Using the Rotaxane Mechanical Bond to Enhance Chemical Reactivity

Rates of cycloreversion for squaraine rotaxane mono(endoperoxides) were enhanced by structural modifications that increased cross-component steric destabilization of the inward directed 9,10-anthracene endoperoxide group. The largest rate enhancements were obtained when the surrounding macrocycle contained two 2,6-pyridine dicarboxamide bridging units, which induced a cavity contraction effect. The precursor fluorescent, near-IR, squaraine rotaxanes are effectively photostable because the mono(endoperoxide) products, formed by reaction with photogenerated singlet oxygen, rapidly cyclorevert back to the original squaraine rotaxane.