Supramolecular Polymers Capable of ... - American Chemical Society

Article Cite This: Acc. Chem. Res. XXXX, XXX, XXX−XXX

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Supramolecular Polymers Capable of Controlling Their Topology Shiki Yagai,*,†,§ Yuichi Kitamoto,† Sougata Datta,† and Bimalendu Adhikari‡ †

Institute for Global Prominent Research (IGPR), Chiba University, 1-33 Yayoi-cho, Inage-ku, Chiba 263-8522, Japan Department of Applied Chemistry and Biotechnology, Graduate School of Engineering, Chiba University, 1-33 Yayoi-cho, Inage-ku, Chiba 263-8522, Japan ‡ Department of Chemistry, Deen Dayal Upadhyay Gorakhpur University, Gorakhpur, Uttar Pradesh 273009, India

Acc. Chem. Res. Downloaded from pubs.acs.org by WEBSTER UNIV on 02/21/19. For personal use only.

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CONSPECTUS: One important class of supramolecular materials is one-dimensionally elongated supramolecular polymers, in which monomers are associated by reversible intermolecular interactions, yielding a fibrous morphology. Unlike frequently reported conventional supramolecular polymers based on, for instance, host−guest interactions, those composed of one-dimensionally stacked π-conjugated molecules can be encoded with high degrees of internal order by cooperative association of the rigid aromatic monomers, endowing such supramolecular polymers with extraordinary properties and functionality. However, their internal order has not yet been exploited to manipulate the complex landscape of well-defined states of the supramolecular polymer backbone, which may induce new functionalities beyond the intrinsic properties of the backbones. This Account will focus on the inceptive phase of our research on supramolecular polymers with high degrees of internal order able to impart intrinsic curvature to their backbones. Initially, we developed a naphthalene molecule functionalized with barbituric acid, which forms uniform toroidal short fibers with diameters of approximately 16 nm via the formation of hydrogenbonded cyclic hexamers (rosettes). As we thought the uniformity of the toroid size to arise from the intrinsic curvature generated upon stacking of the rosettes, we exploited this intrinsic curvature to design continuously curved extended supramolecular polymers by extension of such molecular π-systems. The intrinsic curvature produced by the monomers with more expanded π-systems indeed gave us access to higher-order structures (topologies) ranging from randomly folded to helically folded coils in extended supramolecular polymers. We will discuss the kinetic aspects of the generation of intrinsic curvature for topology control, including the formation of toroidal structures resulting from ring-closing processes. For extended supramolecular polymers with well-defined topologies, we will discuss manipulation of a complex landscape of well-defined states by external stimuli. The incorporation of a photoresponsive azobenzene chromophore in the original naphthalene molecular scaffold allowed us to reversibly destroy or recover the curvature of the main chain through trans−cis photoisomerization. By means of this photocontrollable curvature, we have demonstrated light-induced unfolding of helically folded structures into entirely stretched structures. Furthermore, a direct extension of the π-conjugated core provided us with access to unprecedented supramolecular polymers with emergent time-dependent topology transitions. Molecules with a naphthalene core conjugated with two phenylene units kinetically afforded supramolecular polymers that consist of helically folded and misfolded domains. Upon aging the supramolecular polymer solution, we observed spontaneous folding of the misfolded domains in a time scale of days, eventually obtaining a supramolecular polymer topology analogous to the tertiary structure of proteins. These supramolecular polymers with unrivaled and active topologies provide new prospects for supramolecular polymers as one-dimensional nanomaterials.

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INTRODUCTION Supramolecular polymers (SPs), which represent the most prominent contribution of supramolecular chemistry to polymer science, have attracted remarkable interest over the past few decades owing to their unique physical properties that arise from the reversibility of their main-chain formation.1−3 In particular, the past decade has witnessed a remarkable gain in knowledge on the formation process of SPs through mechanistic and kinetic studies,4−6 which has enabled researchers to prepare SPs with controlled chain lengths and molecular-weight distributions.7−10 Despite such significant © XXXX American Chemical Society

advances, SPs have still not attained the level of complexity of polymers in terms of formation of higher-order structures (Figure 1a). In biological polymers such as polypeptides, higher-order structures are hierarchically organized by intrachain and interchain interactions between the covalently tethered amino-acid backbones, which in turn exert a significant impact on the physical properties and functions of the resulting proteins. In fact, control over the topological Received: December 25, 2018

