Scientists have created photoresponsive supramolecular polymers that can flex and aggregate both within their chains and between different chains.
The characteristics of polymers are influenced by the balance between how their chains fold and how they clump together. For the first time, researchers from Japan have successfully produced folded supramolecular polymers capable of spontaneously aggregating with other chains. By using atomic force microscopy, they demonstrated that this aggregation happens as the main chains unfold. They also found that when light causes an azobenzene unit to change, it leads to the unfolding of the polymer, which speeds up the aggregation between chains.
In the study of polymers, the balance between how chains fold and how they cluster is crucial, affecting their mechanical, thermal, and electrical characteristics. Gaining insight into how folding and aggregation work together offers a great chance to develop and discover new polymeric materials that have custom properties and features.
This concept holds true for non-covalent versions of traditional covalent polymers, known as supramolecular polymers (SPs). These SPs are expected to be useful as innovative polymer materials that respond to external stimuli. While most SPs have a simple one-dimensional linear form that promotes interchain aggregation, there are few examples of SPs that can achieve different complex structures by folding chains. Creating an SP that can fold within chains while also aggregating externally would set a new standard for developing new SP materials with properties that can be adjusted based on these complex structures.
A recent study published in the Journal of the American Chemical Society on July 25, 2024, highlighted a novel folded SP that can naturally aggregate with other chains to form crystalline groups. By employing atomic force microscopy (AFM), the research team illustrated the correlation between folding and aggregation. This study was led by Professor Shiki Yagai from Chiba University, with Kenta Tamaki, a doctoral student from the Graduate School of Science and Engineering at Chiba University, serving as the first author.
“Initially, we discovered a monomer structure that polymerizes into a spiral shape. In this project, we modified the structure of the monomer-driving unit to explore the relationship between the monomer and polymer. Surprisingly, we noticed that the spiral could spontaneously unfold, causing different chains to bunch together. We integrated a photo-switchable molecule, allowing this ‘spontaneous’ unfolding to occur at ‘arbitrary timings’ through light exposure, which laid the groundwork for our research,” stated Prof. Yagai, sharing the inspiration behind this investigation.
To create the new system, the research team chose twistable biphenyl and light-responsive azobenzene units as the core components, which self-assembled into the desired SPs. The SPs initially formed in a folded configuration and gradually rearranged their internal molecular order over the course of half a day, leading to crystalline aggregation. The incorporation of azobenzene units facilitated photo-induced unfolding, significantly accelerating this process by loosening the intrachain stability among the folded sections.
The researchers noted that when they allowed the folded SP solution to sit at 20oC for several days, the polymers naturally transitioned in structure and precipitated. When they examined the precipitate using AFM, they saw a distinct intermediate phase that resembled the merging of curving chains transitioning into straight fibril structures. This intriguing observation reminded the researchers of the interchain aggregation often seen in biological processes, such as when proteins misfold and form amyloid fibrils.
Moreover, the team uncovered the reasons for this structural transformation. This process stemmed from internal molecular organization caused by changes in the biphenyl unit shape, along with interchain organization resulting from the alignment of outer aliphatic tails on the main chains. This process resembles the crystallization found in traditional covalent polymers. The team validated this mechanism using the photoisomerization of the azobenzene unit; upon exposing the folded SP solution to UV light, the conformational changes in the azobenzene units prompted immediate unfolding of the main chains and significantly enhanced interchain aggregation.
This study provides fresh insight into the phenomena of folding and aggregation. The mesoscale SPs, formed by the self-assembly of numerous molecules, can act as a valuable model system for investigating molecular-level dynamics between individual main chains. This opens doors to new possibilities for innovation within the field of materials science.
“Traditionally, these phenomena have been studied through spectroscopic or macroscopic observations, which reflect averaged behaviors across the entire system. Therefore, developing more observable mesoscale models is expected to greatly contribute to advancements in materials science. We hope these findings will inspire new developments in meso-scale molecular assemblies with substantial higher-order structures,” concluded Prof. Yagai.
