Article pubs.acs.org/IC
Switchable Nonlinear Optical and Tunable Luminescent Properties Triggered by Multiple Phase Transitions in a Perovskite-Like Compound Lin Zhou, Xuan Zheng, Ping-Ping Shi, Zainab Zafar, Heng-Yun Ye, Da-Wei Fu,* and Qiong Ye* Ordered Matter Science Research Center, Jiangsu Key Laboratory for Science and Applications of Molecular Ferroelectrics, Southeast University, Nanjing 211189, People’s Republic of China S Supporting Information *
ABSTRACT: A new perovskite-like inorganic−organic hybrid compound [Et3(n-Pr)P][Cd(dca)3] (1) (where [Et3(n-Pr)P]+ is the propyltriethylphosphonium cation and dca is a dicyanamide ligand) was discovered to undergo three reversible phase transitions at 270 K (T1), 386 K (T2), and 415 K (T3), respectively. The variable-temperature single-crystal X-ray structural analyses reveal that these sequential phase transitions originate from the deformations of the [Cd(dca)3]− frameworks and the concomitant reorientations of the [Et3(n-Pr)P]+ guest cations. It is found that 1 possesses a sensitive nonlinear optical (NLO) switching at T2 with a large contrast of ∼40 within a narrow temperature range of ∼7 K. Furthermore, 1 shows intriguing photoluminescence (PL) property, and the PL intensity suffers a plunge near T3. The multiple phase transitions, switchable NLO and tunable luminescent properties simultaneously exist in this inorganic−organic perovskite-like hybrid compound, suggesting its great potential application in molecular switches and photoelectric field.
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cation, B is the metal ion, and X is the bridging ligand, e.g., X−, CN−, HCOO−, N3−) have been discovered with remarkable multifunctional properties such as photovoltaic, optical, electric, and magnetic properties.23−25 Constructing perovskite-like hybrids that contain motional or flexible moieties is a promising strategy for obtaining phasetransition materials. Recently, the metal−organic frameworks with various structures based on the longer and twisty dca ligand, which is distinct from the above-mentioned linear or short ones (X−, CN−, HCOO−, N3−), were noticed.26−31 Cation templating has been found to play a significant role in determining the anionic structure. By combining with proper organic cations, consequently, novel inorganic−organic hybrids with diverse ABX3 perovskite-like structures and multiple phase transitions can be achieved.32 Encouraged by previous work, we expect to utilize rotatable or motional guest cations within the host frameworks to tune their phase transition behavior, for the design and syntheses of new switchable materials. In this context, a flexible organic phosphonium cation, [Et3(n-Pr)P]+, with appropriate size and shape, was selected as a template for the formation of a three-dimensional (3D) perovskite-like structure. Herein, we present a new metal−organic perovskite-like compound, [Et3(n-Pr)P][Cd(dca)3] (1), which has been characterized by differential scanning calorimetry (DSC),
INTRODUCTION The physical or chemical properties of switchable materials can be reversibly converted between two or more stable states under external stimuli, such as light, heat, pH, pressure, electric and magnetic fields,1−8 etc. Notably, nonlinear optical (NLO) switches, including the second harmonic generation (SHG) switch, have been widely studied for their potential applications in optical communication, photoelectric devices, and information processing.9−11 Although photoswitching of NLO properties has been achieved in liquid phase through reversible chemical modifications, such as photochromism and redox reaction,12−14 the solid-state NLO switches still remain uncommon, because of the difficulty of reversibly controlling the noncentrosymmetric alignment of NLO moieties in a collective manner. Recently, an effective strategy to design new NLO switches is to construct and screen phase-transition materials associated with the transformation from a centrosymmetric structure to a noncentrosymmetric one. Several simple organic salts, inorganic−organic hybrids and a recent plastic crystal have been reported to be potential NLO switches.15−19 Among them, inorganic−organic hybrids afford an important opportunity to integrate phase transition with multiple switching responses.20−22 Compared to the pure organic or inorganic compounds, inorganic−organic hybrids combine the best aspects of inorganic frameworks and organic molecules into a monocrystal structure to achieve multifunctional characteristics. Particularly, the hybrids with ABX3-type perovskite structures (where A is the molecular © 2017 American Chemical Society
Received: October 18, 2016 Published: March 6, 2017 3238
DOI: 10.1021/acs.inorgchem.6b02508 Inorg. Chem. 2017, 56, 3238−3244
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duration, 1.6 MW peak power, 10 Hz repetition rate). The instrument model is Ins 1210058, INSTEC Instruments, while the laser is Vibrant 355 II, OPOTEK. Photoluminescence Measurements. The single-crystal samples of 1 were used in temperature-dependent photoluminescence measurements. The PL spectra were recorded using a LabRAM HR800 Raman system with 325 nm He−Cd continuous wave laser in the temperature range from 213 K to 440 K. Measurements below room temperature were performed in a INSTEC HCP621 V stage with a high-precision temperature controller (Model mk1000) and a liquid nitrogen cooling system (Model LN2-SYS).
