Temperature-Triggered Dielectric-Optical Duple Switch Based on an

Jul 14, 2016 - ABSTRACT: Molecular optical-electrical duple switches (switch “ON” and. “OFF” bistable states) represent a class of highly desi...
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Temperature-Triggered Dielectric-Optical Duple Switch Based on an Organic−Inorganic Hybrid Phase Transition Crystal: [C5N2H16]2SbBr5 Chen-Yu Mao, Wei-Qiang Liao, Zhong-Xia Wang, Zainab Zafar, Peng-Fei Li, Xing-Hui Lv, and Da-Wei Fu* Ordered Matter Science Research Center, College of Chemistry and Chemical Engineering, Southeast University, Nanjing 211189, P. R. China S Supporting Information *

ABSTRACT: Molecular optical-electrical duple switches (switch “ON” and “OFF” bistable states) represent a class of highly desirable intelligent materials because of their sensitive switchable physical and/or chemical responses, simple and environmentally friendly processing, light weights, and mechanical flexibility. In the current work, the phase transition of 1 (general formula R2MX5, [C5N2H16]2[SbBr5]) can be triggered by the order−disorder transition of the organic cations at 278.3 K. The temperature-induced phase transition causes novel bistable optical-electrical duple characteristics, which indicates that 1 might be an excellent candidate for a potential switchable optical-electrical (fluorescence/dielectric) material. In the dielectric measurements, remarkable bistable dielectric responses were detected, accompanied by striking anisotropy along various crystallographic axes. For the intriguing fluorescence emission spectra, the intensity and position changed significantly with the occurrence of the structural phase transition. We believe that these findings might further promote the application of halogenoantimonates(III) and halogenobismuthates(III) in the field of optoelectronic multifunctional devices.

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instance, Tarasiewicz et al.32 and Jakubas and Sobczyk33 reported the crystal structures of [n-C4H9NH3]2[SbBr5] and [n-C5H11NH3]2[SbBr5], which exhibit interesting phase transition properties. The imidazolium analogues have also been reported with order−disorder phase transition characters.34,35 However, most investigations in this group have focused on simple monoamines or their derivatives, whereas in comparison, diammonium derivatives have received much less attention.36−38 Encouraged by this strategy, we successfully synthesized a new zigzag-chain organic−inorganic hybrid compound, [C5N2H16]2SbBr5 (1), which undergoes a structural phase transition at 278.3 K, accompanied by novel switchable opticalelectrical (fluorescence/dielectric) responses, in which reports of fluorescence bistability are extremely rare. The emissive behaviors of compound 1 changed with the temperature, making 1 a potential material for luminescent switches, optical temperature indicators, and photoelectric devices.39−43 In addition, compared with other phase transition materials in halogenoantimonates(III) and halogenobismuthates(III), the combination of the electrical and optical properties of compound 1 will shed light on the search for new optoelectronic multifunctional materials.44−46 Differential scanning calorimetry (DSC) measurements, dielectric measurements, and variable-temperature structural analysis have been

INTRODUCTION Phase transition materials, whose physical properties can be switched between two or more states, have been widely applied in the fields of data storage, optoelectronic devices, signal processing, and others.1−6 As one of the best candidates for such materials, organic−inorganic hybrid compounds have gradually become the research focus because of their multiple advantages.7−16 However, controlling the crystal structure and then regulating its physical properties with external stimuli, such as temperature, light, and pressure, are still significant challenges in this process because of the lack of deep knowledge about the relationship between the phase transition and the crystal structure. Studies on halogenoantimonates(III) and halogenobismuthates(III), in which various structures with special properties have been achieved, are excellent approaches to solving this problem.17−19 Most of these crystal structures consist of distorted MX6 octahedra, either isolated or sharing corners, faces, or edges.20−23 The type of the halogen, with its specific radius and electronegativity, plays an important role in the structure of this group.24−26 The organic cations in these compounds are always dynamically disordered with different ordered phases because of the freezing of the orientational motion of these cations.27−29 The relatively weak hydrogenbonding interactions between the organic and inorganic moieties provide freedom of the molecular motion of the cations, which can contribute to the order−disorder transition of the cations.30,31 Many phase transition materials in this family have been found based on different kinds of amines. For © XXXX American Chemical Society

Received: May 5, 2016

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DOI: 10.1021/acs.inorgchem.6b01107 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry performed to investigate the phase transition and dielectric properties. The emission spectra of 1 at different temperatures were recorded to understand more about its fluorescence properties.