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DOI: 10.1021/acs.accounts.8b00660 Acc. Chem. Res. XXXX, XXX, XXX−XXX

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Accounts of Chemical Research

Figure 1. Topological features of (a) conventional SPs and (b) SPs composed of rosettes of barbiturate aromatics.

strategically created SPs with higher-order secondary and tertiary structures by designing and redesigning the monomer molecules and adopting a suitable polymerization method through in-depth understanding of the supramolecular polymerization process and kinetics.

features of synthetic polymers has led to new properties and functions, which is one of the current trends in polymer research.11−13 In SPs, as the main chain itself is formed by noncovalent bridging of monomeric units, further control over higher-order structures of the main chain by noncovalent interactions remains challenging. Another important feature of SPs in addition to main-chain reversibility is good compatibility with functional π-conjugated molecules that may impart specific optical and electronic functions.14 Namely, π−π-stacking interactions can be used as the major driving force for the formation of the main chain. As π−π-stacking interactions between simple aromatics are not strong enough for the formation of “polymeric” assemblies,15 the use of a combination of multiple interactions such as π−πstacking plus hydrogen-bonding interactions is often required.16−18 These additional interactive sites can be introduced in such a way that π−π-stacks are stabilized directly by additional noncovalent interactions (i.e., in a direction parallel to the stacking axis)19,20 or indirectly through the noncovalent expansion of the π-surface (i.e., in a direction orthogonal to the stacking axis).16,18 In any case, π−π-staked molecules are dynamic in terms of conformational motion as well as rotational and/or translational movement between stacks, while high degrees of internal order, i.e., organization of monomers in the assembly, arise from the cooperative association of these functionalized aromatic monomers.21 These unique features endow SPs with extraordinary properties and functionality.21 However, this high degree of internal order has not yet been exploited to generate and dynamically control well-defined states in the landscape of the supramolecular polymer backbone, which may give rise to functionalities beyond the intrinsic properties of the backbone. In this Account, we describe our recent endeavors to introduce and control higher-order topological complexity in SPs for the development of advanced materials (Figure 1b). On the basis of serendipitously obtained circular SPs, we have

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CYCLIZED SUPRAMOLECULAR POLYMERS We will first explain the concept of “cyclized SPs” to avoid any confusion on the behalf of the readers. If a monomer is adequately flexible or rigid but geometrically constrained, cyclic “oligomers” are often formed preferentially at low concentrations as a result of enthalpic or kinetic reasons, thus hindering the formation of extended SPs.22,23 As the monomer concentration is increased to a critical concentration, defined as the effective molarity (EM), which is determined by the ratio of the rate constants of interchain and intrachain association,24,25 supramolecular polymerization involving interchain interactions predominates over cyclic oligomerization. In this sense, cyclic oligomers that consist of several monomers cannot be considered as “cyclized SPs” in terms of molecular weight. The “cyclized SPs” in this Account refer to relatively large aggregates (with degree of polymerization comparable to those of a polymer) that present a closed structure where two ends are linked. For example, Lee and co-workers have reported the formation of toroidal nanoaggregates in aqueous media via the supramolecular polymerization of amphiphilic πconjugated molecules.26 In 2008, we serendipitously discovered that discotic supermacrocyclic assemblies that bear πconjugated units, formed by complementary hydrogen bonding between melamine and cyanuric acid derivatives, stack to form similar toroidal nanoaggregates in nonpolar solvents.27 In this report, we did not refer to these toroidal nanoaggregates as cyclized SPs. However, this result essentially suggests that SPs with lengths of around 100 nm (corresponding to the circumference of the toroids) cyclize into discrete toroidal nanostructures. Although such short aggregates might be B


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Figure 2. (a,b) Schematic representation of the supramolecular polymerization of naphthalene-based barbiturates (a) 1 and (b) 2 via the formation of rosettes together with atomic force microscopy (AFM) images of the SPs. (c) Schematic representation of the time-dependent self-sorting of 1 and 2 using AFM images. In the AFM image of the self-sorted state (right), the contrast of the upper and lower halves is focused on the linear and toroidal SPs, respectively. Adapted with permission from refs 31 and 32. Copyright 2012 Wiley-VCH and 2016 The Royal Society of Chemistry.

