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

Abstract

Two[ 2]-rotaxanes each comprising a -cyclodextrin as the rotor, andw ithe ither3 ,3 0 -difluoro-o r3 ,3 0 -dichlorostilbene as the axle and trinitrophenylamino substituents as the blocking groups at the 4-and 4 0 -positions, were prepared and their structures analyzed in solution and the solid state using 1 HN MR spectroscopy and X-ray crystallography, respectively.W ithe achr otaxane,i n solution the stilbene rotates freely within the cyclodextrin annulus. In the solid state the 3,3 0 -dichlorostilbene-based rotaxane adopts two very similar conformations, each having the chlorines in the anti,anti-orientation. By comparison, the3 ,3 0 -difluorostilbene-based rotaxane adopts anti,anti-, anti,syn-a nd 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 as ingle axis to give apolar,non-centrosymmetric crystal.