Design,Synthesis And Electroluminescence Of Efficient Phosphorescent Tetradentate Pt(Ⅱ) Complexes

Organic light emitting diodes（OLEDs）have been widely used in the new generation of lighting and displays due to their unique advantages such as auto-luminescence,fast response,ultra-thin,broad color gamut,wide viewing angle and flexibility.Of course,an important prerequisite for all these applications is to produce efficient and stable electroluminescent devices.Emissive materials are the most critical functional component among OLEDs materials and have therefore been the hot research topic all the time.At present,the common strategy to develop OLED materials is firstly to develop novel stable and efficient luminescent materials and then to explore the corresponding functional layer materials（host materials,charge injection and transport materials,electrode materials,etc.）.Although the fluorescent OLED demonstrated relatively long lifetime,the maximum external quantum efficiency（EQE）is around 5%.For thermally activated delayed fluorescence（TADF）materials-based OLEDs,although the exciton utilization efficiency can approach 100%,there are still many problems remaining such as low color purity and high efficiency roll-off.So these issues should be addressed before commercialization of TADF materials.The heavy metal chelating complexes（Ir,Pt,Os,etc）phosphorescence materials are generally considered to be the most promising emissive materials for commercial applications.It is worth noting that the rigid tetradentate Pt（Ⅱ）complexes have obvious advantages in suppressing the molecular vibration and rotation,thus increasing the electroluminescence efficiency.Therefore,this paper focus on the study of highly efficient tetradentate Pt（Ⅱ）complexes with simple but rational design strategy.1.ZPt1,ZPt2 and ZPt3 were designed and synthesized based on the tetradentate Pt（N^C^C^N）complexes.By introducing rigid pyrazolo[1,5-f]phenanthridine,these three complexes showed good thermal stability（above 440℃）and high quantum yields（ZPt1:0.70;ZPt2:0.24;ZPt3:0.34）.In addition,the introduction of different coordination groups through the linkage of oxygen can regulate the luminescence spectrum.The OLEDs based on the complexes ZPt1,ZPt2 and ZPt3 exhibit strong electroluminescence from blue to yellowish green.The OLEDs based on these three Pt（Ⅱ）complexes have achieved high device efficiency.The electroluminescent devices of ZPt1 showed the highest electroluminescence performance with the maximum current efficiency（CE）,power efficiency（PE）and EQE of 58.0 cd A^-1/51.6 lm W^-1/16.4%,respectively,and the driving voltage of the device is below 3.8 V.It is worth noting that all the materials exhibit good spectral stability at different current densities.2.The tetradentate Pt（Ⅱ）complexes 1,2,3 and 4 were designed and synthesized with a same N^C^C^N coordination set,where different N-heterocycle rings such as pyridine,quinoline,3-isoquinoline and 1-isoquinoline are used for complexes 1,2,3and 4,respectively.The effect of N-heterozygous groups on the photo-properties and electroluminescence of Pt（Ⅱ）complexes were systemically studied.The T_ds（decomposition temperatures）of complexes 1,2,3 and 4 are 429 ^oC,420 ^oC,463 ^oC and432 ^oC,respectively,which allow the device fabrication with vacuum deposition method.1,2,3 and 4 are orange-yellow,red,yellow and deep red emitters,respectively.The OLED based on complex 1 showed the highest electroluminescence performance with the maximum CE,PE,and EQE of 35.0 cd A^-1,27.9 lm W^-1,12.6%,respectively,and the maximum brightness around 6504 cd m^-2.Complex 4 is a red phosphorescent material with CIE of（0.72,0.28）,which can meet the requirements for high quality OLED display.