Pyridine-Carbonitrile-Carbazole-Based Delayed Fluorescence Materials with Highly Congested Structures and Excellent OLED Performance

Exciplex, Not Heavy-Atom Effect, Controls the Triplet Dynamics of a Series of Sulfur-Containing Thermally Activated Delayed Fluorescence Molecules

The efficiency of thermally activated delayed fluorescence (TADF) in organic materials relies on rapid intersystem crossing rates and fast conversion of triplet (T) excitons into a singlet (S) state. Heavy atoms such as sulfur or selenium are now frequently incorporated into TADF molecular structures to enhance these properties by increased spin-orbit coupling [spin orbit coupling (SOC)] between the T and S states. Here a series of donor-acceptor (D-A) molecules based on 12H-benzo[4,5]thieno[2,3-a]carbazole and dicyanopyridine is compared with their nonsulfur control molecules designed to probe such SOC effects. We reveal that unexpected intermolecular interactions of the D-A molecules with carbazole-containing host materials instead serve as the dominant pathway for triplet decay kinetics in these materials. In-depth photophysical and computational studies combined with organic light emitting diode measurements demonstrate that the anticipated heavy-atom effect from sulfur is overshadowed by exciplex formation. Indeed, even the unsubstituted acceptor fragments exhibit pronounced TADF exciplex emission in appropriate carbazole hosts. The intermolecular charge transfer and TADF in these systems are further confirmed by detailed time-dependent density functional theory studies. This work demonstrates that anticipated heavy-atom effects in TADF emitters do not always control or even impact the photophysical and electroluminescence properties.

Effects of Electron-Withdrawing Strengths of the Substituents on the Properties of 4-(Carbazolyl-R-benzoyl)-5-CF3-1H-1,2,3-triazole Derivatives as Blue Emitters for Doping-Free Electroluminescence Devices

The synthesis of four 4-(carbazolyl-R-benzoyl)-5-CF3-1H-1,2,3-triazoles with extra groups ((3-methyl)-phenyl-, 4-fluorophenyl-, quinolinyl-, or (3-trifluoromethyl)-phenyl-) in the acceptor fragment has been reported. The effects of substituents with different electron-withdrawing strengths on the thermal, electrochemical, photophysical, and electroluminescence properties of the synthesized compounds are discussed. The results of X-ray analyses and density functional theory (DFT) calculations support unusual molecular packing and electronic properties. The compounds are capable of glass formation with glass transition temperatures ranging from 54-84 °C. Ionization potentials of the compounds are in the range of 5.98-6.22 eV and electron affinities range from 3.09 to 3.35 eV. Under ultraviolet excitation, the neat films of the compounds exhibit blue emission with photoluminescence quantum yields ranging from 18 to 27%. The films of selected compounds are used for the preparation of host-free light-emitting layers of organic light-emitting diodes with very simple device structures and an external quantum efficiency of 4.6%.

Dihydroindenofluorenes as building units in organic semiconductors for organic electronics

This review aims to discuss organic semiconductors constructed on dihydroindenofluorene positional isomers, which are key molecular scaffolds in organic electronics. Bridged oligophenylenes are key organic semiconductors that have allowed the development of organic electronic technologies. Dihydroindenofluorenes (DHIFs) belong to the family of bridged oligophenylenes constructed on a terphenyl backbone. They have proven to be very promising building blocks for the construction of highly efficient organic semiconductors for all OE devices, namely organic light emitting diodes (OLEDs), phosphorescent OLEDs, organic field-effect transistors (OFETs), solar cells, etc.

Highly Twisted Thermally Activated Delayed Fluorescence (TADF) Molecules and Their Applications in Organic Light-Emitting Diodes (OLEDs)

Thermally activated delayed fluorescence (TADF) materials have attracted great potential in the field of organic light-emitting diodes (OLEDs). Among thousands of TADF materials, highly twisted TADF emitters have become a hotspot in recent years. Compared with traditional TADF materials, highly twisted TADF emitters tend to show multi-channel charge-transfer characters and form rigid molecular structures. This is advantageous for TADF materials, as non-radiative decay processes can be suppressed to facilitate efficient exciton utilization. Accordingly, OLEDs with excellent device performances have also been reported. In this Review, we have summarized recent progress in highly twisted TADF materials and related devices, and give an overview of the molecular design strategies, photophysical studies, and the performances of OLED devices. In addition, the challenges and perspectives of highly twisted TADF molecules and the related OLEDs are also discussed.

