We report the synthesis and organic light-emitting devices (OLEDs) made from a series of 1-phenyl- and 3-phenylisoquinolinyliridium complexes in which the phenyl group is linked to the C1 and C3 carbons of isoquinoline, respectively. These linkage isomers show distinct differences in their photophysical and electroluminescence (EL) properties, including the magnitude of phosphorescent lifetimes and photoluminescence (PL) and EL emission wavelengths, as well as the phenomenon of triplet-triplet (T-T) annihilation. Complexes of these two families show a strong absorption band in the region 440-490 nm assignable to spin-forbidden (MLCT)-M-3 (metal-ligand charge-transfer) bands. The extinction coefficients of these bands are similar to those of spin-allowed (MLCT)-M-1 bands, indicative of an anomalously strong spin-orbital coupling. Upon excitation, 1-phenylisoquinolinyliridium complexes exhibit a single phosphorescent emission band in the red region (595-631 nm). All of these red phosphors show outstanding EL performance with negligible T-T annihilation because of short phosphorescent lifetimes (1.04-2.46 mu s in CH2Cl2) and good emission quantum yields. One representative, [Ir(5-f-1piq)(2)(acac)] (acac=acetylacetonate) (3) (5-f-1piqH = 5-fluoro-1-phenylisoquinoline), is not only the brightest at low voltages (1883 cd m(-2) at 7.1 V; 8320 cd m-2 at 9.0 V) but also shows a eta(ext) value of. 6.50% at high current (J = 400 mA cm(-2)). The maximum brightness is 38 218 cd m-2 (x = 0.68, y = 0.31) with the full width at half maximum (FWHM) only 50 nm at 8 V. In contrast, 3-phenylisoquinolinyliridium complexes show phosphorescent emissions in the yellow region (534-562 nm) but with a long phosphorescent lifetime (3.90-15.6 mu s in CH2Cl2). Most of these yellow phosphors suffer T-T annihilation in the EL performance. The exception is [Ir(3-piq)(2)(acac)] (5) (3-piqH=3-phenylisoquinoline), which has a relatively short lifetime 3.90 mu s in CH2Cl2. Complex 5 achieves an external efficiency (eta(ext)) value of 5.27% at J = 20 mA cm(-2) and maintains a eta(ext) value of 3.58 at J = 400 mA cm(-2) with a maximum brightness of 65 448 cd m(-2) (x = 0.49, y = 0.51).