Regulating Reversible Phase Transition Behaviors by Poly-H/F Substitution in Hybrid Perovskite-Like 2[CH2FCH2NH3]·[CdCl4].

The molecular design and regulation has shown bright future for constructing smart molecular materials such as ferroelectrics, dielectric switches, electro-optic effect, and so forth. Here, by poly-H/F substitution in a simple organic-inorganic hybrid 2[CH2FCH2NH3]·[CdCl4], 1 (CH2FCH2NH3 = fluorine ethylamine cation), we obtained two novel hybrids, namely, 2[CHF2CH2NH3]·[CdCl4], 2 (CHF2CH2NH3 = 2,2'-difluorine ethylamine cation) and 2[CF3CH2NH3]·[CdCl4], 3 (CF3CH2NH3 = 2,2',2″-trifluorine ethylamine cation). Further investigations show that compounds 1, 2, and 3 experience solid reversible phase transitions with temperatures at 294, 319, and 329 K respectively. These unique phase transitions were confirmed by their remarkable dielectric and heat anomalies around the phase transition temperatures. X-ray single-crystal diffraction analyses before and after the phase transitions show that the order-disorder motions of F atoms and the twist motions from the 2D [CdCl4]2- framework lead to these solid reversible phase transitions. Also, the Hirshfeld surface calculation of compounds 1, 2, and 3 suggests that the increasing ratio of the F···F interaction from the intermolecular interaction makes a major contribution for the substantial increase of their phase transition temperatures.

Introduction

Molecular phase transitions materials are very popular in the recent years because they exhibit a lot of special physical properties such as ferroelectrics, switchable dielectrics, electro-optic effect, nonlinear optic properties, and so forth.[1−10] Especially, they show great advantages such as light weight, friendly environment (not contain lead), structural flexibility, and easy synthesis as compared with the traditional ceramic materials.[11−17]

Although much great progress has been made on the molecular phase transitions materials, there still exist a lot of difficulties on how to obtain ideal molecular materials such as with high phase transitions temperatures and excellent switchable dielectric properties from an existing molecule. It is encouraging that scientists have recently developed a few unique molecular phase transition materials where their crystal structures, symmetry, band gap, and phase transition temperatures can be controlled and tuned by regulating the host of guest molecules.[18−24] This special exploration much benefited to search the perfect performance and further satisfied our update smart materials.[23] Utilizing the idea of molecule design, modification, and assembling, scientists can precisely construct their target products from an existing molecular phase transition systems.[4,23,25] For example, professor Xiong et al. designed and synthesized a high Tc multiaxial molecular ferroelectric [3-oxoquinuclidinium]ClO4 by introducing directional hydrogen-bonding interactions in [quinuclidinium]ClO4, and through this method, they successfully reduced the symmetry from the cubic centrosymmetric Pm3̅m space group in [quinuclidinium]ClO4 to the orthorhombic polar Pna21 space group in [3-oxoquinuclidinium]ClO4.[26] In addition, in our previous work, we have discovered that the guest molecules and the anions play a great influence on the nonferroelectric molecule phase transition system [Cu(1,10-phenothroline)2SeO4·(ethane-1,2-diol)]. With replacement of ethane-1,2-diol by propanel-1,3-diol in such a system, the new compound [Cu(1,10-phenothroline)2SeO4·(ethane-1,3-diol)] possesses lower symmetry and becomes a new ferroelectric with high Tc 325 K.[27] Another significant case is that using the strategy of H/F substitution to the organic–inorganic perovskite ferroelectric (pyrrolidinium)CdCl3 with a low Tc of 240 K,[24] Xiong successfully explored two high Tc homochiral perovskite ferroelectrics, (R)- and (S)-3-F-(pyrrolidinium)CdCl3, of which Tc reaches 303 K, enhancing 63 K.[23] Inspired by their previous outstanding work, we are devoted to designing and constructing molecular phase transition systems using polyfluorine amine as the building block and analyzing the influence of the number of substituted fluorine atoms on their physical properties including the structure symmetry, band gap, phase transition temperatures, and so forth, and obtaining the ideal phase transition materials.

