Abstract
Abstract The electron density distribution of a light-induced molecular excited state, i.e. the high spin metastable state of [Fe(phen)2(NCS)2], was determined from steady-state photocrystallographic measurements. We defined the experimental conditions under which the accuracy of the measured diffraction data is compatible with an electron density analysis. These include: (i) a large structural and electronic contrast between high spin (HS) and low spin (LS) states, (ii) an efficient photoconversion under light irradiation and (iii) slow relaxation of the HS metastable state. Multipolar modeling of the electron density yielded a deformation density and 3d-orbital populations for Fe(II) characteristic of a high spin (t 2g 4 e g 2) electron configuration and support the assumption of significant σ-donation and π-backbonding of the Fe—N interactions. The electron density distribution in the intermolecular regions confirms anisotropic intermolecular interactions with possibly a layer topology parallel to the orthorhombic (ab) plane, related to the system cooperativity.
Highlights
Single crystal X-ray diffraction under optical excitation, termed photocrystallography, is a promising and rapidly developing field
We have recently addressed this point in a separate publication l13]; we briefly discuss below its main conclusions, together with a justification for the parameters selected in our present X-ray diffraction experiment, keeping in mind that the ultimate goal is to get a d~ta set of an accuracy suitable for an electron density (ED) analysis
The electron density distribution of a light-induced, molecular metastable state was determined from steady-state Xray diffraction measurements under light excitation
Summary
Single crystal X-ray diffraction under optical excitation, termed photocrystallography, is a promising and rapidly developing field (see e.g. [1,2,3,4]). At very low temperature, [Fe(phenh(NCShJ exhibits the well-known Light-Induced Excited Spin State Trapping phenomenon ("LIESST"), by means of which a HS metastable state can be efficiently (completely) populated using filtered white light or a He-Ne laser excitation source [17 -19j. This process consists of optical pumping from the thermodynamically stable LS state to an intermediate 1MLCT state (Metal Ligand Charge Transfer), which in turn rapidly decays to a quintet HS state through intersystem crossings [20,21] (Fig. 1).
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