Synthesis of Asymmetrically Arranged Dendrimers with a Carbazole Dendron and a Phenylazomethine Dendron

Asymmetrically arranged dendrimers with a carbazole dendron and a phenylazomethine dendron were synthesized by the combination of Ullmann reaction and a dehydration reaction in the presence of titanium tetrachloride. Stepwise complexation in the phenylazomethine dendron unit within these dendrimers and SnCl2 suggests a gradient in the electron density associated with the imine groups. The complexation of the dendrimer changes the HOMO/LUMO energy gap of the dendrimer. We show the dendrimers with higher generations have the larger HOMO values. The most electron-rich molecule, Cz3-DPA3, has the highest HOMO value of 5.35 eV and, accordingly, is expected to have the lowest barrier for the hole injection from the ITO electrode (4.6 eV) in OLEDs. However, for the HOMO energy levels of the carbazole dendrimer complex with SnCl2, the energy levels of the carbazoles did not change based on almost the same redox potentials as those of the dendrimers themselves. Using Cz3-DPA3 as a hole-transport layer (HTL), only complexation with metal ions results in the enhanced maximum luminescence from 4041 to 10 640 cd/m2 by only complexing with SnCl2 under the nonoptimized conditions. A complexation leads to a high EL efficiency because of the p-type-doped structure of the dendrimers as a hole-transport layer.