**2. Anthocyanidin molecular structure**

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 calculations will be included in the next section.

## **2.1 Anthocyanidins structure**

*Solvents, Ionic Liquids and Solvent Effects*

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

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 anthocya-

benefits and are commonly used colorants in food industry [9, 10].

nins 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

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

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,

particular for processes resistant to biological treatment [27].

**188**

[25, 26].

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**.


#### **Table 1.**

*Six more common anthocyanidins with their variants.*

The core of an anthocyanidin is a 15-carbon structure forming two aromatic rings (A and B in **Figure 1**) joined by a third ring (C) that contains an oxygen atom that provides the molecule positive charge. The presence of two C〓C bonds in the C ring distinguishes anthocyanidins from other flavonoids and imparts a positive charge to the molecule, which results to be a cation (known as flavylium) in its stable form at low pH [28].

The phenylbenzopyrylium core of anthocyanins is typically modified by the addition of a wide range of chemical groups through hydroxylation, acylation, and methylation. In this section, structural data obtained with DFT geometry calculations are included as displayed in the next paragraphs.

#### **2.2 Structure parameters for selected anthocyanidins using DFT**

Structure calculations are needed in DFT methodology because every analysis by this methodology needs first of all relaxed geometries able to provide fundamental data for the molecules ground states. A ball-stick model was used to represent each of the constituent atoms (**Figure 2**).

To obtain molecular initial parameters, molecular database Chemical Entities of Biological Interest (ChEBI) [29] was consulted and three selected anthocyanidin molecules were downloaded from this database. Three of the more common anthocyanidin variants were selected for DFT calculations. These anthocyanidin models were used as initial input data for our DFT calculations. Within this section, our DFT results corresponding to geometry parameters for the selected three anthocyanidins, cyanidin, malvidin, and peonidin, respectively, are included. Bond length values, angles, and dihedral angles obtained from DFT calculations are shown in **Table 2**.

In general, C-C bond length found with the theoretical methodology used within this work is near to the typical value for the case of benzene; it is known that bonds have the same length of 140 pm. Benzene C-C bond length average value is between the generally known length of single and double C-C bonds of 154.0 and 134 pm, respectively. In average, for selected molecules, C-C bond length within this work is 139.9 pm.

**191**

**Table 2.**

**Figure 2.**

*(a) cyanidin, (b) malvidin, and (c) peonidin.*

C(2)-C(1′) 1.436 1.444 1.447 O(1)-C(2)-

C(3)-C(4) 1.382 1.388 1.390 C(3)-C(2)-

C(4)-C(10) 1.403 1.399 1.397 O-C(3′)-

C(5)-C(6) 1.376 1.376 1.375 H-C(5′)-

C(5)-C(10) 1.427 1.435 1.436 O-C(4′)-

C(6)-C(7) 1.412 1.408 1.409 O-C(4′)-

C(7)-C(8) 1.395 1.398 1.398 C(8)-C(9)-

C(8)-C(9) 1.386 1.382 1.381 O(1)-C(9)-

C(9)-C(10) 1.409 1.421 1.423 C(8)-C(9)-

C(1′)-C(2′) 1.422 1.414 1.409 C(5)-C(10)-

C(1′)-C(6′) 1.414 1.407 1.411 C(9)-O(1)-

C(2′)-C(3′) 1.377 1.381 1.387

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

*Selected anthocyanidin structure in their pristine form after geometry relaxation using DFT methodology,* 

**Parameter Cyanidin Malvidin Peonidin Parameter Cyanidin Malvidin Peonidin** O(1)-C(2) 1.350 1.347 1.345 C(3′)-C(4′) 1.422 1.418 1.420 O(1)-C(9) 1.358 1.359 1.358 C(4′)-C(5′) 1.396 1.407 1.399 C(2)-C(3) 1.420 1.407 1.404 C(5′)-C(6′) 1.383 1.395 1.384

C(1′)-C(6′)

C(1′)-C(2′)

C(4′)-C(5′)

C(4′)-C(3′)

C(3′)-C(2′)

C(5′)-C(6′)

C(10)-C(4)

C(10)-C(5)

O(1)-C(2)

C(4)-C(3)

C(2)-C(1′)

*Three selected anthocyanidins' geometric parameters, bond length, and bond angles in Å and °, respectively.*

