Complexities in the Molecular Spin Crossover Transition

Abstract

International audienceVariable-temperature studies of the electronic structures of four different Fe(II) spin crossover molecules, [Fe(H2B(pz)2)2(bipy)] (pz = pyrazol-1-yl, bipy = 2,2′-bipyridine), [Fe(H2B(pz)2)2(phen)], [Fe(phen)2(NCS)2] (phen = 9,10-phenantroline), and [Fe(PM-AzA)2(NCS)2] (PM-AzA = 4-phenyldiazenyl-N-(pyridin-2-ylmethylene)aniline) by X-ray absorption spectroscopy (XAS), combined with electrical properties studies of the [Fe(PM-AzA)2(NCS)2] single crystal are presented. We show that both the XAS signature of the spin state of powdered samples and the dielectric permittivity of the [Fe(PM-AzA)2(NCS)2] single crystal change at significantly lower temperatures than the magnetometry, structure, and resistivity indicators of a spin crossover transition. The changes in electronic structure are in agreement with the expectations from density functional theory (DFT) results for the different molecular electronic structures associated with the high-spin and low-spin states. These findings suggest that the electronic structure phase ordering process does not simply follow the spin transition

Nonvolatile voltage controlled molecular spin state switching

“…21,22 The spin crossover transition temperature of the resulting powder, measured by magnetometry, was found to be about 160 K, in agreement with previous work. 19,[22][23][24][25][26][27] This temperature dependent spin crossover transition, for [Fe{H 2 B(pz) 2 } 2 (bipy)], is also evident in X-ray absorption, as indicated in Fig. 2(a).…”

“…In the final product, the measured width and gap between the electrodes were 10 lm and 260 nm, respectively. X-ray diffraction, 29,30 Mossbauer spectroscopy, 24 scanning tunneling microscopy, 10,22 X-ray photoemission, 23 inverse photoemission, 19,27 X-ray absorption, 14,18,19,26,30 and of course magnetometry 19,[22][23][24][25][26][27]30 have all been used successfully to determine the spin state of [Fe{H 2 B(pz) 2 } 2 (bipy)]. X-ray absorption spectroscopy has the advantage of rapidity, distinct signature of the low spin state (Fig.…”

Five 2,6-Di(pyrazol-1-yl)pyridine-4-carboxylate Esters, and the Spin States of their Iron(II) Complexes

“…Experimental data for the crystal structures are listed in Tables 3 and 4. Diffraction data for L 5 , L 6 , and L 7 • 1 2 MeCN were collected at station I19 of the Diamond synchrotron (λ = 0.6889 Å). The other crystals were measured with an Agilent Supernova dual-source diffractometer using monochromated Cu-K α (λ = 1.5418 Å) or Mo-K α (λ = 0.7107 Å) radiation.…”

“…Spin-crossover (SCO) compounds are an important class of molecular switches which are widely studied in molecular materials chemistry [1][2][3]. The SCO switching event changes a number of physical properties in a material including its colour, magnetic moment [4], volume [5], dielectric properties [6][7][8][9], conductivity [9][10][11][12], and fluorescence [13][14][15]. Solid or mesophase phase transitions and isotropic melting can also be coupled to spin transitions in soft materials containing SCO centres [16,17].…”

Spin-crossover and the LIESST effect in [Fe Co1‒(bpp)2][BF4]2 (1.00 ≤x≤ 0.77). Comparison with bifunctional solid solutions of iron and cobalt spin-crossover centers

“…A particular goal of spin-crossover researchers is to produce multifunctional compounds, that use spin-crossover switching to modulate another property of a material [10]. For example, materials showing fluorescence [11][12][13][14][15], resistivity [16][17][18][19][20] and dielectric [21][22][23][24] that responds to their thermal spin-transition are now well-established.…”