dielectric, variable-temperature single-crystal X-ray diffraction, SHG, and photoluminescence (PL) measurements. As expected, 1 undergoes three structural phase transitions at 270, 386, and 415 K sequentially. Accompanying the phase transitions, the switchable SHG and photoluminescent behaviors were remarkably discovered.
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EXPERIMENTAL SECTION
Synthesis. All the chemical reagents were obtained from commercial sources and used without further purification. Preparation of Propyltriethylphosphonium Bromide. Dehydrated dichloromethane (200 mL) was added into a dry flask under nitrogen at room temperature. Triethylphosphine (8 mL, 60 mmol) and propyl bromide (4.53 mL, 50 mmol) then were added to this solution via syringe, respectively. The mixed solution was warmed to 40 °C and stirred for 3 d. The solvent was evaporated to yield the propyltriethylphosphonium bromide as white solid. The solids were washed with diethyl ether and dried at 40 °C. Synthesis of 1. An aqueous solution of Na(dca) (0.5342 g, 6.0 mmol) was added to a stirred MeOH solution of Cd(NO3)2·4H2O (0.6169 g, 2.0 mmol) and propyltriethylphosphonium bromide (0.4823 g, 2.0 mmol) at room temperature for 30 min. The resulting solution was filtered and allowed to stand in air at room temperature for 2 weeks, yielding colorless crystals. Anal. Calcd (%) for C15H22CdN9P: C, 38.18; H, 4.70; N, 26.72. Found: C, 38.21; H, 4.66; N, 26.69. The formation of compound 1 was certified by the IR spectrum measured on a spectrometer (Shimadzu, Model IR-60) (see Figure S1 in the Supporting Information). The peaks at ∼1343, 2232, and 2973 cm−1 correspond to the characteristic C−N, CN, and C−H vibrations, respectively. The purity of 1 was confirmed by comparing the experimental powder X-ray diffraction (PXRD) pattern at room temperature with the simulated data based on the crystal structure at 293 K (vide inf ra). X-ray Crystallography. Variable-temperature X-ray diffraction data of compound 1 were collected using a Rigaku Saturn 724 diffractometer with Mo Kα radiation (λ = 0.71073 Å). Data processing including empirical absorption correction was performed using the Crystalclear software package (Rigaku, 2005). The crystal structures were solved by direct methods and then refined by full-matrix least-squares refinements on F2 using the SHELXLTL software package (SHELX-97). All nonhydrogen atoms were refined anisotropically and the positions of all hydrogen atoms were generated geometrically. The asymmetric units and the packing views were drawn with DIAMOND Visual Crystal Structure Information System Software. Crystallographic data and structure refinement are listed in Table S1 in the Supporting Information. Powder X-ray Diffraction. Powder X-ray diffraction (PXRD) data for compound 1 was measured on a PANalytical X’Pert PRO X-ray diffractometer at 298−433 K. Diffraction patterns were collected in the range of 2θ = 5°−50° with a step size of 0.02°. DSC and TGA Measurement. DSC measurement was performed on a Netzsch Model DSC 200 F3 instrument by heating and cooling the crystalline sample of compound 1 (4.5 mg) with a rate of 10 K/min over the temperature range of 213−423 K. The sample was placed in aluminum crucible under nitrogen at atmospheric pressure. Thermogravimetric analysis (TGA) was performed on a Netzsch Model TG 209 F3 instrument. The measurements were collected in nitrogen flow from 323 K to 903 K at a rate of 10 K/min, and the result shows that the sample begins to decompose at ∼550 K (Figure S2 in the Supporting Information). Dielectric Constant Measurements. The powder-pressed pellet of compound 1 pasted with silver conducting glue was used in dielectric measurements. The temperature-dependent dielectric constant was measured on a Tonghui Model TH2828A impedance analyzer at the frequencies of 0.5 kHz, 1 kHz, 5 kHz, 10 kHz, 100 kHz and 1 MHz, with the applied AC field fixed at 1 V. SHG Switching Measurement. SHG switching experiment was carried out on powder samples, using an unexpanded laser beam with low divergence (pulsed Nd:YAG at a wavelength of 1064 nm, 5 ns pulse
