Ultrafast optical response in quasi-one dimensional halogen-bridged mixed-valence complexes [pt(en)2] [ptbr2(en)2](c104)4 and [pd(en)2] [pdcl2(en)2](c104)4

Ultrafast optical response in two quasi-one-dimensional halogen-bridged mixed-valence complexs [Pt(en)2] [PtBr2(en)2] (004)4 (en=ethylenediamine) (abbreviated as PtBr) and [Pd(en)2] [PdCl2(en)2] (C1O4)4 (as PdCl) has been investigated by femtosecond absorption spectroscopy at room temperature by pumpprobe spectroscopy. The photoinduced absorption around 1.3 eV and the bleaching from 1.5 eV to 2.7 eV were observed in PtBr. Both consist of a fast-decay component due to STEs and a slow-decay component due probably to polaron pairs. The former decays exponentially with the time constant of 1.4 ± 0.2 ps. The latter decays as erf(fβ) with β = -0.22 ± 0.02, indicating the geminate recombination of an electron polaron and a hole polaron after moving freely along the chain. The deviation of β from the ideal random-walk model (β = -0.5) is explained by introducing the effect of potential barrier between the polarons hindering the recombination. A pumpprobe absorption spectrum of PdCl is obtained from the reflection spectrum by the Kramers-Kronig relations. The time dependence of the transient photoinduced absorption around 1.7 eV and the bleaching from 1.9 eV to 2.5 eV were calculated to be described with three components. They correspond to free excitons with lifetime of about 800 fs, self-trapped excitons with lifetime of about 3 ps, and polaron pairs which hardly relax within 100 ps.