ANALYSIS OF THE ESR LINEWIDTH IN PRISTINE TRANSPOLYACETYLENE

The cw ESR linewidth was studied in detail as a function of frequency and temperature in pristine trans-(CH)x and (CD)x prepared by the Shirakawa technique. It was demonstrated that all the obtained data could be interpreted by quasi-one-dimensional diffusive motion of the neutral soliton with a diffuse-trap soliton model. An anomalous line broadening below 6 MHz was observed only in trans-(CH)x. The origin was ascribed to a crossover from "unlike spins" to "like spins" as a relation between electron and nuclear spins. Such an interpretation gives the maximum spin density ρmax0.1 of the delocalized neutral soliton. The temperature dependence of the motion-narrowed ESR linewidth T2′-1 was found to be consistent with the phase memory time TM-1 in terms of the diffuse-trap model. It is discussed that direct evidence of the diffuse-trap model is given by multiple-quantum-spin coherence experiments, although the original authors concluded that all the solitons were trapped both at 4.2 and 300 K. The temperature dependence of the diffusion rage D along the polymer chain was found to follow a T2 law. This behavior is consistent with that deduced from the NMR T1 data. With this agreement between the two resonance methods, validity of the applied diffuse-trap model was reconfirmed. Another interpretation for D derived from ESR T1 was discussed in connection with the present interpretation. The temperature dependence of the cutoff frequency 1/τ was interpreted by exchange coupling between the solitons at low temperatures and a phonon-assisted hopping through the charged solitons at high temperatures, as discussed by Kivelson and Heeger. © 1995 The American Physical Society.