SHORT COMMUNICATION The First Ru II Bipyridyl-Capped Cyclodextrin: Evidence of Electron-Transfer Through the Cavity Dominique Armspach,* [a] Dominique Matt,* [a] and Anthony Harriman [b] Keywords: Cyclodextrins / Ruthenium / N ligands / Electron transfer / Inclusion compounds The narrow rim of an α-cyclodextrin has been AD-capped with a photoactive ruthenium(II) tris(2,29-bipyridyl) fragment giving a diastereomeric mixture of complexes that bind Introduction Supramolecular assemblies displaying vectorial photoin- duced electron-transfer (ET) reactions have attracted much attention in recent years from the view point of designing efficient artificial photosynthetic systems and light-driven molecular machines. [125] In order to gain a better under- standing of through-space ET processes, it is necessary to closely position the reactants in a rigid manner but to avoid either direct orbital contact or a connecting organic frame- work. Here, we describe the synthesis and photophysical properties of a water-soluble α-cyclodextrin (α-CD) host for which a photoactive ruthenium(II) centre is held, for the first time, at a fixed distance above the receptor. Our ap- proach involves capping the α-CD with a difunctionalized 2,29-pyridyl ligand before subsequent metallation. The re- sulting complex is expected to behave as a metallo-receptor which, upon illumination, will transfer an electron between the excited metal centre and a redox-active substrate trapped in the cavity. Photoactive Ru II (bipy) centres ap- pended to cyclodextrin receptors have been reported previ- ously, but these systems contain flexible Ru2CD con- nectors, thus preventing full geometrical control of the ET process. [6] Results and Discussion The key precursor for our study is 6A,6D-diamino- 6A,6D-dideoxy-hexadeca-O-methyl-α-CD (2) which allows effective capping with diacid chlorides and good solubility in organic solvents. Compound 2 was synthesised in two steps from the dimesylate 1 [7] according to Scheme 1. Mac- rocyclisation of 2 under high dilution with 4,49-bis(chloro- carbonyl)-2,29-bipyridine in the presence of triethylamine [a] Groupe de Chimie Inorganique Mole´culaire, Universite´ Louis Pasteur, UMR 7513 CNRS, 1 rue Blaise Pascal, F-67008 Strasbourg Cedex, France E-mail: armyou@chimie.u-strasbg.fr dmatt@chimie.u-strasbg.fr [b] Department of Chemistry, University of Newcastle, Newcastle upon Tyne NE1 7RU, U.K. Eur. J. Inorg. Chem. 2000, 114721150  WILEY-VCH Verlag GmbH, D-69451 Weinheim, 2000 143421948/00/060621147 $ 17.501.50/0 1147 1,4-benzoquinone in water. Steady-state and time-resolved emission studies have been used to probe electron-transfer processes occurring inside and outside the cavity. afforded the ‘‘exo’’-bipy ligand 3 in ca. 50% yield which, on treatment with the solvent complex [Ru(bipy) 2 (Me 2- CO) 2 ][BF 4 ] 2 , gave an equimolar mixture of the diastereo- meric complexes 4a/4b [8] which was not resolved (see Ex- perimental Section). Unlike ligand 3, these complexes are soluble in water as well as in organic solvents. Both 1 H NMR and 13 C NMR spectroscopy revealed the diastereomeric nature of the ruthenium complexes 4a/4b. Each individual compound retains the C 2 symmetry of the ligand. Complexes 4a/4b undergo chemical shift changes in D 2 O upon addition of varying amounts of 1,4-benzoqui- none (BQ) indicative of bimolecular association (Figure 1). A titration procedure involving the measurement of chem- icals shifts for aqueous solutions containing excess BQ con- firmed the 1:1 stoichiometry of the resultant supramolecu- lar assembly and indicated an association constant (K a ) of 9 ± 3  21 . [9] The CIS [10] (complexation-induced shift on 100% complexation) values obtained from these 1 H NMR spectra are listed in Table 1 for nonoverlapping probes and provide clear indication for the inclusion of BQ into the CD cavity. Amongst the most significant changes are those experienced by the H-3,39 protons (Δδ max 51 0.22, vs. 10.05 for H-5,59 and 10.03 ppm for H-6,69) of the 4,49- dicarbonyl-2,29-bipyridyl fragment, the only 2,29-bipyridyl (bipy) protons that point towards the cavity interior. Inter- estingly, some of the CD MeO-3 protons [10] which also point towards the interior of the cavity are strongly upfield shifted (Δδ max 520.20 and 20.23) whereas others are slightly downfield shifted or remain unchanged (Δδ max 5 10.05 and 20.01) (Figure 1), suggesting that a particular guest orientation within the CD cavity is favoured, perhaps because of some structural distortion imposed by the bipy cap. Furthermore, the chemical shift change found for the H CD -5 protons [11] (Δδ max 510.13) is consistent with me- dium immersion of BQ. [10] Luminescence from the metal complex in 4 is quenched upon addition of BQ but Stern-Volmer plots show pro- nounced positive deviations from linearity without reaching a plateau at high quinone concentrations (Figure 2). This situation arises from a combination of both diffusional and static emission quenching processes, as confirmed by time-