Unusual high-temperature reversible phase transition containing dielectric and nonlinear optical switches in host-guest supramolecular crown ether clathrates

Polymorphism and phase transformation tuned luminescence and mechanistic insights in nonconventional luminophores

Nonconventional luminophores (NCLs) are attracting significant attention for their unique properties and applications. However, the lack of a comprehensive mechanistic understanding impedes their further development. Particularly, a recurring assumption that impurities are responsible for the luminescence has hindered progress. To elucidate the emission mechanism, we report tunable intrinsic emission from highly purified gemini aliphatic quaternary ammonium salts (GAQASs), leveraging their polymorphism. We demonstrate that polymorphism-dependent luminescence arises from distinct molecular packings and consequent varied clustering states. Specifically, denser ion clustering enhances charge transfer and recombination, heavy atom effects and conformational rigidity, thereby accelerating radiative triplet decay and intersystem crossing, while suppressing nonradiative triplet decay, ultimately leading to enhanced phosphorescence. Furthermore, GAQAS crystals undergo irreversible phase transformations upon heating, which partially disrupt intermolecular interactions, thus allowing for tunable emission. This polymorphism and phase transformation regulated luminescence in GAQASs strongly suggests that intrinsic factors, rather than impurities, are responsible for the observed emission, and are consistent with the clustering-triggered emission mechanism. Our findings establish a direct link between molecular packing, electronic structure and luminescent properties in NCLs. This study advances the mechanistic understanding of NCL luminescence, demonstrating an effective strategy for tunable emission via polymorphism and phase transformation.

Anionic modulation induces molecular polarity in a three-component crown ether system

Three-component crown ether phase change materials are characterized by a structural phase change in response to external stimuli such as temperature and electric or magnetic fields, resulting in significant changes in physical properties. In this work, we designed and synthesized two novel host-guest crown ether molecules [(PTFMA)(15-crown-5)ClO4] (1) and [(PTFMA)(15-crown-5)PF6] (2), through the reaction of p-trifluoromethylaniline (PTFMA) with 15-crown-5 in perchloric acid or hexafluorophosphoric acid aqueous solution. Compound 1 undergoes a structural change from the non-centrosymmetric space group (P21) to the centrosymmetric space group (P21/c) with increasing temperature. This transformation is accompanied by a switchable second harmonic generation (SHG) signal, a noticeable dielectric response, and a reversible phase transition in differential scanning calorimetry (DSC). For compound 2, there are reversible phase transitions accompanied by a dielectric response, but it does not exhibit a switchable SHG signal. The difference in properties between the two compounds may be due to the polarity modulation of the anion, providing new ideas for obtaining crown ether complexes with SHG response properties.

Plastic phase transitions in tris(hydroxymethyl)aminomethane perchlorate

Organic crystal materials with metal-free feature and intrinsically low molecular mass are highly desirable for applications in flexible smart devices. Here, we reported a plastic crystal, tris(hydroxymethyl)aminomethane perchlorate (Tris-HClO4), which crystallizes in the R3̄ space group at room temperature and undergoes plastic phase transition at 369 K, showing a large entropy gain of 70.5 J mol-1 K-1, much higher than its fusion entropy gain (12.9 J mol-1 K-1). PXRD measurement indicates that it has cubic lattice symmetry in the high-temperature phase. Moreover, it exhibits excellent dielectric permittivity switching properties and robust cyclic stability. This work could be the pathway for chemical designing multifunctional switchable materials with the motive of combining the idea of symmetry breaking and plastic phase transition.

Supramolecular crystals of Mn(15-crown-5)(MnCl4)(DMF) with dielectric phase transition, high quantum yield and phase transition-induced luminescence enhancement behavior

The supramolecular crystals, Mn(15-crown-5)(MnCl4)(DMF), (1; 15-crown-5 = 1,4,7,10,13-pentaoxacyclopentadecane), were synthesized via a self-assembly strategy under ambient conditions. Comprehensive characterization of the crystals involved microanalysis for C, H, and N elements, thermogravimetric (TG) analysis, differential scanning calorimetry (DSC) and single-crystal X-ray diffraction techniques. The results reveal that 1 undergoes a two-step thermotropic and isostructural phase transition at around 217 K and 351 K upon heating. All three phases belong to the same space group (P212121) with analogous cell parameters. These two phase transitions primarily involve the thermally activated ring rotational dynamics of the 15-crown-5 molecule, with only the transition at ca. 351 K being associated with a dielectric anomaly. 1 exhibits intense luminescence with a peak at ∼600 nm and a high quantum yield of 68%. The mechanisms underlying this intense luminescence are likely linked to low-symmetry ligand fields. Additionally, 1 displays phase transition-induced luminescence enhancement behavior, and the possible mechanism is further discussed.

