Perfluorinated Porphyrins: Design Strategies for Efficient Sensitization of Photoelectrosynthetic Cells

Porphyrin sensitizers play a pivotal role in the development of dye-sensitized photoelectrochemical (DSPEC) devices thanks to their strong UV-Vis absorption, excellent electrochemical and photochemical stability, and adjustable electronic properties through structural modifications.[1] By carefully tuning the porphyrin structure, the ground-state potential of the sensitizers can be optimized to facilitate photo-oxidation processes at the anodic counterpart of DSPEC devices. Perfluorinated porphyrin sensitizers for SnO2-based photoanodes are promising candidates as dyes in DSPECs for water-splitting and HBr splitting applications.[2][3] The pentafluoro phenyl rings at the meso positions of the porphyrin core were demonstrated to guarantee a sufficiently positive ground-state oxidation potentials to promote the hole-transfer to water-oxidation catalysts or to directly promote thermodynamically demanding reactions, such as the oxidization of bromide to bromine. It was also observed that the introduction of a benzothiadiazole (BTD), as strong electron-acceptor unit, at the -pyrrolic position of perfluorinated porphyrin core improves the charge-separation character for n-type sensitization. Recently, a new set of perfluorinated ZnII-porphyrin-based sensitizers has been developed with the specific purpose of promoting alcohol oxidation at the photoanode counterpart of a DSPEC. The photoactivated process initiates an oxidative cascade involving the oxidation of the stable organic radical 2,2,6,6-tetramethylpiperidin-1-oxyl (TEMPO) (Figure 1).[4] A4 β-pyrrolic and A3B meso-substituted zinc porphyrins, equipped with both donating amine groups and BTD spacers have been rationally synthesized to optimize the donor-acceptor system. The impact of the molecular design on electronic properties of porphyrins has been comprehensively explored through a combination of electrochemical, spectroscopic, computational, and photophysical analyses. The key parameters which govern the performances of porphyrin-based DSPEC devices have been also evaluated. The insights gained from these studies pave the way for the development of efficient and stable DSPEC systems for energy conversion and storage applications.