A dynamic coating like Iron Man’s armor? The movies of superheroes of the Marvel studios make us dream in particular by the futuristic prowess which let us glimpse of “lively” materials, which take any form on order and are able to self -scare. Far from science fiction, for the past twenty years, researchers have been developing adaptive materials, but these lack versatility … Another interesting path is perhaps that of molecular engines. As part of Philippe Schiel’s thesis, the teams of Nicolas Giuseppone and Jean-Marie Lehn (CNRS and University of Strasbourg), and their colleagues have succeeded in synthesizing a “dynamic” material in which a large number of molecular engines assemble and disassemble, controlled by light.
The idea of a material whose elementary components are capable of assembling in large numbers in an orderly way is reminiscent, in chemistry, the process of polymerization. Small molecules, monomers, bind each other to form long chains: polymers. The drawback is that the links, of the covalent type, are very strong. Once established, they are difficult to break, which is a drawback for the adaptability envisaged here, requiring connections that form or break up on order. In general, these covalent links are an obstacle for recycling polymers and other options are sought.
In particular, there is another type of polymers, called “supramolecular”, in which monomers are connected by non -covalent links (for example, hydrogen bonds). These connections are low and reversible – enough to provide the reversibility sought. These supramolecular polymers have already proven themselves in recyclable or self -repairing systems. However, they are often obtained by receipt methods, with heating followed by controlled cooling. This process is restrictive.
It is by chance that Philippe Schiel and his colleagues discovered a method of obtaining supramolecular polymers which only requires light excitation and molecular engines as monomers. A molecular engine is a molecule that transforms a chemical, bright or thermal energy supply into mechanical work. For example, in the living cells, by transforming ATP, Kinesin is capable of transporting organelles or vesicles along the microtubules. These organic machines inspired the chemists who imagined and synthesized in the laboratory. This led, in 1999, to the creation by Ben Ferringa of the first unidirectional rotary molecular engine fed by the light which led to the development of motorized objects, such as the first molecular car (the Nanocar).
In his thesis work, Philippe Schiel studied rotation in a constrained environment of amphiphone molecular motors (with two parts, hydrophobic and hydrophilic, which are in rotation compared to the other) distributed over a layer of water when he observed an unexpected behavior: after illumination with ultraviolet light, the engines occupied a lower surface. This result indicated that the engines had changed configuration and that they had reached a stable state, different from their original provision.
To understand what is happening, the researchers analyzed their system by microscopy by atomic force and by X -ray diffraction. They found that the molecular motors had polymerized in long lines of molecules and that they were connected by weak bonds.
Why do engines assemble between them? They are made up of four “arms”: two long hydrophilic chains and two hydrophobic chains. In the absence of light, the engines are in a stable state. Their chains extend disorderly in their favorite environment, water or air. The absorption of the energy provided by the UV rays leads to the rotation of the engines: to turn, the latter act like skaters on ice and lift their arms, which reduces their moment of inertia and makes them gain speed. The arms roll up according to an axis perpendicular to the water-to-air interface. The engines are then in an excited state. To return to a more stable state (less energy), they are approaching each other and binds thanks to the establishment of non -covalent links due to attractive stacking forces (between aromatic cycles).
The molecular engines, activated by UV light alone, thus form a supramolecular polymer with a highly structured organization. This system has a self -repair capacity: if the material is damaged, it regains its disorderly conformation; It is then enough to light it with UV light to reconstruct its structure. The discovery of this dynamic opens the way to better control at the nanometric scale of the design of dynamic materials. Iron Man’s armor may not be the objective to be achieved, but these results promote the development of active materials, capable of adapting to their environment, self-retarfare, or responding to the current problems of recycling polymers.
