Centrosymmetric and Non-centrosymmetric Packing of Aligned Molecular Fibers in the Solid State Self Assemblies of Cyclodextrin-based Rotaxanes

Two [2]-rotaxanes each comprising α-cyclodextrin as the rotor, and with either 3,3′-difluoro- or 3,3′-dichloro-stilbene as the axle and trinitrophenylamino substituents as the blocking groups at the 4- and 4′-positions, were prepared and their structures analyzed in solution and the solid state using ¹H NMR spectroscopy and X-ray crystallography, respectively. With each rotaxane, in solution the stilbene rotates freely within the cyclodextrin annulus. In the solid state the 3,3′-dichlorostilbene-based rotaxane adopts two very similar conformations, each having the chlorines in the anti,anti-orientation. By comparison, the 3,3′-difluorostilbene-based rotaxane adopts anti,anti-, anti,syn- and syn,syn-orientations of the substituents. The crystal packing of each rotaxane displays aligned molecular fibers, which are centrosymmetrically orientated in the case of the difluoride due to the head-to-head/tail-to-tail alignment of the cyclodextrins. By contrast, all of the cyclodextrins in the dichloride are aligned head-to-tail along a single axis to give a polar, non-centrosymmetric crystal.