naphthalene moieties for the SPs of 1 and 2, respectively.31 Reflecting dramatically different self-assembly modes, mixtures of 1 and 2 ultimately afforded self-sorted linear and cyclized SPs, although at the initial stage, the two components kinetically coassemble via formation of heteromeric rosettes (Figure 2c).32 Structural analyses in the liquid crystalline state revealed that 1 and 2 organize into hexagonal and rectangular columnar lattices, respectively, by formation of hexameric structures.31 Hence, we postulated the formation of hydrogen-bonded hexameric supermacrocycles (rosettes) by barbituric acid units (Figure 2a,b). Although organic compounds that contain a barbituric acid unit had not yet been reported to afford rosette motifs, we confirmed the formation of rosettes from a series of barbituric acid derivatives conjugated with oligo(alkylthiophene) groups by scanning tunneling microscopy (STM) studies.33,34 We then structurally optimized the rosettes of 1 and 2 by molecular mechanics calculations and discovered an important geometrical difference between the rosettes that may provide a rational explanation for the topological differences in their respective SPs. As a result of significant intramolecular steric hindrance, the naphthalene rings of 2 adopt a twisted geometry in the rosette, exhibiting a watermill-like structure (Figure 2b). This allows stacking of the rosettes with only rotational displacement to generate a rigid columnar structure with internal helical ordering. Twodimensional wide-angle X-ray diffraction analysis of the oriented thin film of 2 corroborated this model.35 In contrast, a relatively lower degree of twisting of the naphthalene rings

outside the boundaries of the definition for SPs by Meijer et al. in their seminal review,1 here we would like to call them SPs according to the general definition of polymers (>100 monomers), as they contain several hundreds of discotic assemblies (equivalent to thousands of monomers). The structural similarity of barbituric acid to cyanuric acid28,29 led us to prepare various π-conjugated compounds that contain barbituric acid units, which are also capable of forming rosettes by complementary hydrogen bonding with melamine derivatives. Despite the significant stability of these π-conjugated barbiturates in common organic solvents, their mixtures with melamine derivatives resulted in the elimination of the barbituric acid group to produce aromatic aldehyde precursors within a day via a retro-Knoevenagel reaction. Later on, we surprisingly found that some barbiturates self-assemble as a single component in nonpolar solvents, such as methylcyclohexane (MCH), leading to nanoaggregates with unprecedented topological features.30,31 For instance, when a hot MCH solution of naphthalene barbiturate 1 was cooled naturally to room temperature (∼15 °C/min), cyclized SPs were obtained in >90% yield relative to the monomer (Figure 2a). These cyclized structures were rather uniform in diameter (∼20 nm), suggesting the presence of “intrinsic curvature” (so far, we have used the term “spontaneous curvature”) generated by the supramolecular polymerization of 1. In contrast, the regioisomeric naphthalene barbiturate 2 furnished linearly extended SPs under the same conditions (Figure 2b).31 Furthermore, spectroscopic results suggested different stacking modes for 1 and 2, i.e., J- and H-type stacking of the C


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Accounts of Chemical Research was observed for the rosettes of 1, which exhibit a windmilllike structure (Figure 2a). Accordingly, the rosettes stack with not only rotational but also translational displacement, which may be the origin of the intrinsic curvature upon supramolecular polymerization of 1 (Figure 2a). The unique stacking mode of the rosettes of 1 was further corroborated by combined small-angle neutron scattering (SANS) and X-ray scattering (SAXS) analyses of the internal structure of the cyclized SPs in methylcyclohexane-d14,36 in which the SAXS signals provide information on regions of high electron density (π-conjugated core), whereas the SANS data highlight the 1Hrich regions of the assemblies (aliphatic shell) in the d-rich solvent (Figure 3).

