Isotope Effect in the Magneto-Optoelectronic Response of Organic Light-Emitting Diodes Based on Donor-Acceptor Exciplexes

Achieving 9% EQE in light-emitting electrochemical cells via a TADF-sensitized fluorescence strategy

Light-emitting electrochemical cells (LECs) are appealing for cost-effective, large-area emission applications; however, their luminescence efficiency is significantly limited by exciton annihilation caused by high concentration polarons. Here, we present thermally activated delayed fluorescence (TADF) sensitized fluorescence LECs (TSF-LECs) that achieve a record 9% EQE. The TADF sensitizers with rapid reverse intersystem crossing (RISC) rates can effectively convert triplet excitons to singlet excitons in LECs, thereby establishing a more efficient overall energy transfer pathway. Importantly, magneto-electroluminescence measurements indicate that the additional RISC route in TSF-LECs significantly suppresses the annihilation of triplet excitons and thus reduces exciton loss under high concentration polaron conditions. Compared to LECs without a sensitizer, TSF-LECs exhibit improved EQE and luminance, extended operational lifetimes, and suppressed efficiency roll-off. A flexible display prototype based on TSF-LECs was further fabricated, capable of stably displaying high-brightness preset patterns for extended periods. The exploration of the exciton dynamics in high concentration polaron environments offers valuable insights for future developments in high-efficiency LEC technology.

All-Electrical Readout of Coherently Controlled Spins in Silicon Carbide

Spin defects in silicon carbide are promising candidates for quantum sensing applications as they exhibit long coherence times even at room temperature. However, spin readout methods that rely on fluorescence detection can be challenging due to poor photon collection efficiency. Here, we demonstrate coherent spin control and all-electrical readout of a small ensemble of spins in a SiC junction diode using pulsed electrically detected magnetic resonance. A lock-in detection scheme based on a three stage modulation cycle is implemented, significantly enhancing the signal-to-noise ratio. This technique enabled observation of coherent spin dynamics, specifically Rabi spin nutation, spin dephasing, and spin decoherence. The use of these protocols for magnetometry applications is evaluated.

Enhancing operational stability of OLEDs based on subatomic modified thermally activated delayed fluorescence compounds

The realization of operationally stable blue organic light-emitting diodes is a challenging issue across the field. While device optimization has been a focus to effectively prolong device lifetime, strategies based on molecular engineering of chemical structures, particularly at the subatomic level, remains little. Herein, we explore the effect of targeted deuteration on donor and/or acceptor units of thermally activated delayed fluorescence emitters and investigate the structure-property relationship between intrinsic molecular stability, based on isotopic effect, and device operational stability. We show that the deuteration of the acceptor unit is critical to enhance the photostability of thermally activated delayed fluorescence compounds and hence device lifetime in addition to that of the donor units, which is commonly neglected due to the limited availability and synthetic complexity of deuterated acceptors. Based on these isotopic analogues, we observe a gradual increase in the device operational stability and achieve the long-lifetime time to 90% of the initial luminance of 23.4 h at the luminance of 1000 cd m-2 for thermally activated delayed fluorescence-sensitized organic light-emitting diodes. We anticipate our strategic deuteration approach provides insights and demonstrates the importance on structural modification materials at a subatomic level towards prolonging the device operational stability.

Alkali-metal bases in catalytic hydrogen isotope exchange processes

The preparation of compounds labelled with deuterium or tritium has become an essential tool in a range of research fields. Hydrogen isotope exchange (HIE) offers direct access to said compounds, introducing these isotopes in a late stage. Even though the field has rapidly advanced with the use of transition metal catalysis, alkali-metal bases, used as catalysts or under stoichiometric conditions, have also emerged as a viable alternative. In this minireview we describe the latest advances in the use of alkali-metal bases in HIE processes, showcasing their synthetic potential as well as current challenges in the field. It is divided in different sections based on the isotope source used, emphasizing their benefits, disadvantages and limitations. The influence on the choice of alkali-metal in these processes as well as their possible mechanistic pathways are also discussed.

