Thermal and light-induced
spin transition in iron compounds
P. Gütlich et al.
Institut für Anorganische Chemie und Analytische Chemie
Johannes Gutenberg-Universität Mainz
Staudingerweg 9, D-55099 Mainz, Germany
E-mail: guetlich@uni-mainz.de
http://ak-guetlich.chemie.uni-mainz.de
Thermal spin transition (spin crossover) is a widely observed phenomenon in
coordination chemistry of 3d transition metal ions with d4 up to d7 electron
configuration. It was first observed in iron(III) compounds around 1930 by the
italian researchers Cambi and coworkers, but is now well established and
extensively studied in many iron(II) compounds, last but not least by application of
the powerful tool of Mössbauer spectroscopy. Spin crossover in iron(II) compounds
may be observed if the ligand field strength is such that the difference between the
lowest “vibronic” energy levels of the high spin state 5T2 and the low spin state 1A1
state is comparable with thermal energy kBT (kB = Boltzmann constant). The spin
transition behaviour can be influenced by chemical alteration, e.g. ligand
replacement, change of non-coordinating anion and solvent molecule, substitution
of spin state changing metal by another metal, e.g. substitution of iron by zink. It
can also be influenced by physical perturbation such as irradiation with light,
application of pressure or a magnetic field.
For a comprehensive coverage of spin crossover research see:
- Gütlich, P.; Hauser, A.; Spiering, H.Angew. Chem. Int. Ed. Engl. 1994, 33, 2024.
- Gütlich, P.; Goodwin, H.A. (Eds.), Spin Crossover in Transition Metal
Compounds, Top. Curr. Chem. Vol 233, 234, 235, Springer, Berlin Heidelberg New
York
The influence of the ligand molecules on the spin state of the central iron(II) ions is
demonstrated with the following two examples and their temperature dependent
Mössbauer spectra shown in the next picture.
[FeII(phen)3]X2
LS
[FeII(phen)2(NCS)2]
HS
300 K
300K
188 K
240 K
LS
„Tuning“ the
Ligand Field
by Ligand
Replacement
186 K
184 K
80 K
180 K
5K
77 K
LS
[FeII(phen)3]X2 (phen = 1.10-phenanthroline) is a typical low spin compound with the
characteristic Mössbauer spectra as shown on the left; the isomer shift is ca. 0.2 mm s-1
(relative to -iron) and the quadrupole splitting ca. 0.5 mm s-1, nearly independent of
temperature. If one of the relatively strong phen ligands, which occupies two
coordination positions of the octahedron, is replaced by two monofunctional NCSgroups, the average ligand field strength becomes weaker than the mean spin pairing
energy and the compound [FeII(phen)2(NCS)2] adopts high spin (HS) character at room
temperature. The Mössbauer spectrum at 300 K shows the typical features of an iron(II)
HS compound with isomer shift of ca. 1 mm s-1 and large quadrupole splitting of ca. 3
mm s-1. However, the compound [FeII(phen)2(NCS)2] fulfils the condition for thermal
spin crossover to occur, viz. ΔEHL  kBT. On lowering the temperature, the compound
changes spin state from high spin to low spin near 180 K as is well documented by the
Mössbauer spectra as a function of temperature, which was first reported by I. Dezsi et
al. in 1967. Since then more than 200 spin crossover compounds of iron(II) and
iron(III) have been studied by Mössbauer spectroscopy (see e.g. P. Gütlich, H.A.
Goodwin (eds.), Spin Crossover in Transition Metal Compounds, Springer Series
“Topics in Current Chemistry, Vol. 233, 234, 235, Berlin Heidelberg 2004).
[Fe(ptz)6](BF4)2, where ptz stands for the ligand molecule 1-propyl-tetrazole as shown
in the picture, is another iron(II) coordination compound exhibiting thermal spin
crossover with a spin transition temperature T1/2 of ca. 135 K. The HS phase is nearly
colourless, the LS phase is red. Thus the thermally induced spin transition can be easily
followed by optical spectroscopy in the uv/visbile range. The 57Fe Mössbauer spectra
clearly indicate the transition near 135 K between the HS phase (quadrupole doublet
shown in red) and the LS phase (singlet shown in blue). Whereas the quadrupole
doublet is typical for iron(II) in the HS state, the singlet of the LS state is rarely
observed and points at a quasi regular octahedral surroundings at the iron centre.
