No abstract

Helical folding of randomly coiled linear polymers is an essential organization process not only for biological polypeptides but also for synthetic functional polymers. Realization of this dynamic process in supramolecular polymers (SPs) is, however, a formidable challenge because of their inherent lability of main chains upon changing an external environment that can drive the folding process (e.g., solvent, concentration, and temperature). We herein report a photoinduced reversible folding/unfolding of rosette-based SPs driven by photoisomerization of a diarylethene (DAE). Temperature-controlled supramolecular polymerization of a barbiturate-functionalized DAE (open isomer) in nonpolar solvent results in the formation of intrinsically curved, but randomly coiled, SPs due to the presence of defects. Irradiation of the randomly coiled SPs with UV light causes efficient ring-closure reaction of the DAE moieties, which induces helical folding of the randomly coiled structures into helicoidal ones, as evidenced by atomic force microscopy and small-angle X-ray scattering. The helical folding is driven by internal structure ordering of the SP fiber that repairs the defects and interloop interaction occurring only for the resulting helicoidal structure. In contrast, direct supramolecular polymerization of the ring-closed DAE monomers by temperature control affords linearly extended ribbon-like SPs lacking intrinsic curvature that are thermodynamically less stable compared to the helicoidal SPs. The finding represents an important concept applicable to other SP systems; that is, postpolymerization (photo)reaction of preorganized kinetic structures can lead to more thermodynamically stable structures that are inaccessible directly through temperature-controlled protocols.

Supramolecular polymers have emerged in the last decade as highly accessible polymeric nanomaterials. An important step toward finely designed nanomaterials with versatile functions, such as those of natural proteins, is intricate topological control over their main chains. Herein, we report the facile one-shot preparation of supramolecular copolymers involving segregated secondary structures. By cooling non-polar solutions containing two monomers that individually afford helically folded and linearly extended secondary structures, we obtain unique nanofibers with coexisting distinct secondary structures. A spectroscopic analysis of the formation process of such topologically chimeric fibers reveals that the monomer composition varies gradually during the polymerization due to the formation of heteromeric hydrogen-bonded intermediates. We further demonstrate the folding of these chimeric fibers by light-induced deformation of the linearly extended segments.

Beyond phenomenon, self‐assembly of synthetic molecules, is now becoming an essential tool to design supramolecular materials not only in the thermodynamically stable state but also in kinetically trapped states. However, an approach to design complex self‐assembly processes comprising different types of self‐assembled states remains elusive. Herein, an example of such systems is demonstrated based on a unique supramolecular polymer mediated by supermacrocyclization of hydrogen‐bonding π‐conjugated molecules. By adding an aromatic solvent into nonpolar solutions of the monomer, spontaneous nucleation triggered by supermacrocyclization was suppressed so that isothermal supramolecular polymerization could be achieved from kinetically formed topological variants and amorphous agglomerates to afford helicoidal structures hitherto obtainable only with very slow cooling of a hot solution. By increasing the proportion of aromatic solvent further, another self‐assembly path was found, based on competing extended hydrogen‐bonded motifs affording crystalline nanowires.

Supramolecular polymerization of two regioisomeric naphthalene-azobenzene dyads bearing barbituric acid and tri(dodecyloxy)phenyl units were studied in nonpolar solvent. Supramolecular polymers of the two compounds differ considerably in their topologies. While the isomer of which azobenzene unit was introduced along to its longer molecular axis formed randomly coiled supramolecular polymers with intrinsic curvature, the other of which azobenzene unit was introduced along to its shorter molecular axis formed linearly extended supramolecular polymers. Furthermore, due to their different degrees of geometrical changes accompanied with photoisomerization of the azobenzene unit, only the former showed a large topological change upon photo-irradiation. The different structural and photoresponsive properties can be attributed to the distinct geometries of supermacrocyclic hydrogen-bonded intermediates (rosettes).