n-Type Conjugated Dendrimers: Convergent Synthesis, Photophysics, Electroluminescence, and Use as Electron-Transport Materials for Light-Emitting Diodes

Three new electron-acceptor and light-emitting conjugated dendrimers, [G1-4Q], [G1-6Q], and [G2-12Q], based on a benzene core, poly(phenylenevinylene) dendrons, and diphenylquinoline peripheral groups have been synthesized, characterized, and used as emissive and electron-transport materials in efficient lightemitting diodes. All three dendrimers emit blue light ((lambda)max = 414 nm) in solution with high fluorescence quantum yields in the 0.69-0.87 range and fluorescence lifetimes of 1.4-1.6 ns. In thin films the dendrimers emit yellow light with poor fluorescence efficiency, suggesting aggregation and excimer formation in the solid state. The dendrimers showed quasi-reversible electrochemical reduction with a formal potential of -2.0 V (vs SCE) and derived LUMO levels of 2.5-2.6 eV, implying that injected electrons are localized in the periphery of the dendrimers. As the emissive materials in light-emitting diodes, the dendrimers showed yellow electroluminescence the brightness and efficiency of which increased with generation and number of electron-acceptor peripheral groups. The performance of bilayer light-emitting diodes using the dendrimers as the electron-transport layers and poly(2-methoxy-5-(2ʹ-ethyl-hexyloxy)-1,4-phenylene vinylene) as the emissive layer increased with generation and number of electron-acceptor peripheral groups, reaching a maximum external efficiency of 5.0%, a power efficiency of 1.3 lm/W, and a brightness of up to 2000 cd/m^2 in ambient air using aluminum cathode. These results demonstrate that electron-acceptor and electron-transport dendrimers can be created by functionalization of the dendrimer periphery. The new dendrimers are promising n-type semiconductors for organic electronic and optoelectronic devices.