Roman Trattnig

Designed Suppression of Aggregation in Polypyrene: Toward High-Performance Blue-Light-Emitting Diodes

2010 WILEY-VCH Verlag Gm Pyrene is one of the most important and thoroughly investigated organic chromophores. Among the attractive features of pyrene is its exceptionally long fluorescence lifetime, the sensitivity of its excitation spectra to microenvironment changes, and its high propensity for forming excimers. This excimer formation has been utilized over the last 50 years in the investigation of water-soluble polymers, making pyrene, by far, the most frequently applied dye in fluorescence-labeled polymers. Despite its chemical stability and high charge carrier mobility, its strong tendency to form excimers leads to a red-shift of its emission as well as a decrease in its fluorescence efficiency in condensed media, which has prohibited its use as an emissive material in organic light-emitting devices (OLED)s. Since the report of the first double-layer thin-film OLED by the Kodak Company in 1987, OLEDs have attracted enormous attention in the scientific community thanks to their high technological potential for the next generation of fullcolor-flat-panel displays and lighting applications. Whether polymers or small molecules, to date only redand greenemitters have shown sufficient efficiencies and lifetimes to be of commercial value. Here, we present a novel non-aggregating polypyrene, which can be synthesized via a simple three-step chemical route from pyrene in good yields. Our poly-7tert-butyl-1,3-pyrenylene (3) shows a high solid-state fluorescence quantum yield with blue emission, excellent solubility and stability, no aggregation in thin films, and good electro-optical performance in single-layer OLEDs. The use of pyrene, a large conjugated aromatic system, as emitting material in OLED applications has been limited, due to aggregation between planar pyrene molecules. The formation of p-aggregates/excimers causes the quenching of fluorescence, resulting in low solid-state fluorescence quantum yields. In recent years, there has been an increasing interest in the use of pyrene units in OLEDs, including oligothiophenes with pyrenyl side groups, bipyrenylbenzene molecules, as well as pyrene-carbazole and pyrene-fluorene systems, due to their emissive properties combined with high charge carrier mobility. However, the pyrene derivatives that have been reported so far as efficient blue-emitters for OLED applications still present some degree of aggregation in the solid state. A successful effort in the prevention of aggregation in small molecules was achieved with tetrasubstituted, highly sterically hindered pyrenes, which can emit blue light in solution as well in the solid state and with high quantum yield. The well-known 1,3,6,8-tetraphenylpyrene, for example, has been used in OLEDs, organic field-effect transistors (OFET)s, as well in organic light-emitting field-effect transistors (OLEFET)s. Additional tetrasubstituted systems including different phenyl derivatives or pyridyl units at the 1-, 3-, 6-, and 8-positions have been reported as well. Recently, we have reported the suppression of aggregation in a strongly twisted multichromophoric dendrimer made up exclusively from pyrene units substituted at the 1-, 3-, 6-, and 8-positions, which revealed a very high fluorescence quantum yield relative to unsubstituted pyrene. Furthermore, 1,10-bipyrenyl and linear 1,6-disubstituted oligopyrenes have also been investigated. In comparison to small molecules, conjugated organic polymers have the advantage of being applicable in larger display sizes and lighting devices at much lower manufacturing costs via solution-based deposition techniques. Only a small number of investigations concerning the attachment of pyrene to the polymeric chain or the use of pyrene along the polymeric backbone were reported as new materials for molecular electronics. Polypyrenes formed via electrochemical polymerization of pyrenes by a 1 10 coupling were described to give insoluble and unprocessable film or alternatively soluble materials with few repeat units. Thereby, the low degree of polymerization is presumed to be a consequence of the low solubility caused by the strong self-assembly of pyrene segments. In contrast, we present a simple chemical route to a highly soluble and processable polypyrene, 3, with a defect-free structure and the highest degree of polymerization reported up to now for polymers made up exclusively from pyrene units. Compound 3 was synthesized in a simple three-step route as depicted in Scheme 1. Pyrene was first mono-tert-butylated to afford 2-tert-butylpyrene (1), which was then treated with bromine (2 equivalents) in CH2Cl2 at 78 8C to provide the 1,3-dibromo-7-tert-butylpyrene (2) in 89% yield. The use of tert-butyl groups was strategic in order to selectively obtain the 1,3-dibromo pyrene derivative. The polymerization ofmonomer 2 was carried out in a Yamamoto coupling with a Ni(0) catalyst, followed by end-capping with bromobenzene. After precipitation in a mixture of HCl and methanol (1:1) and subsequent removal

Core, shell, and surface-optimized dendrimers for blue light-emitting diodes.

