Halogen-bridged Mixed-valence Complexes Research Articles

Heat Capacity of the Halogen-Bridged Mixed-Valence Complex Pt2(dta)4I (dta = CH3CS2-)

Heat capacity of the halogen-bridged mixed-valence complex Pt2(dta)4I (dta = CH3CS2-) has been measured in the temperature region between 6 and 386 K. The complex exhibited a phase transition of order−disorder type at Ttrs = 373.4 K, from being arranged in an ordered helical form of four dta ligand planes around the central Pt−Pt axis to dynamically jumping between two orientations. Neither latent heat nor distinct thermal hysteresis was observed for the phase transition, suggesting that the phase transition is of higher-order rather than of first-order. The transition enthalpy and entropy were determined to be ΔtrsH = 1.91 ± 0.02 kJ mol-1 and ΔtrsS = 5.25 ± 0.07 J K-1 mol-1, respectively. The transition entropy close to R ln 2 (= 5.76 J K-1 mol-1), where R is the gas constant, implies that the twisting motion of the four dta ligands takes place in a synchronized way. No thermal anomaly was detected around 300 and 90 K, where a Mott transition from the one-dimensional metallic phase to the semiconducting phase and a spin-Peierls-like transition have been expected. The reason for the absence of these transitions in calorimetry is discussed briefly.

Valence Localization and its Ordering in a Mixed-Valence MMX-Chain Complex, Pt2(dta)4I (dta = CH3CS2 −)

The temperature dependencies of UV-VIS-NIR, IR, and Raman spectra of an MMX-type halogen-bridged mixed-valence complex Pt2(dta)4I (dta = CH3CS2 −) and the monovalent dimers Pt2(dta)4 and Pt2(dta)4I2 were investigated. The former undergoes a metal-insulator transition at 300 K, below which a characteristic splitting of C=S stretching mode was observed. The valence localization and its ordering are considered to occur coupled to the metal-insulator transition; -Pt2.5+-Pt2.5+-I-Pt2.5+-Pt2.5+-I- is for the metallic phase, while -Pt2+-Pt3+-I-Pt2+-Pt3+-I- for the insulating one.

Ultrafast Photoluminescence Dynamics in a Quasi-one-dimentional Halogen-bridged Mixed-valence Complex [Pt(en)2][Pt(en)2Cl2](ClO4)4

Abstract Ultrafast dynamics of luminescence from the unrelaxed self-trapped excitons in a quasi-one-dimensional halogen-bridged mixed-valence complex [Pt(en)2][Pt(en)2Cl2](ClO4)4 (en = ethylenediamine) has been investigated for the first time by femtosecond up-conversion method. The photoluminescence lifetime increases from 270 fs to 1.2 ps with wavelength from 570 nm to 660 nm. At longer wavelengths than 660 nm a component with a much longer lifetime than 10 ps was also observed.

The metallic state in a MMX-chain complex, Pt 2(dta) 4I

Transport, optical, magnetic, and structural properties of the MMX-type halogen-bridged mixed-valence complex, Pt 2(dta) 4I (dta = CH 3CS 2 −) were investigated. This complex exhibits metallic conduction between 300 and 340 K, which is the first observation in halogen-bridged 1-D transition-metal complexes. Below 300 K, a metal-semiconductor transition was observed. This salt undergoes an order-disorder phase transition at 340 K. The mixed-valence state is discussed.

Mixed-Valence States in MMX-Chains Complex, Pt2(dta)4I

Abstract Physical and structural properties of the MMX-type halogen-bridged mixed-valence complex, Pt2(dta)4I (dta = CH3CS2 −) were investigated. This complex exhibits metallic conduction above room temperature, which is the first observation in halogen-bridged 1-D transition-metal complexes. Below 300 K, the metal-semiconductor transition was observed. The mixed-valence state of this compound is discussed.

Model of electron-proton correlation in quasi-one-dimensional halogen-bridged mixed-valence complexes: Role of proton motion.

The role of interchain hydrogen (H) bonds in one-dimensional electron systems is studied in halogen-bridged mixed-valence transition-metal complexes. A cluster theory is developed to predict the proton-induced ordering among charge-density-wave (CDW) chains in terms of pseudospin formalism for proton motion on H bonds. It is pointed out that the electron-proton coupling can control the CDW state which is produced from the competition between electron-electron and electron-phonon coupling.

The Electroabsorption in Quasi-One-Dimensional Halogen-Bridged Mixed-Valence Platinum Complexes

The electroabsorption spectra of the halogen-bridged mixed-valence complexes, (HMPC's) (en=ethylenediamine) are calculated using Maxwell equations and dipole approximation, taking into account of the ground state and odd and even charge-transfer (CT) excitons, and only one phonon is involved. The modulation mechanism is quantitatively explained by the electric-field-induced mixing between the odd and the even CT excitons. It agrees with the measured data.

Ultrafast optical response in a quasi-one-dimensional halogen-bridged mixed-valence complex [Pd(en)2] [PdCl2(en)2] (ClO4)4

The ultrafast optical response in a quasi-one-dimensional halogen-bridged mixed-valence complex [Pd(en)2] [PdCl2(en)2] (ClO4)4 (en = ethylenediamine) has been investigated by the use of a femtosecond absorption spectrum, calculated from a pump—probe reflection spectrum at room temperature by the Kramers—Kronig relations. The time dependence of the transient photoinduced absorption around 1.7 eV and the bleaching from 1.9 to 2.5 eV were calculated for three decay components. They are free excitons with a lifetime of about 800 fs, self-trapped excitons with a lifetime of about 3 ps, and polaron pairs which relax within a 100 ps time period.

