Synthesis, characterization and optoelectronic properties of iridium complexes bearing nonconjugated six-membered chelating ligands

We report the synthesis, characterization and the optoelectronic properties of a series of four new luminescent iridium(III) complexes 1-4, of the form [Ir(C^{^}N)₂(N^{^}N)]PF₆ [where C^{^}N is the nonconjugated benzylpyridinato (bnpy) and N^{^}N is a neutral diimine ancillary ligand] with the goal of investigating the impact of the methylene spacer between the coordination moieties of the C^{^}N ligand on the optoelectronic properties of the complexes. The crystal structures of 1-3 illustrate two possible orientations of the methylene unit of the bnpy ligand. The formation of these two separate conformers is a result of the conformational flexibility of the bnpy ligand. In complexes 3 and 4, mixtures of the two conformers were observed by 1H-NMR spectroscopy in CDCl3 at room temperature, whereas only a single conformer is detected for 1 and 2. Detailed DFT calculations corroborate ROESY experiments, accounting for the presence and relative populations of the two conformers. The optoelectronic properties of all four complexes, rationalized by the theoretical study, demonstrate that the interruption of conjugation in the C^{^}N ligands results in a reduced electrochemical gap but similar triplet state energies and lower photoluminescence quantum yields compared to the reference complexes R1-R4. Depending on the nature of the N^{^}N ligand, we observe (1) marked variations of the ratio of the conformers at ambient temperature and (2) phosphorescence ranging from yellow to red.