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3D Noncentrosymmetric Ba(II)/Li(I)–Imidazolecarboxylate Coordination Polymers: SHG and Blue Fluorescence Ying Song, ChenSheng Lin, Qi Wei, Zhao-Feng Wu, and Xiao-Ying Huang Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.6b01331 • Publication Date (Web): 03 Oct 2016 Downloaded from http://pubs.acs.org on October 8, 2016

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Crystal Growth & Design

3D Noncentrosymmetric Ba(II)/Li(I)– Imidazolecarboxylate Coordination Polymers: SHG and Blue Fluorescence Ying Song,a Chen–Sheng Lin,a Qi Wei,a Zhao–Feng Wua and Xiao–Ying Huang*a a

State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, Fujian, 350002, P. R. China

ABSTRACT: Presented are the syntheses, crystal structures, properties, and density functional theory (DFT) studies of three novel noncentrosymmetric (NCS) coordination polymers (CPs), namely, [Ba0.375Li(4,5-ImDCH)0.75(4,5-ImDCH2)0.25]·0.125H2O (1), Ba(4,5-ImDCH)(DMF) (2) and Li2(4,5-ImDC-Me) (3), where 4,5-ImDCH3 = 4,5-imidazoledicarboxylate acid; 4,5ImDCH2-Me = 1-methy-4,5-imidazoledicarboxylate acid; DMF = N,N’-Dimethylformamide. Single–crystal X–ray structural analyses found that all the structures feature a three–dimensional (3D) framework. The simplified structures of 1 and 3 belong to the dia topology, while 2 is characterized with a one–dimensional (1D) metal–carboxylate ribbon. The SHG responses of 1– 3 are about 5, 1.5, and 3 times that of KH2PO4 (KDP), respectively. Thrillingly, compound 1 is highly stable in air. The theoretical studies including electronic structures and optical properties of 2 and 3 confirm the experimental results. What is more, 1–3 emit bright blue light upon the

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excitation of 365 nm UV light with the quantum yields (QY, Φ) of 14.68%, 16.26% and 25.87%, respectively.

INTRODUCTION Noncentrosymmetric (NCS) materials are of special importance due to their inherent properties such as pyroelectricity, piezoelectricity, ferroelectricity and nonlinear optics (NLO). They play a fundamental role in such areas as telecommunication, optical switch and information storage.1 In addition to the inorganic and organic NCS compounds, the coordination polymers (CPs), as a new category of crystalline materials, have drawn substantial attention for designing NCS structures owing to their designability and combined properties of inorganic and organic components.2 There are several strategies proven to be effective to construct the NCS CPs. The most straightforward way is considered to be the utilization of chiral ligand to build chiral thus NCS structures.2f, g, 3 By using the dipolar chromophores, Lin et al. systematically designed a series of NCS structures with particular topologies.2a–c, 2e While Xiong et al. obtained a new category of NCS tetrazole CPs by the reaction of R–CN, NaN3 with Zn/Cd(II) via an in situ hydrothermal method.2h, i, 4 Recently Du et al. found that the combination of alkali or alkaline earth metal ions with Cd(II) inclined to produce NCS CPs with a high chance.2d, f, 5 In our previous study, we also found that the cooperation of Ba(II) and Li(I) ions with a N–containing carboxylate ligand (pyrazole-3,5-dicarboxylic acid) could yield NCS structures with a high tendency;6 these compounds showed moderate second harmonic generation (SHG) response. In order to obtain compounds with stronger SHG response, in this work we selected another two N–containing carboxylate ligand, that is, 4,5-imidazoledicarboxylate acid (4,5-ImDCH3) and


2

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its derivative 1-methyl-4,5-imidazoledicarboxylate acid (4,5-ImDCH2-Me) (Scheme 1), to construct NCS Ba(II)/Li(I)–CPs, where the –N and –COO sites of the ligand may form an effective donor–acceptor system which is believed to have the potential of increasing the polarity of the final structure.2g, 7 Herein, the solvothermal reactions of 4,5-ImDCH3 or 4,5-ImDCH2-Me ligands with Ba(II)/Li(I)

ions

yielded

three

NCS

CPs,

namely

[Ba0.375Li(4,5-ImDCH)0.75(4,5-

ImDCH2)0.25]·0.125H2O (1), Ba(4,5-ImDCH)(DMF) (2) and Li2(4,5-ImDC-Me) (3). They are all 3D frameworks and the structural types of 1–3 could be classified as I3O3, I1O3, and I3O3, respectively.8 Both the frameworks of 1 and 3 can be simplified as dia net and 2 features a 1D metal–carboxylate ribbon. The intensities produced by the crystal powder of 1–3 exhibited SHG efficiencies of about 5, 1.5, and 3 times that of the KDP standard, respectively. Superior to many main group metal based CPs, compound 1 could be stable in air for at least two years. In addition, the fluorescent experiments showed that compounds 1–3 emitted ligand–centered bright blue light under UV light, and the values of quantum yields (QY, Φ) were 14.68%, 16.26% and 25.87% for 1–3, respectively. EXPERIMENTAL SECTION Materials and methods. All the analytical grade chemicals employed in this study were commercially available and used without further purification. Microwave syntheses were carried out in a Biotage Initiator Microwave Synthesizer (power range 0–400 W at 2.45 GHz). Powder X–ray diffraction (PXRD) patterns were recorded in the angular range of 2θ = 5–65° on a Miniflex II diffractometer using CuKα radiation. Thermogravimetric analyses were carried out with a NETZSCH STA 449F3 unit at a heating rate of 10 °C /min under a nitrogen atmosphere. Elemental analyses (EA) for C, H, N were performed on a German Elementary Vario EL III


