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Three 2D organic–inorganic hybrid perovskites ferroelastics with dielectric switch. This work provides a new case for the preparation of 2D perovskite ferroelastic materials and the possibility for the application in actuators.

A liquid phase diffusion method has been developed. Adopting this method, large-sized lead-free hybrid perovskite (1,5-pentanediamine)BiI5 single-crystals were obtained, exhibiting a high X-ray response sensitivity of 8120 μC Gyair−1 cm−2 at −80 V.

A microporous structural molecular rotor ferroelectric with fast polarization switching (25 kHz) originates from the unique mechanism of fast static–rotating–static motional process showing potential application in non-volatile memory devices.

A crown-ether clathrate employing a rare-earth ion as the central metal ion was developed, and shown to display switchable dielectric behaviours around a temperature of about 230 K.

Deuteration of hydrogen-bonded phase transition crystals can increase the transition temperatures due to the isotope effect. But rare examples show the opposite trend that originates from the structural changes of the hydrogen bond, known as the geometric H/D isotope effect. Herein, we report an organic crystal, diethylammonium hydrogen 1,4-terephthalate, exhibits a reversible structural phase transition and dielectric switching. Structural study shows the cations reside in channels formed by one-dimensional hydrogen-bonded anionic chains and undergo an order-disorder transition at around 206 K. The deuterated counterpart shows an elongation of the O···O hydrogen bond by about 0.005 A. This geometric isotope effect releases the internal pressure of the anionic host on the cation guests and results in a downward shift of the phase transition temperature by 10 K.

Crystalline molecular rotors constitute a new class of stimuli-responsive molecular materials owing to inherent molecular dynamics. However, beyond the molecular level, the role of molecular packings on the bulk structures and related properties has yet to be fully understood. Herein, we report a crystalline molecular rotor showing solvent-induced structural transformation and luminescence response. The molecular rotor has a dumbbell shape with two plates as the stators and one axial bridging ligand as the rotator. The crystals adopt solvated and desolvated forms with strikingly different packing structures. The solvated forms can easily transform into the desolvated form. During the structure transformation, the butterfly-like conformation of the stator undergoes a drastic dihedral angle change of about 30°, resulting in a luminescent change of about 10 nm. These findings afford a new aspect for functional molecular rotor materials.

Host–guest molecular crystals are classical types of molecular materials widely applied for fluorescent sensors, absorption, separation, etc. Their significance lies in deciphering the intermolecular interactions in crystal engineering and expanding the scope of pharmaceuticals and chemical-responsive smart materials. Here, we report three flexible host–guest molecular solvate crystals, 1·CH2Cl2, 1·dioxane, and 1·DMF, that crystallize with a much lower boiling point guest dichloromethane (DCM, 313 K), and high boiling points guest dioxane (374 K) and dimethylformamide (DMF, 426 K). Surprisingly, thermogravimetric and crystallographic characterizations show that 1·CH2Cl2 has a relatively high desolvation onset temperature of 390 K, which is much higher than that of 1·dioxane (305 K) and 1·DMF (307 K). Crystal structure analysis shows that this high stability originates from the confinement and steric effects of the trapped guest solvent molecules in the crystal cavities. Powder X-ray diffraction shows that the three compounds' desolvated powder can self-trap the CH2Cl2 (DCM) molecules, returning to the 1·CH2Cl2 state. Moreover, spectroscopic characterization presents an emission redshift of about 30 nm for 1·dioxane and 1·DMF compared to that of the former. These results are attributed to the flexible host plate's dihedral angle that undergoes a drastic change of about 50° which leads to the greatly enhanced conjugation degree of the host plate.

Functional dynamic molecular crystals have drawn increasing interest in exploring next-generation flexible and smart materials. Molecular rotors, as a typical type of dynamic material, are good candidates that can exhibit bulk properties and functionalities. Herein, we report a molecular rotor crystal as a model system to show a unique structural phase transition-related ferroelasticity. The molecular rotor is dumbbell shaped containing a freely rotating axial rotator and multiple peripheral tert-butyl groups on the two plates with restricted motions. The crystal undergoes a ferroelastic structural phase transition at 263 K with unconventional inverse temperature symmetry breaking (ITSB), i.e., a higher-symmetric low-temperature paraelectric phase (point group mmm) vs. a lower-symmetric high-temperature ferroelastic phase (point group 2/m). Combined crystallographic and NMR spectroscopy studies reveal that unequal motions of the peripheral tert-butyl rotators and anisotropic steric repulsion among the molecules are the key cooperative intermolecular interactions to drive a concerted molecular movement to result in the unique ferroelastic phase transition with ITSB. Our study may open avenues for designing and exploring new types of dynamic functional materials.

