High-Potential Porphyrins Supported on SnO₂and TiO₂Surfaces for Photoelectrochemical Applications

We report CF 3 -substituted porphyrins and evaluate their use as photosensitizers in water-splitting dye-sensitized photoelectrochemical cells (WS-DSPECs) by characterizing interfacial electron transfer on metal oxide surfaces. By using (CF 3 ) 2 C 6 H 3 instead of C 6 F 5 substituents at the meso positions, we obtain the desired high potentials while avoiding the sensitivity of C 6 F 5 substituents to nucleophilic substitution, a process that limits the types of synthetic reactions that can be used. Both the number of CF 3 groups and the central metal tune the ground and excited-state potentials. A pair of porphyrins bearing carboxylic acids as anchoring groups were deposited on SnO 2 and TiO 2 surfaces, and the interfacial charge-injection and charge-recombination kinetics were characterized by using a combination of computational modeling, terahertz measurements, and transient absorption spectroscopy. We find that both free-base and metalated porphyrins inject into SnO 2 and that recombination is slower for the latter case. These findings demonstrate that (CF 3 ) 2 C 6 H 3 -substituted porphyrins are promising photosensitizers for use in WS-DSPECs.