A hybrid molecular rotor crystal with dielectric relaxation and thermochromic luminescence

Journal Pre-proof A hybrid molecular rotor crystal with dielectric relaxation and thermochromic luminescence Mei Shi, Shan-Shan Yu, Hui Zhang, Shao-Xian Liu, Hai-Bao Duan PII:

S0022-2860(19)31759-4

DOI:

https://doi.org/10.1016/j.molstruc.2019.127650

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MOLSTR 127650

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Journal of Molecular Structure

Received Date: 18 November 2019 Revised Date:

22 December 2019

Accepted Date: 25 December 2019

Please cite this article as: M. Shi, S.-S. Yu, H. Zhang, S.-X. Liu, H.-B. Duan, A hybrid molecular rotor crystal with dielectric relaxation and thermochromic luminescence, Journal of Molecular Structure (2020), doi: https://doi.org/10.1016/j.molstruc.2019.127650. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier B.V.

A hybrid molecular rotor crystal with dielectric relaxation and thermochromic luminescence Mei Shi,a Shan-Shan Yu,a Hui Zhang,a Shao-Xian Liu,a Hai-Bao Duan*a,b

a

School of Environmental Science, Nanjing Xiaozhuang University, Nanjing 211171,

P.R.China b

Department of Chemistry and Biochemistry, University of California, Los Angeles,

California 90095-1569, United States

Tel.: +86 25 86178260 Fax: +86 25 86178260

E-mail: [email protected]

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Abstract Herein, we reported a hybrid crystals, [C3-Mim][PbBr3] (1) where C3-Mim+ = 1-propyl-3-methylimidazolium, this compound shows two steps solid-solid phase transition at 256 and 359 K, respectively. The first phase transition is related to the strong thermal vibrations of the crystal lattice, and the alkyl chain rotation motion on a single axis of organic cations can be observed from the temperature dependent crystal structure. The second step phase transition were proved by temperature dependent powder X-ray diffraction and accompanied by remarkable dielectric relaxation. In addition, the temperature-dependent photoluminescent indicated two emission bands from organic and inorganic component have different sensitivity to temperature, leading to 1 showing photoluminescent thermochromism, which may have potential application in the thermosensitive devices.

Keywords: Haloplumbate-based hybrids; molecular motion; crystal structure; dielectric relaxation; thermochromic luminescence

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Introduction Molecular motion in crystal is one of effective methods to construct solid-solid phase transition materials, and the physical properties of a material may change massively across solid-solid phase transition [1-3]. For example vanadium dioxide (VO2) undergoes a metal-to-insulator (MI) transition at 341 K and shows dramatic changes in electrical resistivity and infrared transmission across this phase transition [4]. Some displacive type phase transition compound, such as barium titanate is widely used as capacitors, thermistors and self-regulating electric heating systems [5]. With the objective of exploring structures that display phase transition characteristics and interesting physical properties. The amphidynamic crystals built with dynamic mobile components and static elements has received much attention in the design of usable artificial molecular machine [6-9]. Molecular rotational motion plays a crucial role in artificial molecular machine, in which mobile component is rotated in free volume by an external stimulus. For example, polar groups reorientation motion in amphidynamic crystal has been used to created ferroelectric materials [10], and some other functions of materials related to molecular motions in crystals have been reported [11-13]. However, only limited types of molecular motions have been explored for the development of functional materials with the desired properties. Recently, rapid rotation and translation motion ionic liquids (IL) such as pyridinium or imidazolium based IL have been reported to induce dielectric relaxation properties [14-17], and associated to the dynamical glass transition for this family of compounds, and the longitudinal and transverse relaxation time have also been reported by using 1H and 13C NMR technique [18]. As a continuation of our work on molecular machines with rotator or molecular motion and

haloplumbate-based hybrids with tunable structures [19], herein, we report

a new phase transition hybrid compound using 1-propyl-3-methylimidazolium ionic liquid as cation and [PbBr3]- as anion. This compound shows two steps solid-solid phase

