2H NMR characterization of phenylene ring flip motion in poly(p‐phenylene vinylene) films

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Abstract
Solid‐state 2H quadrupole echo nuclear magnetic resonance (NMR) spectra and measurements of 2H spin lattice relaxation times have been obtained for films of poly(p‐phenylene vinylene) deuterated in phenylene ring positions (PPV‐d4). NMR line shapes show that all the phenylene rings of PPV undergo 180° rotational jumps about the 1,4 ring axis (“ring flips”) at 225°C. The temperature dependence of the 2H line shapes show that the jump motion is thermally activated, with a median activation energy, Ea = 15 kcal/mol, and a distribution of activation energies of less than ±2 kcal/mol. The jump rate was also determined from the magnitude of the anisotropic T2 relaxation associated with 2H line shapes and from the curvature of inversion recovery intensity data. The experimental activation energy for jumps is comparable to the intramolecular potential barrier for rotation about phenylene vinylene bonds. 2H NMR provides a method for determining the phenylene‐vinylene rotational barrier in pristine PPV, and may potentially be used to study conjugation in conducting films.