Synthesis and magnetic behaviour of polyradical: poly(1,3-phenyleneethynylene) with π-toporegulated pendant stable aminoxyl and imine N-oxide–aminoxyl radicals

Abstract

The palladium-catalysed polycondensation of N-tert-butyl-N-(3,5-diethynylphenyl)aminoxyl with 2-(3,5-diiodophenyl)-4,4,5,5-tetramethyl-3-oxido-2-imidazolin-3-ium-1-yloxyl in pyridine–triethylamine at room temperature afforded poly(1,3-phenyleneethynylene) with pendant aminoxyl and imine N-oxide–aminoxyl radicals as a light blue powder in 78–83% yields. The number-average molecular weights ( _n) of the polyradical determined by GPC were 2670–3030 and the spin concentrations determined by EPR were 1.91–2.02 × 10²¹ spins g⁻¹. Both solution (CH₂Cl₂) and powder EPR measurements of the polyradicals gave a hyperfine-smeared single line spectrum, suggesting that spin-exchange narrowing took place due to the high spin concentrations of the polyradicals. The magnetic susceptibility (χ) measurements of the polyradicals were carried out on a superconducting quantum interference device (SQUID) magnetometer in the temperature range of 1.8–300 K. The χ Tvs. T plots showed that the polyradicals were a paramegnetic species yielding a weak antiferromagnetic coupling (θ = −1.5 K) below 30 K. The absence of the through-bond ferromagnetic interaction is accounted for by masking of the interaction by the stronger through-space antiferromagnetic interactions.