Transition Metal–organic Coordination Polymers Containing 6-(1H-imidazol-1-yl)-2(1H)-Pyridinone: Synthesis, Structure and Fluorescent Sensing for Enoxacin

Article abstract of DOI:10.1002/aoc.5566

Three metal–organic coordination polymers, [M(2,6-bip)₂] (M = Zn (1), Ni (2)) and [Cu (tfbdc)(2,6-bipH)₂]?2H₂O (3), were obtained with the similar solvothermal reaction systems of bivalent transition metal salt, 6-(1H-imidazol-1-yl)-2(1H)-pyridinone (2,6-bipH) and 2,3,5,6-tetrafluoroterephthalic acid (H₂tfbdc). The three coordination polymers show different framework features, namely 3D (1), 2D (2) and 1D (3) structures, which are resulted from different single-metal ion “node” and “linker” coordination modes. The [ZnO₂N₂] tetrahedron in 1 and [NiO₂N₄] octahedron in 2 as a node, is formed from Zn sp³ and Ni d²sp³ hybrid orbits, respectively, while the linker, 2,6-bip, adopts the μ₂:η¹η¹η⁰ in 1 and μ₂:η¹η¹η¹ in 2 coordination modes. PLATON calculation suggests that 1 possesses the micropore structure with a pore volume of 14.2%. In 3, the [CuO₂N₂] parallelogram as a node is derived from Cu dsp³ hybrid orbital, while the linkers of 2,6-bipH and tfbdc adopt the μ₁:η¹η⁰η⁰ and μ₁:η¹η⁰/μ₁:η¹η⁰ coordination modes, respectively. Intriguingly, Zn(d¹⁰)-centre coordination polymer 1 shows strong blue emission, which are derived from the π* → π transition of the 2,6-bipH ligand. Moreover, 1 exhibits high stability and strong luminescence in both water and ethanol. At this point, the performance of 1 is studied as a chemical sensor for detecting fluoroquinolones drug. The results show that 1 can detect enoxacin in ethanol solution with a low detection limit of 3.61 × 10^?5 M. The luminescent sensing mechanisms were investigated from experimental methods and theoretical calculation in detail as well.

Full text of DOI:10.1002/aoc.5566

Received: 12 November 2019
DOI: 10.1002/aoc.5566
Revised: 3 January 2020
Accepted: 13 January 2020
F U L L P A P E R
Transition Metal–organic Coordination Polymers
Containing 6-(1H-imidazol-1-yl)-2(1H)-Pyridinone:
Synthesis, Structure and Fluorescent Sensing for Enoxacin
Shanshan Li | Xin Zhou | Qiwei Xu | Dongmei Liu | Ruifang Cui | Xia Li
Department of Chemistry, Capital Normal
Three metal–organic coordination polymers, [M(2,6-bip)2] (M = Zn (1), Ni
University, Beijing, 100048, China
(2)) and [Cu (tfbdc)(2,6-bipH)2]•2H2O (3), were obtained with the similar
Correspondence
Xia Li, Department of Chemistry, Capital
solvothermal reaction systems of bivalent transition metal salt, 6-(1H-
imidazol-1-yl)-2(1H)-pyridinone (2,6-bipH) and 2,3,5,6-tetrafluoroterephthalic
acid (H2tfbdc). The three coordination polymers show different framework
features, namely 3D (1), 2D (2) and 1D (3) structures, which are resulted
from different single-metal ion “node” and “linker” coordination modes. The
[ZnO2N2] tetrahedron in 1 and [NiO2N4] octahedron in 2 as a node, is
formed from Zn sp3 and Ni d2sp3 hybrid orbits, respectively, while the
linker, 2,6-bip, adopts the μ2:η1η1η0 in 1 and μ2:η1η1η1 in 2 coordination
modes. PLATON calculation suggests that 1 possesses the micropore struc-
ture with a pore volume of 14.2%. In 3, the [CuO2N2] parallelogram as a
node is derived from Cu dsp3 hybrid orbital, while the linkers of 2,6-bipH
and tfbdc adopt the μ1:η1η0η0 and μ1:η1η0/μ1:η1η0 coordination modes,
respectively. Intriguingly, Zn(d10)-centre coordination polymer 1 shows
strong blue emission, which are derived from the π* ! π transition of the
2,6-bipH ligand. Moreover, 1 exhibits high stability and strong luminescence
in both water and ethanol. At this point, the performance of 1 is studied as
a chemical sensor for detecting fluoroquinolones drug. The results show that
1 can detect enoxacin in ethanol solution with a low detection limit of
3.61 × 10−5 M. The luminescent sensing mechanisms were investigated from
experimental methods and theoretical calculation in detail as well.
Normal University, Beijing, 100048,
China.
Email: xiali@cnu.edu.cn
Funding information
Laboratory Open Project Fund of Capital
Normal University, Grant/Award
Number: LIP18009S047; National Natural
Science Foundation of China, Grant/
Award Number: 21471104; Beijing
Education Commission, Grant/Award
Number: KZ201910028038
K E Y W O R D S
Enoxacin, luminescent sensing, metal–organic coordination polymers, structure
1 | INTRODUCTION
separation, catalysis, luminescence and chemical sensing,
etc.[1–7] The structures of CPs vary from 1D to 3D
Metal–organic coordination polymers (CPs) are prepared
by inorganic building units and linkers through coordina-
tion bonds, which show a variety of structures and
unique physical/chemical properties. These materials
have already shown potential applications in adsorption,
extended frameworks, including open or porous struc-
tures.[8,9] The CPs are often obtained by means of second-
ary building units (SBUs) and multitopic ligands, the
connections between linkers and metal ions could extend
in space and form the coordination frameworks.[10,11]
Appl Organometal Chem. 2020;e5566.
wileyonlinelibrary.com/journal/aoc
© 2020 John Wiley & Sons, Ltd.
