X‐Ray Near‐Edge Absorption Study of Temperature‐Induced Low‐Spin‐to‐High‐Spin Change in Metallo‐Supramolecular Assemblies

Abstract

X-ray absorption near the iron K edge (XANES) was used to investigate the characteristics of temperature-induced low-spin-to-high-spin change (SC) in metallo-supramolecular polyelectrolyte amphiphile complexes (PAC) containing FeN(6) octahedra attached to two or six amphiphilic molecules. Compared to the typical spin-crossover material Fe(phen)(2) (NCS)(2) XANES spectra of PAC show fingerprint features restricted to the near-edge region which mainly measures multiple scattering (MS) events. The changes of the XANES profiles during SC are thus attributed to the structure changes due to different MS path lengths. Our results can be interpreted by a uniaxial deformation of FeN(6) octahedra in PAC. This is in agreement with the prediction that SC is originated by a structural phase transition in the amphiphilic matrix of PAC, but in contrast to Fe(phen)(2) (NCS)(2), showing the typical spin crossover being associated with shortening of all the metal-ligand distances.

Thermally induced structural rearrangement of the Fe(ii) coordination geometry in metallo-supramolecular polyelectrolytes

“…The equatorial bonds expand continuously to reach a value of 2.02 Å at maximum temperature. Thus, in the HT state the FeN 6 pseudooctahedron exhibits a distortion, which may be related to a spin crossover in the Fe(II) complexes from low spin to high spin as reported previously for Fe-PAC 6,7 and other Fe(II)-complexes. 35 This distortion also affects the MLCT transition around 590 nm, causing a colour change from blue to green.…”

“…32) as well as with the Fe-PAC in the low spin state. 6,7 However, the XANES spectra of Fe-MEPE in the HT state differ in details from those of the corresponding Fe-PAC in the high-spin state 6,7 or the photoexcited Fe(tpy) 2 . 32 Although in all the cases a strong first whiteline peak is present with the second one reduced to a shoulder, the exact positions and intensities of the features are in each case slightly different.…”

“…4,5 The corresponding Fe-PACs show an unusual temperature induced spin crossover. 6,7 The modularity of self-assembly permits the introduction of different metal ions, ligands and counter ions, giving rise to a plethora of materials with a large range of properties. 8,9 These macromolecular assemblies which utilize metal ion coordination, open a new dimension to the field of polymer materials.…”