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instrument. Fourier transform infrared (FT–IR) spectra were taken on a Nicolet Magna 750 FT– IR spectrometer in the 4000–400 cm–1 region by using KBr pellets. Optical diffuse reflectance spectra were measured at room temperature with a Varian Cary 500 Scan UV–visible system. A BaSO4 plate was used as a standard (100% reflectance). The absorption (F(R)) data were calculated from reflectance spectra using the Kubelka–Munk function: F(R) = (1 − R2)/2R, where F(R) is the absorption coefficient and R is the reflectance.9 Emission and excitation spectra of the compounds were recorded on a PerkinElmer LS55 luminescence spectrometer. The quantum yield measurements were performed on a FLS920 produced by Edingburgh Insruments by means of an integrating sphere. The absolute error on the quantum yield values is about ±1%. The powder SHG responses were measured on powdered samples by using the experimental method adapted from that reported by Kurtz and Perry.10 1064 nm radiation generated by a Q– switched Nd:YAG solid–state laser was used as the fundamental frequency light. The samples of 1–3 were ground and sieved into six or seven distinct particle size ranges: 25–57, 58–108, 110– 150, 150–210, 212–270, and 270–380 µm for 1, 45–57, 58–74, 75–109, 110–150, 150–210 and 212–270 for 2, and 25–45, 45–53, 58–74, 75–105, 106–114, 114–150 and 150–212 for 3, which were pressed into a disk with diameter of 8 mm that was put between glass microscope slides and secured with tape in a 1 mm thick aluminum holder, respectively. Syntheses of compounds 1–3. All the three compounds were synthesized by solvothermal methods. Typically, a mixture of reactants and solvents was sealed in a stainless steel reactor with a 28 (or 20) mL Teflon liner and heated in an oven for several days, followed by natural cooling to room temperature. The resulting crystals of 1–3 were washed with anhydrous ethanol followed by drying in the air and selected by hand. The yields were calculated based on BaCl2·2H2O for 1 and 2, and LiNO3 for 3, respectively. The highly crystalline single–phase of 2


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with a well–defined morphology could also be quickly prepared through microwave (MW)– assisted synthesis within 20 min. [Ba0.375Li(4,5-ImDCH)0.75(4,5-ImDCH2)0.25]·0.125H2O (1). The reaction of a mixture of BaCl2·2H2O (0.080 g, 0.33 mmol), LiNO3 (0.023 g, 0.33 mmol), 4,5–ImDCH3 (0.156 g, 1 mmol), LiOH·H2O (0.042 g, 1 mmol), 4 mL DMF and 0.5 mL pyridine at 150 ˚C for 53 hrs resulted in light–yellow block–like crystals with a yield of ~ 20.0 mg, 10.4%. Elemental analysis (%): calc. for C5H2.5Ba0.375LiN2O4.125: C, 27.93%; H, 1.17%; N, 13.03%; Found: C, 28.05%; H, 1.32%; N, 13.01%. Ba(4,5-ImDCH)(DMF) (2). Conventional solvothermal synthesis: The reaction of a mixture of BaCl2·2H2O (0.078 g, 0.32 mmol), LiNO3 (0.021 g, 0.30 mmol), 4,5-ImDCH3 (0.101 g, 0.65 mmol), LiOH·H2O (0.006 g, 0.14 mmol), and 4 mL DMF at 140 ˚C for 2.5 days resulted in light–yellow block–like crystals with a yield of ~ 95.0 mg, 81.4%. MW–assisted hydrothermal synthesis: A mixture of BaCl2·2H2O (0.082 g, 0.34 mmol), LiNO3 (0.021 g, 0.30 mmol), 4,5-ImDCH3 (0.107 g, 0.69 mmol), LiOH·H2O (0.039 g, 0.93 mmol), and 4 mL DMF was placed in a Biotage MW vial (Vmax = 20 mL). The MW absorption level of DMF was set to high. The reaction mixture was heated with continuously magnetic stirring at 150 ˚C and was maintained at that temperature for 20 min. After the solution was cooled, white powders were collected by filtration, washed with ethanol and dried in air, yield: ~ 110.0 mg, 88.8%. Elemental analysis (%): calc. for C8H9BaN3O5: C, 26.36%; H, 2.49%; N, 11.53%; Found: C, 25.57%; H, 2.27%; N, 11.58%.


