The Advent of Electrically Conducting Double-Helical Metal–Organic Frameworks Featuring Butterfly-Shaped Electron-Rich π‑Extended Tetrathiafulvalene Ligands

To
diversify metal–organic framework (MOF) structures beyond traditional
Euclidean geometries and to create new charge-delocalization pathways
beneficial for electrical conductivity, we constructed a novel double-helical
MOF (dhMOF) by introducing a new butterfly-shaped electron-rich π-extended
tetrathiafulvalene ligand equipped with four benzoate groups (ExTTFTB).
The face-to-face oriented convex ExTTFTB ligands connected by Zn2(COO)4 paddlewheel nodes formed ovoid cavities
suitable for guest encapsulation, while π–π-interaction
between the ExTTFTB ligands of neighboring strands helped create new
charge-delocalization pathways in iodine-mediated partially oxidized
dhMOF. Iodine vapor diffusion led to oxidation of half of the ExTTFTB
ligands in each double-helical strand to ExTTFTB•+ radical cations, which putatively formed intermolecular ExTTFTB/ExTTFTB•+ π-donor/acceptor charge-transfer chains with
the neutral ExTTFTB ligands of an adjacent strand, creating supramolecular
wire-like charge-delocalization pathways along the helix seams. In
consequence, the electrical conductivity of dhMOF surged from 10–8 S/m up to 10–4 S/m range after
iodine treatment. Thus, the introduction of the electron-rich ExTTFTB
ligand with a distinctly convex π-surface not only afforded
a novel double-helical MOF architecture featuring ovoid cavities and
unique charge-delocalization pathways but also, more importantly,
delivered a new tool and design strategy for future development of
electrically conducting stimuli-responsive MOFs.