Journal of Nuclear and Radiochemical Sciences, Vol. 3, No. 2, pp. A1-A9, 2002

Accounts
Thermal and Optical Switching of Iron(III) Complexes
Shinya Hayami,*, a Osamu Sato,b Katsuya Inoue,c Yasuaki Einaga,d and Yonezo Maedaa
aDepartment of Chemistry, Faculty of Sciences, Kyushu University, 6-10-1 Hakozaki, Higashi-ku, Fukuoka
812-8581, Japanb
Kanagawa Academy of Science and Technology, KSP Bldg. East 412, 3-2-1 Sakado, Takatsu-ku, Kawasaki
213-0012, Japanc
Institute for Molecular Science, Myodaiji, Okazaki 444-8585, Japand
Department of Chemistry, Faculty of Science and Technology, Keio University, 3-14-1 Hiyoshi, Yokohama
223-8522, Japan
Received: October 18, 2002; In Final Form: December 3, 2002
Binuclear iron(III) spin-crossover complexes, [Fe 2(salten)2(az)](BPh4)2 (1) and [Fe2(salten)2(cc)](BPh4)2 (2)(H2salten = 2,2'-[iminobis(3,1-propanediylnitrilomethylidyne)]bis-phenol, az = 4,4'-azobis-pyridine, and cc = 4,4'(1,2-ethenediyl)bis-pyridine), and mononuclear iron(III) spin-crossover complexes, [Fe(pap) 2]ClO4 · H2O (3)(Hpap = 2-(2-pyridylmethyleneamino)phenol) and [Fe(qsal) 2 ]NCSe ⋅ CH 2 Cl 2 (4) (Hqsal = 2-[(8quinolinylimino)methyl]-phenol), were synthesized and characterized by single-crystal X-ray diffraction,
Mössbauer spectra, magnetic susceptibilities, and electronic spectra. The structures of 1 and 2 were determined in
the low-spin state, at 100 K and the high-spin states at 298 K. The complexes 1 and 2 exhibited the spin-crossover
behavior; gradual and rapid spin interconversion was observed by means of Mössbauer spectroscopy at 293 K.
The structure of 3 in the high-spin state was determined at 293 K. The complex 3 exhibited the abrupt spin transition with thermal hysteresis (T1/2↑ = 180 K and T1/2↓ = 165 K). The time dependence of the magnetism and thefrozen-in effect were observed for the complex 3. Furthermore, light-induced excited spin state trapping effectwas observed for the first time for iron(III) complexes. The structure of 4 in the low-spin state was determined at
200 K. The complex 4 exhibited a wide thermal hysteresis loop of 180 K (T1/2↑ = 392 K and T1/2↓ = 212 K) in thefirst cycle, although the hysteresis loop observed for the first cycle is apparent ones. Following the first loop, it
shows a two-step spin transition in warming mode (T1/2(S1)↑ = 215 K and T1/2(S2)↑ = 282 K) and a one-step spintransition in cooling mode (T1/2↓ = 212 K).

Introduction
The first transition metal complexes with d4, d5, d6, and d7
electron configurations are usually classified into two categories according to the strength of ligand field against themean spin-pairing energy, i.e., high-spin (HS) and low-spin(LS) complexes. Iron(III) ions are transition metal ions withfive d electrons. When an iron(III) is coordinated by six donor
atoms of ligands, the complex forms an octahedral conformation. The octahedral ligand field splits according to the
Tanabe-Sugano diagram. 1 If the ligand field strength iscomparable to the mean spin-pairing energy, the complexes
exhibit a spin-crossover (SC) behavior between HS (6A1) andLS (2T2) states by external perturbations (temperature, pressure, or light). Since Cambi first observed the SC phenomenon for tris (dithiocarbamato) iron(III) complexes as the spinisomers in 1931,2 the phenomenon has been found very often
in complexes of iron(II),3 iron(III),4 and cobalt(II).5 The spintransition curves of the SC behaviors exist in the various forms
in solids because of long-range cooperative interactions. In
particular, a number of spin-crossover iron(II) and iron(III)complexes have been studied because of the validity of
Mössbauer spectroscopy.3, 4, 6
Recently, there has been a great interest in developing novel
molecular compounds whose physical properties can becontrolled by illumination. 7−12 In 1984, Decurtins et al.
observed a light-induced LS → HS transition, during which
the molecules can be quantitatively trapped in the excited HS*Corresponding author. E-mail: hayascc@mbox.nc.kyushu-u.ac.jp.
FAX: +81-92-642-2607.

state at sufficiently low temperatures (T < 50 K). 13 The
phenomenon is called light-induced excited spin state trapping(LIESST), the existence of the light-induced HS state for theiron(II) SC complexes was verified with optical, Mössbauer,and magnetic susceptibility measurements.14−21 The discoveryof this LIESST effect suggested that SC complexes could be
used as optical switches. In the meantime a number of iron(II)complexes with light-induced long-live metastable HS states at
low temperatures have been found. The difference in metalligand bond length ∆rºHL and the zero-point energy difference∆EºHL between the two spin states for iron(III) SC complexes
are smaller than those of iron(II) complexes, and therefore thelifetime of the light-induced HS state in the iron(III) SC
complexes is very short even at 5 K.18
Furthermore, ligand-driven light-induced spin changes (LDLISC) in the iron(II) SC complexes have been reported byZarembowitch et al.22 The spin transition of LD-LISC is classified in a new category, which consists in varying the ligand
field strength through photochemical cis-trans isomerization of
the ligand. The phenomenon has been observed by using 4styrylpyridine derivatives at relatively high temperature as 140
K.
However, the number of the optically switchable molecularsolids reported until now has been quite small. This is a reason
why the strategy to achieve photo-induced switching in a solidstate system has not been clarified yet. One of the requisites of
optically switchable compounds is that they have nearly
degenerate electronic states. Furthermore, it is known that thestructural change accompanied with the switching phenomena
should not be too large, because steric effects often prevent thephotochemical transformation. For example, the azobenzene© 2002 The Japan Society of Nuclear and Radiochemical Sciences
Published on Web 12/28/2002

