Unravelling the link between proton conduction and dielectric relaxation by modulating hydrogen-bonded motif in zincophosphate-based coordination polymers: mechanistic insights supported by molecular dynamics simulation

Hydrogen bonds play a pivotal role in governing both proton conduction and dielectric properties in functional materials. However, the direct mechanistic relationship between these two properties, as mediated by hydrogen bonds, remains poorly understood. Here, we address this gap by investigating the role of hydrogen-bonded motifs in coordination polymers (CPs), focusing on how their structural dynamics influence both proton transport and dielectric relaxation. To this end, two CPs, {(H3tren)2[Zn3(PO4)4]·6H2O} (ZnPO4-H3tren-H2O) and {(H3tren)2[Zn3(PO4)4]·2H2ta} (ZnPO4-H3tren-H2ta, tren = tri(2-aminoethyl)amine and H2ta = terephthalic acid), featuring analogous host frameworks but distinct hydrogen-bonded networks, were rationally designed and synthesized by modulating the guest molecules through substituent effects. Despite their structural similarity, ZnPO4-H3tren-H2O and ZnPO4-H3tren-H2ta exhibit markedly different proton conductivities of 4.55×10⁻4 and 3.41×10⁻3 S cm⁻1, respectively, at 353 K and ~97% relative humidity (RH). The nearly one-order-of-magnitude difference is attributed to the dissociation of the H2ta molecule, which provides a more acidic proton source. Moreover, we found that the pronounced non-Debye relaxation behavior at low temperatures in ZnPO4-H3tren-H2O leads to an increased activation energy for proton conduction, in contrast to the relaxation-free behavior of ZnPO4-H3tren-H2ta. The difference is attributed to variations in the dynamics of their hydrogen-bonded motifs. Furthermore, dielectric relaxation of H3tren3+ ions at high temperatures was also observed in both materials. Molecular dynamics simulations corroborate these findings, capturing the distinct dynamic behaviors of water clusters and H3tren3+ ions. Beyond fundamental insights, both CPs exhibit high dielectric constants and moderate conductivities under ambient conditions, highlighting their potential as dispersed-phase components in electrorheological fluids. This study unveils a mechanistic link between dielectric relaxation and proton conduction, offering design principles for multifunctional materials that integrate proton conductivity with desirable dielectric properties.