A Chiral Thermochromic Ferroelastic with Seven Physical Channel Switches

The Halogenation Effect Induces a Variety of Switchable Phase Transition and Second-Harmonic-Generation Materials

Halogen engineering offers a means of enhancing the physical properties of materials by fine-tuning the rotational energy barrier and dipole moment, which proved to be effective in achieving switchable phase transitions and optical responses in materials. In this work, by substituting the methyl group in ligand N-ethyl-1,5-diazabicyclo[3.3.0]octane (CH3CH2-3.3.0-Dabco) with halogen atoms X (Cl or Br) and then contining to react it with FeBr3 in a HBr aqueous solution, we successfully synthesized three kinds of organic-inorganic hybrid switchable phase-change materials, [CH3CH2-3.3.0-Dabco]FeBr4 (1), [ClCH2-3.3.0-Dabco]FeBr4 (2), and [BrCH2-3.3.0-Dabco]FeBr4 (3), which were fully characterized by single-crystal X-ray diffraction and variable-temperature powder X-ray diffraction. Compared to compound 1, compounds 2 and 3 show two pairs of reversible phase transitions, dielectric anomalies, and a second-harmonic-generation effect, which are successfully induced due to the halogen substitution. This study offers an effective molecular design strategy for the exploration and construction of iron halide organic-inorganic hybrid materials with temperature-adjustable physical properties.

Remarkable Second Harmonic Generation Response in (C5H6NO)+(CH3SO3)-: Unraveling the Role of Hydrogen Bond in Thermal Driven Nonlinear Optical Switch

Heat-activated second harmonic generation (SHG) switching materials are gaining interest for their ability to switch between SHG on and off states, offering potential in optoelectronic applications. The novel nonlinear optical (NLO) switch, (C5H6NO)+(CH3SO3)- (4-hydroxypyridinium methylsulfonate, 4HPMS), is a near-room-temperature thermal driven material with a strong SHG response (3.3 × KDP), making it one of the most potent heat-stimulated NLO switches. It offers excellent contrast of 13 and a high laser-induced damage threshold (2.5 × KDP), with reversibility > 5 cycles. At 73 °C, 4HPMS transitions from the noncentrosymmetric Pna21 room temperature phase (RTP) to the centrosymmetric P21/c phase, caused by the rotation of the (C5H6NO)+ and (CH3SO3)- due to partially thermal breaking of intermolecular hydrogen bonds. The reverse phase change exhibits a large 50 °C thermal hysteresis. Density functional theory (DFT) calculations show that (C5H6NO)+ primarily dictates both the SHG coefficient (dij) and birefringence (▵n(Zeiss) = 0.216 vs ▵n(cal.) = 0.202 at 546 nm; Δn(Immersion) = 0.210 vs ▵n(cal.) = 0.198 at 589.3 nm), while the band gap (Eg) is influenced synergistically by (C5H6NO)+ and (CH3SO3)-. Additionally, 4HPMS-RTP also exhibits mechanochromism upon grinding as well as an aggregation-enhanced emission in a mixture of acetone and water.

Molecular orbital breaking in photo-mediated organosilicon Schiff base ferroelectric crystals

Ferroelectric materials, whose electrical polarization can be switched under external stimuli, have been widely used in sensors, data storage, and energy conversion. Molecular orbital breaking can result in switchable structural and physical bistability in ferroelectric materials as traditional spatial symmetry breaking does. Differently, molecular orbital breaking interprets the phase transition mechanism from the perspective of electronics and sheds new light on manipulating the physical properties of ferroelectrics. Here, we synthesize a pair of organosilicon Schiff base ferroelectric crystals, (R)- and (S)-N-(3,5-di-tert-butylbenzylidene)-1-((triphenylsilyl)oxy)ethanamine, which show optically controlled phase transition accompanying the molecular orbital breaking. The molecular orbital breaking is manifested as the breaking and reformation of covalent bonds during the phase transition process, that is, the conversion between C = N and C-O in the enol form and C-N and C = O in the keto form. This process brings about photo-mediated bistability with multiple physical channels such as dielectric, second-harmonic generation, and ferroelectric polarization. This work further explores this newly developed mechanism of ferroelectric phase transition and highlights the significance of photo-mediated ferroelectric materials for photo-controlled smart devices and bio-sensors.

Optical Phenomena in Molecule-Based Magnetic Materials

Since the last century, we have witnessed the development of molecular magnetism which deals with magnetic materials based on molecular species, i.e., organic radicals and metal complexes. Among them, the broadest attention was devoted to molecule-based ferro-/ferrimagnets, spin transition materials, including those exploring electron transfer, molecular nanomagnets, such as single-molecule magnets (SMMs), molecular qubits, and stimuli-responsive magnetic materials. Their physical properties open the application horizons in sensors, data storage, spintronics, and quantum computation. It was found that various optical phenomena, such as thermochromism, photoswitching of magnetic and optical characteristics, luminescence, nonlinear optical and chiroptical effects, as well as optical responsivity to external stimuli, can be implemented into molecule-based magnetic materials. Moreover, the fruitful interactions of these optical effects with magnetism in molecule-based materials can provide new physical cross-effects and multifunctionality, enriching the applications in optical, electronic, and magnetic devices. This Review aims to show the scope of optical phenomena generated in molecule-based magnetic materials, including the recent advances in such areas as high-temperature photomagnetism, optical thermometry utilizing SMMs, optical addressability of molecular qubits, magneto-chiral dichroism, and opto-magneto-electric multifunctionality. These findings are discussed in the context of the types of optical phenomena accessible for various classes of molecule-based magnetic materials.