A2

HayamiJ. Nucl. Radiochem. Sci., Vol. 3, No. 2, 2002

relaxation from a metastable state to a ground state should be
developed. We propose the introduction of strong intermolecular interactions in molecular compounds. It is thought thatthe cooperativity resulting from the intermolecular interaction
operates to increase the activation energy for the relaxation
processes, enabling the observation of a long-lived metastablestate produced by illumination as is well-known for iron(II)spin-crossover system such as [Fe(ptz)6](BF4)2.16, 17 That is,when the metastable molecular orientation reverts to the stableone, a large stress field might be built up in the compounds
because of the presence of a strong binding energy between
molecules. Hence, even if the structural change is small, therelaxation through tunneling will be substantially prevented.
In this paper, we describe our recent approaches for thermaland optical switching iron(III) complexes. We have preparediron(III) SC complexes of which the structural change between
HS and LS states is small and reported about binuclear
iron(III) SC complexes 1 and 2 with photoisomerization
ligands (Chart 1) and the mononuclear iron(III) complexes 3
and 4 (Chart 2) exhibiting LIESST effect and large thermal
hysteresis.

derivatives, which are representative photochromic molecules,
do not undergo trans-cis photo-isomerization in solid state dueto the steric hindrance. This means that compounds withnearly degenerate electronic states and with only small structural changes between the metastable state and the stable state
are appropriate as the materials of optical switching. However,even if these requirements are satisfied, most of thecompounds never show photo-induced switching behavior
with long-lived metastable states. Although it is possible toproduce a metastable state using such compounds by illumination, the metastable state rapidly relaxes back to the stable statethrough tunneling effects due to the small structural change.
Some examples were observed in iron(III) SC complexes.23−26
The excitation of a ligand-to-metal charge transfer (LMCT)
band induces the spin transition from LS to HS states.
However, the photo-induced iron(III) HS states rapidly revert
to the original LS states through tunneling even at 5 K,
because the relaxation probability from the metastable to thestable states by tunneling increases abruptly with decrease instructural change. Hence, in order to develop a variety of optically switchable molecular solids, strategies to prevent rapid

Results and Discussion

N
H

N

Iron(III) Spin-Crossover Compounds with PhotoIsomerization Ligands. The crystal structures of 1 and 2 were
determined by X-ray diffraction at both 298 and 100 K.27, 28
The space groups (P21 / c for 1 and P21 / a for 2) are retained
for both of the HS and LS states (Table 1). Each iron atom issurrounded by a nitrogen atom of pyridine and by two oxygen
atoms and three nitrogen atoms of a pentadentate salten ligand.

N

OH

HO

H2 salten

N
N

N

N

N

N

az

N

N

N

OH

OH

N
cc

Hpap

Chart 1. Structure of the ligands for H2salten, az, and cc. H2salten ispentadentate, and az and cc are bidentate ligands.

Hqsal

Chart 2. Structure of the ligands for Hpap and Hqsal. Hpap and
Hqsal are tridentate ligands.

TABLE 1: Crystallographic Data for 1 – 4Complex
Empirical formula
Formula weight
Crystal dimension / mm
Crystal system
Space group
a/Å
b/Å
c/Å
α/º
β/º
γ/º
V / Å3Z
Dcalcd / gcm−3µ(MoKα) / cm−1
R1

1

2

3

C98H94O4N10B2Fe2
1609.20
0.20 × 0.30 × 0.10
monoclinic
P21/c (#14)
16.422(7)16.005(6)16.565(8)C100H96O4N8B2Fe2
1607.220.20 × 0.20 × 0.10
monoclinic
P21/a (#14)
16.554(8)15.877(3)16.823(3)108.47(3)109.20(2)4129(2)
2
1.294
4.11
0.075

4175(1)
2
1.278
4.06
0.051

C24H20O7N4Cl1Fe1
567.740.30 × 0.30 ×0.03
triclinicP1 (#2)
10.947(6)11.741(6)10.054(5)107.12(3)91.96(1)80.51(3)1217(1)
2
1.548
7.81
0.075

4
C34H24O2N5Se1Cl2Fe1
740.310.40 × 0.40 × 0.03
triclinicP1 (#2)
11.3501(2)13.9252(5)10.127(1)98.327(6)107.055(4)91.896(2)1509.3(2)
2
1.629
19.24
0.047

Thermal and Optical Switching of Iron(III) Complexes

The bond lengths and angles of the compounds corresponding
to the spin equilibrium state at 298 K and those of the LS state
at 100 K are listed in Table 2. All of the cell parameters, a, b,
and c, for 1 and 2 decrease with increasing in temperature andthe volumes of the unit cell at 298 K decrease 4.15% and2.66% than those at 100 K for 1 and 2, respectively.
The ORTEP views of the complexes 1 and 2 at 100 K areshown in Figures 1 and 2, respectively. The binuclear iron(III)complex 1 in which trans-form azobis(4-pyridine) (az) acts asa bridging ligand is centro-symmetric; the diazo moiety sits on
the center of symmetry. The moiety of diazo (N=N) of thecation of 1 sits on a center of symmetry and two nitrogenatoms of the diazo moiety (N(5) and N’(5)) are in a transgeometry. The iron atom of the binuclear complex is in a
pseudo-octahedral coordination with a trans geometry for thetwo salicylideneiminato moieties (N(1) and N(3)), in which thebasal plane are composed of the N2 O 2 donors of these twomoieties and the two axial positions are occupied by thenitrogen atom of the 3,3’-diaminodipropylamine moiety andby the nitrogen atom of az. The basal plane defined by Fe,
O(1), O(2), N(1), and N(3) has a slight tetrahedral distortion:
two oxygen atoms (O(1) and (O(2)) being in trans geometry toeach other, imino nitrogen atoms (N(1) and N(3)) being intrans geometry, and amino and pyridine nitrogen atoms (N(2)
and N(4)) being in trans geometry. The mean bond distancesfor Fe-O (1.89 Å) and Fe-N (2.03 Å) at 298 K are intermediate
between the typical values of the bond distances for HS
complexes, Fe-O (1.91 Å) and Fe-N (2.16 Å), and those for LS
complexes, Fe-O (1.88 Å) and Fe-N (1.98 Å), at room temperature.
The structure of 2 is similar to that of 1 and is the binuclear
iron(III) complex bridged by 4,4'-(1,2-ethenediyl)bis-pyridine(cc). The vinylene group is in trans geometry (C(26) andC’(26)), too. The values of the bond distances of Fe-O (1.89
Å) and Fe-N (2.02 Å) are intermediate between the typicalvalues of the bond distances for HS and those for LS
complexes, too.

J. Nucl. Radiochem. Sci., Vol. 3, No. 2, 2002

A3

N(2)
O(1)
Fe

N(3)
O(2)

N(1)
N(4)

N(5)
N’(5)

N’(4)
Fe’

Figure 1. ORTEP view of [Fe2(salten)2(az)]2+ (12+) at 100 K showing
50% probability displacement ellipsoids. All hydrogen atoms areomitted for clarity.