We present a novel core-shell-surface multifunctional structure for dendrimers using a blue fluorescent pyrene core with triphenylene dendrons and triphenylamine surface groups. We find efficient excitation energy transfer from the triphenylene shell to the pyrene core, substantially enhancing the quantum yield in solution and the solid state (4-fold) compared to dendrimers without a core emitter, while TPA groups facilitate the hole capturing and injection ability in the device applications. With a luminance of up to 1400 cd/m(2), a saturated blue emission CIE(xy) = (0.15, 0.17) and high operational stability, these dendrimers belong to the best reported fluorescence-based blue-emitting organic molecules.

Molecular Triangles: Synthesis, Self-Assembly, and Blue Emission of Cyclo-7,10-tris-triphenylenyl Macrocycles

A set of cyclo-7,10-tris-triphenylenyl macrocycles have been prepared by a Yamamoto cyclotrimerization protocol. In these novel macrocycles, three triphenylene units are covalently linked to each other, resulting in the formation of triangular-shaped molecules. The fully planar derivative revealed pronounced self-assembly behavior. NMR spectroscopy was used to determine the association constant in solution. 2D wide-angle X-ray scattering was applied to the study of the liquid crystallinity of this new discotic mesogen in the bulk state. Furthermore, nonplanar, laterally substituted derivatives were successfully tested as blue emitters in organic light-emitting diodes owing to their unique optoelectronic properties and their high stability. In this case, substitution with sterically demanding phenyl groups was efficiently used to suppress intermolecular packing, thus preventing undesired quenching effects.

Deep blue polymer light emitting diodes based on easy to synthesize, non-aggregating polypyrene

Thorough analyses of the photo- and devicephysics of poly-7-tert-butyl-1,3-pyrenylene (PPyr) by the means of absorption and photoluminescence emission, time resolved photoluminescence and photoinduced absorption spectroscopy as well as organic light emitting devices (OLEDs) are presented in this contribution. Thereby we find that this novel class of polymers shows deep blue light emission as required for OLEDs and does not exhibit excimer or aggregate emission when processed from solvents with low polarity. Moreover the decay dynamics of the compound is found to be comparable to that of well blue emitting conjugated polymers such as polyfluorene. OLEDs built in an improved device assembly show stable bright blue emission for the PPyr homopolymer and further a considerable efficiency enhancement can be demonstrated using a triphenylamine(TPA)/pyrene copolymer.

All solution processed blue multi-layer light emitting diodes realized by thermal layer stabilization and orthogonal solvent processing

Herein we report on the fabrication and the properties of two highly efficient blue light emitting multilayer polymer light emitting diodes (PLEDs). The first device structure combines a thermally stabilized polymer with a material processed from an orthogonal solvent, allowing for the fabrication of a triple layer structure from solution. The well known poly(9,9-dioctyl-fluorene-co-N-(4-butylphenyl)-diphenylamine) (TFB), which can be stabilized in a bake-out procedure, was used as a hole transporting layer. A novel pyrene – triphenylamine (PPyrTPA) copolymer was used as emissive layer. The stack was finalized by a poly(fluorene) - derivative with polar side-chains, therefore being soluble in a polar solvent which allows for the deposition onto PPyrTPA without redissolving. The resulting PLED showed bright-blue electroluminescence (CIE1931 coordinates x=0.163; y=0.216) with a high efficiency of 1.42 cd/A and a peak luminescence of 16500 cd/m². The second presented device configuration comprises a thermally stabilized indenofluorene – triphenylamine copolymer acting as hole transporter, and an emissive copolymer with building blocks specifically designed for blue light emission, effective charge carrier injection and transport as well as for exciton generation. This multilayer PLED led to deep-blue emission (CIE1931 x=0.144; y=0.129) with a remarkably high device efficiency of 9.7 cd/A. Additionally, atomic force microscopy was carried out to investigate the film morphology of the components of the stack and x-ray photoemission spectroscopy was performed to ensure a full coverage of the materials on top of each other. Ultraviolet photoemission spectroscopy confirmed the desired type-II band level offsets on the individual interfaces.