English

• 0.   Introduction

  The uncontrolled release of iron ions into air and water sources leading to several types of environmental pollutions are human health hazards^[1-3]. Therefore, it is important to develop a simple and efficient method for the detection of Fe³⁺. Among many reported sensing approaches, fluorescence sensing has received extensive attention attributed to its high sensitivity, low detection limit, and fast detection speed^[4-5]. Therefore, a fluorescence sensor can be developed to detect lowconcentration of Fe^3+[6-9].

  Coordination polymers (CPs) are fascinating materials that are both fundamentally important and technologically relevant^[10-11] and have been extensively studied, not only because of their diverse structures, but also for their wide application prospects in the fields of molecular storage, luminescence, and magnetism^[12-14]. Currently, luminescent coordination polymers (LCPs) as fluorescent probes have become one of the research hotspots, which can effectively monitor the presence of iron ions based on the fluorescence quenching mechanism^[15].

  The application of the mixed ligands strategy has been proved an effective method to construct a variety of CPs^[16-21]. In this work, a new Tb³⁺ CP with terephthalic acid (1, 4 - bdc) and 1, 10 - phenanthroline (phen), [Tb(1, 4-bdc)_1.5(phen) (H₂O)]_n (1), was synthesized and structurally determined using mixed ligands strategy. The resulting CP exhibited high luminescence sensing performance for Fe³⁺ with a fluorescence quenching mechanism.

  1.   Experimental

  1.1   Reagents and characterization

  Terbium nitrate hexahydrate, 1, 10 - phenanthroline, and terephthalic acid were of commercial grade and used without further purification. Powder X - ray diffraction (PXRD) patterns were recorded on a Rigaku D/Max-2500 diffractometer (Cu Kα, λ=0.154 06 nm, 2θ =5°-50°, U=40 kV, I=40 mA), having a graphite monochromator. Simulation of the PXRD pattern was carried out by the single - crystal data and diffraction - crystal module of the Mercury program version 1.4.2. Elemental analyses for C, H, and N were performed on a PerkinElemer240C analyzer. FT - IR spectra of the powder samples were collected on a Nicolet IS10 infrared spectrum radiometer in a wavenumber range of 4 000-400 cm^-1 using the KBr pellets. The luminescence spectra and luminescence lifetimes properties were measured with the FS5 fluorescence spectrometer (Edinburgh Instruments).

  1.2   Synthesis of the title complex

  1, 4-H₂bdc (8 mg, 0.05 mmol), Tb(NO₃)₃·6H₂O (21 mg, 0.05 mmol), phen (9 mg, 0.05 mmol) and H₂O/ DMAC/EtOH (5 mL, 1∶1∶1, V/V) were sealed in a clear glass vial (10 mL), heated at 95 ℃ for 72 h, and then cooled slowly to room temperature. The colorless and sticklike crystals were obtained and washed with ethanol with a yield of 20% (based on Tb³⁺). Anal. Calcd. for C₂₄H₁₆N₂O₇Tb(%): C, 47.77; H, 2.65; N, 4.64. Found (%): C, 47.81; H, 2.77; N, 4.83. IR (KBr pellet, cm^-1): 2 819(w), 2 671(w), 2 545(w), 1 678(s), 1 396(s), 1 390(s), 1 387(s), 1 286(s), 1 020(m), 933(m), 781(m), 728(vs).

  1.3   XRD structural determination

  Diffraction data of 1 were collected on a Bruker D8 venture diffractometer with graphite - monochromator at 293(2) K with Mo Kα radiation (λ =0.071 073 nm). The crystal data was solved by direct methods using the SHELXS-2014 program and refined by fullmatrix least-squares methods on F² using the program SHELXL-2014. Metal atoms in the complex were located from the E - maps while non - hydrogen atoms were located in successive difference Fourier syntheses and refined with anisotropic thermal parameters on F². Hydrogen atoms of the ligands were generated theoretically onto the specific atoms and refined with fixed thermal factors. The crystallographic data and experimental details for structural analyses are summarized in Table 1. Selected bond lengths for the complex are listed in Table 2.

