Light-emitting poly(dendrimer)s

Abstract

Organic light-emitting diodes (OLEDs) have great potential for displays and lighting applications. For large area displays the ideal materials would be both phosphorescent and solution processible. These requirements mean that the materials need to be able to be patterned and the most advanced method for forming pixelated displays is inkjet printing. Light-emitting phosphorescent dendrimers have given high efficiency monochrome displays with the emitting layer deposited by spin-coating. However, the viscosity of the dendrimer solutions is insufficient for inkjet printing. We report the development of a new class of light-emitting materials, namely poly(dendrimers) in which a green emissive phosphorescent dendrimer is attached to a poly(styrene) backbone. Free radical polymerization of a dendrimer-styrene monomer gave a poly(dendrimer) with a weight average molecular weight of 24000 and a polydispersity of 3.6. A dilute solution of the dendrimer had a viscosity 15% higher than the neat solvent. Comparison of the photophysical studies of the poly(dendrimer) versus a model monomer dendrimer showed that the PL spectrum was broader and red-shifted, and the PL quantum yield around 50% lower. This was attributed to intermolecular interactions of the emissive dendrimers, which are held closely together on the polymer backbone.

Efficient Phosphorescence by Reducing Intrachain Chromophore Interactions in Dendrimer-Containing Polymers

“…In principle, either conjugated or nonconjugated polymer backbones could be used. However, the use of conjugated polymers such as poly(fluorene) is problematic for phosphorescent emitters as they generally have low triplet energies, enabling back-transfer from the phosphorescent emitter to the polymer, leading to a reduction in the photoluminescence quantum yield (PLQY). , As a result, we have taken the approach of using a nonconjugated polymer backbone to create phosphorescent poly(dendrimers) to provide solutions with increased viscosity for inkjet printing applications. ,, The use of nonconjugated polymer backbones with small molecule complex side chains has been reported, ,− although the solubility of the homopolymers has tended to be lower than that of their dendronized equivalents . The homopolymer poly(dendrimers) we have developed have had good PLQYs in spite of there being a dendrimer on every monomer unit, although not as high as the structurally similar individual dendrimers.…”

“…Thus, dendrimers can be designed so that intermolecular interactions that lead to the quenching of the luminescence can be avoided while still allowing charge transport to occur through a film. Although dendrimer solutions are suitable for spin-coating, unlike conjugated polymers, the viscosities tend to be low, , and below those necessary for inkjet printing. , …”