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

Stable narrowband blue OLEDs by modulating frontier molecular orbital levels

Energy level alignment of frontier molecular orbital (FMO) is essential for controlling charge carrier and exciton dynamics in organic light-emitting diodes (OLEDs). However, multiple resonance (MR) emitters with exceptional narrowband luminescence typically suffer from inadequate FMO levels. Herein, a conventional blue MR prototype with a shallow highest occupied molecular orbital (HOMO) level of -5.32 eV is initially employed to reveal the charge carrier and exciton dynamics. Severe hole trapping by its shallow HOMO significantly hinders its transport. More importantly, trapped carriers induce direct exciton formation and recombination at MR emitters in a hyperfluorescent system, leading to triplet accumulation in MR emitters. To resolve these issues, a proof-of-concept wavefunction perturbation strategy is proposed by incorporating cyano motifs at peripheral sites of MR backbone to adjust the energy levels. This approach significantly shifts HOMOs of 0.36 and 0.51 eV without compromising colour purity. The derivative substituting meta-boron position (mCNDB) exhibits a pure-blue emission peaking at 459 nm with a narrow bandwidth of 13 nm. The detrimental carrier trapping effect is eliminated, enhancing external quantum efficiency to exceeding 23%, maintaining around 20% at 1000 cd m-2, and improving the device stability.

Selective Deuteration-Enhanced Phosphorescent Performance in Square-Planar Tetradentate Pt(II) Complexes: Unveiling the Role of Vibration Coupling in Electronic Transitions

Deuteration enhances the organic light-emitting diode (OLED) stability and performance. Theoretical calculations on square-planar tetradentate Pt(II) complexes show that deuteration exerts no electronic effects because deuterium has the same number of electrons as hydrogen. Consequently, the equilibrium structures (ground/excited states), frontier molecular orbital (FMO) distributions and energy levels, natural transition orbitals (NTOs), and spin-orbital coupling (SOC) remain unchanged. Deuteration-doubled mass lowers vibrational frequencies, reducing vibrational energy levels and zero-point energy (ZPE) by about 2.08 kcal/mol per deuteration, while slightly affecting the 0-0 transition energy (E0-0) and reorganization energy. Frequency reduction suppresses nonradiative decay rates (knr), boosting photoluminescence quantum yield (PLQY). Analyses of Huang-Rhys factors (S) and Franck-Condon factors (FC) show significant changes in mid-to-high-frequency vibrational coupling of particle and hole. Altered vibrational wave functions enhance the Herzberg-Teller (HT) transition dipole moment, affecting radiative decay rates (kr). Selective deuteration of the Cz ring effectively increases kr, suppresses knr, and modulates the fine structure of the spectrum, comparable to perdeuteration. C-D bond chemical degradation rate is about 3.94 times slower than C-H, further improving stability. These results establish a framework for optimizing emitters and extending OLED operating lifetimes.

Stable Pincer Gold(III)-TADF Emitters with Extended Donor-Acceptor Separation for Efficient Vacuum-Deposited OLEDs with Operational Lifetime (LT95) up to 3831 h at 1000 cd m-2

Although gold-TADF (thermally activated delayed fluorescence) emitters have attractive prospects as next-generation practical OLED emitters, the performance of OLEDs utilizing gold(I)- and gold(III)-TADF emitters lags behind the requirements of practical applications, and device lifetime has become a bottleneck. Here, novel pincer gold(III)-TADF emitters that are easily fabricated with tunable donor and acceptor ligands are presented. These pincer gold(III)-TADF emitters exhibit an extended molecular π-distance along the transition dipole moment, resulting in a significant reduction in the electron exchange energy between the S1 and T1 excited states, thus narrowing the singlet-triplet energy gap (ΔEST). The combination of small ΔEST and heavy-atom (Au, S) effect greatly enhances spin-flip dynamics and produces efficient TADF (photoluminescence quantum yields up to 90%) with high radiative decay rate constants (kr up to 106 s-1), and short lifetimes (τ less than 1.2 µs) in thin films at room temperature. Vacuum-deposited OLEDs based on these gold(III)-TADF emitters demonstrate impressive stability, achieving i) a high maximum external quantum efficiency (EQEmax) of up to 22.2%, and ii) a record- long operational lifetime (LT95) of 3831 h at an initial luminance of 1000 cd m-2. This excellent durability makes the pincer gold(III)-TADF emitter a promising and competitive alternative to iridium and platinum emitters for practical OLED applications.

Aggregation suppression and enhanced blue emission of perylene in zinc-based coordination polymer glass

Reducing aggregation caused quenching and enhancing stability is crucial in the fabrication of organic light-emitting diodes. Herein, we successfully fabricated blue-emitting coordination polymer glasses using perylene dye and a zinc-based coordination glass. The aggregation of perylene monomers in the solid state was significantly suppressed, and the hybrid glass demonstrated high stability and strong photoluminescent quantum yield (75.5%) under ambient conditions.

