Two Silver Coordination Network Compounds with Colorful

Article pubs.acs.org/IC

Two Silver Coordination Network Compounds with Colorful Photoluminescence Dandan Yang,† Wenlong Xu,† Xiaowei Cao,† Shaojun Zheng,‡ Jiangang He,‡ Qiang Ju,† Zhenlan Fang,*,† and Wei Huang*,† †

Key Laboratory of Flexible Electronics (KLOFE) & Institute of Advanced Materials (IAM), Jiangsu National Synergetic Innovation Center for Advanced Materials (SICAM), Nanjing Tech University (NanjingTech), 30 South Puzhu Road, Nanjing 211816, P.R. China ‡ School of Environmental and Chemical Engineering, Jiangsu University of Science and Technology, 2 Mengxi Road, Zhenjiang 212003, Jiangsu, P.R. China S Supporting Information *

ABSTRACT: The excitation-wavelength-dependent photoluminescence (EWDP) property of flexible organic ligand 1,4-bis(2-methylimidazol-1-yl)butane (Bmib) was observed. Herein, Bmib was chosen as a bridge linker to react with AgX (X = Br and I) to synthesize novel coordination network compounds (CNCs) with interesting EWDP properties. As anticipated, under the same hydrothermal synthesis conditions, two new isomorphic CNCs, i.e. [Ag2(Bmib)Br2]∞ (IAM16-1) and [Ag2(Bmib)I2]∞ (IAM16-2), as the first examples of CNCs showing EWDP properties, have been obtained. The EWDP properties may be attributed to the stretch and rotation of the long -(CH2)4- chains of Bmib and the spatial orientation adjustment of the methyl group of each imidazole ring at different excitation wavelengths. It is a great challenge to point out the emission mechanisms of CNCs merely from the experimental results due to their multiple charge transfer routes. To address this issue, we adopt DFT calculations to pursue in-depth investigation of the emission mechanisms for IAM16-1 and IAM16-2, respectively.

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metries.14 Therefore, the structural diversity of Ag(I)−halogen clusters relatively abundant, including the [AgX] monomeric unit,15 the rhomboid [Ag2X2] dimeric unit,15 the split stair or zigzag [AgX] chain,15 the [Ag2X2]n zigzag double chains,15 the [Ag2X2]n helical chains,11 the wavelike [Ag4X4]n chains,16 and the hexagonal prism-shaped [Ag6X6] cluster unit.17 Proper organic bridging linkers are critical in constructing CNCs, and the selection of flexible ligands has been reported as a popular approach to construct flexible frameworks with versatile properties. The bis(imidazole) ligand with flexible nature of -(CH2)4- spacers as a good representative of nitrogendonor bridging ligand is able to bend and rotate freely when coordinating to metal atoms.17 Combination of the flexibility of the long chain and the spatial orientation of the methyl groups of each imidazole ring may endow creation of various coordination polymers with fascinating properties, such as EWDP properties. Ag(I) ions show a tendency to coordinate to imidazole, and the silver CNCs can inherit and further modify the properties of the bridging organic linker due to the interaction between linker and metal ion. Therefore, controllable modulation of the synthetic conditions can dominate the self-assembly of the

INTRODUCTION Coordination network compounds (CNCs) have attracted widespread attention for their structure diversity1 and the use of their unique electrical and optical properties in optoelectronics devices2 and biological labeling materials.3 The excitationwavelength-dependent photoluminescence (EWDP) property has aroused tremendous research interest owing to their potential applications in photoluminescence (PL),4 optoelectronic and chemical sensor devices,5 and biological labeling.6 EWDP behavior has been observed from the nanostructure metal oxides,4a metallopolymers,7 organic materials,8 coordination compounds,9 semiconductors, and quantum dots so far.10 However, to the best of our knowledge, examples of CNCs with EWDP properties are scarce up to now, although great interest has been focused on the design and construction of novel CNCs with fascinating EWDP properties. Crystal engineering principles are important for the design and controllable synthesis of new materials with targeted properties at the molecular level. It is well-known that construction of coordination frameworks rests on various factors, such as solvent system, temperature, organic ligands, metal ions, and ratio of metal and ligand. Halogen atoms as simple ligands can adopt 1-, 2-, 3-, and 4-coordinated geometries,11 while the closed-shell d10 electronic configuration of Ag(I) can adopt 2-,12 3-,13 4-, and 5-coordinated geo© XXXX American Chemical Society

