**4.1 Computational methods and details**

All calculations were carried out in gas phase and using four different solvents, water, ethanol, n-hexane, and methanol. These solvents were selected because they are used commonly in the process to obtain pigments in the laboratory. PCM (polarizable continuum solvation model) was employed in the present work according to its implementation in G09 program suite. Anthocyanidin geometry was relaxed with B3LYP/6-311+g(d,p), and all of them were built resembling previously reported geometric parameters but a different theoretical method was used during the set of calculations.

Geometry optimizations and vibrational frequency analyses were carried out using DFT with the well-known B3LYP approach, which includes the interchange hybrid functional from Becke in combination with the correlation functional three parameter by Lee-Yang-Parr [44] 6-311+g(d,p) basis set as implemented in the Gaussian09 program package [45]. We selected 6-311+g(d,p) because after running a set of calculations with the selected natural pigments using the reported basis set for similar organic molecules, 6-311+g(d,p) result values were comparable to the different basis sets recommended by the literature. Furthermore, several research works reported that the B3LYP/6-311+g(d,p) theoretical method provides good results with a good level of accuracy for similar organic materials [46–50]. Each geometry optimization was followed by calculations for harmonic vibrational frequencies in order to confirm that a local minimum has been reached. After vibrational frequency results are obtained, the zero-point vibrational energy (ZPVE) and the thermal correction (TC) at 298.15 K were also included to complete these calculations. Energy calculations were performed for all molecules, adiabatic energies were obtained, and with these values, global and local chemical reactivity indexes were evaluated to find the electronic properties and some of its chemical properties such as HOMO, LUMO, gap, ionization potential (IP), electronic affinity (EA), electrophilicity (*ω*), electronegativity (*χ*), and hardness (*η*). All calculations were carried out in gas phase and using four different solvents, water, ethanol, n-hexane, and methanol. These solvents were selected because they are used commonly in the process to obtain pigments in the laboratory. PCM (polarizable continuum solvation model) was employed in the present work according to its implementation in G09 program suite.

Our results are compared with results by other research teams that worked with the selected molecules with other methodologies or experimentally and also the generally accepted TiO2 was used as reference in its bulk presentation [46–50] to gain insight into the pigment application as dyes. Calculations were made for several excited states, but for practical purposes, only first excited states are displayed in the result table. Excited state calculations were carried out using TDDFT with the same theoretical method, B3LYP/6-311g+(d,p). Energy graphs and excited state spectral diagrams were developed using the Chemissian code [51].

#### **4.2 Electronic structure obtained from DFT calculations**

Energy calculations for selected anthocyanidins were carried out with the B3LYP/6311+g(d,p) theoretical model for gas phase and using solvents water, ethanol, n-hexane, and methanol. To the best of our knowledge, this theoretical method has not been reported before for these specific molecules and solvents but other research groups have used other basis sets in their works. HOMO and LUMO molecular orbitals were calculated and these values are displayed in **Table 3**. The importance of molecular orbital calculation relies in the possibility that energy

**195**

**Table 3.**

*solvents.*

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

orbitals in these pigments may overlap with a semiconductor energy orbital such as

Selected anthocyanidins within this work at their ground and excited states match well with the redox level of the electrolyte (−4.85 eV) and the conduction band edge for TiO2 (−4.00 eV) respectively, according to reported literature values

Molecular orbitals were calculated for selected anthocyanidins in gas phase and using solvents water, ethanol, n-hexane, and methanol. LUMO values for anthocyanidins are between −6.856 and −6.624 eV for gas phase LUMO molecular orbital may be the more important contribution from these pigments if used as dye sensitizers. Anthocyanidin LUMO contribution may enable molecular orbital to overlap semiconductor band gap with dye conduction band, and so, it can enable an easier

