Supramolecular Structures in the Presence of Ionic Liquids

Abstract

Supramolecular chemistry is defined as “chemistry beyond the molecule”, bearing on the organized entities of higher complexity that result from the association of two or more chemical species held together by intermolecular forces (Lehn, 1988, 1995). Supramolecular chemistry may be divided into two broad and partially overlapping areas concerning: (1) supermolecules, well-defined and discrete oligomolecular species that result from the intermolecular association of a few components (a receptor and its substrate(s)) following a built-in scheme based on the principles of molecular recognition; (2) supramolecular assemblies, polymolecular entities that result from the spontaneous association of a large undefined number of components into a specific phase having more or less well-defined microscopic organization and macroscopic characteristics depending on its nature (such as micelles, microemulsions, vesicles, films, layers, membranes, mesomorphic phase and so on) (Lehn, 1995). Investigation on supramolecular systems is receiving more and more attention (Beletskaya et al., 2009; Constable, 2008; Descalzo et al., 2006; He et al., 2008b; Metrangolo et al., 2008; Oshovsky et al., 2007). Different from simple inorganic salts (such as NaCl) who often melt at very high temperature, ionic liquids (ILs) are a kind of organic salts that are liquid at or near room temperature, always taking 100 oC as an upper limit. Figure 1 shows the structures of some typical cations and anions of ILs. The imidazolium, especially 1-alkyl-3-methylimidazolium (Cnmim+), is one of the most popular cations. So far, ILs have attracted much attention and been widely used as an attractive class of green solvents in the filelds of chemical reactions, organic and material syntheses, solvent extraction, and electrochemistry because of their negligible vapor pressure, nonflammability, high thermal and chemical stability, high polarity, wide electrochemical window and tunable physicochemical properties (Chiappe & Pieraccini, 2005; Dupont, 2004; Leclercq & Schmitzer, 2009a; Weingartner, 2008; Welton, 1999; Zhao et al., 2002). In the field of supramolecular chemistry, ILs could either participate directly in the assembly of supramolecular organizations, or influence the assembly of various supramolecular structures in a certain way, leading to the appearance of many novel and interesting phenomena. In addition, there exist three-dimensional supramolecular networks with polar and non-polar regions in imidazolium ILs, which can be used as powerful supramolecular receptors (Dupont, 2004; Leclercq & Schmitzer, 2009a). Therefore, this review will summarize the work in the literature, concerning the above novel and