Effect of metal dilution on the light-induced spin transition in[FexZn1−x(phen)2(NCS)2] (phen = 1,10-phenanthroline)
Cherif Balde,a Cedric Desplanches,a Alain Wattiaux,a Philippe Guionneau,a Philipp Gutlichb andJean-Francois Letard*a
Received 7th January 2008, Accepted 19th March 2008First published as an Advance Article on the web 18th April 2008DOI: 10.1039/b800248g
The thermal and light-induced spin transitions in [FexZn1−x(phen)2(NCS)2] (phen =1,10-phenantholine) have been investigated by magnetic susceptibility, photomagnetism and diffusereflectivity measurements. These complexes display a thermal spin transition and undergo thelight-induced excited spin state trapping (LIESST) effect at low temperatures. For each compound, thethermal spin transition temperature, T 1/2, and the relaxation temperature of the photo-inducedhigh-spin state, T(LIESST), have been systematically determined. It appears that T 1/2 decreases withthe metal dilution while T(LIESST) remains unchanged. This behaviour is discussed on the basis of thekinetic study governing the photo-induced back conversion.
Introduction
The spin crossover (SCO) phenomenon encountered in some 3d4–3d7 transition metal compounds has been the subject of manystudies during the last four decades,1 and particularly since thediscovery of the light-induced excited spin state trapping (LIESST)effect.2 Many studies have been dedicated, for instance, to theinfluence of the metal dilution in mixed [FexM1−x] complexes withvarious metals M, on the SCO properties.3–5 It is now admittedthat the matrix plays a major role through a variation of aninternal or chemical pressure.3–5 For instance, Hauser et al.3
have reported that the introduction of foreign metal ions intoa lattice may directly affect, and in some cases stabilize, thephoto-induced metastable HS state. This finding stimulates a truechallenge towards applications in photonic devices; i.e. to identifythe parameters affecting the lifetime of the photo-induced excitedstate with the ultimate goal to operate at room temperature.2
Since 1998, we have created a database of T(LIESST) tem-perature, which represents the limiting value above which thelight-induced magnetic high spin (HS) information is erasedunder specific conditions.6 Using this procedure, we have nowcompared the photomagnetic properties of more than sixty SCOmaterials7,8 and reported an empirical linear relation between thethermal spin transition (T 1/2) and the photo-induced HS state(T(LIESST)) temperatures: i.e. T(LIESST) = T 0 − 0.3T 1/2. Thisrelation has been observed to remain valid for a given family,whatever the change outside the inner coordination sphere, thesalt, the hydration degree and the cooperativity.8
In the present work, we have studied the influence of metaldilution on the T(LIESST) value, as T 1/2 is expected to change. Forthat we have selected one of the most studied iron(II) spin crossovercomplexes, the [Fe(phen)2(NCS)2] (phen = 1,10-phenanthroline),
aICMCB, CNRS, Universite Bordeaux, 87 Av. Doc. A. Schweitzer, F-33608,Pessac, France. E-mail: [email protected] fur Anorganische Chemie und Analytische Chemie, Johannes-Gutenberg Universitat Mainz, Staudingerweg 9, D-55099, Mainz, Germany.E-mail: [email protected]
which undergoes an extremely abrupt thermal spin transitionat 176 K9 with a narrow thermal hysteresis of <1 K.10 Thethermal spin crossover properties of the iron(II) mixed complexes[FexM1−x(phen)2(NCS)2] with M = Co(II), Ni(II), Mn(II) and Zn(II)ions have been originally described by Gutlich and co-workers.4
Moreover, it was earlier reported that the [Fe(phen)2(NCS)2]shows the LIESST phenomenon.11 Here, we will re-investigatethe mixed [FexZn1−x(phen)2(NCS)2] system (with x = 1, 0.73,0.5, 0.32, 0.19 and 0.04), and study the influence on the light-induced HS properties by using a SQUID magnetometer coupledto continuous wave (CW) laser irradiation. The zinc(II) metal hostis selected for (i) the diamagnetic response, (ii) the absence of metalto ligand charge transfer (MLCT) transition in the visible range,due to its d10 electronic contribution, which induced a whitening ofthe sample as the metal dilution increased, and (iii) the close ionicradius between the iron(II) HS state and the zinc(II) ion, whichshould decrease T 1/2 temperature with the metal dilution.4
