Fused π‐Extended Multiple‐Resonance Induced Thermally Activated Delayed Fluorescence Materials for High‐Efficiency and Narrowband OLEDs with Low Efficiency Roll‐Off

Abstract

The simultaneous achievement of multiple‐resonance thermally activated delayed fluorescence (MR‐TADF) materials with strong narrowband emission and efficient reverse intersystem crossing (RISC) process can further promote the advancement of organic light‐emitting diodes (OLEDs). Herein, a new strategy is proposed to achieve two π‐extended MR‐TADF emitters (NBO and NBNP) peaking at 487 and 500 nm via fusing conjugated high‐triplet‐energy units (carbazole, dibenzofuran) into boron‐nitrogen (B/N) framework, aiming to increase charge transfer delocalization of the B/N skeleton and minimize singlet‐triplet energy gap (∆EST). This strategy endows the two emitters with full width at half maximum of 27 and 29 nm, and high photoluminescence efficiencies above 90% in doped films, respectively. Additionally, considerable rate constants of RISC are obtained due to the small ∆EST (0.12 and 0.09 eV) and large spin‐orbital coupling values. Consequently, the OLEDs based on NBO and NBNP show the maximum external electroluminescence quantum efficiency of up to 26.1% and 28.0%, respectively, accompanied by low‐efficiency roll‐off. These results provide a feasible design strategy to construct efficient MR‐TADF materials for OLEDs with suppressed efficiency roll‐off.

Judicious Heteroatom Doping Produces High Performance Deep Blue Multiresonant Thermally Activated Delayed Fluorescence Organic Light-Emitting Diodes

“…An intermediate triplet state was predicted to exist in both compounds, which is acknowledged to facilitate RISC via spin-vibronic coupling in D-A TADF emitters. 34,35,36,37,38 Difference density plots of the S1, S2, T1 and T2 states are shown in Figure 2b. For MesB-DIDOBNA-N each excited state possesses the typical alternating pattern of increasing and decreasing electron density of MR-TADF compounds that is characteristic of an excited state possessing SRCT character.…”

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.…”

High-performance three-coordinated organoboron emitters for organic light-emitting diodes

“…71,[82][83][84][85][86][87][88][89][90] In addition to those BAryl 2 acceptors, boracycles such as dibenzoborole, 1,4-oxaborine, 9,10diboraanthrancene, and borepine have been developed as alternatives in the past few years. 52,[91][92][93][94][95][96][97][98][99][100][101][102] These acceptors of the boracycle structure have been used in building novel emitters for OLEDs, some of which are in the D-A configuration 91,92,98,100,101 and some themselves are used as individual chromophore cores. 52,97,99 Since Noda and Shirota examined three-coordinated boron compounds in OLEDs in 1998, 47,67,68,70,103,104 TAB-based emitters have been developed for OLEDs that rely on diverse luminescence mechanisms, i.e., fluorescence (FL), [105][106][107] phosphorescence (PH), [108][109][110] and thermally-activated delayed fluorescence (TADF).…”

“…Also, TAB-based TADF emitters with strongly twisted D-A structures have been developed robustly in high-performance OLEDs that have both high efficiencies and long lifetimes. 117,118 Besides TABs, other three-coordinated organoboron compounds with B embedded in cycles, e.g., boracycles, 83,100,101 and (un)bonded BN/O-embedded arenes, 99,102,119,120 have been developed and applied mainly in FLOLEDs and TADFOLEDs with superior performance. Many of these novel TADF emitters possess structurally constrained B with rigid planarity in the structure.…”

“…71,82–90 In addition to those BAryl 2 acceptors, boracycles such as dibenzoborole, 1,4-oxaborine, 9,10-diboraanthrancene, and borepine have been developed as alternatives in the past few years. 52,91–102 These acceptors of the boracycle structure have been used in building novel emitters for OLEDs, some of which are in the D–A configuration 91,92,98,100,101 and some themselves are used as individual chromophore cores. 52,97,99…”