Successfully, we developed and investigated another new 2D hybrid metal halide perovskite-type compound with a general formula of AMX4 (A = organic ammonium cation, M = divalent metal, X = chloride ions) 2[CH2FCH2NH3]·[CdCl4], 1 (CH2FCH2NH3 = fluorine ethylamine cation) with reversible phase transition behaviors. Further investigation shows that when 2-fluorine ethylamine were replaced by 2,2′-difluorine ethylamine and 2,2′,2″-trifluorine ethylamine, the phase transition temperatures of 2[CHF2CH2NH3]·[CdCl4], 2 (CHF2CH2NH3 = 2,2′-difluorine ethylamine cation) and 2[CF3CH2NH3]·[CdCl4], 3 (CF3CH2NH3 = 2,2′,2″-trifluorine ethylamine cation) are strikingly enhanced near 25 and 10 K, respectively. Here, we described their interesting reversible phase transitions, switchable dielectrics, crystal structures, and mechanism of increasing phase transition temperatures.

Results and Discussion

For most solid-state reversible phase transitions, especially for the first-order phase transitions, there usually exist remarkable heat anomalies near the phase transition temperatures which include the changes of entropy, enthalpy, specific heat capacity (C), and so forth.[28] Differential scanning calorimetry (DSC) can sensitively capture this information.[29] As shown in Figure , one can clearly discover that a pair of endothermic/exothermic peaks appear at 294.4/289.5 K for the heating/cooling runs for compound 1 (Figure a), while for compound 2, the endothermic/exothermic peaks appear at 319.3/317.2 K, enhancing nearly by 25 K; then, the same situation is also suitable for compound 3; it reaches 328.6/324.8 K for the heating/cooling runs and continues to improve by 8 K. Such a significantly increase of reversible phase transition temperatures indicates that the number of fluorine atoms has played a crucial role in the reversible phase transition because the whole body structures of 2D organic–inorganic hybrids always remained unchanged. As a further step, we calculated their entropy changes (ΔS) using the finite integration method (Supporting Information). Different from the common reversible phase transitions, all of them possess ultra-large entropy changes such as 28.5 J·mol–1·K–1 for compound 1, 38.698 J·mol–1·K–1 for compound 2, and 25.124 J·mol–1·K–1 for compound 3, so large entropy changes combined with the high phase-transition temperatures suggests that it may be used as potential energy storage materials. Subsequently, we calculated the N values with 30.9/26.1 for 1, 105.1/68.783 for 2, and 20.531/20.522 for 3 according to the equation ΔS = R ln N (Boltzmann equation), where R is the gas constant and N represents the ratio of possible orientations (Supporting Information). These large N values suggest that violent molecular motion possibly exists in the phase transition processes. We judge that all the phase transitions of 1, 2, and 3 belong to the first-order phase transitions because the big thermal hysteresis between the cooling and heating runs for 1, 2, and 3 are 5, 2, and 4 K, and in addition, they show ultra-large N and entropy changes values.[30] It should be emphasized that the decomposition temperatures of compounds 1, 2, and 3 locate around 480, 510, and 450 K, respectively, according to the TG–DTA (thermogravimetric analysis and differential thermal analysis) curves (Figure S3), far above those reversible phase transition temperatures.

Figure 1

Heat anomalies at (a) 294.4/289.5 K for compound 1, (b) 319.3/317.2 K for compound 2, and (c) 328.6/324.8 K for compound 3 during their heating/cooling runs.

Permittivity constant anomalies often accompany with the solid-state reversible phase transition behaviors and are also regarded as another effective means to judge the phase transitions.[31] The permittivity constant dependence on temperature under frequency 1 MHz is shown in Figure a–c. Being highly consistent with that of heat anomalies, the remarkable step-like permittivity constant changes also arise around 290, 318, and 327 K for compounds 1, 2, and 3, respectively, suggesting that the increasing number of fluorine atoms has played an important role on the reversible phase transition temperatures of dielectric properties. To further describe the dielectric anomalies at length, we take compound 1 as an example. As depicted in Figure a, a sudden increase of the permittivity constant starts from 283 K (7.2 au) and ends around 292 K (11.2 au) during the heating process (red line); after that, the permittivity constant nearly remains unchanged and belongs to a higher dielectric state, while for the cooling cycle (blue line), the permittivity constants have fallen off a cliff from 285 K (11.0 au) to 281 K (7.3 au) and then almost maintains the lower permittivity state. We have measured the dielectric constant dependence on temperature for 10 cycles, and the results always display the similar dielectric changes as the first time except for a slight decrease of intensity. Compared with compounds 1 and 3, compound 2 displays the most distinct changes of the permittivity constant around the phase transition, in which the dielectric constants increase near 9 au between the low dielectric state (14.5 au) and high dielectric state (23.5 au). Such significant changes of the permittivity constant are predicted to have potential applications in the smart dielectric switches.[32,33]