Received: April 21, 2016

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DOI: 10.1021/acs.inorgchem.6b00999 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry silver halide with the flexible bis(imidazole) ligand to form novel silver CNCs with EWDP properties. As anticipated, under the same synthesis conditions, two new isomorphic CNCs [Ag2(Bmib)Br2]∞ (IAM16-1, IAM is the abbreviation of Institute of Advanced Materials) and [Ag2(Bmib)I2]∞ (IAM162) based on Ag(I)−halogen aggregates with interesting EWDP properties have been first fabricated, applying 1,4-bis(2-methylimidazol-1-yl)butane (Bmib) to react with AgX (X = Br and I). CNCs have multiple charge transfer routes, bringing out a great challenge to point out the emission mechanism of those two CNCs merely from experimental results. In this report, we adopt DFT calculations to pursue in-depth investigations of the luminescence of IAM16-1 and IAM16-2.

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carried out on a NETSCHZ STA 449C thermoanalyzer under N2 (30−800 °C range) at a heating rate of 10 K/min. Emission spectra were measured on an F-4600 FL spectrophotometer instrument. Fluorescence decay curves were recorded on an Edinburgh Analytical instrument FLS920. Optical diffuse reflectance spectra were measured at room temperature with a PerkinElmer Lambda 950 UV−vis spectrophotometer. The instrument was equipped with an integrating sphere and controlled by a personal computer. The samples were ground into fine powder and then pressed onto a thin glass slide holder. A BaSO4 plate was used as a standard (100% reflectance). The absorption spectra were calculated from reflectance spectra using the Kubelka−Munk function: R/S = (1 − R)2/(2R),19 where R is the absorption coefficient, S is the scattering coefficient, which is practically wavelength independent when the particle size is larger than 5 μm, and R is the reflectance. X-ray Crystallography. The single crystals of the two CNCs in the present work were mounted on a glass fiber for the X-ray diffraction analysis. Diffraction data were collected at 296 and 100 K on a Bruker SMART APEX II CCD area-detector diffractometer using graphite-monochromated Mo Kα radiation (λ = 0.71073 Å) from a rotating anode generator, and intensities were corrected for LP factors and empirical absorption using the ψ scan technique. The structures were solved by direct methods, developed by subsequent difference Fourier syntheses, and refined using the Siemens SHELXTL version 5 software package.20 All non-hydrogen atoms were refined with anisotropic displacement parameters. All H atoms on carbons were placed in calculated positions and refined using the riding model. Crystal data as well as details of data collection and refinement for them are summarized in Table 1. The selected interatomic distances and bond angles are given in Table 2. Computational Descriptions. The crystallographic data of IAM16-1 and IAM16-2 determined by single crystal X-ray diffraction were used to calculate their electronic structures. The density functional theory (DFT) calculations using the Perdew−Burke− Ernzerhof (PBE)21 generalized gradient approximation (GGA) were performed on IAM16-1 and IAM16-2 by using the Vienna ab intio simulation package (VASP).22 The ion−electron interaction was modeled with the projector augmented plane wave (PAW) potential.23 The wave functions were expanded by using a plane-wave basis set with an energy cutoff of 420.9 eV. The sampling of the Brillouin zone

EXPERIMENTAL SECTION

Materials and General Methods. The original bridging ligand (Scheme 1) was synthesized according to a reported literature

Scheme 1. Bmib Ligand as Well as Its Possible Coordination Conformations

procedure.18 Other chemicals were obtained from commercial sources, and used without further purification. The IR spectra (KBr pellets) were recorded on an ALPHA FT-IR spectrophotometer in the range 400−1800 cm−1. C, H, and N elemental analyses were determined on a Vario ELcube element analyzer. Powder X-ray diffraction data were recorded on a SmartLab diffractometer (Rigaku, Japan) using Nifiltered Cu Kα radiation (λ = 1.542 Å). Thermal stability studies were

