text: Although van der Waals postulated the intermolecular interaction in 1873, in 1894, Fischer built a philosophical root for supramolecular chemistry. He pointed out that enzyme-protein interactions behave in a "lock-and-key" manner. This interaction is the foundation to host-guest chemistry. With the deeper understanding of the non-covalent interactions, for example, the clear elucidation of DNA structure, chemists started to emphasize the importance of non-covalent interactions. In 1967, Charles J. Pedersen discovered crown ethers, which are ring-like structures capable of chelating certain metal ions. Then, in 1969, Jean-Marie Lehn discovered a class of molecules similar to crown ethers, called cryptands. After that, Donald J. Cram synthesized many variations to crown ethers, on top of separate molecules capable of selective interaction with certain chemicals. The three scientists were awarded the Nobel Prize in Chemistry in 1987 for "development and use of molecules with structure-specific interactions of high selectivity”. In 2016, Bernard L. Feringa, Sir J. Fraser Stoddart, and Jean-Pierre Sauvage were awarded the Nobel Prize in Chemistry, "for the design and synthesis of molecular machines".
  Supramolecular chemistry refers to the chemical systems that contains discrete number chemical components. The strengths of the systems vary from the intermolecular forces to covalent bindings. The researchers mentioned above created and enlarged the area of host-guest interaction, one of the most important concepts of the supramolecular chemistry fields. There are two significant components in the host-guest interaction. One is the "host molecules", which usually have "pore-like" structure that is able to capture some other molecules. The other one is the "guest molecules", which are generally smaller than the host molecules, and capable of binding the host molecules. The driving forces of the interaction might vary in different cases, such as hydrophobic effect, chelate effect, van der Waals force, et al. Different bindings will provide variant properties for the materials, i.e., stimuli-responsiveness, self-healing, matrix rigidification. As a consequence, the host-guest interaction can be applied for self-healing materials, stimuli-responsive materials, room-temperature phosphorescence (RTP), improvement of mechanical properties, et al. The sizes of the host and guest molecules play an essential role in the interactions, and some typical examples of the host interactions will be discussed as follows.
  Host–guest chemistry is a branch of supramolecular chemistry in which a host molecule binds a so-called guest molecule or ion. The two components of the complex interact by non-covalent forces, most commonly by hydrogen-bonding. Binding between host and guest can be highly selective, in which case the interaction is called molecular recognition. Often, a dynamic equilibrium exist between the unbound and the bound states:
  :H ="host", G ="guest", HG ="host–guest complex"
  The "host" component is often the larger molecule, and it encloses the smaller, "guest", molecule. In biological systems, the analogous terms of host and guest are commonly referred to as enzyme and substrate respectively.