# Electronic Properties of Electron-Deficient Zn(II) Porphyrins for HBr Splitting
## 3 Results and Discussion
### Spectroscopic and Electrochemical Properties in Solution
The experimental \(\mathrm{E}^{0^{\prime}}\) values allowed evaluation of the electrochemical HOMO and LUMO energy levels [12] and the electrochemical HOMO-LUMO energy gaps \(\mathrm{E}_{\mathrm{g}}\) (Table S1), which are in good agreement with the spectroscopic \(\mathrm{E}^{0-0}\) (Table 1), confirming the same trend. Differential pulse voltammetry (DPV) experiments allowed better-defined forward and backward waves of the oxidation and reduction processes to be obtained (Figures S2B and S3B), allowed determination of the value of the peak potentials \(\mathrm{E}_{\mathrm{p,la}}\) and \(\mathrm{E}_{\mathrm{p,lc}}\), and calculation of \(\mathrm{E}^{0^{\prime}}\mathrm{{}_{la}}\) and \(\mathrm{E}^{0^{\prime}}\mathrm{{}_{lc}}\) by Equations (4) and (5) (where \(\Delta\)E is the modulation amplitude):  
\[\mathrm{E}^{0^{\prime}}\mathrm{{}_{la}}=\mathrm{E}_{\mathrm{p,la}}+\Delta \mathrm{E/2} \tag{4}\]  
\[\mathrm{E}^{0^{\prime}}\mathrm{{}_{IC}}=\mathrm{E}_{\mathrm{p,lc}}-\Delta \mathrm{E/2} \tag{5}\]  
The HOMO and LUMO energy levels and the HOMO-LUMO energy gaps (\(\mathrm{E}_{\mathrm{g}}\)) derived from DPV measurements are summarized in Table 2.  
DPV data are in perfect correspondence with those obtained by CV (Table S1). The electrochemical measurements confirm a more conjugated character for the \(\beta\)-substituted porphyrin **1**, having a narrower HOMO-LUMO gap. The slight difference between the spectroscopic (Table 1) and the electrochemical (Table 2) energy gaps is due to some issues related to the LUMO energy level. Indeed, while for a neutral molecule the LUMO corresponds to a light-promoted transition, in an electrochemical process it is the addition of an electron to the neutral molecule, generating a radical anion species. Therefore, to better assess the HOMO and LUMO energy levels of **1** and **2**, to match with the energy levels of the components of the photoelectrochemical cell, we calculated the LUMO by Equation (6):  
\[\mathrm{E}_{\mathrm{LUMO}}=\mathrm{E}^{0-0}-\mathrm{E}_{\mathrm{HOMO}} \tag{6}\]  
where \(\mathrm{E}^{0-0}\) is the spectroscopic energy gap (Table 1) and \(\mathrm{E}_{\mathrm{HOMO}}\) is the value obtained by the electrochemical measurements for the first oxidation process (Table 1). For **1** and **2** the calculated LUMO levels are \(-3.56\) eV and \(-3.45\) eV, respectively. \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline \multirow{2}{*}{**Dye**} & **E\({}_{\mathrm{p,la}}\)/V** & **E\({}^{0^{\prime}}\)/V** & **E\({}_{\mathrm{p,la}}\)/V** & **E\({}^{0^{\prime}}\)/V** & **E\({}^{0^{\prime}}\)/V** & **A\(\mathrm{E}^{0^{\prime}}\)/V** \\  & **(Fc\({}^{+}\)[Fc]**)** & **(Fc\({}^{+}\)[Fc]**)** & **(Fc\({}^{+}\)[Fc]**)** & **(Fc\({}^{+}\)[Fc]**)** & **(Fc\({}^{+}\)[Fc]**)** & & **(E\({}_{\mathrm{g,Fc}}\)/eV)** \\ \hline
**1** & 0.80 & 0.86 & \(-1.29\) & \(-1.32\) & \(-5.63\) & \(-3.48\) & \(2.15\) \\
**2** & 0.75 & 0.77 & \(-1.43\) & \(-1.46\) & \(-5.58\) & \(-3.34\) & \(2.24\) \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Electrochemical properties of porphyrins **1** and **2**, and electrochemical HOMO and LUMO energy levels derived from \(\mathrm{E}^{0^{\prime}}\mathrm{{}_{la}}\) and \(\mathrm{E}^{0^{\prime}}\mathrm{{}_{lc}}\) (or \(\mathrm{E}_{\mathrm{p,la}}\) and \(\mathrm{E}_{\mathrm{p,lc}}\)), all referred to the ferrocenium/ferrocene redox couple. As depicted in Figure 3, the energetics of dyes **1** and **2** in solution are suitable to envisage their application in a photoelectrochemical cell for HBr splitting. Indeed, they have an electrochemical LUMO at higher energy than that of TiO\({}_{2}\) and SnO\({}_{2}\) conduction band, offering the exoergonic photoinjection of electrons from the excited states of the dyes into the semiconductor. In particular, the introduction of a BTD moiety in \(\beta\)-pyrrolic position induces a sizable stabilization of the LUMO level, in accordance with the increased electron-withdrawing ability of the acceptor unit [30].