Intermolecular control of thermoswitching and photoswitching phenomena in two spin-crossover polymorphs

Abstract

We discuss here the important role of intermolecular coupling for the thermal-and light-induced molecular state switching in the solid state. Investigations were performed on the two crystalline polymorphs of the spin-crossover [Fe-(PM-BIA) 2 (NCS) 2 ] material. In addition to structural studies at thermal equilibrium, lightinduced phenomena were investigated through photocrystallography, photomagnetic, and dynamical optical measurements. Strong similarities between the thermal-equilibrium and the out-of-equilibrium light-induced transformations are observed in each polymorph: strong cooperative phenomena in one polymorph versus weak cooperative ones in the second polymorph. These different responses of the two crystalline forms of the compound to external perturbations are discussed at the microscopic level in terms of Ising-like model and two-mode description of on-site molecular potentials.

Determination of the molecular structure of the short-lived light-induced high-spin state in the spin-crossover compound [Fe(6-mepy)3tren](PF6)Chakraborty1, Tissot2, Peterhans3 et al. 2013Phys. Rev. B122112View full textAdd to dashboardCite

“…With regard to structural aspects of the light-induced metastable HS state, to date, most reports describe either the structure determination of systems with a very longlived metastable HS state at low temperature by means of conventional x-ray crystallography, [23][24][25] or recently probing short-lived metastable HS states at higher temperatures by means of ultrafast structural studies, [26][27][28][29] where a population of the photoexcited HS state of only a few percents is achieved.…”

Size effects in spin-crossover nanoparticles in framework of 2D and 3D Ising-like breathing crystal field model

“…For simplicity the intermolecular interaction in spincrossover materials is assumed to be the same for all sites in the system and in a phenomenological approach this is described as a ferromagnetic coupling (J > 0) between nearest neighboring spins. This is the simplest way to express the cooperativity between magnetic molecules of spin-crossover material [22][23][24][25].…”

Piezo‐ and Photo‐Crystallography Applied to Spin‐Crossover Materials

“…Diffraction techniques make it possible to understand how electronic and structural degrees of freedom are interconnected, and more importantly to understand how photoinduced states can be stabilized. 94 The recent construction of He cryostats with coupled laser excitation has allowed the development of photo-crystallography of SCO compounds. 64,65,71,84,[94][95][96][97][98][99][100] A nice illustration of the information one can obtain from detailed analysis of the molecular structures of photoinduced states is provided by the comparison of the structural changes in the two polymorphs of [Fe(PM-BiA) 2 (NCS) 2 ], denoted BIA I and BIA II, 94 whose T(LIESST) values are quite different (78 K for BIA I and 35 K for BIA II).…”

“…It has been shown that for weakly cooperative systems, with a homogeneous distribution of photoexcited molecules in the crystal, a gradual evolution of the Bragg peaks occurs when the system transforms. 85,94 For more cooperative systems, the behaviour is different and a phase separation (spatially separated phases) process takes place, as evidenced by X-ray diffraction for the first time in the [Fe(pic) 3 ] 2+ system. 84 This phase separation process was described as indicative of Avrami behaviour, that is growth of the HS phase inside the bulk LS phase (or vice versa).…”