Figure 4. Self-assembly pathways from curvature-generating rosettes to different SPs.

monomers and ∼140 based on the rosettes. An analysis of the results obtained from absorption spectroscopy suggested that the supramolecular polymerization of 1 follows a cooperative mechanism involving nucleation and elongation processes, where the nucleus consists of approximately 20 molecules. Therefore, only a few rosettes (ca. four rosettes) are involved in the formation of the nucleus, which subsequently elongates into circular SPs. Upon careful observation of the AFM image of 1 (Figure 2), we also found a few open-ended curved SP nanofibers, suggesting two possible pathways in the elongation process: one is a kinetically controlled cyclization pathway, responsible for the formation of nanorings, while the other is an extension pathway, leading to curved open-ended SPs (Figure 4). Under kinetic conditions, for instance, by allowing a hot solution to cool down to room temperature either spontaneously or rapidly using an ice bath (quenching), cyclized SPs can be kinetically trapped due to such fast cooling rates. Accordingly, the application of conditions under which the generated SPs can exchange with monomeric species may provide access to extended SPs if such species are thermodynamically more stable. However, when a hot MCH solution of 1 was cooled at a rate of 0.1 °C min −1 (thermodynamically favorable conditions), cyclized SPs were again obtained as the predominant species. The thermodynamic analysis of 1 by means of temperature-dependent absorption experiments revealed a DOP of approximately 270 for 1 at a concentration of 100 μM, which is on the same order with the value obtained from the average circumference of the nanorings visualized by AFM (800). Therefore, 1 is inherently not capable of forming extended SPs because of its low DOP. In other words, the cyclized SPs of 1 are thermodynamic products under the applied conditions. Theoretically, the DOP could be enhanced by increasing the monomer concentration.15 However, the cyclized structures were found to be the predominant species for 1 even in the millimolar regime. This result indicates that higher monomer concentrations increase simultaneously the frequency of nucleation, ultimately nullifying the effect of the concentration on the DOP.

Figure 3. (a) Schematic representation and AFM image of the formation of cyclized SPs from the rosette of 1 with core (orange)− shell (white) morphology. (b) SANS (red triangles) and SAXS (open circles) data for 1 in methylcyclohexane-d14. Blue solid lines indicate the fit to the data, using a toroid form factor and flat background. Adapted with permission from ref 36. Copyright 2016 Wiley-VCH.

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KINETIC CONSIDERATIONS OF THE CYCLIZATIONS The generation of intrinsic curvature upon stacking of the rosettes of 1 endows the resulting SPs with a fascinating toroidal topology. If we can prevent the supramolecular polymer chain from closing into a toroidal structure, a helically folded supramolecular polymer could be expected upon elongation in the same turning direction of the intrinsic curvature (Figure 4), while random changes in the turning direction of the curvature, caused by some defects in the unidirectional stacking of rosettes, would lead to a randomly coiled structure (Figure 4). The cross-sectional diameter of cyclized SPs of 1 is approximately 16 nm, which suggests a degree of polymerization (DOP) of ∼840 based on the D


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Figure 5. (a) Chemical structure of azobenzene-containing 3. (b−d) AFM images of (b) randomly coiled, (c) linearly stretched, and (d) spirally folded SPs of 3 obtained from applying different cooling rates from an MCH solution. Adapted with permission from ref 37. Copyright 2017 Nature Publishing Group.

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THERMODYNAMIC APPROACH TOWARD ELONGATED SPS On the basis of the above discussion, we should in principle be able to obtain extended SPs by employing suitable monomers whose DOP is much higher than that of 1 at the same concentrations.37 Since the DOP depends on the binding (stacking) strength between rosettes, we synthesized a variety of monomers by grafting or extending π-conjugated units without compromising the 2,6-disubstituted naphthalene scaffold responsible for the generation of the intrinsic curvature during supramolecular polymerization. Among the monomers synthesized based on a grafting approach, azobenzene-grafted 3 provided unprecedented SPs in terms of not only extended higher-order topologies but also control over these topologies by external stimuli such as light (Figure 5a). Upon naturally cooling an MCH solution of 3 from 100 °C to room temperature (∼15 °C min−1), randomly coiled elongated SPs with a constant curvature were obtained (Figure 5b). Despite such a significant topological difference, the degree of curvature of 3 is similar to that of 1, suggesting that the grafted azobenzene has no detrimental effect on the specific stacking arrangement of rosettes responsible for the curvature. Temperature-dependent absorption measurements revealed that the elongation enthalpy associated with the supramolecular polymerization of 3 (ΔHe = −108 kJ mol−1) is much higher than that of 1 (ΔHe = −83.4 kJ mol−1). This indicates that the introduction of an azobenzene group as an additional π-surface in the monomer promotes greater stacking and hence enhanced binding strength of the rosettes. Interestingly, when the hot MCH solution of 3 was quenched by abruptly cooling from 90 to 20 °C using an ice bath (>100 °C min−1), we obtained stretched fibers devoid of intrinsic curvature (Figure 5b). These stretched fibers share the same cross-sectional width (9.7 ± 0.3 nm) and thickness (2.7 ± 0.2

