Photoinduced Charge Separation Catalyzed by Manganese Oxides onto a Y-Shaped Branching Acceptor Efficiently Preventing Charge Recombination

Water oxidation through photochemical catalysis is an important chemical reaction in the excited state that leaves behind an O=O bond when the cycle is complete, and the pairs of protons and electrons effect the separation of charges in acceptor molecules such as proteins. There is growing interest in the importance of the interaction between the dynamic charge separation mechanism to create electrons and protons, and the conditions that should be satisfied by the electron-proton acceptors to effect the reaction. Manganese oxides have been used in previous theoretical research work to form a charge separation quantum mechanism.

The conditions that favor the working of the acceptors include the availability of sufficient capacity to allow electron transfer, the ability to prevent destruction of charge separation, and finally the availability of a track-branching function which allows the protons and electrons to be carried separately to their destinations which has not been taken into account in previous research studies due to the use of residues of simple amino acids as acceptors.

Dr. Kentaro Yamamoto and Professor Dr. Kazuo Takatsuka at Fukui Institute for Fundamental Chemistry, Kyoto University focused on the electron-proton accepting system associated with inspired by the functions of Mn₄CaO₅ cluster in photosystem II which has a track-branching function to check its compatibility with the coupled proton electron-wave packet transfer mechanism. They have found out that charge separation can be annihilated through continuous nonadiabatic transition in acceptors to d-d states and have discussed the importance of calcium atoms with this regard. Their work is now published in ChemPhysChem.

In their paper the authors replaced the TyrZ electron acceptor His next to Y_z as a proton acceptor with imidazole, and the Y_z Y_z+P680 electron acceptor with 4-hydroxyaniline, since they are a simpler alternative, after which imidazolate anion and 4-hydroxyanilinium cations were modeled for the respective proton and electron acceptors. This was then followed by photoexcitation of the system. The electron and proton donors in the system are manganese atoms and water respectively. Two systems were adopted in the research which include system A1 that had MnOH and system A2 which had MnCa(OH)₃ in order to examine the role played by calcium atoms in the Y-shaped acceptor.

The researchers used a technique that involved quantum electronic wave packets in order to trace the nuclear and electronic full dimensional nonadiabatic dynamics. Due to the difference in weight between nuclei and electrons, the semiclassical Ehrenfest method was used to accurately estimate the electronic state mixing coefficients.

Path sampling, geometry optimization and vibration analysis were performed in the ground state and the results confirmed that the separation of charges does not occur in this state. Calcium atoms were found to have no major effect in this ground state.

The Y-shaped coupled proton electron-wave packet transfer in the photoexcited state was studied in which the electron-proton pair was shown to be formed in the acceptor which was not the case in ground state dynamics. Once photoexcitation takes place, intramolecular vibrational energy redistribution occurs which comes up against to the quasi-degenerate manifold of conical intersections.

In their study Yamamoto and Takatsuka were able to determine the role of the calcium atoms which included the modulation of electron flow channel, and the suppression of undesirable destruction of electron-proton pairs, and they showed that charge separation on a branched acceptor can occur as a result of photoinduced electron-proton transfer.