Tunable and Switchable Dielectric Constant in an Amphidynamic Crystal

Computational applications for the discovery of novel antiperovskites and chalcogenide perovskites: a review

Novel perovskites pertain to newly discovered or less studied variants of the conventional perovskite structure, characterized by distinctive properties and potential for diverse applications such as ferroelectric, optoelectronic, and thermoelectric uses. In recent years, advancements in computational methods have markedly expedited the discovery and design of innovative perovskite materials, leading to numerous pertinent reports. However, there are few reviews that thoroughly elaborate the role of computational methods in studying novel perovskites, particularly for state-of-the-art perovskite categories. This review delves into the computational discovery of novel perovskite materials, with a particular focus on antiperovskites and chalcogenide perovskites. We begin with a discussion on the computational methods applied to evaluate the stability and electronic structure of materials. Next, we highlight how these methods expedite the discovery process, demonstrating how rational simulations contribute to researching novel perovskites with improved performance. Finally, we thoroughly discuss the remaining challenges and future outlooks in this research domain to encourage further investigation. We believe that this review will be highly beneficial both for newcomers to the field and for experienced researchers in computational science who are shifting their focus to novel perovskites.

Durable Photo-Pyroelectric Detection in a Diamine-Constructed Lead-Free Hybrid Perovskite Ferroelectric

As a subcategory of pyroelectric materials, hybrid perovskite ferroelectrics possess substantial pyroelectric properties and exceptional light absorption characteristics, demonstrating significant potential in the photo-pyroelectric (PPE) detection field. Despite the significant advantages of hybrid perovskite ferroelectric materials for PPE detection, both the lead issue and the weak stability from van der Waals interactions in monoamines have hindered their further application. Here, 1D lead-free ferroelectric (BDA)SbBr5 (1, where BDA is 1,4-butanediammonium) is fabricated to achieve PPE detection. Compound 1 exhibits significant symmetry breaking attributed to the order-disorder transition of organic cations and octahedral distortions. Specifically, compound 1 enables broad-spectrum PPE detection from UV to near-infrared (377-980 nm) and further realizes switchable pyroelectric current after polarization. More importantly, the stability of the pyroelectric current is preserved without degradation over three months, attributed to the hydrogen bonding interactions of butanediamide. Further theoretical calculations of compound 1 reveal a more negative energy of formation than its monoamine homologs (BA)2SbBr5 (where BA is n-butylammonium), which is evidence of its stability. These findings highlight 1 as a promising candidate for high-stability and environmentally friendly PPE wide-spectrum detection, representing a noteworthy advancement in the ferroelectric field.

Image of the solid-state rotary motion encoded in the dielectric response

The future development of advanced molecular systems with controlled rotation requires the development of an effective methodology for assessing the rotational performance of artificial machine components. We identified two patterns of the dielectric behavior for polar rotators in a static non-polar framework of sizable crystal showing relations between the spectral and molecular-level features of solid-state rotary motion. Various functionalization of phenylene rotors with a fluorine atom(s) changed rotational performance from high to low with rotational barriers ranging from 6.06 to 11.84 kcal mol-1. The meta-F-substitution favored rotator-rotator contacts allowing for the implementation of fast rotary motion. Contrary, the presence of rotator-stator contacts inhibited independent rotator dynamics leading to opposite spectral behavior in terms of temperature evolution of loss peak amplitude. Our observations, supported by an analysis based on an asymmetric double well-potential model, show that easily noticeable spectral differences encoded some molecular-level information important for the implementation of rotary motion.

