Decay Processes of a Photo-Excited State in a Strong Electron-Phonon Coupling System Trans-[Pt(en)2Cl2](ClO4)2

Abstract

A quasi one-dimensional compound of halogen-bridged mixed-valence platinum complexes [Pt(en)2][Pt(en)2X2] (ClO4)4 (X=Cl, Br, and I, and en denotes ethylenediamine, hereafter we abbreviate as Pt–X) is known to show a strong charge-transfer (CT) absorption. By the optical excitation of the CT band, one can make a CT exciton on a one-dimensional chain. The CT exciton relaxes, without a barrier, to localized ed exciton (STE) states and the relaxation process was observed as a series of Raman lines followed by a large broad luminescence band. Recent spectroscopic measurement of femtosecond time resolved luminescence by Tomimoto et al. revealed a wave-packet oscillation in an adiabatic potential well associated with a self-trapped exciton. 3) The time constants of cooling from the photo-excited states to the STE states were identified as 0.21 and 0.43 ps for Pt–Cl and Pt–Br, respectively. Recently, optical spectra were measured at a He temperature on a crystal of trans-[Pt(en)2Cl2](ClO4)2 which consists of the same chemical composition as the Pt–Cl. The absorption band observed at the lowest energy of 3.3 eV was due to the d-d transition on a platinum atom. 5) The luminescence spectrum had a Gaussianlike lineshape formed by the superposition of phonon side bands more than 20 series. The energy of phonons observed in the luminescence was quite close to that of a breathing mode of chlorines centered around the platinum ion (44.9meV) observed on the Raman spectrum. On the other hand, no phonon structures were observed on the luminescence of both one-dimensional Pt– Cl and Pt–Br. The electron phonon coupling strength (Huang-Rhys factor) of trans-[Pt(en)2Cl2](ClO4)2 was obtained to be 9.52. These results show that trans[Pt(en)2Cl2](ClO4)2 is a strong electron-phonon coupling system and that one of most suitable materials for the studies of the dynamical decay processes of photo-excited states in a strong electron-phonon coupling system. Special interest is at the point of comparison with onedimensional Pt–X. In this report, time resolved luminescence spectra of trans-[Pt(en)2Cl2](ClO4)2 are for various temperatures and the photo-excited state is shown to be cool-down on the adiabatic potential surface of the excited state at about 1 ns. self-trapp