High-Potential Porphyrins Supported on SnO₂and TiO₂Surfaces for Photoelectrochemical Applications

Abstract

We report CF 3 -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 (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.

Optimization of Photoanodes for Photocatalytic Water Oxidation by Combining a Heterogenized Iridium Water‐Oxidation Catalyst with a High‐Potential Porphyrin Photosensitizer

“…First, the effect of the applied bias was studied for SnO 2 ‐ P , SnO 2 ‐(2:1), SnO 2 ‐(8:1), and SnO 2 ‐(32:1) (Figure ). Biases of 0.2, 0.4, 0.6, and 0.8 V versus NHE were applied to the photoanodes, and the detection of photocurrent upon illumination is indicative of dye photoexcitation and electron injection into the conduction band of SnO 2 , as was confirmed previously for derivatives of P . As the bias increased, the photocurrent also increased.…”

“…First, photoanodes were prepared on TiO 2 surfaces. As the porphyrin in P cannot inject into the conduction band of TiO 2 , which is approximately 500 mV higher than that of SnO 2 , these samples should not be operable. The UV/Vis spectra and CV data of TiO 2 ‐(2:1) are similar to those of SnO 2 ‐(2:1) (Figures S8 and S22); however, TiO 2 ‐(2:1) does not show any current response upon illumination (Figure ), as P is unable to inject electrons into the conduction band; therefore, O 2 evolution is not observed (Figure S28).…”

“…The UV/Vis spectra and CV data of TiO 2 ‐(2:1) are similar to those of SnO 2 ‐(2:1) (Figures S8 and S22); however, TiO 2 ‐(2:1) does not show any current response upon illumination (Figure ), as P is unable to inject electrons into the conduction band; therefore, O 2 evolution is not observed (Figure S28). The second sample was a Zn‐metalated version of P (Zn‐ P ), which should be unable to oxidize Ir as it has a potential of 1.25 V versus NHE . The CA measurements revealed a current response for the illumination of SnO 2 ‐(Zn‐P+Ir) because Zn‐ P can inject ( S 1 =−0.85 V vs. NHE) into the conduction band of SnO 2 (Figure ); however, no O 2 evolution was detected because the oxidation of Ir cannot occur (Figure S28).…”

“…Furthermore, we recently designed new, high‐potential CF 3 ‐substituted porphyrin dye molecules . These porphyrins are preferred over pentafluorophenyl‐substituted porphyrins because they avoid potential nucleophilic attack at the para C−F of the C 6 F 5 group . They are also electrochemically wellbehaved and show reversible features for the formation of the radical cation and dication species.…”

“…They are also electrochemically wellbehaved and show reversible features for the formation of the radical cation and dication species. These porphyrins are appropriate dye molecules for WS‐DSPECs because they can inject electrons into the conduction bands of semiconductor electrodes and their high reduction potentials can result in the oxidization of catalyst molecules . For example, the porphyrin core of P (Scheme ) has an excited‐state potential ( S 1 ) of −0.39 V versus NHE and a potential of 1.53 V versus NHE for the formation of the radical cation species, and these properties allow it to inject electrons into the SnO 2 conduction band upon photoexcitation and also oxidize Ir …”