Halogen-bridged Mixed-valence Metal Complexes Research Articles

Soliton-induced optical absorption of halogen-bridged mixed-valence binuclear metal complexes

Employing the one-dimensional single-band extended Peierls-Hubbard model, we investigate optical conductivity for solitonic excitations in halogen-bridged binuclear metal (MMX) complexes. Photoinduced soliton absorption spectra for MMX chains possibly split into two bands, forming a striking contrast to those for conventional mononuclear metal (MX) analogs, due to the broken electron-hole symmetry combined with relevant Coulomb and/or electron-phonon interactions.

Electron-Spin Dynamics and Structures in Halogen-Bridged 1-D Metal Complexes Studied by Solid NMR

Abstract Magnetic properties characteristic in 1-D systems were investigated in halogen-bridged mixed-valence and averaged-valence metal complexes by applying magnetic resonance techniques. Neutral-solitons and polarons formed from impurity amounts of paramagnetic Pt3+ and Pd3+ sites in 1-D chains were sensitively detected by the NMR relaxation measurement. Their rapid 1-D diffusion comparable to that in trans-polyacetylene was revealed in [MII(en)2][MVIBr2(en)2](ClO4)4 (M: Pt, Pd). From 13C NMR spectra and 1H NMR relaxation studies, [NiIIIX(chxn)2]X2 (X: Cl, Br) was shown to be in an averaged-valence state forming S = 1/2 1-D Heisenberg antiferromagnetic chains. [NiBr(chxn)2]Br2 revealed to have the largest exchange interaction so far reported, was expected to transform into a new state at low-temperatures below 100 K. From studies on thermal motions and phase transitions associated with the 1-D chain-structure in [Pt(en)2][PtX2(en)2](ClO4)4 (X: Cl, Br), the presence of the dynamic disorder of the Pt(en)2 chelate-ring conformation in a highly asymmetric double-well potential undetected by the X-ray diffraction was detected by the NMR relaxation measurement.

Ultrafast dynamics of lattice relaxed states in quasi-one-dimensional MX-chain compounds

Ultrafast relaxation processes of photoexcited states in a quasi-one-dimensional halogen-bridged mixed-valence metal complex [Pt(en) 2][Pt(en) 2I 2](SO 4) 2.6H 2O have been investigated by femtosecond transient absorption spectra measurements. A photoinduced absorption band caused by non-thermalized self-trapped excitons has been found. Oscillation of the absorption band caused by the symmetric Pt–I stretching mode of the self-trapped exciton with a frequency of 103 cm −1 has been observed. The rise time of a polaron absorption band has been measured to be 1.2 ps. This corresponds to the decay time, 1.1 ps, of a photoinduced absorption band which oscillates with a frequency of 74 cm −1.

Intrinsic inhomogeneity due to nonlinearity

Presence of strong nonlinearity in a material, either explicit, or effective (resulting from the interaction of coupled degrees of freedom), can lead to intrinsic inhomogeneity even in a periodic extended system. We briefly discuss two examples where such a breaking of the translational symmetry due to nonlinearity (i) determines the optical features (in particular the resonance Raman scattering spectra) of a quasi-one-dimensional halogen-bridged mixed-valence transition metal complex, and (ii) enhances the trapping of a charge in a model of DNA, which is likely to affect the long-range transport properties in this biomolecule.

Visible to Near-Infrared Ultrafast Spectroscopy of a Quasi-One-Dimensional Halogen-Bridged Mixed-Valence Metal Complex [Pt(en)2][Pt(en)2Cl2](ClO4)4

Ultrafast relaxation dynamics is studied in a quasi-one-dimensional halogen-bridged mixed-valence metal complex [Pt(en)2][Pt(en)2Cl2](ClO4)4 by pump−probe spectroscopy. A photoinduced absorption spectrum is measured in a broadband energy region of 0.6−2.6 eV, applying optical parametric amplification. The measurement is performed at both 293 and 4 K. The photoinduced absorption spectrum consists of the three contributions with different time and temperature dependence. One is the absorption band ascribed to the self-trapped exciton (STE), and the second is that due to the spatially uncorrelated neutral-soliton pair. Just after the photoexcitation, the STE is in a nonequilibrium state, and the time required for the thermalization is evaluated to be about 1 ps. The formation time of the third one is coincident with the thermalization time of the STE, and it is attributed to the spatially confined neutral-soliton pair.

UlTRAFAST LATTICE RELAXATION DYNAMICS OF EXCITON IN A QUAISI-1-D METAL-HALOGEN COMPLEX

A quasi-one-dimensional halogen-bridged mixed-valence metal complex is studied by time-resolved pump and probe spectroscopy with sub-5 fs time resolution. Two kinds of oscillatory signals are observed, which are attributed to the wave packet motions both in an electronic ground state and in a self-trapped exciton (STE) state. The onset of the wave packet motion is found to be delayed by about 50 fs, comparing with the ideal wave packet in the electronic excited state. The delay reflects the thermalization process in a free exciton (FE) state and a lattice relaxation process from FE to STE states.

