Using Photoresponsive End-Closing and End-Opening Reactions for the Synthesis and Disassembly of [2]Rotaxanes: Implications for Dynamic Covalent Chemistry

We have synthesized two [2]rotaxanes, each possessing a (Z)-alpha-methylstilbene unit as one of its stoppers, in good yield through the photoisomerization of terminal (E)-alpha-methylstilbene units of dialkylammonium sails in the presence of the crown ether dibenzo[24]crown-8 (DB24C8). The synthesis relies on the formation of pseudorotaxane intermediates through hydrogen bond-guided self-assembly and subsequent end-closing photoisomerization. An (E)-alpha-methylstilbene unit is not sufficiently bulky to prevent dissociation of the DB24C8 unit, whereas a (Z)-alpha-methylstilbene unit acts as a true stopper. We also synthesized these [2]rotaxanes from the (Z)-alpha-methylstilbene-teriminated axle-like salts though thermodynamic covalent chemistry by taking advantage of the reversibility of the photoisomerization. To dissociate the components of the [2]rotaxanes, we performed the reverse end-opening process under UV irradiation (i.e., Z-to-E isomerization of the alpha-methylstilbene termini) in a polar solvent. These rotaxanes are stable at room temperature, but dissociate slowly to their two components at elevated temperatures.