Electroluminescence from two new ruthenium(II) complexes as phosphorescent dopant: Positive effect of swallow-tail bipyridyl ligand

Two new orange-red emitting ruthenium complexes, [Ru(bpy)2(4,4′-bis(3-ethylheptyl)-2,2′-bipyridine)](PF6)2 and [Ru(bpy)2(4-dihexylmethyl-4′-heptyl-2,2′-bipyridine)](PF6)2 with branched alkyl side groups on the bipyridyl ligand have been synthesized and used as dopants in an organic light emitting device. The device that contains the complex containing the 4-dihexylmethyl-4′-heptyl-2,2′-bipyridine ligand 1.0 wt.% doped polymer blends of poly(vinylcarbazole) and 2-tert-butylphenyl-5-biphenyl-1,3,4-oxadiazol as the emitting layer reached to a luminous efficiency of 7 cd/A, and a maximum brightness of 3824 cd/m2 with an electroluminescence spectral peak at 600 nm and Commission Internationale de L’Eclairage chromaticity coordinates of (0.57, 0.39). The swallow-tail alkyl substituent in this complex played a significant role in lowering the tendency to aggregate and presenting a better electroluminescence efficiency than that of the complex derived from the 4,4′-bis(3-ethylheptyl)-2,2′-bipyridine) ligand in a device.

► Two new alkyl substituted ruthenium bipyridyl complexes are synthesized.
► Swallow-tail alkyl substituent lowers the tendency to aggregate.
►  Swallow-tail alkyl substituent increases the electroluminescence efficiency.
► Luminous efficiency of 7 cd/A at a current density of 10 mA/cm2 is obtained.