Chirality-Dependent Second-Order Nonlinear Optical Effect in 1D Organic-Inorganic Hybrid Perovskite Bulk Single Crystal

Manipulation of Chiral Nonlinear Optical Effect by Light-Matter Strong Coupling

Light-matter strong coupling (LMSC) is an intriguing state in which light and matter are hybridized inside a cavity. It is increasingly recognized as an excellent way to control material properties without any chemical modification. Here, we show that the LMSC is a powerful state for manipulating chiral nonlinear optical (NLO) effects through the investigation of second harmonic generation (SHG) circular dichroism. At the upper polariton band in LMSC, in addition to the enhancement of SHG by more than 1 order of magnitude, the responsivity to the handedness of circularly polarized light was largely modified, where sign inversion and increase of the dissymmetry factor were achieved. Quarter waveplate rotation analysis revealed that the LMSC clearly influenced the coefficients associated with chirality in the NLO process and also contributed to the enhancement of nonlinear magnetic dipole interactions. This study demonstrated that LMSC serves as a great platform for controlling chiral and magneto-optics.

Nonlinear Optical Effects of Hybrid Antimony(III) Halides Induced by Stereoactive 5s2 Lone Pairs and Trimethylammonium Cations

Organic-inorganic hybrid metal halides have unique optical and electronic properties, which are advantageous in the study of nonlinear optical materials. To investigate the effect of stereoactive lone pair electrons and the induction of organic cations on the structure of hybrid antimony(III) halides on nonlinear optics, we synthesize two noncentrosymmetric hybrid antimony(III)-based halide single crystals (TMA)3Sb2X9 (TMA = NH(CH3)3+, X = Cl, Br) by a room-temperature slow evaporation method, and their single-crystal structures, phase transition, X-ray photoelectron spectroscopy, and energy-band structure calculations are studied. More importantly, second-harmonic generation results of (TMA)3Sb2X9 (X = Cl, Br) are about 0.7 and 0.8 × KH2PO4(KDP), respectively. Interestingly, (TMA)3Sb2Cl9 single crystals undergo a reversible structural transition from Pc (No. 7) at room temperature to P21/c (No. 14) at 400 K, while the (TMA)3Sb2Br9 single crystals belong to the noncentrosymmetric space group R3c (No. 161), which clarifies the previous results. This work not only deepens the understanding of the role in lone pair electrons and organic cations in the structural induction in antimony-based halide perovskite materials but also provides guidance for subsequent nonlinear optical explorations.

Magneto-chiral Nonlinear Optical Effect with Large Anisotropic Response in Two-Dimensional Halide Perovskite

The chiral organic-inorganic halide perovskites (OIHPs) are vital candidates for superior nonlinear optical (NLO) effects associated with circularly polarized (CP) light. NLO in chiral materials often couples with magnetic dipole (MD) transition, as well as the conventional electric dipole (ED) transition. However, the importance of MD transition in NLO process of chiral OIHPs has not yet been well recognized. Here, the circular polarized probe analysis of second harmonic generation circular dichroism (SHG-CD) provides the direct evidence that the contribution of MD leads to a large anisotropic response to CP lights in chiral OIHPs, (R-/S-MBACl)2PbI4. The thin films exhibit great sensitivity to CP lights over a wide wavelength range, and the g-value reaches up to 1.57 at the wavelength where the contribution of MD is maximized. Furthermore, it is also effective as CP light generator, outputting CP-SHG with maximum g-factor of 1.76 upon the stimulation of linearly polarized light. This study deepens the understanding of relation between chirality and magneto-optical effect, and such an efficient discrimination and generation of CP light signal is highly applicable for chirality-based sensor and optical communication devices.

Nonlinear Optical Activity of a Chiral Organic-Inorganic ([(NH3CH2CH2)3NH])2[MnBr5]Br5 Photoluminescent and Piezoelectric Crystal

The family of Mn-based organic-inorganic hybrids has greatly expanded due to their advantages in applications. They also show superior bright and size-tunable photoluminescence and can be considered a perfect alternative to toxic lead-based compounds. In this work, we present the detailed structural, optical, and electrical characterization of ([(NH3CH2CH2)3NH])2[MnBr5]Br5. The title compound exhibits a unique type of inorganic arrangement created by the trigonal bipyramids. It crystallizes in noncentrosymmetric space group R32, indicating its optical activity, piezoelectricity, and second-order optical nonlinearity proven by the second harmonic of light measurements. The studied crystals exhibit intense photoluminescence originating from the Mn(II) ion 4T1(G) → 6A1 transition. The measured lifetime of the photoluminescence emission is ≤1.5 ms, while the measured quantum yield for both powder and crystal samples reaches ∼70%.

