Light- and Electric-Field-Induced Switching of Thiolated Azobenzene Self-Assembled Monolayer

The reversible isomerization of the azobenzene (AZO) based self-assembled monolayers (SAMs) under external stimuli is the key to their application as molecular switches. To establish the relationship between electronic structure and switching function, AZO and its derivatives with electron donating (NH₂) and withdrawing (NO₂) terminal groups, respectively, are investigated in the light and electric field triggered configuration changes by using density functional theory (DFT) and molecular dynamics (MD) simulation. Using the modified force field, whose parameters are taken from DFT calculations on the ground and first excited states, the non-equilibrium molecular dynamics simulations show the collective structural transitions in Au(111) surface supported AZO SAMs under ultraviolet–visible light and external electric field stimulation. Along MD trajectories, an index function, <i>S</i>, is then defined to depict the SAM switching dynamics between “on” (<i>S</i> = 1) and “off” (<i>S</i> = 0) states. The charge transfer between SAM and surface and dipole interactions under the external electric field are revealed. The joint configuration changes of the AZO molecules in the SAM are also displayed to be able to lift the alkythiol coated mercury droplet in the Au(111)–SAM_AZO//SAM_C12–Hg junction model, in agreement with experimental observations on the photoswitching of the current in molecular junction. In addition to the manipulation of switching by light irradiation, it is predicted that the AZO SAMs, with or without substitutions, may also work as a molecular cargo lifter under the electric field

Site Partition: Turning One Site into Two for Adsorbing CO₂

We propose the concept of site partition to explain the role of guest molecules in increasing CO₂ uptake in metal–organic frameworks and to design new covalent porous materials for CO₂ capture. From grand canonical Monte Carlo simulations of CO₂ sorption in the recently synthesized CPM-33 MOFs, we show that guest insertion divides one open metal site into two relatively strong binding sites, hence dramatically increasing CO₂ uptake. Further, we extend the site partition concept to covalent organic frameworks with large free volume. After insertion of a designed geometry-matching guest, we show that the volumetric uptake of CO₂ doubles. Therefore, the concept of site partition can be used to engineer the pore space of nanoporous materials for higher gas uptake

Stability and Core-Level Signature of Nitrogen Dopants in Carbonaceous Materials

Nitrogen doping is an important strategy in tuning the properties and functions of carbonaceous materials. But the chemical speciation of the nitrogen groups in the sp²-carbon framework has not been firmly established. Here we address two important questions in nitrogen doping of carbonaceous materials from a computational approach: the relative stability of different nitrogen groups and their X-ray photoelectron spectrum (XPS) signatures of the core-level (N 1s) electron binding energies. Four types of nitrogen groups (graphitic, pyrrolic, aza-pyrrolic, and pyridinic) in 69 model compounds have been examined. Computed formation energies indicate that pyrrolic and pyridinic nitrogens are significantly more stable (by about 110 kJ/mol) than graphitic and aza-pyrrolic nitrogens. This stability trend can be understood from the Clar’s sextet rule. Predicted N 1s binding energies show relatively high consistency among each dopant type, thereby offering a guide to identify nitrogen groups. The relative stability coupled with predicted N 1s binding energies can explain the temperature-dependent change in the experimental XPS spectra. The present work therefore provides fundamental insights into nitrogen dopants in carbonaceous materials, which will be useful in understanding the applications of nitrogen-doped carbons in electric energy storage, electrocatalysis, and carbon capture

Comparative Reaction Diagrams for the S_N2 Reaction Formulated According to the Leffler Analysis and the Hammond Postulate

The Hammond Postulate and the Leffler analysis have provided a cornerstone in the understanding of reaction processes in organic chemistry for over 60 years, yet quantitative applications of these methodologies over the range of reactions envisaged in the original works remain elusive. In the present paper, we analyze a series of S_N2 reactions in three solvents that lead to endothermic and exothermic reaction processes, and we show that within the hybridization reaction coordinate the S_N2 reaction is fully consistent with both treatments. We give new presentations of the reaction energies as a function of reaction progress, which allow the generation of unified reaction coordinate diagrams that show a linear relationship between the hybridization metric of reaction progress and the relative energies of the stationary points on the potential surface as a function of structure and solvent as originally envisaged by Leffler and Hammond

Ion-Gated Gas Separation through Porous Graphene

Porous graphene holds great promise as a one-atom-thin, high-permeance membrane for gas separation, but to precisely control the pore size down to 3–5 Å proves challenging. Here we propose an ion-gated graphene membrane comprising a monolayer of ionic liquid-coated porous graphene to dynamically modulate the pore size to achieve selective gas separation. This approach enables the otherwise nonselective large pores on the order of 1 nm in size to be selective for gases whose diameters range from 3 to 4 Å. We show from molecular dynamics simulations that CO₂, N₂, and CH₄ all can permeate through a 6 Å nanopore in graphene without any selectivity. But when a monolayer of [emim]­[BF₄] ionic liquid (IL) is deposited on the porous graphene, CO₂ has much higher permeance than the other two gases. We find that the anion dynamically modulates the pore size by hovering above the pore and provides affinity for CO₂, while the larger cation (which cannot go through the pore) holds the anion in place via electrostatic attraction. This composite membrane is especially promising for CO₂/CH₄ separation, yielding a CO₂/CH₄ selectivity of about 42 and CO₂ permeance of ∼10⁵ GPU (gas permeation unit). We further demonstrate that selectivity and permeance can be tuned by the anion size, pore size, and IL thickness. The present work points toward a promising direction of using the atom-thin ionic liquid/porous graphene hybrid membrane for high-permeance, selective gas separation that allows a greater flexibility in substrate pore size control

