Photochemical characterization is reported for the mer-stereoisomer of fac-tris[2-(4′,6′-difluorophenyl)pyridine]iridium(III) (2), which has attracted attention as a electrophosphorescent material. The mer-2 has a much smaller emission-quantum yield and a shorter emission lifetime than those of the fac-isomer. The relation of the photophysical properties of mer-2 and one-way photoisomerization to fac-2 is discussed.

A series of cyclometalated iridium complexes with a strong electron-donating and bulky diphenylamino group was prepared and the phosphorescence wavelengths were found to be greatly dependent on the substituent positions: some of them afforded high performance electroluminescence (EL) devices.

We demonstrated efficient organic light-emitting devices (OLEDs) using a phosphorescent host/guest system consisting of bis(2-phenylpyridinato-N,C2′)iridium(acetylacetonate) [(ppy)2Ir(acac)] as a host and a platinum complex (Pt-SA-1) as a guest. The OLED using (ppy)2Ir(acac) film doped with Pt-SA-1 (1wt%) showed an ideal red emission via efficient energy transfer from the host to the guest. The external quantum efficiency of the device was as high as 8.3%. The driving voltage was significantly reduced compared with a device using a conventional host of 4,4′-di(carbazole-9-yl)biphenyl, which resulted from the enhancement of the hole injection from the hole-transport layer to the host.

The α-diimine-type platinum(II) complexes having bulky perfluorinated phenyl groups were prepared and the molecular and crystal structures were revealed by X-ray analysis. They showed phosphorescence and electron-accepting properties. The organic light-emitting diode (OLED) devices showing green luminescence were fabricated.

A series of phosphorescent cyclometalated iridium(III) complexes 4-6 with new ligands of bicyclic phenylvinylpyridines have been synthesized and systematically examined for their photophysical, electrochemical and electroluminescence (EL) properties. All of the bis-and tris-cyclometalated complexes could be characterized by 1 H-NMR, elemental analysis and X-ray crystal structure analysis. The phosphorescence colour of the complexes in solution, the solid state and polymer film is unusually changed depending on the structure of the ligand. EL devices using the complexes exhibited external quantum efficiencies (EQE) of 1.2-8.1% at 100 cd m 22 , and EL colour tuning in the range of 571-635 nm was achieved analogous to PL properties of the phosphors. X-Ray analyses of complexes 4a-c demonstrated that the degree of steric hindrance increases with an increase of the cycloalkane ring size, leading to hypsochromic shifts of emission by the distortion of the phenyl groups. X-Ray analyses also suggested that long iridium-carbon bonds are unfavorable for efficient electroluminescence. In addition, we have found a unique and important aspect for the study of EL devices: the EL devices based on complexes 4c, 6a show moderate EL efficiencies (EQE = 4.0 and 6.1%) although these complexes have no emission in solution at room temperature, indicating that the PL properties of the complexes in solution are not directly related to the EL efficiencies.