Large Increase in External Quantum Efficiency by Dihedral Angle Tuning in a Sky‐Blue Thermally Activated Delayed Fluorescence Emitter

Abstract

which enable internal quantum efficiencies (IQEs) of up to 100%. [2,3] However, they rely on the use of expensive noble metals. [4] Moreover, blue phosphorescent emitters degrade rapidly, which result in low operational lifetimes. [5] As a consequence, blue colors are currently being produced by fluorescent emitters. Those emitters have long been limited to internal quantum efficiencies of 25% but can reach internal quantum efficiencies of up to 62.5% by employing triplet fusion. [6,7] Thermally activated delayed fluorescence (TADF) is a more efficient alternative to classical fluorescence and it can exhibit IQEs up to 100%. [8][9][10][11] In devices, three quarters of excitons are statistically formed in a triplet state, which are nonemissive in case of fluorescent emitters. [12] TADF molecules harness both singlet and triplet excitons by introducing appreciable reverse intersystem crossing (RISC) to convert nonemissive triplet excitons to emissive singlet excitons. [13,14] By spatially separating the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO), the exchange energy is lowered and singlettriplet splitting (ΔE ST = E S1 − E T1 ) is reduced. [8] As a consequence, the singlet state becomes thermally accessible from the triplet state at room temperature. It has also been revealed that the resulting RISC proceeds via vibronic coupling of the triplet charge transfer state ( 3 CT) state with a triplet localized exciton state ( 3 LE), which in turn couples to the singlet manifold. [15] While many TADF molecules have been reported, [16][17][18][19][20][21][22] widely applicable molecular design principles beyond trial-anderror approaches remain scarce. [23][24][25] Recently, Adachi and coworkers [26] and Huang et al. [27] have shown that it is possible to enhance RISC in blue fluorescent molecules with carbazole donors by dihedral angle tuning (Figure 1a,b). The dihedral angle between donor and linker (θ 1 ) and the dihedral angle between linker and acceptor (θ 2 ) can be tuned by attaching methyl groups to the phenylene linker. Advantageously, this can be done without significantly changing emission wavelengths. As a result, deep blue emitters with external quantum efficiencies (EQEs) slightly above 10% were obtained. Herein, we report our latest efforts to test the general validity of the dihedral angle tuning strategy by applying it to a promising Efficient and stable blue emitters for organic light-emitting diodes are urgently needed for next-generation display and lighting applications. The discovery of thermally activated delayed fluorescence (TADF) has revealed a new class of promising candidates. After pairing the iminodibenzyl donor with the triazine acceptor via a phenylene linker, dihedral angle tuning is employed to regulate the difference between the energy levels of singlet and triplet excited states. Enhanced reverse intersystem crossing rates are observed in response to increased methylation at the phenylene linker. This behavior agrees with the density function...

Fast Delayed Emission in New Pyridazine-Based Compounds

“…Among the N -heterocyclic electron-acceptors, 1,3,5-triazine is one of the most employed (Wong and Zysman-Colman, 2017 ; Huang et al, 2019 ; Sharma et al, 2019 ; Wang et al, 2019 ; Woo et al, 2019 ). Other heterocycles used in this role include pyridines (Rajamalli et al, 2017 , 2018a , b , 2019 ), pyrimidines (Komatsu et al, 2016 ; Nakao et al, 2017 ; dos Santos et al, 2019 ), and pyrazines ( Figure 1A ) (dos Santos et al, 2019 ; Kato et al, 2019 ; Liu et al, 2019 ).…”

Nematic Triphenyltriazine Triesters and the Induction of the Columnar Mesophase by Fluorine Substitution

“…Donor-substituted triphenyl-s-triazines such as 3 are a promising major class of thermally activated delayed fluorescence (TADF) emitters, [9][10][11][12][13][14][15][16][17][18][19][20] and parent triphenyl-s-triazine has a high triplet energy of ca. 3.05 eV [21] that allows its consideration as anisotropic host [22][23][24][25][26][27][28] for blue-emitting TADF dopants whose triplet energy should be smaller than the host's to confine the excitons on the guest molecules.…”

A Red Thermally Activated Delayed Fluorescence Emitter Simultaneously Having High Photoluminescence Quantum Efficiency and Preferentially Horizontal Emitting Dipole Orientation

“…To counteract this tradeoff, TADF emitters usually adopt a pre‐twisted charge‐transfer (CT) configuration in D‐A or D‐π‐A systems. [ 15 ] Following this design principle, 100% Φ PL can be achievable for blue and green TADF emitters, [ 11,13,26–30 ] yet near unity Φ PL for red TADF emitters remains a challenge because of their intrinsic quenching mechanism. Red TADF emitters generally suffer more significant nonradiative processes (dominated by the nonradiative internal conversion) between S 1 and the ground state ( S 0 ), largely due to vibrational overlap between the zero‐vibrational levels of S 1 and the higher‐vibrational levels of S 0 governed by the energy gap law.…”