Chemistry Reference
In-Depth Information
Fig. 2.14 Jablonski diagram
depicting potential wells of
the LS and HS sates for an
Fe
II
complex, illustrating the
LIESST and reverse-LIESST
mechanism. These
phenomena refer to the
possibility to reversibly
address spin states of Fe
II
complexes by light irradiation
concepts, the electron configuration changes from t
2g
4
e
g
2
in the HS state to
t
2g
6
in the LS state. This phase transition between paramagnetic and (nearly)
diamagnetic is easily detected by magnetic susceptibility measurements. As the
color changes simultaneously, too, the transition from one spin state to the other is
also easily detected by optical means. The spin transition (ST) can also be afforded
by applying external pressure, magnetic field or by irradiating the material with
light, where green light converts the LS to the HS state and red light the HS to the
LS state. This has become known as Light-Induced Excited Spin State Trapping
(LIESST) and reverse-LIESST [
26
].
The condition that has to be met in order to observe thermal SCO is sketched in
Fig.
2.14
using the term symbols known from ligand field theory. Thermal SCO
may be observed if the ligand field strength of an Fe
II
compound is such that the
difference between the lowest ''vibronic'' energy levels of the HS state
5
T
2
and the
LS state
1
A
1
state is comparable with thermal energy k
B
T (k
B
= Boltzmann
constant). The ST behavior can be influenced by chemical alteration of the
material, e.g. ligand replacement, change of non-coordinating anion and solvent
molecule, substitution of spin state changing metal by another metal (e.g. iron by
zinc). For a comprehensive coverage of chemical and physical influences on the
ST behavior see Refs. [
23
-
25
].
