Carlos Bartual-Murgui

Synergetic effect of host-guest chemistry and spin crossover in 3D Hofmann-like metal-organic frameworks [Fe(bpac)M(CN)4] (M=Pt, Pd, Ni).

The synthesis and characterization of a series of three-dimensional (3D) Hofmann-like clathrate porous metal-organic framework (MOF) materials [Fe(bpac)M(CN)(4)] (M=Pt, Pd, and Ni; bpac=bis(4-pyridyl)acetylene) that exhibit spin-crossover behavior is reported. The rigid bpac ligand is longer than the previously used azopyridine and pyrazine and has been selected with the aim to improve both the spin-crossover properties and the porosity of the corresponding porous coordination polymers (PCPs). The 3D network is composed of successive {Fe[M(CN)(4)]}(n) planar layers bridged by the bis-monodentate bpac ligand linked in the apical positions of the iron center. The large void between the layers, which represents 41.7% of the unit cell, can accommodate solvent molecules or free bpac ligand. Different synthetic strategies were used to obtain a range of spin-crossover behaviors with hysteresis loops around room temperature; the samples were characterized by magnetic susceptibility, calorimetric, Mössbauer, and Raman measurements. The complete physical study reveals a clear relationship between the quantity of included bpac molecules and the completeness of the spin transition, thereby underlining the key role of the π-π stacking interactions operating between the host and guest bpac molecules within the network. Although the inclusion of the bpac molecules tends to increase the amount of active iron centers, no variation of the transition temperature was measured. We have also investigated the ability of the network to accommodate the inclusion of molecules other than water and bpac and studied the synergy between the host-guest interaction and the spin-crossover behavior. In fact, the clathration of various aromatic molecules revealed specific modifications of the transition temperature. Finally, the transition temperature and the completeness of the transition are related to the nature of the metal associated with the iron center (Ni, Pt, or Pd) and also to the nature and the amount of guest molecules in the lattice.

Enhanced porosity in a new 3D Hofmann-like network exhibiting humidity sensitive cooperative spin transitions at room temperature

The porous coordination polymers (PCPs) of general formula {Fe(bpac)[M(CN)4]}·guest (M = Pt, Pd) exhibit larger channels than previously synthesised 3D-Hofmann-like PCP. The channels are partially occupied by uncoordinated guest bpac ligands and labile H2O molecules. These PCPs exhibit very scarce cooperative spin crossover behaviour around room temperature with a large hysteresis loop (up to 49 K) and also display sensitivity to humidity and guest molecules. The inclusion of bpac molecules in the 3D network can be avoided by adding competitive volatile molecules during the crystallization process, affording the guest-free material. The spin crossover behavior of different guest and guest-free materials is also presented.

Spin-crossover metal–organic frameworks: promising materials for designing gas sensors

We present a quantitative study of the sensing behavior of micro-patterned gratings based on the metal–organic framework type spin crossover complex Fe(bpac)[Pt(CN)4] (1) in the presence of vapors of various organic compounds. The gratings of 1 were fabricated by combining a sequential assembly technique and a photolithographic method. The guest absorption driven change of the spin state and the associated variation of the refractive index were in situ monitored by optical diffraction on a grating of 1. Our sensor is characterized by good reversibility, room temperature operation, a low limit of detection (∼30 ppm for molecules of iodobenzene) as well as a linear dynamic range of detection (from 300 to 1500 ppm). The possibility of selectively desorbing the analytes by thermal annealing confers also selectivity to the sensor.

Guest Effect on Nanopatterned Spin-Crossover Thin Films

Nanopatterned thin films of the metal-organic framework {Fe(bpac)[Pt(CN)4]} (bpac=bis(4-pyridyl)acetylene) are elaborated by the combination of a sequential assembly process and a lithographic method. Raman microspectroscopy is used to probe the temperature dependence of the spin state of the iron(II) ions in the films (40-90 nm in thickness), and reveals an incomplete but cooperative spin transition comparable to that of the bulk material. Adsorption/desorption of pyridine guest molecules is found to have a substantial influence on the spin-crossover properties of the thin layers. This interplay between host-guest and spin-crossover properties in thin films and nanopatterns demonstrates the potential ability of using this kind of material as a microsensor.

