Insights into the Electronic Properties and Charge Transfer Mechanism of a Porphyrin Ruthenium-Based Metal-Organic Framework

Metal-organic frameworks (MOFs) have attracted significant attention in the field of solar-driven photocatalysis. Recently, a porphyrin ruthenium-based MOF (Ru-TBP-Zn) has shown highly efficient co-catalyst-free photocatalytic hydrogen evolution reaction (HER) under visible light in neutral water. However, a detailed molecular understanding of the electronic and optical properties is missing. In this work, we have conducted density functional theory (DFT) simulations to study these properties in Ru-TBP-Zn. Our DFT calculations indicate instability in the experimentally reported structure of Ru-TBP-Zn. Such instability is resolved by proposing two structural models in which Cl- or OH- anions are coordinated with the metal backbone of Ru-TBP-Zn. On the basis of these models, the electronic and optical properties in Ru-TBP-Zn are analyzed. Ultraviolet-visible spectral calculations allow the identification of the importance of the charge transfer bands. According to our simulations, two possible charge transfer mechanisms can coexist: the direct photoinduced electron transfer from the porphyrin to the ruthenium upon light absorption and the relaxation from the optically excited porphyrin to the low-lying ligand-to-metal charge-separated state. The interaction energy of the photogenerated electron-hole carriers is computed considering hole-phonon-electron contributions according to the polaron model. Our calculations predict a repulsive electron-hole interaction energy indicating a low electron-hole recombination rate, which is a prerequisite for multi-electron transfer processes such as HER. The understanding of the electronic properties and charge transfer mechanism of Ru-TBP-Zn paves the way for the design of efficient porphyrin-based MOFs for photocatalysis.