Polarity-Tunable Host Materials and Their Applications in Thermally Activated Delayed Fluorescence Organic Light-Emitting Diodes

3D Imaging of Lipid Droplet-Nuclear Membrane Contact Sites and Cirrhotic Lipid Droplet Overexpression

To coordinate cellular physiology, cells rely on the rapid exchange of molecules at specialized organelle-organelle contact sites. Lipid droplets (LDs) and nuclear membrane (NM) contact sites are particularly vital communication hubs, playing key roles in the exchange of signaling molecules, lipids, and metabolites. However, there is still a lack of understanding of the specific morphology of the contact sites. Here, we combine advanced three-dimensional (3D) imaging with a high-brightness fluorescent probe specifically targeting LDs to map the structural landscape of LD-NM contact sites. The probe exhibits exceptional photophysical properties, making it highly suitable for visualizing the changes occurring in LDs during the apoptosis process. In addition, we utilize the advantages of the probe to accurately monitor the overexpression of abnormal LDs in cirrhosis by 3D imaging for the first time. The outcomes of this investigation highlight that the probe has potential as a robust imaging tool to investigate intricate biological functions of LDs and their implications in related diseases.

Stable pure-green organic light-emitting diodes toward Rec.2020 standard

Manipulating dynamic behaviours of charge carriers and excitons in organic light-emitting diodes (OLEDs) is essential to simultaneously achieve high colour purity and superior operational lifetime. In this work, a comprehensive transient electroluminescence investigation reveals that incorporating a thermally activated delayed fluorescence assistant molecule with a deep lowest unoccupied molecular orbital into a bipolar host matrix effectively traps the injected electrons. Meanwhile, the behaviours of hole injection and transport are still dominantly governed by host molecules. Thus, the recombination zone notably shifts toward the interface between the emissive layer (EML) and the electron-transporting layer (ETL). To mitigate the interfacial carrier accumulation and exciton quenching, this bipolar host matrix could serve as a non-barrier functional spacer between EML/ETL, enabling the distribution of recombination zone away from this interface. Consequently, the optimized OLED exhibits a low driving voltage, promising device stability (95% of the initial luminance of 1000 cd m-2, LT95 > 430 h), and a high Commission Internationale de L'Éclairage y coordinate of 0.69. This indicates that managing the excitons through rational energy level alignment holds the potential for simultaneously satisfying Rec.2020 standard and achieving commercial-level stability.

Novel Polarity Fluorescent Probe for Dual-Color Visualization of Lysosomes and Plasma Membrane during Apoptosis

Apoptosis plays a crucial role in the occurrence of cancer and other diseases. Real-time monitoring of the cell apoptosis process has great significance for cell viability and drug screening. Herein, a novel fluorescent probe was constructed based on the fluorescence resonance energy transfer mechanism, which track the sensitivity of polarity changes, as well as detect the drug-induced cell apoptosis process in a dual-color mode. Importantly, the change of cellular microenvironmental polarity makes it possible to dynamically visualize the process of drug-induced cell apoptosis. More significantly, the designed probe targeted the lysosomes in the living cells to give a blue emission, and it accumulated on the plasma membrane to display red fluorescence during the drug-induced cell apoptosis process. Thus, cell viability could be monitored by both the localization and emission colors of the robust probe. We expect that the unique probe can provide a new blueprint for evaluating and screening apoptosis-related drugs.

Phosphine Sulfide-Based Bipolar Host Materials for Blue Phosphorescent Organic Light-Emitting Diodes

Three phosphine sulfide-based bipolar host materials, vizCzPhPS, DCzPhPS, and TCzPhPS, were facilely prepared through a one-pot synthesis in excellent yields. The developed hosts exhibit superior thermal stabilities with the decomposition temperatures (T_d) all exceeding 350 °C and the melting temperatures (T_m) over 200 °C. In addition, their triplet energy (E_T) levels are estimated to be higher than 3.0 eV, illustrating that they are applicable to serve as hosts for blue phosphorescent organic light-emitting diodes (PhOLEDs). The maxima luminance, current efficiency (CE), power efficiency (PE), and external quantum efficiency (EQE) of 17,223 cd m^-2, 36.7 cd A^-1, 37.5 lm W^-1, and 17.5% are achieved for the blue PhOLEDs hosted by CzPhPS. The PhOLEDs based on DCzPhPS and TCzPhPS show inferior device performance than that of CzPhPS, which might be ascribed to the deteriorated charge transporting balance as the increased number of the constructed carbazole units in DCzPhPS and TCzPhPS molecules would enhance the hole-transporting ability of the devices to a large extent. Our study demonstrates that the bipolar hosts derived from phosphine sulfide have enormous potential applications in blue PhOLEDs, and the quantity of donors should be well controlled to exploit highly efficient phosphine sulfide-based hosts.

