# The BCL Method for DSSC: Basis and Applications +
## 4 A Big Challenge: Understanding the Absorption Band of Ruthenium Dyes
### Results  
#### 4.1.1 Absorption Spectra of Ligands  
Absorption spectra of the chromophoric ligands LH and LN show a broad and intense band centered at 317 and 390 nm, respectively, as can be seen in Figure 6, where results from TDDFT calculations on the gas phase, performed with the ADF package, are also displayed.  
The absorption bands of ligands LH (\(\lambda_{\text{max}}\) = 317 nm, \(\varepsilon\) = 90,000) and LN (\(\lambda_{\text{max}}\) = 390 nm, \(\varepsilon\) = 80,000), can be attributed at first to an MLCT process, which appears to be displaced by lower energies when compared to bipyridine (\(\lambda_{\text{max}}\) = 280 nm). This red shift has been attributed to the presence of the aromatic \(\pi\)-conjugated system. The effect is enhanced in the LN ligand, due to the presence of the electron-donating aminoalkyl substituent. An ILCT contribution [14; 15; 16; 17] to the absorption can also be expected. The addition of a small amount of acid to a solution of the ligand diminishes the intensity of the 390 nm band, probably due to the protonation of the amino group which, therefore, can no longer participate in the ILCT transition. This experimental test supports the possibility of an ILCT contribution.  
Figure 5: General Structure for unsubstituted (R = H) and substituted (R = NMe\({}_{2}\)) Chromophoric Ligand (LH, LN) and Ruthenium Complexes. C1: [Ru(bpy(COOH)\({}_{2}\)LH)2\({}^{+}\); C2: [Ru(bpy(COOH)\({}_{2}\)(LNMe\({}_{2}\))]2\({}^{+}\); C3: [Ru(bpy(COOH)(LH)\({}_{2}\)2\({}^{+}\); C4: [Ru(bpy(COOH)(LNMe\({}_{2}\))]2\({}^{+}\).  
A theoretical analysis combining TDDFT calculations with a molecular orbital description, in terms of the three-fragment composition of the ligand, allows us to understand the nature of the observed band. If the fragments are called F (phenyl), V (vinyl) and P (piridyl), it becomes possible to assign an intraligand (IL) character of an excitation when the charge density is displaced from the FV fragment to the P fragment. A \(\pi\)-\(\pi^{\star}\) character in an excitation from V to V, and an intraligand charge transfer character (ILCT) to an excitation started on the amino substituent of the F fragment and ended on the bipiridyl group. Applying this approach to the experimental results, it follows that the LH absorption spectra contains a sum of IL + \(\pi\)-\(\pi^{\star}\) electronic transitions, while the LN ligand is mainly composed by ILCT excitations.  
The three processes depicted in Figure 7 also represent three types of electronic displacement of the electronic cloud that surrounds the carbon skeleton of the mentioned polypiridinic ligands. For the first case (IL), electronic density localized on the FV fragment moves parallel along the XY plane to the bipyridine empty \(\pi^{\star}\) orbitals through a process that can be assimilated to a simple deformation of the electronic density. The intraligand charge transfer process shares some characteristics with the IL since it also occurs along the XY plane, ending in the P fragment. However, they differ in terms of the origin of the excitation; for the case of ILCT, is the origin is located in atomic orbitals of the amino substituent in the F fragment. As a consequence of this electron transfer, an induced dipole moment is created along the molecule axes.  
Figure 6: Experimental absorption spectra of (**a**) LH and (**b**) LN ligands in acetonitrile solvent. TTDFT-corresponding calculations are shown in bars.  
The electron transition, occurring within the FV fragment, involves mainly \(\pi\) and \(\pi^{*}\) molecular orbitals of the vinyl fragment. This excitation occurs parallel to the z axis and perpendicular to the molecular plane. Looking to their directionality, both IL and ILCT occur in the same plane, while \(\pi\)-\(\pi^{*}\) are perpendicular to them. ILCT provide greater directionality than IL, due to the induction of a change in the dipole moment of the molecule.