In this paper, we discuss in the framework of a mechanoelastic model the electronic and mechanical behavior of a single layer of spin crossover molecules self-organized on a substrate. We consider the molecules situated in a face-centered-cubic structure interacting in between and with sites in the substrate by the way of connecting springs with given elastic constants. The main experimental results are reproduced, i.e., typical thermal transitions with their incompleteness of the hysteresis loop, residual fractions after low-temperature relaxations, cooperativity, or kinetic features. However, we prove that the simple model, implying fixed neighbors on the substrate for every spin crossover molecule, leads in some cases to unphysical situations, corresponding to unexpected large curvatures of the spin crossover layer. Therefore, to go further, we allow every spin crossover molecule to change its adsorption site on the substrate at every moment, by connecting to the closest molecules on the substrate. This approach, corroborated with the use of different densities of the sites on the substrate, allows us to simulate further experimental observations, such as the appearance of cracks inside the layer or periodic arrangements of apparent heights of spin crossover molecules on the layer leading to moiré patterns, for which experimental data are also provided.
Spin-crossover molecules present the unique property of having two spin states that can be controlled by light excitation at low temperature. Here, we report on the photoexcitation of [Fe II ((3, ) 2 Pz) 3 BH) 2 ] (Pz = pyrazolyl) ultrathin films, with thicknesses ranging from 0.9 to 5.3 monolayers, adsorbed on Cu(111) substrate. Using Xray absorption spectroscopy measurements, we confirm the anomalous light-induced spin-state switching observed for sub-monolayer coverage and demonstrate that it is confined to the first molecular layer in contact with the metallic substrate. For higher coverages, the well-known light-induced excited spin-state trapping effect is recovered. Combining continuous light excitation with thermal cycling, we demonstrate that at low temperature light-induced thermal hysteresis is measured for the thicker films, while for sub-monolayer coverage, the light enables extension of the thermal conversion over a large temperature range. Mechanoelastic simulations underline that, due to the intermolecular interactions, opposite behaviors are observed in the different layers composing the films.
The embedding of spin-crossover micro- or nanocrystals in various surroundings dramatically changes their functionalities based on first-order spin transitions. The dampening of their internal cooperativity, together with introducing a new kind of interactions occurring at interfaces between spin-crossover particles and their environment, results in spectacular effects, as an enhanced hysteresis with non-cooperative transitions. In this work, we deal with the influence of the embedding matrix on the light-induced thermal hysteresis (LITH) in the case of spin-crossover microparticles of Fe(phen)2(NCS)2. Despite the low cooperativity of this compound, the competition between the continuous photoexcitation towards the metastable high spin state and the relaxation down to low spin ground state leads to a light-induced thermal hysteresis, with a quasi-static width of around 10 K. This unexpected hysteresis is explained by considering a switch-on/cutoff mechanism of the particle–matrix interactions in the framework of a mean-field approach based on negative external pressures, with Gaussian distributed variations and of an Ising-like model with various interactions with the environment. Additional first-order reversal curves measurements and corresponding calculated distributions are in line with relaxations under light and confirm the existence of a non-kinetic LITH.
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