N(2)TABLE 2: Representative Bond Distances and Angles for
the Complexes 1 and 2
Complex1

2

T = 298 K T = 100 K T = 298 K T = 100 K
Bond Distances / ÅFe(1)-O(1)1.893(8) 1.866(4)1.886(5) 1.867(5)Fe(1)-O(2)1.890(9) 1.881(4)1.892(5) 1.889(5)Fe(1)-N(1)2.05(1) 1.963(5)1.989(6) 1.975(6)Fe(1)-N(2)2.02(1) 2.000(5)2.065(6) 2.011(6)Fe(1)-N(3)2.01(1) 1.944(5)2.000(6) 1.952(6)Fe(1)-N(4)2.037(10) 1.988(4)2.045(6) 2.006(6)Bond angles / ºO(1)-Fe(1)-O(2) 176.9(4) 177.8(2)177.7(3) 179.4(2)O(1)-Fe(1)-N(1)89.1(5)88.9(2)90.6(2) 89.8(3)O(1)-Fe(1)-N(2)91.9(4)90.9(2)92.6(3) 89.3(2)O(1)-Fe(1)-N(3)89.8(4)90.1(2)89.9(2) 90.0(2)O(1)-Fe(1)-N(4)88.4(4)89.4(2)87.9(2) 90.7(2)O(2)-Fe(1)-N(1)89.1(5)89.3(2)90.3(2) 89.8(3)O(2)-Fe(1)-N(2)90.7(5)90.4(2)89.6(3) 91.2(2)O(2)-Fe(1)-N(3)92.2(4)91.7(2)89.3(2) 90.4(2)O(2)-Fe(1)-N(4)89.1(4)89.3(2)89.9(2) 88.8(2)N(1)-Fe(1)-N(2)87.3(6)89.4(2)87.3(3) 89.5(3)N(1)-Fe(1)-N(3) 172.8(6) 177.0(2)175.8(3) 178.1(3)N(1)-Fe(1)-N(4)94.6(6)91.3(2)92.3(3) 90.8(3)N(2)-Fe(1)-N(3)85.6(6)87.7(2)88.6(3) 88.6(3)N(2)-Fe(1)-N(4) 178.0(5) 179.2(2)179.3(2) 179.7(3)N(3)-Fe(1)-N(4)92.5(5)91.6(2)91.8(3) 91.1(3)

N(1)

Fe

O(2)
N(3)

O(1)
N(4)

C(26)
C’(26)N ’ (4)Fe’

Figure 2. ORTEP view of [Fe2(salten)2(cc)]2+ (22+) at 100 K showing
50% probability displacement ellipsoids. All hydrogen atoms areomitted for clarity.

HayamiJ. Nucl. Radiochem. Sci., Vol. 3, No. 2, 2002

At 100 K, the arrangements of 1 and 2 units are quite similar
to those at 298 K. The values of the mean bond distances of
Fe-O (1.87 Å) and Fe-N (1.97 Å) are the typical values of thebond distances for LS compounds. The spin transition principally affects the FeN4O2 core geometry. The Fe-O bonds in
LS form are 0.013 Å shorter than those for 1 at 298 K and the
Fe-N bonds in LS form are 0.056 Å shorter than those for 1 at
298 K. The structure of 2 at 100 K is similar to that of 2 at 298
K. The values for the mean bond distances of Fe-O (1.88 Å)
and Fe-N (1.99 Å) are intermediate between the typical valuesof the bond distances for LS compounds. The Fe-O bonds in
LS form are 0.011 Å shorter than those for 2 at 298 K and the
Fe-N bonds are 0.039 Å shorter than those for 2 at 298 K. It
should be noted that a slight conformational change of thechelate rings makes the tetrahedral distortion of the FeO2N2
basal plane and change in the iron-coordination atom bondlengths, and as a result it induces the spin transition.
The temperature dependencies of magnetic susceptibilities
were measured in the temperature range of 1.8 K to 350 K andχmT vs. T plots for 1 and 2 are shown in Figure 3. The valuesof χ m T for 1 and 2 are constant (0.5 cm 3 K mol −1 ) in thetemperature range of 1.8 - 200 K, and the values are thoseobserved for LS complexes. The χmT values increase gradually from 0.5 cm3 K mol−1 at 200 K to ca 2.5 cm3 K mol−1 for 1
and to 1.9 cm3 K mol−1 for 2 at 350 K. The magnetic behavior
for the increasing and decreasing temperature sequences ispractically similar to each other, and the spin transition
between HS and LS states is not associated with a thermal
hysteresis. The results suggest that 1 and 2 are classified as acontinuous type of the spin transition. 18 The value of themagnetic moment of 1 at 350 K is larger than that of 2, i.e. theligand field strength of 2 with cc (vinyl group) is stonger thanthat of 1 with az (azo group). The value of 0.5 cm3 K mol−1 ischaracteristic of an iron(III) complex in a LS state; a value of0.38 cm 3 K mol −1 (spin-only value) would be expected foriron(III) complexes in a LS state. Clearly the complexes 1 and
2 are in the LS state (S = 1/2) below 200 K. The values of χmT
for 1 and 2 at 350 K are smaller than the typial values for HS(S = 5/2) compounds. There are two possibilities for the small
χmT values at 350 K, i.e., the small values suggest that thecomplexes are the mixtures of HS and LS species (S = 1/2 andS = 5/2) or that the complexes show rapid spin-equilibrium
behavior (S = 1/2 ↔ S = 5/2).
The variable temperature Mössbauer spectra of 1 and 2 areshown in Figures 4 and 5, respectively. The typical absorption

for low-spin iron(III) with a large quadrupole of Q.S. = 1.65
mm s−1 doublet and an isomer shift of I.S. = 0.29 mm s−1 isobserved in the spectrum of 1 at 80 K. The spin state expectedfrom the spectrum is agreed with that obtained from themagnetic measurements. The Q.S. of 1 at 95 K, 1.65 mm s−1,
is less than that of the iron(III) complexes with salten ligand
reported previously (2.72 mm s −1); 29 the decrease seems toresult from the change in the electric field gradient due tolattice contribution because the iron atoms have a distorted
octahedral coordination. On the other hand, the spectrum of 1
at 298 K shows a small doublet (Q.S. = 0.886 mm s−1) with I.S.
= 0.28 mm s−1. The spin state expected from the Mössbauer
parameters and the values of χ m T suggests the rapid spin

1.000

(a)
0.990

1.000

Transmittance

A4

(b)
0.992
1.000

(c)
0.990
1.000

(d)
0.980

-4

-2

0

2

4

Velocity / mm s-1
Figure 4. Mössbauer spectra of 1 (a) at 298 K, (b) at 200 K, (c) at
100 K, and (d) at 80 K.

