Electric-field effects on the charge-transfer excitation in quasi-one-dimensional halogen-bridged mixed-valence Pt complexes: [Pt(ethylenediamine${)}_2$] [Pt(ethylenediamine${)}_2$${\mathit{X}}_2$](${\mathrm{ClO}}_4$${)}_4$ (X=Cl,Br,I)

The electroreflectance spectra of the halogen-bridged mixed-valence Pt complexes, [Pt(ethylenediamine${)}_2$][Pt(ethylenediamine${)}_2$ ${\mathit{X}}_2$](${\mathrm{ClO}}_4$ ${)}_4$ (X=Cl, Br, and I), have been measured. Electric-field-induced new absorption bands that can be assigned to optically forbidden charge-transfer (CT) excitons with even parity have been found at a little higher energy than the optically allowed CT excitons with odd parity. The modulation mechanism can be quantitatively explained by the electric-field-induced mixing between the odd and the even CT excitons. The energy separations between the odd and the even excitons of the X=Cl, Br, and I complexes are 0.54, 0.46, and 0.24 eV, respectively. This indicates the large electron-hole attractive energies in the complexes. The halogen-dependent change of the energy separations can be explained in terms of the intersite electron transfer energies.