Excited states of Nb3N2 and Nb3C2: Density functional theory, CASSCF, and MRCI studies

Complete active space self-consistent field (CASSCF) and multireference configuration interaction (MRCI) methods are used to investigate the Nb(3)N(2) and Nb(3)C(2) clusters in order to determine the agreement between multireference methods, density functional theory (DFT), and experiment. These two clusters are ideal candidates to study as the known spectroscopy can serve to validate the computational results, yet there is still room for the calculations to inform further spectroscopic experiments. We find that the MRCI leading configuration for each of the ground states is in agreement with that predicted by DFT but only accounts for up to 70% of the total configuration. CASSCF and DFT geometries are also in general agreement. Transition energies between the neutral and cationic manifolds are found to be poorly predicted by MRCI relative to the computationally cheap DFT method. For Nb(3)C(2) we find that a higher energy isomer may have an electronic transition in the spectral vicinity as the lowest energy isomer.Matthew A. Addicoat and Gregory F. Meth

Triazine functionalized porous covalent organic framework for photo-organocatalytic E–Z isomerization of olefins

Visible light-mediated photocatalytic organic transformation has drawn significant attention as an alternative process for replacing thermal reactions. Although precious metal/organic dyes based homogeneous photocatalysts have been developed, their toxic and nonreusable nature makes them inappropriate for large-scale production. Therefore, we have synthesized a triazine and a keto functionalized nonmetal based covalent organic framework (TpTt) for heterogeneous photocatalysis. As the catalyst shows significant absorption of visible light, it has been applied for the photocatalytic uphill conversion of trans-stilbene to cis-stilbene in the presence of blue light-emitting diodes with broad substrate scope via an energy transfer process

Performance of GFN1-xTB for periodic optimization of metal organic frameworks

Tight-binding approaches bridge the gap between force field methods and Density Functional Theory (DFT). Density Functional Tight Binding (DFTB) has been employed for a wide range of systems including proteins, clays and 2D and 3D materials. DFTB is 2–3 orders of magnitude faster than DFT, allowing calculations containing up to ca. 5000 atoms. The efficiency of DFTB comes via pre-computed integrals, which are parameterized for each pair of atoms, and the requirement for this parameterization has previously prevented widespread use of DFTB for Metal–Organic Frameworks. The GFN-xTB (Geometries, Frequencies, and Non-covalent interactions Tight Binding) method provides parameters for elements up to Z ≤ 86. We have therefore employed GFN-xTB to periodic optimizations of the Computation Ready Experimental (CoRE) database of MOF structures. We find that 75% of all cell parameters remain within 5% of the reference (experimental) value and that bonds containing metal atoms are typically well conserved with a mean average deviation of 0.187 Å. Therefore GFN-xTB provides the ability to calculate MOF structures more accurately than force fields, and ca. 2 orders of magnitude faster than DFT. We therefore propose that GFN-xTB is a suitable method for screening of hypothetical MOFs (Z ≤ 86), with the advantage of accurate binding energies for adsorption applications

O(-)---C interactions and bond formation in 1-naphtholate anions with peri-located electrophilic carbon centres

The first peri interactions between naphtholate oxyanions and electrophilic double bonds are described. Tetramethylguanidine forms crystalline salts with 8-acetyl- and 8-benzoyl-naphthol which show O---C distances for the anions in the range 2.558-2.618 Å and small increases in carbonyl pyramidalities over the corresponding naphthols, whereas DMAP forms only hydrogen bonded complexes. Replacement of the acyl group with an alkene leads to intramolecular O-C bond formation for just the most electron deficient alkenes, with long peri O-C bonds (1.508 and 1.521 Å) observed in one case. However, both cyclised and uncyclised examples can be deprotonated to give cyclic structures according to NMR, and DFT calculations suggest very long peri- O-C bond lengths of 1.540 and 1.622 Å for two of these anions

Pore topology analysis in porous molecular systems

Porous molecular materials are constructed from molecules that assemble in the solid-state such that there are cavities or an interconnected pore network. It is challenging to control the assembly of these systems, as the interactions between the molecules are generally weak, and subtle changes in the molecular structure can lead to vastly different intermolecular interactions and subsequently different crystal packing arrangements. Similarly, the use of different solvents for crystallization, or the introduction of solvent vapour, can result in different polymorphs and pore networks being formed. It is difficult to uniquely describe the pore networks formed, and thus we analyse 1033 crystal structures of porous molecular systems to determine the underlying topology of their void spaces and potential guest diffusion networks. Material-Agnostic topology definitions are applied. We use the underlying topological nets to examine whether it is possible to apply isoreticular design principles to porous molecular materials. Overall, our automatic analysis of a large dataset gives a general insight into the relationships between molecular topologies and the topological nets of their pore network. We show that while porous molecular systems tend to pack similarly to non-porous molecules, the topologies of their pore distributions resemble those of more prominent porous materials, such as metal-organic frameworks and covalent organic frameworks

Electrochemical stimuli-driven facile metal-free hydrogen evolution from pyrene-porphyrin-based crystalline covalent organic framework

