alkene

Article Cite This: Cryst. Growth Des. 2018, 18, 4937−4944

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Series of Mn(II)/Mg(II)/Zn(II) Coordination Polymers with Azo/Alkene Functionalized Ligands Asha P† and Sukhendu Mandal*,† †

School of Chemistry, Indian Institute of Science Education and Research, Thiruvananthapuram, Kerala-695551, India

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ABSTRACT: A series of six novel coordination polymers based on Mn(II)/ Mg(II)/Zn(II) metal ions with azo/alkene functionalized carboxylic acid have been synthesized and well-characterized using several state-of-art techniques. Compound 1, {[Mn2(ABDCA)2(DPE)(DMF)2]·2DMF}n, (where ABDCA = Azobenzene-4,4′-dicarboxylic acid, DPE = 1,2-Di(4-pyridyl)ethylene, and DMF = N,N′-Dimethylformamide) is a three-dimensional structure with threefold interpenetration. Compound 2, {[Mn3(ABDCA)3(DMF)4]·2DMF}n contains Mn3O16 cluster unit which is connected with ABDCA ligands to form a twodimensional layered structure. Compound 3, {[Mg3(ABDCA)3(DMF)4]·4DMF}n contains Mg3O16 clusters, associated with ABDCA ligands, forming the 3D structure. Coordination polymers 4 and 5 ({[Zn(ABDCA)(2,2′-BPy)]·H2O}n, (4), {[Zn(SDCA)(2,2′-BPy]}n, (5)), respectively, (where SDCA = stilbene-4,4′dicarboxylic acid and 2,2′-BPy = 2,2′-Bipyridine) are identical structures with a difference in carboxylate ligand. In both cases, the one-dimensional chains are π-stacked through 2,2′-bipyridine molecules to form the supramolecular three-dimensional structure. Compound 6 ({[Zn(4,4′-BPy)(ABDCA)]·DMF}n (where 4,4′ BPy = 4,4′-Bipyridine) adopts three-dimensional architecture with fivefold interpenetration. Photoluminescence studies showed that all these compounds were emissive due to either intraligand or ligand to metal charge transfer. Optical band gap energy measurements showed that the values are less for ABDCA based compound compared to SDCA based compounds, and this is due to the difference in the HOMO−LUMO gap in the corresponding ligands. The magnetic measurements of compounds 1 and 2 showed that 1 behaves weakly antiferromagnetic while 2 as paramagnetic.

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ature.35 Azobenzene derivatives have gained popularity due to their distinctive photophysical properties and immense utilization in photoresponsive materials, and nonlinear optics.37−39 Azobenzene or stilbene and its derivatives can be used as potential functional materials due to the possibility of trans to cis photoisomerization that results in significant modification in properties, such as color, dielectric constant, dipole moment, etc.21−34,40−42 Here we have used ABDCA and SDCA as ligands to synthesize alkaline earth and transition metal ions based functional materials with a hierarchy of structures. Herein we describe the synthesis, structure, luminescence, and magnetic properties of six new coordination polymers (CPs). The compounds are {[Mn2(ABDCA)2(DPE)(DMF)2]· 2DMF} n, (1), {[Mn 3(ABDCA) 3(DMF) 4]·2DMF} n, (2), {[Mg3(ABDCA)3(DMF)4]·4DMF}n, (3), {[Zn(ABDCA)(2,2′-BPY)]·H2O}n, (4), {[Zn(SDCA)(2,2′-BPy]}n, (5), and {[Zn(4,4′-BPy)(ABDCA)]·DMF}n, (6). All these compounds are synthesized under solvothermal conditions. Compound 1 adopts a three-dimensional structure with three interpenetrating nets with pcu topology. Compounds 2 and 3 have 2-nodal