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EXPERIMENTAL SECTION

Syntheses. In our work, chemicals and solvents were commercially obtained from Aladdin and were chemically pure and used without further purification. The title compound 1 was synthesized through the reaction of Sb2O3 (99.0%) and N-ethyl-1,3-propanediamine (97.0%) in a solution of HBr (40.0%). In particular, SbBr3 solution was obtained by dissolving Sb2O3 (2 mmol, 0.584 g) in HBr. Then, Nethyl-1,3-propanediamine (4 mmol, 0.409 g) was added to the SbBr3 solution under heating and stirring. Yellow block crystals (Figure S1, Supporting Information) were obtained after several days of slow evaporation in a dryer. The purity of the crystals was confirmed by IR spectroscopy (Figure S2, Supporting Information) and powder X-ray diffraction (PXRD) (Figure S3, Supporting Information). The phase transition of 1 was further verified through variable-temperature powder X-ray diffraction measurements (Figure S4, Supporting Information). Single-Crystal X-ray Crystallography. The variable-temperature single-crystal diffraction data were collected using a Rigaku Saturn 924 diffractometer y the ω scan technique with Mo Kα radiation (λ = 0.71073 Å) at 253 and 343 K. The crystal structures were solved through direct methods and refined with the SHELXLTL software package. All non-hydrogen atoms in 1 were refined anisotropically. All hydrogen atoms were located at ideal positions geometrically and refined in “riding” mode. All views of 1 here were drawn using DIAMOND (Brandenburg and Putz, 2005). Crystal data and structure refinements at 253 and 343 K and many other details about the structures are listed in Table S1 (Supporting Information). DSC Measurements. DSC measurements were performed using a Perkin-Elmer Diamond DSC instrument with suitable polycrystalline samples (6.3 mg) of 1. The whole test process was conducted at atmospheric pressure under nitrogen in aluminum crucibles. The polycrystalline samples were subjected to heating and cooling from 200 to 310 K at a rate of 10.0 K/min. Dielectric Measurements. Single crystal and pressed powder were used as samples with silver paint as electrodes for dielectric measurements. The crystal faces were selected according to the hightemperature structure. The dielectric constants ε (ε = ε′ − iε″) were measured on a Tonghui TH2828A precision meter under variable temperatures from 200 to 310 K at frequencies of 5, 10, 100, and 1000 kHz. The applied electric field in the measurements was 1 V. Photoluminescence Measurements. Yellow single-crystal samples of 1 were used for the temperature dependence of photoluminescence measurements on a LabRAM HR800 Raman system with an excitation wavelength of 325 nm in the temperature range from 153 to 363 K. The laser spot size on the samples was about 1 μm with a 50× objective.

Figure 1. DSC curves of compound 1.

Crystal Structures of 1. To understand more details about the structural phase transition of 1, we determined the crystal structures of 1 at 253 K (low-temperature phase, LTP) and 343 K (high-temperature phase, HTP). 1 belongs to the monoclinic crystal system with the space group of P21/c (No. 14) and point group C2h at 253 K. However, as the temperature increases, the crystal structure changes to the orthorhombic crystal system with the space group of Pnma (No. 62) and point group D2h. In addition, the cell parameters of 1 at 253 K are a = 21.426(8) Å, b = 7.889(4) Å, c = 19.961(13) Å, β = 116.79(4)°, V = 3012(3) Å3, and Z = 8 (Table S1, Supporting Information). However, they change obviously in the HTP to the following values: a = 19.227(14) Å, b = 7.894(6) Å, c = 10.118(8) Å, β = 90°, V = 1536(2) Å3, and Z = 4 (Table S1, Supporting Information). In particular, marked changes occur in the c axis and the volume, whose values in the LTP are almost twice those in the HTP. In the LTP, the asymmetric unit of 1 consists of two Sb atoms, 10 Br atoms, and two nonequivalent N-ethyl-1,3propanediamium cations, which are completely ordered (Figure 2a). Each Sb atom, lying in a general position, is octahedrally coordinated by six Br atoms. The adjacent Sb atoms connect to each other through one bridging Br atom, forming a definite [SbBr5]n2− zigzag chain, with the NH3+ groups of the N-ethyl1,3-propanediamium cations pointing toward the cavities of the zigzag chain (Figure 2b). The coordination geometry around the Sb atom can be described as a distorted octahedron, with the Sb−Br bond lengths varying from 2.6186(19) to 2.9519(21) Å (Table S2, Supporting Information), which is consistent the results found for other structurally similar compounds.33,47 The Br−Sb−Br bond angles vary from 85.829(52)° to 96.614(51)° (Table S2, Supporting Information), distorted from the ideal octahedral geometry of 90°. The two independent N-ethyl-1,3-propanediamium cations, placed among the zigzag chains (Figure 2c), link the [SbBr5]n2− zigzag chains through weak N−H···Br hydrogen bonds (Figure S5a, Supporting Information). The donor−acceptor distances vary from 3.414(13) to 3.660(12) Å (Table S3, Supporting Information), indicating relatively weak interactions that provide more freedom of orientational motion to the Nethyl-1,3-propanediamium cations. Compared with those in the LTP, the contents of the asymmetric unit of 1 in HTP comprise one Sb atom, four Br atoms, and one N-ethyl-1,3-propanediamium cation (Figure 3a). The Sb atoms, occupying a special position on an m symmetry plane, are located in distorted octahedra, forming a

