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, 2 J̄ = 48 ± 6 cm - 1, was larger than 2 J̄ = 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.