Helle Svendsen

Photomagnetic Switching of the Complex [Nd(dmf)4(H2O)3(μ‐CN)Fe(CN)5]⋅H2O Analyzed by Single‐Crystal X‐Ray Diffraction

X-ray vision: Single-crystal XRD experiments (see picture) reveal the excited-state structure of the photomagnetic heterobimetallic title complex. The system shows a decrease in all the iron-ligand bond lengths, suggesting that photoexcitation involves a ligand-to-metal charge transfer or a change in the superexchange coupling between the metal centers.

Photomagnetic Switching of Heterometallic Complexes [M(dmf)4(H2O)3(μ-CN)Fe(CN)5]⋅H2O (M=Nd, La, Gd, Y) Analyzed by Single-Crystal X-ray Diffraction and Ab Initio Theory

Single-crystal X-ray diffraction measurements have been carried out on [Nd(dmf)(4)(H(2)O)(3)(mu-CN)Fe(CN)(5)].H(2)O (1; dmf=dimethylformamide), [Nd(dmf)(4)(H(2)O)(3)(mu-CN)Co(CN)(5)].H(2)O (2), [La(dmf)(4)(H(2)O)(3)(mu-CN)Fe(CN)(5)].H(2)O (3), [Gd(dmf)(4)(H(2)O)(3)(mu-CN)Fe(CN)(5)].H(2)O (4), and [Y(dmf)(4)(H(2)O)(3)(mu-CN)Fe(CN)(5)].H(2)O (5), at 15(2) K with and without UV illumination of the crystals. Significant changes in unit-cell parameters were observed for all the iron-containing complexes, whereas 2 showed no response to UV illumination. Photoexcited crystal structures have been determined for 1, 3, and 4 based on refinements of two-conformer models, and excited-state occupancies of 78.6(1), 84(6), and 86.6(7)% were reached, respectively. Significant bond-length changes were observed for the Fe-ligand bonds (up to 0.19 A), the cyano bonds (up to 0.09 A), and the lanthanide-ligand bonds (up to 0.10 A). Ab initio theoretical calculations were carried out for the experimental ground-state geometry of 1 to understand the electronic structure changes upon UV illumination. The calculations suggest that UV illumination gives a charge transfer from the cyano groups on the iron atom to the lanthanide ion moiety, {Nd(dmf)(4)(H(2)O)(3)}, with a distance of approximately 6 A from the iron atom. The charge transfer is accompanied by a reorganization of the spin state on the {Fe(CN)(6)} complex, and a change in geometry that produces a metastable charge-transfer state with an increased number of unpaired electrons, thus accounting for the observed photomagnetic effect.

Helium cryostat synchrotron charge densities determined using a large CCD detector – the upgraded beamline D3 at DESY

A 165 mm Mar CCD detector has been fitted on a large Huber four-circle diffractometer together with a helium cryostat at beamline D3 at Hasylab, DESY in Hamburg. This setup allows fast collection of accurate, short-wavelength, very low temperature X-ray diffraction data for charge-density analysis. As a test example, diffraction data have been collected in 10 h on a hydrogen-bonded network system with 15 unique atoms, and the electron density was modelled with the multipole formalism in an X–N procedure using matching-temperature neutron diffraction data collected at Institut Laue Langevin, Grenoble in France.

Analysis of the photomagnetic properties of cyano-bridged heterobimetallic complexes by X-ray diffraction.

