From Paramagnetism to Pauli Paramagnetism: Spiro-Conjugated One-Dimensional Radical Chain Leads to Metallic-like Behavior

Abstract Unlike nonmagnetic metals that exhibit Pauli-paramagnetic behavior, most nonconjugated radical-containing polymers exhibit paramagnetic behavior due to the lack of delocalization of free electrons in the matrix. Pauli paramagnetism in polymers generally requires the delocalization of radicals through both full π-conjugation and π–π stacking. In this report, a novel series of spiro-conjugated radical-containing polymers was synthesized either by oxidation–coordination polymerization of 2,3,7,8-tetramethoxy-indolo[3,2-b]indole (o-4MeOInIn) or reduction–coordination polymerization of its demethylation product (InInBSQ) with BBr3 under mild conditions. By controlling the molar ratio of o-4MeOInIn and InInBSQ, the zwitterionic radical content can be adjusted from around 56 mol % to nearly 100 mol %. Owing to the spiro-coordinated B–O linkages along the polymer chain, π–π stacking of the polymeric backbones is prohibited, so only the intrachain spin–spin interaction via spiro-conjugation is possible. Interestingly, when the radical content in the polymer is around 56 mol %, only paramagnetic behavior is observed since the radicals are statistically isolated from each other; on the other hand, when the radical content reaches 100 mol %, Pauli-paramagnetic behavior is observed, showing that, although very weak, the spin–spin interaction via spiro-conjugation in a one-dimensional radical chain is enough to induce metallic-like behavior.