190
V. Schünemann
An advantage of using Mössbauer spectroscopy for the characterization of SCO
compounds lies in the fact that this technique is able to discriminate between different
iron sites and/or conformers of the corresponding complex under study. This is
particular important for the investigation of polynuclear SCO complexes. Recently,
we have shown that only the middle iron site of a trinuclear iron linear N1, N2triazole bridged trinuclear Fe(II) complex undergoes a spin transition [55]. The fact
that the two outer iron of the complex are in a 3N3O environment and the inner iron
is in a 6 N coordination lead to slightly different isomer shifts of δ = 1.16 mms
−1
for the outer high spin iron(II) in comparison to the inner iron(II) which undergoes
SCO around 100 K and displays δ = 1.21 mms
−1 in its high spin state (Fig. 4.8).
More examples of this kind can be found in Chap. 6 of this book.
4.3.2 Light Induced Excited Spin State Trapping (LIESST)
For the possible application of SCO molecules in novel storage devices chemists
have started to combine photoactive metal ions with iron ions with suitable ligand
fields which allow SCO. Such a trinuclear complex containing two Pt
II ions and one
Fe
II ion has been prepared by the group of M. Ruben [56].
We have studied this complex by conventional Mössbauer spectroscopy the results
of which will be described in the following. Fig. 4.9 shows the structure of two
conformers of the Pt 2 Fe complex which have been investigated by temperature dependent Mössbauer spectroscopy. The presence of two molecular conformers has been
shown by X-ray crystallography. One conformer (1H) is bent and the other is straight
(1L). A Mössbauer spectrum taken at low temperature of a powder sample of this
compound shows two components. One component with δ = 0.40 mms
−1 and E Q
= 0.75 mms
−1 represents the ferrous low spin iron and the other component with δ
= 1.11 mms
−1 and E Q = 3.43 mms
−1 is characteristic for ferrous high spin iron
(Fig. 4.10a).
When the FePt 2 -complex is irradiated with light at T = 4.2 K, almost all 1L
iron sites can be transferred to the high spin state. This phenomenon is called Light
Induced Excited Spin State Trapping (LIESST) . It has been extensively investigated
by the group of Gütlich and coworkers [52] and is also discussed in Chap. 6 of this
book. In order to investigate the electronic properties of LIESST states Mössbauer
spectroscopy in large applied fields can be used. In order to realize such experiments
a homebuilt sample rod for a high field cryostat in our laboratory [57] has been
equipped with optical fibers as displayed in Fig. 4.11. In this way, Mössbauer spectroscopic measurements can be performed during irradiation of a sample at very low
temperatures (T 4.2 K) and high applied magnetic fields (in our case up to 5 T).
In principle every cryostat suitable for Mössbauer spectroscopy with top loading
sample capabilities can be equipped with such a modification.
Figure 4.12 shows Mössbauer spectra of Fe 2 Pt 2 after 17 h irradiation with red light
at 635 nm using a CW laser with the set-up displayed in Fig. 4.11 [58]. The analysis of
the field dependent data by means of the spin Hamiltonian formalism (see Sect. 4.2.3)
V. Schünemann
An advantage of using Mössbauer spectroscopy for the characterization of SCO
compounds lies in the fact that this technique is able to discriminate between different
iron sites and/or conformers of the corresponding complex under study. This is
particular important for the investigation of polynuclear SCO complexes. Recently,
we have shown that only the middle iron site of a trinuclear iron linear N1, N2triazole bridged trinuclear Fe(II) complex undergoes a spin transition [55]. The fact
that the two outer iron of the complex are in a 3N3O environment and the inner iron
is in a 6 N coordination lead to slightly different isomer shifts of δ = 1.16 mms
−1
for the outer high spin iron(II) in comparison to the inner iron(II) which undergoes
SCO around 100 K and displays δ = 1.21 mms
−1 in its high spin state (Fig. 4.8).
More examples of this kind can be found in Chap. 6 of this book.
4.3.2 Light Induced Excited Spin State Trapping (LIESST)
For the possible application of SCO molecules in novel storage devices chemists
have started to combine photoactive metal ions with iron ions with suitable ligand
fields which allow SCO. Such a trinuclear complex containing two Pt
II ions and one
Fe
II ion has been prepared by the group of M. Ruben [56].
We have studied this complex by conventional Mössbauer spectroscopy the results
of which will be described in the following. Fig. 4.9 shows the structure of two
conformers of the Pt 2 Fe complex which have been investigated by temperature dependent Mössbauer spectroscopy. The presence of two molecular conformers has been
shown by X-ray crystallography. One conformer (1H) is bent and the other is straight
(1L). A Mössbauer spectrum taken at low temperature of a powder sample of this
compound shows two components. One component with δ = 0.40 mms
−1 and E Q
= 0.75 mms
−1 represents the ferrous low spin iron and the other component with δ
= 1.11 mms
−1 and E Q = 3.43 mms
−1 is characteristic for ferrous high spin iron
(Fig. 4.10a).
When the FePt 2 -complex is irradiated with light at T = 4.2 K, almost all 1L
iron sites can be transferred to the high spin state. This phenomenon is called Light
Induced Excited Spin State Trapping (LIESST) . It has been extensively investigated
by the group of Gütlich and coworkers [52] and is also discussed in Chap. 6 of this
book. In order to investigate the electronic properties of LIESST states Mössbauer
spectroscopy in large applied fields can be used. In order to realize such experiments
a homebuilt sample rod for a high field cryostat in our laboratory [57] has been
equipped with optical fibers as displayed in Fig. 4.11. In this way, Mössbauer spectroscopic measurements can be performed during irradiation of a sample at very low
temperatures (T 4.2 K) and high applied magnetic fields (in our case up to 5 T).
In principle every cryostat suitable for Mössbauer spectroscopy with top loading
sample capabilities can be equipped with such a modification.
Figure 4.12 shows Mössbauer spectra of Fe 2 Pt 2 after 17 h irradiation with red light
at 635 nm using a CW laser with the set-up displayed in Fig. 4.11 [58]. The analysis of
the field dependent data by means of the spin Hamiltonian formalism (see Sect. 4.2.3)