Spin-Crossover Nanocrystals DOI:10.1002/ange.200801673 Spin-Crossover Nanocrystals with Magnetic, Optical, and Structural Bistability Near Room Temperature** Ishtvan Boldog, AnaB. Gaspar,* Víctor Martínez, Pablo Pardo-Ibaæez, Vadim Ksenofontov, Ashis Bhattacharjee, Philipp Gütlich, and JosØA. Real* DedicatedtoProfessorJanReedijkontheoccasionofhis65thbirthday Functional molecular materials with switching properties and memory transduction are of considerable interest in view of their potential technological applications. [1] Rational control of the growth of these materials on a nanometric scale is required, however, if they are to be successfully integrated into functional devices. [2] The miniaturization of functional molecular materials and the study of their chemical and physical properties on this scale is a subject of intense research activity in materials science. [3–5] Spin-crossover (SCO) compounds are functional molec- ular materials which possess labile electronic configurations that are switchable between the high-spin (HS) and low-spin (LS) states in response to external stimuli (temperature, pressure, or light). [6] SCO compounds reveal differences in magnetism, optical properties, dielectric constant, color, and structure in the HS and LS states. These physical properties may change drastically in a narrow range of temperature and/ or pressure, and even under irradiation at different wave- lengths, for cooperative transitions exhibiting hysteretic behavior. [7,8] These characteristics have made the SCO phenomenon one of the most interesting examples of bistability in molecular materials. Many publications have underlined the potential applications of iron(II) SCO materi- als as sensors or memory devices. [9] Nanostructuration using the bottom-up synthetic approach is a fundamental requisite to achieve this goal. [2] At this point, it should be stressed that a reduction of the film thickness or particle size of an SCO material could affect its physical properties drastically since electronic bistability depends on the collective behavior of the SCO centers in the lattice. This cooperative mechanism, which is responsible for the occurrence of first-order phase transitions, is crucial for signal and memory generation. Determination of the critical particle size or film thickness that allows the hysteretic behavior to be conserved is therefore an essential aspect of this research. Bottom-up nanostructuration has been successfully applied to the 3D SCO polymer [Fe(pz)Pt(CN) 4 ](1) [10] by layer-by-layer (LbL) epitaxial growth on gold surfaces. The resulting films, [11a,b] which have a thickness of approximately 150 nm, display similar spin crossover to that observed in the “bulk” material. Compound 1 exhibits a sharp first-order spin transition at room temperature with a hysteresis loop 25 K wide. [10b] This spin transition is accompanied by an easily detectable change of color between yellow-orange (HS) and deep-red (LS). [10] In addition, the spin states can be switched by irradiation with a red light pulse within the thermal hysteresis loop. [11b] Water-in-oil microemulsions, also known as reverse micelles, have recently been applied to the synthesis of nanoparticles of the 1D iron(II) SCO polymer [Fe(Htrz) 2 - (trz)](BF 4 ) (Htrz = 1,3,4-1H-triazole). The resulting surfac- tant-coated nanoparticles, which have an average size of less than 20 nm, display a spin transition with thermal hysteresis above room temperature similar to that reported for the “bulk” material. [12a] Previous studies on nanoparticles of this polymer demonstrated suppression of the first-order phase transition in favor of a continuous spin transition for particles of reduced size. [12b,c] Herein we report the successful application of the bottom- up approach for the synthesis of surfactant-free nanometer- sized crystals of [Fe(pz)Pt(CN) 4 ](2 and 3) that display size- dependent magnetic and optical properties. The average size found for nanocrystals of 2·H 2 O and 3·2.5H 2 O is 230  230  55 and 61  61  21 nm, respectively. The syntheses were performed by mixing sodium bis(2- ethylhexyl)sulfosuccinate (NaAOT) stabilized water-in-oil microemulsions containing Fe(BF 4 ) 2 , pyrazine, and K 2 [Pt(CN) 4 ] at constant W ([H 2 O]/[NaAOT]; see Experi- mental Section for details). A distinctive color change from [*] Dr.I.Boldog,Dr.A.B.Gaspar,V.Martínez,Prof.J.A.Real InstitutdeCinciaMolecular DepartamentdeQuímicaInorgànica UniversitatdeValncia Edificid’InstitutsdePaterna ApartatdeCorreus22085,46071Valncia(Spain) Fax:(+ 34)96-3544-855 E-mail:ana.b.gaspar@uv.es jose.a.real@uv.es P.Pardo-Ibaæez InstitutdeCinciadelsMaterials UniversitatdeValncia Edificid’InstitutsdePaterna ApartatdeCorreus22085,46071Valncia(Spain) Dr.V.Ksenofontov,Dr.A.Bhattacharjee,Prof.P.Gütlich InstitutfürAnorganischeundAnalytischeChemie Johannes-Gutenberg-Universität Staudinger-Weg9,55099Mainz(Germany) [**] FinancialsupportfromtheSpanishMinisteriodeEducacióny Ciencia(MEC)(CTQ2007-64727-FEDER),theGeneralitatValenci- ana(ACOMP07/110),andtheEuropeanNetworkofexcellence MAGMANET(contract:NMP3-CT-2005-515767-2)isacknowl- edged.A.B.G.thankstheSpanishMECforaresearchcontract (ProgramaRamónyCajal). SupportinginformationforthisarticleisavailableontheWWW underhttp://dx.doi.org/10.1002/anie.200801673. Angewandte Chemie 6533 Angew. Chem. 2008, 120, 6533–6537 # 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim