**1. Introduction**

Fundamental nature's operations are dominated and regulated by noncovalent interactions. Solvation demonstrates a key role in all these processes as it drastically influences the energetics of host-guest (Ho-G) interactions as well as the supramolecular recognition phenomena. In many occasions solvents can influence and modulate the supramolecular structure of complex systems through various possible interactions with solutes.

In recent years a rapidly increasing interest and development in the field of supramolecular engineering have been observed. Specifically within the scope of modern materials and chemical science, the conducted research is continuously growing [1]. Fundamentally, the target is to create molecular systems by design, intending to regulate the interactions of complex building blocks in the solid and liquid state and to obtain desirable complex systems exhibiting multifunctional properties. Since the recent awarding of the Nobel Prize of chemistry to Jean-Pierre Sauvage, Fraser Stoddart, and Ben Feringa, this area has gained plenty of scientific attention [2]. The microenvironment-dependent complexation of supramolecular complexes provides a conventional tool for creating a vast variety of mechanically interlocked molecules, supramolecular architectures, and molecular machines. Rotaxanes and catenanes, which are at the heart of the development of "molecular machine" chemistry [3], are of principal importance. Under this framework, molecular machines that have been studied until today include molecular motors [4], shuttles [5], muscles [6], pumps [7], elevators [8], etc. Supramolecular chemistry showed an impulsive interest in

molecular-engineered compounds, whereby complexes are formed from small molecular building blocks held together by reversible intermolecular noncovalent interactions such as van der Waals interactions, hydrogen bonding, electrostatic, π-π stacking, and hydrophobic interactions. Their design, control, and function compose new relevant interdisciplinary key enabling areas (KEA) of science and technology. In all these solvents and solvation are demonstrating a fundamentally important role. Ordinarily, solvents are categorized into two main categories cited as polar and nonpolar, whereby their efficacy is often characterized by their dielectric constants. Solvents with a dielectric constant of less than 15 are usually regarded to be nonpolar. Nonpolar solvents contain bonds between atoms with similar electronegativities, such as carbon and hydrogen. Polar solvents have large dipole moments and they comprise bonds between atoms with very different electronegativities, such as oxygen and hydrogen. The aforementioned solvents are additionally divided into polar aprotic and polar protic. The solvation efficacy in a predefined medium pays a key role in thermodynamics and kinetics especially in supramolecular host-guest interactions. This is profoundly correlated to changes in solubility, stability constant, reactivity, redox potential, and some spectral parameters. Host-guest association behaviors can essentially be controlled only by applying diverse solvent system, thus altering by demand their solvation properties. Hence, the solvation environment plays a dynamic role for supramolecular solutes, being able thus to affect the thermodynamics of complex systems. In general, the interactions involved in supramolecular systems are quite weaker than covalent bonds, and thus they can be highly controlled and reversible. Acknowledging the importance of the above, this chapter discusses the impact of solvents in various types of supramolecular systems.
