Luminescent Transition Metal Complexes in Host-Guest Systems

Abstract

Host-guest systems incorporating various luminescent transition metal (TM) complexes as guests with cucurbit[10]uril (Q[10]) as host have been assessed. It has been found that hydrophobic effect is the dominant factor driving the association, while other factors such as overall size and charge of the guest have smaller influence on the binding modes or affinity. A systematic investigation of the effect of the host-guest interaction on the photophysical properties revealed a strong dependence on the electronic structure of the particular TM complex guest.

Archetypal tris-polypyridyl TM complexes Ru-1 {[Ru(bpy)3]2+}, Ru-2 {[Ru(phen)3]2+}, Ir-1 {[Ir(bpy)3]3+} and Ir-2 {[Ir(phen)3]3+} (where, bpy = 2,2′-bipyridine and phen = 1,10-phenanthroline) were studied as guests with the host Q[10]. Larger phen-containing complexes showed greater affinity for Q[10], and changes in emission intensity were greater compared to the corresponding bpy complex. Molecular modeling showed that encapsulation by Q[10] caused distortion of the metal-ligand bonding framework, which, for the Ru(II) complexes, could lead to a reduced energy gap with a higher lying non-emissive state.

Ir(III) and Ru(II) cyclometalated and Re(I) tricarbonyl TM complexes were also studied as a guest with the large host Q[10]. The guest complexes were as follows: Ir-P {[Ir(ppy)2(phen)]+}, Ir-DtBB {[Ir(ppy)2(dtb-bpy)]+}, Ir-bb5 {[{Ir(ppy)2}2(µ-bb5)]2+}, Ru-H {[Ru(bpy)2(ppy)]+}, Re-P {[Re(CO)3(phen)(py)]+}, and Re-TMP {[Re(CO)3(Me4phen)(py)]+} (where ppy = 2-phenylpyridyl, dtb-bpy = 4,4′-di-tert-butyl-2,2′-dipyridyl, bb5 = 1,5-bis[4-(4′-methyl-2,2′-bipyridyl)]-pentane, Me4phen = 3,4,7,8-tetramethyl-1,10-phenanthroline and py = pyridine). The binding preference in mononuclear cyclometalated complexes was not changed by the introduction of hydrophobic groups on the polypyridyl ligand, but the dinuclear complex Irbb5 did show an alternative mode of association for the metal centers. All complexes showed enhancement of emission intensity, with the extent of this being dependent on the nature and proximity of higher lying states, and the orientation of the metal center inside the cavity of Q[10]. Q[10] showed ability to differentiate between diastereomers of Irbb5 by NMR spectroscopy.

The photochemically active guest molecules Ru-1, Ru-3 {[Ru(bpy)2(H2O)2]2+}, Ir-Cl {[Ir(phen)2Cl2]+} and its photoproducts Ir-A {[Ir(phen)2(H2O)2]3+} and Ir-B {[Ir(phen)2(OH)2]+} were investigated with Q[10]. Results showed that Q[10] binding may either enhance or inhibit photochemistry, depending on the particular TM guest complex and the overall reaction mechanism.