High-Temperature Ferroelectricity and Photoluminescence in a Hybrid Organic–Inorganic Compound: (3-Pyrrolinium)MnCl3

Ultrahigh phase transition temperature in a metal–halide perovskite-type material containing unprecedented hydrogen bonding interactions

An ABX₃ perovskite-type compound (N,N-dimethylethanolammonium)CdCl₃ with hydrogen bonding interactions between organic ammonium cations undergoes a phase transition at super-high temperatures.

Efficient modulation of photoluminescence by hydrogen bonding interactions between inorganic [MnBr4]2- anions and organic cations

The different hydrogen bond interactions in two organic-inorganic hybrid manganese halide compounds, namely [A]2[MnBr4] (A = N-butyl-N-methylpyrrolidinium ([P14]+) for (1) and N-butyl-N-methylpiperidinium ([PP14]+) for (2)), lead to distinct photoluminescence quantum yields (81% for 1; 55% for 2). Further applications of luminescent 1 are also developed.

Synthesis and characterization of a new organic–inorganic hybrid ferroelectric: (C4H10N)6[InBr6][InBr4]3·H2O

Compound (C₄H₁₀N)₆[InBr₆][InBr₄]₃·H₂O undergoes a paraelectric–ferroelectric phase transition at 232 K, which triggered by the disorder–order transition of Br atoms in [InBr₄]⁻ anions.

Two low-dimensional metal halides: ionothermal synthesis, photoluminescence, and nonlinear optical properties

An organic–inorganic hybrid metal halide with a chain-like structure has been prepared under ionothermal conditions, which shows a large second harmonic generation (SHG) efficiency.

Achievement of ligand-field induced thermochromic luminescence via two-step single-crystal to single-crystal transformations

We report here a new approach to achieve thermochromic luminescence through thermally induced ligand-field transformations. A one-dimensional chain manganese(ii) complex exhibits thermochromic luminescence from blue-green (502 nm) to red (617 nm) emission in a wide range of temperatures (480-80 K), ascribed to the conversion of a tetrahedronal to trigonal bipyramidal ligand-field via two-step single-crystal to single-crystal transformations.

Halogen substitution effects on optical and electrical properties in 3D molecular perovskites

Both 3D organic-inorganic perovskites ([Et3P(CH2)2Cl][Cd(dca)3] (1) and [Et3P(CH2)2F][Cd(dca)3] (2) [dca = dicyanamide, N(CN)2-]) display two sequentially reversible high-temperature phase transitions and switchable dielectric properties. Through halogen substitution, 1 shows exceptional switching behaviour of second harmonic generation effects and remarkably 2 represents the first above-room-temperature 3D ferroelastic material characterized by two ferroelastic phases.

Doping induced dielectric anomaly below the Curie temperature in molecular ferroelectric diisopropylammonium bromide

A dielectric anomaly induced by doping has been observed at about 340 K in chlorine-doped diisopropylammonium bromide. The dielectric anomaly has a switchable behaviour, which indicates potential applications on switches and sensors. Temperature-dependent Raman spectrum, X-ray diffraction and differential scanning calorimetry do not show any anomaly around the dielectric anomaly temperature, which prove that the dielectric anomaly does not come from structure phase transition and has no specific heat variety. It is assumed that this dielectric anomaly can be attributed to the freezing of ferroelectric domain walls induced by the pinning of point defects.

A facile synthesis of two new IR optical perovskites based on 1,4-diazabicyclo[2,2,2]octane with a high laser damage threshold

The main commercial infrared nonlinear optical (IR NLO) crystals, typically, AgGaS2, have some inherent disadvantages, for example, low laser damage threshold (LDT) or relatively poor stability, which limit their wide application. Here, we discover two new IR optical perovskites based on 1,4-diazabicyclo[2,2,2]octane, namely, (H2dabco)(PbCl3)2 (1) and (H2dabco)(H2PbBr6)·H2O (2). 1 and 2 crystallize in the noncentrosymmetric space groups P43212 and P63mc, respectively, displaying a broad transparent range with high transmission. Particularly, compound 2 exhibits a moderate SHG response of 1.8 times that of KH2PO4 (KDP) at 1064 nm with a high laser-induced damage threshold of 328.8 MW cm-2, indicating that it is a new promising NLO material.

