**1. Introduction**

Alizarin is a stable organic compound, prominently known as a red dye with significant industrial applications, particularly its use in dying textile fabrics. The application in textile coloration industry is inspired by the fact that alizarin is a natural compound often referred a natural dye, initially extracted from the roots of plants of the madder genus [1], before it was synthetically made [2], thus, it is a molecular species from nature, exhibiting and portraying green chemistry properties. The molecular structural framework of alizarin is characterized by the anthraquinone moiety bearing two para-positioned intermolecular hydroxyl groups both on one carbocyclic ring adjacent to the quinone ring. A typical excited state intermolecular proton transfer system (ESIPT), alizarin is a natural dye which has been widely used in pigments, as anticancer agents as well as chemical reagents for use in data recording and storage materials due to its tunable electronic properties [3, 4]. In addition, the natural dye has strong antigenotoxic activity, ascribed to the transfer of ultrafast electrons to TiO2-based materials, which can also perfectly fit as an excellent photosensitizer in dye-sensitized solar cells. Thus, alizarin chromophore has been favored by many researchers, both experimentally and theoretically [5–10].

Ideally, alizarin forms an intramolecular hydrogen bond between a hydroxyl and a carbonyl group, in the ground and excited states, whereby upon photoexcitation, a proton transfer from the hydroxyl to the carbonyl group is observed, which normally results in dual emission bands of the locally excited (LE) and proton-transferred (PT) tautomers [9, 11–13]. Characteristically, this process in known as ESIPT, which is viewed as a very fast photo-tautomerization process taking place along an intramolecular hydrogen bond between two atoms that are significantly tuned by electronic excitation. In recent studies, the practical and applications of the ESIPT mechanism based on their photophysical characteristics and properties have been extensively explored and investigated, especially in laser dyes, OLEDs, molecular switches, fluorescence sensors, and particularly biological systems [14–20]. More importantly, the ESIPT based reactions increase the acidity of the proton donor groups, due to the change of electron density after electron excitation, and the basicity of the acceptor groups is significantly increased to promote the formation of tautomer by intramolecular proton transfer [4, 21–26].

On the other hand, molecular recognition has been the epic center of supramolecular chemistry due to its significant role in biological and environmental systems, through the host-guest interaction chemistry. Consequently, chemosensors are designed for specific target analytes based on their chemical make-up and complementarity towards each other. The impact of sensing biologically important anions such as acetate, cyanide, fluoride, dihydrogen phosphate, etc., have been receiving attention in literature and many industrial applications. A large volume of colorimetric and fluorometric probes for anions such as fluoride (F-), cyanide (CN-), acetate (AcO-), dihydrogen phosphate (H2PO4 − ), hydroxide (OH-) and others have been developed. Hydroxide ions play a very significant role in environmental and physiological systems, thus monitoring its concentration in these systems must be highly prioritized [27–37]. Moreover, the presence of soft (donor) atoms such as oxygen from hydroxyl and quinone groups of the carboxylic ring raises the prospect of dual sensing, for both cations and anions, which stems from the presence of both, the anion receptors (-OH) and the cationphilic groups, through coordination induced interaction [38–41].

Herein, we have conducted a comparative study for the two alizarin-based derivatives, **A3** and **AS3** (**Figure 1**), to investigate their chemosensing properties. The study literally focuses on the effect of the sulfonyl hydroxide group (-SO3H) present in **AS3**, which is the only difference between the two chemical entities. The two entities are highly stable and available commercially, which are very rich in hydroxyl groups, paving ways for possible hydrogen bonding based charge transfer bonding. The two dyes displayed interesting behaviors in the presence of anions and cations, in water-soluble acetonitrile (CH3CN) solvent, with certain commonalities and variations upon interacting. Thus, the two probes (**A3** and **AS3**) can be used as colorimetric/fluorometric probes for discriminating specific cations and anions, with distinctive color changes in organic-aqueous solvents.

*The ESIPT-Steered Molecular Chameleon for Cations and Anions Based on Alizarin… DOI: http://dx.doi.org/10.5772/intechopen.103829*

**Figure 1.** *The molecular 2-D structures of (a) alizarin (***A3***) and (b) alizarin S (***AS3***).*
