Cation-Dependent Intrinsic Electrical Conductivity in Isostructural Tetrathiafulvalene-Based Microporous Metal–Organic Frameworks

Isostructural metal–organic
frameworks (MOFs) M2(TTFTB) (M = Mn, Co, Zn, and Cd; H4TTFTB = tetrathiafulvalene
tetrabenzoate) exhibit a striking correlation between their single-crystal
conductivities and the shortest S···S interaction defined
by neighboring TTF cores, which inversely correlates with the ionic
radius of the metal ions. The larger cations cause a pinching of the
S···S contact, which is responsible for better orbital
overlap between pz orbitals on neighboring
S and C atoms. Density functional theory calculations show that these
orbitals are critically involved in the valence band of these materials,
such that modulation of the S···S distance has an important
effect on band dispersion and, implicitly, on the conductivity. The
Cd analogue, with the largest cation and shortest S···S
contact, shows the largest electrical conductivity, σ = 2.86
(±0.53) × 10–4 S/cm, which is also among
the highest in microporous MOFs. These results describe the first
demonstration of tunable intrinsic electrical conductivity in this
class of materials and serve as a blueprint for controlling charge
transport in MOFs with π-stacked motifs.