Living matter is undeniably the most functional form of matter, displaying sophisticated properties currently unattainable with man-made materials. Nonetheless, the operating range of most biological systems is limited to the conditions compatible with life (i.e. aqueous media, 20-40 °C, biocompatible molecules… etc.) and, most often, these do not reflect the needs of man-made technology. This deficiency can be surmounted by extracting the principles underlying a biological function of interest and, subsequently, devising an artificial chemical framework (i.e. set of molecules and conditions) capable of hosting them. The transfer of biological functions in man-made technology holds strong promise for triggering far-reaching technological innovations (e.g. self-healing materials). Besides, it may also enhance our understanding of the functional and environmental constraints of biological systems.
To host biological-like functions, an artificial chemical framework should enable a precise control of the organization of large and diverse sets of reversibly linked molecules. In addition, this framework should also allow for control over how this organized state adapts to changes in its environment (i.e. chemical or physical stimuli). How could one design such a framework?
The constitution of the chemical species generated following the principles of covalent and coordination chemistry can be made plastic/dynamic via the use of reversible covalent bonds and labile metal ions, respectively. Indeed, species built around dynamic linkages (e.g. reversible imine bonds and labile metal-ligand interactions) can exchange, incorporate and expel building blocks. Consequently, by favouring or preventing the associations of specific building blocks, one can gain control of the organization of complex mixtures of molecules, either at the level of molecules (i.e. molecular level) or at the level of assemblies of molecules (i.e. supramolecular level). A finer control of the organization—and hence the properties—of chemical systems can be achieved by using simultaneously in one system dynamic features at the molecular and the supramolecular levels, thus providing a control of the organization of the system at both the molecular and the supramolecular levels. Prime examples of these so-called constitutional dynamic systems can be found in systems operating via the condensation of amine and aldehyde components into dynamic imine-based ligands around labile metal cations.
Such constitutional dynamic systems have unlocked the path to chemical structures that would have otherwise been inaccessible by traditional synthetic means and from which complex properties reminiscent of biological systems emerged. The range of properties accessible with these systems could be further extended by assembling simultaneously—but selectively—several of these constitutional dynamic architectures within the same reaction mixture by relying on the concept of self-sorting. Self-sorting describes the ability of the individual components of a complex mixture to recognize specific partners during their self-assembling, yielding specific products rather than a statistical collection of products. However, to access the most complex biological functions, the compositional diversity of these constitutional dynamic systems (i.e. the number of architectures simultaneously assembled) will have to be increased even further.
Nevertheless increasing the compositional diversity of constitutional dynamic systems comes at a cost: an “informational cost”. As the system becomes more complex, more delicate structural and interactional information is required to prevent the crossover, within the different architectures assembled, of components participating in dynamic processes taking place in the same domain. This “informational cost” grows rapidly as the number of species assembled through the same type of dynamic processes increases. For this reason, the majority of self-sorting systems involving constitutional dynamic metal-organic architectures known to date occur between architectures sharing one to two organic components and/or built around no more than two different types of metal cations. This limited compositional diversity reflects the need for strategies to simultaneously control the outcome of two (or more) interconnected dynamic processes over several architectures, namely reversible covalent imine bond formation and dynamic metal-ligand coordination.
