| Organic light-emitting diodes(OLEDs)are widely used in flat-pannel displays and solid-state lighting sources.As the third-generation OLED emitters,thermally activated delayed fluorescent(TADF)materials have advantages of low-cost and high-efficiency,becoming one of the hotspots of OLED field.Nowadays,the blue TADF materials with good performance and high color purity are still not as popular as green or red anagolues.In addition,carbazole is a good donor with weak electron-donating ability,which should be suitable to build deep blue TADF molecules.However,many carbazole based compounds do not have TADF feature since the five-membered ring in carbazole is not favorable for spatial separation of frontier molecular orbitals.In principle,the light-emitting properties and performance of the donor-acceptor(D-A)type TADF emitters are determined not only by D and A,but also by the linking style between them.This dissertation aimed to develp novel D-A or D-?-A type TADF materials,with focus on incorporating functional groups on the phenylene bridge to tune the steric conformation,luminescent mechanism,and performance.Novel blue TADF emitters were successfully obtained with carbazole as donor.Similar molecular designing strategies were then used to develop more TADF emitters with emitting color covering almost the whole visible light range.The main works are as follows:(1)A series of D-A-D type blue fluorescent materials,namely CzCF3Ph,BuCzCF3Ph,BiCzCF3Ph,BiCzCNPh were designed and synthesized with trifluoromethylbenzene or cyanobenzene as acceptors and carbazole or substituted carbazole as donors.CzCF3Ph,BuCzCF3Ph,BiCzCF3Ph were normal fluorescence compounds,which exhibited deep blue emission at 442,436,436 nm and CIEs(0.16,0.09),(0.16,0.07),(0.17,0.07)with external quantum efficiencies(?ext)of 1.8-2.4%in OLEDs,respectively.While BiCzCNPh containing cyanobenzene as acceptor showed TADF behavior,which exhibited a maximum ?ext of 7.3%in TADF-OLED.(2)A group of D-?-A type novel emitters were designed and synthesized by using triazine(Trz)as acceptor,carbazole(Cz)or tert-butylcarbazole(BuCz)as donor,and phenylene as ?-bridge.The bulky methyl(Me)or trifluoromethyl(CF3)was incorporated on the bridge to tune the molecular conformation and properties.It was observed that only when electron-whthdrawing CF3(rather than Me)was introduced onto the phenylene bridge and CF3 was close to donor(rather than close to the acceptor),did the obtained molecules have TADF feature.TrzCz-CF3 and TrzBuCz-CF3 exhibited ?exts of 9.4%(15.9 cd A-1)and 14.2%(26.7 cd A-1)in their TADF-OLEDs,respectively.It was concluded that both sufficient acceptor strength and large conformation distorsion,which can be simuteneously achieved by incorporating suitable functional group on bridge,are required to realize TADF feature when carbazole was used as donor.(3)Blue TADF materials,TrzBuCz-Me and TrzBuCz-CN,were synthesized with Trz acceptor and BuCz donor,and Me or cyano(CN)as functional group on the phenylene bridge.In comparison with Me,the incorporation of CN on the ?-bridge not only raised the proportion of TADF by reducing the S1 energy and thus the singlet-triplet energy difference,but also increased the PLQY to as high as 94%,which is 3 times of the CN-free analogue TrzCz-CF3.The doped TADF-OLED of TrzBuCz-CN exhibited high efficiencies of 13.7%,31.0 cd A-1 and 26.3 lm W-1,respectively.(4)A series of D-?-A fluorescent and TADF materials were designed and synthesized with Trz as acceptor,carbazole,9;9-dimethylacridine(DMAC)and phenoxazine(PXZ)as electron donors.Me and CN were used to modify the phenylene bridge.By sequentially varying the donor and introducing the functional group on ?-bridge,not only the luminescence mechanism of these materials was tuned from the normal fluorescence to TADF,but also the color was continuously adjusted from pure blue to orange red.The blue,sky-blue and green TADF-OLEDs exhibited high ?exts of 14.4%(19.7 cd A-1),19.0%(41.1 cd A-1),and 16.5%(51.2 cd A-1),respectively. |
PDF Full Text Request