Journal of Nuclear and Radiochemical Sciences, Vol. 5, No. 1, pp. N1-N3, 2004

Notes
Photo-Induced Spin Transition for an Iron(III) Pyruvic Acid Thiosemicarbazone CompoundShinya Hayami,*, a Koichiro Hashiguchi,b Katsuya Inoue,b and Yonezo Maedaa
aDepartment of Chemistry, Faculty of Science, Kyushu University, 6-10-1 Hakozaki, Higashi-ku, Fukuoka
812-8581, JapanbInstitute for Molecular Science, Myodaiji, Okazaki 444-8585, Japan
Received: December 11, 2003; In Final Form: February 4, 2004
An iron(III) compound [Fe(thpu)(Hthpu)] has exhibited spin transition behavior (T1/2↓ = 225 K and T1/2↑ = 239 K)
with a thermal hysteresis loop (∆T = 14 K). When the sample has been illuminated at 5 K, an increase in the dc
susceptibility was observed. This suggests that the transition from the low-spin state to the high-spin state can beinduced by illumination.

A number of spin-crossover complexes have been studied.1
These compounds are important in the development of electronic devices such as molecular switches. Seventeen years ago
Decurtins et al. observed a light-induced low-spin (LS) → highspin (HS) transition, during which the molecules can be quantitatively trapped in the excited HS state at sufficiently low
temperatures.2 The phenomenon is called light-induced excitedspin state trapping (LIESST), and the existence of the lightinduced HS state for the iron(II) spin-crossover compounds wasverified with optical, Mössbauer, and magnetic susceptibility
measurements. 2 − 7 The discovery of this LIESST effectsuggested that the spin-crossover compounds could be used as
optical switches. In the meantime a number of iron(II)compounds with long-live light-induced metastable HS states
at low temperatures have been found.2−7
Recently, we have succeeded in observing the LIESST
effect for an iron(III) complex [Fe(pap)2]ClO4·H2O (Hpap = 2hydroxyphenyl-(2-pyridyl)-methaneimine) by using the newly
designed ligand which enhanced cooperative behavior andstrong intermolecular interactions for the first time.8 The planar
ligand, pap, with corresponding π electrons, has a potential
ability to interact with neighboring ligands by π-π interactions.
In fact, the presence of a strong intermolecular interaction hasbeen suggested from the single crystal X-ray analysis. Our
purpose is to investigate the intermolecular interactions
between iron(III) complexes and to stabilize long-lived
metastable HS state for illuminated samples by decreasing therate of tunneling. However, the LIESST effect for iron(III)compounds has been observed only in the compounds with π-πintermolecular interaction so far.8, 9 We directed toward toproduce the compounds, which can be switched by illuminationand the metastable HS state can be trapped by introducing
hydrogen bonding.
The iron(III) complex we focused on [Fe(thpu)(Hthpu)] (1),where H 2 thpu is a tridentate Schiff-base derived from thecondensation of pyruvic acid with thiosemicarbazide. The
temperature dependent spin-crossover phenomenon, S = 1 / 2(LS state) ↔ S = 5 / 2 (HS state) of the molecular complex,[Fe(thpu)(Hthpu)] (1) has been extensively studied byHendrickson et al. (Figure 1).10 The compound 1 was preparedby the method described previously (Figure 2).11 The thpu2−(thiol form) and Hthpu− (thione form) ligand is coordinated toa metal ion. Both ligands are obtained by sequentially deprotonating neutral pyruvic acid thiosemicarbazine. Unfortunately,*Corresponding author. E-mail: hayascc@mbox.nc.kyushu-u.ac.jp.
FAX: +81-92-642-2607.

no single crystals of 1 were obtained. The chromium(III)compound [Cr(thpu)(Hthpu)]·H2O was synthesized by Ablov
et al.12 and the crystal structure of the compound was determined by Hendrickson et al.10 The crystallographic study
revealed the structural differences between the two ligands
thpu2− and Hthpu −. The singly deprotonated Hthpu − ligand
possesses a hydrogen atom bound to the nitrogen atom(=N–NH–), whereas there is no hydrogen atom at the nitrogen
position of the doubly deprotonated thpu2− ligand (=N–N=). It
has been inferred that the crystal structure of the compound 1NH2HNNOOH
H2 thpu

NH2S

HN-HS

NO
OHthpu-

NH2
NNOO-

NH2

SH

N-H

NO
O2thpuFigure 1. Derivatives after deprotonations of the H2thpu ligand.

Figure 2. A proposed structure for [Fe(thpu)(Hthpu)] (1).

