Design,Synthesis And Photoelectric Properties Of Thermally Activated Delayed Fluorescent Materials Based On 3,5-Dicyanopyridine Core

Following the cathode ray tubes(CRT)and liquid crystal displays(LCD),organic light emitting diodes(OLEDs)as the third-generation display technology features the characteristics of flexible,foldable,thin,wide viewing angle,etc.,and a great potential in the display and lighting.In recent years,as a new generation of pure organic light-emitting materials,noble metal-free thermally activated delayed fluorescence(TADF)materials can achieve 100%exciton utilization.However,there are still some issues need to be addressed urgently.On the one hand,efficient TADF materials should have sufficiently small singlet–triplet energy gaps(?E_ST)and high photoluminescence quantum yield(PLQY).The?E_ST can be reduced by decreasing the highest occupied molecular orbital(HOMO)and the lowest unoccupied molecular orbital(LUMO)overlap,but PLQY is proportional to the degree overlap of HOMO and LUMO.On the other hand,TADF-OLED devices have a significant efficiency roll-off under high brightness,which is mainly due to exciton annihilation caused by charge accumulation.Aiming at above problems,in the thesis,we used 3,5-dicyanopyridine as acceptor to construct a new type of thermally activated delayed fluorescent material.The main contents of this thesis are as follows:1.In chapter II,we developed two 2,6-dimethyl-3,5-dicyanopyridine-based TADF molecules,Me-PXZ and Me-DMAc.Theoretical calculations demonstrated that both molecules exhibit a double-twisted configuration.Such geometry endows fully separation of HOMO and LUMO leading to a small?E_ST value.Remarkably,both emitters exhibited distinct TADF characteristics and considerable PLQYs in doped films.As a result,high external quantum efficiency(EQE)of 25.8%for Me-DMAC and 21.1%for Me-PXZ are achieved.2.In chapter?,we further investigated the relationship between structures,TADF features and electroluminescent properties.Three emitters,namely Ph-DMAc,Na-DMAc,and 3-DMAc,with similar donor-acceptor skeleton but different side substituents of phenyl,?-naphthyl,and 3-pyridinyl are developed.Theoretical calculations demonstrated that all three molecules have a relatively rigid structure,which is conducive to improving PLQY.Different substituents impart different TADF characteristics.The proportion of delayed component is as high as 60%in Ph-DMAc,and only 8%in Na-DMAc.3-DMAc has the fastest RISC process.Moreover,OLEDs based on the emitters were also fabricated,which could achieve maximum EQE of29.1%,21.2%and 21.5%for Ph-DMAc,Na-DMAc,and 3-DMAc,respectively.3-DMAc-based device exhibits the smallest efficiency roll-off due to its faster RISC process.3.In Chapter?,we selected carbazole with higher triplet energy level as donor.By introducing different numbers of methoxy groups into the carbazole unit,we developed three molecules,Cz-Me,MOCz-Me and DMOCz-Me.Theoretical calculations demonstrated that the LUMO and HOMO of these three molecules have a certain degree of overlap,which is conducive to enhancing the radiative decay rate and thus improving PLQY.The introduction of methoxy group improves the electron-donating ability of the carbazole unit and enhances the intra-molecular charge transfer,thereby reducing the?E_ST.As a result,high EQE of 12.4%for MOCz-Me and 18.6%for DMOCz-Me are achieved.DMOCz-Me-based device exhibits high efficiency and low roll-off,mainly because DMOCz-Me has a higher proportion of delay components and a faster RISC process.We designed and synthesized a series of TADF molecules based on 3,5-dicyanopyridine and its derivatives.The systematic investigation on correlation between molecular structure and properties shows that 3,5-dicyanopyridine has the following advantages:first,it is easy to synthesize,and it can build a variety of performance acceptors through the change of cyanide-containing substrates;second,its structure is relatively rigid,which is conducive to improving the PLQY of the TADF molecule;Third,it has strong electron-deficient properties,can form a strong charge transfer excited state with donor,construct an effective RISC process,and then prepare an efficient organic electroluminescent device.