Supramolecular materials and systems honouring the 2016 Nobel Prize in Chemistry
"The lecture will illustrate the influence of Jean-Pierre Sauvage, Fraser Stoddart and Ben Feringa on the science in the general and that of our group in Eindhoven specifically. Using a historical journey showing some of the highlights, it will become evident that an important role of supramolecular chemistry emerged in the field of materials chemistry. Using nature as a source of inspiration, supramolecular materials became a new field in science. The intriguing prospects of molecular electronics, nanotechnology, biomaterials, and the aim to close the gap between synthetic and biological molecular systems are important ingredients to study the cooperative action of molecules in the self-assembly towards functional supramolecular materials and systems. The design and synthesis of well-defined supramolecular architectures require a balanced choice between covalent synthesis of discrete building blocks and their self-assembly. For synthetic chemists, the non-covalent synthesis of these supramolecular architectures is regarded as one of the most challenging objectives in science: How far can we push chemical self-assembly and can we get control over the kinetic instabilities of the non-covalent architectures made? Where the number of different components is increasing the complexity of the system is increasing as well. Mastering this complexity is a prerequisite to achieve the challenges in creating functional materials and systems."
James Fraser Stoddart
Emergent Applications in NanoScience and Supramolecular Chemistry
The time has come for us to embrace complexity—despite the fact that everyone has their own definition of it—and put much more emphasis into studying mixtures of interacting molecules. An excellent reason for responding positively to the intellectual challenge posed by systems chemistry is that complexity very often gives rise to emergent properties that are not present in the components of a complex mixture but come to light only as a result of interactions between molecules. One example of emergent behavior, which I will highlight, is provided by a class of wholly organic materials based on 1:1 and 2:1 mixtures of neutral aromatic compounds—where donors and acceptors, which also encompass stabilizing hydrogen bonding interactions—form mixed stacks that boast the welcome but elusive property of room temperature ferroelectricity. While the materials’ behavior was unexpected, the molecular basis for it is extremely simple and the superstructure leads directly to the complexity that emerges once the act of crystallization is complete. The result is a material with properties not shared by its components.
Another example is provided by the self-assembly, in aqueous alcohol, of infinite networks of extended structures, which we call CD-MOFs, wherein g-cyclodextrin (g-CD) is linked by coordination to Group IA metal cations to form metal-organic frameworks (MOFs). CD-MOF-1 and CD-MOF-2, which can be prepared on the gram scale from KOH and RbOH, respectively, form body-centered arrangements of (g-CD)6 cubes linked by eight-coordinate alkali metal cations. These CD-MOFs exhibit very different properties than g-CD itself. For a time, I was of the opinion that the nature of the anion accompanying the K+ or Rb+ cation was unimportant. Not so, because if it is AuBr4–, the situation changes quite dramatically. Yet another example of emergent behavior comes to light in the isolation of gold.
We need to come to terms with complex networks that can be periodic, aperiodic or completely random. Complex networks are everywhere to be found: they are all around us. Consider the world-wide web or global stock markets. Reflect on the way birds adopt formations in the sky during migrations or the response of different ecosystems to climate change. In the superorganism formed by certain ant colonies, the ants operate as a unified entity, working together collectively to support the colony. Prediction in the case of complex networks is nigh impossible. Uncertainty rules the roost—and the unexpected is always just lurking round the corner. While research into complex networks is commonplace in mathematics, physics and biology, as well as in computer science, economics and the engineering disciplines, when it comes to creating and understanding complex networks, chemists have been conditioned by their education and training to avoid them. We have an aversion to working with mixtures of molecules, yet complex mixtures no longer constitute an intractable problem with rapidly growing access to modern analytical tools, increasingly enlightened approaches to chemical synthesis—often involving one-step procedures starting from inexpensive and readily available starting materials—and the ability to carry out computations on integrated systems over multiple length scales in time and space.
From Catenanes and Knots to Molecular Machines
The area named "Chemical Topology" is mostly concerned with molecules whose molecular graph is non planar, i.e. which cannot be represented in a plane without crossing points. The most important family of such compounds is that of catenanes. The simplest catenane, a catenane, consists of two interlocking rings. Rotaxanes consist of rings threaded by acyclic fragments (axes). These compounds have always been associated to catenanes although, strictly speaking, their molecular graphs are planar. Knotted rings are more challenging to prepare. One of the most spectacular topologies in this respect is the trefoil knot. Our group has been much interested in knots and, in particular, in their properties in relation to coordination chemistry or chirality.
Separately, the field of artificial molecular machines has experienced a spectacular development, in relation to molecular devices at the nanometric level or as mimics of biological motors. In biology, motor proteins are of the utmost importance in a large variety of processes essential to life (ATPase, a rotary motor, or the myosin-actin complex of striated muscles behaving as a linear motor responsible for contraction or elongation). A few recent examples are based on simple or more complex rotaxanes or catenanes acting as switchable systems or molecular machines. Particularly significant examples include "molecular shuttles" as well as multi-rotaxanes reminiscent of muscles. More recent examples are those of multi-rotaxanes able to behave as compressors and switchable receptors. The molecules are set in motion using electrochemical, photonic or chemical signals. Examples will be given which cover the different approaches used for triggering the molecular motions taking place in various synthetic molecular machine prototypes.
The Joy of Discovery
Exploring across the current frontiers of chemical sciences there is vast uncharted territory to experience the joy of discovery. Far beyond Nature’s design, the creative power of synthetic chemistry provides unlimited opportunities to realize our own molecular world as we experience every day with products ranging from drugs to displays. Among the major challenges ahead in the design of complex artificial molecular systems is the control over dynamic functions and responsive far-from-equilibrium behaviour. A major goal is to gain control over translational and rotary motion. The focus in this lecture is on my discoveries in the world of molecular switches and motors as well as a personal account of my journey beyond the horizon.
Information on http://www.benferinga.com
Molecular Machines: Nature, September 2015
Molecular Switches: Chemistry World, June 2016
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