Pauli Paramagnetism of Stable Analogues of Pernigraniline Salt Featuring Ladder-Type Constitution

Polyaniline derivatives represent one of the most widely
used classes
of conductive polymers. The fundamentally important electronic properties
of pernigraniline salts, the fully oxidized and acid-doped derivatives
of polyanilines, however, are still not well-understood due to their
poor stability and configurational uncertainty. To address these issues
and to synthetically access stable analogues of pernigraniline salts,
ladder-type constitution was imparted into a series of model oligomer
analogues with rigid backbones constituted by up to 27 fused rings.
The syntheses were achieved through iterative cross-coupling reactions
followed by cyclization and oxidation. In contrast to their unstable
non-ladder-type counterparts, these ladder-type pernigraniline-like
molecules all adopt a well-defined all-trans configuration
and demonstrate an excellent chemical stability after protonation,
rendering it possible to reveal the intrinsic electronic and magnetic
properties of molecules resembling pernigraniline. Protonated salts
of these oligomers feature a significant diradicaloid open-shell resonance
contribution. A dominant temperature-independent Pauli paramagnetism
was observed in the solid state, an indication of the delocalization
nature of the polarons in ladder-type analogues of pernigraniline
salt.