Zero- to One-Dimensional Zn24 Supraclusters: Synthesis, Structures and Detection Wavelength

A zinc supracluster [Zn24(ATZ)18(AcO)30(H2O)1.5]·(H2O)3.5 (Zn24), and a 1D zinc supracluster chain {[Zn24(ATZ)18(AcO)30(C2H5OH)2(H2O)3]·(H2O)2.5}n (1-D⊂Zn24) with molecular diameters of 2 nm were synthesized under regulatory solvothermal conditions or the micro bottle method. In an N,N-dimethylformamide solution of Zn24, Fe3+, Ni2+, Cu2+, Cr2+ and Co2+ ions exhibited fluorescence-quenching effects, while the rare earth ions Ce3+, Dy3+, Er3+, Eu3+, Gd3+, Ho3+, La3+, Nd3+, Sm3+, and Tb3+showed no obvious fluorescence quenching. In ethanol solution, the Zn24 supracluster can be used to selectively detect Ce3+ ions with excellent efficiency (limit of detection (LOD) = 8.51 × 10−7 mol/L). The Zn24 supracluster can also detect wavelengths between 302 and 332 nm using the intensity of the emitted light.


Materials and Physical Measurements
All chemicals were bought commercially and used directly after receipt.Elemental analyses were performed using a Perkin-Elmer 240 elemental analyzer (CHN).The FT-IR spectra were captured in the 4000-400 cm −1 region from KBr pellets on a Bio-Rad FTS-7 spectrometer.The SHELXL crystallographic program for molecular structures was used to determine the X-ray crystal structures using an Agilent G8910A CCD diffractometer.Photoluminescence experiments were performed using a Hitachi F-4600 fluorescence spectrophotometer.The power X-ray diffraction (PXRD) patterns were determined using a PANalytical X'Pert 3 power diffractometer (operating at 40 kV and 40 mA) with graphitemonochromatized Cu Kα radiation (λ = 1.54056Å). 1 H NMR spectra were recorded on Bruker AVANCE III 500 instruments.

Synthesis of Zn 24
A mixture of H 2 L 1 (0.5 mmol, 0.1340 g), Zn(CH 3 COO) 2 •2H 2 O (0.5 mmol, 0.1048 g), and ethanol (10 mL) was stirred for 30 min, with the pH adjusted to 6 through the addition of triethylamine.The mixture was then sealed in a 15 mL Teflon-lined stainless-steel vessel, and heated at 353 K for 48 h in an oven, followed by slow cooling to room temperature.Four-cornered golden yellow crystals in a double-cone shape were collected, washed with ethanol, and dried in air.Phase-pure Zn 24 crystals were obtained through manual separation (yield: 66.  S1, KBr, cm −1 ) were as follows: 3445 s, 1578 m, 1400 w, 1165 w, 1101 m, 941 w, 758 w, 685 w, 616 w, and 483 w.

Synthesis of 1-D⊂Zn 24
A mixture of H 2 L 1 (0.5 mmol, 0.1340 g), Zn(CH 3 COO) 2 •2H 2 O (0.5 mmol, 0.1048 g), ethanol (10 mL) and acetonitrile (2 mL) was stirred for 30 min with the pH adjusted to 6 through the addition of triethylamine.The mixture was then sealed in a 20-mL micro bottle capable of autonomously adjusting the reaction pressure and heated at 343 K for 48 h.Subsequently, the micro bottle was slowly cooled to room temperature.Four-cornered golden yellow crystals with a double-cone shape were collected, washed with ethanol and dried in air.Phase-pure crystals of 1-D⊂Zn 24 S1, KBr, cm −1 ) were as follows: 3445 s, 1578 m, 1400 w, 1165 w, 1101 m, 941 w, 758 w, 685 w, 616 w, and 483 w.

