Effect of Stopper Size on Squaraine Rotaxane Stability.

A series of new squaraine rotaxanes have been synthesized with a tetralactam macrocycle and stopper groups of varying sizes and functionalities. In chloroform solvent, the relative size of the stopper group appears to have little influence on the high mechanical stability of the rotaxane structure. There is no evidence for unthreading (sometimes referred to as deslipping), even in the presence of competing chloride salts or elevated temperatures. A difference in rotaxane stability emerges as the polarity of the organic solvent is increased. Squaraine rotaxanes with small stopper groups undergo unthreading in the polar aprotic solvent DMSO. However, a water soluble tetracarboxylic acid derivative was found to be highly stable in aqueous solvents containing serum.