Metal-free three-dimensional perovskite ferroelectrics

Inorganic perovskite ferroelectrics are widely used in nonvolatile memory elements, capacitors, and sensors because of their excellent ferroelectric and other properties. Organic ferroelectrics are desirable for their mechanical flexibility, low weight, environmentally friendly processing, and low processing temperatures. Although almost a century has passed since the first ferroelectric, Rochelle salt, was discovered, examples of highly desirable organic perovskite ferroelectrics are lacking. We found a family of metal-free organic perovskite ferroelectrics with the characteristic three-dimensional structure, among which MDABCO (N-methyl-N'-diazabicyclo[2.2.2]octonium)-ammonium triiodide has a spontaneous polarization of 22 microcoulombs per square centimeter [close to that of barium titanate (BTO)], a high phase transition temperature of 448 kelvins (above that of BTO), and eight possible polarization directions. These attributes make it attractive for use in flexible devices, soft robotics, biomedical devices, and other applications.

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.

A temperature-triggered triplex bistable switch in a hybrid multifunctional material: [(CH2)4N(CH2)4]2[MnBr4]

[ASN]₂[MnBr₄] is a novel multifunctional-material-integrated compound, which simultaneously exhibits prominent dielectric/NLO/fluorescent triple switching triggered by the thermal/electric/optical signal.

The Narrowest Band Gap Ever Observed in Molecular Ferroelectrics: Hexane-1,6-diammonium Pentaiodobismuth(III)

Narrow band gaps and excellent ferroelectricity are intrinsically paradoxical in ferroelectrics as the leakage current caused by an increase in the number of thermally excited carriers will lead to a deterioration of ferroelectricity. A new molecular ferroelectric, hexane-1,6-diammonium pentaiodobismuth (HDA-BiI₅ ), was now developed through band gap engineering of organic-inorganic hybrid materials. It features an intrinsic band gap of 1.89 eV, and thus represents the first molecular ferroelectric with a band gap of less than 2.0 eV. Simultaneously, low-temperature solution processing was successfully applied to fabricate high-quality ferroelectric thin films based on HDA-BiI₅ , for which high-precision controllable domain flips were realized. Owing to its narrow band gap and excellent ferroelectricity, HDA-BiI₅ can be considered as a milestone in the exploitation of molecular ferroelectrics, with promising applications in high-density data storage and photovoltaic conversion.

Visualization of Room-Temperature Ferroelectricity and Polarization Rotation in the Thin Film of Quinuclidinium Perrhenate

Recently, a plastic crystal of quinuclidinium perrhenate (HQReO_{4}) was reported to have the feasibility of controlling the crystallographic orientation in the grown crystal, but the corresponding temperature window is only about 22 K (345-367 K). Such a narrow window and uncertain ferroelectricity at room temperature would extremely limit its application potential. In this report, we prepared a large area high-quality polycrystalline thin film of HQReO_{4} and for the first time observed ferroelectricity in the temperature range from 298 to 367 K. Density functional theory calculations revealed the origin of room-temperature ferroelectricity is ascribed to the collaborative flipping of HQ (protonated quinuclidine) and ReO_{4}^{-}, which is dynamically preferred in the presence of a N─H⋯O hydrogen bond. A local piezoresponse force microscopy measurement was also employed to study the mechanisms of multiaxial polarization rotation and domain dynamics. By extending the ferroelectric temperature window to room temperature and the extraordinary thin-film processability, HQReO_{4} would certainly become a suitable candidate for next generation ferroelectric materials.

Multifunctional Material with Efficient Optoelectronic Integrated Molecular Switches Based on a Flexible Thin Film/Crystal

Switchable materials, due to their potential applications in the fields of sensors, photonic devices, digital processing, etc., have been developed drastically. However, they still face great challenges in effectively inducing multiple molecular switching. Herein organic-inorganic hybrid compounds, an emerging class of hydrosoluble optoelectronic-active materials, welcome a new member with smart unique optical/electrical (fluorescence/dielectric) dual switches (switching ON/OFF), that is, [C₅H₁₃NBr][Cd₃Br₇] (1) in the form of both a bulk crystal and an ultraflexible monodirectional thin film, which simultaneously exhibits fast dielectric/fluorescent dual switching triggered by an optical/thermal/electric signal with a high signal-to-noise ratio of 35 (the highest one in the known optical/dielectric dual molecular switches). Additionally, the exceptional stability/fatigue resistance as well as the fantastic extensibility/compactness of thin films (more than 10000 times folding over 90°), makes 1 an ideal candidate for single-molecule intelligent wearable devices and seamlessly integrated optoelectronic multiswitchable devices. This opens up a new route toward advanced light/electric high-performance switches/memories based on organic-inorganic hybrid compounds.