© 2004 The Japan Society of Nuclear and Radiochemical Sciences
Published on Web 02/28/2004S-Hayami

J. Nucl. Radiochem. Sci., Vol. 5, No. 1, 2004

is similar to that of chromium(III) compound by using X-ray
powder diffraction patterns and IR spectra by Hendrickson etal.10 It is quite likely, thus, that the spin transition for 1 is dueto the cooperative aspect of iron(III) complexes through intermolecular hydrogen bonds.
At room temperature, the thermodynamically stable form of
the compound 1 is a high-spin state. Magnetic properties showthat the value of χmT for 1 is equal to 3.69 cm3 K mol−1 at roomtemperature, which corresponds to what is expected for HS
state in iron(III) compounds (Figure 3). On cooling, the valueof χmT drops abruptly around T1/2↓ = 225 K. The χmT value at
200 K is equal to 0.55 cm3 K mol−1, corresponding to the LS
state. In the warming mode, an abrupt variation of χ m T isobserved around T1/2↑ = 239 K. Fairly abrupt transitions with ahysteresis loop (ca. 14 K) show the presence of the strong
cooperative interaction such as magnetic behavior reported byHendrickson et al.10
Furthermore, Mössbauer spectra for 1 were measured at 80
K, 241 K and 293 K, and are shown in Figure 4. Mössbauer
spectra at 80 K show a wide quadrupole doublet with quadrupole splitting Q.S. = 3.09 and isomer shift I.S. (relative to α-ironfoil at room temperature) = 0.28 mm s−1, showing a LS statefor iron(III) compounds. At 241 K this absorption decreases
and a narrow quadrupole doublet with Q.S.= 0.81 and I.S.=
0.47 mm s−1 appears, supporting the transition from LS to a HS
state at half ratio. A doublet with Q.S. = 0.65 and I.S. = 0.49mm s−1 (line half-width Γ1/2 = 0.35 and 0.34 mm s−1) corresponding to the HS state of iron(III) compounds was observed
at 293 K. It is noticed that line half-widths Γ1/2 ≈ 0.35 mm s−1
for 1 are such small. The iron(III) centers in the HS, if they
are well isolated in molecular solids, can exhibit line halfwidths greater than 1 mm s−1.10, 13 Wignall has reported thatshort paramagnetic relaxation times and narrow Mössbauerline widths were observed for Fe···Fe distances of 6 – 7 Å.13Hendrickson et al. have been revealed by single crystal structure analysis that the Cr···Cr distances in the chromium(III)compound [Cr(thpu)(Hthpu)]·H2 O are probably quite short(< 7 Å).10 It may be thought that there is the strong intermolecular interaction in the compound 1 because the ligands thpu2−and Hthpu− in the compound have hydrogen bonding moieties.
Hendrickson et al. have been also reported that the compound
1 possess an abundance of potential sites for intermolecularhydrogen bonding interactions.10 Therefore we focused on thecompound 1 with strong intermolecular interaction for development of the photo switchable molecular solids.

−2A Hg-Xe lamp (λ ≈ 400–600 nm, 1.5 mW cm ) was used asa light source in the investigation of illumination effects. The
light passes through IR and green filters, and was guided via
an optical fiber into the SQUID. The sample was placed on
the edge of the optical fiber. When the sample was illuminatedat 20 K, an increase in the dc susceptibility was observed(Figure 5). The change in the magnetization persisted forseveral hours at 20 K, even after the illumination was stopped.
This increase suggests that a transition from the LS state to theHS state can be induced by illumination. When the temperature is raised slowly, the energy barrier between HS and LSstates vanishes and light-induced metastable HS state relaxes
to LS state. The relaxation temperature Tc(LIESST)14 for 1 is
1.00

293 K
0.96

Transmittance

N2

1.00

241 K
0.96

1.00

80 K
0.92-4-202

46

Velocity/mm s-1
Figure 4. Mössbauer spectra for 1 at selected temperatures.

0.58

3.50.56

χ m T / cm3 K mol -1

χ mT/cm3 K mol -13.02.5
2.0
1.5

0.54

hν
0.52

1.00.50

0.5200210220230240

250

Temperature/ K
Figure 3. χmT versus T plots for 1. Sample was cooled from 250 K to200 K (▼) and then warmed from 200 K to 250 K (▲) at a rate of 2 K
min−1.

2030

40

50

60

70

Temperature/ K
Figure 5. The χmT versus T plot was recorded in the warming mode(▲) after the sample was exposed to light for 2 hour.

Photo-Induced Spin Transition for an Iron(III)40 K. The achievement of an anomalous long-lived metastablestate is considered to be due to the presence of the strong intermolecular interaction, and the LIESST effect for 1 with intermolecular hydrogen bonds is the second example followingthat for [Fe(pap)2]ClO4 ·H2O with intermolecular π-π interaction.
In summary, we have succeeded in observing the LIESST
effect for an iron(III) compound [Fe(thpu)(Hthpu)] with intermolecular hydrogen bonds. We believe that our approach, i.e.,
the introduction of strong intermolecular interactions to trap
the metastable HS state can be applied in the design of metal
complexes with LIESST effects.
Acknowledgment. This work was supported by Casio
Foundation and Tokuyama Foundation.
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