrated Optoelectronics, College of Electronic Science and Engineering, Jilin University, Qianjin Street
2
699, Changchun 130012, China
*
S Supporting Information
chemiresistive sensing material for the detection of volatile
amines.
ABSTRACT: A new electrically conductive coordination
polymer is synthesized using anthracene-incorporated
organic ligand. The compound exhibits a band-like
structure and long-range π−π stacking of ligand.
Interestingly, the fabricated chemiresisitor using this
coordination polymer shows selective sensing behavior
for volatile amine detection with characteristics of quick
response, good reproducibility, and room-temperature
operation. This study not only presents a rare example of
electrically conductive coordination polymer but also
illustrates the useful application of coordination polymer
for chemiresistor.
The anthracene-incorporated organic ligand of 5,5′-
(
(
anthracene-9,10-diylbis(ethyne-2,1-diyl))diisophthalic acid
L) was synthesized via Sonogashira coupling reaction
18
according to a reported procedure (Supporting Information).
Needle-like crystals of 1 were synthesized by reaction of
Cd(NO ) ·4H O and ligand in DMF at 100 °C. Single-crystal
3
2
2
X-ray diffraction reveals that 1 crystallizes in triclinic space
group P1 with a formula of [Cd(H L) ]·3H O·2DMF. The
̅
2
2
2
asymmetric unit contains two Cd ions, two ligands, two DMF
molecules, and three water molecules (Figure S1 in the
Supporting Information). The two crystallographicaly unique
Cd ions, Cd(1) and Cd(2), in the structure are both
heptacoordinated by oxygen atoms. The Cd−O bond lengths
range from 2.227(2) to 2.501(3) Å. As shown in Figure 1a, the
Cd(1) ion locates on the edge of the band-like framework,
while the Cd(2) ion locates in the middle of the band-like
framework. Two adjacent Cd(2) ions are further connected by
he prospective use of coordination polymers (CPs), also
1
−3
T
known as metal−organic frameworks (MOFs),
in a
4
number of desirable technologies, such as fuel cells,
5
6
7
supercapacitors, electroluminescence, thermoelectrics, and
8
,9
resistive sensing, greatly prompts research on conductive
1
0,11
CPs.
However, CPs are typically poor electrical conductors
due to the poor orbital overlap between metal ions and organic
ligands; hence, a vast majority of CPs do not provide a long-
range charge transport pathway and behave as electrical
1
0,12
insulators.
One possible approach for electrically con-
ductive CPs is long-range charge transport through inorganic
chain units which can be considered as reduced counterparts of
13
semiconductive metal oxides and metal chalcogenides, etc.
Alternatively, long-range charge transport between electro-
active organic ligand through a noncovalent interaction, such
as π−π stacking, is another feasible strategy to realize electrical
14,15
conductivity of CPs.
It is exciting that tens of electrically
conductive CPs have been synthesized on the basis of these
1
0
approaches. Also, some of the conducting CPs display the
potential application of gas sensing which is very important
considering individual health, homeland security, and environ-
1
6,17
mental concerns.
scarcely reported to date owing to the aforementioned poor
However, CP-based chemiresistor is still
Figure 1. Crystal structure of 1. (a) A perspective view shows double-
band framework. (b) The long-range π−π stacking of ligands in the
structure. (c) An overview of the structure. Hydrogen atoms and
DMF molecules are omitted for clarity.
8
−10
electrical conductivity as shown by CPs.
In this work, a new 1D electrically conductive CP 1 is
rationally synthesized using electroactive anthracene-based
ligand. The compound features a unique band-like structure
and shows abundant uncoordinated carboxyl groups in the
framework. Interestingly, 1 is demonstrated to be a useful
Received: September 25, 2017
©
XXXX American Chemical Society
A
DOI: 10.1021/acs.inorgchem.7b02464
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Communication
a Cd−O−Cd bond to form a dimer unit. As shown in Figure
1
c, these band-like chains are further self-assembled through
π−π interaction between ligands (Figure 1b) to construct a
three-dimensional supramolecular structure of 1. The face-to-
face distances between ligands are 3.653 and 3.902 Å,
respectively.
