Molecular Ferroelectric [(NMe2)3PCH2Cl]PbI3 Showing Two-Step Switching Second-Order Nonlinear Optics

Molecular ferroelectrics have garnered significant attention owing to their environmental compatibility, facile synthesis, lightweight nature, and structural versatility. In this study, we present a novel one-dimensional (1D) organic-inorganic hybrid perovskite molecular ferroelectric, [(NMe2)3PCH2Cl]PbI3 (1), which undergoes two successive reversible phase transitions. This compound demonstrates distinct spontaneous polarization in ferroelectric domains, along with two-step switching in second-harmonic generation (SHG) and dielectric responses. Variable-temperature single-crystal X-ray diffraction reveals a symmetry-breaking transition in 1, shifting from the centrosymmetric P63/mmc space group to the polar noncentrosymmetric P63 ferroelectric phase. This transition is primarily driven by an order-disorder transformation of the [(NMe2)3PCH2Cl]+ cation. Furthermore, compound 1 exhibits promising semiconductor characteristic with a bandgap of 2.70 eV. This material's unique high-low-off SHG-switching behavior and robust polarization switching (Ps = 0.3 μC·cm-2) position it as a multifunctional platform for next-generation applications in multistate memory devices, programmable optical modulators, and temperature-responsive sensors, addressing critical limitations of conventional molecular ferroelectrics in stability and functional diversity.

An organic-inorganic hybrid perovskite material [Me3NCMe3]GaCl4 exhibits a two-step off-on-off SHG response with a large temperature interval

Reaction of a quaternary ammonium salt with GaCl3 in concentrated HCl aqueous solution led to an organic-inorganic hybrid perovskite material [Me3NCMe3][GaCl4] (1), which shows a two-step off-on-off second harmonic generation (SHG) response with a rapid and large temperature interval.

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.

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.

Room-Temperature Phase Transition Material with Switchable Second-Order Nonlinear Optical Properties

Phase transition materials with switchable second-order nonlinear optical (NLO) properties have attracted extensive attention because of their great application potential in photoelectric switches, sensors, and modulators, while metal-free organics with NLO switchability near room temperature remain scarce. Herein, we report a hydrogen-bonded metal-free organic crystal, 2-methylpropan-2-aminium 2,2-dimethylpropanoate (1), exhibiting a room-temperature phase transition and favorable NLO switchability. Through investigations on its thermal anomalies, dielectric properties, and crystal structures, we uncover that 1 holds a near-room-temperature phase transition at 303 K from noncentrosymmetric point group C2v to centrosymmetric one D2h, which is attributed to the order-disorder transformations of both tert-butylamine cations and dimethylpropionic acid anions. Accompanied by symmetry change during the phase transition, 1 exhibits reversible and repeatable NLO "on-off" switchability with a desirable switching contrast ratio of ca. 19 between high and low NLO states. This discovery demonstrates a metal-free organic crystal with NLO switching behavior near room temperature, serving as a promising candidate in smart and ecofriendly photoelectric functional materials and devices.

Centrosymmetry-Breaking Morphotropic Phase Boundary: A Pathway to Highly Sensitive and Strong Pressure-Responsive Nonlinear Optical Switches

The quest for high-performance piezoelectric materials has been synonymous with the pursuit of the morphotropic phase boundary (MPB), yet the full potential of MPBs remains largely untapped outside of the realm of ferroelectrics. In this study, we reveal a new class of MPB by creating continuous molecular-based solid solutions between centro- and noncentrosymmetric compounds, exemplified by (tert-butylammonium)1-x(tert-amylammonium)xFeCl4 (0 ≤ x ≤ 1), where the MPB is formed due to disorder of molecular cations. Near the MPB, we discovered an exceptionally sensitive nonlinear optical material in the centrosymmetric phase, capable of activation at pressures as low as 0.12-0.27 GPa, and producing tunable second-harmonic generation (SHG) signals from zero to 18.8 times that of KH2PO4 (KDP). Meanwhile, synchrotron diffraction experiments have unveiled a third competing phase (P212121) appearing at low pressure, forming a triple-phase point near the MPB, thereby providing insight into the mechanism underpinning the nonlinear optical (NLO) switch behavior. These findings highlight the opportunity to harness exceptional physical properties in symmetry-breaking solid solution systems by strategically designing novel MPBs.

