Author Correction: Self-assembled poly-catenanes from supramolecular toroidal building blocks.

An amendment to this paper has been published and can be accessed via a link at the top of the paper.

Self-assembled poly-catenanes from supramolecular toroidal building blocks.

Mechanical interlocking of molecules (catenation) is a nontrivial challenge in modern synthetic chemistry and materials science1,2. One strategy to achieve catenation is the design of pre-annular molecules that are capable of both efficient cyclization and of pre-organizing another precursor to engage in subsequent interlocking3-9. This task is particularly difficult when the annular target is composed of a large ensemble of molecules, that is, when it is a supramolecular assembly. However, the construction of such unprecedented assemblies would enable the visualization of nontrivial nanotopologies through microscopy techniques, which would not only satisfy academic curiosity but also pave the way to the development of materials with nanotopology-derived properties. Here we report the synthesis of such a nanotopology using fibrous supramolecular assemblies with intrinsic curvature. Using a solvent-mixing strategy, we kinetically organized a molecule that can elongate into toroids with a radius of about 13 nanometres. Atomic force microscopy on the resulting nanoscale toroids revealed a high percentage of catenation, which is sufficient to yield 'nanolympiadane'10, a nanoscale catenane composed of five interlocked toroids. Spectroscopic and theoretical studies suggested that this unusually high degree of catenation stems from the secondary nucleation of the precursor molecules around the toroids. By modifying the self-assembly protocol to promote ring closure and secondary nucleation, a maximum catenation number of 22 was confirmed by atomic force microscopy.

Cyclization or bridging: which occurs faster is the key to the self-assembly mechanism of Pd6L3 coordination prisms.

The self-assembly processes of Pd6L3 coordination prisms consisting of cis-protected Pd(II) complexes and porphyrin-based tetratopic ligands with four 3-pyridyl or 4-pyridyl groups (L) were investigated by experimental and numerical methods, QASAP (quantitative analysis of self-assembly process) and NASAP (numerical analysis of self-assembly process), respectively. It was found that contrary to common intuition macrocyclization takes place faster than the bridging reaction in the prism assembly and that the bridging reaction occurring before the macrocyclization tends to produce kinetically trapped species. A numerical simulation demonstrates that the relative magnitude of the rate constants between the macrocyclization and the bridging reaction is the key factor that determines whether the self-assembly leads to the thermodynamically most stable prism or to kinetically trapped species. Finding the key elementary reactions that largely affect the selection of the major assembly pathway is helpful to rationally control the products under kinetic control via modulation of the energy landscape.

Unexpected Self-Assembly Pathway to a Pd(II) Coordination Square-Based Pyramid and Its Preferential Formation beyond the Boltzmann Distribution.

Experimental and theoretical investigations of the self-assembly process of a Pd(II) coordination M6L4 square-based pyramid (SP) were conducted. It was found that the probable self-assembly pathway, in which the dimerization of M2L2 with two M leads to SP, expected from the connectivity of the building blocks is not a major self-assembly pathway to the M6L4 SP. Whether the M6L4 SP is assembled or M2L2 is trapped is determined by an inter- or intramolecular reaction in a chain-like M2L2X, where X is a leaving ligand. The kinetically trapped state where the M6L4 SP is produced from M2L2 beyond the Boltzmann distribution was realized by a concentration-induced process and was kept for at least 2 months at 298 K.

An Aromatic Oligomer Micelle: Large Enthalpic Stabilization and Selective Oligothiophene Uptake.

An aromatic oligomer micelle, featuring both high stability and high uptake ability, was quantitatively formed in water from amphiphilic oligomers, composed of three bent polyaromatic amphiphiles connected alternately by two hydrophilic chains. The well-defined micelle, with a diameter of ca. 2 nm, remains intact even under highly diluted conditions (ca. 3 µM) and at elevated temperature (>130 °C), due to the polyaromatic chelate effect. The thermodynamic studies reveal that large enthalpic gain (ΔH=-110 kJ mol-1 ) is the key for the micelle formation. The oligomer micelle selectively encapsulates unsubstituted oligothiophenes (≥4-mer) to a high degree and the resultant, aqueous host-guest complexes display unusual emission derived from the multiply stacked oligomers. Furthermore, facile uptake and release of unsubstituted polythiophenes can be achieved using the oligomer micelle.

Topological Impact on the Kinetic Stability of Supramolecular Polymers.

