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## Über dieses Buch

This is the first book to comprehensively address the recent developments in both the experimental and theoretical aspects of quasi-one-dimensional halogen-bridged mono- (MX) and binuclear metal (MMX) chain complexes of Pt, Pd and Ni. These complexes have one-dimensional electronic structures, which cause the various physical properties as well as electronic structures. In most MX-chain complexes, the Pt and Pd units are in M(II)-M(IV) mixed valence or charge density wave (CDW) states due to electron-phonon interactions, and Ni compounds are in Ni(III) averaged valence or Mott-Hubbard states due to the on-site Coulomb repulsion. More recently, Pd(III) Mott-Hubbard (MH) states have been realized in the ground state by using the chemical pressure. Pt and Pd chain complexes undergo photo-induced phase transitions from CDW to MH or metal states, and Ni chain complexes undergo photo-induced phase transitions from MH to metal states. Ni chain complexes with strong electron correlations show tremendous third-order optical nonlinearity and nonlinear electrical conductivities. They can be explained theoretically by using the extended Peierls-Hubbard model. For MMX-chain complexes, averaged valence, CDW, charge polarization, and alternating charge polarization states have been realized by using chemical modification and external stimuli, such as temperature, photo-irradiation, pressure, and water vapor. All of the electronic structures and phase transitions can be explained theoretically.

## Inhaltsverzeichnis

### Chapter 1. General Introduction

Abstract
For a long time, quasi-one-dimensional (Q1D) halogen-bridged metal complexes have attracted much attention of chemists and physicists because of significant physical properties based on their Q1D electronic structures such as intense and dichroic charge transfer (CT) bands [1–3], progressive overtones in the resonance Raman spectra [4–7], midgap absorptions attributable to solitons and polarons [8–10], gigantic third-order nonlinear optical susceptibilities [11, 12], spin-Peierls transitions [13], thermochromism in the organic media [14], insertion of 1D chains into artificial peptides [15], current oscillation phenomena [16], and field effect carrier doping [17].
S. Takaishi, M. Yamashita

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

Abstract
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.
Hiroshi Okamoto, Hiroyuki Matsuzaki

### Chapter 3. Ni(III) Mott–Hubbard Compounds

Abstract
For a long time, MX-chain compounds have been extensively studied since Wolffram reported Cl-bridged Pt complex in 1900. To date, more than 300 compounds were reported by combining metal ions (M = Ni, Pd, Pt), bridging halogens (X = Cl, Br, I) in plane ligands (L = ethylenediamine(en), 1R,2R-diaminocyclohexane(chxn), etc.), and counteranions (ClO 4 , BF 4 , X, etc.). However, the number of Ni compounds is very few compared to Pd or Pt compounds. The first Ni complex was reported by Yamashita et al. in 1981. Although there had been much controversy about the electronic state of these compounds, it has been clarified that this complex is in averaged valence state by means of X-ray crystal structure analysis [1]. In the Ni complexes, many attractive physical properties have been reported such as gigantic third-order nonlinear optical susceptibility [2], spin-Peierls transition [3], etc. In this chapter, we introduce structure, physical properties, and recent progresses in the Ni complexes.
S. Takaishi, M. Yamashita

### Chapter 4. Pd(III) Mott–Hubbard Compounds

Abstract
In spite of long history of MX-chain complexes, all Pt or Pd compounds are in CDW state, without exception. We have recently succeeded in realizing Pd(III) MH state by two methods, that is, one is the chemical pressure via long alkyl chains introduced as counteranions, [Pd(en)2Br](C n -Y)2⋅4H2O, and the other is the partial substitution with Ni(III) ions, [Ni1−x Pd x (chxn)2Br]Br2. In both systems, it has been revealed that Pd(III) MH state was realized by the chemical pressure acting in the systems.
S. Takaishi, M. Yamashita

