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New Insights into Self-Assembly of Photoresponsive Molecules

Self-assembly, a fundamental concept in molecular science, involves molecules spontaneously organizing into structured forms, a characteristic that is vital for creating advanced optical and electronic materials. A recent study led by researchers from Japan has shed light on this process, revealing how even a small quantity of residual aggregates can significantly influence the self-assembly of photoresponsive molecules. The team, which was guided by Professor Shiki Yagai from Chiba University’s Graduate School of Engineering and included collaborators from several other institutions, published their findings in Nature Nanotechnology on April 11, 2025.

Recent advancements in research have highlighted the importance of controlling the size and hierarchical organization of self-assembled aggregates. These aggregates can exhibit tailored properties essential for various applications. Nonetheless, self-assembly remains a complex and dynamic phenomenon that demands precise regulation. As Prof. Yagai notes, “The molecules engage in a continuous process of association and dissociation during self-assembly, and even minor impurities or slight variations in conditions can dramatically alter the final structure of the aggregates formed.”

The researchers concentrated on the self-assembly of a chiral, photoresponsive azobenzene known for forming left-handed helical structures. Their investigation led to a surprising discovery: the inclusion of a minimal amount of residual aggregates in the solution could drastically switch the assembly process, resulting in the creation of right-handed helical aggregates. Furthermore, the photoresponsive nature of the molecules allowed for modifications in the timing of assembly through light exposure, enabling the team to precisely control the production of either left-handed or right-handed helical structures.

In their detailed spectroscopic and molecular modeling studies, the team observed that the azobenzene molecule, when dissolved in an organic solvent at room temperature, adopts a scissor-like folded configuration. This structure then evolves into a helical assembly. Prof. Yagai elaborates, “The molecule has a carbon atom bonded to four distinct atomic groups, imparting chirality. These molecules fold similarly to left-handed scissors, twisting to create a left-handed helical stacking of the assembly.”

The ability of these photoresponsive molecules to respond to light plays a pivotal role in their assembly and disassembly. Upon exposure to mild ultraviolet (UV) light, the helical structures disassemble into individual molecules. Subsequent exposure to visible light reinitiates the helical assembly. Interestingly, under certain conditions, right-handed aggregates emerged instead of the expected left-handed ones. When stronger UV light was followed by visible light, the original left-handed assemblies were regenerated.

The research team delved into this mechanism, uncovering that residual left-handed helical aggregates, which persisted undecomposed during weak UV light exposure, serve as nucleation sites. These sites lead to the formation of opposing helical assemblies through a phenomenon termed “secondary nucleation.” Prof. Yagai notes, “This remarkable phenomenon elucidates why meta-stable right-handed aggregates are preferentially generated over left-handed aggregates.”

Further analysis revealed that light intensity plays a critical role in the assembly dynamics. Prof. Yagai remarks, “We discovered that the intensity of visible light influences the timing of assembly. Stronger visible light accelerates assembly and reduces the impact of residual aggregates, while lower intensity amplifies their effect.”

The ability to optimize the intensities of UV and visible light ultimately allowed the researchers to adeptly switch between left- and right-handed helical structures, depending on the presence of residual aggregates. The study also found that the stable left-handed and meta-stable right-handed aggregates exhibited contrasting electron spin polarization, indicating a variation in the electronic properties of the helices formed.

This research not only enhances the understanding of self-assembly mechanisms but also underscores the critical influence of residual aggregates and light manipulation in fabricating innovative functional materials. These insights hold significant promise for advancing material science and developing new applications in the field.

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