Figure 2

Permittivity constant dependence on temperature for (a) compound 1, (b) compound 2, and (c) compound 3.

Structural Difference between before and after Phase Transitions

Most remarkable solid reversible phase transitions also can be reflected by their distinct crystal structural differences between before and after phase transitions. Subsequently, we measured their crystal structures at selected temperatures (before and after phase transitions). To be concise, we labeled the structures of 1 before 290 K (phase transition temperature) 1-LTP, and after that, we named them 1-HTP. In the similar fashion, we named the structures of 2 before 318 K (phase transition temperature) 2-LTP, and after that, we called them 2-HTP. Subsequently, we called the crystal structures of 3 before and after 327 K as 3-LTP and 3-HTP, respectively.

As depicted in Table S1, 1-LTP belongs to the monoclinic crystal system with the P21/c space group, while 1-HTP crystallizes in the orthorhombic crystal system with the Cmce space group. The P21/c space group is part of C2 and has four symmetrical elements, while Cmce belongs to the D2 point group and possess eight symmetrical elements. Also, the symmetry elements decrease significantly from 8 (E, C2, C2′, C2′, i, σ, σ, σ) at 1-HTP to 4 (E, C2, i, σ) at 1-LTP. Such symmetry breaking has been frequently reported in organic–inorganic hybrid phase transition materials by other scientists.[18] Furthermore, other marked change such as the unit cell a-axis decreased sharply from 11.2047(8) in 1-LTP to 7.4525(7) in 1-HTP, while the unit cell parameters b-axis and cell volume V increased abruptly from 7.6515(5) and 607.24(7) in 1-LTP to 21.842(2) and 1233.29(19) Å3 in 1-HTP, nearly reaching three and two times of 1-LTP, respectively. While the c-axis 7.3050(5) in 1-LTP turn to 7.5766(6) Å3 in 1-HTP, almost remaining unchanged.

As shown in Figure , whether before or after the phase transition, both 1-HTP and 1-LTP have the same two dimensional organic–inorganic A2BX4 perovskite structure (here, A is the protonated fluorine ethylamine, B = Cd2+, X represents Cl–). All the cadmium atoms adopt the octahedral geometry which is linked by six Cl– anions and forms an infinite linear [CdCl4]2– structure, and then further crossed each other, resulting in the formation of a negative two-dimensional inorganic sheet. The protonated fluorine ethylamine interspersed between inorganic sheets by intermolecular hydrogen bonds and point charges, packing to three-dimensional layer structures (Figure S4). Nevertheless, great differences can be discovered between 1-HTP and 1-LTP. On the one hand, fluorine ethylamine at high temperature (1-HTP) appears highly disordered which can be viewed as the twofold disorder (Figure b), while at low temperature (1-LTP), fluorine ethylamine is completely ordered (Figure a); on the other hand, the co-ordination geometry of [CdCl6] in 1-HTP is a standard octahedron, while in 1-LTP, it is a seriously distorted one because the Cd–Cl bond distance varied from 2.6743(2) Å in 1-HTP to 2.6574(15) Å and 2.6787(15) Å in 1-LTP. Simultaneously, the 2-D [CdCl4]2– framework in 1-HTP displays a regular square with a side length 5.3486 Å (Figure b), while in 1-LTP, it appears to be an irregular octagon structure (Figure a), and the side lengths has changed to be 5.6950 and 4.9524 Å, respectively, appearing to be seriously distorted (Figure ). These apparent order–disorder changes of fluorine ethylamine guest molecules and distortion of the [CdCl4]2– framework also can be clearly reflected by their packing diagrams in Figure S4a for 1-LTP and Figure S4b for 1-HTP.

