Structure, Luminescent Sensing and Proton Conduction of a Boiling-Water-Stable Zn(II) Metal-Organic Framework

A novel Zn(II) metal-organic framework [Zn4O(C30H12F4O4S8)3]n, namely ZnBPD-4F4TS, has been constructed from a fluoro- and thiophenethio-functionalized ligand 2,2′,5,5′-tetrafluoro-3,3′,6,6′-tetrakis(2-thiophenethio)-4,4′-biphenyl dicarboxylic acid (H2BPD-4F4TS). ZnBPD-4F4TS shows a broad green emission around 520 nm in solid state luminescence, with a Commission International De L’Eclairage (CIE) coordinate at x = 0.264, y = 0.403. Since d10-configured Zn(II) is electrochemically inert, its photoluminescence is likely ascribed to ligand-based luminescence which originates from the well-conjugated system of phenyl and thiophenethio moieties. Its luminescent intensities diminish to different extents when exposed to various metal ions, indicating its potential as an optical sensor for detecting metal ion species. Furthermore, ZnBPD-4F4TS and its NH4Br-loaded composite, NH4Br@ZnBPD-4F4TS, were used for proton conduction measurements in different relative humidity (RH) levels and temperatures. Original ZnBPD-4F4TS shows a low proton conductivity of 9.47 × 10−10 S cm−1 while NH4Br@ZnBPD-4F4TS shows a more than 25,000-fold enhanced value of 2.38 × 10−5 S cm−1 at 40 °C and 90% RH. Both of the proton transport processes in ZnBPD-4F4TS and NH4Br@ZnBPD-4F4TS belong to the Grotthuss mechanism with Ea = 0.40 and 0.32 eV, respectively.


