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

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 (<i>E</i>)- and (<i>Z</i>)-isomers, and the different degrees of protonation of the anion (HSO₄ ^- <i>vs.</i> SO₄ ^2-), 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.