Anthraquinone-based intramolecular charge-transfer compounds: computational molecular design, thermally activated delayed fluorescence, and highly efficient red electroluminescence

Mulifunctional Dendritic Emitter: Aggregation-Induced Emission Enhanced, Thermally Activated Delayed Fluorescent Material for Solution-Processed Multilayered Organic Light-Emitting Diodes

Thermally activated delayed fluorescence (TADF) materials emerged as promising light sources in third generation organic light-emitting diodes (OLED). Much effort has been invested for the development of small molecular TADF materials and vacuum process-based efficient TADF-OLEDs. In contrast, a limited number of solution processable high-molecular weight TADF materials toward low cost, large area, and scalable manufacturing of solution processed TADF-OLEDs have been reported so far. In this context, we report benzophenone-core carbazole dendrimers (GnB, n = generation) showing TADF and aggregation-induced emission enhancement (AIEE) properties along with alcohol resistance enabling further solution-based lamination of organic materials. The dendritic structure was found to play an important role for both TADF and AIEE activities in the neat films. By using these multifunctional dendritic emitters as non-doped emissive layers, OLED devices with fully solution processed organic multilayers were successfully fabricated and achieved maximum external quantum efficiency of 5.7%.

Functional Pyrimidine-Based Thermally Activated Delay Fluorescence Emitters: Photophysics, Mechanochromism, and Fabrication of Organic Light-Emitting Diodes

A new series of molecules, T1-T4, possessing thermally activated delayed fluorescence (TADF) have been strategically designed and synthesized. Molecules T1-T4 contain the dimethyl acridine as the electron donor, which is linked to either symmetrical or unsymmetrical diphenyl pyrimidine as an acceptor. In comparison to the ubiquitous triazine acceptor, the selection of pyrimidine as an acceptor has advantages of facile functionalization and less stabilized unoccupied π orbitals, so that the energy gap toward the blue region can be accessed. Together with acridine donors, the resulting donor-acceptor functional materials reveal remarkable TADF properties. In the solid state, molecules T1-T4 all exhibit intriguing mechanochromism. The crystal structures, together with spectroscopy and dynamics acquired upon application of stressing, lead us to propose two types of structural arrangement that give distinct emission properties, one with and the other without TADF. Upon fabricating organic light-emitting diodes, the T1-T4 films prepared from sublimation all exhibit dominant TADF behavior; this accounts for their high performance: an electroluminescent emission at λ=490 nm, with an external quantum efficiency of 14.2 %, can be attained when T2 is used as an emitter.

Excited state dynamics for hybridized local and charge transfer state fluorescent emitters with aggregation-induced emission in the solid phase: a QM/MM study

Investigation on the excited state dynamics to reveal the AIE and HLCT mechanisms by a QM/MM method.

Theoretical tuning of the singlet–triplet energy gap to achieve efficient long-wavelength thermally activated delayed fluorescence emitters: the impact of substituents

Long-wavelength TADF emitters could be achieved with both small ΔE_ST and high k_r through introducing small electron-withdrawing substituents!

Recent advances in organic thermally activated delayed fluorescence materials

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

Synthesis and optical and electrochemical properties of julolidine-structured pyrido[3,4-b]indole dye

The julolidine-structured pyrido[3,4-b]indole dye ET-1 possesses the ability to act as a calorimetric and fluorescent sensor for Brønsted and Lewis acids.

Achieving a high-efficiency dual-core chromophore for emission of blue light by testing different side groups and substitution positions

The optical and electronic properties of hetero dual core derivatives can be controlled by simply substituting an electron-donating side group.

Thermally activated delayed fluorescence OLEDs with fully solution processed organic layers exhibiting nearly 10% external quantum efficiency

An OLED device with fully solution processed organic layers with new laminatable TADF dendrimers, which exhibits 9.5% external quantum efficiency, is reported.

Adjustment of the solid fluorescence of a chalcone derivative through controlling steric hindrance

A chalcone derivative, ANPEO, exhibits weak emission in the solution state but high fluorescence efficiency in the solid state (66%).

Controlling Singlet-Triplet Energy Splitting for Deep-Blue Thermally Activated Delayed Fluorescence Emitters

The development of efficient metal-free organic emitters with thermally activated delayed fluorescence (TADF) properties for deep-blue emission is still challenging. A new family of deep-blue TADF emitters based on a donor-acceptor architecture has been developed. The electronic interaction between donor and acceptor plays a key role in the TADF mechanism. Deep-blue OLEDs fabricated with these TADF emitters achieved high external quantum efficiencies over 19.2 % with CIE coordinates of (0.148, 0.098).

