| Organic light-emitting diodes（OLED）,as new generation display technology,have put forward higher and higher requirements for the performance of organic light-emitting materials as the display performance continues to improve.As a new generation of organic light-emitting materials,thermally activated delayed fluorescence（TADF）materials have the dual advantages of low cost（no heavy metals）and high efficiency（100%internal quantum efficiency）,and have been a research hotspot in this field.The TADF materials reported in the literatures are mostly constructed from different donor-acceptor groups,and the commonly used donor groups are mostly carbazole,dimethyl acridine,phenoxazine and phenothiazine.It is difficult to obtain TADF luminescence when the carbazole is connected to the acceptor,due to the small dihedral angle between the donor and the acceptor.The methyl group connected to the 1-position of carbazole,the regular tetrahedral configuration will enhance the steric hindrance between the donor and the acceptor,and promote a moderate twist between the donor and the acceptor,which is beneficial to the separation of HOMO and LUMO,and reduce the energy level difference(ΔEST)between the singlet and triplet states of the molecule,and change the molecular luminescence mechanism from ordinary fluorescence emission to thermally activated delayed fluorescence emission.In this thesis,1-methylcarbazole（1-Me Cz）is used as a new donor group to connect with nitrogen-containing heterocycles and aryl ketone acceptors,and two series of TADF molecules are designed and synthesized.The specific research content is as follows:1.Nitrogen-containing heterocyclic thermally activated delayed fluorescence molecules based on 1-methylcarbazole donors:a series of 1-methylcarbazoles were synthesized using triazine,pyrimidine and quinoxaline as acceptor groups,the delayed fluorescence of1-Me Cz-TRZ and 1-Me Cz-Pm follow thermally activated delayed fluorescence mechanism.The electroluminescent device based on 1-Me Cz-TRZ achieves an EQE of 13.07%and a maximum current efficiency of 12.16 cd A-1and the Commission Internationale de L’Eclairage（CIE）coordinates（0.15,0.11）,and doubled the reference molecule Cz-TRZ（EQE is 7.61%）.The maximum EQE of the light-emitting device using 1-Me Cz-Pm as the light-emitting material is 7.53%(10.56 cd A-1)with CIE of（0.17,0.18）.The introduction of a methyl group on the 1-position of carbazole can indeed increase the dihedral angle between the receptors,thereby reducing theΔESTand realizing the effective separation of HOMO and LUMO,creating an advantage for the effective reverse intersystem crossing process to realized TADF emission.2.Aromatic ketone thermally activated delayed fluorescence molecule based on1-methylcarbazole donor:1-methylcarbazole is used as the donor group,and diphenyl ketone and diphenyl ethylenedione are used as acceptors,respectively.Two symmetrical D-A-D molecules,named 1-Me Cz-BP and 1-Me Cz-BZ,were designed and synthesized.Electroluminescent devices were prepared with 1-Me Cz-BP and 1-Me Cz-BZ as doped guests.The electroluminescence wavelengths were 458 nm and 546 nm,and the maximum external quantum efficiency（EQE）was 11.76%and 3.22%,respectively.The experimental results show that the electron-withdrawing ability of the acceptor group increases,as the conjugated structure increases,the emission wavelength red shifts,and the luminous efficiency decreases.If the 1-carbazole phenyl group of 1-Me Cz-BP is replaced with other heteroaromatic rings（oxyfluorene,thiofluorene and 9.9-thiosulfone fluorene）,a series of structurally asymmetric D’-A-D molecules named 1-Me Cz-DBF,1-Me Cz-DBT and 1-Me Cz-DBZP.When testing the luminescence properties of 1-Me Cz-BP and 1-Me Cz-DBT doped in films,the presence of delayed components was detected,and the life of the delayed components decreased with the increase of temperature,indicating the luminescence of materials is thermally activated delayed fluorescence produced by localized radiation transitions. |
