Switchable structures

The editor provides two modes for drawing the structure and the draw mode. This distinction is important for differentiating chemical information from conventional drawings. Both modes are switchable and provide an extensive set of features in the menu bar to create chemical structures and reactions, or just drawings. The number of options can be quite confusing for beginners however, one becomes accustomed to them after a short period of vocational adjustment. 

Ferromagnetic and ferroelectric materials are only two examples of a wider group that contains domains built up from switchable units. Such solids, which are called ferroic materials, exhibit domain boundaries in the normal state. These include ferroelastic crystals whose domain structure can be switched by the application of mechanical stress. In all such materials, domain walls act as planar defects running throughout the solid. 

The transformation from spheres to cyhnders is a peculiar example for the self-adjustment of the molecular conformation. The switching shape can be regarded as an example for the principle of quasi equivalency established by A. Klug for the self-assembly of biomolecules and viruses [145] for the sake of an improved intermolecular packing, the molecules adopt a conformation different from the minimum energy one. This also demonstrates that shape control does not mean a fully constrained structure. Similar to biomolecules, the combination of flexible macromolecules and self-assembly principles is a powerful strategy for preparation of molecules with well-defined but switchable shape [23]. 

Mirkin and coworkers reported on catalytic molecular tweezers used in the asymmetric ring opening of cyclohexene oxide. In this case the early transition metal is the catalyst and rhodium functions as the structural inductor metal. The catalyst consists of two chromium salen complexes, the reaction is known to be bimetallic, and a switchable rhodium complex, using carbon monoxide as the switch. Indeed, when the salens are forced in dose proximity in the absence of CO the rate is twice as high and the effect is reversible [77]. 

Burghardt, S., Hirsch, A., Schade, B., Ludwig, K., Bottcher, C., Switchable supramolecular organization of structurally defined micelles based on an amphiphilic fullerene. Angew. Chem., Int. Ed. 2005, 44, 2976-2979. 

Ionically switchable lumophore-spacer-receptor systems can also be outfitted with an additional PET inactive receptor to enhance guest binding, especially when larger organic structures are involved. For instance, GABA is one of the most important neurotransmitters with the GABA receptor being one of the most studied... 

Figure 17.13 Structure formulas of V-shaped rotaxanes 144+ and 154+, and bistable rotaxanes 164+ and 174+, used to construct switchable electronic junctions for memory and logic function purposes.114 118 119 (Adapted with permission from V. Balzani et al., ChemPhysChem 2008, 9, 202-220. Copyright Wiley-VCH Verlag GmbH Co. KGaA.)...

Electrodes represent an unrivaled platform onto which interfacial supramolecular structures can be assembled. They can be fully characterized before assembly and offer a convenient means to both probe and control the properties of the film. The interest in this area has increased dramatically in recent years because adsorbed monolayers enable both the nature of the chemical functional groups and their topology to be controlled. This molecular-level control allows the effects of both chemical and geometric properties on electron transfer rates to be explored. Moreover, these assemblies underpin technologies ranging from electrocatalysis to redox-switchable non-linear optical materials. 

The covalent connection of two tetra-urea calix[4]arenes via their narrow rims leads to molecules which are bifunctional with respect to their ability to form dimeric capsules. This was used by Rebek and Castellano to form self-assembled polycaps , including also alternating structures based on the heterodimerization of aryl- and tosylureas [42,63]. The idea was later extended to switchable polymers [64]. We were recently able to show that the covalent connection of two tetra-urea calix[4]arenes via a rigid spacer between urea functions also leads to molecules 24, which can form polymeric assemblies (Scheme 5.22) [57,65]. 

K. Gloe, H. Granbaum, M. Wiist, et al., Macrocyclic and open-chain ligands with the redox switchable trithiadiaza-pentalene unit synthesis, structures... 

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