High Performance All Fluorescence White Organic Light Emitting Devices with a Highly Simplified Structure Based on Thermally Activated Delayed Fluorescence Dopants and Host

Thermally activated delayed fluorescence (TADF) emitters of different colors commonly need different hosts, which cause the complexed device structure and low efficiency of all fluorescence white organic light-emitting devices (F-WOLEDs). To solve this, novel concept of employing TADF exciplex as universal host of TADF emitters with different colors was proposed. All blue, green, and orange devices based on the TADF exciplex host show much lower turn-on voltages, and comparable and even higher efficiencies than corresponding devices based on conventional hosts. The two color F-WOLED with extremely simplified device structure was finally fabricated, achieving a white emission with the maximum current efficiency (CE), power efficiency (PE), and external quantum efficiency (EQE) respectively of 50.1 cd A^-1, 63.0 lm W^-1, and 19.0% in the forward-viewing direction without any light out-coupling technology, which is the best performance among reported F-WOLEDs, demonstrating the superiority of the novel concept.

Managing Excitons and Charges for High-Performance Fluorescent White Organic Light-Emitting Diodes

The simultaneous realization of high efficiency, stable spectra, high color rendering index (CRI), and low-efficiency roll-off in a fluorescent white organic light-emitting diode (WOLED) still remains a big challenge. Here, we demonstrate high-performance conventional fluorescent-dopant-based WOLEDs by strategic management of singlet and triplet excitons within an efficient emissive zone. This design consists of two separated red/green sub-EMLs with ultralow doping concentration and a sandwiched sub-EML doped with red and green fluorescent dyes at a relatively high concentration, which can harness all electrogenerated excitons and reduce the energy loss to the utmost extent. Accordingly, the resulting WOLED realizes an external quantum efficiency (EQE) of 18.2% with a maximum power efficiency of 44.6 lm W^-1. At the practical luminance of 1000 cd m^-2 for the lighting source, the EQE still remains as high as 16.2% with a CRI of 82 and stable color spectra. A comprehensive understanding of the device working mechanism is performed to guide design of efficient and stable fluorescent WOLEDs.

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

A series of polarity-tunable host materials were developed based on oligocarbazoles and diphenylphosphine oxide, and their polarities can be tuned through increasing distance of acceptor and donor units. Density functional theory calculations were employed, and photoluminescence spectra in different polar solvents were measured to illustrate different polarities of these host materials. As CZPO has relatively stronger polarity, electroluminescence (EL) spectrum of solution-processed device based on 6 wt % PXZDSO2:CZPO is 7 nm red-shifted relative to that of other host materials based devices. Besides, a comparable impressive external quantum efficiency (EQE) value of 18.7% is achieved for an evaporation-processed yellow device consisting of FCZBn, which is superior to that of the device based on CBP (4,4'-dicarbazolyl-1,1'-biphenyl) (17.0%), and its efficiency roll-off is also obviously reduced, giving an EQE value as high as 16.3% at the luminance of 1000 cd/m². In addition, from CZPO to FCZBn as the polarities of host materials decrease, EL spectra of solution-processed devices based on DMAC-DPS emitter blue-shift constantly from 496 to 470 nm. The current work gives a constructive approach to control EL spectra of organic light-emitting diodes with a fixed thermally activated delayed fluorescence emitter by tuning the polarities of host materials.

A Method for Reducing the Singlet-Triplet Energy Gaps of TADF Materials for Improving the Blue OLED Efficiency

We have successfully synthesized a series of blue thermally activated delayed fluorescence emitters, BPy-pC, BPy-pTC, BPy-p2C, and BPy-p3C, bearing a 4-benzoylpyridine core as the electron-accepting unit and carbazolyl, tert-butylcarbazolyl, dicarbazolyl, and tercarbazolyl groups as the electron-donating units, respectively. The density functional theory calculation shows that all of the compounds have their lowest unoccupied molecular orbitals on the benzoylpyridine moiety. However, the highest occupied molecular orbital (HOMO) of BPy-p3C is widely dispersed to the whole tercarbazolyl group, while the HOMOs of BPy-pC and BPy-pTC are mainly on the carbazolyl and extended to the phenyl ring. As a result, ΔE_ST is reduced from 0.29 eV for BPy-pC to 0.05 eV for BPy-p3C, and the organic light-emitting diodes using these materials as dopants emit blue light and their maximum external quantum efficiencies (EQEs) increase from 4.2% to 23.9% for BPy-pC and BPy-p3C, respectively. The EQE of the BPy-p3C-based device increases 2 times more than that of the BPy-pTC-based device without a significant change in the color coordinates.

