Triptycene-Based 2.5-Dimensional Metal–Organic Frameworks: Atomically Accurate Structures and Anisotropic Physical Properties from Hydrogen-Bonding Bridged Protonated Building Units

Abstract

Recent advances in two-dimensional (2D) π-d conjugated conductive metal–organic frameworks (2D cMOFs) have highlighted their potential as sophisticated, active materials in electrochemical energy storage devices, electrocatalysis, and sensors. However, a lack of high-quality structural characterization severely limits our understanding of their physical properties. Specifically, rapid and irreversible nucleation and aggregation, induced by strong interlayer π-π interactions, have hindered crystal growth in these cMOFs. In this study, by utilizing triptycene-based ligands, 2,3,6,7,14,15-hexahydroxy triptycene (HHTripH₂) and 9,10-dimethyl-2,3,6,7,14,15-hexahydroxy triptycene (HHTripMe₂), we successfully mitigated interlayer π-π interactions and achieved SCXRD quality crystals of two triptycene-based 2D MOFs/Cu₃(TripH₂)₂ and Cu₃(TripMe₂)₂. The crystal structures reveal a protonated catechol ligand and an interlayer hydrogen-bonding-guided stacking motif, with density functional theory (DFT) calculations confirming their semiconducting nature. The steric effect of the two axial methyl groups in the TripMe₂ ligand modifies the structure of Cu₃(TripMe₂)₂ from regular AB stacking to interpenetration, significantly enhancing the stability of the crystals. These high-quality crystals enabled the direct measurement of anisotropic dual proton–electron conduction, governed by thermally activated hopping through hydrogen-bonding networks. ESR and susceptibility measurements indicate that these modifications facilitate hydrogen-bonding-guided One-dimensional (1D) antiferromagnetic behavior in the Cu(cat)₂ secondary building units (SBUs). This study reveals the critical roles of high-quality crystal structures and protonation–deprotonation of coordinating atoms in understanding the properties of 2D MOFs.