Introduction
The luminescent metal-organic framework (LMOF) is a fascinating class of functional materials that has been extensively researched for its inspiring application in plentiful areas, especially chemical sensing of molecules and cation and anion species [1][2][3][4]. The luminescent and sensing property of LMOFs is related to original metal ions, functional ligands and self-assembly processes. To date, LMOF-based sensors have mainly focused on lanthanide-based (e.g., Eu 3+ , Tb 3+ ) metal-organic frameworks largely because of their strong photoluminescence derived from the ligand-to-lanthanide antenna effect [5][6][7][8]. The high cost and almost unalterable emission wavelength of lanthanide LMOFs motivate researchers to develop LMOFs using transition metal ions (e.g., Zn 2+ , Cd 2+ , etc.) as metal nodes [9][10][11]. As some d 10 metal ions (e.g., Zn 2+ ) are proposed to exert less influence on the emission of LMOFs, ligand design is important for designing Zn-LMOF phosphors [12][13][14]. The introduction of an aromatic π-conjugated system into a ligand helps promote the red shift of the emission wavelength to the visible light region and improves the emission intensity. Further, aromatic groups are proposed to boost the interaction between the host framework and substrate [15,16]. Metal ion detection has been focused on extensively because excess use and emission of metal species have caused many issues, such as environmental pollution, health hazards, etc. [17,18]. Nevertheless, it is still a challenge to achieve LMOF-based sensors with high selectivity, sensitivity and recyclability. In particular, only a few examples of LMOF-based sensors of Mn 2+ ions have been reported [19,20].
In recent years, MOF-based proton-conducting materials have received much attention due to their potential application in proton exchange membrane fuel cells [21][22][23][24]. Traditional Nafion materials have the shortcomings of high cost and intolerance of high temperature, which also accelerates the development of candidate materials [25]. MOFs can be constructed by commonly available metal ions and well-designed ligands, and many of them exhibit rather high solvent (i.e., H 2 O) and thermal (over 400 • C) stability [26][27][28]. This feature makes them an advantage in water-assisted proton conducting materials. Hydrated proton conduction is mainly determined by a hydrogen bond network created by appropriate porosity, functional groups on the ligand and proton carriers in the voids. This leads to two kinds of strategies for achieving high proton conductivity: (1) introducing acidic and hydrophilic groups including F, N, O and S atoms to the ligand [29][30][31][32][33]; (2) encapsulating guest molecules (e.g., water, imidazole, histamine, ammonium bromide, etc.) into the voids [34][35][36][37][38].
SCXRD results reveals that ZnBPD-4F4TS crystallizes in the cubic I23 space group with a = 17.232 Å and α = 90 • (Table S1). The asymmetric unit is built from one crystallographically independent Zn 2+ ion, one oxygen and 1/4 BPD-4F4TS 2− anion. The metal node consists of a Zn-O cluster-based secondary building unit (SBU) with a formula of Zn 4 O(COO) 6 ( Figure 1a). Each of these four Zn 2+ ions is located at a tetrahedron center (cyan tetrahedron represents the ZnO 4 polyhedron) and bridged by one µ 4 -O. Each Zn 2+ ion bonds to four oxygen atoms in which one comes from a µ 4 -O and the other three are from three carboxylate groups of BPD-4F4TS 2− . Each linear BPD-4F4TS 2− (Figure 1b) acts as a two-connected bridge to link the SBUs, forming a 3D framework (Figure 1c). Interconnected 1D channels partly filled by thiophene groups run along the a, b and c crystallographic axes (Figure 1d). The coordination environment of the Zn 2+ ion and coordination mode of BPD-4F4TS 2− in ZnBPD-4F4TS are the same as previously reported IRMOF-type MOFs [40], but they show a two-fold interpenetrated aggregation (Figure 1d). The construction of SBUs and linkers can be simply reticulated into a pcu topology (Figure 1e).
Pure phase of as-synthesized ZnBPD-4F4TS was obtained with the solvothermal method according to a consistent powder X-ray diffraction (SCXRD) pattern with the simulated one, which was also verified by FT-IR spectra ( Figure 2 and Figure S4). NH 4 Br@ZnBPD-4F4TS was prepared by immersing ZnBPD-4F4TS in a saturated NH 4 Br solution of ethanol at room temperature for 2 days. The loading amount of NH 4 Br in the voids was further determined to be 2.21 wt% by elemental analysis (EA) measurement. 4F4TS was prepared by immersing ZnBPD-4F4TS in a saturated NH4Br solution of ethanol at room temperature for 2 days. The loading amount of NH4Br in the voids was further determined to be 2.21 wt% by elemental analysis (EA) measurement.

Stability of ZnBPD-4F4TS
The stability of metal-organic frameworks is vital to their practical applications; therefore, the atmospheric, thermal and solvent stabilities have been investigated. ZnBPD-4F4TS was highly air and water stable judging from the unchanged PXRD patterns after exposure to the atmosphere for 21 days and soaking in boiling water for 30 h, respectively (Figure 2c,e). This excellent stability of ZnBPD-4F4TS to water is rare in reported Zn(II)-carboxylate frameworks [26,41]. As-synthesized crystalline ZnBPD-4F4TS was further Soxhlet-extracted by acetone and heated at 100 °C to prepare activated ZnBPD-4F4TS. EA data indicated a formula of [Zn4O(C30H12F4O4S8)3·(H2O)1.5(CH3CN)0.6]n for activated ZnBPD-4F4TS. Consistent diffraction peaks with as-synthesized ZnBPD-4F4TS suggested an intact coordination framework (Figure 2d). The thermogravimetric (TG) curve of activated ZnBPD-4F4TS showed slight weight loss (3.15%) in the initial period, which could be attributed to the loss of a few water molecules from the air and remaining crystallized guest molecules in the pores. This result suggested the coordination framework was thermally stable below 380 °C ( Figure S5). The subsequent large weight loss could be ascribed to decomposition of organic ligands and destruction of coordination bonds, which was supported by the endothermic effect in the differential thermal

Luminescent Properties
With the conjugated thiophene and phenyl units, ZnBPD-4F4TS was expected to possess decent luminescent property. Both excitation and emission spectra of ligand H2BPD-4F4TS and ZnBPD-4F4TS were recorded in the solid state (Figure 3a,b). In the case of ligand H2BPD-4F4TS, it displays one fluorescent emission band centered at 535 nm when excited at 370 nm, which is probably assigned to π or n to π* orbital transitions