Table 1. Crystal Data and Structure Refinement Results for IAM16-1 and IAM16-2 IAM16-1

CNCs temp (K) Empirical formula formula weight crystal system space group Z a (Å) b (Å) c (Å) α (deg) β (deg) γ (deg) V (Å3) ρcalcd (g/cm3) μ (mm−1) GOF R1 (I > 2σ(I))a wR2 (I > 2σ(I))a R1 (all data)b wR2 (all data)b a

296 AgBrC6N2H9 296.93 Monoclinic P21/n 4 9.726(4) 4.619(1) 18.288(7) 90 94.679(9) 90 818.8(5) 2.409 7.267 1.025 0.037 0.084 0.056 0.093

IAM16-2 100 AgBrC6N2H9 296.93 Monoclinic P21/n 4 9.629(1) 4.566(1) 17.937(3) 90 93.883(3) 90 786.8(2) 2.507 7.562 1.047 0.025 0.056 0.033 0.059

296 AgIC6N2H9 343.92 Monoclinic P21/n 4 10.001(1) 4.677(0) 18.658(2) 90 97.196(3) 90 865.8 (1) 2.638 5.823 1.077 0.034 0.090 0.038 0.091

100 AgIC6N2H9 343.92 Monoclinic P21/n 4 9.923(1) 4.650(0) 18.415(1) 90 96.797(2) 90 843.7(1) 2.708 5.975 1.003 0.034 0.109 0.035 0.111

R1 = ∑∥Fo| − |Fc∥/∑|Fo|. bwR2 = {∑[w(Fo2 − Fc2)2/∑[w(Fo2)2]]1/2. B


Article

Inorganic Chemistry Table 2. Selected Bond Lengths (Å) and Angles (deg) for IAM16-1 and IAM16-2a IAM16-1 296 K Br(1)−Ag(1)#1 Br(1)−Ag(1)#2 Br(1)−Ag(1) Ag(1)−N(1) N(1)−Ag(1)−Br(1)#2 N(1)−Ag(1)−Br(1)#1 Br(1)#2−Ag(1)−Br(1)#1 N(1)−Ag(1)−Br(1) Br(1)#2−Ag(1)−Br(1) Br(1)#1−Ag(1)−Br(1)

100 K 2.646(1) 2.794(1) 2.908(1) 2.252(4) 128.12(10) 101.69(10) 116.16(3) 96.81(9) 106.93(2) 103.08(2)

Br(1)−Ag(1)#1 Br(1)−Ag(1)#2 Br(1)−Ag(1) Ag(1)−N(1) N(1)−Ag(1)−Br(1)#2 N(1)−Ag(1)−Br(1)#1 Br(1)#2−Ag(1)−Br(1)#1 N(1)−Ag(1)−Br(1) Br(1)#2−Ag(1)−Br(1) Br(1)#1−-Ag(1)−Br(1)

2.624(0) 2.770(0) 2.890(1) 2.246(2) 131.08(5) 100.99(5) 115.62(1) 96.07(5) 105.97(1) 102.21(1)

IAM16-2 296 K I(1)−Ag(1)#1 I(1)−Ag(1)#2 I(1)−Ag(1) Ag(1)−N(1) Ag(1)−Ag(1)#1 N(1)−Ag(1)−I(1)#2 N(1)−Ag(1)−I(1)#1 I(1)#2−Ag(1)−I(1)#1 N(1)−Ag(1)−I(1) I(1)#2−Ag(1)−I(1) I(1)#1−Ag(1)−I(1) N(1)−Ag(1)−Ag(1)#1 I(1)#2−Ag(1)−Ag(1)#1 I(1)#1−Ag(1)−Ag(1)#1 I(1)−Ag(1)−Ag(1)#1 N(1)−Ag(1)−Ag(1)#2 I(1)#2−Ag(1)−Ag(1)#2 I(1)#1−Ag(1)−Ag(1)#2 I(1)−Ag(1)−Ag(1)#2 Ag(1)#1−Ag(1)−Ag(1)#2

