Design,Synthesis And Properties Of Tristriazolotriazine Acceptor-Blue Thermally Activated Delayed Fluorescence Materials

Organic thermally activated delayed fluorescence is considered as the new generation of electroluminescent materials due to rare metal atom-free and utilization of both singlet and triplet excitons emission.Since the first report of TADF in organic light-emitting diodes in 2009,TADF materials have been attracted much attention by the researchers.Over the past decade,TADF materials have achieved rapid development,and the device efficiency can be comparable to the traditional phosphorescent devices.However,most reported TADF materials were only used for vapor-deposited devices due to their poor solubility and film-forming properties.Compared with vacuum evaporation devices,solution-processed devices have the advantages of simple process preparation,lower cost,and easy controlling of dopant concentration,which is good for realizing large-area display and flexible terminals.Therefore,the development of high-efficiency and solution-processable TADF materials has an important scientific significance.In view of the key scientific problems of solution-processed blue emitter such as few species,low emission efficiency,and single material performance,in this thesis,a series of donor-acceptor（D-A）type TADF Blue materials based on tristriazolotriazine acceptor units were designed and synthesized through reasonable molecular construction.The molecular structure were confirmed by proton nuclear magnetic resonance spectroscopy,carbon spectroscopy,flight time mass spectrometry and X-ray single crystal diffraction.The TADF property is proved via DFT calculation（the molecular frontier orbital,electron cloud density distribution,and singlet and triplet energy levels）.The photophysical,electrochemical and thermodynamic properties were explored through steady/transient state fluorescence spectrometer,cyclic voltammetry and thermogravimetric analysis.Using such TADF materials as the dopants,organic light-emitting diodes were prepared by solution processing method,and then the relationship between molecular structure and device performance were studied in detail.The main research contents of this thesis are as follows:（1）A novel acceptor unit of tristriazolotriazine（TTT）was designed and synthesized.Introduction of the carbazole（Cz）derivatives into TTT moiety,three compounds,named o-TTT-Ph-3Cz、m-TTT-Ph-3Cz and p-TTT-Ph-3Cz,with D₃-A feature were prepared.The influence of the connection position between donor and accept on the material properties was explored.Compounds m-TTT-Ph-3Cz and p-TTT-Ph-3Cz have good thermal stability,and their thermal decomposition temperatures at 5%weight loss are both higher than 380^oC.Although compounds o-TTT-Ph-3Cz,m-TTT-Ph-3Cz and p-TTT-Ph-3Cz do not have TADF property,they exhibit deep blue emission both in solution and doped films.Employing m CPCN as the host material and（o,p,m）-TTT-Ph-3Cz compounds as the dopant,solution-processed OLED were fabricated.These devices present deep blue emission with the maximum emission peaks of<420 nm and the maximum external quantum efficiency(EQE_max)of 3.76%.This research results proved that TTT unit can be used as an acceptor fragment for the blue emitter.（2）Based on the previous results,the molecular structure was optimized through changing the donor units（tert-butylcarbazole,acridine derivatives）.To this end,novel star-shaped emitters of TTT-Ph-3Cz,TTT-Ph-3Ac and TTT-Ph-3BAc with D₃-A feature was constructed.The molecular structure-property relationship was explored in detail.The research results demonstrate that TTT-Ph-3Cz is only a traditional fluorescent material,while TTT-Ph-3Ac and TTT-Ph-3BAc have obvious TADF properties in both solution and solid film.Additionally,compound TTT-Ph-3BAc also shows aggregation-induced emission effect.Using the compounds as the dopants,the solution processable OLEDs were fabricated and present intense blue emissions.The TTT-Ph-3Cz based OLED shows the maximum emission peak at 426 nm with the EQE_max of3.45%.The TTT-Ph-3Ac and TTT-Ph-3BAc based devices display the emission peaks at 480 and 498 nm with the EQE_max of 9.73 and 6.73%,respectively.The research results prove that blue TADF emitters can be obtained by changing the donor unit.（3）In order to further explore the structure-performance relationship ofTTT-based TADF materials,this chapter developed a class of D-A and D₂-A TADF materials by changing the number of donor groups.The influence of the number of donor units on the material property was investigated in detail.Both compounds TTT-Ph-Ac and TTT-Ph-2Ac exhibit clear TADF property.In addition,intense blue emission are observed with the emission peaks at 468 and 471 nm for compounds TTT-Ph-Ac and TTT-Ph-2Ac,respectively.The solution-processed device based on TTT-Ph-Ac has an emission peak of 468 nm and an EQE_max of 3.07%.On the other hand,the solution-processed device based on TTT-Ph-2Ac shows an emission peak of 486 nm and an EQE_max of9.0%.This research result shows that reducing the number of donor units is beneficial to suppress the intramolecular charge transfer effect,which can promote the blue-shifted emission,and then obtain blue emitter with good color saturation.