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 27

and the optical gap energy E CT
decrease and a compound approaches to the CDW-MH phase boundary,
photoresponses are largely changed from those discussed here. A photoexcited
state in the CDW ground state, that is, a M 3 þ pair is converted to a M 3 þ domain
over several tens M 3 þ sites [ 66 , 67 ]. In this case, the lowest photoexcited state of a
M 3 þ pair cannot be regarded as an excitonic state, but the photoresponses should be
regarded as a photoinduced phase transition from CDW to MH state. Such photo-
induced CDW to MH transitions were indeed observed in [Pd(chxn) 2 ][Pd
(chxn) 2 Br 2 ]Br 4 and [Pt(chxn) 2 ][Pt(chxn) 2 I 2 ]I 4 [ 66 , 67 ]. In the latter compound,
the photoinduced CDW to metal transition was also found in the case of strong
photoexcitations. Photoinduced phase transitions in those MX-chain compounds
are discussed in Chap. 5 in detail.

When the bridging halogen displacement d

2.4 Summary

In this chapter, we reviewed the tunability of the CDW states in MX-chain
compounds. By substituting the metals (M ¼ Pt, Pd, and Ni), the bridging halogens

(X Cl, Br, and I), and the ligand molecules and the counter anions surrounding

MX chains, the amplitude of CDW, the degeneracy of CDW, and the optical
gap energy could be widely controlled. On the basis of such controls of the CDW
states, nature of photoexcited states, i.e., excitons, solitons, and polarons, was
investigated. From the comparative studies of the degenerate and nondegenerate
CDW compounds using photoluminescence, photoinduced absorption, and photo-
induced electron spin resonance measurements, the photoinduced gap states were
clearly characterized as spin-solitons and polarons. By comparing the excitation
profiles of the photoinduced absorption signals due to spin-solitons and the
photoluminescence from self-trapped excitons, it was demonstrated that the lumi-
nescence process competes with the dissociation to spin-soliton pairs and the
subsequent nonradiative decay. The detailed analyses of the temperature depen-
dence of the photoluminescence decay time revealed that a conversion from a self-
trapped exciton to a spin-soliton pair occurs through a finite potential barrier, the
magnitude of which depends on the degeneracy of CDW. Such an exciton to a spin-
soliton pair conversion could be explained by the theoretical simulations based
upon the 1D-extended Peierls-Hubbard model. Thus, a spin-soliton pair is the
lowest energy excitation, so that it plays dominant roles on the relaxation process
of photoexcited states in MX-chain compounds having CDW ground states.
On the basis of these fundamental studies on the CDW states, a great deal of
efforts for designs and syntheses of new MX-chain compounds and explorations of
new phenomena have been made. As a result, a variety of novel physical properties
such as gigantic nonlinear optical responses and ultrafast-photoinduced phase
transitions have been discovered. The details of them are reviewed in the following
several chapters from both experimental and theoretical points of view.

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