Dehydration Induced a Structural Transformation into a One-Dimensional Hybrid Perovskite with Second Harmonic Generation and Dual Dielectric Switching.

Hybrid organic-inorganic perovskites with structural transformation have garnered continued interest in recent years for their potential as multifunctional materials in the field of optoelectronics and smart devices. Herein, we report a novel hybrid organic-inorganic halide, [C5NOH12]2[Cd1.5Cl5(H2O)] (1). Remarkably, the centrosymmetric compound 1 undergoes a structural transformation to a novel noncentrosymmetric hybrid perovskite [C5NOH12][CdCl3] (2) after dehydration. Accompanied by the chemical bond cleavage and reorganization, the zero-dimensional (0D) trinuclear cluster in compound 1 transforms into an intriguing one-dimensional (1D) hexagonal perovskite structure in compound 2, generating multiple optoelectronic switching behaviors. It is worth mentioning that compound 2 demonstrates successive structural phase transitions at 353 and 405 K, resulting in switchable second harmonic generation (SHG) and a dual dielectric response. In addition, compounds 1 and 2 both feature blue-light luminescence, with respective photoluminescence lifetimes of 0.73 and 1.42 ns. This work will offer a pioneering approach and expansive potential for the preparation and development of hybrid organic-inorganic perovskite materials with superior properties.

Templating Influence of Regulated Inorganic Framework in Two-Dimensional Ferroelastic Perovskites: (C3 H5 CH2 NH3 )2 [MCl4 ] (M=Mn and Cd).

The remarkable material stability and structural diversity of two-dimensional (2D) organic-inorganic hybrid perovskites (OIHPs) constitute a vast available library of versatile materials. In particular, ferroelastic property, for which the spontaneous strain can be transformed by applying mechanical stress, is very promising for extensive nanotechnological applications. However, integrating ferroelastic property into 2D OIHPs is still in its infancy. Herein, we designed two new 2D OIHPs (C3 H5 CH2 NH3 )2 [MCl4 ] (M=Mn for 1 and Cd for 2), which undergo reversible ferroelastic phase transitions with an Aizu expression 4/mmmFmmm. The templating influence of the more distorted inorganic framework on the disordering of organic cations and the stronger hydrogen bonds has a key role in the striking improvement of Curie temperature from 246 K in 1 to 273 K in 2. Meanwhile, the minimized alteration of structural motif ensures the well maintaining of the ferroelastic performance in the forms of crystals and thin films, as demonstrated by the identifiable evolution of domain structures. This work will provide a fertile new ground for enlarging the limited number of 2D ferroelastic OIHPs with better practical utility.

Two-step nonlinear optical switch in a hydrogen-bonded perovskite-type crystal.

Switchable nonlinear optical (NLO) materials have aroused broad interest on account of their captivating optical and electronic properties. We demonstrate a novel perovskite-type crystal with exceptional hydrogen bond interactions that are associated with the onset of reorientational motions of organic cations and thus induce the occurrence of two successive phase transitions to be a two-step NLO switch. This finding affords an alternative approach for the design and assembly of switchable NLO materials.

(C7H18N2)PbI4: A 2D Hybrid Perovskite Solid-State Phase Transition Material with Semiconducting Properties.

Two-dimensional organic-inorganic hybrid perovskites (OIHPs) have gained attention as a result of their flexibility and adjustability of the structure. However, the large band gap of two-dimensional perovskites limits their application in the photoelectric field. In the present work, we report a two-dimensional organic-inorganic hybrid compound of (C7H18N2)PbI4 (1) with a narrow band gap, which consists of [PbI4]2-n layers and N-(2-aminoethyl)piperidinium cations. 1 exhibits semiconducting properties with a narrow optical band gap of ∼2.02 eV and a photoelectric response with a ratio of photocurrent to dark current of ∼100. In addition, it exhibits a reversible solid-state phase transition at 228 K. This finding should inspire research into more 2D layered OIHPs with the combination of phase transition and semiconductor properties.

Successive Phase Transitions and Dual Dielectric Switching in an Organic-Inorganic Hybrid Perovskite.

Molecular phase transition compounds have become a hot research area in recent years because of their potential as functional materials, such as ferroelectrics, ferroelastics, dielectric switches, etc. However, materials combining switchable dielectric properties and ferroelasticity are still rare. Here, we reported an organic-inorganic hybrid perovskite, [CPtmp][Cd(SCN)3] (1) ([CPtmp]+ is a cyclopentyltrimethylphosphonium cation), with a potential ferroelastic property. This material undergoes three structural phase transitions at 247/226, 335/312, and 349/341 K (upon heating/cooling). The successive phase transitions are mainly caused by the stepwise ordering of [CPtmp]+ cations and the concomitant deformation of [Cd(SCN)3]- anionic chains revealed by structural analyses, which triggers the double-step dielectric switching in 1 as well. These results would inspire further exploration on molecular dielectric switches with ferroelastic properties.