Modifications of Pyridine-3,5-dicarbonitrile Acceptor for Highly Efficient Green-to-Red Organic Light-Emitting Diodes

The strategy of acceptor modification is a powerful technique for tuning the emission color of thermally activated delayed fluorescence (TADF) emitters. In this study, we have successfully designed and synthesized three TADF emitters with donor-acceptor (D-A) structures using a 4-(diphenylamino)-2,6-dimethylphenyl (TPAm) donor and various pyridine-3,5-dicarbonitrile (PC) acceptor units. As a result, three compounds named TPAmbPPC, TPAm2NPC, and TPAmCPPC exhibited greenish-yellow to orange-red emissions with high photoluminescent quantum yields (76-100%) in thin films. Remarkably, a greenish-yellow device based on TPAmbPPC and TPAm2NPC showed a high maximum external quantum efficiency (EQE_max) of 39.1 and 39.0%, respectively. Furthermore, benefiting from the suitable steric hindrance between the acceptor and donor, the nondoped organic light-emitting diodes (OLEDs) based on TPAmbPPC demonstrated an exceptional EQE_max of 21.6%, indicating its promising potential as an efficient emitter for the application of OLED applications. Furthermore, orange-red OLED devices based on TPAmCPPC exhibited a high EQE_max of 26.2%, a CE of 50.1 cd A^-1, and a PE of 52.4 lm W^-1.

Progresses and Perspectives of Near-Infrared Emission Materials with "Heavy Metal-Free" Organic Compounds for Electroluminescence

Organic/polymer light-emitting diodes (OLEDs/PLEDs) have attracted a rising number of investigations due to their promising applications for high-resolution fullcolor displays and energy-saving solid-state lightings. Near-infrared (NIR) emitting dyes have gained increasing attention for their potential applications in electroluminescence and optical imaging in optical tele-communication platforms, sensing and medical diagnosis in recent decades. And a growing number of people focus on the "heavy metal-free" NIR electroluminescent materials to gain more design freedom with cost advantage. This review presents recent progresses in conjugated polymers and organic molecules for OLEDs/PLEDs according to their different luminous mechanism and constructing systems. The relationships between the organic fluorophores structures and electroluminescence properties are the main focus of this review. Finally, the approaches to enhance the performance of NIR OLEDs/PLEDs are described briefly. We hope that this review could provide a new perspective for NIR materials and inspire breakthroughs in fundamental research and applications.

Ornamenting of Blue Thermally Activated Delayed Fluorescence Emitters by Anchor Groups for the Minimization of Solid-State Solvation and Conformation Disorder Corollaries in Non-Doped and Doped Organic Light-Emitting Diodes

Motivated to minimize the effects of solid-state solvation and conformation disorder on emission properties of donor-acceptor-type emitters, we developed five new asymmetric multiple donor-acceptor type derivatives of tert-butyl carbazole and trifluoromethyl benzene exploiting different electron-accepting anchoring groups. Using this design strategy, for a compound containing four di-tert-butyl carbazole units as donors as well as 5-methyl pyrimidine and trifluoromethyl acceptor moieties, small singlet-triplet splitting of ca. 0.03 eV, reverse intersystem crossing rate of 1 × 10⁶ s^-1, and high photoluminescence quantum yield of neat film of ca. 75% were achieved. This compound was also characterized by the high value of hole and electron mobilities of 8.9 × 10^-4 and 5.8 × 10^-4 cm² V^-1 s^-1 at an electric field of 4.7 × 10⁵ V/cm, showing relatively good hole/electron balance, respectively. Due to the lowest conformational disorder and solid-state solvation effects, this compound demonstrated very similar emission properties (emission colors) in non-doped and differently doped organic light-emitting diodes (OLEDs). The lowest conformational disorder was observed for the compound with the additional accepting moiety inducing steric hindrance, limiting donor-acceptor dihedral rotational freedom. It can be exploited in the multi-donor-acceptor approach, increasing the efficiency. Using an emitter exhibiting the minimized solid-state solvation and conformation disorder effects, the sky blue OLED with the emitting layer of this compound dispersed in host 1,3-bis(N-carbazolyl)benzene displayed an emission peak at 477 nm, high brightness over 39 000 cd/m², and external quantum efficiency up to 15.9% along with a maximum current efficiency of 42.6 cd/A and a maximum power efficiency of 24.1 lm/W.

