Proton conduction in three molecular assemblies of bipyridyl–organodisulfonate salts

Abstract

Three new crystalline organic salts (COSs) comprising 4,4′-biphenyldisulfonic acid (bpds) and different bipyridyl derivatives—2,2′-bipyridine (22bpy), 1,2-di(4-pyridyl)ethylene (bpee), and 2,5-di(pyridin-4-yl)-1,3,4-oxadiazole (bpdoz)—have been synthesized and characterized in terms of structure and proton conductivity. The organic salts, [bpds][22bpy] (1), [bpds][bpee]·2H₂O (2), and [bpds][bpdoz]·H₂O (3), form two- or three-dimensional (2D/3D) hydrogen-bonded networks. In each salt, proton transfer from the sulfonic acid group to the bipyridyl nitrogen acceptor results in the formation of ionic heterosynthons. The proton conductivities of the compounds were measured at 90 °C and 95% relative humidity, yielding maximum values of 8.69 × 10⁻⁵ S cm⁻¹ for 1, 1.09 × 10⁻³ S cm⁻¹ and 1.27 × 10⁻⁴ S cm⁻¹ for both 2 and 3. The strong humidity dependence of conductivity, coupled with activation energies of 0.51 eV for 1, and 0.56 and 0.43 eV for 2 and 3, respectively, suggests a proton transport mechanism consistent with the vehicle mechanism, facilitated by crystalline water molecules and extended hydrogen-bonded networks. Notably, the enhanced proton conduction observed in 2 is attributed to its continuous 1D hydrogen-bonded chain composed of –SO₃⁻⋯H₂O linkages, which provides a more efficient pathway for proton mobility. The foregoing results provide not only three new solid-state proton conductors but also a bipyridyl–organodisulfonate strategy for the designing and building proton conducting crystalline organic salts.