A new contender in the race for highly conductive organic materials, organic ionic plastic crystals (OIPCs), display conductivity values up to s =10-3 S cm-1. OIPCs commonly show multiple solid-solid phase transitions, accompanied by a low ?Sf value. Ion conduction of OIPCs, within some or all of the solid phases, is attributed to crystalline vacancies (Schottky defects) and/or nanometer to micron-scale defects (e.g., grain boundaries). We seek to answer two main questions: 1) What are the mechanisms of ion conduction for OIPC cations and anions? 2) Is it possible to induce alignment in the polycrystalline structure of OIPCs? We employ pulsed-field-gradient (PFG) NMR to quantify separate cation and anion diffusion and activation energy (Ea) in these materials. We are beginning OIPC studies with 1,2-bis[N-(N'-hexylimidazolium-d2(4,5))]C2H4 2PF6-. We have measured the cation (1H) and anion (19F) diffusion in the second-to-lowest temperature solid phase, upon fast and slow cooling outside of the magnetic field (B0 = 9.4 T) and upon slow cooling in B0. Our studies suggest a change in transport mechanism (local molecular environment) for the cations upon varying thermal treatment (see figure), evidenced by a change in translational activation energy. Variable temperature 2H solid-state NMR also provides further evidence of a change in microstructure of the material upon heating and cooling in B0. These results are the first of their kind observed for OIPCs.                                                                           

Transport dynamics in a bis-imidazolium-based organic ionic plastic crystal

245th National Meeting (2013)

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