MS36-P34 Evidencing size-dependent cooperative effects on spin cross-over nanoparticles of Hofmann-type [Fe(pz)Pt(CN)4] coordination polymers. Laure Guenee (University of Geneva, Geneva, Switzerland) Teresa Delgado (Department of physical chemistry, University of Geneva, Geneva, Switzerland) Cristian Enachescu (Department of physics, University of Alexandre Ioan Cuza, Lasi, Romania) Antoine Tissot (Institut des matériaux poreux de Paris, ENS,, Paris, France) Andreas Hauser (Department of physical chemistry, University of Geneva, Geneva, Switzerland) Céline Besnard (Laboratory of crystallography, university of Geneva, Geneva, Switzerland)email: laure.guenee@unige.chPhoto-induced processes are of paradigmatic importance in many fields, ranging from biology where light harvesting is the basic process of photosynthesis, to solid-state physics where light can be applied to control the electronic properties of solids. In this context, spin crossover compounds are fascinating photoswitchable materials, with possible applications in sensing or memory devices. However, such materials need to be synthesized as nanoparticles or thin films in order to be incorporated into functional devices. Therefore, the effect of size reduction on the thermo- and photo-switching properties has been abundantly described and modelled within the past few years [1]. However, no direct experimental evidence was made so far on the microscopic origin of the size reduction effects.
Following a previous structural study of the high spin à low spin relaxation dynamics of the porous coordination network [Fe(pz)Pt(CN)4]⋅2.6 H2O in the bulk state [2], nanoparticles of [Fe(pz)Pt(CN)4] were synthesised and the HSàLS relaxation through the light-induced excited spin-state trapping (LIESST) effect is investigated using synchrotron X-ray powder diffraction.
We evidence a clear limiting particle size below which the HSà LS relaxation adopts a different mechanism, indicating a drastic change in cooperative processes. We also provide a qualitative explanation of this observed change in cooperativity upon size reduction within the framework of a mechanoelastical model.

[1] Bousseksou, A.; Molnar, G.; Salmon, L.; Nicolazzi, W., (2011) Chemical Society Reviews, 40 (6), 3313-3335.

[2] Delgado, T.; Tissot, A.; Besnard, C.; Guénée, L.; Pattison, P.; Hauser, (2015). A., Chemistry – A European Journal, 21 (9), 3664-3670.
Keywords: Spin crossover, LIESST, relaxation dynamics