Influence of O-H⋅⋅⋅Pt interactions on photoluminescent response in the (Et4N)2{[Pt(bph)(CN)2][phenylene-1,4-diresorcinol]} framework

Tunable photoluminescence (PL) is one of the hot topics in current materials science, and research performed on the molecular phases is at the forefront of this field. We present the new (Et4N)2[PtII(bph)(CN)2]⋅rez3⋅1/3H2O (Pt2rez3) (bph=biphenyl-2,2'-diyl; rez3=3,3",5,5"-tetrahydroxy-1,1':4',1"-terphenyl, phenylene-1,4-diresorcinol coformer, a linear quaternary hydrogen bond donor) co-crystal salt based on the recently appointed promising [PtII(bph)(CN)2]2- luminophore. Within the extended hydrogen-bonded subnetwork [PtII(bph)(CN)2]2- complexes and rez3 coformer molecules form two types of contacts: the rez3O-H⋅⋅⋅Ncomplex ones in the equatorial plane of the complex and non-typical rez3O-H⋅⋅⋅Pt ones along its axial direction. The combined structural, PL, and DFT approach identified the rez3O-H⋅⋅⋅Pt synthons to be crucial in promoting the noticeable uniform redshift of bph ligand centered (LC) emission compared to the LC emission of the (Et4N)2[PtII(bph)(CN)2]⋅H2O (Pt2) precursor, owing to the direct interference of the phenol group with the PtII-bph orbital system via altering the CT processes within. The high-resolution emission spectra for Pt2 and Pt2rez3 were successfully reproduced at 77 K by using the Franck-Cordon expressions. The possibility to tune PL properties along the plausible continuum of rez3O-H⋅⋅⋅Pt synthons is indicated, considering various scenarios of molecular occupation of the space above and below the complex plane.

Molecular Dynamics of Steroidal Rotors Probed by Theoretical, Spectroscopic and Dielectric Methods

Our study focuses on molecular rotors with fast-moving rotators and their potential applications in the development of new amphidynamic crystals. Steroidal molecular rotors with a dipolar fluorine-substituted phenyl group as the rotator were synthesized and characterized. Three different rotors were investigated with varying numbers of fluorine atoms. A comprehensive analysis was performed using vibrational spectroscopy (Raman, FT-IR), electronic circular dichroism (ECD), and dielectric response to understand the behavior of the investigated model rotors. The results were supported by theoretical calculations using Density Functional Theory (DFT) methods. The angle-dependent polarized Raman spectra confirmed the crystallinity of the samples. Nearly frequency and temperature-independent permittivity suggest low-frequency librational motion of stators. An in-depth analysis of ECD spectra revealed high conformational flexibility in solution, resulting in low ECD effects, while in the solid-state with restricted rotation, significant ECD effects were observed. These findings shed light on the conformational behavior and potential applications of the studied steroidal molecular rotors.

Nonlinear and Emissive {[MIII(CN)6]3-···Polyresorcinol} (M = Fe, Co, Cr) Cocrystals Exhibiting an Ultralow Frequency Raman Response

Optically active functional noncentrosymmetric architectures might be achieved through the combination of molecules with inscribed optical responses and species of dedicated tectonic character. Herein, we present the new series of noncentrosymmetric cocrystal salt solvates (PPh4)3[M(CN)6](L)n·msolv (M = Cr(III), Fe(III), Co(III); L = polyresorcinol coformers, multiple hydrogen bond donors: 3,3',5,5'-tetrahydroxy-1,19-biphenyl, DiR, n = 2, or 5'-(3,5-dihydroxyphenyl)-3,3″,5,5″-tetrahydroxy-1,19:3',1″-terphenyl, TriRB, n = 1) denoted as MDiR and MTriRB, respectively. The hydrogen-bonded subnetworks {[M(CN)6]3-;Ln}∞ of dmp, neb, or dia topology are formed through structural matching between building blocks within supramolecular cis-bis(chelate)-like {[M(CN)6]3-;(H2L)2(HL)2} or tris(chelate)-like {[M(CN)6]3-;(H2L)3} fragments. The quantum-chemical analysis demonstrates the mixed electrostatic and covalent character of these interactions, with their strength clearly enhanced due to the negative charge of the hydrogen bond acceptor metal complex. The corresponding interaction energy is also dependent on the geometry of the contact and size matching of its components, rotational degree of freedom and extent of the π-electron system of the coformer, and overall fit to the molecular surroundings. Symmetry of the crystal lattices is correlated with the local symmetry of coformers and {complex;(coformer)n} hydrogen-bonded motifs characterized by the absence of the inversion center and mirror plane. All compounds reveal second-harmonic generation activity and photoluminescence diversified by individual UV-vis spectral characteristics of the components, and interesting low-frequency Raman scattering spectra within the subterahertz spectroscopic domain. Vibrational (infrared/Raman), UV-vis electronic absorption (experimental and calculated), and 57Fe Mössbauer spectra together with electrospray ionization mass spectrometry (ESI-MS) data are provided for the complete description of our systems.