Lead-free Single-molecule Switching Material with Electric, Optical, Thermal Triple Controllable Multifunction Based on Perovskite-like Crystal and Flexible Thin Film

With the flourishing development of star molecule (CH₃NH₃)PbI₃, organic-inorganic perovskites with multifunction and flexibility have become a worldwide focus. However, the controllable photoelectric switchable material (especially electric, optical, thermal multifunctional switches) still face great challenges, and most of them are ceramic and toxic lead-based series. Herein a lead-free perovskite-like crystal and flexible thin film, ImMC (ImMC = (HIm)₆∙[MnCl₄∙MnCl₆]) (1), with many advantages over inorganic ceramics and lead-based perovskites, performs ideal optical and dielectric duple switching properties simultaneously. The order-disordered HIm (Im = imidazole) cations of α-type occupy two lattice sites corresponding to "Switch-ON/0" and "Switch-OFF/1" states, respectively. Interestingly, the optical and dielectric "ON/OFF or 0/1" switches can be integrated into one single-molecule single/duple channel module with high signal-noise ratio, in which the "ON/OFF" response can be precisely controlled by temperature or/and light wavelength signal to realize automatically multiple switching. In brief, the lead-free multifunctional switch opens up a brand new route and shows the mark of its real genius as a highly desirable material for its advanced applications in highly integrated circuit and ultrahigh-encrypted storage in flexible photoelectric devices.

Tunable electroresistance and electro-optic effects of transparent molecular ferroelectrics

Recent progress in molecular ferroelectrics (MOFEs) has been overshadowed by the lack of high-quality thin films for device integration. We report a water-based air-processable technique to prepare large-area MOFE thin films, controlled by supersaturation growth at the liquid-air interface under a temperature gradient and external water partial pressure. We used this technique to fabricate ImClO₄ thin films and found a large, tunable room temperature electroresistance: a 20-fold resistance variation upon polarization switching. The as-grown films are transparent and consist of a bamboo-like structure of (2,[Formula: see text],0) and (1,0,[Formula: see text]) structural variants of R3m symmetry with a reversible polarization of 6.7 μC/cm². The resulting ferroelectric domain structure leads to a reversible electromechanical response of d₃₃ = 38.8 pm/V. Polarization switching results in a change of the refractive index, n, of single domains, [Formula: see text]. The remarkable combination of these characteristics renders MOFEs a prime candidate material for new nanoelectronic devices. The information that we present in this work will open a new area of MOFE thin-film technologies.

Green-Light-Emitting Diodes based on Tetrabromide Manganese(II) Complex through Solution Process

Highly phosphorescent (Ph₄ P)₂ [MnBr₄ ] as a low-cost and environmentally benign emitting material achieves peak current efficiency of 25.4 cd A^-1 and external quantum efficiency (EQE) of 7.2% for nondoped organic light-emitting diodes, and peak current efficiency of 32.0 cd A^-1 and EQE of 9.6% for doped devices with 20% (Ph₄ P)₂ [MnBr₄ ]:27% TCTA:53% 6DCZPPY as a doping emitting layer.

Dielectric and ferroelectric sensing based on molecular recognition in Cu(1,10-phenlothroline)2SeO4·(diol) systems

The process of molecular recognition is the assembly of two or more molecules through weak interactions. Information in the process of molecular recognition can be transmitted to us via physical signals, which may find applications in sensing and switching. The conventional signals are mainly limited to light signal. Here, we describe the recognition of diols with Cu(1,10-phenlothroline)₂SeO₄ and the transduction of discrete recognition events into dielectric and/or ferroelectric signals. We observe that systems of Cu(1,10-phenlothroline)₂SeO₄·(diol) exhibit significant dielectric and/or ferroelectric dependence on different diol molecules. The compounds including ethane-1,2-diol or propane-1,2-diol just show small temperature-dependent dielectric anomalies and no reversible polarization, while the compound including ethane-1,3-diol shows giant temperature-dependent dielectric anomalies as well as ferroelectric reversible spontaneous polarization. This finding shows that dielectricity and/or ferroelectricity has the potential to be used for signalling molecular recognition.

Molecular recognition is an important biological process where guest and host molecules interact through non-covalent bonding. Ye et al. show that this can be sensed by the dielectric and ferroelectric signals of the final complexes in a series of metal-coordination compounds with different diol molecules.

A near-room-temperature organic–inorganic hybrid ferroelectric: [C6H5CH2CH2NH3]2[CdI4]

The features of synchronously switchable dielectric constant, SHG, and pyroelectric current provide a promising multifunctional switching material applied in the field of electrical/optical switches and sensors at near room temperature.

Polymorphism of a chiral iron(ii) complex: spin-crossover and ferroelectric properties

A chiral iron(ii) complex with two polymorphs, one that undergoes a gradual spin crossover, whilst the other remains in a high-spin state but shows a typical ferroelectric feature, is reported.