Two Cu(I\II) Coordination Polymers for Photocatalytic Degradation of Organic Dyes and Efficient Detection of Fe3+ Ions

In this work, two new copper-based coordination polymers (CPs), {[CuI(H2PO4)(dpe)]5(dpe)0.5(H2O)7}n (Cu-dpe(I)) and {[CuII(HPO4)(dpe)(H2O)](H2O)3}n (Cu-dpe(II)) (dpe = 1,2-di(4-pyridyl)ethylene), had been successfully constructed by hydrothermal method and solvent evaporation method, respectively, and their crystal structures were determined by single crystal X-ray diffraction. The differences of structure and properties between these two coordination polymers were discussed. The synthetic temperature caused their structure differences. They both exhibit excellent photocatalytic activity toward the degradation of methylene blue (MB) under visible light. The degradation rates of Cu-dpe(I) and Cu-dpe(II) to MB in 120 min under visible light were 88.1% and 97.2%, respectively. Some active free radicals such as ·OH, ·O2−, e− played an important role in degradation of MB. In addition, luminescent experiment revealed Cu-dpe(I) exhibits a relatively high sensitive and selective detection of Fe3+ ions in water solution via fluorescence quenching which was caused by competitive absorption of excitation wavelength energy between Fe3+ and Cu-dpe(I).


Introduction
Coordination polymers (CPs) are a kind of functional material which are composed of metal ions and organic ligands, and they can be tuned at the molecular level [1]. CPs have attracted much attention in recent years because of a variety of topology structures and broad application prospects such as gas adsorption/separation [2,3], catalysis [4,5], sensor [6,7] and so on. The design and assembly of some new functional materials is also an important research branch, especially the coordination polymers with d 10 metal ions and conjugated organic ligands are expected to be used as photosensitive materials and have potential applications for photochemistry [8]. Thus, further construct CPs with new functionality are expected.
Over the years, industrialization and urbanization have led to the release of large amounts of organic pollutants into the environment [9]. Organic dyes are one of the common harmful pollutants in sewage, so they must be removed from the wastewater prior to discharge [9][10][11]. Among many degradation methods, photocatalysis of organic dyes is an economical and effective method without secondary pollution [11]. At the same time, CP is a semiconductor with an adjustable band gap and is commonly used as a photocatalysis for the degradation of organic dyes [12]. For example, Cui and co-workers reported two Co(II) coordination polymers using rigid and semi-rigid double (imidazole) ligand showed 91.4% degradation efficiency of methylene blue (MB) within 120 min under UV-vis irradiation [13]. Yang's group fabricated two 3D supramolecular Cd(II) coordination polymers based on aromatic polycarboxylate and semi-rigidity bis(imidazole) ligands that can degrade 85% of MB within 120 min [14].
In addition, the detection of wastewater contamination is another challenge for people. Metal ions play important roles in life and environment, and Fe 3+ acts as a doubleedged sword. Fe 3+ can play an important role in transmitting oxygen and participating in human metabolic activities [15][16][17]. However, deficiency or excess of Fe 3+ ions can lead to a variety of serious dysfunction, including skin diseases, anemia, and decreased immunity [11]. In recent years, luminescent coordination polymers (LCPs) applied as chemical sensors for the detection of Fe 3+ have been investigated extensively [18,19]. Synthesis of LCPs with the exposed Lewis basic sites is a common strategy for detection of metal ions [20][21][22]. This strategy, however, could lower the selectivity toward a special metal ion. This is because various metal ions as Lewis basic acids could interact with the Lewis basic sites of LCPs and thus lower the selectivity [22]. An overlap mechanism, the degree of overlap between the emission spectrum of the material and the fluorescent probes, could effectively improve the selectivity of LCPs. However, most of the reported LCPs have been prepared from noble or rare earth metals [23,24]. Therefore, it is necessary to develop inexpensive, highly sensitive and selective detection for Fe 3+ ions.
In this work, dpe was used as a bridge ligand to construct two different Cu coordination polymers. We successfully synthesized {[Cu I (H 2 PO 4 )(dpe)] 5 (dpe) 0.5 (H 2 O) 7 } n (Cu-dpe(I)) by hydrothermal method and {[Cu II (HPO 4 ) (dpe)(H 2 O)](H 2 O) 3 } n (Cu-dpe(II)) by mechanical milling of CuO, H 3 PO 4 and dpe in the ratio of 1: 1: 1. The crystal structures were determined by single-crystal X-ray diffraction analysis. Organonitrogen species typically behave not only as ligands but also as a reducing agent in the presence of Cu(II) under hydrothermal conditions. Therefore, the starting reagent of Cu(II) (CuO) was reduced to Cu(I) by dpe under 140 °C. This is similar to that of previously reported Cu(I) complexes [25], while the reduction cannot occur if the reaction is at room temperature. More importantly, Cudpe(I) and Cu-dpe(II) can be acted as photocatalysts that can effectively degrade MB at room temperature, and the possible degradation mechanism is further discussed. In addition, Cu-dpe(I) can be used for sensing Fe 3+ ions in aqueous solution by fluorescence quenching, with high selectivity and sensitivity.

