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.
Highlights
Artificial photosynthesis, inspired by nature, with the goal of conversion of solar energy into chemical energy, has attracted growing interest in the past decades.[1]
Owing to a proper molecular assembly of the water oxidation catalyst (WOC) and the dye components in a WOC-dye supramolecular complex, the photooxidation of the dye should be followed by a proton-coupled electron transfer (PCET)[6] process within the water oxidation catalytic cycle: The electron is used for the regeneration of the dye to its initial state while the proton is being transferred to a different direction, into the environment.[7]
The introduction of the second NDI dye in the dye-WOC-dye complex for photocatalytic water splitting provides an extra channel for ET, which enables the sequential event of ET from the WOC to the two separate NDI dyes
Summary
Artificial photosynthesis, inspired by nature, with the goal of conversion of solar energy into chemical energy, has attracted growing interest in the past decades.[1]. In this work we explore how the catalytic cycle could be affected by introducing two excited dye motifs in parallel, and thereby combining two PCET steps without stable intermediates in between For this aim, an additional dye molecule is introduced in the catalyst-dye supramolecular complex 1([(cy) RuIIbpy(H2O)]2+-NDI) (cy = p-cymene, bpy = 2,2’-bipyridine, NDI = 2,6-diethoxy-1,4,5,8-diimide-naphthalene) for photocatalytic water splitting, which has been systematically investigated in silico recently,[7,9c,13] leading to the dye-WOC-dye supramolecular complex 1(NDI1-[(cy)RuIIbpy(H2O)]2+-NDI2) with the total spin S = 0 (denoted as 1(NDI1-[RuII-H2O]2+-NDI2) in Scheme 1, where NDI1 = NDI2 = NDI).
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