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

Solid‐state 2 H quadrupole echo nuclear magnetic resonance (NMR) spectra and measurements of 2 H spin lattice relaxation times have been obtained for films of poly( p ‐phenylene vinylene) deuterated in phenylene ring positions (PPV‐d 4 ). 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 2 H line shapes show that the jump motion is thermally activated, with a median activation energy, E a = 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 T 2 relaxation associated with 2 H 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. 2 H 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.