Boron(III)-Containing Donor-Acceptor Compound with Goldlike Reflective Behavior for Organic Resistive Memory Devices

Achieving efficient and stable blue thermally activated delayed fluorescence organic light-emitting diodes based on four-coordinate fluoroboron emitters by simple substitution molecular engineering

Achieving both high efficiency and high stability in blue thermally activated delayed fluorescence organic light-emitting diodes (TADF-OLEDs) is challenging for practical displays and lighting. Here, we have successfully developed a series of sky-blue to pure-blue emitting donor-acceptor (D-A) type TADF materials featuring a four-coordinated boron with 2,2'-(pyridine-2,6-diyl)diphenolate (dppy) ligands, i.e.1-8. Synergistic engineering of substituents on the phenyl bridge as well as the electronic properties and the attached positions of heteroatom N-donors not only enables fine-tuning of the emission colors, but also modulates the nature and energies of their triplet excited states that are important for the reverse intersystem crossing (RISC). Particularly for the compound with two methyl substituents on the phenyl bridge (compound 8), RISC is significantly facilitated through the vibronic coupling of the energetically close-lying triplet charge transfer (3CT) and the triplet local excited (3LE) states, when compared to analogue 7. Efficient sky-blue to pure-blue OLEDs with electroluminescence peaks (λ EL) at 460-492 nm have been obtained, in which ca. five-fold higher external quantum efficiencies (EQEs) of 18.9% have been demonstrated by 8 than that by 7. Moreover, ca. thirty times longer device operational half-lifetimes (LT50) of 9113 hours for 8 than that for 7 as well as satisfactory LT50 reaching 26 643 hours for 6 at an initial luminance of 100 cd m-2 have also been demonstrated. To the best of our knowledge, these results represent one of the best high-performance blue OLEDs based on tetracoordinated boron TADF emitters. Moreover, the design strategy presented here has provided an attractive strategy for enhancing the device performance of blue TADF-OLEDs.

Molecular-Potential and Redox Coregulated Cathodic Electrosynthesis toward Ionic Azulene-Based Thin Films for Organic Memristors

Organic memristors as promising electronic units are attracting significant attention owing to their simplicity of molecular structure design. However, fabricating high-quality organic films via novel synthetic technologies and exploring unprecedented chemical structures to achieve excellent memory performance in organic memristor devices are highly challenging. In this work, we report a cathodic electropolymerization to synthesize an ionic azulene-based memristive film (PPMAz-Py+Br-) under the molecular-potential and redox coregulation. During the cathodic electropolymerization process, electropositive pyridinium salts migrate to the cathode under an electric field, undergo a reduction-coupling deprotonation reaction, and polymerize into a uniform film with a controllable thickness on the electrode surface. The prepared Al/PPMAz-Py+Br-/ITO devices not only exhibit a high ON/OFF ratio of 1.8 × 103, high stability, long memory retention, and endurance under a wide range of voltage scans, but also achieve excellent multilevel storage and history-dependent memristive performance. In addition, the devices can mimic important biosynaptic functions, such as learning/forgetting function, synaptic enhancement/inhibition, paired-pulse facilitation/depression, and spiking-rate-dependent plasticity. The tunable memristive performances are attributed to the capture of free electrons on pyridinium cations, the migration of the aluminum ions (Al3+), and the form of Al conductive filaments under voltage scans.

Achieving the Reverse Intersystem Crossing in Chalcone Based Donor-Acceptor System through Down-Conversion of Triplet Exciton

In recent years, understanding the mechanism of thermally activated delayed fluorescence (TADF) has become the primary choice for designing high-efficiency, low-cost, metal-free organic light emitting diodes (OLEDs). Herein, we propose a strategically designed chalcone based donor-acceptor system, where intensification of delayed fluorescence with decrease in temperature (300 K to 100 K) is observed; the theoretical investigations of electronic states and orbital characters uncovered a new cold rISC pathway in donor-acceptor system, where rISC occurs through the down-conversation of higher triplet exciton (from T3 ) to lowest singlet state (S1 ), having negative energy splitting, thus no thermal energy is required. The comprehensive research described herein might open-up new avenues in donor-acceptor system over the conventional up-convention of triplet exciton and demonstrates that not necessarily all delayed fluorescence are thermally activated (TADF).

