FULL PAPER DOI: 10.1002/ejic.200900937 Light-Induced Stored Information in Nanoparticles Suzanne M. Neville, [a] Céline Etrillard, [a] Saket Asthana, [a] and Jean-François Létard* [a] Keywords: Spin crossover / Photomagnetism / Nanoparticles / Coordination polymers / Magnetic properties We investigate here the consequence on light-induced and thermally induced spin-crossover (SCO) properties with par- ticle size reduction from the macroscopic to microscale to nanoscale domains. Three samples with distinct particle sizes of the SCO coordination polymer [Fe(NCS) 2 (bpe) 2 ] [bpe = 1,2-bis(4'-pyridyl)ethane] have been prepared by water-in- oil reverse micelle methods. Comparison of the magnetic properties with particle size reduction of these and the origi- nal macroscale slow-grown crystals revealed that the spin transition becomes more gradual, more incomplete and con- comitantly the transition temperature (T 1/2 ) decreases – much Introduction The ability to control the physical properties (i.e., super- paramagnetism, spin crossover, photomagnetism and lumi- nescence) of advanced materials on the molecular level is an important goal for the eventual realization of functional devices. [1–3] Materials that show magnetic switching, such as spin crossover (SCO) where multiple electronic states can be accessed through variation in external stimuli such as temperature, pressure and light, have been identified as a viable class of materials for incorporation into such devices (i.e., information storage, sensing or display devices) - in particular, when they show properties that can be accessed at room temperature. [2,4] The current challenge in this field is to control the cluster size towards the levels required for the achievement of nanoscale devices, whilst retaining the magnetic/cooperative properties. [2] To this end, it has re- cently been demonstrated that monodispersed nanoscale particles of SCO coordination polymers [5–10] can be fabri- cated by exploiting the established micelle techniques uti- lized for Prussian Blue analogues [11] (i.e., the “bottom-up” approach). This is an important step for emerging nano- scale technologies, as the alternate approach for miniaturiz- ing particle size through mechanical processing into smaller particles (i.e., the “top-down” approach) results in gradual, diminished or even nonexistent SCO characteristics. [12,13] Although in an alternate top-down approach, micrometer- [a] Laboratoire des Sciences Moléculaires, ICMCB (CNRS UPR 9048), Université Bordeaux I, 33608 Pessac, France Fax: +33-5-40002649 E-mail: letard@icmcb-bordeaux.cnrs.fr © 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim Eur. J. Inorg. Chem. 2010, 282–288 282 like what is observed in metal dilution studies. Importantly, here, in the first photoinduced magnetic studies on a nano- particle SCO system, we see that even on the nanoscale pho- toconversion of the low spin species to a metastable high- spin state is possible. Furthermore, particle size reduction ap- pears to have little effect on the temperature at which the stored photomagnetic information is erased. These results highlight that light-induced SCO properties are governed by direct metal coordination environment (i.e., on the molecular scale), whereas, thermally induced magnetic properties rely more on crystal packing and ligand field effects. and nanometer-sized patterns of SCO materials have been successfully achieved through a combination of lift-off and multilayer steps. [14] With regard to the bottom-up approach, classically this has been achieved by well-established step-by-step combi- nation of reactants and, indeed, has resulted in many mo- nonuclear, dinuclear and polynuclear materials with inter- esting magnetic properties - including those features re- quired for potential integration into devices (i.e., room-tem- perature transitions, hysteresis loops). [15] More recently, with the identified requirement for control of particle size, the elaboration of nanoparticles of SCO materials by re- verse micelle techniques was developed by Létard et al. [5,7,10] In this way, through variation of the reaction con- ditions, such as oil/water ratio and reactant concentration, the size of the particles can be tailored/controlled. Overall at this stage, in this and further reports by other groups, it appears that with particle size reduction the spin transition nature becomes more gradual, the hysteresis loop size is di- minished and the transition temperature decreases, but im- portantly, the SCO nature is retained in the nanoscale do- main. [5–10] These trends were nicely depicted in a recent sys- tematic study of a series of six different particle-sized ana- logues of [Fe(NH 2 trz) 3 ]Br 2 ·nH 2 O (covering macro- to nano- meter), highlighting the possibility to tune the size of a hys- teresis loop to that required for application through particle size control. [5,10] A further question concerns the effect on the photomag- netic response upon particle size reduction, as photocontrol of magnetic properties at the molecular scale opens new perspectives in information storage. [16] The photomagnetic effect on the molecular scale is an extremely active field of