100 K 2.815(1) 2.891(1) 2.944(1) 2.282(5) 3.297(1) 122.60(13) 102.93(13) 110.09(2) 98.82(14) 111.78(2) 109.60(2) 109.79(13) 127.54(3) 56.35(13) 53.25(2) 127.02(14) 56.93(13) 128.03(3) 54.85(2) 90.35(3)

I(1)−Ag(1)#1 I(1)−Ag(1)#2 I(1)−Ag(1) Ag(1)−N(1) Ag(1)−Ag(1)#1 N(1)−Ag(1)−I(1)#2 N(1)−Ag(1)−I(1)#1 I(1)#2-Ag(1)−I(1)#1 N(1)−Ag(1)−I(1) I(1)#2−Ag(1)−I(1) I(1)#1−Ag(1)−I(1) N(1)−Ag(1)−Ag(1)#1 I(1)#2−Ag(1)−Ag(1)#1 I(1)#1−Ag(1)−Ag(1)#1 I(1)−Ag(1)−Ag(1)#1 N(1)−Ag(1)−Ag(1)#2 I(1)#2−Ag(1)−Ag(1)#2 I(1)#1−Ag(1)−Ag(1)#2 I(1)−Ag(1)−Ag(1)#2 Ag(1)#1−Ag(1)−Ag(1)#2

2.810(1) 2.887(1) 2.926(1) 2.279(5) 3.224(1) 123.19(11) 100.83(11) 109.41(2) 98.07(12) 113.27(2) 111.01(2) 107.57(11) 129.24(2) 56.91(1) 54.11(2) 127.65(12) 57.538(11) 129.73(2) 55.74(2) 92.31(2)

a Symmetry transformations used to generate equivalent atoms: IAM16-1 and IAM16-2, (#1) −x + 3/2, y − 1/2, −z + 3/2; (#2) −x + 3/2, y + 1/2, −z + 3/2.

was done by using 4 × 4 × 3 and 2 × 4 × 3 Monkhorst−Pack24 kpoint grids for IAM16-1 and IAM16-2, respectively. Synthesis of [Ag2(Bmib)Br2]∞ (IAM16-1). AgBr (30 mg, 0.13 mmol) and Bmib (30 mg, 0.2 mmol) were mixed in 10 mL of dimethylformamide (DMF), and then added to a solution of sodium ethoxide in ethanol (0.24 mol/L, 1 mL). After that the mixture was placed in a Parr Teflon-lined stainless steel vessel (20 mL) under autogenous pressure, and stirred at room temperature for 5 h. Then, the mixture was heated at 180 °C for 48 h. This was followed by slow cooling to room temperature at a rate of 6 °C h−1. After being washed with DMF and ethanol, and then air-dried, the yellow sheet crystals of IAM16-1 were obtained in 54.6% yield based on Bmib. Anal. Calcd (%) for Ag2Br2C12N4H18: C, 24.27; H, 3.03; N, 9.43. Found: C, 24.10 ; H, 2.94; N, 9.47. FT-IR for IAM16-1 (solid KBr pellet, v/cm−1): 3117(w), 2929(w), 2854(w), 1654(w), 1591(bw), 1496(s), 1472(m), 1457(m), 1415(m), 1370(m), 1298(m), 1274(s), 1154(m), 1113(m), 1074(w), 1037(w), 1020(w), 998(m), 770(s), 748(vs), 678(shoulder), 667(s), 624(w), 453(m). Synthesis of [Ag2(Bmib)I2]∞ (IAM16-2). AgI (30 mg, 0.13 mmol) and Bmib (30 mg, 0.2 mmol) were mixed in 10 mL of dimethylformamide (DMF), and then added to a solution of sodium ethoxide in ethanol (0.24 mol/L, 1 mL). After that the mixture was placed in a Parr Teflon-lined stainless steel vessel (20 mL) under autogenous pressure, and stirred at room temperature for 5 h. Then, the mixture was heated at 180 °C for 48 h. This was followed by slow cooling to room temperature at a rate of 3 °C h−1. After being washed

with DMF and ethanol, and then air-dried, the yellow sheet crystals of IAM16-2 were obtained in 64.6% yield based on Bmib. Anal. Calcd (%) for Ag2I2C12N4H18: C, 20.93; H, 2.62; N, 8.14. Found: C, 20.78; H, 2.56; N, 8.21. FT-IR for IAM16-2 (solid KBr pellet, v/cm−1): 3110(w), 2929(w), 2854(w), 1662(w), 1582(bw), 1494(s), 1469(m), 1457(m), 1414(m), 1368(m), 1296(m), 1273(s), 1154(m), 1112(m), 1070(w), 1037(w), 1019(w), 995(m), 761(s), 743(vs), 677(shoulder), 665(s), 623(w), 458(m).

