Langmuir 1996,11, 3467-3472
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Langmuir-Blodgett Films of Spin Transition Iron(I1) Metalloorganic Polymers. 1 Iron(I1) Complexes of Octadecyl-1,2,4-triazole
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F. Armand,"??C. Badoux,? P. Bonville,t A. Ruaudel-Teixier,? and 0. Kahn# CEAI DSMlDRECAMIService de Chimie Molkculaire, CE Saclay, 91191 Gif-sur-YvetteCedex, France, CEAlDSMIDRECAMIService de Physique de l%tat Condensk, CE Saclay, 91 191 Gifsur-Yvette Cedex, France, and Laboratoire de Chimie Inorganique, Universitk de Paris-Sud, URA-CNRS 420, 91405 Orsay Cedex, France Received October 31, 1994. In Final Form: May 1, 1995@ A series of iron(I1) metalloorganic polymers of 4-octadecyl-1,2,4-triazole has been synthesized. Their spin transition properties have been checked by Mossbauer spectroscopy, by magnetic susceptibility measurements, and particularly by IR spectroscopy, which was shown to be an easy and sensitive way to characterize spin transition phenomena. Counteranion exchange was used to tune the spin transition temperature. The compound with tosylate counterions was found to present a sharp transition close to room temperature. Comparisonbetween polymers of 4-octadecyl-1,2,4-triazoleand 4-amino-1,2,4-triazole showed that the cooperativity process in those compounds is mainly intramolecular, i.e., between Fer1 centers of the same polymer chain. The iron(I1)amphiphilic compounds were unstable at the air-water interface and direct fabrication of Langmuir-Blodgett films was not achieved. Diffusion of iron(I1)salts led to complexeswith polymer chains that were into Langmuir-Blodgett films of 4-octadecyl-1,2,4-triazole too short to present spin transition properties.
Introduction The use of molecular materials for storage and treatment of information is the challenging issue of molecular Molecular engineering and synthesis on one side and study of the organization and interaction of molecules in the solid state on the other side are the main aspects of the work in this field. In the search to obtain materials exhibiting switching and memory properties, the use of spin transition complexes seems to be particularly a d a ~ t e d . ~Such - ~ molecules can have two different electronic states depending on the value of external parameters like temperature. These two stateslead tovery M e r e n t optical and magnetic properties for the molecule; therefore, easy detection of the molecular state is possible. The corresponding molecular material may present a sharp transition with wide hysteresis if cooperativity is effective.6 Thus, in a given range of external parameters, the molecular assembly may exist under two stable electronic states. Some iron(I1) complexes with iron-nitrogen bonds present spin transition phenomenon.'-15 Recently, en-
' CEA/DSM/DRECAM/Service
de Chimie Molkculaire. CEAIDSMIDRECAMIService d e Physique d e l'Etat Condensh. 8 Laboratoire d e Chimie Inorganique, Universite de Paris-Sud. * Abstract published inAdvanceACSAbstracts, August 1,1995. (1) Molecular Electronic Devices, Eds.; Carter, F. L., Ed.; Marcel Dekker, New York, 1982. Molecular Electronic Devicesll, Eds.; Carter, F. L., Ed.; Marcel Dekker, New York, 1987. (2) Barraud, A.;Kahn, 0.;Launay, J.-P. Sci. Technol. 1989,15,54. (3)Kahn, 0. Molecu1arMagnetism;VCH Publishers: New York, 1993; Chapter 4. (4)Gatteschi, D. Magnetic MolecularMaterials;Kluwer: Dordrecht, 1991. (5)Ban, R. A.; Sivardihre, J. Phys. Rev.B 1972,5,4466. ( 6 ) Kahn, 0.; Launay, J.-P. Chemtronics 1988,3, 140.
(7)Greenaway, A. M.; OConnor, C. J.; Schrock, A.; Sinn, E. Inorg. Chem. 1979,18,2692. (8)Mikami, M.; Konno, M.; Saito, Y.Chem.Phys. Lett. 1979,63,566. (9)Giitlich, P. Struct. Bonding (Berlin), 1981,44,83. (10)Konig, E.; Ritter, G.; Kulshreshtha, S. K. Chem. Rev. 1986,85, 219. ~~.