Tuning Electrochemical Stability of 5,10-Ditolylphenazine-Based Antiaromatic Materials for Unipolar Memristor toward Artificial Synapses Application

Three organic conjugated small molecules, DTA-DTPZ, Cz-DTPZ, and DTA-me-DTPZ comprising an antiaromatic 5,10-ditolylphenazine (DTPZ) core and electron-donating peripheral substituents with high HOMOs (-4.2 to -4.7 eV) and multiple reversible oxidative potentials are reported. The corresponding films sandwiched between two electrodes show unipolar and switchable hysteresis current-voltage (I-V) characteristics upon voltage sweeping, revealing the prominent features of nonvolatile memristor behaviors. The numerical simulation of the I-V curves suggests that the carriers generated by the oxidized molecules lead to the increment of conductance. However, the accumulated carriers tend to deteriorate the device endurance. The electroactive sites are fully blocked in the dimethylated molecule DTA-me-DTPZ, preventing the irreversible electrochemical reaction, thereby boosting the endurance of the memristor device over 300 cycles. Despite the considerable improvement in endurance, the decrement of on/off ratio from 105 to 101 after 250 cycles suggests that the excessive charge carriers (radical cations) remains a problem. Thus, a new strategy of doping an electron-deficient material, CN-T2T, into the unipolar active layer was introduced to further improve the device stability. The device containing DTA-me-DTPZ:CNT2T (1:1) blend as the active layer retained the endurance and on/off ratio (∼104) upon sweeping 300 cycles. The molecular designs and doping strategy demonstrate effective approaches toward more stable metal-free organic conjugated small-molecule memristors.

Water-soluble conjugated polymers for bioelectronic systems

Bioelectronics is an interdisciplinary field of research that aims to establish a synergy between electronics and biology. Contributing to a deeper understanding of bioelectronic processes and the built bioelectronic systems, a variety of new phenomena, mechanisms and concepts have been derived in the field of biology, medicine, energy, artificial intelligence science, etc. Organic semiconductors can promote the applications of bioelectronics in improving original performance and creating new features for organisms due to their excellent photoelectric and electrical properties. Recently, water-soluble conjugated polymers (WSCPs) have been employed as a class of ideal interface materials to regulate bioelectronic processes between biological systems and electronic systems, relying on their satisfying ionic conductivity, water-solubility, good biocompatibility and the additional mechanical and electrical properties. In this review, we summarize the prominent contributions of WSCPs in the aspect of the regulation of bioelectronic processes and highlight the latest advances in WSCPs for bioelectronic applications, involving biosynthetic systems, photosynthetic systems, biophotovoltaic systems, and bioelectronic devices. The challenges and outlooks of WSCPs in designing high-performance bioelectronic systems are also discussed.

Edge electron-rich carbon nitride via π-acceptor frame with high-efficient charge separation for photocatalytic hydrogen evolution and environmental remediation

Carbon nitride (g-C₃N₄) has broad application prospects in photocatalytic hydrogen production, but its photocatalytic efficiency is not ideal because of the rapid recombination of photogenerated electrons and holes. Herein, we developed a green strategy to fabricate hydroxyls and carbon-bridging co-modified g-C₃N₄ (CCN-OH) through a one-pot copolymerization and hydrothermal treatment. Experiments and density functional theory (DFT) calculations illustrated that carbon substitution of partial bridge nitrogen can improve the degree of electron delocalization to enhance the electron supply capacity of g-C₃N₄, and the exsitence of the electron-withdrawing OH group induces electron migration from carbon nitride to hydroxyl group, which further improves the efficiency of photogenerated charge separation. In addition, CCN-OH possess narrower band structure, resulting in an increased visible light utilization efficiency. The as-synthesized CCN-OH9 samples displayed an excellent photocatalytic activity for degradation of tetracycline with apparent reaction rate constant (k) of 0.018 min^-1 and photocatalytic hydrogen evolution of 1880.3 μmol g^-1h^-1, which was respectively 2.2 and 9.8 times higher than that of CN.

Donor-Acceptor Hybrid Heterostructures: An Emerging Class of Photoactive Materials with Inorganic and Organic Semiconductive Components

Just as the heterojunctions in physics, donor-acceptor (D-A) heterostructures are an emerging class of photoactive materials fabricated from two semiconductive components at the molecular level. Among them, D-A hybrid heterostructures from organic and inorganic semiconductive components have attracted extensive attention in the past decades due to their combined advantages of high stability for the inorganic semiconductors and modifiability for the organic semiconductors, which are particularly beneficial to efficiently achieve photoinduced charge separation and transfer upon irradiations. In this review, by analogy with the heterojunctions in physics, a definition of the D-A heterostructures and their general design and synthetic strategies are given. Meanwhile, the D-A hybrid heterostructures are focused on and their recent advances in potential applications of photochromism, photomodulated luminescence, and photocatalysis summarized.