  Table 1

  

  Table 1.  Crystallographic data and structural refinements of complex 1

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  Parameter	1		Parameter	1
  Empirical formula	C24H16N2O7Tb		Z	2
  Formula weight	603.31		D / (g·cm-3)	1.835
  θ range / (°)	2.2-26.5		μ / mm-1	3.29
  Crystal system	Triclinic		F(000)	590
  Space group	P1		Measured reflection	19 438
  a / nm	1.028 53(4)		Independent reflection	6 249
  b / nm	1.081 88(4)		Reflection with I > 2σ(I)	5 415
  c / nm	1.132 75(3)		Rint	0.049
  α / (°)	107.375(3)		R [F2 > 2σ(F2)]	0.034
  β / (°)	91.669(3)		wR (F2)	0.089
  γ / (°)	112.992(4)		Goodness-of-fit (on F2)	1.07
  V / nm3	1.092 04(7)

  Table 2

  

  Table 2.  Selected bond lengths (nm) and bond angles (°) of 1

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  Tb1—O7ⅰ	0.228 7(3)	Tb1—O6ⅰ	0.233 0(3)	Tb1—O1	0.233 4(3)
  Tb1—O4	0.235 9(3)	Tb1—O3	0.237 4(3)	Tb1—O1W	0.247 5(3)
  Tb1—N1	0.257 4(3)	Tb1—N2	0.260 5(4)
  O7ⅰ—Tb1—O6ⅰ	73.30(13)	O7ⅰ—Tb1—O1	84.33(11)	O6ⅰ—Tb1—O1	85.19(11)
  O7ⅰ—Tb1—O4	79.75(11)	O6ⅰ—Tb1—O4	124.39(11)	O1—Tb1—O4	139.42(11)
  O7ⅰ—Tb1—O3	123.92(11)	O6ⅰ—Tb1—O3	81.55(11)	O1—Tb1—O3	142.83(11)
  O4—Tb1—O3	74.67(11)	O7ⅰ—Tb1—O1W	73.81(12)	O6ⅰ—Tb1—O1W	140.84(12)
  O1—Tb1—O1W	70.94(11)	O4—Tb1—O1W	68.83(11)	O3—Tb1—O1W	135.42(11)
  O7ⅰ—Tb1—N1	146.31(13)	O6ⅰ—Tb1—N1	140.11(13)	O1—Tb1—N1	93.16(12)
  O4—Tb1—N1	80.65(11)	O3—Tb1—N1	76.00(11)	O1W—Tb1—N1	73.65(12)
  O7ⅰ—Tb1—N2	142.82(12)	O6ⅰ—Tb1—N2	79.20(13)	O1—Tb1—N2	68.68(11)
  O4—Tb1—N2	137.22(12)	O3—Tb1—N2	74.77(11)	O1W—Tb1—N2	117.52(12)
  N1—Tb1—N2	63.37(12)
  Symmetry code: ⅰ -x+1, -y+1, -z.

  CCDC: 2095406.

  2.   Results and discussion

  2.1   Structural description of 1

  X-ray single-crystal analysis reveals that complex 1 crystallizes in the triclinic space group P1. The asymmetric unit of 1 is composed of one Tb³⁺ ion, one and a half 1, 4-bdc^2- ligands, one phen ligand, and one coordinated water molecule. Tb³⁺ ion is eight-coordinate and surrounded by five oxygen atoms from five 1, 4 - bdc^2-, one oxygen atom from the water molecule, and two nitrogen atoms from one chelating phen molecule (Fig. 1a). As expected, the average distance of Tb—O (0.255 6 nm) is shorter than that of Tb—N (0.257 4 nm). 1, 4-bdc^2- adopt two kinds of coordination modes to bridge three Tb³⁺ ions and four Tb³⁺ ions (Fig. 1b). Neighboring Tb³⁺ ions are bridged by —O—C—O— links from four 1, 4 - bdc^2- into a binuclear unit with Tb…Tb distance of 0.432 1 nm (Fig. 1c). Such units are further bridged by 1, 4-bdc^2- into an infinite 2D plane (Fig. 1d).