High quality nano-patterned thin films of the coordination compound {Fe(pyrazine)[Pt(CN)4]} deposited layer-by-layer

An optimised procedure was developed for the layer-by-layer deposition of the Hofmann clathrate-like coordination compound {Fe(pyrazine)[Pt(CN)4]} either as continuous or as nano-patterned thin films. Characterization of the thickness and topography of the thin films by atomic force microscopy (AFM) and by surface plasmon resonance (SPR) spectroscopy, which also yields the layers refractive index and losses, are reported. We found that the films are of good optical quality and the results of both AFM and SPR experiments are in good agreement with the theoretical predictions of the films thicknesses.

Tunable Spin‐Crossover Behavior of the Hofmann‐like Network {Fe(bpac)[Pt(CN)4]} through Host–Guest Chemistry

A study of the spin-crossover (SCO) behavior of the tridimensional porous coordination polymer {Fe(bpac)[Pt(CN)4]} (bpac=bis(4-pyridyl)acetylene) on adsorption of different mono- and polyhalobenzene guest molecules is presented. The resolution of the crystal structure of {Fe(bpac)[Pt(CN)4]}⋅G (G=1,2,4-trichlorobenzene) shows preferential guest sites establishing π⋅⋅⋅π stacking interactions with the host framework. These host-guest interactions may explain the relationship between the modification of the SCO behavior and both the chemical nature of the guest molecule (electronic factors) and the number of adsorbed molecules (steric factors).

Detection of molecular spin-state changes in ultrathin films by photonic methods

Ultrathin films of molecular spin crossover materials exhibit very appealing properties for a variety of photonic applications because the spin-state switching is accompanied by a spectacular change of the complex refractive index in a wide spectral range. After examining different optical spectroscopic approaches for the detection of spin-state changes in nanometric films, we found that conventional light absorption measurements can be used down to the nanometer thickness if the oscillator strength of the transition is high, which is often the case for charge transfer transitions in the ultraviolet range. Methods based on fluorescence energy transfer provide a straightforward means for detecting spin-state changes in films in the visible wavelength range, even if photobleaching may be a problem for certain luminophores. Alternatively, changes in the refractive index accompanying the spin transition can be conveniently determined by surface plasmon resonance spectroscopy, which can also provide very accurate film thickness determination. Plasmonic effects were also used to investigate spin-crossover films by means of surface-enhanced Raman spectroscopy. We found that this technique can provide information not only on the spin state of the molecules in very thin layers, but also on their chemical composition and structure.

Thermal and pressure-induced spin crossover in a novel three-dimensional Hoffman-like clathrate complex

The synthesis and crystal structure of the interpenetrated metal–organic framework material Fe(bpac)2[Ag(CN)2]2 (bpac = 4,4′-bis(pyridyl)acetylene) are reported along with the characterization of its spin crossover properties by variable temperature magnetometry and Mossbauer spectroscopy. The complex presents an incomplete stepped spin transition as a function of temperature that is modified upon successive thermal cycling. The pressure-induced transition has also been investigated by means of high pressure Raman spectroscopy using a diamond anvil cell. The results show that it is possible to reach the thermally-inaccessible fully low spin state at room temperature by applying hydrostatic pressure to the sample.

Photonic gratings of the metal-organic framework {Fe(bpac)[Pt(CN)4]} with synergetic spin transition and host-guest properties

Surface-relief photonic gratings of the spin-crossover metal-organic framework {Fe(bpac)[Pt(CN)4]} (bpac = bis(4-pyridyl)acetylene) were elaborated by the combination of a sequential assembly process and lithographic methods. Optical diffraction, surface plasmon resonance spectroscopy and Raman micro-spectroscopy were used to investigate the temperature dependence of the spin state of the iron(II) ions and the concomitant change of the refractive index of the grating material. The refractive index change associated with the high spin ((5)T) to low spin ((1)A) transition was found to be as high as Δn = 0.08 ± 0.005, which was attributed to the pronounced mass density difference between the two spin states. While the grating thickness (15-90 nm) had no influence on the spin-crossover properties of the gratings, the adsorption of aromatic guest molecules was found to have a substantial effect both on the spin transition temperature and the completeness of the transition.