We have used single-crystal x-ray diffraction, Raman spectroscopy, and magnetometry to investigate the effect of hydrostatic pressures up to 16 kbar on the molecular spin crossover complex [Fe II (bapbpy)(NCS) 2 ]. A stepped first-order transition from the high-spin (HS) to low-spin (LS) phase was observed upon compression of single-crystal samples. The intermediate phase (IP) is stable between 4 and 11 kbar at room temperature. This phase is characterized by supercell reflections and tripling of the c-axis of the unit cell (C2/c) due to the formation of a periodic [HS-LS-LS] structural motif, as seen in the thermal stepped transition. The pressure-temperature phase diagram reveals an anomalous increase of the thermal hysteresis widths with increasing pressure and the stabilization of the IP across the investigated P-T space.

In this study, we show that 1) different isomers of the same mononuclear iron(II) complex give materials with different spin-crossover (hereafter SCO) properties, and 2) minor modifications of the bapbpy (bapbpy=N6,N6'-di(pyridin-2-yl)-2,2'-bipyridine-6,6'-diamine) ligand allows SCO to be obtained near room temperature. We also provide a qualitative model to understand the link between the structure of bapbpy-based ligands and the SCO properties of their iron(II) compounds. Thus, seven new trans-[Fe{R(2)(bapbpy)}(NCS)(2)] compounds were prepared, in which the R(2)bapbpy ligand bears picoline (9-12), quin-2-oline (13), isoquin-3-oline (14), or isoquin-1-oline (15) substituents. From this series, three compounds (12, 14, and 15) have SCO properties, one of which (15) occurs at 288 K. The crystal structures of compounds 11, 12, and 15 show that the intermolecular interactions in these materials are similar to those found in the parent compound [Fe(bapbpy)(NCS)(2)] (1), in which each iron complex interacts with its neighbors through weak N-H···S hydrogen bonding and π-π stacking. For compounds 12 and 15, hindering groups located near the N-H bridges weaken the N-S intermolecular interactions, which is correlated to non-cooperative SCO. For compound 14, the substitution is further away from the N-H bridges, and the SCO remains cooperative as in 1 with a hysteresis cycle. Optical microscopy photographs show the strikingly different spatio-temporal evolution of the phase transition in the noncooperative SCO compound 12 relative to that found in 1. Heat-capacity measurements were made for compounds 1, 12, 14, and 15 and fitted to the Sorai domain model. The number n of like-spin SCO centers per interacting domain, which is related to the cooperativity of the spin transition, was found high for compounds 1 and 14 and low for compounds 12 and 15. Finally, we found that although both pairs of compounds 11/12 and 14/15 are pairs of isomers their SCO properties are surprisingly different.

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We report here the optical microscopic imaging of a first-order phase transition induced by a nanosecond laser pulse (532 nm) in a single crystal of the molecular spin-crossover complex [FeðbapbpyÞðNCSÞ 2 ]. The transition starts with the formation of a high spin domain in the region irradiated by the focused laser beam, followed by the subsequent growth or contraction of the initial domain. Remarkably, in otherwise identical experimental conditions one can observe either the irreversible transition of the whole crystal or merely the formation of a transient domain-depending on which region of the crystal is excited. This observation as well as the rather slow dynamics suggest that the main control parameter is the inhomogeneous accommodation strain, which destabilizes the photoinduced domain. DOI: 10.1103/PhysRevLett.109.135702 PACS numbers: 64.70.KÀ, 07.60.Pb, 64.60.My, 75.30.Wx Photoinduced phase transition (PIPT) phenomena have received much attention in various types of solids such as organic charge transfer compounds, metal oxides, inorganic complexes, and even in metals [1][2][3][4]. In these materials, the strong electron-lattice or spin-lattice interactions have been shown to play a key role in driving the photoinduced transformation. Experimental evidence indicating that relatively weak photoexcitation can trigger a real macroscopic phase transition has been inferred in these systems from the huge photoconversion efficiencies as well as from the occurrence of characteristic nonlinear phenomena such as a threshold behavior. Here, we report on a conceptually different investigation where the phase transition was triggered by a spatially localized laser pulse within a comparatively large single crystal. Compared to previous studies on PIPT phenomena, our work brings in two new experimental features:(1) only a small part of the crystal is photoexcited by the focused laser beam, and (2) the phase change is not averaged for the whole crystal by a point or spectroscopic array detector but followed instead by an imaging charge-coupled device.For our experiments, we have chosen the molecular spin-crossover compound [Fe II ðbapbpyÞðNCSÞ 2 ] (1) (where bapbpy ¼ N-{6-[6-(pyridin-2-ylamino)pyridin-2-yl]pyridin-2-yl}pyridin-2-amine). Spin-crossover (SCO) materials of transition metal complexes are benchmark examples for PIPT. They exhibit bistability between the so-called low spin (LS) and high spin (HS) electronic configurations, which display strikingly different physical properties, including mass density, magnetic susceptibility, optical density, etc. [5]. Switching between the two molecular spin states can be induced by changing the sample temperature, applying external pressure or an intense magnetic field, and also by light irradiation. Light-induced excited spin state trapping in the solid state was first demonstrated by Decurtins et al. [6]. This so-called ''LIESST effect'' is basically a molecular phenomenon and can be observed only at cryogenic temperatures. On the other hand, several studies aimed a...

Micrometer-sized defects, induced by laser ablation, radically change the spatiotemporal dynamics of a first-order structural phase transition, in this case of a spin crossover material. This type of "domain engineering" is thus based on artificial defects, such as that in the image, which can serve either as nucleation sites or as pinning sites. The subsequent growth of the nucleated domains can also be guided to some extent.

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Optical microscopy measurements have been realized on single crystals of the [Fe(1-bpp)2][BF4]2 complex (1-bpp=2,6-di(pyrazol-1-yl)pyridine). The thermal spin transition around 253 K occurs by a heterogeneous nucleation and growth mechanism and involves a clear phase separation and a small hysteresis. This very abrupt and complete thermal transition is preceded by a premonitory spin conversion, which implies only a small fraction (ca. 2–3 %) of the molecules. This peculiar behavior may be the sign of heterophase fluctuations. The light-induced spin transition from the stable low spin (LS) to the metastable high spin (HS) phase was achieved at 80 K by focusing laser light into a small volume fraction of the crystal. Under continuous irradiation this photo-converted HS “nucleus” then grows and the whole crystal converts to the HS phase providing evidence for a light-induced instability in the system due to long-range elastic interactions. The relaxation of the light-induced metastable HS phase at 83 K follows a sigmoidal decay – typical of cooperative spin crossover systems. Nevertheless the spatial development of this relaxation process appears very homogeneous and no phase separation could be detected within the resolution of the optical microscope.

Nano- and microparticles or polycrystalline powders of the Fe(Me2-bpy)2(NCSe)2spin-crossover complex were easily elaborated from the diamagnetic precursor [Fe(Me2-bpy)3](NCSe)2·S by precipitation in an anti-solvent or by solid-state thermolysis.