"Click" Chemistry Synthesis Of Dendrimers As Host Materials For Electrophosphorescent Devices

Over the past two decades, organic light-emitting diodes (OLEDs) have been one of exciting fields of research due to their applications in flat panel display and solid state lighting. The phosphorescent organic light-emitting diodes (PHOLEDs) based on transition metal complexes are capable to utilize excitons in both singlet and triplet state and may achieve nearly 100% internal quantum efficiency. For the PHOLEDs, the host materials play key roles such as confinement of the triplet excitons on the emitter, charge transport, suppression of concentration quenching.Comparing with small molecular hosts, the polymeric hosts have attracted much attention because polymeric materials can be processed from solution by spin-coating or ink-jet printing. Solution processing is convenient method to reduce the cost of manufacture. As a distinct class of macromolecules, dendrimers are monodisperse and their structure is precisely known, highly-branched and non-planar. Thus, there is little issue of batch to batch reproducibility. However, traditional synthesis of dendrimers is time consuming. Developing modular synthetic strategies for rapid construction of dendrimers are of great interests for synthetic chemists. The use of orthogonal monomers for the preparation of dendrimers has gained increasing attention. Cu-catalyzed azide/alkyne cycloaddition (CuAAC) reaction has been widely used as click reaction in synthesis of functional materials. Hawker's group reported a orthogonal coupling strategy to synthesize dendrimers based on the combination of CuAAC reaction with esterification or etherification reactions. The growth of generation was accelerated and product was in high overall yields. It will be interesting to apply the orthogonal coupling strategy to the synthesis of dendritic materials for OLED applications.In this paper, a series of dendrimers with different functional groups had been synthesized for application as electrophosphorescent host materials. The synthesis was based on orthogonal coupling method by combining CuAAC click reaction with Williamson ether synthesis. The main contents were listed below:1 Synthesis and characterization of orthogonal monomers, surface groups, and coresThe orthogonal monomers for the accelerated preparation of dendrimers were synthesized from 5-hydroxyisophthalic acid. The surface groups, two different carbazoles and one oxadiazole, were synthesized. They served as hole transporting and electron transporting units, respectively. Two kinds of cores were synthesized for the two symmetrical dendrimers. 6-azidohexyl bromide with two different functional groups was synthesized for the bipolar dendrimer. All new chemical structures were identified by NMR and Mass spectra.2 Synthesis and characterization of carbazole dendrimer: 16HCZ-G3-dendrimerHerein we report the synthesis of third-generation carbazole dendrimer, 16HCZ-G3-dendrimer, based on convergent orthogonal coupling approach. All new chemical structures were identified by NMR and Mass spectra. The result indicated orthogonal coupling strategy avoided protection and activation of the focal points in the process of dendrimer synthesis. The growth of generation was accelerated and product was in high overall yields.3 Synthesis, characterization, and electroluminescence properties of carbazole dendrimer: 8CZ-G2-dendrimerA new solution processable dendrimer, with carbazole as hole transporting unit, was efficiently synthesized based on convergent orthogonal coupling approach. The dendrimer showed wide bandgap and has good thermal stability. The carbazole dendrimer was studied as electrophosphorescent host materials. Electrophosphorescent devices with the configuration of ITO/PEDOT:PSS/carbazole dendrimer:Ir(ppy)3/TPBI/LiF/Al were fabricated. The device showed turn-on voltage of 5.67 V, a maximum current efficiency of 15.0 cd/A, a power efficiency of 7.50 lm/W and an external quantum efficiency of 5.55 %.4 Synthesis, characterization, and electroluminescence properties of bipolar dendrimer: 4CZ-(CH2)6-4OXZA new solution processable bipolar dendrimer, with carbazole as a hole transporting unit and oxadiazole as an electron transporting unit, was efficiently synthesized based on convergent approach. This synthetic strategy allowed the preparation of a bipolar dendrimer in only six steps with an overall yield of 61.6% from the starting carbazole and oxadiazole. The dendrimer showed wide bandgap and good thermal stability. The bipolar dendrimer was studied as electrophosphorescent host materials in OLED devices. Electrophosphorescent devices with the configuration of ITO/PEDOT:PSS/bipolar dendrimer:Ir(ppy)3/TPBI/LiF/Al were fabricated. The device showed turn-on voltage of 10.7 V, a maximum current efficiency of 16.8 cd/A, power efficiency of 4.22 lm/W and external quantum efficiency of 5.70%.5 Synthesis, characterization, and electroluminescence properties of dendronized linear polymers: P1(G0-CZ) and P2(G2-CZ)Two new solution processable dendronized linear polymers, with carbazole as a hole transporting unit, were efficiently synthesized based on graft-to approach. The dendronized linear polymers showed wide bandgap and have good thermal stability. The carbazole dendronized linear polymers was studied as electrophosphorescent host materials. Electrophosphorescent devices with the configuration of ITO/PEDOT:PSS/carbazole dendronized polymers:Ir(ppy)3/TPBI/LiF/Al were fabricated. The best device showed turn-on voltage of 4.71 V, a maximum current efficiency of 21.4 cd/A, a power efficiency of 12.7 lm/W and an external quantum efficiency of 6.02 %.