CONSTITUTIONALLY ASYMMETRIC AND CHIRAL [2]PSEUDOROTAXANES

The self-assembly and characterization of a range of chiral pseudorota xanes has been explored using chiroptical methods. The syntheses of (i ) constitutionally asymmetric acyclic hydroquinone-containing polyethe rs and (ii) optically active hydroquinone-containing acyclic polyether s, bearing pairs of methyl or isobutyl groups related to each other in a C-2-symmetric manner within the polyether backbone, are described. The combination of (i) the tetracationic cyclophane cyclobis(paraquat- p-phenylene) tetrakis(hexafluorophosphate), possessing a pi-electron d eficient cavity, and (ii) the linear noncentrosymmetric acyclic polyet hers produces [2]pseudorotaxanes that have been characterized by H-1 N MR, UV/vis and circular dichroism (CD) spectroscopies in solution and by X-ray crystallography in the solid state. The introduction of const itutional asymmetry or chirality gives rise to a number of different g eometries for the [2]pseudorotaxanes in both the solution and solid st ates. in particular, CD-spectroscopic measurements on the optically ac tive [2]pseudorotaxanes have shown that-depending on the positions of the C-2 symmetrically related chiral centers in the polyether chains w ith respect to the hydroquinone rings-the chirality present in the pi- electron rich threadrike guest can induce chirality that is associated with the supramolecular structure as a whole, resulting in a chiral c harge-transfer transition involving not only the pi-donors in the chir al guests but also the pi-acceptors in the achiral host.