Effect of 1,3-adamantane bridging units within the surrounding macrocycle of squaraine rotaxanes

Abstract

A systematic comparison of empty tetralactam macrocycles containing 1,3-benzenedicarboxamide, 2,6-pyridinedicarboxamide, and 1,3-adamantanedicarboxamide bridging units finds that macrocycles with adamantyl bridging units exhibit weaker non-covalent affinity for an encapsulated guest. In the case of interlocked squaraine rotaxanes with macrocycles containing phenylene sidewalls, the structural change induced by the adamantyl bridging units produces a more loosely held rotaxane co-conformation that diminishes the ability of the surrounding macrocycle to protect the encapsulated squaraine dye from attack by nucleophiles. In the case of squaraine rotaxanes with macrocycles containing anthracene sidewalls, there is no obvious change in rotaxane co-conformation. But there is a difference with the corresponding squaraine rotaxane endoperoxide, namely a significantly slower rate of cycloreversion and oxygen release. A series of molecular dynamics simulations provides reasons for the differences in co-conformational mobility. The overall result is a new set of structural strategies to control the chemical and photophysical properties of luminescent squaraine rotaxanes.