The Relationship Between The Molecular Structure And Optoelectronic Properties Of Cyanobenzene Based Thermally Activated Delayed Fluorescent Materials

Thermally activated delayed fluorescence（TADF）materials as pure organic materials can achieve 100%theoretical internal quantum efficiency under electric excitation,having attracted tremendous attention.Up to now,the performance of the OLED devices based on TADF materials（TADF-OLEDs）has been greatly improved,while that of the red TADF-OLEDs is still lower than their phosphorescent counterparts.Besides,the roll-off of TADF-OLED devices is usually severe,impeding their commercialization.To reveal the factors influencing the luminous efficiency of TADF materials and the roll-off characteristics of TADF-OLEDs,this dissertation investigated the relationship between the molecular structure and the photoelectrical properties of cyanobenzene based TADF materials,and proposed molecular design strategies of avoiding the formation of intramolecularπ-πinteraction and introducing peripheral substituent groups to protect molecule’s active sites.The main conclusions are as follows:1.Three D-A-D type TADF materials,DMAC-PN,PXZ-PN,and PTZ-PN,with the same acceptor of o-phthalodinitrile（PN）and different donors(9,9-dimethyl-9,10-dihydroacridine（DMAC）,phenoxazine（PXZ）,and phenothiazine（PTZ）,have been synthesized.PXZ-PN has strong intramolecularπ-πinteraction,resulting in increased D-A dihedral angles and enhanced molecular rigidity,thus its T₁ excitons could not easily convert into S₁ excitons,which is detrimental to the luminous efficiency.While DMAC-PN having no intra-molecularπ-πstacking because of the steric hindrance of the methyl groups shows the highest photoluminescence quantum yield（PLQY）.When using DPEPO instead of CBP as the host,the DMAC-PN based OLED shows a large redshift of the emission peak to 594 nm,while maintains an EQE_max as high as 7.2%,indicating that efficient red TADF-OLEDs can be prepared by doping highly efficient orange TADF emitters into hosts with proper polarity.When adopting DIC-TRz（a TADF material）as the host,the DMAC-PN based OLED shows reduced roll-off and improved lifetime.2.Three TADF molecules（2MeCzPN,2tBuCzPN,2PHCzPN）were synthesized by introducing methyl,tert-butyl and phenyl groups into the 3,6-positions of the carbazoles of 2CzPN,and the effects of the peripheral substituent groups on the molecular electrochemical stability and the intermolecular interactions were studied.The repeated cyclic voltammetry scan test results shows that compared with 2CzPN,the electrochemistry stabilities of 2MeCzPN,2tBuCzPN and 2PHCzPN are greatly improved.Besides,the peripheral phenyl groups can partly inhibit the intermolecular interaction due to its larger steric hindrance than methyl and tert-butyl groups,which helps to reduce the device’s roll-off.3.A green TADF molecule 4MeOCzBN was synthesized by adopting just one cyano group as the acceptor and four methoxyl groups modified carbazoles as the donor.Compared with 4CzIPN,the HOMO and LUMO levels of 4MeOCzBN are pulled up,thus its choice of host materials are broader.Using an exciplex as the host,4MeOCzBN based OLED achieves high efficiency and low roll-off simultaneously,its EQE can maintain as high as 12.4%at 10000 cd/m².