Materials
CuO, dpe, and phosphoric acid (H 3 PO 4 , 85%) were purchased from Sinopharm Chemical Reagent Co., Ltd., (Shanghai, China). All of the chemicals and solvents used in the synthesis were analytical grade and used without further purification.

Synthesis of the Cu-dpe(II)
CuO (0.796 g, 10 mmol), dpe (1.820 g, 10 mmol) and H 3 PO 4 (85%, 670 μL, 10 mmol) were put into a 10 mL Teflon jar with two steel-cored 10 mm Teflon balls. The mixture was ground for 30 min in a GT200 grinder mill operating at 1800 rpm. The powder obtained was soaked in 20 mL water, kept at room temperature and grown for a month to give blue, block-like crystals and blue clarified solution (yield: 72%).

Photocatalytic Degradation of MB
Photocatalytic experiments were carried out in a multichannel photochemical reaction system (PCX50C, Beijing Perfectlight, China) with one 5 W LED lamp as a white-light source. Photocatalyst (25 mg) was added to a quartz glass bottle containing 50 mL of MB solution (10 ppm) [13,26,27]. First, the dark reaction was carried out for 30 min to reach the adsorption equilibrium to avoid the influence of material adsorption on the experiment. After the dark reaction, H 2 O 2 (200 μL) was added to the aqueous solution and the light source was turned on for the photolysis reaction for 120 min. Samples were taken every 20 min, and the residual concentration of MB solution was measured by UV-vis spectrophotometer at 664 nm excitation.

Luminescence Sensing Experiments
First, sample (3 mg) was dispersed in 8 mL of deionized water, and the sample was uniformly dispersed by ultrasound for 2 h. After standing for 2 days, the liquid fluorescence of the sample was measured as a blank experiment. Then, 200 μl M(NO 3 ) x (x is the charge number for each metal ion) solution (1 × 10 -3 mol L −1 , M = K + , Na + , Pb 2+ , Cr 3+ , Cu 2+ , Sr 2+ , Ni 2+ , Zn 2+ , Co 2+ , Cd 2+ and Fe 3+ ) was added dropwise to the sample solution to measure the fluorescence intensity.
All the liquid fluorescence was tested at the excitation wavelength of 381 nm.

Physical Measurements
The powder X-ray diffraction (PXRD) measurements were recorded on a Rigaku RINT 2200 Ultima diffractometer at 40 kV, 30 mA with a Cu-target tube (Cu Kα radiation, λ = 1.5418 Å) at room temperature. The thermogravimetry analysis (TGA) experiments were performed on a Rigaku TG8120 instrument from room temperature to 500 ℃ under flowing N 2 with a heating rate of 10 K min −1 . Sample preparations for the TGA were carried out under air. The Infra-red (IR) Spectroscopy was obtained using a Nicolet ID5 ATR operating in wavelengths of 4000-400 cm −1 at room temperature. Raman spectra of the samples were obtained by using a Renishaw inVia Micro-Raman spectrometer. Scanning electronic microscopy (SEM) images were carried out using Hitachi FESEM-4800, and the complex was dispersed in ethanol by ultrasonication for 30 min, then dripped onto the conducting substrates and dried under ambient conditions for SEM measurements. Ultraviolet-visible (UV-vis) absorption spectra was recorded by using a UV-2600 UV-vis spectrometer with BaSO 4 as reflectance scaffold. The fluorescence spectra were performed on a Hitachi F-7000 fluorescence spectrophotometer at room temperature. Single crystal X-ray diffraction measurements were performed with a Rigaku AFC10 diffractometer with Rigaku Saturn Kappa CCD system equipped with a MicroMax-007 HF / VariMax rotating-anode X-ray generator with confocal monochromated MoKα radiation. Data were processed by a direct method (SIR97) [28] and refined by full-matrix least-squares refinement using the SHELXL-2018 computer program [29]. The hydrogen atoms were positioned geometry and refined using a riding model. The deposited number of Cambridge Crystallographic Data Centre (CCDC) is 912031 (293 K) for Cu-dpe(I) and 2,117,855 (173 K) for Cu-dpe(II). A summary of the related crystallographic data, the main bond length and angle of Cu-dpe(I) and Cudpe(II) was summarized in Table S1.

