Synthesis And Properties Of Thermally Activated Delayed Fluorescent Materials Based On Diphenylsulfone-acridine Moiety

Thermally activated delayed fluorescent(TADF)materials are considered as the third generation of organic electroluminescent materials due to the theoretical internal quantum efficiency(IQE)of 100%and heavy metal atom-free.Since the first report on the TADF materials in organic light-emitting diodes(OLEDs)in 2009 by Adachi's group,TADF materials have attracted much attention by the researchers.To date,TADF materials with red,green and blue have been rapidly developed,and their device efficiency has approached or even exceeded 30%,which is comparable to traditional phosphorescent materials.Nevertheless,TADF materials still have some scientific problems,such as:(1)The small energy gap difference of TADF materials usually leads to weak oscillator strength,and few high-efficiency TADF material have been observed;(2)Relatively long lifetime of the emitter results in triplet-triplet exciton annihilation;(3)The TADF-based solution-processed electroluminescent devices have poor performance.Therefore,the development of efficient and stable TADF materials has an important significance.In order to further explore the relationship between TADF materials and properties,three series of TADF materials were designed and synthesized in this thesis,in which diphenylsulfone was considered as the acceptor(A)unit and 9,9-dimethylacridine as the donor(D)unit,respectively.The molecular structures were confirmed by hydrogen and carbon NMR spectroscopy,time-of-flight mass spectrometry and single crystal X-ray diffraction.The photophysical properties,electrochemical and thermal properties were investigated in detail using density functional theory,steady/transient state fluorescent spectra,cyclic voltammetry and thermogravimetic analysis.The solution-processed devices were fabricated employing these TADF materials as the dopants,and their electroluminescent properties were studied.The main contents of the thesis are as follows:(1)Using diphenyl sulfone as the acceptor,9,9-dimethyl acridine as the donor,and the benzene ring as the?bridge,a series of D-?-A type compounds(DPS-1?DPS-4)were synthesized.The effect of the connection position between the donor/acceptor units on the properties of the materials has been explored.The results show that compounds DPS-1?DPS-4 have good thermal stability with the decomposition temperatures(at 5%weight loss)of>300°C.All compounds exhibit TADF properties in solid films,and their maximum emission peak is in the range of 437?467 nm.In addition,compounds DPS-3 and DPS-4 display mechanochromic luminescence(MCL)properties.Using these compounds as dopants,solution processable blue OLEDs were fabricated.Maximum external quantum efficiency(EQE_max)of 5.26%was achieved for the DPS-3 based device.(2)Based on the molecular structure-property relationship of compounds DPS-1?DPS-4,a series of D-A type compounds DPS-5?DPS-8were prepared via removing the?bridge of the benzene ring to further improve the emission efficiency..Such molecules can effectively reduce intramolecular rotation and suppress non-radiative transitions,which is in favor of improving the emission efficiency.The structure-property relationship of the compounds was systematically studied through steady/transient-state fluorescence spectroscopy.All the four compounds have clear TADF characteristics,and very high photoluminescence quantum yields(PLQY),up to 99.7%,were observed in the solid film.Additionally,this type of D-A molecules also exhibited aggregation-induced emission enhancement(AIEE),room temperature phosphorescence(RTP),and mechanoluminescence(ML)features.Both theoretical calculation and single crystal indicate that the intramolecular through-space charge transfer(TSCT)effect plays a key role on the photophysical property.The solution-processed OLED based on DPS-7 achieved a best performance with the EQE of 20.07%.(3)In order to further study the molecular structure-property relationship of TSCT-based TADF materials,a second donor unit was introduced into compound DPS-6 to regulate the intramolecular charge transfer effect.With this in mind,the D₂-A type compounds DPS-9,DPS-10and DPS-11 were designed and synthesized.The influence of the donor type and connection position on the property was investigated in detail.The research results show that the introduction of the second donor unit is beneficial to improve the thermal stability of the material.The decomposition temperature(at 5%weight loss)of the compounds DPS-9and DPS-10 exceeds 350?.All three compounds exhibit TADF character with very high PLQY(70?98%).Using these compounds as the dopants,solution-processed OLEDs were fabricated,and DPS-9 based device present the best performance with an EQE_max of 21.73%.