English

• 1. INTRODUCTION

  During the past few decades, luminescent metal-organic frameworks (MOFs) or coordination polymers (CPs) have been attracting lots of attention not only because of their intriguing structural motifs but also to their potential applications in chemical sensors, light-emitting devices, biomedicine and so on^[1-5]. Regulation of fluorescent properties is mainly caused by different metal ions and a large number of organic ligands to form MOFs or CPs with various structures^[6-9]. Moreover, the fluorescent performance of MOFs or CPs can be further tuned by encapsulating guest species such as cations, anions, vapors, and dyes into their highly regular channels and controllable pore sizes structures^[10-12].

  In comparison with the well studied fluorescent materials, MOFs or CPs featuring room temperature phosphorescence (RTP) properties have rarely been researched owing to the spin-forbidden for the transition of the lowest triplet excited state (T₁)^{[10, 11, 13-19]}. Benefiting from the long-last lifetime and large stock shifts, phosphorescent materials have been well applied in the fields of biological imaging, chemical sensing, security protection, organic light-emitting diodes (OLEDs) and so on^[20-22]. So far, most of RTP materials are focused on the noble-metal, such as rhodium (Rh), iridium (Ir), platinum (Pt) and palladium(Pd) based complexes^[23]. However, these metals are high-cost and less reserves on the earth. So, design and synthesis of new phosphorescent molecules based on low-cost and abundant metals are important to sustainable development of light-emitting devices, photosensitizers and imaging agents. It has been confirmed that organic chromophore with thio, and/or nitrogen heterocycles groups have the potential to emit phosphorescence owing to the existence of n→p* transition facilitating the spin-forbidden shift of singlet-to-triplet^[11]. In this work, a nitrogen heterocylic phosphorescent ligand 2-propylimidazole (PPIM) was used to construct new types of none noble-metal based coordination polymer with RTP performance by the mixture of 1, 4-naphthalenedicarboxylic acid (1, 4-NDC). One new cadmium(Ⅱ) coordination polymer [Cd(1, 4-NDC)(PPIM)₂]_n (1) has been synthesized under hydrothermal conditions. The title complex exhibits highly prolonged RTP lifetime 452 times in comparison with that of pristine PPIM ligand, which was further investigated by theory calculations.

  2. EXPERIMENTAL

  2.1 Materials and general methods

  All commercially available solvents and chemicals were of analytical grade, and used without further purification. Elemental analyses for carbon, hydrogen, and nitrogen atoms were performed on a Vario EL Ⅲ elemental analyzer (Elementar, Germany). The crystal was determined on a Bruker SMART APEX Ⅱ CCD diffractometer (Madison, WI, USA) equipped with a graphite-monochromatized MoKα radiation (λ = 0.71073 Å). Room temperature photolu-minescence spectra and time-resolved lifetime were conducted on an Edinburgh FLS1000 fluorescence spectrometer equipped with a xenon arc lamp (Xe900), nanosecond flash-lamp (nF900) and a microsecond flashlamp with time-resolved single photon counting-multi-channel scaling (MCS) mode. PXRD patterns were collected on a Bruker D8-ADVANCE X-ray diffractometer with CuKα radiation (λ = 1.5418 Å).

  2.2 Synthesis of [Cd(1, 4-NDC)(PPIM)₂]_n (1)

  A mixture of 1, 4-naphthalenedicarboxylic acid (0.02 g, 0.1 mmol), 2-propylimidazole (0.02 g, 0.2 mmol) andCd(NO₃)₂·4H₂O (0.06 mg, 0.2 mmol) were added to water (12 mL) in a 25 mL Teflon-lined stainless-steel vessel. The mixture was heated at 423 K for 3 days, and then slowly cooled down to room temperature. Colourless block crystals of 1 were obtained (yield: 53% based on cadmium). Anal. Calcd. (%) for C₂₄H₂₆CdN₄O₄: C, 52.71; H, 4.79; N, 10.24. Found (%): C, 53.95; H, 4.71; N, 10.31.

  2.3 Crystal structure determination

  Single-crystal X-ray diffraction analysis of the complex was carried out by Oxford Diffraction SuperNova area-detector diffractometer using mirror optics monochromated MoKα radiation (λ = 0.71073 Å). The structure was solved by direct methods with SHELXS-2014^[24]. The hydrogen atoms were assigned with common isotropic displacement factors and included in the final refinement by use of geometrical restrains. A full-matrix least-squares refinement on F² was carried out using SHELXL-2014^[25]. A total of 32429 reflections were obtained and 4693 unique (R_int = 0.0350) were collected in the range of 3.33 < θ < 25.99° by an ω scan mode, of which 4693 reflections with I > 2σ(I) were used in the succeeding refinement. The final R = 0.0360, wR = 0.0850, and S = 1.076.

  2.4 Periodic density functional theoretical (PDFT) calculations

  All calculations were performed with the periodic density functional theory (DFT) method using Dmol3 module in Material Studio software package^[26]. The initial configuration was fully optimized by Perdew-Wang (PW91)^[27] generalized gradient approximation (GGA) method with the double numerical basis sets plus polarization function (DNP). The Brillouin zone is sampled by 1 × 1 × 1 k-points.