Ultrafast relaxation processes of solitons, polarons, and self-trapped excitons in MX-chain

Ultrafast relaxation processes of photoexcited states of a quasi-one-dimensional halogen-bridged mixed-valence metal complex [Pt(en) 2][Pt(en) 2Cl 2](PF 6) 4 have been investigated by femtosecond transient absorption spectra measurements. It has been found that a considerable part of the neutral soliton pairs is generated directly from photogenerated charge transfer excitons. Charged soliton pairs have been found to be generated from free electron–hole pairs. The rise time and the decay time of the charged soliton absorption band are 2.2 and 6.5 ps, respectively. The polaron pairs are generated from free electron–hole pairs within time resolution.

Wave packet dynamics in a quasi-one-dimensional metal-halogen complex studied by ultrafast time-resolved spectroscopy.

An ultrafast optical response is studied in a quasi-one-dimensional halogen-bridged mixed-valence metal complex [Pt(en)(2)] [Pt(en)2I2] (ClO4)(4) with ultrafast time resolution. Wave packet motions both in the ground and self-trapped exciton (STE) states are observed as oscillatory modulations in the time-resolved reflectivity. The wave packet motion on the STE potential surface begins after about 50 fs with respect to the photoexcitation. This delay is attributed to the lattice relaxation from the free exciton state to the STE state.

Ultrafast relaxation processes of the photoexcited states in MX-chains

Femtosecond transient absorption spectra of a quasi-one-dimensional halogen-bridged mixed-valence metal complex [Pt(en) 2 ][Pt(en) 2 Cl 2 ](PF 6 ) 4 have been measured. It has been found that the neutral solitons are generated from charge transfer excitons within the time resoution (300fsec). It is found that charged solitons are generated by the interband excitation. The rise time and decay time of the charged soliton absorption band are 2.2psec and 6.5psec respectively.

Recent structural studies of PtI

The halogen-bridged mixed-valence metal complexes (MX) have been the object of numerous studies probing the properties of their quasi-one-dimensional M-X sublattice in which the charge disproportionation δ of the metal (M 3−δ/M 3-δ can be tuned from 0 to 1. We have recently investigated the structure of a fully deuterated sample of [Pt(en) 2[Pt(en) 2I 2](ClO 4) 4 at room temperature by using pair-distribution-function analysis X-ray powder diffraction data and regular single crystal diffraction in order to study the relationship between the local structure and the charge density wave strength of this class of materials.

Density-matrix renormalization group studies of metal-halogen chains within a two-band extended Peierls-Hubbard model

The phase diagram of halogen-bridged mixed-valence metal complexes (MX) has been studied using a two-band extended Peierls-Hubbard model employing the recently developed density-matrix renormalization group method. We present the energies, charge- and spin-density distributions, bond orders, and charge-charge and spin-spin correlations, for the ground state, for different parameters of the model. The effects of bond alternation and site-diagonal distortion on the ground-state properties are considered in detail. We observe that the site-diagonal distortion plays a significant role in deciding the nature of the ground state of the system. We find that while the charge-density-wave (CDW) and bond-order-wave (BOW) phases can coexist, the CDW and SDW (spin-density-wave) phases are mutually exclusive in most cases. We have also studied the doped MX chains both with and without bond alternation and site-diagonal distortion in the CDW as well as SDW regimes. We find that the additional charges in the polarons and bipolarons for hole doping are confined to a few sites, in the presence of bond alternation and site-diagonal distortion. For electron doping, we find that the additional charge(s) is (are) smeared over the entire chain length, and although the energetics implies a disproportionation of the negatively charged bipolaron, the charge- and spin-density distributions do not reflect this. A positively charged bipolaron disproportionates into two polarons in the SDW region. There is also bond-order evidence for compression of the bond length for the positively charged polaronic and bipolaronic systems and an elongation of the bonds for systems with negatively charged polarons and bipolarons.

Time-resolved luminescence study of self-trapped-exciton relaxation in quasi-one-dimensional halogen-bridged mixed-valence metal complexes.

The relaxation processes of self-trapped excitons (STE's) in halogen-bridged mixed-valence metal complexes (MX-chain complexes) have been investigated by picosecond time-resolved luminescence measurements. The self-trapped exciton luminescence of [Pt(en${)}_{2}$][Pt(en${)}_{2}$${\mathrm{Cl}}_{2}$](${\mathrm{ClO}}_{4}$${)}_{4}$ (en=ethylenediamine) decays exponentially with a lifetime of 230\ifmmode\pm\else\textpm\fi{}10 psec at 2 K. The STE's are dominantly annihilated nonradiatively by tunneling and thermally activated processes. In an MX complex with ligand disorder caused by the random arrangement of the R,R and S,S-isomer ligand molecules, the STE's exhibit nonexponential decay. The decay dynamics varies with energy across the luminescence band and depends on the photon energy of the excitation laser. This behavior is explained by site-selective generation of STE's and their subsequent hopping relaxation.