(11)Klinig, E.; Kanellakopulos, B.; Powietzka, B.; Goodwin, H. A. Inorg. Chem. 1989,28,3993. (12)Gallois, B.;Real, J.-A.; Hauw, C.; Zarembowitch, J. Inorg. Chem. 1990,29,1152. (13)Zarembowitch, J.; Kahn, 0. New. J. Chem. 1991,15,181.
hancement of the cooperative effect was obtained while using polymers.15-19 For instance, metalloorganic polymers of iron(I1) and triazole derivatives were shown to present a spin transition at room temperature T,from a low temperature diamagnetic state (spin S = 0)to a high temperature paramagnetic state (spins = 2).18J9In some cases a sharp transition with a n hysteresis of 20 K has been obtained. Being able to obtain thin films ofreproducible thickness with spin transition compounds would be of highest interest for device applications. The Langmuir-Blodgett technique appears to be the best way to realize this.20-22 The present work deals with metalloorganic iron(I1) polymers of amphiphilic triazoles. The spin transition phenomenon that takes place in these complexes was characterized and compared with the transition in hydrophilic polymers in order to evaluate the influence of the interchain interactions on the cooperative effect. Langmuir-Blodgett experiments were tried on these complexes and the results are presented here.
Experimental Section 4-Octadecyl-l,!4,4-triazole (ODT). 1-Pentanol and monoformyl hydrazine were dried before use. To a solution of 0.6 g mol) of monoformylhydrazine in 1-pentanol(25mL), 2.5 mL (1.5 x mol) of triethylorthoformate was added. After (14)Zarembowitch, J. Mol. Cryst. Liq. Cryst. 1993,234,247. (15)VreugdenhiI, W.; van Diemen, J.H.; de Graaff, R. A. G.; Haasnoot, J. G.; Reedijk, J.; van der Kraan, A. M.; Kahn, 0.; Zarembowitch, J. Polyhedron 1990,9,2971. (16)Haasnoot, J.G.;Vos, G.; Groeneveld, W. L.Z.Naturforsch. 1977, 326,1421. (17)Lavrenova, L.G.;lkorskii, V. N.; Varnek, V. A.; Oglezneva, I. M.; Larionov, S. V. Koord. Khim. 1990,16,654. (18)Kahn, 0.; Krober, J.; Jay, C. Adv. Mater. 1992,4,718. (19)Kr6ber, J.; Codjovi, E.; Kahn, 0.;Grolihre, F.; Jay, C. J.Am. Chem. SOC.1993,115,9810. (20)Ruaudel-Teixier, A.;Barraud, A.; Coronel, P.; Kahn, 0. Thin Solid Films 1988,160,107. (21)Coronel, P.; Ruaudel-Teixier, A. Mol. Cryst. Liq. Cryst. 1990, 187,319. (22)Ruaudel-Teixier, A. In Lower-Dimensional Systems a n d Molecular Electronics; Metzger, R. M., Eds.; Plenum Press: New York, 1991;p 511.
0743-7463/95/2411-3467$09.00/00 1995 American Chemical Society
Armand et al.
3468 Langmuir, Vol. 11, No. 9, 1995 refluxing 3.5 h, a solution of 2.69 g mol) of octadecylamine in 1-pentanol (25 mL) was added dropwise. The reflux was maintained during 4 h more. The mixture was then concentrated and cooled and a solid precipitated. After filtration, ODT was recrystallized several times from petroleum ether. 1H NMR (CDC13)6 0.87 (t, 3H), 1.24 (m, 30H), 1.8(m, 2H), 4.0 (t, 2H), 8.15 ppm (s, 2H); IR(KBr) 3096 (stretch, YC-H a"), 2954, 2917, 2850 (stretch, YC-H aph), 1536 (stretch, v ~ g )1471, , 1380 (bend, BC-H dlph), 1189, 1078,955 (stretch, vmg), 855 (bend, 6 ~ - H a " ) , 719 (rock, YCHJ,643 cm-l (bend, dmg); mp = 68 "C. Anal. Found: C, 75.12; H, 12.05; N, 12.81. Calcd for C20H39N3: C, 74.71; H, 12.22; N, 13.07. Saltsof[F$(ODT)sln. Fe(Cl)z4HzO,Fe(C104)26H20,Fe(CF3SO&-6HzO, and Fe(SiFsWHz0were commercially available and used without further purification. Fe(tosylate)~-6H2Owas obtained from iron powder and p-toluenesulfonic acid in water. After nitrogen was bubbled through water or ethanol for 1h, 0.5 g of Fe(I1) salt was then added. Small amounts of ascorbic acid (0.1 g) and/or SO2 stream were used to avoid iron(I1) oxidation. The ligand ODT (3.5 moVl mol of iron(I1) salt) was then added, giving almost immediately a precipitate of the [FeI1(ODT)3],salt. In the case of using water as solvent, the reaction mixture was heated at 70 "C. The precipitate was filtrated, washed with ethanol, and dried. [Fell(ODT)s(tosylate)al,. IH NMR (CDCL3)6 1.24 (SI, 1.42 (s), 2.17 (s). IR(aggregates onto fluorine substrates), the bands corresponding to the aliphatic chains are unchanged, the bands corresponding to the ring are shifted: 3128 (stretch, YC-H arom), 1556 (stretch, vnW) 1213, 1204, 1005 cm-l (stretch, vmg). Anal. Found: C, 65.22; H, 9.69; N, 9.25; S, 4.71; Fe, 4.10. Cakd for C74H13lN90eSzFe: C, 65.18; H, 9.61; N, 10.26; S, 4.50; Fe, 3.81. Magnetic Susceptibility Measurements. They were obtained on a SQUID-type magnetometer from SHE. Mtissbauer Absorption Measurements. Mossbauer