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

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

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 visible region.


#### **Table 5.**

*Solvents, Ionic Liquids and Solvent Effects*

(C15H11O6)

(C15H13O5)

(C15H13O6)

**Table 4.**

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,

*Values include ionization potential (IP), electron affinity (EA), electronegativity (χ), chemical hardness (η),* 

**Pigment Solvent IP EA** *χ η ω Ѕ*

<sup>+</sup> Gas phase 10.642 5.154 7.898 2.744 11.439 0.364

<sup>+</sup> Gas phase 10.614 5.296 7.955 2.659 11.899 0.376

<sup>+</sup> Gas phase 10.869 5.34 8.105 2.765 11.879 0.362

Water 6.322 3.802 5.062 1.26 10.165 0.793 Ethanol 6.443 3.838 5.141 1.302 10.147 0.768 n-Hexane 8.608 4.522 6.565 2.043 10.549 0.49 Methanol 6.382 3.825 5.104 1.278 10.189 0.782

Water 6.406 3.878 5.142 1.264 10.462 0.791 Ethanol 6.528 3.921 5.224 1.304 10.469 0.767 n-Hexane 8.647 4.633 6.64 2.007 10.983 0.498 Methanol 6.486 3.906 5.196 1.29 10.466 0.775

Water 6.545 3.881 5.213 1.332 10.199 0.751 Ethanol 6.67 3.922 5.296 1.374 10.209 0.728 n-Hexane 8.85 4.652 6.751 2.099 10.859 0.477 Methanol 6.627 3.908 5.267 1.359 10.205 0.736

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

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

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-hex-

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

ethanol, and methanol were used, IP decreased to values around 6 eV.

highest the *χ* value, the highest its suitability to act as a charge acceptor.

ane solvent effect was less with values around 6 eV.

*electrophilicity index (ω), and chemical softness (Ѕ), all of them in eV.*

*Chemical property results for selected anthocyanidins.*

well as their molecule constituents.

**198**

water and methanol.

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

**Figure 5.**

*Anthocyanidin excited state spectra from results using the TD-DFT scheme for gas phase and solvents water, ethane, n-hexane, and methane corresponding to: (a) cyanidin, (b) malvidin, and (c) peonidin.*

In a general view of absorption results, selected anthocyanidins in gas phase had absorption wavelength between 479.1 and 536.4 nm so, all selected molecules work in the visible part of the electromagnetic spectrum. Cyanidin works in the blue region and displays lower values calculated for wavelength. Malvidin has higher values while cyanidin presents a similar value. These results suggest that there is an effect caused by the small relative angle at B ring considering that these molecules are the simplest regarding their constituents. Addition of solvent shifts the absorption spectrum by increasing its wavelength by <5 nm in the case of water, ethanol, and methanol. For n-hexane solvent, absorption spectrum shifts the wavelength by slightly more than 10 nm.

First excited state values using TDDFT to calculate absorption data are displayed in **Table 5** and absorption spectrum is shown in **Figure 5**. The visible and near-UV regions are the most important for photon-to-current conversion to obtain the microscopic information about the electronic transitions and their corresponding MO properties.