  3. RESULTS AND DISCUSSION

  3.1 Powder X-ray diffraction (PXRD) and thermogravimetric analysis of 1

  In order to check phase purity of complex 1, PXRD experiment was performed. As shown in Fig. 1a, all peaks of powder diffraction measured by the experiment are in good agreement with the simulated PXRD patterns based on the single-crystal diffraction data, which confirmed high phase purity of the as-synthesized samples. Moreover, narrow and intense diffraction peaks demonstrated the high crystallinity of the samples. The thermogravimetric analysis (TGA) curve indicates that complex 1 is thermally stable upon heating to about 310 ℃, and then suffers three consecutive steps of weight loss up to 620 ℃, indicating the removal of organic components (Fig. 1b). The first stage from 310 to 360 ^oC can be considered as the departure of 1, 4-NDC ligands, the second one is between 360 and 450 ^oC due to the release of half of PPIM ligands, and the third stage falls in the 450~620 ^oC range caused by the loss of the other half of PPIM ligands.

  Figure 1

  

  Figure 1.  PXRD patterns (a) and thermogravimetric analysis (b) curve of 1

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  3.2 Description of the crystal structure of [Cd(1, 4-NDC)(PPIM)₂]_n (1)

  Single-crystal X-ray diffraction analysis reveals that complex 1 crystallizes in the monoclinic space group P2₁/n and its structure shows a 1D chain. The asymmetric unit of 1 consists of one crystallographically independent Cd(Ⅱ) ion, one 1, 4-NDC and two PPIM ligands. As shown in Fig. 2a, each Cd(Ⅱ) ion is six-coordinated with two nitrogen atoms from two PPIM molecules (Cd(1)–N(1) = 2.234(3) Å, Cd(1)–N(3) = 2.250(3) Å) and four oxygen atoms from two 1, 4-NDC ligands (Cd(1)–O(1) = 2.265(2) Å, Cd(1)–O(2) = 2.601(2) Å, Cd(1)–O(3A) = 2.386(2) Å, Cd(1)–O(4A) = 2.368(2) Å). The fully deprotonated 1, 4-NDC ligand shows none-plane conformation with large torsion angle between carboxylic group and naphthalene ring of 52.59° due to the bulk spatial resistance of naphthalene ring (Fig. 2b). Each Cd(Ⅱ) atom is connected by 1, 4-NDC ligand via chelate coordination mode to form 1D zigzag polymer chains with a pair of PPIM ligands hanging on two sides (Fig. 2c).

  Figure 2

  

  Figure 2.  (a) Coordination environment of Cd(Ⅱ) ion in 1. (b) Torsion angle between carboxylic group and naphthalene ring of 1, 4-NDC ligand. (c) 1D zigzag chain structure of 1 along the b axis

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  In 1, the PPIM ligands act as hydrogen-bonding donor (Table 1). As a result, the adjacent 1D chains are further linked together to generate a 2D layer (Fig. 3a) along the ab plane by N–H∙∙∙O hydrogen-bonding interactions (N(2)–H(2)∙∙∙O(4A)). Herein, viewed along the b direction, the other kind of PPIM ligands hang on up-and-down sides of the 2D layer. And then, the 2D layers were further extended to the overall 3D supramolecular network by N–H∙∙∙O hydrogenbonding interactions between carboxylate O atoms of 1, 4-NDC ligands and imidazole rings (N(4)–H(4)∙∙∙O(2B)) (Fig. 3b). It can be concluded that the N–H∙∙∙O hydrogenbonding interactions play an important role in forming and stabilizing the network of the title complex.

  Table 1

  

  Table 1.  Hydrogen-bonding Scheme (Distances and Angles) for Complex 1

  DownLoad: CSV

  D–H···A	D–H (Å)	H···A (Å)	D···A (Å)	D–H···A (°)
  N(2)–H(2)∙∙∙O(4A)	0.86	1.96	2.804(4)	168
  N(4)–H(4)∙∙∙O(2B)	0.86	1.95	2.808(4)	172
  Symmetry codes: A = 0.5 – x, –0.5 + y, 1.5 – z; B = 0.5 – x, 0.5 + y, 1.5 – z

  Figure 3

  

  Figure 3.  (a) View of the 2D layer structure of 1 along the c direction by N(2)–H(2)∙∙∙O(4A) hydrogen bonding. (b) View of the 3D supramolecular network of 1 along the b direction extended by N(4)–H(4)∙∙∙O(2B) hydrogen bonding between adjacent layers (Layers show different colors for clarity)

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  3.3 Room-temperature phosphorescent (RTP)

