Electrically Detectable Photoinduced Polarization Switching in a Molecular Prussian Blue Analogue.

Light, a nondestructive and remotely controllable external stimulus, effectively triggers a variety of electron-transfer phenomena in metal complexes. One prime example includes using light in molecular cyanide-bridged [FeCo] bimetallic Prussian blue analogues, where it switches the system between the electron-transferred metastable state and the system's ground state. If this process is coupled to a ferroelectric-type phase transition, the generation and disappearance of macroscopic polarization, entirely under light control, become possible. In this research, we successfully executed a nonpolar-to-polar phase transition in a trinuclear cyanide-bridged [Fe2Co] complex crystal via directional electron transfer. Intriguingly, by exposing the crystal to the wavelength of light─785 nm─without any electric field─we can drive this ferroelectric phase transition to completely depolarize the crystal, during which a measurable electric current response can be detected. These discoveries signify an important step toward the realization of fully light-controlled ferroelectric memory devices.

High-performance Pyroelectric Property Accompanied by Spin Crossover in a Single Crystal of Fe(II) Complex.

Pyroelectric materials hold significant potential for energy harvesting, sensing, and imaging applications. However, achieving high-performance pyroelectricity across a wide temperature range near room temperature remains a significant challenge. Herein, we demonstrate a single crystal of Fe(II) spin-crossover compound shows remarkable pyroelectric properties accompanied by a thermally controlled spin transition. In this material, the uniaxial alignment of polar molecules results in a polarization of the lattice. As the molecular geometry is modulated during a gradual spin transition, the polar axis experiences a colossal thermal expansion with a coefficient of 796×10-6 K-1. Consequently, the material's polarization undergoes significant modulation as a secondary pyroelectric effect. The considerable shift in polarization (pyroelectric coefficient, p=3.7-22 nC K-1cm-2), coupled with a low dielectric constant (ϵ'=4.4-5.4) over a remarkably wide temperature range of 298 to 400 K, suggests this material is a high-performance pyroelectric. The demonstration of pyroelectricity combined with magnetic switching in this study will inspire further investigations in the field of molecular electronics and magnetism.

Fully Optical Control of Polarization Current Direction in a Cyanide-Bridged Trinuclear Complex.

As a remote and non-contact stimulus, light offers the potential for manipulating the polarization of ferroelectric materials without physical contact. However, in current research, the non-contact write-read (erase) process lacks direct observation through the stable current as output signal. To address this limitation, we investigated the photoinduced polarization switching capabilities of the cyanide-bridged compound [Fe2Co] using visible light, leading to the achievement of rewritable polarization. By subjecting [Fe2Co] crystals to alternating irradiation with 785 nm and 532 nm light, the polarization changes exhibited a distinct square wave pattern, confirming the reliability of the writing and erasing processes. Initialization involved exposing specific crystal units to 532 nm light for storing "1" or "0" information, while reading was accomplished by scanning the units with 785 nm light, resulting in brief current pulses for "1" states and no current signal for "0" states. This research unveils new possibilities for optical storage systems, paving the way for efficient and rewritable data storage and retrieval technologies, such as the next-generation memories.

Magnetoelectricity Enhanced by Electron Redistribution in a Spin Crossover [FeCo] Complex.

Molecular-based magnetoelectric materials are among the most promising materials for next-generation magnetoelectric memory devices. However, practical application of existing molecular systems has proven difficult largely because the polarization change is far lower than the practical threshold of the ME memory devices. Herein, we successfully obtained an [FeCo] dinuclear complex that exhibits a magnetic field-induced spin crossover process, resulting in a significant polarization change of 0.45 µC cm-2. Mössbauer spectroscopy and theoretical calculations suggest that the asymmetric structural change, coupled with electron redistribution, leads to the observed polarization change. Our approach provides a new strategy toward rationally enhancing the polarization change.

High-Temperature Metal-Free Molecular Ferroelectrics with Large Spontaneous Polarization.