Exceptional structural phase transition near room temperature in an organic-inorganic hybrid ferroelectric

An organic-inorganic hybrid ferroelectric, (C6H5CH2CH2NH3)2[HgI4], undergoes an exceptional structural phase transition near room temperature, triggered by a flip of half the organic cations and an order-disorder transition of the inorganic anions, and may be regarded as a displacive-type ferroelectric. This finding provides a new structural phase transition mechanism in molecule-based ferroelectrics.

Synchronous Switching of Dielectric Constant and Photoluminescence in Cyanidonitridorhenate-Based Crystals

Switching of multiple physical properties by external stimuli in dynamic materials enables applications in, e.g., smart sensors, biomedical tools, as well as data-storage devices. Among stimuli-responsive materials, inorganic-organic molecular hybrids exhibiting thermal order-disorder phase transitions were tested as promising molecular switches of electrical characteristics, including dielectric constant. We aimed at broadening the multifunctional potential of such hybrid materials towards the switching of not only electrical but also other physical properties, e.g., light emission. We report two ionic salts based on luminescent tetracyanidonitridorhenate(V) anions bearing two different diamine ligands, 1,2-diaminoethane (1) and 1,3-diaminopropane (2), both crystallizing with polar N-methyl-dabconium cations. They exhibit an order-disorder phase transition related to the heating-induced turning-on of the rotation of polar cations. This leads to a unique synchronous switching of the dielectric constant as well as metal-complex-centered photoluminescence, as demonstrated by changes in, e.g., emission lifetime. The roles of organic cations, non-trivial Re(V) complexes, and their interaction in achieving the coupled thermal switching of electrical and optical properties are discussed utilizing experimental and theoretical approaches.

Raman study of pressure-induced phase transitions in imidazolium manganese- hypophosphite hybrid perovskite

By using Raman spectroscopy, we demonstrate that [IM]Mn(H₂POO)₃ is a highly compressible material that undergoes three pressure-induced phase transitions. Using a diamond anvil cell we performed high-pressure experiments up to 7.1 GPa, using paraffin oil as the compression medium. The first phase transition, which occurs near 2.9 GPa, leads to very pronounced changes in the Raman spectra. This behavior indicates that this transition is associated with very large reconstruction of the inorganic framework and collapse of the perovskite cages. The second phase transition, which occurs near 4.9 GPa, is associated with subtle structural changes. The last transition takes place near 5.9 GPa and it leads to further significant distortion of the anionic framework. In contrast to the anionic framework, the phase transitions have weak impact on the imidazolium cation. Pressure dependence of Raman modes proves that compressibility of the high-pressure phases is significantly lower compared to the ambient pressure phase. It also indicates that the contraction of the MnO₆ octahedra prevails over that of the imidazolium cations and hypophosphite linkers. However, compressibility of MnO₆ strongly decreases in the highest pressure phase. Pressure-induced phase transitions are reversible.

A hybrid double perovskite ferroelastic exhibiting the highest number of orientation states

Integrating NH4+ as a B'-site ion within a three-dimensional double hybrid perovskite resulted in a novel high-temperature ferroelastic, (Me3NOH)2(NH4)[Co(CN)6], which uniquely demonstrates a reversible triclinic-to-cubic phase transition at 369 K and offers a record-setting 24 orientation states, the highest ever reported among all ferroelastics.

