First Magnetic Observation of a Spin Crossover in a Langmuir-Blodgett Film

Abstract

For particular transition metal complexes, spin crossover processes [1] occurring between a low-spin (LS) and a highspin (HS) state may be triggered by a change in temperature or pressure or by light irradiation. This striking phenomenon may appear either very gradually, as expected from a thermal equilibrium between the two spin states, or with some abruptness when intersite interactions are strong enough.[2] When the cooperativity is important, thermal hysteresis may appear, leading to new bistable materials.[3]These characteristics of the spin crossover depend greatly on the molecular surroundings and on the structure of the material. Therefore, the organization of the complexes is one of the most important factors in modifying and controlling the spin conversion. Thus, the Langmuir±Blodgett (LB) technique may lead to interesting materials because of its ability to pack and orient molecules in a well-defined fashion along a surface. [4] However, the few attempts to build LB multilayers from iron(II) complexes with ligands based on a phenanthroline or a triazole have illustrated the high chemical instability of such compounds at the gas± water interface. [5,6] One possible way to solve this problem is to modify the sub-phase on which the Langmuir film is obtained. [7] Another possible strategy relies on subtle changes in the chemical structure of the iron complex in order to obtain a perfectly stable monolayer at the gas±water interface. Thus, we have synthesized a semi-fluorinated derivative of a spin crossover complex ( Fig. 1) that shows astonishing stability in the Langmuir film, allowing the formation of perfectly defined LB films of such a molecule. Using a recently developed method to perform magnetic measurements on a LB film with a SQUID susceptometer, [8] we were able to follow, for the first time, the spin crossover process in these multilayers. These measurements demonstrate the strong modification of the spin crossover phenomenon induced by the lamellar organization imposed by the LB technique. Following standard procedures, [9] the synthesis of the ligand [10] (denoted L) 4- (13,13,14,14,15,15,16,16,17,17,18,18, 18-tridecafluorooctadecyl)-4¢-methyl-2,2¢-bipyridine was carried out starting from commercially available 4,4¢-dimethyl-2,2¢-bipyridine and 13, 13,14,14,15,15,16,16,17,17,18, 18,18-tridecafluoroheptadecyl-1-bromide.[11] As already described in the literature, [12] the iron(II) complex [13] was obtained using a solution of Fe(NCS) 2 in methanol. The spin crossover process in the final compound was confirmed by magnetic susceptibility measurements (see below). The LB films were prepared by spreading a chloroform solution of the complex onto a 10 ±2 M KNCS aqueous solution. Compression isotherms yield a molecular area of ca. 63 A 2 at the collapse pressure. As already observed for a similar compound, this value is slightly smaller than that evaluated from the crystal structure [14] of Fe(bipy) 2 (NCS) 2 , where bipy stands for 2,2¢-bipyridine. Distortion of the complex due to the packing...

Template Assembly of Spin Crossover One‐Dimensional Nanowires

“…Both SQUID and Raman data therefore agree in confirming a SCO for the nanowires with a smaller LS fraction at low temperature in the nanowire sample compared with the bulk powder. This behavior shows that nanoassembly affects the thermal SCO profile of 1 , as previously observed with nanoparticles of Fe II triazole10b,d and pyrazine10c, 13d, 30 coordination polymers or SCO LB films 14b. Modulation of the power settings on the laser did not alter the resulting spectra, thus suggesting that the nanowires are more thermally robust than the bulk powder.…”

Spin crossover with thermal hysteresis: practicalities and lessons learnt

“…gathering momentum in multidisciplinary studies aimed at producing SCO materials, rather than powder samples in vials, as this is an essential, but challenging, step towards the wide array of potential applications of SCO. The materials being explored include nanoparticles (NPs) made up of SCO-active complexes (size usually controlled by use of surfactants) 20,21 and SCO complexes immobilised on surfaces, either producing relatively soft materials (typically by Langmuir methods 22 or Fig. 1 For octahedral iron(II), the diamagnetic LS state and the HS state with 4 unpaired electrons.…”

Spin Crossover in a Vacuum-Deposited Submonolayer of a Molecular Iron(II) Complex

“…Typical methods for preparing the latter comprise Langmuir-Blodgett techniques, spin coating, and vacuum deposition, leading to thin or ultrathin films. [5][6][7][8] As the spin state of a molecule influences a multitude of its physical and chemical properties, it can be monitored using a wide variety of methods, e. g. IR, Mößbauer, NMR, Raman, UV/Vis, and X-ray absorption (XAS) spectroscopies, conductometry, dielectrometry, diffractometry, refractometry, and susceptometry. [9][10][11][12][13] Recently, an electron-induced spin-state switching of [Fe II (bpb) 2 (phen)] (bpb: bis(1H-pyrazol-1-yl)borate, phen: 1,10-phenanthroline) in the second molecular layer deposited on Au(111) in ultrahigh vacuum (UHV) was observed by differential tunneling spectroscopy at 5 K, but the molecules of the first layer could not be switched.…”