For molecules with solvents water, ethanol, and methanol caused similar effect

**Pigment Solvent H-L HOMO LUMO λ***<sup>e</sup>* **EEP λ***<sup>h</sup>* **HEP**

<sup>+</sup> Gas phase 2.664 −9.288 −6.624 0.318 5.525 0.344 10.361 Water 2.824 −6.452 −3.628 0.262 4.064 0.284 6.038 Ethanol 2.816 −6.528 −3.712 0.264 4.102 0.288 6.155 n-hexane 2.712 −7.916 −5.204 0.295 4.818 0.324 8.284 Methanol 2.818 −6.501 −3.683 0.263 4.089 0.267 6.115

<sup>+</sup> Gas phase 2.539 −9.24 −6.701 0.371 5.666 0.452 10.162 Water 2.823 −6.532 −3.709 0.294 4.172 0.46 5.946 Ethanol 2.81 −6.61 −3.8 0.295 4.216 0.462 6.066 n-hexane 2.657 −7.975 −5.318 0.335 4.968 0.479 8.169 Methanol 2.815 −6.583 −3.768 0.294 4.201 0.461 6.024

<sup>+</sup> Gas phase 2.691 −9.465 −6.774 0.364 5.703 0.498 10.371 Water 2.955 −6.668 −3.713 0.293 4.173 0.527 6.019 Ethanol 2.945 −6.748 −3.803 0.294 4.217 0.527 6.142 n-hexane 2.815 −8.166 −5.351 0.328 4.98 0.533 8.316 Methanol 2.948 −6.72 −3.772 0.294 4.202 0.527 6.100

*Selected anthocyanidins' energy results using DFT (B3LYP/6311+g(d,p)) in gas phase and with different* 

in the molecular orbitals' energy and because of these solvents' value shift in around 3 eV. These three solvents had a similar effect in HOMO molecular orbital with similar shift magnitude around 3 eV. Then n-hexane causes a smaller shift in molecular orbitals with <1.5 eV in both HOMO and LUMO. HOMO and LUMO molecular orbitals and additional energy levels are displayed in **Figures 3** and **4**. HOMO-LUMO energy difference is a good approximation to the material's band gap. For the selected anthocyanidins, energy gap was between 2.539 and 2.881 eV in

HOMO and LUMO are involved in the electronic transitions because the photoinduced electron transfers from the dye excited state to the semiconductor surface. It has been reported in the literature that dye sensitizer energy levels for HOMO and LUMO are required to match the potential of the electrolyte redox and the conduc-

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

TiO2 or ZnO for photocatalytic applications.

charge transfer process in DSSC applications.

gas phase with malvidin having the narrower gap.

*H-L is HOMO-LUMO gap energy band. All units are in eV.*

[46–50].

(C15H11O6)

(C15H13O5)

(C15H13O6)

tion band edge level of a semiconductor such as TiO2 [46].

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

orbitals in these pigments may overlap with a semiconductor energy orbital such as TiO2 or ZnO for photocatalytic applications.

HOMO and LUMO are involved in the electronic transitions because the photoinduced electron transfers from the dye excited state to the semiconductor surface. It has been reported in the literature that dye sensitizer energy levels for HOMO and LUMO are required to match the potential of the electrolyte redox and the conduction band edge level of a semiconductor such as TiO2 [46].

Selected anthocyanidins within this work at their ground and excited states match well with the redox level of the electrolyte (−4.85 eV) and the conduction band edge for TiO2 (−4.00 eV) respectively, according to reported literature values [46–50].

Molecular orbitals were calculated for selected anthocyanidins in gas phase and using solvents water, ethanol, n-hexane, and methanol. LUMO values for anthocyanidins are between −6.856 and −6.624 eV for gas phase LUMO molecular orbital may be the more important contribution from these pigments if used as dye sensitizers. Anthocyanidin LUMO contribution may enable molecular orbital to overlap semiconductor band gap with dye conduction band, and so, it can enable an easier charge transfer process in DSSC applications.