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transition. The relationship between the phase transition and dielectric relaxation as well as thermochromic luminescence properties were discussed. The white needle-shaped hybrid crystals, [C3-Mim][PbBr3] (1) where C3--Mim+ = 1-propyl-3-methylimidazolium), by slow evaporation of the dimethylformamide (DMF) solution containing equimolar PbBr2 and [C3-Mim]Br at ambient temperature over 20 days. Hybrid crystal 1 is thermally stable up to ca. 603 K (Figure S1). The purity of 1 was examined by powder X-ray diffraction (Figure S2). Experimental section Chemicals and materials All reagents and chemicals were purchased from commercial sources and used without further purification. [C3-Mim]Br were synthesized according to the similar procedure described in the literature [19]. Synthesis of 1 A mixture of PbBr2 (2 mmol) and [C3-Mim]Br (2 mmol) with molar ratio of 1:1 in DMF (50 mL) was heated under reflux with stirring for 8h. After the clear solution was formed, which was stirred for 2 h and filtered to remove insoluble compounds. The filtrate was evaporated at ambient temperature for 25 days to produce white needle-shaped crystals. The crystal was washed with DMF. Chemical characterization and physical measurements Thermogravimetric (TG) experiments were performed with a TA2000/2960 thermogravimetric analyzer at a warming rate of 10 K / min under a nitrogen atmosphere. Powder X-ray diffraction (PXRD) measurements were performed on Bruker D8 diffractometer with Cu-Kα radiation (λ = 1.5418 Å). Differential scanning calorimetry (DSC) experiments were carried out on a Pyris 1 power-compensation differential scanning calorimeter with the heating and cooling rate of 10 K/min. Temperature and frequency dependent dielectric constant, ε’, dielectric loss, tan (δ), and electric modulus, 4

M’’, measurements were carried out employing Concept 80 system; the powdered pellet was coated by gold films on the opposite surfaces and sandwiched by the copper electrodes and the ac frequencies span from 1 to 107 Hz. Fluorescence spectra were recorded on a Fluorolog Tau-3 Fluorometer for the powdered crystalline sample at selected temperatures. UV-visible absorption spectra were recorded on a PerkinElmer LAMBDA 950 UV/Vis/NIR spectrophotometer for solid sample at ambient temperature. X-ray single crystallography Single-crystal X-ray diffraction data were collected for 1 at 100 and 300 K using graphite monochromated Mo Ka (λ= 0.71073 A) radiation on a CCD area detector (Bruker-SMART). Data reduction and absorption corrections were performed with the SAINT and SADABS software packages [20], respectively. Structures were solved by a direct method using the SHELXL-97 software package [21]. The non-hydrogen atoms were anisotropically refined using a full-matrix least-squares method on F2. All hydrogen atoms were placed at the calculated positions and refined as riding on the parent atoms. The details of data collection, structure refinement and crystallography are summarized in Table 1. Results and discussion Differential scanning calorimetry (DSC) To confirm the phase transition behaviors of 1, differential scanning calorimetry (DSC) is performed on the polycrystalline samples in the temperature ranges of 223-383K. As shown in Figure 1, two endothermic peaks appeared in the first heating process. Two peaks are located at ca. 256 and 359 K, respectively, which means two step phase transition were observed in the investigated temperature region. Three phase are named the low-temperature phase (LTP) below 256 K (T1), the intermediate temperature phase (ITP) between 256 and 359 K (T2), and the high-temperature phase (HTP) above 359 K (T3). The entropy changes (∆S) from T1 to T2 is very small and estimated to be

