Synthesis And Characterization Of Blue Dendritic Hosts And Phosphorescent Materials Featuring Thermally Activated Delayed Fluorescence Effect

Dendritic light-emitting materials have three-dimensional topological structure,excellent solution processing performance and rich adjustable luminescent properties,and is one of the representative material systems for developing solution-processed organic electroluminescent devices.Compared with the dendritic green and red materials,the development of blue light-emitting dendritic materials is relatively lagging behind and their luminous efficiency,color purity and stability are still not enough for commercial applications.In this paper,for the issue of high driving voltage and low power efficiency for the blue light-emitting dendritic materials,we developed the blue dendritic hosts and phosphorescent materials with thermally activated delayed fluorescence(TADF)effect utilizing the the advantages of small singlet-triplet energy gap(?Est)and bipolar transport capability of TADF materials and realized efficient blue organic light-emitting diodes(OLEDs)with low driving voltage and high power efficiency.1.Blue TADF host materials based on fused phenylimidazole acceptorWe designed and synthesized two host materials AcCz-IP and AcAc-IP by using fused phenylimidazole as the acceptor and acridine/carbazole hybrid and oligoacridine as the donor.It is found that both AcAc-IP and AcCz-IP exhibit typical TADF effects.AcAc-IP and AcCz-IP have relative low singlet energy levels while maintaining a high triplet energy level of approximately 2.6 eV,thus they exhibit higher HOMO levels(-5.15 eV)and lower LUMO levels(-2.40 eV)which is beneficial for carrier injection and transport.When AcCz-IP and AcAc-IP used for solution-processed blue phosphorescent device,AcCz-IP and AcAc-IP based device achieve low tum-on voltages of 2.7 V and 2.4 V,with maximum power efficiency of 25.6 1m/W,25.4 1m/W,respectively.2.Triphenylphosphine-oxide based dendritic blue TADF hostsWe designed and synthesized three dendritic host materials 3CzCz-PO,3AcCz-PO and 3AcAc-PO by using the non-conjugated acceptor triphenylphosphine oxide as the central core and introducing the oligocarbazole,acridine/carbazole hybrid and oligoacridine.It is found that 3CzCz-PO,3AcCz-PO and 3AcAc-PO have high triplet energy levels(2.87-2.90 eV),which are suitable used as the hosts for blue phosphorescent dopants.With increasing electron-donating ability from the peripheral dendron,the singlet-triplet gap(?Est)is found to be gradually reduced from 0.35 eV for 3CzCz-PO to 0.22 eV for 3AcCz-PO and 0.20 eV for 3AcAc-PO As a result,3AcAc-PO exhibits an obvious thermally activated delayed fluorescence(TADF)with a lifetime of 1.0 ?s.When using 3AcAc-PO as the host for the blue phosphor,the corresponding solution-processed device exhibits a turn-on voltage as low as 2.6 V with maximum power efficiency of 46.4 lm/W,which is superior to the traditional non-TADF host H2(3.0 V,29.6 lm W-1).3.Blue phosphorescent dendrimers based on imidazole ligandWe introduce TADF host into the periphery of phenylimidazole Ir complexes,three novel TADF dendrons based Ir dendrimers Ir-CzCz-PO,Ir-AcCz-PO and Ir-AcAc-PO have been designed and synthesized.The photophysical characterization indicate that the dendritic blue phosphorescent materials exhibit emission from the central Ir core in film state due to the complete energy transfer.And for Ir-CzCz-PO,Ir-AcCz-PO and Ir-AcAc-PO,the film state photoluminescence quantum efficiency(PLQY)are 0.47,0.73 and 0.78,respectively.When these Ir dendrimers are used to prepare solution-processed nondoped device,Ir-AcAc-PO achieves the best nondoped device performance,the corresponding solution-processed device exhibits a turn-on voltage as low as 2.9 V with maximum power efficiency of 31.0 lm/W,which is superior to the Ir-CzCz-PO which does not have the TADF dendrons,the turn-on voltage is reduced by 0.7 V and the power efficiency is improved by 93%.