PHOTOINDUCED DEFECTS IN THE BROMIDE-BRIDGED PLATINUM LINEAR-CHAIN [PT(EN)2][PT(EN)2BR2](CLO4)4 (WITH EN=ETHYLENEDIAMINE)

Photoinduced defects in the bromide-bridged platinum linear chain [Pt(en)2][Pt(en)2Br2](ClO4)4 (PtBr, where en=ethylenediamine) yield Raman, absorption, and electron paramagnetic resonance (EPR) signals that have been examined as a function of temperature in order to determine the kinetics of the decay of these metastable defects in an intermediate-strength low-dimensional charge-density-wave (CDW) solid. Both diamagnetic and paramagnetic defects are photogenerated with the initial excitonic transient primarily decaying to form electron bipolarons and hole polarons. The photogenerated Raman signal is indicative of the former and the EPR signal of the latter. The type and stability of these photoinduced defects are dependent on the sample history, with PtBr crystals grown in the orthorhombic phase (above 30-degrees-C) leading to greater variety and stability of defects as compared with those photoinduced in crystals grown in the monoclinic phase (below 10-degrees-C). The temperature dependence of the photogenerated spectroscopic signals reveals complicated decay routes, with the eventual recombination of valence defects mediated by lattice pinning potentials of various strengths, interconversion of defect states (including the possible "melting" of electron bipolarons into electron polarons) and a strengthening of the CDW as the crystals are warmed toward room temperature. The observation of bromide hyperfine splitting in the EPR data and the reversible intensity changes in the Raman defect signals indicates that these defects are spatially quite disperse, with ten or greater platinum sites revealed in the EPR signal, and demonstrates that the weakening of the CDW in PtBr relative to the strong CDW solid PtCl has a significant effect on the ground- and defect-state properties.