Figure 3

Molecular structure of (a) 1-LTP and (b) 1-HTP which highlights the change of intermolecular hydrogen bonds and fluorine atoms.

Figure 4

(a) Irregular framework of 1-LTP and (b) regular square framework of 1-HTP.

Molecular structure of (a) 1-LTP and (b) n class="Chemical">1-HTP which highlights the change of intermolecular hydrogen bonds and fluorine atoms.

(a) Irregular framework of 1-LTP and (b) regular square framework of n class="Chemical">1-HTP.

Different from 1, both 2-LTP and 2-HTP belong to the same orthorhombic crystal system but with different space groups, namely, Pbca and Cmce, respectively. Because they originate the same D2 point group with eight symmetrical elements, no symmetry breaking occurs during phase transition. However, there exist some significant crystallographic changes, for example, the unit cell b and c [7.4027(9), 23.054(3)] in 2-LTP has exchanged to 23.412(2), 7.5566(7) in 2-HTP (Table S2), and such displacement indicates that a possible rotation movement was experienced in phase transition. Similar to compound 1, 2-LTP and 2-HTP have two-dimensional organic–inorganic A2BX4 perovskite structures.[34] As shown in Figure a,b, at high temperature, C1, C2, F1, and F2 in 2,2′-difluorine ethylamine are seriously disordered in 2-HTP, which can be split as the twofold disorder (highlighted in Figure b), while all the atoms in 2-LTP are completely ordered (Figure a), showing an order–disorder motion in the transition process. In addition, the coordination geometry of [CdCl6] has also changed from slight octahedron in 2-HTP to a seriously distorted one in 2-LTP, and the regular square 2-D [CdCl4]2– framework in 2-HTP has turned into an irregular distorted structure because the side length between Cl···Cl atoms have been changed from the unified 5.3465 Å in 2-HTP to 6.0599 and 4.5924 Å in 2-LTP (Figure ). These clear changes suggest that both the order–disorder motion of F atoms and the twist motion from [CdCl4]2– frameworks lead to its solid reversible phase transition which can be also reflected by the packing diagram of 2-LTP (Figure S5a) and 2-HTP (Figure S5b).[17,29]

Figure 5

Molecular structure of (a) 1-LTP and (b) 1-HTP which highlights the change of order–disorder states between 1-HTP and 1-LTP.

Figure 6

(a) Irregular framework of 2-LTP and (b) regular square framework of 2-HTP.

Molecular structure of (a) 1-LTP and (b) n class="Chemical">1-HTP which highlights the change of order–disorder states between 1-HTP and 1-LTP.

(a) Irregular framework of 2-LTP and (b) regular square framework of n class="Chemical">2-HTP.

As a further step, when we replaced 2-fluorine ethylamine by 2,2′,2″-trifluorine ethylamine, compound 3 undergoes a higher reversible phase transition temperature (329 K) than that of compound 2. As listed in Table S3, both 3-LTP and 3-HTP crystallized in the same symmetrical crystal system monoclinic but with different space groups, where 3-LTP belongs to C2/c and 3-HTP is the P21/c space group. They have the same C point group with four symmetrical elements (E, C2, i, and σ), and no symmetry breaking happened in the phase transition too. Nevertheless, some distinct changes can be also easily discovered between 3-LTP and 3-HTP. According to the crystallographic Table S3, the most remarkable changes between 3-LTP and 3-HTP is that the unit cell length a and cell volume V have increased from 13.603(4) Å and 718.4(5) Å3 in 3-HTP to 25.831(6) Å and 1390.6(6) Å3 in 3-LTP, which is nearly a doubled increase. Another striking change is that the β angle has changed from 105.740(16) in 3-HTP to 90.462(5) in 3-LTP, showing that a significant twist motion occurs in the phase transition (Table S3).

As further depicted in Figure a,b, one can discover that all the fluorine and nitrogen atoms, especially F1 atoms in 3-HTP, are obviously more disordered than those in 3-LTP, exhibiting the characteristics of order–disorder type phase transition. In addition, a slightly distorted motion can be discovered from the 2-D [CdCl4]2– framework between 3-LTP and 3-HTP, such as the Cd–Cl–Cd angles has changed from 152.914 to 154.087°(Figures S6 and S7). What is more, when viewed from the b-axis direction, large deviation angles have been slipped between the neighbor layers along the a-axis direction between 3-HTP (Figure b) and 3-LTP (Figure a), according to their packing diagrams. This larger shift also affords a crucial interpretation for the space group transformed from C2/c in 3-LTP to P21/c in 3-HTP.