Enhanced Efficiency of Exciplex Emission from a 9-Phenylfluorene Derivative

The exciplex-thermally activated delayed fluorescence (exciplex-TADF) system is an excellent candidate for the fabrication of high-efficiency organic light-emitting diodes (OLEDs) because of its more easily achieved small singlet-triplet energy splitting (ΔE_ST) and doping control. However, exciplex-TADF is still faced with the problems of low external quantum efficiency (η_ext) and unclear effect of structure modification in electron acceptors. Herein, we provide a steric hindrance increase strategy to obtain high-efficiency exciplex emissions. Through introducing a 9-phenylfluorene group into N-ethylcarbazole of the dicyano-substituted 9-phenylfluorene, an electron acceptor material with increased steric hindrance is obtained, which helps the exciplex harvest a larger driving force and higher emission efficiencies. Encouragingly, the obtained OLED displays a maximum η_ext of 25.8%, which is one of the best efficiency values among reported exciplex-OLEDs, simultaneously possessing excellent current efficiency of 83.6 cd A^-1 and power efficiency of 93.7 lm W^-1. It is expected that this work will offer a new avenue for designing electron acceptors for highly efficient exciplex emissions.

Degradation Analysis of Organic Light-Emitting Diodes through Dispersive Magneto-Electroluminescence Response

Understanding the stability and degradation of organic light-emitting diodes (OLEDs) under working conditions is a significant area of research for developing more effective OLEDs and further improving their performance. However, studies of degradation processes by in situ noninvasive methods have not been adequately developed. In this work, tris-(8-hydroxyquinolino) aluminum (Alq3)-based OLED degradation processes have been analyzed through the investigation of the device dispersive magneto-electroluminescence (MEL(B)) response measured at room temperature. By studying the change in the MEL(B) response during the device degradation under different external stimuli, such as exposing the device to the atmosphere and prolonged illumination by a strong visible light source, we have gained insight into the microscopic spin-dependent phenomena that control the recombination of e-h polaron pairs in the device. We found that the device degradation leads to a shorter e-h polaron lifetime, smaller dispersive parameter, and broader lifetime distribution function that shows increased disorder in the active layer. This study could offer a potential tool that may be beneficial for assessing the degradation of OLED devices based on various active layers.

Organoboron Reagent-Controlled Selective (Deutero)Hydrodefluorination

(Deuterium-labeled) CF₂ H- and CFH₂ -moieties are of high interest in drug discovery. The high demand for the incorporation of these fluoroalkyl moieties into molecular structures has witnessed significant synthetic progress, particularly in the (deutero)hydrodefluorination of CF₃ -containing compounds. However, the controllable replacement of fluorine atoms while maintaining high chemoselectivity remains challenging. Herein, we describe the development of a selective (deutero)hydrodefluorination reaction via electrolysis. The reaction exhibits a remarkable chemoselectivity control, which is enabled by the addition of different organoboron sources. The procedure is operationally simple and scalable, and provides access in one step to high-value building blocks for application in medicinal chemistry. Furthermore, density functional theory (DFT) calculations have been carried out to investigate the reaction mechanism and to rationalize the chemoselectivity observed.

Highly selective single and multiple deuteration of unactivated C(sp3)-H bonds

Selective deuteration of unactivated C(sp3)-H bonds is a highly attractive but challenging subject of research in pharmaceutical chemistry, material science and synthetic chemistry. Reported herein is a practical, highly selective and economical efficient hydrogen/deuterium (H/D) exchange of unactivated C(sp3)-H bonds by synergistic photocatalysis and hydrogen atom transfer (HAT) catalysis. With the easily prepared PMP-substituted amides as nitrogen-centered radical precursors, a wide range of structurally diverse amides can undergo predictable radical H/D exchange smoothly with inexpensive D2O as the sole deuterium source, giving rise to the distal tertiary, secondary and primary C(sp3)-H bonds selectively deuterated products in yields of up to 99% and excellent D-incorporations. In addition to precise monodeuteration, this strategy can also achieve multideuteration of the substrates contain more than one remote C(sp3)-H bond, which opens a method to address multi-functionalization of distal unactivated C(sp3)-H bonds.

