Tris-cyclometalated iridium(III) complexes of carbazole(fluorenyl)pyridine ligands: Synthesis, redox and photophysical properties, and electrophosphoreseent light-emitting diodes


Bettington S., TAVASLI M. , Bryce M. R. , Beeby A., Al-Attar H., Monkman A. P.

CHEMISTRY-A EUROPEAN JOURNAL, vol.13, no.5, pp.1423-1431, 2007 (Journal Indexed in SCI) identifier identifier identifier

  • Publication Type: Article / Article
  • Volume: 13 Issue: 5
  • Publication Date: 2007
  • Doi Number: 10.1002/chem.200600888
  • Title of Journal : CHEMISTRY-A EUROPEAN JOURNAL
  • Page Numbers: pp.1423-1431
  • Keywords: carbazole, cyclometallation, fluorene, fluorescence, organic light-emitting devices, pyridine, CARBAZOLE COMPOUNDS, TRIPLET EMITTERS, HOST MATERIALS, IR COMPLEXES, DEVICES, POLYMERS, BLUE, EMISSION, GREEN, PHOSPHORESCENCE

Abstract

Using ligands synthesized by Suzuki cross-coupling methodology, new phosphorescent homoleptic tris-cy-clometalated complexes have been obtained, namely fac-[Ir(Cz-2-Fl(n)Py)(3)] (1d-f) and fac-[Ir(Cz-3-Fl(n)Py)(3)] (2d-f), which are solution-processible triplet emitters (Cz denotes N-hexylcarbazole, n is the number of 9,9'-dihexylfluorene (Fl) units (n=0,1,2) and Py is pyridine). In all cases, Py and Fl are substituted at the 2- and 2,7-positions, respectively, and Cz moieties are substituted by either Py or Fl at the 2- or 3-positions, in series I and 2, respectively. The oxidation potential of 1d studied by cyclic voltammetry (E-1/2(ox) = 0.14 V versus Ag/AgNO3, CH2Cl2) is less positive (i.e. raised HOMO level) compared to that of the isomer 2d (E-1/2(ox) = 0.30 V), where the Cz-nitrogen is meta to the Ir center. Ligand-centered oxidations occur at more positive potentials, leading to 7+ oxidation states with good chemical reversibility and electrochemical quasi-reversibility, for example, for 2f E-pa(ox) =0.45 (1e), 0.95 (3e), 1.24V (3e). Striking differences are seen in the solution-state photophysical data between complexes [Ir(Cz-2-Py)(3)] (1d) and [Ir(Cz-3-Py)(3)] (2d), in which the Cz moiety is bonded directly to the metal center: for the latter there is an 85 nm blue-shift in emission, a decrease in the luminescence lifetime and an increase in the PLQY value. Organic light emitting devices were made by spin-coating using polyspirobifluorene:bis(triphenyl)diamine (PSBF:TAD) copolymer as host and the complexes Id or 2d as dopants. Turn-on voltages are low (3-4 V). With Id orange light is emitted at lambda(max) = 590 nm with an EQE of 1.3 % (at 7.5 mA cm(-2)) and an emission intensity (luminance) of 4354 cdm(-2) (at 267 mA m(-2)). The green emission from 2d devices 500 nm) is due to the reduced electron-donating ability of the carbazole unit in 2d. Recording the EL spectra of the Id device at 6 V (current density, 1.00 mA cm(-2)) established that the time to half brightness was about 9 h under continuous operation with no change in the spectral profile, confirming the high chemical stability of the complex.