Solid-to-solid polymorphic phase transitions in two isostructural Bi(III) complexes with 1-phenylethyl-N-ethylthiosemicarbazide and halogens

Two isostructural (in room temperature) complexes of Bi(III) with halogens and sulfur ligands have been investigated in terms of the solid-to-solid phase transitions indicated by temperature. Both chloride and bromide (X) complexes of the general formula (µ₂-X)-(BiX₂L₂)₂ exhibit some phase transitions between 100 and 333 K, which, apart from the numerous similarities, show significant differences, which have been noted and analyzed in detail in this paper by using different techniques, i.e., powder and single crystal diffraction or DSC. The obtained results have also been collated with those obtained for solid solutions of both complexes.

Enhanced proton conductivity by guest molecule exchange in an acylamide-functionalized metal-organic framework

Metal-organic frameworks (MOFs) as types of proton conductive materials have attracted much attention. Here, an acylamide-functionalized 3D MOF, [Ni₃(TPBTC)₂(stp)₂(H₂O)₄]·2DMA·32H₂O, has been successfully constructed via combining Ni(NO₃)₂, TPBTC (TPBTC = benzene-1,3,5-tricarboxylic acid tris-pyridin-4-ylamide) and 2-H₂stp (2-H₂stp = 2-sulfoterephthalic acid monosodium salt) under solvothermal conditions. Single-crystal X-ray diffraction revealed that there are uncoordinated guest DMA molecules in the pores of the compound. On removal of guest DMA molecules, the proton conductivity of the compound increased to 2.25 × 10^-3 S cm^-1 at 80 °C and 98% RH which is about 110 times that of the original material. It is hoped that this work can provide essential insight for designing and obtaining improved crystalline-state proton conducting materials by considering the influences of guest molecules on proton conduction properties of porous materials.

Mechanistic View on the Order-Disorder Phase Transition in Amphidynamic Crystals

We combine temperature-dependent low-frequency Raman measurements and first-principles calculations to obtain a mechanistic understanding of the order-disorder phase transition of 2,7-di-tert-butylbenzo[b]benzo[4,5]thieno[2,3-d]thiophene (ditBu-BTBT) and 6,13-bis(triisopropylsilylethynyl) pentacene (TIPS-pentacene) semiconducting amphidynamic crystals. We identify the lattice normal modes associated with the phase transition by following the position and width of the Raman peaks with temperature and identifying peaks that exhibit nonlinear dependence toward the phase transition temperature. Our findings are interpreted according to the "hardcore mode" model previously used to describe order-disorder phase transitions in inorganic and hybrid crystals with a Brownian sublattice. Within the framework of this model, ditBu-BTBT exhibits an ideal behavior where only one lattice mode is associated with the phase transition. TIPS-pentacene deviates strongly from the model due to strong interactions between lattice modes. We discuss the origin of the different behaviors and suggest side-chain engineering as a tool to control polymorphism in amphidynamic crystals.

K2CdGe3S8: A New Infrared Nonlinear Optical Sulfide

A quaternary metal chalcogenide, namely K2CdGe3S8 (I), is obtained through a high-temperature solid-state approach. Compound I crystallizes with the non-centrosymmetric space group P212121. It features a 2D layer structure with [CdGe3S8] layers consisting of tetrahedral GeS4 and CdS4 units, and counter K+ embedded between the layers. The compound exhibits a powder second-harmonic generation (SHG) response of ~0.1 times that of KH2PO4 (KDP) with phase-matchable character at the laser wavelength of 1064 nm. Remarkably, it has a wide band gap (3.20 eV), which corresponds to a favorable high laser-induced damage threshold of 6.7 times that of AgGaS2. In addition, the calculated birefringence (Δn) is 0.039 at the wavelength of 1064 nm, which satisfies the Δn criteria for a promising infrared NLO material.

Multi-step phase transition crystal with dielectric constant bistability and temperature-dependent conductivity

Here, we report the crystal structures, phase transitions, and thermal, dielectric, and conducting properties of an ion-pair compound [C4-bmim][Ni(mnt)2] (1). 1 undergoes a three-step phase transition with four phases before melting. A two-step dielectric constant bistability is also realized by the structural phase transition in 1 occurring among phases I, II, and III, which is due to the in-plane oscillations of the alkyl chain and crystal-to-mesophase transition, respectively. Moreover, 1 exhibits rare temperature-dependent conducting properties accompanying structural phase transitions, and conductivity is very high with 0.00186 S cm-1 at 413 K. The conduction properties of phase III (mesophase) arise from the dipole molecular motion.