Easy-to-Implement Hydrogen Isotope Exchange for the Labeling of N-Heterocycles, Alkylkamines, Benzylic Scaffolds, and Pharmaceuticals

Facilitating access to deuterated and tritiated complex molecules is of paramount importance due to the fundamental role of isotopically labeled compounds in drug discovery and development. Deuterated analogues of drugs are extensively used as internal standards for quantification purposes or as active pharmaceutical ingredients, whereas tritiated drugs are essential for preclinical ADME studies. In this report, we describe the labeling of prevalent substructures in FDA-approved drugs such as azines, indoles, alkylamine moieties, or benzylic carbons by the in situ generation of Rh nanoparticles able to catalyze both C(sp2)-H and C(sp3)-H activation processes. In this easy-to-implement labeling process, Rh nanocatalysts are formed by decomposition of a commercially available rhodium dimer under a deuterium or tritium gas atmosphere (1 bar or less), using the substrate itself as a surface ligand to control the aggregation state of the resulting metallic clusters. It is noteworthy that the size of the nanoparticles observed is surprisingly independent of the substrate used and is homogeneous, as evidenced by transmission electron microscopy experiments. This method has been successfully applied to the one-step synthesis of (1) deuterated pharmaceuticals usable as internal standards for MS quantification and (2) tritiated drug analogues with very high molar activities (up to 113 Ci/mmol).

Long-lived spin-polarized intermolecular exciplex states in thermally activated delayed fluorescence-based organic light-emitting diodes

Spin-spin interactions in organic light-emitting diodes (OLEDs) based on thermally activated delayed fluorescence (TADF) are pivotal because radiative recombination is largely determined by triplet-to-singlet conversion, also called reverse intersystem crossing (RISC). To explore the underlying process, we apply a spin-resonance spectral hole-burning technique to probe electroluminescence. We find that the triplet exciplex states in OLEDs are highly spin-polarized and show that these states can be decoupled from the heterogeneous nuclear environment as a source of spin dephasing and can even be coherently manipulated on a spin-spin relaxation time scale T₂* of 30 ns. Crucially, we obtain the characteristic triplet exciplex spin-lattice relaxation time T₁ in the range of 50 μs, which far exceeds the RISC time. We conclude that slow spin relaxation rather than RISC is an efficiency-limiting step for intermolecular donor:acceptor systems. Finding TADF emitters with faster spin relaxation will benefit this type of TADF OLEDs.

Detecting triplet states in opto-electronic and photovoltaic materials and devices by transient optically detected magnetic resonance

Triplet excited states in organic semiconductor materials and devices are notoriously difficult to detect and study with established spectroscopic methods. Yet, they are a crucial intermediate step in next-generation organic light emitting diodes (OLED) that employ thermally activated delayed fluorescence (TADF) to upconvert non-emissive triplets to emissive singlet states. In organic photovoltaic (OPV) devices, however, triplets are an efficiency-limiting exciton loss channel and are also involved in device degradation. Here, we introduce an innovative spin-sensitive method to study triplet states in both, optically excited organic semiconductor films, as well as in electrically driven devices. The method of transient optically detected magnetic resonance (trODMR) can be applied to all light-emitting materials whose luminescence depends on paramagnetic spin states. It is thus an ideal spectroscopic tool to distinguish different states involved and determine their corresponding time scales. We unravel the role of intermediate excited spin states in opto-electronic and photovoltaic materials and devices and reveal fundamental differences in electrically and optically induced triplet states.