Bulk Photovoltaic Effect Induced by Non-Covalent Interactions in Bilayered Hybrid Perovskite for Efficient Passive X-Ray Detection

Hydrogen bonding as a multifunctional tool has always influenced the structure of hybrid perovskites. Compared with the research on hydrogen bonding, the study of halogen-halogen interactions on the structure and properties of hybrid perovskites is still in its early stages. Herein, a polar bilayered hybrid perovskite (IEA)2FAPb2I7 (IEA+ is 2-iodoethyl-1-ammonium, FA is formamidinium) with iodine-substituted spacer is successfully constructed by changing the configuration of interlayer cations and regulating non-covalent interactions at the organic-inorganic interface, which shows a shorter interlayer spacing and higher density (ρ = 3.862 g cm-3). The generation of structure polarity in (IEA)2FAPb2I7 is caused by the synergistic effect of hydrogen bonding and halogen-halogen interactions. Especially, as the length of the carbon chain in organic cations decreases, the I---I interaction in the system gradually strengthens, which may be the main reason for the symmetry-breaking. Polarity-induced bulk photovoltaics (Voc = 1.0 V) and higher density endow the device based on (I-EA)2FAPb2I7 exhibit a high sensitivity of 175.6 µC Gy-1 cm-2 and an ultralow detection limit of 60.4 nGy s-1 at 0 V bias under X-ray irradiation. The results present a facile approach for designing polar multifunctional hybrid perovskites, also providing useful assistance for future research on halogen-halogen interactions.

Multiple Functions of l-Thioproline in the Synthesis of Chiral Metal Bromides Showing Second-Harmonic-Generation Responses

Three new homochiral metal bromides, namely, (l-Htp)2Cu2Br4 (1), (l-Htp)(l-tp)CdBr3 (2), and (l-tp)2ZnBr2 (3), were prepared using l-thioproline as the chiral template. These compounds feature dimeric, chainlike, and monomeric structures. Their second-harmonic-generation (SHG) efficiencies are 0.1, 0.3, and 2.0 times that of KH2PO4, respectively. Density functional theory calculations were performed to reveal the origin of the SHG response of compound 3.

Boosted Second Harmonic Generation of a Chiral Hybrid Lead Halide Resonant to Charge Transfer Exciton from Metal Halide Octahedra to Ligand

Chiral hybrid organic-inorganic metal halides (HOMHs) offer an ideal platform for the advancement of second-order nonlinear optical (NLO) materials owing to their inherent noncentrosymmetric structures. The enhancement of optical nonlinearity of chiral HOMHs could be achieved by matching the free exciton and/or self-trapped exciton energy levels with desired NLO frequencies. However, the current scarcity of resonance modes and low resonance ratio hamper the further improvements of NLO performance. Herein, we propose a new resonant channel of charge transfer (CT) excited states from metal halide polyhedra to organic ligand to boost the second-order optical nonlinearity of chiral HOMHs. The model lead halide (C7H10N)PbBr3 (C7H10N=1-ethylpyridinium) exhibits a drastically enhanced second harmonic generation in resonance to the deep CT exciton energy, with intensity of up to 111.0 times that of KDP and 10.9 times that of urea. The effective NLO coefficient has been determined to be as high as ~40.2 pm V-1, balanced with a large polarization ratio and high laser damage threshold. This work highlights the contribution of organic ligands in the construction of a resonant channel for enhancing second-order NLO coefficients of metal halides, and thus provides guidelines for designing new chiral HOMHs materials for advanced nonlinear photonic applications.

Multifunctional Chiral d10-Metal Coordination Polymers: Tunable Photoluminescence and Efficient Second-Harmonic Generation with Circular Dichroic Response

A series of homochiral coordination polymers (HCPs), [M2(SIAP)2(bpy)2] [M(S)] and [M2(RIAP)2(bpy)2] [M(R)] (M = Zn or Cd, SIAP or RIAP = (S,S)- or (R,R)- 2,2'-(isophthaloylbis(azanediyl))di-propionic acid, bpy = 4,4'-bipyridine), is successfully synthesized through solvothermal reactions, self-assembling d10 metal cations, chiral dicarboxylic ligands, and π-conjugated bipyridyl ligands. The HCPs crystallize in the extremely rare triclinic chiral space group, P1, and present 3D framework structures attributed to the strong intermolecular interactions, such as hydrogen bonds and π-π stacking. Due to the unique crystal structures, the title compounds reveal efficient photoluminescence emission across a broad visible range, with significant brightness and color tuning by varying the excitation wavelength. Moreover, they exhibit efficient phase-matched second-harmonic generation (SHG) with very high laser-induced damage thresholds, essential for high-power nonlinear optical (NLO) applications. Intriguingly, the title compounds exhibit a measurable contrast in the SHG response under right- and left-handed circularly polarized excitation, thereby providing a unique case of SHG circular dichroism from the chiral centers of SIAP2- or RIAP2- ligand packed in the noncentrosymmetric environment. These exceptional attributes position these HCPs as promising candidates for multifunctional materials, with potential applications ranging from NLO devices to tailored luminescent systems with polarization control.

1D Cu(I)-based chiral organic-inorganic hybrid material with second harmonic generation and circular polarized luminescence

In recent years, organic-inorganic hybrid materials have demonstrated exceptional performance in nonlinear optics, attracting widespread attention. However, there are relatively few examples of coordination compounds synthesized with Cu as the metal center that exhibit excellent nonlinear optical properties. In this study, we successfully synthesized a pair of enantiomers named R/S-Cu2I2 by reacting chiral ligands with CuI. The crystal structure reveals a one-dimensional copper-iodide chain structure built by Cu2I2 clusters, and its ordered arrangement in space provides not only a strong second harmonic generation (SHG) signal (1.24 × KDP) but also a large birefringence (0.15@1064 nm). Under excitation at 395 nm, the crystals exhibit red fluorescence peaked at 675 nm. The CD spectra of R/S-Cu2I2 show a distinct mirror-symmetric Cotton effect, and their CPL signals are corresponding and opposite in the emission range, with a maximum glum of approximately ±2.5 × 10-3. Theoretical calculations using density functional theory were also carried out to enhance our understanding of the correlation between their structures and optical properties.

