# The BCL Method for DSSC: Basis and Applications +
## 5 The \(\mathbf{I_{T}}\) Parameter: A Measure of the Electronic Density on the Anchor Group
### Results
#### 5.1.2 Analisys of Complexes  
In Table 3, the optical and electrochemical values obtained for the set of complexes under study are summarized. The acceptor behavior of the unsubstituted ppl model ligand can be observed when comparing complex 6 with 5: the presence of a ppl ligand, instead of a dmbpy ligand, causes an increase in oxidation potential from 1.09 V to 1.24 V. Electron withdrawing groups R on R\({}_{2}\)-ppl reduce the electronic density in the ruthenium atom and, consequently, the oxidation potential continues to increase. The reduction process is more sensitive to this effect, decreasing E\({}_{1/2}\). In all cases, the first and second reduction processes were assigned to the R\({}_{2}\)ppl acceptor ligand. In complex **2**, the third process was assigned to the dmbpy ligand. In complex **1**, the reduction process is irreversible; therefore, it is not possible to compare it with the other complexes. The effect of the anchor group on the maximum absorption of the MLCT band of the complexes can be evidenced by taking the value of complex 5 with R = H and \(\lambda_{\text{max}}\) = 450 nm as a reference. The presence of the COOEt and CN groups causes an hypsochromic shift in the maximum of the band to 442 and 446 nm, respectively, while when R = OH, the maximum undergoes a bathochromic shift towards 457 nm. These results show that the anchor group can influence the absorption spectrum through its donor or acceptor capacity. In other words, what is being observed is that there is a connection between the absorption spectrum and the anchor group, so that part of the absorbed energy is directed towards it. This is especially true for complexes with R = COOMe, COOH and CN. Molecular Orbitals  
The diagram in Figure 18 shows the energy levels and the composition of HOMO and LUMO for the entire series of complexes. The HOMO orbital of these compounds is composed mainly of metal atom orbitals, and only a small contribution from ligands (3%) is observed. The energy of this orbital has the meaning of an ionization potential and, therefore, linearly correlates (0.98) with the oxidation potential, as shown in Figure 19a. The influence of electron donor substituents causes a progressive stabilization of energy which is reflected in an increase in the oxidation potential. \begin{table}
\begin{tabular}{l c c c c c} \hline \hline \multicolumn{1}{c}{**Complexes**} & \multicolumn{1}{c}{**Oxidation**} & \multicolumn{1}{c}{**Reduction**} & \multicolumn{1}{c}{**Absorption**} & \multicolumn{1}{c}{**Assignment**} \\  & & E\({}_{1/2}\)/V & E\({}_{1/2}\)/V & \(\lambda_{\text{max}}\)/nm & **Assignment** \\ \hline \([\text{Ru(dmbpy)}_{2}\) ppI(OH)\({}_{2}]^{2+}\) & **(1)** & 1.22 & \(-\)0.95, \(-\)1.11 & **457** & MLCT \\ \hline \([\text{Ru(dmbpy)}_{2}\) ppI(COOEt)\({}_{2}]^{2+}\) & **(2)** & 1.28 & \(-\)0.89, \(-\)1.19, \(-\)1.47 & **442** & MLCT \\ \hline \([\text{Ru(dmbpy)}_{2}\) ppI(COOH)\({}_{2}]^{2+}\) & **(3)** & 1.29 & \(-\)0.87 & **450** & MLCT \\ \hline \([\text{Ru(dmbpy)}_{2}\) ppI(CN)\({}_{2}]^{2+}\) & **(4)** & 1.39 & \(-\)0.86 & **446** & MLCT \\ \hline \([\text{Ru(dmbpy)}_{2}\) ppI\(]^{2+}\) & **(5)** & 1.24 & \(-\)1.23 & **450** & MLCT \\ \hline \([\text{Ru(dmbpy)}_{3}]^{2+}\) & **(6)** & 1.09 & \(-\)1.47 & **560** & MLCT \\ \hline \end{tabular}
\end{table} 
Table 3: Electrochemical and UV–Vis absorption properties of \([\text{Ru(dmbpy)}_{2}\text{R}_{2}\text{ppl}]\) complexes in acetonitrile. The LUMO orbital is centered on the R\({}_{2}\)ppl ligand and its composition is a mixture of the B, Q and R fragments; the predominance of one or the other depends on the nature of the R component. For example, the electron donor substituent R = OH localizes electronic density in the bipyridine ring of ppl, while electron-acceptor substituents such as CN and COOH involve the participation of the QR fragments.