ABSTRACT
Structural rigidity and the preorganization of thread binding sites are shown to have a major influence on template efficiency in the synthesis of hydrogen bond-assembled rotaxanes. Preorganization is so effective, in fact, that with good hydrogen bond acceptors (amides) a "world record" yield of 97% for a [2]rotaxane is obtained. The truly remarkable feature of this efficient template, however, is that it allows even poor hydrogen bond acceptors (esters) to be used to prepare hydrogen bond-assembled rotaxanes, despite the presence of competing hydrogen bonding groups (anions) which bind the key intermediates at least 10000x more strongly than single, unorganized, ester groups! The structures of the rotaxanes are established unambiguously in solution by (1)H NMR spectroscopy and in the solid state by X-ray crystallography. As a series they provide unique experimental information regarding the nature of amide-ester hydrogen bonding interactions; in particular they suggest that in CDCl(3), amide-ester NH...O=C hydrogen bonds are approximately 1 kcal mol(-)(1) weaker than the corresponding amide-amide interactions.
ABSTRACT
A rotaxane is described in which a macrocycle moves reversibly between two hydrogen-bonding stations after a nanosecond laser pulse. Observation of transient changes in the optical absorption spectrum after photoexcitation allows direct quantitative monitoring of the submolecular translational process. The rate of shuttling was determined and the influence of the surrounding medium was studied: At room temperature in acetonitrile, the photoinduced movement of the macrocycle to the second station takes about 1 microsecond and, after charge recombination (about 100 microseconds), the macrocycle shuttles back to its original position. The process is reversible and cyclable and has properties characteristic of an energy-driven piston.