Optical Second Harmonic Generation of Low-Dimensional Semiconductor Materials

In recent years, the phenomenon of optical second harmonic generation (SHG) has attracted significant attention as a pivotal nonlinear optical effect in research. Notably, in low-dimensional materials (LDMs), SHG detection has become an instrumental tool for elucidating nonlinear optical properties due to their pronounced second-order susceptibility and distinct electronic structure. This review offers an exhaustive overview of the generation process and experimental configurations for SHG in such materials. It underscores the latest advancements in harnessing SHG as a sensitive probe for investigating the nonlinear optical attributes of these materials, with a particular focus on its pivotal role in unveiling electronic structures, bandgap characteristics, and crystal symmetry. By analyzing SHG signals, researchers can glean invaluable insights into the microscopic properties of these materials. Furthermore, this paper delves into the applications of optical SHG in imaging and time-resolved experiments. Finally, future directions and challenges toward the improvement in the NLO in LDMs are discussed to provide an outlook in this rapidly developing field, offering crucial perspectives for the design and optimization of pertinent devices.

Chiral Viologen-Derived Water-Stable Small Band Gap Lead Halides: Synthesis, Characterization, and Optical Properties

Chiral organic-inorganic metal halides (OIMHs) are attractive for their potential applications in chiral optoelectronics and spintronics, such as circular polarized light emitters, detectors, and chiral-induced spin selectivity. Here, we report three pairs of chiral OIMHs with great water stability constructed from chiral viologens. These OIMHs contain either 1D or 0D structures, however, with small band gaps around 2 eV. Circular dichroism (CD) spectroscopy on transparent thin films of two OIMH pairs showed a wide CD response covering most of the visible light range. Although the chiral center is not directly attached to the pyridinium in these chiral viologens, the chirality is still successfully transferred into both the band gap and the exciton absorption ranges. Liquid and solid CD studies of the chiral viologens further indicate that the chiral induction inside these OIMHs is possibly through chiral crystallization. This work demonstrated the design strategy of water-stable, small band gap chiral OIMHs through chiral viologens. These low-dimensional chiral materials may provide an interesting system to investigate chiral induction, and their broad CD response may enable their potential application as circular photodetectors with a wide detection range.

Unique Perovskitizer N─Pb Bond Switching Induced Polar Photovoltaic Effect in Trilayered Hybrid Perovskite

Polar photovoltaic effect (PPE) has attracted great attention in regulating desired optoelectronic properties, which can be driven by order-disorder and displacive phase transitions. Bond-switching is also a feasible method to induce PPE, but such investigation is very rare. Lead-halide hybrid perovskite (LHHP) is an outstanding photodetection material; lead atoms possess rich coordination modes to provide possibilities to construct switchable bonds. Here, a unique perovskitizer N─Pb bond-switching is disclosed to induce polar photovoltage in the emerging LHHP, PA2MHy2Pb3Br10 (1, PA = n-propylamine, MHy = methylhydrazine). Interestingly, the perovskitizer MHy+ provides 2s2 lone pair while the Pb atom affords empty d orbitals, which coordinate with each other to generate a flexible N─Pb bond. Further, the introduction of N─Pb bonds results in a high distortion of the PbBr6 octahedron to form local polarity and further orientation to induce spontaneous polarization. More importantly, such a flexible N─Pb bond switching mechanism drives a notable PPE and controllable polarized photo-response, a polarization ratio up to 9.7 at the polar phase in striking contrast with the non-polar phase (1.03). The work provides the first demonstration of bond-switching to induce polar phase transition and polar photovoltage in the photoconductive hybrid perovskites for photoelectric applications.

Ligand-Induced Chirality in ClMBA2 SnI4 2D Perovskite

Chiral perovskites possess a huge applicative potential in several areas of optoelectronics and spintronics. The development of novel lead-free perovskites with tunable properties is a key topic of current research. Herein, we report a novel lead-free chiral perovskite, namely (R/S-)ClMBA2 SnI4 (ClMBA=1-(4-chlorophenyl)ethanamine) and the corresponding racemic system. ClMBA2 SnI4 samples exhibit a low band gap (2.12 eV) together with broad emission extending in the red region of the spectrum (∼1.7 eV). Chirality transfer from the organic ligand induces chiroptical activity in the 465-530 nm range. Density functional theory calculations show a Rashba type band splitting for the chiral samples and no band splitting for the racemic isomer. Self-trapped exciton formation is at the origin of the large Stokes shift in the emission. Careful correlation with analogous lead and lead-free 2D chiral perovskites confirms the role of the symmetry-breaking distortions in the inorganic layers associated with the ligands as the source of the observed chiroptical properties providing also preliminary structure-property correlation in 2D chiral perovskites.