Emerging Spintronic Materials and Functionalities

The explosive growth of the information era has put forward urgent requirements for ultrahigh-speed and extremely efficient computations. In direct contrary to charge-based computations, spintronics aims to use spins as information carriers for data storage, transmission, and decoding, to help fully realize electronic device miniaturization and high integration for next-generation computing technologies. Currently, many novel spintronic materials have been developed with unique properties and multifunctionalities, including organic semiconductors (OSCs), organic-inorganic hybrid perovskites (OIHPs), and 2D materials (2DMs). These materials are useful to fulfill the demand for developing diverse and advanced spintronic devices. Herein, these promising materials are systematically reviewed for advanced spintronic applications. Due to the distinct chemical and physical structures of OSCs, OIHPs, and 2DMs, their spintronic properties (spin transport and spin manipulation) are discussed separately. In addition, some multifunctionalities due to photoelectric and chiral-induced spin selectivity (CISS) are overviewed, including the spin-filter effect, spin-photovoltaics, spin-light emitting devices, and spin-transistor functions. Subsequently, challenges and future perspectives of using these multifunctional materials for the development of advanced spintronics are presented.

Perovskite Smart Windows: The Light Manipulator in Energy-Efficient Buildings

Uncontrolled sunlight entering through windows contributes to substantial heating and cooling demands in buildings, which leads to high energy consumption from the buildings. Recently, perovskite smart windows have emerged as innovative energy-saving technologies, offering the potential to adaptively control indoor solar heat gain through their impressive sunlight modulation capabilities. Moreover, harnessing the high-efficiency photovoltaic properties of perovskite materials, these windows have the potential to generate power, thereby realizing more advanced windows with combined light modulation and energy harvesting capabilities. This review summarizes the recent advancements in various chromic perovskite materials for achieving light modulation, focusing on both perovskite structures and underlying switching mechanisms. The discussion also encompasses device engineering strategies for smart windows, including the improvement of their optical and transition performance, durability, combination with electricity generation, and the evaluation of their energy-saving performance in building applications. Furthermore, the challenges and opportunities associated with perovskite smart windows are explicated, aimed at stimulating more academic research and advancing their pragmatic implementation for building energy efficiency and sustainability.

On-demand engineerable visible spectrum by fine control of electrochemical reactions

Tunability of optical performance is one of the key technologies for adaptive optoelectronic applications, such as camouflage clothing, displays, and infrared shielding. High-precision spectral tunability is of great importance for some special applications with on-demand adaptability but remains challenging. Here we demonstrate a galvanostatic control strategy to achieve this goal, relying on the finding of the quantitative correlation between optical properties and electrochemical reactions within materials. An electrochromic electro-optical efficiency index is established to optically fingerprint and precisely identify electrochemical redox reactions in the electrochromic device. Consequently, the charge-transfer process during galvanostatic electrochemical reaction can be quantitatively regulated, permitting precise control over the final optical performance and on-demand adaptability of electrochromic devices as evidenced by an ultralow deviation of <3.0%. These findings not only provide opportunities for future adaptive optoelectronic applications with strict demand on precise spectral tunability but also will promote in situ quantitative research in a wide range of spectroelectrochemistry, electrochemical energy storage, electrocatalysis, and material chemistry.

A High Working Temperature Multiferroic Induced by Inverse Temperature Symmetry Breaking

Molecular-based multiferroic materials that possess ferroelectric and ferroelastic orders simultaneously have attracted tremendous attention for their potential applications in multiple-state memory devices, molecular switches, and information storage systems. However, it is still a great challenge to effectively construct novel molecular-based multiferroic materials with multifunctionalities. Generally, the structure of these materials possess high symmetry at high temperatures, while processing an obvious order-disorder or displacement-type ferroelastic or ferroelectric phase transition triggered by symmetry breaking during the cooling processes. Therefore, these materials can only function below the Curie temperature (Tc), the low of which is a severe impediment to their practical application. Despite great efforts to elevate Tc, designing single-phase crystalline materials that exhibit multiferroic orders above room temperature remains a challenge. Here, an inverse temperature symmetry-breaking phenomenon was achieved in [FPM][Fe3(μ3-O)(μ-O2CH)8] (FPM stands for 3-(3-formylamino-propyl)-3,4,5,6-tetrahydropyrimidin-1-ium, which acts as the counterions and the rotor component in the network), enabling a ferroelastoelectric phase at a temperature higher than Tc (365 K). Upon heating from room temperature, two-step distinct symmetry breaking with the mm2Fm species leads to the coexistence of ferroelasticity and ferroelectricity in the temperature interval of 365-426 K. In the first step, the FPM cations undergo a conformational flip-induced inverse temperature symmetry breaking; in the second step, a typical ordered-disordered motion-induced symmetry breaking phase transition can be observed, and the abnormal inverse temperature symmetry breaking is unprecedented. Except for the multistep ferroelectric and ferroelastic switching, this complex also exhibits fascinating nonlinear optical switching properties. These discoveries not only signify an important step in designing novel molecular-based multiferroic materials with high working temperatures, but also inspire their multifunctional applications such as multistep switches.

