Unprecedented direct spatiotemporal imaging of a reversible interface propagation in a switchable cooperative Fe(III) spin crossover material

Marcos A. Bento; Nour el islam Belmouri; André Fernandes; David Ndrio; Guillaume Bouchez; Longhe Li; Liliana P. Ferreira; Hermínio P. Diogo; João Rocha; Francesc Viñes; Adrià Gil; Kamel Boukheddaden; Paulo N. Martinho

doi:10.1039/D5QI01876E

Unprecedented direct spatiotemporal imaging of a reversible interface propagation in a switchable cooperative Fe(iii) spin crossover material

Marcos A. Bento,

^a Nour el islam Belmouri,

^b André Fernandes,^a David Ndrio,

^c Guillaume Bouchez,^b Longhe Li,

^b Liliana P. Ferreira,^de Hermínio P. Diogo,

^f João Rocha,

^g Francesc Viñes,

^c Adrià Gil,

^h Kamel Boukheddaden

*^b and Paulo N. Martinho

*^a

Author affiliations

* Corresponding authors

^a Centro de Química Estrutural, Institute of Molecular Sciences, Departamento de Química e Bioquímica, Faculdade de Ciências, Universidade de Lisboa, Campo Grande, 1749-016 Lisboa, Portugal
E-mail: pnmartinho@ciencias.ulisboa.pt

^b Groupe d'Etudes de la Matière Condensée, CNRS UMR 8635, Université Paris Saclay-Versailles, 45 Avenue des Etats Unis 78035, Versailles, France
E-mail: kamel.boukheddaden@uvsq.fr

^c Departament de Ciència de Materials i Química Física & Institut de Química Teòrica i Computacional (IQTCUB), Universitat de Barcelona, c/Martí i, Franquès 1-11, 08028 Barcelona, Spain

^d Department of Physics, University of Coimbra, 3004-516 Coimbra, Portugal

^e Centro de Física Teórica e Computacional, Faculdade de Ciências, Universidade de Lisboa, Portugal

^f Centro de Química Estrutural, Institute of Molecular Sciences, Instituto Superior Técnico, Universidade de Lisboa, Portugal

^g Department of Chemistry, CICECO-Aveiro Institute of Materials, University of Aveiro, Portugal

^h Departament de Química, Física i Ciències Ambientals i del Sòl, ETSEA – Escola Tècnica Superior d'Enginyeria Agrària, Universitat de Lleida, Av. de l'alcalde Rovira Roure, 191, Lleida, Catalunya, Spain

Abstract

We report the unprecedented observation of reversible interface propagation in the switchable cooperative Fe(III) spin crossover compound [Fe(3-EtO-salEen)₂]NO₃, where 3-EtO-salEen is 3-ethoxy-N-ethyl-N-(2-aminoethyl)salicylaldiminate. This complex exhibits a spin transition from low-spin to high-spin states, triggered by temperature around 345 K. Magnetic susceptibility measurements reveal an abrupt transition with a thermal hysteresis of 5 K. Differential scanning calorimetry confirms both endothermic (T_on = 344.50 K and T_max = 345.61 K, Image ID:d5qi01876e-t1.gif and Image ID:d5qi01876e-t2.gif ) and exothermic (T_on = 344.40 K, T_max = 345.49 K, Image ID:d5qi01876e-t3.gif and Image ID:d5qi01876e-t4.gif ) peaks corresponding to the spin transition temperatures. Cryogenic optical microscopy reveals rapid interface propagation, with velocities (v_HS→LS ≃ 1672 μm s⁻¹ and v_LS→HS ≃ 850 μm s⁻¹) significantly higher than those observed in traditional Fe(II)-based spin crossover materials. The high-speed propagation is attributed to the combined effect of the reduced volume change and lower elastic energy barriers associated with the transition in Fe(III) systems and the high values of the transition temperatures. Hirshfeld surface analysis reveals that the cooperative nature of the transition is influenced by non-conventional hydrogen bonding and π–π interactions. This study represents the first report of reversible interface propagation in Fe(III) complexes, providing valuable insights into the fundamental mechanisms that govern spin crossover transitions in these systems.