Supramolecular cis-“Bis(Chelation)” of [M(CN)6]3− (M = CrIII, FeIII, CoIII) by Phloroglucinol (H3PG)

Studies on molecular co-crystal type materials are important in the design and preparation of easy-to-absorb drugs, non-centrosymmetric, and chiral crystals for optical performance, liquid crystals, or plastic phases. From a fundamental point of view, such studies also provide useful information on various supramolecular synthons and molecular ordering, including metric parameters, molecular matching, energetical hierarchy, and combinatorial potential, appealing to the rational design of functional materials through structure–properties–application schemes. Co-crystal salts involving anionic d-metallate coordination complexes are moderately explored (compared to the generality of co-crystals), and in this context, we present a new series of isomorphous co-crystalline salts (PPh4)3[M(CN)6](H3PG)2·2MeCN (M = Cr, 1; Fe, 2; Co 3; H3PG = phloroglucinol, 1,3,5-trihydroxobenzene). In this study, 1–3 were characterized experimentally using SC XRD, Hirshfeld analysis, ESI-MS spectrometry, vibrational IR and Raman, 57Fe Mössbauer, electronic absorption UV-Vis-NIR, and photoluminescence spectroscopies, and theoretically with density functional theory calculations. The two-dimensional square grid-like hydrogen-bond {[M(CN)6]3−;(H3PG)2}∞ network features original {[M(CN)6]3−;(H3PG)4} supramolecular cis-bis(chelate) motifs involving: (i) two double cyclic hydrogen bond synthons M(-CN⋅⋅⋅HO-)2Ar, {[M(CN)6]3−;H2PGH}, between cis-oriented cyanido ligands of [M(CN)6]3− and resorcinol-like face of H3PG, and (ii) two single hydrogen bonds M-CN⋅⋅⋅HO-Ar, {[M(CN)6]3−;HPGH2}, involving the remaining two cyanide ligands. The occurrence of the above tectonic motif is discussed with regard to the relevant data existing in the CCDC database, including the multisite H-bond binding of [M(CN)6]3− by organic species, mononuclear coordination complexes, and polynuclear complexes. The physicochemical and computational characterization discloses notable spectral modifications under the regime of an extended hydrogen bond network.

A series of dynamic single crystals of [MII(en)3]SO4 (M = Ni, Mn, and Cd) shows tunable dielectric properties and anisotropic thermal expansion

A series of dynamic single crystals with a chemical formula of [M^II(en)₃]SO₄ (en = ethylene and M^II = Ni^II, Mn^II, and Cd^II) was synthesized. As the temperature decreases, these materials exhibit dielectric switching in the vicinity of the phase transition point accompanied by anisotropic thermal expansion in the cell parameters as a consequence of the order-disorder structural change of SO2-4 in a cavity surrounded by five [M^II(en)₃]²⁺ complex cations. Because the variation of metal centers with different ionic radii changes the shape of the complex cation, which affects the distribution of hydrogen-bond interactions around the SO2-4, the dynamic motion of SO2-4 is substantially tuned. Correspondingly, the dielectric properties and anisotropic thermal expansion of materials were largely shifted, especially in the single crystals of [Mn^II(en)₃]SO₄, whose structural change is distinctly different from the crystals of Ni(II) and Cd(II). The detailed structural mechanism accounting for the different physical properties of these materials was discussed.

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.