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RESULTS AND DISCUSSION Crystal Structure. The crystal structure of 1 at 293 K was determined initially, which belongs to the chiral space group P212121 (Table S1 in the Supporting Information). The asymmetric unit of 1 contains one [Et3(n-Pr)P]+ cation and one [Cd(dca)3]− anion, while two of the three dca ligands are disordered over two positions (Figure S3 in the Supporting Information). Each Cd(II) atom is coordinated to six terminal N atoms of the dca ligands, forming a distorted octahedral geometry. The Cd−N bond distances range from 2.288(5) Å to 2.364(5) Å and the cis-N−Cd−N bond angles range from 86.02(18)° to 94.6(2)° (Table S2 in the Supporting Information). The dca ligands show approximately C2v symmetry with the middle C−N−C bond angles ranging from 109.8(11)° to 121.5(5)°, of which the one in the ordered dca ligand is close to 120°. The NC−N bond angles of the ordered dca ligand are 173.9(7)° and 173.6(6)°; however, those in the disordered dca ligands range from 151.6(14)° to 167.0(10)°. The structure of 1 is an analogue of ABX3 perovskite: octahedrally coordinated Cd(II) centers are connected via dca bridges to form the cubic [Cd(dca)3]− network, with [Et3(n-Pr)P]+ cations occupying the A-cation site (see Figure 4b, presented later in this work). Note that the organic cations seem to be highly size- and shape-selective in the templated formation of the [M(dca)3]− (M = metal ion) network with perovskite-like structural type.32 For compound 1, the [Et3(n-Pr)P]+ cation adopts a distorted tetrahedron geometry, with an average P−C distance of 1.808 Å and C−P−C angles ranging from 106.3(4)° to 112.9(4)°. In the [Cd(dca)3]− cage, the [Et3(n-Pr)P]+ cation has its propyl group pointing to a dca edge which extends outward, to better fit the confined space of the cage. Thermal and Dielectric Analyses. The disordered roomtemperature phase of 1 suggests the possibility of phase transition, since a decrease in temperature may easily turn the disordered state into an ordered one with a lower lattice symmetry. To detect whether a phase transition occurs in response to the external temperature stimulus, DSC measurement was performed on 1 with the heating and cooling rate of 10 K/min in the temperature range of 213−423 K. (See Figure 1.) For the DSC runs, three pairs of reversible heat anomalies were observed at 258/270 K, 379/386 K, and 411/415 K (cooling/ heating), indicating that 1 undergoes three reversible phase transitions at T1 = 270 K, T2 = 386 K and T3 = 415 K. The sharp peaks at T1 and T2 reveal the first-order type of the two phase transitions,17,33,34 while the round peaks at T3 correspond to a second-order phase transition.35,36 On the basis of the DSC curves, the entropy changes (ΔS) were estimated to be ∼12.06 J mol−1 K−1 for the phase transition at T1, 22.11 J mol−1 K−1 for the phase transition at T2, and 0.34 J mol−1 K−1 for the one at T3, respectively. According to the Boltzmann equation, ΔS = R ln(N ) 3239
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unchanged at different frequencies, indicating that there is no relaxation of any type in the measured frequency range.24,40,41 Second Harmonic Generation (SHG) Switching Property. Considering that the SHG effect is sensitive to the symmetry change in the phase transition process, the SHG response of 1 is measured in a warming and cooling cycle (Figure 3).15,42 As expected, the SHG signal of 1 is detectable at room
Figure 1. DSC curves of 1.
