Managing Intersegmental Charge‐Transfer and Multiple Resonance Alignments of D₃‐A Typed TADF Emitters for Red OLEDs with Improved Efficiency and Color Purity

Abstract

diodes (OLEDs) since they permit full exciton utilization with pure organic chemical structures. [1] The most widely used molecular strategy for TADF emitters is to employ highly twisted donoracceptor (D-A) structures in order to separate the highest occupied molecular orbitals (HOMOs) and the lowest unoccupied molecular orbitals (LUMOs). [2] While such molecules can facilely be developed by combining various donors and acceptors, enabling extremely small singlet-triplet energy splitting (∆E ST ) and efficient reverse intersystem crossing (RISC), they have an obvious side effect on fluorescence radiation. Particularly when the singlet non-radiative decay process is significant, low efficiencies would be realized even the RISC process is effective. [3] Another common weakness of D-A typed emitters is their poor color purities. This is because their geometries undergo large molecular relaxations during intersegmental charge-transfer (ICT) processes. [4] Efforts of developing red TADF emitters clearly demonstrate the influences of both the adverse factors. According to the bandgap law, singlet non-radiative decay rates (k IC ) would be magnified in the red emission region and even surpass the corresponding fluorescence radiation rates (k r ). [5] Thus, highly twisted D-A structured red emitters may lead to TADF but often show unsatisfactory efficiencies. On the other hand, it is very common for D-A typed molecules that their emission maxima are over 600 nm, while their broad emission spectra still cover evident green and even blue regions. [6] This would eventually give rise to low x coordinates of Commission Internationale de l'Eclairage (CIE), which are far from meeting the demands of display technologies.The recently proposed strategy of using multiple resonance (MR) typed frameworks for TADF molecular design is believed to be an ideal alternative approach to address the above issues. [7] Unlike conventional D-A typed TADF emitters that propose ICT transitions, MR-TADF emitters precisely exploit the opposite resonance effects of electron-withdrawing and electron-donating atoms/groups mainly in the same rigid π-conjugated frameworks to achieve HOMO-LUMO separations Thermally activated delayed fluorescence (TADF) emitters induced by the multiple resonance (MR) effect have garnered considerable attention. However, it is difficult to develop MR-TADF emitters that maintain high color purities in the red region. In this work, the importance of excited state alignments of MR-based donor-acceptor (D-A) molecules in determining their preferring characteristics is clarified. By using the newly designed molecule mBDPA-TOAT whose apparent excited states show hybridization of MR and intersegmental charge-transfer features as an emitter in an organic light-emitting diode (OLED), a high external quantum efficiency of 17.3% is achieved with a full width at half-maximum of 45 nm (154 meV) and Commission Internationale de L'Éclairage coordinate of (0.61, 0.39). This work demonstrates when introducing D-A typed structures, features...

Influence of charge transfer strength on emission bandwidth for multiple-resonance emitters via systematically tuning the acceptor–donor assembly

“…, gallium nitrides (micro-LEDs) and CdS/ZnS or CdSe/ZnS quantum dots (QD-LEDs). 4,5 Accordingly, MR-TADF emitters with panchromatic wavelength tuning from blue 4–18 through green 19–27 to red 28–30 have sprung up and rushed. The key to high color purity of MR-TADF emitters, from the perspective of electronic structure and computational simulations, 5,31 seems to lie in the strong ortho -positioned transition orbitals, i.e.…”

Efficient narrowband electroluminescence based on a hetero-bichromophore thermally activated delayed fluorescence dyad

“…S5 in the ESI†), which indicates that the weak long-range ICT interactions between the two MR segments does not have a significant influence on the FWHM. 54 The fluorescence spectra in different polar solvents were also investigated (Fig. 3b and Fig.…”

Boron‐, Sulfur‐ andNitrogen‐DopedTridecacyclic Aromatic Emitters with Multiple Resonance Effect for Narrowband Red Emission†

“…For instance, by employing boron (B) and nitrogen (N) as electron‐deficient and electron‐rich atoms, respectively, [ 23 ] B, N‐doped PAH MR‐TADF emitters are developed to exhibit both narrowband emission and high photoluminescence quantum yield (PLQY) near unity. [ 24‐32 ] Recently, carbonyl, nitrogen (C=O, N)‐doped PAH MR‐TADF emitters were designed by inserting carbonyl and nitrogen atoms as electron‐deficient and electron‐rich atoms in PAH skeletons, [ 33‐38 ] giving narrowband emissions with external quantum efficiency (EQE) over 20%. Lately, we have developed boron, sulfur (B, S)‐doped PAH as a kind of nitrogen‐free MR‐TADF emitter by incorporating B and S atoms in PAH skeleton, where S atoms not only serve as electron donors to generate MR effect with B atoms, but also play crucial roles in enhancing SOC and promoting RISC process by heavy‐atom effect.…”