# Electronic Properties of Electron-Deficient Zn(II) Porphyrins for HBr Splitting
## 3 Results and Discussion
### Spectroscopic and Electrochemical Properties in Solution  
Porphyrins **1** and **2** were investigated by UV-vis absorption and emission spectroscopies in toluene solution. The \(\varepsilon\)-normalized spectra are reported in Figure 2, and the corresponding data in Table 1.  
The UV-vis absorption spectrum of **1** (Figure 2A) shows the typical pattern of \(\beta\)-pyrrolic porphyrins, with a broadened Soret or B band (at 430 nm) and two well-defined Q bands (at 555 and 595 nm). The broadening of the B band is the result of the electronic and steric interaction between the porphyrin ring and the BTD moiety of the anchoring unit in the \(\beta\)-pyrrolic position, rather than an additional absorption peak due to the BTD unit itself [30]. Moreover, the spectrum is red-shifted in comparison to that of **2**, confirming for the \(\beta\)-substituted molecular architecture a more favorable conjugation and charge transfer between the porphyrin core and the anchoring unit, with increased light-harvesting [31]. The UV-vis absorption spectrum of **2** (Figure 2A) displays a sharp and intense B band (at 415 nm) and apparently only one Q band (at 546 nm), with a pattern more similar to that of a _meso_-substituted porphyrin [12]. However, a second Q band featuring a very low intensity can be detected at 580 nm (Figure 2B). The emission spectra (Figure 2B) are a mirror copy of the absorption of the Q bands. Although, at first sight, **2** seems to be an exception, the absorption intensity of its Q band at higher energy is likely affected by the broadening of the B band. Minimal Stokes shifts between the peak of the absorption band at lower energy and the peak of the emission band at higher energy are observed, confirming the rigidity of these systems and reduced molecular distortions during the absorption and emission processes. The comparison between the UV-vis absorption and emission spectra allowed us to assess the E\({}^{0-0}\) value, which corresponds to the spectroscopic HOMO-LUMO energy gap (see Table 1). In agreement with the enhanced push-pull character imparted by the presence of the BTD unit, the HOMO-LUMO gap of **1** is lower than that of **2**. The characterization of the excited states of **1** and **2** in solution was completed by the measurement of the lifetime by the single photon counting technique using a pulse laser source at 442 nm (see Experimental Section). From the convolution fit of the decay profiles (Figure S1), comparable \(\tau\) values were in the range of 1.3-1.4 (Table 1). The energy levels of the oxidation and reduction processes and the chemical and electrochemical reversibility of **1** and **2** were evaluated by cyclic voltammetry (CV) experiments in order to state their implementation in a photoelectrochemical cell for efficient HBr splitting. The CV patterns are reported in Figures S2A and S3A, and their key features are summarized in Table S1. For porphyrin **2** one oxidation peak and two reduction peaks are observed in the potential window investigated (Figure S3A), whereas for porphyrin **1** a third reduction peak is evident due to the presence of the electron-rich BTD unit (Figure S2A). In particular, the clear presence of only one oxidation peak reflects the electron-poor features of this class of perfluorinated porphyrins, where oxidation processes are less accessible. The oxidation peak and the first reduction peak of both dyes are reversible or quasi-reversible from both the electrochemical and chemical point of view, allowing the calculation of formal potentials \(\mathrm{E}^{0^{\prime}}\mathrm{{}_{la}}\) and \(\mathrm{E}^{0^{\prime}}\mathrm{{}_{IC}}\) for the first oxidation (anodic) and first reduction (cathodic) processes, respectively, in the operating solvent (formal potentials \(\mathrm{E}^{0^{\prime}}\) approximate standard potentials \(\mathrm{E}^{0}\) under the assumption of neglecting activity coefficients).