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consist of two Fe
II
ions bridged by three triazole ligands in a 1,2-fashion. The
coordination spheres of both iron ions are completed by two NCS anions and one
monodentate 1,2,4-triazole ligand. The monodentate triazole ligands are connected
through the non coordinating N atom to the mononuclear iron unit by hydrogen
bonds from its coordinated water molecule. Magnetic susceptibility measurements
indicate a ST only for the iron ions in the dinuclear units, centered at around
T
1/2
= 111 K. The transition takes place within a relatively narrow T range. The
mononuclear iron ion with relatively weak ligand field strength of the FeN
4
O
2
core
remains in the HS state even at very low temperature, yielding a ratio of 4:1 of LS to
HS iron ions. Temperature-dependent Mössbauer spectroscopy and magnetic sus-
ceptibility measurements confirm these results as seen from Figs.
2.27
and
2.28
[
46
].
2.3.2.7 Spin Transition in Metallomesogens
Metallomesogens belong to a class of metal-containing compounds which exhibit
liquid-crystal (LC) properties. The possibility of combining LC properties with
SCO behavior and accompanying change of color and magnetism in advanced
functional materials has been a major objective of research in this field in recent
years. In addition, the possibility of tuning the physical (mesomorphic, optical,
57
Fig. 2.28
Fe Mössbauer spectra of [Fe
2
L
5
(NCS)
4
]
2
[FeL
2
(NC-
S)
2
(H
2
O)
2
]
H
2
O[
44
]. Above the ST temperature of ca. 110 K the spectra show three doublets
(red, green, orange) characteristic of Fe
II
in HS state. The green doublet refers to the central Fe
II
site, which remains in the HS state over the whole temperature range under study. The red and
orange doublets with nearly equal intensity arise from the HS- Fe
II
ions in the outer dinuclear
units; the slight differences in quadrupole splitting point at slightly different molecular
distortions. These two doublets decrease in intensity on lowering the temperature at the favor of a
new signal (singlet shown in blue) with increasing intensity arising from the four LS-Fe
II
ions in
the dinuclear units after thermal spin transition
Variable
temperature