For molecules with solvents water, ethanol, and methanol caused similar effect in the molecular orbitals' energy and because of these solvents' value shift in around 3 eV. These three solvents had a similar effect in HOMO molecular orbital with similar shift magnitude around 3 eV. Then n-hexane causes a smaller shift in molecular orbitals with <1.5 eV in both HOMO and LUMO. HOMO and LUMO molecular orbitals and additional energy levels are displayed in **Figures 3** and **4**.

HOMO-LUMO energy difference is a good approximation to the material's band gap. For the selected anthocyanidins, energy gap was between 2.539 and 2.881 eV in gas phase with malvidin having the narrower gap.


#### **Table 3.**

*Solvents, Ionic Liquids and Solvent Effects*

the set of calculations.

**4.1 Computational methods and details**

implementation in G09 program suite.

**4.2 Electronic structure obtained from DFT calculations**

All calculations were carried out in gas phase and using four different solvents, water, ethanol, n-hexane, and methanol. These solvents were selected because they are used commonly in the process to obtain pigments in the laboratory. PCM (polarizable continuum solvation model) was employed in the present work according to its implementation in G09 program suite. Anthocyanidin geometry was relaxed with B3LYP/6-311+g(d,p), and all of them were built resembling previously reported geometric parameters but a different theoretical method was used during

Geometry optimizations and vibrational frequency analyses were carried out using DFT with the well-known B3LYP approach, which includes the interchange hybrid functional from Becke in combination with the correlation functional three parameter by Lee-Yang-Parr [44] 6-311+g(d,p) basis set as implemented in the Gaussian09 program package [45]. We selected 6-311+g(d,p) because after running a set of calculations with the selected natural pigments using the reported basis set for similar organic molecules, 6-311+g(d,p) result values were comparable to the different basis sets recommended by the literature. Furthermore, several research works reported that the B3LYP/6-311+g(d,p) theoretical method provides good results with a good level of accuracy for similar organic materials [46–50]. Each geometry optimization was followed by calculations for harmonic vibrational frequencies in order to confirm that a local minimum has been reached. After vibrational frequency results are obtained, the zero-point vibrational energy (ZPVE) and the thermal correction (TC) at 298.15 K were also included to complete these calculations. Energy calculations were performed for all molecules, adiabatic energies were obtained, and with these values, global and local chemical reactivity indexes were evaluated to find the electronic properties and some of its chemical properties such as HOMO, LUMO, gap, ionization potential (IP), electronic affinity (EA), electrophilicity (*ω*), electronegativity (*χ*), and hardness (*η*). All calculations were carried out in gas phase and using four different solvents, water, ethanol, n-hexane, and methanol. These solvents were selected because they are used commonly in the process to obtain pigments in the laboratory. PCM (polarizable continuum solvation model) was employed in the present work according to its

Our results are compared with results by other research teams that worked with the selected molecules with other methodologies or experimentally and also the generally accepted TiO2 was used as reference in its bulk presentation [46–50] to gain insight into the pigment application as dyes. Calculations were made for several excited states, but for practical purposes, only first excited states are displayed in the result table. Excited state calculations were carried out using TDDFT with the same theoretical method, B3LYP/6-311g+(d,p). Energy graphs and excited state spectral diagrams were developed using the Chemissian

Energy calculations for selected anthocyanidins were carried out with the B3LYP/6311+g(d,p) theoretical model for gas phase and using solvents water, ethanol, n-hexane, and methanol. To the best of our knowledge, this theoretical method has not been reported before for these specific molecules and solvents but other research groups have used other basis sets in their works. HOMO and LUMO molecular orbitals were calculated and these values are displayed in **Table 3**. The importance of molecular orbital calculation relies in the possibility that energy

**194**

code [51].

