LIU Ye-Kun ZHOU Xin-Hui

a (Queen Mary University of London Engineering School, Northwestern Polytechnical University, Xi'an 710072, China)

b (Key Laboratory for Organic Electronics and Information Displays & Jiangsu Key Laboratory for Biosensors, Institute of Advanced Materials (IAM), Jiangsu National Synergetic Innovation Center for Advanced Materials (SICAM), Nanjing University of

Posts & Telecommunications, Nanjing 210023, China)

ABSTRACT A new terbium(III) metal-organic framework [Tb(HL)(H2O)] (TbL) based on a new synthetic ligand 9-(2,6-dicarboxy-pyridin-4-yl)-9H-carbazole-3,6-dicarboxylic acid (H4L) has been synthesized under solvothermal conditions. Its structure was determined by single-crystal X-ray diffraction analysis, and further characterized by powder X-ray diffraction analysis and IR spectra. The title complex crystallizes in trigonal space group P3212 with a = b = 13.6491(11), c = 32.345(3) ?, γ = 120°, V = 5218.5(10) ?3, C42H20N4O17Tb2, Mr = 1170.46, Dc = 1.117 g/cm3, μ(MoKα) = 2.065 mm-1, F(000) = 1698, GOF = 1.054, Z = 3, the final R = 0.0384 and wR = 0.0771 for 5223 observed reflections (I ＞ 2σ(I)). In TbL, the tri-bridged binuclear Tb2 units are bibridged by two carbazole-3,6-dicarboxylate moieties to lead to the homochiral parallel arranged helical chains, which are further connected by 2,6-pyridinedicarboxylate moieties to produce the chiral neutral 3D framework. There are there kinds of 1D channels in the framework with the channel space occupying 53.3% of the total volume. TbL exhibits intense characteristic green emission of Tb3+ ions.

Keywords: metal-organic frameworks (MOFs), terbium(III), carbazole, photoluminescence;

1 INTRODUCTION

In the past two decades, metal-organic frame- works (MOFs) have drawn lots of attention from chemists and material scientists due to their fascinating structural aesthetics, rich optical, electrical and magnetic phenomena as well as their promising applications in gas storage and separation, catalysis, sensors, semiconductor devices, multifer- roic materials, etc[1-14]. They can always bring us various kinds of surprises. For example, in 2017, Yaghi group reported a porous metal-organic framework [Zr6O4(OH)4(fumarate)6]}, MOF-801, which is capable of harvesting 2.8 liters of water per kilogram of MOF-801 daily at relative humidity levels as low as 20%, and requires only the input of energy from natural sunlight[7]. More recently, MOFs materials have been combined with another star material perovskite. In 2019, Fei group reported the intrinsic white-light-emitting MOFs [Pb2X3+][-O2C(C6H4)CO2-]2[(CH3)2NH2+]3(X = Cl, Br, or I) with excellent stability, tunability and moisture resistance, in which the structurally deformable haloplumbate units often observed in organolead halide perovskites have been success- fully incorporated into MOFs[15].

Metal-organic frameworks are made up of metal ions or metal clusters and organic ligands, in which the types of metal ions are very limited when compared with the organic ligands. The structures and properties of the complexes are affected or even directly determined by organic ligands. Therefore, the design and syntheses of organic ligands play a vital role in the development of MOFs. The fluorene and carbazole molecules and their derivatives are excellent electron donors and have been used extensively in organic photoelectric functional mate- rials[16,17]. Our group has been committed to the syntheses and performance studies of coordination compounds with the fluorene- and carbazole-based ligands for ten years[2,18-30]. We have synthesized eight fluorene- and carbazole-based ligands succes- sively, which are 9,9-dimethylfluorene-2,7-dicar- boxylic acid, 9,9-dimethylfluorene-2,7-diphosphonic acid, 4,4?-(9,9-dimethyl-9H-fluorene-2,7-diyl)diben- zoic acid, 9-methyl-9-hydroxy-fluorene-2,7-dicar- boxylic acid, 9-fluorenone-2,7-dicarboxylic acid, 9-(pyridin-4-yl)-9H-carbazole-3,6-dicarboxylic acid, 9-(pyridin-4-yl)-9H-carbazole-3,6-dicarboxylic acid and 9H-carbazole-2,7-dicarboxylic acid, respectively. In addition, lanthanide ions possess excellent optical, electrical and magnetic properties such as rare earth catalysts, single-ion and single-molecule magnets, photo-emitting materials, and so on[31-35].

