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(a) NEXAFS spectra for a series of 3d metal OEPs sorted by their atomic number Z. The lowest energy peaks correspond to the to LUMO transition. The structure of an OEP is shown in the inset. (b) Comparison between experimental (left axis) and calculated (right axis) values for the maximum of the to LUMO transition peak of the series of OEPs. Notice that the axis is shifted 3.5 eV between calculated and experimental values. The systematic shift of the LUMO peak with the atomic number Z is reproduced by the calculation.
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Frontier orbitals of the OEPs. The orbitals are labeled according to the symmetry group. The order of the orbitals from HOMO to HOMO-3 corresponds to MnOEP. This order varies depending on the considered OEP (Ref. 7). The LUMO is mainly determined by the orbitals of the aromatic rings, while the HOMOs are dominated by the 3d orbitals of the metal.
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(a) Plot of the three energies contributing to the to LUMO transition observed in NEXAFS, i.e., the LUMO (blue diamonds, left axis), the core level (red squares, right axis), and the core electron-hole interaction (with negative sign, green circles, left axis). These energies are plotted for a series of 3d metal OEPs, sorted by their atomic number Z. They are obtained by a LDA calculation for the optimized geometry with in the core level and 0.5 e in the LUMO [following the transition state model proposed by Slater (Ref. 23)]. The core level shift dominates the shift of the to LUMO transition. (b) Bader charge on the nitrogen atoms vs the position of the level for the series of 3d metal OEPs. A turnaround of the charge transfer near the middle of the series scrambles the sequence of atomic numbers in this plot. The different charges on the N atoms along the OEP series cause a shift of the core levels and thus explain the shift of the to LUMO transitions in NEXAFS (see Fig. 1).
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Porphyrins are widely used as dye molecules in solar cells. Knowing the energies of their frontier orbitals is crucial for optimizing the energy level structure of solar cells. We use near edge x-ray absorption fine structure(NEXAFS) spectroscopy to obtain the energy of the lowest unoccupied molecular orbital (LUMO) with respect to the core level of the molecule. A systematic energy shift of the to LUMO transition is found along a series of 3d metal octaethylporphyrins and explained by density functional theory. It is mainly due to a shift of the level rather than a shift of the LUMO or a change in the electron-hole interaction of the core exciton.
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