### Chapter 5. Photoinduced Phase Transitions in MX-Chain Compounds

Abstract
One-dimensional (1D) electron system is good arena for the exploration of photoinduced phase transitions (PIPTs). This is because photocarrier generations and/or charge-transfer (CT) excitations by lights can stimulate instabilities inherent to 1D nature of electronic states through strong electron–electron interaction and electron (spin)–lattice interaction. Halogen (X)-bridged transition metal (M) compound (the MX-chain compound) examined here, which are prototypical 1D electronic systems with strong electron–electron interaction and electron–lattice interaction are promising candidates for realizing characteristic PIPTs. In this chapter, we review dynamical aspects of several typical PIPTs observed in the MX-chain compound (1) a photoinduced insulator–metal transition in the bromine-bridged nickel-chain compound, [Ni(chxn)2Br]Br2 (chxn = cyclohexanediamine), (2) a photoinduced charge-density-wave phase to Mott–Hubbard phase transitions, and (3) a photoinduced insulator–metal transition in the halogen-bridged palladium-chain compound, [Pd(chxn)2Br]Br2, and platinum-chain compound, [Pt(chxn)2I]I2. Dynamics and mechanism of these PIPTs are discussed on the basis of the experimental results obtained by femtosecond pump–probe reflection spectroscopy.
Hiroyuki Matsuzaki, Hiroshi Okamoto

### Chapter 6. Nonlinear Electrical Conductivity, Current Oscillation and Its Control in Halogen-Bridged Nickel(III) Compounds

Abstract
In this chapter, we introduce the nonlinear conducting behaviors in halogen-bridged nickel(III) compounds. Clear negative differential resistance behaviors are observed in [Ni(chxn)2Br]Br2 and [Ni(chxn)2Cl]Cl2. Utilizing these negative differential resistance behaviors, spontaneous current oscillations are achieved by being assisted with the external resistors and capacitor. Varying the circuit constants of the external components, we controlled the period of oscillations.
Hideo Kishida, Arao Nakamura

### Chapter 7. Third-Order Optical Nonlinearity of Halogen-Bridged Nickel(III) Compounds

Abstract
In this chapter, we review third-order optical nonlinearity of halogen (X)-bridged nickel (Ni) chain compounds, which are one-dimensional Mott–Hubbard insulators. Electroreflectance measurements revealed that a Ni–Br chain compound, [Ni(chxn)2Br]Br2 (chxn = cyclohexanediamine), has large third-order nonlinear susceptibility $${\chi^{(3)}}$$, which reaches 9 × 10−5 esu. Detailed analyses of the $${\rm Im} {\chi^{(3)}}$$ spectra showed that odd-parity and even-parity charge transfer states are nearly degenerated. As a result, the transition dipole moment between these two excited states is very large, leading to the enhancement of $${\chi^{(3)}}$$. Third-harmonic generation (THG) spectroscopy revealed that another odd-parity state, which corresponds to the continuum, is located above the nearly degenerate odd- and even-parity charge transfer states and plays an important role on the THG process. To evaluate $${\chi^{(3)}}$$ values corresponding to the all-optical switching process, a thin film sample of the Ni–Br chain compound was fabricated. $${\rm Re} {\chi^{(3)}}$$ and $${\rm Im} {\chi^{(3)}}$$ values measured by the Z-scan method were also found to be very large, exceeding 10−9 esu. All-optical switching using two-photon absorption processes demonstrated that THz repetition of on–off switching is possible in the thin film sample of the Ni–Br chain compound. These results show the high potential of Ni–X chain compounds as prospective nonlinear optical materials.
Hideo Kishida, Hiroshi Okamoto

### Chapter 8. Theory of MX Chain Compounds

Abstract
The theory of MX chains is described with emphases on the charge-density-wave (CDW) states, the Mott-insulator states, and the mutual conversions between them.
Kaoru Iwano

### Chapter 9. Crystal Structures and Properties of MMX-Chain Compounds Based on Dithiocarboxylato-Bridged Dinuclear Complexes