nm) of the main chains, suggesting that they stem from an identical self-assembly process of 3. The stretched fibers completely transformed into randomly coiled SPs when the quenched solution was kept for 24 h at 20 °C. This spontaneous generation of curvature more clearly demonstrates the existence of intrinsic curvature. The AFM image of the randomly coiled fibers showed meandering main chains resulting from turning of many adjacent curvature domains in opposite directions where we can consider the presence of some defects in the internal order responsible for the curvature (Figure 5b). The turning direction of the curvatures is directly determined by the direction of the “rotational displacement” between rosettes (Figure 4), which is the key factor determining the topology of the SPs. In order to form a helically folded structure, accordingly, the rosettes must rotate consistently in a uniform direction during the elongation of the SP (Figure 4). This can be realized under more thermodynamic supramolecular polymerization conditions, e.g., much slower cooling, given that such long-distance uniform molecular packing attained under thermodynamic conditions maximizes the enthalpic gain. Moreover, the intermolecular forces along to the helix axis (interactions between the helical pitches) provide additional thermodynamic stabilization for helically folded structures. We thus cooled a heated MCH solution of 3 at a very slow rate (0.1 °C min−1), which provided SPs with spirally folded domains (Figure 5d). Although the random-to-spiral topology transition suggests that slow cooling is favorable to maintain the same turning direction of curvatures at longer distances, more thermodynamically favorable conditions were necessary to further extend such domains unidirectionally to obtain helically folded structures. Since slower cooling (e.g., 0.01 °C min−1) is impractical, we therefore changed the solvent system from MCH to MCH/chloroform (CHCl3), wherein the polar CHCl3 acts as a good solvent that may E


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Figure 6. (a−d) AFM images capturing the unfolding process of helically folded SPs of 3 upon exposure to UV irradiation. (e) Molecular models of trans-3 and cis-3. (f,g) Molecular models of rosettes composed of only trans-3 (f) and five trans-3 and one cis-3 (g). (h) Schematic representation of the photoreversible generation of the curvature. Adapted with permission from ref 37. Copyright 2017 Nature Publishing Group.

molecules, thus inducing a direct conformational change of the main chain. This direct mechanism is difficult to verify, although it may occur in other photoresponsive SPs. In our study, we ruled out the indirect mechanism by mixing separately prepared SPs with and without curvature. When an MCH solution of spirally folded SPs was mixed with fully unfolded SPs, obtained upon exposure to UV irradiation, and kept for a few days, the coexistence of folded and unfolded SPs was observed without averaging out of their structures. When a cis-3-rich CHCl3 solution was injected into an MCH solution of spirally folded SPs, a similar result was obtained. These results indicate an insignificant monomer exchange rate at a practical time scale. This excludes the role of monomer exchange or reorganization in the mechanism of the unfolding of helically folded SPs. Hence, the change in the internal order of the main chain owing to the trans-to-cis isomerization of azobenzene (Figure 6e) is responsible for the UV-induced unfolding of SPs. It is worth noting that the isomerization of only 30% of trans-azobenzene is sufficient for complete loss of the curvature. This isomerization ratio corresponds to the presence of approximately two cis-azobenzenes in the sixmembered rosette (Figure 6f,g). The bent cis-arms of these two cis-azobenzene units likely act as a “bulge”, contributing to a linear alignment of rosettes along the axial direction of the SP (Figure 6h). We also studied light-induced refolding of the unfolded SPs by means of visible light to trigger cis-to-trans isomerization. At a photostationary state (PSS) with approximately 11% cisazobenzene, randomly coiled structures were observed by AFM imaging (Figures 7a,b), suggesting that the intrinsic

effectively improve the reversibility of the supramolecular polymerization. Finally, we found that the supramolecular polymerization of 3 in MCH/CHCl3 (85/15, v/v) at a cooling rate of 0.1 °C min−1 afforded SPs, wherein major segments were helically folded and minor sections were misfolded (Figure 6a).

