Light and protonation-controlled complex formation between sulfate ions and a stiff-stilbene based bis(cyclopeptide)

Abstract

Anion–ligand coordination has been used to generate a number of supramolecular structures. Of particular interest is the transformation between different types of complexes using various stimuli. While there are multiple examples where this has been achieved with metal–ligand coordination complexes through incorporation of molecular photoswitches, the same has not yet been realized with anion–ligand coordination-driven assemblies. In this study, a sulfate-binding bis(cyclopeptide) with a photoswitchable stiff-stilbene linker is presented. Its (E)- and (Z)-isomers, and the different degrees of protonation of the anion (HSO₄⁻vs. SO₄²⁻), give rise to different assembly states. The accessible products have 1 : 1, 1 : 2 and 2 : 2 host–guest stoichiometries and can be interconverted by light irradiation and acid/base addition, resulting in a highly controllable responsive system that demonstrates the potential of sulfate coordination-driven supramolecular assembly.