Higher-Temperature Dielectric Molecular Motor Induced by Unusual Chair-to-Rotator Motion

2D lead-free organic–inorganic hybrid exhibiting dielectric and structural phase transition at higher temperatures

A novel switchable molecular dielectric material [3-3-difluorocyclobutylammonium]2CdCl4 was synthesized. It shows a reversible phase transition at 353.95 K and rapid switching and reversibility between high and low dielectric states for several cycles.

The Role of Fluorine-Substituted Positions on the Phase Transition in Organic-Inorganic Hybrid Perovskite Compounds

Although research on organic-inorganic hybrid perovskites (OIHPs) has grown exponentially in the past two decades, the high phase transition temperature of OIHP materials is still one of the insurmountable difficulties. Herein, a series of A₂BX₄ type OIHP materials [(2,n-DFBA)₂PbCl₄] (n = 3, for 1; n = 4, for 2; n = 5, for 3; n = 6, for 4) have been prepared by reactions of double-substituted difluorobenzylamine (difluorobenzylamine = DFBA) with lead chloride in concentrated HCl aqueous solution. It was found the OIHP compounds 1-3 proceed a switchable phase transition with phase transition temperatures (T_c) at 449 K (1), 462 K (2) and 500 K (3), higher than that of the parent compound [(BA)₂PbCl₄] (BA = benzylammonium) at 438 K, but compound 4 exhibits no phase transition. A crystal structure analysis elucidated that the organic template ligands DFBA lead in the inorganic part in compounds 1-3 to a two-dimensional (2D) perovskite structure, while that in compound 4 leads to a one-dimensional (1D) chain structure. The different double-substituted positions of fluorine atoms on benzylamine have important influences on the phase transition in compounds 1-4.

Target Designing Phase Transition Materials through Halogen Substitution

Crystalline materials have received extensive attention due to their extraordinary physical and chemical properties. Among them, phase transition materials have attracted great attention in the fields of photovoltaic, switchable dielectric devices, and ferroelectric memories, etc. However, many of them suffer from low phase transition temperatures, which limits their practical application. In this work, we systematically designed crystalline materials, (TMXM)₂ PtCl₆ (X=F, Cl, Br, I) through halogen substitution on the cations, aiming to improving phase transition temperature. The resulting phase transition of (TMXM)₂ PtCl₆ (X=F, Cl, Br, I) get a significant enhancement, compared to the parent compound [(CH₃ )₄ N]₂ PtCl₆ ((TM)₂ PtCl₆ ). Such phase transition temperature enhancement can be attributed to the introduction of halogen atoms that increase the potential energy barrier of the cation rotation. In addition, (TMBM)₂ PtCl₆ and (TMIM)₂ PtCl₆ have a low symmetry and crystallize in the space group C₂ /c and P2₁ 2₁ 2₁ , respectively. This work highlights the halogen substitution in designing crystal materials with high phase transition temperature.

Stepwise Dielectric Switching Occurs in Two Photo-Responsive Complexes Possessing Two-Dimensional Structures

Two organic-inorganic hybrid complexes, (CH₃NH₃)Na[Fe(CN)₅NO]·H₂O (1) and (CH₃NH₃)₂[Fe(CN)₅NO] (2), which exhibit stepwise dielectric switching as well as photo-induced structural transformation, are synthesized and examined. In these two compounds, the photo-responsive complex anions, [Fe(CN)₅NO]^2-, connected by Na⁺ through N-Na coordination bonds or CH₃NH₃⁺ through N···H-N hydrogen bonds, form two-dimensional structures. One organic cation, CH₃NH₃⁺, that resides in the intralaminar cavity and plays a role as a template, undergoes a temperature-controlled order-disorder structural phase transition. As the frozen-thawed state change of the polar organic cations modifies the polarizability of materials, stepwise dielectric switching is observed at the phase transition temperature. Furthermore, the photo-induced linkage isomerism of [Fe(CN)₅NO]^2- building block survives in the new compounds at the low-temperature range, which is verified by variable-temperature IR spectra after photo-irradiation. The coexistence of switchable dielectric properties and photo-induced structural variation suggests multiple optical-electric roles of the present materials.

Unique Design Strategy for Dual Phase Transition That Successfully Validates Dual Switch Implementation in the Dielectric Material

Dual phase transition/switch materials are a critical cornerstone of information storage and sensing. However, they are difficult to design successfully, and compared with materials showing single-switchable phase transitions, the dual ones retain many challenges by far. Therefore, the significance of a general strategy is far greater than an accidental success. Here, an efficient strategy combining branchlike Et₃R and trunklike benzylamine analogues successfully validates dual-switch implementation in the dielectric materials. This inevitable success is based on our treelike analogue mentioned above in which amines with multiple branches can achieve a temperature-induced phase change. Exactly, (BCDA)₂ZnBr₄ [BCDA = benzyl-(2-chloroethyl)dimethylammonium] proves the regularity and undergoes two reversible phase transitions at 295.4 and 340.8 K, respectively. Variable-temperature single-crystal X-ray diffraction revealed that the generation of double phase transitions is caused by progressive changes of treelike BCDA⁺ as the temperature rises. Because the permittivity ε' of (BCDA)₂ZnBr₄ abruptly changed near the phase-transition temperatures, such physical properties make it have latent applicability. In short, the success of our strategy will inspire researches to discover more interesting dual phase transition/switch materials.

Semiconducting crystalline inorganic-organic hybrid metal halide nanochains

One-dimensional (1D) inorganic-organic metal halide hybrids at the molecular level, which can be considered as arrays of nanochains isolated by organic components, have shown remarkable optical and electric properties. This review summarizes their reported structural types and shows how to modify their band gaps and optical and electric properties.

Halogen substitution regulates the phase transition temperature and band gap of semiconductor compounds

(CH₃CH₂NH₃)₃BiX₆ and (CH₂ClCH₂NH₃)₃BiX₆ (X = Cl, Br) obtained by halogen substitution not only realize the adjustment of the phase transition in a relatively wide temperature range, but also optimize the semiconductor performance. This will promote the exploration and construction of semiconductor materials with tunable temperatures and lower band gaps.

Anion-Regulated Molecular Rotor Crystal: The First Case of a Stator-Rotator Double Switch with Relaxation Behavior

Molecular rotational motion is crucial in artificial molecular machines and is expected to be very significant for the development of an electronic information molecular machine as mentioned in the 2016 Nobel Prize. However, controlling multiple motor modes is a huge challenge. Here, we report a case in which the structural phase transition effectively triggers multiple motor modes by regulating the rotational speed of the cation and/or anion. A novel switchable crystalline supramolecular rotor, [(cyclohexylammonium)(18-crown-6)] FSO₃ (1), exhibits prominent temperature-dependent double switching behavior at 157.9 and 389.1 K induced by the variation of the rotational speed of the FSO₃^- anion (which acts as a super miniature rotator) in response to temperature. Moreover, it exhibits significant relaxation behavior and excellent pyroelectric switch characteristics. To the best of our knowledge, this might be the first discovery of the stator-rotator double switch with a relaxation effect, which could be a promising candidate for a slow/fast responsive double switch over a wide temperature range.

Triggering the dynamics of a carbazole-p-[phenylene-diethynyl]-xylene rotor through a mechanically induced phase transition

A crystalline molecular machine with several solid phases where only one is able to show intramolecular rotation.