Results
The [FexZn1−x(phen)2(NCS)2] (with x = 1, 0.73, 0.5, 0.32, 0.19and 0.04) complexes have been prepared along the proceduredescribed by Gutlich and co-workers,4 but slightly modified. Asa matter of fact, depending on the preparation of the sample,the thermal spin transition can be more or less gradual, withvarying HS residue at low temperature.9 In order to obtainan abrupt thermal spin transition with minimal HS residueat low temperature, [Fe(phen)2(NCS)2] can be extracted from[Fe(phen)3](NCS)2 with dry acetone.12 Alternatively, one cansynthesize the [Fe(phen)2(NCS)2] starting from FeSO4, KSCN and1,10-phenanthroline and further purify it by Soxhlet techniques(see Experimental section). In the following discussion the iron(II)fractions x were calculated from the iron and zinc concentrationdetermined by elemental analysis. Fig. 1(a) shows the discrepan-cies between the found and the calculated values. Fig. 1(b) showsroom-temperature powder diffractograms in the range 10–50◦ (2h)of pure Fe (x = 1) and pure Zn (x = 0) compounds. The presence
Fig. 1 (a) Iron fraction x obtained from elemental analysis vs. ironfraction calculated from the iron/zinc ratio used for the synthesis. Thefull line corresponds to the hypothetical situation where x(found) =x(calc.). (b) Room-temperature X-ray powder diffraction patterns of[FexZn1−x(phen)2(NCS)2] (x = 1 and x = 0)
of well-defined peaks proves that these samples consist of well-crystallized phases. Moreover, all the (hkl) reflections positionsand relative intensities match for the two compounds. The Fe andZn compounds are thus isomorphous, justifying the character ofsolid solution of [FexZn1−x(phen)2(NCS)2].
The temperature dependence of the magnetic susceptibility forthe mixed crystal system [FexZn1−x(phen)2(NCS)2] (with x = 1,0.73, 0.5, 0.32, 0.19 and 0.04) is shown in Fig. 2(a). The magnetic
Table 1 Mossbauer data of the metal diluted [FexZn1−x(phen)2(NCS)2]systems at 4.2 K with DEQ the quadrupole splitting and d the isomershift (relative to metallic iron). The iron concentration in the non-enrichedsample of [Fe0.04Zn0.96(phen)2(NCS)2] was too low to give a sufficientlygood signal to noise ratio
response was expressed in the form of the vMT vs. T curves,where vM stands for the molar magnetic susceptibility and T thetemperature. Based on these data and considering that the zinc(II)metal ions has diamagnetic response in regard to the filled 3d10
level, (t2g)6(eg)4, the iron(II) HS molar fraction, c HS, may be directlydeduced from the vMT product through c HS = vMT/(vMT)HT where(vMT)HT is the high-temperature limit, typically at 290 K. Fig. 2(b)reports the deduced evolution of c HS as a function of temperature.As a test, the level of the HS fraction deduced at low temperaturewas also determined by Mossbauer spectroscopy at 4.2 K (Table 1),and both techniques deliver comparative data. The informationgiven by Fig. 2(b) is in fact close to the original studies of Gutlichet al.;4 i.e. when x decreases (i) the spin crossover regime becomesmore gradual, (ii) the thermal spin transition is lowered, and (iii)the residual HS fraction at low temperature increases. All that canbe explained in the following way:
(i) The increase of the gradual character with the metal dilutionreflects the progressive loss of cooperativity, as expected from theincreasing distance between active iron(II) metal centers in thediluted [Zn] lattice.1b
(ii) The shift of the thermal spin transition towards lowertemperature can be understood through change of internalpressure. The radius of zinc(II) ion (r = 74 pm) is close to theionic radius of iron(II) HS ion (r = 78 pm) and higher than theionic radius of iron(II) LS ion (61 pm).13 Thus, if iron(II) LS ions arehighly diluted in a [Zn] lattice, the zinc lattice induces a ‘negative’pressure on the Fe(II) site leading to an increase of the Fe–N bondlengths (decrease of the crystal field potential) which favours theHS state.1b,14 On the contrary, the effect of zinc lattice is negligible
Fig. 2 Evolution as function of x of the vMT signal (a) and of the HS fraction (b) vs. T curves.