Anisotropic fluid with phototunable dielectric permittivity

Dielectric permittivity, a measure of polarisability, is a fundamental parameter that dominates various physical phenomena and properties of materials. However, it remains a challenge to control the dielectric permittivity of materials reversibly over a large range. Herein, we report an anisotropic fluid with photoresponsive dielectric permittivity (200 < ε < 18,000) consisting of a fluorinated liquid-crystalline molecule (96 wt%) and an azobenzene-tethered phototrigger (4 wt%). The reversible trans-cis isomerisation of the phototrigger under blue and green light irradiation causes a switch between two liquid-crystalline phases that exhibit different dielectric permittivities, with a rapid response time (<30 s) and excellent reversibility (~100 cycles). This anisotropic fluid can be used as a flexible photovariable capacitor that, for example, allows the reversible modulation of the sound frequency over a wide range (100 < f < 8500 Hz) in a remote manner using blue and green wavelengths.

Light stimuli are widely used to control material properties, yet it remains challenging to reversibly photocontrol the dielectric permittivity. Nishikawa et al. achieve this goal in an anisotropic fluid via its liquid crystal phase transition induced by isomerization of an azobenzene-tethered phototrigger.

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.

Near-Infrared Phosphorescent Hybrid Organic-Inorganic Perovskite with High-Contrast Dielectric and Third-Order Nonlinear Optical Switching Functionalities

Hybrid organic-inorganic perovskites providing integrated functionalities for multimodal switching applications are widely sought-after materials for optoelectronics. Here, we embark on a study of a novel pyrrolidinium-based cyanide perovskite of formula (C4H10N)2KCr(CN)6, which displays thermally driven bimodal switching characteristics associated with an order-disorder phase transition. Dielectric switching combines two features important from an application standpoint: high permittivity contrast (Δε' = 38.5) and very low dielectric losses. Third-order nonlinear optical switching takes advantage of third-harmonic generation (THG) bistability, thus far unprecedented for perovskites and coordination polymers. Structurally, (C4H10N)2KCr(CN)6 stands out as the first example of a three-dimensional stable perovskite among formate-, azide-, and cyanide-based metal-organic frameworks comprising large pyrrolidinium cations. Its stability, reflected also in robust switching characteristics, has been tracked down to the Cr3+ component, the ionic radius of which provides a large enough metal-cyanide cage for the pyrrolidinium cargo. While the presence of polar pyrrolidinium cations leads to excellent switchable dielectric properties, the presence of Cr3+ is also responsible for efficient phosphorescence, which is remarkably shifted to the near-infrared region (770 to 880 nm). The presence of Cr3+ was also found indispensable to the THG switching functionality. It is also found that a closely related cobalt-based analogue doped with Cr3+ ions displays distinct near-infrared phosphorescence as well. Thus, doping with Cr3+ ions is an effective strategy to introduce phosphorescence as an additional functional property into the family of cobalt-cyanide thermally switchable dielectrics.

Near-room-temperature dielectric switch and thermal expansion anomaly in a new hybrid crystal: (Me2NH2)[CsFe(CN)5(NO)]

A new hybrid crystal, (Me2NH2)[CsFe(CN)5(NO)], featuring a double-layered nitroprusside-based inorganic framework with cubic-like cages encapsulating organic cations, undergoes a near-room-temperature phase transition accompanying with dielectric switch and thermal expansion anomaly....

High-Temperature Phase Transition Containing Switchable Dielectric Behavior, Long Fluorescence Lifetime, and Distinct Photoluminescence Changes in a 2D Hybrid CuBr4 Perovskite

A novel organic-inorganic hybrid perovskite crystal, [ClC₆H₄(CH₂)₂NH₃]₂CuBr₄ (1), having experienced an invertible high-temperature phase transition near T_c (the Curie temperature T_c = 355 K), has been successfully synthesized. The phase-transition characteristics for compound 1 are thoroughly revealed by specific heat capacity (C_p), differential thermal analysis, and differential scanning calorimetry tests, possessing 16 K broad thermal hysteresis. Multiple-temperature powder X-ray diffraction analysis further proves the phase-transition behavior of compound 1. Moreover, compound 1 exhibits a significant steplike dielectric response near T_c, revealing that it can be deemed to be a promising dielectric switching material. The variable-temperature fluorescence experiments show distinct photoluminescence (PL) changes of compound 1. Further investigation and calculation disclose that the fluorescence lifetime of compound 1 can reach as long as 55.46 μs, indicating that it can be a potential PL material. All of these researches contribute a substitutable avenue in the design and construction of neoteric phase-transition compounds combining high Curie temperature and PL properties.

