Primary Role of the Electrostatic Contributions in a Rational Growth of Hysteresis Loop in Spin-Crossover Fe(II) Complexes Mikae ¨ l Kepenekian, †,‡ Boris Le Guennic, † and Vincent Robert* ,† UniVersite ´ de Lyon Laboratoire de Chimie, Ecole Normale Supe ´rieure de Lyon, CNRS, 46 alle ´e d’Italie, F-69364 Lyon, France, and Laboratoire de Reconnaissance Ionique et de Chimie de Coordination, CEA-INAC/LCIB (UMRE 3 CEA-UJF), 17 rue des Martyrs, F-38054 Grenoble Cedex 9, France Received April 21, 2009; Revised Manuscript Received July 3, 2009; E-mail: vincent.robert@ens-lyon.fr Abstract: We report a comprehensive analysis of the hysteresis behavior in a series of well-characterized spin-crossover Fe(II) materials. On the basis of the available X-ray data and multireference CASSCF (complete active space self-consistent field) calculations, we show that the growth of the hysteresis loop is controlled by electrostatic contributions. These environment effects turn out to be deeply modified as the crystal structure changes along the spin transition. Our theoretical inspection demonstrates the synergy between weak bonds and electrostatic interactions in the growth of hysteresis behavior. Quantitatively, it is suggested that the electrostatic contributions significantly enhance the cooperativity factor while weak bonds are determinant in the structuration of the 3D networks. Our picture does not rely on any parametrization but uses the microscopic information to derive an expression for the cooperativity parameter. The calculated values are in very good agreement with the experimental observations. Such inspection can thus be carried out to anticipate the hysteresis behavior of this intriguing class of materials. 1. Introduction The constant development and characterization of sophisti- cated materials holding switchable physical properties stems from their possible applications to electronic devices such as thermal sensors, optical switches, and information storage media. 1,2 A prerequisite to generate memory effects is the presence of bistable units within the crystal structure. Striking examples are to be found in spin-crossover (SCO) Fe(II) complexes, such as 1 ) [Fe(pm-pea)(NCS) 2 ] (pm-pea ) N-2′- pyridylmethylene-4-(phenylethynyl)aniline) and analogues 2 ) [Fe(pm-bia)-(NCS) 2 ] (pm-bia ) N-2′-pyridylmethylene-amino- biphenyl) and 3 ) [Fe(pm-aza)-(NCS) 2 ] (pm-aza ) N-2′- pyridylmethylene-4-(phenylazo)aniline) (see Figure 1). 3-6 In such materials, the transition occurs between a low-spin state (LS, S ) 0) and a high-spin state (HS, S ) 2). While compounds 1, 2, and 3 belong to the same family characterized by a N 6 coordination sphere, hysteretic behavior has been recently reported in other environments such as N 4 O 2 . 7-10 In particular, compound 4 ) [Fe(3-MeO,5-NO 2 -sal-N(1,4,7,10))] was fully characterized and exhibits a LS to HS transition. 9 Over the past decade, much effort has been dedicated to the synthesis of cooperative materials to grow thermal hysteresis, using not only covalent linkers to form coordina- tion polymers 1,11-13 but also van der Waals interactions between the transiting units. In spite of promising results, 1 such as the possibility to photoinduce reversible spin transi- tion 14 with attractive applications in multilayer materials, 15 the former strategy has not led to the expected breakthroughs in the generation of large hysteresis loops at room temper- ature. An alternative route consists in the establishment of communication networks to generate supramolecular interac- tions through extended aromatic structures held by the ligands. π-Stacking is indeed considered to play a crucial † Ecole Normale Supe ´rieure de Lyon. ‡ CEA-INAC/LCIB. (1) Kahn, O.; Martinez, C. J. Science 1998, 279, 44–48. (2) Le ´tard, J.-F.; Guionneau, P.; Goux-Capes, L. Top. Curr. Chem. 2004, 235, 221–249. (3) Le ´tard, J.-F.; Guionneau, P.; Codjovi, E.; Lavastre, O.; Bravic, G.; Chasseau, D.; Kahn, O. J. Am. Chem. Soc. 1997, 119, 10861–10862. (4) Guionneau, P.; Le Gac, F.; Lakhoufi, S.; Kaiba, A.; Chasseau, D.; Le ´tard, J.-F.; Ne ´grier, P.; Mondieig, D.; Howard, J. A. K.; Le ´ger, J.- M. J. Phys.: Condens. Matter 2007, 19, 326211. (5) Le ´tard, J.-F.; Guionneau, P.; Rabardel, L.; Howard, J. A. K.; Goeta, A. E.; Chasseau, D.; Kahn, O. Inorg. Chem. 1998, 37, 4432–4441. (6) Guionneau, P.; Le ´tard, J.-F.; Yufit, D. S.; Chasseau, D.; Bravic, G.; Goeta, A. E.; Howard, J. 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Ed. 2006, 45, 5786–5789. Published on Web 07/24/2009 10.1021/ja9031677 CCC: $40.75  2009 American Chemical Society 11498 9 J. AM. CHEM. SOC. 2009, 131, 11498–11502 Downloaded by ECOLE NORMALE SUPERIEURE LYON on September 10, 2009 | http://pubs.acs.org Publication Date (Web): July 24, 2009 | doi: 10.1021/ja9031677