Halogen-Regulated Tc and X-ray Radiation Detection in 2D Hybrid Perovskite Ferroelastic Semiconductor

Organic-inorganic hybrid perovskites (OIHPs) have received particular attention due to their characteristic structural tunability and flexibility. These features make OIHPs behave with excellent modifications on macroscopic properties, such as ferroicity or semiconductor performances, etc. Herein, we report two 2D hybrid stibium-based halide perovskite (C3H7N)3Sb2X9 (X = Br, 1; Cl, 2) ferroelastic semiconductor possessing dual switching properties of dielectric and second harmonic generation (SHG). Notably, these two hybrids exhibit halogen-regulated ferroelasticity and semiconductor properties. There is a significant difference in Curie temperature (Tc) and X-ray radiation detection sensitivity (S), i.e., the ΔTc and ΔS are 38 K and 87 μC Gyair-1 cm-2, respectively. Meanwhile, crystals 1 and 2 do not show dark current drift in cyclic measurements of different radiation doses with stable switching ratios of 30 and 10, separately. Meanwhile, these results were proven by scientific experimental results and density functional theory (DFT) calculations. Our work presents a facile and practical method to regulate macroproperties on the molecular level, providing a new vision to develop hybrid perovskite ferroic-photoelectric materials.

Tuning ferroelectric phase transition temperature by enantiomer fraction

Tuning phase transition temperature is one of the central issues in phase transition materials. Herein, we report a case study of using enantiomer fraction engineering as a promising strategy to tune the Curie temperature (TC) and related properties of ferroelectrics. A series of metal-halide perovskite ferroelectrics (S-3AMP)x(R-3AMP)1-xPbBr4 was synthesized where 3AMP is the 3-(aminomethyl)piperidine divalent cation and enantiomer fraction x varies between 0 and 1 (0 and 1 = enantiomers; 0.5 = racemate). With the change of the enantiomer fraction, the TC, second-harmonic generation intensity, degree of circular polarization of photoluminescence, and photoluminescence intensity of the materials have been tuned. Particularly, when x = 0.70 - 1, a continuously linear tuning of the TC is achieved, showing a tunable temperature range of about 73 K. This strategy provides an effective means and insights for regulating the phase transition temperature and chiroptical properties of functional materials.

A role of intermolecular interaction modulating thermal diffusivity in organosuperelastic and organoferroelastic cocrystals

Although the finding of superelasticity and ferroelasticity in organic crystals has been serendipitous, an increasing number of organic crystals with such deformation properties have been witnessed. Understanding the structure-property relationship can aid in the rational selection of intermolecular interactions to design organic crystals with desired superelastic or ferroelastic properties. In this study, we investigated the mechanical deformation in two cocrystals, prepared with the parent compound, 1,4-diiodotetrafluorobenzene with two coformers, 1,2-bis(4-pyridyl)ethane and pyrene. The parent compound and coformers were chosen to introduce distinct weak interactions such as halogen bonds and C-H⋯F, and π⋯π interactions in the crystal structure. The two cocrystals exhibited different mechanical deformations, superelasticity, and ferroelasticity, respectively. The single-crystal X-ray diffraction and energy framework analysis of the crystal structure of the cocrystals revealed that both deformations were caused by mechanical twinning. Interestingly, a difference in the extent of deformation was observed, modulated by a combination of strong and weak intermolecular interactions in the superelastic cocrystal, and only weak interaction in the ferroelastic one. In this comparison, the superelastic cocrystal exhibited higher thermal diffusivity than the ferroelastic cocrystal, indicating the presence of symmetrical and relatively robust intermolecular interactions in the superelastic cocrystal.

The halogen substitution strategy of inorganic skeletons triggers dielectric and band gap regulation of hybrid perovskites

Organic-inorganic hybrid perovskites (OIHPs) with dielectric switching functions have aroused comprehensive scientific interest, benefitting from their promising applications in sensors and information storage. However, to date, most of these materials discovered thus far possess a single function and are limited in their applicability, failing to meet the requirements of diverse applications. Moreover, the discovery of these materials has been largely serendipitous. Building multifunctional OIHPs with dielectric switching and semiconductors remains a daunting task. In this context, by introducing [C7H16N]+ as cations and in combination with lead halide with semiconducting properties, two OIHPs [C7H16N]PbI3 (1) and [C7H16N]PbBr3 (2) ([C7H16N]+ = (cyclopropylmethyl) trimethylammonium) have been successfully designed. They have dielectric switching properties close to 253 and 279 K and semiconducting behavior with band gaps of 2.67 and 3.22 eV. The phase transition temperature increased by 26 K through halogen substitution. In summary, our findings in this study provide insights into the application of the halogen substitution regulation strategy and open up new possibilities for designing perovskite semiconductors with dielectric switching functionality.