Boosting Organic Afterglow Performance via a Two-Component Design Strategy Extracted from Macromolecular Self-Assembly

Because intersystem crossing and phosphorescence decay are spin-forbidden in organic systems, it is challenging to obtain high-performance organic afterglow materials. Inspired by two-component design strategy from macromolecular self-assembly, here we report the utilization of synthetic polymers to control the excited state properties of difluoroboron β-diketonate (BF₂bdk) and deuterated BF₂bdk compounds for the fabrication of room-temperature organic afterglow materials. The polymer component can interact with BF₂bdk excited states by dipole-dipole interactions, lower BF₂bdk S₁ levels with insignificant effect on T₁ levels, reduce ΔE_ST, and thus enhance intersystem crossing of BF₂bdk excited states. The polymer component can also suppress intramolecular motion of BF₂bdk triplets and protect BF₂bdk triplets from oxygen quenching. The obtained BF₂bdk-polymer afterglow materials exhibit emission lifetimes up to 2.2 s and high photoluminescence quantum yields under ambient conditions, display excellent processability and flexibility, and can function as efficient donors for excited state energy transfer to construct red afterglow materials.

Nanofiber Architecture Engineering Implemented by Electrophoretic-Induced Self-Assembly Deposition Technology for Flash-Type Memristors

Electrophoretic deposition (EPD) has been recognized as a promising large-scale film preparation technology for industrial application. Inspired by the conventional EPD method and the crystal diffusion growth strategy, we propose a modified electrophoretic-induced self-assembly deposition (EPAD) technique to control the morphologies of organic functional materials. Here, an ionic-type dye with a conjugated skeleton and strong noncovalent interactions, celestine blue (CB), is chosen as a module molecule for EPAD investigation. As expected, CB molecules can assemble into different nanostructures, dominated by applied voltage, concentration effect, and duration. Compared to a nanopillar layered packing structure formed by the traditional spin-coating method, the EPAD approach can produce a nanofiber structure under a fixed condition of 10 V/10 min. Intriguingly, a memristor device based on a pillar-like nanostructure exhibits WORM-type behavior, while a device based on nanofibers presents Flash memory performance. The assemble process and the memory mechanism are uncovered by molecular dynamics simulations and density-functional theory (DFT) calculations. This work endows the typical EPD technique with a fresh application scenario, where an in-depth study on the growth mechanism of nanofibers and the positive effect of unique morphologies on memristor performance are offered.

Water Molecular Bridge-Induced Selective Dual Polarization in Crystals for Stable Multi-Emitter

In the solid state, the molecular polarization of donor–acceptor (D–A) molecules can be implemented in a simple way via the use of an external polarizing source (e.g., an electric field). However, internal chemical polarization approaches are less studied due to difficulties related to controlling the charge-separation orientation in the solid state. Herein, a series of D–A molecules with both a proton donor and an acceptor were designed. Water-based molecular bridges were then established in their crystal structures, which firmly and alternately connected the proton donor of one molecule and the acceptor of another via an intermolecular H-bond network. In this way, the selective dual polarization of a phenolic hydroxyl group and a pyridinyl group could be achieved, owing to the strengthening of the charge-separation orientation upon the simultaneous deprotonation and protonation of the D–A molecules. This effect led to a 3–5-fold amplification of the molecular dipole moment in the crystal form relative to the monomeric state. On this basis, multi-excitation and multi-emission characteristics were achieved in these charge-separated crystals, endowing them with the ability to visually detect the energy of a light source, covering a wide range of the UV-Vis spectral region. This work provides a practical chemical approach for developing intrinsically polarized systems that can exhibit stable but distinct molecular photophysical properties.

In the solid state, the molecular polarization of donor–acceptor (D–A) molecules can be implemented by internal chemical polarization approaches.

Synthesis and n-Type Semiconducting Properties of Bis(dioxaborin) Compounds Containing a π-Extended 2,2'-Bithiophene Structure

Bis(dioxaborin) compounds containing π-conjugated systems have been studied as n-type semiconductors for organic field-effect transistors (OFETs). In this study, with the aim of investigating the effect of the extension of the π-conjugation on the n-type semiconducting properties and stability of bis(dioxaborin) compounds, we synthesized new compounds containing 2,2'-bithiophene derivatives extended with an olefin or an acetylene spacer. The absorption maxima of the compounds containing olefin spacers were greatly red-shifted compared with those of the original compound without a π-spacer. The newly synthesized compounds exhibited high electron affinity, and the olefin spacers effectively reduced the on-site Coulomb repulsion in the two-electron reduction of the compounds. An OFET fabricated using one of these compounds having a layer-by-layer crystal structure exhibited n-type semiconductor behavior with a low threshold voltage, most likely due to the small on-site Coulomb repulsion. The electron-transporting properties were investigated by theoretical calculations based on the Marcus theory.

