CHEMPHYSCHEM 2002, No. 12 ¹ 2002 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 1439-4235/02/03/12 $ 20.00+.50/0 1045

A Comparative Investigation of SpinCrossover in Bulk Materials, LamellarCdPS3 Layers, and Polyelectrolytes ina Series of Ferric Complexes:[FeIII((X-sal)2trien)]Y

Archana Jaiswal,[a] Se¬bastien Floquet,[b]

Marie-Laure Boillot,[b] and Pierre Delhae¡s*[a]

KEYWORDS:

EPR spectroscopy ¥ intercalations ¥ ligand design ¥ magneticproperties ¥ spin crossover

Transition metal complexes, with mainly iron or cobalt as activecenters, it is well known that electronic spin crossover phenom-ena can occur. These are induced by external stimuli, such astemperature, pressure, or light.[1] These phenomena are asso-ciated with changes in magnetic responses due to a transitionof the metallic center between low-spin (LS) and high-spin(HS) states that often manifest in optical characteristics useful forpotential electronic devices.

Spin crossover is related to several mecha-nisms that give rise to different types oftransition behavior ranging from a gradualprocess (spin equilibrium) to more coopera-tive mechanisms including hysteresis or mem-ory effects.[2] The presence of molecular inter-actions and sometimes polymeric associationsin these systems[3] has been found to be a keyparameter to favor a molecular-type bistabil-ity.[4] It turns out that in these electronicsystems the properties that are switchedunder the effect of an external constraintmay be tuned by the environment of theactive complex.[5]

Following this line we have already inves-tigated a series of iron(II) complexes in differ-ent surroundings, for example a photomag-netic effect, the so-called Light-Induced Excit-ed Spin State Trapping (LIESST) effect, hasbeen observed on Langmuir ± Blodgett filmsincorporating FeII spin-crossover centers.[6] In

the present study we have investigated a series of ferriccomplexes of hexadentate ligands derived from salicylaldehyde(X-sal) and triethylenetetramine (trien): [FeIII((X-sal)2trien)]Y,where Y is a monoanion. These complexes show a gradual spincrossover both in solution and in the solid state.[7] The mainadvantages for this series of coordination complexes aretwofold:� FeIII is EPR active, both in the low-spin (S�1/2) and the high-

spin (S� 5/2) state allowing a dynamic study.[8]

� The active centers are cationic species, therefore by changingeither the counterion or the anionic matrix and its associatedelectrostatic interactions, these host effects and associateddilutions can be investigated particularly well by EPRspectroscopy.[9]

The purpose of this work is to study the magnetizationbehavior and EPR spectra of a new series of compounds recentlysynthesized[10] based on [Fe((3,5-Cl2-sal)2trien)]� ionic species,which exhibit thermal spin crossover around room temperature.A comparison between different polycrystalline samples, speciesintercalated in lamellar CdPS3

[11] and samples encapsulatedwithin polyelectrolytes by metathesis or using the layer-by-layer(LbL) technique[12] is presented in this work. Finally, using theadvantages of the EPR technique, a comparative investigationcharacterizes the occurrence of the spin crossover in this seriesof new complexes.

Static Magnetic Properties : The temperature dependencies, inthe range 4 ±350 K, of the static magnetizations have beenmeasured. From these experiments, �MT versus T of the aboveactive cationic species with different surroundings, where �M isthe molar magnetic susceptibility, reveals different temperaturedependencies (Figure 1).

First, for polycrystalline samples the nitrate salt is in a stablehigh-spin state. In contrast, the ferric complexes associated with

[a] Dr. P. Delhae¡s, Dr. A. JaiswalCentre de Recherche Paul PascalCNRS–Universite¬ de Bordeaux I115 Av. Albert Schweitzer, 33600 Pessac (France)Fax: (�33) 556-845-600E-mail : delhaes@crpp.u-bordeaux.fr

[b] Dr. S. Floquet, Dr. M.-L. BoillotLaboratoire de Chimie InorganiqueUMR 8613, ICMMO, Universite¬ Paris-Sudba√timent 420, 91405 Orsay (France)

Figure 1. Temperature dependence of the �M T product for [FeIII((3,5-Cl2-sal)2trien)]Y complexes, whereY is either NO3

� (�), PF6� (�), ClO4

� (�), PSS (�), PSSMA (�), or CdPS3(�).

1046 ¹ 2002 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 1439-4235/02/03/12 $ 20.00+.50/0 CHEMPHYSCHEM 2002, No. 12

PF6� and ClO4

� ions, which both presentmonoclinic crystal packing, show a gradualtransformation characteristic of a thermo-dynamic spin equilibrium.[10]

A characteristic temperature T1/2 , whichdepends on thermodynamic parameters[2]

can be given. T1/2 equals 170 and 290 Kfor the PF6

� and ClO4� salts, respectively.

