A series of red-light-emitting ionic iridium complexes: Structures, excited state properties, and application in electroluminescent devices

A series of ionic diiminoiridium complexes [Ir(piq-C∧N) ₂(L-N∧N)](PF₆) were prepared, where piq-C∧N is 1-phenylisoquinolinato and L-N∧N are bidentate N-coordinating ligands: 2,2′-bipyridine (bpy), 4,4′-dimethyl-2,2′-bipyridine (mbpym), 5,5′-bis(thiopen-2-yl)-2,2′-bipyridine (tbpyt), and 5,5′-bis(9,9-dioctylfluoren-2-yl)-2,2′-bipyridine (FbpyF). X-ray diffraction studies of [Ir(piq)₂(mbpym)](PF₆) revealed that the iridium center adopts a distorted octahedral geometry. All complexes exhibited intense and long-lived emission at room temperature. The substituents on the 2,2′-bipyridine moieties influence the photophysical and electrochemical properties. The excited states were investigated through theoretical calculations together with photophysical and electrochemical properties. It was found that the excited state of the [Ir(piq) ₂(FbpyF)](PF₆) complex can be assigned to a mixed character of ³LC (π_N∧N→π *_N∧N), ³MLCT, ³LLCT (π _C∧N→π*_N∧N), and ³LC (π_C∧N→π*_C∧N). In addition, the alkylfluorene-substituted complex, [Ir(piq)₂(FbpyF)](PF₆), had relatively high quantum efficiency and good film-forming ability, and it was expected to be a good candidate for lighting and display applications. A nondoped, single-layer device that incorporates this complex as a light-emitting layer was fabricated and red phosphorescence was obtained.