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relax into a self-trapped state. This is in contrast to the relaxation process for the CT
exciton state (M
3+
,M
3+
M
4+
,M
2+
) in Peierls-distorted Pd and Pt MX chains. Pt and
Pd MX chains show large Stokes shifts due to the strong electron-lattice interaction
because bridging halide ions X
are quite sensitive to the charge of M ions. On the
other hand, in the case of [Ni(chxn)
2
Br]Br
2
, because the bridging bromine, which is
neutral (Br
0
) when the LMCT occurs, is insensitive to the charge of the Ni ions, it
showed almost no Stokes shift, indicating that the electron-lattice interaction (
S
)ofthis
system is small or suppressed. More detailed studies have been recently made on the
time dependence of the photoluminescence [
22
] as well as theoretical research [
23
,
24
].
The peaks at 1.3 and 1.4 eV gradually disappeared with an increase in the
temperature. This shows that the deactivation process changes from luminescence
to thermal relaxation. The peak at 1.3 eV slightly shifted to a lower energy with an
increase in the temperature. We attributed the shift to an increase in the Ni-Br
distance with an increase in the temperature, resulting in a decrease in the separa-
tion of Br
4
p
z
and Ni
3+
3d
z
2
orbitals.
!
References
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