These different behaviors reveal the influ-ence of counterion size and associatedelectrostatic interactions on the spin con-version process.

Second, for the samples either interca-lated in the two-dimensional lamellar hostmaterial CdPS3 or encapsulated in polyelec-trolytes (PSS and PSSMA), the trapped para-magnetic blocks undergo a very limitedtemperature-induced spin conversion,which is not complete even at the highestexperimental temperature. For the interca-lated compound we observe a quasifrozenlow-spin state, because the host ±guestinteractions are strong, favoring the stabili-zation of the LS state, as observed in otherseries.[10, 13]

Finally, for the polymeric samples differ-ent curves are observed after the firstthermal treatment (heating to 350 K). Inthe case of the PSS compound, a smallirreversible increase of the high-spin con-tent (around 10%) is detected after heatingto temperatures between 293 and 423 K.This change appears reproducible in furthertemperature cycles. Indeed, the role ofwater molecules in such behavior indicatedby the removal of residual water (6.5 wt.%)at 423 K as detected by differential scanningcalorimetric (DSC; heating rate 3 Kmin�1)measurements. The result is in agreementwith previous results which have shownthat the presence of water is rather stabiliz-ing the LS state.[14]

It turns out that a large HS residualfraction is observed at all temperaturesmainly for both samples containing poly-electrolyte. Only a small fraction of thesecomplexes undergo a spin transformationthat should occur at higher temperatures (T1/2�300 K) thanthose observed for polycrystalline samples. Molecular interac-tions, including hydrogen bonding,[14] and also the polymerstructure and its entanglement effect are expected to play a rolein quenching the HS species, because the FeIII complexes shouldbe distributed in different surroundings.

Electron Spin Resonance Investigations : The above observa-tions were further confirmed by EPR experiments. Figure 2represents the EPR spectra of the different materials. Distinct HSand LS signals are present in several cases, indicating that the

spin state conversion rates are slow compared with thespectroscopic frequencies, as already known from Mˆssbauerspectroscopy[8] or ultrasonic relaxation.[15]

Consequently, the resonance lines for both spin states can beanalyzed independently. The low-spin ferric centers in a pseudo-octahedral environment typically exhibit one resonance line atany temperature located near g� 2.17 with a g-factor anisotropydue to spin ±orbit coupling, which is responsible for theobserved shoulder at high resonance field (see Figure 2). Forthe high-spin ferric systems several allowed resonance lines are

Figure 2. Room-temperature and liquid 4He-temperature X-band EPR spectra of [FeIII((3,5-Cl2-sal)2trien)]Ycomplexes, where Y is a) PF6

�, b) ClO4�, c) CdPS3 (––) or LbL deposition (- - - -), or d) PSS (––) or PSSMA

(- - - -).

CHEMPHYSCHEM 2002, No. 12 ¹ 2002 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 1439-4235/02/03/12 $ 20.00+.50/0 1047

determined by classical rhombic type zero-field splitting com-bined with the Zeeman interaction. In a perfect rhombicsymmetry, an intense central line is expected at g�4.3.[16] Thisis the case in this series where the only intense line is observedaround this g value (see Figure 2a). Indeed, we note for thenitrate salt a signal at g�4.5 with a linewidth �H�760 G atroom temperature, typical of such a situation. Such a line is alsoobserved at liquid 4He temperature for two samples containingthe polyelectrolytes. This line indicates the presence of a residualHS species, as already evidenced by SQUID experiments. Aspointed out by Federer and Hendrickson by studying the effectof grinding,[17] both HS and LS species are randomly distributedwithin a crystallite for a gradual transition. Our experimentsconfirm this point and further show that the position of theresonance line for the LS state (g�2.17) is not shifted during thespin transition. Only its linewidth is sensitive to the amount ofrandomly distributed HS population.

Taking advantage of the presence of an homogeneous(Lorentzian-type) low-spin resonance line in this series ofcompounds, we have investigated the temperature depend-encies of these linewidths (see Figure 3). At first glance, thetemperature dependencies of these linewidths appear verysimilar to those already presented in Figure 1 for the product�MT. For the thermal cycles, the small difference observed whileraising or decreasing the temperature of the encapsulatingpolyelectrolytes (see above) was not investigated for thelinewidth behavior.

Spin Transition Analysis : We propose an analysis that relatesthe static susceptibility with the dynamic response. This analysisis based upon the magnetic dipolar interactions betweenneighboring spins as the main source of spin ± spin relaxationeffects in these magnetic insulators.[18] At a given temperatureduring a spin crossover phenomenon, a low-spin ferric ion isstatistically surrounded by either HS and LS state neighbors. Anincrease in the high-spin population induces an increasing staticdipolar broadening of the LS resonance linewidth.

