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.

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...

The tetrapyridyl ligand bbpya (bbpya=N,N-bis(2,2'-bipyrid-6-yl)amine) and its mononuclear coordination compound [Fe(bbpya)(NCS)2 ] (1) were prepared. According to magnetic susceptibility, differential scanning calorimetry fitted to Sorai's domain model, and powder X-ray diffraction measurements, 1 is low-spin at room temperature, and it exhibits spin crossover (SCO) at an exceptionally high transition temperature of T1/2 =418 K. Although the SCO of compound 1 spans a temperature range of more than 150 K, it is characterized by a wide (21 K) and dissymmetric hysteresis cycle, which suggests cooperativity. The crystal structure of the LS phase of compound 1 shows strong NH⋅⋅⋅S intermolecular H-bonding interactions that explain, at least in part, the cooperative SCO behavior observed for complex 1. DFT and CASPT2 calculations under vacuum demonstrate that the bbpya ligand generates a stronger ligand field around the iron(II) core than its analogue bapbpy (N,N'-di(pyrid-2-yl)-2,2'-bipyridine-6,6'-diamine); this stabilizes the LS state and destabilizes the HS state in 1 compared with [Fe(bapbpy)(NCS)2 ] (2). Periodic DFT calculations suggest that crystal-packing effects are significant for compound 2, in which they destabilize the HS state by about 1500 cm(-1) . The much lower transition temperature found for the SCO of 2 compared to 1 appears to be due to the combined effects of the different ligand field strengths and crystal packing.

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.

Coordination of the ligand bapbpy (1, bapbpy = N,N′-di(pyrid-2-yl)-2,2′-bipyridine-6,6′-diamine), of one of its four dimethyl-substituted analogues 2−5 (R 2 bapbpy = N,N′-di(methylpyrid-2-yl)-2,2′-bipyridine-6,6′-diamine), or of one of its three bis(iso)quinoline analogues 6−8 (R 2 bapbpy= N,N′-di(quinolyl)-2,2′bipyridine-6,6′-diamine), to Fe(NCSe) 2 , afforded eight new iron(II) compounds of the type [Fe(R 2 bapbpy)(NCSe) 2 ] (9−16). Three of these compounds (11, 13, and 16) were structurally characterized by single crystal X-ray diffraction, which showed similar molecular geometry and packing compared to their thiocyanate analogues. Magnetic susceptibility measurements were carried out for all iron compounds and revealed thermal spin-crossover (SCO) behavior for compounds 9, 11, 13, 15, and 16. Compounds 11, 13, 15, and 16 show an increased transition temperature compared to the thiocyanate analogues. [Fe(bapbpy)(NCSe) 2 ] (9) shows a gradual, one-step SCO, whereas its thiocyanate analogue [Fe(bapbpy)-(NCS) 2 ] is known for its cooperative two-step SCO. To discuss the influence of Sto-Se substitution on the cooperativity of the SCO, heat capacity measurements were carried out for compounds 9, 11, 13, 15, and 16, and fitted to the Sorai domain model. The number n of like-spin SCO centers per interacting domain, which is a quantitative measure of the cooperativity of the spin transition, was found to be high for compounds 11 and 15, and low for compounds 9, 11, and 13. Compound 15 is one of the few known SCO compounds that is more cooperative than its thiocyanate analogue. Altogether, X-ray diffraction, calorimetry, and magnetic data give a consistent structure−property relationship for this family of compounds: hydrogen-bonding networks made of intermolecular N−H•••Se interactions are of paramount importance for the cooperativity of the SCO.