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https://doi.org/10.1002/aoc.5566

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LI ET AL.
The basic SBUs are composed of metal ions or clusters,
such as the single-metal ion “node”, binuclear clusters,
trinuclear metal clusters, or polynuclear clusters.[12,13]
The strong coordination bond between SBUs and linkers
can make the frameworks rigid, stable and amenable to
further development for industrial applications.[14]
Various design and synthesis approaches were developed
to construct CPs, and the variability of lanthanide or
transition metal–ligand combinations led to numerous
CPs materials.[15,16] CPs have a diversity of structures
since multiple coordination environments of transition
metal ions. For example, Zn, Cu, Cd, Ni, and Co can
adopt four-, five-, or six-coordinate geometries, leading to
H2tfbdc, like 1,4-BDC, is also utilized as the linker to
form open frameworks. Furthermore, H2tfbdc is a fluori-
nated aromatic dicarboxylic acid. Fluorinated organic
ligands can improve the luminescence of the coordina-
tion polymers by reducing the fluorescence quenching
effect due to the vibrational C-H bonds.[44] The hydroxyl-
ation of N-heterocyclic ligand, 6-(1H-imidazol-1-yl)-2
(1H)-Pyridinone (2,6-bipH) contains N/O-donor atoms
with a π-conjugated system, which can act as a linker and
bridge metal atoms in the construction of coordination
frameworks.
Considering these points, H2tfbdc/2,6-bipH as ligands
and transition metal Zn/Ni/Cu with different construc-
tions to synthesize coordination polymers. So, three new
coordination frameworks, [M(2,6-bip)2] (M = Zn(1),
Ni(2)) and [Cu (tfbdc)(2,6-bipH)2]•2H2O(3) were obtained
under hydrothermal reaction conditions. Zn, Ni, and Cu
adopt different constructions, Zn sp3, Ni d2sp3, and Cu
dsp2 hybrid orbitals, the [ZnO2N2] tetrahedron, [NiO2N4]
octahedron, and [CuO2N2] parallelogram are formed,
which can be incorporated into the single-metal ion node
as building unit. Linker 2,6-bipH adopt different coordina-
tion modes in 1–3. The combinations of single-metal ion
node and the linkers create the coordination polymer for-
mation of the 3D (1), 2D (2), and 1D (3). Intriguingly,
Zn(d10)-centre coordination polymer 1 exhibits strong
linker-based luminescence. Thus, coordination polymer
1 as a luminescence sensor for detecting antibiotics was
studied. Luminescence sensing for EX was realized by
luminescence quenching effect and the sensing mecha-
nism was investigated in detail as well, which indicates a
potential application in chemical sensing.
the
extended
CPs
from
1D
to
3D
dimensionality[8–12,14,17–20] However, CPs based on the
transition metal ions without unpaired electrons, espe-
cially those with d10 configurations of Zn/Cd, can yield
linker-based high emission.[11,21] So, the Zn/Cd-
luminescence coordination polymers were studied for
luminescence sensing of the analytes.[11,22–24] In recent
years, the luminescence has been widely employed in
analytical,
environment,
and
biomedical
applications.[1,4,13,25–27] Luminescent CPs have been
reported as fluorescent sensors for the detection of
anions, cations, organic small molecules and gas
molecules.[11,22–25,28,29]
Enoxacin (EX) is an important synthetic antibiotic,
which belongs to fluoroquinolones (FQs). FQs are
mainly used for the treatment of respiratory tract
infections and special infections caused by gram-
negative and gram-positive bacteria.[30] However, their
abused usage can lead to potential residual in animal-
derived food matrices, causing adverse reactions and
reducing the efficiency of human infection treatment.
Therefore, the development of effective FQs detecting
method is urgent. Compared with instrumental analy-
sis method,[31–34] fluorescence FQ detection may be a
promising method due to its high sensitivity and sim-
ple sample preparation procedures. Therefore, it is
important to develop luminescent CPs as the sensor
for EX detection.
Multi-functional organic ligands play an important
role in the luminescence of CPs. To the best of
our knowledge, organic ligands with a π-conjugated sys-
tem can enhance the stability of CPs, and improve the
luminescence properties due to their conjugation effects
and π-π* transitions.[13,35,36] Aromatic compounds, such
as aromatic polycarboxylic acid and/or N-donor auxiliary
ligands are used in synthesis of the coordination poly-
mers with good luminescence properties.[2,12,37–44]
1,4-benzenedicarboxylic acid (1,4-BDC) is widely used in
CPs,[39,42,44] and its derivative ligand 2,3,5,6-tetrafluo-
roterephthalic acid (H2tfbdc) is seldom investigated.[42–44]
2 | EXPERIMENTAL
2.1 | Materials and physical
measurements
Zn (CH3COO)2, NiSO4•6H2O, CuCl2, 2,6-bipH and
H2tfbdc were commercially available. The C, H, N ele-
mental analysis data of 1–3 were determined by the Vario
EL III elemental analyzer of Elementar Company of
Germany. Infrared (IR) spectra were obtained on a
Bruker Tensor 37 spectrophotometer (KBr pellet). Experi-
mental powder X-ray diffraction was measured on a
PANalytical X'Pert PRO MPD diffractometer with CuKα
radiation. Fluorescence spectra of 1 in solid/liquid state
were determined by Hitachi F-7000 fluorescence spectro-
photometer. The thermal stability of coordination poly-
mers 1–3 by thermogravimetric analyses (TGA) were
measured on a HCT-2 differential thermal analyzer (air
atmosphere, a heating rate of 10 ꢀC•min−1). UV–vis

LI ET AL.