Electrochemical phosphorylation of arenols and anilines leading to organophosphates and phosphoramidates

A practical phosphorylation for generating organophosphates and phosphoramidates via electrochemical dehydrogenative cross-coupling of P(O)H compounds with arenols and anilines is disclosed. This method involves using inorganic iodide salts as both redox catalysts and electrolytes in an undivided cell without the addition of oxidants or bases. A preliminary mechanistic study suggests that radicals are not involved in this process. This method is green and eco-friendly and has good functional group tolerance, high yields and broad substrate scope, with the potential for practical synthesis.

Organic Resonance Materials: Molecular Design, Photophysical Properties, and Optoelectronic Applications

Organic optoelectronic molecules with resonance effects are a striking class of functional materials that have witnessed booming progress in recent years. Various resonances induced by particularly constructed molecular structures can effectively influence key photophysical processes to afford particular optoelectronic properties of the organic resonance materials. The charge transport behaviors were tuned to be dynamic and self-adaptive; emission spectra were made to be very narrow with high color purity; optical bandgaps were significantly reduced, and intersystem crossing was greatly promoted. Therefore, great success has been achieved in various optoelectronic devices by using organic resonance materials to function as smart host materials with high triplet energies, highly luminescent emitters with high quantum yields and narrow emission bands, efficient organic afterglow molecules, and sensitive fluorescent probes. In this Perspective, material design principles, molecular structures and properties, and device performance of organic resonance materials are highlighted and future directions and challenges for this series of amazing materials are discussed.

Electrochemical Oxidative Dehydrogenative Phosphorylation of N-Heterocycles with P(O)-H Compounds in Imidazolium-Based Ionic Liquid

We report a direct and green electrochemical oxidative cross-dehydrogenative coupling reaction of N-heterocycles with hydrogen phosphoryl compounds under external oxidant-free conditions. Various phosphorylation products of substituted carbazoles and indoles are assembled in modest to excellent yields. A hydrogen release process is preliminarily demonstrated and H₂ is the sole byproduct. An imidazolium based ionic liquid is selected as the optimal electrolyte.

Efficient fluorescent OLEDS based on assistant acceptor modulated HLCT emissive state for enhancing singlet exciton utilization

Phenylamine phenanthroimidazole based bipolar compounds with donor-acceptor (D-A) architecture namely, 4-(1-(2,3-dihydrobenzo[b][1,4]dioxin-5-yl)-6,9-di(pyren-4-yl)-1H-phenanthro[9,10-d]imidazol-2-yl)-N,N-diphenylaniline (DDPPPA) and 4'-(1-(2,3-dihydrobenzo[b][1,4]dioxin-5-yl)-6,9-di(pyren-4-yl)-1H-phenanthro[9,10-d]imidazol-2-yl)-N,N-diphenyl-[1,1'-biphenyl]-4-amine (DDPBA) have been synthesized with highly fluorescent pyrene moieties at C6- and C9-positions. The C6 and C9 modification enhanced the thermal, photochemical and electroluminescent properties. Both molecules were employed as blue emitters in non doped organic light emitting devices (OLEDs) and show high performances due to hybridized local and charge-transfer properties. An OLED with DDPPPA/DDPBA emissive layer shows deep-blue emission with maximum external quantum efficiency (η ex), current efficiency (η c) and power efficiency (η p) of 5.7/6.0%, 10.5/12.0 cd A-1 and 8.3/9.2 lm W-1, respectively. Both devices show high singlet exciton utilizing efficiency (η s) of DDPPPA-31.33% and DDPBA-35.29%. The doped device m-MTDATA:DDPPPA/m-MTDATA:DDPBA shows maximum efficiencies of η c -7.4/8.23 cd A-1; η p -5.8/6.13 lm W-1; η ex -4.72/5.63% (5 wt%):η c -8.36/9.15 cd A-1; η p -6.32/6.65 lm W-1; η ex -4.86/5.45% (10 wt%):η c -9.58/10.02 cd A-1; η p -7.8/8.25 lm W-1; η ex -5.96/6.25% (20 wt%). The doped device based on TAPC host TAPC:DDPPPA/TAPC:DDPBA exhibits maximum efficiencies of η c -9.60/10.03 cd A-1; η p -7.81/8.26 lm W-1; η ex -5.96/6.25% (20 wt%).