In this work, we continued to synthesize a new carbazole-based ligand 9-(2,6-dicarboxy-pyridin- 4-yl)-9H-carbazole-3,6-dicarboxylic acid (H4L). Based on this ligand, a metal-organic framework [Tb(HL)(H2O)] (TbL) was obtained by solvother- mal synthesis method. Here, we report the syntheses of H4L ligand and MOF TbL, structure and pro- perty of TbL.

2 EXPERIMENTAL

2. 1 Materials and general methods

Unless otherwise indicated, chemicals and solvents were purchased from commercial suppliers or purified by standard techniques, and all reactions were carried out in standard operation. Diethyl 4-iodopyridine-2,6-dicarboxylate and 3,6-diiodo- 9H-carbazole (1) were prepared according to pre- vious literatures[36,37]. Reactions were monitored by TLC on silica gel plates (GF254).1H NMR and13C NMR spectra were recorded on Bruker Avance instruments (400 MHz and 100 MHz, respectively) and internally referenced to tetramethylsilane signal or residual protio solvent signals (s = singlet, d = doublet, dd = double doublet, t = triplet, q = quartet, m = multiplet). Flash column chromatography was carried out using silica gel (200～300 mesh) eluting with ethyl acetate/petroleum ether or dichloro- methane/methanol. FT-IR spectra were measured in the range of 400～4000 cm-1with a PerkinElmer- Spectrum on KBr pellets. Powder X-ray diffraction patterns (PXRD) data of all samples were collected in the 2θrange of 5～50° with a scan step width of 0.02° on a Bruker D8 Advance A25 diffractometer (Cu-Kα,λ= 1.5418 ?). Photoluminescence spectra were carried out using a RF-5301PC spectrofluoro- photometer at the room temperature.

2. 2 Preparation of 2

To the mixture of 3,6-diiodo-9H-carbazole (1, 4.2 g, 10.0 mmol), diethyl 4-iodopyridine-2,6-dicar- boxylate (3.5 g, 10.0 mmol), copper(I) iodide (190 mg, 1.0 mmol),L-proline (230 mg, 2.0 mmol) and K2CO3(276 mg, 2.0 mmol) in Schlenk flask was added 20 mL dimethyl sulfoxide (DMSO). The flask was degassed in vacuum and backfilled with argon and heated to 90 °C for 24 h. After cooling to room temperature, the reaction mixture was poured into water and extracted with dichloromethane. The combined organic phases were dried over Na2SO4and the solvent was removed afterwards. Puri- fication of the residue by flash chromatography afforded the desired product 2 as a light yellow solid (4.73 g, 74% yield). 8.72 (s, 2H), 8.40 (s, 2H), 7.76 (d,J= 8.4 Hz, 2H), 7.45 (d,J= 8.8 Hz, 2H), 4.41 (dd,J= 14.0, 6.8 Hz, 4H), 1.35 (t,J= 6.8 Hz, 6H).13C-NMR (100 MHz, DMSO):δ(ppm) 163.5, 150.2, 145.9, 138.1, 135.2, 129.7, 124.9, 124.1, 112.1, 85.1, 61.8, 14.0.

2. 3 Preparation of 3

A mixture of the diethyl 4-(3,6-diiodo-9H-car- bazol-9-yl)pyridine-2,6-dicarboxylate (2, 0.64 g, 1 mmol) and dried copper(I) cyanide (0.45 g, 5 mmol) in dry N-methyl pyrrolidone (NMP) (5 mL) was heated in a sealed tube at 140 °C overnight. After cooling to room temperature, a mixture of water (18 mL), HCl (6 mL) and FeCl3(1.45 g, 9 mmol) was poured into the reaction mixture and the solution was stirred for 1 h. The resulting brown precipitate was filtered and washed with water. The solid was re-dissolved in dichloromethane (DCM) and washed with water. The combined organic phases were dried over Na2SO4and the solvent was removed at reduced pressure to give the crude product which was further purified by flash chromatography to give the desired product 3 as a white solid (0.13 g, 30% yield). 8.96 (s, 2H), 8.52 (s, 2H), 7.97 (d,J= 10.8 Hz, 2H), 7.77 (d,J= 11.2 Hz, 2H), 4.42 (dd,J= 14.0, 6.8 Hz, 4H), 1.34 (t,J= 7.2 Hz, 6H).13C-NMR (100 MHz, CDCl3):δ(ppm) 163.7, 150.5, 145.1, 142.1, 131.0, 126.5, 125.4, 122.9, 119.5, 111.8, 104.0, 61.8, 14.1.

