Pyrazine-Based Blue Thermally Activated Delayed Fluorescence Materials: Combine Small Singlet–Triplet Splitting With Large Fluorescence Rate

Abstract

Metal-free thermally activated delayed fluorescence (TADF) emitters have emerged as promising candidate materials for highly efficient and low-cost organic light-emitting diodes (OLEDs). Here, a novel acceptor 2-cyanopyrazine is selected for the construction of blue TADF molecules via computer-assisted molecular design. Both theoretical prediction and experimental photophysical data indicate a small S 1 -T 1 energy gap (Δ E ST ) and a relative large fluorescence rate ( k F ) in an o -phenylene-bridged 2-cyanopyrazine/3,6-di- tert -butylcarbazole compound (TCzPZCN). The k F value of 3.7 × 10 7 s −1 observed in a TCzPZCN doped film is among the highest in the TADF emitters with a Δ E ST smaller than 0.1 eV. Blue TADF emission is observed in a TCzPZCN doped film with a short TADF lifetime of 1.9 μs. The OLEDs using TCzPZCN as emitter exhibit a maximum external quantum efficiency (EQE) of 7.6% with low-efficiency roll-off. A sky-blue device containing a derivative of TCzPZCN achieves an improved EQE maximum of 12.2% by suppressing the non-radiative decay at T 1 .

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 ). Missing from this family of heterocycles is pyridazine (Pydz), which is investigated in this work as an acceptor in three novel D-A compounds ( Figure 1B ).…”

High-Efficiency Diphenylpyrimidine Derivatives Blue Thermally Activated Delayed Fluorescence Organic Light-Emitting Diodes

“…A E ST of 4,6-PhPMAF and 2,6-PhPMAF dopants had been calculated to 0.27 and 0.17 eV, respectively. The small E ST can contribute to improved internal quantum efficiency by up-converted triplet excitons (Liu et al, 2019). However, the small E ST value of faster RISC from triplet to singlet can result in short or similar TADF lifetime (Zhang et al, 2019).…”

“…Improvement in device efficiency has been reported for device based on TADF molecules that the azasiline donor unit-based intermolecular charge-transfer exhibited deep blue TADF dye of high efficiency of 22.3% in mixed co-host (Kwon et al, 2015), and the methoxy substituents to replace tert-butyl substituents on the carbazole donors have been found to decrease the singlettriplet state energy splitting ( E ST ), as well as long lifetime and reducing the device efficiency roll-off (Wu et al, 2014;Shizu et al, 2015). Triplet energy levels and reduction potentials of various acceptor cyano-substituted pyrazines were reported with combined small singlet-triplet splitting and large fluorescence rate (Liu et al, 2019). This molecular design can be accomplished with acridine, carbazole, and phenoxazine as electron donor units and/or sulfone, phosphine oxide, dimesitylboryl, and polycyclic borazine as electron acceptor units (Ganesan et al, 2018).…”

Planar and Rigid Pyrazine Based TADF Emitter for Deep Blue Bright Organic Light Emitting Diodes

“…The related N-heterocycle, pyrazine, has thus far not been explored for blue TADF emitter design, though the pyrazine has been used within an electron-acceptor design to tune its electron affinity and the corresponding energies of the emitters. [14][15][16] Recently, Duan et al reported a series of blue TADF emitters bearing pyrazine as an acceptor and benzofuro-carbazoles or benzothieno-carbazoles as donor moieties. [17] Amongst the molecules in the study, BFCZPZ2 possessed a moderately small ΔEST (0.31 eV in PPT) and high photoluminescence quantum yield (PLQY) (91% in PPT).…”