where R is the gas constant and N is the ratio of the number of respective geometrically distinguishable orientations, the values of N(T1), N(T2), and N(T3) are calculated as 4.27, 15.34, and 1.04, which suggest the order−disorder feature of the phase transitions at T1 and T2, and a complicated phase transition at T3.37−39 For convenience, we designated the phase at 415 K as the δ-phase. The temperature-dependent complex dielectric permittivity measurements of 1 were performed on the polycrystalline samples, to detect dielectric responses accompanied by the phase transitions. As illustrated in Figure 2, during the cooling process, the real part (ε′) of the complex dielectric permittivity at 1 MHz decreases gradually from 285 K to 270 K. Subsequently, it shows a steplike reduction from 3.3 to 3.1 near T1. In the heating mode, the appearance of the corresponding steplike increase of ε′ signifies the reversible phase transition at T1. Upon heating from room temperature, ε′ exhibits two obvious slopes near T2 and T3, respectively, in accordance with the two observed in the cooling run. The three pairs of dielectric anomalies in the vicinities of the phase transition temperatures are consistent with the DSC results. The temperature-dependent ε′ and dielectric loss (tan δ = ε″/ε′, where ε″ represents the imaginary part of the complex dielectric constant) of 1 obtained at different frequencies are shown in Figure S4 in the Supporting Information. The values of ε′ and dielectric loss increase with decreasing frequency, which is a general characteristic of dielectric materials. In addition, the inflection point temperatures of ε′ and dielectric loss are almost
Figure 3. Temperature-dependent SHG signal obtained above room temperature. A superior switching contrast of ∼40 is achieved in the vicinity of T2.
temperature, since it crystallizes in the noncentrosymmetric space group P212121 at 293 K. The powder SHG intensity at room temperature is estimated to be ∼0.12 times that of KDP (Figure S5a in the Supporting Information). As the temperature increases, the SHG signal maintains until 382 K, corresponding to the SHG-on state. From 386 K to 437 K, the SHG activity vanishes with only very weak noise errors (i.e., the SHG-off state), and then recovers rapidly without any recession when temperature dips below 379 K again (also, low-temperature SHG-switching behavior can be found in Figure S5b in the Supporting Information). Such bistable NLO states can be switched within a narrow temperature range of ∼7 K, implying its high sensitivity to the change of temperature at ∼386 K. Moreover, 1 exhibits a remarkable NLO switching contrast (defined as the ratio of SHG intensities at the SHG-on to SHGoff states) of at least ∼40, higher than many solid-state NLO switches.18,19 Figure S6 in the Supporting Information shows that 1 exhibits a high repeatability of SHG switching with at least 4 cycles, and the intensities of its SHG signal still maintain their
Figure 2. Temperature-dependent dielectric constant measured at 1 MHz: (a) from 250 K to 290 K and (b) from 330 K to 430 K. 3240
DOI: 10.1021/acs.inorgchem.6b02508 Inorg. Chem. 2017, 56, 3238−3244
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Inorganic Chemistry initial level without obvious fatigue after a long-time switching experiment. Note that the switching temperature of SHG signal is consistent with the second phase transition temperature (T2), suggesting that the SHG switching is closely related to the structural phase transition at T2, and the crystal structures of both the γ- and δ-phases are centrosymmetric. Furthermore, the discontinuous plunge of the SHG signal near T2 manifests that the phase transitions should be first-order ones, coinciding with the DSC analysis. Analyses of Crystal Structures and Origins of Phase Transitions. In order to explore the mechanisms of the structural phase transitions and understand the above-mentioned SHG switching behavior, the crystal structures of the α-, γ-, and δ-phases were characterized at 223, 397, and 427 K, respectively. The single-crystal diffraction data suggest that 1 crystallizes in the centrosymmetric space group P21/c at 223 K (α-phase), centrosymmetric space group Ibam at 397 K (γ-phase), and centrosymmetric space group Cmca at 427 K (δ-phase), in good agreement with the SHG results. The structure at 293 K (βphase) has been described in detail above. Although the diffraction data of γ- and δ-phases are not very good, because of the existence of severe disorder, the comparison between the α- and β-phases can give insight into the origins of phase transitions. For the first phase transition at T1, the unit-cell parameters change remarkably from the β-phase to the α-phase (a = 10.642(2) Å, b = 16.472(3) Å, c = 12.470(3) Å, β = 90° at 293 K and a = 10.498(8) Å, b = 16.500(3) Å, c = 15.763(9) Å, β = 128.040(2)° at 223 K). The reduction in temperature gives rise to the change of β angle from 90° to 128.040(2)°, accompanied by the