Stability of ZnBPD-4F4TS
The stability of metal-organic frameworks is vital to their practical applications; therefore, the atmospheric, thermal and solvent stabilities have been investigated. ZnBPD-4F4TS was highly air and water stable judging from the unchanged PXRD patterns after exposure to the atmosphere for 21 days and soaking in boiling water for 30 h, respectively (Figure 2(c,e)). This excellent stability of ZnBPD-4F4TS to water is rare in reported Zn(II)carboxylate frameworks [26,41]. As-synthesized crystalline ZnBPD-4F4TS was further Soxhlet-extracted by acetone and heated at 100 • C to prepare activated ZnBPD-4F4TS. EA data indicated a formula of [Zn 4 O(C 30 H 12 F 4 O 4 S 8 ) 3 ·(H 2 O) 1.5 (CH 3 CN) 0.6 ] n for activated ZnBPD-4F4TS. Consistent diffraction peaks with as-synthesized ZnBPD-4F4TS suggested an intact coordination framework (Figure 2(d)). The thermogravimetric (TG) curve of activated ZnBPD-4F4TS showed slight weight loss (3.15%) in the initial period, which could be attributed to the loss of a few water molecules from the air and remaining crystallized guest molecules in the pores. This result suggested the coordination framework was thermally stable below 380 • C ( Figure S5). The subsequent large weight loss could be ascribed to decomposition of organic ligands and destruction of coordination bonds, which was supported by the endothermic effect in the differential thermal analysis (DTA) curve ( Figure S5). The consistent PXRD pattern of NH 4 Br@ZnBPD-4F4TS with ZnBPD-4F4TS indicated that the host framework remained stable after immersion in NH 4 Br solution ( Figure S6b).

Luminescent Properties
With the conjugated thiophene and phenyl units, ZnBPD-4F4TS was expected to possess decent luminescent property. Both excitation and emission spectra of ligand H 2 BPD-4F4TS and ZnBPD-4F4TS were recorded in the solid state (Figure 3a,b). In the case of ligand H 2 BPD-4F4TS, it displays one fluorescent emission band centered at 535 nm when excited at 370 nm, which is probably assigned to π or n to π* orbital transitions [42,43]. After coordination to form ZnBPD-4F4TS, it shows a similar but blue-shifted emission peak at 518 nm when excited at 370 nm, showing a green emission of crystals (Figure 3c). This blue shift of ZnBPD-4F4TS in comparison to free ligand is probably attributed to the metal-ligand coordination interaction and deprotonated effect of the dicarboxylic acid [44,45]. Accordingly, Commission International De L'Éclairage (CIE) coordinates change from (0.297, 0.396) for the ligand to (0.264, 0.403) for ZnBPD-4F4TS (Figure 3d). In addition, the emission spectrum of NH 4 Br@ZnBPD-4F4TS was also obtained and found to be quite similar to that of as-synthesized ZnBPD-4F4TS ( Figure S7).

Metal Sensing and Mechanism
Excellent water stability, free-standing thiophenethio-functions in the pores and potential accessible pores inspired us to investigate the chemical sensing performance of ZnBPD-4F4TS in aqueous media. As-synthesized crystals of ZnBPD-4F4TS were first immersed in 500 ppm (based on metal species) metal chloride or nitrate solutions (Pb 2+ , Pd 2+ , Co 2+ , Fe 3+ , Ni 2+ , Hg 2+ , Cu 2+ , Cd 2+ , Pt 2+ , Mn 2+ and Ag + ) for 2 h at 80 • C and then selected for photography (Figure 4a). All of these crystals show no obvious color change, though some exhibit a semi-transparent appearance. When irradiated under 365 nm UV light, these crystals exhibit similar emission colors but with variously decreased brightness in comparison to as-synthesized ZnBPD-4F4TS. Emission spectra of bulk samples were also measured in the wavelength range of 380 to 720 nm, and they show different extents of reduction in luminescence intensity (monitored at 518 nm) after immersion in various metal ion solutions (Figure 4b). In particular, the case of Mn 2+ exhibits the strongest luminescence quenching (Figure 4c). PXRD patterns of ZnBPD-4F4TS were found to be unchanged after exposure to various metal ion solutions, suggesting an intact framework structure ( Figure S8).

