Effects of intervalence charge transfer interaction between π-stacked mixed valent tetrathiafulvalene ligands on the electrical conductivity of 3D metal-organic frameworks

Dynamic Interchain Motion in 1D Tetrathiafulvalene-Based Coordination Polymers for Highly Sensitive Molecular Recognition

The application of electrically conductive 1D coordination polymers (1D CPs) in nanoelectronic molecular recognition is theoretically promising yet rarely explored due to the challenges in their synthesis and optimization of electrical properties. In this regard, two tetrathiafulvalene-based 1D CPs, namely [Co(m-H2TTFTB)(DMF)2(H2O)]n (Co-m-TTFTB), and {[Ni(m-H2TTFTB)(CH3CH2OH)1.5(H2O)1.5]·(H2O)0.5}n (Ni-m-TTFTB) are successfully constructed. The shorter S···S contacts between the [M(solvent)3(m-H2TTFTB)]n chains contribute to a significant improvement in their electrical conductivities. The powder X-ray diffraction (PXRD) under different organic solvents reveals the flexible and dynamic structural characteristic of M-m-TTFTB, which, combined with the 1D morphology, lead to their excellent performance for sensitive detection of volatile organic compounds. Co-m-TTFTB achieves a limit of detection for ethanol vapor down to 0.5 ppm, which is superior to the state-of-the-art chemiresistive sensors based on metal-organic frameworks or organic polymers at room temperature. In situ diffuse reflectance infrared Fourier transform spectroscopy, PXRD measurements and density functional theory calculations reveal the molecular insertion sensing mechanism and the corresponding structure-function relationship. This work expands the applicable scenario of 1D CPs and opens a new realm of 1D CP-based nanoelectronic sensors for highly sensitive room temperature gas detection.

Rare Guest-Induced Electrical Conductivity of Zn-Porphyrin Metallacage Inclusion Complexes Featuring π-Donor/Acceptor/Donor Stacks

Charge-transfer (CT) interactions between co-facially aligned π-donor/acceptor (π-D/A) arrays engender unique optical and electronic properties that could benefit (supra)molecular electronics and energy technologies. Herein, we demonstrate that a tetragonal prismatic metal-organic cage (MOC18+) having two parallel π-donor tetrakis(4-carboxyphenyl)-Zn-porphyrin (ZnTCPP) faces selectively intercalate planar π-acceptor guests, such as hexaazatriphenylene hexacarbonitrile (HATHCN), hexacyanotriphenylene (HCTP), and napthanelediimide (NDI) derivatives, forming 1:1 πA@MOC18+ inclusion complexes featuring supramolecular π-D/A/D triads. The π-acidity of intercalated π-acceptors (HATHCN ≫ HCTP ≈ NDIs) dictated the nature and strength of their interactions with the ZnTCPP faces, which in turn influenced the binding affinities (Ka) and optical and electronic properties of corresponding πA@MOC18+ inclusion complexes. Owing to its strongest CT interaction with ZnTCPP faces, the most π-acidic HATHCN guest enjoyed the largest Ka (5 × 106 M-1), competitively displaced weaker π-acceptors from the MOC18+ cavity, and generated the highest electrical conductivity (2.1 × 10-6 S/m) among the πA@MOC18+ inclusion complexes. This work demonstrates a unique through-space charge transport capability of πA@MOC18+ inclusion complexes featuring supramolecular π-D/A/D triads, which generated tunable electrical conductivity, which is a rare but much coveted electronic property of such supramolecular assemblies that could further expand their utility in future technologies.

An Octacarboxylate-Linked Sodium Metal-Organic Framework with High Porosity

Alkali metal-based metal-organic frameworks (MOFs) with permanent porosity are scarce because of their high tendency to coordinate with solvents such as water. However, these MOFs are lightweight and bear gravimetric benefits for gas adsorption related applications. In this study, we present the successful construction of a microporous MOF, designated as HIAM-111, built solely on sodium ions by using an octacarboxylate linker. The structure of HIAM-111 is based on 8-connected Na4 clusters and exhibits a novel topology with an underlying 32,42,8-c net. Remarkably, HAM-111 possesses a robust and highly porous framework with a BET surface area of 1561 m2/g, significantly surpassing that of the previously reported Na-MOFs. Further investigations demonstrate that HIAM-111 is capable of separating C2H2/CO2 and purifying C2H4 directly from C2H4/C2H2/C2H6 with high adsorption capacities. The current work may shed light on the rational design of robust and porous MOFs based on alkali metals.