INTRODUCTION The design and production of metal−organic frameworks (MOFs) or porous coordination polymers (PCPs) is at the forefront of materials research due to their vast applications. The formation of the desired structure could be controlled by selecting the linkers and metal ions with utmost care.1−5 However, most of the time our efforts go in vain and the products formed will be unexpected. For the past two decades, several attempts were made to design and build novel, porous MOFs or PCPs for diverse applications like gas storage,6 drug delivery,7,8 heterogeneous catalysis,9,10 and sensing.11−13 The unique properties like high surface area,14 tunable porosity,15 and thermal16 and chemical stability17 make the MOFs outstanding candidates for applications in day-to-day life. Metal ions and the organic ligands take part in the MOF structure formation, and these retain their inherent properties in the end structure.18 This provides us with the opportunity to keenly select the precursors, keeping the application in mind. A large number of MOFs or CPs were reported using various carboxylate ligands with transition metal ions.19,20 However, very few structures with azo-functionalized (ABDCA) or stilbene dicarboxylic (SDCA) ligands are reported.21−34 The success of the formation of MOFs or CPs by using ABDCA ligand is limited.35,36 It might be due to the instability of ABDCA under solvothermal methods at a higher temper© 2018 American Chemical Society

Received: February 13, 2018 Revised: July 7, 2018 Published: July 17, 2018 4937

DOI: 10.1021/acs.cgd.8b00243 Cryst. Growth Des. 2018, 18, 4937−4944

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filtration. The filtered crystals were dried in ambient condition for further use. Elemental analysis (%) (C33H27Mg1.50N7O10) (Mw = 718.08) Calculated: C, 56.92; H, 3.48; N, 11.11. Found: C, 56.52; H, 3.32; N, 11.41. IR data (KBr, cm−1): 3394, 1597, 1555, 1399, 1304, 805, 708, 631, 502. Yield: 48% (Calculated based on the ABDCA ligand). Compound {[Zn(ABDCA)(2,2′-BPy)]·H2O}n, 4. 4 was produced using the solvothermal method at 100 °C. Zn(NO3)2·6H2O (0.030 g, 0.1 mmol), ABDCA (0.027 g, 0.1 mmol), and 2,2′-bipyridine (0.016g, 0.1 mmol) were mixed in a glass vial. The mixture was dissolved in 10 mL of DMF through sonication. Then, the mixture was heated at 100 °C for 5 days. After being cooled to room temperature, orange block crystals of 4 were collected by filtration. The filtered crystals were dried in ambient condition for further use. Elemental analysis (%) (C12H8N2O3Zn0.5) (Mw = 260.89) Calculated: C, 55.24; H, 3.07; N, 10.78. Found: C, 55.05; H, 3.12; N, 10.23. IR data (KBr, cm−1): 3394, 3050, 1607, 1555, 1382, 1313, 1011, 865, 768, 631. Yield: 50% (Calculated based on the ABDCA ligand). Compound {[Zn(2,2′-BPy)(SDCA)]}n, 5. 5 was produced using the solvothermal method at 100 °C. Zn(NO3)2·6H2O (0.030 g, 0.1 mmol), 2,2′-bipyridine (0.016g, 0.1 mmol), and SDCA (0.027 g, 0.1 mmol) were dissolved in 10 mL of DMF and were mixed by sonication in a glass vial. Then, it was heated at 100 °C for 5 days. White block crystals of 5 were isolated at room temperature. Crystals were collected through filtration followed by washing through DMF. The air-dried crystals were used for further studies. Elemental analysis (%) (C12H8N2O2Zn0.50) (Mw = 244.89) Calculated: C, 64.02; H, 3.69; N, 5.77. Found: C, 62.25; H, 3.60; N, 5.90. IR data (KBr, cm−1): 3390, 1672, 1603, 1535, 1406, 1165, 1105, 1002, 650. Yield: 54% (Calculated based on the SDCA ligand). Compound {[Zn(4,4′-BPy)(ABDCA)]·DMF}n 6. 6 was synthesized through solvothermal method at 100 °C. Zn(NO3)2·6H2O (0.030 g, 0.1 mmol), 4,4′-bipyridine (0.016g, 0.1 mmol), and ABDCA (0.027 g, 0.1 mmol) were mixed in a glass vial and then dissolved in 10 mL of DMF. Then, it was heated at 100 °C for 5 days. After being cooled to room temperature, orange block crystals of 6 were collected by filtration. The filtered crystals were dried in ambient condition for further use. Elemental analysis (%) (C13.50H8N2.50O2.50Zn0.50) (Mw = 277.91) Calculated: C, 58.34; H, 2.88; N, 12.65. Found: C, 58.23; H, 2.54; N, 12.89. IR data (KBr, cm−1): 3422, 1555, 1391, 1304, 1209, 1089, 865, 795, 701, 631. Yield: 50% (Calculated based on the ABDCA ligand). Crystallographic Data Collection and Refinement. Single crystal diffraction data were collected on a Bruker AXS Smart Apex CCD diffractometer. The X-ray generator was operated at 50 kV and 35 mA using a Mo Kα (λ = 0.71073 Å) radiation. The data were reduced using SAINTPLUS43 and an empirical absorption correction was applied using the SADABS program.44 The crystal structure was determined by direct methods using SHELXS2014/2018 and refined using SHELXL2014/2018 present in the SHELXTL v 6.1445 package. In compound 2, disorder of one of the azo-benzene rings was modeled. In the case of compounds 2 and 3, the electron density in the void spaces was modeled as disorder DMF molecules. We have used SQUEEZE to prove the correctness of the atomic positions in the framework. The SQUEEZE structure is devoid of any solvent molecules, and formula matches with elemental analysis on the evacuated compound 3. From the difference Fourier map, all the nonhydrogen atoms were located and then refined anisotropically. HFIX was used to fix all the hydrogen atoms. The WINGX46,47 package of programs were used for full-matrix least-squares structure refinement against F2. In Table S1 we have provided all the crystallographic and structure refinement data for compounds 1−6. Tables S2−S7 contain selected bond lengths for all the compounds. CCDC 1823569− 1823574 contain crystallographic data for all the compounds.

