Time-resolved luminescence study of self-trapped-exciton relaxation in quasi-one-dimensional halogen-bridged mixed-valence metal complexes

The relaxation processes of self-trapped excitons (STE’s) in halogen-bridged mixed-valence metal complexes (MX-chain complexes) have been investigated by picosecond time-resolved luminescence measurements. The self-trapped exciton luminescence of [Pt(en${)}_2$][Pt(en${)}_2$ ${\mathrm{Cl}}_2$](${\mathrm{ClO}}_4$ ${)}_4$ (en=ethylenediamine) decays exponentially with a lifetime of 230±10 psec at 2 K. The STE’s are dominantly annihilated nonradiatively by tunneling and thermally activated processes. In an MX complex with ligand disorder caused by the random arrangement of the R,R and S,S-isomer ligand molecules, the STE’s exhibit nonexponential decay. The decay dynamics varies with energy across the luminescence band and depends on the photon energy of the excitation laser. This behavior is explained by site-selective generation of STE’s and their subsequent hopping relaxation.