Photoswitchable Dynamic Combinatorial Libraries: Coupling Azobenzene Photoisomerization with Hydrazone Exchange

The novel azobenzene-based monomer 1 was prepared, equipped with the necessary functionality to undergo simultaneous dynamic exchange processes: hydrazone exchange and photoisomerization. Acid-promoted hydrolysis of the azobenzene building block produced a dynamic combinatorial library of cyclic oligomers, while multibuilding block libraries were also generated upon addition of proline-based monomers. Libraries equilibrated under thermal conditions were dominated by trans isomers of the azobenzene macrocycles, whereas light-induced isomerization resulted in a conformational change of the library members to their corresponding cis-azo form. In the presence of a pentaproline template, a slower rate of thermal relaxation of the cis-azobenzene species 1(c) was observed, resulting in stabilization and amplification of this receptor due to favorable binding interactions. The facile identification and application of such photoswitchable receptors have the potential to allow for greater control over molecular recognition events.