Luminescent Zn(II) and Cd(II) coordination polymers based on naphthalene tetracarboxylic acid and 4,4'-bipyridine for sensing of nitrobenzene, Fe3+ and Cr2O7 2− ions

Abstract Two luminescent Zn(II) and Cd(II) metal-organic frameworks (MOFs) containing 1,4,5,8-naphthalene tetracarboxylic acid (H4L’) and 4,4'-bipyridine (4bpy), [ZnL(4bpy)0.5(DMA)]n (1) and [CdL(4bpy)0.5]n (2) (L = 1,8-monoanhydride-1,4,5,8-naphthalenetetracarboxylate, DMA = N,N-dimethyl acetamide), were synthesized and characterized through X-ray diffraction, IR, and UV-Vis spectroscopy. 1 shows a 1D linear chain structure, while 2 shows a 2D baluster structure. Both show multiple H bonding and π-stacking interactions. Both compounds show ligand-based luminescence in the range 400-500 nm with contributions from both ligands. The luminescence is strongly dependent on the presence of potentially coordinating molecules. Luminescence quenching experiments show good sensitivities in detecting nitrobenzene (NB), Fe3+ and Cr2O7 2−.


Powder X-ray diffraction studies
Based on samples (20.0 mg) of 1 and 2 of as-synthesized materials, after sensing of organic molecules, and after sensing of ions measured in air, the powder X-ray diffraction (PXRD) patterns were recorded on a Rigaku D/Max-2500 diffractometer, operated at 40 kV and 100 mA, using a Cu-target tube and a graphite monochromator. The intensity data were recorded by continuous scan in a 2h/h mode from 5 to 50 with a step size of 0.02 and a scan speed of 8 min À1 .

Single crystal XRD studies
The structures of 1 and 2 were determined by a Bruker APEX II CCD single crystal diffractometer at 293(2) K. Determination of unit cell parameters and data collection were performed with Mo-Ka radiation at a wavelength of 0.71073 Å using the x-scan technique. The structures were solved by direct methods and refined by full matrix least-squares on F 2 using SHELXS and SHELXL programs [32][33][34][35][36]. The metal atoms in each compound were located from the E-maps, and other non-hydrogen atoms were located in successive difference Fourier syntheses and refined with anisotropic thermal parameters on F 2 . The hydrogen atoms were added in calculated positions, riding on the attached atoms and refined with fixed thermal factors. The crystallographic data are listed in Table S1, and selected bond lengths are given in Table S2.

Syntheses and characterizations
Solvothermal synthesis methods have been universally used to construct diverse structures, though their mechanisms are not completely clear [37,38]. In particular, some in situ reactions, such as oxidation and reduction, substitution, hydrolysis and dehydration, are often observed in solvothermal reactions [37,38]. In this work, H 4 L' underwent a dehydration reaction and formed the H 2 L ligand (chart 1), which was found in 1 and 2. Compounds 1 and 2 both are stable in air. All the characterizations employed crystalline samples.

Single crystal structures
Compound 2 possesses an interesting 2D baluster structure composed of helix chains as posts and 4bpy as connectors ( Figure S1 and Table S3), which has been reported before [18]. Different from 2, in 1 each asymmetric unit includes one Zn II , one L ligand from a H 4 L' ligand via a dehydration reaction, half a 4bpy ligand and one coordinated DMA molecule. That is to say, each Zn II is coordinated by two O atoms [O1, O3D, symmetry code: D: -x, -y þ 1, -z þ 1] from two different L ligands in the l 1 -g 1 :g 0 and l 1 -g 1 :g 0 bis-monodentate coordination mode, one N atom [N1] from one 4bpy ligand and one O atom [O8] from one DMA molecule, showing a distorted tetrahedral coordination geometry (Figure 1a). Then the adjacent Zn II ions are connected through L bridges to result in a binuclear [ZnL 2 (DMA)] unit. Different from 2, because of existence of coordinated DMA molecules, the adjacent binuclear units are interlinked by 4bpy ligands to only form a 1D chain structure (Figure 1b). Further, these 1D chains are assembled into a 3D supramolecular network by intermolecular C-HÁÁÁO H-bonding [the Chart 1.  (Figure 1c and Table S3).
The structure differences of 1 and 2 are as follows. First, the carboxylate groups of L ligands exhibit different coordination modes, a l 1 -g 1 :g 0 and l 1 -g 1 :g 0 bis-monodentate coordination mode in 1 and a l 3 -g 1 :g 2 and l 2 -g 1 :g 1 chelating/bridging-bridging coordination mode in 2. Second, Zn II shows a distorted tetrahedral coordination geometry in 1, while in 2, Cd II displays a distorted octahedral coordination geometry. Finally, because of existence of DMA molecules in 1, it has a different 1D linear chain structure from 2. In summary, besides the coordinated DMA molecules, the structure differences of 1 and 2 may be attributed to the coordination configuration of M II .
This work gives a good comparison between two compounds with different metals.