Effect of interchain interactions on nonlinear lattice relaxation processes in MX chains

Abstract The effect of interchain interactions on the stability of solitons and polarons in halogen-bridged mixed-valence metal complexes has been studied within the one-dimensional extended Peierls-Hubbard model. The adiabatic potential-energy surfaces have been computed within a mean-field approximation. It turns out that in the weak electron-phonon coupling case, both solitons and polarons exist stably and solitons form the soliton-antisoliton bound state, i.e., breather, for small interchain interactions, while only polarons exist stably for large interchain interactions.

Ground State and Band Gap of MX complex11The project supported by National Natural Science Foundation of China and Advanced Materials Committee of China

Using the one-band model of halogen-bridged mixed-valence transition metal complex (MX complex), within mean field theory, we have proved for fixed electron-phonon coupling, the band gap reaches the minimum at the boundary of the charge density wave state and the spin density wave state in the phase diagram. The analytic form of the ground state energy and the band gap are also obtained.

Effects of e-e Interaction on the CDW of MX Complex

We have considered the effects of Hubbard repulsion on the charge-density-wave (CDW) state of halogen-bridged mixed-valence transition metal complex (MX complex) within mean field theory (MFT), and taken electron correlation into account by second-order perturbation relative to the mean field state. It is seen correlation reduces the dimerization and narrows the band gap further compared with MFT.

Photo-generation and annihilation processes of mid-gap states in quasi-one-dimensional halogen-bridged mixed-valence metal complexes

Photo-generation and annihilation processes of the mid-gap states in quasi-one-dimensional halogen-bridged mixed-valence metal complexes (HMMC's) have been investigated by the excitation spectra measurements of photo-induced absorption (PA) bands and excitation power density dependence measurements of the PA bands. One kind of PA bands are efficiently generated from the CT-excitons. On the other hand, another kind of PA bands are efficiently generated from higher energy excited states. The generation efficiencies of the PA bands are proportional to the square root of the excitation light power density. This indicates that the mid-gap states which cause PA bands are mobile and are annihilated by the collision among them.

Relaxation Processes of Photoexcited States in Halogen-Bridged Mixed-Valence Metal Complexes

Abstract Excitation spectra of the self-trapped exciton luminescence bands and those of photo-induced absorption bands of halogen-bridged mixed-valence metal complexes have been measured. The excitation spectra of luminescence bands begin to decrease from energies smaller than the peak energies of the charge transfer exciton absorption bands. The excitation spectra of photo-induced absorption bands begin to rise from the same energies. These show the competition between the self-trapped excitons and the photo-induced defect states in their generation processes.

Control of CDW state in halogen-bridged metal complexes

Abstract Chemical modifications of one-dimensional (1D) halogen-bridged mixed-valence metal complexes have been made in order to control the amplitude of the charge density wave (CDW) state in -M-X-chains (M = Pt, Pd or Ni, and X = Cl, Br or I). By changing chemical parameters, such as strength of hydrogen-bonds between the ligands and the counter anions, and the ionic radii of M and X, the characteristic diagram of the amplitude of CDW has been obtained as a function of the M-X-M distances. This provides a new basic idea for understanding the 1D electronic state of the M-X compounds, especially as related to dynamical phenomena originated in the charge transfer instability in the ground state and the photo-excited state.

Nonlinear lattice-relaxation process of excitons in quasi-one-dimensional halogen-bridged mixed-valence metal complexes: Self-trapping, solitons, and polarons.

Self-trapped excitons, solitons, and polarons of a one-dimensional extended Peierls-Hubbard model are investigated, in order to clarify the lattice-relaxation paths of photogenerated excitons in halogen-bridged mixed-valence metal complexes. This theory takes into account the lattice distortion of halogen ions in a direction perpendicular to the chain, as well as in a parallel direction. It is mainly based on the adiabatic approximation for phonons and the mean-field theory for interelectronic interactions, but is also reinforced by taking into account the electron-hole correlation to obtain the exciton. Potential surfaces relevant to the relaxation of the exciton are clarified in terms of various nonlinear excitations. This result can explain rather diverse experiments from a unified point of view, such as photoabsorption and luminescence spectra, the Stark effect, and electron-spin-resonance data. The origin of the photoinduced absorption is concluded to be polarons.

Electron-phonon coupling and the charge transfer absorption band of halogen-bridged mixed-valence metal complexes

Electron-phonon coupling and the charge transfer absorption band of halogen-bridged mixed-valence metal complexes