In the past two decades, numerous molecular ferroelectrics have been reported. However, metal-free molecular ferroelectrics with high working temperatures and large spontaneous polarizations are still uncommon. Herein, we present two metal-free molecular ferroelectrics prepared from monoprotonated hexamethylenetetramine (HMTA), namely (HMTAH)Cl and (HMTAH)Br, which crystallize in a polar point group of 3m. In these crystals, the polar HMTAH+ organic cations can be reoriented 180° along the polar axis because of the quasispherical molecular geometry. As a result of the large shift of the positively charged protonated N atoms, these compounds demonstrate large spontaneous polarizations with values of 8.3 and 8.1 µC cm-2 and high working temperatures of 390 and 435 K, respectively. The ferroelectric property of these compounds is characterized with second-harmonic generation, ferroelectric hysteresis loop, and pyroelectric current measurements.

Macroscopic Polarization Change of Mononuclear Valence Tautomeric Cobalt Complexes Through the Use of Enantiopure Ligand.

The crystallization of a complex having electron transfer properties in a polar space group can induce the polarization switching of a crystal in a specific direction, which is attractive for the development of sensors, memory devices, and capacitors. Unfortunately, the probability of crystallization in a polar space group is usually low. Noticing that enantiopure compounds crystallize in Sohncke space groups, this paper reports a strategy for the molecular design of non-ferroelectric polarization switching crystals based on the use of intramolecular electron transfer and chirality. In addition, this paper describes the synthesis of a mononuclear valence tautomeric (VT) cobalt complex bearing an enantiopure ligand. The introduction of enantiomer enables the crystallization of the complex in the polar space group (P21 ). The polarization of the crystals along the b-axis direction is not canceled out and the VT transition is accompanied by a change in the macroscopic polarization of the polar crystal. Polarization switching via electron transfer is realized at around room temperature.

Behavior Evolution of Droplets Suspended in Castor Oil under Alternating Current Electric Field.

Electric fields, which can promote the approach of droplets and break the liquid film, are extensively used in the separation of the water phase in water-in-oil emulsions. However, there is an evolution of droplet behavior under an electric field. After the two droplets meet with each other, the electric force becomes undesirable, which would even cause breakup of the merged droplet. When the electric field strength E reaches a particular value, the final behavior of droplets is made, which goes against coalescence, and there are lots of behavior evolution types. Several research studies have studied on whether droplets coalesce and the critical condition, but few works have focused on the classification and mechanism of non-coalescence behaviors. In this paper, the behavior evolution of two single droplets suspended in castor oil under an alternating current electric field is studied by a high-speed camera. Six distinct behavior evolution modes are observed and summarized: coalescence, bounce, partial coalescence, partial rupture, coalescence-rupture, and rupture. The behavior evolution mode is influenced by the initial separation distance s0 between droplets and the electric field strength. Moreover, there exist critical electric field strengths among different behavior evolution modes. As E gradually increases, two water droplets go through coalescence, partial coalescence, and coalescence-rupture in sequence when s0 is small and coalescence, bounce, partial rupture, and rupture when s0 is large. The mechanisms of behavior evolution are revealed by investigating the confrontation between electric force and capillary force in the condition with liquid bridge or pressure difference from the surrounding fluid and electric force in the condition without a liquid bridge. In addition, a cone-dimple mode of water droplets in castor oil is found, demonstrating the rationality of electric force theory.

Photoinduced Persistent Polarization Change in a Spin Transition Crystal.

Using light as a local heat source to induce a temporary pyroelectric current is widely recognized as an effective way to control the polarization of crystalline materials. In contrast, harnessing light directly to modulate the polarization of a crystal via excitation of the electronic bands remains less explored. In this study, we report an FeII spin crossover crystal that exhibits photoinduced macroscopic polarization change upon excitation by green light. When the excited crystal relaxes to the ground state, the corresponding pyroelectric current can be detected. An analysis of the structures, magnetic properties and the Mössbauer and infrared spectra of the complex, supported by calculations, revealed that the polarization change is dictated by the directional relative movement of ions during the spin transition process. The spin transition and polarization change occur simultaneously in response to light stimulus, which demonstrates the enormous potential of polar spin crossover systems in the field of optoelectronic materials.