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RESULTS AND DISCUSSION Structure Description of IAM16-1 and IAM16-2. Single crystal X-ray diffraction studies of the yellow crystalline solid at 296 K reveal that IAM16-1 and IAM16-2 are isostructural, and both of them crystallize in the monoclinic space group P21/n with one-half Bmib ligand, one Ag(I) atom, and one halogen anion (Br− and I− for IAM16-1 and IAM16-2, respectively) in the crystallographic minimum asymmetric unit. Herein, we discuss the structure of IAM16-1 in detail and only mention pertinent points of IAM16-2 for comparison. The coordination environment of IAM16-1 is shown in Figure 1a. Bimb ligand displays the most stable trans-conformation (Scheme 1),25 and adopts a bis(monodentate) bridging coordination mode26 with two imidazole N atoms coordinating to two crystallographic identified Ag(I) ions. At 296 K, the dihedral angle of the two C


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Inorganic Chemistry

These layers are packed in an ABAB fashion (Figure 2), and then generate a three-dimensional supramolecular framework

Figure 2. Illustration showing the ABAB stacking fashion of the layers.

by van der Waals interactions while not bridged by the Bimb linker through coordination of the Ag and the N atoms of the Bmib. The probable reason for this favorable arrangement may be that the two methyl groups are juxtaposed to the N atoms of the imidazole ring, and thus they exhibit steric hindrance that not only can restrict another Bimb ligand bonding to the Ag centers, but also can restrain the occurrence of interpenetration among the motifs to yield a 3-dimensional framework.26 At present, Bmib has been widely used in the creation of various coordination polymers through a mixed-ligands strategy incorporating an imidazole-based colinker with different carboxylic acid ligands.30 However, to the best of our knowledge, this is the first example of silver CNCs only based on Bmib according to the Cambridge Crystal Structural Database. Luminescence Properties of IAM16-1 and IAM16-2. The interesting luminescence feature shown by the Bmib, IAM16-1, and IAM16-2 is the EWDP properties (Figures 3 and 4). At 296 K, the PL spectral evolutions of Bmib and IAM16-1 at different excitation wavelengths (λex = 336, 351, 366, 381, 396, and 420 nm, respectively) are very similar to those of quantum dots (QDs), generally shifting to longer wavelength when the excitation wavelength increases.6a These bands are red-shifted, and become narrower and more and more symmetric along with increasing excitation wavelength (Figure 3). For both Bmib and IAM16-1, the intensity of emission bands strengthens upon excitation wavelengths in the range of 336 to 366 nm, and then declines when the excitation wavelengths are larger than 366 nm (Figure 3a−b). Noticeably, the intensity weakening tendency of IAM16-1 is more obvious than that of Bmib. When excited at 336 nm, the blunt band without the distinguishing peak of IAM16-1 covers the whole range of all bands at excitation wavelengths in the range of 351 to 420 nm, and is broader than that of Bmib (Figure 3c−d). Figure 4 presents the PL spectral evolution of the assynthesized IAM16-2 crystals with different excitation wavelengths. The as-synthesized IAM16-2 crystals exhibit an emission band with a peak centered at 493 nm when excited at 336 nm. This almost symmetric band becomes broad and then is separated into two overlapped bands when photoexcitation wavelengths are in the range from 351 to 396 nm. These two overlapped emission bands exhibit no distinguishing shift, but show variations of their intensity. The intensity of the low wavelength band is enhanced, while that of the high

Figure 1. For IAM16-1: (a) perspective view of the coordination environments of Ag and Bmib ligand, with the crystallographic minimum asymmetric unit highlighted; (b) perspective view of the 2D infinite layer with the 1-D corrugated double stranded stair-like inorganic [AgBr]n chains.

