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

Abstract

Metal-organic frameworks (MOFs) have attracted significant attention in the field of solar-driven photo-catalysis. 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. Certainly, a system with such features is of great interest for the design of MOF-based photocatalysts. In this work, we have conducted Density Functional Theory (DFT) simulations to provide insights into the unique electronic and optical properties of Ru-TBP-Zn. To do so, we propose two structural models that resolve the coordination of the ruthenium atoms in the metal backbone of Ru-TBP-Zn, both in agreement with the experimental observations. UV/Vis spectra calculations allow identifying the importance of the chargetransfer bands. According to our simulations, two possible charge transfer mechanism can co-exist: the direct photo-induced electron transfer from the porphyrin to the ruthenium upon light absorption, and the relaxation of the visible-light active excited states of the porphyrin to the low-lying ligand-to-metal charge transfer states. Analysis of the photo-generated charge carriers predicts a repulsive interaction energy indicating a low electron-hole recombination rate, required for multi-electron transfer processes such as HER. The understanding of the electronic properties and charge transfer mechanism in Ru-TBP-Zn paves the way for designing efficient porphyrin-based MOFs for photocatalysis.

Metal–Organic Frameworks: Molecules or Semiconductors in Photocatalysis?

“…The molecular picture, away from conductivity through continuous pathways of strong bonding, relies on charge hopping rather than band conductivity (the bands are flat), and therefore relies on the density and dynamics of those hopping sites [8–11] . The physical system can be surprisingly complex—there are cases where computation at the highest level reveals the workings of a MOF photocatalyst to depend on a dynamical, molecular picture in solution [12] . But several concepts from band theory that are relevant for photocatalysis, such as band alignment, translate as naturally to such molecular MOFs as band theory does to coordination chemistry [13] .…”

“…Computed values for E int show a significant difference between MIL‐125 (−0.04 eV) and NH 2 ‐MIL‐125 (+0.46 eV), [116] which can be used to explain the different lifetimes observed by ultrafast spectroscopy techniques [117] . The same approach was applied to explain the photocatalytic activity of a ruthenium‐based (porphyrin) MOF, Ru‐TBP‐Zn [12] . Depending on the level of theory used, we think this approach may work to include (polaronic) effects of the solvent and the (flexible) framework.…”

Toward Optimal Photocatalytic Hydrogen Generation from Water Using Pyrene-Based Metal–Organic Frameworks

“…This is a result of the excitonic effects (originated from the electron−hole Coulombic interactions) in MOFs. 29 On the other hand, Ti-TBAPy's first excitation is an LMCT excitation from the pyrene to the Ti orbitals, as it was reported as well by Cadiau et al 21 The experimental measurement of the optical band gaps in Al-, In-, and Sc-TBAPy MOFs was determined via Tauc plots (see Figure S6). The experimental optical band gaps are 2.68, 2.63, and 2.58 eV for Al-TBAPy, In-TBAPy, and Sc-TBAPy, respectively.…”

Metal–Organic Frameworks: Molecules or Semiconductors in Photocatalysis?

“…[8][9][10][11] Thep hysical system can be surprisingly complex-there are cases where computation at the highest level reveals the workings of aMOF photocatalyst to depend on ad ynamical, molecular picture in solution. [12] But several concepts from band theory that are relevant for photocatalysis,s uch as band alignment, translate as naturally to such molecular MOFs as band theory does to coordination chemistry. [13] We treat those concepts first, before looking at the perspectives from band theory and semiconductor physics.…”

“…[117] Thes ame approach was applied to explain the photocatalytic activity of ar uthenium-based (porphyrin) MOF,Ru-TBP-Zn. [12] Depending on the level of theory used, we think this approach may work to include (polaronic) effects of the solvent and the (flexible) framework. These would include the screening of charges and spins.…”