Synthesis And Optoelectronic Properties Of Thermally Activated Delayed Fluorescent Materials

Organic light-emitting diodes(OLEDs)with highly potential applications in display and lighting have become a focus for the last thirty years.Electrophosphorescent devices with the capability of harvesting both singlet and triplet excitons and realize 100%exciton utilization efficiencies,which are much superior than traditional electrofluorescent devices with limited 25%exciton utilization efficiencies,have made critical breakthrough in elevating device performances.However,the high cost of heavy-metal complexes and the instable defects of blue phosphors still extremely restrict their further applications.In recent years,OLEDs employing purely organic thermally activated delayed fluorescence(TADF)dyes with the ability of harvesting triplet exciton via an environmentally heat mediated reverse intersystem crossing(RISC)process and subsequent emission of delayed fluorescence,which can also realize 100%exciton utilization efficiency without involving noble metal,have attracted the focus.The design principles of TADF dyes require both small singlet-triplet energy gaps(?ESTSS)to ensure large rate constants of RISC procedure(kRISCS)and large rate constants of the radiative singlet(KrSS)to guarantee a high photoluminescence quantum yields(PLQYs).However,hitherto the quantity of the reported TADF dyes meeting all these requirements is still insufficient,which has hampered the development of this research domain.With the intension of resolving these challenges,in this thesis,we designed and synthesized a series of TADF dyes via modified molecular design tactics,and systematically investigated their properties.The results demonstrated that optoelectronic properties and device performances of these TADF dyes were optimized,providing new insights into the realizing of high-efficiency TADF materials and devices.In chapter 1,it starts with a brief introduction of the background and history of OLEDs,the luminescent mechanism,construction and performance evaluation parameters of OLEDs.Then,the introduction of functional layers in OLEDs with an emphasized review on the research progress of TADF dyes is elaborated.Finally,the main design tactics and contents of this thesis are illustrated.In chapter 2,three green TADF dyes,i.e.PXZPM,PXZMePM and PXZPhPM were constructed on the basis of pyrimidine/phenoxazine through substituent modification on the 2-position of pyrimidine.The introduction of substituent to the 2-position of pyrimidine unit,results in improved thermal stabilities,reduced ?ESTS and delayed fluorescent(DF)lifetimes,and enhanced PLQYs,with maintained frontier molecular orbital(FMO)energy levels and electronic energy levels.The OLED based on phenyl substituted PXZPhPM obtained the best device performance with a CEmax(maximun current efficiency)of 80.0 cd/A,a PEmax(maximum power efficiency)of 73.7 lm/W,and a EQEmax(maximum external quantum efficiency)of 24.6%,which are basically comparable to the OLEDs on the basis of green heavy-metal complexes.In chapter 3,four light-blue TADF dyes,i.e.Py1,Py2,Py3 and Py4,were constructed on the basis of pyridine/phenoxazine via controlling the peripherally phenyl number on the pyridine unit.With the step-by-step increase of phenyl numbers,the LUMO distributions of the TADF molecules are gradually extended,accompanied with gradually enhanced thermally stabilities and increased PLQYs in degassed toluene.However,excessive phenyl will leads to the increase of ?ESTS and DF lifetimes,and subdued TADF nature,accompanied with enlarged acceptor planarity and ehanced molecular packing leading to a decreased film state PLQY.The OLED based on Pyl demonstrated the best device performance with a EQEmax of 7.9%.In chapter 4,three green TADF dyes,i.e.o-PXZP,m-PXZP and p-PXZP were constructed on the basis of phenanthroline/phenoxazine through isomer engineering.The change of the linking position of the electron-donating phenoxazine groups on the phenanthroline unit results in