Various conventional reactions in polymer chemistry have been translated to the supramolecular domain, yet it has remained challenging to devise living supramolecular polymerization. To achieve this, ...self-organization occurring far from thermodynamic equilibrium--ubiquitously observed in nature--must take place. Prion infection is one example that can be observed in biological systems. Here, we present an 'artificial infection' process in which porphyrin-based monomers assemble into nanoparticles, and are then converted into nanofibres in the presence of an aliquot of the nanofibre, which acts as a 'pathogen'. We have investigated the assembly phenomenon using isodesmic and cooperative models and found that it occurs through a delicate interplay of these two aggregation pathways. Using this understanding of the mechanism taking place, we have designed a living supramolecular polymerization of the porphyrin-based monomers. Despite the fact that the polymerization is non-covalent, the reaction kinetics are analogous to that of conventional chain growth polymerization, and the supramolecular polymers were synthesized with controlled length and narrow polydispersity.
A bis(squaraine) dye equipped with alkyl and oligoethyleneglycol chains was synthesized by connecting two dicyanomethylene substituted squaraine dyes with a phenylene spacer unit. The aggregation ...behavior of this bis(squaraine) was investigated in non‐polar toluene/tetrachloroethane (98:2) solvent mixture, which revealed competing cooperative self‐assembly pathways into two supramolecular polymorphs with entirely different packing structures and UV/Vis/NIR absorption properties. The self‐assembly pathway can be controlled by the cooling rate from a heated solution of the monomers. For both polymorphs, quasi‐equilibrium conditions between monomers and the respective aggregates can be established to derive thermodynamic parameters and insights into the self‐assembly mechanisms. AFM measurements revealed a nanosheet structure with a height of 2 nm for the thermodynamically more stable polymorph and a tubular nanorod structure with a helical pitch of 13 nm and a diameter of 5 nm for the kinetically favored polymorph. Together with wide angle X‐ray scattering measurements, packing models were derived: the thermodynamic polymorph consists of brick‐work type nanosheets that exhibit red‐shifted absorption bands as typical for J‐aggregates, while the nanorod polymorph consists of eight supramolecular polymer strands of the bis(squaraine) intertwined to form a chimney‐type tubular structure. The absorption of this aggregate covers a large spectral range from 550 to 875 nm, which cannot be rationalized by the conventional exciton theory. By applying the Essential States Model and considering intermolecular charge transfer, the aggregate spectrum was adequately reproduced, revealing that the broad absorption spectrum is due to pronounced donor‐acceptor overlap within the bis(squaraine) nanorods. The latter is also responsible for the pronounced bathochromic shift observed for the nanosheet structure as a result of the slip‐stacked arranged squaraine chromophores.
The self‐assembly of a newly synthesized bis(squaraine) dye was investigated revealing a supramolecular polymorphism dependent on the cooling rate. Fast cooling leads to helical nanorods, while slow cooling leads to more organized nanosheets.
Abstract
Connecting molecular-level phenomena to larger scales and, ultimately, to sophisticated molecular systems that resemble living systems remains a considerable challenge in supramolecular ...chemistry. To this end, molecular self-assembly at higher hierarchical levels has to be understood and controlled. Here, we report unusual self-assembled structures formed from a simple porphyrin derivative. Unexpectedly, this formed a one-dimensional (1D) supramolecular polymer that coiled to give an Archimedean spiral. Our analysis of the supramolecular polymerization by using mass-balance models suggested that the Archimedean spiral is formed at high concentrations of the monomer, whereas other aggregation types might form at low concentrations. Gratifyingly, we discovered that our porphyrin-based monomer formed supramolecular concentric toroids at low concentrations. Moreover, a mechanistic insight into the self-assembly process permitted a controlled synthesis of these concentric toroids. This study both illustrates the richness of self-assembled structures at higher levels of hierarchy and demonstrates a topological effect in noncovalent synthesis.
Far‐from‐equilibrium thermodynamic systems that are established as a consequence of coupled equilibria are the origin of the complex behavior of biological systems. Therefore, research in ...supramolecular chemistry has recently been shifting emphasis from a thermodynamic standpoint to a kinetic one; however, control over the complex kinetic processes is still in its infancy. Herein, we report our attempt to control the time evolution of supramolecular assembly in a process in which the supramolecular assembly transforms from a J‐aggregate to an H‐aggregate over time. The transformation proceeds through a delicate interplay of these two aggregation pathways. We have succeeded in modulating the energy landscape of the respective aggregates by a rational molecular design. On the basis of this understanding of the energy landscape, programming of the time evolution was achieved through adjusting the balance between the coupled equilibria.
Finding the right balance: The energy landscape of a supramolecular polymerization in which the supramolecular assembly transforms from a J‐aggregate to an H‐aggregate over time has been modulated by a rational molecular design. Based on this, kinetic control over pathway complexity was achieved through adjusting the balance between the coupled equilibria.