Single crystal synchrotron X-ray diffraction measurements have been carried out on [Nd(DMF)(4)(H(2)O)(3)(μ-CN)Fe(CN)(5)]·H(2)O (DMF = dimethyl-formamide), 1; [Y(DMF)(4)(H(2)O)(3)(μ-CN)Fe(CN)(5)]·H(2)O, 2; [Ce(DMF)(4)(H(2)O)(3)(μ-CN)Fe(CN)(5)]·H(2)O, 3; [Sm(DMF)(4)(H(2)O)(3)(μ-CN)Fe(CN)(5)]·H(2)O, 4; [Tb(DMF)(4)(H(2)O)(3)(μ-CN)Fe(CN)(5)]·H(2)O, 5; [Yb(DMF)(4)(H(2)O)(3)(μ-CN)Fe(CN)(5)]·H(2)O, 6; and [Nd(DMF)(4)(H(2)O)(3)(μ-CN)Co(CN)(5)]·H(2)O, 7, at 15(2) K with and without UV illumination of the crystals. Significant changes in unit cell parameters are observed for all of the iron-containing complexes, while compound 7 shows no response to UV illumination. These results are consistent with previous results and are furthermore reproduced by powder synchrotron X-ray diffraction for compounds 1 and 7. Photoexcited crystal structures have been determined for 1-6 from refinements of two-conformer models, and excited state occupancies in the range 80-94% are found. Significant bond length changes are observed for the Fe-ligand bonds (up to 0.06 Å), the cyano bonds (up to 0.02 Å), and the lanthanide-ligand bonds (up to 0.1 Å). On the contrary, powder X-ray diffraction on the simple compound K(3)Fe(CN)(6), 8, upon UV illumination does not show any structural changes, suggesting that the photomagnetic effect requires the presence of both the transition metal and the lanthanide ion. Photomagnetic measurements show an increase in magnetization of the excited state of 1 of up to 3%, which is much diminished compared with previously published values of 45%. Furthermore, they show that the isostructural complex [La(DMF)(4)(H(2)O)(3)(μ-CN)Fe(CN)(5)]·H(2)O, 9, exhibits identical magnetic responses in the UV-induced excited crystal structure.

On the significance of Bragg reflections

Recently Henn & Meindl [Acta Cryst. (2010), A66, 676-684] examined the significance of Bragg diffraction data through the descriptor W = (I(1/2))/(σ(I)). In the Poisson limit for the intensity errors W equals unity, but any kind of data processing (background subtraction, integration, scaling, absorption correction, Lorentz and polarization correction etc.) introduces additional error as well as remaining systematic errors and thus the significance of processed Bragg diffraction data is expected to be below the Poisson limit (W(Bragg) < 1). Curiously, it was observed by Henn & Meindl for several data sets that W(Bragg) had values larger than one. In the present study this is shown to be an artefact due to the neglect of a data scale factor applied to the standard uncertainties, and corrected values of W(Bragg) applied to Bragg data on an absolute scale are presented, which are all smaller than unity. Furthermore, the error estimation models employed by two commonly used data-processing programs {SADABS (Bruker AXS Inc., Madison, Wisconsin, USA) and SORTAV [Blessing (1997). J. Appl. Cryst. 30, 421-426]} are examined. It is shown that the empirical error model in SADABS very significantly lowers the significance of the Bragg data and it also results in a very strange distributions of errors, as observed by Henn & Meindl. On the other hand, error estimation based on the variance of a population of abundant intensity data, as used in SORTAV, provides reasonable error estimates, which are only slightly less significant than the raw data. Given that modern area detectors make measurement of highly redundant data relatively straightforward, it is concluded that the latter is the best approach for processing of data.

A photo-induced excited state structure of a hetero-bimetallic ionic pair complex, Nd(DMA)4(H2O)4Fe(CN)6·3H2O, analyzed by single crystal X-ray diffraction

The excited state crystal structure of the ionic complex (Nd(DMA)(4)(H(2)O)(4)-Fe(CN)(6)·3H(2)O (DMA = dimethylacetamide) has been determined at 15 K upon UV illumination by single crystal X-ray diffraction. Significant structural changes are observed around the Fe site in the excited state. These changes are similar to those observed for a related molecular compound exhibiting photomagnetic properties.

Photomagnetic complexes. Structures of excited states

Photoinduced magnetization is an interesting property in materials, with special focus on the development of new memory devices. We have synthesized a range of complexes with the general formula M1(DMF)4(H2O)3(m-CN)M2(CN)5.H2O (M1M2DMF), where M1 is a rare earth metal and M2 a transition metal. For NdFeDMF it has been shown that the material has a significant photoinduced magnetization. Upon UV irradiation the material stays in the excited state for several hours. The mechanism causing this large change in the magnetic susceptibility is not yet understood, and in order to understand the nature of the electronic transition it is crucial to know exactly which electrons are involved in the process. For that reason electron density studies are of great importance. The long lived metastable state makes it possible not only to study electron densities in the ground state but also in the excited state as well without the need for time resolution. As a first step towards excited state electron densities we have measured the structure in the excited state of NdFeDMF as well as four other complexes, which show the same behavior upon UV-illumination.