Measuring and Reporting Electrical Conductivity in Metal–Organic Frameworks: Cd

Electrically conductive metal–organic frameworks (MOFs) are emerging as a subclass of porous materials that can have a transformative effect on electronic and renewable energy devices. Systematic advances in these materials depend critically on the accurate and reproducible characterization of their electrical properties. This is made difficult by the numerous techniques available for electrical measurements and the dependence of metrics on device architecture and numerous external variables. These challenges, common to all types of electronic materials and devices, are especially acute for porous materials, whose high surface area make them even more susceptible to interactions with contaminants in the environment. Here, we use the anisotropic semiconducting framework Cd₂(TTFTB) (TTFTB⁴⁻ = tetrathiafulvalene tetrabenzoate) to benchmark several common methods available for measuring electrical properties in MOFs. We show that factors such as temperature, chemical environment (atmosphere), and illumination conditions affect the quality of the data obtained from these techniques. Consistent results emerge only when these factors are strictly controlled and the morphology and anisotropy of the Cd2(TTFTB) single-crystal devices are taken into account. Most importantly, we show that depending on the technique, device construction, and/or the environment, a variance of 1 or even 2 orders of magnitude is not uncommon for even just one material if external factors are not controlled consistently. Differences in conductivity values of even 2 orders of magnitude should therefore be interpreted with caution, especially between different research groups comparing different compounds. These results allow us to propose a reliable protocol for collecting and reporting electrical properties of MOFs, which should help improve the consistency and comparability of reported electrical properties for this important new class of crystalline porous conductors.United States. Department of Energy. Office of Basic Energy Sciences (Award DE-SC0006937)National Science Foundation (U.S.) (Award 1122374

Electrochemical oxygen reduction catalysed by Ni3(hexaiminotriphenylene)2

Control over the architectural and electronic properties of heterogeneous catalysts poses a major obstacle in the targeted design of active and stable non-platinum group metal electrocatalysts for the oxygen reduction reaction. Here we introduce Ni[SUBSCRIPT 3](HITP)[SUBSCRIPT 2] (HITP=2, 3, 6, 7, 10, 11-hexaiminotriphenylene) as an intrinsically conductive metal-organic framework which functions as a well-defined, tunable oxygen reduction electrocatalyst in alkaline solution. Ni[SUBSCRIPT 3](HITP)[SUBSCRIPT 2] exhibits oxygen reduction activity competitive with the most active non-platinum group metal electrocatalysts and stability during extended polarization. The square planar Ni-N[SUBSCRIPT 4] sites are structurally reminiscent of the highly active and widely studied non-platinum group metal electrocatalysts containing M-N[SUBSCRIPT 4] units. Ni[SUBSCRIPT 3](HITP)[SUBSCRIPT 2] and analogues thereof combine the high crystallinity of metal-organic frameworks, the physical durability and electrical conductivity of graphitic materials, and the diverse yet well-controlled synthetic accessibility of molecular species. Such properties may enable the targeted synthesis and systematic optimization of oxygen reduction electrocatalysts as components of fuel cells and electrolysers for renewable energy applications.United States. Department of Energy. Office of Basic Energy Sciences (Award DESC0006937

High Electrical Conductivity in Ni 3 (2,3,6,7,10,11-hexaiminotriphenylene) 2 , a Semiconducting Metal–Organic Graphene Analogue

Reaction of 2,3,6,7,10,11-hexaaminotriphenylene with Ni2+ in aqueous NH3 solution under aerobic conditions produces Ni3(HITP)2 (HITP = 2,3,6,7,10,11-hexaiminotriphenylene), a new two-dimensional metalorganic framework (MOF). The new material can be isolated as a highly conductive black powder or dark blue-violet films. Two-probe and van der Pauw electrical measurements reveal bulk (pellet) and surface (film) conductivity values of 2 Scm -1 and 40 Scm -1, respectively, both records for MOFs and among the best for any coordination polymer.Chemistry and Chemical Biolog

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

Isostructural metal–organic frameworks (MOFs) M[subscript 2](TTFTB) (M = Mn, Co, Zn, and Cd; H[subscript 4]TTFTB = 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[subscript –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.United States. Department of Energy. Office of Basic Energy Sciences (Award DE-SC0006937)National Science Foundation (U.S.). Graduate Research Fellowship Program (Award 1122374)David & Lucile Packard Foundation (Fellowship

Highly Coplanar Very Long Oligo(alkylfuran)s: A Conjugated System with Specific Head-To-Head Defect

Well-defined monodisperse conjugated oligomers, which have planar backbones and are free from the disturbance of substituents, attract broad interest. Herein, we report a series of symmetrical, isomerically pure oligofurans, namely, the 16-mer <b>16F-6C</b>_<b>6</b> together with the related <b><i>n</i>F-</b><b>2</b><b>C</b>_<b>6</b> (<i>n</i> = 4, 6, 8). Through computational studies and detailed spectroscopic and X-ray characterization, for the first time, we show that the planarity of the furan backbone is almost unaffected by the head-to-head defect which is known to cause considerable twists in its oligo- or poly­thiophene analogues. We present that the properties of these rigid oligo­(alkyl­furan)­s are strongly influenced by the conjugation length. As the longest monodisperse α-oligofuran synthesized to date, <b>16F-6C</b>_<b>6</b> was observed to be stable and highly fluorescent. Experimental and computational studies of the redox states of these oligo­(alkyl­furan)­s reveal that <b>16F-6C</b>_<b>6</b> has singlet biradical (polaron-pair) character in the doubly oxidized ground state: the open-shell singlet (⟨<i>S</i>₂⟩ = 0.989) is 3.8 kcal/mol more stable than the closed-shell dication