Two‐Channel Model for Electron Transfer in a Dye‐Catalyst‐Dye Supramolecular Complex for Photocatalytic Water Splitting

Abstract

To improve the performance of dye‐sensitized photoelectrochemical cell (DS‐PEC) devices for splitting water, the tailoring of the photocatalytic four‐photon water oxidation half‐reaction represents a principle challenge of fundamental significance. In this study, a Ru‐based water oxidation catalyst (WOC) covalently bound to two 2,6‐diethoxy‐1,4,5,8‐diimide‐naphthalene (NDI) dye functionalities provides comparable driving forces and channels for electron transfer. Constrained ab initio molecular dynamics simulations are performed to investigate the photocatalytic cycle of this two‐channel model for photocatalytic water splitting. The introduction of a second light‐harvesting dye in the Ru‐based dye‐WOC‐dye supramolecular complex enables two separate parallel electron‐transfer channels, leading to a five‐step catalytic cycle with three intermediates and two doubly oxidized states. The total spin S=1 is conserved during the catalytic process and the system with opposite spin on the oxidized NDI proceeds from the Ru=O intermediate to the final Ru‐O2 intermediate with a triplet molecular 3O2 ligand that is eventually released into the environment. The in‐depth insight into the proposed photocatalytic cycle of the two‐channel model provides a strategy for the development of novel high‐efficiency supramolecular complexes for DS‐PEC devices with buildup and conservation of spin multiplicity along the reaction coordinate as a design principle.

Strategies to Enhance the Rate of Proton‐Coupled Electron Transfer Reactions in Dye‐Water Oxidation Catalyst Complexes

“…Earlier constrained AIMD simulations in our group have shown that electron transfer in 2 is strongly inhibited once the CÀ NÀ CÀ C torsional mode at the WOC-dye interface is fixed. [25,26] The results presented in this paper provide a rationale behind this observation: The energy gap between the two adiabatic states in the region of the transition state is around 0.1 eV, which is indeed comparable to the energy associated with the torsional mode discussed in ref. [26] The resonance between the torsional mode frequency and the electronic energy gap between the reactant and product states, leads to a deterministic conversion pathway from the reactant to the product.…”

“…[25,26] The results presented in this paper provide a rationale behind this observation: The energy gap between the two adiabatic states in the region of the transition state is around 0.1 eV, which is indeed comparable to the energy associated with the torsional mode discussed in ref. [26] The resonance between the torsional mode frequency and the electronic energy gap between the reactant and product states, leads to a deterministic conversion pathway from the reactant to the product. These results show that vibronic coupling is an important mechanistic aspect to It is observed that for the reactant structure, the first excited state is a charge transfer excitation from the ruthenium catalyst to the NDI, while for the product structure, the excitation corresponds to a charge transfer from the NDI to the ruthenium center.…”

Noble-Metal-Free Heterojunction Photocatalyst for Selective CO2 Reduction to Methane upon Induced Strain Relaxation

“…68 The conservation of spin of an intermediate may also affect the selectivity. 69 However, including these two effects is beyond the scope of the current study and will be a subject of future investigations. Furthermore, the IR peak corresponding to the *CO intermediate is not observed.…”

“…Another interesting finding to note is that there is a precise matching between the bottom of the CB and *CH 2 O intermediate, which may enhance the reaction rate by vibronic coupling . The conservation of spin of an intermediate may also affect the selectivity . However, including these two effects is beyond the scope of the current study and will be a subject of future investigations.…”

An energy decomposition and extrapolation scheme for evaluating electron transfer rate constants: A case study on electron self-exchange reactions of transition metal complexes

“…The former can be evaluated by investigating the seam-of-crossing (SX) region between the initial and final diabatic states. This can be done by optimizing an SX structure or by a confined sampling such as the blue-moon or umbrella sampling on the corresponding reaction coordinate [26][27][28] . Molecular dynamics simulations, such as surface hopping, are useful to account for the latter dynamics effects 9,[29][30][31][32] .…”