In Situ Chiral Template Approach to Synthesize Homochiral Lead Iodides for Second-Harmonic Generation

Homochiral halide perovskites have gained increasing attention because of their fascinating optoelectronic properties and prospective applications in laser technologies. However, the limited choice of chiral organic templates severely restricts their structural diversity and second-harmonic generation (SHG) effects. Here, we present an in situ chiral template approach for the synthesis of one-dimensional (1D) homochiral lead iodides. A chiral imine (L-ipp) template was generated in situ by reacting L-proline (L-pro) and acetone under ambient conditions. Notably, L-ipp can cooperate with L-pro to direct the formation of a homochiral lead iodide with dual chiral templates, which is unprecedented in crystalline metal halides. The homochiral lead iodide containing both L-ipp and L-pro shows a strong SHG response of 8.0 times that of KH2 PO4 (8.0×KDP). The SHG efficiency is one of the largest values reported to date for any homochiral lead halides under 1064 nm laser irradiation. A comparative study shows that homochiral 1D lead iodides containing either L-ipp or L-pro exhibit relatively weak SHG responses (≤1.0×KDP). This work demonstrates the advantage of using two different chiral templates over a single chiral template in enhancing the SHG responses of halide materials.

From C4H7N2Ge0.4Sn0.6Br3 to C6H11N2Ge0.4Sn0.6Br3: Effective Modulation of the Second Harmonic Generation Effect and Optical Band Gap by Planar π-Conjugated Organic Cation Size

In the search for nonlinear optical (NLO) materials with excellent overall performance, we have devoted ourselves to organic-inorganic hybrids consisting of anionic groups containing stereochemically active lone-pair (SCALP) electron cations and organic planar π-conjugated group cations. Accordingly, in this paper, two novel organic-inorganic hybrid metal halides, C4H7N2Ge0.4Sn0.6Br3 (I) and C6H11N2Ge0.4Sn0.6Br3 (II), have been synthesized. The powder second-harmonic technique shows that both C4H7N2Ge0.4Sn0.6Br3 and C6H11N2Ge0.4Sn0.6Br3 have moderately strong second-order nonlinear optical effects, which are about 2.03 (I) and 1.16 (II) times that of KH2PO4 (KDP), respectively. They also have different optical band gaps of 2.75 (I) and 2.88 eV (II) due to the different sizes of the organic cations, and their photoluminescent and thermal properties were also investigated. This work provides new structural insights for the design and modulation of organic-inorganic hybrid halide materials with multiple excellent optical properties.

Antimony Bromide Organic-Inorganic Hybrid Compound with a Long-Chain Diamine Showing Switchable Phase Transition and Second-Harmonic Generation Properties

Organic-inorganic hybrid metal halides have attracted significant attention in recent years due to their excellent optoelectronic properties and potential applications in solar cells. Herein, the organic-inorganic hybrid molecule [N,N-dimethyl-1,3-propanediamine]SbBr5 (1) was synthesized by reacting a long-chain organic diamine N,N-dimethyl-1,3-propanediamine with SbBr3 as a metal halide precursor in HBr aqueous solution. Compound 1 possesses a one-dimensional chainlike structure with the second-harmonic generation switch and two continuous phase transitions above room temperature. The band gap of compound 1 is about 2.62 eV, exhibiting a semiconductive property, which may have important implications for the development of new optoelectronic devices.

Synthesis and structure-dependent optical properties of two new organic-inorganic hybrid antimony(III) chlorides

Two organic-inorganic hybrid antimony(III) chlorides, namely (C9H26N3)2Sb4Cl18 (1) and (C9H26N3)SbCl6 (2), were prepared using a facile solvent evaporation method. They have low-dimensional structures constructed from SbCl6 octahedra and triply protonated N,N,N',N'',N''-pentamethyldiethylenetriamine cations. The organic cations exhibit different conformations in the two compounds. Compound 1 crystallizes in the centrosymmetric space group P1̄, while compound 2 crystallizes in the noncentrosymmetric space group Pca21. Notably, compound 2 exhibits a moderate second harmonic generation (SHG) response of about 1.3 times that of KH2PO4 (KDP) under 1064 nm laser irradiation. Meanwhile, this compound displays green-yellow luminescence under 342 nm ultraviolet light irradiation, indicating its potential as a bifunctional optical material. Theoretical calculations based on density functional theory were also performed to gain insights into the correlation between their structures and optical properties.