planes through imidazole rings of the same Bmib ligand equals 180°, indicating that the two planes are parallel to each other. Each tetracoordinated Ag(I) ion, possessing a distorted tetrahedral coordination environment, is linked to three μ3halogen (Br− or I−) ions and one terminal imidazole N atom from Bmib with the following separations: Ag−N, 2.252(4); Ag−Br, 2.646(1)2.908(1) Å for IAM16-1; Ag−N, 2.282(5); Ag−I, 2.815(1)2.944(1) Å for IAM16-2, which are comparable to those in related AgI−X compounds.27 The selected bond lengths and angles for IAM16-1 and IAM16-2 are listed in Table 2. Noticeably, the diagonal distance between Ag−Ag of IAM16-1 [3.428 (1) Å] is comparable to the sum of the van der Waals radii of two silver atoms (3.44 Å),28 while the intermetallic separation of Ag−Ag of IAM16-2 [3.297(1) Å] is shorter than the sum of the van der Waals radii of two silver atoms. This suggests that the week Ag−Ag bonding is present in IAM16-1, while slightly stronger Ag−Ag bonding exists in IAM16-2.29 The combination of Ag and μ3-Br− or I− generates a 1-D centrosymmetric corrugated double-stranded stair-like inorganic [AgX]n chain with the arrangement of alternating silver and halogen atoms (Figure 1b). This chain is composed of distorted rhombus [Ag2Br2] dimers, in which the dihedral angle between the plane defined by Ag1b, Br1, and Ag1 and that through Ag1, Br1b, and Ag1b is 2.134 (32), while the dihedral angle between the plane through Br1, Ag1, and Br1b and that through Br1b, Ag1b, and Br1 is 2.782 (37). The corresponding two dihedral angles between the planes of the [Ag2I2] dimer of IAM16-2 are 1.089 (18) and 1.575 (22), respectively, which is smaller than that of IAM16-1. This indicates that the rhombus [Ag2Br2] dimer is more distorted than the rhombus [Ag2I2] counterparts. Both sides of the parallel [AgBr]n skeleton are decorated by Bmib acting as “side arms” through the terminal N atoms of the imidazole ring to generate a 2-dimesnsional layer (Figure 1b). D


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Inorganic Chemistry

Figure 3. Solid-state emission spectra of Bmib (a = measured, c = normalized) and IAM16-1 (b = measured, d = normalized) at various excitation wavelengths from 336 to 420 nm at ambient temperature.

Figure 4. Solid-state emission spectra of IAM16-2 at various excitation wavelengths from 336 to 381 nm (a = measured, c = normalized), and from 381 to 420 nm (b = measured, d = normalized) at ambient temperature.

were affected by the coordination between silver halide aggregates and the N atom of Bmib. To address this issue, the PL spectra of Bmib and these two CNCs at 77 K have been obtained. The spectral bands of Bmib at 77 K are gradually redshifted, similar to that at 296 K, except the intensities continuously strengthen at excitation wavelengths ranging from 336 to 420 nm (Figure S1). However, the emission spectral evolution of IAM16-1 at 77 K (Figure S1), which is different from that of IAM16-1 at 296 K, shows significant distinctions from that of Bmib at both 296 and 77 K. The emission spectral evolution of IAM16-2 at 77 K (Figure S2) is similar to that at 296 K, accompanied by changes of relative intensities. Noticeably, the emission band of IAM16-1 excited by 351 nm at 77 K is quite similar to that of IAM16-2, excited by 366/381 nm at 296 K and 381/396 nm at 77 K. The peak

wavelength band shows tiny changes upon the increasing excitation wavelengths in the range from 351 to 381 nm (Figure 4a). Noticeably, when the excitation wavelengths are larger than 381 nm, the intensity of both bands deceases, and finally these two bands merge into one band peaked at 488 nm (Figure 4b). The normalized PL spectra (Figure 4c−d) show the relative intensity of each band. The dominant and shoulder bands of the two overlapped bands are exchanged to the shoulder and dominant bands, respectively. The disparities of the PL spectral evolutions between IAM16-1 and IAM16-2 may result from the different ligand field strengths of the halogen anions and different distances of Ag−Ag. The high similarity of the PL spectral evolution of Bmib and IAM16-1 upon different excitation wavelengths makes it a little difficult to elucidate whether the EWDP properties of IAM16-1 E


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Inorganic Chemistry

Figure 5. Band structure of IAM16-1 (left) and IAM16-2 (right) calculated using DFPT within the generalized gradient approximation.