Metal Sensing and Mechanism
Excellent water stability, free-standing thiophenethio-functions in the pores and potential accessible pores inspired us to investigate the chemical sensing performance of ZnBPD-4F4TS in aqueous media. As-synthesized crystals of ZnBPD-4F4TS were first immersed in 500 ppm (based on metal species) metal chloride or nitrate solutions (Pb 2+ , Pd 2+ , Co 2+ , Fe 3+ , Ni 2+ , Hg 2+ , Cu 2+ , Cd 2+ , Pt 2+ , Mn 2+ and Ag + ) for 2 h at 80 °C and then selected for photography ( Figure 4a). All of these crystals show no obvious color change, though some exhibit a semi-transparent appearance. When irradiated under 365 nm UV light, these crystals exhibit similar emission colors but with variously decreased brightness in comparison to as-synthesized ZnBPD-4F4TS. Emission spectra of bulk samples were also measured in the wavelength range of 380 to 720 nm, and they show different extents of reduction in luminescence intensity (monitored at 518 nm) after immersion in various metal ion solutions (Figure 4b). In particular, the case of Mn 2+ exhibits the strongest luminescence quenching (Figure 4c). PXRD patterns of ZnBPD-4F4TS were found to be unchanged after exposure to various metal ion solutions, suggesting an intact framework structure ( Figure S8).

Proton Conduction
The proton carrier is one of the vital factors for improving proton conductivity. This motivated us to optimize the as-synthesized framework by encapsulating protonic guests (i.e., NH4Br) into framework voids through a post-synthetic strategy. After ZnBPD-4F4TS

Proton Conduction
The proton carrier is one of the vital factors for improving proton conductivity. This motivated us to optimize the as-synthesized framework by encapsulating protonic guests (i.e., NH 4 Br) into framework voids through a post-synthetic strategy. After ZnBPD-4F4TS was immersed in NH 4 Br/EtOH solution for 2 days, NH 4 Br@ZnBPD-4F4TS was separated, washed and dried. The ac impedance measurements were carried out with compacted pellets of ZnBPD-4F4TS and NH 4 Br@ZnBPD-4F4TS, respectively. The corresponding Nyquist plots are shown at different temperatures at 90% relative humidity (RH) (Figure 5a,c). In a typical measurement, the proton conductivity of a sample is determined by the high frequency region with the following equation: where l is the thickness (mm) and S is the cross-sectional area (mm 2 ) of the pellet, while R (Ω) can be calculated from the Nyquist impedance plots [46,47]. Accordingly, the corresponding proton conductivity was obtained and is listed in Table S2. It is exciting that NH 4 Br@ZnBPD-4F4TS (2.38 × 10 −5 S·cm −1 ) exhibits a 25,000-fold increased proton conductivity compared to the original ZnBPD-4F4TS (9.47 × 10 −10 S·cm −1 ) at 40 • C and 90% RH. This high and sharply increased proton conductivity of NH 4 Br@ZnBPD-4F4TS highlights that manipulation of protonic guests serves as an effective strategy to promote proton transport in the framework. Further research reveals that ZnBPD-4F4TS possesses increasing proton conductivity varying from 9.47 × 10 −10 (40 • C) to 4.19 × 10 −9 (80 • C) S·cm −1 when the temperature increases under 90% RH ( Figure S9). Similarly, the conductivity of NH 4 Br@ZnBPD-4F4TS increases from to 2.38 × 10 −5 (40 • C) to 7.87 × 10 −5 (80 • C) S·cm −1 ( Figure S9). Both of the metal-organic frameworks remain stable after proton conduction measurements according to the unchanged PXRD patterns ( Figures S6 and S10).
Molecules 2021, 26, x FOR PEER REVIEW 7 of 12 ( Figure S9). Both of the metal-organic frameworks remain stable after proton conduction measurements according to the unchanged PXRD patterns (Figures S6 and S10).

