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Issue 17, 2018
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Collision induced charge separation in ground-state water splitting dynamics

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Abstract

In one type of photocatalytic dynamics of water splitting formally summarized as2H2O + 4 + MH → 2H2O + M*H → 4H+ + 4e + O2 + MHthe catalytic center M mainly composed of Mn oxides (clusters) along with supporting molecules like proteins is directly photoexcited and discharges electrons and protons. The mechanism can be comprehended in terms of the coupled proton electron-wavepacket transfer (CPEWT). In another type proposed in the literature, M is not directly photoexcited, and instead, lights are absorbed somewhere other than M, thereby creating complicated sequential steps (a ladder) of oxidation–reduction potential, thus sucking electrons successively from one molecular site to the next, and the final place providing electrons and protons is the catalytic center M. During charge separation dynamics, M is assumed to remain in the electronic ground state, and this type can be schematically summarized asMH+ + Ω+ → M + H+ + Ω+ewhere Ω indicates a cation species (a hole carrier) at the site of photon absorption. It is widely believed that the latter mechanism is responsible for water splitting in plants and cynobacteria, and M in photosystem II (PSII) is known to include Mn4CaO5. However, difficult questions about this mechanism of ground-state charge separation in the latter reaction arise as to whether it is quantum mechanically possible and what is it, if indeed possible? Besides, the time-constant for this reaction reported in the literature is so long, actually far longer than the time-scale for energy dissipation for inter- and intra-molecular vibrational energy redistribution, that the quantum mechanical coherence of the reaction should not be able to be maintained. More seriously, we wonder how protons and electrons can be isolated in the ground state, if any, and how they can be transferred unidirectionally (with no return)? We address these fundamental questions affirmatively by proposing a general chemical principle; collision induced charge separation dynamics in the ground state.

Graphical abstract: Collision induced charge separation in ground-state water splitting dynamics

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Publication details

The article was received on 24 Jan 2018, accepted on 04 Apr 2018 and first published on 06 Apr 2018


Article type: Paper
DOI: 10.1039/C8CP00520F
Citation: Phys. Chem. Chem. Phys., 2018,20, 12229-12240
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    Collision induced charge separation in ground-state water splitting dynamics

    K. Yamamoto and K. Takatsuka, Phys. Chem. Chem. Phys., 2018, 20, 12229
    DOI: 10.1039/C8CP00520F

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