The structural dynamics of two pairs of [2]rotaxanes were compared using variable-temperature NMR. Each rotaxane had a surrounding tetralactam macrocycle with either 2,6-pyridine dicarboxamide or isophthalamide bridging units. Differences were observed in two types of rotational processes: spinning of the phenylene wall units in the surrounding macrocycle of squaraine rotaxanes and macrocycle pirouetting in xanthone rotaxanes. The rotaxanes with macrocycles containing 2,6-pyridine dicarboxamide bridges exhibited higher rotational barriers due to a cavity contraction effect, which disfavored macrocycle breathing.

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.

Squaraine rotaxanes with macrocycles containing 1,3-adamantanedicarboxamide bridging units have altered chemical and luminescent properties due to a more loosely held rotaxane co-conformation.

No abstract