# Electronic Properties of Electron-Deficient Zn(II) Porphyrins for HBr Splitting
## 3 Results and Discussion
### Transient Absorption Spectroscopy
the dynamics are multiexponential due to surface heterogeneity, with at least one fast component in the 0-2 ms interval, and a long component extending from 2 to 20 ms. Upon application of anodic potential, withdrawal of electrons, percolating through the SnO\({}_{2}\) network to the electron collector, occurs, reducing electron density within the sensitized film and slowing down, as a consequence, recapture by the oxidized dye, which in the presence of an inert electrolyte (0.1 M HClO\({}_{4}\)), constitutes the only recombination pathway. As a result, both the initial amplitude of the CS state and that of the long component increase. Overall, upon 0.8 V versus Pt (approximately 0.5 V versus SCE) bias, approximately half of the initial population of the CS states survives after approximately 10 ms from the initial excitation (Figure 12 and Figure S7). Figure 11: Transient absorption spectra of SnO\({}_{2}\)**-1** (**A**) and SnO\({}_{2}\)**-2** (**B**) at different delays from the 532 nm ns-laser excitation recorded at OCP in 0.1 M HClO\({}_{4}\); 1 M\(\Omega\) oscilloscope resistance. The dynamics in the presence of Br\({}^{-}\) allow quantification of the regeneration efficiency of the porphyrins, i.e., the ability to produce Br\({}_{3}\)\({}^{-}\) and hydrogen. As a Br\({}^{-}\) electrolyte, we used the 0.1 M HBr/0.3 M NaBr formulation to keep the pH constant (hence the flat band potential of the metal oxide photoanode), while introducing a total 0.4 M Br\({}^{-}\), which is in large excess with respect to the surface concentration of photooxidized dye. Nevertheless, even under such conditions, we observed the incomplete recovery of the \(\Delta\)A traces (Figure 13), indicating that the electron transfer from Br\({}^{-}\) to the oxidized dye was slow compared to recombination, partly explaining the small photocurrents and IPCE discussed before. In 0.4 M Br, the dependence of the transient kinetics on the applied bias was similar to that described before in the case of the blank electrolyte, confirming that recombination of photoinjected electrons with the oxidized dyes is still the dominant charge loss pathway, despite the presence of the electron donor (Figure S4). Pseudo-first-order regeneration and recombination rate constants evaluated at 0.8 V versus Pt, well within the plateau region (i.e., that corresponding to the maximum charge extraction rate from the sensitized SnO\({}_{2}\)) of the J/E curves (Figure 8), were found to be 1.4 10\({}^{2}\) and 1.9 10\({}^{2}\) s\({}^{-1}\) for **1** and 3.9 10\({}^{1}\) and 2.2 10\({}^{2}\) s\({}^{-1}\) for **2**. This corresponds to regeneration efficiencies \(\eta_{\mathrm{reg}}\) of the order of 40% for **1** and 15% for **2**. It should be considered that the evaluation based on \(\tau_{1/2}\) is relatively rough, but is only available here since the time duration of decays exceed the current capabilities of our spectrometric apparatus to frame the complete time evolution of the recombination/regeneration kinetics. Nevertheless, the indication of a sluggish regeneration of the oxidized dye clearly emerges in both cases, and may contribute to explaining the better performance of **1** with respect to **2**. This latter trend is also consistent with the larger driving force for Br\({}^{-}\) oxidation available to **1** (0.37 eV) with respect to **2** (0.3 eV) for Br\({}^{-}\) oxidation (E\({}^{0}\) Br\({}^{-}\)/Br\({}_{2}\) is 0.86 V versus SCE in water). In addition, when considering the elementary step in the monoelectronic oxidation of Br\({}^{-}\)/Br-(1.68 V versus SCE), the one that is likely to be followed at the molecular level in the sensitized photoanode [8], the driving force becomes clearly unfavorable to efficient regeneration, and only by working under sufficient Br\({}^{-}\) concentration could one obtain a small overlap between the porphyrin\({}^{+}\) states and the redox electrolyte to observe some degree of charge transfer and Br\({}_{3}\)\({}^{-}\) formation.