A luminescent tricarbonylchlororhenium(I) complex featuring a flexible "crown ether" ligand. Manipulation of photoexcited state properties via binding of small cations

A luminescent complex, [Re(CO)(3)(Cl)(LL)] where LL is 1,11-bis(4-pyridylca rboxy)-3,6,9-trioxaundecane, was prepared, and the crystal structures of tw o solvates were determined. The flexible polyether chain of the chelating l igand adopts an all gauche conformation about the aliphatic C-C bonds, and does not appear to introduce any appreciable constraints on the geometry at the octahedral metal center. The cavity formed by the "crown ether" has a diameter of ca. 7 Angstrom. Binding of cations causes a decrease of the lum inescence lifetime of the (ReLL-)-L-II excited state, consistent with an "e nergy gap" effect and with a rapid excited-state equilibrium between bound and unbound moieties. Binding constants were determined for several alkali metal ions, and for the ammonium cation. The chemical affinity of the crown -functionalized ligand is not specific for any of the cations studied, but, instead, is similar for a range of cations. Comparative studies with a com plex containing ester-functionalized pyridine ligands, but lacking the poly ether linkage necessary to form the crown, also showed excited state bindin g of small cations, indicating that the anionic ester pyridyl fragment, rat her than the polyether fragment, is responsible for the binding.