Up and down the dumbbell: Photoisomerization of a stilbene rotaxane results in relocation of the cyclodextrin macrocycle in just one direction (see scheme). This behavior, and that of related rotaxanes, sheds light on the mechanism of shuttling in molecular machines.

Highly fluorescent stilbene and tolan cyclodextrin [2]rotaxanes have been synthesised in good yield using aqueous Suzuki coupling, and the crystal structure of one of these rotaxanes has been determined.A rotaxane is a supramolecular assembly of a dumbbell locked through the cavity of a macrocycle. 1 The formation of rotaxanes provides a means of stabilising dumbbell-shaped chromophores, by shielding them from the external environment. This type of encapsulation can also enhance the fluorescence efficiency. 2 Hydrophobic binding is a convenient way of directing rotaxane formation, provided the dumbbell can be synthesised in water. Recently we reported the synthesis of poly-p-phenylene rotaxanes and polyrotaxanes using aqueous Suzuki coupling, although in this case the [2]rotaxane (1a in Table 1) was only obtained in low yield (4%). 3 While exploring the scope of this route to rotaxanes, we discovered that the right combinations of aryl iodide stopper, diboronic acid core and macrocycle components give highly fluorescent cyclodextrin encapsulated stilbenes and tolans, such as 1g, in high yield. Previous stilbene rotaxanes have been prepared by aromatic nucleophilic substitution 4 and by slipping macrocycles over pre-formed dumbbells. 5 Six new rotaxanes 1b-g have been prepared by reacting bulky water-soluble aryl iodides 2a and 2b with diboronic acids 3a-c 6 in the presence of cyclodextrins (a-CD and b-CD; ca. 5 eq.), in aq. sodium carbonate containing palladium(II) acetate, as summarised in Scheme 1 and Table 1. † As expected, the 5-iodoisophthalic acid stopper 2b is too narrow to form rotaxanes with b-CD, and the biphenyl diboronic acid core 3a is too bulky to form rotaxanes with a-CD. Apart from these exceptions, all combinations of aryl iodides, diboronic acids and macrocycles yield rotaxanes. The lower yields obtained with the 1-iodonaphthalene-3,6-disulfonate stopper 2a can be attributed to an unfavourable interaction between the bound CD and the inwardly pointing H8 of the naphthalene. The tetracarboxylate Table 1 Yields for rotaxane synthesis, and fluorescence behaviour of rotaxanes and dumbbells a Scheme 1 This journal is

Die Hantel rauf und runter: Der Cyclodextrin‐Makrocyclus bewegt sich bei Photoisomerisierung des Stilben‐Rotaxans in genau eine Richtung (siehe Schema). Mit diesem Verhalten ermöglichen die beschriebenen und ähnliche Rotaxane Rückschlüsse auf Mechanismen des Hin‐und‐Herbewegens bei molekularen Maschinen.