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spectra were obtained on a 2550 UV–visible spectropho-
tometer (Shimadzu, Kyoto, Japan).
coordinates of the hydrogen atoms are determined by the
theoretical hydrogenation method, and the full-matrix
least-squares method is used to correct the hydrogen
atoms and the non-hydrogen atoms by the isotropic and
anisotropic temperature factors respectively. The
crystallographic data and selected bond lengths and
angles of the coordination polymers 1–3 are listed in
Tables 1 and 2, respectively.
2.2 | Synthesis of coordination polymers
1–3
Coordination polymer 1: The materials, 2,6-bipH
(0.2 mmol), H2tfbdc (0.2 mmol), Zn (CH3COO)2
(0.1 mmol), NaOH (0.1 ml, 2 mol•L−1), water (10 ml) and
ethanol (2 ml) were placed in ꢀa 25 ml Teflon Lined Parr
autoclave and heated to 140 C for 72 hr, then it was
cooled to room temperature. Colourless acicular crystals
were obtained. Elemental analysis (%) Calcd for
C16H12N6O2Zn: C, 49.81; H, 3.11; N, 21.79. Found: C,
49.87; H, 3.05; N, 21.82. Selected IR (KBr pellet, cm−1):
3420 (w), 3113 (w), 1612 (s), 1548 (m), 1505 (m), 1471 (s),
1387 (w), 1286 (m), 891 (w), 786 (w), 734 (w), 686 (w),
654 (w), 560 (w), 442 (w).
3 | RESULTS AND DISCUSSIONS
3.1 | Crystal structure
Structure of 1. The single crystal X-ray diffraction study
reveals that 1 possesses a 3D structure responding to
ꢀ
trigonal system and the space group R3. As depicted
in Figure 1a, the asymmetric unit consists of one Zn
(II) ion and two 2,6-bip ligands. The coordination
environment of Zn (II) ion is a [ZnO2N2] tetrahedron
using the Zn sp3 hybrid orbitals. The Zn (II) center is
coordinated by two O atoms (O1 and O2) from two dif-
ferent 2,6-bip ligands with bond distances of 1.957
(4) Å and 1.981(4) Å, respectively, while two N atoms
(N3A and N6B) from two different 2,6-bip ligands with
bond distances of 2.029(5) Å and 2.035(5) Å. The bond
angles at the Zn (II) center (O-Zn-O, O-Zn-N, N-Zn-N)
are in the range of 95.42(18) ꢀ-133.39(17) ꢀ. The
H2tfbdc ligand is not coordinated to Zn (II) ion. The
2,6-bip ligand as a linker adopts the μ2:η1η1η0 coordi-
nation mode in which the N atom (on the imidazole
ring) and the O atom on the 2,6-bip ligand act as coor-
dinating atoms. The dihedral angle between the imid-
Coordination polymer 2: The 2,6-bipH (0.2 mmol),
H2tfbdc (0.2 mmol), NiSO4•6H2O (0.3 mmol), NaOH
(0.2 ml, 2 mol•L−1), water (8 ml) and ethanol (2 ml) were
placed in a 25 ml Teflon Lined Parr autoclave and heated
ꢀ
to 120 C for 72 hr. After that the reaction mixture was
cooled to room temperature. Pale-green block crystals
were obtained. Elemental analysis (%) Calcd for
C16H12N6NiO2: C, 50.69; H, 3.17; N, 22.18. Found: C,
50.48; H, 3.13; N, 22.11. IR (KBr pellet, cm−1): 3606 (w),
3436 (w), 2923 (s), 2376 (m), 1612 (m), 1461 (s), 1109 (w),
721 (w), 420 (w).
Cooedination polymer 3: A mixture of 2,6-bipH
(0.2 mmol), H2tfbdc (0.2 mmol), CuCl2 (0.1 mmol),
NaOH (0.2 ml, 1 mol•L−1), water (8 ml) and ethanol
(2 ml) were placed in a 25 ml Teflon Lined Parr auto-
ꢀ
azole ring and the pyridone ring is 10.049(495) . The
ꢀ
clave and heated to 100 C for 72 hr. It was cooled to
[ZnO2N2] units as nodes are bridged by 2,6-bip linkers
with a Zn … Zn distance of 6.276 (13) Å to from the
3D structure. The Right/Left-Zn-(2,6-bip) helical
chains exist in the structure due to the deformation of
the ligand (Figure 1b). In the helical chain, a repeated
unit consist of three Zn (II) centers and three 2,6-bip
ligands with the pitch of 14.5084(19) Å and 16.5660
(51) Å. The helical chains are bridged by 2,6-bip
ligands resulting in the 3D framework of 1 (Figure 1c).
PLATON[47] calculation suggests that the coordination
polymer has micropore with the pore volume of 14.2%.
Structure of 2. Coordination polymer 2 is a 2D network
structure and crystallizes in monoclinic system and
space group P21/c with one Ni (II) and two 2,6-bip
ligands in the asymmetric unit (Figure 2a). The
H2tfbdc molecule does not participate in the coordina-
tion with Ni (II). The arrangement of coordination
atoms around the Ni (II) ion displays a [NiO2N4] dis-
torted octahedron, which is formed from Ni d2sp3
room temperature. Pale-green block crystals were
obtained.
