Magnetic-resonance studies in undopedtrans-polyacetylene,(CH)x

Abstract

We present room-temperature magnetic-resonance studies of undoped trans-${\left(\mathrm{CH}\right)}_x$, including EPR, proton nuclear relaxation, and dynamic nuclear-polarization experiments. We show that oxygen has drastic effects on the spin dynamics. The results are explained by assuming two kinds of spins: diffusive and oxygen-trapped localized spins. Evidences for the existence of these localized spins are provided by inhomogeneous nuclear-relaxation and dynamic nuclear-polarization experiments in samples contaminated by air. The EPR line is very sensitive to oxygen. Taking great care to avoid the presence of oxygen, a trans-${\left(\mathrm{CH}\right)}_x$ sample has been prepared with the narrowest EPR linewidth reported so far ($\Delta H=0.35\mathrm{}$ G). We conclude that a large part of the commonly reported linewidth ($\Delta H\simeq 1$ G) comes from a small amount of oxygen-trapped localized spins, the width of which is not motional narrowed. A theoretical expression for the EPR line shape of the diffusive spins is given, taking into account the broadening by both hyperfine and dipolar spin-spin interactions and the narrowing by one-dimensional spin diffusion. Analyzing the EPR line shape up to 100 times the linewidth, we show that it is possible to extract from the line the contribution of the diffusive spins. The result is quite consistent with that obtained by nuclear-relaxation measurements; i.e., the spin diffusion is very fast and highly one dimensional.