Beauty in Chemistry

Mechanically interlocked objects are ubiquitous in our world. They can be spotted on almost every scale of matter and in virtually every sector of society, spanning cultural, temporal, and physical boundaries the world over. From art to machinery, to biological entities and chemical compounds, mechanical interlocking is being used and admired every day, inspiring creativity and ingenuity in art and technology alike. The tiny world of mechanically interlocked molecules (MIMs), which has been established and cultivated over the past few decades, has connected the ordinary and molecular worlds symbolically with creative research and artwork that subsumes the molecular world as a miniaturization of the ordinary one. In this review, we highlight how graphical representations of MIMs have evolved to this end, and discuss various other aspects of their beauty as chemists see them today. We argue that the many aspects of beauty in MIMs are relevant, not only to the pleasure chemists derive from their research, but also to the progress of the research itself.

Chemistry is a central science because all the processes that sustain life are based on chemical reactions, and all things that we use in everyday life are natural or artificial chemical compounds. Chemistry is also a fantastic world populated by an unbelievable number of nanometric objects called molecules, the smallest entities that have distinct shapes, sizes, and properties. Molecules are the words of matter. Indeed, most of the other sciences have been permeated by the concepts of chemistry and the language of molecules. Like words, molecules contain specific pieces of information that are revealed when they interact with one another or when they are stimulated by photons or electrons. In the hands of chemists, molecules, particularly when they are suitably combined or assembled to create supramolecular systems, can play a variety of functions, even more complex and more clever than those invented by nature. The wonderful world of chemistry has inspired scientists not only to prepare new molecules or investigate new chemical processes, but also to create masterpieces. Some nice stories based on chemical concepts (1) show that there cannot be borders on the Earth, (2) underline that there is a tight connection among all forms of matter, and (3) emphasize the irreplaceable role of sunlight.

What makes a given object look beautiful to the observer, be it in the macroscopic world or at the molecular level? This very general question will be briefly addressed at the beginning of this essay, in relation to contemporary molecular chemistry and biology, leading to the general statement that, most of the time, beauty is tightly connected to function as well as to the cultural background of the observer. The main topic of the present article will be that of topologically non-trivial molecules or molecular ensembles and the fascination that such species have exerted on molecular or solid state chemists. Molecules with a graph identical to Kuratowski’s K₅ or K_3,3 graphs are indeed highly attractive from an aesthetical viewpoint, but perhaps even more fascinating and beautiful are molecular knots. A general discussion will be devoted to these compounds, which are still considered as exotic species because of the very limited number of efficient synthetic strategies leading to their preparation. Particularly efficient are templated approaches based either on transition metals such as copper(I) or on organic groups able to form hydrogen bonds or acceptor–donor stacks. A particularly noteworthy property of knots, and in particular of the trefoil knot, is their topological chirality. The isolation of both enantiomers of the trefoil knot (3₁) could be achieved and showed that such species have fascinating chiroptical properties. Finally, various routes to more complex and beautiful knots than the trefoil knot, which is the simplest non-trivial knot, will be discussed in line with the remarkable ability of transition metals to gather and orient in a very precise fashion several organic components in their coordination spheres, thus leading to synthetic precursors displaying geometries which are perfectly well adapted to the preparation of the desired knots or links.

There exist molecules, whose shape is reminiscent of a cage, that are able to include either metal ions or anions or both. In contrast to what happens in the macroscopic world, where a kinetic barrier prevents the escaping of the guest from the cage, the inclusion–extrusion of an ion from a molecular cage is in most cases thermodynamically controlled and the ion can get in or out of the cage at will. This gives the basis for highly selective ion recognition processes by cage-shaped ligands or receptors for metal ions and anions. Nobody in everyday life would say that a cage (for birds or wild animals), even if nicely designed and splendidly decorated, was beautiful and appealing, due to the consciousness of its reprehensible function. This does not happen in chemistry and we admire the ingenuity and skilfulness of synthetic chemists for the design of cage-shaped polycyclic hosts, made for the inclusion of a variety of guests, but also capable of generating in the viewer emotion and gratification of aesthetical origin. We have tried to outline, in this chapter, the development of cages in metal coordination chemistry and in anion coordination chemistry, over the last 50 years.