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0.66 J K−1 mol−1. The small values of ∆S indicate that the phase transitions at ca. 256 K are probably related to the strong thermal vibrations of the crystal lattice. In order to reveal the mechanisms of two step phase transitions, temperature dependent crystal structure of 1 was performed for T1 and T2 phase. However, we failed to obtain details of the crystal structure of T3 phase due to the poor quality of the single crystal X-ray diffraction data above the phase transition, and variable-temperature powder X-ray diffraction was used to investigated T3 phase. Crystal structure of 1 The compound 1 crystallized in a same monoclinic space group P21/n at 100 K (T1 phase) and 300 K (T2 phase), and the asymmetric unit contained one Pb2+ ions, three Brions, and one monovalent cation of 1-propyl-3-methylimidazolium (Figure 2a). As example of T1 phase, for its inorganic part, the Pb-Br bond lengths change from 2.825 Å to 3.367 Å, and as a result, highly distorted octahedron were formed (Figure 2b). It is noted that the Pb-Br2 bond length (3.367 Å) is very longer in the PbBr6 octahedron. Each distorted PbBr6 octahedron shares faces by three µ2–Br atoms with other two PbBr6 octahedron forming an infinite one dimension (1D) chain. The cations adopt the bent conformation, alkyl chain slightly disrupted close to the imidazole ring with an almost completely trans-planar conformation.There exists C-H...Br hydrogen bond interaction between the inorganic and the organic portion. From T1 to T2 phase, the single crystal structure of 1 is quite different. As shown in following: (1) the axis length of a, b nearly not change, however, c-axis elongated by 13.7% from 100 K to 300 K. In addition, the value of β angle also elongated from 94.264(3)O at 100 K to 115.407(3)O at 300 K. It is obvious that a structure fluctuation has been occurred from T1 to T2 phase. (2) For the organic cation component, the alkyl chain rotation motion on a single axis can be observed from the Figure 2c and 2d. Interestingly, The bond length of C4-C5 (1.528 Å) at 100 K is abnormal larger than the value of 1.475 Å at 300 K. In addition, the angle of C4-C5-C6 is obvious decrease from 74.54O at 100 K

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to 114.44O at 300 K. (3) For the inorganic chain part, very strong thermal vibrations were occurred with the temperature increased, which can be observed from variation of the Pb-Br bond lengths (Figure 2c and 2d). PbBr6 octahedron in the T1 phase is more distorted than in the T2 phase. It is noted that order-disorder transition were not observed from 100 to 300 K. Variable-temperature powder X-ray diffraction (PXRD) of 1 were taken to further verify the structural phase transition from T2 to T3 phase (Figure 2e and 2f). From 303 to 333 K, the position of the diffraction peak is almost unchanged demonstrating no structure phase transition. However, when the temperature increased to 383 K, the low angle diffraction peaks at 10.6° and 11.1° were disappeared, and two new diffraction peaks at 8.6°, 13.3° were observed. In addition, some high angle peaks showed some shift and splits. Thus, an obvious phase transition were occurred from 333 to 383 K, which is quite consistent with the DSC measurement results. Dielectric properties The dynamic motion of dipole molecule or solid-solid phase transition may accompany of dielectric relaxation. In order to investigated the relationship between the phase transition and dielectric properties. The temperature dependent dielectric properties of 1 were investigated in the temperature range of 213-393 K. The permittivities ε′ data shows obvious dispersion in the low frequency region (Figure S3), which can be ascribed to sample-electrode interface polarization. In order to reduce electrode polarization effect at the low frequency region. The electric modulus (M*) is used to analyze the dielectric relaxation processes occurring in 1.

M ' (ω ) =

ε' ε ' + ε "2 2

M ' ' (ω ) =

ε '' ε ' + ε "2 2

(1)

As shown in Figure 3a and 3b, in the temperature rang of 213-293 K, no dielectric relaxation was observed. With the temperature increased to 303 K, an obvious dielectric peak was occurred in low frequency region. The dielectric peak shift to high frequency

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region with the temperature increased, which is typical dielectric relaxation. Th activation energy (Ea) of the relaxation can be calculated from Arrhenius law as our described previously. The Ea is 0.575 eV in the temperature range 303-373 K. From the M''-T curve (Figure 3c), in the frequency range of 102-104 Hz, the dielectric anomalies was observed, and the anomalies shows frequency dependent properties. It is noted that dielectric anomalies also was occurred in the following cooling processes. These results indicated that the frequency of dipole motion is in 102-104 Hz range. According to the above studies, it becomes more and more clear that compound 1 undergoes two steps phase transition, thermal vibrations of the crystal lattice did not induce dielectric relaxation, and the dielectric relaxation may be ascribed to the second step structure phase transition. UV-vis absorption and fluorescence spectra

