Topologically Tunable Conjugated Metal-Organic Frameworks for Modulating Conductivity and Chemiresistive Properties for NH3 Sensing.

Electrically conductive metal-organic frameworks (cMOFs) have garnered significant attention in materials science due to their potential applications in modern electrical devices. However, achieving effective modulation of their conductivity has proven to be a major challenge. In this study, we have successfully prepared cMOFs with high conductivity by incorporating electron-donating fused thiophen rings in the frameworks and extending their π-conjugated systems through ring-closing reactions. The conductivity of cMOFs can be precisely modulated ranging from 10-3 to 102 S m-1 by regulating their dimensions and topologies. Furthermore, leveraging the inherent tunable electrical properties based on topology, we successfully demonstrated the potential of these materials as chemiresistive gas sensors with an outstanding response toward 100 ppm NH3 at room temperature. This work not only provides valuable insights into the design of functional cMOFs with different topologies but also enriches the cMOF family with exceptional conductivity properties.

Adjusting the Stacking Model of Two-Dimensional Covalent Organic Frameworks for Volatile Acid Sensing via Spatial Effects.

Covalent organic frameworks (COFs) are polymer networks with a precise structure and permanent porosity, making them an attractive platform for the detection of volatile analytes due to their chemical stability and accessible active sites. In this study, based on electron-rich N,N,N',N'-tetrakis(4-aminophenyl)-1,4-benzenediamine moiety, two 2D COFs with different topological structures and stacking models were designed by the strategy of spatial effect. The conductivity of the AB-stacked COF-NUST-20 was an order of magnitude higher than that of the AA-stacked COF-NUST-30. With the protonation of the imine bond, both COFs exhibited a strong, rapid, and reversible visible color change in response to corrosive HCl vapor. In addition, the AB-stacked COF-NUST-20, which facilitates both interlayer and intralayer charge transfer, shows better sensing performance. These findings demonstrate the usefulness of all-aromatic 2D COFs as real-time responsive chemosensors and provide insight into the design of sensing materials with high sensitivity.

Three-dimensional covalent organic frameworks based on a π-conjugated tetrahedral node.

The construction of three-dimensional (3D) covalent organic frameworks (COFs), especially fully conjugated 3D COFs, is a long-standing challenge. Herein, we report a saddle-like, π-conjugated cyclooctatetrathiophene (COTh) as a tetrahedral node to construct fully conjugated 3D COFs. The present work enriches the structural diversities and potential applications of 3D COFs.

Synthesis of All Thiophene-Based [7]Helicenes and Trithienothiepines with Isomeric Location of Sulfur Atoms Based on Intramolecular Selectivity of Deprotonation.

Three unsymmetric thiophene-based [7]helicenes, namely, endo-exo-UH-1, endo-top-UH-2, and exo-top-UH-3, with different isomeric locations of sulfur atoms in two terminal thiophene rings were efficiently synthesized using dithieno[2,3-b:3',2'-d]thiophene (bb-DTT), dithieno[2,3-b:2',3'-d]thiophene (bt-DTT), and dithieno[2,3-b:3',4'-d]thiophene (bs-DTT) as building blocks via Suzuki cross-coupling and intramolecular cyclization reactions. Aside from these racemic [7]helicenes, two novel heterocyclic isomers, namely, trithienothiepines TTTP-1 and TTTP-2, were simultaneously obtained during the intramolecular cyclization. Two novel deprotonations of bi-DTTs and cyclization for synthesizing target compounds showed high selectivity and efficiently constructed both UHs and TTTPs. X-ray crystallographic analyses revealed that the UHs have typical helical molecular structures. The isomeric location of sulfur atoms in the two terminal thiophene rings in endo-exo-UH-1, endo-top-UH-2, exo-top-UH-3, and TTTP-1 allowed multiple intermolecular interactions, such as S···S, S···C, and S···H interactions, resulting in different crystal-packing patterns. Moreover, the absorption behaviors of these [7]helicenes, TTTP-1, and TTTP-2 were examined and theoretically calculated. Results indicated that the isomeric location of sulfur atoms plays a key role in tuning intramolecular π-electronic conjugation.

Highly Robust Non-Noble Alkaline Hydrogen-Evolving Electrocatalyst from Se-Doped Molybdenum Disulfide Particles on Interwoven CoSe2 Nanowire Arrays.

Developing efficient non-noble and earth-abundant hydrogen-evolving electrocatalysts is highly desirable for improving the energy efficiency of water splitting in base. Molybdenum disulfide (MoS2 ) is a promising candidate, but its catalytic activity is kinetically retarded in alkaline media due to the unfavorable water adsorption and dissociation feature. A heterogeneous electrocatalyst is reported that is constructed by selenium-doped MoS2 (Se-MoS2 ) particles on 3D interwoven cobalt diselenide (CoSe2 ) nanowire arrays that drives the hydrogen evolution reaction (HER) with fast reaction kinetics in base. The resultant Se-MoS2 /CoSe2 hybrid exhibits an outstanding catalytic HER performance with extremely low overpotentials of 30 and 93 mV at 10 and 100 mA cm-2 in base, respectively, which outperforms most of the inexpensive alkaline HER catalysts, and is among the best alkaline catalytic activity reported so far. Moreover, this hybrid catalyst shows exceptional catalytic performance with very low overpotentials of 84 and 95 mV at 10 mA cm-2 in acidic and neutral electrolytes, respectively, implying robust pH universality of this hybrid catalyst. This work may provide new inspirations for the development of high-performance MoS2 -based HER electrocatalysts in unfavorable basic media for promising catalytic applications.

