Ir-III and Ru-II Complexes Containing Triazole-Pyridine Ligands: Luminescence Enhancement upon Substitution with beta-Cyclodextrin

Novel 2-(1-substituted-1H-1,2,3-triazol-4-yl)pyridine (pytl) ligands have been prepared by click chemistry and used in the preparation of heteroleptic complexes of Ru and Ir with bipyridine (bpy) and phenylpyridine (ppy) ligands, respectively, resulting in [Ru(bpy)(2)(pytl-R)]Cl-2 and [Ir(ppy)(2)-(pytl-R)]Cl (R=methyl, adamantane (ada), beta-cyclodextrin (beta CD)). The two diastereoisomers of the Ir complex with the appended beta-cyclodextrin, [Ir-(ppy)(2)(pytl-beta CD)]Cl, were separated. The [Ru(bpy)(2)(pytl-R)]Cl-2 (R=Me, ada or beta CD) complexes have lower lifetimes and quantum yields than other polypyridine complexes. In contrast, the cyclometalated Ir complexes display rather long lifetimes and very high emission quantum yields. The emission quantum yield and lifetime (Phi = 0.23, tau = 1000 ns) of [Ir(ppy)(2)(pytl-ada)]Cl are surprisingly enhanced in [Ir(ppy)(2)(pytl-beta CD)]Cl (Phi=0.54, tau = 2800 ns). This behavior is unprecedented for a metal complex and is most likely due to its increased rigidity and protection from water molecules as well as from dioxygen quenching, because of the hydrophobic cavity of the beta CD covalently attached to pytl. The emissive excited state is localized on these cyclometalating ligands, as underlined by the shift to the blue (450 nm) upon substitution with two electron-withdrawing fluorine substituents on the phenyl unit. The significant differences between the quantum yields of the two separate diastereoisomers of [Ir(ppy)(2)(pytl-beta CD)]Cl (0.49 vs. 0.70) are attributed to different interactions of the chiral cyclodextrin substituent with the A and A isomers of the metal complex.