Light-switchable Janus [2]rotaxanes based on α-cyclodextrin derivatives bearing two recognition sites linked with oligo(ethylene glycol).

Two Janus [2]rotaxanes, 5a and 5b, with α-cyclodextrin (α-CD) derivatives substituted on the 6-position with two recognition sites (azobenzene and heptamethylene (C7)) that were linked with linkers of different lengths (oligo(ethylene glycol) with a degree of polymerization equal to 2 or approximately 21) were synthesized and characterized. 2D ROESY NMR spectroscopy and circular dichroism (cd) spectra demonstrated that the recognition site of the α-CD moiety was switched by photoisomerization of the azobenzene moiety in 5a and 5b. The different size changes of 5a and 5b in hydrodynamic radius (R(H)) owing to the different length of linker between two recognition sites were observed by pulse-field-gradient spin-echo NMR spectroscopy. The kinetic results indicated that the different length of linker had no or a weak effect for the photoisomerization process of 5a and 5b.