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Publication details for Professor Andy Monkman
Bettington, S, Tavasli, M, Bryce, MR, Beeby, A, Al-Attar, H & Monkman, AP (2007). Tris-cyclometalated iridium(III) complexes of carbazole(fluorenyl)pyridine ligands: Synthesis, redox and photophysical properties, and electrophosphoreseent light-emitting diodes. Chemistry-a European Journal 13(5): 1423-1431.- Publication type: Journal papers: academic
- ISSN/ISBN: 0947-6539
- Keywords: carbazole; cyclometallation; fluorene; fluorescence; organic light-emitting devices; pyridineCARBAZOLE COMPOUNDS; TRIPLET EMITTERS; HOST MATERIALS; IR COMPLEXES; DEVICES; POLYMERS; BLUE; EMISSION; GREEN; PHOSPHORESCENCE
Author(s) from Durham
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.