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.