Tuning of Electronic Structures of Quasi-One-Dimensional Bromo-Bridged PdII–PdIV Mixed-Valence Complexes by Substituting Counter Anions

Abstract

Quasi-one-dimensional halogen-bridged mixed-valence complexes (MX chains) have been attracting much attention because they show very interesting physical properties, such as intense and dichroic inter-valence charge-transfer bands, progression in the resonance Raman spectra, luminescence spectra with large Stokes-shifts, large third-order nonlinear optical properties, midgap absorptions attributable to solitons, polarons, etc.1 Theoretically, these MX chains are considered to be extended Peierls-Hubbard systems, where electron-phonon interaction (S), electron transfer (T), on-and neighbor-site Coulomb repulsion energies (U and V, respectively) compete or cooperate with each other. The Pt and Pd complexes take charge density wave (CDW) or M II -M IV mixed-valence states due to S, where the bridging halides are displaced from the midpoints between the neighboring two metal ions (Fig. 1a). Therefore, these Pt and Pd complexes are class II type for mixed-valence compounds in the Robin and Day classification.3 In these MX chain complexes, the CDW amplitudes can be tuned by varying metal ions, bridging halide ions, in-plane ligands, and counter anions. 4 Moreover, the dimensionalities of the CDW can be controlled by using the intra-and inter-chain hydrogen-bond networks. 5On the other hand, Ni complexes take Ni III Mott-Hubbard states due to the strong U, where the bridging halides are located at the midpoints between neighboring two Ni atoms (Fig. 1b). Very strong antiferromagnetic interactions among spins located on the Ni III d z 2 orbitals through the bridging halide ions are observed in these complexes.6 Therefore, the Ni complexes are class III type for mixed-valence compounds in the Robin and Day classification. These complexes have also been of recent interest in applied science because a gigantic third-order nonlinear optical susceptibility (%10 À4 e.s.

Temperature- and Pressure-Induced Charge-Density-Wave to Mott–Hubbard Phase Transitions in Quasi-One-Dimensional Bromine-Bridged Pd Compounds

“…The relationship between and L for several Pd-Br compounds with the (en)-ligand are also shown in the same figure, including [Pd(en) 2 Br](ClO 4 ) 2 with relatively large (¼ 0:2028 A). 18,23) As seen in Fig. 2(b), is proportional to L in wide ranges of L and independent of the temperature.…”