when the iron(II) HS ions are highly diluted. Consequently, theT 1/2 is shifted towards lower temperature with increasing metaldilution.
(iii) The increase of the residual HS fraction at low temperatureis also a consequence of the negative pressure occurring on theiron(II) LS ions in the highly diluted [Zn] lattice. The HS state isin fact stabilized by the [Zn] lattice.
The thermal spin-crossover regime can also be monitored byfollowing the visible spectrum of the sample as a function oftemperature, measured by diffuse absorption. Fig. 3(a) illustratesthat with the metal diluted x = 0.5 compound. The broad diffuseabsorption band at 800–850 nm corresponds to a d–d transitionof the HS iron(II) centre, while the absorption in the 500–650 nmregion can be assigned to both d–d and MLCT (metal to ligandcharge transfer) transitions of the LS iron metal centre. For theZn(II) metal ion, the d–d and MLCT transitions are nonexistentin the visible range. Consequently, any change of the signal can bedirectly used to monitor the thermal SCO transition at the surfaceof the sample. The increase of the diffuse absorption in the 600–700 nm range from room temperature down to 100 K correspondsto the increasing population of the LS state following thermalSCO.
Fig. 3 (a) Changes in the diffuse absorption spectra of the mixed[Fe0.5Zn0.5(phen)2(NCS)2] system upon cooling and upon light irradiation.(b) Evolution of the diffuse reflectivity signal recorded at 647 nm underthe same conditions as function of x.
The same reflectivity experiment can also monitor any light-induced phenomena occurring at the surface of the sample. Whenthe temperature is sufficiently low that relaxation of the photo-
induced high-spin state is slow, the light intensity at the surfaceof the sample can be used to tune the spin-state of the complex.15
In this sense, the diffuse absorption band at 647 nm (Fig. 3(a))which increases along the thermal spin transition and reverselydecreases at lowest temperature, from 100 to 10 K (Fig. 3(a)), is aclear indication that the LIESST phenomenon occurs.
The low→high spin photoconversion can also be investigated inbulk condition using a SQUID magnetometer connected to a CWoptical source. For all complexes, a drastic increase of the magneticsignal under light irradiation was observed at 10 K. Fig. 4 showsthe typical T(LIESST) curve recorded for two samples, x = 0 and0.5. In this procedure, the irradiation is maintained until the signalis saturated, then the light is switched off and the temperatureslightly increased at 0.3 K min−1. The minimum of the dvMT/dTvs. T curve defines the limiting temperature T(LIESST), abovewhich the light-induced magnetic high-spin information is erasedin a SQUID cavity.6–8
Fig. 4 Magnetic and photomagnetic properties recorded of apolycrystalline sample of pure [Fe(phen)2(NCS)2] (a) and of the[Fe0.5Zn0.5(phen)2(NCS)2] system (b). � = data recorded in the cooling andwarming mode without irradiation; � = data recorded with irradiation at10 K; � = T(LIESST) measurement, data recorded in the warming modeat a rate of 0.3 K min−1 with the laser turned off after irradiation for 1 h.The solid line through the T(LIESST) measurement shows the fit generatedfrom the deduced experimental thermodynamic parameters (Ea, k∞ andEa*) and with a zero-field splitting value of 15 cm−1 and a g factor of 2.18.
