EPR of pernigraniline base: The structure of neutral solitons

EPR hyperfine spectra of pernigraniline base (PNB) powder and solutions provide a direct measure of the internal structure of neutral soliton defects, ${\mathit{S}}^0$. The π-electron spin density resides largely on a central nitrogen, ‖${\mathrm{\rho }}_{\mathit{N}}$‖≊0.36, and adjacent phenyl rings where ‖${\mathrm{\rho }}_{\mathit{C}}$‖≊0.125 and ‖${\mathrm{\rho }}_{\mathit{C}}$‖≲0.04 for carbons meta and ortho to the nitrogen center, respectively. Calculations based on a Hubbard model yield an estimate of the on-site Coulomb repulsion U∼2.5 eV similar to that of ${\mathit{S}}^0$ in trans-polyacetylene. The smaller diffusion coefficient for ${\mathit{S}}^0$ in PNB (${\mathit{D}}_{\mathrm{\parallel }}$<${10}^{\mathrm{-}6}$ ${\mathrm{cm}}^2$ ${\sec }^{\mathrm{-}1}$) is contrasted to that of solitons in trans-polyacetylene (${\mathit{D}}_{\mathrm{\parallel }}$∼${10}^{\mathrm{-}4}$ ${\mathrm{cm}}^2$ ${\sec }^{\mathrm{-}1}$).