# The BCL Method for DSSC: Basis and Applications +
## 3 Mapping Electronic Density in the Ligand Excited State
### Results  
Cyclic voltammetry measurements were carried out for the four ligands, permitting us to determine the corresponding reduction potentials. The results obtained are displayed in Figure 3a together with the calculated energy of the SOMO.  
When compared to ppl, it can be seen that the introduction of a quinone group diminishes the reduction potential in different extents, with aqphen the most easily reduced. Looking to the SOMO-calculated energy a contrasting tendency is observed, confirming that the ligand with a higher electron affinity is the one with a lower reduction potential.  
The calculated electrophilicity index, displayed in Figure 3b, shows that quinonic groups increase the acceptor capacity of the polypirydinic ligands, especially in the case of aqphen and aqphen, which possess the higher capacity to accept electronic charge.  
In a second series of experiments, the free radicals of the five ligands under study were prepared \"in situ\" by electrochemical reduction in the cavity of an EPR instrument. In parallel, simulated spectra were generated using standard software [12]. The potential applied corresponded to the one observed for the first monoelectronic wave obtained from the cyclic voltammetric experiments. The signal of the free radical obtained was registered and the coupling of the unpaired electron with N and H nucleus measured through the calculation of the hyperphyne-coupling constants. According to these values, a simulated ESR spectrum was generated, as shown in Figure 4a.  
Figure 4b displays the nucleus that interacts with the unpaired electron according the EPR data, and Figure 4c shows the Fukui function, for the different fragment components. For the simple case of ppl, according to EPR, the spin density is localized on the nitrogen and on the carbon skeleton of the phenanthroline and pyrazine fragments.  
On the other hand, the Fukui function shows changes in the spin density of the nitrogen and carbon atoms of pyrazine, while for phenanthroline only, carbon atoms are affected. In aqphen, the ppl ligand is coupled with a naphtoquinone group. In this case, the EPR spectrum displays only one signal: the free electron does not interact with nitrogen or carbon. The Fukui function reveals that the electron density is localized mainly on the carbon and oxygen atoms of the quinonic group.  
Figure 3: (**a**) Values for the reduction potentials of the ligands, and for the calculated energy of the SOMO. (**b**) Electrophilicity Index.  
For the pptd ligand, the quinone moiety is in _meta_ position instead of _para_, as in napphen. This structural change is reflected in the EPR spectrum: the Fukui function for this ligand shows a high value for the pyrazine fragment, with contributions from carbon and nitrogen atoms. It is interesting to note that the change in the relative position of the quinone groups caused the charge to be redirected and stored on the nitrogen atoms of pyrazine.  
As a whole and looking at the above results, the effect of the quinonic group in increasing the electron acceptor capacity of the ligands is evidenced. Additionally, it can be concluded that the electronic distribution is different depending on if the quinone groups are located in _meta_ or _para_ positions: in aqphen and napphen, the highest electron density is in the quinonic part. This is clearly reflected by the first reduction potential that appears at less negative potentials and the increasing value of the electrophilicity.