The Electroluminescent Materials Based On Spiro Aromatic Unit And Diphenyltriazine: Design, Synthesis And Device Performance

Organic light-emitting diodes(OLEDs)possess the advantages of self-emission,highbrightness,vivid-color,quick response speed and large-area fabrication by solution processing.Therefore,OLEDs are regarded as the latest generation of information display and solid-state lighting technologies.The emission layer of an OLED device is generally composed of a host material and an emitting material.Rigid molecular structure and stable optical and electrochemical properties are common requirements for host and emitting materials.Meanwhile,the high triplet energy level of host material is the key property to ensure efficient energy transfer from host molecule to guest molecule;and blue light emitting materials are indispensable for realizing full-color display and white light illumination.To meeting abovementioned requirements,the spiro[fluorene-9,9’-xanthene](SFX)with three-dimensional configuration is selected as the donor(D),and the diphenyltriazine unit with electron transport characteristics is utilized as the acceptor(A),to construct A-D-A conjugated blue-light emitting materials and D-A conjugated host materials.At the 2’,7’-positions or 3’,6’-positions of the xanthene moiety of SFX,two diphenyltriazine units were symmetrically introduced by benzene-ring bridging,and the A-D-A conjugated blue light emitting materials,p TRZSX and m TRZSX,were prepared.Through basic structural characterization,theoretical simulation,photophysical and electrochemical properties analysis,and performance testing of solution-processed OLED devices,the effects of substitution positions on the optoelectronic properties of the emitters were investigated.The fluorescence emission peaks of m TRZSX and p TRZSX are located at 379 and 385 nm,and their HOMO/LUMO energy levels are-6.01 e V/-2.61 e V and-6.13 e V/-2.80 e V,respectively.Because the diphenyltriazine units of p TRZSX molecule are at the para-positions of the oxygen atom in xanthene,it shows a relatively red-shifted emission and a slightly lower energy level,moreover,the two diphenyltriazine units are close to the fluorene end of SFX,which reduces the planar feature of the A-D-A conjugated skeleton and then greatly improves the solubility of the material.Undoped OLED devices with p TRZSX as the emitting layer were fabricated by solution processing.In the electroluminescence spectrum of the device,besides anticipative the blue-light peak of p TRZSX at 410 nm,an unexcepted emission peak appears around 600 nm,originating from the electroplex forming between the electron transporting layer(Tm Py PB)and the emitting layer.Thus,the device is white light emission.This provides a novel method for realizing single emitting-layer white OLEDs devices with simple architecture.For obtaining high triplet energy hosts,three D-A conjugated molecules were synthesized by mono-substituting the 2-position,2’-position and 3’-position of spiro[fluorene-9,9’-xanthene] with diphenyltriazine,namely SF-TRZ,SX2’-TRZ and SX3’-TRZ.Due to the shortened conjugate size,their energy gaps are widened,and the triplet-levels are lifted.Among them,the triplet energy levels of SX2’-TRZ and SX3’-TRZ are 2.82 e V and 2.86 e V,respectively,both of which are higher than the fluorene-endmodified SF-TRZ(2.48 e V),due to the blocking effect of oxygen atoms on the conjugation.In addition,single crystal structure analysis showed that the substitution on xanthene moiety further weakened the intermolecular aggregation.Therefore,SX2’-TRZ and SX3’-TRZ were used as the host materials with the delayed fluorescence emitter 4Cz IPN to fabricate OLED devices by vacuum evaporation.The thermally activated delayed fluorescence OLED devices based on SX2’-TRZ and SX3’-TRZ have maximum external quantum efficiencies of 18.7% and 23.0%,respectively.The excellent device performance of them is mainly attributed to their high triplet energy levels and electron-transport capability.Since the diphenyltriazine unti in SX3’-TRZ is located at the meta-position of the oxygen atom of xanthene moiety,the relatively weakened push-pull electron effect upraises the triplet energy level.Furthermore,the D-A conjugated skeleton beyond fluorene-end contains its planar feature,which is beneficial to improve carrier transport capability.Hence,SX3’-TRZ-based device has a better performance than SX2’-TRZ-based device.Combining the binary structures and abundant modification sites of spiro[fluorene-9,9’-xanthene] with the n-type characteristics of diphenyltriazine unit,make it a promising material system for electroluminance application.