The bulk sample of 1 was first characterized by powder X-
ray diffraction (PXRD), where the experimental pattern
matches well with the simulated one, indicating good purity
of the synthesized material (Figure S2). Thermogravimetric
(
TG) analysis under air atmosphere displays a two-step weight
loss where the coordinated DMF and water molecules (expt
3.1 wt %; calcd 13.0 wt %) are removed before 240 °C,
1
followed by organic ligand decomposition (expt 68.1 wt %;
calcd 69.7 wt %) at ca. 370−530 °C (Figure S3). PXRD study
suggests the residual is CdO after complete removal of organic
ligand at 550 °C (Figure S4). Fourier transform infrared
(
FTIR) spectroscopy (Figure S5) exhibits a characteristic peak
−
1
from the alkynyl group in the ligand at 2200 cm . The strong
peaks at ca. 1545 and 1436 cm− are due to vibrations of the
1
−
1
carboxylate groups in the ligand, and the peak at ca. 759 cm
can be ascribed to the out-of-plane bending vibration of the
C−H bond in benzene groups of the ligand. The vibration of a
−
1
coordinated H O molecule is observed at ca. 3375 cm . The
2
−
1
intense peak for carbonyl stretching at ca. 1650 cm (v
)
Figure 2. (a) The optical image of 1 (the contacts are made by silver
resins, and the length between contacts is ca. 360 μm). (b) Two-
probe method for conductivity measurement where I, V, L, W, and T
represent working current and voltage, and length, width, and
thickness of sample, respectively. (c) J−E curves of 1 measured from
different crystal samples.
CO
suggests the presence of a DMF molecule. In addition, weak
peaks around 3000 cm− would associate with the vibrations of
1
19,20
C−H bonds in DMF molecule.
It is intriguing that long-range π−π stacking of anthracene-
based ligands is observed in 1, which provides a potential
charge transport pathway in the structure. The crystal indexing
experiment suggests that the stacking of ligand in the structure
is parallel to the long axis of the crystal (Figure S6). As shown
in Figure 2, the electrical conductivity of solvated 1 was
measured by two-probe method using single-crystal sample
where it was immobilized between two blocks of conducting
silver resin to support Ohmic contact. The electrically
conductive performances of different crystals were measured
by sweeping voltage under ambient conditions (Supporting
Information). To compare the electrical conductivity in various
crystals, I−V curves are transformed into J−E curves, i.e.,
current density (J) versus electric field strength (E), by
considering the length and the cross-sectional area of the
conduction channel for each sample. As shown in Figure 2c,
the tested crystals exhibit very similar conductive behavior.
The obtained average electrical conductivity of 1 is about 1.27
Organic amines were first chosen for the sensing of volatile
organic compounds (VOCs), owing to their high risk for
human health, universal usage in production and experiment,
as well as high volatility. As shown in Figure 3a, the sensor is
capable of ethylenediamine (EDA) detection where the
resistance of the sensor significantly decreases in the presence
of EDA vapor. The response value becomes larger as the gas
concentration increases and reaches saturation at high EDA
−
6
−1
×
10 S cm . This value is very close to that shown by other
electrically conductive coordination polymers, including
2
1
22
[
[Ni(6-MP) ·2H O] , [Cu Br(IN) ]
{Cu(TANC)}-
2
2
n
2
2 n,
2
3
24
F ] , [{Rh (acam) } I] ·6nH O, and [Ni(6-ThioG)2·
0
.5 n
2
4
2
n
2
2
1
2
H O] .
2 n
Structural analysis suggests that half of the carboxyl (C(
O)OH) groups in 1 are uncoordinated; hence, they might
serve as antenna to interact with guest molecules leading to
desirable sensing applications. With 1 showing high electrical
conductivity, the use of 1 for chemiresistive sensing is
preferred. The chemiresistor was then fabricated by coating a
finely ground sample on an alumina tube (Supporting
Information). A static system was applied to investigate the
gas sensing behavior of the fabricated sensor under air
Figure 3. Gas sensing performance of the MOF sensor. (a) Dynamic
resistance variation at different EDA concentrations. (b) Reproduci-
bility curve for EDA (900 ppm) sensing. (c) Responses for different
VOCs at 900 ppm. (d) Relationship between response and gas
concentration for volatile amines.
25
atmosphere (30% RH) at room temperature (Figure S7).
The sensing performance is evaluated on the basis of response
values calculated from variation of its resistance.