Manipulation of Organic Afterglow by Thermodynamic and Kinetic Control

The fabrication of room-temperature organic phosphorescence and afterglow materials, as well as the transformation of their photophysical properties, has emerged as an important topic in the research field of luminescent materials. Here, we report the establishment of energy landscapes in dopant-matrix organic afterglow systems where the aggregation states of luminescent dopants can be controlled by doping concentrations in the matrices and the methods of preparing the materials. Through manipulation by thermodynamic and kinetic control, dopant-matrix afterglow materials with different aggregation states and diverse afterglow properties can be obtained. The conversion from metastable aggregation state to thermodynamic stable aggregation state of the dopant-matrix afterglow materials to leads to the emergence of intriguing afterglow transformation behavior triggered by thermal and solvent annealing. The thermodynamically unfavorable reversible afterglow transformation process can also be achieved by coupling the dopant-matrix afterglow system to mechanical forces.

TADF-Type Organic Afterglow

We report a highly efficient dopant-matrix afterglow system enabled by TADF mechanism to realize afterglow quantum yields of 60-70 %, which features a moderate rate constant for reverse intersystem crossing (k_RISC ) to simultaneously improve afterglow quantum yields and maintain afterglow emission lifetime. Difluoroboron β-diketonate (BF₂ bdk) compounds are designed with multiple electron-donating groups to possess moderate k_RISC values and are selected as luminescent dopants. The matrices with carbonyl functional groups such as phenyl benzoate (PhB) have been found to interact with and perturb BF₂ bdk excited states by dipole-dipole interactions and thus enhance the intersystem crossing of BF₂ bdk excited states. Through dopant-matrix collaboration, the efficient TADF-type afterglow materials have been achieved to exhibit excellent processability into desired shapes and large-area films by melt casting, as well as aqueous afterglow dispersions for potential bioimaging applications.

Irreversible Resistive State Switching in Devices with a Homoleptic Cobalt(II) Complex Active Layer

Molecules with bi-stable electronic transport behaviour have been in upfront research topics of the molecular semiconductor devices in the past few decades due to the use of such materials in resistive data storage devices. Transition metal complexes (TMC) are expected to be potential candidates in regard to the tunable and manifold redox behaviour expecting multiple bulk transport states. Finding alternate mechanisms in such devices with TMC as the active layer materials would revoke the multifaceted approach to the functional gain. We have succeeded in demonstrating write once-read many (WORM) type of resistive memory device using a homoleptic Cobalt(II) (Co(II)) complex with large on/off current ratio ensuring the easy readout process at lower voltage. The advantage of this device was the turn on voltage was found to be the low (<2.7 V) operational voltage and the success ratio of the devices were more than 83%. The durability of the stored data was found to be more than 35,000 seconds which ensures the stability of the bistable state in the fabricated devices. Such ambient stable, solution processable devices are important for the large-scale printable devices. The manuscript describes the preparation, optical and electrochemical characterisation of the metal complex used along with a detailed mechanistic investigations and electrical characterisation of memory device obtained from a stable cobalt complex.

Toward Highly Robust Nonvolatile Multilevel Memory by Fine Tuning of the Nanostructural Crystalline Solid-State Order

Organic resistive memory (ORM) offers great promise for next-generation high-density multilevel-cell (MLC) data storage. However, the fine tuning of crystalline order among its active layer still remains challenging, which largely restricts ORM behavior. Here, an exceptional solid-state transition from disordered orientations to highly-uniform orientation within the ORM layer is facilely triggered via molecular strategic tailoring. Two diketopyrrolopyrrole-based small molecular analogues (NI₁ TDPP and NI₂ TDPP) are demonstrated to display different symmetry. The asymmetric NI₁ TDPP shows an irregular solid-state texture, while the centro-symmetric NI₂ TDPP conforms to an ordered out-of-plane single-crystalline pattern that aligns with the foremost charge transportation along the substrate normal, and exhibits excellent MLC memory characteristics. Moreover, this highly oriented pattern guarantees the large-area film uniformity, leading to the twofold increase in the yield of as-fabricated ORM devices. This study reveals that the solid-state crystalline nanostructural order of organic materials can be controlled by reasonable molecular design to actuate high-performance organic electronic circuits.

