Chemistry Reference
In-Depth Information
Fig. 2.16 The variable
temperature
57
Fe Mössbauer
spectra of [Fe(ptz) 6 ](BF 4 ) 2 ,
reveals a thermally induced
SCO with a transition
temperature on warming T 1/2
:
of ca. 135 K. Above this
temperature the spectra show
a quadrupole doublet
characteristic for the HS state
of Fe II . Near the transition
temperature the quadrupole
doublet disappears on further
cooling at the favor of a
singlet which is typical of the
LS state [ 28 ]
[Fe(ptz) 6 ](BF 4 ) 2 (ptz = 1-propyl-tetrazole), is another model Fe II coordination
compound exhibiting thermal SCO with a transition temperature on warming T 1/ : of
ca. 135 K. The 57 Fe Mössbauer spectra of Fig. 2.16 , recorded on warming from
98 K, clearly confirm the transition at this temperature between the LS phase (singlet
shown in dark grey) and the HS phase (quadrupole doublet shown in grey).
2.3.2.2 Light Induced Spin State Switching
With [Fe(ptz) 6 ](BF 4 ) 2 it was observed for the first time that the ST can also be
induced by irradiating the crystals with light; green light converts the LS state to the
HS state, which can have very long lifetimes, e.g. on the order of days at temperatures
below ca. 20 K [ 26 ]. Figure 2.17 shows a single crystal of [Fe(ptz) 6 ](BF 4 ) 2 (size ca.
3 9 3cm 2 ), which is colorless at room temperature. The absorption spectrum shows
only a weak absorption band at ca. 12,000 cm -1 . At 80 K the crystal has changed
totally to the LS state by thermal SCO and is now red and absorbs relatively strongly
at 18,000 and 26,000 cm -1 . Irradiating the crystal at ca. 10 K converts the LS state to
the metastable HS state. The optical spectrum of the white spot (ca. 1 mm in
diameter) is practically identical to the one recorded at 300 K.
Mössbauer spectroscopy is ideally suited to follow the light-induced spin state
conversion in [Fe(ptz) 6 ]BF 4 ) 2 as exemplified in Fig. 2.18 . A polycrystalline
sample of [Fe(ptz) 6 ](BF 4 ) 2 was cooled to 15 K. Before irradiation, the sample is in
the LS state and shows the typical Mössbauer spectrum of Fe II -LS (upper left).
After irradiating with green light (Xe lamp with filters or 514 nm band of an Ar
ion laser) at 15 K the sample is quantitatively converted to the metastable HS state
(middle left). The asymmetry in the intensity of the two components of the
 
Search WWH ::




Custom Search