Enhancement of Mobile Protons Induced by Ammonia in Isostructural Hydrogen-Bonded Organic Frameworks

Superprotonic conductivity is achieved by simple compositional tuning of hydrogen-bonded organic frameworks (HOFs). We compare two isostructural HOFs, H 2 O_DOBDC [(C 8 H 6 O 6 )(H 2 O)] and NH 4 + _DOBDC [(C 8 H 5 O 6 )(NH 4 + )], each undergoing a reversible transformation from monoclinic to triclinic symmetry with incorporation of additional lattice water molecules under high temperature and humidity conditions. The ammonium-containing triclinic phase, NH 4 + _DOBDC-TS [(C 8 H 5 O 6 )(NH 4 + )(H 2 O)], displays the highest conductivity, reaching 2.55 × 10 –3 S cm –1 at 363 K and 95% relative humidity, in contrast to 1.15 × 10 –6 S cm –1 for H 2 O_DOBDC-TS [(C 8 H 6 O 6 )(H 2 O) 2 ]. 1 H-Pulsed-field-gradient nuclear magnetic resonance ( 1 H-PFG-NMR) reveals similar diffusion coefficients for both NH 4 + _DOBDC-TS and H 2 O_DOBDC-TS. However, the mobile proton concentrations are significantly different, calculated as 4.88 × 10 25 ions m –3 for NH 4 + _DOBDC-TS and 1.23 × 10 23 ions m –3 for H 2 O_DOBDC-TS. This distinction accounts for the significantly enhanced conductivity in the ammonium-based structure. These findings underscore the pivotal role of guest ions in modulating hydrogen-bonded networks and establish a design strategy for soft crystalline proton conductors.