MATERIALS CHEMISTRY COMMUNICATION Non-classical FeII spin-crossover behaviour leading to an unprecedented extremely large apparent thermal hysteresis of 270 K: application for displays Yann Garcia, Petra J. van Koningsbruggen, Epiphane Codjovi, Rene� Lapouyade, Olivier Kahn* and Louis Rabardel L aboratoire des Sciences Mole�culaires, Institut de Chimie de laMatie`re Condense�e de Bordeaux, UPR CNRS no. 9048, 33608 Pessac, France [Fe(hyetrz)3](anion)2·3H2O [hyetrz=4-(2¾-hydroxyethyl)- 1,2,4-triazole, anion=3-nitrophenylsulfonate] is a novel linear polynuclear FeII spin-crossover compound. The low-spin to high-spin transition accompanied by a pronounced thermochromic eect occurs at 370 K in a very abrupt way. Just before this temperature, the three non-coordinated water molecules are removed.The dehydrated high-spin form remains stable down to ca. 100 K, where it transforms into a new low-spin form, implying that this material shows an apparent thermal hysteresis width of 270 K. Applications of amount of ascorbic acid was heated and added under stirring this material are discussed. to a methanolic solution (10 ml) containing 9.1 mmol (1.03 g) of hyetrz prepared from monoformyl hydrazine, triethylorthoformateand 2-ethanolamine according to the method described by Bayer et al.21 A white precipitate was formed immediately which was filtered, washed with methanol and dried in air.Iron(II) spin-crossover materials have acquired increasing inter- The compound changes from white to pink during the drying est during the last decade.Evidently, the fast developments in process, due to the 1A1g�1T1g d–d transition at 520 nm of the advanced electronic technology may require compounds show- compound in the LS state. The compound changes to white ing bistability behaviour on the molecular scale.1 A fascinating upon heating to ca. 370 K, because the spin-allowed d–d example of molecular bistability is represented by FeII spin- transition of lowest energy of the compound in the HS state, crossover compounds, which show a transition from the high- 5T2g�5Eg, occurs at the limit of the visible and IR regions.spin state (HS, S=2) to the low-spin state (LS, S=0) on Surprisingly, subsequent cooling to room temperature leaves cooling, upon increasing pressure, or by light irradiation.2–9 the white colour of the compound unaected.Since this The use of such materials as molecular-based memory devices compound shows a thermochromic eect, the FeII spin trans- and displays has been investigated.10,11 This type of application ition has been studied optically using a device described requires abrupt spin transitions involving a large thermal previously.7,21 This device records the change in intensity of hysteresis as well as an associated thermochromic the absorption band at 520 nm, and therefore allows one to eect.3,7,9,10,12,13 The occurrence of abrupt transitions involving follow the spin crossover in a remarkably simple and reliable hysteresis is related to cooperativity.Although the mechanism way. The results of the optical measurements displayed in of this cooperativity is not yet fully understood, it is commonly Fig. 1 reveal a very abrupt LS�HS transition taking place at accepted that these interactions may become extremely import- 370 K, after which the compound is further heated to 400 K. ant when the active spin-crossover sites are covalently linked Subsequent cooling shows the HS�LS transition to occur at by conjugated ligands.The cooperativity may even be 100 K, yielding an extremely wide apparent hysteresis of 270 K. enhanced by hydrogen bonding interactions within the crystal lattice.7,9 Indeed, in the linear polynuclear FeII spin-crossover compounds of general formula [Fe(NH2trz)3 ](anion)2·xH2O (NH2trz=4-amino-1,2,4-triazole; anion=NO3-,9,14,15 ClO4-,16 BF4-,10,16 I-,17 Br-,10,16 CH3SO3-18), in which the FeII ions are linked by triple N1,N2-1,2,4-triazole bridges,19 relatively large thermal hysteresis (up to about 35 K) was observed.Within this family of compounds, the derivatives containing tosylate20 and related aromatic sulfonate anions show an exceptionally large apparent hysteresis loop up to 80 K, owing to the synergy between FeII spin-crossover behaviour and a dehydration–rehydration process.Modifying the 1,2,4-triazole ligand yielded a novel polynuclear FeII spincrossover compound showing unprecedented behaviour, and oering a new opportunity to use such compounds in display devices. [Fe(hyetrz)3](anion)2·3H2O [hyetrz=4-(2¾-hydroxyethyl)- 1,2,4-triazole, anion=3-nitrophenylsulfonate] was synthesized Fig. 1 Optical detection of the spin transition for the couple as follows.A methanolic solution (20 ml) containing 3 mmol [Fe(hyetrz)3](3-nitrophenylsulfonate)2·3H2O–[Fe(hyetrz)3 ](3-nitrophenylsulfonate) 2 (1.72 g) of [Fe(H2O)6](3-nitrophenylsulfonate)2 and a small J. Mater. Chem., 1997, 7(6), 857–858 857A second heating experiment reveals a LS�HS transition with by heating the sample as described above, or by secondary pumping at a pressure of 10-6 mbar.TC(=115 K. Additional heating and cooling cycles show that this hysteresis of 15 K is maintained. The unprecedented wide apparent hysteresis loop of 270 K for [Fe(hyetrz)3](3-nitrophenylsulfonate)2·xH2O implies that Thermogravimetry (see Fig. 2) carried out with the same velocity of heating (1 K min-1) as for the optical measurements we are dealing with a compound using its memory eect only once; after it has been addressed