Oligo(fluorenyl)pyridine ligands and their tris-cyclometalated iridium(iii) complexes: synthesis, photophysical properties and electrophosphorescent devices

Abstract

The new extended tri- and penta-fluorenylpyridine ligands Fl₃Py 2 and Fl₅Py 3 and their tris-cyclometalated iridium(III) complexes Ir[Fl₃Py]₃5 and Ir[Fl₅Py]₃6 have been synthesised and their properties compared with the known iridium(III) complex Ir[Fl₁Py]₃4. The lowest energy (emissive) excited states of the complexes 4–6 are dominated by ligand centred (LC) π→π* triplet states, as observed for their uncomplexed ligands 1–3. The emission maximum of complex 4 is ∼546 nm with a triplet lifetime of 2.8 µs. For complexes 5 and 6 the emission maxima are both ∼566 nm with triplet lifetimes of 7.4 µs and 7.8 µs, respectively. Devices made from poly(9,9′-spirobifluorene) (PSF) as the host and doped with complexes 4–6 show good stability; the EL spectra are unchanged after repeated operation over several days. Devices containing complexes 5 and 6 exhibit higher external quantum efficiency (EQE) values. Turn-on voltages of ∼3 V, giving an EQE of 2.8% at a current density of 30 mA cm⁻², with a power efficiency of 4.3 lm W⁻¹ and electroluminescence (EL) intensity of 25 000 cd m⁻² at 550 mA cm⁻² were observed for ITO/PEDOT ∶ PSS/PSF ∶ 6/Ca/Al devices.