Cocrystalline Polymer Films Exhibiting Second-Order Nonlinear Optical Properties

Tailored polymer materials exhibiting high-glass transition temperatures, cross-linked matrices, and/or strong intermolecular interactions containing electric-field poled nonlinear optical (NLO) chromophores are promising materials for applications in optical telecommunication, high-performance computing, and data transmission. Although the current design parameters have led to significant advances in NLO materials, we introduce an alternative, yet highly effective, approach in which a NLO chromophore is cocrystallized with a polymer, forming a noncentrosymmetric hybrid host-guest complex. Specifically, poly(ethylene oxide) (PEO) and 2-chloro-4-nitroaniline (CNA) will cocrystallize and exhibit second harmonic generation (SHG) activity due to the formation of a noncentrosymmetric cocrystalline unit cell where the chromophore exhibits acentric alignment. Furthermore, the hybrid PEO/CNA films exhibit interesting SHG activity at elevated temperature in which SHG intensity decreases to zero when the cocrystal orientation randomizes due to sample melting. Aligning and maintaining a cocrystalline domain orientation via the formation of hybrid host-guest complexes, while imparting SHG properties, is an innovative approach for creating materials exhibiting SHG properties.

Solid solutions of flexible host-guest supramolecules for tuning molecular motion and phase transitions

By utilizing a supramolecular complex rather than an individual molecule as a deformable and elastic substitutional component, we put forward a solid-solution strategy and demonstrate an example of how two related yet non-isostructural crystalline host-guest compounds can form molecular solid solutions. Interestingly, such a strategy can effectively and continuously modulate the molecular motion and phase transition in them, as revealed by the variable-temperature/frequency dielectric responses.

Mechanism of Unusual Isosymmetric Order-Disorder Phase Transition in [Dimethylhydrazinium]Mn(HCOO)3 Hybrid Perovskite Probed by Vibrational Spectroscopy

[DMHy]Mn(HCOO)₃ (DMHy⁺ = dimethylhydrazinium cation) is an example of an organic–inorganic hybrid adopting perovskite-like architecture with the largest organic cation used so far in the synthesis of formate-based hybrids. This compound undergoes an unusual isosymmetric phase transition at 240 K on heating. The mechanism of this phase transition has a complex nature and is mainly driven by the ordering of DMHy⁺ cations and accompanied by a significant distortion of the metal–formate framework in the low temperature (LT) phase. In this work, the Density Functional Theory (DFT) calculations and factor group analysis are combined with experimental temperature-dependent IR and Raman studies to unequivocally assign the observed vibrational modes and shed light on the details of the occurring structural changes. The spectroscopic data show that this first-order phase transition has a highly dynamic nature, which is a result of balanced interplay combining re-arrangement of the hydrogen bonds and ordering of DMHy⁺ cations. The tight confinement of organic cations forces simultaneous steric deformation of formate ions and the MnO₆ octahedra.

Adjusting Rotational Behavior of Molecular Rotors by a Rational Tuning of Molecular Structure

Many crystalline molecular rotors have been developed in the past decades. However, manipulating the rotational gesture that intrinsically controls the physical performance of materials remains a challenge. Herein, we report a series of crystalline rotors whose rotational gestures can be modulated by modifying the structures of molecular stators. In these dynamic crystals, the ox^2- (ox^2- = oxalate anion) behave as molecular rotators performing axial-free rotation in cavities composed of five complex cations, [M^II(en)₃]²⁺ (en = ethylenediamine). The structure of [M^II(en)₃]²⁺ that serves as a molecular stator can be tuned by varying the metal center with different ionic radii, consequently altering the chemical environment around the molecular rotator. Owing to the quasi-transverse isotropy of ox^2- and multiple hydrogen-bond interactions around it, the molecular rotator exhibits unusual motional malleability, i.e., it can rotate either longitudinally in the compound of Zn^II, or with a tilt angle of 42° in the compound of Fe^II, or even laterally in the compound of Cd^II. The atypical dynamic behavior demonstrated here provides a new chance for the development of exquisite crystalline molecular rotors with advanced tunable functionalities.