Figure 7

Molecular structure of (a) 3-LTP and (b) 3-HTP.

Molecular structure of (a) 3-LTP and (b) n class="Chemical">3-HTP.

Based on the analysis of above results, it can be seen that when we replaced fluorine ethylamine with 2,2′-difluorine ethylamine, the space group changed from P21/c in 1-LTP to Pbca in 2-LTP, and then, when 2,2′-difluorine ethylamine was further replaced by 2,2′,2″ trifluorine ethylamine, the space group decreased from Pbca to C2/c in 3-LTP, showing that the symmetry of the guest molecule have an important influence on the space group of our product compounds.[35] In addition, with increasing number of fluorine in guest molecules, the reversible phase transition temperatures are improved from 294 K in compound 1, to 319 K in compound 2, then to 328 K in compound 3, and such substantial increases indicate that the fluorine atoms may play a crucial role for the intermolecular interactions (Figure ).

Figure 8

Packing diagram of (a) 3-LTP and (b) 3-HTP viewed along the b-axis.

Packing diagram of (a) 3-LTP and (b) n class="Chemical">3-HTP viewed along the b-axis.

Once again, the variable-temperature powder X-ray diffractometry (PXRD) patterns of title compounds further confirmed their interesting solid reversible phase transitions. Take compound 2 (Tc = 319.3 K) as an example, in comparison with those at 298.15 K, we can easily observe that two Bragg diffraction peaks at ∼33.1° and another one at ∼43.2° vanished when the temperature is higher than that at 319.3 K and then appeared again when it was cool down to 298.15 K. (As shown in Figure .)

Figure 9

Variable-temperature PXRD patterns of 2.

To deeply disclose the mechanism for the gradual decrease of reversible phase transition temperatures among compounds 1, 2, and 3, we analyze their near-interaction forces for crystal structures by Hirshfeld surface calculation.[36−38] This calculation is based on the definition of accessibility and affords important information of intermolecular interaction with the aid of CrystalExplorer software.[38] Consequently, we import the crystal structures of compounds 1, 2, and 3 into CrystalExplorer software and analyze the Hirshfeld surface result. The dnorm surface picture and the exploded view of the fingerprint for 1-LTP, 2-LTP, and 3-LTP were depicted in Figures S8–S10. Obviously, the ratios of F···H/H···F, F···Cl interactions occupy the 30.2%, 0.4% of the total hirshfeld surface for compound 1, while the percentages of F···H/H···F and F···Cl/Cl···F interactions for compound 2 are 42.4, 2.0%, and for compound 3 are 19.5 and 3.5%, respectively. Obviously, the F···Cl/Cl···F interactions gradually increased with the increasing of fluorine atoms, especially, as shown in Figure which represents the ratio of F···F interaction of 2D fingerprint plots in compound 1, 2, and 3. These results suggest that the ratio of the F···F interaction quickly improves from 0.5% for 1 (Figure a) to 8.1% for 2 (Figure b) to 37.0% for 3 (Figure c), with the increase of fluorine atoms, and it exerts the primary role for intermolecular interactions.[39] In combination of their crystal structures analysis, we can attentively infer that compound 3 needs to overcome more energy for the reversible phase transition contributes to the enhancement of F···F interaction, so it should undergo the highest temperature phase transition among the three compounds, and such work has been addressed by Wolters and Bickelhaupt.[39] According to the abovementioned results and the recent work reported by Xiong et al.,[23] we roughly proposed the following explanation for the improving phase transition temperatures with the increase of fluorine atoms. On the one hand, the barrier of rotation of the fluorinated organic molecules is raised, resulting in a remarkable increase in Tc.[23] On the other hand, the van der Waals forces including the dispersion force, induction force, and orientation force are improved, leading to the synergistic increase in Tc.

Figure 10

2D fingerprint plots of (a) 1-LTP, (b) 2-LTP, and (c) 3-LTP highlighting the ratio of F···F interaction.