Zero-Dimensional Sn-Based Enantiomeric Phase-Transition Materials with High-Tc and Dielectric Switching

The combination of chirality and phase-transition materials has broad application prospects. Therefore, based on the quasi-spherical theory and the thought strategy of introducing chirality, we have successfully synthesized a pair of chiral enantiomeric ligands (R/S)-triethyl-(2-hydroxypropyl)ammonium iodide, which can be combined with a tin hexachloride anion to obtain a pair of new organic-inorganic hybrid enantiomeric high-temperature plastic phase-transition materials: (R/S)-[CH3 CH(OH)CH2 N(CH2 CH3 )3 ]2 SnCl6 (1-R/1-S), which have a high temperature phase transition of Tc =384 K, crystallize in the P21 chiral space group at room temperature, and have obvious CD signals. In addition, compounds 1-R and 1-S have a good low-loss dielectric switch and broadband gap. This work is conducive to the research into chiral high-temperature reversible plastic phase-transition materials, and promotes the development of multi-functional phase-transition materials.

Large Phase-Transition Temperature Enhancement Achieved in a Layered Lead Iodide Hybrid Crystal by H/F Substitution

Organic-inorganic hybrid metal halides with structural flexibility and solution processability have been widely investigated for different application scenarios. However, the effective construction of phase-transition materials with a high phase-transition temperature (Ttr) for potential practical applications remains a great challenge, and reports on the regulation of Ttr with significant enhancement have been rare. In this manuscript, we have realized a large Ttr increase of 148 K in a layered hybrid lead iodide crystal (4-FTMBA)4Pb3I10 (4-FTMBA = 4-fluoro-N,N,N-trimethylbenzenaminium) by the H/F substitution strategy. Compared to the parent (TMBA)4Pb3I10 (TMBA = N,N,N-trimethylbenzenaminium), H/F substitution preserves the structural framework and crystal symmetry in (4-FTMBA)4Pb3I10. The introduction of heavier fluorine will significantly increase the motion barrier for the order-disorder transition, resulting in the remarkably improved Ttr. Temperature-dependent crystal structures, Raman spectra, and dielectric analyses well support the phase-transition behavior. In addition, evident thermochromism with a tunable direct band gap in (4-FTMBA)4Pb3I10 has been observed using UV-vis spectra. To the best of our knowledge, the achieved Ttr enhancement of 148 K by H/F substitution is the highest among the organic-inorganic hybrid lead halide phase-transition materials. This finding would greatly inspire the rational design of functional materials with high performance.

High-Temperature Ferroelasticity and Photoluminescence in a 2D Monolayer Perovskite Compound: (C5NH8Br)2PbBr4

Hybrid organic-inorganic perovskites (HOIPs) have attracted much attention due to their excellent properties and easy synthesis. As far as we know, most documented ferroelastics mainly focus on the 3D (three-dimensional) perovskites, the 2D monolayer perovskite ferroelastics are rarely reported before. In this work, we synthesized a 2D lead-based perovskite (C₅NH₁₃Br)₂PbBr₄ (1) (C₅NH₁₃Br = 5-bromoamylamine cation) by introducing flexible chain organic cations. The evolution of ferroelastic domains observed by a polarized light microscope confirms that compound 1 undergoes a ferroelastic phase transition at 392/384 K. In addition, its direct band gap is 2.877 eV. Interestingly, the material emits an attractive blue light (quantum yield 5.06%) under UV light. Three structural descriptors are introduced to quantitatively analyze the relationship between structural distortion and the shape of emission peak. This work provides a way to design multifunctional perovskite-type materials.

An anomalous ferroelastic phase transition arising from an unusual cis-/anti-conformational reversal of polar organic cations

Hybrid ferroelastics have attracted increasing attention for their potential application as mechanical switches. The sporadically documented anomalous ferroelastic phase transitions, i.e., ferroelasticity that appears at a high-temperature phase rather than a low-temperature phase, are of particular interest but are not well understood at the molecular level. By judiciously choosing a polar and flexible organic cation (Me₂NH(CH₂)₂Br⁺) with cis-/anti- conformations as an A-site component, we obtained two new polar hybrid ferroelastics, A₂[MBr₆] (M = Te for 1 and Sn for 2). These materials undergo distinct thermal-induced ferroelastic phase transitions. The larger [TeBr₆]^2- anions anchor the adjacent organic cations well and essentially endow 1 with a conventional ferroelastic transition (P2₁ → Pm2₁n) arising from a common order-disorder transition of organic cations without conformational changes. Moreover, the smaller [SnBr₆]^2- anions can interact with the adjacent organic cations in energetically similar sets of intermolecular interactions, enabling 2 to undergo an anomalous ferroelastic phase transition (P2₁2₁2₁ → P2₁) arising from an unusual cis-/anti-conformational reversal of organic cations. These two instances demonstrate the importance of the delicate balance of intermolecular interactions for inducing anomalous ferroelastic phase transitions. The findings here provide important insights for seeking new multifunctional ferroelastic materials.