π-Conjugated Materials Derived From Boron-Chalcogenophene Combination. A Brief Description of Synthetic Routes and Optoelectronic Applications

Functional materials composed of Boron-chalcogenophene conjugates have emerged as promising ensemble featuring commendable optoelectronic properties. This review describes the categories, synthetic routes and optoelectronic applications of a range of boron-chalcogenophene conjugates. Conjugation and linking of different types of tri- and tetra-coordinated boron moieties with chalcogenophenes have remained an important strategy for constructing a range of functional materials. Synthetic protocols have been devised to efficiently prepare such chemically robust conjugates, often exhibiting a myriad of photophysical properties, redox capabilities and also solid-state behaviors. Tin-boron and silicon-boron exchange protocols have been efficiently adapted to access these boron-chalcogenophenes. Few other commonly used methods namely, hydroboration of alkynes as well as electrophilic borylations are also mentioned. The chemical and electronic properties of such boron-chalcogenophene conjugates are directly influenced by the strong Lewis acid character of trivalent boranes which can further alter the intra- and inter- molecular Lewis acid-base interactions. Apart from the synthetic protocols, recent advances in the application of these boron-chalcogenophene conjugates towards analyte sensing, organic electronics, molecular switches and several other aspects will be discussed in this review.

Fabrication of One-Dimensional Organic Nanofiber Networks via Electrophoretic Deposition for a Nonvolatile Memory Device

Despite many advanced growth methodologies for organic nanofibers (ONFs), the lack of efficient and scalable ONF-based film preparation technologies has long been a hindrance in their practical application in organic electronic devices. Here, a typical cathode electrophoretic deposition (C-EPD) technology was developed to controllably produce ONFs and their corresponding thin films. Using the solvent effect and an external electric field force during the C-EPD process, a one-dimensional ONF network was formed, which exhibits compact molecular packing and superior optoelectronic properties in the thin-film state. Prototype sandwich-structure memory devices based on these ONF films exhibited a binary nonvolatile memory performance significantly superior than that of the bulk materials. This study provides an efficient and scalable ONF fabrication technology for high-performance electronic devices in various potential applications.

New bis-photochromic compounds based on diarylimidazoles: Synthesis and multistimuli-responsive optical properties

Three new bis-photochromic molecules containing two 4,5-diarylimidazole units were synthesized and characterized. We investigated their photophysical properties and pH response characteristics in acetonitrile solutions and polymethylmethacrylate (PMMA) membranes. All compounds displayed significant pH responses for the imidazole ring in the molecular structure, which could be potentially used as pH probes. Under ultraviolet light, these compounds show photochromism and 'turn-off' fluorescence properties either in solution or in PMMA film. The mechanism of this phenomenon may be due to photoisomerization, that is the conversion of the 4,5-diarylimidazole units open-ring form to the closed-ring form. Moreover, such systems as logic gates, high density (3D) data storage systems and optical equivalents of field-effect transistors are considered necessary for optical computing.

Synthesis, Photophysical, Photochromic, and Photomodulated Resistive Memory Studies of Dithienylethene-Containing Copper(I) Diimine Complexes

A series of dithienylethene-containing copper(I) diimine complexes have been synthesized and structurally characterized. Systematic studies on their photophysics, electrochemistry, and photochromism have been carried out. The photoinduced color changes of the copper(I) complexes have been achieved by photoexcitation into the metal-to-ligand charge-transfer (MLCT) absorption bands, indicating the photosensitization of light-induced cyclization by the ³MLCT excited state. In addition, by an increase in either the steric bulkiness around the copper(I) center or the structural rigidity of the complexes, the quantum efficiencies of photoluminescence and photocyclization can be effectively enhanced because of suppression of the flattening distortion of the complexes at the MLCT excited state. Furthermore, one of the complexes has been employed as an active component in the fabrication of solution-processed resistive memory devices. Notable lowering of the switching threshold voltage of the binary memory devices has been realized through photocyclization of the dithienylethene-containing copper(I) system.