Applications of Palladium-Catalyzed C-N Cross-Coupling Reactions

Pd-catalyzed cross-coupling reactions that form C–N bonds have become useful methods to synthesize anilines and aniline derivatives, an important class of compounds throughout chemical research. A key factor in the widespread adoption of these methods has been the continued development of reliable and versatile catalysts that function under operationally simple, user-friendly conditions. This review provides an overview of Pd-catalyzed N-arylation reactions found in both basic and applied chemical research from 2008 to the present. Selected examples of C–N cross-coupling reactions between nine classes of nitrogen-based coupling partners and (pseudo)aryl halides are described for the synthesis of heterocycles, medicinally relevant compounds, natural products, organic materials, and catalysts.

Highly Efficient Thermally Activated Delayed Fluorescence in Dinuclear Ag(I) Complexes with a Bis-Bidentate Tetraphosphane Bridging Ligand

A series of highly emissive neutral dinuclear silver complexes [Ag(PPh₃)(X)]₂(tpbz) (tpbz = 1,2,4,5-tetrakis(diphenylphosphanyl)benzene; X = Cl (1), Br (2), I (3)) was synthesized and structurally characterized. In the complexes, the silver atoms with tetradedral geometry are bridged by the tpbz ligand, and the ends of the molecules are coordinated by a halogen anion and a terminal triphenylphosphine ligand for each silver atom. These complexes exhibit intense white-blue (λ_max = 475 nm (1) and 471 nm (2)) and green (λ_max = 495 nm (3)) photoluminescence in the solid state with quantum yields of up to 98% (1) and emissive decay rates of up to 3.3 × 10⁵ s^-1 (1) at 298 K. With temperature decreasing from 298 to 77 K, a red shift of the emission maximum by 9 nm for all these complexes is observed. The temperature dependence of the luminescence for complex 1 in solid state indicates that the emission originates from two thermally equilibrated charge transfer (CT) excited states and exhibits highly efficient thermally activated delayed fluorescence (TADF) at ambient temperature. At 77 K, the decay time is 638 μs, indicating that the emission is mainly from a triplet state (T₁ state). With temperature increasing from 77 to 298 K, a significant decrease of the emissive decay time by a factor of almost 210 is observed, and at 298 K, the decay time is 3.0 μs. The remarkable decrease of the decay time indicates that thermal population of a short-lived singlet state (S₁ state) increases as the temperature increases. The charge transfer character of the excited states and TADF behavior of the complexes are interrogated by DFT and TDDFT calculations. The computational results demonstrate that the origin of TADF can be ascribed to ^1,3(ILCT + XLCT+ MLCT) states in complexes 1 and 2 and ^1,3(XLCT) states mixed with minor contributions of MLCT and ILCT in complex 3.

Controlling Synergistic Oxidation Processes for Efficient and Stable Blue Thermally Activated Delayed Fluorescence Devices

Efficient sky-blue organic light-emitting diodes (OLEDs) employing thermally activated delayed fluorescence (TADF) display a three orders of magnitude increase in lifetime, which is superior to those of controlled phosphorescent OLEDs used in this study. The combination of electro-oxidation and photo-oxidation of the TADF emitters in their triplet excited-states is suppressed through molecule design and device engineering.

Modulation of Exciton Generation in Organic Active Planar pn Heterojunction: Toward Low Driving Voltage and High-Efficiency OLEDs Employing Conventional and Thermally Activated Delayed Fluorescent Emitters

Organic light-emitting diodes (OLEDs) combining low driving voltage and high efficiency are designed by employing conventional and thermally activated delayed fluorescence emitters through modulation of excitons generated at the planar p-n heterojunction region. To date, this approach enables the highest power efficiency for yellow-green emitting fluorescent OLEDs with a simplified structure.

The Importance of Vibronic Coupling for Efficient Reverse Intersystem Crossing in Thermally Activated Delayed Fluorescence Molecules

Factors influencing the rate of reverse intersystem crossing (k_rISC ) in thermally activated delayed fluorescence (TADF) emitters are critical for improving the efficiency and performance of third-generation heavy-metal-free organic light-emitting diodes (OLEDs). However, present understanding of the TADF mechanism does not extend far beyond a thermal equilibrium between the lowest singlet and triplet states and consequently research has focused almost exclusively on the energy gap between these two states. Herein, we use a model spin-vibronic Hamiltonian to reveal the crucial role of non-Born-Oppenheimer effects in determining k_rISC . We demonstrate that vibronic (nonadiabatic) coupling between the lowest local excitation triplet (³ LE) and lowest charge transfer triplet (³ CT) opens the possibility for significant second-order coupling effects and increases k_rISC by about four orders of magnitude. Crucially, these simulations reveal the dynamical mechanism for highly efficient TADF and opens design routes that go beyond the Born-Oppenheimer approximation for the future development of high-performing systems.