From a Collapse-Prone, Insulating Ni-MOF-74 Analogue to Crystalline, Porous, and Electrically Conducting PEDOT@MOF Composites

Electrically conductive porous metal-organic frameworks (MOFs) show great promise in helping advance electronics and clean energy technologies. However, large porosity usually hinders long-range charge transport, an essential criterion of electrical conductivity, underscoring the need for new strategies to combine these two opposing features and realize their diverse potentials. All previous strategies to boost the conductivity of porous MOFs by introducing redox-complementary guest molecules, conducting polymers, and metal nanoparticles have led to a significant loss of frameworks' porosity and surface areas, which could be otherwise exploited to capture additional guests in electrocatalysis and chemiresistive sensing applications. Herein, we demonstrate for the first time that the in situ oxidative polymerization of preloaded 3,4-ethylenedioxythiophene (EDOT) monomers into the polyethylenedioxythiophene (PEDOT) polymer inside the hexagonal cavities of an intrinsically insulating Ni2(NDISA) MOF-74 analogue (NDISA = naphthalenediimide N,N-disalicylate), which easily collapses and becomes amorphous upon drying, simultaneously enhanced the crystallinity, porosity, and electrical conductivity of the resulting PEDOT@Ni2(NDISA) composites. At lower PEDOT loading (∼22 wt %), not only did the Brunauer-Emmett-Teller surface area of the PEDOT@Ni2(NDISA) composite (926 m2/g) more than double from that of evacuated pristine Ni2(NDISA) (387 m2/g), but also its electrical conductivity (1.1 × 10-5 S/cm) soared 105 times from that of the pristine MOF, demonstrating unprecedented dual benefits of our strategy. At higher PEDOT loading (≥33 wt %), the electrical conductivity of Ni2(NDISA)⊃PEDOT composites further increased modestly (10-4 S/cm), but their porosity dropped precipitously as large amounts of PEDOT filled up the hexagonal MOF channels. Thus, our work presents a simple new strategy to simultaneously boost the structural stability, porosity, and electrical conductivity of intrinsically insulating and collapse-prone MOFs by introducing small amounts of conducting polymers that can not only reinforce the MOF scaffolds and prevent them from collapsing but also help create a much coveted non-native property by providing charge carriers and charge transport pathways.

Redox-Active Metal-Organic Frameworks with Three-Dimensional Lattice Containing the m-Tetrathiafulvalene-Tetrabenzoate

Metal-organic frameworks (MOFs) constructed by tetrathiafulvalene-tetrabenzoate (H₄TTFTB) have been widely studied in porous materials, while the studies of other TTFTB derivatives are rare. Herein, the meta derivative of the frequently used p-H₄TTFTB ligand, m-H₄TTFTB, and lanthanide (Ln) metal ions (Tb³⁺, Er³⁺, and Gd³⁺) were assembled into three novel MOFs. Compared with the reported porous Ln-TTFTB, the resulted three-dimensional frameworks, Ln-m-TTFTB ([Ln₂(m-TTFTB)(m-H₂TTFTB)_0.5(HCOO)(DMF)]·2DMF·3H₂O), possess a more dense stacking which leads to scarce porosity. The solid-state cyclic voltammetry studies revealed that these MOFs show similar redox activity with two reversible one-electron processes at 0.21 and 0.48 V (vs. Fc/Fc⁺). The results of magnetic properties suggested Dy-m-TTFTB and Er-m-TTFTB exhibit slow relaxation of the magnetization. Porosity was not found in these materials, which is probably due to the meta-configuration of the m-TTFTB ligand that seems to hinder the formation of pores. However, the m-TTFTB ligand has shown to be promising to construct redox-active or electrically conductive MOFs in future work.

An electrically conductive metallocycle: densely packed molecular hexagons with π-stacked radicals

Electrical conduction among metallocycles has been unexplored because of the difficulty in creating electronic transport pathways. In this work, we present an electrocrystallization strategy for synthesizing an intrinsically electron-conductive metallocycle, [Ni6(NDI-Hpz)6(dma)12(NO3)6]·5DMA·nH2O (PMC-hexagon) (NDI-Hpz = N,N'-di(1H-pyrazol-4-yl)-1,4,5,8-naphthalenetetracarboxdiimide). The hexagonal metallocycle units are assembled into a densely packed ABCABC… sequence (like the fcc geometry) to construct one-dimensional (1D) helical π-stacked columns and 1D pore channels, which were maintained under the liberation of H2O molecules. The NDI cores were partially reduced to form radicals as charge carriers, resulting in a room-temperature conductivity of (1.2-2.1) × 10-4 S cm-1 (pressed pellet), which is superior to that of most NDI-based conductors including metal-organic frameworks and organic crystals. These findings open up the use of metallocycles as building blocks for fabricating conductive porous molecular materials.

Direct synthesis of a stable radical doped electrically conductive coordination polymer

K-ONDI, a directly synthesized coordination polymer, contains NDI˙− radicals that are stable in air and in common organic solvents. Benefiting from π–π interactions and unpaired electrons, K-ONDI exhibits an electrical conductivity of 10−6 S cm−1.