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RESULTS AND DISCUSSION Phase Transitions of 1. The phase transition of 1 was first confirmed by DSC measurements in the temperature range of 200−310 K (Figure 1). Two reversible anomalies upon heating and cooling indicate that 1 undergoes a reversible phase transition at around Tc = 278.3 K. The broad and small peaks and the narrow thermal hysteresis probably suggest the occurrence of a continuous second-order phase transition. The average entropy change ΔS was estimated to be 3.713 J (K mol)−1 for the phase transition at Tc. From the Boltzmann equation, ΔS = R ln (N), where R is the gas constant and N represents the proportion of numbers of geometrically distinguishable orientations. The value of N was calculated as 1.56, indicating a complicated phase transition process. B

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Figure 2. (a) Asymmetric unit of 1 with atom labeling at 253 K. Thermal ellipsoids for all atoms are shown at the 40% probability level. (b) [SbBr5]n2− zigzag chain. The NH3+ groups of the N-ethyl-1,3propanediamium cations point toward the cavities of the zigzag chains. (c) Packing diagram of 1 along the b axis at 253 K. All hydrogen atoms are omitted for clarity.

Figure 3. (a) Asymmetric unit of 1 with atom labeling at 343 K. Thermal ellipsoids for all atoms are shown at the 40% probability level. (b) [SbBr5]n2− zigzag chain. The NH3+ groups of N-ethyl-1,3propanediamium cations point to the cavities of the zigzag chains. The blue plane represents the mirror plane. (c) Packing diagram of 1 along the b axis at 343 K. All hydrogen atoms are omitted for clarity.

definite [SbBr5]n2− zigzag chain through the bridging Br atoms similar to that in the LTP (Figure 3b). The Sb−Br bond lengths in the distorted octahedra vary from 2.6608(16) to 3.0622(15) Å, and the Br−Sb−Br bond angles range from 86.349(29)° to 96.085(29)°, similar to the values in the LTP (Table S2, Supporting Information). The N-ethyl-1,3-propanediamium cations are orientationally disordered over two positions related to an m symmetry plane perpendicular to the b axis. The N1 and C4 atoms are distributed equally over this mirror plane and thus occupy the same crystallographic sites (Figure 3a,b). The relatively larger anisotropic displacement parameters show that the N-ethyl-1,3-propanediamium cations at the two positions are still disordered. Thus, the molecular motions of the cations in the HTP become more vigorous. Except for disordered N-ethyl-1,3-propanediamium cations, the packing diagram is similar to that of the LTP (Figure 3c). The donor−acceptor N···Br distances of the weak hydrogen bonds range from 3.443(16) to 3.731(18) Å, contributing to the orientational motion of the cations (Figure S5b and Table S3, Supporting Information). Dielectric Properties of 1. The anomaly in the dielectric permittivity around the phase transition point can further verify the occurrence of the phase transition. The temperature

dependence of the real part (ε′) for the polycrystalline sample of 1 depicted in Figure 4a clearly shows steplike anomalies at Tc. The steplike anomalies and the small hysteresis between heating and cooling modes correspond well to the results of the DSC measurements. In the heating mode, ε′ increases gradually from 12.75 to 13.9 as the temperature is increased from 200 to 250 K, which is consistent with the low-dielectric state. As the sample is heated further, ε′ increases to a maximum value of 18.4 at about 280 K and subsequently exhibits a plateau, which corresponds to the high-dielectric state. Such a change in the behavior of ε′ indicates that 1 is a potential switchable dielectric material. In addition, the temperature-dependent measurements of ε′ of dielectric permittivity upon heating taken at 5, 10, 100, and 1000 kHz are shown in the inset, which also display obvious steplike anomalies in the vicinity of Tc. Moreover, no prominent frequency-dependent property was observed. Temperature-dependent measurements of the real part (ε′) of the dielectric peranomalymittivity of 1 with frequencies of 100 and 1000 kHz along various crystallographic axes were found to exhibit marked anisotropy (Figure 4b−d). The crystal faces were selected according to the structure in the HTP. C

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Figure 4. Temperature dependences of the real part (ε′) of the dielectric permittivity of the following samples: (a) Polycrystalline sample measured at 1000 kHz upon heating and cooling (inset: measured at different frequencies upon heating), and (b−d) single-crystal samples along the (b) a, (c) b, and (d) c axes. Inset in panel d: Large-size yellow crystal in which the crystal axes are based on the structure in the HTP.