Deuterated oxazines are bright near-infrared fluorophores for mitochondrial imaging and single molecule spectroscopy

Bright near-infrared fluorophores are in demand for microscopy. We showcase a deuterated oxazine being 23% brighter vs. ATTO700. With a longer lifetime of 1.85 nanoseconds, we find the best-in-class SulfoOxazine700-d10 to stain mitochondria for confocal microscopy, and demonstrate unaffected diffusion properties in single molecule fluorescence correlation spectroscopy.

Unlocking the potential of hydrogen deuterium exchange via an iterative continuous-flow deuteration process

Labelled compounds bearing hydrogen isotopes are keystones in diverse areas constituting a multi-billion dollar global market including drugs, diagnostics, biology, toxicology and smart materials. While hydrogen deuterium exchange (HDE) methods hold promise as relevant tools for the late-stage and one-step preparation of deuterium-labelled compounds, they often fall short in achieving sufficient isotopic purity combined either with a site-selectivity or with a full deuteration process, highlighting the need for further development and optimisation. This report pinpoints an approach to unlock the potential of HDE using the concept of iterative runs in continuous-flow technology (recirculation process). This closed-loop process grants access now to deuterated compounds with high isotopic purities, labelled at a precise site or perdeuterated on demand, in a fast, productive, and environmentally friendly way.

Selective Hydrogen Isotope Exchange Catalysed by Simple Alkali-Metal Bases in DMSO

Dedicated to Proferssor Robert E. Mulvey on the occasion of his 65th birthday. Isotope Exchange processes are becoming the preferred way to prepare isotopically labelled molecules, avoiding the redesign of multistep synthetic protocols. In the case of deuterium incorporation, the most used strategy has employed transition metals, that offer high reactivity under mild reaction conditions. Despite their success, the trade-off is that these metals are precious, so expensive, and often exhibit high toxicity. Therefore, alternative transition-metal-free protocols would be a welcome addition to this field. In this report we show how the simple bases NaHMDS (HMDS=hexamethyldisilazide) and NaCH2SiMe3 can efficiently and selectively promote deuteration of a wide range of C(sp2)-H and C(sp3)-H bonds in DMSO-d6, providing an easy and direct access to deuterated compounds. Heterocycles, fluoroarenes, N-heterocyclic carbenes, amides and other aromatic molecules could be deuterated under mild conditions using catalytic amounts of base. Mechanistic studies along with the isolation and characterisation of reaction intermediates have flagged up the importance of the metalated substrate and metalated solvent in solution, establishing an equilibrium between these compounds is crucial for the success of this approach. An alkali-metal effect was observed, with heavier alkali-metal amides being more reactive at room temperature, but their lower stability at higher temperatures made sodium bases the optimal reagents for Hydrogen Isotope Exchange.

The degradation mechanism of multi-resonance thermally activated delayed fluorescence materials

1,4-Azaborine-based arenes are promising electroluminescent emitters with thermally activated delayed fluorescence (TADF), offering narrow emission spectra and high quantum yields due to a multi-resonance (MR) effect. However, their practical application is constrained by their limited operational stability. This study investigates the degradation mechanism of MR-TADF molecules. Electroluminescent devices incorporating these compounds display varied operational lifetimes, uncorrelated with excitonic stability or external quantum efficiency roll-off. Bulk electrolysis reveals significant instability in the radical cationic forms of MR-TADF compounds, with device lifetime linked to the Faradaic yield of oxidation. Comprehensive chemical analyses corroborate that the degradation byproducts originated from intramolecular cyclization of radical cation, followed by hydrogen atom transfer. The mechanism is further supported by enhanced stability observed in a deuterated MR-TADF emitter, attributed to a secondary kinetic isotope effect. These findings provide insights into the stabilizing effects of deuteration and mechanism-driven strategies for designing MR-TADF compounds with improved operational longevity.

High-Performance Narrowband Pure-Green OLEDs with Gamut Approaching BT.2020 Standard: Deuteration Promotes Device Efficiency and Lifetime Simultaneously

In order to fulfill the demand for ultrahigh definition organic light-emitting diodes (OLEDs), pure-green emitters with Commission Internationale de l'Éclairage (CIE) y-coordinate over 0.71 are in urgent demand. Meanwhile, the high device efficiency, small efficiency roll-off, and operational lifetime also remain challenging issues. In this work, a series of narrowband pure-green fluorescent emitters based on a double-boron (B) doped polycyclic aromatic hydrocarbons (PAHs) framework fused with naphthalene units is reported. The newly designed emitters realize green emission with peaks of 512-521 nm and extremely narrow full-width at half-maxima (FWHMs) of 16-17 nm in toluene solution. Utilizing these emitters in a phosphor-sensitized fluorescence (PSF) system, the resulting OLEDs exhibit pure green emission with peaks in the range of 514-526 nm and very narrow FWHMs of 19-20 nm. Notably, the devices based on the partially deuterated emitter, DBN-NaPh-d, not only achieve a maximum external quantum efficiency (EQEmax) as high as 35.2%, small efficiency roll-off, along with a CIEy of 0.74 but also showcase impressive operational stability with a long lifetime (LT50) of over 3000 h at an initial luminance of 1000 cd m-2. This work represents one of the highest device efficiencies and superior device stability among fluorescence emitter-based OLEDs.