Single-Crystal X-ray Diffraction
The single-crystal data of the Zn 24 and 1-D⊂Zn 24 complexes were collected using a SuperNova (single source at offset) Eos with graphite monochromatic Mo-Kα radiation (λ = 0.71073 Å) in the ω scan mode in the ranges of 3.16 • ≤ θ ≤ 25.01 • and 3.30 • ≤ θ ≤ 25.01 • , respectively.Raw frame data were integrated using the SAINT program [35].The Zn 24 and 1-D⊂Zn 24 structures were solved with direct methods using SHELXS [35] and refined with full-matrix least-squares on F 2 using SHELXL-2018 within the Olex2 GUI [36].Empirical absorption correction using spherical harmonics was implemented in the SCALE3 ABSPACK scaling algorithm.All non-hydrogen atoms were refined anisotropically.All hydrogen atoms to carbon atoms were positioned geometrically and refined as riding atoms.Calculations and graphics were performed with SHELXTL [35].The computer programs used in this study were CrysAlis PRO (Agilent Technologies, Version 1.171.37.35 released 13-08-2014 CrysAlis171.NET compiled 13 August 2014), SHELXL [35], and Olex2 [36].The crystallographic details of Zn 24 and 1-D⊂Zn 24 are provided in Table 1.Selected bond lengths and angles for Zn 24 and 1-D⊂Zn 24 are listed in Tables S1 and S2.

Structural and Synthetic Details
Herein, we investigated the effects of ligand, reaction temperature, counterbalance anion, ligand/metal ion molar ratio, solvent, and synthetic method on the self-assembly of supraclusters (Scheme 2).The synthetic strategy for the Hatz system is depicted in Scheme 2. First, a mixture of Zn(OAc) 2 •2H 2 O (0.2 mmol), 4-bromo-2-[(1H-tetrazol-5- ylimino)-methyl]-phenol (H 2 L 1 , 0.2 mmol), and anhydrous ethanol (10 mL) was poured into a Teflon-lined autoclave (20 mL).The autoclave was cooled slowly to room temperature after heating at 80 • C for 2 days.Four-cornered golden yellow Zn 24 crystals in a doublecone shape were collected via filtration.In the Zn 24 supracluster, Hatz is produced by the decomposition of H 2 L 1 .To understand the role of H 2 L 1 in the synthesis, we used various salicylaldehyde-derived Schiff bases (H 2 L 2 -H 2 L 5 ) instead of 4-bromo-2-[(1H-tetrazol-5ylimino)-methyl]-phenol (H 2 L 1 ) to conduct the same experiment, but we could not obtain the Zn 24 supercluster or analog.Similarly, if only H 2 L 1 was replaced by Hatz, the Zn 24 supercluster or analog was not obtained.Through previous experiments, we can draw the conclusion that although 5-bromosalicylicaldehyde does not participate in coordination in the 24-atom Zn cluster, 5-bromosalicylicaldehyde is an essential raw material for the synthesis of the Zn 24 cluster.Thus, we speculate that 5-bromosalicylicaldehyde may act as a template.