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LIGHT-INDUCED TOPOLOGY CHANGES Given the helically folded supramolecular fibers from azobenzene-containing 3, we were interested in studying their photoresponsive behavior. Upon exposing a solution of a helically folded SP to irradiation with UV light, 20% of the trans-azobenzene moieties isomerized into the cis-isomer. This isomerization resulted in the formation of randomly coiled fibers, which was confirmed by AFM (Figure 6b,c). Further increasing the population of the cis-isomer to 30% afforded stretched SP fibers devoid of curvature (Figure 6d). The morphological changes observed by AFM for spin-coated MCH solutions on a highly oriented pyrolytic graphite (HOPG) substrate were also supported by the occurrence of the same topological changes in solution, which was confirmed by SAXS analysis. The light-induced unfolding of SPs may occur via a direct or indirect pathway.38 In the indirect pathway, the isomerization of photochromic molecules that typically occurs on the subpicosecond to picosecond time scale proceeds in the monomeric state, as induced by monomer exchange between aggregates.39 In the direct pathway, on the other hand, photochromic moieties that are embedded in the SP chains isomerize and change the packing arrangement of the F


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Figure 7. AFM images showing the photoinduced refolding of (a) stretched SPs into (b) a randomly coiled state upon exposure to weak visible light and (c) its relaxation to a randomly coiled state upon aging. (d) Schematic representation of the recovery of the curvature upon exposure to visible light and aging. Adapted with permission from ref 37. Copyright 2017 Nature Publishing Group.

Figure 8. (a) Molecular structure of 4. (b) Energy landscape of SPs of 4 prepared under fast (left side) and slow cooling (right side). (c−e) AFM images showing the self-folding process of SPs of 4 prepared by cooling a hot MCH solution of 4 (c = 5 × 10−6 M) at a cooling rate of 1.0 °C min−1. (f) AFM image of fully misfolded SP prepared by cooling at a rate of 10 °C min−1. Adapted with permission from ref 39. Copyright 2018 American Association for the Advancement of Science.

curvature was almost regained upon cis-to-trans isomerization. However, the original helically folded state could not be recovered even after aging the PSS solution to thermally reduce the population of cis-isomer to 5% (Figures 7c,d). This

result suggests that the random-coil structures are unable to fold back into a thermodynamically stable helical structure via cis-to-trans isomerization. This occurs because cis-to-trans isomerization proceeds randomly across the fibers, and the G


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Accounts of Chemical Research recovery of curvature occurs with nonuniform direction at different segments of the fiber, thus resulting in randomly folded fibers.