IR spectra
For 1 and 2, the asymmetric and symmetric stretching vibrations of carboxylate groups appeared at 1570-1580 cm À1 (for 1, at 1580 cm À1 ; for 2, at 1570 cm À1 ) and 1400-1390 cm À1 (for 1, at 1390 cm À1 ; for 2, at 1400 cm À1 ), respectively. The characteristic aromatic peaks of the naphthalene and pyridine rings in 1 and 2 were distributed at 1400 $ 1600 cm À1 (for 1, at 1580, 1430 and 1390 cm À1 ; for 2, at 1570, 1450 and 1400 cm À1 ). The adjacent C-H absorption peaks of the naphthalene and pyridine rings were distributed at 1180, 1030, 816, 770 and 639 cm À1 for 1 and 1180, 1020, 820, 776 and 628 cm À1 for 2. Moreover, the characteristic peak at 1770 cm À1 in 1 and 2 can be due to the anhydride group. The characteristic peak at 1740 cm À1 in 1 may be attributed to DMA molecules. The IR spectra of 1 and 2 were consistent with single-crystal analysis ( Figure S2) [39,40].

Thermal stability
Thermogravimetric analysis (TGA) experiments were conducted to determine the thermal stabilities of 1 and 2, which is an important aspect for MOFs. 1 and 2 are similar ( Figure  S3). They are stable up to 102 C and 85 C, respectively, followed by a consecutive step of weight loss, corresponding to the release of one L ligand (for 1, calcd: 55.20%; observed: 49.35%; in the range of 102-470 C; for 2, calcd: 59.87%; observed: 61.72%; in the range of 85-518 C). Finally, additional weight loss does occur up to 850 C.

Selective luminescence sensing of small molecules
It is imperative to detect small molecules in water for environmental protection. Thus, several small molecule analytes, such as ACN, DMA, DMF, Dox, EA, EtOH, H 2 O, IPA, MeOH, NB and THF, have been investigated. The PXRD analysis confirmed that the frameworks of 1 and 2 remained almost unchanged after testing ( Figure S4). As shown in Figure 4, the luminescence intensity was mainly dependent on small molecules. Especially, compared to others, the luminescence intensities almost disappeared completely upon treatment with NB. In other words, NB exhibits the most significant quenching effect, which is very significant because of the harmfulness of NB [9][10][11][12].       The NB concentration gradient has been used to further investigate the quenching effects ( Figure 5). We found luminescence intensities almost disappeared completely at 0.0500 vol % in 1 and 2, which also shows that the luminescence quenching effect of 1 and 2 is dependent on NB.
Because of existence of overlaps between UV-Vis absorption peaks of NB and emission peaks of 1 and 2, the luminescence quenching mechanism may be ascribed to the highly deficient electron nature of NB. Due to rich electron skeletons, ligands H 4 L' and 4bpy can enrich electrons of compounds. Thus, electrons may easily transfer from electron-rich frameworks to electron-deficient NB [63][64][65][66][67].
In general, the luminescence quenching mechanism of ions may be divided into four kinds [85][86][87][88][89][90]: (1) structural collapse; (2) ion exchange between analytes and structures; (3) interactions between ions and structures; (4) luminescence energy transfer (LET). The PXRD patterns confirm that the structures of 1 and 2 remain unchanged after testing ( Figure S4), showing that the luminescence quenching was not due to the structures collapse. IR spectra of the samples recovered after testing experiments by washing and drying did not provide any evidence about interactions between structures and analytes [91]. However, there are partial overlap between UV-Vis absorption peaks of Fe 3þ or Cr 2 O 7 2À ions and luminescence emission peaks of 1 and 2. Thus, the luminescence quenching mechanism of ions may be ascribed to the energy transfer from the structures (donors) to the analytes (acceptors) [69,92,93].

Conclusion
Treatment of 1,4,5,8-naphthalenetetracarboxylic acid (H 4 L) and 4,4 0 -bipyridine (4bpy) with Zn II and Cd II gave two compounds, showing a 1D linear chain structure and a 2D baluster structure, respectively. The results reveal that the coordination configuration of M II plays an important role in formation of the final structures. Both compounds show ligand-based luminescence in the range 400-500 nm with contributions from both ligands. The fluorescence is strongly dependent on the presence of potentially coordinating molecules. Luminescence quenching experiments showed good sensitivities in detecting nitrobenzene (NB), Fe 3þ and Cr 2 O 7 2À . This work broadens the horizon for design and synthesis of aromatic complexes with good luminescence sensing performance.

Disclosure statement
No potential conflict of interest was reported by the authors.

Funding
This work was financially supported by Anyang Normal University College Student Innovation Fund Project (No. 202110479087) and the Key grant Project of Science and Technology of Anyang (No. 13).