Elemental
analysis
(%)
Calcd
for
C24H18CuF4N6O8: C, 43.80; H, 2.74; N, 12.77.
Found: C, 43.40; H, 2.75; N, 12.79. IR (KBr pellet,
cm−1): 3551(w), 3396(w), 3142(w), 1616(s), 1548(m),
1503(m), 1469(s), 1377(w), 1278 (m), 989(w), 876(w),
799(w), 745(w), 648(w), 613(w), 515(w).
2.3 | X-ray crystallographic study
Crystals of suitably sized and transparent coordination
polymers 1–3 were picked up and diffracted using an X-
ray diffractometer with Mo Kα (λ = 0.071073 nm) (1, 3)
or Cu Kα (λ = 0.154184 nm) (2) monochromatized by a
graphite monochromator as a diffracted light source at
room temperature. The crystal structure is solved by the
SHELXS 97 and SHELXL 97 programs.[45,46] The

4 of 13
LI ET AL.
TABLE 1 Crystal data and structural refinement for coordination polymers 1–3
Coordination polymers
1
2
3
Empirical formula
C16H12N6O2Zn
385.69
C16H12N6NiO2
379.03
C24H18CuF4N6O8
657.98
Formula weight
Temperature/K
296.15
173.00(10)
1.54184
Monoclinic
P21/c
293(2)
Wavelength/Å
0.71073
Trigonal
0.71073
Triclinic
Pī
Crystal system
ꢀ
R3
space group
a/Å
b/Å
c/Å
α/ꢀ
β/ꢀ
23.238(3)
23.238(3)
16.566(5)
90
7.38530(10)
10.5505(2)
9.34560(10)
90
7.449(12)
7.998(12)
11.002(17)
103.26(5)
95.70(4)
90.58(5)
634.5(17)
1
90
96.7670(10)
90
γ/ꢀ
V/Å3
120
7747(3)
18
723.123(19)
2
Z
Dcalcd (gꢁcm−3
)
1.488
1.448
1.741
1.722
Absorption coefficient
(mm−1
F(000)
Crystal size/mm3
2.172
0.954
)
3528
388
333
0.34 x 0.14 x 0.11
0.25 x 0.21 x 0.11
6.352–73.454
0.36 x 0.25 x 0.18
2.618–25.004
θ range for data collection/ 1.592–24.994
ꢀ
Limiting indices
-27 < =h < =27, −27 < =k < =26,
−19 < =l < =19
-9 < =h < =8, −12 < =k < =8,
−6 < =l < =11
−8 < =h < =8, −9 < =k < =9,
−13 < =l < =13
Reflections
17216/3040 [R (int) = 0.0964]
4127/1403 [R (int) = 0.0427]
13127/2221 [R (int) = 0.0319]
collected/unique
Data/restraints/parameters 3040/0/226
1403/0/115
2221/8/200
Goodness-of-fit on F2
1.047
1.040
1.027
Final R indices
R1 = 0.0516, wR2 = 0.1799
R1 = 0.0447, wR2 = 0.1236
R1 = 0.0466, wR2 = 0.1674
[I > 2sigma(I)]
R indices (all data)
R1 = 0.0812, wR2 = 0.2019
R1 = 0.0459, wR2 = 0.1249
R1 = 0.0506, wR2 = 0.1702
Largest diff. Peak and
1.834and −0.368
0.576and −0.842
0.338 and − 0.525
hole(e. Å−3
)
CCDCNO.
1832414
1958466
1958467
ꢀ
is 63.50(5) to 116.50(5) ꢀ, and the N-Ni1-N bond
hybrid orbitals. In the [NiO2N4], four N atoms (N1,
N3, N1A, N3B) from four different 2,6-bip ligands
and two O atoms (O1, O1A) from two 2,6-bip ligands
are attached to Ni (II) ion. The Ni-N bond distances
range from 2.0908(15) to 2.1410(16) Å, and the Ni-O
bond distance is 2.1025(13) Å. The distances of Ni-
O1A/N1A are 2.1025(13)/2.1410(16) Å, slightly
larger, which can be ascribed to the tension of the
quaternion chelating ring. The N1, N1A, O1 and
O1A are located in the equatorial plane, while N3
and N3B occupy the axial positions. The O-Ni1-O
bond angle is 180.0ꢀ, the O-Ni1-N bond angle range
ꢀ
ꢀ
angle range is 89.99(6) to 180.00(3) . The 2,6-bip
ligand adopts the μ2:η1η1η1 coordination mode,
which is different from that in 1. One N atom on
the imidazole ring coordinates with the Ni (II) ion,
while one N atom and one O atom on the pyridone
ring chelate the Ni (II) ion. The dihedral angle of
the imidazole ring and the pyridone ring in 2,6-bip
ligand is 29.46 ꢀ. The two adjacent Ni (II) ions are
bridged by 2,6-bip ligand to form a 1D chain with
the Ni … Ni distance of 7.696 Å. The 1D chains are
connected by 2,6-bip ligands to form a 2D network

LI ET AL.