5H-Benzo[d]Benzo[4,5]Imidazo[2,1-b][1,3]Thiazine as a Novel Electron-Acceptor Cored High Triplet Energy Bipolar Host Material for Efficient Solution-Processable Thermally Activated Delayed Fluorescence Organic Light-Emitting Diodes

Organic entities that can transport electrons are seldom available to develop adequate bipolar host materials applicable for solution-processable thermally activated delayed fluorescence (TADF)-organic light-emitting diodes (OLEDs). Therefore, the introduction of new electron-affine entities that plausibly demonstrate high triplet energy (E T) is of urgent need. In this contribution, we introduced benzimidazo[1,2-a][3,1]benzothiazine (BBIT) as a novel electron-affine entity and developed two new bipolar host materials, CzBBIT and 2CzBBIT. Both host materials exhibit high E T of 3.0 eV, superior thermal robustness with the thermal decomposition temperature of up to 392°C, a glass transition temperature of up to 161°C, and high solubility in common organic solvents. Consequently, the solution-processable OLEDs fabricated using a recognized IAcTr-out as the green TADF emitter doped into CzBBIT as the host, realized a maximum external quantum efficiency (EQE) of 23.3%, while the 2CzBBIT:IAcTr-out blend film-based device displayed an EQE of 18.7%. These outcomes corroborated that this work could shed light on the scientific community on the design of new electron-affine entities to establish the effective use of bipolar host materials toward proficient solution-processable TADF-OLEDs.

Delayed Blue Fluorescence via Upper-Triplet State Crossing from C-C Bonded Donor-Acceptor Charge Transfer Molecules with Azatriangulene Cores

We report the synthesis and structural and photophysical characterization of two series of molecules with functionalized azatriangulene electron donor cores and three pendant electron acceptor units. The presented donor and acceptor units are joined by C-C bonds, instead of the usual C-heteroatom bonds often found in thermally activated delayed fluorescence (TADF) emitters. The effects of the donor-acceptor strength and donor-acceptor dihedral angle on the emission properties are assessed. The data establish that the singlet-triplet energy gap is >0.3 eV and that delayed emission is present in only specific host matrices, irrespective of host polarity. Specific host behavior is atypical of many TADF materials, and we suggest the delayed emission in this work does not occur by a conventional vibronically coupled TADF mechanism, as the ΔE ST value is too large. Detailed photophysical analysis and supporting density functional theory calculations suggest that some presented azatriangulene molecules emit via an upper-triplet state crossing mechanism. This work highlights that several different mechanisms can be responsible for delayed emission, often with highly similar photophysics. Detailed photophysical analysis is required to establish which delayed emission mechanism is occurring. Our results also highlight a clear future direction toward vibronically coupled C-C bonded TADF materials.

Feasible Modification of PEDOT:PSS by Poly(4-styrenesulfonic acid): A Universal Method to Double the Efficiencies for Solution-Processed Organic Light-Emitting Devices

A feasible, universal, and low-cost strategy for solution-processed organic light-emitting diodes (OLEDs) was provided to significantly enhance the electroluminescent performances. The commercially available poly(4-styrenesulfonic acid) (PSSA) aqueous solution was mixed into poly(styrene sulfonic acid)-doped poly(3,4-ethylenedioxythiphene) (PEDOT:PSS) to modify its chemical and physical properties. The corresponding work function can be easily elevated from 5.04 to 5.63 eV. The modification of PEDOT:PSS by PSSA is found to be a universal method to demonstrate highly efficient OLEDs with different solution-processed host/emitter combinations, covering phosphorescent and thermally activated delayed fluorescence devices. The benchmarking solution-processed OLEDs based on 2,4,5,6-tetrakis(carbazol-9-yl)-1,3-dicyanobenzene and bis[2-(4,6-difluorophenyl)pyridinato-C²,N](picolinato)iridium(III) achieved the maximum external quantum efficiencies of 26.6 and 22.4%, respectively, simply by modifying PEDOT:PSS with PSSA, corresponding to the improvement factors of 2.7 and 2.2. It is confirmed that such performances originate simultaneously from reduced interfacial fluorescence quenching, elevated work function, and reduced lateral conduction of the commonly used PEDOT:PSS (Clevios P VP Al 4083).

