**3. Nano-mechanical motions affected by solvents**

#### **3.1 Rotaxanes and catenanes**

Rotaxanes, pseudorotaxanes, and catenanes are prominent members of the supramolecular family of compounds. They may be composed of both organic and inorganic (macro)molecules mechanically linked together. One of the latter parts may exhibit the ability to move in relation to the rest of the other parts. Due to this effect, many of these systems have been proposed for molecular machinery applications. In the case of rotaxanes bulky substitutes, the so-called stoppers are integrated in these systems preserving the stability of the supramolecular assembly. Pseudorotaxanes consist of the same structural units as rotaxanes, but do not include the aforementioned stoppering bulky substituents. In contrast, catenanes consist of two or more macrocyclic molecules tied together, forming chain-like supramolecular assemblies (**Figure 3**). The stability in all these systems is achieved through various types of weak interactions such as van der Waals forces, hydrophobic effects, hydrogen bonds, donor acceptor interactions, etc. Today a large number of scientific works have been published following the pioneering synthesis of the first rotaxane by Wasserman in 1960 [17]. Additional scientific support has been provided by a stream of publications by pioneering researchers such as Luttringhaus [18], Wasserman [17], Harrison [19], and Schill [20]. All of them dealt with the creation of functional molecular devices of high complexity and specialization.

Today multiple supramolecular structures have been created through the inclusion of a variety of linear axle-like molecules in the cavities of macromolecules such as crown ethers, cyclophanes, cyclodextrins (CD), etc. In this way numerous supramolecular systems exhibiting diverse one-, two-, and three-dimensional (1D, 2D, and 3D) architectures have been reported. The methodologies leading to catenanes and rotaxanes after assembling and pseudorotaxane formation are illustrated in the schematic representation of **Figure 3**.

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**Figure 4.**

*Solvent Effects in Supramolecular Systems DOI: http://dx.doi.org/10.5772/intechopen.86981*

*From pseudorotaxanes to rotaxanes and catenanes.*

**3.2 Molecular shuttles**

**Figure 3.**

locked systems is reviewed.

Interlocking a part of a linear molecule of a rotaxane into the cavity of a macrocycle molecule is associated with a series of complex interaction phenomena. An important function that many of the aforementioned systems can undergo is that of molecular shuttling. This often happens when a macrocycle trapped onto a linear component (axle) is capable of moving reversibly between two or more Regions on the axle (often called stations), in response to external stimuli (e.g., electrochemical

As already mentioned the main forces that hold these supramolecular structures together are relatively weak, and therefore the systems can undergo the described shuttling movement under mild external changes in a fully controlled manner [21]. Across all the potential driving forces, all the kind of energy inputs, and all aforementioned parameters, it is noteworthy that a simple change in solvent polarity can be harvested in order to induce a controllable molecular machine function (**Figure 4**). In this section the stimulating role of solvents on the function of inter-

One of the key/pioneering contributions in the field of solvent effects on the (multi)functional behavior of rotaxanes has been made by Leigh and coworkers. By applying chemistry similar to that occurring in natural systems and specifically

*Schematic illustration of controllable switching of [2]rotaxanes through external stimuli.*

stimuli, irradiation, heating/cooling, and/or solvent polarity changes) [21].

*Solvent Effects in Supramolecular Systems DOI: http://dx.doi.org/10.5772/intechopen.86981*

**Figure 3.**

*Solvents, Ionic Liquids and Solvent Effects*

Large solvent effects are also encountered in supramolecular complexation (SC) involving ionic and neutral, e.g., hydrophobic, entities. These effects are largely dependent of the nature of the target guest molecule for a given host molecule. Noteworthy, ionic and neutral SC often exhibits opposite solvent polarity dependencies. Two characteristic such examples are the SC of aromatic hydrophobic molecules by a cyclophanes and that of potassium ions by the crown ether 18-crown-6. For instance, Smithrud and Diederich observed five orders of magnitude higher association constant in water compared to the solvent carbon disulfide for a cyclophane/ pyrene SC system [13]. The hydrophobic 3D cyclophane developed by Smithrud and Diederich involved a large cavity accessible to solvent molecules, and the huge *Ka* determined in water was attributed to the solvophobic effect. In simple words pyrene prefers to be encapsulated in the hydrophobic cavity of the cyclophane instead of interacting with water. The effect becomes less and less important as one moves from water to apolar solvents [13]. The opposite effect is observed for the SC of potassium ions by ether 18-crown-6 [14]. In that case the association constant becomes larger in solvents of lower polarity, e.g., log*Ka* (H2O) = 2.0, whereas logKa (acetone) = 6.0 [14]. Interestingly, the log*Ka* exhibits a linear dependence to the surface tension of

the medium as well as to other parameters/properties of solvents [15, 16].

characteristic molecular machines and switches.

**3.1 Rotaxanes and catenanes**

**3. Nano-mechanical motions affected by solvents**

molecular devices of high complexity and specialization.

schematic representation of **Figure 3**.

The above-described examples are fundamental for the development of complex supramolecular systems with possibilities of external control and the design of molecular machines. Focus of the next section is the effect of solvents on some

Rotaxanes, pseudorotaxanes, and catenanes are prominent members of the supramolecular family of compounds. They may be composed of both organic and inorganic (macro)molecules mechanically linked together. One of the latter parts may exhibit the ability to move in relation to the rest of the other parts. Due to this effect, many of these systems have been proposed for molecular machinery applications. In the case of rotaxanes bulky substitutes, the so-called stoppers are integrated in these systems preserving the stability of the supramolecular assembly. Pseudorotaxanes consist of the same structural units as rotaxanes, but do not include the aforementioned stoppering bulky substituents. In contrast, catenanes consist of two or more macrocyclic molecules tied together, forming chain-like supramolecular assemblies (**Figure 3**). The stability in all these systems is achieved through various types of weak interactions such as van der Waals forces, hydrophobic effects, hydrogen bonds, donor acceptor interactions, etc. Today a large number of scientific works have been published following the pioneering synthesis of the first rotaxane by Wasserman in 1960 [17]. Additional scientific support has been provided by a stream of publications by pioneering researchers such as Luttringhaus [18], Wasserman [17], Harrison [19], and Schill [20]. All of them dealt with the creation of functional

Today multiple supramolecular structures have been created through the inclusion of a variety of linear axle-like molecules in the cavities of macromolecules such as crown ethers, cyclophanes, cyclodextrins (CD), etc. In this way numerous supramolecular systems exhibiting diverse one-, two-, and three-dimensional (1D, 2D, and 3D) architectures have been reported. The methodologies leading to catenanes and rotaxanes after assembling and pseudorotaxane formation are illustrated in the

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*From pseudorotaxanes to rotaxanes and catenanes.*
