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Issue 40, 2013
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Porphyrin adsorbed on the (10[1 with combining macron]0) surface of the wurtzite structure of ZnO – conformation induced effects on the electron transfer characteristics

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Abstract

Electron transfer at the adsorbate–surface interface is crucial in many applications but the steps taking place prior to and during the electron transfer are not always thoroughly understood. In this work a model system of 4-(porphyrin-5-yl)benzoic acid adsorbed as a corresponding benzoate on the ZnO wurtzite (10[1 with combining macron]0) surface is studied using density functional theory (DFT) and time-dependent DFT. Emphasis is on the initial photoexcitation of porphyrin and on the strength of coupling between the porphyrin LUMO or LUMO + 1 and the ZnO conduction band that plays a role in the electron transfer. Firstly, ZnO wurtzite bulk is optimized to minimum energy geometry and the properties of the isolated ZnO (10[1 with combining macron]0) surface model and the porphyrin model are discussed to gain insight into the combined system. Secondly, various orientations of the model porphyrin on the ZnO surface are studied: the porphyrin model standing perpendicularly to the surface and gradually brought close to the surface by tilting the linker in a few steps. The porphyrin model approaches the surface either sideways with hydrogen atoms of the porphyrin ring coming down first or twisted in a ca. 45° angle, giving rise to π-interactions of the porphyrin ring with ZnO. Because porphyrins are closely packed and near the surface, emerging van der Waals (vdW) interactions are examined using Grimme's D2 method. While the orientation affects the initial excitation of porphyrin only slightly, the coupling between the LUMO and LUMO + 1 of porphyrin and the conduction band of ZnO increases considerably if porphyrin is close to the surface, especially if the π-electrons are interacting with the surface. Based on the results of coupling studies, not only the distance between porphyrin and the ZnO surface but also the orientation of porphyrin can greatly affect the electron transfer.

Graphical abstract: Porphyrin adsorbed on the (10 [[1 with combining macron]] 0) surface of the wurtzite structure of ZnO – conformation induced effects on the electron transfer characteristics

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Publication details

The article was received on 19 Apr 2013, accepted on 21 Aug 2013 and first published on 10 Sep 2013


Article type: Paper
DOI: 10.1039/C3CP51685G
Citation: Phys. Chem. Chem. Phys., 2013,15, 17408-17418
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    Porphyrin adsorbed on the (10[1 with combining macron]0) surface of the wurtzite structure of ZnO – conformation induced effects on the electron transfer characteristics

    M. Niskanen, M. Kuisma, O. Cramariuc, V. Golovanov, T. I. Hukka, N. Tkachenko and T. T. Rantala, Phys. Chem. Chem. Phys., 2013, 15, 17408
    DOI: 10.1039/C3CP51685G

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