Free Halogen Substitution of Chiral Hybrid Metal Halides for Activating the Linear and Nonlinear Chiroptical Properties

Halogen substitution has been proven as an effective approach to the band gap engineering and optoelectronic modulation of organic-inorganic hybrid metal halide (OIHMH) materials. Various high-performance mixed halide OIHMH film materials have been primarily obtained through the substitution of coordinated halogens in their inorganic octahedra. Herein, we propose a new strategy of substitution of free halogen outside the inorganic octahedra for constructing mixed halide OIHMH single crystals with chiral structures, resulting in a boost of their linear and nonlinear chiroptical properties. The substitution from DMA4[InCl6]Cl (DMA = dimethylammonium) to DMA4[InCl6]Br crystals through a facile antisolvent vaporization method produces centimeter-scale single crystals with high thermal stability along with high quantum yield photoluminescence, conspicuous circularly polarized luminescence, and greatly enhanced second harmonic generation (SHG). In particular, the obtained DMA4[InCl6]Br single crystal features an intrinsic chiral structure, exhibiting a significant SHG circular dichroism (SHG-CD) response with a highest reported anisotropy factor (gSHG-CD) of 1.56 among chiral OIHMH materials. The enhancements in both linear and nonlinear chiroptical properties are directly attributed to the modulation of octahedral distortion. The mixed halide OIHMH single crystals obtained by free halogen substitution confine the introduced halogens within free halogen sites of the lattice, thereby ensuring the stability of compositions and properties. The successful employment of such a free halogen substitution approach may broaden the horizon of the regulation of structures and the optoelectronic properties of the OIHMH materials.

H/F Substitution Achieved Enantiomeric Organic Inorganic Hybrid Perovskites and Trigonal Structure [DMFP]3(CdBr3)(CdBr4)

Organic-inorganic hybrid perovskites (OIHPs) have been emerging as a hot research topic due to their potential applications in energy storage, semiconductors, and electronic devices. Herein, we systematically investigated the synthesis and phase transition behaviors of the enantiomeric OIHPs, (R) and (S)-N,N-dimethyl-3-fluoropyrrolidinium cadmium bromide ([DMFP][CdBr3]), and the hybrid trigonal structure [DMFP]3 (CdBr3)(CdBr4). The enantiomers have a mirror-symmetric structure and enhanced solid-state phase transition points of 417 and 443 K, in contrast to the nonfluorinated parent compound, N,N-dimethyl-pyrrolidinium cadmium bromide ([DMP][CdBr3], 385 K). Moreover, racemic H/F substitution on the pyrrolidinium cations leads to the formation of a trigonal compound, showing above-room-temperature structural phase transition and dominant ferroelasticity. This work discovers chiral enantiomeric OIHPs through H/F substitution, demonstrating a useful chemical synthesis strategy for exploring novel phase transition materials.

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

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

Circular polarization-resolved ultraviolet photonic artificial synapse based on chiral perovskite

Circularly polarized light (CPL) adds a unique dimension to optical information processing and communication. Integrating CPL sensitivity with light learning and memory in a photonic artificial synapse (PAS) device holds significant value for advanced neuromorphic vision systems. However, the development of such systems has been impeded by the scarcity of suitable CPL active optoelectronic materials. In this work, we employ a helical chiral perovskite hybrid combined with single-wall carbon nanotubes to achieve circularly polarized ultraviolet neuromorphic vision sensing and imaging. The heterostructure demonstrates long-term charge storage as evidenced by multiple-pulsed transient absorption measurements and highly sensitive circular polarization-dependent photodetection, thereby enabling efficient CPL-resolved synaptic and neuromorphic behaviors. Significantly, our PAS sensor arrays adeptly visualize, discriminate, and memorize distinct circularly polarized images with up to 93% recognition accuracy in spiking neural network simulations. These findings underscore the pivotal role of chiral perovskites in advancing PAS technology and circular polarization-enhanced ultraviolet neuromorphic vision systems.

Nucleation-mediated growth of chiral 3D organic-inorganic perovskite single crystals

Although their zero- to two-dimensional counterparts are well known, three-dimensional chiral hybrid organic-inorganic perovskite single crystals have remained difficult because they contain no chiral components and their crystal phases belong to centrosymmetric achiral point groups. Here we report a general approach to grow single-crystalline 3D lead halide perovskites with chiroptical activity. Taking MAPbBr3 (MA, methylammonium) perovskite as a representative example, whereas achiral MAPbBr3 crystallized from precursors in solution by inverse temperature crystallization method, the addition of micro- or nanoparticles as nucleating agents promoted the formation of chiral crystals under a near equilibrium state. Experimental characterization supported by calculations showed that the chirality of the 3D APbX3 (where A is an ammonium ion and X is Cl, Br or mixed Cl-Br or Br-I) perovskites arises from chiral patterns of the A-site cations and their interaction with the [PbX6]4- octahedra in the perovskite structure. The chiral structure obeys the lowest-energy principle and thereby thermodynamically stable. The chiral 3D hybrid organic-inorganic perovskites served in a circularly polarized light photodetector prototype successfully.

Chiral Hybrid Germanium(II) Halide with Strong Nonlinear Chiroptical Properties

Due to the pronounced anisotropic response to circularly polarized light, chiral hybrid organic-inorganic metal halides have been regarded as promising candidates for the application in nonlinear chiroptics, especially for the second-harmonic generation circular dichroism (SHG-CD) effect. However, designing novel lead-free chiral hybrid metal halides with large anisotropy factors and high laser-induced damage thresholds (LDT) of SHG-CD remains challenging. Herein, we develop the first chiral hybrid germanium halide, (R/S-NEA)3 Ge2 I7 ⋅H2 O (R/S-NGI), and systematically investigated its linear and nonlinear chiroptical properties. S-NGI and R-NGI exhibit large anisotropy factors (gSHG-CD ) of 0.45 and 0.48, respectively, along with a high LDT of 38.46 GW/cm2 ; these anisotropy factors were the highest values among the reported lead-free chiral hybrid metal halides. Moreover, the effective second-order nonlinear optical coefficient of S-NGI could reach up to 0.86 pm/V, which was 2.9 times higher than that of commercial Y-cut quartz. Our findings facilitate a new avenue toward lead-free chiral hybrid metal halides, and their implementation in nonlinear chiroptical applications.

