Liquid Crystalline Phase Transition Induces Spin Crossover in a Polyelectrolyte Amphiphile Complex

Abstract

Self-assembly of Fe(2+) or Ni(2+) ions and the ditopic ligand 6,6',6''-bis(2-pyridyl)-2,2':4',4'':2'',2'''-quaterpyridine (btpy) through coordinative binding results in rodlike metallosupramolecular coordination polyelectrolytes (Fe-MEPE or Ni-MEPE). Sequential self-assembly with dihexadecyl phosphate (DHP) via electrostatic interactions between MEPE and DHP leads to the corresponding polyelectrolyte amphiphile complex (PAC) with liquid crystalline properties. The MEPE rods are embedded in between the interdigitated DHP layers. Upon heating above room temperature, the Fe-PAC shows an irreversible spin-crossover (SCO) from a diamagnetic low-spin (LS) to a paramagnetic high-spin (HS) state accompanied by a color change from dark blue to pale blue. The SCO is nearly complete (95%) and directly associated with the structure changes induced by the melting of the amphiphilic matrix. The original Fe-PAC architecture does not reassemble upon cooling and remains in a disordered frozen HS state. However, dissolving the heated PAC induces reassembly, and the original dark blue, diamagnetic, ordered material is completely recovered. In comparison to Fe-PAC, Ni-PAC shows the same lamellar structure and the same temperature depended structure changes but has a constant magnetic moment. In contrast to Fe-PAC, in neat Fe-MEPE the SCO depends on the history of the sample and in particular on the amount of included solvent as thermogravimetric analysis, differential scanning calorimetry (DSC), and magnetic measurements indicate. Solid MEPE does not have liquid crystalline properties, and, therefore, the induced structure changes upon heating are constrained by the solid-state architecture, and thus, the SCO in Fe-MEPE is incomplete.

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

“…[6] Our XANES results on PACs confirm the hypothesis suggested by quite recent UV/Vis data. [28] The authors observed upon heating an increase in intensity of the band of free bis-terpyridine ligands which indicates a change in the coordination geometry and at least a partial disassembly of the MEPE backbones. Such a strong distortion of the metallo-supramolecular architecture is probably responsible of the changes in the MS events and thus the changes we observed in the XANES profiles upon heating.…”

Hierarchical assemblies of coordination supramolecules

“…Reattach the coronal chains to the micellar core can be realized. [54]. It is very interesting to find that the phase transition of the amphiphiles can tailor the magnetic properties of the MEPE [52,54].…”

“…[54]. It is very interesting to find that the phase transition of the amphiphiles can tailor the magnetic properties of the MEPE [52,54]. Generally, in the presence of ligand, the crystal field around the metal ions is very strong so that the metal ions are forced into the low-spin state (LS) [55].…”

Spin crossover in liquid (Mg,Fe)O at extreme conditions

“…The phenomenon has been observed in solids, liquids, liquid crystals, and complexes in solution [18] and is believed to enable a wide array of technological applications from switching-devices to sensors [17]. Spin crossover has been shown to accompany a solid-to-liquid phase transition in some * eero.holmstrom@aalto.fi systems [19], and a different threshold temperature for spin crossover in the solid phase compared to a liquid crystal phase of the same complex has been reported [20].…”