Substitution Conformation Balances the Oscillator Strength and Singlet-Triplet Energy Gap for Highly Efficient D-A-D Thermally Activated Delayed Fluorescence Emitters

Four donor-acceptor-donor region isomers (2,3-TXO-PhCz, 2,6-TXO-PhCz, 2,7-TXO-PhCz, and 3,6-TXO-PhCz) are designed. The substitution positions of the two PhCz units significantly impact the photophysical properties of the isomers, especially for the singlet-triplet energy splitting (Delta E-ST) and oscillator strength (f ). 2,3-TXO-PhCz exhibits weak emission due to the large steric hindrance of the two PhCz units. While 2,6-TXO-PhCz, 2,7-TXO-PhCz, and 3,6-TXO-PhCz all exhibit strong emission. The four emitters possess small Delta E-ST of 0.01-0.24 eV; the corresponding f values are 0.064, 0.107, 0.026, and 0.134. Consequently, the photoluminescence quantum yields (PLQYs) of the doped films in CBP host are: 62.1% for 2,3-TXO-PhCz, 83.8% for 2,6-TXO-PhCz, 89.0% for 2,7-TXO-PhCz, and 85.4% for 3,6-TXO-PhCz. Although notable divergences of f and Delta E-ST exist between 2,6-TXO-PhCz and 2,7-TXO-PhCz, similar PLQY of doped film in the CBP host, and exciton utilization and external quantum efficiency (EQE) of the corresponding devices can be achieved: 2,6-TXO-PhCz and 2,7-TXO-PhCz endow the organic light-emitting devices with high EQE of 23.2% and 24.4%. This proximity can be attributed to the synergistic effect of f and Delta E-ST. This finding highlights the beneficial role of the different linking positions on the acceptor unit in facilitating the adjustment of f and Delta E-ST in order to improve the device efficiency.