Characterization of Coordination Polymers
Single-crystal X-ray diffraction analysis indicates Cudpe(I) belongs to the monoclinic crystal system with space group C2/c (Table S1). The crystal structure of Cu-dpe(I) contains Cu(I) metal centers, dpe ligands, and monovalent H 2 PO 4 − anions. In a triangular coordination geometry, each Cu(I) center is coordinated with two nitrogens from the dpe ligand and one oxygen from the phosphate group, as shown in Fig. 1a. Each dpe ligand bridges two Cu + ions equatorially to build the extended one-dimensional (1D) chain. The crystal structure of Cu-dpe(II) contains Cu(II) metal centers, one dpe ligand, one HPO 4 2− anion and one water molecule. Cu-dpe(II) belongs to the monoclinic crystal system with space group C2, the metal center Cu is the chiral center. Cu(II) ion adopts five-coordination geometry, each Cu(II) center is coordinated with two nitrogen atoms from two dpe ligands and three oxygen atoms from two phosphate groups and a water molecule, as shown in Fig. 1c. The whole structure is extended into a two-dimensional layered structure. The layers are filled with guest water molecules. These water molecules stabilize the layered structure of the whole molecule by piling up with each other. The PXRD patterns of Cu-dpe(I) and Cu-dpe(II) at room temperature are identical to the simulated pattern from the single crystal structure (Fig. 2a). Cu-dpe(I) is an orange, rod-shaped crystal, and Cu-dpe(II) is a green block-like crystal, as shown in the inset of Fig. 2b and 2c, respectively. Thermogravimetric analysis (TGA) profile of Cu-dpe(I) shows no clear weight loss below 200 °C but a gradual decrease in weight above 200 °C, as shown in Fig. S2. The first ca. 6% weight loss is due to the release of guest water molecules (calcd. 6.53%) from 80 to 200 °C. The second weight loss from 250 to 310 °C is around 26%, which is attributed to the absence of free dpe ligands in the structure. The thermal stability of Cu-dpe(II) is worse than that of Cu-dpe(I). From 64 °C to 95 °C, a weight loss of about 17%, which is mainly attributed to the removal of the guest water molecules (calcd. 13.06%) and coordinate water molecules (calcd. 4.35%). When the temperature rises to 150 °C, the main framework of the compound gradually collapses. Infra-red (IR) and Raman spectra of Cu-dpe(I) and Cu-dpe(II) were shown in figure S3. In IR, the characteristic band shown at 1073 cm −1 is confirmed by the antisymmetric and symmetric Cu-O(P) stretching, while the stretching vibrations of P-OR group is in the range of 1000 to 400 cm −1 [30,31]. The stretching vibrations of C-N on the aromatic Fig. 1 a The 1D chain structure of Cu-dpe(I) view along z axis. b The 3D structure of Cu-dpe(I). c The 3D structure of Cu-dpe(II). Cu(I), Cu(II), P, N, O and C atoms are shown in brown, dark blue, yellow, light blue, red, and gray, respectively. H atoms have been omitted for clarity. Guest molecules were also omitted Fig. 2 a PXRD patterns of Cudpe(I) and Cu-dpe(II). b The SEM and the photograph (inset) of single crystal Cu-dpe(I). c The SEM and photograph (inset) of single crystal Cudpe(II) ring of dpe is at 1583 and 1420 cm −1 [32]. A strong Raman band located at 1635 cm −1 is observed, which is the stretching vibrations of v(C = N) [33].
Complexes with a d 10 metal center have attracted extensive interest due to their excellent photoluminescence properties. In addition, N-pyridyl ligands form a charge transfer interaction with the metal center relatively easily due to the presence of both n-and π-electrons [34,35]. UV-vis absorption spectra of Cu-dpe(I) and Cu-dpe(II) in the wavelength of 200 to 1000 nm were observed (Fig. S4). The dpe has a wide absorption band from 200 to 420 nm, which is attributed to the π-π* transition of large conjugated π-electron systems [36]. There is a broad metal-to-ligand charge transfer (MLCT) (d → π*) in the visible region around 430 nm [37]. As a result, Cu-dpe(I) exhibits a broader absorption band and a significant red-shift compared to the free dpe ligands. The absorption band of Cu-dpe(II) extends to the near infrared (NIR) range compared with the dpe ligand, and the complex has better light absorption performance in the range of 200-1000 nm. The adsorption band of Cudpe(II) shows a significant blue shift compared with that of the ligand, which is different from that of Cu-dpe(I). This can be attributed to Cu(II) framework altering the inherent electronic structure of dpe or internal charge transfer of dpe (n → π* and π → π*) [38]. In addition, band gaps of approximate 2.05 and 3.03 eV for Cu-dpe(I) and Cu-dpe(II) respectively are obtained from the Tauc plot, as shown in Fig. S4b. CPs with a band gap from 1.91 eV to 3.15 eV are appropriate candidates as photocatalysts [39,40]. Therefore, Cu-dpe(I) and Cu-dpe(II) can be considered as ideal photocatalytic materials.
We investigated the solid-state fluorescence of Cudpe(I), Cu-dpe(II), and dpe ligand at room temperature to further understand their fluorescence properties. The maximum emission peaks are observed at 366 nm for dpe ligand (λ ex = 332 nm) while 388 nm for Cu-dpe(I) (λ ex = 247 nm) and 399 nm for Cu-dpe(II) (λ ex = 247 nm) (Fig. S5). In contrast to the dpe ligand, Cu-dpe(I) shows a red-shift (22 nm) and Cu-dpe(II) shows a red-shift (33 nm) while they also show an intense emission intensity. The red shift of the emission peak may be due to the metal-to-ligand charge transfer [41]. The intense emission intensity originates from the coordinated N-donor ligands to Cu(I/II) centers, which could enhance the conformational rigidity of the ligands in compound and thereby reducing the energy loss [19,42]. This effect thus results in higher fluorescence intensity in Cu(I/ II) complex.

