Daniel A. Poulsen

Site isolation in phosphorescent bichromophoric block copolymers designed for white electroluminescence.

White organic light-emitting diodes (WOLEDs) have attracted great attention for their potential use in full color displays and solid-state lighting applications due to several advantages, such as low cost and flexibility. To date, the most efficient WOLEDs have used small phosphorescent molecules in multilayer structured devices prepared by high vacuum vapor deposition. The key issue in these systems is that the phosphorescent emission produced by each individual metal complex Ir(III) or Pt(II), is narrow, thus requiring simultaneous emission from more than one color phosphor to illuminate across the visible region. Typically this is achieved through a combination of either three different chromophores emitting blue, green, and red, or of two different ones emitting green/blue and orange/red. If more than one phosphorescent emitter is present in a device, the electroluminescent color may be affected by the energy transfer (both Förster and Dexter) between emitters. Vapor deposition enables isolation of the various emitters to minimize the energy transfer and achieve the desired goal of multiple emission using techniques such as patterning, stacking, layered isolation, and exciton management. Because polymeric materials can be solution-processed, they constitute an interesting option for application in OLEDs due to their potential to reduce cost and increase scalability. Another advantage is that a single polymer chain can bear multiple functional groups, each contributing to the tuning of properties. For example, successful demonstrations of polymer WOLEDs have been based on blends of fluorescent polymers, polymers incorporating multiple fluorescent emitters in their side chains or their backbone and fluorescent polymers doped with small molecule phosphorescent emitters. However, these devices are generally fluorescent systems with limited internal quantum efficiencies or doped phosphorescent systems with poor stability. Furthermore, the occurrence of energy transfer limits the amount of low energy dopant that can be incorporated into these polymers, which affects their intrinsic efficiency. There have been some efforts to suppress this energy transfer using dendrimers for site isolation, but ultimately multilayer structures that can isolate phosphorescent emitters are needed. Unfortunately, this is extremely difficult to achieve with solution processing as the deposition of a layer must not affect any previously deposited layers. Block copolymers allow hierarchical supramolecular control over the spatial location of their functional component blocks as well as various nanoscale objects. This design flexibility has been exploited in the efficient fabrication of novel functional materials, such as nanostructured solar cells, photonic bandgap materials, highly efficient catalysts, and high-density magneticstorage media. Therefore, block copolymers have the unique potential to spontaneously achieve phosphorescent emitter isolation through self-assembly. Herein, we have explored their use as active materials for WOLEDs in which phosphorescent emitter isolation can be achieved. We have exploited the use of triarylamine (TPA) oxadiazole (OXA) diblock copolymers (TPA-b-OXA), which have been used as host materials due to their high triplet energy and charge-transport properties enabling a balance of holes and electrons. These coil–coil type TPA-b-OXA diblocks can produce various morphologies with controlled domain spacings ranging from 10–50 nm. By incorporating two different colored phosphorescent Ir(III) emitters (green–blue and orange–red emissive pendant styryl heteroleptic Ir(III) complexes) randomly into each different block, we have been able to produce a block-copolymer system, (TPA-r-Blue)-b-(OXA-r-Red), which can deliver site isolation of the two emitters. As a result of site isolation these diblock copolymers can be targeted to suppress energy transfer from high to lower energy emitters, which generally occurs at distances below 10 nm. With these block copolymers, we demonstrate a seld-assembled single layer solution processed WOLED that provides improved white color balance, and efficiency. Furthermore, by varying the molecular weight (MW) of (TPA-r-Blue)-b-(OXA-r-Red) and the ratio of blue to red emitters, we have investigated the effect of domain spacing on the electroluminescence spectrum and device performance. Polymers containing heavy metal complexes have been demonstrated previously for similar Ir(III) complexes through incorporation of ancillary ligand then post polymerization complex formation, or through the post polymerization attachment of preformed Ir(III) complexes. Unfortunately, these strategies are unsuitable since they do not allow incorporation of

Measuring reversible photomechanical switching rates for a molecule at a surface

We have used single-molecule-resolved scanning tunneling microscopy to measure the photomechanical switching rates of azobenzene-derived molecules at a gold surface during exposure to UV and visible light. This enables the direct determination of both the forward and reverse photoswitching cross sections for surface-mounted molecules at different wavelengths. In a dramatic departure from molecular behavior in solution-based environments, visible light does not efficiently reverse the reaction for azobenzene-derived molecules at a gold surface.

Self-patterned molecular photoswitching in nanoscale surface assemblies

Photomechanical switching (photoisomerization) of molecules at a surface is found to strongly depend on molecule-molecule interactions and molecule-surface orientation. Scanning tunneling microscopy was used to image photoswitching behavior in the single-molecule limit of tetra-tert-butyl-azobenzene molecules adsorbed onto Au(111) at 30 K. Photoswitching behavior varied strongly with surface molecular island structure, and self-patterned stripes of switching and nonswitching regions were observed having approximately 10 nm pitch. These findings can be summarized into photoswitching selection rules that highlight the important role played by a molecules nanoscale environment in determining its switching properties.

Modular small-molecule directed nanoparticle assembly

We demonstrate a simple and versatile approach to produce hierarchical assemblies of nanoparticles by combining block copolymers and small molecules. Directed nanoparticle assemblies can be achieved with a variety of nanoparticles and small molecules without modification of the nanoparticle ligands. This novel approach opens up new routes toward the fabrication of nanoparticle-based functional devices using a “Bottom-Up” approach.