The shapes of all the T(LIESST) curves are almost identical.The vMT product first increases upon warming from 10 K dueto the zero-field splitting of the high-spin iron(II) ion and reachesa plateau near 30 K. The comparison of the vMT value at themaximum of the T(LIESST) curve and the magnetic response
Table 2 Magnetic and photomagnetic properties of the mixed [FexZn1−x(phen)2(NCS)2] system. T 1/2 is the temperature at which the sample contains50% of LS and HS molecules after thermal spin transition. T(LIESST) is the temperature at which the light-induced HS information was erased duringwarming from 10 K at a rate of 0.3 K min−1. For x = 1 and x = 0.5, kinetic parameters as defined in eqn (2) and (3) are reported
x T 1/2/K T(LIESST)/K k0/s−1 k∞/s−1 Ea/cm−1 Ea*/cm−1
recorded at room temperature confirms that even in bulk conditionfor all the [FexZn1−x(phen)2(NCS)2] complexes a quantitativephotoconversion occurs. The different T(LIESST) values arecollected in Table 2. Interestingly, while the T 1/2 temperature hasbeen found to decrease with the metal dilution, the T(LIESST)temperature remains almost constant. In other words, for thefirst time we found an exception to the empirical T(LIESST) =T 0 − 0.3T 1/2 relation7,8 (Fig. 5). It seems that the sensitivity ofthe thermal spin transition on changing the internal pressure bymetal dilution is higher than on T(LIESST) properties. The sameconclusion is obtained if we compare the change of the diffusereflectivity signal at 647 nm as a function of the metal dilution(Fig. 3(b)). The thermal spin transition is clearly shifted towardslower temperature while the light-induced properties as functionof the metal dilution remain unchanged.
Fig. 5 Evolution of the T(LIESST) vs T 1/2. The T 0 lines (100, 120 and150 K) refer to previous works.7,8
The fact that the T(LIESST) temperature as function ofthe metal dilution remains almost constant suggests that thekinetics are also certainly similar. We have compared the re-laxation processes of the pure [Fe(phen)2(NCS)2] and of themixed [Fe0.5Zn0.5(phen)2(NCS)2] system at different temperaturesbetween 10 K and the highest temperatures accessible with ourSQUID set up, which are close to the T(LIESST) values.
Fig. 6 displays the kinetics recorded using the SQUID mag-netometer in the 30–64 K temperature range, where the HS–LSstate relaxation is thermally activated. The strong deviation ofthese relaxation curves from a single exponential is striking, andthey can be modelled using a sigmoidal law, consistent with theself-accelerated behaviour predicted for cooperative systems. Thiscooperativity arises from the large difference in metal–ligand bondlengths between high-spin and low-spin states, resulting in elastic
Fig. 6 Time dependence of the pure [Fe(phen)2(NCS)2] (a) and diluted[Fe0.5Zn0.5(phen)2(NCS)2] complexes at various temperatures for the highspin molar fraction generated by light irradiation in the temperature range30–63 K. Solid lines are sigmoidal least-squares fits (Table 2).
interactions caused by the change in internal pressure as the spintransition proceeds.16 Thus, the height of the activation barrier toLIESST relaxation changes as a function of c HS (the molar fractionof HS molecules at a given temperature), and the relaxation ratekHL*(T , c HS) depends exponentially on both c HS and T (eqn (1)and (2)), where a(T) (= Ea*/kBT) is the acceleration factor at agiven temperature.
∂c HS/∂T = −kHL*c HS (1)
kHL*(T , c HS) = kHL(T)exp[a(T)(1 − c HS)] (2)
kHL(T) = k0 + k∞exp[−EA/kBT ] (3)
Eqn (3) supposes that the evolution of the HS fraction is thesummation of two processes:17 a tunnelling process,16 independent
of temperature, and a thermally activated process. Least squaresfits of the data for x = 0 and x = 0.5 were performed usinga sigmoidal model where kHL(T) and a(T) were refined as freeparameters. The calculated curves are shown as solid lines in Fig. 6and the kinetic parameters k0, k∞, Ea and Ea* are reported intable 2.
