Tuning the structure and the magnetic properties of metallo-supramolecular polyelectrolyte-amphiphile complexes.

Self-assembly of Fe(2+) ions and the rigid ditopic ligand 1,4-bis(2,2':6',2''-terpyridin-4'-yl)benzene results in metallo-supramolecular coordination polyelectrolytes (MEPE). Sequential self-assembly of MEPE and dialkyl phosphoric acid esters of varying chain length via electrostatic interactions leads to the corresponding polyelectrolyte-amphiphile complexes (PAC), which have liquid-crystalline properties. The PACs have a stratified architecture where the MEPE is embedded in between the amphiphile layers. Upon heating above room temperature, the PACs show either a reversible or an irreversible spin-crossover (SCO) in a temperature range from 360 to 460 K depending on the architecture of the amphiphilic matrix. As the number of amphiphiles per metal ion is increased in the sequence 1:2, 1:4, and 1:6, the temperature of the SCO is shifted to higher values whereas the amphiphile chain length does not have a significant impact on the SCO temperature. In summary, we describe in this article how the structure and the magnetic response function of PACs can be tailored through the design of the ligand and the composition. To investigate the structure and the magnetic behavior, we use X-ray scattering, X-ray absorption spectroscopy, differential scanning calorimetry, faraday-balance, and superconducting quantum interference measurements in combination with molecular modeling.

X-ray near-edge absorption study of temperature-induced low-spin-to-high-spin change in metallo-supramolecular assemblies.

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.

Liquid crystalline phase transition induces spin crossover in a polyelectrolyte amphiphile complex.

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.

Inducing spin crossover in metallo-supramolecular polyelectrolytes through an amphiphilic phase transition.

A phase transition in an amphiphilic mesophase is explored to deliberately induce mechanical strain in an assembly of tightly coupled metal ion coordination centers. Melting of the alkyl chains in the amphiphilic mesophase causes distortion of the coordination geometry around the central transition metal ion. As a result, the crystal field splitting of the d-orbital subsets decreases resulting in a spin transition from a low-spin to a high-spin state. The diamagnetic-paramagnetic transition is reversible. This concept is demonstrated in a metallo-supramolecular coordination polyelectrolyte-amphiphile complex self-assembled from ditopic bis-terpyridines, Fe(II) as central transition metal, and dialkyl phosphates as amphiphiles. The magnetic properties are studied in a Langmuir-Blodgett multilayer. The modularity of this concept provides extensive control of structure and function from molecular to macroscopic length scales and gives access to a wide range of new molecular magnetic architectures such as nanostructures, thin films, and liquid crystals.