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SELF-FOLDING Not only grafting an additional chromophore in the structure of 1, as applied to 3, but also direct extension of the πconjugation of its naphthalene core was recently found to be a useful molecular design to obtain elongated SPs with intrinsic curvature. Among the monomers synthesized accordingly, monomer 4 exhibited an unprecedented supramolecular polymer system, wherein initially formed misfolded structures self-fold to yield a topology analogous to the tertiary structure of proteins (Figure 8a,b).40 Slow cooling (1.0 °C min−1) of a hot MCH solution of 4 afforded randomly coiled (“misfolded”) SP fibers with minor amounts of helically folded domains, as observed by AFM (Figure 8c). Upon aging the resulting solution at room temperature, we observed to our surprise spontaneous folding (self-folding) of the misfolded domains into helical domains, which occurred on a time scale of days (Figure 8d). The process of self-folding lasted for about 5−7 days, whereby eventually fully bundled helices, reminiscent of the tertiary structure of proteins, were obtained in quantitative yield (Figure 8e). This higher-order topology is composed of fully bundled helices with uniform curvature (radius: 13 ± 1 nm) and a persistent length (170−340 nm), interlinked by “turn segments” (dotted squares). No further topological alterations were observed even after prolonged aging, suggesting that a thermodynamic equilibrium was reached after 7 days. By cooling the same MCH solution of 4 at a much faster cooling rate (e.g., 10 °C/min), we obtained fully misfolded shorter SPs with more inhomogeneous curvature radii (14 ± 4 nm) (Figure 8f). Evidently, the internal ordering in the SP backbone is lower in the misfolded domains compared to that of helical domains, as the misfolded domains exhibit a higher deviation (ca. 4 nm) from their average curvature radius, in contrast to the helical domains (ca. 1 nm). Probably, the higher degree of internal order in the backbone and interactions between the helical pitches renders the helical domains less dynamic compared to the misfolded domains. The fully misfolded fibers do not exhibit a topological transition into a more ordered structure, suggesting that they are kinetically trapped species. Hence, the coexistence of misfolded and helical domains in a single chain is crucial for the self-folding of SPs of 4 as the termini of the helical domains can guide the folding of tethered misfolded chains by repairing the defects in the main chains. Having obtained fully folded and fully misfolded SPs, we were interested in the thermodynamic aspects of these assemblies. By monitoring their thermal depolymerization by temperature-dependent absorption spectroscopy, we discovered that these SPs depolymerize upon heating via completely different mechanisms, i.e., a cooperative (nucleation−elongation)41,42 mechanism for the fully folded species and an isodesmic mechanism for the misfolded species (Figure 9a).43 A fitting analysis of these depolymerization curves with the respective models provided an elongation enthalpy of −171 kJ mol−1 for the fully folded SPs and −128 kJ mol−1 for the fully misfolded SPs. The strength of the π−π-stacking interactions should not differ significantly in these SPs, as reflected in the nearly identical shape of their absorption spectra. Thus, the different elongation enthalpy values obtained for the fully

Figure 9. (a) Cooling and heating curves for fully misfolded (brown dots) and fully folded (red dots) SPs of 4 (c = 5 × 10−6 M) in MCH. The black solid curve was obtained from fitting the experimental data to the isodesmic and cooperative models. (b) Changes in the heating curves of freshly prepared SPs of 4 in MCH upon aging the solution at different times at 20 °C. Adapted with permission from ref 39. Copyright 2018 American Association for the Advancement of Science.

folded and fully misfolded SPs can be attributed to the presence of van der Waals interactions between SP chains surrounded by long alkyl chains at higher-order levels, i.e., interactions between the helical pitches (within a helix) to stabilize the secondary structures, and between the helices to stabilize the tertiary structures. This might be a rare example wherein depolymerization processes of SPs revealed by spectroscopic analysis are reflected well in their higher-order structural features. It should also be worth investigating how the freshly prepared SPs of 4, which consist of misfolded and folded domains, would thermally depolymerize. The depolymerization curve measured immediately after cooling at 1.0 °C min−1 is a combination of those of the fully folded and fully misfolded SPs (Figure 9b). This result suggests the occurrence of a domain-selective depolymerization of SP chains, i.e., the misfolded domains initially depolymerize isodesmically at lower temperatures, while the remaining folded domains depolymerize cooperatively at higher temperatures. As selffolding proceeds, the isodesmic region gradually attenuates, H


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but hold the potential for exploration of new directions in supramolecular polymer as well as polymer research. Mimicking the secondary and tertiary structures of proteins and their conformational transitions with our synthetic SPs may provide valuable insight into the principles underpinning biological systems, which is also a promising approach to realize or improve biological functions using SPs.

and eventually, a highly cooperative depolymerization curve was observed after 7 days. A depolymerization curve similar to that of the freshly prepared SPs could be obtained when we blended separately prepared solutions of fully folded and fully misfolded SPs. However, the resulting mixtures did not show any evolution over time, suggesting that the fully misfolded SPs do not self-fold, neither via an external template mechanism nor by depolymerization−polymerization via monomer exchange. Accordingly, self-folding occurs only when misfolded and folded domains exist in the same SP chain. Although further molecular-level mechanisms for self-folding are currently under investigation, we believe that the degrees of internal order in folded and misfolded domains differ due to the involvement of molecular-level structural defects (e.g., conformational differences) in the misfolded domains, which can be repaired from the edge of folded domains toward the main chain.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Shiki Yagai: 0000-0002-4786-8603 Notes

The authors declare no competing financial interest.