  The photophysical properties of the pristine 1, 4-NDC, PPIM and 1 were investigated by steady-/transien-state and time-resolved photoluminescence (PL) spectroscopy in the solid state at room temperature. As shown in Fig. 4a, excited by 366 nm, 1 displays cyan fluorescence emission at 436 nm with the color coordinate of (0.249, 0.271). Upon excitation at 370 nm by a microsecond flash lamp (100 Hz), the phosphorescence emission peaked at 600 nm with two shoulders of 515 and 558 nm can be detected (color coordinate (0.454, 0.456)). In contrast, the free 1, 4-H₂NDC and PPIM emit cyan fluorescence with the main peaks at 492 and 485 nm, respectively (λ_ex = 360 nm). The phosphorescent spectra of PPIM have a main peak at 690 nm (Fig. 4b). The phosphorescence decay curves (Fig. 4c) give a longer lifetime value of 724 μs for 1, which is about 452 times as long as that of the pristine PPIM (1.6 μs) (Fig. 4d). The phosphorescent emission for free 1, 4-H₂NDC can not be detected. This results indicate that the phosphorescent ligand PPIM can be tightly fixed in the rigid matrix of coordination polymer through strong coordination bonds and hydrogen-bonding interactions, which highly reduce the nonradiative loss of triplet excitons and promote long-last phosphorescent emission in comparison with the free PPIM ligand.

  Figure 4

  

  Figure 4.  Normalized PL spectra of 1 (a) free PPIM (b) measured under fluorescence (black) and phosphorescence (red) modes as well as PL decay curves of 1 (c) and free PPIM (d) in solid state at room temperature

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  3.4 Periodic density functional theoretical (PDFT) calculations

  The periodic density functional theoretical (PDFT) calculations were carried out to better demonstrate the photophysical properties of 1. The theoretical mode was selected by the crystallographic information file (cif) of 1 directly. It can be obviously observed that the electronic clouds exclusively distribute on the PPIM ligands from HOMO-7 to HOMO-4. The HOMO-3 to HOMO and LUMO up to LUMO+3 are mainly located on 1, 4-NDC ligands. Herein, the good separation of molecular orbitals between 1, 4-NDC and PPIM ligands is benefit to restrain the recombination of electro and hole, prolonging the phosphorescent emission (Fig. 5). Herein, the large spatial separation of molecular orbitals between PPIM donor and 1, 4-NDC acceptor can restrain electronic coupling^[28], promoting the efficient separation of electron and hole for long lifetime phosphorescent emission^[14]. This result distinctly differs from those of cluster based MOFs, in which molecular orbitals are well separated between the metal clusters and organic ligands^[10].

  Figure 5

  

  Figure 5.  Distributions of the highest occupied molecular orbitals (HOMOs) and lowest unoccupied molecular orbitals (LUMOs) of 1

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  4. CONCLUSION

  In summary, one new Cd(Ⅱ) based coordination polymer [Cd(1, 4-NDC)(PPIM)₂]_n (1) has been synthesized based on the mixture of poly-carboxylate ligand 1, 4-naphthalenedi-carboxylic acid (1, 4-NDC) and 2-propylimidazole (PPIM). Crystal structure reveals that 1 exhibits a 3D supramolecular network extended by N–H···O hydrogen bond from 1D infinite zigzag chain. Fixed in the coordination polymer matrix through strong coordination bond, the phosphorescent life-time of free PPIM ligand can be highly enhanced more than 450 times. Further theoretical calculations indicate that complete separation of HOMOs and LUMOs between 1, 4-NDC and PPIM can efficiently prevent the recombination of electron-hole, facilitating long-last phosphorescent emission. Therefore, the present work provides a new strategy to construct none noble-metal based RTP materials by the strategy of mixture-ligands in coordination polymer system.

  
  
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• 

  Figure 1  PXRD patterns (a) and thermogravimetric analysis (b) curve of 1

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  Figure 2  (a) Coordination environment of Cd(Ⅱ) ion in 1. (b) Torsion angle between carboxylic group and naphthalene ring of 1, 4-NDC ligand. (c) 1D zigzag chain structure of 1 along the b axis

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  Figure 3  (a) View of the 2D layer structure of 1 along the c direction by N(2)–H(2)∙∙∙O(4A) hydrogen bonding. (b) View of the 3D supramolecular network of 1 along the b direction extended by N(4)–H(4)∙∙∙O(2B) hydrogen bonding between adjacent layers (Layers show different colors for clarity)

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  Figure 4  Normalized PL spectra of 1 (a) free PPIM (b) measured under fluorescence (black) and phosphorescence (red) modes as well as PL decay curves of 1 (c) and free PPIM (d) in solid state at room temperature

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  Figure 5  Distributions of the highest occupied molecular orbitals (HOMOs) and lowest unoccupied molecular orbitals (LUMOs) of 1

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  Table 1.  Hydrogen-bonding Scheme (Distances and Angles) for Complex 1

  D–H···A	D–H (Å)	H···A (Å)	D···A (Å)	D–H···A (°)
  N(2)–H(2)∙∙∙O(4A)	0.86	1.96	2.804(4)	168
  N(4)–H(4)∙∙∙O(2B)	0.86	1.95	2.808(4)	172
  Symmetry codes: A = 0.5 – x, –0.5 + y, 1.5 – z; B = 0.5 – x, 0.5 + y, 1.5 – z

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•