Boron(iii) β-diketonate-based small molecules for functional non-fullerene polymer solar cells and organic resistive memory devices

A class of acceptor-donor-acceptor chromophoric small-molecule non-fullerene acceptors, 1-4, with difluoroboron(iii) β-diketonate (BF2bdk) as the electron-accepting moiety has been developed. Through the variation of the central donor unit and the modification on the peripheral substituents of the terminal BF2bdk acceptor unit, their photophysical and electrochemical properties have been systematically studied. Taking advantage of their low-lying lowest unoccupied molecular orbital energy levels (from -3.65 to -3.72 eV) and relatively high electron mobility (7.49 × 10-4 cm2 V-1 s-1), these BF2bdk-based compounds have been employed as non-fullerene acceptors in organic solar cells with maximum power conversion efficiencies of up to 4.31%. Moreover, bistable resistive memory characteristics with charge-trapping mechanisms have been demonstrated in these BF2bdk-based compounds. This work not only demonstrates for the first time the use of a boron(iii) β-diketonate unit in constructing non-fullerene acceptors, but also provides more insights into designing organic materials with multi-functional properties.

Nanometer-Scale Uniform Conductance Switching in Molecular Memristors

One common challenge highlighted in almost every review article on organic resistive memory is the lack of areal switching uniformity. This, in fact, is a puzzle because a molecular switching mechanism should ideally be isotropic and produce homogeneous current switching free from electroforming. Such a demonstration, however, remains elusive to date. The reports attempting to characterize a nanoscopic picture of switching in molecular films show random current spikes, just opposite to the expectation. Here, this longstanding conundrum is resolved by demonstrating 100% spatially homogeneous current switching (driven by molecular redox) in memristors based on Ru-complexes of azo-aromatic ligands. Through a concurrent nanoscopic spatial mapping using conductive atomic force microscopy and in operando tip-enhanced Raman spectroscopy (both with resolution <7 nm), it is shown that molecular switching in the films is uniform from hundreds of micrometers down to the nanoscale and that conductance value exactly correlates with spectroscopically determined molecular redox states. This provides a deterministic molecular route to obtain spatially homogeneous, forming-free switching that can conceivably overcome the chronic problems of robustness, consistency, reproducibility, and scalability in organic memristors.

Synthesis and Photovoltaic Properties of Boron β-Ketoiminate Dyes Forming a Linear Donor-π-Acceptor Structure

Organoboron complexes are of interest as chromophores for dye sensitizers owing to their light-harvesting and carrier-transporting properties. In this study, compounds containing boron β-ketoiminate (BKI) as a chromophore were synthesized and used as dye sensitizers in dye-sensitized solar cells. The new dyes were orange or red crystals and showed maximum absorptions in the 410-450 nm wavelength region on titanium dioxide substrates. These electrodes exhibited maximum efficiencies of over 80% in incident photon-to-current conversion efficiency spectra, suggesting that the continuous process of light absorption-excitation-electron injection was effectively performed. Open-circuit photovoltages were relatively high owing to the large dipole moments of the BKI dyes with a linear molecular structure. Thus, a maximum power conversion efficiency of 5.3% was successfully observed. Comparison of BKI dyes with boron β-diketonate dyes revealed certain differences in solution stability, spectral properties, and photovoltaic characteristics.

Highly Robust Organometallic Small-Molecule-Based Nonvolatile Resistive Memory Controlled by a Redox-Gated Switching Mechanism

Although organic small-molecule-based memory devices (OSMDs) have been demonstrated to show great potential for the application in next-generation data-storage technology, progress toward their further development has been hugely hindered by the ambiguity of their electrical switching mechanism. Thus, purposely fabricating OSMDs with a definite switching behavior is very urgent. Here, we reported a redox-gated nonvolatile rewritable memory device using an organometallic small molecule as an active material. By introducing the redox-active ferrocene into an organic skeleton, the target small molecule exhibits reliable and robust FLASH-type bistable electrical characteristics with a clear redox-controlled switching mechanism, which leads to low operational voltages, good endurance, and long retention. Our study offers a proof-of-concept strategy to design controllable OSMDs with excellent performances.