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Li2(4,5-ImDC-Me) (3). The reaction of a mixture of LiNO3 (0.014 g, 0.20 mmol), 4,5ImDCH2-Me (0.055 g, 0.32 mmol), LiOH·H2O (0.020 g, 0.48 mmol), 3 mL DMF and 0.4 mL pyridine at 150 ˚C for 2 days resulted in colorless block–like crystals with a yield of ~ 6.0 mg, 33.0%. Elemental analysis (%): calc. for C6H4Li2N2O4: C, 39.60%; H, 2.22%; N, 15.39%; Found: C, 39.53%; H, 2.40%; N, 15.43%. Single–crystal structural determination. Suitable single crystals of compounds 1–3 were carefully selected under an optical microscope and glued to thin glass fibers. Data collections were performed on an Oxford Xcalibur Eos diffractometer equipped with graphite– monochromated MoKα radiation (λ = 0.71073 Å) at room temperature for 2 and on a SuperNova CCD diffractometer at 100(2) K (MoKα) for 1 and 295(2) K (CuKα, λ = 1.54178 Å) for 3. The structures were solved by direct methods and refined by full–matrix least–squares on F2 by using the program package SHELX–97.11 All non–hydrogen atoms were refined anisotropically. The positions of hydrogen atoms attached to C and N in 1–3 were generated geometrically with assigned isotropic thermal parameters, and allowed to ride on their respectively parent atoms before the final cycles of least–squares refinements, while those in lattice water molecule in 1 were located from the difference–Fourier maps. The empirical formulae were confirmed by the thermogravimetric analyses (TGA) and elemental analyses (EA) results. Particularly, in 1, the Ba2+ ion (Ba1) with an occupancy of 0.75 is disordered with one H2O (O5) and one H+ of NH (N1) and the restraint of “EADP Ba1 O5” was applied. What is more, the soft restraints of DFIX and DANG were applied on the H2O molecule of 1 to keep its geometry and atomic displacement parameters reasonable. Details of crystallographic data and structural refinement parameters are summarized in Table 1. CCDC nos. 1473018 (1), 1473019 (2) and 1473020 (3) contain the supplementary crystallographic data for compounds 1–3, respectively.


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Computational Descriptions. Energy band structures and optical properties of 2 and 3 were accomplished by using the density functional theory (DFT) calculations with CASTEP code12 provided by the Material Studio package (Because the crystal structure of 1 is statistically disordered, so it is improper to do theoretical calculations). Interaction of the electrons with ion cores was represented by the norm–conserving pseudo potentials, and the valence electrons were treated as Ba, 5s25p66s2; O, 2s22p4; N, 2s22p3; C, 2s22p2; Li, 2s1; H, 1s1. Generalized gradient approximation (GGA) in the scheme of Perdew−Burke−Eruzerhof (PBE) was used to describe the exchange and correlative potential of electron−electron interactions.13 The k point of first Brillouin zone was sampled as the 2×2×2 Monkhorst−Pack scheme.14 Energy cutoff and precision were set to be 600 eV and 2.0×10−6 eV/atom for 2, 750 eV and 1.0×10−6 eV/atom for 3, respectively. The X–ray crystal structure data were used without further optimization. The theoretical studies of optical properties in terms of the complex dielectric function ε(ω) = ε1(ω) + iε2(ω) are given by i pcv (k ) p vcj (k ) 8π 2 h 2 e 2 ε (ω ) = 2 ∑ k ∑ cv ( f c − f v ) δ [ Ecv (k ) − hω ] m Veff E vc2 ij 2

where δ[Ecv(k) − ℏω] = δ[Ec(k) − Ev(k) − ℏω] indicates the energy difference between the conduction and valence bands at the k point with absorption of a quantum ℏω. The fc and fv represent the Fermi distribution functions of the conduction and valence bands, respectively. The term pcvi(k) denotes the momentum matrix element transition from the energy level c of the conduction band to the level v of the valence band at the k point in the Brillouin zones and Veff is the volume of the unit cell. The m, e, and ℏ are the electron mass, charge, and Plank’s constant, respectively. Then the first–order susceptibility at low frequency region is given by χ(1)(ω)ii = [ε(ω)i − 1]/4π, and the second–order susceptibilities can be expressed in terms of the first–order susceptibilities as follows:


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χ ijk( 2) (−ω3 ;ω1 ,ω2 ) = F ( 2) χ ii(1) (ω3 ) χ (jj1) (ω1 ) χ kk(1) (ω2 ) Where F characterizes the nonlinearity of the response and it is estimated to be 0.755 and 0.786 esu for 2 and 3, respectively. The SHG component dij is equal to half of the corresponding χij value for the consequence of historical convention. RESULTS AND DISCUSSION Synthesis. The title compounds 1–3 were synthesized by solvothermal methods using DMF as the main solvent. By comparison, there are already five compounds based on Ba/Sr(II) and 4,5ImDCH3 ligand reported which were prepared using water as the solvent; for details, see Table S1 in the Electronic Supporting Information (ESI).15 By the MW–assisted solvothermal method, highly crystalline pure phase of 2 with higher yield could be synthesized conveniently by modifying the reactant ratios, as confirmed by PXRD (Figure S9, ESI). Compared with conventional heating, the MW route produced highly phase– pure materials of compound 2 in a much shorter time. As shown in Figure 1, which presents the photograph of crystals synthesized by conventional heating and the SEM image of the microcrystals obtained by MW heating, the respective morphologies of the fully crystalline sample prepared by the two methods are very similar, but the crystals obtained by MW heating are in a micrometer (or submicrometer) level due to the higher number of nucleation sites thus more homogeneous, which further shows the efficiency of the synthesis by the MW approach. Crystal structure descriptions. [Ba0.375Li(4,5-ImDCH)0.75(4,5-ImDCH2)0.25]·0.125H2O (1). Compound 1 belongs to the Fdd2 space group featuring a 3D condensed framework with I3O3 connectivity (the definition of ImOn is described in the ESI).8 The asymmetric unit contains one formula unit. It deserves to be specially noted that in the asymmetric unit, the Ba2+ centre is not in its full site occupancy and is