B
DOI: 10.1021/acs.inorgchem.7b02464
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Inorganic Chemistry
Communication
concentrations. Such behavior is similar to that shown by
chemiresistors fabricated from metal oxides. It is believed that
more amine molecules are in contact with the sensor under
higher gas concentration, resulting in an increase of response,
whereas after a dynamic equilibrium between gas and sensor is
pound features a band-like framework and long-range π−π
stacking of ligand in the structure. The CP is then investigated
for sensing VOCs, making use of the uncoordinated carboxyl
groups in the structure as an antenna. The fabricated
chemiresistor shows selective sensing behavior for amine
detection with the characteristics of a rapid response, good
reproducibly, and room-temperature operation. This work
demonstrates that conductive CPs are promising materials for
gas sensing applications, and we are hopeful that more and
more conductive MOFs could be synthesized and applied for
useful electrical applications.
26
achieved, the sensor comes into a saturation state.
The response time at moderate EDA concentration is less
than 3 s, and the corresponding recovery time is less than 10 s,
where the response time and the recovery time are defined as
time required decreasing the saturated resistance to its 10%
and time required increasing the resistance to 90% saturation
27,28
value, respectively.
These response times are comparable
and even shorter than that needed by commercialized
ASSOCIATED CONTENT
Supporting Information
■
2
9
chemiresistors of metal oxides. Figure 3b shows the
performance of the sensor based on instantaneous supply
and cut of EDA, where the sensor shows good response
repeatability. PXRD and SEM study suggests that amine
treatment does not change the structure and the morphology
of 1 (Figures S2 and S8). Besides, the sensor is stable enough
to survive for two months under ambient conditions (Figure
S9).
*
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.7b02464.
Detailed experimental procedures, analytical data, SEM
images, PXRD patterns, UV−vis spectra, IR spectra,
scheme of sensing, and X-ray crystallographic data
(PDF)
Beyond EDA, the chemiresistive sensing for other VOCs
with different functional groups and polarities was further
studied under air atmosphere. As shown in Figure 3c, the
sensor is highly selective for volatile amine detection, showing
a more significant response than for other VOCs. It is believed
that a strong hydrogen bonding interaction between the
carboxyl group and amine plays a crucial role for the selectivity.
The hydrogen bond is universal in various chemical and
Accession Codes
CCDC 1517717 contains the supplementary crystallographic
data for this paper. These data can be obtained free of charge
via www.ccdc.cam.ac.uk/data_request/cif, or by emailing
data_request@ccdc.cam.ac.uk, or by contacting The Cam-
bridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
3
0−33
biological systems,
where polar groups, such as −NH and
2
3
4,35
−OH groups,
can get very close to each other and
AUTHOR INFORMATION
Corresponding Authors
E-mail: xinghz223@nenu.edu.cn.
E-mail: gaoyuan@jlu.edu.cn.
experience an unusually strong electrostatic interaction.
Actually, the utilization of a hydrogen bonding interaction to
achieve chemical selectivity has widely been reported in fields
■
*
*
36,37
of molecular recognition and sensing.
As compared with
VOCs including formaldehyde, acetone, and methanol, etc.,
organic amine shows larger polarity due to the more
electronegative amino group. Hence it might interact with a
carboxyl group in a relatively strong hydrogen bond.
ORCID
Hongzhu Xing: 0000-0001-7179-0394
Author Contributions
§
S.W. and J.L. contributed equally.
The sensing performances of different volatile amines are
displayed in Figure 3d. The efficiencies for chosen amines are
in the trend EDA > 1,3-diaminopropane ≥ 1,2-propanedi-
amine > ethylamine > diethylamine ≥ triethylamine. This can
be understood according to the characteristic of the hydrogen
bond; i.e., the strength of the hydrogen bond is directionally
affected. The primary amine possesses a polar amino group on
the side of the molecule, which makes it easier to spatially
interact with carboxylate groups through the hydrogen bond.
In contrast, secondary and tertiary amines, such as diethyl-
amine and triethylamine, with an amino group in the middle of
the molecule, would interact with the MOF structure in weaker
hydrogen bonding. By the same argument, it is rational that a
linear EDA molecule exhibits a higher sensing response than
bending 1,2-diaminopropane and 1,3-propanediamine mole-
cules. Beyond the hydrogen bonding interaction, the proton
from the neutral carboxyl group may transmit to amine to form
an ionic species. This might contribute the high response for
amine sensing through an electrostatic force, and the ionic
species may affect the electrical conductivity in the MOF
compound. In contrast, VOCs such as acetone, CO, and
ethanol, etc., cannot provide this possibility.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work is supported by the National Natural Science
Foundation of China (21473024 and 61520106003) and
Science and Technology Research Project of Jilin Province
(JJKH20170906KJ).
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