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.
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