Structurally Encoded Mixed Proton-Electron Transport in Tetrathiafulvalene-Based Lanthanide MOFs

A family of 2D tetrathiafulvalene tetracarboxylate (TTFTC)-based lanthanide (LnIII = Dy III, Er III, and Yb III) metal-organic frameworks (MOFs) is reported as a robust platform for mixed proton-electron conduction. Electronic transport arises from partially oxidized, π-stacked TTFTC columns with a fixed hole population imposed by framework stoichiometry, leading to narrow-band semiconducting behavior. The charge distribution within the π-stacks is preserved over a wide humidity range, resulting in only minor, compound-dependent variations in electronic conductivity. In parallel, proton transport is promoted by dense hydrogen-bond networks formed by coordinated water molecules anchored to rigid Ln6 cluster walls, enabling an efficient proton conduction that remains operative across a broad range of humidity and temperature conditions (Ea < 0.4 eV at RH 80%), consistent with a dominant Grotthuss-type mechanism. Among the series, Er6TTFTC5 displays the highest room-temperature single-crystal electronic conductivity reported to date for a TTF-based MOF (1.0 × 10-2 S cm-1, 4-probe), together with a proton conductivity of 3.7 × 10-3 S cm-1 under humid conditions (80 °C, 80% RH). To enable direct comparison, both electronic and proton conductivities were evaluated under identical conditions, and an effective ambipolar conductivity (σamb) was derived from the independently measured contributions. The resulting values rank among the highest reported for structurally defined proton-electron mixed conductors (2.3 × 10-3, 7.2 × 10-4, and 5.9 × 10-4 S cm-1 at 80% RH and 80 °C for Er6TTFTC5, Yb6TTFTC5, and Dy6TTFTC5). Subtle variations across the lanthanide series are attributed to differences in framework rigidity and intermolecular interactions, which modulate transport properties without altering the underlying, structurally encoded conduction pathways.