  Figure 1

  

  Figure 1.  Crystal structure of complex 1: (a) coordination environment; (b) two kinds of coordination mode of 1, 4-bdc^2-; (c) binuclear unit; (d) 2D plane where phen molecules are omitted for clarity

  Symmetry code: ^ⅰ-x+1, -y+1, -z

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  2.2   PXRD analysis and stability in organic solvents

  The purity of the crystalline samples of 1 was confirmed by PXRD and the results shown in Fig. 2a confirm that the PXRD pattern of the as-synthesized complex matched well with the simulated one based on the single-crystal diffraction data. To explore its stability in different solvents, crystal samples of 1 were immersed in several common organic solvents, which were ethylene glycol (EG), N-methyl-2-pyrrolidone (NMP), MeOH, EtOH, dimethyl sulfoxide (DMSO), N, N dimethylformamide (DMF), N, N - dimethylacetamide (DMAC), N, N-diethylformamide (DEF), and CH₂Cl₂, for 24 h. As shown in Fig. 2b, the peak positions of the sample after soaking in different organic solvents and experimental patterns were in good agreement with each other, proving that 1 has good stability in the above organic solvents.

  Figure 2

  

  Figure 2.  (a) View of simulated and experimental PXRD patterns of 1; (b) PXRD patterns of 1 soaked in different solvents

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  2.3   Fluorescence spectra and sensing performance

  2.3.1   Luminescent properties

  The solid - state luminescence performance of 1 was studied at room temperature and the results are shown in Fig. 3. As shown in Fig. 3a, the luminescence spectrum of 1 showed a characteristic emission of Tb(Ⅲ) with an excitation wavelength of 310 nm. The emission peaks located at 489, 544, 585, 613 nm are ascribed to the ⁵D₄ → ⁷F_j (j=6, 5, 4, 3) transition of Tb³⁺. To better understand its luminescent properties, the luminescence lifetime was also studied. Under 310 nm excitation wavelength, the luminescence lifetime was studied by monitoring the most intense peak in the whole spectrum of 1 (⁵D₄ → ⁷F₅). The observed luminescent lifetime determined from Fig. 3b was 708 µs.

  Figure 3

  

  Figure 3.  (a) Emission spectra of 1 in solid-state; (b) Decay curve of 1 at room temperature

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  2.3.2   Sensing performance for metal ions

  To investigate the sensing ability of different metal ions, the ground powder of complex 1 (3 mg) was introduced into an ethanol solution (3 mL) of M(NO₃)_m(M^m+=Ni²⁺, Mn²⁺, Zn²⁺, K⁺, Cd²⁺, Ca²⁺, Mg²⁺, Fe³⁺, 1 mmol·L^-1). The resulting solution was homogenized under ultrasonication for 5 min, and then the fluorescence responses were recorded at room temperature (Fig. 4a). The luminescence intensity at 544 nm showed a slight decrease with the addition of Ni²⁺, Mn²⁺, Zn²⁺, K⁺, Cd²⁺, Ca²⁺, and Mg²⁺. On the contrary, the addition of Fe³⁺ prominently decreased the luminescence intensity (almost 0), indicating that complex 1 can selectively detect Fe³⁺ by fluorescence"turn - off", which also can even be observed by naked eyes (Fig. 4b).

  Figure 4

  

  Figure 4.  (a) Emission spectra of 1 in ethanol solutions with different metal ions; (b) Luminescence intensity at 544 nm of 1 in different ethanol solutions of various metal ions

  Inset: photographs of 1 under UV lamp after addition of different metal ions

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  To better understand the sensitivity to Fe³⁺ ions of complex 1, a luminescence quantitative titration experiment was carried out. The luminescence intensity of complex 1 descended gradually with increasing the concentration of Fe³⁺ ions (Fig. 5). Quantitatively, the calculated value of K_sv was 8.39 × 10³ L·mol^-1 using the Stern-Volmer equation at low concentration with a linear correlation coefficient value (R²) of 0.995^[22-26] (Fig. 6). According to LOD=3σ/k^[27-30], the calculated detection limit was 0.017 µmol·L^-1. The limit of detection of compound 1 was much superior to those CPs based sensor materials reported in the literature (Table 3).