Boosting external quantum efficiency to 38.6% of sky-blue delayed fluorescence molecules by optimizing horizontal dipole orientation

To achieve high electroluminescence efficiency, great efforts are devoted to enhancing photoluminescence quantum yield (Φ_PL) and exciton utilization of luminescent molecule, while another important factor, light out-coupling efficiency (η_out), receives less attention in molecule design. Here, we focus on horizontal dipole orientation engineering of the molecule to increase η_out and external quantum efficiency (η_ext). A series of tailor-made luminescent molecules consisting of an electron-accepting carbonyl core plus double electron-donating groups of spiro[acridine-9,9′-fluorene] and carbazole derivatives [e.g., 1,3,5-tri(carbazol-9-yl)benzene] are developed and systematically investigated. These molecules hold distinguished merits of strong sky-blue delayed fluorescence with excellent Φ_PL values, large horizontal dipole ratios, and balanced bipolar carrier transport, which furnish record-high η_ext values of up to 26.1 and 38.6% in nondoped and doped sky-blue organic light-emitting diodes (OLEDs), respectively. Moreover, the state-of-the-art nondoped hybrid white OLED and all-fluorescence single-emitting layer white OLED are also realized, demonstrating great potentials in OLED industry of these molecules.

Triarylamine-Pyridine-Carbonitriles for Organic Light-Emitting Devices with EQE Nearly 40

Highly efficient thermally activated delayed fluorescence (TADF) molecules are in urgent demand for solid-state lighting and full-color displays. Here, the design and synthesis of three triarylamine-pyridine-carbonitrile-based TADF compounds, TPAPPC, TPAmPPC, and tTPAmPPC, are shown. They exhibit excellent photoluminescence quantum yields of 79-100% with small ΔE_ST values, fast reverse intersystem crossing (RISC), and high horizontal dipole ratios (Θ_//  = 86-88%) in the thin films leading to the enhancement of device light outcoupling. Consequently, a green organic light-emitting diode (OLED) based on TPAmPPC shows a high average external quantum efficiency of 38.8 ± 0.6%, a current efficiency of 130.1 ± 2.1 cd A^-1 , and a power efficiency of 136.3 ± 2.2 lm W^-1 . The highest device efficiency of 39.8% appears to be record-breaking among TADF-based OLEDs to date. In addition, the TPAmPPC-based device shows superior operation lifetime and high-temperature resistance. It is worth noting that the TPA-PPC-based materials have excellent optical properties and the potential for making them strong candidates for TADF practical application.

Identification of the Key Parameters for Horizontal Transition Dipole Orientation in Fluorescent and TADF Organic Light-Emitting Diodes

In organic light-emitting diodes (OLEDs), horizontal orientation of the emissive transition dipole moment (TDM) can improve light outcoupling efficiency by up to 50% relative to random orientation. Therefore, there have been extensive efforts to identify drivers of horizontal orientation. The aspect ratio of the emitter molecule and the glass-transition temperature (Tg ) of the films are currently regarded as particularly important. However, there remains a paucity of systematic studies that establish the extent to which these and other parameters control orientation in the wide range of emitter systems relevant for state-of-the-art OLEDs. Here, recent work on molecular orientation of fluorescent and thermally activated delayed fluorescent emitters in vacuum-processed OLEDs is reviewed. Additionally, to identify parameters linked to TDM orientation, a meta-analysis of 203 published emitter systems is conducted and combined with density-functional theory calculations. Molecular weight (MW) and linearity are identified as key parameters in neat systems. In host-guest systems with low-MW emitters, orientation is mostly influenced by the host Tg , whereas the length and MW of the emitter become more relevant for systems involving higher-MW emitters. To close, a perspective of where the field must advance to establish a comprehensive model of molecular orientation is given.

