## Materials and Methods
### Synthesis and Characterization
A comparison between the HOMO values of the parent species **1e** and the derivative **1b** confirms that the presence of the \(-\)Et substituent on the bpy ligand induces a stabilization of the HOMO. Despite the trend observed in anodic potential values, the HOMO in **1a** is lower in energy than in **1b** and in **1e**. Such apparently anomalous behavior can be ascribed to the mixed \(-\)I/+M electron effect of the \(-\)OMe substituent. The UV-Vis spectra in CH\({}_{2}\)Cl\({}_{2}\) (Figure 3 and Table 3) is evidence of the presence of higher energy bands, attributed to ligand-centered transitions (p\(\rightarrow\)p*) located on the bpy\({}^{6\text{R}}\) ligand. Furthermore, d\({}_{\text{Pt}}\)\(\rightarrow\)p* metal-to-ligand charge transfer _mixed with intra-ligand_ transitions are present at  
Figure 3: Cyclic voltammetry curves (on the left) and UV-Vis spectra (on the right) of [Pt(bpy\({}^{6\text{OMe}}\)-H)(Me)(DMSO)] (**1a**, blue) and [Pt(bpy\({}^{6\text{Et}}\)-H)(Me)(DMSO)] (**1b**, red). \begin{table}
\begin{tabular}{l c c c c c} \hline  & \(\lambda\) (nm) * & **E\({}_{\text{g,opt}}\) (eV) *b** & **E\({}_{\text{ex}}\) (V) *c** & **HOMO (eV) *d** & **LUMO (eV) *e** \\ \hline
[Pt(bpy\({}^{6\text{OMe}}\)-H)(Me)(DMSO)] **1a** & 255, 352 & 3.03 & 1.29 & \(-\)5.59 & \(-\)2.56 \\ \hline
[Pt(bpy\({}^{6\text{Et}}\)-H)(Me)(DMSO)] **1b** & 249, 309, 340 & 3.14 & 1.4\(-\)1.5 (broad) & \(-\)5.4 \(\alpha\).f & \(-\)2.3 \(\alpha\).f \\ \hline
[Pt(bpy\({}^{6\text{Et}}\)-H)(Me)(DMSO)] **1e** & 276, 307, 380 & 2.99 & 0.72 & \(-\)5.13 & \(-\)2.14 \\ \hline \end{tabular}  
* In CH\({}_{2}\)Cl\({}_{2}\) solution; concentration about \(7\times 10^{-5}\) M. b The Energy-gap (E\({}_{\text{g,opt}}\)) value was determined from the \(\lambda_{\text{onset}}\) in the UV\(-\)Vis spectrum. c Potential values reported vs. Ag/AgCl in CH\({}_{2}\)Cl\({}_{2}\)-TEAPF\({}_{6}\) 0.1 M solvent system. Concentration about \(2.5\times 10^{-3}\) M. d Calculated from the anodic onset potential value. e Calculated from the equation LUMO(eV) = HOMO + E\({}_{\text{g,opt}}\)[77]. f Due to the scarce reproducibility of the voltammetric response, only an approximate evaluation of the HOMO can be calculated.  
\end{table} 
Table 3: Optical and electrochemical data. lower energy. In addition, a blue shift in the MLCT (metal to ligand charge transfer) band going from bpy to bpy\({}^{6\text{OMe}}\) to bpy\({}^{6\text{E}}\) is observed, suggesting an increasing s-donor effect according to this trend. Finally, the energy gap values (E\({}_{\text{g,opt}}\)) were calculated from the UV-Vis spectra, showing an increasing trend along the series **1b**\(>\)**1a**\(>\)**1e**. The LUMO energies obtained from the experimental E\({}_{\text{g,opt}}\) and HOMO values [77] suggest that the increasing in E\({}_{\text{g,opt}}\) is due to a stabilization both of the highest occupied and of the lower unoccupied molecular orbitals. The frontier orbitals of the investigated compounds are depicted in Figure 4. Within the same series (e.g., HOMO for **1a-e**) both the HOMO and LUMO are qualitatively similar. The HOMO of protonated species **3a-e** (see later) is mainly localized on the platinum atom.