Constructing Efficient Red Thermally Activated Delayed Fluorescence Emitters Through Modulating Excited State Energy Level Alignments And Their Application Investigations

The research highlights in this thesis are the design and development of novel red emitters based on thermally activated delayed fluorescence（TADF）mechanism.It focuses on the issue of red TADF emitters with slow reverse intersystem crossing（RISC）rates（kRisc）to date.Our work centers on the contribution of local excited（LE）triplet states（³LE_A）of the acceptor（A）fragments to the overall k_RISC of red TADF emitters.As results,a series of red TADF materials with high k_RISC were designed and synthesized,and the following research findings were obtained:（1）By changing the degree of conjugation expansion inside the A fragment,the ³LE_A energy level was regulated without affecting the charge transfer（CT）properties of the whole molecule,thus improving the RISC rate of the material.The A fragment of pDPBPZ-PXZ is a hexagonal ring,while the control molecule pDPAPQ-PXZ contains a pentagonal ring.The former has a more complete conjugation expansion,resulting in the energy level of ³LE_A dramatically reduced,being closer to their CT levels.In this sense,the ³LE_A of pDPBPZ-PXZ is effectively involved in the RISC process which greatly enhances the spin-orbit coupling（SOC）and obtains a higher k_RISC.In contrast,the ³LE_A level of pDPAPQ-PXZ is far above the CT levels and thus hardly participates in the up-conversion process.Eventually,the OLED device based on pDPBPZ-PXZ successfully achieved an external quantum efficiency（EQE）of 20.3%,which is almost double that of pDPAPQ-PXZ.These results demonstrate the feasibility of improving the k_RISC by changing the conjugation inside the A fragment,realizing fine-tuned ³LE_A levels while fixed CT levels,which is of important guiding significance for constructing high-performance red TADF emitters.（2）Leveraging the auxiliary acceptor strategy that has identical A core,donor fragments and molecular construction modes,was proposed,in which the conjugation degree of the overall molecule can be unchanged,thus ensuring the same ³LE_A level.Then,by regulating peripheral second acceptors of the A core to change the CT strength of the three materials,the ¹CT and ³CT level positions can be strategically regulated.Among them,mDPBPZ-DPXZ has the smallest ΔDEST,and its ³LE_A is exactly located between 1CT and 3CT,thus giving rise to an ideal excited state level arrangement for triplet exciton un-conversion.Benefiting from such favorable level arrangement,mDPBPZ-DPXZ obtained a faster k_RISC(1.54×10⁶ s^-1),a higher photoluminescent quantum yield（93%）and a higher EQE of 21.6%,significantly better than the other two materials.These results reveal that fine-tuning the level arrangement between CTs and LE without affecting the position of the ³LE_A level can be achieved based on the above-mentioned method.It demonstrates the great potential to enhance the orbital coupling and improve the up-conversion rate towards developing highly efficient red TADF materials in the future.