Harnessing Light to Create Unstable Molecular Fits
Harnessing Light to Create Unstable Molecular Fits
In a groundbreaking study led by Professor Alberto Credi of the University of Bologna, a team has successfully demonstrated how light can be used to induce a molecular "fit" that would otherwise be impossible under normal conditions. By combining photochemical reactions with self-assembly processes, the researchers have inserted a filiform molecule into a ring-shaped molecule in a high-energy configuration, far from thermodynamic equilibrium.
The researchers discovered that by applying light energy to an aqueous solution, they could prevent a molecular self-assembly reaction from reaching its thermodynamic minimum. This results in a product distribution that deviates from what would be observed at equilibrium. Prof. Credi highlighted that this behavior, which is crucial in many biological systems, is often not explored in artificial molecules due to its complexity. The simplicity and versatility of their approach, along with the use of visible light—an abundant and sustainable energy source—suggests potential applications in technology and medicine. The self-assembly of molecular components at the nanoscale is fundamental to nanotechnology, as it exploits molecules' natural tendency to reach thermodynamic equilibrium. However, living organisms operate through chemical processes that occur away from equilibrium, driven by external energy. Reproducing such behavior in artificial systems is a significant challenge, but success in this area could lead to the creation of materials capable of responding to external stimuli, such as smart drugs or active materials.
In this study, the team used cyclodextrins—hollow, water-soluble molecules with a truncated cone shape—and azobenzene derivatives, which change shape under light exposure. The interaction between these components in water leads to the formation of supramolecular complexes, where the filiform azobenzene molecule is inserted into the cyclodextrin cavity.
These complexes come in two forms: complex A, which is more stable, and complex B, which forms more quickly. Without light, only the thermodynamically favored complex A is observed. However, when the solution is irradiated with visible light, the azobenzene changes from an extended form to a bent one, causing the complex to dissociate. The light can also revert the azobenzene back to its extended form, allowing the components to reassemble. Under continuous illumination, complex B becomes the dominant form, but when the light is turned off, complex A slowly reappears.
This innovative self-assembly mechanism, coupled with photochemical reactions, shows how light can be used to create and maintain unstable molecular products. This opens up possibilities for new methods in chemical synthesis and the development of dynamic materials and devices, such as nanomotors, that function under non-equilibrium conditions, much like living organisms. The study is the result of a collaboration between the University of Bologna, the University of Coruña in Spain, and the Isof-Cnr Institute in Bologna.