Measurement of the Ligand Field Spectra of Ferrous and Ferric Iron Chlorides Using 2p3d RIXS

Abstract

Ligand field spectra provide direct information about the electronic structure of transition metal complexes. However, these spectra are difficult to measure by conventional optical techniques due to small cross sections for d-to-d transitions and instrumental limitations below 4000 cm. 2p3d resonant inelastic X-ray scattering (RIXS) is a second order process that utilizes dipole allowed 2p to 3d transitions to access d-d excited states. The measurement of ligand field excitation spectra by RIXS is demonstrated for a series of tetrahedral and octahedral Fe(II) and Fe(III) chlorides, which are denoted Fe(III)-T, Fe(II)-T, Fe(III)-O, and Fe(II)-O. The strong 2p spin-orbit coupling allows the measurement of spin forbidden transitions in RIXS spectroscopy. The Fe(III) spectra are dominated by transitions from the sextet ground state to quartet excited states, and the Fe(II) spectra contain transitions to triplet states in addition to the spin allowed Γ →Γ transition. Each experimental spectrum is simulated using a ligand field multiplet model to extract the ligand field splitting parameter 10Dq and the Racah parameters B and C. The 10Dq values for Fe(III)-T, Fe(II)-T, and Fe(III)-O are found to be -0.7, -0.32, and 1.47 eV, respectively. In the case of Fe(II)-O, a single 10Dq parameter cannot be assigned because Fe(II)-O is a coordination polymer exhibiting axially compressed Fe(II)Cl units. TheT → E transition is split by the axial compression resulting in features at 0.51 and 0.88 eV. The present study forms the foundation for future applications of 2p3d RIXS to molecular iron sites in more complex systems, including iron-based catalysts and enzymes.

NHC-Based Iron Sensitizers for DSSCs

“…narrow photoinduced absorption (PIA) at 330 nm characteristic of metal-centered 5 T2 for time delays <0.5 ps [25]. The energy of this PIA is almost identical to that of 5 T2 in Fe(bpy)3, meaning that it is situated only 0.5-0.6 eV above S0 [36]. We recall here that 5 T2 is thus too low in energy to serve for electron injection in the CB of TiO2.…”

“…This is clearly observed in the time-resolved differential ∆A(t) spectra in acetonitrile upon excitation at 515 nm (Figure 10b), by the ultrafast sub-100-fs decay of the MLCT ESA in the 400-600 nm range and the concomitant rise of the narrow photoinduced absorption (PIA) at 330 nm characteristic of metal-centered 5 T 2 for time delays <0.5 ps [25]. The energy of this PIA is almost identical to that of 5 T 2 in Fe(bpy) 3 , meaning that it is situated only 0.5-0.6 eV above S 0 [36]. We recall here that 5 T 2 is thus too low in energy to serve for electron injection in the CB of TiO 2 .…”

Resonant Inelastic X-ray Scattering (RIXS) Studies in Chemistry: Present and Future

“…These are due to the spin-allowed 5 Γ 5 Γ transitions between the e/t 2g and t 2 /e g orbitals in T d /O h symmetry. This is also where the information from Fe 2p3d RIXS can now be compared in detail with results from optical measurements by UV/Vis absorption and magnetic circular dichroism measurements and we refer to the original paper by Hahn et al for this discussion (Hahn et al, 2017). As for Fe III , the Fe II RIXs spectra in Figure 9b also show spin-forbidden transitions ( 5 Γ 3 Γ for Fe II ) at energy transfers of above 1 eV.…”

Resonant Inelastic X-ray Scattering (RIXS) Studies in Chemistry: Present and Future

“…These are due to the spin-allowed 5 Γ 5 Γ transitions between the e/t 2g and t 2 /e g orbitals in T d /O h symmetry. This is also where the information from Fe 2p3d RIXS can now be compared in detail with results from optical measurements by UV/Vis absorption and magnetic circular dichroism measurements and we refer to the original paper by Hahn et al for this discussion(Hahn et al, 2017). As for Fe III , the Fe II RIXs spectra in…”

“…The schematic representation of Fe 2p3d RIXS in Figure 9a (Hahn et al, 2017). Copyright 2017 American Chemical Society.…”