Four-Coordinate Boron Emitters with Tridentate Chelating Ligand for Efficient and Stable Thermally Activated Delayed Fluorescence Organic Light-Emitting Devices

A new class of four-coordinate donor-acceptor fluoroboron-containing thermally activated delayed fluorescence (TADF) compounds bearing a tridentate 2,2'-(pyridine-2,6-diyl)diphenolate (dppy) ligand has been successfully designed and synthesized. Upon varying the donor moieties from carbazole to 10H-spiro[acridine-9,9'-fluorene] to 9,9-dimethyl-9,10-dihydroacridine, these boron derivatives exhibit a wide range of emission colors spanning from blue to yellow with a large spectral shift of 2746 cm^-1 , with high PLQYs of up to 96 % in the doped thin film. Notably, vacuum-deposited organic light-emitting devices (OLEDs) made with these boron compounds demonstrate high performances with the best current efficiencies of 55.7 cd A^-1 , power efficiencies of 58.4 lm W^-1 and external quantum efficiencies of 18.0 %. More importantly, long operational stabilities of the green-emitting OLEDs based on 2 with half-lifetimes of up to 12 733 hours at an initial luminance of 100 cd m^-2 have been realized. This work represents for the first time the design and synthesis of tridentate dppy-chelating four-coordinate boron TADF compounds for long operational stabilities, suggesting great promises for the development of stable boron-containing TADF emitters.

Difluoroboron Chelation to Quinacridonequinone: A Synthetic Method for Air-Sensitive 6,13-Dihydroxyquinacridone via Boron Complexes

This study aims to perform the chelation of difluoroboron (BF₂ ) to quinacridonequinone (QQ). The resulting dark green solid was determined to be QA-BF₂ , which is a BF₂ complex of 6,13-dihydroxyquinacridone (QA-OH), and not QQ-BF₂ , which is a BF₂ complex of QQ. This result indicated that QQ-BF₂ was first generated as an O,O-bidentate chelate, which immediately underwent a two-electron reduction to produce QA-BF₂ . This compound was converted to air-sensitive QA-OH by undergoing hydrolysis in argon. Since QA-OH has a strong electron-donating property, it easily produced QQ via air oxidation in the solution. QA-OH also acts as a reducing reagent for quinones. The crystal packing of QA-OH is a herringbone type with short π⋅⋅⋅π contacts, and a good hole mobility has been suggested by theoretical calculations. Herein, a new synthetic method from QQ to QA-OH using BF₂ chelation and hydrolysis was proposed. QA-BF₂ and QA-OH are useful organic functional pigments and reducing reagents.

Direct covalent modification of black phosphorus quantum dots with conjugated polymers for information storage

It has long been recognized that a small switching bias window, which is defined as the difference between the switch-on and switch-off voltages (Δ|VON - VOFF|), and a high ON/OFF current ratio would be greatly favorable to reduce the power consumption of memory devices and to decrease the information misreading rate in digital memory devices. In contrast to two-dimensional BP nanosheets, zero dimensional BP quantum dots (BPQDs) show more exciting physical and chemical properties. By using newly synthesized poly[(9,9-dioctyl-9H-fluorene)-alt-(4-(9H-carbazol-9-yl)aniline)] (PFCz-NH2) as the synthetic precursor, a highly soluble diazotated polymer, PFCz-N2+BF4-, was successfully synthesized and used to react with BPQDs under aqueous conditions to give the first conjugated polymer covalently functionalized BPQDs (PFCz-g-BPQDs). The as-prepared Al/PFCz-g-BPQDs/ITO device exhibits excellent nonvolatile rewritable memory performance, with a large ON/OFF current ratio (>107) and low switch-on/off voltages (-0.89/+1.95 V). In contrast, the Al/PFCz-NH2 : BPQDs blend/ITO device also shows a rewritable memory effect, but its ON/OFF current ratio and Δ|VON - VOFF| value are found to be 3 × 103 and 5.47 (Δ|+2.53-2.94|), respectively. This work, which offers an easy one-step strategy for direct covalent functionalization of BPQDs, opens a way to explore more applications of BPQDs.

Boron-based stimuli responsive materials

Representative types of boron-based molecular systems that respond to external stimuli such as temperature, pressure, light, or chemicals (oxygen, acid, base etc.) are described in this review article. The boron molecules are classified according to their operating mechanisms, with emphasis on systems, which are based on switchable boron-donor bonds and switchable excited states.

Pyrene-affixed triazoles: a new class of molecular semiconductors for robust, non-volatile resistive memory devices

Bistable resistive switching in devices with molecular semiconductor with pyrene appended triazole as active stratum.

Amphiphilic Carbazole-Containing Compounds with Lower Critical Solution Temperature Behavior for Supramolecular Self-Assembly and Solution-Processable Resistive Memories

The self-organization and resistive memory performances of a series of newly synthesized water-soluble amphiphilic carbazole derivatives have been explored. Temperature-dependent UV/Vis absorption spectroscopy has been conducted to study the isodesmic self-assembly mechanism of the carbazole-containing compounds. This class of compounds also exhibits interesting lower critical solution temperature properties, which are sensitive to concentration and ionic additives. One of the compounds has been solution-processed and utilized as an active material in the engineering of resistive memory devices, exhibiting a switching voltage of about 3.9 V, a constant ON/OFF current ratio of 10⁶ , and a long retention time of 10⁴  s. The present work demonstrates the versatile potential applications of water-soluble amphiphilic carbazole-containing compounds in supramolecular chemistry and resistive memory devices.

