1 I NTRODUCTION
1.1 General Introduction and History
The field of supramolecular chemistry focusses on the construction of highly complex, functional systems held together by intermolecular forces and is referred to as the “chemistry beyond the molecule”.[1] For a long time, chemists solely concentrated on connecting atoms by covalent bonds, whereas the field of supramolecular chemistry investigates non-covalent interactions between mol-ecules.[2] Examples for these weak and reversible interactions include hydrogen bonds, metal co-ordination, van-der-Waals forces (dispersion), π-π stacking interactions, hydrophobic effects and electrostatic interactions (e.g. ion-ion, ion-dipole).[3]
In nature, a vast number of examples can be found, in which non-covalent interactions play an important role. For example, the DNA (deoxyribonucleic acid) double helix, which carries our ge-netic information, is stabilized by complementary hydrogen bonds between the nucleobases and π-π stacking interactions.[4] Proteins and enzymes are folded into unique three-dimensional struc-tures due to stabilizing non-covalent forces (e.g. hydrogen bonds, ionic interactions).[5] Due to dis-persion interactions, lizards are able to climb up a straight wall.[6] These biological examples serve as an inspiration and motivation for chemists in all fields. Supramolecular chemistry is an interdis-ciplinary research area, which includes aspects from fields such as physics, biochemistry, biology, crystallography as well as (in-)organic and computational chemistry.[3,7]
Figure 1.1 Schematic representation of a) Dibenzo-[18]crown-6 complex by Pederson[8] b) [2.2.2]cryptand complex by Lehn[9] and c) spherand-6 complex by Cram.[10]
The field of supramolecular chemistry is one of the most vigorous and fast developing areas of chemistry with the foundation being laid in the early 1960s with the discovery of crown ethers[8] and cryptands[9] by Pedersen and Lehn. The discovered compounds showed an unexpected and di-verse ability to bind alkaline and alkaline earth metals (e.g. Li, Na, K, Rb, Cs) and led to the reali-zation, that small, complementary molecules can be designed to recognize each other by
non-covalent interactions. Soon after, in 1979, Cram studied the metal binding ability of rigid and pre-organized spherands[10] and found a stronger metal binding ability than that of the crown ether or cryptand systems.
When in 1987 the Nobel Prize in Chemistry was awarded jointly to Donald J. Cram, Jean-Marie Lehn and Charles J. Pederson for “their development and use of molecules with structure-specific interactions of high selectivity” the phrase “chemistry beyond the molecule” was coined by Jean-Marie Lehn in the Nobel lecture in Stockholm.[1] At that time, supramolecular chemistry has been accepted as an interdisciplinary but independent field of contemporary research. In the following years, the field of supramolecular chemistry experienced a rapid expansion and the number of supramolecular systems and unique structures increased continuously. With the rising interest and development of new analytical techniques,[11] novel chemical systems with high diversity and com-plexity were developed.
A fascinating approach was performed by Sauvage in order to from a so-called catenane[12] (see Figure 1.2a), which consist of two (or more) mechanically interlocked molecules that cannot be separated without breaking a bond.[13,14,15] Wassermann already synthesized a similar system in an earlier study. However, the synthesis by pure organic chemistry was tedious and nerve-wrack-ing.[16] Sauvage’s strategy improved the yield and accessibility of catenanes by using a metal ion (e.g. copper) as a template. With this strategy, fascinating new intertwined structures like links and knots could be formed. Some stunning examples are the Borromean rings[17], tre[18]- or pentafoil[19]
knots and the Solomon’s knot[20] (see Figure 1.2b-c).
Figure 1.2 Schematic representation of different links and knots: a) Catenane, b) Borromean rings, c) trefoil knot, and d) Solomon knot.
Another class of supramolecular structures are the so-called “Rotaxanes”,[13] which consists of a dumbbell shaped molecule, which is threaded through a macrocycle. High yields of rotaxanes could be obtained by preorganizing the components utilizing non-covalent interactions (e.g. hydrogen bonds, metal coordination, coulomb interactions etc.). In 1991, Stoddard showed, that the ring can be moved along the rod[21] (see Figure 1.3a). Controlled movement or switching of the macrocycle along the thread was achieved by applying an external trigger, for example: oxidation/reduction, irradiation with light, pH variation or addition of a chemical trigger (e.g. metals, crown ether).[13] With the use of these principles, the formation of a shuttlebus rotaxane[22] or the mimicry of a molecular muscle[23] (see Figure 1.3b) was realized.
3
Figure 1.3 Schematic representation of a) molecular shuttle[22] and b) molecular muscle.[23]
In 2004, Stoddard could enhance the complexity of the rotaxane chemistry by synthesizing a mo-lecular machine that behaves similar to an elevator (see Figure 1.4).[24] The supramolecular as-sembly consists of a platform-like component interlocked with a trifurcated ring-like component and operated by pH variation. The traveled distance of this nano elevator is 0.7 nm.
Figure 1.4 Schematic representation of a molecular elevator.[24] Addition of base causes the platform (red) to move to the lower level. Addition of acid results in a lift of the platform to the upper level.
Feringa introduced the first unidirectional molecular rotor in 1999.[25] Irradiation with light resulted in a rotation around a central bond and by variation of temperature, the direction of this rotation was controlled. In 2011, this principle helped in the synthesis of the smallest car on earth. This 4-wheeled “vehicle” is only 1 nm long and can move in one direction along a copper-surface.[26]
In 2016, the achievements of Jean-Pierre Sauvage, Sir Frazer Stoddard and Bernard L. Feringa were awarded jointly with the Nobel Prize in chemistry for “the design and synthesis of molecular machines”. This shows, that the fascinating field of supramolecular chemistry is of high impact and of broad interest.[27] The developed molecular machines and other supramolecular structures are only of fundamental research interest, but applications in medicine, computer science, as smart materials or as energy storage devices are just one-step away.