The solid state absorption spectra of 1 at room temperature is shown in Figure S5. It contains one sharp absorption band and a broad absorption band below 400 nm. The origin of the first high energy band are duo to cation π* ← π transitions, while the second low energy band are due to electron transition of valence band to conducting band within the inorganic chain according to our previously work [19]. The optical band gap is estimated to 3.65 eV for the inorganic chain, demonstrating the semiconductor character. The temperature dependent emission spectra is displayed in Figure 4a, 1 exhibits thermochromism properties arises from dual emission bands. A weak emission band center located at 429 nm at 300 K with excitation wavelength of 320 nm. This emission band center is not changed with the temperature decreased (Figure 4b), which is different from haloplumbate-based inorganic-organic hybrids in our previously work. The emission intensity of 429 nm shows exponential decay with the temperature increased (Figure 4b), and can be assigned as electron transition of π* ← π within the imidazole rings. This assignment is consistent with the previous reports on the related haloplumbate hybrids [18,19]. The low energy emission band center located at 630 and 667 nm,

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respectively. Similar with emission band 429 nm, the emission band center is not changed and the intensity shows obvious decrease with the temperature increased. The low energy band emission can be assigned to the valence band within the one dimensional inorganic [PbBr3]- chain.The temperature dependent luminescence intensity (peak area) and intenisty ration of dual emission band are shown in Figure 4c and Figure S6. The intensity of dual emission bands is very sensitive to the temperature, thus, the emission color is also changed from 10 to 300 K. The CIE (x, y) chromaticity diagram coordinates of 1 are shown in Figure 4d. At 10 K, the CIE coordinate is (0.5457, 0.3892). At 300 K, the CIE coordinate is (0.4655, 0.4215), and emission color shift to yellow. Thus, 1 have potential applications as thermochromic luminescence. Conclusion In summary, we have successfully designed and crystal structure characterized a novel haloplumbate-based hybrid crystal with

two steps solid-solid phase

transition at 256 and 359 K, respectively. The first step phase transition are related to the strong thermal vibrations of the crystal lattice and the alkyl chain rotation motion of organic cation component. The second step phase transition was

confirmed by temperature dependent PXRD and probably ascribed to the orderdisorder transition. Interesting, the second step phase transition accompanied by remarkable dielectric relaxation. In addition, the temperature-dependent photoluminescent

indicated

that

this

compound

shows

thermochromic

luminescent properties. The results reported here prove an available avenue in the design of solid-solid phase transition materials with tunable physical properties and novel promising strategies to engineer the dynamics of crystalline rotators. Supporting information. Additional figures, power XRD and TG curves and other

electronic format See ESI. Crystallographic data (excluding structure factors) for the structures for 1 at room temperature have been deposited with the Cambridge

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Crystallographic Data Centre as supplementary publication no. CCDC 1592799. Copies of the data can be obtained free of charge via www.ccdc.cam.ac.uk/conts/retrieving.html or by application to The Director, CCDC, 12 Union Road, Cambridge CB2 1EZ, UK; fax: (+44) 1223 336 033; [email protected]. Acknowledgments. The authors thanks Natural Science Foundation of High Learning

Institutions of JiangSu Province and National Nature Science Foundation of China for their financial support (grant No: 13KJD150002, 21201103 and 21301093).

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Notes and References

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19. Y. B.Tong, L. T. Ren, H. B. Duan, J. L. Liu, X. M. Ren. An amphidynamic inorganic-organic

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1,5-bis(1-methylimidazolium)pentane exhibiting multi-functionality of a dielectric anomaly and temperature-dependent dual band emissions. Dalton Trans., 44 (2015) 17850-17858. 20. Software packages SMART and SAINT, Siemens Analytical X-ray Instrument Inc., Madison, WI, 1996.