Designing dynamic coordination bonds in polar hybrid crystals for a high-temperature ferroelastic transition

Ferroelastic materials have gained widespread attention as promising candidates for mechanical switches, shape memory, and information processing. Their phase-transition mechanisms usually originate from conventional order-disorder and/or displacive types, while those involving dynamic coordination bonds are still scarce. Herein, based on a strategic molecular design of organic cations, we report three new polar hybrid crystals with a generic formula of AA'RbBiCl6 (A = A' = Me3SO+ for 1; A = Me3SO+ and A' = Me4N+ for 2; A = A' = Me3NNH2+ for 3). Their A-site cations link to the [RbBiCl6]n2n- inorganic framework with lon topology through Rb-O/N coordination bonds, while their significantly different interactions between A'-site cations and inorganic frameworks provide distinct phase-transition behaviour. In detail, the strongly coordinative A'-site Me3SO+ cations prevent 1 from a structural phase transition, while coordinatively free A'-site Me4N+ cations trigger a conventional order-disorder ferroelastic transition at 247 K in 2, accompanied by a latent heat of 0.63 J g-1 and a usual "high → low" second-harmonic-generation (SHG) switch. Interestingly, the A'-site Me3NNH2+ cations in 3 reveal unusual dynamic coordination bonds, driving a high-temperature ferroelastic transition at 369 K with a large latent heat of 18.34 J g-1 and an unusual "low → high" SHG-switching behaviour. This work provides an effective molecular assembly strategy to establish dynamic coordination bonds in a new type of host-guest model and opens an avenue for designing advanced ferroelastic multifunctional materials.

Nonlinear Optical Switching in a Tin-Based Multilayered Halide Perovskite Activated by Stereoactive Lone Pairs and Confined Rotators

In recent years, there has been a surge in research enthusiasm on searching for solid-state nonlinear optical (NLO) switching materials in halide perovskites owing to their exceptional structural flexibility, compositional diversity, and broad property tenability. However, the majority of reported halide perovskite NLO switching materials contain toxic elements (e.g., Pb), which raise significant environmental concerns. Herein, we present a novel lead-free multilayered halide perovskite NLO switching material, (BA)2(EA)2Sn3Br10 (1, where BA is butylammonium and EA is ethylammonium). Driven by the stereochemically active lone-pair electrons of the Sn2+ cation and the cage-confined effect of EA rotators, 1 undergoes a phase transition with symmetry breaking from P4/mnc to Cmc21, which gives rise to a highly efficient modulation of the quadratic NLO property (0.7 times that of KH2PO4) at a high temperature of 353 K. Furthermore, crystallographic investigation combined with theoretical calculations reveals that the efficient modulation of NLO properties in 1 stems from the synergistic effects between stereochemically active lone pair-induced octahedral distortions and order/disorder transformation of organic cations. This study opens up an instructive avenue for designing and advancing environmentally friendly solid-state NLO switches in halide perovskites.

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.

Two quasi-spherical molecules [1,4-diazabicyclo(3.2.2)nonane]X (X = ClO4, ReO4) exhibit switchable phase transition, dielectric and second-harmonic-generation properties

Two quasi-spherical molecules [3.2.2-Hdabc]X (1,4-diazabicyclo[3.2.2]nonane = 3.2.2-dabcn, X = ClO4, ReO4) with a high phase transition temperature exhibited switchable phase transition as well as dielectric and SHG properties.

Pressure-Driven Two-Step Second-Harmonic-Generation Switching in BiOIO3

Materials with multi-stabilities controllable by external stimuli are potential for high-capacity information storage and switch devices. Herein, we report the observation of pressure-driven two-step second-harmonic-generation (SHG) switching in polar BiOIO 3 for the first time. Structure analyses reveal two pressure-induced phase transitions in BiOIO 3 from the ambient noncentrosymmetric phase (SHG-high) to an intermediate noncentrosymmetric phase (SHG-intermediate) and then to a centrosymmetric phase (SHG-off). The three-state SHG switching is inspected by in-situ high-pressure powder SHG and polarization-dependent single-crystal SHG measurements. Local structure analyses based on the in-situ Raman spectra and X-ray absorption spectra reveal that the SHG switching are caused by the step-wise suppression of lone-pair electrons on the [IO 3 ] - units. The dramatic evolution of the functional units under compression also leads to subtle changes of the optical absorption edge of BiOIO 3 . Materials with switchable multiple stabilities provide a state-of-art platform for next-generation switch and information storage devices.

