Synthesis And Electroluminescence Of Thermally Activated Delayed Fluorescence Materials Based On Triazine-carbazole

Organic light-emitting diodes（OLEDs）have become the third generation of displays due to their fast response,low power consumption,and flexible display.Thermally activated delayed fluorescence（TADF）material is a new type of pure organic OLED emitter,which has been a hot research topic in recent years due to its wide color gamut,low cost and theoretical internal quantum efficiency up to 100%.In this thesis,a series of TADF materials were designed and synthesized with N₃,N₃,N₆,N₆-tetraphenyl-9H-carbazole-3,6-diamine（Cz DP）as the electron donor and triazine as the electron acceptor.Cz DP was used as the electron donor,in which the diphenylamino disperses the HOMO of the emitter,so that the highest occupied molecular orbital（HOMO）and the lowest unoccupied molecular orbital（LUMO）of the molecule are separated from each other.This results in a small single-triplet energy splitting(ΔE_ST),which allows the molecule to acquire TADF properties without relying on a highly twisted structure.Theπ-bridge between donor and acceptor was varied from phenylene to electron-withdrawing pyridyl,cyanophenylene or trifluoromethylphenylene,which can change the conformation of the emitters and then affect the excited state properties including k _RISC and k_r.The specific research content is as follows:（1）A series of novel D-π-A type TADF emitters,namely TRZCz DP,TRZCz DP-Py,TRZCz DP-CF_3（O）,TRZCz DP-CN and TRZCz DP-CF₃,were developed with2,4-diphenyl-1,3,5-triazine as electron acceptors and Cz DP as electron donors.Incorporation of electron-withdrawing pyridyl,trifluoromethyl or cyano on or as theπ-bridge enhanced the triazine acceptor strength,not only leading to bathchromatic shift of the TADF,but also changing molecular conformation.The electroluminescent devices based on TRZCz DP-CN and TRZCz DP-CF₃ achieved maximum EQE of 10.64%and 8.79%,with the Commission International ede L’Eclairage（CIE）coordinates of（0.49,0.49）and（0.42,0.53）,respectively.The reason for the inefficiency of the emitters may be due to the large energy gaps between³LE and ³CT,and between ³LE and ¹CT of the emitters.（2）Based on the above research,A series of novel D-π-A type thermally activated delayed fluorescence（TADF）emitters,namely DTPTCz DP,DTPTCz DP-Py,DTPTCz DP-CN and DTPTCz DP-CF₃,were developed with 2,4-di-tert-butyl-1,3,5-triazine as electron acceptors and Cz DP as electron donors.The replacement of the phenyl group on the triazine ring with tert-butyl group can reduce the conjugate degree of the triazine ring and then reduces the electron-withdrawing ability of the acceptor,resulting in a blue shift of the TADF and a change in the energy level and distribution of the excitation states for TADF emitters.Incorporation of electron-withdrawing pyridyl,trifluoromethyl or cyano on or as theπ-bridge enhanced the triazine acceptor strength,not only leading to bathchromatic shift of the TADF,but also reducing the singlet and triplet energy splitting(ΔE_ST)and increasing the reverse intersystem crossing rate constant(k _RISC).Particularly DTPTCz DP-CN exhibited high radiation rate constant k_r>10⁷ s^-1 and k _RISC>10⁶ s^-1 simultaneously.The greenish-blue organic light-emitting diode of DTPTCz DP realized a maximum EQE of 20.13%while the pale-yellow device of DTPTCz DP-CN exhibited an EQE of 17.81%.