The Mechanical Bond: A Work of Art - Beauty in Chemistry - page 22

personal pleasure rather than in the context of rigorous aesthetic formalisms. We
take for granted that beauty has a place in chemistry because we know, as chemists,
that chemists are passionate in their vocation and take great pleasure in their work.
Moreover, despite a lack of formal aesthetic training among most chemists - and the
longstanding stigma associated with discussing beauty in the scientific literature - at
least 2% of chemistry papers mention [ 17 ] aesthetic values as a justification for
studying a molecule. Classic examples include a variety of synthetic Platonic and
Archimedean objects [ 18 ], such as cubane [ 19 ], dodecahedrane [ 20 ], buckminster-
fullerene [ 21 ], and many metal-ligand coordination complexes and cages [ 22 - 24 ]
that have been appreciated [ 10 ] for their symmetry, simplicity, uniformity, and
harmony: “simply beautiful and beautifully simple.” On the other hand, natural
products and their corresponding organic transformations have been admired for the
beauty in their elegance, complexity, and sophistication [ 25 ]. For perspectives on
beauty in experimental chemistry, see [ 26 ] and [ 27 ].
Molecular nanotechnology has uncovered yet another way to address beauty in
chemistry: miniaturization. Chemists now frequently “miniaturize” everyday
objects by constructing them to varying and sometimes quite liberal degrees of
approximation, using molecules as their building blocks. Our affinity for miniaturi-
zation is a consequence of two factors: (1) the development of supramolecular
chemistry [ 28 ], which has made this kind of miniaturization possible; and (2) the
emergence of a paradigm shift in molecular representations, in which molecules are
portrayed more ambiguously so as to resemble their macroscopic analogs. In other
words, the vision of a miniaturized world has been catalyzed in part by the beautiful
new ways of representing molecules, which deliberately blur the lines between the
molecular world and the macroscopic one. We refer to these graphical representations
as cartoons, illustrations, and - in this chapter - art.
In this regard, mechanically interlocked molecules (MIMs) are of particular
interest because they have played a central role in molecular nanotechnology and
the aforementioned paradigm shift to more artistically disposed figures and
schemes in the literature. Moreover, the mechanical bond is ubiquitous in the
macroscopic world, but has been, until recently, challenging to introduce into
molecules. Simply defined, MIMs are molecules with two or more components
that are not covalently connected, but cannot be separated without breaking a
covalent bond. The inseparability of the components is what makes them molecules
instead of supermolecules. Cartoon representations of two prevalent types of MIMs
are shown in Fig. 2 . A catenane is a molecule with two or more interlocking rings,

Fig. 2 Graphical
representations of the
structures of mechanically
interlocked molecules
(MIMs): a catenane ( left ) and
a rotaxane ( right )

Catenane Rotaxane