# Electronic Properties of Electron-Deficient Zn(II) Porphyrins for HBr Splitting
## 3 Results and Discussion
### Spectroscopic and Photoelectrochemical Characterization of the Sensitized Photoanodes  
SnO\({}_{2}\) photoanodes, treated with TiCl\({}_{4}\), have been sensitized by soaking in a 0.1 mM dye solution. It is worth noting that the conduction band edge of SnO\({}_{2}\) should allow for a large driving force for electron injection from the excited states of the two dyes. Indeed, SnO\({}_{2}\), or SnO\({}_{2}\)/TiO\({}_{2}\) core-shell nanostructures, have been used as semiconductive scaffolds in dye-sensitized devices for solar energy conversion, particularly when the excited states of some sensitizers were too poorly reduced to allow for an ultrafast injection into TiO\({}_{2}\)[34; 35; 36]. At the same time, the hydrophilic properties of TiO\({}_{2}\) may enhance the higher uptake of the porphyrinic dyes via formation of ester-like bonds between COOH in the linker and OH groups at the metal oxide surface, resulting in better coverage and light harvesting. For the sake of notation simplicity, we will refer to the TiCl\({}_{4}\)-treated SnO\({}_{2}\) photoanodes dyed with the respective porphyrins as SnO\({}_{2}\)-**1** and SnO\({}_{2}\)-**2**, as this is the only type of semiconductor substrate used in the present study. The UV-vis spectra of SnO\({}_{2}\)-**1** and SnO\({}_{2}\)-**2** are reported in Figure 6, showing that light harvesting can be extended at \(\lambda\) > 650 nm with the \(\beta\) porphyrin **1**. Consistent with the absorption spectra recorded in solution and with the calculations, the presence of an increased charge transfer (CT) character broadens the B band, which bears the contribution of multiple electronic transitions (at least 6, according to Figure 5) and, thus, extends from 400 to 500 nm. Two distinct Q bands centered at 560 and 600 nm, respectively, cover the red part of the visible spectrum. Further, **1** achieves similar maximum maximum absorbance (approximately 0.7) but its main bands are narrower compared to **2**, as a result of nearly degenerate transitions localized on the tetrapyrrolic cycle, analogous to those of a symmetric porphyrin, showing a single Q band at 560 nm with two shoulders at 520 and 590 nm and the Soret band centered at 430 nm. The photoanodic performance was initially evaluated in the presence of the presence of 0.1 M ascorbic acid (AA; pH5), which is easy to oxidize and performs as an efficient hole scavenger. Under such conditions, the regeneration efficiency (\(\eta_{\rm{neg}}\)) of the oxidized sensitizer should be nearly quantitative and the photocurrent should, thus, be limited by the light harvesting efficiency (\(\rm{LHE}=1-10^{-A}\)) and by the charge injection yield (\(\rm{\Phi_{inj}}\)). Current/potential (J/E) characteristics registered under Air Mass (AM)1.5G (0.1 W/cm\({}^{2}\)) in the presence of both continuous and shuttered illumination are reported in Figure 7. The photocurrent onset is observed at \(-\)0.15 V versus SCE and increases upon anodic polarization, consistent with the n-type properties of the semiconductor/liquid electrolyte junction (SCLE)), up to 3 mA/cm\({}^{2}\) in the case of **1** at 0.6 V versus SCE. A well-defined photoanodic plateau is, however, not observed, as the dark current starts to rise at 0.4 V versus SCE, due to the electrochemical oxidation of AA, probably through pinholes in the mesoporous SnO\({}_{2}\) film. Photoanodic transients match reasonably well with the shape of the steady state curve and allow quantification of precisely the magnitude of the net photocurrent; approximately 2.85 mA/cm\({}^{2}\) with SnO\({}_{2}\)-**1**, which is approximately 40% higher than that found for SnO\({}_{2}\)-**2** at equivalent voltage, as a probable result of broader light harvesting and better injection from the former. The encouraging results obtained in the presence of the ascorbic acid as a scavenger prompted us to test the photoanodes for the photoinduced HBr splitting, which is the main target of our study. We decided to operate at pH 1 (0.1 M HBr) in order to avoid the formation of bromine oxo-anions.