DOI: 10.1002/chem.201000433 Two-Step Thermal Spin Transition and LIESST Relaxation of the Polymeric Spin-Crossover Compounds FeACHTUNGTRENNUNG(X-py) 2 [Ag(CN) 2 ] 2 (X = H, 3-methyl, 4-methyl, 3,4-dimethyl, 3-Cl)** J. Alberto Rodríguez-Velamazµn,* [a] Chiara Carbonera, [a, b] Miguel Castro, [a] Elías Palacios, [a] Takafumi Kitazawa, [c] Jean-FranÅois LØtard, [b] and Ramón Burriel [a] Introduction Materials that undergo variation of their physical properties under external perturbations such as temperature, light, pressure, and electric and magnetic fields are potential can- didates for applications in information storage devices, mo- lecular switches and sensors. Spin-crossover (SCO) com- pounds, most commonly Fe II complexes, in which the spin state of the transition metal can be switched in a controlled manner between low-spin (LS) and high-spin (HS) states, belong to this class of materials. [1, 2] Interaction with light has attracted much attention follow- ing the discovery of light-induced excited spin state trapping (LIESST) in 1984. [3] Irradiation of an SCO compound at low temperature with precise wavelengths induces formation a metastable HS state from the LS state. The latter can be subsequently recovered either by the reverse LIESST effect, which occurs if irradiation at a different wavelength is per- formed, or by thermal relaxation if the temperature of the sample is sufficiently increased. Since the latter process de- pends on the heating rate, a characteristic temperature, [4] de- noted TACHTUNGTRENNUNG(LIESST), has been experimentally defined to de- scribe the stability of the induced HS state. A systematic study has shown a correlation between the temperature cor- responding to the HS–LS thermal transition T 1/2 and T - ACHTUNGTRENNUNG(LIESST), which is estimated from the maxima of the d(cT)/dT curve, where c is the magnetic susceptibility and T the temperature. [4] Abstract: In the series of polymeric spin-crossover compounds FeACHTUNGTRENNUNG(X- py) 2 [Ag(CN) 2 )] 2 (py = pyridine, X = H, 3-Cl, 3-methyl, 4-methyl, 3,4-dimethyl), magnetic and calorimetric measure- ments have revealed that the conver- sion from the high-spin (HS) to the low-spin (LS) state occurs by two-step transitions for three out of five mem- bers of the family (X = H, 4-methyl, and X = 3,4-dimethyl). The two other compounds (X = 3-Cl and 3-methyl) show respectively an incomplete spin transition and no transition at all, the latter remaining in the HS state in the whole temperature range. The spin- crossover behaviour of the compound undergoing two-step transitions is well described by a thermodynamic model that considers both steps. Calculations with this model show low cooperativity in this type of systems. Reflectivity and photomagnetic experiments reveal that all of the compounds except that with X = 3-methyl undergo light-induced ex- cited spin state trapping (LIESST) at low temperatures. Isothermal HS-to-LS relaxation curves at different tempera- tures support the low-cooperativity character by following an exponential decay law, although in the thermally activated regime and for aX = H and X = 3,4-dimethyl the behaviour is well described by a double exponential function in accordance with the two- step thermal spin transition. The ther- modynamic parameters determined from this isothermal analysis were used for simulation of thermal relaxation curves, which nicely reproduce the ex- perimental data. Keywords: calorimetry · iron · mag- netic properties · spin crossover [a] Dr. J. A. Rodríguez-Velamazµn, Dr. C. Carbonera, Dr. M. Castro, Dr. E. Palacios, Prof. Dr. R. Burriel Instituto de Ciencia de Materiales de Aragón (ICMA) CSIC - Universidad de Zaragoza, 50009 Zaragoza (Spain) E-mail : jarv@unizar.es [b] Dr. C. Carbonera, Prof.Dr. J.-F. LØtard Laboratoire des Sciences MolØculaires Institut de Chimie de la Matire CondensØe de Bordeaux (ICMCB) CNRS UPR No 9048, 33608 Pessac (France) [c] Prof. Dr. T. Kitazawa Department of Chemistry, Faculty of Science, Toho University Miyama, Funabashi, Chiba, 274-8510 (Japan) [**] LIESST: light-induced excited spin state trapping. Chem. Eur. J. 2010, 16, 8785 – 8796 # 2010 Wiley-VCH Verlag GmbH&Co. KGaA, Weinheim 8785 FULL PAPER