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.

同构金属有机框架（metal–organic frameworks, MOFs）M₂(TTFTB)（其中M=Mn、Co、Zn和Cd；H₄TTFTB=四硫富瓦烯四苯甲酸（tetrathiafulvalene tetrabenzoate））的单晶电导率，与相邻四硫富瓦烯（tetrathiafulvalene, TTF）核所定义的最短S···S相互作用之间存在显著相关性，且该相互作用与金属离子的离子半径呈负相关。阳离子半径越大，S···S接触距离越被压缩，这正是相邻S、C原子的pz轨道间实现更优轨道重叠的关键原因。密度泛函理论（Density functional theory）计算表明，这些轨道与该类材料的价带密切相关，因此对S···S距离的调控可显著影响能带色散，并间接影响材料电导率。其中阳离子半径最大、S···S接触距离最短的镉基同构物，展现出最高的电导率σ=2.86(±0.53)×10⁻⁴ S/cm，该值同时也是微孔金属有机框架中最高的电导率之一。本研究成果首次证实了该类材料具备可调谐本征电导率的特性，同时为调控具有π堆叠基元的金属有机框架中的电荷传输提供了指导蓝图。