Molecular Design Strategies for Color Tuning of Blue TADF Emitters

New thermally activated delayed fluorescence (TADF) blue emitter molecules based on the known donor-acceptor-donor (D-A-D)-type TADF molecule, 2,7-bis(9,9-dimethylacridin-10-yl)-9,9-dimethylthioxanthene-S,S-dioxide (DDMA-TXO2), are reported. The motivation for the present investigation is via the use of rational molecular design, based on DDMA-TXO2, to elevate the organic light emitting diode (OLED) performance and obtain deeper blue color coordinates. To achieve this goal, the strength of the donor (D) unit and acceptor (A) units have been tuned with methyl substituents. The methyl functionality on the acceptor was also expected to modulate the D-A torsion angle in order to obtain a blue shift in the electroluminescence. The effect of regioisomeric structures has also been investigated. Herein, we report the photophysical, electrochemical, and single-crystal X-ray crystallography data to assist with the successful OLED design. The methyl substituents on the DDMA-TXO2 framework have profound effects on the photophysics and color coordinates of the emitters. The weak electron-donating methyl groups alter the redox properties of the D and A units and consequently affect the singlet and triplet levels but not the energy gap (ΔE_ST). By systematically manipulating all of the aforementioned factors, devices have been obtained with acceptor-substituted III with a maximum external quantum efficiency of 22.6% and Commission Internationale de l'Éclairage coordinates of (0.15, 0.18) at 1000 cd m^-2.

Rational Molecular Design of Dibenzo[a,c]phenazine-Based Thermally Activated Delayed Fluorescence Emitters for Orange-Red OLEDs with EQE up to 22.0

The design and synthesis of highly efficient thermally activated delayed fluorescence (TADF) emitters with an electroluminescence wavelength beyond 600 nm remains a great challenge for organic light-emitting diodes (OLEDs). To solve this issue, three TADF molecules, xDMAC-BP (x = 1, 2, 3), are developed in combination with the rigid planar dibenzo[a,c]phenazine (BP) acceptor core and different numbers of 9,9-dimethylacridan (DMAC) donors. All these emitters possess stable internal charge transfer and a large dihedral angle between the donors and planar BP core. The emission wavelength can be regulated from 541 to 605 nm by increasing the number of the donor DMAC units because of the controllable tuning of the intramolecular charge transfer effect and the molecular geometrical structure. The photoluminescence quantum yields of these emitters are improved from 42 to 89% with the increase in the number of DMAC units. The orange-red OLEDs employing the xDMAC-BP emitters exhibit maximum external quantum efficiency (EQE) of 22.0% at 606 nm, which is the highest EQE of the previously reported TADF OLEDs exceeding 600 nm.

Unveiling the Role of Langevin and Trap-Assisted Recombination in Long Lifespan OLEDs Employing Thermally Activated Delayed Fluorophores

Recent research studies on noble-metal-free thermally activated delayed fluorescent (TADF) materials have boosted the efficiencies of organic light-emitting diodes (OLEDs) to unity. However, the short lifespan still hinders their further practical application. Carrier recombination pathways have been reported to have a significant influence on the efficiencies of TADF devices, though their effects on device lifetimes remain rarely studied. Here, we have designed and synthesized five pyrimidine or pyrazine/carbazole isomers as hosts for TADF OLEDs to explore the inherent role of Langevin recombination (LR) and trap-assisted recombination (TAR) in device lifetimes. It is revealed that for LR dominant devices, lifetimes would increase by reducing the host triplet energy levels, whereas for TAR dominant devices, lifetimes are insensitive to the host triplet excitons as recombination mainly takes place on dopants. Still, LR dominant devices are favored as they offer more room for optimization. We further apply this concept in designing a stable LR dominant blue TADF device, achieving a long LT50 (lifespan up to 50% of the initial luminance) of 269 h and high external quantum efficiency of 17.9% at 1000 cd m^-2 simultaneously.

