# Electronic Properties of Electron-Deficient Zn(II) Porphyrins for HBr Splitting
## 3 Results and Discussion
### Spectroscopic and Photoelectrochemical Characterization of the Sensitized Photoanodes
Figure 8 shows the J/E curves recorded for SnO\({}_{2}\)-**1** and SnO\({}_{2}\)-**2** photoanodes under AM 1.5 G (0.1 W/cm\({}^{2}\))  
Figure 6: Electronic spectra of SnO\({}_{2}\)-**1 (left)** and SnO\({}_{2}\)-**2 (right)**. Spectra were recorded in transmission mode against a blank SnO\({}_{2}\) photoanode for background correction. Figure 7: J/E curves in the presence of 0.1 M AA under AM 1.5G illumination. Illumination from the FTO side, potential sweep rate 10 mV/s. illumination. The position of the onset potential, estimated to be 0.05 V from the analysis of the chopped curves, is approximately 0.2 V more anodic than that obtained for ascorbic acid oxidation and can be ascribed to the Nernstian shift of the flat band voltage of SnO\({}_{2}\) (59 mV \(\times\)\(\Delta\)pH) due to the pH difference of AA compared to HBr. Nevertheless, compared to the electrochemical oxidation of Br\({}^{-}\) at SnO\({}_{2}\) working electrodes, which is onset at approximately 0.8 V versus SCE, the photoactive interface is able to store approximately 0.6 V in the form of photovoltage. The limiting photocurrent, which corresponds to the maximum rate of photoxodiation of Br\({}^{-}\) at the SCLEJ is already found starting from approximately 0.3 V versus SCE but values are approximately 10 times lower than in the AA case, reaching up 0.3 and 0.23 mA/cm\({}^{2}\) at 0.4 V, respectively, for SnO\({}_{2}\)-**1** and SnO\({}_{2}\)-**2**. Large overshootings in the activation region of the curves, between 0.1 and 0.25 V versus SCE (Figure 5), coupled to the observation that J/Es collected by illumination through the electron collector (i.e., from the FTO side) gave substantially better results than those acquired by electrolyte side excitation (see Figure S4 for a comparison), indicated a substantial amount of electronic recombination, probably due to inefficient regeneration of the photogenerated oxidized sensitizer. This indication is also corroborated by the approximately 10-fold reduction of photocurrents recorded with HBr with respect to AA present in comparable concentrations. The photoaction spectra (IPCE versus \(\lambda\)) recorded in 0.1 M HBr under 0.6 V versus SCE, a potential well into the plateau region of the respective J/E, are reported in Figure 9. Although maximum IPCE values are fairly small, with maximum values in the order of 2.5%, consistent with the modest photocurrents recorded under white light illumination, the photoaction spectra are in good agreement with the absorption profiles recorded on the sensitized thin films, testifying to the involvement of the excited state of each dye in the sensitization process. In particular, the photoaction spectra also point to the superiority of **1**, yielding higher photoconversion than **2** in correspondence of its broader B band with the added benefit of a red shifted response originating from Q band excitation extending up to 700 nm. Figure 8: J/E curves of SnO\({}_{2}\)-**1** (**A**) and SnO\({}_{2}\)-**2** (**B**) photoanodes recorded in 0.1 M HBr (pH 1) under AM 1.5 G (0.1 W/cm\({}^{2}\)) illumination through the FTO side. The corresponding absorbed photon to current efficiencies (APCE or IQE (Inner Quantum Yield) = IPCE(\(\lambda\))/LHE(\(\lambda\)) with LHE(\(\lambda\)) = (1\(-\)10\({}^{-\mathrm{A}(\lambda)}\)), where A(\(\lambda\)) is the photoelectrode absorbance at a given wavelength \(\lambda\)), have been calculated and reported and are shown in Figure S5. The APCE spectra are quite constant in the visible range with the best values again found with **1**, giving maximum values of approximately 3%, compared with less than 2.5% in the case of **2**.