Figure 6. Electron-density distribution of the highest occupied frontier orbitals of IAM16-1 (a) and IAM16-2 (c) and the lowest unoccupied frontier orbitals of IAM16-1 (b) and IAM16-2 (d).

species,34 size distribution for the nanocrystallites,4b level splitting by doped ions,35 etc. Herein, the EWDP phenomenon of Bmib, IAM16-1, and IAM16-2 may be attributed to the heterogeneity of them, originating from the flexibility of -(CH2)4- long chains and the spatial orientation of the methyl group of Bmib at different excitation wavelengths. The time decay curves of IAM16-1 and IAM16-2 can be fitted to diexponential decay (correlation constants >0.95) with multicomponent fluorescent lifetimes (τ1 = 1.414 ns, 50.73%, τ2 = 5.552 ns, 49.27% and τ1 = 1.061 ns, 46.89%, τ2 = 4.579 ns, 53.11% (Figure S3), respectively), implying that two charge transfer transitions exist in them. The emission of Bmib arises from an intraligand charge transfer π−π* excited state, while the natures of the excited states in IAM16-1 and IAM16-2 are more difficult to be assigned due to the polarizability of the halogen anions, the effect of the heavy metal center, and the strong interaction existing between ligands and metal atoms. For the purpose of obtaining the light-emission mechanisms of these two

values of emission spectra as well as their according emission energies, irradiated by distinct excitation wavelengths at 296 and 77 K, are listed in Table S1, respectively. Furthermore, for IAM16-1 and IAM16-2, single crystal X-ray diffraction studies of the same yellow crystal at 100 and 296 K reveal that the distances between Ag and coordination atoms (Br−/I−/N), and the separations between Ag−Ag at 100 K become shorter than that at 296 K (Table 2), demonstrating the coordination bonds are stronger at 100 K than that at 296 K. Consequently, the EWDP properties of these two CNCs are affected by the interactions between silver halide aggregates and Bmib. These results suggest that the EWDP properties of Bmib, IAM16-1, and IAM16-2 are attributed to their structures, while are not due to different oligomers in their as-synthesized samples. The EWDP phenomenon has been observed for GQDs,26,31 solubilized carbon nanotubes,32 and suspended silicon nanocrystals,33 while that of CNCs is first reported herein. The proposed mechanisms of the EWDP of these nanoscale materials are incomplete solvation of the excited state of the F


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Inorganic Chemistry

Thermogravimetric Analysis (TGA) and X-ray Powder Diffraction (XRPD). The structures of IAM16-1 and IAM16-2 were characterized via XRPD (Figure S5−6), FT-IR spectra (Figure S7), and TGA (Figure S8). The XRPD patterns measured for the as-synthesized samples of them are in good agreement with the XRPD patterns simulated from the singlecrystal X-ray data. The thermogravimetric analysis (TGA) of them in a flow of nitrogen atmosphere shows no weight loss up to 234 and 258 °C, respectively. The above results of TGA indicate that they have good thermal stability and can be envisaged as good candidates for new low energy colorful photoluminescent materials.