by increasing temperature to reveals a continuous loss of mass starting at room temperature.This decrease in mass proceeds rapidly in the temperature the value of TC(, leading to an abrupt LS�HS transition involving a colour change from pink to white, it remains in range 325–340 K, after which it continues in a much smoother fashion.At 370 K the percentage mass lost is in exact agreement this white HS state in the absence of extreme humidity, provided it is not cooled to very low temperature. At room with the removal of all three lattice water molecules from [Fe(hyetrz)3](3-nitrophenylsulfonate)2·3H2O. Consequently, temperature [Fe(hyetrz)3](3-nitrophenylsulfonate)2 in the HS state and under normal atmospheric conditions does not when the spin transition occurs, no lattice water molecules are present in the compound. Upon cooling no change in the mass rehydrate, in contrast to [Fe(NH2trz)3 ](tosylate)2.20 The information induced by the abrupt spin transition is retained. of the sample is observed, which indicates that the compound is not rehydrated. Therefore, this material may be of use in applications in which detection of a specific temperature is required in a very simple The magnetic properties have also been investigated.The results are in good agreement with the optical data of Fig. 1. and accurate way. The spin-crossover material then acts as a thermal sensor giving an optical response only when TC( is In addition, the magnetic data indicate that the LS�HS transition at 370 K for the starting material is essentially reached, for instance as an alert when the temperature exceeds an upper limit.More interestingly, this thermal addressing complete, while the HS�LS transition for the dehydrated compound at 100 K is incomplete; about 15% of the FeII ions accompanied by an abrupt and stable optical response may alsobe used in single-use (orone-shot) displays.Further physical remain in the HS state below 100 K.These results can be interpreted as follows. At room tempera- studies along with detailed exploration of implementation in devices of [Fe(hyetrz)3](3-nitrophenylsulfonate)2·xH2O and ture the thermodynamical stable state for the hydrated compound [Fe(hyetrz)3](3-nitrophenylsulfonate)2·3H2O is the LS related materials are in progress.state. Evidently, this LS state is stabilized by the hydrated nature of this modification. Indeed, studies on mononuclear References FeII spin-crossover compounds have already revealed that the 1 O. Kahn and J. P. Launay, Chemtronics, 1988, 3, 140. low-spin state may be stabilized by interactions with lattice 2 Pding, 1981, 44, 83. water molecules.22–25 Upon heating, the compound loses all 3 J. Zarembowitch and O. Kahn, New J. Chem., 1991, 15, 181. its lattice water molecules, yielding just below 370 K the 4 E.Ko� nig, Prog. Inorg. Chem., 1987, 35, 527. dehydrated [Fe(hyetrz)3 ](3-nitrophenylsulfonate)2 analogue in 5 J. G. Haasnoot, in Magnetism: A Supramolecular Function, ed.a LS state. However, at this temperature this LS state is a O. Kahn, Kluwer Academic Publishers, Dordrecht, 1996, p. 299. 6 P. Gu� tlich, A. Hauser and H. Spiering, Angew. Chem., Int. Ed. metastable state, and transforms in an exceptionally abrupt Engl., 1994, 33, 2024. fashion to the HS state. Further cooling of [Fe(hyetrz)3 ](3- 7 O. Kahn and E.Codjovi, Philos. T rans. R. Soc. L ondon, A, 1996, nitrophenylsulfonate)2 reveals the HS�LS transition for the 354, 359. dehydrated compound taking place at 100 K. Subsequent 8 O. Kahn,MolecularMagnetism, VCH, New York, 1993. heating shows the LS�HS transition with TC(=115 K. This 9 O. Kahn, E. Codjovi, Y. Garcia, P. J. van Koningsbruggen, hysteresis of 15 K is now stable and can be considered as a R.Lapouyade and L. Sommier, in Molecule-Based Magnetic Materials, ed. M. M. Turnbull, T. Sugimoto and L. K. Thompson, genuine hysteresis, whose origin is governed by cooperative ACS Symp. Ser. No. 644, American Chemical Society,Washington interactions. Evidently, the crucial feature allowing the occur- DC, 1996, p. 298. rence of the apparent hysteresis of 270 K results from the 10 O.Kahn, J. Kro�ber and C. Jay, Adv. Mater., 1992, 4, 718. stabilization of the LS state by water molecules, which ceases 11 C. Jay, F. Grolie`re, O. Kahn and J. Kro�ber,Mol. Cryst. L iq. Cryst., only when the water molecules are removed leading to the 1993, 234, 255. formation of the metastable low-spin state of [Fe(hyetrz)3](3- 12 J. Kro�ber, J.-P. Audie`re, R. Claude, E.Codjovi, O. Kahn, J. G. Haasnoot, F. Grolie`re, C. Jay, A. Bousseksou, J. Linare`s, nitrophenylsulfonate)2. The LS [Fe(hyetrz)3](3-nitrophenyl- F. Varret and A. Gonthier-Vassal, Chem.Mater., 1994, 6, 1404. sulfonate)2·3H2O compound, after being cycled once, can be 13 J. Kro�ber, E. Codjovi, O. Kahn, F. Grolie`re and C. Jay, J. Am. reconstructed at room temperature from the HS [Fe(hyetrz)3] Chem.Soc., 1993, 115, 9810. (3-nitrophenylsulfonate)2 compound by placing the latter in a 14 L. G. Lavrenova, V. N. Ikorskii, V. A. Varnek, I. M. Oglezneva very humid (i.e. a water saturated) atmosphere. The reverse and S. V. Larionov, Koord. Khim., 1986, 12, 207. process requires the removal of lattice water molecules either 15 L. G. Lavrenova, V. N. Ikorskii, V.A. Varnek, I. M. 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Fig. 2 Thermogravimetric analysis for [Fe(hyetrz)3 ](3-nitrophenylsulfonate) 2·xH2O Communication 7/01242J; Received 24th February, 1997 858 J. Mater. Chem., 1997,