Rational Molecular Design Overcoming the Long Delayed Fluorescence Lifetime and Serious Efficiency Roll-Off in Blue Thermally Activated Delayed Fluorescent Devices

Recent synthetic strategies for the construction of functionalized carbazoles and their heterocyclic motifs enabled by Lewis acids

This article demonstrates recent innovative cascade annulation methods for preparing functionalized carbazoles and their related polyaromatic heterocyclic compounds enabled by Lewis acid catalysts. Highly substituted carbazole scaffolds were synthesized via Lewis acid mediated Friedel-Crafts arylation, electrocyclization, intramolecular cyclization, cycloaddition, C-N bond-formations, aromatization and cascade domino reactions, metal-catalyzed, iodine catalyzed reactions and multi-component reactions. This review article mainly focuses on Lewis acid-mediated recent synthetic methods to access a variety of electron-rich and electron-poor functional groups substituted carbazole frameworks in one-pot reactions. Polyaromatic carbazole and their related nitrogen-based heterocyclic compounds were found in several synthetic applications in pharma industries, energy devices, and materials sciences. Moreover, the review paper briefly summarised new synthetic strategies of carbazole preparation approaches will assist academic and pharma industries in identifying innovative protocols for producing poly-functionalized carbazoles and related highly complex heterocyclic compounds and discovering active pharmaceutical drugs or carbazole-based alkaloids and natural products.

Carbazolylgold(iii) complexes with thermally activated delayed fluorescence switched on by ligand manipulation as high efficiency organic light-emitting devices with small efficiency roll-offs

A series of carbazolyl ligands has been designed and synthesized through the integration of various electron-donating and electron-accepting motifs, including electron-donating 4-(diphenylamino)aryl and electron-accepting cyano and diphenylphosphine oxide moieties, for the development of a new class of gold(iii) complexes, where the energies of their triplet intraligand and ligand-to-ligand charge transfer excited states can be manipulated for the activation of thermally activated delayed fluorescence (TADF). Upon excitation, these complexes show high photoluminescence quantum yields of up to 80% in solid-state thin films, with short excited state lifetimes down to 1 μs. Vacuum-deposited and solution-processed organic light-emitting devices based on these complexes demonstrate promising electroluminescence (EL) performance with maximum external quantum efficiencies of 15.0% and 11.7%, respectively, and notably small efficiency roll-off values of less than 1% at the practical luminance brightness level of 1000 cd m⁻². These distinct EL performances are believed to be due to the occurrence of multichannel radiative decay pathways via both phosphorescence and TADF that significantly shorten the emission lifetimes and hence reduce the occurrence of the detrimental triplet–triplet annihilation in the gold(iii) complexes.

Switch on of TADF can be achieved by tuning the excited state energy levels via ligand manipulation of the carbazolylgold(iii) C^C^N complexes. The resulting OLEDs show maximum EQEs of over 11% and efficiency roll-offs of down to less than 1%.

Effective Design Strategy for Aggregation-Induced Emission and Thermally Activated Delayed Fluorescence Emitters Achieving 18% External Quantum Efficiency Pure-Blue OLEDs with Extremely Low Roll-Off

High-color purity organic emitters with a simultaneous combination of aggregation-induced emission (AIE) and thermally activated delayed fluorescence (TADF) characteristics are in great demand due to their excellent comprehensive performances toward efficient organic light-emitting diodes (OLEDs). In this work, two D-π-A-structure emitters, ICz-DPS and ICz-BP, exhibiting AIE and TADF properties were developed, and both the emitters have narrow singlet (S₁)-triplet (T₁) splitting (ΔE_ST) and excellent photoluminescence (PL) quantum yields (Φ_PL), derived from the distorted configurations and weak intra/intermolecular interactions, suppressing exciton annihilation and concentration quenching. Their doped OLEDs based on ICz-BP provide an excellent electroluminescence external quantum efficiency (η_ext) and current efficiency (η_C) of 17.7% and 44.8 cd A^-1, respectively, with an η_ext roll-off of 2.9%. Their nondoped OLEDs based on ICz-DPS afford high efficiencies of 11.7% and 30.1 cd A^-1, with pure-blue emission with Commission Internationale de l'Éclairage (CIE) coordinates of (0.15, 0.08) and a low roll-off of 6.0%. This work also shows a strategy for designing AIE-TADF molecules by rational use of steric hindrance and weak inter/intramolecular interactions to realize high Φ_PL values, fast reverse intersystem crossing process, and reduced nonradiative transition process properties, which may open the way toward highly efficient and small-efficiency roll-off devices.