1.00

3

(a)

1

TransmittanceχmT / cm3 K mol-1

4

2

2

1

0.98
1.00

(b)
0.97

0
0

100

200

300

400

Temperature / K
Figure 3. χ m T versus T plots for 1 (▲) and 2 (■). Sample was
warmed from 5 K to 350 K at a rate of 2 K min−1.

-4

-2

0

2

4

Velocity / mm s-1
Figure 5. Mössbauer spectra of 2 (a) at 298 K and (b) at 80 K.

Thermal and Optical Switching of Iron(III) Complexes

J. Nucl. Radiochem. Sci., Vol. 3, No. 2, 2002

conversion between HS and LS states. The spectra for rapid
spin interconversion between HS and LS states have beentheoretically calculated by Maeda et al.30 It is expected thatthe relaxation spectra with the end components (the spectra for
HS or LS species) of an antisymmetric doublet are not reliably
analyzed, and the spin interconversion rate for 1 could not becalculated from the spectra. However, it is expected that the
spin interconversion rate between HS and LS states for 1 isfaster than the inverse of the lifetime of the excited Mössbauer
nuclear state (107 s −1 ) at 298 K. The values of the bonddistances of Fe-O (1.89 Å) and Fe-N (2.03 Å) are intermediate
between the typical values of the bond distances for HS andthose for LS complexes. The Mössbauer spectra of 2 aresimilar to those of 1; the spectra are composed of a large
doublet (Q.S. = 1.53 mm s−1) at 80 K have a small doublet(Q.S. = 0.94 mm s−1) at 298 K. The spin state at 298 K estimated from the Mössbauer parameters (I.S. = 0.19 mm s−1) andthe values of χmT shows the rapid spin-equilibrium between
HS and LS states.
The temperature dependence of the electronic spectra wasmeasured in acetonitrile. The binuclear complexes 1 and 2
showed thermochromic behavior. Figure 6 shows the temperature dependence of the spectra for 1. At 60 ºC, the spectrum of
1 exhibits absorption bands at 405 and 470 nm with extinction
coefficients (ε) of 2190 and 1740 L mol−1 cm−1, respectively
attributable to ligand-to-metal charge transfer (LMCT) bands.
Hauser et al. reported that the bands with the extinction coefficients on the order of 103 L mol − 1 cm − 1 for iron(III)compounds correspond to the LMCT transitions. 3 1 On
lowering the temperature, the absorption at 405 nm decreases
in intensity and that at 470 nm shifts to a longer wavelength.
The spectral change is similar to those of the spin-crossovercomplexes with salten reported previously.29 The temperaturedependence of the electronic spectrum is due to the spin-equilibrium between HS and LS states. The spectral changes areassociated with two isosbestic points at 365 and 490 nm.
Figure 7 shows the temperature dependence of the spectra for
2. At 60 ºC, the spectrum exhibits bands at 400 and 470 nm;
the absorption at 400 nm decreases in intensity with decreasing
temperature and that at 470 nm shifts to a longer wavelength
accompanied by the decrease in intensity with lowering thetemperature. The temperature dependence of the electronic
spectra is also due to the spin-equilibrium between HS and LS
states. The spectrum changes with a set of isosbestic points at
370 and 520 nm.
Iron(III) Spin Transition Compounds Exhibiting
LIESST Effect. Strong intermolecular interaction can be

A5achieved by using π−π interactions as well as by using bridging ligands.20, 21, 32−35 The iron(III) compound we focused on is[Fe(pap)2]ClO4 ⋅H2O (3).36, 37 The planar ligand, pap, with corresponding π electrons, has a potential ability to interact withneighboring ligands by π−π interactions. In fact, the presenceof a strong intermolecular interaction is suggested from thesingle crystal X-ray analysis. Single crystals suitable for X-ray
diffraction of 3 were isolated by slow diffusion in methanolic
solutions. The complex 3 crystallizes in the triclinic P1 spacegroup at room temperature (Table 1). The X-ray structure of 3
revealed that the iron(III) ion has an octahedral coordination
with an N4O2 donor set (Figure 8(a)). Its electronic state ishence situated just in the critical region between high-spin andlow-spin states, i.e., nearly degenerate. The bond lengths of
Fe-O(1), Fe-O(2), Fe-N(1), Fe-N(2), Fe-N(3), and Fe-N(4) for
3 are 1.93, 1.93, 2.11, 2.20, 2.14, and 2.14 Å, respectively.
The values of the bond lengths are consistent with those typical for HS iron(III) compounds. Two tridentate ligands arefound to be perpendicular to one another. [Fe(pap)2]+ cations
form short contacts (3.50 Å) with neighboring molecules
through the pyridine and phenyl rings in the ligands on an abplane, providing a 2-D sheet extended by the π-π interactions(Figure 8(b)). The 2-D sheets align along c axis. The watermolecules and perchlorate ions exist between the 2-D sheets
and are hydrogen-bonded to each other. The molecular packing of 3 is very tight. This suggests that there are strong intermolecular interactions in the molecular assembly. It should be
noted that the structure of complex 3 has no remarkable features at the single molecule level compared to other typicaliron(III) spin-crossover complexes.
The HS to LS conversion was followed by measurement of
the magnetic susceptibility (Figure 9) and Mössbauer spectra(Figure 10) as a function of temperature. At room temperature, the value of χmT for 3 is equal to 3.9 cm3 K mol−1. The
Mössbauer spectrum for 3 at room temperature shows a small
doublet (Q.S. = 1.11 mm s−1; I.S. = 0.36 mm s−1) which can be
assigned to the HS state of iron(III). On cooling, the value ofχmT drops abruptly around T1/2↓ = 165 K. The χmT value at
100 K is equal to 0.51 cm3 K mol−1. The Mössbauer spectrumat 13 K for 3 shows a large doublet (Q.S. = 3.08 mm s−1; I.S. =0.11 mm s−1) which can be assigned to the LS state of iron(III).
In the warming mode, an abrupt variation of χmT was observed
around T 1 / 2 ↑ = 180 K. Fairly abrupt transitions with ahysteresis loop (ca. 15 K) support the presence of the strong
cooperative interaction. Another important point is that thiscomplex exhibits a frozen-in effect19, 37−39 which is observed forthe materials with large cooperativity. If the sample is rapidly

2500

3500
3000

ε / L mol-1 cm-1

ε / L mol-1 cm-1

2000

1500

60 ºC
40 ºC

1000

20 ºC
500

0
200

400

500

2000
1500

60 ºC
40 ºC

1000

20 ºC

0 ºC

300

2500

500

600

700

800

λ / nmFigure 6. Temperature dependence of the electronic spectra of 1 inacetonitrile solution at the temperature range between 60 ºC - 0 ºC.

