Optical properties of halogen-bridged mixed-valence complexes, [m(En)₂][ptx₂(en)₂](cio₄)₄, (m=pt, pd and ni; x = c1, 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][PtX₂(en)₂](CIO₄)₄, (X=C1, 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.