Inversion of Molecular Chirality Associated with Ferroelectric Switching in a High-Temperature Two-Dimensional Perovskite Ferroelectric

Controlling molecular chirality by external stimuli is of great significance in both fundamental research and technological applications. Herein, we report a high-temperature (384 K) molecular ferroelectric of a Cu(II) complex whose spontaneous polarization can be switched associated with flipping of molecular chirality. In this two-dimensional perovskite structure, the inorganic layer is separated by (NH₃(CH₂)₂SS(CH₂)₂NH₃)²⁺ organic cations skewed in a chiral conformation (P- or M-helicity in an individual crystal). As the stereodynamic disulfide bridge determines the molecular dipole moment along the polar axis, the chiral organic cation can be converted to its enantiomer as a consequence of an electric field-induced shift of the S-S moiety relative to its screw axis during the ferroelectric switching. The variation of the molecular chirality is examined with single-crystal X-ray diffraction and circular dichroism spectra. The simultaneous switching of molecular chirality and spontaneous polarization in this perovskite ferroelectric may lead to novel chiral electronic phenomena.

Linear optical afterglow and nonlinear optical harmonic generation from chiral tin(IV) halides: the role of lattice distortions

The striking chemical variability of hybrid organic-inorganic metal halides (HOMHs) endows them with fascinating optoelectronic properties. The inorganic skeletons of HOMHs are often flexible and their lattice deformations could serve as an effective factor for enabling the functionalities of HOMHs. Here, the linear and nonlinear optical properties of zero-dimensional (0D) tin(IV) halides have been tuned by structural distortion facilitated by the chiral amines. Enantiopure α-methylbenzyl ammoniums (XMBA, X = Cl, F) effectively transfer their chirality to the inorganic scaffolds when forming the tin(IV) halides, which enables polar arrangements in their crystals and leads to outstanding second-order nonlinear optical performances. In contrast, the racemic mixture of R- and S-FMBA results in the formation of HOMHs with room temperature phosphorescence. The lower lattice deformation in (rac-FMBA)₂SnCl₆ restrains the non-radiative decay from electron-phonon coupling and facilitates the photoluminescence. Meanwhile, the marked π-π interaction stabilizes the T₁ state for phosphorescent emission. These distinct linear and nonlinear optical properties denote the important role that the lattice distortion plays in tuning the optical properties of low-dimensional HOMHs, and offer a promising perspective of 0D tin(IV) halides for applications in optoelectronic materials and devices.

1D Chiral Lead Bromide Perovskite with Superior Second-Order Optical Nonlinearity, Photoluminescence, and High-Temperature Reversible Phase Transition

Multifunctional materials are an attractive research area. Organic-inorganic hybrid perovskites are widely used in the design of these materials due to their rich properties and flexible composition. It is easy to obtain more photoelectric properties by introducing chiral groups as ligands. In this work, we synthesized chiral one-dimensional organic-inorganic hybrid perovskites, namely (R/S-3-HP)PbBr₃ (1R/1S) (3-HP=3-hydroxy-piperidine). The enantiomer compounds undergo reversible phase transition at 349/336 K. Under the excitation light of 339 nm, 1R and 1S have a wide emission peak at 635 nm, showing orange light. In addition, the indirect bandgap is 3.29 eV and the SHG intensity is comparable to that of KDP. This work provides a way to design multifunctional chiral perovskite materials.

Design, synthesis, and characterization of a new hybrid organic-inorganic perovskite with a high-Tc dielectric transition

Phase change materials (PCMs) have drawn increasing attention for their promising applications in thermal switches, data communication, and energy storage. Because of the complexity of the interactions between molecules, it is still a challenge to design PCMs with a desired high phase transition temperature (T_c). In this study, a one-dimensional hybrid perovskite of (TEACCl)PbBr₃ (1, TEACCl = Et₃NCH₂Cl) was successfully designed and synthesized with a T_c = 390 K. Disordering of TEACCl⁺ on the heating process is the origin of the structural phase transition of 1 from the P2₁/c to P6₃/mmc structure. It is noted that the phase transition is associated with an excellent switchable dielectric property, which indicates that 1 has the potential to be applied to sensor equipment. After calculation, 1 is an infrequent indirect bandgap semiconductor with an energy gap of 3.57 eV. Moreover, 1 exhibits strong red fluorescence under irradiation of UV light. This work will provide guidance for designing high T_c switching materials.

