Photoluminescence and Electroluminescence of d6 Metal−Organic Conjugated Oligomers: Correlation of Photophysics and Device Performance

The photophysical and electroluminescent device properties of a series of d 6 transition metal complexes that feature an oligo(aryleneethynylene) (OAE) “ligand” have been investigated. The metals are coordinated η 2 - to the OAE via a 2,2‘-bipyridine moiety that is in the “core” of the oligomer, and the metals used are η 2 -Ir(ppy) 2 +, η 2 -Ru(bpy) 2 2+, and η 2 -Os(bpy) 2 2+ (Ir-2, Ru-2, and Os-2, respectively, where ppy = 2-phenylpyridine and bpy = 2,2‘-bipyridine). The photoluminescence spectra and excited-state decay parameters for the metal−OAEs indicate that at ambient temperature Ru-2 and Os-2 feature a lowest 3 MLCT excited state; however, in Ir-2 the lowest excited state is 3 π,π* localized on the OAE. Electroluminescent (EL) devices that contain the metal−OAEs dispersed at 5 wt % in a poly(vinyl carbazole):2-(4- tert -butylphenyl)-5-(4-biphenylyl)-1,3,4-oxadiazole matrix exhibit electroluminescence at wavelengths that correlate well with the photoluminescence from the complexes in a rigid glass at low temperature. The EL efficiency of the devices varies in the order Ir-2 > Ru-2 ≫ Os-2. Factors that may contribute to the difference in EL efficiencies are discussed.