Creating competing charge-transfer pathways for water oxidation photocatalysis

Abstract

Replacing fossil fuels with renewable energy requires scalable and economic catalysts. However, solar conversion to chemical energy is heavily dependent on rare-earth metal-based photosensitizers and catalysts. Thus, it is necessary to build photocatalytic dyads that consist of first-row transition metals. However, Fe(II) polypyridyls, unlike their Ru(II) and Ir(III) analogues, show weak octahedral field splitting, which changes the photophysics to be unfavorable for catalysis. In this thesis, we focused on coupling metal-to-ligand charge transfer (MLCT) states with metal-to-metal charge transfer (MMCT) states to improvethe water oxidation catalytic activityof Prussian blue analogue(PBA) based photocatalysts. We first synthesised two of the most commonly studied Fe(II) polypyridyls, Fe(bpy)2(CN)2 and K2Fe(bpy)(CN)4. Then we coupled these chromophores with [Co(bpy)(H2O)n]2+ and [Zn(bpy)(H2O)n]2+ groups to prepare FeCo and FeZn compounds. FeZn compounds are used as a reference to investigate Fe cyanopolypyridyls in a network system since Zn ions are not expected to exhibit MMCT. With FeCo systems, it isaimed to investigate the role of the MMCT on the decay dynamics of the MLCT process. Characterization studies show that the local environment of Fe sites is affected by the coordination of Co and Zn ions to the cyanide bridge, resulting in a stabilization of the t2g and eg levels. We performed a series of photocatalytic experiments revealing that the highest catalytic activity is achieved for systems that possess MMCT processes directly coupled with the MLCT process through cyanide bridging group. Ultrafast transient absorption experiments indicate that the photophysics of the MLCT process is significantly different in Fe and FeCo compounds since 5MC is stabilized in energy compared to the 3MC state, and MMCT serves as an alternate relaxation pathway for the excited MLCT state. Finally, we combined our experimental findings to establish Jablonski diagrams for Fe, FeZn, and FeCo compounds and further proposed a mechanism for the photocatalytic water oxidation process. This thesis indicates that the cyanide bridge could provide an ideal platform to couple different charge transfer processes due to its rigid and short nature. The results also suggest that combining charge transfer processes could be a viable pathway to favor the decay dynamics of MLCT states for photocatalytic applications.