Photocatalytic Degradation of MB
Based on the above characterizations, Cu-dpe(I) and Cudpe(II) are potential semiconductor catalytic materials. Since Cu-dpe(I) has d 10 metal center and conjugated organic bonding group, it is expected to be used as photosensitive materials. The surface areas of Cu-dpe(I) and Cu-dpe(II) are 1.290 m 2 /g and 1.317 m 2 /g respectively, which are quite limited. Therefore, the degradation of dye should be attributed to photocatalytic degradation rather than adsorption. Several studies have shown that photocatalytic degradation can be performed using non-porous CPs. Accordingly, we evaluated the photocatalytic degradation of MB for these two compounds. The change of MB photocatalytic degradation rate with reaction time under different conditions was studied (Fig. 3a) This indicates that the catalyst has good photocatalytic activity, and the degradation effect of Cu-dpe(II) is slightly higher than that of Cu-dpe(I).
In order to further compare the photocatalytic degradation efficiency, the kinetic behaviors of the photocatalysts were investigated, and the photodegradation reactions were all in accordance with the pseudo-first-order reaction kinetic model (Fig. 3b). The relationship between ln(C 0 /C) and time is shown in Fig. 3b. Among them, the rate constants K of degradation reactions of Cu-dpe(I) and Cu-dpe(II) containing H 2 O 2 to degrade MB were 0.0147 and 0.0286 min −1 , respectively, which were higher than those of the other three groups (MB, MB + H 2 O 2 , MB + catalyst). This indicates that the reaction rate of the catalyst with H 2 O 2 is higher than that of any other three components.
It seemed that the degradation mechanism should be associated with a photo-Fenton-like process. We did free radical trapping experiments to investigate the main actives in the photocatalytic reaction. (Fig. 3c). The main actives were removed by introducing CCl 4 (e − trapping agent), isopropyl alcohol (IPA, ·OH trapping agent), N 2 (·O 2− trapping agent), and EDTA (h + trapping agent). The results show that ·OH is the key active group for the degradation of MB for Cu-dpe(I). For Cu-dpe(II), ·OH plays a major role in the degradation process, ·O 2− plays the secondary role in the reaction rate, followed by e − has a slight promotion effect on the degradation reaction. H 2 O 2 is the precursor of ·OH, which is an effective and highly active oxidizing substance. Fig. 3 a Photocatalytic degradation of MB. b Kinetic fitting curve of degradation reaction. c Free radical trapping experiment of Cu-dpe(I) and Cu-dpe(II). d Photocatalytic reaction mechanism of the MB solu-tion on CPs. e Cycling catalytic degradation reaction for Cu-dpe(I) and Cu-dpe(II). f The PXRD patterns for photocatalyst before and after MB degradation In the photocatalytic process of coordination polymers, ultraviolet and visible light can induce the coordination polymers to produce oxygen or nitrogen-metal charge transfer by driving electrons from the highest occupied molecular orbital (HOMO) to the lowest unoccupied molecular orbital (LUMO), which strongly requires an electron to return to its stable state. Thus, H 2 O 2 or H 2 O captures an electron and then reduces to form ·OH which can effectively decompose MB [1]. While e − can be transferred to the surface of the catalyst to form ·O 2− with O 2 , and further capture H 2 O to form ·OH. Therefore, inhibition of e − and ·O 2− in Cu-dpe(II) will lead to poor degradation effect [1]. As shown in Fig. 3e, Cudpe(I) and Cu-dpe(II) retained effective MB degradation after three cycles. The complexes possess good structural stability after three cycles of MB photodegradation.