Spontaneous self-formation of molecular ferroelectric heterostructures.

A new phase of diisopropylammonium perchlorate (DIPAP) forms during freeze-drying or heat treatment, which generates the heterostructure with its original ferroelectric phase. There is no composition fluctuation in the DIPAP molecular ferroelectric heterostructures, but there is an interface between the two phases of DIPAP. The formation of the new phase resembles that of martensite in alloys. A large internal bias field that is almost 2.5 times of the coercive field was found in the molecular ferroelectric heterostructures, which is comparable to that of doped triglycine sulfate. The large internal bias field will promote the ability of the DIPAP heterostructure to adsorb PM2.5 under light. The spontaneous self-formation of molecular ferroelectric heterostructures may help improve the performance of molecular ferroelectric devices.

Enhanced lasing behavior enabled by guided-mode resonance structure embedded with double waveguide layers.

By doping a laser dye into two polyurethane layers in a guided-mode resonance (GMR) structure, we observed that the lasing emission can be enhanced by approximately fivefold. The structure comprises, from top to bottom, a grating layer, four alternating layers of polyurethane and Ta2O5, and a bottom substrate. Two different GMR wavelengths can be generated because of the two films of Ta2O5 serving as waveguide layers. The enhancement of the lasing emission is achieved by matching both the absorption and emission wavelength of the laser dye with the two GMR wavelengths. When the absorption wavelength matches the GMR wavelength, the formation of high intensity near the polyurethane layer serves to efficiently excite the laser dye. Additionally, as the emission wavelength overlaps with the GMR wavelength, the extraction of lasing intensity can be further increased in the preferred directions owing to the high reflection efficiency and directivity of the GMR. Moreover, we found that the linewidth is reduced to approximately 1.06 nm, and the estimated threshold is approximately 0.92mJ/cm2 when both excitation and extraction resonances occur in the waveguide structure.

The growth mechanism and ferroelectric domains of diisopropylammonium bromide films synthesized via 12-crown-4 addition at room temperature.

Diisopropylammonium bromide (DIPAB) has attracted great attention as a molecular ferroelectric with large spontaneous polarization and high Curie temperature. It is hard to grow the ferroelectric phase DIPAB because of the appearance of the non-ferroelectric phase DIPAB at room temperature. Here, a ferroelectric thin film of DIPAB was successfully fabricated on a Si substrate using a spin coating method from aqueous solution via 12-crown-4 addition at room temperature. The ferroelectric DIPAB film with a thickness of hundreds of nanometers is distributed discontinuously on the substrate in narrow strips. The direction of polarization is along the narrow strip. Piezoresponse force microscopy (PFM) shows that the ferroelectric films have two kinds of domain structures: noncharged antiparallel stripe domains and charged head-to-head (H-H)/tail-to-tail (T-T) type domains. 12-crown-4 has been proved to play important roles in forming the H-H/T-T type domains. The Chynoweth method shows that the DIPAB films synthesized in this way show a better pyroelectric effect than DIPAB crystals.

Relaxation of ferroelectric thin films of diisopropylammonium perchlorate.

Molecular ferroelectric thin films are highly desirable for their easy and environmentally friendly processing, light weight, and mechanical flexibility. A thin film of diisopropylammonium perchlorate (DIPAP) processed by a spin-coating method shows a good roughness and textured structure with (101) orientation in the ferroelectric phase with a space group of P1. Simultaneously, the thin film shows ferroelectricity and ferroelectric relaxivity above room temperature, which is completely different from crystals. These properties make DIPAP a candidate in sensing, data storage, electro-optics, and molecular/flexible electronics.