A [2+2] Schiff base type condensation between 5, 10,15, 20 tetrakis(4 aminophenyl)porphyrin (TAP) and 1,3,6,8 tetrakis (4 formylphenyl) pyrene (TFFPy) under solvothermal condition yields a crystalline, quasi two dimensional covalent organic framework (SB PORPy COF). The porphyrin and pyrene units are alternatively occupied in the vertex of 3D triclinic crystal having permanent micro-porosity with moderately high surface area (~869 m2g-1) and promising chemical stability. The AA stacking of the monolayers give a pyrene bridged conducting channel. SB PORPy COF has been exploited for metal free hydrogen production to understand the electrochemical behavior using the imine based docking site in acidic media. SB PORPy-COF has shown the onset potential of 50 mV and the Tafel slope of 116 mV dec-1. We expect that the addendum of the imine based COF would not only enrich the structural variety but also help to understand the electrochemical behavior of these class of materials

A crystalline, 2D polyarylimide cathode for ultrastable and ultrafast Li storage

Organic electrode materials are of long‐standing interest for next‐generation sustainable lithium‐ion batteries (LIBs). As a promising cathode candidate, imide compounds have attracted extensive attention due to their low cost, high theoretical capacity, high working voltage, and fast redox reaction. However, the redox active site utilization of imide electrodes remains challenging for them to fulfill their potential applications. Herein, the synthesis of a highly stable, crystalline 2D polyarylimide (2D‐PAI) integrated with carbon nanotube (CNT) is demonstrated for the use as cathode material in LIBs. The synthesized polyarylimide hybrid (2D‐PAI@CNT) is featured with abundant π‐conjugated redox‐active naphthalene diimide units, a robust cyclic imide linkage, high surface area, and well‐defined accessible pores, which render the efficient utilization of redox active sites (82.9%), excellent structural stability, and fast ion diffusion. As a consequence, high rate capability and ultrastable cycle stability (100% capacity retention after 8000 cycles) are achieved in the 2D‐PAI@CNT cathode, which far exceeds the state‐of‐the‐art polyimide electrodes. This work may inspire the development of novel organic electrodes for sustainable and durable rechargeable batteries

Morphological evolution of two-dimensional porous hexagonal trimesic acid framework

Hexagonal single crystal structure (Form II) of trimesic acid (TMA) has been isolated by dissolving the interpenetrated Form I of TMA in tetrahydrofuran. Form II (hexagonal) was converted to Form I (interpenetrated) at room temperature through some intermediate structures. A detailed time-dependent FESEM study shows that the external morphology of Form II (hexagonal) is a hollow hexagonal tube that mimics its crystal structure. The block-shaped (morphology) of Form I (interpenetrated) was converted to the hollow hexagonal tube through some intermediate morphologies which are corresponding to particular crystal structures. Here, we have established a strong correlation between crystal structures with the morphology. These hollow tubes have been employed for Rhodamine B dye adsorption studies and showed an uptake of 82%, much more significant than Form I (interpenetrated) (39%) due to the presence of a pore channel in the crystal structure

Control of crystallinity of vinylene-linked two-dimensional conjugated polymers by rational monomer design

The interest in two-dimensional conjugated polymers (2D CPs) has increased significantly in recent years. In particular, vinylene-linked 2D CPs with fully in-plane sp2-carbon-conjugated structures, high thermal and chemical stability, have become the focus of attention. Although the Horner-Wadsworth-Emmons (HWE) reaction has been recently demonstrated in synthesizing vinylene-linked 2D CPs, it remains largely unexplored due to the challenge in synthesis. In this work, we reveal the control of crystallinity of 2D CPs during the solvothermal synthesis of 2D-poly(phenylene-quinoxaline-vinylene)s (2D-PPQVs) and 2D-poly(phenylene-vinylene)s through the HWE polycondensation. The employment of fluorinated phosphonates and rigid aldehyde building blocks is demonstrated as crucial factors in enhancing the crystallinity of the obtained 2D CPs. Density functional theory (DFT) calculations reveal the critical role of the fluorinated phosphonate in enhancing the reversibility of the (semi)reversible C−C single bond formation

Outstanding Charge Mobility by Band Transport in Two-Dimensional Semiconducting Covalent Organic Frameworks

[Image: see text] Two-dimensional covalent organic frameworks (2D COFs) represent a family of crystalline porous polymers with a long-range order and well-defined open nanochannels that hold great promise for electronics, catalysis, sensing, and energy storage. To date, the development of highly conductive 2D COFs has remained challenging due to the finite π-conjugation along the 2D lattice and charge localization at grain boundaries. Furthermore, the charge transport mechanism within the crystalline framework remains elusive. Here, time- and frequency-resolved terahertz spectroscopy reveals intrinsically Drude-type band transport of charge carriers in semiconducting 2D COF thin films condensed by 1,3,5-tris(4-aminophenyl)benzene (TPB) and 1,3,5-triformylbenzene (TFB). The TPB–TFB COF thin films demonstrate high photoconductivity with a long charge scattering time exceeding 70 fs at room temperature which resembles crystalline inorganic materials. This corresponds to a record charge carrier mobility of 165 ± 10 cm(2) V(–1) s(–1), vastly outperforming that of the state-of-the-art conductive COFs. These results reveal TPB–TFB COF thin films as promising candidates for organic electronics and catalysis and provide insights into the rational design of highly crystalline porous materials for efficient and long-range charge transport