Luminescence Sensing Properties
Copper-based LCPs, including Cu(II) and Cu(I), are interesting alternative fluorescence complexes. Cu(II) absorbs light of the longer wavelengths in the visible to near-infrared spectral region [43], which indicates a low possibility of overlap mechanism for detection of metal ions [22]. Cu(I), on the other hand, has a d 10 electronic configuration and thus no d-d transitions. This could be effective to improve the selectivity of LCPs toward Fe 3+ through a possible overlap mechanism. However, there are rare examples of the Cu(I) complexes for the detection of Fe 3+ ions [44] and no reported with an overlap mechanism.
The above results clearly indicate Cu-dpe(I) is a promising photoluminescent CPs and thus maybe useful in the detection of metal ions. We first measured the fluorescence response of Cu-dpe(I) (λ ex = 381 nm) and Cu-dpe(II) (λ ex = 322 nm) to various solvents (Acetonitrile, Methanol, Ethanol, DCM, Acetone, DMF, THF, and H 2 O) (Fig. 4a,  4b). It shows that Cu-dpe(I) has an obvious fluorescence response to pure water. However, Cu-dpe(II) has no specific fluorescence response to many kinds of solutions. Therefore, it is not suitable for using as a fluorescent probe. Due to excellent fluorescence properties of Cu-dpe(I), we further explored the reaction of Cu-dpe(I) in aqueous solutions for the detection of metal ions. We measured the emission spectra of Cu-dpe(I) in aqueous solutions containing trace amounts of various metal ions. As shown in Fig. 4c, Fe 3+ exhibited a drastic quenching effect on the luminescence of Cu-dpe(I), while no significant influence was observed for other metal ions.
The concentration-dependent luminescence behavior of Fe 3+ ion was performed to further assess the sensitivity of Cu-dpe(I) to Fe 3+ . As shown in Fig. 4d, the fluorescence intensity of Cu-dpe(I) in the aqueous solution gradually decreased as the Fe 3+ concentrations increased from 20 to 200 μM. The emission was almost completely quenched when the concentration of Fe 3+ was up to 200 μM. The Stern-Volmer equation (I 0 /I = K sv [M] + 1) [45] was used to analyze the corresponding quenching coefficient of Fe 3+ . Here, I 0 is the original fluorescence intensity, I is the fluorescence intensity, and [M] is the molar concentration of Fe 3+ (μmol L −1 ). A linear relationship was observed when Fe 3+ ion ([M]) was at low concentrations (inset of Fig. 4e). As a result, the quenching constant K sv was estimated to be 1.25 × 10 4 M −1 , which is comparable to those of other reported Fe 3+ quenching coefficients (Table S3) [16,[46][47][48][49][50]. In addition, we measured the fluorescence behaviors of Cudpe(I) in both metal ions solutions of M n+ with and without Fe 3+ (M = K + , Na + , Pb 2+ , Cr 3+ , Cu 2+ , Sr 2+ , Ni 2+ , Zn 2+ , Co 2+ , and Cd 2+ ) to further understand the selectivity of Cudpe(I) towards Fe 3+ . As shown in Fig. S7, the fluorescence intensity of Cu-dpe(I) was sharply decreased when Fe 3+ ions were added into the solutions. Furthermore, a control experiment was conducted with M n+ /Fe 3+ mixed ions. These results indicate Cu-dpe(I) has excellent selectivity and sensitivity for Fe 3+ ions in aqueous solution.
Currently, the luminescent quenching of Fe 3+ ions can be attributed to the collapse of the frameworks, cation exchange, and competitive absorption between the Fe 3+ ions and the LCPs (overlap mechanism) [22,51]. As the PXRD results shown in Fig. S8, there are no changes in the crystal structure of Cu-dpe(I) after being immersed in the Fe 3+ ions solution for 24 h, suggesting the quenching is not related to the structural collapse. Moreover, it is hard for neutral LCPs to capture Fe 3+ ions through a cation exchange mechanism [52]. Therefore, the resonance energy transfer may be another mechanism for the quenching effect [53]. If the emission spectrum of the fluorophore (donor) has a certain degree of overlap with the absorption of the analyte (acceptor), resonance energy transfer from the donor to the acceptor can be observed [54]. As shown in Fig. 4f, a large overlap between the absorption spectrum of Fe 3+ ions solution and the excitation wavelength of compound Cu-dpe(I) is observed, while not for the other metal ion solutions. We thus confirm that this high selectivity for Fe 3+ ions is contributed by the competitive absorption.