Spatially Correlated Chirality in Chiral Two-Dimensional Perovskites Revealed by Second-Harmonic-Generation Circular Dichroism Microscopy

Understanding the chiral mechanism of chiral hybrid perovskites is a prerequisite for developing relevant chiroptoelectronic applications. Although conventional circular dichroism (CD) spectroscopy can be used to characterize chirality in chiral perovskites, it has a low signal-to-noise ratio and can provide only information about macroscopic chirality. Herein, with the aim of revealing the microscopic chiral mechanism in chiral perovskites, we utilize a spacer cation alloying strategy to construct chiral two-dimensional perovskites. For the first time, we demonstrate second-harmonic-generation CD microarea imaging in chiral perovskite thin films to unveil their spatially correlated chirality. In combination with theoretical calculations, it is revealed that the spatially correlated chirality is caused by localized out-of-plane supramolecular orientations. This work will not only advance the understanding of the mechanism of chiroptical activity in chiral perovskites but also provide inspiration for the rational design and synthesis of perovskites for chirality-related nonlinear optoelectronic devices.

Halogen Bonding Assembles Anion⋅⋅⋅Anion Architectures in Non-centrosymmetric Iodate and Bromate Crystals

Single crystal X-ray diffraction of iodate and bromate salts shows that the I and Br atoms in IO3 - and BrO3 - anions form short and linear O-I/Br⋅⋅⋅O contacts with the O atoms of nearby anions. Non-centrosymmetric systems are formed wherein anions are orderly aligned into supramolecular 1D and 2D networks. Theoretical evidences, namely the outcome of QTAIM and NCIplot studies, prove the attractive nature of these contacts and the ability of iodate and bromate anions to act as robust halogen bond (HaB) donors. The HaB is proposed as a general and effective assisting tool to control the architecture of acentric iodate salts.

Centimeter-Sized Piezoelectric Single Crystal of Chiral Bismuth-Based Hybrid Halide with Superior Electrostrictive Coefficient

The lead-free hybrid perovskite piezoelectrics possess advantages of easy processing, light weight, and low-toxicity over inorganic ceramics. However, the lack of understanding in structure-property relationships hinders exploration of new molecular piezoelectric crystals with excellent performances. Herein, by introducing chiral α-phenylethylammonium (α-PEA⁺ ) cations into bismuth-based hybrid halides, centimeter-sized (R-α-PEA)₄ Bi₂ I₁₀ and (S-α-PEA)₄ Bi₂ I₁₀ single crystals with a superior piezoelectric voltage coefficient g₂₂ of 309 mV m N^-1 , are obtained. Structural rigidity in crystals leads to a remarkable electrostrictive coefficient Q₂₂ of 25.8 m⁴ C^-2 , nearly 20 times higher than that of poly(vinylidene fluoride) (PVDF), which is beneficial to improve piezoelectricity with the synergistic effect of chirality. Moreover, the as-grown crystals show outstanding phase stability from 173 K to ≈470 K. This work suggests a design strategy based on rigidity and chirality to exploit novel piezoelectrics among hybrid metal halides.

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

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

Second-order nonlinear optical properties of copper-based hybrid organic-inorganic perovskites induced by chiral amines

Recently, chiral hybrid organic-inorganic perovskites (HOIPs) are drawing wide attention due to their intrinsic noncentrosymmetric structures which result in fascinating properties such as ferroelectronics and second-order nonlinear optics (NLO). However, previous research mainly focused on chiral lead-based halide perovskites ignoring that the toxic Pb element is harmful to humans and the environment. Herein, we successfully synthesized block-like (R-/S-NEA)₂CuCl₄ (NEA = 1-naphthylethylamine) and needle-like (R-/S-CYHEA)₆Cu₃Cl₁₂ (CYHEA = 1-cyclohexylethylamine) single crystals, which crystallize in the Sohncke P2₁ and I2 space group, respectively. Each pair of chiral perovskite enantiomers shows mirror circular dichroism (CD) signals. The thin films show an efficient second harmonic generation (SHG) response and the NLO coefficients of (R-NEA)₂CuCl₄ and (R-CYHEA)₆Cu₃Cl₁₂ are 11.74 and 3.04 pm V^-1, respectively, under 920 nm excitation with Y-cut quartz as a reference, which shows that the chiral amine has a significant effect on the SHG behavior. The high SHG response of (R-NEA)₂CuCl₄ is perhaps due to the rigidity of the aromatic amine, which leads to highly asymmetrical space groups. Our results provide guidelines for designing and tuning the SHG response in chiral HOIPs.

