Diarylethene-Powered Light-Induced Folding of Supramolecular Polymers

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.

Visualizing molecular weights differences in supramolecular polymers

“…Atomic force microscopy (AFM) is another powerful tool that may be used to determine the morphology of self-assembled materials ( 74 ). Thus, [ M1 ·Zn(OTf) 2 ] n was analyzed at different monomer concentrations via tapping-mode AFM ( 75 ). Consistent with the images captured through SEM, microsized particles were observed by AFM over the 0.08 mM to 0.16 mM monomer concentration regime, while fibrous species of increasing size were seen as the monomer concentration was increased past 0.20 mM; this was accompanied by a decrease in the number of visible particles ( Fig.…”

Using transient equilibria (TREQ) to measure the thermodynamics of slowly assembling supramolecular systems

“…Heating curves are typically used together with the concentrationdependent Te approach described above 12,[23][24][25] , although sometimes cooling scans have been employed instead. 11,[20][21][22] Interestingly, in their seminal 2003 review, Mergny and Lacroix point out that "analysis of the concentration dependency of the denaturation profile only is seriously flawed" and urge "great caution about conclusions reached solely by analysis of the heating curves, a recurrent theme in the literature", when thermal hysteresis is present. 10 To gain a clearer picture of the magnitude of the problem, we simulated TH data using GS parameters matching our polyA-CA system at different values of cT and analyzed the resulting concentration dependent Te values.…”

“…A large number of slowly assembling supramolecular systems have been described in the literature, with only a subset referenced in this study. [11][12][13][14][15][16][17][18][19][20][21][22][23][24][25][26][27] This field is expected to expand in the coming years, since slow, nonequilibrium, nucleated assembly is a living polymerization process. The advantages of living polymers in supramolecular chemistry are an area of active research, with benefits already evident in the level of control they give over fibre length and monodispersity.…”

“…Common examples include amyloid fibrils, viral capsids, and a variety of self-assembling non-biological small molecules. [11][12][13][14][15][16][17][18][19][20][21][22][23][24][25][26][27] Interest in these kinds of slowly assembling supramolecular systems has grown in recent years, since they allow the size distributions of the resulting fibres to be tightly controlled. 24,[26][27][28] Current thermodynamic analyses rely on systems reaching equilibrium before the measurement is taken.…”

Fluorescent supramolecular polymers of barbiturate dyes with thiophene-cored twisted π-systems

“…1b ). 10 However, the luminescence efficiency of these soft supramolecular polymers is at most 10% due to the excimer formation of π-conjugated units, which act as the primary interactive cites for the polymerization. In the present study, we addressed one-dimensionally nanostructured luminescent materials by utilizing direct supramolecular polymerization of barbiturates through crystalline tape-like hydrogen-bonding motif.…”