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Researchers at Kumamoto University and Nagoya University have developed a new class of two-dimensional (2D) metal-organic frameworks (MOFs) using triptycene-based molecules, marking a breakthrough in the quest to understand and enhance the physical properties of these promising materials. The work is in the Journal of the American Chemical Society.

This innovation opens new possibilities for multifunctional applications in gas/molecular sensors, electrochemical energy storage, and spintronic devices.

Two-dimensional (2D) conductive metal-organic frameworks (MOFs) have drawn increasing attention for their unique properties—like high electron and proton conductivity and unusual magnetic behaviors—that set them apart from conventional MOFs. However, the field has long been held back by challenges in growing large, high-quality crystals and a lack of clarity about how molecular structures relate to material performance.

To tackle these issues, Associate Professor Zhongyue Zhang from the Faculty of Advanced Science and Technology at Kumamoto University, in collaboration with Professor Kunio Awaga's team at Nagoya University at the time of the research, turned to triptycene-based linkers.

Unlike traditional flat, π-conjugated ligands that promote fast crystal growth and stacking, triptycene has a rigid 3D shape that suppresses interlayer interactions. This allows crystals to grow more slowly and reach larger sizes, making them suitable for detailed structural and functional studies.

Using a slow diffusion method in sealed glass tubes—rather than standard solvothermal techniques—the team successfully synthesized two new MOFs: Cu₃(TripH₂)₂ and Cu₃(TripMe₂)₂.

These reached crystal sizes exceeding 0.3 mm, sufficient for single-crystal X-ray diffraction and precise measurements of electronic, magnetic, and proton transport properties.

Structural analysis revealed that the catechol groups coordinating the metal ions remain fully protonated—an unusual and experimentally verified feature that stabilizes the layered framework via hydrogen bonding.

While previous theories suggested protonation might affect electronic properties, this study provides the first experimental evidence of such protonated states in MOFs.

Measurements on these large single crystals showed high directional conductivities, with electron and proton transport significantly stronger along the vertical (a-axis) direction. The data suggest a possible cooperative mechanism for charge and proton hopping between different arms of the triptycene units.

Electron paramagnetic resonance (EPR) and magnetization studies further revealed one-dimensional antiferromagnetic coupling along the same axis, enabled by interlayer hydrogen bonds. This is in sharp contrast to the frustrated in-plane magnetic behavior seen in other 2D MOFs.

Because these materials exhibit strong electronic and magnetic correlations in the interlayer direction—despite having no continuous structural connections there—the researchers propose a new term to describe them: "2.5-dimensional" (2.5-D) MOFs.

"This study demonstrates how a simple change in molecular geometry can overcome longstanding barriers in MOF research," said Professor Zhongyue Zhang of Kumamoto University.

"By enabling high-quality single crystals, we not only clarified fundamental structure–property relationships but also unlocked new potential for next-generation MOFs in real-world devices."

The findings pave the way for further developments in MOF-based technologies, including zinc-ion batteries, molecular sensors, and quantum information systems.