Charge separation and successive reconfigurations of electronic and protonic states in a water-splitting catalytic cycle with the Mn4CaO5 cluster. On the mechanism of water splitting in PSII.

Kentaro Yamamoto,Kazuo Takatsuka

doi:10.1039/d0cp00443j

Abstract

Much insight into the basic mechanisms of photoexcited and collision-induced ground-state water splitting has been accumulated in our nonadiabatic electron wavepacket dynamics studies based on a building-block approach reaching up to systems of binuclear Mn oxo complexes. We here extend the study to a ground-state water-splitting catalytic cycle with tetranuclear Mn oxo complex Mn4CaO5, or Mn3Ca(H2O)2(OH)4-OH-Mn(4)(H2O)2, where Mn3Ca(H2O)2(OH)4 is fixed to a skewed cubic structure by μ-hydroxo bridges and is tied to the terminal group Mn(4)(H2O)2. We show using the method of real-time nonadiabatic electron wavepacket dynamics that four charge separation steps always take place only through the terminal group Mn(4)(H2O)2 alone, thereby producing 4 electrons and 4 protons which are transported to the acceptors. Each of the three charge separation steps is followed by a reloading process from the skewed cubic structure, by which electrons and protons are refilled to the vacant terminal group so that the next charge separation dynamics can resume. After the fourth charge separation an oxygen molecule is generated. It is emphasized that the mechanisms of O2 generation should depend on the multiple channels of reloading.

Highlights

Photoinduced water-splitting2H2O + 4hn - 4H+ + 4eÀ + O2 (1)is a fundamental process of conversion of photon energy into some other form such as chemical energy
In any water-splitting scheme, the main products should be protons and electrons, and oxygen molecules and/or peroxides as a possible precursor of O2 are by-products produced as a result of charge separation H+ + eÀ in water splitting
We examine the role of Mn reduction processes to possibly generate an oxygen molecule and thereby complete one circuit of the catalytic cycle

Summary

Introduction

Photoinduced water-splitting2H2O + 4hn - 4H+ + 4eÀ + O2 (1)is a fundamental process of conversion of photon energy into some other form such as chemical energy. As for water-splitting in photosystem II (PSII), complete understanding of the catalytic cycle with the Mn4CaO5 cluster being the unique catalyst is one of the ultimate goals.[1,2,3,4,5,6,7,8,9,10] Yet there are still quantum mechanical mysteries involved despite many experimental and theoretical studies. A major difference between biological and artificial watersplitting systems is that the catalytic reaction of the former is widely believed to take place in the electronic ground state of Mn4CaO5, while the catalysts used in the latter are usually directly photoexcited. The main characteristics of these types of splitting are summarized, which shows the essential differences between the mechanisms The main characteristics of these types of splitting are summarized in Table 1, which shows the essential differences between the mechanisms

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Conclusion