Proton Conduction Mechanism
The obtained conductivities of ZnBPD-4F4TS and NH4Br@ZnBPD-4F4TS at various temperatures are plotted against temperature in the form of ln(σT) against 1000/T ( Figure  5b,d), and the activation energy (Ea) of proton transport can be estimated using the Arrhe-

Proton Conduction Mechanism
The obtained conductivities of ZnBPD-4F4TS and NH 4 Br@ZnBPD-4F4TS at various temperatures are plotted against temperature in the form of ln(σT) against 1000/T (Figure 5b,d), and the activation energy (E a ) of proton transport can be estimated using the Arrhenius equation: The E a value is calculated to be 0.40 and 0.32 eV for ZnBPD-4F4TS and NH 4 Br@ZnBPD-4F4TS, respectively. Generally, the proton transport process is classified into Grotthuss [48] (E a < 0.4 eV) and vehicle [49] (E a > 0.4 eV) mechanisms according to the E a value. Therefore, both of these materials can be attributed to the Grotthuss mechanism. It indicates that proton transport is achievable through the hydrogen bond network of proton carriers. Both of the H 2 O molecules and electronegative S atoms in the framework help form the hydrogen bonds to promote proton transport. NH 4 Br@ZnBPD-4F4TS shows a 25,000-fold improvement in conductivity compared to ZnBPD-4F4TS that might originate from the more favorable hydrogen bond channels among H 2 O, thiophene and NH 4 Br species.

General Procedure
Starting materials, reagents and solvents were purchased from commercial sources (J&K, Aldrich and Acros) and used without further purification. Elemental analysis (EA) was performed with a Vario Micro CUBE CHN elemental analyzer (Elementar, Germany). FT-IR spectra were obtained using a Avatar 360 spectrophotometer (Thermo Nicolet, The United States). Nuclear magnetic resonance (NMR) spectra were recorded at 298 K on a 400 MHz superconducting magnet high-field NMR spectrometer (Bruker, The Swiss), with working frequencies of 400 MHz for 1 H, 376 MHz for 19 F. Chemical shifts are reported in ppm relative to the signals corresponding to the residual non-deuterated solvents, with tetramethylsilane (TMS) as the internal standard. Thermogravimetric (TG) analyses were carried out in a nitrogen stream using Thermal analysis equipment (STA 6000) (PerkinElmer, The United States) with a heating rate of 10 • C/min. Powder X-ray diffraction (PXRD) data were collected in reflection mode at room temperature on a Smart Lab diffractometer (Rigaku, Japan) with a mixture of Cu-Kα1 (λ = 1.54056 Å) and Cu-Kα2 (λ = 1.5418 Å) radiation. Fluorescence spectra were measured on a FluoroMax-4 fluorescence spectrometer (HORIBA Jobin Yvon, France) at room temperature.

Synthesis of [Zn 4 O(H 2 BPD-4F4TS) 3 ] n (ZnBPD-4F4TS)
The ligand H 2 BPD-4F4TS (5 mg, 0.0065 mmol) and zinc nitrate hexahydrate (5 mg, 0.017 mmol) were weighed into a glass tube 8 mm in diameter, and then a mixed solvent of water and acetonitrile (0.8 mL, v/v = 1:1) was added. The mixture was sonicated for 5 min to form a clear solution. Then, the glass tube nozzle was melted and sealed at high temperature, and the glass tube was heated at 140 • C for 36 h, followed by natural cooling to room temperature, during which light yellow truncated cube-like crystals (2.4 mg, 14% based on H 2 BPD-4F4TS) were formed.