Phosphonium-Based One-Dimensional Perovskite with Switchable Dielectric Behaviors and Phase Transitions.

The one-dimensional (1D) ABX3-type perovskite [(CH3)3PCH2F]CdCl2Br (1) has been obtained on the basis of the design of an organic-inorganic hybrid. Strikingly, it experiences sequential phase transitions at around 295 and 336 K, respectively. Given the noticeable steplike dielectric anomalies in the vicinity of 295 K, 1 is identified as a promising dielectric-switchable material. According to the single-crystal structure analysis, the order-to-disorder transformation of the [(CH3)3PCH2F]+ cation is the main reason for the phase transitions and the change of space group from the orthorhombic Pnma (No. 62) to the hexagonal P63/m (No. 176). This design of a perovskite structure will inspire more advances in the ever-growing field of switchable functional materials.

A one-dimensional switchable dielectric material with Pd uptake function: [(CH2)3NH2S]2BiCl5.

A semiconductor material [(CH2)3NH2S]2BiCl5 (1) exhibits dielectric switching and Pd uptake functions. The thioether group in 1 provides an opportunity for Pd uptake. After 1 adsorbs Pd, the dielectric switching disappears, so that Pd adsorption can be monitored by dielectric measurements. Moreover, the material becomes an insulator after adsorbing Pd.

A Three-Dimensional Molecular Perovskite Ferroelastic with Two-Step Switching of Quadratic Nonlinear Optical Properties Tuned by Molecular Chiral Design.

Molecular perovskite materials have recently attracted extraordinary interest from the academic community owing to their excellent multifunctional properties. Nevertheless, although massive efforts have been made, molecular ferroelastics with three-dimensional (3D) perovskite structures are still rare. Herein, we report two 3D organic-inorganic hybrid perovskites [(2-hydroxy-propyl)-tripropyl-ammonium][Mn(dca)3] (1) and [(2-hydroxy-1-methyl-ethyl)-tripropyl-ammonium] [Mn(dca)3] (2) [dca = dicyanamide, N(CN)2]. The different position of the chiral center results in a tremendous difference in the properties. Compound 1 displays only one phase transition; however, intriguingly, 2 has three phase transitions and represents ferroelastic behavior with exceptional two-step switching of quadratic nonlinear optical (NLO) properties. To the best of our knowledge, this is the first molecular ferroelastic with two-step switching of quadratic NLO properties. The results demonstrate that the molecular chiral design works, and this finding opens up a new avenue to designing multifunctional molecular perovskite materials.

Phase Transition and Band Gap Regulation by Halogen Substituents on the Organic Cation in Organic-Inorganic Hybrid Perovskite Semiconductors.

In the last decade, hybrid materials have received widespread attention. In particular, hybrid lead halide perovskite-type semiconductors are very attractive owing to their great flexibility in band gap engineering. Here, by using precise molecular modifications, three one-dimensional perovskite-type semiconductor materials are designed and obtained: [Me3 PCH2 X][PbBr3 ] (X=H, F, and Cl for compounds 1, 2, and 3, respectively). The introduction of a heavier halogen atom (F or Cl) to [Me4 P]+ increases the potential energy barrier required for the tumbling motion of the cation, hence achieving the transformation of the phase transition temperature from low temperature (192 K) to room temperature (285 K) and high temperature (402.3 K). Moreover, the optical band gaps reveal a broadening trend with 3.176 eV, 3.215 eV, and 3.376 eV along the H→F→Cl series, which is attributed to the formation of the structural distortion.

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.

Three-Dimensional Metal-Free Molecular Perovskite with a Thermally Induced Switchable Dielectric Response.

Temperature-responsive materials with switching physical properties have been widely researched. Among them, the switchable dielectric perovskite materials show potential applications in the electrical and electronic industries and even the intelligence industries. However, perovskite oxides and hybrid organic-inorganic perovskites, as the most representative switchable dielectric materials, are limited by bad biocompatibility. Herein, we report temperature-dielectric-responsive metal-free perovskite (H2dabco)(NH4)[BF4]3 constructed by the strategy of substituting the B site in the general formula ABX3 (doubly protonated 1,4-diazabicyclo[2.2.2]octane = H2dabco). Meanwhile, structurally similar hybrid material (H2dabco)Rb[BF4]3 was designed as a control. They exhibit similar phase-transition characteristics and dielectric response behaviors around 333 K. More interestingly, the ordered-disordered transformation of their organic "spherical" cations (H2dabco) was deemed to produce their phase transitions and dielectric response switching. Given its ability to generate a dielectric response, (H2dabco)(NH4)[BF4]3 will show the potential application of metal-free perovskite in a future thermal sensing device.