experiments on the isotope 57Fe(Ig= Y2, Z, = 312, Eo = 14.4 keV) were performed using a standard Wo*:Rh y-ray source and an electromagneticdrive with triangularvelocity sweep. The spectra were taken a t various temperatures from liquid helium to ordinary temperature (notemperature cycle experiment has been performed). Infrared Spectroscopy. IR spectra were obtained with a Perkin-Elmer 1720X Fourier transform spectrometer. The temperature measurements were performed with a CEA-CENG BT 3001301 temperature regulating system that was connected with the copper walls of the heating cavity. The regulating temperature and the sample temperature being always slightly different (by around 1deg), the temperature of the experiment was determined directly on the sample. In order to avoid counterion exchange with KBr, the iron complexeswere deposed from chloroform solutions on silica substrates of3 mm thickness. The temperature was increased by steps of 5 "C and we waited 1h to obtain an homogeneous atmosphere before each measurement. Langmuir-Blodgett Experiments. The metalloorganic polymers with c104-,CF3S03-, or tosylate as counterions were spread from chloroform solutions of concentration approximately 5x mol*L-l,at the surface of pure Millipore water (reverse osmosis deionization). The Langmuir trough, made of special nonporous PTFE (ATEMETAtrough, Model LB 105),was 0.4 m2 in size. 100 monolayers could be transferred at 26 mN/m onto calcium fluoride at a speed of 5 m d m i n on the upstroke and 400 m d m i n on the down stroke.
+
Results and Discussion 1. Synthesis. Compounds ODT and [Fen(ODT)&salts were synthesized accordingto the route depicted in Scheme 1. ODT was prepared in two steps in one pot and anhydrous conditions were necessary to obtain good yield. Iron(I1)metalloorganic polymers were obtained by reaction of the ODT ligand with diverse iron salts: Fe(C1)~.4Hz0, Fe(C104)~6H20(precursor of polymer I), Fe(CF3S03)26HzO (precursor of polymer IT), Fe(SiF6).4HzO (precursor of polymer 1111,and Fe(tosylate)&HzO (precursor of polymer IV). In order to avoid iron(I1)oxidation, the solvents, water or ethanol, were deoxygenated with nitrogen bubbling before use, and ascorbic acid andlor SO2 stream were
Scheme 1
Fell(Xz)
1 employed during the complexation reaction. In the case of using water as solvent, the reaction mixture had to be slightly heated because of the insolubility of ODT ligand. IR was a very efficient way to check the complexation: the absorption bands corresponding to the aliphatic chains were unchanged but the bands concerning the aromatic ring showed a shift to higher frequencies and there was presence of Fe-N bands between 300 and 200 cm-l.16 2. Spin Transition Measurements. The change of the iron(I1)spin state can be directly observed by magnetic susceptibility measurements. However, as spin transition phenomenon induces modifications in the complex structure, Mossbauer and IR spectroscopies are also efficient methods for its o b s e r v a t i ~ n . ~To ~ , characterize ~~ the temperature T,,the steepness, and the eventual hysteresis of the spin transition that takes place in the synthesized amphiphilic metalloorganic polymers, magnetic susceptibility measurement and Mossbauer and IR spectroscopies were used. First, from the spin transition curve, we evaluate the effect of the solvent used in the synthesis (water or ethanol) on the length of the polymer chains. Then, the effect of the counteranion (Ci04-, CF3S03-, SiFs2-, CH3PhS03-) on the spin transition characteristics is presented. Finally, we also discuss how the long aliphatic chains affect the spin transition phenomenon from the point of view of cooperativity. 2.1. Role of the Solvent on the Length of the Polymer Chains. Polymer I, [Fe(ODT)3(C104)21n, was synthesized in water and ethanol; in both cases, Mossbauer spectroscopy was used as characterization method. The spectrum of the compound presented a n evolution when temperature was increased from liquid helium to room temperature. This evolution from a quadrupole doublet with a small splitting a t low temperature to a quadrupole doublet with a large splitting a t high temperature (see Figure 1) is characteristic of a low spin (LS)-high spin (HS) transition or iron(I1). The presence of a quadrupole splitting for the LS form indicates clearly that the iron(I1) (23)Takemoto, J. H.; Hutchinson, B. B. Inorg. Nucl. Chem. Lett. 1972,8, 769. (24)Grandjean, F.;Long, G. J.; Hutchinson, B. B.; Ohlhausen, L.; Neill, P.; Holcomb, J. D. Inorg. Chem.1989,28,4406.
