Ultrafast dynamical study of self-trapped excitons in ladder type of halogen-bridged Pt complexes

The properties and the dynamics of the excited states in out-of-phase ladder-type halogen-bridged Pt complexes (Pt-Br-out and Pt-I-out) are studied using femtosecond luminescence spectroscopy. In both materials, Gaussian-shaped broad luminescence bands with large Stokes shifts are found around 0.8 eV and assigned to self-trapped excitons (STE). The wave-packet oscillations with periods of 290 and 306 fs were observed in Pt-Br-out and Pt-I-out, respectively. The higher oscillation frequency in Pt-I-out compared with the one-dimensional (1D)-type Pt-I is ascribed to the smaller lattice relaxation in the ladder system. Although the lifetime of the STE is almost the same as that of the 1D-type Pt-Br, the lifetime in Pt-I-out is one order of magnitude longer than that in the 1D-type Pt-I. This is ascribed to the phase locking of the charge density wave due to the interchain interaction.