Two-Dimensional Organic-Inorganic Perovskite Ferroelectric Semiconductors with Fluorinated Aromatic Spacers.

Two-dimensional (2D) organic-inorganic perovskites (OIPs), with improved material stability over their 3D counterparts, are highly desirable for device applications. It is their considerable structural diversity that offers an unprecedented opportunity to engineer materials with fine-tuning functionalities. The isosteric substitution of hydrogen by an electronegative fluorine atom has been proposed as a useful route to improve the photovoltaic performance of 2D OIPs, whereas its valuable role in developing ferroelectricity is still waiting for further exploration. Herein, for the first time we applied fluorinated aromatic cations in extending the family of 2D OIP ferroelectrics, and successfully obtained [2-fluorobenzylammonium]2PbCl4 as a high-performance ferroelectric semiconductor. The failures in the nonferroelectric [4-fluorobenzylammonium]2PbCl4 and [3-fluorobenzylammonium]2PbCl4 demonstrate that the selective introduction of fluorine in correct structural positions is particularly essential. This work represents an unprecedented proof-of-concept in the use of fluorinated aromatic cations for the targeted design of excellent 2D OIP ferroelectrics, and is believed to inspire the future development of low-cost, high-efficiency, and stable device applications.

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.

H/F substituted perovskite compounds with above-room-temperature ferroelasticity: [(CH3)4P][Cd(SCN)3] and [(CH3)3PCH2F][Cd(SCN)3].

An organic-inorganic perovskite compound [(CH3)4P][Cd(SCN)3] (1) and its fluorine-substituted product [(CH3)3PCH2F][Cd(SCN)3] (2) exhibit ferroelastic phase transitions above room temperature. The very close van der Waals radii of H and F atoms ensure isomorphism of the crystal structures. However, the higher phase transition temperature, stronger ferroelastic spontaneous strain value and dielectric properties of 2 can possibly be explained by differences in the electronegativity between F and H atoms.

3D Organic-Inorganic Perovskite Ferroelastic Materials with Two Ferroelastic Phases: [Et3 P(CH2 )2 F][Mn(dca)3 ] and [Et3 P(CH2 )2 Cl][Mn(dca)3 ].

Organic-inorganic hybrid perovskite-type multiferroics have attracted considerable research interest owing to their fundamental scientific significance and promising technological applications in sensors and multiple-state memories. The recent achievements with divalent metal dicyanamide compounds revealed such malleable frameworks as a unique platform for developing novel functional materials. Herein, two 3D organic-inorganic hybrid perovskites [Et3 P(CH2 )2 F][Mn(dca)3 ] (1) and [Et3 P(CH2 )2 Cl][Mn(dca)3 ] (2) (dca=dicyanamide, N(CN)2 - ) are presented. Accompanying the sequential phase transitions, they display a broad range of intriguing physical properties, including above room temperature ferroelastic behavior, switchable dielectricity, and low-temperature antiferromagnetic ordering (Tc =2.4 K for both 1 and 2). It is also worth noting that the spontaneous strain value of 1 is far beyond that of 2 in the first ferroelastic phase, as a result of the precise halogen substitution. From the point view of molecular design, this work should inspire further exploration of multifunctional molecular materials with desirable properties.

Toward the Targeted Design of Molecular Ferroelectrics: Modifying Molecular Symmetries and Homochirality.

Although the first ferroelectric discovered in 1920 is Rochelle salt, a typical molecular ferroelectric, the front-runners that have been extensively studied and widely used in diverse applications, such as memory elements, capacitors, sensors, and actuators, are inorganic ferroelectrics with excellent electrical, mechanical, and optical properties. With the increased concerns about the environment, energy, and cost, molecular ferroelectrics are becoming promising supplements for inorganic ferroelectrics. The unique advantages of high structural tunability and homochirality, which are unavailable in their inorganic counterparts, make molecular systems a good platform for manipulating ferroelectricity. Remarkably, based on the Neumann's principle and the Curie symmetry principle defining the group-to-subgroup relationship, we have found some outstanding high-temperature molecular ferroelectrics, like diisopropylammonium bromide (DIPAB) with a large spontaneous polarization up to 23 µC/cm2 ( Fu, D. W.; et al. Science 2013 , 339 , 425 ). However, their application potential is severely limited by the uniaxial nature, leading to major issues in finding proper substrates for thin-film growth and achieving high thin-film performance. Inspired by the commercialized inorganic ferroelectrics like Pb(Zr, Ti)O3 (PZT), where the multiaxial nature contributes greatly to the optimized ferroelectric and piezoelectric performance, developing high-temperature multiaxial molecular ferroelectrics is an imminent task. In this Account, we review our recent research progress on the targeted design of multiaxial molecular ferroelectrics. We first propose the "quasi-spherical theory", a phenomenological theory based on the Curie symmetry principle, to modify the spherical cations to a low-symmetric quasi-spherical geometry for acquiring the highly symmetric paraelectric phase and the polar ferroelectric phase of multiaxial ferroelectrics simultaneously. Besides the sizes and weights of the cation and anion, the intermolecular interactions are particularly crucial for decelerating the molecular rotation at low temperature to reasonably induce ferroelectricity. It means that the momentums of the cation and anion should be matched, so we describe the "momentum matching theory". In particular, introducing homochirality, a superiority of molecular materials over the inorganic ones, was demonstrated as an effective approach to increase the incidence of ferroelectric crystal structures. Thanks to the striking chemical variability and structure-property flexibility of molecular materials, our research efforts outlined in this Account have led to and will further motivate the richness and the application exploration of high-temperature, high-performance multiaxial molecular ferroelectrics, along with the implementation and perfection of the targeted design strategies.