Iron(II) Metalloorganic Polymer Films
-4 -3 -2 -1 0 1 2
v
3
Langmuir, Vol.11,No. 9, 1995 3469
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("A
Figure 1. Mossbauer spectra of [Fe(ODT)3(C104)&synthesized in water, as a function of temperature. The quadrupole doublet withthe small splittingwhich dominatesin the low temperature region refers to the LS state; the doublet with large splitting whose intensity increases with increasing temperature refers to the HS state of iron(I1).
ion is located in a noncubic symmetry center. Also, we have seen that even at low temperature there was a residue of HS iron(I1). This result might have been due to a too quick cooling of the sample leading to the trapping of HS iron(I1) form; however, slow cooling of the sample gave exactly the same percentage of HS spin residue at 4.2 K. Therefore, this HS residue certainly corresponds to the iron(I1) with water ligands that are located at the end of the polymeric chains (see Scheme 1)and that are in the HS state in the whole temperature range. In Figure 2 is presented the HS form fraction of iron(I1) in polymer I as a function of temperature using the Mossbauer spectroscopy results. The two curves that correspond to polymer I synthesized in water and in ethanol gave similar transition temperature ((T,)I = 200 "C); this value was confirmed by susceptibility measurements. Therefore, it seems that the solvent used in the synthesis has very little impact on the spin transition temperature. However, the percentage of HS iron(I1) residue at low temperature is larger for compounds synthesized in ethanol than for compounds synthesized in water. This might indicate that polymers synthesized in water were made up of longer chains and therefore of a smaller percentage of chain ending monomer than those synthesized in ethanol. 2.2. Effect of the Counteranion. Any slight change in the polymer structure like anion replacement or introduction of a substituent on the triazole ligand can be used as a tool to control the temperature T , of the spin transition. In our search for room temperature spin transition compounds, we have used the anion replacement method to modify T,. Polymer [Fe(ODT)s(Cl)& exhibited degradation with time and therefore was not used. The results obtained with the other polymers are presented in Table 1. From Mossbauer spectroscopy and magnetic susceptibility measurements, approximate spin transition temperatures
0
100
200
300
Temperature ("K) Figure 2. Spin transition curve for polymer I, [Fe(ODT)S(C104)2In,derived from the Mossbauer spectra taken at increasing temperatures. Curve a corresponds to the case of polymer I synthesized in water and curve b to the case of polymer I synthesized in ethanol. were determined for each polymer: (T,)IGZ 200 K, (T,)II= 250 K,(T,)III GZ 300 K, and (T,)w = 315 K. For the first two polymers, the transition was rather slow (AT1 = 115" and AT11 = 100') without hysteresis, while polymer I11 provided a n example of a compound with a slow transition (AT111= 100")and a rather large hysteresis (AT,)III = 15". Because its sensitivity decreases drastically above room temperature due to the decrease of the Lamb-Mossbauer factor, the Mossbauer technique was used only with compounds presenting low-temperature spin-transition. On the other hand, for complexes with spin transition temperature slightly above room temperature, like complex IV, IR was found to be a perfect characterization method. Indeed, on the transition from low spin to high spin, d-type electrons ofFe(I1)get into antibinding orbitals which results in a n increase of the Fe-N distance; therefore, the vibration bands of the Fe-N bond but also some vibrations of the cycle are shifted at the moment of the transition. The Fe-N band that is directly influenced by the spin transition was at very low frequency and not easily detectable (absorption of substrates, . . .I. A temperature study of the complex nT, however, showed reversible and reproducible modifications in IR spectrum between 4000 and 800 cm-l. In particular, the 1415 and 1105 cm-l bands at low temperature shifted to 1400 and 1090 cm-', respectively, at a temperature above the spin transition (see Figures 3 and 4). Careful measurements of the temperature revealed a n abrupt transition (ATw = 10') with a hysteresis of (AT& = 8" as is shown in Figure
3470 Langmuir, Vol. 11, No. 9, 1995
Armand et al.
