Metallocyclodextrins as building blocks in noncovalent assemblies of photoactive units for the study of photoinduced intercomponent processes

Cyclodextrin cups have been employed to build supramolecular systems consisting of metal and organic photoactive/redox-active components; the photoinduced communication between redox-active units assembled in water via noncovalent interactions is established. The functionalization of a β-cyclodextrin with a terpyridine unit, ttp-βCD, is achieved by protection of all but one of the hydroxyl groups by methylation and attachment of the ttp unit on the free primary hydroxyl group. The metalloreceptors [(β-CD-ttp)Ru(ttp)][PF6]2, [(β-CD-ttp-)Ru(tpy)][PF6]2, and [Ru(β-CD-ttp)2][PF6]2 are synthesized and fully characterized. The [(β-CD-ttp)Ru(ttp)][PF6]2 metalloreceptor exhibits luminescence in water, centered at 640 nm, from the 3MLCT state with a lifetime of 1.9 ns and a quantum yield of Φ = 4.1 x 10-5. Addition of redox-active quinone guests AQS, AQC, and BQ to an aqueous solution of [β-CD-ttp)Ru(ttp)]2+ results in quenching of the luminescence up to 40%, 20%, and 25%, respectively. Measurement of the binding strength indicates that, in saturation conditions, 85% for AQS and 77% for AQC are bound. The luminescence quenching is attributed to an intercomponent electron transfer from the appended ruthenium center to the quinone guest inside the cavity. Control experiments demonstrate no bimolecular quenching at these conditions. A photoactive osmium metalloguest, [Os(biptpy)(tpy)][PF6], is designed with a biphenyl hydrophobic tail for insertion in the cyclodextrin cavity. The complex is luminescent at room temperature with an emission band maximum at 730 nm and a lifetime of 116 ns. The osmium(III) species are formed for the study of photoinduced electron transfer upon their assembly with the ruthenium cyclodextrin, [(β-CD-ttp)Ru(ttp)]2+. Time-resolved spectroscopy studies show a short component of, 10 ps, attributed to electron transfer from Ru(II) to Os(III) giving an electron transfer rate 9.5 x 109 s-1.