0
200

0 ºC
300

400

500

600

700

800

λ / nmFigure 7. Temperature dependence of the electronic spectra of 2 inacetonitrile solution at the temperature range between 60 ºC - 0 ºC.

A6HayamiJ. Nucl. Radiochem. Sci., Vol. 3, No. 2, 2002
C(3)

C(4)

4.0
C(2)

3.5

C(1)
O(1)

C(5)

C(22)

C(6)
C(24)

C(21)

N(1)
Fe
N(4)

C(7)

C(20)
C(19)

N(2)

O(2)
N(3)

C(8)

C(18)

C(13)

C(17)

C(9)

C(10)

(a)

C(12)

χmT / cm3 K mol-1

C(23)

3.0
2.5
2.0
1.5

C(14)
C(11)
C(15)

C(16)

1.0
0.5
0

50

100

150

200

250

300

Temperature / K
Figure 9. χmT versus T plots for 3. Sample was cooled from 300 K to
5 K (▽) and then warmed from 5 K to 300 K (△) at a rate of 2 K
min − 1 . Sample was warmed at a rate of 2 K min − 1 after it was
quenched to 5 K (▲).

(b)

1.00

(a)
Figure 8. Molecular structures in the high-spin state at 298 K. (a)ORTEP view of [Fe(pap)2]+ (3+) showing 50% probability displacement ellipsoids. (b) Structure of the π−π stucking between
complexes. All hydrogen atoms are omitted for clarity.

1.00

(b)
Transmittancequenched in liquid helium, whereby the sample temperaturedrops from room temperature to 4.2 K within a few seconds,
the HS state may be “frozen-in” and a transition to LS state is
not observed even in a few days. The relaxation rate from HS
to LS is quite slow. These experimental results support thatstrong cooperative interactions operate in this compound.
It has been reported that a broad band (from 500 to 800 nm)
in UV-vis spectrum for the LS iron(III) complex can be attributed to the spin-allowed LMCT transition.23, 24 Furthermore,
Hauser et al. reported that the excitation of the LMCT band
results in the transient generation of HS iron(III) fractions.23, 24
Hence, in order to excite the LMCT band of complex 3, light(540 - 550 nm) passing through a combination of IR cut-off
and green filters was guided via an optical fiber into a SQUID
magnetometer. The sample was placed on the edge of an
optical fiber. When the sample was illuminated at 5 K, theincrease of the magnetization value was observed. 40 Thissuggests that the transition from LS to HS was induced by illumination. The change in the magnetization persisted for
periods of at least ten hours after the illumination. Figure 10
shows a series of Mössbauer spectra that were recorded at 13
K before and after the excitation with light. In order toenhance the Mössbauer spectra, 57 Fe-enriched sample wasused. The spectrum in Figure 10(b) measured before illumination at 13 K shows a large doublet (Q.S. = 3.08 mm s−1; I.S. =0.11 mm s − 1 ), representing the LS state. The spectrum
measured after illumination for 15 min shows a small doublet(Q.S. = 1.14 mm s−1, I.S. = 0.44 mm s−1), representing the HS
state. Thus, it has been confirmed that the LS state changed to
the HS state by illumination. These indicate clearly that thevisible light illumination of complex 3 induces the LMCT,
followed by relaxation to a metastable HS state. When the

0.80

0.94
1.00

(c)
0.98

1.00

(d)
0.98

-4

-2

0

2

4

Velocity / mm s-1
Figure 10. 57Fe Mössbauer spectra for 1. (a) Spectrum at 298 K, (b)spectrum at 13 K before illumination, (c) spectrum at 13 K after illumination and (d) spectrum at 13 K after thermal treatment at 130 K.

Thermal and Optical Switching of Iron(III) Complexes

J. Nucl. Radiochem. Sci., Vol. 3, No. 2, 2002

C(4)

C(3)
C(2)

C(5)

The two tridentate ligands in the complexes are found to benearly perpendicular to one another: the least-square plane of
the slightly distorted qsal ligands for 4 makes a dihedral angleof 79.49° and 80.44° with one of the other qsal ligands. No
counter anion NCSe− and no solvent molecules are located in
the first co-ordination sphere. The crystal structure of the solvated 4 could not be determined in the HS form because thespin transition induces cracks in the crystal, making an X-ray
single crystal study impossible.
The temperature dependence of the magnetic susceptibility
for the single crystal of 4 was measured at a rate of 2 K min−1
and the result is shown in the form of the χ m T vs. T plot
(Figure 12). The χmT value for 4 is equal to 0.36 cm3 K mol−1
at 150 K, which is in the range of the values expected for the
LS iron(III) ions. As the temperature is increased over 150 K,
the χmT product remains constant from 150 K to 350 K, andthen abruptly increases around T1/2↑ = 392 K. The χmT value
at 400 K is 3.8 cm3 K mol−1, showing that the spin transition
from the LS to the HS state is induced. The spin transition is
directly related to the removal of solvent molecules.
Thermogravimetry (Figure 13) carried out with the same4

