Rotaxane molecules

Besides self-assembly on the surface of metals, another example of self-assembly is the formation of Langmuir-Blodgett monolayers on the surface or at the interface of liquids.47,48 One approach to molecular-based electronic devices that has made use of self-assembled Langmuir-Blodgett films is the crossbar-based defect-tolerant approach to molecular computing 49 Rotaxane molecules that depend on physical dislocations within the molecule to produce switching functionality have been designed such that they will... 

FIGURE 5.9. A rotaxane molecule used as a component of a Langmuir-Blodgett film in the crossbar-based defect-tolerant approach to molecular computing. 

One rotaxane molecule is shown in Figure 5.9. This molecule is not expected to demonstrate bistable switching, but is a candidate for a molecular switch that can be configured one time. Note that both ends of the molecule are non-polar hydrocarbon moieties while the center of the molecule is polar. Molecules that have both a non-polar and a polar end, for easier formation of Langmuir-Blodgett films, have been synthesized and tested in devices.51... 

Examples of cyclodextrin-based [2]rotaxanes, which reflect the diversity of possible structures, are depicted in Figure 6.3. Aliphatic as well as aromatic compounds can complex with CDs. Different end groups can be used in the rotaxanes to prevent the dethreading of the CD, even within one rotaxane molecule as in 2 [32]. 

It is worth noting that no alignment of the rotaxane molecules with respect to the cantilevers is required to observe a bending effect because only the component of the contraction that is parallel to the long axis of the cantilever contributes effectively to the bending. 

Two-terminal devices might seem more natural for the molecular-scale systems than three-terminal ones because of the technological difficulties in manipulating small structures. Furthermore, chemical assembly of molecular devices usually results in a periodic structure. This observation resulted in the idea to have a two-terminal switch, electronically reconfigurable, where a relatively high voltage (e.g. —2V or +2V in [62], which uses a 2-catenane-based molecule) (Fig. (7a)) is applied to close or open the switch, but a relatively low voltage to read (M). 1 V) [60]. These molecular switches [62], a mono-layer of rotaxane molecules, are not field-activated but can be described as small electro-chemical cells, which are characterized by... 

Figure 17.9 Schematic representation of the surface-bound photoswitchable rotaxane 10.104 Such a device is capable of transducing an optical signal into an electronic signal by means of the photocontrolled ring shuttling in the rotaxane molecules. (Adapted with permission from V. Balzani et al., ChemPhysChem 2008, 9, 202-220. Copyright Wiley-VCH Verlag GmbH Co. KGaA.)...

The binding of ammonium ion, NH4+, by 18-crown-6 was demonstrated early in the history of macrocycle chemistry. The O-H-N in crowns or N-H-N interaction in azacrowns has been characterized by a variety of techniques, including NMR, calorimetry, and X-ray crystal structure analysis. Recent studies in this area have shown that both quaternary and secondary ammonium salts can form complexes with crowns. In the latter case, a rotaxane molecule was prepared by treatment of a dibenzylammonium salt with dibenzo-24-crown-8 and other macrocycles, including pyrido-24-crown-8. The solid-state structure of the pseudo-rotaxane structure obtained with pyrido-24-crown-8 is shown in the left panel of Figure 23. 

Several macrocyclic compounds have been prepared using Wittig reactions including tetraepoxyannulenes, which utilized the ylide derived from phos-phonium salt (57), and rotaxanes, molecules in which one or more macrocyclic components are trapped around a rod section of a dumbbell-shaped molecule, obtained by treating terephthaldehyde derivatives with dibenzylic bis(triphenyl-phosphonium)-stoppered [2]rotaxane (58). Aromatic triesters have been obtained by the (ethoxycarbonylmethylene)triphenylphosphorane-catalysedcyclo-trimerisation of ethyl propynoate. ... 

Table 4. Third order NLO susceptibilities and molecular second hyperpolarizabilities of simple catenane and rotaxane molecules as determined by the optical third harmonic generation technique...

Let us assume that we can set up a monolayer (2D array) of switchable rotaxane molecules on an appropriate planar surface and that we have a suitable means to switch individual molecules in known positions. Furthermore, let us assume that switching takes place by an electron-transfer mechanism, such as benzidine oxidation to take a familiar case. In this case, if... 

The cellular unit that is active toward the contraction of skeletal muscles, known as the sarcomere, is comprised of alternatively stacked filaments of the proteins actin and myosin. During muscle contraction, the protein filaments slide past each other as a result of a rowing action of the surface myosin heads (Figure 6.92a). Hence, an effective biomimetic approach would entail the design of a linear architecture that features sliding components that will respond to a chemical stimulus. This approach has recently been demonstrated with the design of a rotaxane molecule that exhibits redox-controlled contraction and extension of the molecular architecture, in response to a chemical or electrochemical stimulus (Figure 6.92b). ... 

If one can incorporate one binding site into the axis, then of course, having two of them is not a fundamental problem, and if the ring prefers to bind one under one kind of condition, and the other if the condition is changed, we have a true molecular switch. For instance, a change in pH could be used to switch a rotaxane molecule between its two states, if it is designed accordingly. 

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