High-Performance Solution-Processed Red Thermally Activated Delayed Fluorescence OLEDs Employing Aggregation-Induced Emission-Active Triazatruxene-Based Emitters

Two novel red thermally activated delayed fluorescence (TADF) emitters [triazatruxene (TAT)-dibenzo[a,c]phenazine (DBPZ) and TAT-fluorine-substituted dibenzo[a,c]phenazine (FDBPZ)] were developed by incorporating TAT as the electron donor (D) and DBPZ or FDBPZ as the electron acceptor (A). Both compounds showed aggregation-induced emission behaviors and bright red emission in neat films. Benefited from the rigid and large planar conjugated structure of TAT and DBPZ, TAT-DBPZ and TAT-FDBPZ realized high photoluminescence quantum yields in solid states. Meanwhile, the large steric hindrance between TAT and DBPZ segments produced small singlet-triplet energy splitting (ΔE_ST), leading to short delayed fluorescence lifetimes and high reverse intersystem crossing (RISC) rate (>10⁶ s^-1) for both compounds. The solution-processable doped organic light-emitting diodes (OLEDs) based on TAT-DBPZ achieved a high external quantum efficiency (EQE) of 15.4% with a red emission peak at 604 nm, which was one of the highly efficient solution-processable red TADF OLEDs. TAT-FDBPZ-based doped devices also showed a red emission peak at 611 nm with a maximum EQE of 9.2% and low-efficiency roll-off ratios of 1.0% at 100 cd m^-2 and 19% at 1000 cd m^-2. Furthermore, their solution-processable nondoped devices displayed EQEs of 5.6 and 2.9% with the red-shifted emission peaks at 626 and 641 nm, respectively. These results indicate the huge potential of utilization of TAT as the donor unit to achieve highly efficient and low-efficiency roll-off solution-processable red TADF OLEDs.

Electroabsorption Spectroscopy as a Tool for Probing Charge Transfer and State Mixing in Thermally Activated Delayed Fluorescence Emitters

Solid-state electroabsorption is demonstrated as a powerful tool for probing the charge transfer (CT) character and state mixing in the low-energy optical transitions of two structurally similar thermally activated delayed fluorescent (TADF) materials with divergent photophysical and device performances. The Liptay model is used to fit differentials of the low-energy absorption bands to the measured electroabsorption spectra, with both emitters showing CT characteristics and large changes in dipole moments upon excitation despite the associated absorption bands appearing to be structured. High electric fields then reveal transfer of oscillator strength to a state close to the CT in the better performing molecule. With supporting TDDFT-TDA and DFT/MRCI calculations, this state showed ππ* characteristics of a local acceptor triplet that strongly mixes with the σπ* of the CT. The emitter with poor TADF performance showed no evidence of such mixing.

Thermally Activated Delayed Fluorescence Materials: Towards Realization of High Efficiency through Strategic Small Molecular Design

Thermally activated delayed fluorescence (TADF) is one of the most intriguing and promising discoveries towards realization of highly-efficient organic light emitting diodes (OLED) utilizing small molecules as emitters. It has the capability of manifesting all excitons generated during the electroluminescent processes, consequently achieving 100 % of internal quantum efficiency. Since the report of the first efficient OLED based on a TADF small molecule in 2012 by Adachi et al., the quest for optimal TADF materials for OLED application has never stopped. Various TADF molecules bearing different design concepts and strategies have been designed and produced, with the aim to boost the overall performances of corresponding OLEDs. In this minireview, the general principles of TADF molecular design based on three basic categories of TADF species: twisted intramolecular charge transfer (TICT), through-space charge transfer (TSCT) and multi-resonance induced TADF (MR-TADF) are discussed in detail. Several key aspects with respect to each category, as well as some effective methods to enhance the efficiency of TADF materials and corresponding OLEDs from the molecular engineering perspectives, are summarized and discussed to exhibit a general landscape of TADF molecular design to a wide variety of scientific researchers within this particular disciplinary area.