Molecular Dynamics and Near-Tg Phenomena of Cyclic Thioethers

This article presents the synthesis and molecular dynamics investigation of three novel cyclic thioethers: 2,3-(4'-methylbenzo)-1,4,7,10-tetrathiacyclododeca-2-ene (compound 1), 2,3,14,15-bis(4',4″(5″)-methylbenzo)-1,4,7,10,13,16,19,22,25-octathiacyclotetracosa-2,14-diene (compound 2), and 2,3,8,9-bis(4',4″(5″)-methylbenzo)-1,4,7,10-tetrathiacyclododeca-2,8-diene (compound 3). The compounds exhibit relatively high glass transition temperatures (Tg), which range between 254 and 283 K. This characteristic positions them within the so-far limited category of crown-like glass-formers. We demonstrate that cyclic thioethers may span both the realms of ordinary and sizeable molecular glass-formers, each featuring distinct physical properties. Furthermore, we show that the Tg follows a sublinear power law as a function of the molar mass within this class of compounds. We also reveal multiple dielectric relaxation processes of the novel cyclic thioethers. Above the Tg, their dielectric loss spectra are dominated by a structural relaxation, which originates from the cooperative reorientation of entire molecules and exhibits an excess wing on its high-frequency slope. This feature has been attributed to the Johari-Goldstein (JG) process. Each investigated compound exhibits also at least one intramolecular secondary non-JG relaxation stemming from conformational changes. Their activation energies range from approximately 19 kJ/mol to roughly 40 kJ/mol. Finally, we analyze the high-pressure molecular dynamics of compound 1, revealing a pressure-induced increase in its Tg with a dTg/dp coefficient equal to 197 ± 8 K/GPa.

The Synthesis, Structure, and Dielectric Properties of a One-Dimensional Hydrogen-Bonded DL-α-Phenylglycine Supramolecular Crown-Ether-Based Inclusion Compound

A novel hydrogen-bonded supramolecular crown-ether-based inclusion compound, [(DL-α-Phenylglycine)(18-crown-6)]+[(CoCl4)0.5]-(1), was obtained via evaporation in a methanolic solution at room temperature using DL-α-phenylglycine, 18-crown-6, cobalt chloride (CoCl2), and hydrochloric acid. Its structure, thermal properties, and electrical properties were characterized via elemental analysis, single-crystal X-ray diffraction, variable-temperature infrared spectroscopy, thermogravimetric analysis, differential scanning calorimetry, and variable temperature-variable frequency dielectric constant testing. The compound was a monoclinic crystal system in the C2 space group at low temperature (100 K) and room temperature (293 K). Analysis of the single crystal structure showed that [(CoCl4)0.5]- presented an edge-sharing ditetrahedral structure in the disordered state, while the protonated DL-α-phenylglycine molecule in the disordered state and intramolecular hydroxyl group (-OH) underwent dynamic rocking, causing a significant stretching motion of the O-H···Cl-type one-dimensional hydrogen bond chain. This resulted in dielectric anomalies in the three axes of the crystal, thus showing significant dielectric anisotropy.

An organic-inorganic hybrid material [Me3NCH2CH2F]FeBr4 exhibits three-step SHG on/off

A novel solid-state second harmonic generation (SHG) organic-inorganic hybrid switch [Me3NCH2CH2F]FeBr4 (1) exhibits genuine three-step "on-off-on-off" SHG-switching above-room temperature, which has potential applications in multi-step optical devices.

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.

Two host-guest grown ether supramolecules show switchable phase transition, dielectric and second-harmonic generation effect

The excellent properties of host-guest crown ether inclusions in phase transition, dielectric and second-order nonlinear optical properties have attracted much attention. In this paper, we successfully designed and prepared two novel host-guest crown ether supramolecules [(DFBA)(15-crown-5)]X (X = ClO₄^-, 1; ReO₄^-, 2) by reactions of 2,6-difluorobenzylamine (DFBA) with 1,4,10,13-pentaoxacyclopentadecane (15-crown-5) in HClO₄, or HReO₄ aqueous solution. By the introduction of difluoro-substituted benzylamine as a guest cation, the phase transition temperatures are greatly increased to 377 K for 1 and 391 K for 2. More importantly, the space group of 1 has changed from centrosymmetric (CS) P2/c to the non-centrosymmetric (NCS) Pca2₁ in 2 when substituting perchlorate (ClO₄^-) with the larger and heavier perrhenate (ReO₄^-), which leads to 2 showing a switchable and stable second-harmonic generation (SHG) effect. According to the principle of momentum matching between a cation and anion, the perrhenate group increases the energy barrier of the molecular thermal motion, which not only significantly increases the phase transition temperature of 2 but also causes it to be frozen and crystallized in a NCS space group at room temperature. This research demonstrates that a polar molecule can adjust the suitability of anions and cations inside the crystal by practical chemical means.

