Bridged diiridium complexes for electrophosphorescent OLEDs: synthesis, X-ray crystal structures, photophysics, and devices

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-C-2, N']-bis(mu-chloro)diiridium(III) [Ir(flpy)(2)Cl](2) (complex 3) and tetrakis[9,9-dihexyl-2-(pyridin-2-yl)fluorene-C-2,N']-bis(mu-isocyanate) diiridim(III) [Ir(flpy)(2)NCO](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 (3)pi-pi*/(MLCT)-M-3 states at similar to 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 e(g) gap which reduces the probability of non-radiative decay processes. Spin-coated organic light emitting devices ( OLEDs) made from the host -polymerpoly(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 to 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.