Guest-tuned proton conductivity of a porphyrinylphosphonate-based hydrogen-bonded organic framework

Abstract

Hydrogen-bonded organic frameworks (HOFs), similar to their MOF analogues, exhibit great potential in proton conduction applications. Herein, a porous HOF namely [(NiH₄TPPP)(Me₂NH₂)₄(DMF)(H₂O)₄] (UPC-H5) was synthesized from phosphonate-based porphyrinato nickel (NiH₈TPPP), and its proton conductivity is regulated through a two-step guest change. Firstly, immersing UPC-H5 in CH₂Cl₂ to exchange lattice solvent molecules for 24 h followed by heating under vacuum afforded the lattice solvent molecule-free HOF [(NiH₄TPPP)(Me₂NH₂)₄] (UPC-H5a) with the pristine framework still retained. Secondly, exposing UPC-H5a to vapors of 25% aqueous ammonia for 24 h at room temperature gave a new derivative UPC-H5a@NH₃·H₂O with the molecular formula [(NiH₄TPPP)(Me₂NH₂)₂(NH₄)₂(H₂O)₄] according to elemental and thermal analyses. At 30 °C and 95% R.H., the proton conductivity of UPC-H5, UPC-H5a, and UPC-H5a@NH₃·H₂O amounts to 5.59 × 10⁻⁴, 7.00 × 10⁻³, and 1.47 × 10⁻² S cm⁻¹, respectively, which increases to 1.85 × 10⁻³, 3.42 × 10⁻², and 1.59 × 10⁻¹ S cm⁻¹ at 80 °C and 99% R.H., clearly showing the effect of guest regulation on the proton conductivity of the HOF-based materials. In addition, this result is also helpful towards understanding the important role of guests in the formation of their proton conduction pathways.