Bettington, S., Tavasli, M., Bryce, M.R., Batsanov, A.S., Thompson, A.L., Al Attar, H.A., Dias, F.B. & Monkman, A.P. (2006). Bridged diiridium complexes for electrophosphorescent OLEDs synthesis, X-ray crystal structures, photophysics, and devices. Journal of materials chemistry 16
Author(s) from Durham
Results are presented which challenge the accepted view that dinuclear transition metal–ligand complexes are unsuitable for organic light-emitting device (OLED) applications due to their low luminescence quantum efficiencies. We establish for the first time that halo- and pseudo-halo-bridged diiridium(III) species are viable electrophosphorescent dopants in OLEDs. New cyclometalated chloro- and isocyanate-bridged diiridium(III) complexes, viz. tetrakis[9,9-dihexyl-2-(pyridin-2-yl)fluorene-C2,N′]-bis(μ-chloro)diiridium(III) [Ir(flpy)2Cl]2 (complex 3) and tetrakis[9,9-dihexyl-2-(pyridin-2-yl)fluorene-C2,N′]-bis(μ-isocyanate)diiridim(III) [Ir(flpy)2NCO]2 (complex 4) were obtained in high yield from the 9,9-dihexyl-2-(pyridin-2-yl)fluorene (flpyH) ligand 1. The X-ray crystal structures are described for 3 and the monomeric complex Ir(flpy)2NCO(DMSO) (5) which was obtained from 4. The solution-state photophysical properties of complexes 3 and 4 are characterised by emission from mixed 3π–π*/3MLCT states at [similar]545–550 nm. Complex 4 displays a solution-state photoluminescence quantum yield which is five times that of the dichloro-bridged analogue 3. This is ascribed to an increase in the ligand-LUMO/metal eg gap which reduces the probability of non-radiative decay processes. Spin-coated organic light emitting devices (OLEDs) made from the host polymer poly(9,9-bis-2-ethylhexylfluorene-2,7-diyl) (PF2/6) end-capped with bis-(4-methylphenyl)phenylamine (PF2/6am4) doped with 12.5 wt% of the complexes 3 and 4 show good stability: turn-on voltages are low (<4 V) with maximum EL intensities of [similar]1300 and 13 000 cd m−2, and peak external quantum efficiencies (EQE) of 0.1 and 0.8%, at ca. 400 and 60 mA cm−2, respectively.