Crystal Structures of Zn24 and 1-D⸦Zn24
Single crystal analysis confirmed Zn24 to have a 0D wheel-like coordination supracluster of the monoclinic crystal system with P21/n space group consisting of 24 Zn II atoms, 30 acetate groups, 1.5 coordinated water molecules, 3.5 lattice water molecules and 18 Atz ligands derived from H2L 1 (Figure 1a).The Zn24 cluster was stabilized by 5-amino-1,2,3,4tetrazole, which binds along the wheel of the cluster core (Figure 1b), bridging the four neighboring zinc atoms.Acetate groups further stabilized the cluster through 18 µ2:ƞ 1 :ƞ 1acetate bridging two zinc atoms.The 12 Zn ions in the inner ring of the wheel (inner red ring shown in Figure 1a) coordinated with four N atoms from four different Atz ligands and two O atoms from two syn-syn-µ2:ƞ 1 :ƞ 1 -acetate bridging groups to form a distorted octahedral geometry.By contrast, the 12 Zn atoms in the outer ring (outer red ring shown in Figure 1a) coordinated with two N atoms from two different Atz ligands.They also coordinated with two or three O atoms from one syn-syn-µ2:ƞ 1 :ƞ 1 -acetate bridging group and one µ1:ƞ 1 :ƞ 1 -acetate terminal group or one µ1:ƞ 1 -acetate ligand, as well as one coordinated water molecule, to form a distorted tetragonal pyramidal, or a trigonal bipyramidal or an octahedral geometry.Close inspection of the nanosized wheel-like conformation showed approximate wheel dimensions of 8.912 × 20.491 × 9.747 Å (inner ring diameter × According to the ring structure of Zn 24 , the Zn 24 cluster can form a 1D chain or 2D network through bridging ligands.H 2 L 1 and Zn(CH 3 COO) 2 •2H 2 O were selected as the starting materials.By tuning the reaction temperature, solvent, ligand/metal salt molar ratio, and synthetic methods, the optimal synthesis conditions for the 1D Zn supracluster chain 1-D⊂Zn 24 were determined as follows: reaction temperature, 70 • C; solvent, anhydrous ethanol (8 mL), and acetonitrile (2 mL); H 2 L 1 /Zn(CH 3 COO) 2 •2H 2 O molar ratio, 2:1; and reactor, micro bottle (autonomously adjusting the reaction pressure).
To obtain a 2D supracluster network, we replaced acetic acid with various carboxylic acids such as oxalic acid, malonic acid, succinic acid, and terephthalic acid.However, these experiments were unsuccessful.

Luminescent Properties
Numerous studies have demonstrated the good fluorescence of clusters, especially those of Zn(II) and Cd(II) ions with closed d subshells [37,38].In recent years, Zn(II) clusters have been widely used in luminescent probes due to their desirable advantages in

Luminescent Properties
Numerous studies have demonstrated the good fluorescence of clusters, especially those of Zn(II) and Cd(II) ions with closed d subshells [37,38].In recent years, Zn(II) clusters have been widely used in luminescent probes due to their desirable advantages in detection and promising applications in biological and environmental systems [39].Luminescent probes and complexes can selectively detect various sizes of molecules or ions through their adjustable porosity [40].
In this paper, the luminescent properties (the phase purity of Zn 24 has been checked using PXRD patterns, Figure S7) of Zn 24 were investigated in different solvents with concentrations of 1 × 10 −6 mol•L −1 (Figure 3).Upon photoexcitation at 404, 382, 426 and 418 nm in water, N,N-dimethylformamide (DMF), DMSO, and ethanol solvent, Zn 24 exhibited green, blue, green and green luminescent emission bands with fluorescence maxima at 496, 459, 507, and 508 nm, respectively.These results predominantly originated from the metal-to-ligand charge-transfer excited state [41,42].Furthermore, Zn 24 exhibited a qualitative change in its luminescence due to the interaction between metal ions and ligands, and it had a stronger fluorescence intensity in DMF and ethanol solutions.Although Zn 24 also had a stronger fluorescence intensity in DMSO, we did not consider DSMO for Zn 24 luminescent probes due to its high toxicity.Thus, we discussed the Zn 24 complex as a luminescent probe for highly selective sensing in DMF and ethanol.
centrations of 1 × 10 −6 mol•L −1 (Figure 3).Upon photoexcitation at 404, in water, N,N-dimethylformamide (DMF), DMSO, and ethanol solv green, blue, green and green luminescent emission bands with fluo 496, 459, 507, and 508 nm, respectively.These results predominantly metal-to-ligand charge-transfer excited state [41,42].Furthermore, Zn tative change in its luminescence due to the interaction between met and it had a stronger fluorescence intensity in DMF and ethanol solut also had a stronger fluorescence intensity in DMSO, we did not cons luminescent probes due to its high toxicity.Thus, we discussed the Z minescent probe for highly selective sensing in DMF and ethanol.
The Zn24 (1 mg, with a final concentration of 1 mg/mL in ethano centrations of Ce 3+ (from 1 × 10 −2 to 1 × 10 −8 mol/L) were added to the s temperature.The fluorescence spectrum was taken at its excitation w nm).

