Chemistry Reference
In-Depth Information
magnetic) properties of metallomesogens is significantly extended, since the
organic ligand of these systems can be varied. Liquid crystalline materials in
which a SCO center is incorporated into the mesogenic organic skeleton establish a
separate class of compounds for which an interplay of structural transitions and
liquid crystallinity is expected. This may lead to advantages in practical applica-
tions, for example, processing SCO materials in the form of thin films, enhance-
ment of ST signals, switching and sensing in different temperature regimes, or
achievement of photo- and thermochromism in metal-containing liquid crystals.
The change of color in coexistence with liquid crystallinity is certainly a phe-
nomenon of particular interest in the field of materials sciences.
Galyametdinov et al. [
47
] reported on temperature-dependent Mössbauer and
magnetic susceptibility measurements of an Fe
III
compound which exhibits liquid
crystalline properties above and thermal ST below room temperature. This was the
first example of SCO in metallomesogens. Later, different families of Fe
II
and Co
II
systems were also investigated [
48
-
57
]. The question whether the solid-liquid
crystal phase transition provokes the spin-state change in SCO metallomesogens
has been addressed in several series of Fe
II
systems employing a variety of
physical measurements [
54
, and references therein]. In all these studies
57
Fe
Mössbauer spectroscopy has been extremely helpful, e.g. in controlling the com-
pleteness of ST in both the high and low temperature regions, where v
M
T data are
often not reliable due to calibration difficulties. Also, one can unambiguously
decide whether a significant decrease of the v
M
T vs. T plot towards lower tem-
peratures is due to SCO or zero-field splitting. An example is the study of the one-
dimensional (1D) 1,2,4-triazole-based compound [Fe(C
10
-tba)
3
](4-MeC
6
H
4-
SO
3
)
2
nH
2
O, with C
10
-tba = 3,5-bis(decyloxy)-N-(4H-1,2,4-triazol-4-yl)benz-
amide, n = 1or0[
55
,
56
]. This system exhibits a spin state change on warming as
a result of solvent release with a concomitant change of color between white (HS
state) and purple (LS state) [
58
]. The magnetic properties of the pristine compound
(n = 1) and the dehydrated sample (n = 0) are depicted in the form of v
M
T vs.
T plots in Fig.
2.29
.
At 300 K the value of v
M
T = 0.20 cm
3
K mol
-1
indicates that the compound is
in the LS (purple) state. The Mössbauer spectrum recorded at 4.2 K is in agree-
ment with the magnetic data, i.e. the HS population is 4.8 % and the LS population
95.2 % (Fig.
2.30
a). Upon heating v
M
T increases abruptly within a few degrees,
reaching the value of 3.74 cm
3
K mol
-1
at 342 K. This clearly shows that a spin
state change from LS to HS has occurred. The thermo-gravimetric analysis (TGA)
of this system showed that dehydration takes place in the same temperature region
where the spin state change occurs. The magnetic susceptibility of the dehydrated
compound (n = 0) was recorded in a temperature loop, i.e. from 375 K down to
10 K and then up again to 375 K. The dehydrated complex reveals incomplete
SCO, accompanied by hysteresis and color change (from purple in the LS state to
white in the HS state), in the temperature region of 250-300 K. Around 50 % of
Fe
II
sites have changed the spin state as can be inferred from the value of v
M
T at
200 K. The Mössbauer spectrum recorded at 200 K (Fig.
2.30
b) yields 49.3 % of
Fe
II
in the LS state and 50.7 % in the HS state. The further decrease of the