An elastic single crystal composed of one-dimensional chiral coordination polymers.

A single crystal composed of one-dimensional coordinated polymers, [CdCl2(1-methyl-2-pyridone)]n, has been synthesized and characterized. This compound exhibits outstanding elastic bending due to the molecular spring nature of the CdCl2 coordination framework and weak intermolecular interactions between the coordination chains. Owing to the helical arrangement of organic ligands surrounding the coordination structure, the compound crystallizes in a chiral space group. As a result, it displays compelling circular dichroism spectra and second harmonic generation properties.

Enantiospecific total synthesis of (+)-tanikolide via a key [2,3]-Meisenheimer rearrangement with an allylic amine N-oxide-directed epoxidation and a one-pot trichloroisocyanuric acid N-debenzylation and N-chlorination.

The enantiospecific total synthesis of the δ-lactonic marine natural product (+)-tanikolide (1), isolated from Lyngbya majuscula , was achieved using a [2,3]-Meisenheimer rearrangement as the key reaction. During this rearrangement, we discovered that the allylic amine N-oxide could direct the m-CPBA double-bond epoxidation to the syn position. The resulting syn product 8 underwent epoxide ring opening under the m-CBA conditions to give the five- and six-membered cyclic ether amine N-oxides, which we further treated with Zn and conc. HCl to obtain the reduced bisbenzyl tertiary amines 23 and 22, respectively. When 23 and 22 were treated with trichloroisocyanuric acid (TCCA) in dichloromethane, oxidation at the benzyl position occurred, forming iminium ions. These intermediates were trapped by intramolecular reaction with the hydroxyls, and the resulting intermediates were then oxidized or shifted to afford 25 and 24, respectively. The entire one-pot process involves N-debenzylation, N-chlorination, and hemiacetal oxidation. The amine N-oxide-directed epoxidation complements Davies' ammonium-directed epoxidation. Thus, TCCA N-debenzylation is described for the first time and might be a useful N-debenzylation technique.

Energy conversion and storage via photoinduced polarization change in non-ferroelectric molecular [CoGa] crystals.

To alleviate the energy and environmental crisis, in the last decades, energy harvesting by utilizing optical control has emerged as a promising solution. Here we report a polar crystal that exhibits photoenergy conversion and energy storage upon light irradiation. The polar crystal consists of dinuclear [CoGa] molecules, which are oriented in a uniform direction inside the crystal lattice. Irradiation with green light induces a directional intramolecular electron transfer from the ligand to a low-spin CoIII centre, and the resultant light-induced high-spin CoII excited state is trapped at low temperature, realizing energy storage. Additionally, electric current release is observed during relaxation from the trapped light-induced metastable state to the ground state, because the intramolecular electron transfer in the relaxation process is accompanied with macroscopic polarization switching at the single-crystal level. It demonstrates that energy storage and conversion to electrical energy is realized in the [CoGa] crystals, which is different from typical polar pyroelectric compounds that exhibit the conversion of thermal energy into electricity.

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

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

Fabrication of Ag3PO4/polyaniline-activated biochar photocatalyst for efficient triclosan degradation process and toxicity assessment.

Triclosan (TCS) is a typical environmental pollutant, which seriously threatens the health of humans and organisms. A novel strategy of biochar/Ag3PO4/polyaniline (PANI) composite photocatalyst was synthesized by a facile chemical precipitation method to efficiently degrade TCS. XRD, Raman, ESR, etc. were used to reveal the effective associations among physiochemistry, photochemistry and photocatalytic properties of the composite. It was proved the synergistic effects of biochar (T-Bio) and PANI resulted in the decrease of Ag3PO4 particle size, the enhancement of adsorption, the improvement of light utilization, the increase of photogenerated carrier separation and the promotion of reactive species. The photocatalytic mechanism showed h+ was the main active species, O2- and OH played minor roles. Under the irradiation of visible light, the optimal photocatalyst (1.0% T-Bio/AP/1.0% PANI) displayed excellent photocatalytic activity with the removal rate of 85.21% for TCS within 10 min, and the apparent rate constant K' was 2.38 times of Ag3PO4. 11 main intermediates for TCS degradation were identified, and their toxicity was significantly reduced. The possible degradation pathways were proposed. This work is the first systematic study on the degradation behavior of TCS by Ag3PO4-based photocatalyst, and it provides a new approach to fabricate photocatalysts with synergistic effects and amazing photocatalytic activity by biochar.

