Acyclic Tripodal Receptors as Ultrahigh Superprotonic Conductors

Solid-state proton-conductors (SSPCs) with high conductivity and excellent chemical stability are the key requirements for proton exchange membrane fuel-cell systems. Herein, inspired by the acyclic supramolecular chemistry, we inaugurate a new class of proton-conducting tripodal receptor based hydrogen-bonded platforms, namely, Tripod-COOH-P and Tripod-NO₂-P, with differently functionalized tails. The receptors act as anchors for the intrinsically crystallized proton carriers (H₃PO₄ and H₂PO₄ ^-) through multiple host-guest interactions and thus eventually address leaching issues with externally doped phosphoric acid from SSPCs. Distinct variation in structural engineering through "complementary docking strategy" coupled with striking variation in H-bonded architectures (continuous versus closed loops) marked for three orders higher proton conductivity of Tripod-COOH-P (1.5 × 10^-2 S cm^-1) than that for Tripod-NO₂-P (3.08 × 10^-5 S cm^-1) at 80 °C under 80% RH. Quantum-chemical simulation revealed a smaller proton transfer energy barrier (E_PT) in Tripod-COOH-P (13.7 kcal mol^-1 and 1.7 kcal mol^-1 for channel-a and b, along with an energetically favourable "quantum tunnelling effect" in channel-b) than in Tripod-NO₂-P (24.9 kcal mol^-1). Such a unique framework engineering served as an effective tool for flaunting tripodal receptor Tripod-COOH-P as a new zoner ultrahigh proton conducting platform (beyond > 10^-2 S cm^-1).