Charge Transfer Exciton in Halogen-Bridged Mixed-Valent Pt and Pd Complexes: Analysis Based on the Peierls-Hubbard Model

Polarized reflection and luminescence have been measured for the single crystals of [MA2][MX2A2](C1O4)4 (M=Pt, Pd, X=C1, Br, I and A=ethylenediamine, cyclohexanediamine). The strong absorption bands due to the charge-transfer (CT) exciton transitions between the mixed-valent metal ions have been investigated in detail in the visible or infrared energy regions. The dependence of the CT excitation energies on the species M and X is shown to be consistent with the prediction by the Peierls-Hubbard model which incorporates the effect of the electron-electron correlation on inter-metal sites. The oscillator strength of the CT excitons are observed to be enhanced by substituting heavier halogen ions. This enhancement is interpreted by a halogen-linked super-transfer mechanism. The unusually large values of the oscillator strength can be qualitatively explained in terms of the trimer CT model.