While the general concept of steric speed bumps has been demonstrated in rotaxane shuttles and macrocycle threading systems, the sensitivity of speed bump effects has not been evaluated as a function of structural geometry. Values of K and k for macrocycle threading in water are reported for a series of homologous squaraine dyes with different substituents (speed bumps) on the flanking chains and two macrocycles with different cavity sizes. Sensitivity to a steric speed bump effect was found to depend on (a) structural location, being lowest when the speed bump was near the end of a flanking chain, and (b) macrocycle cavity size, which was enhanced when the cavity was constricted. This new insight is broadly applicable to many types of molecular threading systems.
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