*Selected anthocyanidins' energy results using DFT (B3LYP/6311+g(d,p)) in gas phase and with different solvents.*

**Figure 3.**

*Molecular orbitals for selected anthocyanidins cyanidin, malvidin, and peonidin corresponding to (a) gas phase, (b) water, (c) ethanol, (d) n-hexane, and (e) methanol. H-L gap energy units are shown in eV.*

#### **Figure 4.**

*HOMO and LUMO molecular orbital charge distributions using B3LYP/6-311 + g(d,p), corresponding to: (a) cyanidin, (b) malvidin, and (c) peonidin.*

**197**

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

When solvents either water, ethanol, n-hexane, or methanol are added, H-L values shift slightly for all selected pigments. Malvidin in its gas phase has a lower value for gap energy, and with addition of solvents, H-L increases in all cases but malvidin with n-hexane is the narrower. Solvent addition has a more noticeable

Overall, H-L values are similar in magnitude for all selected pigments when using either solvent water, ethanol, n-hexane, or methanol. The H-L shift in all cases is <10% if compared with their H-L values for its respective gas phase. Malvidin and peonidin presented the bigger shift with 11 and 10%, respectively, with the exception of malvidin using n-hexane which had a shift of 5%. Among selected solvents, water caused the bigger H-L shift and n-hexane caused the smaller shift. Energy gap of anthocyanidins has few variations with ~0.3 eV as the mean difference between variants. Overall, planarity and the relative angle among rings have small contribution to gap energy results and predominates their family common features to determine the H-L parameter. Intramolecular reorganization energies were calculated to find the required energy for the molecule to go from neutral to ionized state (as cation if charge is lost and anion if charge is accepted). Also, these calculations help understand the inverse process when the ionized molecule becomes neutral and these two different

Values as low as possible are desirable for reorganization energies so the available energy is used in the charge transfer process instead of using the energy in reorganization processes in such a way that *λ* should be as low as possible in order to avoid wasting solar energy instead of taking advantage of sunlight during the energy transferring process. Overall, solvent addition helps the pigment decrease *λ*, and display similar values for water, ethanol, and methanol. Solvent n-hexane also helps decrease *λ* values but with less impact than the other solvents. From selected anthocyanidins, cyanidin presented lower electron reorganization energy (*λe*) using

For the hole reorganization energy (*λh*), again cyanidin had the lower values but now with methanol solvent followed by water and methanol with near values but not as close as for *λh*. Hole extraction potential (HEP) and the electron extraction potential (EEP) were calculated, and the results overall present the higher values for gas phase and the value for each case is decreased when any of the selected solvents are added. When n-hexane solvent decreases around 8 eV and with water, ethanol, and methanol as solvents HEP is around 6 eV. For EEP, a similar effect occurs but the values decrease in less than around 1 eV when water, ethanol, and methanol solvents are used and around 0.5 eV when n-hexane is used. The three variant anthocyanidins had similar values in gas phase with <0.1 eV of difference.

Reorganization energies show that malvidin is the best choice for sensitization applications. Electron energy *λe* indicates clearly that cyanidin with methanol is the

For hole energy *λh* also cyanidin with the same solvents is the best choice; this behavior with *λ* values may be attributed to its molecular planarity. EEP and HEP are not as clear as *λ*; in the case of these two parameters, malvidin with solvent water is the best choice but only with slight differences for the same solvent in other

Conceptual DFT was used to calculate the chemical properties of these three selected anthocyanidin variants. Chemical property results are shown in **Table 4**.

effect for water solvent if compared with ethanol and methanol.

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

processes relate to the charge transfer process.

best choice followed by water and ethanol.

molecules like cyanidin and peonidin.

**4.3 Chemical properties calculated from DFT results**

solvent water but ethanol and methanol had similar values.

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

When solvents either water, ethanol, n-hexane, or methanol are added, H-L values shift slightly for all selected pigments. Malvidin in its gas phase has a lower value for gap energy, and with addition of solvents, H-L increases in all cases but malvidin with n-hexane is the narrower. Solvent addition has a more noticeable effect for water solvent if compared with ethanol and methanol.