5 of 13
TABLE 2 Selected bond lengths [Å] and angles [ꢀ] for coordination polymers 1–3
1
Zn(1)-O(2)
1.981(4)
1.957(4)
Zn(1)-N(3)A
2.029(5)
Zn(1)-O(1)
Zn(1)-N(6)B
2.035(5)
109.87(18)
97.25(18)
107.3(2)
O(2)-Zn(1)-N(3)A
O(2)-Zn(1)-N(6)B
O(1)-Zn(1)-O(2)
2
95.42(18)
112.12(19)
133.39(17)
O(1)-Zn(1)-N(3)A
O(1)-Zn(1)-N(6)B
N(3)A-Zn(1)-N(6)B
Ni(1)-O(1)A
2.1025(13)
2.1025(13)
2.1410(16)
180.0
Ni(1)-N(1)A
2.1410(16)
2.0909(15)
2.0908(15)
89.62(6)
90.38(6)
90.01(6)
90.01(6)
89.99(6)
89.99(6)
180.0
Ni(1)-O(1)
Ni(1)-N(3)B
Ni(1)-N(1)A
Ni(1)-N(3)
O(1)-Ni(1)-O(1)A
O(1)-Ni(1)-N(1)
O(1)A-Ni(1)-N(1)
O(1)-Ni(1)-N(1)A
O(1)A-Ni(1)-N(1)A
N(1)-Ni(1)-N(1)A
N(3)B-Ni(1)-O(1)A
N(3)-Ni(1)-O(1)A
3
N(3)B-Ni(1)-O(1)
N(3)-Ni(1)-O(1)
N(3)-Ni(1)-N(1)
N(3)B-Ni(1)-N(1)A
N(3)B-Ni(1)-N(1)
N(3)-Ni(1)-N(1)A
N(3)-Ni(1)-N(3)B
63.50(5)
116.50(5)
116.50(5)
63.50(5)
180.00(3)
90.38(6)
89.62(6)
Cu(1)-O(3)
1.944(4)
1.944(4)
180.0
Cu(1)-N(3)
1.976(4)
1.976(4)
89.11(15)
89.11(15)
180.0
Cu(1)-O(3)A
Cu(1)-N(3)A
O(3)-Cu(1)-O(3)A
O(3)A-Cu(1)-N(3)
O(3)-Cu(1)-N(3)A
O(3)-Cu(1)-N(3)
O(3)A-Cu(1)-N(3)A
N(3)-Cu(1)-N(3)A
90.89(15)
90.89(15)
Symmetry transformations used to generate equivalent atoms:
For 1: A: -x + y + 2/3, −x + 4/3, z + 1/3; B: y-1/3, −x + y + 1/3, −z + 4/3
For 2: A: x + 1/2, −y + 1/2, z + 1/2; B: -x + 2, −y, −z + 1
For 3: A: -x, −y, −z + 1
structure, in which the [NiO2N4] acts as a node, and
the 2,6-bip ligand acts as a linker (Figure 2b).
to form a 1D chain along the c-axis with the Cu …
Cu distance of 11.002 Å and the Cu … Cu … Cu
bond angle of 180 (Figure 3b). In tfbdc, the car-
ꢀ
Structure of 3. Coordination polymer 3 is a 1D chain
structure, belonging to the triclinic system, and the
space group Pī. The asymmetric unit of 3 contains
one Cu (II) ion, one tfbdc ligand, two 2,6-bipH
ligands, and two free H2O molecules. As shown in
Figure 3a, Cu (II) ion adopts a [CuO2N2] parallelo-
gram coordination geometry by Cu dsp3 hybrid
orbitals. The Cu (II) ion is surrounded by two O
atoms (O3, O3A) from two tfbdc ligands and two N
atoms (N3, N3A) from two 2,6-bipH ligands. The
Cu-O and Cu-N bond lengths are 1.944(4) and
1.976(4) Å, respectively. The O-Cu1-O/N-Cu1-N
boxyl group on the benzene ring is twisted, with a
dihedral angle of 54.12ꢀ between the carboxyl plane
and the benzene ring. The 2,6-bipH as a mono-
dentate ligand adopts the μ1:η1η0η0 coordination
mode, and only one N atom on the imidazole ring
coordinate with Cu (II) ion. The hydroxyl group of
the 2,6-bipH ligand is not deprotonated. The dihe-
dral angle of the imidazole ring and the pyridone
ꢀ
ring is 6.49 . The H atom on the imidazole ring of
the 2,6-bipH ligand forms a hydrogen bond with
the F atom of tfbdc. The C8-H8 … F2 hydrogen
bond distance is 3.1998(7) Å, and the bond angle is
121.417(2) ꢀ. A hydrogen bond O1-H1 … O1A is
also formed between the hydroxyl groups of the
ꢀ
bond angle is 180.0 , and the O-Cu1-N bond angle
range is 89.11(15) ꢀ~90.89(15) ꢀ. Unlike complexes
1 and 2, [CuO2N2] acts as a node while the tfbdc
ligand acts as linker in 3. The tfbdc ligand bridges
Cu (II) ions by a μ1:η1η0/μ1:η1η0 coordination mode
2,6-bipH ligands with the hydrogen bond distance
ꢀ
of 2.6623(6) Å and the bond angle of 85.58 (2)
.

6 of 13
LI ET AL.
adopt different constructions, tetra-coordinated Zn,
hexa-coordinated Ni, and tetra-coordinated Cu. The
coordination number of Ni is greater than that of Zn
and Cu, which can be ascribed to the larger radius of Ni
than those of Zn and Cu. Zn/Ni/Cu use different hybrid
orbitals, sp3 for Zn, d2sp3 for Ni, and dsp2 for Cu to form
[ZnO2N2] tetrahedron, [NiO2N4] octahedron, and
[CuO2N2] parallelogram, which acts as the single-metal
ion “node” in structure. The linker 2,6-bipH has three
coordination atoms, which can adopt different coordina-
tion modes, namely μ2:η1η1η0 in 1, μ2:η1η1η1 in 2, and
μ1:η1η0η0 in 3. However, the H2tfbdc ligand only coordi-
nates with Cu (II) ion in 3, H2tfbdc ligand may play the
role of template in the synthesis of 1 and 2. We can con-
clude that differences in the constructions of center
metal and ligand coordination modes result in the for-
mation of different dimensional structures, e.g. 3D
framework (1), 2D network (2), and 1D chain (3) struc-
tures. The presence of hydrogen bonds in 3 leads the for-
mation of high-dimensional supermolecular structure. It
is interesting to note that the high-dimensional coordi-
nation polymer 1 features a micropore with the pore vol-
ume of 14.2% (based on PLATON calculation).
