# Electronic Properties of Electron-Deficient Zn(II) Porphyrins for HBr Splitting
## 1 Introduction  
In view of replacing traditional fossil fuels and reducing the resulting emissions of carbon dioxide, several strategies based on solar energy conversion and storage have been pursued [1; 2]. In particular, along with the widespread photoinduced water splitting reaction [3; 4], the splitting of hydrohalic acids (HX) in photoelectrosynthetic cell (PEC) set-ups has grown in interest among the scientific community [5; 6; 7]. Among the possible reactions, HBr splitting is the most promising [8; 9], since the oxidation reaction yields Br\({}_{2}\), a liquid product easy to transport for its further use in fuel cells [10].  
Photoanodes for bromide oxidation include TiO\({}_{2}\) or SnO\({}_{2}\)/TiO\({}_{2}\) core-shell nanostructures functionalized with ruthenium polypyrid based photosensitizers [11], as well as WO\({}_{3}\), SnO\({}_{2}\), and Sb-doped SnO\({}_{2}\) decorated with aggregated dyes belonging to the family of perylene diimides, reaching up to 1 mA/cm\({}^{2}\) photocurrent [9].  
Being the primary light-harvesting pigments in photosynthesis, porphyrins have great potential in view of developing devices for solar energy conversion, due to their strong visible absorbance and synthetic tunability. Indeed, porphyrins display two sets of visible transitions, namely: (i) the Soret band, due to the excitation to the S\({}_{2}\) excited state, usually located at approximately 400 nm and characterized by high extinction coefficients (\(\varepsilon=10^{5}\) M\({}^{-1}\) cm\({}^{-1}\)); and (ii) broader and less intense (\(\varepsilon=10^{3}\)-\(10^{4}\) M\({}^{-1}\) cm\({}^{-1}\)) Q-bands at longer wavelengths (500-600 nm), due to excitations to the first singlet excited state (S\({}_{1}\)). Furthermore, the photophysical and electrochemical properties of porphyrins can be tuned upon metalation or the introduction of tailored substituents [12; 13; 14], making themexcellent candidates for dye-sensitized regenerative solar cells [15; 16], as well as for water splitting (when combined with suitable catalysts) [17; 18].  
In this work, we investigate the use of perfluorinated Zn(II) porphyrins in photoelectrosynthetic set-ups for HBr splitting, with the exploitation of two novel species (namely porphyrin **1** and **2** in Figure 1), with structural features aimed at optimizing the charge separation in the resulting dye-sensitized photoanode. In particular, while maintaining the functionalization of the _meso_-position with perfluorophenyl groups, known to increase the oxidation potential of the resulting dyes [18], we have introduced conjugated electron withdrawing linkers, terminating in cyanoacrylic groups for the anchoring to the semiconductor surface. Either \(\beta\) (in the case of **1**) or _meso_ (in the case of **2**) positions have been functionalized in order to evaluate their impact on the sensitization efficiency in terms of directionality of the charge injection from the excited state. In addition, porphyrin **1** displays also an additional acceptor unit, namely the benzothiadiazole (BTD), which also contributes to the absorption of longer wavelengths. In this contribution we have explored the electronic and spectroscopic properties of such species applied to the sensitization of n-type SnO\({}_{2}\) substrates.