Although directional chain reactions are common in nature’s self-assembly processes and in covalent polymerizations, it has been challenging to perform such processes in artificial one-dimensional self-assembling systems. In this paper, we describe a system, employing perylene bisimide (PBI) derivatives as monomers, for selectively activating one end of a supramolecular polymer during its growth and, thereby, realizing directional supramolecular polymerization. Upon introduction of a solution containing only a single PBI monomer into the microflow channel, nucleation was induced spontaneously. The dependency of the aggregation efficiency on the flow rate suggested that the shear force facilitated collisions among the monomers to overcome the activation energy required for nucleation. Next, by introducing a solution containing both monomer and polymer, we investigated how the shear force influenced the monomer–polymer interactions. In situ fluorescence spectra and linear dichroism revealed that growth of the polymers was accelerated only when they were oriented under the influence of shear stress. Upon linear motion of the oriented polymer, polymer growth at that single end became predominant relative to the nucleation of freely diffusing monomers. When applying this strategy to a two-monomer system, the second (less active) monomer reacted selectively at the forward-facing terminus of the first polymer, leading to the creation of a diblock copolymer through formation of a molecular heterojunction. This strategyfriction-induced activation of a single end of a polymershould be applicable more generally to directional supramolecular block copolymerizations of various functional molecules, allowing molecular heterojunctions to be made at desired positions in a polymer.
One major concern in supramolecular chemistry is how to place different intermolecular interactions in a desired position, especially at the terminal ends, of 1D structures. A solution to the problem is co-assembly in microflow. We demonstrate that kinetic co-assembly of two kinds of amphiphilic oligo(p-phenylenevinylene) molecules with different amide groups result in metastable nanofibers where stronger hydrogen-bonding interactions are regularly inserted as stabilizing wedges. It is found that decomposition of the nanofibers from the ends is suppressed at the wedges, leading to the creation of discrete 1D structures with capped ends (length dispersity L w /L n � 1.2), which act as a micrometer-sized building blocks that can be used for further hierarchical assembly.
The anisotropic properties of one-dimensional (1D) supramolecules have generally been the sole way to input molecular information along a structure of high density. Although the chain reaction of a synthetic polymer (e.g., in radical polymerization) does realize anisotropic polymer elongation, it has remained challenging to induce such properties in artificial 1D self-assembling systems. Herein, by employing J-aggregate nanofibers of TPPS — a sort of self-assembling porphyrin — as a model, we describe a system in which linear momentum of laminar flow facilitates directional supramolecular elongation of the flowing nanofibers. In situ fluorescence and linear dichroism (LD) measurements revealed that the elongation of the J-aggregate nanofibers could be accelerated only when they were oriented in the flow direction. Moreover, linear transport of the oriented nanofibers along the stream disrupted the isotropic reactivity at their two termini; one terminus could be activated selectively, resulting in directional nanofiber elongation. The shear rate gradient of the laminar flow induced collisions of the TPPS monomer units at the end of one terminus of the nanofibers. This strategy should be applicable more generally to supramolecular 1D elongation (supramolecular polymerization) of various functional molecules, regardless of their chemical properties, thereby extending the frontiers of supramolecular chemistry.
The front cover artwork is provided by group of Dr. Munenori Numata at the Kyoto Prefectural University (Japan). The image shows that the dynamics of supramolecular polymerization can be visualized in a space‐resolved manner along a microflow channel, similar to a snapshot taken of the assembly dynamics. Read the full text of the Article at 10.1002/syst.202000006.
Despite rapid advances in supramolecular chemistry, only limited attention has been paid to regulating the self‐assembling field, such as by studying diffusional dynamics or the hydrodynamic properties of the solvents that always surround the self‐assembling molecules. Herein, we demonstrate that a proton gradient generated in laminar flow can facilitate the acid‐base reaction, leading to effective self‐assembly of TPPS, a type of porphyrin. In situ fluorescence microspectroscopies reveal that the acid‐base reaction followed by self‐assembly of TPPS proceed under a uniform diffusional environment in the laminar flow, leading to the creation of discrete J‐aggregate fibers (length dispersity Lw/Ln <1.2). Furthermore, we can successfully evaluate the time required for nucleation to be 2.9 ms. In sharp contrast to conventional solution chemistry, the nucleation process of TPPS finishes at a much earlier stage, implying that nucleation might no longer be the rate‐determining step in laminar flow. The present self‐assembly system in laminar flow provides a way to create a variety of supramolecular architectures under reproducible kinetic conditions, and a comprehensive strategy to construct a kinetic database for self‐assembly dynamics.
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