2D fingerprint plots of (a) 1-LTP, (b) n class="Chemical">2-LTP, and (c) 3-LTP highlighting the ratio of F···F interaction.

Conclusions

In conclusion, by poly-H/F substitution in the new organic-inorganic hybrid 2[CH2FCH2NH3]·[CdCl4], 1, we obtained two new hybrids, 2[CHF2CH2NH3]·[CdCl4], 2 and 2[CF3CH2NH3]·[CdCl4], 3, with higher reversible phase transition temperatures. This research suggests that the poly-H/F substitution exerts the primary role on the crystal structures and phase transition behaviors. We believe such investigation will pave a new way for searching the multifunctional materials such as dielectric switches.

Experimental Section

Synthesis of Compounds 1, 2, and 3

All the reagents and solvents are commercially obtained from Aladdin Company. As shown in Scheme , in a 50 mL beaker, 2 mmol (0.190 g) 2-fluoroethylamine hydrochloride and 1 mmol CdCl2·4H2O (0.255 g) were solved in 10 mL of absolute methanol, and 1.5 mL of concentrated hydrochloride acid (12 mol/L) was added to the solution, and then, the mixture was stirred for half an hour and filtrated. The colorless lamellar crystals of 1 were obtained after one week of slow evaporation. Elemental analysis for 1 (C4H14F2N2CdCl4), calcd (%) C, 12.56; H, 3.69; N, 7.33. Found (%), C, 12.60; H, 3.50; N, 7.31. With the similar procedure, we obtained the sheet crystals of 2 and 3 with replacement of 2-fluoroethylamine hydrochloride with 2,2′-difluorine ethylamine and 2,2′,2″-trifluorine ethylamine respectively. Elemental analysis for 2 (C4H12F4N2CdCl4), calcd (%), C, 11.48; H, 2.89; N, 6.70. Found (%), C, 11.50; H, 2.90; N, 6.68. For 3, (C4H10F6N2CdCl4), calcd (%), C, 10.57; H, 2.22; N, 6.17. Found (%), C, 10.61; H, 2.20; N, 6.15. All the compounds were initially judged by IR spectra in Figure S1. Also, all the products of title compounds are highly purified, which was further confirmed by the comparison between the simulated and experimental PXRD patterns (Figure S2).

Scheme 1

Preparation of Title Compounds 1, 2, and 3

Singe-Crystal Structure Determination

We measured the single crystal X-ray diffraction of compounds 1, 2, and 3 on a D8 QUEST (Bruker) diffractometer using the Cu Kα radiation. According to the DSC results, we collected the data by θ–2θ scanning before and after their phase transition temperatures and then corrected via Lp absorption. Their structures were solved using the direct method using OLEX-2 software package. All H atoms were located from difference Fourier maps and refined with isotropic temperature parameters. The crystallographic information of compounds 1, 2, and 3 has been deposited in the CIF format in the Cambridge Crystallographic Database Centre, as given in the Supporting Information, CCDC 1964748 for 1-LTP, 1964749 for 1-HTP, 1964750 for 2-LTP, 1964751 for 2-HTP, 1964757 for 3-LTP, and 1964755 for 3-HTP. Crystallographic data and structure refinements of compounds at different temperatures for compounds 1, 2, and 3 are given in Tables S1–S3, respectively.

DSC and Thermogravimetric Analysis

We measured the thermal properties of compounds 1, 2, and 3 by DSC using a TA Q2000 DSC instrument. The DSC were recorded both on cooling and heating runs, and the powdered samples were determined at the rate of temperature changes of 10 K/min. The indium standard was used for the temperature and enthalpy calibration. As a further step, thermogravimetric analysis measurement was performed on a TA Instruments STD2960 system from room temperature to 700 K in the nitrogen atmosphere at a rate of 10 K/min (Figure S3).

Dielectric Property

Dielectric measurements were carried out with powder samples in the form of tablets which were used as electrodes by coating silver conduction on both of its surfaces compactly. The dielectric constant dependence on temperatures of title compounds was confirmed with an Agilent or a model TH2828A impedance analyzer under the frequency 1 MHz in the process of heating and then cooling. The temperatures were controlled within the range of 270–420 K.