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 consis ting of metal and organic photoactive/ redox-active components; the photoin duced communication between redox-active units assembled in water via nonco valent interactions is established. The functionalization of a beta -cyclod extrin with a terpyridine unit, ttp-PCD, is achieved by protection of all b ut one of the hydroxyl groups by methylation and attachment of the ttp unit on the free primary hydroxyl group. The metalloreceptors [(beta -CD-ttp)Ru (ttp)][PF6](2), [(beta -CD-ttp)Ru(tpy)][PF6](2), and [Ru(beta -CD-ttP)(2)][ PF6](2) are synthesized and fully characterized. The [(beta -CD-ttp)Ru(ttp) ][PF6](2) metalloreceptor exhibits luminescence in water, centered at 640 n m, from the (MLCT)-M-3 state with a lifetime of 1.9 ns and a quantum yield of Phi = 4.1 x 10(-5). Addition of redox-active quinone guests AQS, AQC, an d BQ to an aqueous solution of [(beta -CD-ttp)Ru(ttp)](2+) results in quenc hing 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 t he quinone guest inside the cavity. Control experiments demonstrate no bimo lecular quenching at these conditions. A photoactive osmium metalloguest, [ Os(biptpy)(tpy)][PF6], is designed with a biphenyl hydrophobic tail for ins ertion in the cyclodextrin cavity. The complex is luminescent at room tempe rature with an emission band maximum at 730 nm and a lifetime of 116 ns. Th e osmium(III) species are formed for the study of photoinduced electron tra nsfer upon their assembly with the ruthenium cyclodextrin, [(beta -CD-ttp)R u(ttp)](2+). Time-resolved spectroscopy studies show a short component of 1 0 ps, attributed to electron transfer from Ru(II) to Os(III) giving an elec tron transfer rate 9.5 x 10(9) s(-1).