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

With regard to the artificial molecular motor that was recognized with the 2016 Nobel Prize, this success proves the great scientific significance of rotary motor-type motion at the molecular level, which has been expected to play an invaluable role in the development of electronic information molecular materials. However, designing electronic information-critical high-temperature molecular motors has always been a huge challenge. Since we discovered [(CH3)3NCH2Cl]MnCl3, this cation rotation pattern with a motor-type motion structure has continued to attract our attention. Considering a strategy that combines molecular machines with dielectric theory, ( N, N-dimethylpiperidinium)CdCl3, the new dielectric molecular motor material that exhibits superior physical properties, could be considered to be an excellent dielectric switch based on its electric field and temperature. Crystal structure analyses reveal that the reversible phase transition is mainly induced by the unusual chair-to-rotator motion of cations. Because of the unprecedented leaping structural transition from P63/ mmc to P21/ c and the rotating motor-type motion structure, the material exhibits remarkable anisotropy and outstanding dielectric switching characteristics. These findings open a new avenue for the design and assembly of novel molecular motor materials in the field of electronic information.

A molecular perovskite solid solution with piezoelectricity stronger than lead zirconate titanate.

Piezoelectric materials produce electricity when strained, making them ideal for different types of sensing applications. The most effective piezoelectric materials are ceramic solid solutions in which the piezoelectric effect is optimized at what are termed morphotropic phase boundaries (MPBs). Ceramics are not ideal for a variety of applications owing to some of their mechanical properties. We synthesized piezoelectric materials from a molecular perovskite (TMFM) x (TMCM)1- x CdCl3 solid solution (TMFM, trimethylfluoromethyl ammonium; TMCM, trimethylchloromethyl ammonium, 0 ≤ x ≤ 1), in which the MPB exists between monoclinic and hexagonal phases. We found a composition for which the piezoelectric coefficient d 33 is ~1540 picocoulombs per newton, comparable to high-performance piezoelectric ceramics. The material has potential applications for wearable piezoelectric devices.

Fluorine Substitution Induced High Tc of Enantiomeric Perovskite Ferroelectrics: ( R) - and ( S) -3-(Fluoropyrrolidinium)MnCl3.

The past decade has witnessed much progress in designing molecular ferroelectrics, whose intrinsic mechanical flexibility, structural tunability, and easy processability are desirable for next-generation flexible and wearable electronic devices. However, an obstacle in expanding their promising applications in nonvolatile memory elements, capacitors, and sensors is effectively modulating the Curie temperature ( Tc). Here, taking advantage of fluorine substitution on the reported molecular ferroelectric, (pyrrolidinium)MnCl3, we present enantiomeric perovskite ferroelectrics, namely, ( R) - and ( S) -3-(fluoropyrrolidinium)MnCl3. The close van der Waal's radii and the similar steric parameters between H and F atoms ensure the minimum disruption of the crystal structure, while their different electronegativity and polarizability can trigger significant changes in the physical and chemical properties. As expected, the Tc gets successfully increased from 295 K in (pyrrolidinium)MnCl3 to 333 K in these two homochiral compounds. Such a dramatic enhancement of 38 K signifies an important step toward designing high- Tc molecular ferroelectrics. In the light of the conceptually new idea of fluorine substitution, one could look forward to a continuous succession of new molecular ferroelectric materials and technology developments.

Molecular design of high-temperature organic dielectric switches.

A design strategy of reducing the molecular symmetry was used to obtain a series of picrate-based high-temperature phase transition compounds. Their dielectric switching behaviours accompanied by phase transitions can be attributed to the order-disorder transitions of the cations and the displacements of both cations and anions.