# Electronic Properties of Electron-Deficient Zn(II) Porphyrins for HBr Splitting
## 4 Conclusions  
In this work, we have explored the electronic properties of fluorinated porphyrins intended to be applied in sensitized n-type PECs for HBr splitting. Thus, the effort was to increase the ground state oxidation potential of the dyes in order to access energy-demanding redox reactions, to obtain adsorption stability in an aqueous environment thanks to hydrophobic properties, and to provide the ability to bind metal oxides for photoinduced charge injection. These requirements led to the design of Zn(II) porphyrins, carrying either 4 or 5 _meso_ pentafluorophenyl moieties as electron acceptor groups, and a further electron withdrawing branch inserted in either the \(\beta\) or _meso_ position. The latter is tasked with binding the metal oxide, and with mediating the electronic coupling for charge injection from the excited state. DFT and TDDFT calculations showed that delocalization of the excited states on the electron withdrawing link was partially successful with the BTD electron acceptor, however the electronic density on the cyano-acrylic moiety remained comparatively small with respect to that localized in the ring. Nevertheless, in the presence of aqueous AA as a hole scavenger, the \(\beta\) design **1** was able to produce a 40% superior photocurrent with respect to the _meso_ design **2**, ostensibly due to better injection and broader light harvesting. TA recorded on the sensitized photoanodes in contact with an inert electrolyte revealed indeed, in both cases, the formation of a long lived charge separated as a consequence of photoinduced injection, but the persistence of some unquenched emission at early delays indicated that injection was in neither cases optimal. Finally, both dyes could be successfully employed in the splitting of HBr, where red photons, up to 700 nm, could be harvested and converted, and again, **1** performed better than **2**, although the photoanodic current associated with such process was decreased by an approximately 10-fold factor with respect to the AA case. On the basis of TA results, the large drop photocurrent when Br\({}^{-}\) was used as electron donor was ascribed to inefficient regeneration of the oxidized sensitizer as a result of insufficient driving force for Br\({}^{-}\) oxidation. This study will, thus, serve as a guide for the development of better designs for dye-sensitized PECs, which may profit from the advantages and shortcomings outlined with these two model sensitizers. **Supplementary Materials:** The following are available online at [[http://www.mdpi.com/2076-3417/9/13/2739/s1](http://www.mdpi.com/2076-3417/9/13/2739/s1)]([http://www.mdpi.com/2076-3417/9/13/2739/s1](http://www.mdpi.com/2076-3417/9/13/2739/s1)). Figure S1: Fluorescence decay of porphyrin 1 (A) and 2 (B) in toluene solution, bold black line (\(\mathrm{\mu_{exc}}\) = 442 nm; \(\mathrm{\lambda_{em}}\) = 642 nm). Convolution fit, red line. Weighted residuals are shown under the decay curves. Figure S2: CV (**A**) and DPV (**B**) of porphyrin **1** on glassy carbon electrode, in DMF+0.1 M TBAP, at 0.2 V-s1. Figure S3: CV (**A**) and DPV (**B**) of porphyrin **2** on glassy carbon electrode, in DMF+0.1 M TBAP, at 0.2 V-s1. Figure S4: Net photocurrents (i.e., after the subtraction of the dark contribution) recorded for SnO\({}_{2}\)-1 (**A**) and SnO\({}_{2}\)-2 (**B**) photoanodes in 0.1 M HBr (pH 1) under 1 sun (0.1 mW/cm\({}^{2}\)) AM 1.5 G illumination from both the back side of the  
Figure 13: Kinetic evolution of SnO\({}_{2}\)(e\({}^{-}\))-**1**(h\({}^{+}\)) (**A**) and SnO\({}_{2}\)(e\({}^{-}\))-**2**(h\({}^{+}\)) (**B**) charge-separated states after the ns laser excitation at 532 nm in 0.1 M HClO\({}_{4}\) (black traces) and 0.1 M HBr + 0.3 M NaBr (red traces) under 0.8 V versus Pt applied bias; 1 M\(\Omega\) oscilloscope resistance. Traces have been normalized for unity. electrode (i.e., the FTO side) and the front side (i.e., from the electrolytic solution). Average curves of 5-7 electrodes are reported.