## 3 Results and Discussion
### Characterization of Monomers
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Compound** & \multicolumn{2}{c}{**E\({}_{\bf ox}\) (V)**} & \multicolumn{2}{c}{**E\({}_{\bf ox,_{onset}}\) (V)**} \\ \hline  & **CH\({}_{2}\)Cl\({}_{2}\)/TEAPF\({}_{6}\)** & **CH\({}_{3}\)CN/TEAPF\({}_{6}\)** & **CH\({}_{2}\)Cl\({}_{2}\)/TEAPF\({}_{6}\)** & **CH\({}_{3}\)CN/TEAPF\({}_{6}\)** & **HOMO (eV)** \\  & **0.1 M** & **0.1 M** & **0.1 M** & **0.1 M** & **0.1 M** \\ \hline
2,2\({}^{\prime}\):5\({}^{\prime}\),2\({}^{\prime\prime}\)-terthiophene (**2**) & 1.13 & 1.05 & 0.97 & 0.95 & \(-\)5.66 \\
5-Br-terthiophene (**3**) & 1.21 & 1.13 & 0.99 & 0.92 & \(-\)5.63 \\
5-ethynyl-terthiophene (**5-ET**) & 1.15 & 1.12 & 0.93 & 0.91 & \(-\)5.62 \\
3\({}^{\prime}\)-ethyl-terthiophene (**3\({}^{\prime}\)-ET**) & 1.26 & 1.19 & 1.00 & 1.01 & \(-\)5.72 \\ \hline \hline \end{tabular} Potential values referred to Ag/AgCl. HOMO values referred to electrochemical responses in CH\({}_{3}\)CN/0.1 M TEAPF\({}_{6}\).  
\end{table} 
Table 1: Electrochemical data of the terthiophene derivatives in CH\({}_{3}\)CN/0.1 M TEAPF\({}_{6}\) and CH\({}_{2}\)Cl\({}_{2}\)/0.1 M TEAPF\({}_{6}\) solvent system. Figure 2: Cyclic voltammetry responses of 2,2\({}^{\prime}\):5\({}^{\prime}\),2\({}^{\prime\prime}\) –terthiophene (**a**), 5\(-\)Br\(-\)2,2\({}^{\prime}\):5\({}^{\prime}\),2\({}^{\prime\prime}\) –terthiophene (**b**), and 5\(-\)ethynyl\(-\)2,2\({}^{\prime}\):5\({}^{\prime}\),2\({}^{\prime\prime}\) –terthiophene (**c**) in CH\({}_{2}\)Cl\({}_{2}\)/0.1 M TEAPF\({}_{6}\), potential scan rate: 100 mV/s. The black arrow indicates the starting potential and the scan direction. The analysis of UV-Vis data suggests that the presence of -Br as a substituent on the C-5 atom of the terthiophene skeleton slightly affects the maximum absorbance wavelength, whereas an -ethynyl fragment on the same position of the ring results in a red shift in the \(\lambda_{\text{max}}\) value, as well as a decrease in the Eg value. Furthermore, the comparison between **3'- ET** and **5-ET** suggests a slight lowering in Eg when the ethynyl substituent is in a peripheral position instead of on the central thiophene ring. Experimental results were compared with theoretical calculations using the TD-DFT approach. Initially, a level RHF/STO-3G was tested to obtain an equilibrium geometry of the reference molecules that is the quaterthiophene [46]. Then, different functional/basis set couples were adopted and compared, as reported in Table 3. According to such a preliminary study, PBE0 functional with split-valence (def2-SVP) basis set shows the strongest agreement with the proposed structure, although the time required is quite long. Therefore, the PBE0/def2-SVP level was selected to calculate the singlet excited states. The second step was the choice of the proper structure of terthiophene for the relative position of heteroatoms (i.e., _cis-cis_, _cis-trans_, or _trans-trans_). From a general point of view, the structure of 2,2',5',2''-terthiophene has a conformational variability due to the free rotation of aromatic rings. For this reason, a tridimensional map has been calculated to observe the changing energies as a function of dihedral angles S-C-C-S in 2,2',5',2''-terthiophene (Figure 3). The map evidences a symmetric behavior with respect to the diagonal, suggesting an equivalence of the external aromatic rings. Moreover, the _trans-trans_ configuration (with 180\({}^{\circ}\) dihedral angles) shows the lowest energy. Hence, although the situation in a solution phase is certainly more dynamic, the _trans-trans_ configuration was selected for the optimizations adopted here.