Ultrafast optical response in a quasi-one-dimensional halogen-bridged mixed-valence complex [Pt(en) 2][PtBr 2(en) 2](ClO 4) 4

Ultrafast optical response in a quasi-one-dimensional halogen-bridged mixed-valence complex [Pt(en) 2][PtBr 2(en) 2](ClO 4) 4 (en=ethylenediamine) has been investigated by femtosecond absorption spectroscopy at room temperature using pump-probe spectroscopy. The photo-induced absorption around 1.3 eV and the bleaching from 1.5 eV to 2.7 eV were observed. Both consist of a fast-decay component due to STEs and a slow-decay component due to polaron pairs. The former decays exponentially with the time constant of 1.4±0.1 ps. The latter decays as erf( t β) with β =−0.22±0.02, indicating the geminate recombination of an electron polaron and a hole polaron after moving freely along the chain. The deviation of β from the ideal random-walk model ( β= −0.5) is explained by introducing the effect of potential barrier between the polarons hindering the recombination.

Halogen-bridged mixed-valence complexes as paradigms of strongly interacting low-dimensional systems: Ground state

The microscopic parameters of a representative halogen-bridged transition metal complex -[Pt(en) 2][Pt(en) 2Cl 2](ClO 4) 4- are extracted from the literature optical data on the basis of a simple phenomenological model. Proper account of electron-molecolar vibration coupling allows one to reproduce satisfactorily the experimental data, clarifying the relevance of this interaction in respect of the electron-electron ones. The ground state of mixed-valence complexes is rationalized through Valence Bond calculations which solve exactly the extended Hubbard model for finite clusters with alternating on-site energies. The calculations allow a sound comparison with other half-filled systems like polyacetylene (CH) x or charge-transfer salts.

Optical Properties of Halogen-Bridged Mixed-Valance Complexes, [M(en)2][PtX2(en)2](ClO4)4, (M=Pt, Pd and Ni; X=Cl, Br and I): Effects of Metal-Alternation

The effect of metal alternation (Pt and M) in linear-chain halogen-bridged mixed-valence complexes, [M(en) 2 ][PtX 2 (en) 2 ](ClO 4 ) 4 , (X=Cl, Br and I; M=Pt, Pd and Ni; en=ethylenediamine), has been investigated using polarized reflection measurements. The electronic absorption spectra of these materials, obtained by applying the Kramers-Kronig relation to the reflectivity data, show two distinctive charge-transfer exciton bands associated with the inter-metal transitions, “M–Pt”, and the transitions from halogen ions to metal ions, “X–Pt”. On substitution of M=Pt by Pd or Ni, the absorption intensity of the “X–Pt” exciton was enhanced in contrast to the reduction of the “M–Pt” exciton intensity. The intensity borrowing between these two excitons is consistent with a numerical calculation based on a trimer model of the CT exciton transitions.

Three photo-induced absorption bands of one-dimensional halogen-bridged mixed-valence complex, [Pt(en) 2][PtBr 2(en) 2](SO 4) 2.6H 2O

Weak absorption bands below the low-energy absorption edge of the intervalence charge-transfer (IVCT) transition in one-dimensional halogen-bridged mixed-valence complex [Pt(en) 2][PtBr 2(en) 2](SO 4) 2.6H 2O have been investigated at 4.2K for the light polarized parallel to the Pt chain axis. In this complex, we found that three weak absorption bands (A-, B-, C-bands) were observed below the IVCT band. It is the first example of a non-doped halogen-bridged mixed-valence complex having three weak absorption bands. The absorption intensity of the C-band is much weaker than those of the A- and B-bands, but it is remarkably enhanced by the Au-doping. The A-, B- and C-bands have the photo-induced absorption effect, and its effect depends sensitively on the crystal quality and on the condition of the crystal growth, and is enhanced by the Au-doping. The photo-induced absorption decays during several minutes since stopping the irradiating light in the region of the IVCT band. From these behaviors of the A-, B-, and C-bands, we discuss the relation between the origin of these bands and the mismatch of the valence alternation.

Charge transfer excitons in halogen-bridged mixed valence complexes

The charge transfer (CT) excitons in [Pt(en) 2][Pt(en) 2X 2](ClO 4) 4, (X=Br and Cl), single crystals have been studied by the electro-reflectance (ER) spectroscopy. By the Kramers-Kronig transformation, the electric field modulation spectra of imaginary part of the dielectric constant were obtained, in which a new absorption band is observed in the higher energy side of the optically allowed CT exciton. Assuming that the modulation mechanism is due to a mixing of parity-allowed and forbidden CT exciton states, modulation efficiency of those two excitons were calculated. It was found that there is a good consistence between the experimental and calculated results in both crystals.

Linear-chain halogen-bridged mixed-valence complexes of palladium: infrared, electronic, Raman, and resonance Raman study

Etude d'une serie de complexes a valence mixte du type [Pd(II)(LL) 2 ] [Pd(IV)(LL) 2 X 2 ](ClO 4 ) 4 . Ils possedent des proprietes optiques identiques a celles des complexes analogues de Pt mais leur conductivite est plus elevee