Kinetically formed metastable molecular assemblies have attracted increasing interest especially in the field of supramolecular polymers. In most cases, metastable assemblies are ensemblies of aggregates based on the same supramolecular motif but with different lengths or sizes, and therefore their kinetic stabilities are experimentally indistinguishable. Herein, we demonstrate a topological effect on kinetic stabilities in a complex mixture of metastable supramolecular polymers. Our azobenzene-incorporated monomer upon heating in nonpolar solvent at ambient temperature kinetically forms complex mixtures of supramolecular polymers with cyclized and open-ended randomly coiled topologies. Upon further heating, we obtained thermodynamically stable twisted fibrils organizing into crystalline fibers. Through the direct visualization of the complex supramolecular polymer mixtures by atomic force microscopy, we demonstrate that the cyclized supramolecular polymer has superior kinetic stability compared to the open-ended species toward the thermal transformation into twisted fibrils. Since the superior kinetic stability of the cyclized species can be attributed to the absence of aggregate termini, we could convert them fully into the thermodynamic species through photoinduced opening of the cyclized structures.

Photoresponsive Circular Supramolecular Polymers: A Topological Trap and Photoinduced Ring-Opening Elongation.

Topological features of one-dimensional macromolecular chains govern the properties and functionality of natural and synthetic polymers. To address this issue in supramolecular polymers, we synthesized two topologically distinct supramolecular polymers with intrinsic curvature, circular and helically folded nanofibers, from azobenzene-functionalized supramolecular rosettes. When a mixture of circular and helically folded nanofibers was exposed to UV light, selective unfolding of the latter open-ended supramolecular polymers was observed as a result of the curvature-impairing internal force produced by the trans-to-cis photoisomerization of the azobenzene. This distinct sensitivity suggests that the topological features of supramolecular polymers define their mechanical stability. Furthermore, the exposure of circular supramolecular polymers in more polar media to UV irradiation resulted in ring opening followed by chain elongation, thus demonstrating that the circular supramolecular polymer can function as a topological kinetic trap.

Simultaneous SAXS and SANS Analysis for the Detection of Toroidal Supramolecular Polymers Composed of Noncovalent Supermacrocycles in Solution.

Molecular self-assembly primarily occurs in solution. To better understand this process, techniques capable of probing the solvated state are consequently required. Small-angle scattering (SAS) has a proven ability to detect and characterize solutions, but it is rarely applied to more complex assembly shapes. Here, small-angle X-ray and neutron scattering are applied to observe toroidal assemblies in solution. Combined analysis confirms that the toroids have a core-shell structure, with a π-conjugated core and an alkyl shell into which solvent penetrates. The dimensions determined by SAS agree well with those obtained by (dried-state) atomic force microscopy. Increasing the number of naphthalene units in the molecular building block yields greater rigidity, as evidenced by a larger toroid and a reduction in solvent penetration into the shell. The detailed structural analysis demonstrates the applicability of SAS to monitor complex solution-based self-assembly.

Hydrogen bond-directed supramolecular polymorphism leading to soft and hard molecular ordering.

Transformation of metastable supramolecular stacks of hydrogen-bonded rosettes composed of an ester-containing barbiturated naphthalene into crystalline nanosheets occurs through the rearrangement of hydrogen-bonding patterns. The involvement of the ester group in the crystalline hydrogen-bonded pattern is demonstrated, guiding us to a new molecular design that can afford supramolecular polymorphs with soft and hard molecular packing.

Supramolecular copolymerization driven by integrative self-sorting of hydrogen-bonded rosettes.

Molecular recognition to preorganize noncovalently polymerizable supramolecular complexes is a characteristic process of natural supramolecular polymers, and such recognition processes allow for dynamic self-alteration, yielding complex polymer systems with extraordinarily high efficiency in their targeted function. We herein show an example of such molecular recognition-controlled kinetic assembly/disassembly processes within artificial supramolecular polymer systems using six-membered hydrogen-bonded supramolecular complexes (rosettes). Electron-rich and poor monomers are prepared that kinetically coassemble through a temperature-controlled protocol into amorphous coaggregates comprising a diverse mixture of rosettes. Over days, the electrostatic interaction between two monomers induces an integrative self-sorting of rosettes. While the electron-rich monomer inherently forms toroidal homopolymers, the additional electrostatic interaction that can also guide rosette association allows helicoidal growth of supramolecular copolymers that are comprised of an alternating array of two monomers. Upon heating, the helicoidal copolymers undergo a catastrophic transition into amorphous coaggregates via entropy-driven randomization of the monomers in the rosette.

Self-Sorting of Rosette-Forming Naphthalene Barbiturates into Distinct Toroidal Assemblies.