As observed in the case of a homogeneous resonance line, theintrinsic linewidth behavior is associated with a motionallynarrowed dipolar interaction between neighbors as also ob-served in doped FeII compounds.[19] Therefore, we can write sucha mechanism as Equation (1), since the Curie constant is given byEquation (2); �B is the Bohr magneton.

�H(T)�g�B[S(S�1)]1/2(�ir�6i �1/2� (�MT)1/2 (1)

�MT�g2�2BS(S�1) (2)

If we assume now that this type of interaction is bothdominant and temperature-independent, the only parametersto be taken into account are the respective spin values. Thestatistical probability of neighboring HS states increases with thetemperature. On a macroscopic scale, which is equivalent to theclassical molecular field approximation, this probability is givenby the respective molar fractions of HS and LS states, nHS and nLS

(nHS�nLS� 1).The HS molar fraction can be determined from the magnetic

measurements (see Figure 1), Equation (3), where CLS and CHS arethe product of the temperature and molar magnetic suscepti-bilities for each spin state.

nHS(T) � �MT � CLS

CHS � CLS

(3)

For the S� 5/2 and 1/2 spin states of the ferric ion, wecan assume CHS� (�MT)HS� 4.37 cm3mol�1 K�1 and CLS� (�MT)LS�0.5 cm3mol�1K�1. From Equation (3), we get Equation (4), and,from Equation (1), Equation (5) follows.

�MT � nHS(T)CHS�nLS(T)CLS (4)

�H(T)2� [nHS(T)CHS�nLS(T)CLS] (5)

The relationship given by Equation (5) is used to compare allthe spin crossover totally or partially observed for these differentcompounds.

Figure 4 shows a parameter homogeneous to �MTversus the square of the EPR linewidth. An identicalbehavior is found for both the polycrystalline salts (ClO4

�

and PF6�) and also the lamellar one in the LS state. This

result confirms the validity of the proposed relaxationmechanism, although a linear relationship is not strictlyobserved at high temperature. Our approximation of arelaxation mechanism that is not temperature-depend-ent is not completely valid. Moreover, it turns out thata rather different behavior is observed mainly for thePSS polyelectrolyte, but not for its analog PSSMA. For agiven HS molar fraction, the EPR line for the PSS sampleis broader than expected. We suppose that a secondbroadening mechanism is responsible, which could bedue to the polymer disorder acting on the paramag-netic complexes or a nonstatistical distribution of HS FeIII

sites. Moreover, a preliminary study using the LbLtechnique with PSS, described in the ExperimentalSection, shows a narrow but weak line characteristic ofa pure LS state at liquid-helium temperature. This line

Figure 3. Temperature dependencies of the linewidth �H of the main resonance lineobserved at g� 2.17 for resonance of the low-spin ferric center in [FeIII((3,5-Cl2-sal)2trien)]Ycomplexes, where Y is NO3

� (�), PF6� (�), ClO4

� (�), PSS (�), PSSMA (�), or CdPS3(�).

1048 ¹ 2002 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 1439-4235/02/03/12 $ 20.00+.50/0 CHEMPHYSCHEM 2002, No. 12

disappears as soon as the temperature is raised (see Figure 2c).This result appears rather similar to those observed for the CdPS3

host compound than to those in bulk polymers. The effect ofsurroundings is confirmed to be different in amorphouspolymers as compared with those present in more organizedsystems.

In crystals, a counterion size effect acts like an internalpressure which shifts the spin crossover. This effect is confirmedby studies under hydrostatic pressure as already shown.[20] Inpolyelectrolytes, not only the electrostatic interactions actingoutside the first coordination sphere, but also steric andmechanical effects due to the polymer chains are efficient inlimiting the spin transition, which is partially blocked over a widetemperature range.

In conclusion of this study on ferric complexes with anhexadendate ligand, a description of the spin crossover has beenestablished including both static and dynamic magnetic re-sponses. This description could be extended to other similar FeIII-type compounds.[10] The method relies the use of low-spin ferricspecies as an intrinsic spin label for its linewidth to test themagnetic environment. A unique relationship is found as long asa statistical distribution of spin states is present in theinvestigated compounds. Furthermore, a variation of counter-ions shows that the use of lamellar host lattices and in particularpolyelectrolytes can modify the spin crossover behavior. Byvarying both the electrostatic and thermomechanical properties,the occurrence of the thermal spin conversion is modified. Thenext step will be to control thermal spin conversion particularlyin the case of sharp spin transitions, where a different behaviorshould be observed that is related to the presence of domainsrather than a statistical distribution of the two kinds of ferric spin.Finally, it should be mentioned that these two ways ofprocessing, using polymers and the layer-by-layer technique,could be a valuable approach to understand the surroundingsand dilution effects in thin films for future applications in thisfield.

