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

The new extended tri- and penta-fluorenylpyridine ligands Fl(3)Py 2 and Fl(5)Py 3 and their tris-cyclometalated iridium(III) complexes Ir[Fl(3)Py](3) 5 and Ir[Fl(5)Py](3) 6 have been synthesised and their properties compared with the known iridium( III) complex Ir[Fl(1)Py](3) 4. The lowest energy (emissive) excited states of the complexes 4-6 are dominated by ligand centred (LC) (3)pi ->pi* triplet states, as observed for their uncomplexed ligands 1-3. The emission maximum of complex 4 is similar to 546 nm with a triplet lifetime of 2.8 mu s. For complexes 5 and 6 the emission maxima are both similar to 566 nm with triplet lifetimes of 7.4 mu s and 7.8 mu s, 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 similar to 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.