A Highly Efficient Red-Emitting Ruthenium Complex with 3,5-Difluorophenyl Substituents

Five 3,5-difluorophenyl-substituted ruthenium complexes [Ru(bpy)₂ (dfpbpy)(ClO₄ )₂ ] (Ru-Fbpy-bpy; bpy=bipyridine, dfpbpy=5,5'-di(3,5-difluorophenyl)-2,2'-bipyridine), [Ru(dpp)₂ (dfpbpy)(ClO₄ )₂ ] (Ru-Fbpy-dpp; dpp=4,7-diphenyl-1,10-phenanthroline), [Ru(dpp)₂ (dfpphen)(ClO₄ )₂ ] (Ru-Fphen-dpp; dfpphen=5,5'-di(3,5-difluorophenyl)-1,10-phenanthroline), [Ru(dpp)₂ (4,7-dfpphen)(ClO₄ )₂ ] (Ru-Fdpp-dpp; 4,7-dfpphen=4,7-di(3,5-difluorophenyl)-1,10-phenanthroline), and [Ru(4,7-dfpphen)₃ (AsF₆ )₂ ] (Ru-Fdpp) were synthesized, and their photophysical, electrochemical, and electroluminescent properties were studied systematically. The introduction of the electron-withdrawing group 3,5-difluorophenyl leads to a redshift of the emission of the ruthenium complexes. In addition, the 3,5-difluorophenyl substituent extends the π conjugation of the ligand, and thus facilitates the metal-to-ligand charge-transfer process of the complexes. All the complexes display orange-red phosphorescent emissions centered at 638, 638, 624, 614, and 605 nm, respectively. Density functional theory calculations show that the lowest unoccupied molecular orbital of Ru-Fbpy-dpp, Ru-Fphen-dpp, and Ru-Fdpp-dpp are all distributed on the 3,5-difluorophenyl-substituted ligand. By using these complexes as emitters, highly luminescent single-layer devices are obtained. The optimized device based on Ru-Fdpp exhibits the highest luminous efficiency and power efficiency of 4.19 cd A^-1 and 1.46 lm W^-1 , respectively.