Influence of Pd(II) Adsorption on High-Temperature Ferroelastic Phase Transition in (2-Amino-2-thiazolinium)PbBr3

Ferroelastic materials have received special attention because of their great promise for mechanical switches, piezoelectric sensors, and data storage applications. Here, we report a novel ferroelastic semiconducting hybrid organic-inorganic perovskite (C₃H₇N₂S)PbBr₃ (1) [(C₃H₇N₂S)⁺ is 2-amino-2-thiazolinium] with a ferroelastic phase transition at 395 K and an optical band gap of 3.43 eV. 1 has a one-dimensional BaNiO₃-type structure and undergoes a high-temperature ferroelastic phase transition with an Aizu notation of mmmF2/m. Meanwhile, 1 exhibits dielectric switch near the phase transition temperature. By introducing the thioether group, the motion of the molecules or ions of 1 is hindered after the sorption of Pd(II) metal ions, which leads to the disappearance of the high-temperature ferroelastic phase transition and dielectric switch. This is the first reported ferroelastic semiconductor material with Pd(II) adsorption property, by studying the influence of Pd(II) adsorption on high-temperature ferroelastic phase transition, it may be enlightening to further uncover the mechanism of phase transition or the origin of ferroelasticity, which represents an important step toward multifunctional applications of lead-hybrid perovskite-based ferroelastic materials.

Three-Dimensional Perovskite Phase Transition Materials with Switchable Second Harmonic Generation Properties Introduced by Homochiral (1S,4S)-2,5-Diazabicyclo[2.2.1]-heptane

Switchable second harmonic generation (SHG) materials have potential applications in information storage, signal processing, and so on because they can switch between SHG-on and SHG-off states. In this work, we designed and synthesized three organic-inorganic hybrid Rb halide three-dimensional (3D) perovskite materials [1S,4S 2,5-2.2.1-H₂dabch]RbX₃·0.5H₂O (X = Cl, 1; Br, 2; I, 3) based on the chiral 1S,4S-2,5-diazabicyclo[2.2.1]heptane (1S,4S-2,5-2.2.1-dabch). The selection of homochiral organic cations ensures that the compounds 1∼3 crystallize in the noncentrosymmetric and chiral space group P2₁2₁2₁, which further leads to reversible SHG responses of the three compounds. Through differential scanning calorimetry (DSC) and dielectric measurements, it revealed that the phase transition point of the compounds 1∼3 increased with RbCl, RbBr, and RbI. This is because the hydrogen interaction H···X between the inorganic framework [RbX₃]_n and the organic cation [1S,4S-2,5-2.2.1-H₂dabch]²⁺ is increased with the order of I > Br > Cl. This study can provide an effective molecular design strategy for the exploration and construction of temperature-tunable SHG switching materials.

Integrated Reversible Thermochromism, High Tc , Dielectric Switch and Narrow Band Gap in One Multifunctional Ferroic

Organic-inorganic Hybrid (OIH) materials for multifunctional switchable applications have attracted enormous attention in recent years due to their excellent optoelectronic properties and good structural tunability. However, it still remains challenging to fabricate one simple OIH compound with multi-functionals properties, such as dielectric switching, thermochromic properties, semiconductor characteristics and ferroelasticity. Under this context, we successfully synthesized [2-(2-fluorophenyl)ethan-1- ammonium]₂ SnBr₆ (compound 1), which has a higher phase transition temperature of 427.7 K. Additionally, it exhibits a semiconducting property with an indirect band gap of 2.36 eV. Combining ferroelastic, narrow band gap, thermochromic, and dielectric properties, compound 1 can be considered as a rarely reported multi-functional ferroelastic material, which is expected to give inspiration for broadening the applications in the smart devices field.

Chlorine Substitution in Spirocyclic Derivatives Triggers SHG Response in Noncentrosymmetric Crystal

Organic-inorganic hybrid phase transition materials with switchable properties have important potential applications in smart devices such as dielectric switches and storage. Nevertheless, it remains challenging to modify molecular structures efficiently to obtain materials simultaneously possessing multiple responsive properties. Herein, different from ordinary halogen substitutions in Metal Halide, we report a halogen regulation design of organic molecular strategy: (ASD) 2 MnBr 4 (ASD=5-azonia-spiro [4.5] decane) to (CASD) 2 MnBr 4 (CASD=8-chloro-5-azonia-spiro [4.5] decane). After organic molecular halogen regulation, the SHG response was excited and the dielectric phase transition temperature ( T c ) has also been greatly improved. Furthermore, under the irradiation of UV lamp, both showed green light with quantum yields above 50%. This work is of great significance for further exploration of multifunctional molecular switch materials through halogen modification strategies.

Ferroelasticity in Organic-Inorganic Hybrid Perovskites

Molecular ferroelastics have received particular attention for potential applications in mechanical switches, shape memory, energy conversion, information processing, and solar cells, by taking advantages of their low-cost, light-weight, easy preparation, and mechanical flexibility. The unique structures of organic-inorganic hybrid perovskites have been considered to be a design platform for symmetry-breaking-associated order-disorder in lattice, thereby possessing great potential for ferroelastic phase transition. Herein, we review the research progress of organic-inorganic hybrid perovskite ferroelastics in recent years, focusing on the crystal structures, dimensions, phase transitions and ferroelastic properties. In view of the few reports on molecular-based hybrid ferroelastics, we look forward to the structural design strategies of molecular ferroelastic materials, as well as the opportunities and challenges faced by molecular-based hybrid ferroelastic materials in the future. This review will have positive guiding significance for the synthesis and future exploration of organic-inorganic hybrid molecular ferroelastics.

