Synthesis And Photoelectric Property Investigation Of Thermally Activated Delayed Fluorescent Dendrimers

Dendrimers are a kind of monodisperse and highly branched macromolecules,which are composed of central nucleus,branching unit and peripheral functional groups.The introduction of specific chemical units at specific locations in dendritic structures makes it possible to perform particular functionality at the molecular level.Therefore,it has a wide application prospect in many fields such as catalytic membrane,sensor technology,fluorescence diagnosis,photodynamic therapy,especially organic light-emitting diode（OLEDs）.Recently,dendrimers have also become a kind of important materials for solution-processed OLEDs after traditional organic small molecules and polymers.In this paper,a series of multi-functional emissive dendrimers have been designed and synthesized by constructing functional dendritic structure,which can achieve 100%theoretical exciton utilization and thermally activated delayed fluorescence（TADF）.This work focuses on the intrinsic structural characteristics of dendrimers and some important issues related to their electroluminescent characteristics in OLED,including the generation of dendrimers,the transport of carriers,the electroluminescence properties and the energy transfer process.1.Four color-adjustable TADF dendrimers were designed and synthesized based on benzonitrile carbazole by adjusting the method of connecting branches to the core and the number of peripheral branches.Due to the energy gradient structure of the core-dendron system and efficient exciton utilization,fully solution-processed OLED devices based on these dendrimers exhibit low turn-on voltage and high device efficiency.It is worth mentioning that the maximum external quantum efficiency(EQE_max)based on the 5Cz BN-2Cz device exceeds 20%,which is comparable to the efficiency of vacuum-deposited TADF OLED.2.By changing the length of the flexible chain,dendrimers with different numbers of carbon chains（n=3,6,9）were designed and synthesized.These dendrimers with different lengths exhibited different fluorescence properties in aggregated states.The shorter the carbon chain,the stronger the encapsulation and the higher the fluorescence quantum yield.Through the preparation of single-carrier devices,the carrier transport properties of different dendrimers were studied,and it was found that the flexible chain could reduce the charge transport rate to some extent,thus optimizing the structure of dendrimers and improving the device performance.By tuning the length of flexible chains,the photophysical and electroluminescent properties in films can be systematically regulated.The dendrimer5Cz BN-3CSP connected by a propane flexbile chain demonstrated highest PLQY of 91.2%,and the fully solution-processed OLEDs based on 5Cz BN-3CSP show excellent device performance of the maximum external quantum efficiency of 21.7%,the maximum current efficiency of 64.0 cd A^-1.the maximum power efficiency of 50.3 lm W^-1,which is among the highest results of the reported solution-processed TADF OLEDs.3.By designing a series of molecules transitioning from small molecules to dendritic macromolecules,the functional advantages of this core-dendron structure and the mechanism of aggregation induced luminescence（AIE）were investigated.These compounds possess luminescent core and multifunctional branches,and exhibit aggregation induced delayed fluorescence（AIDF）properties by inhibiting singlet oxygen（¹O₂）generation and inhibiting triplet-triplet annihilation（TTA）.Under the protection of functional branches,the dendrimers exhibit strong luminescent properties,high photoluminescence quantum yield and exciton utilization.In addition,because the branches and the electron transport materials can form the exciplex,the fully solution processed OLED devices based on these dendrimers show a low turn-on voltage and excellent device performance of the current efficiency is 61.4 cd A^-1,the power efficiency is 42.8 lm W^-1.4.Blue guest dendrimer and host dendrimer were synthesized by changing the design of the core group.An optimization strategy of TADF-sensitized fluorescence（TSF）was proposed to improve the electroluminescence efficiency of solution-processed blue OLEDs.TADF sensitizers and fluorescent emitters with core-dendron structures mainly go through the Forster energy transfer process.While the Dexter electron transfer process,which mainly occurs between triplet excitons,is inhibited due to spatial separation.In addition,the core can be fully encapsulated by introducing carbazole dendron with high band gap,which also inhibits the aggregation caused quenching（ACQ）of fluorescence core.This solution-processed OLED achieves efficient blue emission,CIE coordinates are（0.15,0.14）,and EQE_maxis up to 10.16%.This TSF strategy promotes the development of solution-processed blue OLED to a greater extent.