finely tuned and optimized molecular thermal stabilities,FMO energy levels,electronic energy levels,?ESTS,DF lifetimes and PLQYs of the TADF dyes,followed by successively refined device performances and efficiency roll-offs of the OLEDs employing these emitters.The device on the basis of meta-linking compound of m-PXZP obtained the best performance with a CEmax of 63.0 cd/A,a PEmax of 56.9 lm/W and a EQEmax of 18.9%,which are all higher than that of the devices based on the ortho-and para-linking compounds of o-PXZP and m-PXZP,and comparable to the green phosphorescent devices.In chapter 5,a series of green TADF dyes,i.e.o-PXZS02,m-PXZS02,23'PXZS02,24'PXZS02 and 34'PXZS02,were constructed through isomer engineering based on phenyl-sulfone/phenoxazine and the the prototype molecule of PXZ-DPS.The change of the linking position of the donors finely adjusted the thermal stabilities,FMO energy levels,electronic energy levels,PLQYs,AESTS and DF lifetimes of the TADF dyes,accompanied with refined solubility in chlorobenzene suitable for solution-processed device applications.The isomer of 34'PXZS02 exhibits the highest PLQY of 49%.The solution-processed device employing 34'PXZS02 as emitter acquired the best device performance with a CEmax of 13.1 cd/A and a EQEmax of 4.5%.In chapter 6,two blue TADF dyes,i.e.o-ACS02 and m-ACS02,were constructed through isomer engineering based on phenyl-sulfone/9,9-dimethyl-9,10-dihydroacridine and the prototype molecule of DMAC-DPS.The o-ACS02 and m-ACS02 demonstrate reserved fine TADF nature and relatively high PLQYs.Furthermore,the unique aggregation induced emission(AIE)property and refined neat film homogeneity of m-ACS02 render it suitable for solution-processed undoped OLEDs.The solution-processed undoped OLEDs based on m-ACS02 realized a EQEmax of 17.2%,which is among the highest device performances of solution-processed ones and comparable to the sky-blue undoped vapor-deposited ones.In addition,the inserting of an exciton blocking layer between the emissive layer and the electron-transporting layer can finely confine the exciton distribution,and thus switch the heterojunction emission with interfacial exciplex feature to homojunction emission only from the guest emitter.In chapter 7,two orange-red TADF dyes,i.e.27ACFO and 36ACFO,were constructed through isomer engineering based on fluorenone/9,9-dimethyl-9,1O-dihydroacridine.Both the two emitters exhibit excellent thermal properties with decomposition temperatures(Tds)of 402 and 400 ?,and glass transition temperatures(Tgs)of 153 and 131 ?,respectively.Photophysical measurements reveal that 27ACFO with a large ?EST of 0.38 eV exhibits subdued TADF nature while 36ACFO with a small ?EST of 0.13 eV shows distinct TADF nature.The 36ACFO demonstrated relatively higher PLQY of 31%in doped film.The OLEDs based on 36ACFO acquired the best device at a doping concentration of 1.5 wt%with a CEmax of 24.2 cd/A,a PEmax of 20.0 lm/W and a EQEmax of 10.8%,which is at the high level of orange-red TADF-OLEDs.In chapter 8,excited state intramolecular proton transfer(ESIPT),which can enhance the emissive characterisics of TADF dyes,is integrated into TADF molecules through employing ?-diketone with enolic form tautomer as electron-accepting unit,and two proof-of-concept ESIPT-TADF emitters of PXZPDO and ACPDO are constructed.Replacing the P-diketone unit with dimethyl substituted?-diketone,two contrast molecules of PXZDMePDO and ACDMePDO were also synthesized and explored.Photophysical measurements reveal that the PXZPDO and ACPDO exhibit ESIPT and TADF features simultaneously,while the PXZDMePDO and ACDMePDO show just TADF feature.The higher PLQYs,larger DF ratios and kRIscs of PXZPDO and ACPDO than those of PXZDMePDO and ACDMePDO,verify the enhancement effect of ESIPT procedure to TADF nature.The OLED based on PXZPDO acquired a EQEmax of 18.8%and the OLED on the basis of ACPDO acquired a EQEmax of 23.9%,which are much higher than the OLEDs employing the contrast molecules of PXZDMePDO and ACDMePDO as emitters.