8

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disordered with one H2O and one H+ of N–H in the 4,5-ImDCH2– ligand. As a result, there exist two types of structures in 1 (Scheme S1 in the ESI). One is the heterometallic BaLi–based structure, giving rise to a formula of [Ba0.5Li(4,5-ImDCH)] with a probability of 75%; and the other is the single metal ion Li–based structure with a formula of [Li(4,5-ImDCH2)]·H2O with a probability of 25% (denoted as structure 1’). The crystal structure of the former is described here in detail; the structure description of 1’ is deposited in the ESI. The Ba2+ ion (Ba1) is eight–coordinated by four carboxylic O atoms from four 4,5-ImDCH2– ligands and two N and two O atoms from another two ligands in a chelating mode, resulting in a [BaO6N2] trigonal dodecahedron (Figure 2a). The Li+ ion (Li1) adopts tetrahedral coordination geometry occupied by the O atoms from three ligands, one of which chelates to Li+ with its two O atoms from different carboxylate groups (Figure 2b). The 4,5-ImDCH2– ligand adopts a (k1-k2µ3)-(k2-k2-µ4)-µ6 coordination mode (Figure 2c). Obviously, the three structural components, Ba2+, Li+ and 4,5-ImDCH2–, all feature noncentrosymmetric coordination modes. The framework of 1 is rather complicated so we shall describe this structure in a simplified polyhedral way in the text. As shown in Figure 2d, there exists edge–shared coordination polyhedra of trinuclear [LiBaLi] heterometallic unit in the structure which could be viewed as the secondary building unit (SBU) of 1. The unit contains one Ba1 in m symmetry with two Li1 in both sides. The Ba1 and Li1 are interconnected by µ2–O1 and µ2–O2; as a result, the [BaO6N2] and [LiO4] polyhedra are edge–shared. Then, adjacent [LiBaLi] units are interconnected with each other through µ2–O3 with their polyhedra corner–shared, as shown in Figure 2e. With this linking mode, each unit connects another four adjacent units by the linking of 4,5-ImDCH2– ligands via sharing corners between [BaO6N2] and [LiO4] polyhedra (Figure 2f) to form the final 3D framework (Figure S3).


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Topologically, when ignoring the 4,5-ImDCH2– ligands and regarding each trinuclear unit as a 4–connected node, the structure of 1 could be simplified as a typical dia (diamond) topology, Figure 2g. Ba(4,5-ImDCH)(µ2-DMF) (2). Compound 2 belongs to the space group of Cc featuring a 3D framework constructed by 4,5-ImDCH2– ligands connecting Ba2+ ions with I1O3 connectivity. The asymmetric unit contains one formula unit. The Ba2+ ion is nine–coordinated by four 4,5ImDCH2– ligands and two DMF molecules, as depicted in Figure 3a; one ligand coordinates with one terminal carboxylate O atom, and the other three chelating ligands adopt different fashion: I. chelating with two O atoms of one carboxylate group, II. chelating with two O atoms from two carboxylate groups, III. chelating with one carboxylate O atom and one N atom. Thus a [BaO8N] trigonal tetrakaidecahedron is formed. The 4,5-ImDCH2– ligand adopts a (k1-k1-µ2)-(k1-k2-µ3)-µ4 coordination mode (Figure 3b). The DMF molecule in a µ2–fashion links two Ba2+ ions with different Ba–O bond distances. Therefore, the three structural components, Ba2+, 4,5-ImDCH2– and DMF, all feature noncentrosymmetric coordination modes. The Ba2+ ions are interlinked by 4,5-ImDCH2– ligands and DMF molecules in a “tri–bridge” fashion through µ2–O3 and µ2–O4 from a carboxylate group of 4,5-ImDCH2– and µ2–O from a DMF molecule, thus forming a 1D infinite chain along the c axis, Figures 3c and 3d. Notably in the chain the [BaO8N] polyhedra share faces while the DMF molecules are arranged along one side of the chain. Further, each chain connects to adjacent four same chains by 4,5-ImDCH2– ligands to form a 3D framework with the coordinated DMF molecules oriented towards the tunnels along the c axis, Figure 3e and S4. Li2(4,5-ImDC-Me) (3). Compound 3 belongs to the Fdd2 space group featuring a 3D condensed framework with I3O3 connectivity. The asymmetric unit contains one formula unit


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including two independent Li+ ions and one 4,5-ImDC-Me2- ligand. The two independent Li+ ions adopt tetrahedral coordination geometries. Li1 is surrounded by three 4,5-ImDC-Me2ligands, one of which chelates to Li1 with its two O atoms from different carboxylate groups, and the other two coordinate to Li1 in a monodentate mode, forming a [Li1O4] polyhedron (Figure 4a). Li2 is also surrounded by three ligands, two of which adopt a monodentate mode, and another one chelates to Li2 with its O and N atoms, giving a [Li2O3N] polyhedron (Figure 4b). The 4,5-ImDC-Me2- ligand presents a (k1-k2-µ3)-(k2-k2-µ4)-µ6 coordination mode (Figure 4c). There exist tetranuclear circular [Li4] units in the structure which could be viewed as the SBU of 3. In the unit, Li1 and Li2 are interlinked by the di–bridge of carboxylate group (O1, O2) and µ2–O4 and the mono–bridge of µ2–O3 in a Li1–Li2–Li1–Li2 fashion (Figure 4d), notably, the [Li1O4] and [Li2O3N] tetrahedra share vertex with each other. Then, adjacent SBUs are interconnected with each other through µ2–O2, making their tetrahedra corner–shared, as shown in Figure 4e. With this linking mode, each unit connects another four adjacent units (Figure 4f) to form the final 3D framework (Figure S5). Topologically, when ignoring the 4,5-ImDC-Me2– ligands and regarding each [Li4] unit as a 4– connected node, the structure of 3 could be simplified as a typical dia topology similar with that of 1, Figure 4g. In compounds 1 and 2, the coordination number of Ba2+ is 8~9 and the two carboxylate groups in the ligands are all deprotonated. And they feature the I3O3 and I1O3 structural types, respectively.