Thermal spin transition in [Fe(ptz)6](BF4)2
followed by optical spectroscopy
C3H7
N
300 K
N
N
N
Six ptz molecules
coordinate via N atoms
to the iron center and
form the twofold positive
cationic complex ion. Two
BF4¯ serve as counter ions.
1A →1T
1
2
1A →1T
1
1
80 K
The complex compound [Fe(ptz)6](BF4)2 is in the high spin (HS) state 5T2 (t2g4eg2) at room temperature
down to ca. 135 K. The optical spectrum of the colorless crystal shows a weak absorption band for the
5T →5E transition. At 135 K, thermal spin transition to the low spin (LS) state 1A (t 6) takes place
2
1 2g
on further cooling with dramatic color change from white (HS) to red (LS). The optical spectrum shows
two spin-allowed (but parity-forbidden) singlet-singlet absorption bands.
E. W. Müller, J. Ensling, H. Spiering, P. Gütlich, Inorg. Chem., 22(14), 2074-8 1983
The 57Fe Mössbauer spectra of [Fe(ptz)6](BF4)2
recorded as a function of temperature clearly indicate the
spin transition between HS state (quadrupole doublet shown
in red) and LS state (singlet shown in blue) at ca. 135 K. The
HS quadrupole doublet is mainly caused by noncubic
electron distribution in the (Jahn-Teller-active) t2g4eg2
configuration. The singlet of the LS state arises from the t2g6
points at a quasi regular octahedral surroundings at the iron
centre. The spin transition occurs with hysteresis as shown by
the magnetic susceptibility measurements, from which the
effective magnetic moment can be derived.
Rel. Transmission (%)
Thermal spin transition in [Fe(ptz)6](BF4)2 followed by
Mössbauer spectroscopy and magnetic susceptilibility measurements
155 K
HS
136 K
98 K
LS
v (mm/s)
E. W. Müller, J. Ensling, H. Spiering, P. Gütlich,
Inorg. Chem., 22(14), 2074-8 1983
Mössbauer spectroscopy is ideally suited to follow the light-induced spin state
conversion in this system as exemplified in the next viewgraph
A polycrystalline sample of [Fe(ptz)6](BF4)2 was cooled to 15 K. Before irradiation the
sample is in the LS state and shows the typical Mössbauer spectrum of the LS state
(upper left). After irradiating with green light (xenon lamp with filters or 514 nm band
of an Ar ion laser) at 15 K the sample is quantitatively converted to the metastable HS
state (middle left). The asymmetry in the intensity of the two components of the
quadrupole doublet is due to the plate-like shape of the crystals (texture effect). Thermal
relaxation on a 15 minute-timescale sets in at 50 K (lower left and upper right: the
sample was heated for 15 minutes at 50 K and then cooled to the measuring temperature
of 15 K in two runs). Thermal relaxation to the stable LS state is complete at 10 K. On
further heating to 150 K the sample undergoes again thermal spin transition at 135 K to
the (now stable) HS state.
This photophysical phenomenon was first observed by S. Decurtins, P. Gütlich, C. P.
Köhler, H. Spiering, A. Hauser, Chem. Phys. Lett., 105(1), 1-4 1984 and became known
as “Light-Induced Excited Spin State Trapping” (LIESST). The processes involved
in the LIESST effect are well understood on the basis of ligand field theory (Gütlich, P.;
Hauser, A.; Spiering, H. Angew. Chem. Int. Ed. Engl. 1994, 33, 2024. The next
viewgraph explains the mechanisms of LIESST and reverse-LIESST.
Relative Transmission (%)
Light-Induced Spin Crossover in [Fe(ptz)6](BF4)2
15 K
no light
50 K
(30 ')
15 K
green light
100 K
50 K
(15 ')
150 K
LS
HS
5T
2
5E
15 K
green light
Light-Induced Excited Spin State Trapping
(LIESST)
e
The mechanism of LIESST and Reverse-LIESST
is explained on the basis of ligand field theory. Green
ligth (e.g. 514 nm from Ar ion laser) affords the
spin-allowed (but parity-forbidden) transition 1A1 →
1T , . Preferred relaxation occurs via two
12
subsequent intersystem crossing processes 1T1,2 →
3T → 5T , which are favoured by spin-orbit coupling.
1,2
2
The metastable HS state 5T2 has an extremely long
lifetime (typically minutes near 50 K). Backconversion
from the metastable HS state to the LS state can be
achieved by irradiation with red light (e.g. 820 from
Kr laser), whereby the first transition step 5T2 → 5E is
followed by two intersystem crossing processes
5E → 3T → 1A back to the stable LS state.
1,2
1
tg2
g