framework with topology {39.412}. In compound 4, zinc ions are connected with carboxylate ligands to form the onedimensional zigzag chain. 2,2′-Bipyridine molecule hangs from the metal center. This arrangement leads to the π-stacking supramolecular structure. The structure of the compound 5 is identical to that of compound 4 where SDCA replaces ABDCA. Compound 6 consists of three-dimensional framework with fivefold interpenetrating nets. All the compounds were well characterized through several techniques. The luminescence and optical band gap energy were measured. All these compounds were luminescent due to ligand-based charge transfer. The optical band gap varies in these structures due to the difference in ligand−metal bonding connectivity and structural arrangement. Compounds 1 and 2 show magnetic activity in which 1 is weakly antiferromagnetic, and 2 is paramagnetic.

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EXPERIMENTAL SECTION

Materials and Physical Measurements. All the reagents used were commercially available and used as provided without additional purification. [Mn(NO3)2·4H2O], [Mg(NO3)2·6H2O], [Zn(NO3)2· 6H2O], Azobenzene-4,4′-dicarboxylic acid (ABDCA), Stilbene-4,4′dicarboxylic acid (SDCA), 4,4′-Bipyridine (4,4′-Bpy), 2,2′-Bipyridine (2,2′-Bpy), and 1,2-Di(4-pyridyl)ethylene (DPE) were purchased from Sigma-Aldrich. X’pert PRO (PANalytics) powder diffractometer (equipped with Cu Kα radiation (1.5405 Å)) was used for powder X-ray diffraction data. Vario MICRO cube elemental analyzer was employed for elemental analysis (C, H, and N). FT-IR spectra were recorded using KBr pellets in the range of 4000−400 cm−1, in FT-IR prestige-21 (Shimadzu) spectrometer. TGA was carried out on an SDT Q600 (Shimadzu) analyzer with a heating rate of 10 °C per minute in N2 atmosphere. UV−vis spectroscopy was done at room temperature in the solid state set up on a UV-3800 SHIMADZU UV−vis-NIR spectrophotometer. The emission spectra were collected using the Horiba scientific Fluoromax-4 spectrophotometer with a tungsten lamp. Synthesis. Compound {[Mn2(ABDCA)2(DPE)(DMF)2]·2DMF}n, 1. 1 was synthesized by solvothermal method at 100 °C. Mn(NO3)2· 4H2O (0.027 g, 0.1 mmol), ABDCA(0.027 g, 0.1 mmol), and DPE (0.018 g, 0.1 mmol) were mixed and dissolved in 10 mL of DMF. Then, after sonication the glass was kept at 100 °C for 5 days. After being cooled to room temperature, orange block crystals of 1 were collected by filtration. Crystals were dried in ambient conditions and used for further studies. Elemental analysis (%) (C26H27N5O6Mn) (Mw = 560.46) Calculated: C, 55.72; H, 4.82; N, 12.55. Found: C, 55.06; H, 4.70; N, 12.48. IR data (KBr, cm−1): 3364, 1705, 1598, 1386, 1306, 1213, 1068, 801, 629, 489. Yield: 64% (Calculated based on the ABDCA ligand). Compound {[Mn3(ABDCA)3(DMF)4]·2DMF}n, 2. 2 was produced using the solvothermal method at 100 °C. Mn(NO3)2·4H2O (0.027 g, 0.1 mmol) and ABDCA (0.027 g, 0.1 mmol) were mixed and dissolved in 10 mL of DMF in a glass vial by sonication. Then, it was kept at 100 °C for 5 days. Orange block crystals of 2 were isolated at room temperature. Crystals were collected through filtration followed by washing with DMF. The air-dried crystals were used for further studies. Elemental analysis (%) (C30H33Mn1.50N6O9) (Mw = 704.03) Calculated: C, 51.18; H, 4.69; N, 11.98. Found: C, 50.98; H, 4.71; N, 11.45. IR data (KBr, cm−1): 4025, 3470, 1651, 1298, 929, 823, 590, 477. Yield: 48% (Calculated based on the ABDCA ligand). Compound {[Mg3(ABDCA)3(DMF)4]·4DMF}n, 3. 3 was produced using the solvothermal method at 100 °C. Mg(NO3)2·6H2O (0.026 g, 0.1 mmol) and ABDCA (0.027 g, 0.1 mmol) were mixed in a glass vial and then dissolved in 10 mL of DMF through sonication. Then, the mixture was heated at 100 °C for 5 days. After being cooled to room temperature, orange block crystals of 3 were collected by