By

comparison,

in

the

reported

Ba-4,5-ImDC

structure,

[Ba(4,5-

ImDCH2)2(H2O)4]·2H2O15a, four sites of the ten–coordinated Ba2+ centre are occupied by H2O molecules which may contribute to its layered structure nature that is formed by the mono– deprotonated 4,5-ImDCH2– ligands connecting single Ba2+ ion nodes, giving an I0O2 structure.


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What is more, there are four Sr-4,5-ImDC frameworks with various degrees of deprotonation of 4,5-ImDCH3 ligands showing I0O3, I1O3 and I3O3 connectivities, respectively.15b It is worth noting that, 1 represents the first example of heterometallic CP constructed from 4,5-ImDCH3 ligand with Ba(II) and Li(I). NLO Properties. Considering their NCS structural nature, the SHG responses of 1–3 were investigated on the powder samples by using the Kurtz–Perry method.10 An approximate estimation was carried out on a pulsed Q–switched Nd:YAG laser at a wavelength of 1064 nm. The intensity of the green light (λ = 532 nm) produced by the crystal powder of 1–3 is about 5, 1.5 and 3 times that of a KDP marker in the same particle size, respectively. Moreover, the SHG effect increases with particle size and plateaus at a maximum value when the particle size is large enough, revealing phase–matching behaviour for 1–3, Figure 5. The second–order susceptibility χ(2)eff of a KDP powder sample is about 0.36 pm/V, so the derived second–order susceptibility χ(2)eff for 1–3 are 1.80, 0.54 and 1.08 pm/V. Interestingly, when we deeply analyze the structure of 1, we find that it shows enhanced polarity along the c axis. As depicted in Scheme 2, the coordination polyhedron of Ba2+ is very aberrant. Detailedly, the coordination atoms are distributed rather unbalanced in the spaces around Ba2+ centre and inclined to one side, which makes the negative charge centre (centre of the coordination atoms, denoted as X) and the positive charge centre (Ba2+) rather separated (the distance of Ba–X is 0.5 Å) forming a typical dipole. Because the two centers have the same x and y coordinates of –0.25 and 0.25, respectively, the dipoles are rightly along the c axis. Coincidently, these dipoles have the same direction in the whole structure. So an efficient polar system along the c axis is formed. By comparison, the negative charge centre of Li+ coordination polyhedron (denoted as Y) is very close to Li+ center (Li–Y = 0.06 Å). Furthermore, the Li+ ion


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has a very little polarizability compared to Ba2+ which has very large radius, so we suppose that the polarity around Li+ is negligible. This may explain the relatively large SHG response of 1 compared with that of 2 and 3. Theoretical analyses. Electronic Structures. The density functional theory (DFT) studies were carried out for 2 and 3 to study their energy band structures and optical properties. It can be seen from the band structures (Figure 6) that, for 2, the highest occupied valence band (HVB) and the lowest unoccupied conduction band (LCB) are located at the G and Z points. These features of the band structure indicate that 2 has flat valence band (VB), sparse conduction band (CB), and indirect band gap. While for 3, its HVB and LCB are located between the X and G points which are in different sites, so it has flat VB, sparse CB, and indirect band gap. The calculated results reveal that the band gap of 2 and 3 are 3.395 and 3.818 eV, respectively, which are consistent with the experimental value (4.0 and 4.2 eV), Fig. S9. Figure 7 schematically illustrates the total density of states (TDOS) and partial density of states (PDOS) of 2 and 3. For 2, with energy ranging from −9 eV to the Fermi level, the VB region is dominated by the O–2p, C–2P, N–2P and H–1s states mixed with minor O–2s, C–2s, N–2s states and negligible Ba–5p6s states, whereas the CB (from 3 to 8 eV) is derived mainly from the C–2P, N–2P, O–2p states and a small mixture of C–2s, N–2s, O–2s, H–1s and Ba–6s states. Comparing the charge distributions nearby the Fermi level between the 4,5-ImDCH2– ligand and DMF molecule, the contributions of the 4,5-ImDCH–s and p states are more than that of DMF–s and p states in the VB region. Moreover, the 4,5-ImDCH–p state makes more contribution than the 4,5ImDCH–s state. Thus, it is obvious that the optical diffuse reflectance spectra is mainly ascribed to the charge transfers from the occupied 4,5-ImDCH and DMF states to their unoccupied states


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and Ba–6s states. For 3, with energy ranging from −8 eV to the Fermi level, the VB region is dominated by the O–2p, C–2P, N–2P and H–1s states mixed with minor O–2s, C–2s, N–2s states and negligible Li–2s states, whereas the CB (from 3 to 7 eV) is derived mainly from the C–2P, N–2P, O–2p states and a small mixture of Li–2s with negligible H–1s states. Totally, the 4,5ImDC-Me–p state makes more contribution than the 4,5-ImDC-Me–s state. Thus, it is obvious that the optical diffuse reflectance spectra is mainly ascribed to the charge transfers from the occupied 4,5-ImDC-Me states to their unoccupied states and Li–2s states. For an intuitionistic view of the electronic structures of 2 and 3, the charge densities in the HVB and LCB region are shown in Figure 8, respectively. Theoretical Analyses of the Optical Properties. The space group of 2 belongs to class m, so there are ten nonvanishing tensors of second–order susceptibility. In the low–energy region and under the restriction of Kleinman’s symmetry, only six independent SHG tensor component (d11, d12, d13, d15, d24, d33) are considered. As shown in Figure 9a, the theoretical values of tensor components for 2 are 0.53, 0.55, 0.61, 0.57, 0.59 and 0.65 pm/v at a wavelength of 1064 nm (1.167 eV) which are consistent with the experiment value of 0.54 pm/v. The space group of 3 falls into class mm2, thus there are five nonvanishing tensors of second–order susceptibility. In the low–energy region and under the restriction of Kleinman’s symmetry, only the independent d31, d32 and d33 SHG tensor component are considered. The theoretical values of tensor components are 0.42, 0.63, 0.55 pm/v at 1.167 eV which are close to that of the experiment value of 1.08 eV (Figure 9b). Fluorescence spectra. The photoluminescence (PL) properties of the title compounds in the solid state were investigated at room temperature. As shown in Figure 10a, the emission bands are centred at 405 nm, 450 nm, 410 nm for 1, 2 and 3 when excited by 340 nm, 410 nm and 340


