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

Abstract

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.