Abstract
In this chapter, a comprehensive study of the syntheses, crystal structures, and properties of the series of one-dimensional (1D) halogen-bridged mixed-valence dimetal complexes, MMX-chain compounds, based on the dithiocarboxylato-bridged dinuclear complexes, [Pt2(RCS2)4I] (R = Me (1), Et (2), n-Pr (3), n-Bu (4), n-Pen (5), and n-Hex(6)) and [Ni2(RCS2)4I] (R = Me (7), Et (8), n-Pr (9), and n-Bu (10)) are described. The evolution from 1D halogen-bridged metal complex, MX-chain compounds, to MMX-chain compounds has produced a variety of electronic states and subtle balance of solid-state properties originating from the charge–spin–lattice coupling and the fluctuation of these degrees of freedom. With increasing the internal degrees of freedom originating from the mixed-valence diplatinum unit, the Pt MMX-chain compounds except for 3 show relatively high electrical conductivity of 0.84–43 S cm–1 at room temperature and exhibit metallic conducting behavior with T M–S = 205–324 K. These compounds at room temperature are considered to take the valence-ordered state close to an averaged-valence (AV) state of –Pt2.5+–Pt2.5+–I–. The analyses of the diffuse scattering observed in the metallic state of 2 revealed that the metallic state has appeared by the valence fluctuation accompanying the dynamic valence-ordering state of the charge-density-wave (CDW) type of –Pt2+–Pt2+–I–Pt3+–Pt3+–I–. On the other hand, the metallic Pt MMX-chain compounds become insulators with lowering temperature due to the lattice dimerization originating from an effective half-filled metallic band. The synchrotron radiation crystal structure analysis of 2 at 48 K revealed that the valence-ordered state in the LT phase is the alternate charge-polarization (ACP) state of –Pt2+–Pt3+–I–Pt3+–Pt2+–I–. Furthermore, the elongation of the alkyl chains introduces increasing motional degrees of freedom in the system. Interplay between electronic degrees of freedom and molecular dynamics is also expected to cause an intriguing structural phase transition accompanying an electronic and/or magnetic transition never observed for [M2(MeCS2)4I] (M = Pt (1), Ni (7)). With the elongation of alkyl chains in dithiocarboxylato ligands, the compounds 35 undergo two phase transitions at near 210 K and above room temperature, indicating the existence of the LT, RT, and HT phases. The periodicity of crystal lattice in the RT phase of 35 along 1D chain is threefold of a –Pt–Pt–I– unit, and the structural disorders have occurred for the dithiocarboxylato group and the alkyl chain belonging to only the central dinuclear units in the threefold periodicity. In the HT phase, the dithiocarboxylato groups of all the dinuclear units in 35 are disordered and the lattice periodicities in 3 and 4 change to onefold of a –Pt–Pt–I– period. Ikeuchi and Saito have revealed from the heat capacity measurements that the entropy (disorder) reserved in alkyl groups in the RT phase is transferred to the dithiocarboxylate groups with the RT–HT phase transition [50–52]. Whereas, the lattice periodicity of 4 in the LT phase changes to twofold periodicity being assigned to the ACP state similar to the LT phase of the compound 2 and the dithiocarboxylate groups of all the diplatinum units are ordered. Furthermore, accompanying to the RT–LT phase transition, the compound 4 exhibits the paramagnetic–nonmagnetic transition originating from the regular electronic Peierls transition. These facts suggest that the dynamics (motional degrees of freedom) of the dithiocarboxylato ligands and bridging iodine atoms affects the electronic and magnetic systems through the electron–lattice interaction.
On the other hand, unlike the metallic Pt MMX-chain compounds, all the Ni MMX-chain compounds are Mott–Hubbard semiconductor due to the strong on-site Columbic repulsion on the nickel atom. The room-temperature crystal structures of the compounds 710 indicate their valence states to be an averaged-valence (AV) state or a charge-polarization (CP) state of –Ni(2.5−δ)+–Ni(2.5+δ)+–I–Ni(2.5−δ)+–Ni(2.5+δ)+–Ni(2.5−δ)+–I– ($$\delta \ll 0.5$$) close to an averaged-valence state. With the elongation of the alkyl chains in dithiocarboxylato ligands, the periodicity of crystal lattice in 9 and 10 along 1D chain in the RT phase is threefold of a –Ni–Ni–I– unit by the same origin as the diplatinum compounds 35, and furthermore, the lattice periodicity of 9 changes to onefold in the LT phase with a first-order phase transition at 205.6 K. The high temperature magnetic susceptibilities of 810 can be described by an S = 1/2 1D Heisenberg antiferromagnetic chain model with the very large exchange coupling constant |J|/k B ranging from 898(2) to 939(3) K. Furthermore, the compounds 8 and 9 undergo a spin-Peierls (SP) transition at relatively high T sp = 47 and 36 K, respectively, which are accompanied by superlattice reflections corresponding to twofold of a –Ni–Ni–I– period below T sp. The synchrotron radiation crystal structure analysis of 8 at 26 K revealed that the valence-ordered state changes from the CP state in the RT phase to the ACP state in the SP phase. These facts demonstrate that the electronic system of the Ni MMX-chain compounds in which the on-site Columbic repulsion U plays a dominant role in determining the electronic system is hardly affected by the molecular dynamics.
Minoru Mitsumi