180.0 151.4 150.1

180.0 149.3 149.1

180.0 179.3 177.4

180.0 175.9 178.5

180.0 177.0 176.7

180.0 178.0 177.7

180.0 176.3 175.9

180.0 178.8 178.7

180.0 179.9 179.4

180.0 179.3 179.5

180.0 179.1 179.0

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

**Figure 1.** *Structure of anthocyanidins in their pristine form in correlation with* **Table 1***.*

*Solvent Effects on Dye Sensitizers Derived from Anthocyanidins for Applications in Photocatalysis DOI: http://dx.doi.org/10.5772/intechopen.87151*

#### **Figure 2.**

*Solvents, Ionic Liquids and Solvent Effects*

stable form at low pH [28].

of the constituent atoms (**Figure 2**).

tions are included as displayed in the next paragraphs.

**2.2 Structure parameters for selected anthocyanidins using DFT**

The core of an anthocyanidin is a 15-carbon structure forming two aromatic rings (A and B in **Figure 1**) joined by a third ring (C) that contains an oxygen atom that provides the molecule positive charge. The presence of two C〓C bonds in the C ring distinguishes anthocyanidins from other flavonoids and imparts a positive charge to the molecule, which results to be a cation (known as flavylium) in its

The phenylbenzopyrylium core of anthocyanins is typically modified by the addition of a wide range of chemical groups through hydroxylation, acylation, and methylation. In this section, structural data obtained with DFT geometry calcula-

Structure calculations are needed in DFT methodology because every analysis by this methodology needs first of all relaxed geometries able to provide fundamental data for the molecules ground states. A ball-stick model was used to represent each

To obtain molecular initial parameters, molecular database Chemical Entities of Biological Interest (ChEBI) [29] was consulted and three selected anthocyanidin molecules were downloaded from this database. Three of the more common anthocyanidin variants were selected for DFT calculations. These anthocyanidin models were used as initial input data for our DFT calculations. Within this section, our DFT results corresponding to geometry parameters for the selected three anthocyanidins, cyanidin, malvidin, and peonidin, respectively, are included. Bond length values, angles, and dihedral angles obtained from DFT calculations are shown in

In general, C-C bond length found with the theoretical methodology used within this work is near to the typical value for the case of benzene; it is known that bonds have the same length of 140 pm. Benzene C-C bond length average value is between the generally known length of single and double C-C bonds of 154.0 and 134 pm, respectively. In average, for selected molecules, C-C bond length within

*Structure of anthocyanidins in their pristine form in correlation with* **Table 1***.*

**190**

**Figure 1.**

**Table 2**.

this work is 139.9 pm.

*Selected anthocyanidin structure in their pristine form after geometry relaxation using DFT methodology, (a) cyanidin, (b) malvidin, and (c) peonidin.*


#### **Table 2.**

*Three selected anthocyanidins' geometric parameters, bond length, and bond angles in Å and °, respectively.*

Our results for C-C bonds in average for selected anthocyanidins are within the range of 1.346–1.444 Å with <0.1 Å of difference between the larger and the shorter bonds for all cases. Literature reports for geometries include different methodologies such as B3LYP/6-31G(d) and B3LYP/6-31+G(d,p) [30–33]. All reports are in agreement that B3LYP reaches accurate results for this kind of molecules and overall C-C bond lengths are in good agreement with our results.

Dihedral angles are a good indication of the planarity in a structure; for anthocyanidins, we focused more in analyzing planarity among the three rings that form the molecule skeleton within each anthocyanidin. Also, the literature reports torsion angle as a parameter related to dihedral angles and this value may be used as a factor that helps differentiate anthocyanidins and their electronic structure behavior [30]. Dihedral values show that cyanidin is a planar molecule, selected values are 180°, and in general, all dihedrals are planar or differ with <1°. Peonidin presents more dihedrals that deviate from 180° but only a couple of dihedrals deviate by more than 5°. This last observation occurs for all the selected anthocyanidins; only a couple of dihedrals deviate in a significant amount from planarity but this small difference in the planarity determines the molecule character and its chemical properties. Then, only a few dihedrals indicate a nonplanar structure; these correspond to the relative angle variation observed in the B ring compared with the rest of the structure. These situations occur in all selected structures except in cyanidin which is a planar structure as shown by its dihedral values.