An elegant way to test the validity of the kinetic parametersis to reproduce the experimental T(LIESST) curve.8,17 For thatit is necessary to carefully take into account the time and thetemperature dependencies. The main difficulty of this simulationis to estimate satisfactorily the rate constant k0 for relaxationin the quantum mechanical tunnelling region. For this, weconsider that the complete kinetic measurement recorded at thelowest temperature can be regarded as an upper limit for the k0
value (Table 2). Fig. 4 shows the calculated T(LIESST) curvededuced from the kinetics parameters listed in Table 2. Theperfect agreement between the experimental and the simulatedT(LIESST) curves (see Fig. 4) provides some confidence aboutthe kinetics parameters determined in this study which are slightlydifferent from those previously reported by Lee et al.18 using theL-edge absorption technique.
Concluding remarks
We have reported the thermal and photochemical spin-crossoverproperties of [FexZn1−x(phen)2(NCS)2] systems. Although thethermal spin transition temperatures are lowered with increasingmetal dilution, the T(LIESST) temperatures are almost constant.This surprisingly leads to a deviation of the T(LIESST) =T 0 − 0.3T 1/2 law observed previously in many different iron(II)families.7,8 In other words, the thermal spin transition appears tobe sensitive to the change of internal pressure in the lattice whilethe T(LIESST) limit temperature remains unaffected, at least inthe range of internal pressures generated by metal dilution in thisstudy.
Previous studies on the thermal spin crossover phenomenonhave shown that any change of pressure in the lattice by chemicalmetal dilution or by external stimulus affects the Gibbs free energythrough a change of the molecular volume of reaction givingrise to the additional term pDV HL.3 The shift of T 1/2 towardslower temperature with increasing zinc metal dilution is a directconsequence of the negative pressure generated in the Zn host.Concerning the LIESST properties, Hauser3 has evidenced anenhancement of the tunnelling rate by changing the size of thehost ion (zinc, cadmium, ...) due to a vertical displacement of theHS and LS potential wells relative to each other. More precisely, inthe case of zinc dilution (negative pressure effect), the HS potentialis well shifted to lower energies relative to the LS state, boththe barrier height and thickness for the tunnelling process areincreased and the relaxation rate is reduced. As a consequenceof that one should expect an increase of the T(LIESST) value,in agreement with the decrease of T 1/2, according to the empiricalT(LIESST) = T 0 − 0.3T 1/2 relation.7,8 But this is not the case, fromthe present work the T(LIESST) limit temperature experimentallyremains constant with increasing metal dilution. Similar resultsare also found for metal dilution with manganese(II) and nickelions in [FexM1−x(bpp)2](NCSe)2 (with bpp = 2,6-bis(pyrazol-3-yl)pyridine) complexes and will be reported in a forthcomingpaper.19
In fact, the T(LIESST) temperature is defined by the thermallyactivated regime, i.e. a tunnelling from thermally populated vibra-tional levels of the HS state.8 The study of the kinetics parameterson [FexZn1−x(phen)2(NCS)2] systems has shown that when x variesfrom 1 to 0.5, the cooperativity factor Ea* decreases from 90 to45 cm−1. As cooperativity arises from elastic interactions betweeniron centres, it is not surprising that the strength of cooperativitydecreases with increasing dilution and thus increasing distancebetween the interacting iron centres. The changes in k∞ and Ea areless drastic: Ea is increased by only 7% while k∞ is almost multipliedby 4, an apparently large change. According to eqn (3), an increaseof k∞ will lead to a decrease of T(LIESST), whereas an increaseof Ea will lead to an increase of T(LIESST). As conclusion, itseems that the metal dilution through the increase of k∞ and Ea
parameters causes a kind of compensation leading practically toa constant T(LIESST) values.
