High-resolution solid-state NMR studies of imidazole-based proton conductors: Structure motifs and chemical exchange from H-1 NMR

High-resolution solid-state H-1 NMR under fast magic angle spinning is used for the first time to study proton conductivity. The materials of interest, ethylene oxide tethered imidazole heterocycles (Imi-nEO), are characterized by variable temperature experiments, as well as 2D homonuclear double quantum (DQ) NMR and 2D exchange spectroscopy. Quantum chemical calculations provide a full assignment and understanding of the H-1 chemical shifts, based on a single-crystal structure obtained for Imi-2EO. Three types of hydrogen-bonded N-H-1 resonances are observed by H-1 MAS NMR at 30 kHz. Double quantum NMR experiments identify those hydrogen-bonded protons that are mobile on the time scale of the experiment, and thereby, those which are able to participate in charge transport. Characterized by their spin-spin relaxation (T-2*) behavior, the local mobility of these protons as a function of temperature is compared to the conductivity of the materials. Homonuclear H-1 2D DQ MAS spectra provide evidence for locally ordered domains within all the Imi-nEO materials. Disordered (mobile) and ordered components in Imi-2EO dramatically differ in their H-1 spin-lattice relaxation times. 2D NOESY spectra show no evidence of chemical exchange processes between the ordered and disordered domains. These results indicate that the highly ordered regions of the materials do not (or only poorly) contribute to proton conductivity, which is rather taking place in the disordered regions. Molecules in the disordered domains are in a state of dynamic or fluctuating hydrogen-bonding, allowing for Grotthus mechanism proton transport, while molecules in the ordered domains do not experience exchange, and do not participate in long-range proton conductivity. At the interface between these regimes a small number of molecules undergo slow exchange. With increasing temperature, this exchange becomes fast on the NMR time scale, and the final chemical shift of 12.5 ppm in Imi-5EO implies the persistence of strongly and weakly hydrogen-bonded domains, which reorganize rapidly to support the proton transport process.