Achieving Tunable Monomeric TADF and Aggregated RTP via Molecular Stacking

Organic emitters with both thermally activated delayed fluorescence (TADF) and room-temperature phosphorescence (RTP) have attracted widespread interest for their intriguing luminescent properties. Herein, a series of triphenylamine-substituted isoquinoline derivatives possessing monomeric TADF and aggregated RTP properties are reported. As the molecules exhibited various forms of π-π and charge transfer (CT) stacking with different intensities, inter/intramolecular CT can be meticulously modulated to achieve tunable TADF-RTP. Aggregated phosphorescence originates from intermolecular CT initiated by CT dimers, whereas monomeric TADF is facilitated by intramolecular CT enhanced by π-π dimers. Leveraging the properties of these molecules, luminescent materials with tunable TADF-RTP properties in multistates are obtained by molecular substitution position alignment, dealing with different solvents, grinding, adjusting concentration, changing polymer matrix, photoactivation, and heat treatment. This work is critical for a deeper understanding of construction and regulation of the TADF-RTP dual-channel emission, enabling the development of advanced optoelectronic devices with tailored emission properties.

Nitrogen-Embedding Strategy for Short-Range Charge Transfer Excited States and Efficient Narrowband Deep-Blue Organic Light Emitting Diodes

The development of deep-blue organic light-emitting diodes (OLEDs) featuring high efficiency and narrowband emission is of great importance for ultrahigh-definition displays with wide color gamut. Herein, based on the nitrogen-embedding strategy for modifying the short range charge transfer excited state energies of multi-resonance (MR) thermally activated delayed fluorescence (TADF) emitters, we introduce one or two nitrogen atoms into the central benzene ring of a versatile boron-embedded 1,3-bis(carbazol-9-yl)benzene skeleton. This approach resulted in the stabilization of the highest occupied molecular orbital energy levels and the formation of intramolecular hydrogen bonds, and thus systematic hypsochromic shifts and narrowing spectra. In toluene solution, two heterocyclic-based MR-TADF molecules, Py-BN and Pm-BN, exhibit deep-blue emissions with high photoluminescence quantum yields of 93 % and 94 %, and narrow full width at half maximum of 14 and 13 nm, respectively. A deep-blue hyperfluorescent OLED based on Py-BN exhibited a maximum external quantum efficiency of 27.7 % and desired color purity with Commission Internationale de L'Eclairage (CIE) coordinates of (0.150, 0.052). These results demonstrate the significant potential for the development of deep blue narrowband MR-TADF emitters.

Expanding Multiple-Resonance Structure of a Double-Borylated Skeleton by Fusing with Indolocarbazole a Multiple-Resonance Donor for Narrowband Green Emission

Double-borylated multiple-resonance (MR) skeletons are promising templates for high performance, while the chemical design space is relatively limited. Peripheral segments are often used to decorate/fuse MR skeletons and modulate the photophysics but they can also cause unwanted spectral broadening. Herein, a narrowband MR emitter ICzDBA by fusing an MR-featured donor segment indolocarbazole into a double-borylated MR skeleton is developed. In ICzDBA, the nitrogen atom located away from the core benzene ring can also contribute to the generation of the overall MR-featured distribution through the long-range conjugation effect, along with the other boron/nitrogen atoms on the phenyl center. Thus, ICzDBA in toluene displays a narrowband emission peaking at 507 nm with a full width at half maximum of merely 20 nm (0.09 eV). Moreover, organic light-emitting diode devices using ICzDBA emitter exhibit ultrapure green emission with Commission Internationale de l'Eclairage (CIE) coordinates of (0.27, 0.70) and a high external quantum efficiency of 32.5%. These results manifest the importance of MR characters of peripheral decorations/fusions in preserving the narrowband features of MR skeletons, which provides a solution for further expanding MR structures with well-maintained narrowband characters.

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.

Effects of Deuterium Isotopes on Pt(II) Complexes and Their Impact on Organic NIR Emitters

Insight into effect of deuterium isotopes on organic near-IR (NIR) emitters was explored by the use of self-assembled Pt(II) complexes H-3-f and HPh-3-f, and their deuterated analogues D-3-f and DPh-3-f, respectively (Scheme 2). In vacuum deposited thin film, albeit having nearly identical emission spectral feature maximized at ~810 nm, H-3-f and D-3-f exhibit remarkable difference in photoluminescence quantum yield (PLQY) of 29 % and 50 %, respectively. Distinction in PLQY is also observed for HPh-3-f (800 nm, 50 %) and DPh-3-f (798 nm, 67 %). We then elucidated the theoretical differences in the impact on near-infrared (NIR) luminescence between Pt(II) complexes and organic small molecules upon deuteration. The results establish a general guideline for the deuteration on NIR emission efficiency. From a perspective of practical application, NIR OLEDs based on D-3-f and DPh-3-f emitters attain EQEmax of 15.5 % (radiance 31,287 mW Sr-1  m-2 ) and 16.6 % (radiance of 32,279 mW Sr-1  m-2 ) at 764 nm and 796 nm, respectively, both of which set new records for NIR OLEDs of >750 nm.