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Biographies

CONCLUDING REMARKS AND PERSPECTIVE Although the primary structure of supramolecular polymers (SPs) can be designed infinitely via molecular design of their monomeric units, control over the higher-order structures of SPs, similar to the topological features of biopolymers, has been barely explored. In this Account, we have reviewed our initial endeavors toward the construction of such SPs with well-defined topological features, from cyclized (toroidal) to randomly coiled or helically folded SPs, and their stretched analogues. Although these sequential topological changes partially rely on the structural modification of π-conjugated cores to produce intrinsic curvature and ensure a high DOP, topological control at such higher-orders can be realized by understanding and controlling the kinetics of supramolecular polymerization. We have also successfully manipulated the “unfolding” landscape of SPs using light as an external stimulus by incorporating photoresponsive azobenzene units. Only recently, we have succeeded in preparing cyclized SPs with the azobenzene-incorporated monomer based on kinetically controlled supramolecular polymerization exploiting cis-totrans isomerization of the azobenzene unit44−46 and utilizing the cyclized SPs to demonstrate light-induced ring-opening and subsequent elongation of SP chains.47 By introducing a variety of photochromic chromophores, we may be able to also manipulate “folding” landscape of SP backbones. We also found that the introduction of a twistable rigid π-conjugated core can realize the complex folding landscape of the SP backbone. If we successfully implement this specific feature to a supramolecular copolymer system, more enticing manipulation in biomimetic higher-order structures must be realized. Our research group has already undertaken these projects and developed further exciting SPs featuring unprecedented controllability in the complex landscape of their backbones. Some of these SPs indeed exhibit unique properties and functions that unequivocally originate from their unique topological features. In this context, some more interesting projects are currently in progress, and we look forward to sharing our findings in the very near future. By indepth understanding of the structure−property relationships behind these intriguing phenomena, we would be able to know the importance of topology on the physical properties and hence functions of SPs, which have not been addressed so far. From a materials science point of view, the topologydependent physical properties of one-dimensional supramolecular materials at the mesoscopic region are unknown

Shiki Yagai was born in 1975 in Yamanashi (Japan) and studied at Ritsumeikan University, where he received his PhD in 2002. Soon after, he became an Assistant Professor at Chiba University, where he was promoted to Associate Professor in 2010. In July 2017, he was appointed as a Full Professor at the Institute for Global Prominent Research, Chiba University. His research interests focus on the selfassembly of functional dyes and π-conjugated systems as well as on the resulting soft materials. Yuichi Kitamoto was born in 1986 in Iwate (Japan) and studied at Tohoku University, where he received his PhD in 2014 and continued to work as a postdoctoral fellow. In 2017, he joined the research group of Prof. Yagai at Chiba University. His research interests include supramolecular polymers, the design of boron-containing functional materials, and the development of advanced methods for molecular recognition. Sougata Datta was born in 1987 in West Bengal (India). He received his PhD (2015) in organic chemistry from the Indian Institute of Science, Bangalore. In 2016, he moved to the group of Prof. Peter J. Stang at the University of Utah (USA) as a SERB-Indo-US postdoctoral fellow. In 2019, he joined the research group of Prof. Yagai at Chiba University. His research interests focus on supramolecular polymers, molecular gels, liquid crystals, and organoplatinum(II)-based supramolecular systems. Bimalendu Adhikari was born in 1983 in West Bengal (India). He received his PhD at IACS (India) in 2012. Subsequently, he carried out postdoctoral research in the group of Prof. Heinz-Bernhard Kraatz at the University of Toronto and in the group of Prof. Yagai at Chiba University as a JSPS postdoc fellow. In 2016, he moved to IISER Mohali (India) as an INSPIRE faculty, and currently, he is an Assistant Professor of chemistry at DDU Gorakhpur University. His research interests include supramolecular polymers, gels, peptides, and nanomaterials.

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ACKNOWLEDGMENTS The authors thank Dr. Deepak D. Prabhu, Mr. Keisuke Aratsu, and Mr. Atsuhito Suzuki for helpful discussion and for their continued support during the preparation of nice drawings for the figures.

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Supramolecular Polymers Capable of ... - American Chemical Society

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