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RESULTS AND DISCUSSION Structural Descriptions of Compounds 1−6. Crystal Structure of {[Mn2(ABDCA)2(DPE)(DMF2]·2DMF}n, 1. Com4938

DOI: 10.1021/acs.cgd.8b00243 Cryst. Growth Des. 2018, 18, 4937−4944

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pound 1 crystallizes in the triclinic system with P1̅ space group(Table S1). The asymmetric unit consists of one Mn atom, one ABDCA, a half-molecule of DPE ligand, and one coordinated DMF molecule. One molecule of lattice DMF resides in the pore. The Mn atom takes on a distorted octahedral geometry with four of the ABDCA oxygen atoms, one nitrogen atom of DPE spacer and an oxygen atom of the coordinated DMF molecule. The Mn−O bond distances fall in the range of 2.106−2.326 Å (av. 2.194 Å) (Table S2). The distorted MnO5N units are connected via Mn−O−C−O−Mn bonds to form a dimeric unit (Figure 1a). The dimeric units

space group (Table S1). The asymmetric unit consists of one and a half of Mn atoms, one and a half of ABDCA molecules, two coordinated DMF molecules, and one lattice DMF molecule. Mn(1) has a distorted octahedral geometry with four oxygen atoms of ABDCA anion and two oxygen atoms of coordinated DMF molecules (Figure 2a). Similarly, Mn(2)

Figure 2. (a) The manganese trimer and (b) two-dimensional layered structure of compound 2.

adopts an indistinct octahedral geometry of six ABDCA oxygen atoms (Figure 2a). The Mn−O bond distances fall in the range of 2.096−2.269 Å (av. 2.160 Å) (Table S3). Mn(1)O6 octahedral unit is symmetry generated and corner-wise linked with the Mn(2)O6 unit to form the trimeric unit, Mn3O16. These trimers are linked with the ABDCA ligands forming a layered architecture (Figure 2b). The topological study identified that the coordination polymer has a topological network with a 2-nodal 7,14-c net with stoichiometry (7-c) 2(14-c) (Figure S2). It can be represented by the Schläfli symbol {324.448.516.63}{39.412}2 (TD10 = 979) (see the SI).48 Crystal Structure of {[Mg3(ABDCA)3(DMF)4]·4DMF}n, 3. Compound 3 crystallizes in the monoclinic system with C2/c space group (Table S1). The asymmetric unit consists of one and a half Mg atoms, one and a half ABDCA molecules, two coordinated DMF molecules, and two lattice disorder DMF molecules. Both Mg(1) and Mg(2) atoms assume a distorted octahedral geometry in which Mg(1) is bonded to six oxygen atoms of ABDCA anions and Mg(2) connected to four oxygen atoms of ABDCA anions and oxygen atoms of two coordinated DMF molecules. The Mg−O bond distances fall in the range of 1.985−2.185 Å (Table S4). Mg(1)O6 octahedral unit is symmetry generated and linked with the Mg(2)O6 octahedral unit through corners to form the trimeric unit, Mg3O16 (Figure 3a). The trimeric units are connected to the ABDCA ligands, forming a 3D architecture (Figure 3b). The topological study shows that the structure has a 2-nodal 7,14-c net (Figure S3), which can be represented by the Schlä fl i symbol {318.436.522.614.7}{39.412}2 (TD10 = 6287) (see the SI).48 Crystal Structure of {[Zn(ABDCA)(2,2′-BPy)]·H2O}n, 4. Compound 4 crystallizes in the monoclinic system with space group, C2/c (Table S1). The asymmetric unit consists of one Zn atom, one ABDCA molecule, one 2,2′-bipyridine ligand, and one lattice H2O molecule. Zn metal ion adopts distorted octahedral geometry with four oxygen atoms of