The main result of the present study, viz. metal dilution decreasesT 1/2 but leaves the T(LIESST) temperature practically unchangedappears to contradict the inverse energy gap law introduced byHauser:3,14 the lower the thermal spin transition temperature(T 1/2), the slower is the HS → LS relaxation after LIESST. Thisexperience eventually instigates to attempt to prepare metal-diluted spin crossover material with both T 1/2 and T(LIESST)occurring in the room temperature region, an exciting challengetowards applications in devices.
Experimental
Materials
The preparation of [FexZn1−x(phen)2(NCS)2] has been describedearlier by Gutlich and co-workers.4 The synthesis of the neat ironcompound [Fe(phen)2(NCS)2] was done by reacting 0.77 g (2.77mmol) of FeSO4·7H2O and 0.539 g (5.55 mmol) of KSCN in dryand freshly distilled methanol (6 ml). The precipitated K2SO4 wasfiltered off. The solution was added to 1 g (5.55 mmol) of 1,10-phenanthroline in 3 ml of methanol. Red powder formed from thesolution. The precipitate was filtered off, washed with methanoland diethyl ether and dried under vacuum. The compound isfurther purified by the Soxhlet technique during 48 h. The dilutedcompounds [FexZn1−x] were synthesized according to the sameprocedure, replacing the metal salt by mixtures of iron(II) sulfateand zinc(II) sulfate in given ratio. The iron fraction (x) values werecalculated from the iron and zinc atomic percentage determinedby quantitative analysis.
Magnetism and photomagnetism
The photomagnetic measurements were performed using a Spec-trum Physics Series 2025 Kr+ laser (k = 532 nm) coupled viaan optical fibre to the cavity of a MPMS-55 Quantum DesignSQUID magnetometer. The optical power at the sample surfacewas adjusted to 5 mW cm−2, and it was verified that this resultedin no change in magnetic response due to heating of the sample.Photomagnetic samples consisted of a thin layer of compoundwhose weight was obtained by comparison of the thermal spincrossover curve with that of a more accurately weighed sample ofthe same material. Our previously published standardized methodfor obtaining LIESST data was followed.6–8 After slowly coolingto 10 K the sample, now in the low spin state, was irradiated and
the change in magnetism followed. When the saturation point wasreached the laser was switched off and the temperature increased ata rate of 0.3 K min−1. The magnetization was measured every 1 K.T(LIESST) was determined from the minimum of a dvMT/dT vs.T plot for the relaxation process.
Other measurements
Mossbauer measurements were performed using a constant accel-eration HALDER-type spectrometer with a room temperature57Co/Rh source in transmission geometry. The polycrystallineabsorbers containing about 10 mg cm−2 of iron were used to avoidexperimental broadening of the peaks. The spectra were recordedat 293 and 4.2 K using a variable temperature cryostat. The velocitywas calibrated using pure iron metal as the standard material.The refinement of Mossbauer spectra has been done assuming adistribution of hyperfine fields.
Powder X-ray diffraction data were recorded using a PANalyt-ical X′Pert MPD diffractometer with Bragg–Brentano geometry,Cu Ka radiation and a backscattering graphite monochromator.
The measurement of the diffuse absorption spectra and reflectiv-ity signal were performed by using a custom-built set-up equippedwith a SM240 spectrometer (Opton Laser International). Thisequipment allows to record both the diffuse absorption spectrawithin the range of 500–900 nm at a given temperature and thetemperature dependence (5–290 K) of the reflectivity signal at aselected wavelength (±2.5 nm). The diffuse reflectance spectrumwas calibrated with respect to charcoal activated (Merck) as blackstandard and barium sulfate (BaSO4, Din 5033, Merck) as whitestandard.
Acknowledgements
The authors would like to thank the Alliance bilateralDFG/CNRS action, the ANR Fast-switch (NT05-3_45333) andthe Aquitaine Region for supporting the platform of photomag-netism.
References
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