transition of crystal system from orthorhombic to monoclinic. In the α-phase, the thermal ellipsoids of most atoms are accordingly smaller than those in the β-phase (Figure S3), corresponding to a definitely ordered phase. With the temperature decreasing from 293 K to 223 K, the swaying motions of the dca ligands are frozen, while the previously disordered dca ligands become ordered and only occupy one position, respectively. The conformational changes of the dca ligands also result in the structural variation of the anion framework. The key bond distances and angles (Cd−N distances, N−Cd−N angles, and dca) are supplied in Table S2 in the Supporting Information. For instance, at 223 K, the C−N−C bond angles in the dca ligands vary within a narrower range of 116.2(16)°− 126.5(14)°, in contrast with those at 293 K, and the NC−N bonds of the disordered dca ligands are more linear with bond angles closer to 180°. The order−disorder transformation of dca ligands leads to the deformation of the [Cd(dca)3]− framework, as shown in Figure 4. During the transition from β-phase to αphase, the symmetry elements of the crystallographic point group are changed from D2 (E, 3C2) to C2h (E, C2, i, σh) with the deformation of the [Cd(dca)3]− framework. Meanwhile, the reorientation of the [Et3(n-Pr)P]+ cation was also found to accompany the phase transition. As shown in Figure 5, when overlapping the cage units of β- and α-phases, it is obvious that the [Et3(n-Pr)P]+ cations exhibit totally different orientations, because of the host−guest interaction, to adapt to the change of the anion cage. Generally, the reorientation of the guest cation and the deformation of the anion framework together trigger the transition from the β-phase to the α-phase. The crystal structures of γ- and δ-phases are so disordered that no high-quality diffraction data were obtained, being an indicator of the intense molecular motions at high temperature. The cell
Figure 4. Crystal structures of 1 at (a) 223 K and (b) 293 K. Graphs on the right show a single extra-framework cation in space-filling representation to illustrate the relationship between cation shape and network topology. Hydrogen atoms are omitted for clarity.
parameters of γ- and δ-phases are similar (γ-phase: orthorhombic, Ibam, a = 17.25(4) Å, b = 17.57(4) Å, c = 16.79(3) Å; δ phase: orthorhombic, Cmca, a = 17.056(4) Å, b = 16.624(4) Å, c = 17.063(4) Å), and the cell volumes are approximately double those in the α- and β-phases. The space groups Ibam and Cmca are both centrosymmetric, which is consistent with the abovementioned fact that the SHG signal disappears in the γ- and δphases. Although only limited information was available for the crystal structures in the γ- and δ-phases, the anion frameworks have been roughly refined. As shown in Figure 6, eight neighboring Cd(II) ions of each anion cage in the four phases are compared to provide information on the structural changes taking place in the course of the phase transitions.30,41 It is the remarkable shifts of the Cd···Cd distances and the Cd···Cd···Cd angles that demonstrate the obvious deformations of the anion frameworks in the four phases (Figure 6). Significantly, all these changes, especially for the γ- and δ-phases, imply the flexibility of the [Cd(dca)3]− framework, which should play a key role in the origins of the sequential phase transitions.24,43 Because of the failure of obtaining the complete crystal structures of the γ- and δ-phases, the variable-temperature PXRD measurements were performed on 1 at 353, 403, and 433 K to further verify the phase transitions at T2 and T3 (Figure 7).44−46 The experimental PXRD pattern obtained at 353 K matches well with the simulated one based on the crystal structure of the βphase. As the temperature rises, the significant differences in the measured PXRD patterns are evidence of the emergences of the γ- and δ-phases. Specifically, the diffraction peaks at 9.88°, 12.17°, 12.86°, 13.57°, 14.18°, and 16.64°, which existed in the βphase, disappeared in the γ- and δ-phases during the heating process. Meanwhile, new diffraction peaks at 12.97° and 13.10° emerged in the γ-phase, while one vanished in the δ-phase. Moreover, two diffraction peaks at 10.95° and 10.93° splitted into three from the β-phase to the γ-phase, and then restored to two peaks in the δ-phase. Similar changes were observed in the peak at 15.34°. These findings demonstrate the occurrence of the phase transitions, coinciding with the DSC and structural analyses. 3241
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Figure 5. Overlap of single cage units of the α- and β-phases, where the anion frameworks are drawn in blue and pink, respectively.
exhibit a wide and intense band in the visible region (see Figure 8 and Figure S8 in the Supporting Information). The PL behavior
Figure 8. Variable-temperature emission spectra of 1 under 325 nm excitation above room temperature; inset shows temperature-dependent PL intensities at 525 and 573 nm.