2. 4 Preparation of H4L

A mixture of diethyl 4-(3,6-dicyano-9H-car- bazol-9-yl)pyridine-2,6-dicarboxylate (3, 88 mg, 0.2 mmol) and KOH (0.22 g, 4 mmol) in water (1 mL) and ethanol (5 mL) was heated at reflux for 3 d. The reaction mixture was cooled at room temperature, diluted with water (20 mL) and acidified with HCl 2M topH = 1. The resulted precipitate was filtered, washed with water and crystallized from ethanol to give pure H4L as a off-white solid (58.8 mg, 70% yield).1H-NMR (400 MHz, CDCl3):δ(ppm) 8.93 (s, 2H), 8.43 (s, 2H), 8.07 (dd,J= 8.8, 1.6 Hz, 2H), 7.65 (d,J= 8.8 Hz, 2H) 3.82～3.17 (m, 4H).13C-NMR (100 MHz, CDCl3): δ(ppm) 167.3, 165.0, 150.9, 145.8, 142.3, 128.5, 124.5, 124.3, 123.3, 123.0, 110.0.

Scheme 1. Synthetic route for the preparation of H4L

2. 5 Preparation of TbL

H4L (0.04 mmol, 0.0172 g) and Tb(NO3)3·6H2O (0.04 mmol, 0.0180 g) were dissolved in 2 mL N,N-dimethylformamide (DMF), and then 2 mL deionized water was added. The mixture was sealed in a 15 mL Teflon-lined stainless-steel vessel, and heated at 180 °C for 72 h. After cooling to room temperature, the colorless crystals were obtained, washed by DMF, and then put in a glass desiccator. IR data (KBr, cm-1): 3428 (b, m), 1602 (s), 1538 (w), 1395 (s), 1136 (w), 1023 (w), 784 (w), 741 (w), 645 (w).

2. 6 X-ray crystallography

X-ray diffraction data of the single crystal of TbL were collected on a Bruker Smart Apex CCD area detector diffractometer (Germany) using graphite-monochromated Mo-Kαradiation (λ= 0.71073 ?). Absorption corrections were applied using the SADABS supplied by Bruker[38]. Structures were solved by direct methods and refined by full-matrix least-squares method proto- cols onF2using the program SHELXL-2014/7 program package[39]. The positions of metal atoms and their first coordination spheres were located from direct-method E maps; other non-hydrogen atoms were found using alternating difference Fourier syntheses and least-squares refinement cycles and, during the final cycles, were refined anisotropically. The hydrogen atoms were placed in calculated positions and refined as riding atoms with a uniform value of Uiso. Because the isolated solvents within the channels were not crystallogra- phically well-defined, the data were treated with the SQUEEZE routine within PLATON[40]. The selected bond lengths and bond angles are listed in Table 1.

Table 1. Selected Bond Lengths (?) and Bond Angles (°)

3 RESULTS AND DISCUSSION

The compound TbL is obtained under the solvo- thermal condition.

