Structures and Optical Properties of Pt and Pd Compounds with Charge-Density-Waves - Material Designs and New Physical Properties in MX-and MMX-Chain Compounds - page 26

Fig. 2.15 Schematic diagram
of the relaxation process of
the photoexcited state in
Pt-Br-Pt-II (Reprinted figure
with permission from [ 5 ])

E exc > E CT e-h pair

E exc ∼ E CT CT exciton

self-trapped
exciton

spin-soliton
pair

polaron
pair

nonradiative
recombination

luminescence

pair (the singlet B u state) to a spin-soliton pair is essentially forbidden within the
noninteracting electron model, due to the charge configuration symmetry in the
bond ordered wave (BOW) ground state [ 64 , 65 ]. Additional interactions (i.e., a
second nearest-neighbor hopping [ 64 ]) which break the charge configuration sym-
metry, permit the conversion of the electron-hole pair to a spin-soliton pair. On the
other hand, the MX-chain compounds are not charge configuration symmetric from
the nature of CDW, so that photogeneration of spin-solitons is possible to occur.
Moreover, in contrast to trans -(CH) x , the excitonic effect is important in MX-chain
compounds. Thus, the generation process of spin-solitons from the photoexcited
states in MX-chain compounds is considerably different from that in trans -(CH) x .

2.3.4 Overall Dynamics of Photoexcited States

The relaxation process of photoexcited states in Pt-Br-Pt-II was summarized in
Fig. 2.15 . Polarons are not generated from the CT exciton, since the energy of a
polaron pair is much higher than the CT exciton. Polarons can be generated only
from the higher energy excited states corresponding probably to the electron-hole
continuum. CT excitons are relaxed to STEs. A part of STEs decay by PL, and the
other parts are relaxed to spin-soliton pairs and then decay nonradiatively. CT
excitons with large excess energies dissociate into spin-soliton pairs before
stabilizing as STEs, since the generation efficiency of spin-solitons increases with
increase of the excitation photon energy at around E CT as seen in Fig. 2.10 .This
process is also shown by the dashed-dotted line in Fig. 2.15 .
As mentioned in Sect. 2.2.2, Pt and Pd compounds having halogen ions as the
counter anions show the 2D-ordered CDW states. In a typical example of such
compounds, [Pt(chxn) 2 ][Pt(chxn) 2 Br 2 ]Br 4 , it was also demonstrated that
photogenerations of solitons were suppressed [ 37 ] and only the photogeneration
of polarons was observed. In general, in the compounds having 2D-ordered CDW
states, PL intensities were relatively larger than that in the 1D CDW compounds as
seen in Fig. 2.8 , consistent with the result of the hetero-metal compound, Pt-Br-Pd.