The enhancement mechanism of ultra-active Ag3PO4 modified by tungsten and the effective degradation towards phenolic pollutants.

A novel strategy of W modification was applied to overcome the disadvantages of Ag3PO4. Ultra-active Ag3PO4 with different W doping ratios were successfully synthesized by facile chemical precipitation method, among which 0.5%W-AP showed the best results. Meanwhile, the stability and yield were enhanced. XRD, Raman and ESR etc. were employed to investigate the morphology, structure and optical properties of samples. It was proved W6+ entered into the Ag3PO4 lattice, occupied the position of P5+ and doped in the form of WO42-. The significant improvement of photocatalytic performance of W doped Ag3PO4 was attributed to the change of morphology, the decrease of particle size, the increase of crystallinity, the shrink of band gap energy and the reduction of photo-induced carriers recombination rate with W doping. The photocatalytic mechanism analysis showed h+ was the main oxidative species in the photocatalytic process, •O2- and •OH played minor roles. Under visible light irradiation, the impacts of the important operating parameters on the typical phenolic pollutants, phenol and bisphenol A, were evaluated with 0.5%W-AP. It was confirmed that 68% and 82% of phenol and bisphenol A were respectively degraded within 15 min and 40 min under optimized photocatalytic parameters: 0.4 g/L catalyst dosage, 20 mg/L pollutant concentration, pH 5.7 and 125 mW/cm2 irradiation intensity, and the corresponding K' were 2.14 and 5.50 times of undoped samples. This work provides a new approach for effective degradation towards phenolic pollutants by Ag3PO4 with ultra-high photocatalytic activity, high applicability and enhanced stability and yield.

Manipulating electron redistribution to achieve electronic pyroelectricity in molecular [FeCo] crystals.

Pyroelectricity plays a crucial role in modern sensors and energy conversion devices. However, obtaining materials with large and nearly constant pyroelectric coefficients over a wide temperature range for practical uses remains a formidable challenge. Attempting to discover a solution to this obstacle, we combined molecular design of labile electronic structure with the crystal engineering of the molecular orientation in lattice. This combination results in electronic pyroelectricity of purely molecular origin. Here, we report a polar crystal of an [FeCo] dinuclear complex exhibiting a peculiar pyroelectric behavior (a substantial sharp pyroelectric current peak and an unusual continuous pyroelectric current at higher temperatures) which is caused by a combination of Fe spin crossover (SCO) and electron transfer between the high-spin Fe ion and redox-active ligand, namely valence tautomerism (VT). As a result, temperature dependence of the pyroelectric behavior reported here is opposite from conventional ferroelectrics and originates from a transition between three distinct electronic structures. The obtained pyroelectric coefficient is comparable to that of polyvinylidene difluoride at room temperature.

Pyroelectricity and field-induced spin-flop in (4-(Aminomethyl)pyridinium)2 MnCl4 · 2H2O.

Large single crystals of (4-(Aminomethyl)pyridinium)2 MnCl4 · 2H2O (1) were grown by slow evaporation of solution. The crystal structure was solved to be Pi, which belongs to the central symmetric space group. But small pyroelectric current was detected, as well as a ferroelectric hysteresis loop. The pyroelectric and the ferroelectric properties were attributed to the strain caused by defects. Temperature-dependent magnetic curves and the M-H curve show that 1 is antiferromagnetic ordering below 2.5 K. A field-induced spin-flop is observed in the antiferromagnetic ordering state.