Overall, H-L values are similar in magnitude for all selected pigments when using either solvent water, ethanol, n-hexane, or methanol. The H-L shift in all cases is <10% if compared with their H-L values for its respective gas phase. Malvidin and peonidin presented the bigger shift with 11 and 10%, respectively, with the exception of malvidin using n-hexane which had a shift of 5%. Among selected solvents, water caused the bigger H-L shift and n-hexane caused the smaller shift. Energy gap of anthocyanidins has few variations with ~0.3 eV as the mean difference between variants. Overall, planarity and the relative angle among rings have small contribution to gap energy results and predominates their family common features to determine the H-L parameter.

Intramolecular reorganization energies were calculated to find the required energy for the molecule to go from neutral to ionized state (as cation if charge is lost and anion if charge is accepted). Also, these calculations help understand the inverse process when the ionized molecule becomes neutral and these two different processes relate to the charge transfer process.

Values as low as possible are desirable for reorganization energies so the available energy is used in the charge transfer process instead of using the energy in reorganization processes in such a way that *λ* should be as low as possible in order to avoid wasting solar energy instead of taking advantage of sunlight during the energy transferring process. Overall, solvent addition helps the pigment decrease *λ*, and display similar values for water, ethanol, and methanol. Solvent n-hexane also helps decrease *λ* values but with less impact than the other solvents. From selected anthocyanidins, cyanidin presented lower electron reorganization energy (*λe*) using solvent water but ethanol and methanol had similar values.

For the hole reorganization energy (*λh*), again cyanidin had the lower values but now with methanol solvent followed by water and methanol with near values but not as close as for *λh*. Hole extraction potential (HEP) and the electron extraction potential (EEP) were calculated, and the results overall present the higher values for gas phase and the value for each case is decreased when any of the selected solvents are added.

When n-hexane solvent decreases around 8 eV and with water, ethanol, and methanol as solvents HEP is around 6 eV. For EEP, a similar effect occurs but the values decrease in less than around 1 eV when water, ethanol, and methanol solvents are used and around 0.5 eV when n-hexane is used. The three variant anthocyanidins had similar values in gas phase with <0.1 eV of difference.

Reorganization energies show that malvidin is the best choice for sensitization applications. Electron energy *λe* indicates clearly that cyanidin with methanol is the best choice followed by water and ethanol.

For hole energy *λh* also cyanidin with the same solvents is the best choice; this behavior with *λ* values may be attributed to its molecular planarity. EEP and HEP are not as clear as *λ*; in the case of these two parameters, malvidin with solvent water is the best choice but only with slight differences for the same solvent in other molecules like cyanidin and peonidin.

## **4.3 Chemical properties calculated from DFT results**

Conceptual DFT was used to calculate the chemical properties of these three selected anthocyanidin variants. Chemical property results are shown in **Table 4**.

*Solvents, Ionic Liquids and Solvent Effects*

**196**

**Figure 4.**

**Figure 3.**

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

*HOMO and LUMO molecular orbital charge distributions using B3LYP/6-311 + g(d,p), corresponding to: (a)* 

*Molecular orbitals for selected anthocyanidins cyanidin, malvidin, and peonidin corresponding to (a) gas phase, (b) water, (c) ethanol, (d) n-hexane, and (e) methanol. H-L gap energy units are shown in eV.*


*Values include ionization potential (IP), electron affinity (EA), electronegativity (χ), chemical hardness (η), electrophilicity index (ω), and chemical softness (Ѕ), all of them in eV.*

#### **Table 4.**

*Chemical property results for selected anthocyanidins.*

Ionization potential (IP) is the needed energy to extract an electron from a neutral molecule in order to form a cation. This property is related with the stiffness of the electronic cloud. In regard to reactivity, the cloud is more reluctant to participate in electron transfer. Then, a lower ionization potential value is desirable so there is a higher molecular potential to serve as an electron donor. The molecule with the lower IP was malvidin in its gas phase but with solvent addition, IP decreased in all cases. Although water, ethanol, and methanol cause a similar effect in IP magnitude, it was water used as solvent in cyanidin, the variant with the lower IP value among all variants. IP in gas phase was around 11 eV for selected anthocyanidins and when water, ethanol, and methanol were used, IP decreased to values around 6 eV.

