To examine the substituent effect, 5'-substituted 2,4-diphenylquinoline (dpq) ligands and their iridium complexes, Ir(dpq)(2)(acac), Ir(dpq-5F)(2)(acac), Ir(dpq-5CH(3))(2)(acac) and Ir(dpq-5OCH3)(2)(acac), were synthesized and prepared, and the photophysical properties were investigated for red electro-phosphorescent materials. The 5'-substituted phenyl ring generated 3'-positon and 5'-position isomers. However, the 5'-position of the phenyl ring is more stable than the 3'-position due to steric effects. The maximum emission wavelengths of Ir(dpq)(2)(acac), Ir(dpq-5F)(2)(acac), Ir(dpq-5CH(3))(2)(acac) and Ir(dpq-5OCH(3))(2)(acac) were found at 612, 606, 616, 606 nm, respectively. The emission spectrum of Ir(dpq-5CH(3))(2)(acac) was red shifted compared to that of Ir(dpq)(2)(acac) because the methyl group substituted on the 5'-position of the phenyl ring is an electron donating group. Ir(dpq-5F)(2)(acac) shows highly efficient and blue-shifted luminescence because the electron withdrawing group (-F) substitute on the 5'-position of the phenyl ring lowered the HOMO level slightly. Hence MLCT coupling is increased and blue shifted luminescence by 6 nm was observed compared to Ir(dpq)(2)(acac). Especially, Ir(dpq-5OCH(3))(2)(acac) shows blue-shifted phosphorescence due to the effect of the electron withdrawing methoxyphenyl group onto the iridium atom. This destabilized the MLCT state and leads to an increased in the emission energy.