Engineering the Macrocyclic Donor Structures towards Deep-Blue Thermally Activated Delayed Fluorescence Emitters

Deep-blue thermally activated delayedfluorescence (TADF) moleculespresent promising potential in organic light-emitting diodes (OLEDs),especially for display applications. Here, an efficient molecularengineering approach to modifying the donor or acceptor features ofthe D-& pi;-A-configured TADF molecules for deep-blueemission is reported. By introducing oxygen and sulfone as a bridgeunit onto the macrocyclic donor, two emitters, c-ON-MeTRZ and c-NS-MeTRZ, are synthesized and characterized,respectively. The reduced donor strength of c-ON-MeTRZ and c-NS-MeTRZ as compared to that of the model molecule c-NN-MeTRZ leads to blue-shifted emissions with high photoluminescencequantum yields (PLQYs) and retains TADF characters, while the newemitter c-NN-MePym with the most blue-shifted emissiononly exhibits a pure fluorescent nature because of the electron-acceptingfeature of pyrimidine that is insufficient for inducing the TADF property.In the presence of macrocyclic donors, these new emitters show highhorizontal dipole ratios (& UTheta;(//) = 85-89%), whichare beneficial for improving the light out-coupling efficiency. Deep-blueTADF OLEDs incorporating c-ON-MeTRZ as an emitter dopedin the mCPCN host achieves a high maximum external quantum efficiency(EQE(max)) of 30.2% together with 1931 Commission Internationalede I'Eclairage (CIE) coordinates of (0.14, 0.13), while thecounter device employing c-NS-MeTRZ as a dopant givesEQE(max) of 15.4% and CIE coordinates of (0.14, 0.09). TheEQE(max) of c-ON-MeTRZ- and c-NS-MeTRZ-based devices can be significantly improved to 34.4 and 29.3%, respectively,with a polar host DPEPO, which stabilizes the charge transfer (CT)S-1 state to give lower & UDelta;E (ST) for improving the reverse intersystem crossing process. The efficientTADF character, high PLQYs, and high anisotropic emission dipole ratioswork together to render the superior electroluminescence (EL) efficiencies.Based on the detailed characterizations of physical properties, theoreticalanalyses, and comprehensive study on the corresponding devices, aclear structure-property-performance relationship has beensuccessfully established to verify the effective molecular designstrategy of modulating the macrocyclic donor characters for efficientdeep-blue TADF emitters.