# The BCL Method for DSSC: Basis and Applications +
## 4 A Big Challenge: Understanding the Absorption Band of Ruthenium Dyes
### Results
#### 4.1.2 Absorption Spectra of Complexes  
The absorption spectra for the ruthenium complexes studied in this work show two characteristic bands: The first appears in the UV region and shows high \(\varepsilon\) values. This band can be associated with the described absorbance of the chromophoric ligand (LH or LN), although red shifted due to the effect of coordination to the metallic center. The second broad and less intense band is located in the visible region and, according to the literature is mainly assigned to an MLCT band, also considering the fact that it appears at lower energy. Regarding solar cell devices, the second band is more relevant in the injection process and, therefore, is interesting to analyze. Additionally, since the energy absorbed comes from the visible region, it could be expected that modifications of the structure of the complexes should result in a change of the energy and intensity of the band, allowing us to improve the solar cell device. A summary of their electrochemical and optical properties of C1-C4 complexes is shown in Table 1.  
The results displayed in Table 1 enable these complexes to be characterized. It can be highlighted that complexes C1-C4 exhibit enhanced absorption capacity, possessing a broad absorption band in the visible region with a high extinction molar coefficient in accordance with the proposed original design. However, in the hypothetical case that most of the absorbed energy could be delivered to the semiconductor, the injected energy should relate directly to the amount of absorbed energy and, therefore, depend on the molar extinction coefficient. Using this parameter as a predictor index, the set of complexes can be ordered in the following way: C4 > C3 = C2 > C1, where C4 should be the most suitable for DSSC. This crude prediction needs to be examined with more detail, and it becomes necessary to  
Figure 7: Types of electronic transition ocurring wihin the diferents fragments of LH and LN ligands. (IL: intraligand; ILCT: intraligand charge transfer.) disclose the nature of the absorption bands. For example, MLCT absorption that displaces electronic density to the ligand should be more relevant in DSSC than ILCT absorption. To achieve this goal, a theoretical methodology was developed, based on TDDFT calculations combined with a transition density analysis (TDA), which allowed the participation of the different types of electronic transitions (ILCT, MLCT, LL, IL) to be quantified within the absorption band. The first step of the proposed methodology consisted of expressing the composition of the molecular orbitals as the sum of several fragments, labeled Ru, LH, LN and Bp according to their main component, as shown in the energy diagram of molecular orbitals in Figure 8.  
Figure 8: Molecular orbital energies and their composition in terms of basic fragments: Ru, LH, Bp and LNMe\({}_{2}\). The theoretical MO calculations show consistency with the electrochemical experimental results for the complexes (Table 1). It is noteworthy that a correlation coefficient of 0.95 was found when comparing the first electrochemical oxidation potential with the calculated energy of the HOMO orbital for the series of complexes under study. This relationship opens the possibility to employ theoretical information to gain insight on the trend of electrochemical experimental results. From the results in Figure 8 and Table 1, the assignment of the irreversible oxidation process to the amino group can be corroborated as complexes 2, 4 possess a HOMO orbital, related to the first oxidation, centered on the dimethylamino group. The same result is obtained by a fragment analysis performed for complexes C1 to C4, in an analogous way as described before for the free ligands. In C1, the first five occupied MO of higher energy contain d pure orbitals and mixture of d orbitals with orbitals from the chromophoric ligand.