Thiadizoloquinoxaline-Based N-Heteroacenes as Active Elements for High-Density Data-Storage Device

A novel thiadiazoloquinoxaline (TQ)-based donor-acceptor (D-A)-type N-heteroacene (Py-1-TQ) has been demonstrated for promising applications in organic multilevel resistive memory devices. Compared with its counterparts (Py-0-TQ and Py-2-TQ), which show flash-type binary memory behaviors, Py-1-TQ exhibits excellent nonvolatile write-once-read-many-times-type ternary memory effects with high ON2/ON1/OFF current ratios (10^5.8:10^3.4:1), which can be attributed to the different electron-withdrawing abilities between the pyrazine unit and TQ species that can induce stepwise D-A charge-transfer processes. These results suggest that TQ-based N-heteroacenes can be potentially useful in ultrahigh-density data-storage devices through the rational D-A tuning.

Discrimination of saturated alkanes and relevant volatile compounds via the utilization of a conceptual fluorescent sensor array based on organoboron-containing polymers

A conceptual sensor array for the efficient discrimination and fast detection of saturated alkanes and commonly found volatile solvents is reported.

This work reports a conceptual sensor array for the highly discriminative analysis of 20 clinically and environmentally relevant volatile small organic molecules (VSOMs), including saturated alkanes and common solvents, in the air at room temperature. For the construction of the sensor array, a four coordinated, non-planar mono-boron complex and four relevant polymers are synthesized. Based on the polymers and the use of different substrates, 8 fluorescent films have been fabricated. Integration of the film-based sensors results in the sensor array, which demonstrates unprecedented discriminating capability toward the VSOMs. Moreover, for the signal molecule of lung cancer, n-pentane, the response time is less than 1 s, the experimental detection limit is lower than 3.7 ppm, and after repeating the tests over 50 times no observable degradation was observed. The superior sensing performance is partially ascribed to the tetrahedral structure of the boron centers in the polymers as it may produce molecular channels in the films, which are a necessity for fast and reversible sensing. In addition, the polarity of the micro-channels may endow the films with additional selectivity towards the analytes. The design as demonstrated provides an effective strategy to improve the sensing performance of fluorescent films to very challenging analytes, such as saturated alkanes.

AIE-active boron complexes based on benzothiazole–hydrazone chelates

A new family of benzothiazole–hydrazone-based monoboron and bisboron complexes with aggregation-induced emission (AIE) was developed.

An all-in-one memory cell based on a homopolymer with a pyrene side chain and its volatile and nonvolatile resistive switch behaviors

Memory devices based on PMPPE exhibit a rewritable ternary memory behaviour (0, 1, 2, three conductivity states).

Deriving highly oriented organic nanofibers and ternary memory performance via salification-induced effects

A novel pyridine-based organic conjugated molecule (NCPy) has been successfully synthesized and converted into an organic pyridinium salt (NCPy-salt). Highly oriented organic nanofibers and high-quality films were obtained by transforming the molecule to a salt form. This transformation also resulted in a unique ternary resistive switching memory behavior.

Poly(3,4-ethylenedioxythiophene)-Poly(styrenesulfonate) Interlayer Insertion Enables Organic Quaternary Memory

Herein, for the first time, quaternary resistive memory based on an organic molecule is achieved via surface engineering. A layer of poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT-PSS) was inserted between the indium tin oxide (ITO) electrode and the organic layer (squaraine, SA-Bu) to form an ITO/PEDOT-PSS/SA-Bu/Al architecture. The modified resistive random-access memory (RRAM) devices achieve quaternary memory switching with the highest yield (∼41%) to date. Surface morphology, crystallinity, and mosaicity of the deposited organic grains are greatly improved after insertion of a PEDOT-PSS interlayer, which provides better contacts at the grain boundaries as well as the electrode/active layer interface. The PEDOT-PSS interlayer also reduces the hole injection barrier from the electrode to the active layer. Thus, the threshold voltage of each switching is greatly reduced, allowing for more quaternary switching in a certain voltage window. Our results provide a simple yet powerful strategy as an alternative to molecular design to achieve organic quaternary resistive memory.