  Figure 5

  

  Figure 5.  Emission spectra of 1 in Fe³⁺ solution with various concentrations

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  Figure 6

  

  Figure 6.  Stern-Volmer plot of 1 in Fe³⁺ solution with various concentrations

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  Table 3

  

  Table 3.  Comparison of sensing performance of compounds 1 for Fe³⁺ ions with other reported CPs

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  CP	LOD / (µmol·L-1)	Ksv / (L·mol-1)	Reference
  [Eu2(L)2(DMF)4]n	3.69	1.34×105	[22]
  [Tb(L)(H2O)3]n	1.79	8.39×104	[23]
  {[Tb(μ6-H2cpboda)(μ2-OH2)2]·H2O}n	0.84	6.50×104	[24]
  Eu-MOF	0.027	2.45×104	[25]
  Eu-BPDA	0.9	1.25×104	[26]
  [Eu(atpt)1.5(phen)(H2O)]n	45	7.60×103	[27]
  [Zn(modbc)2]n(Zn-CP)	0.57	7.20×103	[28]
  534-MOF-Tb	0.13	5.51×103	[29]
  [Zn2(cptpy)(btc)(H2O)]n	0.433	5.46×103	[30]
  [Tb(1, 4-bdc)1.5(phen)(H2O)]n (1)	0.017	8.39×103	This work

  The PXRD patterns of 1 before and after the Fe³⁺ treatment proved that the crystal structure of 1 remained stable (Fig. 2b), so the luminescence quenching was not caused by the decomposition of the framework. We further obtained the UV - Vis spectra of all the identified metal ions (1 mmol·L^-1 in ethanol). In Fig. 7, it can be seen that the absorption band of Fe³⁺ was in 300 - 400 nm region, which overlaps with the excitation wavelength of 1 (310 nm), indicating that there may be a competitive energy absorption progress. And other metal ions had negligible absorption on 310 nm. So, the quenching mechanism of 1 for Fe³⁺ may be attributed to competitive energy absorption, which is a common quenching mechanism in CPs sensing for Fe^3+[31-32].

  Figure 7

  

  Figure 7.  UV-Vis spectra of different metal ions used for the detection experiments

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  Seven kinds of interference substances were used to selectively detect the identified iron ions. As shown in Fig. 8, initially, with the addition of other metal ions, the fluorescence intensity of 1 showed a negligible change (left column). However, with the addition of Fe³⁺, the fluorescence intensity decreased rapidly (right column). The decrease in fluorescence intensity indicates that it can detect Fe³⁺ ions even in the presence of other metal ions.

  Figure 8

  

  Figure 8.  Fluorescence response of 1 to Fe³⁺ in the presence of different ions

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  3.   Conclusions

  A 2D coordination polymer, [Tb(1, 4-bdc)_1.5(phen) (H₂O)]_n (1), was synthesized by the solvothermal method and the luminescent emission spectrum and luminescence lifetime were investigated. Compound 1 exhibited good sensitivity and selectivity for detecting Fe³⁺ ions by fluorescence quenching mechanism with K_sv=8.39× 10³ L·mol^-1 and limit of detection of 0.017 µmol·L^-1.

  
  
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      Yin J C, Li N, Qian B B, Yu M H, Chang Z, Bu X H. Highly Stable Zn-MOF with Lewis Basic Nitrogen Sites for Selective Sensing of Fe³⁺ and Cr₂O₇^2- Ions in Aqueous Systems[J]. J. Coord. Chem., 2020, 73:  2718-2727. doi: 10.1080/00958972.2020.1830976

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  Figure 1  Crystal structure of complex 1: (a) coordination environment; (b) two kinds of coordination mode of 1, 4-bdc^2-; (c) binuclear unit; (d) 2D plane where phen molecules are omitted for clarity

  Symmetry code: ^ⅰ-x+1, -y+1, -z

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  Figure 2  (a) View of simulated and experimental PXRD patterns of 1; (b) PXRD patterns of 1 soaked in different solvents

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  Figure 3  (a) Emission spectra of 1 in solid-state; (b) Decay curve of 1 at room temperature

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  Figure 4  (a) Emission spectra of 1 in ethanol solutions with different metal ions; (b) Luminescence intensity at 544 nm of 1 in different ethanol solutions of various metal ions

  Inset: photographs of 1 under UV lamp after addition of different metal ions

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  Figure 5  Emission spectra of 1 in Fe³⁺ solution with various concentrations

