Light-responsive molecules form shape-shifting 2D nanosheets

Professor Shiki Yagai, Kenta Tamaki (Graduate Student at the time of research, now an Assistant Professor at Nagoya University), and their colleagues at the Institute for Advanced Academic Research (IAAR) at Chiba University, together with Professor Masaki Kawano from the School of Science at the Institute of Science Tokyo and the Exploratory Research Center on Life and Living Systems of the National Institutes of Natural Sciences, in collaboration with Université Paris-Saclay (France), RIKEN, and Nagoya University, announced that they have discovered that "two-dimensional nanosheets" transform into completely different structures such as one-dimensional nanofibers (thin string-like forms) or three-dimensional nanocrystals (stacked thick masses) when irradiated with light of varying intensities.

The sheet-like structures form through the self-assembly of "photochromic molecules," which change shape and color in response to light. The group also succeeded in elucidating the mechanism by observing the structural changes in real time using high-speed atomic force microscopy (AFM). This research is expected to contribute to the development of next-generation functional materials. The findings were published in Chem on November 18.

Biological molecules function in "out-of-equilibrium states" that deviate from thermodynamically stable "equilibrium states" by constantly exchanging energy with their surroundings. In recent years, research has been advancing to incorporate such mechanisms into functional materials made from synthetic molecules.

Photochromic molecules change shape or color in response to light energy, with many studies having reported switching between equilibrium and out-of-equilibrium states by incorporating them into constituent molecules. However, research to elicit different responses based on light intensity had not yet been conducted.

In this study, the research group succeeded in selectively creating three types of assembly states with different dimensionalities depending on the intensity of irradiated light. This materialized through combining the structural changes of photochromic molecules (photoisomerization) with the structural diversity of molecular assemblies (supramolecular polymorphism).

Specifically, they designed and synthesized self-assembling molecule 1, incorporating an "azobenzene" photochromic molecule that bends in response to ultraviolet light.

Yagai and his colleagues have previously developed self-assembling molecules that exhibit various supramolecular polymorphisms.

When molecule 1 was dissolved in an organic solvent, it formed two-dimensional nanosheets, the most energetically stable structure. When strong ultraviolet light was applied to these nanosheets, they transformed into one-dimensional nanofibers. When weak ultraviolet light was applied, the decomposition and growth of the nanosheets competed with each other, resulting in transformation into three-dimensional nanocrystals.

In revealing this mechanism, observation with high-speed AFM confirmed the transformation from two-dimensional nanosheets to one-dimensional nanofibers proceeds selectively from specific edges of the nanosheets. High-speed AFM can observe moving materials in solution with nanometer spatial resolution and sub-second temporal resolution.

Furthermore, the researchers succeeded in single-crystal structure analysis of molecule 1. They revealed the mechanism of structural molecular changes occurring from the edges where the azobenzene moiety is exposed and strongly susceptible to photoisomerization. Under weak ultraviolet light irradiation, both two-dimensional nanosheets transforming into three-dimensional nanocrystals and nanosheets disappearing were observed, revealing that this change occurs through a classical crystal growth mechanism called light-induced Ostwald ripening.

For the first time, it was demonstrated that adjusting light intensity can create multiple new out-of-equilibrium states.

This is expected to lead to the development of new technologies that use light to achieve dimensional control of various organic materials.

Yagai commented: "Light is an energy source that drives chemical reactions necessary for life activities, as seen in photosynthesis. In this research, we utilized that light energy to control the 'assembly state of molecules itself.' This is the first time anyone has shown that the shape of artificial molecular materials changes to different states depending on light intensity, and we expect this to be an important step toward the development of smart materials in the future."

Journal Information
Publication: Chem
Title: Light-intensity-dependent out-of-equilibrium processes toward dimensionally distinct nanopolymorphs
DOI: 10.1016/j.chempr.2025.102818