Detection Wavelength
The solid-state fluorescence spectra of Hatz were obtained at a slit width of 5 nm and

Detection Wavelength
The solid-state fluorescence spectra of Hatz were obtained at a slit width of 5 nm and The Zn 24 (1 mg, with a final concentration of 1 mg/mL in ethanol) and different concentrations of Ce 3+ (from 1 × 10 −2 to 1 × 10 −8 mol/L) were added to the sample tube at room temperature.The fluorescence spectrum was taken at its excitation wavelength (λ = 402 nm).
The fluorescence spectra of the Zn 24 -Ce 3+ system for various concentrations of Ce 3+ are shown in Figure 6.The fluorescence intensity at 504 nm progressively increased as the concentration of Ce 3+ decreased.In addition, we quantitatively analyzed the quenching efficiency through the Stern-Volmer equation: I o /I = K sv [C] + l, where I o and I are the respective emission intensities before and after adding Ce 3+ , while C is the concentration of Ce 3+ in ethanol solution.The quenching efficiency, of Zn 24 was −1.68 × 10 M −1 (Figure S5).According to the limit of detection (LOD) = 3δ/K sv (Figure S6), we calculated an LOD of 8.51 × 10 −7 mol/L, which was lower than the reported LOD of the Ln-MOF [41].

Detection Wavelength
The solid-state fluorescence spectra of Hatz were obtained at a slit width of 5 nm and an excitation wavelength of 402 nm, while the solid-state fluorescence spectra of Zn24 were obtained at an excitation wavelength of 302-332 nm (Figures 7 and S4).Under the same test conditions, the fluorescence spectrum of Hatz peaked at 615 nm, while that of Zn2 peaked at 502 nm.The luminous color changed from red to blue-green, and the fluores cence intensity of Zn24 was more than 200 times that of Hatz.The full-type Zn 2+ metal ion

Detection Wavelength
The solid-state fluorescence spectra of Hatz were obtained at a slit width of 5 nm and an excitation wavelength of 402 nm, while the solid-state fluorescence spectra of Zn 24 were obtained at an excitation wavelength of 302-332 nm (Figure 7 and Figure S4).Under the same test conditions, the fluorescence spectrum of Hatz peaked at 615 nm, while that of Zn 24 peaked at 502 nm.The luminous color changed from red to blue-green, and the fluorescence intensity of Zn 24 was more than 200 times that of Hatz.The full-type Zn 2+ metal ion in Zn 24 has an extra-nuclear d 10 electron, and did not undergo a d-d transition, leading to a significant enhancement in luminous intensity.At the same time, the deprotonated tetrazolium ring is an electron-deficient conjugated ring that causes the electron migration (M→L) of zinc ions to the tetrazolium ring [42,43].As a result, the luminous color changed from red to blue-green, and the fluorescence intensity of Zn 24 was more than 200 times that of Hatz.The fluorescence intensity of Zn 24 decreased linearly with the increase in excitation wavelength (Figures 7 and 8).Between 302 and 332 nm, the wavelength of the excitation light can be determined by detecting the intensity of the emitted light.aterials 2023, 13, x FOR PEER REVIEW in Zn24 has an extra-nuclear d 10 electron, and did not undergo a d a significant enhancement in luminous intensity.At the same tim trazolium ring is an electron-deficient conjugated ring that cause (M→L) of zinc ions to the tetrazolium ring [42,43].As a result, the from red to blue-green, and the fluorescence intensity of Zn24 w that of Hatz.The fluorescence intensity of Zn24 decreased linearly citation wavelength (Figures 7 and 8).Between 302 and 332 nm excitation light can be determined by detecting the intensity of th

Hirshfeld Surface Analysis of the Complex Zn24
Hirshfeld surface analysis [44] is a useful tool for describing the surface cha tics of molecules, and was performed to visualize the different intermolecular inte in crystal structures by employing 3D molecular surface contours.Figure 9 disp findings of the Zn24 Hirshfeld surface study.The middle shape index ranges from to 1.000 Å, whereas the range of the dnorm surface on the left is −1.238 to 1.570 Å. Th of the curvature curvedness was −4.000 to 0.400 Å.In Figure 9, the dnorm surface Zn24 is colored from light to dark red spots to represent the interaction force of the c Zn24 from weak to strong.

