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 eta(2)- to the OAE via a 2,2'-bipyridine moiety that is in the core of the oligomer, and the metals used are eta(2)-Ir(ppy)(2)(+), eta(2)-Ru(bpy)(2)(2+), and eta(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 (MLCT)-M-3 excited state; however, in Ir-2 the lowest excited state is 3pi,pi* 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 much greater than Os-2. Factors that may contribute to the difference in EL efficiencies are discussed.