Amorphous Poly-2,7-fluorene Networks Dirk Marsitzky, Jaqueline Murray, J. Campbell Scott, and Kenneth R. Carter* IBM Almaden Research Center, NSF Center for Polymeric Interfaces and Macromolecular Assemblies, 650 Harry Road, San Jose, California 95120-6099 Received March 27, 2001. Revised Manuscript Received August 20, 2001 We have developed a novel strategy for the synthesis of emitters, based on amorphous poly-2,7-fluorene (2,7-PF) networks which exhibit high glass transition temperatures and good photoluminescence properties and are suitable for incorporation into multilayer light- emitting diodes (LEDs). The materials design includes nickel(0)-mediated copolymerization of 2,7-dibromo-9,9-dihexylfluorene with a tetrafunctional spiro-compound, 2,2′,7,7′-tetra- bromo-9,9-spirobifluorene, and 4-bromostyrene as end-capping reagent. The introduction of the spiro-bifluorene leads to a 90° branch of the growing 2,7-PF chain, the number of which can be controlled via the feed ratio of monomers used. The termination of the growing network with 4-phenylethenyl end groups limits the molecular weight, hence keeps the material soluble and processable. The oligomer solution can be cast into thin films and thermally cross-linked leading to new amorphous 2,7-PF networks. The amorphous 2,7-PF networks were incorporated in bilayer and trilayer LED devices which emitted colorfast blue light. Introduction Since their discovery, 1 electroluminescent organic materials have attracted a good deal of industrial interest driven by the huge market potential of this novel technology. 2 While small molecule-based full-color electroluminescent devices are beginning to appear, 3 conjugated polymer-based displaysshighly desirable because of their ease of fabrication and the possibility of building flexible displayssstill suffer from the lack of a long-term stable blue emitting material. The shortcomings of these high band-gap polymersstypical poly-para-phenylene (PPP) derivativessarise from their intrinsic structural features: the rigid backbone of these materials leads, via π-π-interaction, to the formation of aggregates and excimers upon thermal treatment or passage of current, resulting in quenching of the highly efficient emission of the single polymer chains that is observed in solution. 4 Among the PPP derivatives, poly-2,7-fluorene (2,7-PF) containing two planarized benzene rings per monomer unit, is being considered by several research and industrial groups as the most promising blue-light- emitting material due to the chemical and thermal stability, high photoluminescence quantum efficiency, and ease of property tunability through structural modification of the 9-position of the fluorene ring. 5 The ideal high band-gap material for light-emitting diodes (LEDs), however, would be a highly amorphous polymer exhibiting good solubility and having almost no ten- dency to aggregate in the solid state. Two main concepts have been applied to control and stabilize the amor- phous state in solid thin films of conjugated polymers: the first approach involves lowering the crystallization tendency of the rodlike macromolecules by introducing bulky solubilizing groups in the side chain 6 or at the end of the polymer chains, 7 synthesizing copolymers, 8 or introducing saturated spacers in the polymer main chain. 9 A second approach to maintain the amorphous state at elevated temperatures is to introduce spiro- linkages into a molecular structure. 10 The previous studies have shown that solubility can be enhanced, crystallization tendency can be minimized and T g can be increased by synthesizing a spiro-bifluorene contain- ing a tetrahedral bonding atom at the center of two biphenylene units. This structure maintains a 90° angle between the connected conjugated units via a σ-bonded network and was shown to operate as an emissive layer in organic LED structures. 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(c) Lupo, D.; Salbeck, J.; Schenk, H.; Stahlin, T.; Stern, R.; Wolf, A. U.S. Patent 5,840,217, 1995. 4285 Chem. Mater. 2001, 13, 4285-4289 10.1021/cm010282h CCC: $20.00 © 2001 American Chemical Society Published on Web 10/13/2001