Domain memory effect in the organic ferroics

Shape memory alloys have been used extensively in actuators, couplings, medical guide wires, and smart devices, because of their unique shape memory effect and superelasticity triggered by the reversible martensitic phase transformations. For ferroic materials, however, almost no memory effects have been found for their ferroic domains after reversible phase transformations. Here, we present a pair of single-component organic enantiomorphic ferroelectric/ferroelastic crystals, (R)- and (S)-N-3,5-di-tert-butylsalicylidene-1-(1-naphthyl)ethylamine SA-NPh-(R) and SA-NPh-(S). It is notable that not only can their ferroic domain patterns disappear and reappear during reversible thermodynamic phase transformations, but they can also disappear and reappear during reversible light-driven phase transformations induced by enol-keto photoisomerization, both of which are from P1 to P21 polar space groups. Most importantly, the domain patterns are exactly the same in the initial and final states, demonstrating the existence of a memory effect for the ferroic domains in SA-NPh-(R) and SA-NPh-(S). As far as we are aware, the domain memory effect triggered by both thermodynamic and light-driven ferroelectric/ferroelastic phase transformations remains unexplored in ferroic materials. Thermal and optical control of domain memory effect would open up a fresh research field for smart ferroic materials.

High-Endurance Magneto-Electronic Switchable Molecular Electronic Crystal

Electrically switchable magnetic and electronic properties are promising for quantum sensing and information technology. Here, we report an electrically driven magnetic and electronic phase transition in molecular electronic crystal, potassium-7,7,8,8-tetracyanoquinodimethan, with the magneto-electric switching over 10⁵ cycles at room temperature. Electron spin resonance study reveals the cooperative transition between spin and charge degrees of freedom. In addition, the mechanistic spectroscopy studies suggest the charges in an inhomogeneous conductor-insulator mixed state. The findings shown here suggest electrically controlled ordering in strongly correlated molecular crystal leads to dynamic magneto-electric switching, paving the way for developing molecular-based memory and switching devices.

H/F Substitution induced switchable coordination bonds in a cyano-bridged hybrid double perovskite ferroelastic

A three-dimensional cyano-bridged double perovskite ferroelastic [(CH₃)₃NCH₂F]₂[KFe(CN)₆] was constructed by introducing unprecedented switchable C-F-K coordination bonds. H/F substitution not only preserves the basic structure of the parent [(CH₃)₄N]₂[KFe(CN)₆] but also affords an m3̄mF2/m-type ferroelastic phase transition.

Mixed-halide copper-based perovskite R2Cu(Cl/Br)4 with different organic cations for reversible thermochromism

Mechanically exfoliated flakes of mixed-halide Cu-based perovskite crystals, R2Cu(Cl/Br)4, with three alkyl chains exhibit reversible thermochromic behavior with differences in crystal lattice behavior depending on the organic spacer used.

Dual-channel control of ferroelastic domains in a host–guest inclusion compound

By replacing [PF6]− with the larger [TFSA]−, the phase transition temperature is increased from 305 K to 342 K in a host–guest inclusion ferroelastic crystal, [(3,4-DFA)(18-crown-6)][TFSA], which can realize dual-channel (thermal and stress) control of ferroelastic domains.

Ferroic phase transition molecular crystals

Ferroic phase transition molecular crystals (FPTMCs), i.e., ferroelectrics and ferroelastics, are an important family of functional molecular materials, having merits of easy synthesis, structural tunability and flexibility, and biocompatibility. Both...

Thermally stimuli-responsive materials with transformable double channels of nonlinear optical and dielectric

Organic-inorganic hybrid materials have received extensive attention and in-depth research in the past few decades due to their superior properties and potential applications in storage, sensing, dielectric switches, actuators and...

Tellurate Polymorphs with High-performance Nonlinear Optical Switch Property and Wide Mid-IR Transparency

Here, we report two tellurate polymorphs, α- and β-Li2HfTeO6, undergoes a fast thermally induced phase transition that originates from symmetry breaking of HfO6 and TeO6 octahedra, behaves as a potential...

Manipulating Selective Metal-to-Metal Electron Transfer to Achieve Multi-Phase Transitions in an Asymmetric [Fe2Co]-Assembled Mixed-Valence Chain

Manipulation of multi-functions in molecular materials is promising for future switching and memory devices, although is currently difficult. Herein, we assembled the asymmetric {Fe2Co} unit into a cyanide-bridged mixed-valence chain {[(Tp)Fe(CN)3]2Co(BIT)}·2CH3OH (1) (Tp = hydrotris(pyrazolyl)borate and BIT = 3,4-bis-(1H-imidazol-1-yl)thiophen), which showed reversible multi-phase transitions accompanied by the photo-switchable single-chain magnet property and dielectric anomalies. Variable temperature X-ray structural studies revealed thermo-and photo-induced selective electron transfer (ET) between the Co and one of the Fe ions. Alternating-current magnetic susceptibility studies revealed that 1 displayed on and off of the single-chain magnet behavior by alternating 946-nm and 532-nm light irradiations. A substantial anomaly in dielectric constant was discovered during the electron transfer process, which is uncommon in similar ET complexes. These findings illustrate that 1 provided a new platform for multi-phase transitions and multi-switches adjusted by selective metal-to-metal ET.