Figure 6. Cd···Cd distances (Å) and Cd···Cd···Cd angles (deg) within a Cd8(dca)12 cage of (a) the α-phase at 223 K, (b) the β-phase at 293 K, (c) the γ-phase at 397 K, and (d) the δ-phase at 427 K.
of 1 can probably be attributed to the metal−ligand charge transfer (MLCT).47 As depicted in the inset of Figure 8, the PL intensities at 525 and 573 nm are drawn, as a function of temperature. With the temperature increasing, the emission bands have no obvious shift, but the PL intensity declines. Similarly, below room temperature, the PL intensity increases upon cooling (Figure S8). That is, the PL intensity is stronger at lower temperature. The frozen molecular motions at low temperature increase the rigidity of the framework and reduce the energy dissipation by radiationless decay to result in the enhancive luminescence.48,49 Interestingly, there is a sharp variation of PL intensity between 410 K and 420 K, which might be related to the phase transition at T3 (415 K). Moreover, the peak intensity at 525 nm is stronger than that at 573 nm below T3, but they are almost equal above T3. These results suggest that the emission intensity can be tuned by the external temperature.
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Figure 7. Variable-temperature PXRD patterns of 1.
CONCLUSION [Et3(n-Pr)P][Cd(dca)3] (1), which is a hybrid inorganic− organic compound with perovskite-like structure, has been successfully synthesized and studied by crystal structure analyses and property characterizations. It exhibits three reversible phase transitions and four different phases (α-, β-, γ-, and δ-phases) in a wide range of temperature (200−490 K), resulting from the
Photoluminescence Property. In the further study of the physical properties of 1, the crystals of it was found to emit weak light under 365 nm UV lamp, as shown in Figure S7 in the Supporting Information. The photoluminescence spectra of 1 were measured on the crystal samples from 213 K to 440 K. Under the excitation wavelength of 325 nm, the emission spectra 3242
DOI: 10.1021/acs.inorgchem.6b02508 Inorg. Chem. 2017, 56, 3238−3244
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Inorganic Chemistry deformations of the [Cd(dca)3]− framework and the concomitant reorientations of the [Et3(n-Pr)P]+ guest cations. Three pairs of dielectric anomalies appear with the occurrence of sequential phase transitions, and accompanying the second phase transition, symmetry breaking from noncentrosymmetric to centrosymmetric structure occurs. The attractive switching between “SHG-on” and “SHG-off” states with a large contrast of ∼40 makes 1 a potential temperature-controlled NLO switch. In addition, 1 shows intriguing luminescence properties, which can be tuned by external temperature. This compound represents a new class of multifunctional inorganic−organic perovskite-like hybrids, and such phase transition materials with bistable features provide us with an effective pathway to explore new switchable materials. To modulate the structures, phase transitions and physical properties, further studies with analogue compounds assembled by different phosphonium cations are in progress.
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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.inorgchem.6b02508. IR spectrum, TG curves, frequency dependence of dielectric constant and dielectric loss, asymmetric units of compound 1, PL spectra, tables of the crystallographic data, and selected bond distances and angles (PDF) Crystallographic data for CCDC No. 1497309, from the Cambridge Crystallographic Data Centre (CIF) Crystallographic data for CCDC No. 1497310, from the Cambridge Crystallographic Data Centre (CIF) Crystallographic data for CCDC No. 1497311, from the Cambridge Crystallographic Data Centre (CIF) Crystallographic data for CCDC No. 1497312, from the Cambridge Crystallographic Data Centre (CIF)
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AUTHOR INFORMATION
Corresponding Authors
*E-mail:
[email protected] (D.-W. Fu). *E-mail:
[email protected] (Q. Ye). ORCID
Qiong Ye: 0000-0002-3532-5388 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS Y.Q. thanks the Fundamental Research Funds for the Central Universities. This work was financially supported by the Project 973 (No. 2014CB848800) and National Natural Science Foundation of China (No. 21471032 and No. 21422101).
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