Ir-III and
Ru-II Complexes Containing Triazole-Pyridine Ligands: Luminescence
Enhancement upon Substitution with beta-Cyclodextrin
Felici M (Felici, Marco) Contreras-Carballada P (Contreras-Carballada,
Pablo), Vida Y (Vida, Yolanda), Smits JMM (Smits, Jan M. M.), Nolte RJM
(Nolte, Roeland J. M.), De Cola L (De Cola, Luisa), Williams RM
(Williams, Rene M.), Feiters MC (Feiters, Martin C.)
Abstract:
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.