Solvent-Induced Structural Transformation and Luminescence Response in a Dumbbell-Shaped Crystalline Molecular Rotor

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.

The spectroscopic study of phase transitions in the series of cyanide perovskites

The series of MeA₂KFe(CN)₆, where MeA = CH₃NH₃⁺, (CH₃)₂NH₂⁺, (CH₃)₃NH⁺ and (CH₃)₄N⁺, has been studied by IR and Raman spectroscopy as the function of temperature in order to elucidate the mechanisms of the phase transitions. The order-disorder process has been confirmed in all cases. Different models have been proposed based on the dynamic effects observed in the spectra. The crystal containing (CH₃)₂NH₂⁺ cations constitutes a model with melt-like thermal behavior and strongly temperature-influenced hydrogen bonding. In the case of sample with (CH₃)₄N⁺ an unperturbed rotation of these cations is observed, while in the crystals with methyl- and trimethylammonium cations the hydrogen bonds acting as positional stabilizers prevent the organic cation from a completely free motion. Additionally, the statistical disorder of dimethyl- and trimethylammonium cations has been confirmed by the thermal evolution of the FWHM of the related bands.

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.

Improving proton conduction of the Prussian blue analogue Cu3[Co(CN)6]2·nH2O at low humidity by forming hydrogel composites

Composites of Prussian blue analogue (PBA) adsorbed imidazole-acetic acid with polyvinyl alcohol hydrogel show excellent water-retention capacity and fast proton conduction at 25% RH in 298–353 K, herein X is the mass ratio of PBA to hydrogel.

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.

Steroidal Molecular Rotors with 1,4-Diethynylphenylene Rotators: Experimental and Theoretical Investigations Toward Seeking Efficient Properties

Properly designed molecular rotors with sizable stators and a fast-moving rotator could provide efficient building blocks for amphidynamic crystals. Herein, we report the synthesis of steroidal compounds 1, 2, and 3 and their deuterated analogues 1D, 2D, and 3D envisioned to work as molecular rotors. The obtained compounds were characterized by attenuated total reflection-infrared, Raman, and circular dichroism (CD) spectroscopy measurements. The interpretation of spectra was supported by theoretical calculations using density functional theory methods. The analysis of the most characteristic bands confirmed different molecular dynamics of the rotors investigated. Angle-dependent polarized Raman spectra showed the crystallinity of some samples. Electronic CD (ECD) spectra of compounds 1–3 and their relevant deuterated analogues 1D–3D are identical. The increase of the band intensity with lowering the temperature shows that the equilibrium is shifted to the thermodynamically most stable conformer. ECD spectra simulated at the TDFFT level of theory for compound 3 were compared with experimental results. It was proved that conformer 3a, with a torsion angle of +50°, exhibits the best agreement with the experimental results. Simulated vibrational CD and IR spectra for conformer 3a and its deuterated analogue 3Da also display good agreement with experimental results. In light of our comprehensive investigations, we evidenced that steroidal compounds 1, 2, and 3 can work as molecular rotors.

Rotational dynamics of the imidazolium ion in cyanide-bridged dielectric framework materials

Reorientation of organic cations in the cubic interstices of cyanoelpasolite molecular perovskites results in a variety of structural phase transitions, but far less is known about these cations' dynamics. We report quasielastic neutron scattering from the materials (C₃H₅N₂)₂K[M^III(CN)₆], M = Fe,Co, which is directly sensitive to the rotation of the imidazolium ion. The motion is well described by a circular three-site hopping model, with the ion rotating within its plane in the intermediate-temperature phase, but tilting permanently in the high-temperature phase. Thus the two rhombohedral phases, which are crystallographically rather similar, have markedly different dynamics. The activation energy of rotation is about 10 kJ mol^-1 and the barrier between orientations is 6 kJ mol^-1. Our results explain two anomalous features in these materials' dielectric constants.