Development Of High-performance Light-emitting Materials And Host Materials For OLEDs By Regulating Electron Donor-acceptor Connection

Organic light-emitting diodes（OLEDs）are self-luminous light sources with the advantages of ultra-thin thickness,flexibility,wide viewing angle,high resolution,high contrast and fast motion picture response time.In the past two decades,the research of OLEDs in academia and industry has maintained a high degree of enthusiasm.In recent years,electron donor-acceptor（D-A）type organic light-emitting materials for non-doped and doped devices have attracted much attention and developed rapidly.However,there are still some challenges.For example,the efficiencies of doped OLEDs have serious roll-offs and the development of host materials is far behind that of luminescent materials.This paper first introduces the basic photophysical knowledge of OLEDs,and reviews the development history of luminescent materials and host materials for OLEDs.Based on the research background,this paper focus on the development of high-performance D-A type luminescent materials and host materials.The luminescent materials and host materials with different D-A connection forms are designed and synthezied.Their structure-property relationships are systematically studied and their applications on OLEDs are evaluated.The work of the paper includes the following five parts:In chapter 2,this work studies the structure-activity relationship of naphthalene-based D-A type materials.The electron donor phenoxazine/carbazole and electron acceptor triazine are modified at the C1 and C8 positions of naphthalene.u-PXZTRZ and u-Cz TRZ are designed and synthesized to study the effect of charge transfer pathways on luminescent behaviors.The stronger TSCT of u-PXZTRZ results in the efficient separation of frontier orbitals and exhibits delayed fluorescenc（DF）.As a result,the maximum external quantum efficiency of the prepared doped OLEDs using u-PXZTRZ as emitter is 3.72%;There are both TSCT and through-bond charge transfer pathways in u-Cz TRZ,which leads to bluer emission.Due to balanced carrier transport capacity,the maximum efficiencies of phosphorescent OLEDs（Ph OLEDs）using u-Cz TRZ as host are 36.4%,117.9 cd A^-1 and 82.5 lm W^-1.In chapter 3,in order to further explore the effect of TSCT on the photoluminescence and electroluminescence（EL）properties of DF materials.Using o/p-terphenyl as the molecular backbone,phenoxazine/phthathiazine as electron donor and triazine as electron acceptor to control the intensity of the charge transfer state to achieve thermally activated delayed fluorescence,the conmparative V-shaped（v-PXZTRZ and v-PTZTRZ）and linear（l-PXZTRZ and l-PTZTRZ）molecules are designed and synthesized.Compared with linear molecules,the TSCT brought by the edge-to-face configurations of phenoxazine/phenothiazine and triazine in V-type molecules can accelerate the reverse intersystem crossing（RISC）in films.The aggregation-induced delayed fluorescence effect of these molecules is observed,which show high photoluminescence quantum yield and exciton utilization efiiceiency.As a result,the non-doped delayed fluorescent OLEDs（DF-OLEDs）based on V-type molecules have maximum efficiencies of 9.9%,30.9 cd A^-1 and 26.5 lm W^-1 with an efficiency roll-off of 3.9%.In chapter 4,to expand the application of o/p-terphenyl skeleton in host materials,two new host materials,v-Cz TRZ and l-Cz TRZ,are synthesized by introducing carbazole and triazine into the o/p-terphenyl skeleton.The lowest singlet（S₁）states of the molecules exhibit local excitation characteristics,and the molecules have solid-state emission peaks at 437 nm and 458nm.Due to the twisted comformation,the lowest triplet（T₁）levels of v-Cz TRZ and l-Cz TRZ are 2.57 e V and 2.34 e V,respectively.As a result,the orange Ph OLEDs using l-Cz TRZ as host shows the maximum efficiencies of 41.3%,135.5 cd A^–1 and 135.7 lm W^–1,with a small efficiency roll-off of 2.1%.In chapter 5,to further improve the T₁ level of host materials,benzophenone derivatives with ortho-and para-modifications,o-BPPh Cz and p-BPPh Cz,are synthesized.The S₁ states of o-BPPh Cz and p-BPPh Cz have identical local excitation characteristics with emission peaks at 436 nm and 437 nm,respectively.The T₁ levels of o-BPPh Cz and p-BPPh Cz are 2.67 e V and2.54 e V,respectively.As a result,the orange Ph OLEDs using o-BPPh Cz as host show the maximum efficiencies of 35.5%,115.3 cd A^–1 and 129.3 lm W^–1,with an efficiency roll-off of9.5%.In chapter 6,to develop highly efficient DF-OLEDs,C6 and C7 positions of quinazoline are modified with acridine type donors to create four luminescent molecules named 6-QZL-DPAC,6-QZL-SFAC,7-QZL-DPAC and 7-QZL-SFAC.It is easier to induce the generation of charge transfer states at the C7 position compared with the C6 position.The twisted molecular configurations can weaken the intermolecularπ–πinteraction in film state,and lead to aggregation-induced emission effect.The resulting smallΔE_ST can promise DF property.As a result,the maximum efficiencies of the doped OLEDs based on 7-QZL-SFAC reach 33.2%,107.7 cd A^–1 and 120.8 lm W^–1.