Multichannel Control of Multiferroicity in Single-Component Homochiral Organic Crystals

A ferroelectric/ferroelastic is a material whose spontaneous polarization/strain can be switched by applying an external electric field/mechanical stress. However, the optical control of spontaneous polarization/strain remains relatively unexplored in crystalline materials, although photoirradiation stands out as a nondestructive, noncontact, and remote-controlled stimulus beyond stress or electric field. Here, we present two new organic single-component homochiral photochromic multiferroics, (R)- and (S)-N-3,5-di-tert-butylsalicylidene-1-4-bromophenylethylamine (SA-Ph-Br(R) and SA-Ph-Br(S)), which show a full ferroelectric/ferroelastic phase transition of 222F2 type at 336 K. Under photoirradiation, their spontaneous polarization/strain can be switched quickly within seconds and reversibly between two ferroelectric/ferroelastic phases with the respective enol and trans-keto forms triggered by structural photoisomerizations. In addition, they possess a superior acoustic impedance characteristic with a value of ∼2.42 × 10⁶ kg·s^-1·m^-2, lower than that of polyvinylidene fluoride (PVDF, (3.69-4.25) × 10⁶ kg·s^-1·m^-2), which can better match human tissues. This work realizes for the first time that multiple ferroic orders in single-component organic crystals with ultralow acoustic impedance can be simultaneously controlled and coupled by three physical channels (electric, stress, light fields), suggesting their great potential in multichannel data storage, optoelectronics, and related applications compatible with all-organic electronics and human tissues.

In Situ Observation of Ferroelastic Domain and Phase Transition in a Three-Dimensional Molecular Crystal

Massive efforts have been devoted to designing molecular ferroic materials by molecular modification. For molecular ferroelastic, previous work is focused on the temperature-dependent ferroelastic domains, however, few are related to controlling the ferroelastic domain by the stress. Inspired by the "quasi-spherical theory" and fluorination effect, we designed a more flexible (MedabcoF)²⁺ (MedabcoF=1-fluoro-4-methyl-1,4-diazoniabicyclo[2.2.2]octane) cation by introducing a methyl group and a fluorine atom at the two symmetrical ends of the Dabco (1,4-diazoniabicyclo[2.2.2]octane) and synthesized a hybrid 3D perovskite (MedabcoF)Rb(BF₄ )₃ (1) which displays three reversible phase transitions accompanying dual ferroelastic behavior. Besides, it not only exhibits ferroelastic domains switching by the thermal stimulation, and the sensitive reaction of in situ domains under the stress of it is also realized. This work not only achieves a force-controlled ferroelastic domain but develops a more profound comprehension of the relationship between the thermal motion behavior of guest cations and the intriguing properties of materials.

A Photoluminescent Lead Bromide Hybrid Perovskite Molecular Ferroelastic Semiconductor with Sequential High-Tc Phase Transitions

Organic-inorganic hybrid lead halide perovskites have attracted great interest for their use in promising optoelectronic applications. However, reports of photoluminescent perovskite molecular ferroelastic semiconductors with sequential high-T_c phase transitions have been scarce. In this work, a one-dimensional lead bromide hybrid perovskite [N,N-dimethylethanolammonium]PbBr₃ has been synthesized, undergoing high-T_c sequential phase transitions at around 351 and 444 K, higher than those of most previously discovered hybrid perovskite phase transition materials. The specific intermolecular hydrogen bond between cationic molecules provides the greatest contribution to its high T_c by increasing the barrier of molecular motion under the temperature stimuli. The prominent ferroelastic domain evolution is visually observed under orthogonally polarized light. In addition, [N,N-dimethylethanolammonium]PbBr₃ exhibits semiconducting and orange light emission characteristics. This finding opens up an avenue for designing high-performance ferroelastic materials and provides great motivation for discovering new multifunctional materials for the next generation of smart devices.

Giant Optical Activity and Second Harmonic Generation in 2D Hybrid Copper Halides

Hybrid organic-inorganic metal halides have emerged as highly promising materials for a wide range of applications in optoelectronics. Incorporating chiral organic molecules into metal halides enables the extension of their unique optical and electronic properties to chiral optics. By using chiral (R)- or (S)-methylbenzylamine (R-/S-MBA) as the organic component, we synthesized chiral hybrid copper halides, (R-/S-MBA)₂ CuCl₄ , and investigated their optical activity. Thin films of this material showed a record anisotropic g-factor as high as approximately 0.06. We discuss the origin of the giant optical activity observed in (R-/S-MBA)₂ CuCl₄ by theoretical modeling based on density functional theory (DFT) and demonstrate highly efficient second harmonic generation (SHG) in these samples. Our study provides insight into the design of chiral materials by structural engineering, creating a new platform for chiral and nonlinear photonic device applications of the chiral hybrid copper halides.