Magnetism,optical absorbance,and 19F NMR spectra of nafion films with self‐assembling paramagnetic networks

Magnetization, optical absorbance, and ¹⁹F NMR spectra of Nafion transparent films as received and doped with Mn²⁺, Co²⁺, Fe²⁺, and Fe³⁺ ions with and without treatment in 1H‐1,2,4‐triazole (trz) have been studied. Doping of Nafion with Fe²⁺ and Co²⁺ and their bridging to nitrogen of triazole yields a hybrid self‐assembling paramagnetic system that exhibits interesting magnetic and optical properties. These include spin crossover phenomena between high‐spin (HS) and low‐spin (LS) states in Nafion‐Fe²⁺‐trz and Nafion‐Co²⁺‐trz accompanied by thermochromic effects in the visible range induced by temperature. A large shift of the magnetization curve induced by a magnetic field in the vicinity of the HS ? LS, ～220 K, observed for Nafion‐Fe²⁺‐trz has a rate of ～6 K/kOe, which is about three orders of magnitude larger than that in bulk spin crossover Fe²⁺ materials. Selective response of ¹⁹F NMR signals on doping with paramagnetic ions demonstrates that NMR can be used as spatially resolved method to study Nafion film with paramagnetic network. Both chemical shift and width of ¹⁹F NMR signals show that SO groups of Nafion, Fe or Co ions, and nitrogen of triazole are bonded whereas they form a spin crossover system. Based on a model of nanosize cylinders proposed for Nafion [K. Schmidt‐Rohr and Q. Chen, Nat Mater (2008), 75], we suggest that paramagnetic ions are located inside these cylinders, forming self‐assembling magnetically and optically active nanoscale networks. © 2011 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 50: 129–138, 2012