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nm wavelengths, respectively. All of the emissions are located in the blue light region (Figure S10) and the quantum yields (QY) values are 14.68%, 16.26% and 25.87% for 1–3 (Table S2), respectively, which are even comparable to luminescent rare–earth–ion–containing CPs.16 The PLs of the free ligands of 4,5-ImDCH3 and 4,5-ImDCH2-Me showed emission maxima at 460 nm (Ex = 390 nm) and 425 nm (Ex = 360 nm), respectively (Figure 10b). The QY values of 1–3 are 2 ~ 4 times that of the free ligands, demonstrating the emission of the organic ligands are obviously enhanced when assembled into hybrid frameworks (Table S2).17 Considering that the Ba2+/Li+ ions are difficult to be oxidized or reduced, the PLs of the title compounds are neither based on metal–to–ligand charge transfer (MLCT) nor ligand–to–metal charge transfer (LMCT). Instead, they shall probably be assigned to the ligand–centred (intraligand, π→π*) emission.17–18 CONCLUSIONS. As an extention of our recently developed strategy for constructing NCS CPs, we obtained another three NCS compounds constructed from dipolar imidazolecarboxylate ligands with Ba(II) or/and Li(I) ions. 1 and 3 feature similar dia topology and 2 has an open framework, all of which are three dimensional networks. Especially, compound 1 shows strong SHG response, phase– matching ability and high stability. 1–3 emit bright blue light upon the excitation of UV light with high quantum yields. Our ongoing work will continue to construct NCS CPs with higher SHG response and stability, and simultaneously to gain deeply understanding of the strategy for further increasing the chance to obtain NCS structures.


15


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FIGURES

Figure 1. (a) Photographs of crystals of 2 synthesized by conventional heating, and (b) SEM images of the microcrystals of 2 obtained by MW heating.

Figure 2. (a–c) The coordination modes of Ba2+, Li+ and 4,5-ImDCH2– in 1; (d) the trinuclear [LiBaLi] unit; (e) the linking mode of two adjacent trinuclear [LiBaLi] units by 4,5-ImDCH2– ligands; (f) the corner–sharing linking mode of the [LiBaLi] SBUs with four adjacent SBUs; the C and N atoms of 4,5-ImDCH2– ligands are omitted for a simplified view; (g) the diamond topology of 1. All of the H atoms are omitted for clarity.


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Figure 3. (a–b) The coordination modes of Ba2+ and 4,5-ImDCH2– in 2; (c) the 1D chain along the c axis; (d) the face–shared polyhedra in the chain; (e) the 3D structure viewed along the c axis showing the DMF molecules in a space–filling mode in 2. All of the H atoms are omitted for clarity.


17


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Figure 4. (a–c) The coordination modes of Li+ and 4,5-ImDC-Me2– in 3; (d) the tetranuclear circular [Li4] unit; (e) the linking mode between two adjacent tetranuclear [Li4] SBUs by 4,5ImDC-Me2– ligands; (f) the corner–sharing linking mode of the SBU with four adjacent SBUs; the C and N atoms of 4,5-ImDC-Me2– ligands are omitted for a simplified view; (g) the diamond topology of 3. All of the H atoms are omitted for clarity.


18

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Figure 5. (a–c) Particle size dependence of the SHG intensity for 1–3. Insets: Oscilloscope trace of the SHG signal of each compound and KDP in the particle size of 150–212 µm.


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Figure 6. Electronic band structures for 2 (a) and 3 (b).


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Figure 7. Total and partial densities of states for 2 (a) and 3 (b). The Fermi level is set at 0 eV.


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Figure 8. The calculated charge distributions of the HVB and LCB near the Fermi level for 2 (a– b) and 3 (c–d). The isosurfaces value is 0.001 e·Å–3.


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Figure 9. Calculated frequency–dependent SHG tensor component dij for 2 (a) and 3 (b). Frequency: 1064 nm (1.167 eV).


23


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Figure 10. Solid–state fluorescence spectra of 1–3 (a) and the free ligands (b) at room temperature. The solid and dashed lines represent the emission and excitation spectra of each compound, respectively.

Scheme 1. Structural diagrams of the 4,5-ImDCH3 and 4,5-ImDCH2-Me ligands utilized in this work.


