Ultrafast dynamics of lattice relaxation of excitons in quasi-one-dimensional halogen-bridged platinum complexes

Abstract

The temporal evolution of the photoexcited state in quasi-one-dimensional (1D) halogen-bridged platinum complexes $[\mathrm{Pt}(\mathrm{en}{)}_{2}][\mathrm{Pt}(\mathrm{en}{)}_{2}{X}_{2}]({\mathrm{ClO}}_{4}{)}_{4}$ (abbreviated as $\mathrm{Pt}\ensuremath{-}X,$ $X=\mathrm{C}\mathrm{l},$ Br or I), has been comprehensively studied by femtosecond time-resolved luminescence spectroscopy. In Pt-Cl, new short-lived hot luminescence is found in the low-energy side of a self-trapped exciton (STE) luminescence band. The overall behavior of the STE luminescence band within 2 ps is well explained by the vibrational relaxation of the STE. The behavior is reproduced by a model calculation based on wave packet propagation on an interaction mode composed of frequency-dispersed bulk phonons. This model is also applied to the previous results in Pt-Br. For both Pt-Cl and Pt-Br, the frequency spectra of phonons which compose the interaction mode have been estimated. In Pt-I, the STE luminescence decays much faster than those in Pt-Cl and Pt-Br, showing existence of more effective nonradiative decay channel.