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21. G. M. Sheldrick, SHELX-97, Program for the refinement of crystal structure, University of Göttingen, Germany, 1997.

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Table 1 Crystal and structural refinement data for 1 at 100 and 300 K Compound 1 at 100 K 1 at 300 K Empirical formula C7H13Br3N2Pb C7H13Br3N2Pb Formula weight 572.11 572.11 Temperature/K 100 300 Crystal system monoclinic monoclinic Space group P21/n P21/n a/Å 9.5586(3) 9.6466(6) b/Å 8.0414(3) 8.0309(5) c/Å 17.1199(7) 19.4892(10) α/° 90 90 β/° 94.264(3) 115.407(3) γ/° 90 90 3 1312.27(8) 1363.8(14) Volume/Å Z 4 4 3 ρcalcg/cm 2.896 2.786 -1 µ/mm 21.955 21.125 F(000) 1024.0 1024.0 3 Crystal size/mm 0.12 × 0.11 × 0.1 0.12 × 0.1 × 0.09 MoKα (λ = 0.71073) Radiation MoKα (λ = 0.71073) 2Θ range for data 4.736 to 49.996 4.25 to 49.558 collection/° -9 ≤ h ≤ 11, -9 ≤ k ≤ 6, -18 ≤ l -10≤ h ≤ 12, -10≤ k ≤ 10, -25≤ Index ranges ≤ 20 l ≤ 23 Reflections collected 8085 11437 2314 [Rint = 0.0584, Rsigma = 3117 [Rint = 0.1172, Rsigma = Independent reflections 0.0555] 0.1012] Data/restraints/paramete 2314/6/120 3117/0/121 rs Goodness-of-fit on F2 1.120 1.018 Final R indexes [I>=2σ R1 = 0.0373, wR2 = 0.0900 R1 = 0.0686, wR2 = 0.1689 (I)] Final R indexes [all R1 = 0.0409, wR2 = 0.0920 R1 = 0.0945, wR2 = 0.1849 data] Largest diff. peak/hole / 1.32/-3.13 3.553/-2.672 e Å-3 a

R1 = ∑||Fo| -|Fc||/|Fo|, wR2 = [∑w(∑Fo2- Fc2)2/∑w(Fo2)2]1/2

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Figure 1. DSC curves for compound 1 in the heating process showing heat anomaly at 256 and 359 K. (a)

(b)

(c)

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(d)

(e)

(f)

Figure 2 (a) Molecular structure of 1 at 100 K (the symmetric codes: A = 1.5-x, -1.5-y, 0.5+z); (b) packing diagram viewed along the b-axis directionand and one dimension inorganic chain; (c) and (d) slected the bond length and angle of 1 at 100 K and 300 K, respectively, showing structure change; and variable-temperature PXRD profiles of 1 in the ranges of (e) 2θ = 5–50° and (f) 2θ = 5–22°. (a)

(b)

17

(c)

Figure 3 (a) and (b) Frequency dependencies of the Imaginary parts of the complex modulus (M’’) in the temperature rang of 213-283 K and 293-373 K, respectively; (c) temperature dependent M’’of 1 at selected frequency showing dielectric relaxation. (a)

(b)

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(c)

(d)

Figure 4 Temperature dependent emission spectra at solid state (a) with dual emission bands; (b) high energy emission band center located at 429 nm; (c) the temperature dependent luminescence intenisty ration of dual emission band, I1 and I2 are high enery band intensity and low enery band intensity, respectively, and (d) CIE chromaticity diagram showing the fluorescence color of 1 at selected temperatures.

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Two steps solid-solid phase transition at 256 and 359 K, respectively ; Dielectric relaxation in selected frequency region; Dual emission and thermochromic luminescence properties;

Author contributions Hai-Bao Duan conceived the ideal for this work, designed experiments and revised the manuscript. Mei Shi and Shan-Shan Yu performed the experiments, analysis the data and wrote the manuscript. Hui Zhang and Shao-Xian Liu assisted with sample characterization and provided some suggestions.