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Solution-Processible Conjugated Electrophosphorescent Polymers

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journal contribution
posted on 09.06.2004, 00:00 by Albertus J. Sandee, Charlotte K. Williams, Nicholas R. Evans, John E. Davies, Clare E. Boothby, Anna Köhler, Richard H. Friend, Andrew B. Holmes
We report the synthesis and photophysical study of a series of solution-processible phosphorescent iridium complexes. These comprise bis-cyclometalated iridium units [Ir(ppy)2(acac)] or [Ir(btp)2(acac)] where ppy is 2-phenylpyridinato, btp is 2-(2‘-benzo[b]thienyl)pyridinato, and acac is acetylacetonate. The iridium units are covalently attached to and in conjugation with oligo(9,9-dioctylfluorenyl-2,7-diyl) [(FO)n] to form complexes [Ir(ppy-(FO)n)2(acac)] or [Ir(btp-(FO)n)2(acac)], where the number of fluorene units, n, is 1, 2, 3, ∼10, ∼20, ∼30, or ∼40. All the complexes exhibit emission from a mixed triplet state in both photoluminescence and electroluminescence, with efficient quenching of the fluorene singlet emission. Short-chain complexes, 1113, [Ir(ppy-(FO)n-FH)2(acac)] where n = 0, 1, or 2, show green light emission, red-shifted through the FO attachment by about 70 meV, but for longer chains there is quenching because of the lower energy triplet state associated with polyfluorene. In contrast, polymer complexes 1821 [Ir(btp-(FO)n)2(acac)] where n is 5−40 have better triplet energy level matching and can be used to provide efficient red phosphorescent polymer light-emitting diodes, with a red shift due to the fluorene attachment of about 50 meV. We contrast this small (50−70 meV) and short-range modification of the triplet energies through extended conjugation, with the much more substantial evolution of the π−π* singlet transitions, which saturate at about n = 10. These covalently bound materials show improvements in efficiency over simple blends and will form the basis of future investigations into energy-transfer processes occurring in light-emitting diodes.

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