Figure 1. (a) Manganese dimer, (b) three-dimensional architecture, and (c) threefold interpenetration in compound 1.

are connected with ABDCA and DPE linkers, forming a 3D structure with threefold interpenetration (Figures 1b,c, and S1a−c). Topological investigation with the TOPOS software identified that the coordination polymer has a topological network with a uninodal 7-c net, which can be represented by the Schläfli symbol {36.48.56.6}(TD10 = 1946) (see the SI).48 Crystal Structure of {[Mn3(ABDCA)3(DMF)4]·2DMF}n, 2. Compound 2 crystallizes in the monoclinic system with P21/n 4939

DOI: 10.1021/acs.cgd.8b00243 Cryst. Growth Des. 2018, 18, 4937−4944

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ABDCA ligand was replaced by SDCA in compound 5. The asymmetric unit made up of one Zn atom, one 2,2′-bipyridine molecule, and one SDCA molecule. The Zn atom adopts a distorted octahedral geometry with four oxygen atoms of SDCA and two nitrogen atoms of 2,2′-bipyridine spacer (Figure 5). The Zn−O and Zn−N bond distances fall in the

Figure 5. Three-dimensional supramolecular architecture of compound 5. Figure 3. (a) Trimeric unit and (b) three-dimensional connectivity in compound 3.

range of 1.941−1.990 Å (av. 1.965 Å) and 2.043−2.064 Å (av. 2.054 Å), respectively (Table S6). The zinc ions and SDCA linkers connected to form the zigzag chain structure. 2,2′Bipyridine molecule hangs from the Zn center. The chains are stacked through π-stacking interactions by bipyridine molecules to form the 3D supramolecular architecture like compound 4. Crystal Structure of {[Zn(4,4′-BPy)(ABDCA)]·DMF}n, 6. Compound 6 crystallizes in the monoclinic system with C2/c space group (Table S1). The asymmetric unit is made up of half Zn atom, half-molecule of 4,4′-bipyridine, half-molecule of ABDCA ligand, and half-molecule of lattice DMF. The Zn atom adopts a distorted tetrahedral geometry with two oxygen atoms of ABDCA and two nitrogen atoms of 4,4′-bipyridine spacer. The average Zn−O and Zn−N bond lengths are 1.943 and 2.062 Å, respectively (Table S7). Zinc ions are linked through ABDCA ligands to form the one-dimensional wire-like structure which in turn was connected by 4,4′-bipyridine molecule to form the three-dimensional structure. The disordered DMF molecules reside in the pores (Figures 6a and S4). The topological study shows that the structure consists of a 3D framework with five interpenetrating nets (Figure 6b). The coordination polymer has a topological network with a uninodal 4-c net, which can be represented by the Schläfli symbol {66} (TD10 = 981) (see the SI).48 Powder X-ray Diffraction and Infrared Spectroscopy. The powder X-ray diffraction study was used to check the phase purity of all the compounds. The powder X-ray diffraction spectra of all the compounds were precisely matched with the simulated data, which indicates the phase purity of the compounds in bulk (Figure S5). The infrared spectroscopic studies of all the compounds showed the presence of the carboxylic groups, azo/alkene groups, and the solvent molecule (DMF or H2O) in the structures (Figure S6 and Table S8). Thermal Stability. The thermal stability of all the compounds has been studied using thermogravimetric analysis (Figure S7). Compounds 1 and 2 have both lattice and coordinated DMF molecules. The initial weight loss in both

