Dual molecular interaction-triggered stable proton conductive channels in heteroatom-embedded covalent organic frameworks

Abstract

Two covalent organic frameworks (COFs), TZ-COF and TP-COF, were synthesized from triformylphloroglucinol (Tp) building blocks by varying the heteroatom-embedded linkers with either pyridine or triazine rings, and then characterized systematically. Pyridine scaffolds present in TP-COF act as an active site for binding an external proton source (i.e., H₃PO₄; PA) via hydrogen bonding interaction to accommodate them within the pores of the COF skeleton. The density functional theory calculation revealed a synergistic ionic hydrogen bonding interaction between PA and pyridinic nitrogen/carbonyl oxygen. This interaction leads to a dense amount of phosphoric acid tightly bound along the 1D pore channels in PA@TP-COF (PA-doped TP-COF), as compared to PA@TZ-COF (PA-doped TZ-COF), which has hydrogen bonding between PA and carbonyl oxygen groups. As a result, the proton conductivity of PA@TP-COF is enhanced up to 7.9 × 10⁻³ S cm⁻¹ under anhydrous conditions at 140 °C. Furthermore, the proton conductivity increases to 1.2 × 10⁻² S cm⁻¹ under humidified conditions (80 °C and 95% RH), and its initial proton conductivity is complemented to 1.8 × 10⁻² S cm⁻¹ after 96 h under prolonged exposure to the same temperature and humidity. The confined hydrated hydrogen bonds account for the action of the newly formed proton pathway in the 1D channels.