Design, synthesis and host-guest chemistry of rhodamine derivatives and their transition metal complexes

Abstract

New classes of luminescent transition metal complexes, including the systems of rhenium(I) tricarbonyl diimine, cyclometalated iridium(III) diimine, cyclometalated rhodium(III) diimine, ruthenium(II) diimine and ruthenium(II) terpyridine complexes tethered with rhodamine moieties, have been synthesized. The X-ray crystal structure of one of the cyclometalated rhodium(III) diimine complexes with rhodamine pendants, [Rh(SPLC2N2)(ppy)2](PF6) has been determined. The molecular structure of [Rh(SPLC2N2)(ppy)2](PF6) showed a distorted octahedral geometry and the rhodamine moiety was found in a spirolactam closed-ring form. All of them were found to exhibit emission in fluid solution. The emissions of rhenium(I) tricarbonyl diimine and ruthenium(II) polypyridyl complexes are derived from the triplet metal-to-ligand charge transfer (3MLCT) excited state, i.e. from dπ orbital of the rhenium(I) or ruthenium(II) metal center to the π* orbital of the diimine ligand; while that of cyclometalated iridium(III) diimine complexes is (3MLCT) [dπ(Ir) → π*(N^N)] and that of cyclometalated rhodium(III) diimine complexes involves the (3IL) [π → π*(N^C)] excited state, probably mixed with (3MLCT) [dπ(Rh) → π*(N^C)] character. The cation-binding properties of these complexes toward alkali, alkaline-earth and transition-metal cations were investigated by electronic absorption and emission spectrophotometries. Some of them were found to exhibit new low-energy absorption and emission bands, characteristic of the rhodamine absorption and emission, with high selectivity and sensitivity for certain transition metal cations.

A series of rhodamine-appended Schiff base organic compounds has also been synthesized in order to explore their capability as luminescent lanthanide ion sensors. The lanthanide ion binding properties of one of the compounds in acetonitrile were examined. Upon addition of lanthanide ions, new intense low-energy electronic absorption and emission bands were also observed, characteristic of the rhodamine absorption and emission, demonstrating its lanthanide ion sensing behaviour.