Experimental Section

Synthesis : Among this series of hexadendate ligands ((X-sal)2trien)2�

derived from salicylaldehyde and triethylenetetramine, the initialligand H2((3,5-Cl2-sal)2trien) (1) has been selected and synthesizedfollowing the procedure already reported for various substitutedsalicyladehydes[7] [Fe((3,5-Cl2-sal)2trien)]PF6, [Fe((3,5-Cl2-sal)2trien)]-NO3 and [Fe((3,5-Cl2-sal)2trien)]ClO4 have been prepared followingstandard procedures.[10]

[Fe((3,5-Cl2-sal)2trien)]0.22Cd0.89PS3 was obtained by ion exchangebetween (NMe4)2xCd1�xPS3 and the hexafluorate salt.[11] Completeintercalation is confirmed by IR spectroscopy, X-ray powder diffrac-tion, and elemental analysis which have confirmed the givenstoichiometries.[10]

[Fe((3,5-Cl2-sal)2trien)]PSS and [Fe((3,5-Cl2-sal)2trien)]PSSMA wereformed by metathesis from [Fe((3,5-Cl2-sal)2trien)]NO3 salts and thepoly(sodium 4-styrenesulfonate) (PSS; Aldrich, Mw� 70000) andpoly(styrenesulfonic acid-co-maleic acid) sodium salt (PSSMA; Al-drich, Mw� 20000), respectively. Aqueous solutions of above poly-electrolytes were added to the equimolar solution of [Fe((3,5-Cl2-sal)2trien)]NO3 prepared in water/methanol (1/1 v/v) under stirring.Resulting black precipitates were filtered off, washed with purewater, and dried in vacuo before use. The adopted stoichiometry is1:1, that is one molecular complex per repeat unit of the polymerinvolved is achieved, which clearly defines the molecular mass formagnetic measurements.

Thin films of [Fe((3,5-Cl2-sal)2trien)] species together with PSS andpoly(allylamine hydrochloride) (PAH; Aldrich, Mw� 15000) by LbLdeposition.[11] As usual, quartz substrates were used for thedeposition of these films. Polyelectrolyte solutions were preparedin deionized water, whereas [Fe((3,5-Cl2-sal)2trien)]� species weredeposited from a solution of [Fe((3,5-Cl2-sal)2trien)]NO3 in water/methanol (1/1). The substrates were first coated with single layer ofpoly(ethylenimine hydrochloride) (PEI ; Aldrich, Mw�25000) bydipping them in a 10�3 M aqueous solution of PEI for 30 minfollowed by rinsing with pure deionized water. The PSS/[Fe((3,5-Cl2-sal)2trien)]/PAH LbL assembly was obtained by repeated sequentialdipping of the above PEI-coated substrate in a 1 mM aqueoussolution of PSS, then a 30 mM solution of [Fe((3,5-Cl2-sal)2trien)]NO3 in

Figure 4. [nHS(T)CHS�nLS(T)CLS] versus the square of the linewidth �H2 of the main resonance line observed at g� 2.17 for [FeIII((3,5-Cl2-sal)2trien)]Y complexes, where Yis either PF6

� (�), ClO4� (�), PSS (�), PSSMA (�), or CdPS3(�).

CHEMPHYSCHEM 2002, No. 12 ¹ 2002 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 1439-4235/02/03/12 $ 20.00+.50/0 1049

water/methanol (1/1) and finally a 1 mM aqueous PAH solution. Thedipping time was 25 min for each solution to achieve completeadsorption. After each deposition step the substrate was carefullyrinsed twice with pure deionized water.

Physical Techniques : The magnetic susceptibility was measuredbetween 2 and 350 K with a SQUID magnetometer (MPMS-5,Quantum Design) in a magnetic field of 5 kG. EPR spectra (ESP300E, Bruker) were recorded at 10 GHz (X-band) equipped with aliquid 4He temperature accessory (Oxford). All EPR experiments werecarried out increasing the temperature from 5 K to room temper-ature.

The authors thank for their help Dr. E. Rivie¡re for several magneticexperiments with a SQUID magnetometer and Dr. G. Sigaud forcalorimetric measurements. They acknowledge the financial sup-port of the European Union in the frame of the TMR network™Thermal and Optical Switching of Spin States∫ (TOSS ERB-FMRX-CT98-0199).

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Received: December 17, 2001 [Z348]Revised: June 6, 2002