Chemistry Reference
In-Depth Information
4
×
100
×
10
3
×
1000
2
1
0
1.0
1.2
1.4
1.6
1.8
2.0
2.2
2.4
2.6
Photon energy (eV)
Fig. 2.5 Photoluminescence and Raman bands in [Pt(en)
2
][Pt(en)
2
Cl
2
](ClO
4
)
4
at 2 K. The
excitation photon energy (2.54 eV) is indicated by the
arrow
. (Reprinted figure with permission
from [
11
])
Dynamics of STE was studied by time-resolved PL measurements [
60
-
62
].
Figure
2.7
shows the time evolutions of PL at several probe energies in [Pt(en)
2
]
[Pt(en)
2
Br
2
](ClO
4
)
4
-I. The oscillatory structures were observed and attributed to
the wave-packet motion in the potential curve as shown by the arrow in Fig.
2.6b
.
A similar oscillation was also detected in [Pt(en)
2
][Pt(en)
2
Br
2
](PF
6
)
4
by using
transient absorption technique [
63
].
Next, we discuss how the PL properties depend on materials. The energies of PL
(
E
lm
) for various MX compounds were also listed in Table
2.1
and plotted against
the distortion parameter
d
in Fig.
2.4b
[
5
].
E
lm
is 40-50 % of
E
CT
for the Pt or Pd
compounds and 50-60 % for the heterometal compounds with M
¼
Pt and Pd,
showing large Stokes shifts in common. On the other hand, the efficiency of PL is
considerably changed by the decrease in gap energy
E
CT
. In Fig.
2.8
, the relative
intensities of PL for the 2.4-eV excitation measured at 2 K are plotted for various
MX compounds [
5
]. The intensity of PL decreases by more than four orders of
magnitude when
E
CT
decreases from 2.7 to 1.4 eV. This result suggests that with
decrease of
E
CT
, that is, the halogen distortion
, the STE becomes unstable.
Another feature in Fig.
2.8
is that the PL intensity in the compounds with nonde-
generate CDW [the heterometal compounds (diamonds) and the Pt compounds with
the 2D ordered CDW (solid circles)] is relatively larger than those of the Pt
compounds with 1D CDW. This is related to the fact that nonradiative recombina-
tion of photoexcited states via soliton formations is suppressed in nondegenerate
CDW compounds. This point is discussed in the following subsections.
d
2.3.2 Photogeneration of Solitons and Polarons
In this section, we review the studies of solitons and polarons in the MX-chain
compounds. The most effective method to detect solitons and polarons is a
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