An organic–inorganic hybrid double perovskite-type cage-like crystal, (MA)2KBiCl6 (MA = methylammonium cation, 1), was synthesized. It undergoes structural phase transitions at 336/326 K (heating/cooling). Variable-temperature single-crystal structural analyses reveal that the change of the dynamics of the MA and the deformation of the anionic cage-like framework contribute to the structural phase transition. This order–disorder transition of the MA causes dielectric transitions between the low- and high-dielectric states, making it a type of switchable dielectric material. Furthermore, dielectric anisotropic properties and reversibility of the dielectric switching of this crystal under temperature stimuli are also identified. The results reveal that hybrid double perovskite-related halides provide a promising platform for achieving switchable dielectric properties, and more hybrid perovskites can be prepared by using different organic amines as the A-site cation.

Sliding ferroelectricity is a recently observed polarity existing in two-dimensional materials. However, due to their weak polarization and poor electrical insulation in these materials, all available experimental evidence till now are indirect, with most based on transport properties in the nanoscale or piezoresponse force microscopy. We report the direct observation of sliding ferroelectricity, using a high-quality amphidynamic single crystal, (15-Crown-5)Cd$_3$Cl$_6$, which possesses a large band-gap and so allows direct measurement of P-E hysteresis. This coordination polymer is a van der Waals material, which is composed of inorganic stators and organic rotators as measured using XRD and NMR characterisation. From DFT calculations, we find that after the freezing of rotators an electric dipole is generated in each layer driven by the geometric mechanism, meanwhile a comparable ferroelectric polarization originates from the interlayer sliding. The net polarization of these two components can be directly measured and manipulated. Our finding provides insight into low-dimensional ferroelectrics, especially the controlling of synchronous dynamics of rotating molecules and sliding layers in solids., 21 pages, 5 figures

Invited for the cover of this issue are Le-Ping Miao, Chao Shi, Yi Zhang and co-workers at Jiangxi University of Science and Technology. The image depicts the structure diagrams of the 3D hybrid rare-earth double perovskite compounds. The phase transition temperatures of the two compounds were indicated by the "ice and fire", respectively. It implies the increase of the phase transition temperature of the compounds. Read the full text of the article at 10.1002/chem.202103913.

A novel electron donor unit based on phenothiazine has been designed and constructed. The target compound 10-octyl-3,7-di (thiophen-2-yl)-10H-phenothiazine was easily synthesized in high yields with a milder and efficient route via the Suzuki coupling reaction. Optical spectra show that, from the solution to the solid state, both the absorption and fluorescence spectra of the target compound are small blue-shifted. Moreover, the fluorescence spectra exhibit the emission maxima of the target compound ranging from 485 nm to 496 nm, which belongs to blue-green light region. As expected, the target compound should be a promising donor to construct D-A type materials for applications in organic light-emitting diodes (OLEDs).

A novel D-A type molecule based on carbazole central core and aldehyde terminal groups has been designed and synthesized. Optical properties of the resulting compounds were examined by means of UV-vis and fluorescence spectroscopies. It shows that, the absorption and fluorescence spectra of M2 are large red-shifted relative to M1 in both states, whereas the absorption maximum and emission maxima of M1 and M2 are small blue-shifted, from solution to the solid state. The fluorescence spectra exhibit that the emission maxima of the compounds which range from 419 nm to 472 nm belong to blue light region. As expected, the target compounds should be a promising pure blue light-emitting material for applications in organic light-emitting diodes (OLEDs).

A novel D-A type molecule based on central fluorene unit and aldehyde group termini has been designed and constructed. Optical spectra show that, from the solution to the solid state, the absorption and fluorescence spectra of the compounds are red-shifted. Moreover, a red-shift of the absorbance maxima and the emission maxima of the compounds were observed due to the electron-accepting groups. Blue light-emitting properties with purity color range at 412–468 nm were also obtained. Thus, the target compound is expected to be a promising pure blue light-emitting material for applications in organic light-emitting diodes (OLEDs).