Tuning the photophysical properties of cationic iridium(iii) complexes containing cyclometallated 1-(2,4-difluorophenyl)-1H-pyrazole through functionalized 2,2′-bipyridine ligands: Blue but not blue enough

Research output: Contribution to journalArticlepeer-review

Authors

  • Henk J. Bolink
  • Edwin C. Constable
  • Manuel Delgado
  • Daniel Häussinger
  • Catherine E. Housecroft
  • Mohammad K. Nazeeruddin
  • Markus Neuburger
  • Enrique Ortí
  • Gabriel E. Schneider
  • Daniel Tordera
  • Roché M. Walliser
  • Jennifer A. Zampese

Colleges, School and Institutes

External organisations

  • Instituto de Ciencia Molecular, Universidad de Valencia
  • Universidad de Valencia
  • Fundació General de la Universitat de Valencia (FGUV)
  • University of Basel
  • EPFL SB ISIC LPI
  • CH B2 425
  • Bâtiment CH Station 6

Abstract

Four new heteroleptic iridium(iii) complexes in the family [Ir(dfppz) 2(N^N)]+, where Hdfppz = 1-(2,4-difluorophenyl)-1H- pyrazole and N^N = 6-phenyl-2,2′-bipyridine (1), 4,4′-(di-tert- butyl)-6-phenyl-2,2′-bipyridine (2), 4,4′-(di-tert-butyl)-6, 6′-diphenyl-2,2′-bipyridine (3) and 4,4′-bis(dimethylamino)-2, 2′-bipyridine (4), have been synthesized as the hexafluoridophosphate salts and fully characterized. Single crystal structures of ligand 3 and the precursor [Ir2(dfppz)4(μ-Cl)2] have been determined, along with the structures of the complexes 4{[Ir(dfppz) 2(1)][PF6]}·3CH2Cl2, [Ir(dfppz)2(3)][PF6]·CH2Cl2 and [Ir(dfppz)2(4)][PF6]·CH2Cl 2. The role of inter- and intramolecular face-to-face π-stacking in the solid state is discussed. In the [Ir(dfppz)2(N^N)]+ (N^N = 1-3) cations, the phenyl substituent in ligands 1, 2 or 3 undergoes hindered rotation on the NMR timescale at 298 K in solution and the systems have been studied by variable temperature NMR spectroscopy. Acetonitrile solutions of [Ir(dfppz)2(N^N)][PF6] (N^N = 1-3) exhibit similar absorption spectra arising from ligand-based transitions; absorption intensity is enhanced on going to [Ir(dfppz)2(4)][PF6] and the spectrum extends further into the visible region. Acetonitrile solutions of the complexes are blue emitters with λem = 517, 505, 501 and 493 nm for N^N = 1, 2, 3 and 4, respectively (λexc = 280-310 nm). The redox behaviours of [Ir(dfppz)2(N^N)][PF6] (N^N = 1-3) are similar, and the introduction of the electron-donating NMe2 substituents onto the N^N ligand shifts the metal-centred oxidation to less positive potentials. Theoretical calculations predict a mixed metal-to-ligand/ligand-to-ligand charge transfer (MLCT/LLCT) character for the emitting triplet state in agreement with the broad and unstructured character of the emission bands. The NMe2 substituents enlarge the HOMO-LUMO gap and blue-shifts the emission of [Ir(dfppz)2(4)]+ that is centred on the ancillary ligand. These complexes, when processed into a thin film and sandwiched between two electrodes, lead to very low voltage operating electroluminescent devices. No additional components are needed, which demonstrates their electron and hole transport abilities in conjunction with the luminescent properties.

Details

Original languageEnglish
Pages (from-to)1073-1087
Number of pages15
JournalDalton Transactions
Volume42
Issue number4
Publication statusPublished - 28 Jan 2013

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