Highly Anisotropic Second-Order Nonlinear Optical Effects in the Chiral Lead-Free Perovskite Spiral Microplates

Chiral metal halide perovskites with intrinsic asymmetric structures have drawn increased research interest for the application of second-order nonlinear optics (NLO). However, designing chiral perovskites with the features of a large NLO coefficient, high laser-induced damage thresholds (LDT), and environmental friendliness remains a major challenge. Herein, we have synthesized two chiral hybrid bismuth halides: (R/S-MBA)₄Bi₂Br₁₀ spiral structure microplates, templated by chiral (R/S)-methylbenzylamine (R/S-MBA). The as-grown chiral lead-free perovskite spiral microplates exhibit a recorded second harmonic generation (SHG) effect with a large effective second-order NLO coefficient (d_eff) of 11.9 pm V^-1 and a high LDT of up to 59.2 mJ cm^-2. More importantly, the twisted screw structures show competitive circular polarization sensitivity at 1200 nm with an anisotropy factor (g_SHG-CD) of 0.58, which is about 3 times higher than that of reported Pb-based chiral perovskites. These findings provide a new platform to design multifunctional lead-free chiral perovskites for nonlinear photonic applications.

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

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

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

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

Spin Selectivity in Chiral Hybrid Cobalt Halide Films with Ultrasmooth Surface

Introducing chirality into low-dimensional hybrid organic-inorganic halides (HOIHs) creates brand-new opportunities for HOIHs in spintronics and spin-related optoelectronics owing to chirality-induced spin selectivity (CISS). However, preparing smooth films of low-dimensional HOIHs with small roughness is still a great challenge due to the hybrid and complex crystal structure, which severely inhibits their applications in spintronic devices. Exploring new lead-free chiral HOIHs with both efficient spin selectivity and excellent film quality is urgently desired. Here, cobalt-based chiral metal halide crystals (R/S-NEA)₂ CoCl₄ constructed by 0D [CoCl₄ ] tetrahedrons and 1-(1-naphtyl)ethylamine (NEA) are synthesized. The orderly configuration of NEA molecules stabilized by noncovalent CH···π interaction endows (NEA)₂ CoCl₄ with good film-forming ability. (NEA)₂ CoCl₄ films exhibit strong chiroptical activity (g_CD  ≈ 0.05) and significant spin-polarized transport (CISS efficiency up to 90%). Furthermore, ultrasmooth films (roughness ∼ 0.3 nm) with enhanced crystallinity can be achieved by incorporating tiny amount tris(8-oxoquinoline)aluminum that has analogous conjugated structure to NEA. The realization of highly efficient spin selectivity and sub-nanometer roughness in lead-free chiral halides can boost the practical process of low-dimensional HOIHs in spintronics and other fields.

Engineering hydrogen bonding to align molecular dipoles in organic solids for efficient second harmonic generation

Considering nearly infinite design possibilities, organic second harmonic generation (SHG) molecules are believed to have long-term promise. However, because of the tendency to form dipole-antiparallel crystals that lead to zero macroscopic polarization, it is difficult to design a nonlinear optical (NLO) material based on organic molecules. In this manuscript, we report a new molecule motif that can form asymmetric organic solids by controlling the degree of hydrogen bonding through protonation. A conjugated polar organic molecule was prepared with a triple bond connecting an electron-withdrawing pyridine ring and an electron-donating thiophene ring. By controlling the degree of hydrogen bonding through protonation, two different crystal packing motifs are achieved. One crystallizes into the common dipole-antiparallel nonpolar P1̄ space group. The second crystallizes into the uncommon dipole-parallel polar P1 space group, in which the molecular dipoles are aligned along a single axis and thus exhibit a high macroscopic polarization in its solid-state form. Due to the P1 polar packing, the sample can generate second harmonic light efficiently, about three times the intensity of the benchmark potassium dihydrogen phosphate. Our findings show that crystal engineering by hydrogen bonding in a single molecular backbone can be used for controlling the macroscopic NLO properties.

Biferroelectricity of a homochiral organic molecule in both solid crystal and liquid crystal phases

Ferroelectricity, existing in either solid crystals or liquid crystals, gained widespread attention from science and industry for over a century. However, ferroelectricity has never been observed in both solid and liquid crystal phases of a material simultaneously. Inorganic ferroelectrics that dominate the market do not have liquid crystal phases because of their completely rigid structure caused by intrinsic chemical bonds. We report a ferroelectric homochiral cholesterol derivative, β-sitosteryl 4-iodocinnamate, where both solid and liquid crystal phases can exhibit the behavior of polarization switching as determined by polarization–voltage hysteresis loops and piezoresponse force microscopy measurements. The unique long molecular chain, sterol structure, and homochirality of β-sitosteryl 4-iodocinnamate molecules enable the formation of polar crystal structures with point group 2 in solid crystal phases, and promote the layered and helical structure in the liquid crystal phase with vertical polarization. Our findings demonstrate a compound that can show the biferroelectricity in both solid and liquid crystal phases, which would inspire further exploration of the interplay between solid and liquid crystal ferroelectric phases.

Ferroelectricity normally exists in either solid crystals or liquid crystals. Here, the authors report a homochiral organic compound which shows ferroelectricity in both solid crystal and liquid crystal phases.