24

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Scheme 2. Dipoles in 1 along the c axis. The yellow ball represents the negative charge centre of the coordinated atoms around Ba2+ ion. Table 1. Crystallographic data and structural refinement details for 1–3. 1

2

3

Empirical formula

C5H2.50Ba0.375LiN2O4.125

C8H9BaN3O5

C6H4Li2N2O4

Formula Mass

215.03

364.52

181.99

Crystal system

orthorhombic

monoclinic

orthorhombic

Space group

Fdd2

Cc

Fdd2

a/Å

16.0667(6)

10.696(2)

15.3790(4)

b/Å

16.3694(5)

16.0212(6)

15.9122(4)

c/Å

9.8196(8)

7.0551(9)

12.6196(3)

α/°

90

90

90

β/°

90

115.21(2)

90

90

90

90

V/Å

2582.6(2)

1093.8(3)

3088.19(13)

Z

16

4

16

γ/° 3


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T/K

100(2)

295(2)

295(2)

Flack parameter

0.00(2)

0.01(4)

0.0(2)

F(000)

1656

696

1472

θ(max.)

27.10

26.72

76.30

ρcalcd/g cm–3

2.212

2.214

1.566

µ/mm–1

2.387

3.646

1.096

Measured refls.

5738

2607

7269

Independent refls.

1409

1661

1365

No. of parameters

119

159

128

Rint

0.0409

0.0311

0.0369

a

R indices [I>2σ(I)]

0.0227, 0.0526

0.0325, 0.0764

0.0288, 0.0763

R indices (all data)

0.0259, 0.0543

0.0363, 0.0787

0.0291, 0.0766

GOF

1.007

1.005

1.013

[a] R1 = ∑║Fo│–│Fc║/∑│Fo│, wR2 =

[∑w(Fo2–Fc2)2/∑w(Fo2)2]1/2.

ASSOCIATED CONTENT Supporting Information. More structural details, figures, PXRD patterns, thermal analyses, IR, UV–vis spectra, and photographs of the luminescence. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. Fax: (+86)591–63173145. Notes The authors declare no competing financial interest. ACKNOWLEDGEMENTS


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We acknowledge the NNSF of China (no. 21521061 and no. 21403233) and the 973 program (no. 2012CB821702) for financial support. REFERENCES (1)

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Mater. Sci. 1986, 16, 203–243. (c) Keszler, D. A. Curr. Opin. Solid State Mater. Sci. 1999, 4, 155–162. (d) Ok, K. M.; Chi, E. O.; Halasyamani, P. S. Chem. Soc. Rev. 2006, 35, 710–717. (2)

(a) Evans, O. R.; Lin, W. B. Chem. Mater. 2001, 13, 3009–3017. (b) Evans, O. R.; Lin,

W. B. Acc. Chem. Res. 2002, 35, 511–522. (c) Evans, O. R.; Xiong, R. G.; Wang, Z. Y.; Wong, G. K.; Lin, W. B. Angew. Chem., Int. Ed. 1999, 38, 536–538. (d) Lin, J. D.; Long, X. F.; Lin, P.; Du, S. W. Cryst. Growth Des. 2010, 10, 146–157. (e) Lin, W. B.; Wang, Z. Y.; Ma, L. J. Am. Chem. Soc. 1999, 121, 11249–11250. (f) Tian, C.; Zhang, H.; Du, S. CrystEngComm 2014, 16, 4059–4068. (g) Wang, C.; Zhang, T.; Lin, W. B. Chem. Rev. 2012, 112, 1084–1104. (h) Xiong, R. G.; Xue, X.; Zhao, H.; You, X. Z.; Abrahams, B. F.; Xue, Z. L. Angew. Chem., Int. Ed. 2002, 41, 3800–3803. (i) Zhao, H.; Qu, Z. R.; Ye, H. Y.; Xiong, R. G. Chem. Soc. Rev. 2008, 37, 84– 100. (3)

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Q. A.; Yu, J. C.; Gao, J. K.; Rao, X. T.; Yang, X. L.; Cui, Y. J.; Wu, C. D.; Zhang, Z. J.; Xiang, S. C.; Chen, B. L.; Qian, G. D. Cryst. Growth Des. 2010, 10, 5291–5296. (c) Saha, R.; Biswas, S.; Mostafa, G. CrystEngComm 2011, 13, 1018–1028. (4)

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2003, 42, 3969–3971. (b) Xie, Y. R.; Zhao, H.; Wang, X. S.; Qu, Z. R.; Xiong, R. G.; Xue, X. A.; Xue, Z. L.; You, X. Z. Eur. J. Inorg. Chem. 2003, 3712–3715. (c) Ye, Q.; Li, Y. H.; Song, Y.


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M.; Huang, X. F.; Xiong, R. G.; Xue, Z. L. Inorg. Chem. 2005, 44, 3618–3625. (d) Ye, Q.; Tang, Y. Z.; Wang, X. S.; Xiong, R. G. Dalton Trans. 2005, 1570–1573. (5)

(a) Du, F.; Zhang, H.; Tian, C.; Du, S. Cryst. Growth Des. 2013, 13, 1736–1742. (b) Liu,

Y.; Zhang, H.; Tian, C.; Lin, P.; Du, S. CrystEngComm 2013, 15, 5201–5204. (c) Zhang, H.; Lin, P.; Shan, X.; Han, L.; Du, S. CrystEngComm 2014, 16, 1245–1248. (6)

Song, Y.; Lin, C.–S.; Zhang, M.–J.; Wei, Q.; Feng, M.–L.; Huang, X.–Y. CrystEngComm

2015, 17, 3418–3421. (7)

(a) Wampler, R. D.; Begue, N. J.; Simpson, G. J. Cryst. Growth Des. 2008, 8, 2589–

2594. (b) Di Bella, S. Chem. Soc. Rev. 2001, 30, 355–366. (8)

Cheetham, A. K.; Rao, C. N. R.; Feller, R. K. Chem. Commun. 2006, 4780–4795.