Although living supramolecular polymerization (LSP) has recently been realized, the scope of the monomer structures applicable to the existing methods is still limited. For instance, a monomer that ...spontaneously nucleates itself cannot be processed in a manner consistent with LSP. Herein, we report a new method for such a "reactive" monomer. We use a 'dummy' monomer which has a similar structure to the reactive monomer but is incapable of one-dimensional supramolecular polymerization. We show that in the presence of the dummy monomer, the reactive monomer is kinetically trapped in the dormant state. In this way, spontaneous nucleation of the reactive monomer is retarded; yet, addition of seeds of a supramolecular polymer can initiate the supramolecular polymerization in a chain growth manner. As a result, we obtain the supramolecular polymer of the reactive monomer with a controlled length, which is otherwise thermodynamically inaccessible. We believe that this concept will expand the scope of LSP for the synthesis of other functional supramolecular polymers, and thus lead to a variety of applications.
New method of living supramolecular polymerization is demonstrated. Spontaneous nucleation of a reactive monomer is suppressed by using a 'dummy' monomer. Addition of seeds can initiate supramolecular polymerization in a chain-growth manner.
Recent developments in kinetically controlled supramolecular polymerization permit control of the size (i.e., length and area) of self‐assembled nanostructures. However, control of molecular ...self‐assembly at a level comparable with organic synthetic chemistry and the achievement of structural complexity at a hierarchy larger than the molecular level remain challenging. This study focuses on controlling the aspect ratio of supramolecular nanosheets. A systematic understanding of the relationship between the monomer structure and the self‐assembly energy landscape has derived a new monomer capable of forming supramolecular nanosheets. With this monomer in hand, the aspect ratio of a supramolecular nanosheet is demonstrated that it can be controlled by modulating intermolecular interactions in two dimensions.
Design on a small scale! Molecular self‐assembly in solution, with rational molecular design and under kinetic control, has achieved supramolecular nanosheets with controlled area and aspect ratio.
Despite recent advances in mechanistic understanding and controlled‐synthesis methodologies regarding synthetic supramolecular assemblies, it has remained challenging to capture the molecular‐level ...phenomena in real time, thus hindering further progress in this research field. In this study, we applied high‐speed atomic‐force microscopy (AFM), which has extraordinary spatiotemporal resolution (1 nm and sub‐100 ms), to capture dynamic events occurring during synthetic molecular self‐assembly. High‐speed AFM permitted the visualization of unique dynamic behavior, such as seeded growth and self‐repair in real time. Furthermore, scanning‐probe AFM permitted the site‐specific manipulation and functionalization of a molecular self‐assembly. This powerful combination of bottom‐up and top‐down approaches at the molecular level should enable targeted syntheses of unprecedented functional nanoarchitectures.
Up to speed: The extraordinary spatiotemporal resolution of high‐speed AFM permitted visualization of the seeded growth and self‐repair of supramolecular polymers in real time, and scanning‐probe AFM permitted the site‐specific manipulation and functionalization of a molecular self‐assembly (see picture). This combination of bottom‐up and top‐down approaches at the molecular level should enable the targeted synthesis of functional nanoarchitectures.
Despite substantial effort devoted in the history of supramolecular chemistry, synthetic supramolecular systems still lag behind biomolecular systems in terms of complexity and functionality. This is ...because biomolecular systems function in a multicomponent molecular network under out-of-equilibrium conditions. Here we report two-component supramolecular assemblies that are metastable and thus show time-dependent evolution. We found that the systems undergo either self-sorting or coassembly in time depending on the combination of components. Interestingly, this outcome, which had been previously achievable only under specific conditions, emerged from the two-component systems as a result of synergistic or reciprocal interplay between the coupled equilibria. We believe that this study sheds light on the similarity between synthetic and biomolecular systems and promotes better understanding of their intricate kinetic behaviors.
Self-assembled, surfactant-free organic nanoparticles of an oligofluorene derivative with high colloidal stability were prepared in aqueous medium. The blue emission of the nanoparticles was tuned to ...white through fluorescence resonance energy transfer (FRET) by encapsulating an orange-red emitting dye (1 mol%) within the nanoparticle scaffold.
Molecular self-assembly under kinetic control has many opportunities leading to novel phenomena and applications. However, a mechanistic insight into the intricate kinetic behavior is still lacking. ...Herein, we demonstrate that even a subtle change in the molecular structure affects the kinetic behavior of metastable supramolecular assembles. Interestingly, the kinetic process influenced the size of the final outcomes (in this study, nanosheet structures) in a manner opposite to that predicted by the thermodynamic model. Our findings illustrate the difference between thermodynamically and kinetically controlled molecular self-assemblies. We believe that this study will expand the potential applications of molecular self-assemblies.
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•Transformation kinetics of metastable supramolecular assembles was investigated.•We found that only a subtle alternation in molecular structure affects the kinetics.•We found that the size of the final outcome was determined kinetically.
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