An Exceptional Thermally Induced Four-State Nonlinear Optical Switch Arising from Stepwise Molecular Dynamic Changes in a New Hybrid Salt

Switching materials in channels of nonlinear optics (NLOs) are of particular interest in NLO material science. Numerous crystalline NLO switches based on structural phase transition have emerged, but most of them reveal a single-step switch between two different second-harmonic-generation (SHG) states, and only very rare cases involve three or more SHG states. Herein, we report a new organic-inorganic hybrid salt, (Me₃ NNH₂ )₂ [CdI₄ ], which is an unprecedented case of a reversible three-step NLO switch between SHG-silent, -medium, -low, and -high states, with high contrasts of 25.5/4.3/9.2 in a temperature range of 213-303 K. By using the combined techniques of variable-temperature X-ray single-crystal structural analyses, dielectric constants, solid-state ¹³ C nuclear magnetic resonance spectroscopy, and Hirshfeld surface analyses, we disclose that this four-state switchable SHG behavior is highly associated with the stepwise-changed molecular dynamics of the polar organic cations. This finding demonstrates well the complexity of molecular dynamics in simple hybrid salts and their potential in designing new advanced multistep switching materials.

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.

Molecule-based nonlinear optical switch with highly tunable on-off temperature using a dual solid solution approach

Nonlinear optical switches that reversibly convert between on/off states by thermal stimuli are promising for applications in the fields of photoelectronics and photonics. Currently one main drawback for practical application lies in the control of their switch temperature, especially for the temperature range near room temperature. By mixed melting treatment, here we describe an alloy-like nonlinear optical switch with tunable switch temperature via a dual solid solution approach within the coordination polymer system. We initially prepare a coordination polymer (i-PrNHMe2)[Cd(SCN)3], which functions as a high-contrast thermoresponsive nonlinear optical switch originating from a phase transition at around 328 K. Furthermore, by taking advantage of a synergistic dual solid solution effect, the melt mixing of it with its analogue (MeNHEt2)[Cd(SCN)3], which features an unequal anionic chain templated by an isomeric ammonium, can afford coordination polymer solid solutions with switch temperatures that are tunable in a range of 273-328 K merely by varying the component ratio.

[C5 H12 N]SnCl3 : A Tin Halide Organic-Inorganic Hybrid as an Above-Room-Temperature Solid-State Nonlinear Optical Switch

Nonlinear optical (NLO) switches driven by a solid-state structural phase transition have attracted extensive attention; however, above-room-temperature solid-state NLO switch materials are still sparse. Herein, we report an above-room-temperature tin halide organic-inorganic hybrid quadratic NLO switchable material, N-methylpyrrolidinium trichloride stannite ([C₅ H₁₂ N]SnCl₃ , MPSC). The MPSC crystal exhibits a phase-matchable NLO property that is 1.1 times that of KH₂ PO₄ (KDP) and NLO switching behavior, changing from a high second harmonic generation (SHG) response to a low SHG response at 383 K, thereby demonstrating its prospective applications in the field of nonlinear optics. Variable-temperature crystal structural analysis combined with theoretical calculations revealed that the large NLO response stems from the inorganic SnCl₃ moiety, whereas the high-performance NLO switching properties mainly originate from the order/disorder transformation of the N-methylpyrrolidinium. This work provides a new approach to designing and exploring new high-performance quadratic NLO switches involving tin halide organic-inorganic hybrids.

Above room-temperature dielectric and nonlinear optical switching materials based on [(CH3)3S]2[MBr4] (M = Cd, Mn and Zn)

Three organic–inorganic hybrid compounds based on [Me₃S]⁺ cations exhibit sensitive dielectric and nonlinear optical switching characteristics above room temperature.

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

A novel high-temperature dielectric and quadratic nonlinear optical switching material, 3,4-difluoroanilinium 18-crown-6 perchlorate, was synthesized. It exhibits two successive structural phase transitions at 347 K and 240 K, respectively. Gradually increased SHG-active characteristics are also identified.

A lead-free perovskite-like hybrid with above-room-temperature switching of quadratic nonlinear optical properties

We report a new lead-free perovskite-like hybrid, composed of hexamethyleneimine and one-dimensional [SbCl5]n2- polyanionic zigzag chains, which shows above-room-temperature switching of quadratic nonlinear optical (NLO) properties induced by the unique order-disorder conformational changes of the organic cation.

Room-temperature optic-electric duple bistabilities induced by plastic transition

A plastic crystal simultaneously exhibits room-temperature bistabilities in two physical channels: dielectric and nonlinear optics.