3.2 | Thermal stability of coordination
polymers 1–3
The phase purity of 1, 2 and 3 were confirmed by pow-
der X-ray diffraction (PXRD) analysis. The powder X-
ray diffraction (PXRD) patterns of synthesized samples
1–3 fit well with the simulation plots, indicating that
all coordination polymers are in pure phase (Figure 4
and Figure S1). In order to measuring their thermal sta-
bility, samples 1–3 were subjected to thermogravimetric
analysis (TGA) at a heating rate of 10 ꢀC•min−1. As
shown in Figure S2, the TGA curves of coordination
polymers 1 and 2 below 275 ꢀC and 300 ꢀC, respec-
tively, are substantially stable, indicating that 1 and
ꢀ
ꢀ
2 are stable under 275 C for 1 and 300 C for 2. When
the temperatures further rise form 275 ꢀC for 1 and
300 ꢀC for 2, the frameworks of 1 and 2 begin to
decompose. The TGA curves show that the total loss is
72.32% for 1 and 79.84% for 2. The final residuals of
1 and 2 after thermal decomposition are ZnO (calcu-
lated value: 79.00%) for 1 and NiO (calculated value:
80.30%) for 2. For 3, the first loss is observed on the
FIGURE 1 The structure of 1: (a) the coordination
environment of the Zn (II) ion; (b) Left-type Zn-(2,6-bip) helical
chain (left) and Right-type Zn-(2,6-bip) helical chain (right), (c) 3D
framework
The hydrogen bonds connect the 1D chains to con-
struct a 3D supramolecular structure (Figure 3c).
ꢀ
TGA curve between the temperature of 100 and 150 C,
and the weight loss is about 5.81%, corresponding to
the loss of water molecules (calculated value: 5.47%).
The TGA curve starts a second drop at 190 C with the
total loss of 87.74%, indicating that the final residual is
CuO (calculated value: 87.84%).
Discussions. The three synthesized coordination
polymers show different structures from 3D to 1D,
which are resulted from the different single-metal ion
“node” and “linker” coordination modes. Zn/Ni/Cu
ꢀ

LI ET AL.
7 of 13
FIGURE 2 The structure of 2: (a) the
coordination environment of the Ni (II) ion;
(b) 2D network
3.3 | Luminescence properties
As Zn-CP with d10 electronic construction often shows
interesting luminescence properties, while Cu (II) and Ni
(II)-CPs with open shells often show quenching lumines-
cence.[3] So, the luminescence properties of 1 and free
2,6-bipH ligand were studied (Figure 5). The spectrum
shows that 2,6-bipH ligand exhibits a broad emission
band with the maximum emission intensity at 428 nm
upon the excitation at 352 nm. The spectrum of 1 displays
strong blue emission band from 375 to 500 nm upon the
excitation at 340 nm. The maximum emission is at
412 nm, which is close to that of the free 2,6-bipH ligand.
The center Zn (II) with d10 electronic configuration
couldn't accept π electron easily.[48] Therefore, the blue
emission of 1 can be attributed to the π* ! π transition
of the 2,6-bipH ligand. Compared with 2,6-bipH ligand,
1 shows much stronger emission and a significant blue
shift of 16 nm for maximum emission. Emission intensi-
ties of the free ligand and ligand within the coordination
FIGURE 4 The PXRD patterns of coordination polymer 1
polymer are different significantly. Additionally, the
ligand within the coordination polymer may enforce a
molecular conformation of the ligand, resulting in an
FIGURE 3 The structure of 3:
(a) the coordination environment of the
Cu (II) ion; (b) 1D chain structure;
(c) 2D supramolecular structure and the
hydrogen bonds

8 of 13
LI ET AL.
increase in the non-radiation deactivating effect, and fur-
ther contribute to stronger emission.[3,48–50] The good
luminescence properties of 1 makes it possible in the field
of luminescence detection.
excitation wavelength of 340 nm. As shown in Figure 6,
an emission band of 1 is observed in all suspensions, and
the maximum value is at 412 nm with relatively high
intensity, indicating that 1 maintains good luminescence
properties in these solvents. When coordination polymer
1 is dispersed in ethanol, the strongest luminescence can
be obtained, compared with all other suspensions. This
means that 1 has potential luminescence detection appli-
cation in solvent systems.
