Conformational dynamics of phenyhlene rings in poly(p-phenylene vinylene) as revealed by ~13C magic-angle-spinning exchange nuclear magnetic resonance experiments

摘要

Poly(p-phenylene vinylene) (PPV) has shown a great potential for electro-optical applications due to its electroluminescent and semiconducting properties. Such properties are directly related with the polymer chain conformation and dynamics. Then, it is important to understand in detail the local chain motions. In this work, three ~13C solid-state magic-angle-spinning (MAS) exchange NMR techniques were used to study conformational dynamics of phenylene rings in PPV. The standard 2D MAS exchange experiment was used to identify exchange processes between equivalent and nonequivalent sites. Centerband-only detection of exchange (CODEX) experiments were applied to determine the amplitude of the phenylene ring flips and small-angle oscillations. Additionally, a new version of the CODEX technique, which allows for the selective observation of segments executing exchange between non-equivalent sites, is demonstrated that applied to determine the flipping fractions and the activation energies of the phenylene ring rotations. It was found that, at - 15 deg C, (26 (+-) 3)% of the rings undrego 180 deg flips in the millisecond time scale, with average impression of (30 (+-) 5) deg and activation energies of (23 (+-) 3) kJ/mol. Other (31 (+-) 10)% of the rings perform only small-angle oscillations with an average amplitude of (9 (+-) 2) deg. These results corroborate previous experimental data and agree with recent ab initio calculations of potential energies barriers in phenylenevinylene oligomers.