Photoinduced Charge-Transfer State of 4-Carbazolyl-3-(trifluoromethyl)benzoic Acid: Photophysical Property and Application to Reduction of Carbon-Halogen Bonds as a Sensitizer

The photoinduced persistent intramolecular charge-transfer state of 4-carbazolyl-3-(trifluoromethyl)benzoic acid was confirmed. It showed a higher catalytic activity in terms of yield and selectivity in the photochemical reduction of alkyl halides compared to the parent carbazole. Even unactivated primary alkyl bromides could be reduced by this photocatalyst. The high catalytic activity is rationalized by considering the slower backward single-electron transfer owing to the spatial separation of the donor and acceptor subunits.

High-Efficiency WOLEDs with High Color-Rendering Index based on a Chromaticity-Adjustable Yellow Thermally Activated Delayed Fluorescence Emitter

A chromaticity-adjustable yellow thermally activated delayed fluorescence (TADF) material, PXZDSO2 as a triplet harvester provides a rational device concept, giving two-color and three-color pure organic white organic light-emitting diodes (WOLEDs) with unprecedented color-rendering index of 95 and external quantum efficiency of 19.2%.

Highly efficient blue thermally activated delayed fluorescent OLEDs with record-low driving voltages utilizing high triplet energy hosts with small singlet-triplet splittings

The high driving voltage of blue organic light-emitting diodes (OLEDs) based on emitters with thermally activated delayed fluorescence (TADF) remains a constraint for their portable application. A major reason for this is that the high triplet (T1) of the host required to match the blue TADF emitters would always lead to inferiority in terms of carrier injection. Therefore, a suitable host should possess not only a high T1 but also a relatively low singlet (S1) for improved carrier injection, indicating that small singlet-triplet splittings (ΔESTs) are highly desired. Here, four carbazolyl benzonitrile derivatives are facilely prepared in a one-step approach with restrained conjugate lengths to maintain high triplet energies while their highly twisted structures spatially separate the frontier orbital distribution to achieve relatively low ΔESTs. Meanwhile, the charge transporting mobilities of these hosts are effectively tuned by the different linker types of the host moieties. Consequently, high-triplet-energy hosts with favorable carrier injection/transporting abilities are realized, endowing blue TADF devices with a maximum external quantum efficiency of 21.5%, a maximum power efficiency of 42.0 lm W-1 and an ultra-low onset voltage of 2.8 V. It is noteworthy that a driving voltage of 4.9 V is achieved at a practical luminance of 1000 cd m-2, which is the lowest among the doped blue TADF OLEDs reported until now. This work suggests that manipulation of the molecular topologies not only leads to the flexible and feasible design of novel bipolar host materials, but also affords a promising method for fine-tuning physical properties and thus obtaining state-of-the-art device performances.

Ultrapure Blue Thermally Activated Delayed Fluorescence Molecules: Efficient HOMO-LUMO Separation by the Multiple Resonance Effect

Ultrapure blue-fluorescent molecules based on thermally activated delayed fluorescence are developed. Organic light-emitting diode (OLED) devices employing the new emitters exhibit a deep blue emission at 467 nm with a full-width at half-maximum of 28 nm, CIE coordinates of (0.12, 0.13), and an internal quantum efficiency of ≈100%, which represent record-setting performance for blue OLED devices.

Structure-Performance Investigation of Thioxanthone Derivatives for Developing Color Tunable Highly Efficient Thermally Activated Delayed Fluorescence Emitters

Thioxanthone derivatives consisting of undecorated carbazole as an electron donor and thioxanthone (TXO) or 9H-thioxanthen-9-one-S,S-dioxide (SOXO) as an electron acceptor in a donor-acceptor (D-A) or donor-acceptor-donor (D-A-D) structure were developed as thermally activated delayed fluorescence emitters to fabricate highly efficient fluorescent organic light emitting diodes. Their emission color was successfully tuned from blue to yellow by changing the sulfur atom valence state of the thioxanthone unit to tune intramolecular charge transfer effect. Their thermal, electrochemical, photophysical, and electroluminescent properties, and theoretical calculations were systematically investigated to illustrate the molecular structure and property relationships. Maximum external quantum efficiency (EQE) of 13.6% with Commission Internationale de L'Eclairage coordinates of (0.37, 0.57) was achieved for green light emission CzSOXO consisting of SOXO and carbazole in a D-A structure. Blue light emission CzTXO and DCzTXO consisting of TXO and carbazole in a D-A and D-A-D structure could also give EQE values exceeding 11%. Their efficiency roll-off with increasing current density was simulated by adopting triplet-triplet annihilation model, indicating that the TXO derivatives suffer more severe efficiency roll-off because of their relatively long delayed fluorescence lifetime (τ(D)).

