Chemistry Reference
In-Depth Information
complete at 100 K. On further heating to 150 K the sample undergoes again
thermal ST at 135 K to the (now stable) HS state.
This photophysical phenomenon has become known as LIESST. The processes
involved in the LIESST effect are well understood on the basis of ligand field
theory [
23
,
24
]. Figure
2.14
explains the mechanisms of LIESST and reverse-
LIESST. The energy level scheme shows in the uppermost part the distribution of
the six valence electrons of Fe
II
over the five d-orbitals split in an octahedral ligand
field into the subgroups t
2g
and e.
g
resulting in the two spin states LS (left) and HS
(right). The corresponding energy potentials are drawn in the lower part of the
scheme. The complex molecules in the HS state are bigger than those in the LS
state due to the fact that the antibonding e
g
orbitals are partially occupied in the HS
state, whereas in the LS state they are empty. Thus the HS potentials are placed in
positions of larger metal-donor atom distances as compared to the LS potentials.
Green light (514 nm) excites the LS state (
1
A
1
) to the
1
T
1
and
1
T
2
states (spin-
allowed, but parity-forbidden), which decay fast via the spin triplet states
3
T
1,2
to the
5
T
2
state. This double intersystem crossing decay path is favored by spin-orbit
coupling over the direct decay path back to
1
A
1
. Decay of the
5
T
2
state to the
1
A
1
state is forbidden, the metastable HS state is trapped until radiationless thermal
relaxation sets in by nonadiabatic multiphoton processes [
23
,
24
]. Light-induced
back conversion of the metastable LIESST state is possible by irradiating the sample
with red light, thereby undergoing again double intersystem crossing processes
similar to the LS to HS conversion with green light (reverse-LIESST) [
29
].
Replacing propyl by methyl in the tetrazole molecule yields [Fe(mtz)
6
](BF
4
)
2
with mtz = 1-methyl-1H-tetrazole which undergoes thermal ST around 74 K. The
crystal structure at room temperature shows two lattice sites called A and B that only
differ slightly by their Fe-N bond lengths (Fig.
2.19
a) [
30
]. Mössbauer spectroscopy
between room temperature and 160 K shows only one HS site. However, on further
cooling, two sites with equal population become clearly distinguishable. Upon
cooling to 60 K, the HS quadrupole doublet (green) attributed to B is not affected.
However, a new signal (blue) appears at 85 K which is characteristics of the LS state
indicating a SCO of the A site. At 60 K, the transition of the A site ions is complete
while the B site ions fully remain in the HS state with ca. 50 % population evaluated
from the resonance area fractions assuming equal Lamb-Mössbauer factors for both
HS and LS states of Fe
II
(Fig.
2.19
b).
The [Fe(mtz)
6
](BF
4
)
2
compound with thermal ST at A site ions but HS behavior at
B site ions is an interesting case for LIESST effect studies (Fig.
2.20
). When the sample
is irradiated with green light at 20 K, where all A site ions have turned to the LS state
but all B site ions are still in the HS state, a complete LS(A) ? HS(A) photo-con-
version is observed. At around 65 K, thermal relaxation back to the LS state is
observed for the A site ions. When the sample is irradiated with red light at 20 K, the
signal corresponding to B site ions disappears almost totally, and a new resonance
signal appears which is characteristic for Fe
II
ions in the LS state. This is the first
observation of a light induced excitation of a stable HS state into a long lived meta-
stable LS state. Again, upon warming to ca. 65 K, complete thermal relaxation is
observed back to the stable HS(B) state [
31
].
