**1. Introduction**

The discovery, by Antoine Villiers, of the biosynthetic cyclic oligosaccharides-based seminatural products consisting of 6, 7, and 8 chiral glucose units, arranged in a donut shape and connected *via* α-1,4-gycosidic bonds, is generally symbolized as α-, β-, and γ-cyclodextrins (or ACDs, BCDs, and GCDs), respectively. They seem to be the most investigated macrocyclic host molecules in the realm of supramolecular chemistry a study of the noncovalent interactions. Naturally, they are being obtained from the enzymatic degradation of one of the most indispensable polysaccharides, that is, potato starch in the bacteria (**Figure 1**) [1]. Sometimes, they also dubbed as the enzyme-modified starch derivatives. These macrocyclic systems comprise the lipophilic inner cavities as well as hydrophilic outer surfaces of the particular interest. Interestingly, the cyclodextrins (CDs) are produced "hundreds-of-thousands" of tons every year by means of environmentally friendlier, simple yet effective techniques and methods. The CDs belong to a family of "cage molecules" in which the core of their structures is unruffled of a very stable hydrophobic cavity, having the distinctive property of encapsulating the hydrophobic entities by virtue of the invaluable host-guest supramolecular interactions. The driving strengths that operate in the inclusion complex formation are van der Waals and electrostatic interactions besides the hydrogen bonding forces. Generally, the complex formation by the CDs depends on the shape and size of the cavities of CDs, chemical nature of the guests, expulsion of the high-energy H2O molecules, and CD-CD aggregation. More importantly, their vibrant properties can easily be altered significantly through their ability of forming the inclusion complexes and also by means of their apposite functionalizations, as they contain a groups of primary as well as secondary hydroxyl functionalities at the two rims (**Figure 2**) [2]. The chemical structures of the most popular cyclodextrins, i.e. α-, β-, and γ-CDs, are depicted in **Figure 3**. As shown in **Figure 3**, the CDs have "truncated cone shape" rather than the perfect cylindrical structures because of the chair conformations of the glucopyranose units present in these types of cyclic systems. The toroidal structure of the CDs contains a panel of secondary hydroxyl groups on the wider rim, whereas the primary hydroxyl groups are present at the narrower rim side. The hydrophobic cavity is clearly displayed with an arrow inside the truncated cone as displayed in **Figure 3**. On the other hand, different structural

### **Figure 1.**

*Parent as well as the modified CDs of the pharmaceutical importance: [RAMEB: randomly methylated β-CD; HP-β-CD: hydroxy propyl β-CD; HE-β-CD: hydroxy ethyl β-CD; DIME-β-CD: heptakis-2,6-dimethyl-β-CD; TRIME-β-CD: heptakis-2,3,6-trimethyl)-β-CD].*

### **Figure 2.**

*Various possible functionalization sites to generate a variety of modified CDs.*

and physiochemical properties of the cyclodextrins are tabulated in **Table 1**—just to provide a quick glance to the readers so as to compare and have adequate knowledge of about these versatile parameters.

Remarkably, their unique "molecular encapsulation" signatures had already been immensely exploited in a myriad of industrial products, technologies, and analytical services as well. The fascination toward the researchers and industrialists worldwide could be inspected from their diverse potential applications in pharmacy, dyeing, food, medicine, biology, biomedicine, biotechnology, beverage industry, organic solar cells (OSCs), nanotechnology, environmental protection, wastewater treatment, conducting polymeric materials, semiconductors, supercapacitors, agrochemistry, remediation, "cosmetology and hygiene," catalysis, drug carriers, and ligands engineering, besides their usage in the chiral chromatographic separations (**Figure 4**) [3]. Moreover, the CDs had also been used as the crucial "bricks" in assembling the vital supramolecular architectures of the meticulous importance, such as catenanes,

*Introductory Chapter: Historical Background – Fundamental Structural and Physiochemical… DOI: http://dx.doi.org/10.5772/intechopen.107434*

#### **Figure 3.**

*Structures of α/β/γ-CDs (top), and wrong and correct pictures of CDs (bottom).*


#### **Table 1.**

*Different structural as well as physiochemical properties of the cyclodextrins.*

#### **Figure 4.** *Pictorial representation of diverse potential applications of the CDs.*

rotaxanes, polyrotaxanes, supramolecular polymeric assemblies, and so forth [4]. Commercial products entailing the CDs, used in our daily lives, are displayed in **Figure 5** [5]. In this particular book based on the CDs, we intended to showcase the new frontiers in this emerging arena with an intention to aware the readers where this wonderful field presently stands, and where it might go in years to come, though fully matured. We anticipated that this new package in the form of book based on the CDs chemistry would be much informative to the researchers working in both academia and industry. Surely, it will also be very helpful to the undergraduate and postgraduate students in addition to the young minds planning to enter into the ever-booming area of research.

*Introductory Chapter: Historical Background – Fundamental Structural and Physiochemical… DOI: http://dx.doi.org/10.5772/intechopen.107434*