### Chapter 10. POP-Type MMX-Chain Compounds with Binary Countercations and Vapochromism

Abstract
Pyrophosphito-bridged diplatinum complex is one of the most studied dinuclear complexes in a paddle-wheel structure. The chemistry of pyrophosphito-bridged diplatinum complex began at the discovery of the Pt(II)–Pt(II) complex, K4[Pt2(pop)4]⋅2H2O (pop = P2H2O 5 2− ) by Roundhill et al. in 1977 [1]. Since [Pt2(pop)4]4− shows intense long-lived phosphorescence, photochemistry and excited-state chemistry of [Pt2(pop)4]4− has been attracted much attention (see following early reviews and references therein [2, 3]). Although no chemical bond exist between two Pt(II) atoms in the ground state, 5dσ* → 6pσ transition should induce the bonding character between them. This excited-state structure has been confirmed by several optical methods [4–9]. The excited state of [Pt2(pop)4]4− is powerful one-electron reductant, therefore, it can be used as a photochemical catalyst for converting ethanol to acetaldehyde and hydrogen [10] and for the transfer hydrogenation of alkenes and alkynes [11, 12]. The Pt(III)–Pt(III) complex, [Pt2(pop)4H2]4−, is the active species of the catalytic reaction [13].
Hiroaki Iguchi, Shinya Takaishi, Masahiro Yamashita

### Chapter 11. Photoinduced Phase Transitions in MMX-Chain Compounds

Abstract
In this chapter, we deal with the photoinduced phase transition (PIPT) in the MMX-chain compound, which is another 1D electronic systems with strong electron–electron interaction and electron–lattice interaction. A characteristic PIPT between a diamagnetic charge-density-wave state and a paramagnetic charge polarization state appeared in the iodine-bridged binuclear platinum compound, R4[Pt2(pop)4I]nH2O (pop = P2O5H 2 2− ). The mechanism of the PIPT is discussed on the basis of the experimental results obtained by several kinds of optical spectroscopies.
Hiroyuki Matsuzaki, Hiroshi Okamoto

### Chapter 12. Theory of MMX-Chain Compounds

Abstract
Mechanisms of a variety of charge and lattice ordered phases observed in MMX compounds are theoretically studied by using a one-dimensional two-band three-quarter-filled extended Peierls–Hubbard model. In R4[Pt2(pop)4I]nH2O [R = Na, K, NH4, (CH3(CH2)7)2NH2, etc., pop = P2O5H 2 2− ] containing charged MMX chains, three electronic phases are suggested by experiments. We find that the variation of the electronic phases originates not only from competition between site-diagonal electron–lattice and electron–electron interactions but also from competition between short-range and long-range electron–electron interactions. On the other hand, in Pt2(RCS2)4I (R = CH3, n-C4H9) containing neutral MMX chains, a site-off-diagonal electron–lattice interaction and the absence of counterions are found to be crucial to produce the alternate-charge-polarization phase. The optical conductivity spectra are also studied, which directly reflect the electronic phases. A photoinduced transition has been found in a MMX compound, R4[Pt2(pop)4I]nH2O (R = (C2H5)2NH2). Its mechanism is theoretically studied by solving the time-dependent Schrödinger equation. Above a threshold in the photoexcitation intensity, a transition takes place from the charge-density-wave phase to the charge-polarization phase. The threshold-intensity dependence on the relative stability of these phases is explained qualitatively by their diabatic potentials. However, the transition in the opposite direction is hardly realized and needs careful consideration of different charge transfer processes.
Kenji Yonemitsu

### Backmatter

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