Functional Structural Disorder Governing Efficient Proton Transport in Non-Porous Phosphonic-Acid-Based HOFs

Efficient proton transport in hydrogen-bonded organic frameworks (HOFs) is widely assumed to require permanent porosity and well-defined conduction channels. This work demonstrates that dynamic structural disorder can play a central role in governing proton dynamics, even in essentially non-porous systems. We report the synthesis and comparative analysis of three positional isomers of pyridinediyldiphosphonic acid 2,6-, 2,5-, and 3,5- - providing a systematic platform for investigating structure-property relationships. Among them, the 2,5-isomer exhibits an outstanding proton conductivity of $1.88\times {10}^{-3}$  S  ${\mathrm{cm}}^{-1}$ at room temperature and 97% relative humidity, despite its essentially non-porous nature (BET $<3.8$   $m^2$   $g^{-1}$ ). A humidity-driven decrease in activation energy from 0.46 to 0.15 eV reveals a transition from vehicle-assisted transport to a Grotthuss-type proton-hopping mechanism. Crystallographic analysis demonstrates pronounced positional and dynamic disorder, while ATR-FTIR spectroscopy reveals a Zundel-type continuum indicative of highly polarized and delocalized protons. We show that structural disorder enables adaptive reorganization of hydrogen-bond networks upon hydration, generating efficient proton-conduction pathways without permanent pores. These findings challenge the prevailing paradigm that permanent porosity is essential for proton-conducting HOFs and establish dynamic disorder as a powerful design principle for solid-state proton conductors.