Unparalleled Ease of Access to a Library of Biheteroaryl Fluorophores via Oxidative Cross-Coupling Reactions: Discovery of Photostable NIR Probe for Mitochondria

The development of straightforward accesses to organic functional materials through C-H activation is a revolutionary trend in organic synthesis. In this article, we propose a concise strategy to construct a large library of donor-acceptor-type biheteroaryl fluorophores via the palladium-catalyzed oxidative C-H/C-H cross-coupling of electron-deficient 2H-indazoles with electron-rich heteroarenes. The directly coupled biheteroaryl fluorophores, named Indazo-Fluors, exhibit continuously tunable full-color emissions with quantum yields up to 93% and large Stokes shifts up to 8705 cm(-1) in CH2Cl2. By further fine-tuning of the substituent on the core skeleton, Indazo-Fluor 3l (FW = 274; λem = 725 nm) is obtained as the lowest molecular weight near-infrared (NIR) fluorophore with emission wavelength over 720 nm in the solid state. The NIR dye 5h specifically lights up mitochondria in living cells with bright red luminescence. Typically, commercially available mitochondria trackers suffer from poor photostability. Indazo-Fluor 5h exhibits superior photostability and very low cytotoxicity, which would be a prominent reagent for in vivo mitochondria imaging.

Design and Synthesis of New Circularly Polarized Thermally Activated Delayed Fluorescence Emitters

This work describes the first thermally activated delayed fluorescence material enabling circularly polarized light emission through chiral perturbation. These new molecular architectures obtained through a scalable one-pot sequential synthetic procedure at room temperature (83% yield) display high quantum yield (up to 74%) and circularly polarized luminescence with an absolute luminescence dissymmetry factor, |glum|, of 1.3 × 10(-3). These chiral molecules have been used as an emissive dopant in an organic light emitting diode exhibiting external quantum efficiency as high as 9.1%.

"Rate-limited effect" of reverse intersystem crossing process: the key for tuning thermally activated delayed fluorescence lifetime and efficiency roll-off of organic light emitting diodes

The rate constant of reverse intersystem crossing was found to be the “rate-limited step” in thermally activated delayed fluorescence lifetime governing.

Issues concerning excited state lifetime (τ_TADF) tuning of thermally activated delayed fluorescence (TADF) materials are critical for organic light emitting diode (OLED) applications and other specific fields. For TADF-OLEDs, employing emitters with a short τ_TADF gives rise to suppressed singlet–triplet annihilation (STA) and triplet–triplet annihilation (TTA), leading to reduced efficiency roll-off at practical relevant brightness (100 and 1000 cd m^–2 for display and illumination applications, respectively). Through molecular design, exciton dynamic process rate constants including fluorescence (k_F), intersystem crossing (k_ISC), internal conversion (k_IC) and reverse intersystem crossing (k_RISC) are selectively altered, affording four representative TADF emitters. Based on lifetime and quantum yield measurements, k_F, k_ISC, k_IC and k_RISC are calculated for four emitters and their interrelationship matches corrected time-dependent density functional theory simulation. Among them, even with a small k_F, low photoluminescence quantum efficiency (Φ) and large k_ISC, molecules with a small singlet–triplet splitting energy (ΔE_ST) and lowest charge transfer triplet excited state (³CT) eventuate in shortening the τ_TADF. Herein, k_RISC, which is inversely proportional to ΔE_ST, turns out to be the rate-limited factor in tuning the τ_TADF (“rate limited effect” of the RISC process). As revealed by flexible potential surface scanning, PyCN–ACR exhibited a moderate k_F, reduced k_IC and enlarged k_RISC, resulting in a short τ_TADF and a moderate Φ with orange-red emission. OLEDs containing PyCN–ACR as the emitting guest achieved orange-red TADF-OLEDs with an emission peak at 590 nm and the best external quantum efficiencies (EQEs) of 12.4%/9.9%/5.1% at practical luminances of 100/1000/10 000 cd m^–2.