Metalated Ir(III) Complexes Based on the Luminescent Diimine Ligands: Synthesis and Photophysical Study

A series of novel diimine (N^∧N) ligands containing developed aromatic [2,1- a]pyrrolo[3,2- c]isoquinoline system have been prepared and used in the synthesis of Ir(III) luminescent complexes. In organic solvents, the ligands display fluorescence which depends strongly on the nature of solvents to give moderate to strong orange emission in aprotic solvents and shows a considerable blue shift and substantial increase in emission intensity in methanol. Insertion of electron-withdrawing and -donating substituents into peripheral phenyl fragment has nearly no effect onto emission parameters. The ligands were successfully used to prepare the metalated [Ir(N^∧C)₂(N^∧N)]⁺ complexes (where N^∧C = phenylpyridine (N^∧C-1), p-tolylpyridine (N^∧C-2), 2-(benzo[ b]thiophen-2-yl)pyridine (N^∧C-3), 2-benzo[ b]thiophen-3-yl)pyridine (N^∧C-4), and methyl 2-phenylquinoline-4-carboxylate (N^∧C-5)) using standard synthetic procedures. The complexes obtained display moderate to strong phosphorescence in organic solvents; the emission characteristics is determined by the nature of emissive triplet state, which varies substantially with the variations in the structure and donor properties of the C- and N-coordinating functions in metalating ligands. TD-DFT calculations show that for complexes 1, 2, and 4 the emission originates from the mixed ³MLCT/³LLCT excited states with the major contribution from the aromatic moiety of the diimine ligand, whereas in 3 the emissive triplet manifold is mainly located at the N^∧C ligand to give structured emission band typical for the ligand centered (LC) excited state. In the case of 5, the phosphorescence may be also assigned to the mixed ³MLCT/³LLCT excited state; however, the major contribution is attributed to the aromatic moiety of the metalating N^∧C ligand.