Design And Synthesis Of Novel Thermally Activated Delayed Fluorescence Emitters For Highly Efficient Non-doped Organic LightEmitting Diodes

This thesis centered interest on developing novel thermally activated delayed fluorescence(TADF)emitters for highly efficient non-doped organic light-emitting diodes(OLEDs).Some breakthroughs of fighting against the drawbacks of the non-doped OLEDs have been listed below:(1)Terpyridine(TPy)derivatives have been widely used in organic optoelectronics,supramolecular self-assembly and so on due to their suitable electron-withdrawing as well as abundant intra-and inter-molecular hydrogen bonds.A serious of novel TADF emitters.named as ToPy-Ph-PXZ.TmPy-Ph-PXZ,and TpPy-Ph-PXZ,were thus designed and synthesized.Owing their similar donor-?-acceptor(D-?-A)molecular fameworks,they exhibited similar unimolecular photophysical and electrochemical properties.Nevertheless,their molecular packing models exhibited significantly difference in single crystal states which were well consistent with their intra-and inter-molecular mutual hydrogen bonds.Unlike the others,intermolecular hydrogen bonds in TmPy-Ph-PXZ can not only help to strictly restrict the annoying ?-? stacking of the electron-rich phenoxazine segments but also aid to obtain high fluorescence quantum yield as rigid intermolecualr fameworks contribute to the inhibited non-radiation transitions.Together with TADF natural,TmPy-Ph-PXZ successfully realized the maximum external quantum efficiency(EQE)of 22.6%in non-doped OLEDs.All results confirmed that continuous and ordered intermolecular hydrogen bonds together with TADF natural was a novel design strategy of constructing highly efficient non-doped OLEDs.(2)Peripheral pyridine rings in TmPy-Ph-PXZ have been replaced with the pyrimidine segment and a novel novel TADF emitter 10-(4-(2,6-di(pyrimidin-5-yl)pyridin4-yl)phenyl)-10H-phenoxazine(DPmP-PXZ)was thus synthesized,adopting classical twisted D-?-A molecular architecture.Owing enhanced electron-withdrawing,DPmP-PXZ exhibited a typical bathochromic-shift.Benifite from the unique intermolecular hydrogen bonds of the single crystal state,effectively restricted exciton annihilations,balanced charge transfer,as well as high out-coupling efficiency,were realized simultaneously.Together with the thermally activated delayed fluorescence(TADF)natural,the non-doped device utilizing DPmP-PXZ as the emitter,exhibited a yellow emission with a peak at 560 nm and CIE of(0.44,0.54).Excellent maximum Current Effciecy(CE),Power Efficiency(PE)and EQE of 70.0 cd A-1?90.1 lm W-1 and 21.8%were realized without any out-coupling sheet.And an ultralow EQE roll-off(RO)factor of 7.8%was obtained at 1000 cd m-2.Moreover,an impressive EQE of 7.6%at an ultra-high luminance over 70,000 cd·m-2 was realized,which was among the best performance of yellow OLEDs.(3)With an aim to exploit novel highly efficient orange to red TADF emitters for non-doped OLEDs,two novel TADF emitters named as DPyPm-PXZ and PyPmPm-PXZ have been designed and synthesized.As anticipated,their charge-tansfer(CT)typed absorbtion and emission bands exhibited typical bathochromic-shift which were well consistent with the tendency of their electron-withdraw groups.Further detection of intermolecular mutual interactions in single crystal states,both emitters could form continuous and ordered intermolecular hydrogen bonds which greatly restrict the non-radiation transitions.As in return,the non-doped devices exhibited an orange to red emission with a peak at 576 nm and 596 nm and CIE of(0.50,0.49)and(0.56,0.44)for DPyPm-PXZ and PyPmPm-PXZ,respectively.Besides,ultra-high maximum EQEs of 18.8%and 11.3%for DPyPm-PXZ and PyPmPm-PXZ were obtained without any outcoupling sheet.To our knowledge,these are the highest among ever reported orange to red non-doped devices.Moreover,the device retained high efficiencies as luminance increases with negligible efficiency roll-off.All results suggested that applying continuous and ordered intermolecular hydrogen bonds to constract high efficient orange to red TADF emitters for non-doped OLEDs was a wise method.(4)A TADF emitter 10-(4-(4-(4,6-diphenyl-1,3,5-triazin-2-yl)phenoxy)phenyl)-9,9-dimethyl-9,10-dihydroacridine(DMAC-o-TRZ)was designed and synthesized by applying novel D-Spacer-A molecular model.Introduction of space-enough and conjugation-forbidden diphenyl ether linkage helped to strongly suppress the intramolecular CT transition.Thus it exhibited stable local excited state in single-molecular state and then intermolecular CT transition gradually acted as the vital radiation channel with the increasing concentration of DMAC-o-TRZ.Intermolecular CT transition not only endows DMAC-o-TRZ with TADF characteristics but also helps to inhibit the annoying concentration quenching effect.As in return,the concentrations of DMAC-o-TRZ greatly determine the device performance.For instance,the maximum EQEs of DMAC-o-TRZ 5 wt%,42 wt%and 100 wt%doped devce were 3.0%,15.5%,and 14.7%,respectively.Moreover,excellent maximum CE,PE,and EQE of 30.0 cd A-1,21,2 1m W-1 and 11.3%were realized at 1000 cd m-2 for the non-doped device.All results prove the feasibility of D-Spacer-A molecules to develop intermolecular CT transition TADF emitters for efficient non-doped OLEDs.