Divalent Osmium Complexes: Synthesis, Characterization, Strong Red Phosphorescence, and Electrophosphorescence

We report new divalent osmium complexes that feature strong red metal-to-ligand-charge-transfer
(MLCT) phosphorescence and electrophosphorescence. The general formula of the complexes is Os(II)(N−N)2L−L, where N−N is either a bipyridine or a phenanthroline and L−L is either a phosphine or an
arsine. New polypyridyl ligands synthesized are 4,4‘-di(biphenyl)-2,2‘-bipyridine (15) and 4,4‘-di(diphenyl
ether)-2,2‘-bipyridine (16), and the 1,10-phenanthroline derivatives synthesized are 4,7-bis(p-methoxyphenyl)-1,10-phenanthroline (17), 4,7-bis(p-bromophenyl)-1,10-phenanthroline (18), 4,7-bis(4‘-phenoxybiphen-4-yl)-1,10-phenanthroline (19), and 4,7-bis(4-naphth-2-ylphenyl)-1,10-phenanthroline (20). 4,4‘-Diphenyl-2,2‘-bipyridine (21) and 4,7-diphenyl-1,10-phenanthroline (22) were also used in these studies. Strong π-acid
ligands used were 1,2-bis(diphenylarseno)ethane (23), cis-1,2-bis(diphenylphosphino)ethylene (24), and
cis-1,2-vinylenebis(diphenylarsine) (25). Ligand 25 is used for the first time in these types of luminescent
osmium complexes. These compounds feature strong MLCT absorption bands in the visible region and
strong red phosphorescent emission ranging from 611 to 651 nm, with quantum efficiency up to 45% in
ethanol solution at room temperature. Red organic light-emitting diodes (OLEDs) were successfully fabricated
by doping the Os(II) complexes into blend of poly(N-vinylcarbazole) (PVK) and 2-tert-butylphenyl-5-biphenyl-1,3,4-oxadiazole (PBD). Brightness over 1400 cd/m2 for a double-layer device has been reached, with a
turn-on voltage of 8 V. The maximum external quantum efficiency was 0.64%. Commission Internationale
de l'Eclairage (CIE) chromaticity coordinates (x, y) of the red electrophosphorescence from the complexes
are (0.65, 0.34), which indicates pure red emission.