Unidirectional Threading Synthesis of Isomer-Free [2]Rotaxanes

The threading of an α‐cyclodextrin (α‐CyD) by an unsymmetrical dumbbell generally results in two isomeric [2]rotaxanes differing in the orientation of the α‐CyD. In this work, two methods have been developed for the unidirectionally threading an α‐CyD to obtain isomer‐free [2]rotaxanes. These methods use the Suzuki coupling of a boronic acid derivative and a halide in aqueous alkaline solution. The conformations of the two unidirectional [2]rotaxanes‐R3 and R4 were determined by 2D 1H ROESY NMR spectra. The optical spectral studies revealed that each of the two [2]rotaxanes can proceed with E/Z photoisomerization and shuttling motions of the α‐CyD ring on the thread under alternating irradiation at 330 and 275 nm, accompanied by fluorescence intensity changes at 530 nm. The induced circular dichroism (ICD) spectra of another two analogous [2]rotaxanes R1 and R2 were also studied. Distinctive ICD signal changes resulting from the photoisomerization with respect to the movements of α‐CyD were detected. This demonstrates that, besides the fluorescence, ICD signal is another way to identify the shuttling motions of α‐CyD in these [2]rotaxanes. Switch to shuttle: [2]Rotaxane‐based molecular shuttles were synthesized by two different unidirectional threading approaches of α‐cyclodextrin (see scheme). The relative switching processes of these shuttles were also investigated.