High-Potential Porphyrins Supported on SnO2 and TiO2 Surfaces for Photoelectrochemical Applications

We report CF3-substituted porphyrins and evaluate their use as photosensitizers in water-splitting dyesensitized photoelectrochemical cells (WS-DSPECs) by characterizing interfacial electron transfer on metal oxide surfaces. By using (CF3)2C6H3 instead of C6F5 substituents at the meso positions, we obtain the desired high potentials while avoiding the sensitivity of C6F5 substituents to nucleophilic substitution, a process that limits the types of synthetic reactions that can be used. Both the number of CF3 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 SnO2 and TiO2 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 SnO2 and that recombination is slower for the latter case. These findings demonstrate that (CF3)2C6H3-substituted porphyrins are promising photosensitizers for use in WS-DSPECs.