Table 1. Effect of the Counteranion on the Spin Transition Characteristics of [Fe11(ODT)s(X2-)],Polymers synthesis characterization % HS residue at counteranion solvent method low temperature T,t, T,C (K) A F (K) AT,. (K) Clod- (polymer I) water Mossbauer 15 210 115 ethanol Mossbauer 42 195 115 ethanol magn suscept 35 180 125 0 CF3S03- (polymer 11) water Mossbauer 19 250 100 35 310,295 100 15 magn suscept SiFs2- (polymer 111) ethanol 4 318,310 ethanol IR 10 8 CH3PhS03- (polymer J W Halftransitiontemperature. Steepness ofthe transitionAT= [(Tie% under HS - Tim over HS md+ ( T I Munder HS - T i m Over HS ,,,in)&]/ 2. Hysteresis width AT, = T,t - Tcb. ~~
IO0
%T
10
0 ,do0
I
1
!am
1000
0
10
20
30
40
50
Temperature
60
70
80
90
100
(OC)
Figure 6. Spin transition curve derived from the evolution with temperature of the 1415/1400cm-I absorption band for complex IV.
.... I.'"
... 1.1"
..-.
I
1.11
..." I"."
cm"
Figure 4. Evolution of the 1400 cm-I IR band absorbance of
complex IV, [Fe(ODT)3(CH3PhS03-)2]., with temperature. Progressive disappearance of the high spin 1400 cm-l IR band and appearance of the low spin 1415 cm-l IR band on cooling.
5. The exact same hysteresis cycle was obtained when the heating-cooling process was repeated. Changing the counteranions has induced rather important variations on the spin transition temperature T,. Although no rule could really be found to relate T, with the counteranion type, we managed to obtain metalloorganic polymers with T, close to room temperature.
2.3. Effect of the Long Aliphatic Chains. By introducing long aliphatic chains on the triazole ligands, we clothed the hydrophilic spin transition polymer with a hydrophobic coat. Thus, it seems a good approximation to consider that polymer chains were maintained far enough away from each other to neglect the cooperative effect among them. The three-dimensional cooperativity that takes place in iron(I1) complexes of hydrophilic triazoles becomes here a unidimensional cooperative process; consequently, this should drastically diminish the hysteresis and the steepness of the spin transition. In order to discuss the effect of the hydrophobic coating of the polymer chains on the spin transition characteristics, we synthesized a n hydrophilic polymer from 4-amino1,2,4-triazoleand Fe(CH3PhSO& salt in a ethanol-water solvent mixture. The obtained polymer, polymer V, was studied by IR spectroscopy. The spin transition characteristics were the following: (T,th = 41 "C, ( T J h= 35 "C, ATv = 20",(AT& = 6". This indicates even less coop-
Iron(II) Metalloorganic Polymer Films
Langmuir, Vol. 11, No. 9, 1995 3471
40
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Figure 7. IR spectra of (a)[Fe(ODT)3(CH3PhS03-)21,polymer aggregates obtained by evaporationof chloroform solutions on CaFz substrate. (b) [Fe(ODT)3(CHsPhS03-)21,after LB technique deposition on CaFz substrate, the polymer decomposed and only the ligand was deposited. (c) After introductionof the substrate during 2 days in saturated solution of Fer1(CH3PhS03-)26Hz0.