Positively Charged Hyperbranched Polymers with Tunable Fluorescence and Cell Imaging Application

Fluorescence-tunable materials are becoming increasingly attractive because of their potential applications in optics, electronics, and biomedical technology. Herein, a multicolor molecular pixel system is realized using a simple copolymerization method. Bleeding of two complementary colors from blue and yellow fluorescence segments reproduced serious multicolor fluorescence materials. Interestingly, the emission colors of the polymers can be fine-tuned in the solid state, solution phase, and in hydrogel state. More importantly, the positive fluorescent polymers exhibited cell-membrane permeable ability and were found to accumulate on the cell nucleus, exhibiting remarkable selectivity to give bright fluorescence. The DNA/RNA selectivity experiments in vitro and in vivo verified that [tris(4-(pyridin-4-yl)phenyl)amine]-[1,8-dibromooctane] has prominent selectivity to DNA over RNA inside cells.

High efficiency solution-processed blue electrophosphorescent device with a bipolar host material based on diphenylphosphine oxide unit

A new bipolar host material BCz-BPO for solution-processed blue phosphorescent OLED was designed and synthesized. The device showed a maximum luminance efficiency of 30.1 cd A⁻¹, which was about three times higher compared with that of BCz-PO.

Solution-processed thermally activated delayed fluorescence organic light-emitting diodes using a new polymeric emitter containing non-conjugated cyclohexane units

A new solution-processable polymeric emitter containing non-conjugated cyclohexane units was developed for high-performing TADF-OLEDs.

Aromatically C6- and C9-Substituted Phenanthro[9,10-d]imidazole Blue Fluorophores: Structure-Property Relationship and Electroluminescent Application

In this study, a series of aromatically substituted phenanthro[9,10-d]imidazole (PI) fluorophores at C6 and C9 (no. 6 and 9 carbon atoms) have been synthesized and systematically characterized by theoretical, thermal, photophysical, electrochemical, and electroluminescent (EL) studies. C6 and C9 modifications have positive influences on the thermal properties of the new materials. Theoretical calculations suggest that the C6 and the C9 positions of PI are electronically different. Theoretical and experimental evidences of intramolecular charge transfer (ICT) between two identical moieties attaching to the C6 and the C9 positions are observed. Photophysical properties of the fluorophores are greatly influenced by size and conjugation extent of the substituents as well as linking steric hindrance. It is found that the C6 and C9 positions afford moderate conjugated extension compared to the C2 modification. Moreover, ICT characteristics of the new fluorophores increase as the size of the substituted aromatic group, and are partially influenced by steric hindrance, with the anthracene and the pyrene derivatives having the strongest ICT excited properties. EL performances of the fluorophores were evaluated as host emitters or dopants in organic light-emitting devices (OLEDs). Most of the devices showed significantly improved efficiencies compared to the OLED using the nonmodified emitter. Among all the devices, a 5 wt % TPI-Py doped device exhibited excellent performances with an external quantum efficiency >5% at 1000 cd/m² and a deep-blue color index of (0.155, 0.065), which are comparable to the most advanced deep-blue devices. Our study can give useful information for designing C6/C9-modificated PI fluorophores and provide an efficient approach for constructing high-performance deep-blue OLEDs.

Design of encapsulated hosts and guests for highly efficient blue and green thermally activated delayed fluorescence OLEDs based on a solution-process

The molecular aggregation and exciton–polaron interaction of the TADF host–guest system were successfully restricted by efficient molecular encapsulation.

Recent advances in organic thermally activated delayed fluorescence materials

Thermally activated delayed fluorescence: harvesting dark triplet excitons to generate bright emissive singlet excitons.

Dipolar vinyl sulfur fluorescent dyes. Synthesis and photophysics of sulfide, sulfoxide and sulfone based D–π–A compounds

New vinyl sulfides and sulfoxides were obtained in good yields using the Horner–Wadsworth–Emmons (HWE) reaction.

Modeling electrochromic poly-dioxythiophene-containing materials through TDDFT

Here we report the first colormetric model using mPW1PBE/cc-PVDZ with CPCM to predict the chemical properties of three electrochromic polymers in which the largest UV-Vis absolute peak maximum difference when compared to experiment was 14 nm.