2. Structures and Optical Properties of Pt and Pd Compounds with Charge-Density-Waves

In this chapter, we review electronic structures and physical properties in MX-chain compounds having the charge-density-wave (CDW) ground states. By substituting the metals (M = Pt, Pd, and Ni), the bridging halogens (X = Cl, Br, and I), the ligand molecules, and the counteranions surrounding the MX chains, the amplitude of CDW, the degeneracy of CDW, and the optical gap energy corresponding to the energy of charge-transfer (CT) exciton can be widely controlled. On the basis of these controls, nature of photoexcited states including excitons, solitons, and polarons were investigated using photoluminescence spectroscopy, photoinduced absorption (PA) spectroscopy, and photoinduced electron spin resonance measurements. Comparisons of the PA spectra experimentally obtained with those theoretically expected, photogenerations of spin-solitons and polarons were demonstrated. After the resonant excitation of the CT excitons, the radiative decay from the self-trapped exciton competes with the nonradiative decay via the spin-soliton formation and subsequent nonradiative recombination. By referring to the theoretical analyses based upon the one-dimensional extended Peierls–Hubbard model, we discuss the overall view of the relaxation process of photoexcited states related to solitons, polarons, and excitons in MX-chain compounds.