10
0
0
100
200
300
400
Area per monomer unit (A21 Figure 6. l7-A isotherm for a monomolecular layer of polymers I, 11, and IV.
erativity effect in polymer V than in polymer IV. Because these two compounds are different one to the other only by the nature of the substituent on the triazole ligand, this result tends to show that the cooperativity in those polymers of iron(I1) is mainly along the polymer chain. 3. Langmuir-Blodgett Experiments. Metalloorganic polymers I, 11,and IV were soluble in chloroform and therefore were used for Langmuir-Blodgett (LB) experiments. The ODT ligand and the three types of polymers gave stable Langmuir films and reproducible ll-A isotherms. The area per molecule for a solid Langmuir layer of ODT was approximately 25 A2;however, the area per repeating unit (Fe(ODT13)that was obtained for solid monomolecular layers of the polymer was around 100-120 AZ(see Figure 6), which is much larger than the cross section area for three ODT molecules. This result suggests a tilted arrangement of the ligands in the iron(I1) complex orland a loose packing of the polymeric chains in the monolayer. By use of the vertical dipping method, one hundred layers of polymer IV were deposited on upstrokes and downstrokes onto fluorine (CaF2) substrates that were made hydrophobic by predeposition of five fatty acid monolayers. IR measurements on such substrates indicated that not the polymer but the amphiphilic triazole ODT ligand was deposited (see spectra a and b of Figure 7). The metalloorganic polymer that is rather stable in chloroform solutions was destroyed at the air-water interface. The tail-to-tail head-to-head Y-type structure of the ODT LB film was confirmed by X-ray diffraction measurements: the period of 43 A obtained along the normal to the substrate is much longer than the length
Figure 8. Schematic representation of a Langmuir-Blodgett film of ODT after diffusion of Fen(CH3PhS03-)2-6H20.X- stands for tosylate ion.
of one ODT molecule determined from molecular models. Diffusion experiments were carried out on substrates covered with Y type LB films of ODT. The substrate was introduced at room temperature into a saturated solution of Fe”(CH3PhS03-)2*6H20which contained a catalytic amount of ascorbic acid. After 2 days, we observed the formation of a complex (see Figure 8) whose IR spectrum presents similar features with the one of the original
3472 Langmuir, Vol. 11, No. 9, 1995
[Fe(ODT)g(CH3PhS03-)2]ncomplex (see spectra a and c of Figure 7). In particular, the positions of the triazole ring vibrations clearly indicate a n almost overall complexation by iron(I1): for instance the 1536 cm-' band shifted to 1556 cm-l. However, the presence of the bands at 1400 and 1090 cm-l suggests that this compound is high spin a t room temperature. No characteristic change of spin transition phenomenon was observed in the IR spectrum by heating or cooling, A n explanation of this result could be the following: the iron(I1)located a t the end of the spin transition polymer chains are high spin in the whole temperature range. For this reason, polymer chains must be a t least of three iron(I1) centers to lead to a slow spin transition and four iron(I1) centers to lead to a spin transition with cooperativity process. The absence of a remarkable spin transition in the LB films of ODT after diffusion of iron(I1) suggests that the ODT ligands are not mobile enough in the LB film and lead to very short polymer chains of no more than three metal centers.
Conclusion This work on spin transition polymers of iron(I1) and amphiphilic triazoles suggests several conclusions in
Armand et al. relation to the fundamental characteristics of such compounds: (i) water was found to be a good solvent for the synthesis of iron(I1) triazole polymers in the search to obtain compounds exhibiting large cooperativity; (ii) counteranion exchange was a fruitful way to tune the spin transition temperature; (iii) the cooperativity in the spin transition seems to be mainly a n intrapolymer chain process. IR spectroscopy revealed as a sensitive method that could easily characterize the spin transition phenomenon in small amounts of material and particularly in LB films. The fabrication of LB films directly with amphiphilic polymers seems to be very difficult considering the instability of the compounds in aqueous conditions; however, the first results directed to build the metalloorganic polymer by using a diffusionmethod are promising.
Acknowledgment. We thank Professor Delhaes of CRPP in Bordeaux for the magnetic susceptibility measurements. LA9408533
