| Thermally activated delayed fluorescence（TADF）emitters,as the core material for the third-generation of organic light emitting diodes（OLEDs）,can realize 100%internal quantum efficiency（IQE）theoretically without using any d8 heavy metals.Up till now,although the maximum external quantum efficiencies（EQEs）of TADF-OLEDs have exceeded 25%in the whole visible region,even 30%in both blue and green color gamut,there are still many problems to be solved.For instance,the development of red,especially deep-red,TADF emitters have lagged well behind those of their blue and green counterparts.Also,researchers have gradually realized that the regulation of supramolecular structures on the optoelectronic properties of solid emitters can not be ignored.The in-depth study on the relationship between supramolecular structures and optoelectric properties will definitely usher the further development of TADF materials.As for red TADF materials,pyrazine is a well-deserved hotspot,since almost all the highly efficient acceptor core reported so far are based on the pyrazine derivatives.Therefore,in this thesis,we employed pyrazine ring as core to synthesize a series of thermally activated delayed fluorescence compounds through reasonable molecular design,some of which had special luminescent properties,such as polymorphism,high contrast mechanochromic luminescene,etc.The main research contents are as follows:1.In Chapter Ⅱ,an unsymmetric donor-acceptor（D-A）type molecule TPA-DQP was designed and synthesized based on diquinoxalino[2,3-a:2’,3’-c]phenazine（DQP）acceptor and triphenylamine（TPA）.TPA-DQP exhibited not only remarkble TADF characteristics but high contrast mechanochromic luminescent（MCL）behavior(Δλmax=130 nm).Through thorough analysis,we deduced that such large-photoluminescence-shift MCL phenomena resulted from reversible phase transitions between two polymorphs and an amorphous state.The single crystal XRD analyses revealed that the conversion of molecular packing modes and intermolecular interactions are responsible for the reversible phase transition process and thereby tune their TADF properties.The best performing OLED device utilizing TPA-DQP demonstrated maximum EQE of 18.3%with an emission peak at 676 nm,which is in line with the best device efficiency for deep-red TADF emitters.2.In Chapter Ⅲ,the effects of different donor/acceptor on TADF properties were well investigated.Four asymmetric D-A TADF molecules were synthesized utilizing different units such as carbazole（CZ）,tert-butylcarbazole（t CZ）,diphenylamine（DPA）and 9,9-dimethylacridine（DMAC）as donors and the DQP as acceptor.By using methyl-modified DQP（MDQP）as the acceptor and DPA as the donor,another D-A TADF molecule was obtained.Based on these five emitters,a series of doped OLEDs with a traditional electron-transporting type host TPBi were fabricated.These devices realized yellow,orange and red electroluminance separately at a doping concentration of 10 wt%.There is no significant correlation between the efficiency roll-off of the device and the delayed lifetime of these molecules,besides their unconspicuous△EST difference.Therefore,we deduced that the unipolar host material of the of these devices led to the imbalance of carrier transport,which further hampered the performance of the devices and led to faster efficiency roll-offs.3.In Chapter Ⅳ,two highly symmetrical D-A-D type TADF emitters were synthesized by coupling DQP acceptor and two donor units（TPA/DPA）.Both of the two resulting molecules（DDPA-DQP and DDPA-DQP）could realize deep-red emission.DFT calculations revealed that DDPA-DQP have more twisted structures than DTPA-DQP.Therefore,the calculated oscillator strength for DDPA-DQP was lower than that of DTPA-DQP,which was verified by the following photophysical property measurements.We found that the fluorescence quantum yield of DDPA-DQP doped films was lower than that of DTPA-DQP,and the internal conversion process of S1 excitons in DDPA-DQP doped films was more competitive with their radiative transition process.In this sense,the energy loss caused by non-radiative transition in DDPA-DQP was much more severe than that in DTPA-DQP.Doped OLEDs adopting these two compounds as luminescent layers could both achieve deep red emission.The maximum EQE of DDPA-DQP device was11.3%,with the corresponding CIE coordinates of（0.64,0.35）.While the emission peak of DTPA-DQP-based device located at 643 nm,with CIE coordinates of（0.65,0.35）,and the EQE maxima reaching to 17.3%,which is among the best device performance with similar CIE coordinates.4.In Chapter Ⅴ,a new red TADF emitter,named ACPP-TPA,was developed by coupling a rigid acenaphtho[1,2-b]pyrido[3,4-e]pyrazine（ACPP）core and two typical TPA units.Three polymorphs of this emitter were cultivated under different conditions and named as Y,O,and R after their luminance.Among them,O and R belonged to thermally and kinetically stable states separately,while Y was the metastable state both thermally and kinetically.Therefore,the three polymorphs could converted reversibly to each other under certain stimuli.These three polymorphs also showed significantly different photoluminance quantum yields as well as TADF properties.The single crystal structures showed that molecular aggregation in Y,O,R increases step in step due to the distinct conformation,hence bathochromic shifts in emission peaks.Furthermore,consecutiveπ-πinteractions existed only in cystal Y.And the interruptedπ-πinteractions in crystal O and R made for their higher photoluminescence quantum yields and longer delayed lifetimes than Y.Finally,ACPP-TPA-based OLEDs with different doping concentrations were sucessfully bulit.EQEs of the doped devices were not as outstading as their highly efficient counterparts,but the efficiency roll-off was improved greatly due to their short delayed lifetimes.In addition,this emitter showed good potential in nondoped devices.The maximum EQE of ACPP-TPA-based non-doped OLED reached to 6.9%with corresponding CIE coorinates of（0.64,0.36）. |
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