ABDCA and two nitrogen atoms of 2,2′-bipyridine ligand. The Zn−O and Zn−N bond distances fall in the range of 2.003− 2.395 Å (av. 2.185 Å) and 2.065−2.074 Å (av. 2.07 Å), respectively (Table S5). The zinc ions are linked with the ABDCA ligands forming the zigzag chain structure. 2,2′Bipyridine molecule hangs from the Zn center (Figure 4a). The chains are stacked through π-stacking interactions by bipyridine molecules and forming the 3D supramolecular architecture (Figure 4b). Crystal Structure of {[Zn(2,2′-BPy)(SDCA)]}n, 5. The structures of compounds 4 and 5 are identical, in which

Figure 4. (a) One-dimensional zigzag chain and (b) threedimensional supramolecular structure in compound 4. 4940

DOI: 10.1021/acs.cgd.8b00243 Cryst. Growth Des. 2018, 18, 4937−4944

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shifted compared to the emissions from corresponding sodium salts (Figure S9b). This is because of the greater possibility of cofacial interligand interactions of the acid ligand in the solid state.29 Compounds 1−4 and 6, which consist of ABDCA ligand in the framework, show the characteristic peak arising from its sodium salt. Similarly, compound 5 exhibits luminescence comparable to that of Na2SDCA. By studying the emission behavior of both classes of compounds and comparing with the free acid ligand, the absence of interligand interactions in the corresponding MOF structures was confirmed. The luminescence purely arises from the ligandcentered π to π* or n to π* transitions (Figure S9c). These characteristics also specify the nonexistence of charge transfer transitions between the ligands and the metal ions. Optical Band Gap Measurements. Understanding the origin of band gap for inorganic−organic hybrid compounds is very important for their applications in the optoelectronic industry. Most of these hybrid materials are insulators in nature. The synthesis of semiconductor hybrid materials is in high demand. It was proved that HOMO is located on the organic ligand and LUMO mainly on the metal node,49,50 so conjugation and functionalization of the organic ligand may affect the HOMO position. These types of hybrid structures exhibit flat conduction and valence bands which lead to the indirect band gap.51 Here, we have synthesized hybrid materials with conjugated and different functionalized (azodicarboxylic acid vs stilbene dicarboxylic acid) organic ligands. Diffuse reflectance spectra of the solid samples have been collected using a UV−vis-NIR spectrophotometer, to know the effect of ligands on the optical band gap energy. The band gap energy was calculated by the Kubelka−Munk equation. The indirect band gap energy of the compounds obtained from the Tauc plots ranges from 1.78 to 3.14 eV (Figure S10 and Table S9). We have collected the band gap of the Na salts of ABDCA and SDCA, and those were 1.89 and 3.84 eV, respectively. The −NN− group in ABDCA makes it more electronegative compared to SDCA with −CC− bond, and as a result, the HOMO−LUMO positions are quite different in this ligand, which is manifested in their band gap values. Even though compounds 2 and 3 have similar bonding connectivity, they differ in their metal center and structural architecture. This is reflected in their band gap energy values (1.78 and 1.86 eV, for compounds 2 and 3, respectively). Similarly, the structures of 4 and 5 are identical but they differ in ligand connectivity, and as a result, they have quite different band gap energy (1.84 and 3.14 eV, for compounds 4 and 5, respectively). Magnetic Measurements. The magnetic properties of compounds 1 and 2 were measured (Figure 7). Magnetic susceptibility χ(T) as a function of temperature was measured at an applied field of H = 1 and 0.5 T for compound 1 and H = 1 T for compound 2. The magnetic isotherm (M vs H) was measured at T = 2.1 K by varying H from 0 to 9 T. In compound 1 the χ(T) vs T plot exhibits a broad maximum at lower temperatures indicating a short-range antiferromagnetic ordering and is a unique feature of low dimensional magnetic compounds. For compound 2, the χ(T) increases as temperature decreases following Curie−Weiss behavior of a paramagnetic material. We have checked the Curie−Weiss fitting to the 1/χ vs T plot at a higher temperature using the formula