A unique narcissistic self-sorting system is composed of two structurally similar mono- and bisnaphthalene-based supramolecular monomers. Upon cooling the pure mono- and bisnaphthalene monomers in a nonpolar solvent, toroidal and helicoidally elongated supramolecular polymers were obtained, respectively, through the nucleation-elongation mechanism. When a 1 : 1 mixture of these monomers was similarly cooled, we obtained self-sorted mixtures of smaller and larger toroidal supramolecular polymers with diameters of 15 and 26 nm, respectively. Absorption spectroscopic studies revealed that the mononaphthalene monomers prevents the bisnaphthalene monomers from forming nuclei, leading to a less cooperative polymerization process.

Self-folding of supramolecular polymers into bioinspired topology.

Folding one-dimensional polymer chains into well-defined topologies represents an important organization process for proteins, but replicating this process for supramolecular polymers remains a challenging task. We report supramolecular polymers that can fold into protein-like topologies. Our approach is based on curvature-forming supramolecular rosettes, which affords kinetic control over the extent of helical folding in the resulting supramolecular fibers by changing the cooling rate for polymerization. When using a slow cooling rate, we obtained misfolded fibers containing a minor amount of helical domains that folded on a time scale of days into unique topologies reminiscent of the protein tertiary structures. Thermodynamic analysis of fibers with varying degrees of folding revealed that the folding is accompanied by a large enthalpic gain. The self-folding proceeds via ordering of misfolded domains in the main chain using helical domains as templates, as fully misfolded fibers prepared by a fast cooling rate do not self-fold.

Hydrogen-bonded rosettes comprising π-conjugated systems as building blocks for functional one-dimensional assemblies.

Hydrogen-bonded supermacrocycles (rosettes) are attractive disk-shaped noncovalent synthons for extended functional columnar nanoassemblies. They can serve not only as noncovalent monomer units for supramolecular polymers and discrete oligomers in a dilute solution but also as constituent entities for soft matters such as gels and lyotropic/thermotropic liquid crystals. However, what are the merits of using supramolecular rosettes instead of using expanded π-conjugated covalent molecules? This review covers the self-assembly of photochemically and electrochemically active π-conjugated molecules through the formation of supramolecular rosettes via directional complementary multiple hydrogen-bonding interactions. These rosettes comprising π-conjugated covalent functional units stack into columnar nanoassemblies with unique structures and properties. By overviewing the design principle, characterization, and properties and functionalities of various examples, we illustrate the merits of utilizing rosette motifs. Basically, one can easily access a well-defined expanded π-surface composed of multi-chromophoric systems, which can ultimately afford stable extended nanoassemblies even in a dilute solution due to the higher association constants of supermacrocyclized π-systems. Importantly, these columnar nanoassemblies exhibit unique features in self-assembly processes, chiroptical, photophysical and electrochemical properties, nanoscale morphologies, and bulk properties. Moreover, the stimuli responsiveness of individual building blocks can be amplified to a greater extent by exploiting rosette intermediates to organize them into one-dimensional columnar structures. In the latter parts of the review, we also highlight the application of rosettes in supramolecular polymer systems, photovoltaic devices, and others.

Light-induced unfolding and refolding of supramolecular polymer nanofibres.

Unlike classical covalent polymers, one-dimensionally (1D) elongated supramolecular polymers (SPs) can be encoded with high degrees of internal order by the cooperative aggregation of molecular subunits, which endows these SPs with extraordinary properties and functions. However, this internal order has not yet been exploited to generate and dynamically control well-defined higher-order (secondary) conformations of the SP backbone, which may induce functionality that is comparable to protein folding/unfolding. Herein, we report light-induced conformational changes of SPs based on the 1D exotic stacking of hydrogen-bonded azobenzene hexamers. The stacking causes a unique internal order that leads to spontaneous curvature, which allows accessing conformations that range from randomly folded to helically folded coils. The reversible photoisomerization of the azobenzene moiety destroys or recovers the curvature of the main chain, which demonstrates external control over the SP conformation that may ultimately lead to biological functions.

Self-sorting regioisomers through the hierarchical organization of hydrogen-bonded rosettes.

The self-assembly of two regioisomeric hydrogen-bonding naphthalenes was studied in mixed states in different polarity solvents. The regioisomers co-assemble to form heteromeric rosettes in chloroform. Upon injecting this solution into methylcyclohexane the heteromeric rosettes kinetically form amorphous aggregates, which over time differentiate into thermodynamically stable distinct nanostructures through self-sorting.