3.4 | Luminescence properties of 1 in
solvents
When coordination polymer 1 is immersed in water or
ethanol, the powder X-ray diffraction (PXRD) patterns
still fit well with the simulation plot, which indicates
1 has good chemical stability in solvents (Figure 4). The
luminescence properties of 1 in common solvents are fur-
ther discussed. Coordination polymer 1 powder sample
(3 mg) was dispersed in solvents (3 ml) to make suspen-
sions by ultrasonication, where the used solvents include
ethanol, acetonitrile, water, dimethyl sulfoxide (DMSO),
acetone, N,N-dimethylformamide (DMF), trichloro-
methane and methanol. The luminescence spectra of
these suspensions are obtained at the maximum
3.5 | Luminescent sensing
Considering the good luminescence properties and stabil-
ity of 1, it can be used as a luminescent sensor. To test the
capability of 1 in sensing different antibiotics, its lumines-
cence properties were tested in ethanol solution with the
different antibiotics, including ciprofloxacin (CX),
pefloxacin (PX), levofloxacin (LX), falofloxacin (FX), and
enoxacin (EX) (Figure S3). The emission spectra of sus-
pensions (3 mg 1 in 3 ml 0.01 M ethanol solution of the
different antibiotics) were measured at the excitation
wavelength of 340 nm. As shown in Figure 7, different
antibiotics have different degrees of influence on the
luminescence intensity of 1, compared with the blank
ethanol suspension. The emission of all suspensions is
reduced. The luminescence quenching efficiency of 1 by
FX, LX, PX and CX is 34.05%, 61.74%, 76.02%, and
82.47%, respectively. And the position of maximum emis-
sion shows a red shift with different degree. It is worth
noting that the luminescence quenching efficiency of
1 by EX is 88.20%. Therefore, the detection result of EX is
better than those of other antibiotics. Thus, coordination
polymer 1 can be used as the luminescence sensor to
detect EX. To further explore the quenching effect of
luminescence of 1 with EX, luminescence titration experi-
ment was carried out by adding EX step-by-step to the
coordination polymer 1 ethanol suspension, and the
emission intensity at 412 nm was monitored. When the
FIGURE 5 The solid state emission spectra of 1 and the free
ligand
FIGURE 6 Emission spectra
(a) and the transition intensity (b) of
1 dispersed in different solvents (λex
=340 nm)

LI ET AL.
9 of 13
TABLE 3 Comparison of other methods for detection EX
Method
HPLC
LOD
Ref
1.09 μg•L−1
40 μg•L−1
[32]
[34]
[33]
[31]
[30]
UHPLC
RLS-HPLC
ECL
5.1 ng•mL
0.02 μg•mL−1
3.2 × 10−7 mol•L−1
ECL
detection limit is 3.61 × 10−5 M. The results show that
1 can be used as a luminescent sensor to detect EX.
3.6 | Luminescence quenching
mechanism
FIGURE 7 Luminescent spectra of 1 upon the addition of
different antibiotics (CXEXFXLXPX, 10–2 M) ethanol solutions
In order to clearly understand the effect of EX on the
luminescence of 1, the possible quenching mechanisms
were explored. Firstly, the simulated PXRD patterns and
the one after being immersed in EX fit well (Figure 4).
The result indicates that the 3D structure of 1 remains
intact when adding EX. Therefore, the luminescence
quenching of 1 shows no obvious connections to the col-
lapse of the framework.[52] In order to further explore the
quenching mechanism of 1 in the presence of EX, the
UV–vis absorption spectrum of the EX solution was mea-
sured. The absorption spectrum of EX solution and the
emission spectrum of the 1 are compared. It could be
seen that they have no overlaps (Figure 9). Therefore, the
energy transfer 1 to EX is excluded. It is observed by the
UV–vis absorption spectrum of EX that there is strong
absorption at 262 nm and 336 nm. It is worth noting that
the absorption of EX at 336 nm coincides with the excita-
tion spectrum of 1 partially. The reason for luminescence
quenching may be that the absorption of EX at 336 nm
inhibits the absorption of excitation energy of 1. To verify
this mechanism, the effect of EX on the luminescence of
1 was examined at an excitation wavelength of 380 nm
(which is out of the range of EX absorption). The results
show that the luminescence quenching also occurred
volume of EX ethanol solution(10−3 molꢁL−1) increases
from 0 to 270 μL, it can be observed that the emission
intensity of 1 is gradually reduced (Figure 8a). The lumi-
nescence quenching efficiency can be analyzed by the
Stern−Volmer (SV) equation I0/I-1 = KSV[A][51] (KSV, the
quenching constant; [A], the molar concentration of EX;
I0 and I, the luminescence intensities before and after the
addition of EX, respectively). As depicted in Figure 8b,
the SV plot is almost linear, and the calculated KSV is
4.2259 × 104 M−1 with a linear fit coefficient value of
0.9903. Moreover, the detection limit (3σ/KSV) of 1 toward
EX is calculated (σ: standard error for three repeated
luminescence measurements of blank solution,
σ = 0.509) to be 3.61 × 10−5 M.
The sensing performances of other method toward
EX are listed in Table 3. These methods, including HPLC
(high
performance
liquid
chromatography)/ECL
(Electrochemiluminescence)/RLS-HPLC (high perfor-
mance liquid chromatography technique incorporating
resonance light scattering)/FL-CPs, can be used to detect
EX. Remarkably, 1 is represented to be the first CPs-
based luminescence sensor for EX detection. The
FIGURE 8 (a) Luminescent spectra of
1 ethanol solutions with different
concentrations of EX (λex = 340 nm);
(b) The sterm-volmer plot of 1 for EX
(λex = 340 nm)

10 of 13
LI ET AL.
TABLE 4 The LUMO and HUMO energies of the
fluoroquinolones
CX
EX
PX
FX
LX
HOMO/ev
LUMO/ev
−3.399
−2.305
−5.77
−1.438
−5.780
−1.399
−5.631
−1.421
−5.398
−1.278
transfer from LUMO orbit of 2,6-bipH ligand to the
LUMO orbit of EX much easier. Therefore, the lumines-
cence quenching occurs. Therefore, both the PET process
and the absorption of EX at excitation wavelength
(336 nm) contribute to the luminescence quenching of 1.
Discussion. Coordination polymers 1–3 are the first
synthesized complexes containing the 2,6-bipH ligand,
and such complexes are not reported in the literatures.