Management of Locally Excited States for Purine-based TADF Emitters: A Method to Reduce Device Efficiency Roll-Off

The programmed arylation of purine has been developed to construct a series of efficient thermally activated delayed fluorescent (TADF) materials. The corresponding organic light-emitting diodes (OLEDs) exhibit external quantum efficiency as high as 16.0% alongside small efficiency roll-off. Intriguingly, this work proves that the good management of localized states is an efficient way to reduce device efficiency roll-off and is crucial for the future design of high-performance OLEDs.

Speculative assessment, molecular composition, PDOS, topology exploration (ELF, LOL, RDG), ligand-protein interactions, on 5-bromo-3-nitropyridine-2-carbonitrile

Computational calculations of 5-bromo-3-nitropyridine-2-carbonitrile (5B3N2C) on molecular structure and on energy are implemented using the 6-311++G(d,p) basis set by DFT/B3LYP method. The UV-Vis spectrum of 5B3N2C was obtained by TD-DFT with chloroform as a solvent. The analysis of molecular electrostatic potential (MEP) and frontier molecular orbital (FMO) were used to evaluate, the entire electron density and organic reactive sites of 5B3N2C. The electron-hole conversions were conjointly deliberated. Donor-acceptor interactions (NBO) analysis examines the intra-and intermolecular charge transfer, hyper conjugate interaction of the compound. The orbital molecular contributions are evaluated by density of states (DOS and PDOS). To discern the reactivity of the molecule, topology analyses were done. The biological prominence of the 5B3N2C molecule was investigated in a pertinent study of molecular docking with target protein 3CEJ exhibiting the centromere associated protein inhibitor property. Molecular Dynamics simulations were done to assess the stability of the complex. 5B3N2C physiochemical parameters were also compared to those of widely viable medications Ispinesib and Lonafarnib.

A Brief History of OLEDs-Emitter Development and Industry Milestones

Organic light-emitting diodes (OLEDs) have come a long way ever since their first introduction in 1987 at Eastman Kodak. Today, OLEDs are especially valued in the display and lighting industry for their promising features. As one of the research fields that equally inspires and drives development in academia and industry, OLED device technology has continuously evolved over more than 30 years. OLED devices have come forward based on three generations of emitter materials relying on fluorescence (first generation), phosphorescence (second generation), and thermally activated delayed fluorescence (third generation). Furthermore, research in academia and industry toward the fourth generation of OLEDs is in progress. Excerpts from the history of green, orange-red, and blue OLED emitter development on the side of academia and milestones achieved by key players in the industry are included in this report.

Fused tetracyclic tris[1,2,4]triazolo[1,3,5]triazine as a novel rigid electron acceptor for efficient thermally activated delayed fluorescence emitters

Tris[1,2,4]triazolo[1,3,5]triazine, a new acceptor based on a fused triazole and triazine moiety, is utilized to construct D₃–A star-shaped tristriazolotriazine derivatives, named 3,7,11-tris(4-(10H-phenoxazin-10 yl)phenyl)tris([1,2,4]triazolo)[1,3,5]triazine (TTT-PXZ) and 3,7,11-tris(4-(9,9-dimethylacridin-10(9H)yl)phenyl)tris([1,2,4])triazolo[1,3,5]triazine (TTT-DMAC). Both TTT-PXZ and TTT-DMAC emitters feature TADF activities and AIEE properties. Consequently, solution processed OLEDs based on TTT-PXZ green emitters exhibited good performances, with an external quantum efficiency (EQE) of up to 6.2%.

Tris[1,2,4]triazolo[1,3,5]triazine, a new acceptor based on a fused triazole and triazine moiety, has been utilized to construct two new star-shaped TADF emitters.

Heteroatom-bridged heterofluorenes: a theoretical study on molecular structures and optoelectronic properties

A double heteroatom introduction strategy was proposed and theoretically investigated to construct highly rigid π-conjugated small molecules and polymers for optoelectronic applications.