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Materials presenting a stable and reversible switch of physical properties in the solid state are of major interest either for fundamental interests or potential industrial applications. In this context, the design of metal complexes showing a light-induced crossover from one spin state to another, leading to a major change of magnetic and optical properties, is probably one of the most appealing challenges. The so-denoted spin-crossover materials undergo, in some cases, a reversible photoswitch between two magnetic states, but, unfortunately, lifetimes of the photomagnetic states for compounds known so far are long enough only at low temperatures; this prohibits any applications. We have measured and collected the temperatures above which the photomagnetic effect disappears for more than sixty spin-crossover compounds. On the basis of this large data base, a correlation between the nature of the coordination sphere of the metal and the photomagnetic lifetime can be drawn. Such correlation allows us to propose here a general guideline for the rational design of materials with long-lived photomagnetic lifetimes. This result clearly opens the way towards room-temperature photonic materials, based on the spin-crossover phenomenon, which will be of great interest for future communication devices.

The [FeLn(NCS)2] iron(II) spin-crossover complexes cover a wide range of magnetic behaviour. Owing to the large number of known structural and magnetic data, this series is perfectly adapted to the investigation of the structure-magnetic properties relationship. In this paper we propose a new structural parameter, denoted Theta, which is used to correlate the features of the spin-crossover phenomena with the distortion of the iron environment. In particular, this parameter has shed light on the role of such distortion on the limiting temperature of photo-inscription, known as T(LIESST). A strong dependence of T(LIESST) on Theta is clearly demonstrated. The stronger the distortion the higher the T(LIESST) value. This structure-property dependence represents, for instance, a powerful tool to estimate the highest potential T(LIESST) value for a series of complexes. This limit in the [FeLn(NCS)2] series is estimated to be around 120 K, which probably prevents their use in any industrial application.

OctaDist, a program for calculating three common octahedral distortion parameters, is presented and the calculation of the trigonal distortion parameter, Θ is standardized for the first time.

No abstract

We report a two-dimensional Hofmann-like spin-crossover (SCO) material, [Fe(trz-py){Pt(CN)}]·3HO, built from [FePt(CN)] layers separated by interdigitated 4-(2-pyridyl)-1,2,4,4H-triazole (trz-py) ligands with two symmetrically inequivalent Fe sites. This compound exhibits an incomplete first-order spin transition at 153 K between fully high-spin (HS-HS) and intermediate high-spin low-spin (HS-LS) ordered states. At low temperature, it undergoes a bidirectional photoswitching to HS-HS and fully low-spin (LS-LS) states with green and near-IR light irradiation, respectively, with associated T(LIESST = Light-Induced Excited Spin-State Trapping) and T(reverse-LIESST) values of 52 and 85 K, respectively. Photomagnetic investigations show that the reverse-LIESST process, performed from either HS-HS or HS-LS states, enables access to a hidden stable LS-LS state, revealing the existence of a hidden thermal hysteresis. Crystallographic investigations allowed to identify that the strong metastability of the HS-LS state originates from the existence of a strong elastic frustration causing antiferroelastic interactions within the [FePt(CN)] layers, through the rigid NC-Pt-CN bridges connecting the inequivalent Fe sites. The existence of the stable LS-LS state paves the way for a multidirectional photoswitching and allows potential applications for electronic devices based on ternary digits.

The crystal structures in both irradiated and nonirradiated states of a photoinduced molecular switch based on the spin-crossover phenomenon are presented. From the structural point of view, the light-induced metastable high-spin state of the spin-crossover complex [Fe(phen)2(NCS)2] (phen = 1,10-phenanthroline) shows significant differences with the low-spin state but also with the thermally induced high-spin state.

The [Fe(PM-BiA)(2)(NCS)(2)] complex, where PM is N-2-pyridylmethylene and BiA is 4-aminobiphenyl, crystallizes in two polymorphs. The two phases, denoted (I) and (II), undergo a spin-crossover when the sample is cooled and present distinct spin-transition features as (I) shows a very abrupt spin transition, while (II) exhibits a gradual transition. The two forms of the complex are used to investigate the correlations that exist between the spin-transition features and structural features. This article presents the crystal structures of polymorph (II) at room temperature (high spin) and at 120 K (low spin), including a comparison with those of polymorph (I). This study reveals that the packing, in a first approximation, is similar in both forms. In order to look at the crystal structures in more detail, a new angular parameter, denoted theta(NCS), as well as a particular type of intermolecular hydrogen-bond interaction, which involves the S atoms of the NCS ligands, are investigated. Interestingly, this angle and this intermolecular interaction can be directly connected to the cooperativity of the spin transition. Such a result is extended to all the SCO iron(II) complexes belonging to the same family of the general formula [Fe(PM-L)(2)(NCS)(2)].