Figure 6. (a) Three-dimensional architecture and (b) fivefold interpenetrating nets in compound 6.

cases is due to the loss of lattice DMF molecules and was followed by elimination of coordinated DMF molecules up to ∼300 °C. In the case of compound 3, both lattice solvent and coordinated DMF molecules were eliminated by ∼200 °C and the compound was stable up to ∼550 °C before organic struts started decomposing. For compounds 1 and 2, framework decomposed at a lower temperature (∼450 °C) compared to compound 3. In the case of compound 4, the lattice water molecule slowly released until ∼250 °C followed by removal of the organic struts. Compound 5 was stable up to ∼250 °C and then the framework started degrading. For compound 6, the framework exhibits a stepwise decomposition. The lattice DMF get released up to ∼200 °C and after that it started losing the organic struts of the framework. Photoluminescence Properties. The luminescence property of the MOFs is interesting due to their application in sensing and other photophysics related applications. The rigid arrangement of organic ligands in the MOFs/CPs structure helps to control and modify the luminescent behavior of the chromophores.13 The luminescence spectra of the MOF structure differs from that of the free ligand (H2L) due to lack of interligand interactions. The UV−vis spectra and emission spectra of all the compounds are given as Figure S8. The absorption spectra of all the incorporated ligands are provided as Figure S9a. The sodium salts of ABDCA and SDCA are considered noninteracting chromospheres with very few interligand interactions. The free acid ligand emissions are considerably red-

χ (T ) = χ0 + 4941

c T + θCW DOI: 10.1021/acs.cgd.8b00243 Cryst. Growth Des. 2018, 18, 4937−4944

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Figure 7. (1a,2a) Magnetic susceptibility (χ) vs temperature (T). (1b,2b) 1/χ vs T plots. (1c,2c) Magnetization isotherm M vs H of compounds 1 and 2, respectively.

where χ0 is temperature-independent and includes contributions from core diamagnetic susceptibility and Van-Vleck paramagnetic susceptibility. The second term is the Curie− Weiss law with C being the Curie constant and θCW being the characteristic Curie−Weiss temperature. Based on the C−W equation the θCW values of compounds 1 and 2 are 10 and 5 K, respectively. The mean field theory states that the positive value of θCW indicates a predominant antiferromagnetic superexchange between the spin.51 These θCW values indicate that compound 1 behaves as weakly antiferromagnetic and compound 2 is paramagnetic. The calculated effective magnetic moments of compounds 1 and 2 are 6.32 μB, which is in good agreement with the spin-only magnetic moment of Mn2+(5.92 μB).52

structure with threefold interpenetration. Compounds 2 and 3 are two-dimensional and three-dimensional structures, respectively. Compounds 4 and 5 are one-dimensional chains with 2,2-bipyridine hanging from the metal center. These structures are stabilized by π-stack interaction through bipyridine molecules. Compounds 6 adopts a three-dimensional structure with fivefold interpenetration. The luminescence properties of these as-synthesized compounds purely arise from the ligandcentered π to π* or n to π* transition. Optical band gap energy measurements showed that the band gap of the as-synthesized compounds ranges from 1.83 to 3.14 eV. The band gap energies are different in ABDCA compared to SDCA due to their different HOMO−LUMO energy gaps. Magnetic measurements showed that compound 1 is weakly antiferromagnetic and compound 2 is paramagnetic.

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CONCLUSIONS Six new compounds were synthesized by a solvothermal method using azo- and stilbene dicarboxylic acid and Mg, Mn, and Zn metal ions. Compound 1 has a three-dimensional 4942

DOI: 10.1021/acs.cgd.8b00243 Cryst. Growth Des. 2018, 18, 4937−4944

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.cgd.8b00243. Materials and methods, crystallographic data tables, structures, PXRD, IR, TGA, absorption, emission and the solid state optical band gap energy spectra for all the six coordination polymers (PDF) SHELXL-2014 data (TXT) Accession Codes

CCDC 1823569−1823574 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Sukhendu Mandal: 0000-0002-4725-8418 Funding

Science and Engineering Research Board (SERB), Govt. of India, through a grant EMR/2016/007501. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge Mr. Subhrajyoti Bhandary for help regarding the disorder treatment of compound 2. P.A. acknowledges CSIR for SRF.

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REFERENCES

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