Positive Cooperativity in the Template-Directed Synthesis of Monodisperse Macromolecules

Two series of oligorotaxanes R and R′ that contain −CH2NH2+CH2– recognition sites in their dumbbell components have been synthesized employing template-directed protocols. [24]Crown-8 rings self-assemble by a clipping strategy around each and every recognition site using equimolar amounts of 2,6-pyridinedicarboxaldehyde and tetraethyleneglycol bis(2-aminophenyl) ether to efficiently provide up to a [20]rotaxane. In the R series, the −NH2+– recognition sites are separated by trismethylene bridges, whereas in the R′ series the spacers are p-phenylene linkers. The underpinning idea here is that in the former series, the recognition sites are strategically positioned 3.5 A apart from one another so as to facilitate efficient [π···π] stacking between the aromatic residues in contiguous rings in the rotaxanes and consequently, a discrete rigid and rod-like conformation is realized; these noncovalent interactions are absent in the latter series rendering them conformationally flexible/nondiscrete. Although in th...