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(10) Kurtz, S. K.; Perry, T. T. J. Appl. Phys. 1968, 39, 3798–3813. (11) Sheldrick, G. M. SHELX 97, Program for Crystal Structure Solution and Refinement, University of Göttingen: Germany 1997. (12) Clark, S. J.; Segall, M. D.; Pickard, C. J.; Hasnip, P. J.; Probert, M. J.; Refson, K.; Payne, M. C. Z. Kristallogr. 2005, 220, 567–570. (13) Perdew, J. P.; Burke, K.; Ernzerhof, M. Phys. Rev. Lett. 1996, 77, 3865–3868. (14) Dudarev, S. L.; Botton, G. A.; Savrasov, S. Y.; Humphreys, C. J.; Sutton, A. P. Phys. Rev. B 1998, 57, 1505–1509. (15) (a) Starosta, W.; Leciejewicz, J.; Premkumar, T.; Govindarajan, S. J. Coord. Chem. 2007, 60, 313–318. (b) Zhang, X. F.; Deng, Z. P.; Huo, L. H.; Feng, Q. M.; Gao, S. Eur. J. Inorg. Chem. 2012, 5506–5514.


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(16) (a) Cui, Y. J.; Yue, Y. F.; Qian, G. D.; Chen, B. L. Chem. Rev. 2012, 112, 1126–1162. (b) Allendorf, M. D.; Bauer, C. A.; Bhakta, R. K.; Houk, R. J. T. Chem. Soc. Rev. 2009, 38, 1330– 1352. (17) Furman, J. D.; Warner, A. Y.; Teat, S. J.; Mikhailovsky, A. A.; Cheetham, A. K. Chem. Mater. 2010, 22, 2255–2260. (18) Zhang, Q. F.; Hao, H. G.; Zhang, H. N.; Wang, S. N.; Jin, J.; Sun, D. Z. Eur. J. Inorg. Chem. 2013, 1123–1126.


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For Table of Contents Use Only

The efficient cooperation of dipolar imidazolecarboxylate ligand with Ba(II) and/or Li(I) ions afforded three NCS CPs with strong and phase-matching SHG response and bright blue fluorescence.


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TOC figure 45x27mm (300 x 300 DPI)



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Figure 1. (a) Photographs of crystals of 2 synthesized by conventional heating, and (b) SEM images of the microcrystals of 2 obtained by MW heating. 83x45mm (300 x 300 DPI)


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Figure 2. (a–c) The coordination modes of Ba2+, Li+ and 4,5-ImDCH2– in 1; (d) the trinuclear [LiBaLi] unit; (e) the linking mode of two adjacent trinuclear [LiBaLi] units by 4,5-ImDCH2– ligands; (f) the corner– sharing linking mode of the [LiBaLi] SBUs with four adjacent SBUs; the C and N atoms of 4,5-ImDCH2– ligands are omitted for a simplified view; (g) the diamond topology of 1. All of the H atoms are omitted for clarity. 83x87mm (300 x 300 DPI)



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Figure 3. (a–b) The coordination modes of Ba2+ and 4,5-ImDCH2– in 2; (c) the 1D chain along the c axis; (d) the face–shared polyhedra in the chain; (e) the 3D structure viewed along the c axis showing the DMF molecules in a space–filling mode in 2. All of the H atoms are omitted for clarity. 82x133mm (300 x 300 DPI)


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Figure 4. (a–c) The coordination modes of Li+ and 4,5-ImDC-Me2– in 3; (d) the tetranuclear circular [Li4] unit; (e) the linking mode between two adjacent tetranuclear [Li4] SBUs by 4,5-ImDC-Me2– ligands; (f) the corner–sharing linking mode of the SBU with four adjacent SBUs; the C and N atoms of 4,5-ImDC-Me2– ligands are omitted for a simplified view; (g) the diamond topology of 3. All of the H atoms are omitted for clarity. 83x106mm (300 x 300 DPI)



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Figure 5. (a–c) Particle size dependence of the SHG intensity for 1–3. Insets: Oscilloscope trace of the SHG signal of each compound and KDP in the particle size of 150–212 µm. 176x140mm (300 x 300 DPI)


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Figure 6. Electronic band structures for 2 (a) and 3 (b). 83x127mm (300 x 300 DPI)



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Figure 7. Total and partial densities of states for 2 (a) and 3 (b). The Fermi level is set at 0 eV. 176x108mm (300 x 300 DPI)


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Figure 8. The calculated charge distributions of the HVB and LCB near the Fermi level for 2 (a–b) and 3 (c– d). The isosurfaces value is 0.001 e∙Å–3. 163x149mm (300 x 300 DPI)



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Figure 9. Calculated frequency–dependent SHG tensor component dij for 2 (a) and 3 (b). Frequency: 1064 nm (1.167 eV). 83x131mm (300 x 300 DPI)


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Figure 10. Solid–state fluorescence spectra of 1–3 (a) and the free ligands (b) at room temperature. The solid and dashed lines represent the emission and excitation spectra of each compound, respectively. 83x140mm (300 x 300 DPI)



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Scheme 1. Structural diagrams of 4,5-ImDCH3 and 4,5-ImDCH2-Me ligands utilized in this work. 75x24mm (300 x 300 DPI)


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Scheme 2. Dipoles in 1 along the c axis. The yellow ball represents the negative charge centre of the coordinated atoms around Ba2+ ion. 65x79mm (300 x 300 DPI)


Li (I

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