Poly(phenylenevinylene)-attached phenoxyl radicals: Ferromagnetic interaction through planarized and π-conjugated skeletons

Chemically stable poly[2-(3,5-di-tert-butyl-4-oxyphenyl)-1,4(p)-phenylenevinylene], poly[4-(3,5-di-tert-butyl-4-oxyphenyl)-1,3(m)-phenylenevinylene], and poly[4-(3,5-di-tert-butyl-4-oxyphenyl)-1,2(o)-phenylenevinylene] were synthesized via the polymerization of 4-bromo-2-(3,5-di-tert-butyl-4-acetoxyphenyl)styrene, 5-bromo-2-(3,5-di-tert-butyl-4-acetoxyphenyl)styrene, and 2-bromo-4-(3,5-di-tert-butyl-4-acetoxyphenyl)styrene, respectively, using a palladium catalyst. The o- and p-polyradicals, even with their spin concentration of 0.6 spin/unit, displayed S values of 4/2 to 5/2 (S = 5/2 for the o-polyradical with a degree of polymerization of 17 and a spin concentration of 0.68), while the m-polyradical has a singlet ground state. A long-range intramolecular through-bond ferromagnetic exchange interaction was realized between the pendant unpaired electrons for the o- and p-polyradicals, in accordance with theoretical predictions based on the molecular connectivity. The average value of the exchange coupling constant for the o-polyradical, 2J̄ = 48 ± 6 cm^-1, was larger than 2J̄ = 32 ± 2 cm^-1 for the p-polyradical, while they were almost coincident with or much larger than the exchange coupling constants for the corresponding diradical model compounds. Both force field calculations and spectroscopic results indicated a less hindered steric structure with a planarized π-conjugated skeleton for the o-polyradical.