χmT / cm3 K mol-1

temperature is raised to 150 K for a few minutes and is again
lowered to 13 K, the metastable state is found to have relaxedback to the LS ground state (Q.S. = 3.08 mm s−1; I.S. = 0.11
mm s−1). The metastable state could be maintained for a longtime, provided that the sample was kept below ca. 70 K. The
achievement of an anomalous long-lived metastable state isconsidered to be due to the presence of the strong intermolecular interaction. We believe that this approach may be valuable in developing novel optically switchable molecularcompounds.
Iron(III) Spin Transition Compound with Wide Thermal
Hystresis. Iron(III) complex [Fe(qsal) 2 ]NCSe ⋅ CH 2 Cl 2 (4)exists in the LS state at room temperature when they are synthesised in MeOH/CH2Cl2 solutions. X-ray analysis was successfully carried out for the compound 4 at 230 K. Figure 11
shows that when the complexe 4 is in the LS state it crystallizes in a triclinic P1 space group (Table 1). The single crystal
X-ray analysis of 4 reveales that the iron(III) atoms are octahedrally co-ordinated by four nitrogen atoms and two oxygen
atoms of two qsal ligands, i.e., an N4O2 donor set. The valuesof the bond lengths (1.88 Å for Fe-O and 1.97 Å for Fe-N) areconsistent with those for typical LS iron(III) compounds. The
Fe-O distances are shorter than the Fe-N distances, which
induce a pronounced distortion of the FeN4O2 octahedron. The
qsal ligands form slightly distorted planes. The dihedral
angles between the phenyl ring and Fe-O(1)-N(1) chelate ring,
between the other phenyl ring and Fe-O(2)-N(3) chelate ring,
between the quinoline ring and Fe-N(1)-N(2) chelate ring, andbetween the other quinoline ring and Fe-N(3)-N(4) chelate ringfor 4 are equal to 2.43°, 9.52°, 0.68°, and 3.66°, respectively.

A7

3

2

1

C(1)

C(6)

C(19)

O(1)

C(20)

C(18)
C(7)

C(22) C(21)
C(23)

C(17)
O(2)
Fe

N(1)
C(9)

C(8)

200

C(31) N(4)
C(30)
C(29)

C(16)

C(12)

250

300

350

400

Temperature / K
N(2)

C(28)
C(27)

C(10)

C(11)

0
150

N(3)
C(24) C(25)

C(32)

C(26)

C(15)

C(14)

(a)

Figure 12. χmT versus T plots for 4. The sample was warmed from 5
K to 400 K (▲) and cooled from 400 K to 5 K (▼) in the first cycle,
and then the sample was warmed from 5 K to 400 K (△) and cooled
from 400 K to 5 K (▽) in the second cycle at a rate of 2 K min−1.

C(13)Mass change / mg

23.0

(b)
Figure 11. Molecular structures in the low-spin state at 200 K. (a)ORTEP view of [Fe(qsal)2]+ (4+) showing 50% probability displacement ellipsoids. (b) Structure of the π−π stucking between complexes.
All hydrogen atoms and solvent molecule are omitted for clarity.

22.5

CH2Cl2

22.0
21.5
21.0
320

360

400

440

Temperature / K
Figure 13. Thermogravimetric analysis for 4. (2.356 mg of sample)

A8

HayamiJ. Nucl. Radiochem. Sci., Vol. 3, No. 2, 2002
1.00

Transmittance

(a)

0.70

1.00

(b)

0.85

-4

-2

0

2

4

Velocity / mms-1
Figure 14. 57Fe Mössbauer spectra at room temperature (a) beforeand (b) after annealing at 400 K for 4. There is a small amount of LS
form in the HS form after annealing, indicating that solvent molecules
are still retained.

heating rate (2 K min −1 ) as for the magnetic susceptibility
measurements revealed a continuous loss of mass, starting atroom temperature. The decrease in mass proceeds rapidly in thetemperature range 380 K - 410 K. At 420 K, the mass lost is inagreement with the removal of a CH2Cl2 molecule. Elemental
analysis also supports the desolvation. On cooling, the χmTvalue for the non-solvated compound 4 was almost constantfrom 400 K to 220 K, then abruptly dropped at around T1/2↓ =212 K, showing that the HS moieties were restored to the LS
state. The first hysteresis loop of 180 K (T1/2↑ = 392 K and
T 1/2 ↓ = 212 K) is an apparent one because the desolvation
occurs at elevated temperature. It should be noted that a similarlarge apparent hysteresis has been reported for [Fe(hyetrz)3](3nitrophenylsulfonate)2 ⋅3H2O by Garcia et al.41, 42
A second heating experiment revealed that the spincrossover takes place in a two-step manner.43−51 The so-called“step 1” and “step 2” are centred around the temperatures
T1/2(S1)↑ = 215 K and T1/2(S2)↑ = 282 K, respectively. The
magnetic behavior shows that the fraction of the LS to HS spintransition in the upper step “step 2” is about twice as high asthat in the lower step “step 1”. Additional thermal cycles did
not modify the thermal hysteresis loop. The χmT value at the
inflection point at around 250 K is 1.6 cm3 K mol−1. When thehysteresis widths (∆T) in “step 1” and “step 2” are defined as
the differences of T 1/2 (S1)↑ and T 1/2 ↓ and of T 1/2 (S2)↑ andT1/2↓, respectively, they are estimated to be 3 K (step 1) and 70
K (step 2). In order to confirm the transition of the SC,
Mössbauer spectra for 4 were measured at room temperature
before and after annealing at 400 K (Figure 14). The complex
was prepared with using 57Fe to enhance the Mössbauer signal.
Before annealing, a large doublet with Q.S. = 2.47 mm s−1 and
I.S. = 0.11 mm s−1 for 4 was observed at room temperature,
which corresponded to the LS state of iron(III) compounds.
After being annealed at 400 K, a broad singlet (Γexp = 2.21 mm
s −1 ) with Q.S. = 0.46 mm s −1 and I.S. = 0.23 mm s −1 wasobserved at room temperature, showing that the iron(III) turns
from the LS state to the HS state. This is consistent with themagnetic properties described above.
It should be noted that several examples of the two-step spintransition have been reported. Compared with these examples,the present compounds have the following important characteristics; (i) the spin transition shows a hysteresis loop, and (ii)the two-step spin crossover is observed only in the warming