A lead bromide organic-inorganic hybrid perovskite material showing reversible dual phase transition and robust SHG switching

A one-dimensional organic-inorganic hybrid perovskite material [3.3.0-dabco]PbBr₃ (1) was synthesized by the reaction of 1,5-diazabicyclo[3.3.0]octane (3.3.0-dabco) with PbBr₂ in concentrated HBr aqueous solution. Differential scanning calorimetry, dielectric measurements, and variable-temperature structural analyses revealed that compound 1 exhibits two successive structural phase transitions from P2₁2₁2₁ to Pbcm at 387 K (T₁) and then to P6/mmc at 436 K (T₂), accompanied by two pairs of dielectric anomalies with a clear one at T₁ and an unobvious one at T₂. In addition, compound 1 shows a robust second harmonic generation (SHG) effect between SHG-OFF and SHG-ON states during its centrosymmetric to non-centrosymmetric symmetry breaking phase transition at T₁.

Structural phase transition in a charge-transfer compound: tropylium hexafluoridoantimonate(V)–1,4-dimethylnaphthalene (1/1)

Molecular motion in crystals has attracted much attention for the development of stimuli-responsive materials. The most studied are molecules with few atoms or highly symmetrical molecules. To develop molecules with new motion characteristics, we synthesized a charge-transfer compound, namely, tropylium hexafluoridoantimonate(V)–1,4-dimethylnaphthalene (1/1), (C7H7)[SbF6]·C12H12, and studied its structural phase transition. In this compound, the tropylium cation and the 1,4-dimethylnaphthalene molecule have planar geometry, but the latter has low symmetry. They are stacked as a one-dimensional chain structure through π–π charge-transfer interactions. Weak intermolecular interactions and planar molecular geometry result in a large degree of freedom of in-plane motion. Upon heating, due to the in-plane rotation of the molecules, the compound undergoes an order–disorder structural phase transition (phase-transition temperature = 334 K). The space group of the room-temperature phase is P21/m and the space group of the high-temperature phase is P4/mmm. This phase transition is accompanied by significant dielectric anomalies. The current investigation shows that the structural features of the title compound can be used to construct functional materials with phase transitions, such as molecular ferroelectrics.

Unusual high-temperature host–guest inclusion compound-based ferroelectrics with nonlinear optical switching and large spontaneous polarization behaviours

Host–guest inclusion compound-based ferroelectrics [Hcta-(18-crown-6)]+[BF4]− (1) and [Hcta-(18-crown-6)]+[ClO4]− (2) with a high Curie temperature (Tc = 403/394 K) and large spontaneous polarization (Ps = 5.7/4.7 μC cm−2) are reported.

Structural phase transition and dielectric relaxation in an organic–inorganic hybrid compound: [(CH3)3NH]4[Fe(SCN)6]Cl

A new hybrid compound undergoes a structural phase transition accompanied by the thermal hysteresis of dielectric bistability as well as anisotropic dielectric relaxation along the a-, b-, and c-axis.

Room-temperature dielectric switching in a host–guest crown ether inclusion complex

Using the “momentum matching” theory, we have designed a new host–guest crown ether inclusion complex, which exhibits prominent room temperature bistable dielectric switching.

Enhancing switchable dielectric property for crystalline supramolecular rotor compounds by adding polar components

Two new compounds were obtained by assembling the [(2-methoxy-5-nitro-anilinium)(18-crown-6)]+ cation with non-polar PF6- and polar SO3CF3- anions, respectively. Benefiting from its polar anion, the SO3CF3- compound reveals a more significant dielectric switching behaviour during phase transition, demonstrating an effective strategy to enhance the dielectric property by adding polar components.

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

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

Conformational analysis and molecular dynamics of glass-forming aromatic thiacrown ethers

The dielectric properties, glass transition temperature and molecular dynamics of thiacrown ethers are strongly dependent on the thiacrown ring type.

Above Room Temperature Reversible Phase Transition Induces Distinct Dielectric and Nonlinear Optical Switching Response Behavior in Crown-Ether-Based Supramolecular Clathrate

Stimuli-responsive materials with coexisting nonlinear optical (NLO) and dielectric properties are technologically important, which enable simultaneous conversion of optoelectronic properties between different states under external stimuli. By rationally screening guest cations (C6H5NF2)+ in the crown-ether inclusion system, we synthesized a crown-ether supramolecular compound [(C6H5NF2)(18-crown-6)][PF6] (1). Differential scanning calorimetry (DSC) showed that 1 undergoes a reversible phase transition above room temperatures (305 K/292 K), with a thermal hysteresis of 13 K. Temperature-dependent dielectric and NLO measurements show that the compound exhibits two distinct switching response behaviors. Structural analysis indicates that the order–disorder change of the host molecule 18-crown-6 and the guest organic cation during the phase transition induces the dielectric and NLO switching behavior of the compound.

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.

High quantum yield and unusual photoluminescence behaviour in tetrahedral manganese(ii) based on hybrid compounds

High quantum yields and unusual luminescence characteristics were successfully achieved by regulating the organic cations in tetrahedral manganese(ii) complexes.