Pyridinium-pentacyanoferrate complexes as novel iron photosensitizers: synthesis, photophysics, and function characterization towards light-driven water oxidation

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Replacing precious metal-containing photosensitizers (PSs), e.g., benchmark ruthenium complexes, with iron PSs, has been a long-standing interest for the advancement of low-cost and scalable water splitting cells. Iron PSs, however, have historically posed significant challenges for applications due to the extremely short-lived nature of the excited states, which were considered usually in the ps range. The corresponding rapid excited-state decay of these systems, once optically excited to their metal-to-ligand charge transfer (1MLCT) state, is due to the availability of energetically low-lying metal-centered (3,5MC*) states, which constitute efficient deactivation channels for the excited states. Another critical issue related to the practical implementation of an iron PS into a photoelectrochemical electrode is the design of a proper ligand, which cannot only increase the excited state lifetime of the iron PS but also connect it efficiently to a catalyst and the semiconductor. In this respect, the cyanide ligand can serve as a short bridging ligand connecting metal ions. Furthermore, it is a strong electron-donating ligand that can destabilize MC* states and hinder the rapid deactivation of the excited states in the corresponding complexes. Previous studies on pyridyl-cyanoferrate complexes indicate that increasing the number of cyanide ligands around the Fe(II) site effectively increases the MLCT lifetimes by increasing the octahedral splitting, thus, destabilizing the MC* states. However, studies on cyanoferrate photosensitizers have been limited to di-, tri-, and tetra-cyanoferrate complexes with pyridyl ligands, and their utilization in photocatalytic applications has not been explored until now. Pyridinium-pentacyanoferrate complexes hold the potential to address all the problems mentioned above due to the unique architecture of the iron complex. Joint preliminary studies of the applicants indicate that maximizing the number of cyanide ligands up to five and utilizing cationic organic ligands is an efficient strategy to achieve lifetimes long enough to be utilized in the photocatalytic water oxidation process. In this project, the solvent-dependent photophysics of a series of pyridinium-pentacyanoferrate complexes with varying electron acceptor pyridyl and pyridinium ligands will be investigated spectroscopically, while we analyze the complexes functionally concerning their ability to trigger light-driven water oxidation when combined with Co-catalysts.