Several Zn/Ni/Cu-complexes with polycarboxylic acid
and/or N-donor ligands have been synthesized by our
FIGURE 9 Excitation and emission spectra of EX and UV–vis
absorption spectrum of EX
laboratory, such as 3D [Zn (dpdc)(btb)0.5 n
]
and
(Figure 10). The KSV of 1.4759 × 103 M−1 is calculated,
which is less significant than upon 340 nm excitation
(KSV = 4.2259 × 104 M−1). The difference in quenching
constant indicates that the absorption of EX at the excita-
tion wavelength partially affects the luminescence
quenching of 1.[53]
[Cu3(dpdc)3(btb)2]n with [ZnO4N] and [Cu1O5]/
[Cu2O4N2] polyhedra, respectively[57]; 3D [Cu2(O-cpia)
(btb)0.5•(OH)], 3D [Cu3(O-cpia)2(bpy)2], 2D [Ni2(O-cpia)
(phen)•(OH)•H2O]•2H2O
cpia)2(bpy)3•2H2O]•2H2O
and
with
3D
[Ni3(O-
[Cu1O4N]/[Cu2O5],
Another possible mechanism for the luminescence
quenching is photoinduced electron transfer (PET).[54–56]
Since the luminescence of 1 can be attributed to the
π* ! π transition of the 2,6-bipH ligand, the lumines-
cence quenching may be due to the PET between
2,6-bipH ligand and EX. When the lowest-unoccupied-
molecular-orbital (LUMO) energy level of the
fluoroquinolones is lower than that of 2,6-bipH ligand,
electrons can be transferred from the LUMO of the
2,6-bipH ligand to the LUMO of fluoroquinolone antibi-
otics upon excitation to cause luminescence quenching.
To explore the conjecture, the highest-occupied-molecu-
lar-orbital (HOMO) and LUMO of the fluoroquinolones
was calculated (Table 4). The calculated LUMO energy of
2,6-bipH ligand (−1.1 eV) is higher than that of EX
(−1.437 eV) (Figure 11), which makes the electron
FIGURE 11 The calculated LUMO and HUMO energy of
2,6-bipH ligand and EX
FIGURE 10 (a) Luminescent
spectra of 1 in ethanol solution with
different concentrations of EX
(λex = 380 nm); (b) The sterm-volmer
plot of 1 for EX (λex = 380 nm)

LI ET AL.
11 of 13
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[Cu1O3N2]/[Cu2O4], [Ni1O4N2]/[Ni2O6], and [Ni1O4N]/
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of different metal ion (Zn (II), Cu (II) and Ni (II)) config-
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Zn-complexes exhibit ligand-based blue luminescence
and can be used as luminescent sensors, reflecting the
luminescent sensing ability for different analytes, such as
for Fe3+ and Tb3+ ion. Coordination polymers 1–3 have
different structures from these complexes. Especially,
coordination polymer 1 can detect enoxacin in ethanol
solution with a low detection limit.
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4 | CONCLUSIONS
In summary, three coordination polymers, [M(2,6-bip)2]
(M = Zn (1), Ni (2)) and [Cu (tfbdc)(2,6-bipH)2]•2H2O
(3), have been synthesized and characterized. The tfbdc
ligand is bound to Cu ions in 3, whereas it is
uncoordinated to Zn in 1 and Ni in 2, tfbdc ligand may
play the template role in the synthesis of 1 and 2. The
2,6-bip or 2,6-bipH ligand adopts different coordination
modes in the three coordination polymers. The different
polyhedral units of metal ions (tetrahedron for Zn, octa-
hedron for Ni, and parallelogram for Cu) are linked by
ligands to form the diverse structures of the coordination
polymers, 3D architecture (1), 2D network (2), and 1D
chain (3). Interestingly, 1 exhibits high emission in etha-
nol and can be used as a luminescent sensor for the
detection of EX in ethanol solution. The detection limit is
3.61 × 10−5 M. The sensing mechanisms may be attrib-
uted to the PET process and the absorption of EX at
excitation.
[21] Z. W. Zhai, S. H. Yang, M. Cao, L. K. Li, C. X. Du, S. Q. Zang,
Cryst. Growth des. 2018, 18, 7173.
[22] G. C. Liu, Y. Li, J. Chi, N. Xu, X. L. Wang, H. Y. Lin,
Y. Q. Chen, Dyes Pigm. 2020, 174, 108064.
ACKNOWLEDGMENTS
[23] Y. D. Farahani, V. Safarifard, J. Solid State Chem. 2019,
275, 131.
The authors are grateful to the Key project of science and
technology plan of Beijing Education Commission
(KZ201910028038), the National Natural Science Foun-
dation of China (21471104), and Laboratory Open Project
Fund of Capital Normal University (LIP18009S047).
[24] F. Cheng, R. Fu, Y. Wen, Y. Y. Yang, C. Zeng, Y. Zhang,
S. Hu, X. Wu, J. Mater. Chem. C 2018, 6, 12341.
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CONFLICT OF INTEREST
There are no conflicts to declare.
[27] J. Lu, X. H. Xin, Y. J. Lin, S. H. Wang, J. G. Xu, F. K. Zheng,
G. C. Guo, Dalton Trans. 2019, 48, 1722.
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ORCID
Xia Li https://orcid.org/0000-0002-1110-0117
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How to cite this article: Li S, Zhou X, Xu Q,
Liu D, Cui R, Li X. Transition Metal–organic
Coordination Polymers Containing 6-
(1H-imidazol-1-yl)-2(1H)-Pyridinone: Synthesis,
Structure and Fluorescent Sensing for Enoxacin.
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