Redox Activities of Metal–Organic Frameworks Incorporating Rare-Earth Metal Chains and Tetrathiafulvalene Linkers

Abstract

Metal-organic frameworks (MOFs) incorporating lanthanide nodes and tetrathiafulvalene (TTF) linkers offer a viable approach for combining redox activity and magnetism in one material. Four rare-earth lanthanide ions (RE = Tb, Dy, Ho, and Er) were found to form isostructural MOFs consisting of metal chains bridged by redox-active tetrathiafulvalene-tetrabenzoate (TTFTB^4-) whereby the carboxylate moieties act in both anti- anti and syn- syn coordination modes. These materials display tunable redox-active properties and slow magnetic relaxation phenomenon (Er and Dy). While the as-synthesized crystals contain the neutral diamagnetic TTF moiety, using either a solid-solution electrochemical method or iodine oxidation transforms part of the latter to the paramagnetic TTF^•+ radical in a single-crystal-to-single-crystal manner without altering the internal structure of the building chains and the frameworks. This is accompanied by inclusion of I₃^- replacing some of the solvents, as well as changes in the central C-C bond length of TTFTB, a strong EPR response at g ∼ 2, and an enhancement of the reflectance at low energies originating from absorption by the radical.