isomorphic CNCs, we have conducted density functional theory (DFT) calculations on IAM16-1 and IAM16-2 using the Perdew−Burke−Ernzerhof (PBE)21 generalized gradient approximation (GGA) by employing the Vienna ab intio simulation package (VASP).22 The calculated band structures of IAM16-1 and IAM16-2 along high symmetry points of the first Brillouin zone are similar (Figure 5). They all show dispersions between the tops of valence bands (VBs) and the bottoms of conduction bands (CBs). Both the lowest energy (2.800 and 2.755 eV for IAM161 and IAM16-2, respectively) of CBs and the highest energy (0.00 eV for both of them) of VBs are localized at D. Consequently, IAM16-1 and IAM16-2 give calculated band gaps of 2.800 and 2.755 eV (Figure 5), respectively. Their optical gaps obtained from UV−vis diffuse reflective spectra are 2.83 and 3.01 eV (Figure S4), respectively, which are consistent with the color of IAM16-1 and IAM16-2 crystals. The calculated band gaps are comparable to the experimental values, illustrating that the calculated results are reliable. DFT calculated results show that the HOMOs of IAM16-1 and IAM16-2 are primarily composed of 4d orbitals of AgI and lone pair orbitals of halogen anions (Br− or I−) of the 1-D [AgX]n skeletons (Figure 1b), while the LUMOs also mainly consist of 4d orbitals of AgI and lone pair orbitals of halogen anions (Br− or I−), combined with a small distribution of 2p orbitals of C and N of imidazole rings (Figure 6). The calculated total and partial densities of states (DOS) of IAM161 and IAM16-2 (Figure 7) show that the upper part (from −2.0

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CONCLUSIONS

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ASSOCIATED CONTENT

Under the same hydrothermal synthesis conditions, the flexible Bmib, showing EWDP properties, was chosen as bridging linker to react with AgX (X = Br and I). Two isostructural 2-D layer CNCs with EWDP properties, i.e. [Ag2(Bmib)Br2]∞ (IAM161) and [Ag2(Bmib)I2]∞ (IAM16-2) have been successfully synthesized. The possible reason for the EWDP properties of them may be attributed to the heterogeneity originating from the flexibility of -(CH2)4- long chains and the spatial orientation of the methyl group of Bmib at different excitation wavelengths. The EWDP properties are affected by the ligand field strength of the halogen anions, the distances of Ag−Ag, as well as the interactions between silver halide aggregates and Bmib based on DFT calculated and experimental results. The emissive excited states of those two CNCs are primarily attributed to cluster-centered halogen-metal to ligand (XMLCT) charge transfer. In summary, the CNCs constructed by metal and ligand can inherit the properties of a bridging ligand, such as EWDP properties. Therefore, rational selection, design, or modification of bridging linkers to imbue them with fascinating properties is an efficient way to obtain desirable target materials.

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.6b00999. Experimental and simulated X-ray powder diffraction patterns, FT-IR spectra, solid-state emission spectra at 77 K, a table of emission energies of Bmib, IAM16-1, and IAM16-2, time decay curves, TGA profiles, and experimental band gaps obtained from UV−vis diffuse reflective spectra of crystalline IAM16-1 and IAM16-2 (PDF) CIF data for IAM16-1 at 100 K(CIF) CIF data for IAM16-1 at 296 K(CIF) CIF data for IAM16-2 at 100 K (CIF) CIF data for IAM16-2 at 296 K (CIF)

Figure 7. Total and partial density of states for IAM16-1 (left) and IAM16-2 (right), respectively. (The Fermi level is set at 0 eV.)

to Fermi level) of VBs is primarily from Ag-4d and Br-4p/I-5p states. The antibond states formed by C-2p and N-2p states have dominantly contributed to CBs, except the conduction band minimum (CBM), which is mainly formed by Ag-4d and Br-4p/I-5p orbitals. These calculated results illuminate that the emissive excited states of these two CNCs are primarily attributed to cluster-centered halogen-metal to ligand (XMLCT) charge transfer, which is consistent with the presence of week Ag−Ag bonds. The disparities between the PL spectral evolution of Bmib, IAM16-1, and IAM16-2 are mainly assigned to different ligand field strengths of the halogen ions (Br and I), different distances of Ag−Ag, and the bonding between silver halide aggregates and Bmib, according to their EWDP properties and single crystal X-ray diffraction studies. This agrees with the above assignments based on DFT calculations.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest. G


Article

Inorganic Chemistry

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ACKNOWLEDGMENTS This work was supported by National Basic Research Program of China (973 Program, No. 2015CB932200), National Natural Science Foundation of China (61136003, 51173081, 21501089, and 61505077), and Natural Science Foundation of Jiangsu Province, China (BM2012010, BK20150936, and BK20150939).

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