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  Figure 6  Stern-Volmer plot of 1 in Fe³⁺ solution with various concentrations

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  Figure 7  UV-Vis spectra of different metal ions used for the detection experiments

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  Figure 8  Fluorescence response of 1 to Fe³⁺ in the presence of different ions

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  Table 1.  Crystallographic data and structural refinements of complex 1

  Parameter	1		Parameter	1
  Empirical formula	C24H16N2O7Tb		Z	2
  Formula weight	603.31		D / (g·cm-3)	1.835
  θ range / (°)	2.2-26.5		μ / mm-1	3.29
  Crystal system	Triclinic		F(000)	590
  Space group	P1		Measured reflection	19 438
  a / nm	1.028 53(4)		Independent reflection	6 249
  b / nm	1.081 88(4)		Reflection with I > 2σ(I)	5 415
  c / nm	1.132 75(3)		Rint	0.049
  α / (°)	107.375(3)		R [F2 > 2σ(F2)]	0.034
  β / (°)	91.669(3)		wR (F2)	0.089
  γ / (°)	112.992(4)		Goodness-of-fit (on F2)	1.07
  V / nm3	1.092 04(7)

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  Table 2.  Selected bond lengths (nm) and bond angles (°) of 1

  Tb1—O7ⅰ	0.228 7(3)	Tb1—O6ⅰ	0.233 0(3)	Tb1—O1	0.233 4(3)
  Tb1—O4	0.235 9(3)	Tb1—O3	0.237 4(3)	Tb1—O1W	0.247 5(3)
  Tb1—N1	0.257 4(3)	Tb1—N2	0.260 5(4)
  O7ⅰ—Tb1—O6ⅰ	73.30(13)	O7ⅰ—Tb1—O1	84.33(11)	O6ⅰ—Tb1—O1	85.19(11)
  O7ⅰ—Tb1—O4	79.75(11)	O6ⅰ—Tb1—O4	124.39(11)	O1—Tb1—O4	139.42(11)
  O7ⅰ—Tb1—O3	123.92(11)	O6ⅰ—Tb1—O3	81.55(11)	O1—Tb1—O3	142.83(11)
  O4—Tb1—O3	74.67(11)	O7ⅰ—Tb1—O1W	73.81(12)	O6ⅰ—Tb1—O1W	140.84(12)
  O1—Tb1—O1W	70.94(11)	O4—Tb1—O1W	68.83(11)	O3—Tb1—O1W	135.42(11)
  O7ⅰ—Tb1—N1	146.31(13)	O6ⅰ—Tb1—N1	140.11(13)	O1—Tb1—N1	93.16(12)
  O4—Tb1—N1	80.65(11)	O3—Tb1—N1	76.00(11)	O1W—Tb1—N1	73.65(12)
  O7ⅰ—Tb1—N2	142.82(12)	O6ⅰ—Tb1—N2	79.20(13)	O1—Tb1—N2	68.68(11)
  O4—Tb1—N2	137.22(12)	O3—Tb1—N2	74.77(11)	O1W—Tb1—N2	117.52(12)
  N1—Tb1—N2	63.37(12)
  Symmetry code: ⅰ -x+1, -y+1, -z.

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  Table 3.  Comparison of sensing performance of compounds 1 for Fe³⁺ ions with other reported CPs

  CP	LOD / (µmol·L-1)	Ksv / (L·mol-1)	Reference
  [Eu2(L)2(DMF)4]n	3.69	1.34×105	[22]
  [Tb(L)(H2O)3]n	1.79	8.39×104	[23]
  {[Tb(μ6-H2cpboda)(μ2-OH2)2]·H2O}n	0.84	6.50×104	[24]
  Eu-MOF	0.027	2.45×104	[25]
  Eu-BPDA	0.9	1.25×104	[26]
  [Eu(atpt)1.5(phen)(H2O)]n	45	7.60×103	[27]
  [Zn(modbc)2]n(Zn-CP)	0.57	7.20×103	[28]
  534-MOF-Tb	0.13	5.51×103	[29]
  [Zn2(cptpy)(btc)(H2O)]n	0.433	5.46×103	[30]
  [Tb(1, 4-bdc)1.5(phen)(H2O)]n (1)	0.017	8.39×103	This work

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