Turning 2D MOFs into Mixed Ionic-Electronic Conductors via Side Chain Engineering

Despite the wide-ranging use of metal-organic frameworks (MOFs) as either electronic or ionic conductors, mixed electron-ion conductivity in MOFs remains largely unexplored. Here, we report a new methodology for designing new mixed ionic-electronic conductors (MIECs) by tethering ionophilic ethylene glycol (EG) functional groups onto an electrically conductive MOF (cMOF), yielding M₃(HIR₃-TAT)₂ (M = Ni, Cu; TAT = triazatruxene; R = nBu, 1EG, 2EG; referred to as M-R). Leveraging systematic physicochemical variability in the pore microenvironment while preserving in-plane conjugation of the functionalizable TAT core, we orthogonally tune and enhance electronic conjugation through the crystalline framework (in-plane and out-of-plane) and ionic transport through defined abundant pore channels. Upon LiTFSI incorporation, Ni-1EG exhibits the highest room-temperature ionic conductivity (1.1 × 10^-4 S/cm) in the series─nearly an order of magnitude higher than Ni-nBu (4.18 × 10^-5 S/cm), which features a hydrophobic side chain of the same length─while preserving the electronic conductivity (∼5 × 10^-4 S/cm). Consistent with previous reports, the added steric bulk in Ni-2EG reduces electronic conductivity to 2.7 × 10^-4 S/cm, though the drop is minimal compared to aliphatic side chains of similar length. Counterintuitively, despite improved polarity, ionic conductivity significantly decreases in Ni-2EG, to 2.58 × 10^-6 S/cm, likely due to pore blockage, with electronic conductivity largely remaining unchanged at 2.7 × 10^-6 S/cm. This study unlocks the potential of cMOFs as tunable platforms for the decoupled control of ionic and electronic transport via side chain engineering. It opens doors to the unexplored design space of single-phase porous crystalline MIECs with orthogonal integration of functionalities tailored for emerging applications in chemistry, physics, and materials science.