Many systems in biology use the energy of chemical fuels to provide function. Chemical fuels are organic materials that power things such as muscle contraction. This type of behavior is rare in artificial systems. If researchers can enable this behavior in artificial systems, materials could actively control their own functions and heal themselves. This research used difunctional molecular building blocks, meaning they have the same acidic functional group at each end. The researchers treated these building blocks with a simple chemical fuel. Next, they assembled the difunctional building blocks into large rings. These results create new possibilities for producing materials that can adapt and respond like biological systems.
This research shows that scientists can use chemical fuels to assemble molecular structures connected by multiple bonds that can form and break as desired. This science can be applied to systems like cages, greatly increasing the structural space possible in chemically fueled assembly. The result would be materials that can transport and repair themselves. These materials could also adapt to changes in the environment to maintain function. These functions are not possible with existing molecular assembly methods.
Chemical fuels provide energy to generate many important actions in biology. Artificial systems capable of these bio-inspired actions would be an important advance in materials science. However, researchers have a very limited understanding of how to use chemical fuels to achieve this goal. They have accomplished only very simple processes involving formation of a single transient bond between two different chemicals. While researchers have accomplished some remarkable properties, the ability to form many new bonds at the same time will lead to more sophisticated functions. The challenge is that as soon as two or more bonds form in an assembly process, that process also creates unwanted structures.
In this research, scientists showed that difunctional building blocks can select for cyclic products out of many other possible products in a system that is constantly changing. A carbodiimide reagent fueled assembly of diacid compounds effectively into large ring macrocycles by forming two transient anhydride bonds. This process is unselective and both ring and non-ring structures are formed. However, this process self-corrects to preferentially form the macrocycle rings. The non-ring structures fall apart much more quickly. Subsequent regeneration of anhydrides by the fuel eventually favors the more stable macrocycles. Also, components of the structures can exchange and over time this benefits the formation of the rings.
This work was supported by the Department of Energy Office of Science, Basic Energy Sciences program.