Abstract
Theoretical predictions show that depending on the populations of the Fe 3dxy, 3dxz, and 3dyz orbitals two possible quintet states can exist for the high-spin state of the photoswitchable model system [Fe(terpy)2]2+. The differences in the structure and molecular properties of these 5B2 and 5E quintets are very small and pose a substantial challenge for experiments to resolve them. Yet for a better understanding of the physics of this system, which can lead to the design of novel molecules with enhanced photoswitching performance, it is vital to determine which high-spin state is reached in the transitions that follow the light excitation. The quintet state can be prepared with a short laser pulse and can be studied with cutting-edge time-resolved X-ray techniques. Here we report on the application of an extended set of X-ray spectroscopy and scattering techniques applied to investigate the quintet state of [Fe(terpy)2]2+ 80 ps after light excitation. High-quality X-ray absorption, nonresonant emission, and resonant emission spectra as well as X-ray diffuse scattering data clearly reflect the formation of the high-spin state of the [Fe(terpy)2]2+ molecule; moreover, extended X-ray absorption fine structure spectroscopy resolves the Fe–ligand bond-length variations with unprecedented bond-length accuracy in time-resolved experiments. With ab initio calculations we determine why, in contrast to most related systems, one configurational mode is insufficient for the description of the low-spin (LS)–high-spin (HS) transition. We identify the electronic structure origin of the differences between the two possible quintet modes, and finally, we unambiguously identify the formed quintet state as 5E, in agreement with our theoretical expectations.
Highlights
Switchable molecular compounds have significant potential as very high-density devices in the areas of data storage systems, molecular switching, and display devices.[1−3] Promising candidates include transition metal compounds, in particular octahedral FeII complexes, which can exist either in a low-spin (LS) or a high-spin (HS) state, depending on parameters such as temperature or pressure.[4]
On the basis of this it was concluded that the structure for the 5E is in somewhat better agreement with the experimental data than that of the 5B2, this proposed structure contradicts the energetics arising from the density functional theory (DFT) models applied by these authors, which suggest 5B2 as the more stable quintet state
The metalcentered quintet (5E) crosses the 1,3MLCT band somewhat close to its minimum, the coupling between these states was found negligible in a recent theoretical work on [Fe(bipy)3]2+, and it was suggested that the system relaxes via the metal-centered triplet states before the intersystem crossing to the quintet.[26]
Summary
Switchable molecular compounds have significant potential as very high-density devices in the areas of data storage systems, molecular switching, and display devices.[1−3] Promising candidates include transition metal compounds, in particular octahedral FeII complexes, which can exist either in a low-spin (LS) or a high-spin (HS) state, depending on parameters such as temperature or pressure.[4]. It is worth mentioning that the inclusion of outer coordination shells (beyond Fe−N single scattering contributions in the 1−2 Å range in Figure 4) was possible given the superior EXAFS data quality obtained in the experiment for both the LS ground and photoexcited HS state spectra.
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