Narrowing mechanisms of ESR line of (CH)X

Electron Spin Resonance (ESR) on the paramagnetic centers in undoped polyacetylene (CH)X was studied and the effects of the thermal isomerization and the oxidation on the narrowing mechanisms of an ESR line were discussed. The ESR linewidth at 4.2K is found to be proportional to n−1/3 (n: spin density in (CH)X) which means that the static narrowing mechanism was three-dimensional exchange narrowing. The linewidth at room temperature changed with the thermal isomerization and oxidation of (CH)X and was explained by two opposed mechanisms; motion of spins in the conjugated chain and restriction of the motion by cutting off the conjugated chain by cis content or oxidation. The temperature dependence, the thermal isomerization dependence and the oxidation dependence of the ESR linewidth of the undoped polyacetylene can be explained by the three- dimensional exchange narrowing and the motional narrowing mechanisms. Only one kind of spins (neutral solitons) in the conjugated chain is considered to contribute to these mechanisms. On the other hand, Holczer et al. proposed the “two kinds of spin” model to analyze the oxidation dependence of the ESR linewidth at room temperature.5 They assumed the two kinds of spins; the diffusive spins in the polymer chain and the localized spins around the oxygen. They showed the observed linewidth at room temperature is given as, ΔH = CΔHL + (1-C)ΔHD, where C is the relative concentration of the localized spins, and ΔHL and ΔHD, are the linewidths of the localized and diffusive spins respectively. The oxidation dependence of the linewidth at 4.2K can be explained by the “two kinds of spins'’ model as follows. If the motion of the diffusive spin becomes slower and ΔHD approaches to ΔHL at lower temperature, eq. (1) becomes ΔH = ΔHD (=ΔHL). Thus “two kinds of spins” model coincides with “one kind of spins” model. Then the spin density dependence of the linewidth at 4.2K can be explained when AHD is proportional to n−1/3. Further experiment on the temperature dependence of the linewidth at various oxidation stages will be helpful to see the dynamics of the spins in (CH)X.