mode. To the best our knowledge, this kind of hysteresis
behaviour has not been reported previously. Furthermore, thehysteresis width of 70 K in “step 2” is quite wide, comparedwith those reported elsewhere. The abrupt phase transition
with the hysteresis effect suggests that a strong intermolecularinteraction operates in this complex.40
It is useful to compare the hysteresis width of the nonsolvated complexe 4 with those reported previously.20, 21, 41−52
Several compounds with a polymeric structure have beenfound to display a wide thermal hysteresis. Kahn et al.
reported that an iron(II) compound, [Fe(Htrz) 3 - 3 x (4NH2trz)3x](ClO4)2 ⋅ nH2O (x = 0.05) (Htrz = 1,2,4-1H-triazole,
4-NH 2 trz = 4-amino-1,2,4-triazole) which has a polymeric
structure, displays a thermal hysteresis at the range of room
temperature (T1/2↑ = 304 K and T1/2↓ = 288 K).53, 54 However,the polymeric compound is not strictly molecular, as wasconcluded by previous studies.41, 42, 53−55 Several mononuclear
molecules have also exhibited thermal hysteresis loops. The
maximum hysteresis width observed for iron(II) complexes
with intermolecular π-π interactions is 40 K,20, 21, 51, 52 which isnarrower than that observed for the non-solvated complex 4.
Furthermore, it has been reported that the iron(II) compound,[Fe(2-pic) 3 ]Cl 2 ⋅ H 2 O (2-pic = 2-picolylamine), exhibits thewidest thermal hysteresis loop of 91 K with T1/2↑ = 295 K and
T1/2↓ = 204 K.56, 57 The structure of the compound has notbeen fully analysed yet. These previous results show thatthe hysteresis width of 70 K presented here58 is one of thewidest values among the spin-crossover complexes reported so
far.20, 21, 32−55
In summery, we have prepared the binuclear iron(III)complexes 1 and 2 with bridging photoisomerization ligands
az or cc, and the complexes 1 and 2 exhibit spin-crossover
behavior depending on temperature. The Mössbauer spectra
for 1 and 2 show a doublet at 293 K, suggesting the rapid spin
conversion between HS and LS states. The average bondlengths of Fe-O and Fe-N for 1 and 2 are intermediate between
the typical values of the bond lengths for HS and those for LS
complexes. However, it is very difficult to observe cis-transphotoisomerization for the complexes with photoisomerization
ligands in solid state. We have succeeded in observing the
LIESST effect for the iron(III) complex 3 for the first time.
This suggests that the cooperative interactions such as π−πinteractions and hydrogen bonding can prevent the rapid relaxation from the metastable state to the stable state. It is important that our approach, the introduction of intermolecularinteractions to trap the metastable state, can be widely applied
not only to the design of other metal complexes with LIESST
effects, but also in the design of a variety of optically switchable molecular compounds. Furthermore, we have shown thatthe iron(III) complex 4 displays a wide apparent hysteresis in
the first cycle. After the first cycle, they show a two-step transition in the warming mode and a one-step transition in thecooling mode. The hysteresis width in “step 2” after the firstcycle is 70 K, which is one of the largest hysteresis effects
reported so far.
Acknowledgements. This work was supported in part by aGrant-in-Aid for Science Research on Priority Areas (417)from the Ministry of Education, Culture, Sports, Science, andTechnology (MEXT) of the Japanese Government and by a
Joint Research Project for Regional Intensive of Kanagawa
Prefecture.
References(1) T. M. Dunn, Modern Coordination Chemistry, edited by J.
Lewis and R. G. Wilkins (Interscience Pub. New York,1960), Chap. 4.
(2) L. Cambi and A. Cagnasso, Atti Accad. Naz. Lincei 13,

Thermal and Optical Switching of Iron(III) Complexes

809 (1931).
(3) (a) H. A. Goodwin, Coord. Chem. Rev. 18, 293 (1976).
(b) P. Gütlich, Struct. Bonding (Berlin) 44, 83 (1981). (c)P. Gütlich and A. Hauser, Coord. Chem. Rev. 97, 1(1990). (d) E. König, Prog. Inorg. Chem. 35, 527 (1987).
(e) E. König, Struct. Bonding (Berlin) 76, 51 (1991).
(4) Y. Maeda and Y. Takashima, Comments Inorg. Chem. 7,
41 (1988).
(5) J. Zarembowitch, New J. Chem. 16, 255 (1992).
(6) (a) E. König and G. Ritter, Mössbauer Eff. Methodol.
edited by I. J. Gruverman, C. W. Seidel, and D. K.
Dieterly (Plenum, New York, 1974), vol. 9, pp. 3-21. (b)P. Gütlich, Chemical Mössbauer Spectroscopy, edited by
R. H. Herber (Plenum, New York, 1984), pp. 287-337. (c)P. Gütlich, R. Link, and A. X. Trautwein, Mössbauer
Spectroscopy and Transition Metal Chemistry (Springer,
Berlin, 1978).
(7) O. Sato, T. Iyoda, A. Fujishima, and K. Hashimoto,
Science 272, 704 (1996).
(8) Z.-Z. Gu, O. Sato, T. Iyoda, K. Hashimoto, and A.
Fujishima, J. Phys. Chem. 100, 18289 (1996).
(9) V. I. Kudinov, A. I. Kirilyuk, and N. M. Kreines, Phys.
Lett. A 151, 358-364 (1990).
(10) G. Nieva, Phys. Rev. B 46, 14249 (1992).
(11) D. V. Fomitchev, K. A. Bagley, and P. Coppens, J. Am.
Chem. Soc. 122, 532 (2000).
(12) Y. Teki, S. Miyamoto, K. Iimura, M. Nakatsuji, and Y.
Miura, J. Am. Chem. Soc. 122, 984 (2000).
(13) S. Decurtins, P. Gütlich, C. P. Köhler, H. Spiering, and A.
Hauser, Chem. Phys. Lett. 105, 1 (1984).
(14) S. Decurtins, P. Gütlich, K. M. Hasselbach, A. Hauser,and H. Spiering, Inorg. Chem. 24, 2174 (1985).
(15) A. Hauser, Chem. Phys. Lett. 124, 543 (1986).
(16) A. Hauser, J. Chem. Phys. 94, 2741 (1991).
(17) A. Hauser, A. Vef, and P. Adler, J. Chem. Phys. 95, 8710
(1991).
(18) P. Gütlich, A. Hauser, and H. Spiering, Angew. Chem. Int.
Ed. Engl. 33, 2024 (1994).
(19) Th. Buchen, P. Gütlich, and H. A. Goodwin, Inorg. Chem.
33, 4573 (1994).
(20) J.-F. Létard, P. Guionneau, L. Rabardel, J. A. K. Howard,
A. E. Goeta, D. Chasseau, and O. Kahn, Inorg. Chem. 37,
4432 (1998).
(21) J.-F. Létard, J. A. Real, N. Moliner, A. B. Gaspar, L.
Capes, O. Cador, and O. Kahn, J. Am. Chem. Soc. 121,
10630 (1999).
(22) (a) C. Roux, J. Zarembowitch, B. Gallois, and M. Bolte,
New J. Chem. 16, 671 (1992). (b) C. Roux, J. Zarembowitch,
B. Gallois, T. Granier, and R. Claude, Inorg. Chem. 33,
2273 (1994). (c) M.-L. Boillot, C. Roux, J.-P. Audiére, A.
Dausse, and J. Zarembowitch, Inorg. Chem. 35, 3975
(1996).
(23) S. Schenker and A. Hauser, J. Am. Chem. Soc. 116, 5497
(1994).
(24) S. Schenker, A. Hauser, and R. M. Dyson, Inorg. Chem.
35, 4676 (1996).
(25) M. A. Berkgamp, P. Gütlich, T. L. Netzel, and N. J. Sutin,
J. Phys. Chem. 87, 3877 (1983).
(26) I. Lawthers and J. J. Mcgarvey, J. Am. Chem. Soc. 106,4280 (1984).
(27) S. Hayami, K. Inoue, S. Osaki, and Y. Maeda, Chem. Lett.
987 (1998).
(28) S. Hayami, Y. Hosokoshi, K. Inoue, Y. Einaga, O. Sato,and Y. Maeda, Bull. Chem. Soc. Jpn. 74, 2361 (2001).
(29) (a) N. Matsumoto, S. Ohta, C. Yoshimura, A. Ohyoshi, S.
Kohata, H. Okawa, and Y. Maeda, J. Chem. Soc. Dalton
Trans. 2575 (1985). (b) S. Ohta, C. Yoshimura, N.
Matsumoto, H. Okawa, and A. Ohyoshi, Bull. Chem. Soc.
Jpn. 59, 155 (1986).

