Femtosecond Dynamics of the Exciton Self-Trapping Process in a Quasi-One-Dimensional Halogen-Bridged Platinum Complex

Self-trapped-exciton (STE) luminescence of quasi-one-dimensional halogen-bridged platinum complex $\left[\mathrm{Pt}\right(\mathrm{en}{)}_2\left]\right[\mathrm{Pt}\left(\mathrm{en}{)}_2{\mathrm{Cl}}_2\right]({\mathrm{ClO}}_4{)}_4$ is investigated by femtosecond time-resolved luminescence spectroscopy. In the high energy tail of the luminescence band, a fast rise and decay are seen, whereas around the luminescence peak energy a slow rise of about 500 fs is observed. This rise becomes faster at the low energy side of the band. These results are consistently explained by the cooling of prethermalized STEs, following the rapid self-trapping which is estimated to occur within 140 fs.