Theoretical investigations on the unsymmetrical effect of ß-link Zn-porphyrin sensitizers on the performance for dye-sensitized solar cells.

Dye sensitizers play an important role in dye-sensitized solar cells (DSSCs). As a promising strategy for the design of novel porphyrin sensitizers, the asymmetric modification of the porphyrin ring to meso-link porphyrin sensitizer has emerged in recent years, which can improve the light-harvesting properties and enhance the electron distribution. In this work, in order to reveal the essence of the effect of unsymmetrical substitution on the performance of ß-link porphyrin dyes in DSSCs, four kinds of common ß-link porphyrin dyes with different structures are calculated by using density functional theory (DFT) and time-dependent density functional theory (TD-DFT). The electronic structures and optical properties of these studied dyes in dimethylformamide (DMF) are also investigated. The key parameters of the short-circuit current density (Jsc), including light harvesting efficiency (LHE), electron injection driving force (ΔGinject), and intra-molecular charge transfer (ICT) are discussed in detail. In addition, the periodic DFT calculations in the dye-TiO2 systems are also employed to investigate the geometrical and electronic injection process of the different connection types of these studied dyes adsorbed on the periodic TiO2 model with an exposed anatase (101) surface. We expect the present study would deepen the understanding of the alternative function of unsymmetrical substitution and may contribute to future DSSC design.

Effective Augmentation of Complex Networks.

Networks science plays an enormous role in many aspects of modern society from distributing electrical power across nations to spreading information and social networking amongst global populations. While modern networks constantly change in size, few studies have sought methods for the difficult task of optimising this growth. Here we study theoretical requirements for augmenting networks by adding source or sink nodes, without requiring additional driver-nodes to accommodate the change i.e., conserving structural controllability. Our "effective augmentation" algorithm takes advantage of clusters intrinsic to the network topology, and permits rapidly and efficient augmentation of a large number of nodes in one time-step. "Effective augmentation" is shown to work successfully on a wide range of model and real networks. The method has numerous applications (e.g. study of biological, social, power and technological networks) and potentially of significant practical and economic value.

Molecular dynamics simulation of melting and crystallization processes of polyethylene clusters confined in armchair single-walled carbon nanotubes.

The confined interaction is important to understand the melting and crystallization of polymers within single-wall carbon tube (SWNT). However, it is difficult for us to observe this interaction. In the current work, the structures and behaviors of melting and crystallization for polyethylene (PE) clusters confined in armchair single-walled carbon nanotubes ((n,n)-SWNTs) are investigated and examined based on molecular dynamics (MD) simulations. The nonbonded energies, structures, Lindemman indices, radial density distributions, and diffusion coefficients are used to demonstrate the features of melting phase transition for PE clusters confined in (n,n)-SWNTs. The chain end-to-end distance (R(n)) and chain end-to-end distribution are used to examine the flexibility of the PE chain confined in SWNT. The global orientational order parameter (P2) is employed to reveal the order degree of whole PE polymer. The effect of polymerization degree on melting temperature and the influence of SWNT chirality on structure of PE cluster are examined and discussed. Results demonstrate that within the confined environment of SWNT, PE clusters adopt novel co-axial crystalline layer structure, in which parallel chains of each layer are approximately vertical to tube axis. The disordered-ordered transformation of PE chains in each layer is an important structural feature for crystallization of confined PE clusters. SWNTs have a considerable effect on the structures and stabilities of the confined PE clusters.

Some Insight into Stability of Amorphous Poly(ethylene glycol) Dimethyl Ether Polymers Based on Molecular Dynamics Simulations.

Poly(ethylene glycol) dimethyl ether (PEGDME) polymers are widely used as drug solid dispersion reagents. They can cause the amorphization of drugs and improve their solubility, stability, and bioavailability. However, the mechanism about why amorphous PEGDME 2000 polymer is highly stable is unclear so far. Molecular dynamics (MD) simulation is a unique key technique to solve it. In the current work, we systematically investigate structure, aggregate state, and thermodynamic and kinetic behaviors during the phase-transition processes of the PEGDME polymers with different polymerization degree in terms of MD simulations. The melting and glass-transition temperatures of the polymers are in good agreement with experimental values. The amorphous PEGDME2000 exhibits high stability, which is consistent with the recent experiment results and can be ascribed to a combination of two factors, that is, a high thermodynamic driving force for amorphization and a relatively low molecular mobility.