A new series of electroluminescent materials is providing promising candidates for high-resolution, full-color, flat-panel organic light-emitting diode (OLED) displays, according to a team of DuPont scientists.

The group synthesized and characterized a series of iridium complexes with fluorinated aromatic ligands and showed that the emission colors of the materials can be fine-tuned by systematic control of the nature and position of the substituents in the ligands [Chem. Commun., 2001, 1494].

"These findings raise the exciting possibility of color tuning all the way from blue to red," the authors suggest, adding that the materials could provide a unique platform for full-color display devices of the future.

The team prepared the compounds in a one-step, solvent-free process by the reaction of iridium trichloride with fluorinated 2-arylpyridines in the presence of silver trifluoroacetate.

"The work shows that the selective fluorination of the ligand sphere of iridium 2-phenylpyridine complexes considerably improves the optoelectronic performance of the resulting materials," comments Guillermo C. Bazan, chemistry professor at University of California, Santa Barbara. "It is a substantial contribution to the design of phosphorescent molecules for use in light-emitting diodes."

The DuPont team also showed that introduction of fluorine and trifluoromethyl groups into the arylpyridine ligands substantially reduces the self-quenching of luminescence that is characteristic of the nonfluorinated parent compound. Fluorination also improves the processing properties of the materials.

"Fluorination results in enhanced volatility and thermal stability of the materials," says Vladimir V. Grushin, a synthetic chemist at DuPont. "Our compounds are remarkably robust and can be used 'as is' without a host matrix, which makes them attractive candidates for the vacuum-deposition OLED manufacturing process."

As an example of fine-tuning of the emission wavelengths, Grushin points out that one of the compounds, which has a fluorine substituent meta to the iridium atom, emits green light, whereas an isomer with the fluorine in a different position on the aromatic ring emits yellow-orange light.

The new materials are also highly efficient LED materials. The performance of an OLED device based on one of the most efficient of these materials compares well with that of a fluorescent lightbulb, the DuPont researchers observe.

"Our devices are not yet fully optimized," the team notes. "But even without device optimization, our new materials seem to be at least as good as the best emitters that have been reported elsewhere."

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