Axial-Chiral BINOL Multiferroic Crystals with Coexistence of Ferroelectricity and Ferroelasticity

Chirality exists everywhere from natural amino acids to particle physics. The introduction of point chirality has recently been shown to be an efficient strategy for the construction of molecular ferroelectrics. In contrast to point chirality, however, axial chirality is rarely used to design ferroelectrics so far. Here, based on optically active 1,1'-bi-2-naphthol (BINOL), which has been applied extensively as a versatile chiral reagent in asymmetric catalysis, chiral recognition, and optics, we successfully design a pair of axial-chiral BINOL multiferroics, (R)-BINOL-DIPASi and (S)-BINOL-DIPASi. They experience a 2F1-type full ferroelectric/ferroelastic phase transition at a high temperature of 362 and 363 K, respectively. Piezoelectric force microscopy and polarization-voltage hysteresis loops demonstrate their ferroelectric domains and domain switching, and polarized light microscopy visualizes the evolution of stripe-shaped ferroelastic domains. The axial-chiral BINOL building block promotes the generation of the polar structure and ferroelectricity, and the organosilicon component increases the rotational energy barrier and thus the phase transition temperature. This work presents the first axial-chiral high-temperature multiferroic crystals, offering an efficient path for designing molecular multiferroics through the introduction of axial chirality.

Chiral Lead-Free Double Perovskite Single-Crystalline Microwire Arrays for Anisotropic Second-Harmonic Generation

Halide double perovskites present a new branch for versatile optoelectronic devices because of their huge structural compatibility and environmental friendliness, whereas nonlinear optics (NLO) devices remain blank for this fascinating family. Simultaneously, the precise patterning of single-crystalline perovskite microwire arrays remains a challenge for the integration of NLO devices. Herein, we designed lead-free chiral 2D double perovskites with the nonsymmetrical structure presenting second-harmonic generation (SHG). Furthermore, perovskite single-crystalline arrays with regulated geometry, pure orientation, and high crystallinity are prepared using the capillary-bridge confined assembly technique. The efficient SHG originates from the asymmetric crystal structure and high crystallinity of the microwire arrays. Compared with their polycrystalline thin-film counterparts, linearly polarized SHG and a higher SHG conversion efficiency are demonstrated based on microwire arrays. The results not only expand the applications of lead-free double perovskites in the NLO-integrated fields but also provide a viable way for lead-free optoelectronic devices.

Homochiral Hybrid Organic-Inorganic Cadmium Chlorides Directed by Enantiopure Amino Acids

Homochiral cadmium chlorides were prepared under mild conditions using enantiopure amino acids as structure-directing agents. They feature a lacunary hexagonal CdCl₂ lattice as well as a one-dimensional perovskite structure. The coexistence of protonated and zwitterionic amino acids between cadmium chloride chains is quite rare. These compounds are nonlinear optically active solids showing a moderate second-harmonic-generation response. Theoretical calculations were performed to reveal the origin of their nonlinear-optical properties.

Organic Cation Modulation Triggered Second Harmonic Response in Manganese Halides with Bright Fluorescence

Zero-dimensional (0D) hybrid manganese halides have been recently synthesized and exhibited rich functional properties including fluorescence, ferroelectrics, and ferromagnetism. However, few studies on second-harmonic generation (SHG) behaviors of manganese halide crystals have been reported, presumably owing to the d-d transitions. Here, we report three manganese bromides, [TEA]₂MnBr₄ (TEA⁺ = tetraethylammonium; 1), [BTEA]₂MnBr₄ (BTEA⁺ = benzyltriethylammonium; 2), and [BTMA]₂MnBr₄ (BTMA⁺= benzyltrimethylammonium; 3), with linear and nonlinear optical properties via benzyl or ethyl/methyl substitution strategies. They feature 0D structures containing isolated [MnBr₄]^2- anions and quaternary ammonium cations with different sizes inserted for charge balance. They all show green phosphorescence, and 2 possesses good luminescence efficiency with a quantum yield of 97.8%, which is larger than those of 1 (79%) and 3 (72%). Specifically, acentric 1 and 3 present effective SHG responses about 0.48 and 0.59 times that of KDP, respectively. The result throws light on the new properties of the hybrid manganese halides and provides a new way to develop novel nonlinear optical-active organic-inorganic hybrid metal halides.

0D chiral hybrid indium(III) halides for second harmonic generation

Chiral metal halides have shown great potential for application in next generation nonlinear optical (NLO) devices owing to their intrinsic non-centrosymmetry. However, the structures and properties of chiral hybrid indium halides have been rarely reported, especially when it comes to second-harmonic generation (SHG) in NLO. In this work, we have synthesized a pair of new zero-dimensional (0D) chiral hybrid indium halides, (R-MPEA)₆InCl₉ and (S-MPEA)₆InCl₉, and studied their NLO properties. The as-prepared chiral hybrid indium halides crystallize in non-centrosymmetric P3₂21 and P3₁21 space groups, respectively. NLO studies show that 0D chiral hybrid indium halide crystals exhibit strong SHG responses with high polarization ratio and high laser damage threshold (LDT). This work enriches the family of chiral hybrid metal halide materials and offers a feasible strategy for the targeted design and synthesis of intrinsically non-centrosymmetric metal halide materials for NLO applications.