3. 1 Structural description of complex TbL

Single-crystal X-ray analysis on TbL reveals a chiral neutral three-dimensional (3D) framework. In the asymmetric unit of TbL, there are one Tb3+ion, one HL3-ligand and one coordinated H2O molecule (Fig. 1a). The Tb3+ion is eight-coordinated by O(6), O(8) and N(2) from one tri-dentate chelating 2,6-pyridinedicarboxylate group of a HL3-ligand, O(3)#3and O(4)#3from a di-dentate chelated car- boxylate group of another HL3-ligand, O(1)#1and O(2)#2from the third and fourth HL3-ligands, and O(9) from theμ2-H2O molecule. The eight coor- dinated atoms form the distorted bicapped trigonal prismatic coordination geometry around the Tb3+ion. The Tb-O bond distances range from 2.349(5) to 2.418(6) ? and the Tb-N bond distance is 2.485(7) ? in TbL, which are comparable to those reported for other Tb3+complexes[41,42]. The HL3-ligand links four Tb3+ions by 2,6-pyridinedicarboxylate moiety tri-dentate chelating to a Tb3+ion, a carbazole carboxylate group chelating to the second Tb3+ion, another carbazole carboxylate group coordinating to the third and fourth Tb3+ions with each oxygen atom coordinating to a Tb3+ion. In the HL3-ligand, the dihedral angle between the carbazole and pyridine rings is 61.803(2)o. Two carboxylate groups and a water molecule link two Tb3+ions in a tri-bridge mode to form a binuclear Tb2unit with the Tb···Tb distance of 3.861(1) ?. Each two neighboring binuclear Tb2units are bibridged by two car- bazole-3,6-dicarboxylate moieties from two HL3-ligands to lead to a serials of homochiral parallel arranged helical chains extending along thecaxis (Fig. 1b). In each helical chain, there existπ···πstacking interactions between two carbazole-3,6- dicarboxylate moieties as a bibridge with the centroid-to-centroid distance in the range of 3.698(7)～3.779(5) ? (Table 2 and Fig. 1b). These homochiral helical chains are further connected by 2,6-pyridinedicarboxylate moieties to produce the chiral neutral 3D framework of TbL (Fig. 1c and 1d). In the framework, there are three classes of 1D channels. One possesses the hexagonal window and exists within the helical chain; one possesses the triagonal window and exists among the helical chains (Fig. 1c); and the last one possesses the fish-mouth-like window and extends along thea,baxes and [110] direction, respectively (Fig. 1d). The solvent accessible space for the desolvated TbL is 2783.1 ?3per unit cell or 53.3% of the total volume, calculated using the PLATON routine[43].

Fig. 1. (a) View of the asymmetric unit of TbL with thermal ellipsoids drawn at the 30% probability level. (b)Helical chains. Views of the 3D framework along the c (c) and a (d) axes. All hydrogen atoms are omitted for clarity. Symmetry transformation: #1 1 - y, 1 - x, 4/3 - z; #2 -1 + x, -1 + y, z; #3 -x + y, -1 + y, 5/3 - z

Table 2. Defined Rings and Relative Para mete rs of the π···π Interactions for Complex TbL

3. 2 PXRD and IR

In order to check the phase purity of TbL, the PXRD of the compound was recorded at room temperature. As shown in Fig. 2, the peak positions of the simulated pattern closely match those of the experimental one, indicating phase purity of the as-synthesized samples. In IR spectrum of TbL (Fig. S2), the strong absorption peaks locating at around 1602～1395 cm-1are characteristic of the expected absorption for asymmetric and symmetric vibrations of the carboxylate groups. The broad band observed at around 3428 cm-1should result from the stre- tching vibrations of H2O molecules in TbL.

3. 3 Photoluminescence property

As shown in Fig. 3, the solid-state photolumine- scence properties of TbL and free H4L ligand were studied at room temperature. The free H4L ligand emits blue light with the broad emission peak and the emission maximum at 455 nm upon excitation at the maximum excitation wavelength of 378 nm. The emission spectrum of TbL in the solid state excited at 374 nm exhibits the luminescence peaks at 497, 552, 591 and 628 nm, which could be attributed to the characteristic transitions of the Tb3+ion:5D4→7FJ(J= 6, 5, 4 and 3), respectively. In the emission spectrum of TbL, the strong blue light emission in free H4L completely disappear, suggesting that the ligands effectively transfer energy to the Tb(III) ions.

Fig. 2. PXRD patterns of TbL

Fig. 3. Excitation and emission spectra of TbL and H4L

4 CONCLUSION

In conclusion, based on a new synthetic ligand 9-(2,6-dicarboxy-pyridin-4-yl)-9H-carbazole-3,6-di- carboxylic acid (H4L), a metal-organic framework [Tb(HL)(H2O)] (TbL) has been obtained. TbL possesses the chiral neutral 3D framework con- structed by the interconnected homochiral parallel arranged helical chains. Three kinds of 1D channels are observed in the framework. TbL exhibits intense characteristic green emission of Tb3+ions in the solid state at room temperature.