Activation of ZnBPD-4F4TS
To exchange and remove the solvent molecules from the pores of ZnBPD-4F4TS, a thimble (e.g., made from folding filter paper) containing as-synthesized ZnBPD-4F4TS crystals (50 mg) was loaded into the main chamber of a Soxhlet extractor. The Soxhlet extractor was connected to a 250 mL round-bottomed flask including acetone (150 mL) and a magnetic stirring bar, and then equipped with a water condenser. The flask was heated to 100 • C with an oil bath for 3 days. The filter paper was then taken out and the solid was heated at 90 • C under vacuum to give the activated ZnBPD-4F4TS sample.

Synthesis of NH 4 Br@ZnBPD-4F4TS
The solid sample of as-synthesized ZnBPD-4F4TS (30 mg) was added to a small glass bottle containing 4 mL of saturated ammonium bromide in ethanol solution, and soaked at room temperature for 2 days. Afterwards, the resultant solid NH 4

Single Crystal X-ray Crystallography
Single crystal data for ZnBPD-4F4TS were collected using a Bruker APEX-II CCD diffractometer (Bruker, Germany) with an I-mu-S micro-focus X-ray source using Cu Kα radiation (λ = 1.54178). Data were collected at 230.0 K. Reflections were indexed and processed, and the files scaled and corrected for absorption using APEX3 v2018. The space group was assigned and the structure was solved by direct methods using XPREP-2014/2 program and refined by full matrix least squares against F 2 with all reflections using Shelxl2018 using the graphical interface Olex2 [50]. All non-hydrogen atoms were refined with anisotropic thermal parameters, and all hydrogen atoms were included in calculated positions and refined with isotropic thermal parameters riding on those of the parent atoms. The hanging thiophene groups are highly disordered and difficult to resolve. Therefore, their electron peaks were squeezed in the refinement process. The crystallographic data for incomplete ZnBPD-4F4TS, in CIF format, have been deposited with the Cambridge Crystallographic Data Centre as CCDC 2098157. These data can be obtained free of charge from the Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif (accessed on 22 July 2021).

Metal Ion Sensing Experiment
ZnBPD-4F4TS crystals (2 mg) were first introduced into different metal ion aqueous solutions (3 mL, 500 ppm based on metal ion) of MCl x (M x+ = Hg 2+ , Cd 2+ , Ni 2+ , Co 2+ , Mn 2+ , Cu 2+ , Pt 2+ , Pd 2+ , Fe 3+ , Pb 2+ ), AgNO 3 and then heated at 80 • C for 2h. After cooling to room temperature, the bulk samples were centrifuged and washed three times with water. Additionally, they were washed three times with acetone and dried under vacuum. The dried samples were then used for luminescence measurements and the luminescence data were collected. The blank sample was obtained from ZnBPD-4F4TS crystals being immersed in pure water instead of metal ion solution.

Electrochemical Impedance Spectroscopy
The Nyquist plots (Z vs. Z ) of proton-conducting MOF often show a single semicircle at high frequency, representing proton resistivity contributions of the bulk sample. The proton conductivity was deduced from the semicircle by fitting an equivalent circuit which consists of Rs, R1 and W1 in the frequency range from 10 MHz to 1 Hz. Rs corresponds to wire and electrode resistance, R1 is proton resistance and W1 is the resistivity of the grain boundary. Sometimes W1 is not necessary, because the impedance plot of the capacitive tail may not appear in the measured range due to the high magnitude of the resistivity. The water-assisted conductivities of synthesized materials were measured under different relative humidity and temperature conditions and were further fitted with different fitting circuits using the ZView software [51].

Conclusions
In summary, a green emitter (x = 0.264, y = 0.403), ZnBPD-4F4TS, has been constructed from a fluorine-and thiophenethio-functionalized ligand. Exposed to various metal ions, it exhibits different reductions in luminescent intensity. In particular, the luminescence is almost fully quenched when exposed to Mn 2+ ions. Moreover, NH 4 Br-loaded NH 4 Br@ZnBPD-4F4TS shows a more than 25,000-fold enhanced proton conductivity compared to the original ZnBPD-4F4TS at 40 • C and 90% RH, serving as an example of the enhancement of proton conducting material by post-synthetic modification.