Solvent n-hexane also had a decreasing effect in IP values but the values were observed around 8 eV. Cyanidin using water and methanol presented lower IP values and other molecules like malvidin also presented their lower values with water and methanol.

Selected anthocyanidins in gas phase had EA values around 5 eV and with solvents water, ethanol, and methanol, values decreased to around 3 eV while n-hexane effect decreased the EA to around 4 eV. Regarding electronegativity (*χ*), it is calculated to estimate the capacity of molecules to attract electron pairs. The highest the *χ* value, the highest its suitability to act as a charge acceptor.

In general, selected anthocyanidins had *χ* values around 8 eV, and with solvents like water, ethanol, and methanol this value decreased to around 5 eV while n-hexane solvent effect was less with values around 6 eV.

Overall, the chemical properties estimated display some similarity among calculated values which may be attributed to molecular resemblance such as relative angle at ring B, and the differentiator relates to the small structural differences as well as their molecule constituents.

**199**

**Table 5.**

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

Excited states were calculated using the TDDFT scheme as implemented in Gaussian09 using the B3LYP/6311+g(d,p) theoretical method for selected anthocyanidins. B3LYP has been reported as an efficient hybrid functional that has been compared with several other functionals with good results [46–50, 52] to process different anthocyanins and anthocyanidins. For any DSSC to be effective, its absorption spectrum must match the solar irradiation spectrum. The absorption property of the dye determines its light harvesting capability and thus affects the

Our calculations showed that there is a slight difference with experimental values due to solvent effects and variation contributed by measuring methodologies [52, 58–60]. Two main regions in the anthocyanidin UV-Vis spectra have been reported in the literature, the first located between 260 and 280 nm and the second is located at the visible region between 490 and 550 nm. A third peak appears at 310–360 nm [59]; our discussion will focus on the principal peak located in the

> <sup>+</sup> Gas phase 1 2.546 487.1 (522\*) H → L 67% 0.507 Water 1 2.524 491.2 H → L 68% 0.619

Ethanol 1 2.528 490.4 H → L 68% 0.629

n-Hexane 1 2.473 501.4 H → L 69% 0.686 Methanol 1 2.524 491.3 H → L 68% 0.622

<sup>+</sup> Gas phase 1 2.312 536.4 (542\*) H → L 60% 0.24 Water 1 2.434 509.3 H-1 → L 30% 0.604 Ethanol 1 2.481 499.8 H → L 68% 0.591

n-Hexane 1 2.376 521.9 H → L 70% 0.627 Methanol 1 2.431 510.1 H → L 61% 0.601

<sup>+</sup> Gas phase 1 2.401 516.3 (532\*) H → L 67% 0.288

*Excited state absorption results for selected anthocyanidins using TD-DFT.*

Water 1 2.509 494.2 H → L 69% 0.53 Ethanol 1 2.564 483.6 H → L 67% 0.515 n-Hexane 1 2.465 503 H → L 69% 0.535 Methanol 1 2.505 494.9 H → L 69% 0.527

**λ (nm) Transition Contribution f**

H-1 → L 17% H-2 → L 12%

H-1 → L 15% H-2 → L 12%

H-1 → L 17% H-2 → L 12%

H-2 → L 17%

H-1 → L 11%

**(eV)**

**4.4 Excited states for absorption energy calculation using TDDFT**

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

performance of dye sensitizers in DSSCs [53–57].

**Molecule Solvent State ΔE** 

visible region.

(C15H11O6)

(C15H13O5)

(C15H13O6)

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