J. Nucl. Radiochem. Sci., Vol. 3, No. 2, 2002

A9(30) Y. Maeda, H. Ohshio, and Y. Takashima, Chem. Lett. 943(1982).
(31) S. Schenker, A. Hauser, and R. M. Dyson, Inorg. Chem.
35, 4676 (1996).
(32) J. A. Real, E. Andrés, M. C. Muñoz, M. Julve, T. Granier,
A. Bousseksou, and F. Varret, Science 268, 265 (1995).
(33) C.-L. Xie and D. N. Hendrickson, J. Am. Chem. Soc. 109,6981 (1987).
(34) A. J. Conti, C.-L. Xie, and D. N. Hendrickson, J. Am.
Chem. Soc. 111, 1171 (1989).
(35) A. Hauser, J. Adler, and P. Gütlich, Chem. Phys. Lett.
152, 468 (1988).
(36) H. Oshio, K. Kitazaki, J. Mishiro, N. Kato, Y. Maeda, and
Y. Takashima, J. Chem. Soc. Dalton Trans. 1341 (1987).
(37) S. Hayami and Y. Maeda, Inorg. Chim. Acta 255, 181(1997).
(38) G. Ritter, E. König, W. Irler, and H. A. Goodwin, Inorg.
Chem. 17, 224 (1978).
(39) S. M. Nelson, P. D. A. Mcllroy, C. S. Stevenson, E.
König, G. Ritter, and J. Waigel, J. Chem. Soc. Dalton
Trans. 991 (1986).
(40) S. Hayami, Z.-Z. Gu, M. Shiro, Y. Einaga, A. Fujishima,
and O. Sato, J. Am. Chem. Soc. 122, 7126 (2000).
(41) Y. Garcia, P. J. V. Koningsbruggen, E. Codjovi, R.
Lapouyade, O. Kahn, and L. Rabardel, J. Mater. Chem. 7,
857 (1997).
(42) Y. Garcia, P. J. Koningsbruggen, R. Lapouyade, L.
Fournès, L. Rabardel, O. Kahn, V. Ksenofontov, G.
Levchenko, and P. Gütlich, Chem. Mater. 10, 2426 (1998).
(43) H. Köppen, E. W. Müller, C. P. Köhler, H. Spiering, E.
Meissner, and P. Gütlich, Chem. Phys. Lett. 91, 348(1982).
(44) J. A. Real, H. Bolvin, A. Bousseksou, A. Dworkin, O.
Kahn, F. Varret, and J. Zarembowitch, J. Am. Chem. Soc.
114, 4650 (1992).
(45) R. Jacobi, H. Spiering, and P. Gütlich, J. Phys. Chem.
Solids 53, 267 (1992).
(46) D. Boinnard, A. Bousseksou, A. Dworkin, J.-M.
Savariault, F. Varret, and J.-P. Tuchagues, Inorg. Chem.
33, 271 (1994).
(47) T. Buchen, D. Schollmeyer, and P. Gütlich, Inorg. Chem.
35, 155 (1996).
(48) J. A. Real, I. Castro, A. Bousseksou, M. Verdaguer, R.
Burriel, J. Linares, and F. Varret, Inorg. Chem. 36, 455(1997).
(49) H. Spiering, T. Kohlhaas, H. Romstedt, A. Hauser, C.
Bruns-Yilmaz, J. Kusz, and P. Gütlich, Coord. Chem.
Rev. 190-192, 629 (1999).
(50) A. J. Simaan, M.-L. Boillot, E. Rivière, A. Boussac, andJ.-J. Girerd, Angew. Chem. Int. Ed. 39, 196 (2000).
(51) J.-F. Létard, P. Guionneau, E. Codjovi, O. Lavastre, G.
Bravic, D. Chasseau, and O. Kahn, J. Am. Chem. Soc.
119, 10861 (1997).
(52) Z. J. Zhong, J. Q. Tao, Z. Yu, C. Y. Dun, Y. J. Liu, and X.
Z. You, J. Chem. Soc. Dalton Trans. 327 (1998).
(53) J. Kröber, E. Codjovi, O. Kahn, F. Grolière, and C. Jay, J.
Am. Chem. Soc. 115, 9810 (1993).
(54) J. Kröber, J.-P. Audière, R. Claude, E. Codjovi, O. Kahn,
J. G. Haasnoot, F. Grolière, and C. Jay, Chem. Mater. 6,1404 (1994).
(55) P. J. V. Koningsbruggen, Y. Garcia, E. Codjovi, R.
Lapouyade, O. Kahn, L. Fournès, and L. Rabardel, J.
Mater. Chem. 7, 2069 (1997).
(56) M. Sorai, J. Ensling, and P. Gütlich, Chem. Phys. 18, 199(1976).
(57) M. Sorai, J. Ensling, K. M. Hasselbach, and P. Gütlich,
Chem. Phys. 20, 197 (1977).
(58) S. Hayami, Z.-Z Gu, H. Yoshiki, A. Fujishima, and O.
Sato, J. Am. Chem. Soc. 123, 11644 (2001).