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

Abstract

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 ∼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 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 ∼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.