Hirshfeld Surface Analysis of the Complex Zn 24
Hirshfeld surface analysis [44] is a useful tool for describing the surface characteristics of molecules, and was performed to visualize the different intermolecular interactions in crystal structures by employing 3D molecular surface contours.Figure 9 displays the findings of the Zn 24 Hirshfeld surface study.The middle shape index ranges from −1.000 to 1.000 Å, whereas the range of the d norm surface on the left is −1.238 to 1.570 Å.The range of the curvature curvedness was −4.000 to 0.400 Å.In Figure 9, the d norm surface map of Zn 24 is colored from light to dark red spots to represent the interaction force of the complex Zn 24 from weak to strong.One useful supplement for Hirshfeld surface analysis is the 2-D fingerprint plot [45].It quantitatively analyses the nature and type of intermolecular interaction between the molecules inside the crystals.The fingerprint plots can be decomposed to highlight particularly close contacts between the elements (Figure 10).The H•••H interaction is one of the most significant contacts for the Zn24 complex.
The main intermolecular interaction of Zn24 is H  One useful supplement for Hirshfeld surface analysis is the 2-D fingerprint plot [45].It quantitatively analyses the nature and type of intermolecular interaction between the molecules inside the crystals.The fingerprint plots can be decomposed to highlight particularly close contacts between the elements (Figure 10).The H•••H interaction is one of the most significant contacts for the Zn 24 complex.
The main intermolecular interaction of Zn 24 is H•••H contact, which is reflected in the middle of the scattered points of the 2-D fingerprint plots (the percentage of H•••H contacts of Zn 24 is 40.4%).Another main intermolecular interaction of Zn 24 is O•••H interaction, which is represented by double spikes in the bottom left (acceptor and donor) region of the fingerprint plots.Accordingly, we can infer that there are significant N-H•••O hydrogen bonds (Table S3) observed in Zn 24 (

Figure 6 .
Figure 6.Liquid-state fluorescence behavior of Zn24 for different concentrations of Ce 3+ in ethanol.

Figure 6 .
Figure 6.Liquid-state fluorescence behavior of Zn24 for different concentrations of Ce 3+ in ethanol.

Figure 6 .
Figure 6.Liquid-state fluorescence behavior of Zn24 for different concentrations of Ce 3+ in ethanol.

Figure 6 .
Figure 6.Liquid-state fluorescence behavior of Zn 24 for different concentrations of Ce 3+ in ethanol.
•••H contact, which is reflected in the middle of the scattered points of the 2-D fingerprint plots (the percentage of H•••H contacts of Zn24 is 40.4%).Another main intermolecular interaction of Zn24 is O•••H interaction, which is represented by double spikes in the bottom left (acceptor and donor) region of the fingerprint plots.Accordingly, we can infer that there are significant N-H•••O hydrogen bonds (Table S3) observed in Zn24 (the percentage of O•••H contacts of Zn24 is 22.4%).Also, the N•••H contacts play important roles for Zn24.The percentage of N•••H contacts of Zn24 is 10.0%.In addition to those above, the presence of C•••H, C•••O, and N•••O contacts were also observed.These three forces accounted for 4.0%, 1.4% and 1.3% of the total Hirshfeld surface force, respectively.
the percentage of O•••H contacts of Zn 24 is 22.4%).Also, the N•••H contacts play important roles for Zn 24 .The percentage of N•••H contacts of Zn 24 is 10.0%.In addition to those above, the presence of C•••H, C•••O, and N•••O contacts were also observed.These three forces accounted for 4.0%, 1.4% and 1.3% of the total Hirshfeld surface force, respectively.