**1. Introduction**

Organic pigments have raised great interest in late years, may be driven by their potential in renewable energy applications which has been reinvigorated with the invention of dye-sensitized solar cells (DSSCs). Dye-sensitized solar cells (DSCs) are an attractive solar energy conversion technology and present advantages that include low cost of manufacture, ease of fabrication, and modifiable features such as color and transparency [1–5]. First DSSCs employed ruthenium (II)-based dyes in conjunction with iodide-based electrolytes to achieve an 11.9% solar-to-electric power conversion efficiency (PCE) [6]. A new generation of DSSCs based on naturally obtained pigments is a great option as dye sensitizers, and this is the reason of a revitalized interest in these pigments [6, 12]. For example, porphyrin-based dyes have been tested as viable options, and they displayed great flexibility to work as panchromatic sensitizers [6, 13, 14]. It has been reported that porphyrin chromophore has strong light absorption around 400 nm in the blue region which is known as the Soret band or Soret peak and also in the Q-bands which is a region between slightly over 500 and 620 nm, but presents weak absorption in the region between these two features [6].

Then, it may be considered an interesting green option using organic pigments in DSSC technology. Analogously, these principles may be used to decompose chemical pollutants naturally without any contaminant waste. These organic pigments possess environmentally friendly properties, easy accessibility, and high absorption in the visible region which make them good candidates [7]. An alternative organic pigment to porphyrins may be anthocyanidins, a group of flavonoids contained in different parts of plants such as fruits, leaves, and flowers. Anthocyanidins may be considered water-soluble plant pigments that usually carry colors ranging from red to blue [8]. These natural pigments have shown health benefits and are commonly used colorants in food industry [9, 10].

Researchers continue to look for viable alternatives to ruthenium-based dyes and DSSC components in order to increase efficiency [11–15]. Natural pigments represent, in particular, a good option and among them anthocyanins are within our research interest [13–15]. These pigments have shown relevant advantages in DSSC technology, for example, they are metal-free, nontoxic, widely available, and inexpensive. They also have hydroxyl groups that benefit binding with TiO2 and have been shown to be able to inject electrons into the TiO2 conduction band at an ultrafast rate when excited with visible light [16]. There have been several studies on DSSC using anthocyanins as a photosensitizer with promising results [17–21]. The efficiency (*ƞ*) from those studies, however, was generally quite low (0.5–0.6%). Recently, one of these reports [22] was carried out using sealed solar cells with enhanced electrodes (multilayer TiO2 film plus a scattering layer), and anthocyanins contained organic acids demonstrated an efficiency of around 1.0%.

In nature, dyes can absorb visible light to enable plant photochemical processes; many of them are able to inject an electron into the conduction band of the semiconductor which is fundamental for photocatalytic processes [23]. This property is of great interest in dye-sensitized solar cells (DSSC), where dyes are used with a photocatalyst that may be a semiconductor oxide such as TiO2 or ZnO for example [24]. An important consideration relates to prevent the degradation of the dye on a DSSC but this may not be the case for aqueous suspension of dye and photocatalyst. In such case, it may be confusing whether the dye degradation is due to dye sensitization itself or by action of the photocatalyst or under the influence of both factors [25, 26].

In regard to chemical processes, for chemical decoloration, the oxidation method is the most used because of its easy application. This method may be found in the literature as chemical oxidation and advanced oxidation process. Both these methods achieve the degradation of chemical dyes, pesticides among other pollutants, either partially or completely under ambient conditions [27]. The advanced oxidation process may be categorized in photocatalytic oxidation (use of light for activation of catalyst) and Fenton chemistry suitable for treating wastewater in particular for processes resistant to biological treatment [27].

Then, dye-sensitized process may be used in other applications and in this chapter, we will refer to its application in photocatalysis. This process uses light to activate a photocatalyst and represents a potential application to take advantage of sunlight for diverse processes such as gas purification, H2 production, and water treatment. There are limitations for dye-sensitized semiconductors; for example,

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**Table 1.**

*Solvent Effects on Dye Sensitizers Derived from Anthocyanidins for Applications in Photocatalysis*

Among the different natural pigments, anthocyanidins represent an interesting alternative as dye sensitizing naturally obtained pigment. Since dye sensitizer and DSSC advances may be used in solar technology applications such as photocatalysis, this chapter presents interesting information related to anthocyanidins focused on its potential application in renewable energy applications and in particular when used as dye sensitizing pigment in photocatalysis. In particular, the chapter presents information related to an analysis of the effects caused by commonly used solvents to obtain anthocyanidins such as gas phase (as comparative basis), water, ethanol, n-hexane, and methanol and includes discussion on how several electronic properties of interest are subject to different effects in consequence depending on the selected solvent.

In this section, anthocyanidin structural data from published references and also our own results obtained with DFT methodology are presented. For convenience, structural data are presented in this section and computational details used in DFT

Anthocyanidins are natural pigments commonly found in plants with a molecular structure based on the flavylium ion or 2-phenylchromenylium (chromenylium may be referred to as benzopyrylium). These natural pigments are salt derivatives of the 2-phenylchromenylium cation, commonly known as flavylium cation. The more common anthocyanidins and their substitution pattern are shown in **Table 1**. The phenyl group at the 2-position can carry different substituents that determine a particular anthocyanidin. With a positive charge, anthocyanidins differ from other flavonoids. Pigment molecule substituents and features are summarized in **Table 1** with a general interpretation of structural differences amongst variants,

and a general scheme for anthocyanidins is displayed in **Figure 1**.

**Name Chemical formula Substitution pattern Color**

**R1 R2**

<sup>+</sup> OH OH Orange-red

<sup>+</sup> OH OH Blue-red

<sup>+</sup> OCH3 OCH3 Blue-red

<sup>+</sup> H H Orange

<sup>+</sup> OCH3 H Orange-red

<sup>+</sup> OCH3 OH Blue-red

in water purification, the organic dye may be diluted by water or at least erodes or deteriorates from the photocatalyst surface due to continuous interaction with water. Of course, the specific device configuration used during water treatment defines the disposition and interaction of the photocatalyst with the liquid and ultimately defines the severity of the fluid effects on the sensitizer layer. Dye sensitization effect may be a good choice for water treatment, but its effectiveness depends on the device disposition and the sensitizer presentation mainly when it

*DOI: http://dx.doi.org/10.5772/intechopen.87151*

gets in touch with the polluted water.

**2. Anthocyanidin molecular structure**

calculations will be included in the next section.

**2.1 Anthocyanidins structure**

Cyanidin (C15H11O6)

Delphinidin (C16H11O7)

Malvidin (C15H13O5)

Pelargonidin (C15H11O5)

Peonidin (C15H13O6)

Petunidin (C15H12O6)

*Six more common anthocyanidins with their variants.*
