Visible-light driven multistate assembly of hydrogen bonding foldamers

Supramolecular polymers offer tremendous potential to produce new “smart” materials however, there remains a need to develop multistate systems that are responsive to external stimuli. In this work visible-light responsive hydrogen-bonded supramolecular polymers have been successfully constructed using photoresponsive foldamers consisting of two hydrogen bonding motifs (HBMs) connected by a central ortho-tetrafluorinated azobenzene as characterized by DOSY NMR and viscometry. Assembly is strongly influenced by the strength of association between the HBMs Foldamer I, incorporating a triply hydrogen-bonded heterodimer, was found to switch between a monomeric pseudo-cycle and supramolecular oligomer through intermolecular hydrogen bonding interactions between the HBMs. Foldamer II, based on the same photoresponsive scaffold, and incorporating a quadruply hydrogen-bonded homodimer was found to form a supramolecular polymer which was dependent upon the ring-chain equilibrium and thus dependent upon both concentration and photochemical stimulus. Finally, foldamer III, incorporating a quadruply hydrogen-bonded heterodimer represents the first photoswitchable AB type hydrogen-bonded supramolecular polymer. Depending on the concentration and photostationary state, four different assemblies dominate for both foldamers II and III, demonstrating for the first time the ability to achieve multistate supramolecular assembly triggered by light. Importantly, these molecular transitions also resulted in physical changes to the solution whereby the increased molecular weight conferred a measurable increase in viscosity. This latter observations has significant implications with respect to development of advanced responsive materials.