Search results for "crossover"
showing 10 items of 658 documents
Guest Modulation of Spin-Crossover Transition Temperature in a Porous Iron(II) Metal Organic Framework: Experimental and Periodic DFT Studies
2014
The synthesis, structure, and magnetic properties of three clathrate derivatives of the spin-crossover porous coordination polymer {Fe(pyrazine)[Pt(CN)(4)]} (1) with five-membered aromatic molecules furan, pyrrole, and thiophene is reported. The three derivatives have a cooperative spin-crossover transition with hysteresis loops 14-29 K wide and average critical temperatures T-c=201 K (1.fur), 167 K (1.pyr), and 114.6 K (1.thio) well below that of the parent compound 1 (T-c=295 K), confirming stabilization of the HS state. The transition is complete and takes place in two steps for 1.fur, while 1.pyr and 1.thio show 50% spin transition. For 1.fur the transformation between the HS and IS (mi…
Spin- and phase transition in the spin crossover complex [Fe(ptz)6](BF4)2 studied by nuclear inelastic scattering of synchrotron radiation and by DFT…
2006
Abstract Nuclear inelastic scattering (NIS) spectra of [Fe(ptz) 6 ](BF 4 ) 2 (ptz = 1- n -propyl-tetrazole) have been measured for five phases differing in spin state and crystallographic structure. Different spectral patterns have been found for the low-spin and high-spin phases and are described in terms of normal coordinate analysis of the complex molecule. For both low-spin and high-spin phases the conversion from ordered to disordered phase results in splitting of the observed NIS bands. Packing becomes visible in the NIS spectra via coupling of the Fe–N stretching vibrations with those of the terminal n -propyl groups. The DFT-based normal coordinate analysis also reveals the characte…
High-Spin → Low-Spin Relaxation in [Fe(bpp)2](CF3SO3)2 H2O after LIESST and Thermal Spin-State Trapping—Dynamics of Spin Transition Versus Dynamics o…
1996
The iron(II) complex [Fe(bpp)2]-(CF3SO3)2 H2O (bpp = 2,6-bis(pyrazolyl-3-yl)pyridine) shows a thermal spin transition associated with a hysteresis of approximately 140 K width. The transition temperatures T1/2 (where the fraction of HS species γHS = 0.5) are 147 K and ≈285 K in the cooling and heating directions, respectively. The compound shows the LIESST and reverse-LIESST effects at low temperatures. The relaxation of the metastable HS states generated by LIESST was observed quantitatively at temperatures between 77.5 and 85 K by Mossbauer spectroscopy. Metastable HS states can also be generated by rapid cooling of the sample. The relaxation of the metastable HS states formed by thermal …
Unprecedented multi-stable spin crossover molecular material with two thermal memory channels.
2013
et al.
Formation of local spin-state concentration waves during the relaxation from photoinduced state in a spin-crossover polymer
2017
The complex relaxation from the photoinduced high-spin phase (PIHS) to the low-spin phase of the bimetallic two-dimensional coordination spin-crossover polymer [Fe[(Hg(SCN)3)2](4,4′-bipy)2]nis reported. During the thermal relaxation, commensurate and incommensurate spin-state concentration waves (SSCWs) form. However, contrary to the steps forming at thermal equilibrium, associated with long-range SSCW order, the SSCWs forming during the relaxation from the PIHS phase correspond to short-range order, revealed by diffuse X-ray scattering. This is interpreted as resulting from the competition between the two types of SSCW order and another structural symmetry breaking, due to ligand ordering,…
Spin crossover star-shaped metallomesogens of iron(II).
2014
Three new types of spin crossover (SCO) metallomesogens of Fe-II based on symmetric tripod ligands and their magnetic and structural properties are reported here. These were obtained by condensation of tris(2-aminoethyl)-amin (tren) with the aldehyde derived from 3-alkoxy-6-methylpyridine (mpyN, N (number of carbon atoms in n-alkyl chains) = 8, 18), 1-alkyl-1H-imidazole (imN, N = 4, 16, 18, 20, 22), or 1-alkyl-1H-benzimidazole (bimN, N = 6, 14, 16, 18, 20). A complex derived from 1-octadecyl-1H-naphtho[2,3-d]imidazole (nim18) retains the high spin state at any temperature. Single crystals of the short-chain complexes were investigated by a combination of X-ray crystallography, magnetic meas…
Spin crossover in metallomesogens
2009
Abstract In this review article are illustrated the strategies developed in order to achieve interplay/synergy between spin transition and liquid crystal transition. The synthesised Fe(II) metallomesogens exhibit different types of interplay between both phase transitions. A classification according to the analysis of the magnetic and structural data has led to the separation of three types of interplay, namely: type i systems with coupled phase transitions, subdivided into three groups a , b and c (in a the structural changes associated with the Cr ↔ LC drive the spin transition while in b these structural changes influence the spin state of the metallic centers but they are not the drivin…
Cooperative phenomena and light-induced bistability in iron(II) spin-crossover compounds
1999
In iron(II) spin-crossover compounds, the transition from the 1A1 low-spin state at low temperatures to the 5T2 high-spin state at elevated temperatures is accompanied by a large increase in metal-ligand bond lengths. The resulting elastic interactions may be pictured as an internal pressure which is proportional to the concentration of the low-spin species. Because pressure stabilises the low-spin state relative to the high-spin state this results in a positive feedback. Thermal transition curves in neat iron(II) spin-crossover compounds are thus invariable much steeper than in diluted mixed crystals, and the high-spin→low-spin relaxation following the light-induced population of the high-…
Clathration of Five-Membered Aromatic Rings in the Bimetallic Spin Crossover Metal–Organic Framework [Fe(TPT)2/3{MI(CN)2}2]·G (MI = Ag, Au)
2014
Six clathrate compounds of the three-dimensional spin crossover metal−organic framework formulated [Fe(TPT)2/3{MI (CN)2}2]· nG, where TPT is 2,4,6-tris(4-pyridyl)-1,3,5-triazine, MI = Ag or Au and G represent the guest molecules furan, pyrrole and thiophene, were synthesized using slow diffusion techniques. The clathrate compounds were characterized by single-crystal X-ray diffraction at 120 and 300 K, thermogravimetric analysis and thermal dependence of the magnetic susceptibility. All compounds crystallize in the R3̅ m trigonal space group. The FeII defines a unique [FeN6] crystallographic site with the equatorial positions occupied by four dicyanometallate ligands while the axial positio…
Mössbauer thermal scan study of a spin crossover system
2010
Programmable Velocity equipment was used to perform a Mössbauer Thermal Scans to allow a quasi-continuous temperature study of the magnetic transition between the low-spin and a high-spin configurations in [Fe(Htrz)2(trz)](BF4) system. The material was studied both in bulk as in nanoparticles sample forms.