Conclusions
In summary, two novel copper-based coordination polymers have been successfully synthesized and structurally characterized. We confirmed that divalent copper can be hydrothermally reduced to monovalent copper under 140 °C, with the coexistence of pyridyl ligand and phosphoric acid. More importantly, both of them show efficient photocatalytic activity for the degradation of MB, and Cu-dpe(II) can degrade MB up to 97.2% in 120 min under visible light. Some active free radicals such as ·OH, ·O 2− , e − play important roles in Fig. 4 a Emission spectra of Cu-dpe(I) to different solvents. The inset represents photographs of Cu-dpe(I) in pure methanol (3000 μL methanol, right) and methanol/water mixtures (300 μL water and 3000 μL methanol, left) (λ ex = 381 nm). b Emission spectra of Cudpe(II) to different solvents (λ ex = 322 nm). c Fluorescence quenching of Cu-dpe(I) in the presence of different metal ions in the water solutions (λ ex = 381 nm). d Fluorescence spectra of Cu-dpe(I) in a water solution with different Fe 3+ concentrations (λ ex = 381 nm, [Fe 3+ ] = 1 × 10 −3 mol L −1 ). e Stern-Volmer plot for Cu-dpe(I) with different concentrations of Fe 3+ in an aqueous solution. Inset is the linear correlation plot of Cu-dpe(I) at low Fe 3+ concentrations. f The UV-vis absorption spectra of various metal ions aqueous solutions and the excitation spectrum of Cu-dpe(I) degradation of MB. Moreover, Cu-dpe(I) also possesses a relatively high selectivity and sensitivity for Fe 3+ ions in aqueous solution. The experiments indicate that the competitive absorption of excitation wavelength energy between Fe 3+ and Cu-dpe(I) leading to fluorescence quenching. This metal complex, to the best of our knowledge, is the first example of Cu(I) LCPs for the detection of Fe 3+ ions with an overlapping mechanism. This work provides a promising approach for designing coordination polymers based on the Cu LCPs for MB degradation and Fe 3+ ion sensors.