Obvious steplike anomalies were recorded along the a, b, and c axes. At 1000 kHz, the maximum of ε′ is 13.5, 34, and 17.1 along the a, b, and c axes, respectively. The values of ε′ along the a and c axes are distinctly smaller than that along the b axis. The striking dielectric anisotropy can be attributed to the orientational motion of the N-ethyl-1,3-propanediamium cations. It can be seen from the crystal structure in the HTP that the disordered N-ethyl-1,3-propanediamium cations take two positions along the b axis (Figure 3a,b). Therefore, the dynamic behavior of the N-ethyl-1,3-propanediamium cations leads to the intense dielectric along the b axis and the relatively weaker anomalies along the a and c axes. Optical Properties of 1. Aside from the switchable dielectric properties, the temperature-induced phase transition also caused novel molecular bistable optical (fluorescence) switch characteristics that had never been reported. As shown in Figure 5, after excitation of the sample at 325 nm, the emission spectra at 363 and 153 K were recorded. Meanwhile, it can be seen from inset b in Figure 5 that the intensity of the emission spectrum changed significantly with the occurrence of the structural phase transition, which shows bistable properties in the two phases (switch “ON” for low temperature, switch “OFF” for high temperature). At 363 K, a relatively weak emission peak can be observed at 510 nm, which undergoes a red shift as the temperature decreases. At 153 K, the intensity of the emission spectra is 250 times higher than that at 363 K, and the position of the emission peak shifts to a longer wavelength of 653 nm (red shift of 143 nm). The weaker intensity of the emission spectrum at 363 K can be attributed to the vigorous

Figure 5. Emission spectra of 1 at 153 and 363 K for anexcitation wavelength of 325 nm. Insets: (a) Emission spectum of 1 at 363 K. (b) Intensities of emission peaks at different temperatures (switch ON for low temperature, switch OFF for high temperature).

thermal motion of the organic cations in the HTP, which could induce the energy loss. The fluorescence properties are related to the emission originated from the 3P1−1S0 transition of Sb3+ in the [SbBr5]n2− zigzag chain.48−51 We have been firmly confirmed that this original example can play an important role in optoelectronic devices such as solar cells, photoelectric sensor, and light-emitting diodes.

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CONCLUSIONS In conclusion, we have successfully synthesized a new zigzagchain organic−inorganic hybrid phase transition compound, D

DOI: 10.1021/acs.inorgchem.6b01107 Inorg. Chem. XXXX, XXX, XXX−XXX

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[C5N2H16]2SbBr5 (1), with novel bistable optical-electrical (fluorescence/dielectric) duple properties. The systematic characterization of the structure and physical properties confirmed that 1 undergoes a second-order structural phase transition at 278.3 K. The origin of the phase transition is associated with the order−disorder transition of the N-ethyl1,3-propanediamium cations. The switching in dielectric and fluorescence responses indicate that 1 might be an excellent candidate for a potential switchable optical-electrical material. We believe that this novel example will shed light on the search for new optoelectronic multifunctional materials.

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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.6b01107. CIF data for 1 at 253 and 343 K have been deposited at the Cambridge Crystallographic Data Centre (CCDC) (CCDC 1473501 and CCDC 1473502, respectively). They can be obtained free of charge from the CCDC at www.ccdc.cam.ac.uk/data_request/cif. Picture of crystals; IR spectrum; variable-temperature PXRD patterns; hydrogen-bonding interactions; crystal data and structure refinements, bond distances and angles, and hydrogen-bond geometries of the crystal structures at 253 and 343 K (PDF) CIF data for 1 at 253 K (CIF) CIF data for 1 at 343 K (CIF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by Project 973 (2014CB848800), the National Natural Science Foundation of China (21371032, 21522101), the Outstanding Young Teachers of Southeast University Research Fund (2242015R30025), and the Jiangsu Province NSF (BK20130600).

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DOI: 10.1021/acs.inorgchem.6b01107 Inorg. Chem. XXXX, XXX, XXX−XXX