Chiral Interconversions of Pd and/or Au Bis-Metalated [32]Octaphyrins(1,0,1,0,1,0,1,0) Involving Hückel and Möbius Macrocyclic Topologies: A Theoretical Prediction

Several Pd and/or Au bis-metalated [32]­octaphyrins­(1,0,1,0,1,0,1,0) were theoretically designed with rich conformations of Hückel or Möbius topology. The conformations and hence properties of macrocycles were tuned by twisting the active pyrrolic ring either clockwise (through multistep reactions with several Hückel and Möbius macrocyclic intermediates) or anticlockwise (via a direct Hückel–Hückel chiral interconversion). The encapsulated metal atoms, M₁, M₂ = Pd, Au, give different impacts on these two reaction processes. Facile occurrences of chiral interconversions between two enantiomers of bis-metalated octaphyrins were predicted with the largest activation barrier less than 40 kcal/mol. Some Au-coordinated octaphyrins (M₁ = Au) were demonstrated to be thermodynamically stable with large negative nucleus-independent chemical shift (NICS) values, which are comparable to those of the synthetic Pd-coordinated complexes. The free-base [32]­octaphyrins­(1,0,1,0,1,0,1,0) display the characteristic absorption spectra with distinct sharp Soret-like bands. After metalation, the Soret-like bands are red-shifted in different degrees along with the appearance of rather weak Q-like band. The heterometal-coordinated complexes (i.e., M₁ ≠ M₂) show stronger and more splitting absorptions than the homometal-coordinated ones with M₁ = M₂. The hyperpolarizabilities sharply augment with the metalation in Hückel systems due to the destruction of the centrosymmetry and the increase in polarizability by coordinated metal atoms

Separation and Sequential Recovery of Tetracycline and Cu(II) from Water Using Reusable Thermoresponsive Chitosan-Based Flocculant

Coexistence of antibiotics and heavy metals is typically detected in water containing both organic and inorganic contaminants. In this work, a flocculation method using a reusable thermoresponsive chitosan-based flocculant (CS-<i>g</i>-PNNPAM) was applied for separation and sequential recovery of tetracycline (TC) and Cu­(II) from water. High synergistic removal rates of both TC and Cu­(II) from water (>90%) were reached. Interactive effects among targeted water temperature (<i>T</i>₁), stock solution temperature (<i>T</i>₂), and flocculant dosage on flocculation performance were assessed using response surface methodology. To optimize flocculation, operation strategies of adjusting <i>T</i>₂ and dosage according to <i>T</i>₁ based on the interactive effects were given through mathematical analyses. The flocculation mechanism as well as interfacial interactions among CS-<i>g</i>-PNNPAM, TC, and Cu­(II) were studied through experimental investigations (floc size monitoring, X-ray photoelectron spectroscopy, and UV spectra) and theoretical calculations (density functional theory and molecular dynamics simulations). Coordination of Cu­(II) with TC and the flocculant promoted flocculation; switchable interactions (H bonds and hydrophobic association) of the TC–flocculant at different temperatures were key factors affecting operation strategies. When these interactions were weakened step by step, TC and Cu­(II) were sequentially recovered from flocs using certain solutions. Meanwhile, the flocculant in flocs was regenerated and found reusable with high flocculation efficiency

Photoexcited Single-Electron Transfer for Efficient Green Synthesis of Cyclic Carbonate from CO₂

It is attractive but challenging to produce high-value-added cyclic carbonates at ambient condition by the 100% atom-economic photocatalytic cycloaddition of CO2, which is limited by the insufficient understanding of the catalytic mechanism. Here, by taking Mg-MOF-74 as a model system, we propose the photoexcited catalyst can generally activate CO2 via single-electron-transfer mechanism, leading to the rapid formation of •CO2– radical. Subsequently, beneficial for the activation of inert CO2, the energy barrier of the rate-determining step (RDS) of the whole cycloaddition, i.e., the CO2 attacking step, significantly decreases, resulting in the feasible synthesis of cyclic carbonates under ambient condition. With the combination of synchrotron radiation in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), in situ electron spin resonance (ESR) spectroscopy, and the density functional theory calculation, the reaction process and corresponding key intermediates are systematically investigated, revealing the CO2 activation to be the most energy consumption steps in cyclic carbonate production, rather than the ring-opening of epoxide, thus furnishing new insights into photocatalytic CO2 cycloaddition

Interactions between Antibiotics and Graphene-Based Materials in Water: A Comparative Experimental and Theoretical Investigation

Complex interactions between antibiotics and graphene-based materials determine both the adsorption performance of graphene-based materials and the transport behaviors of antibiotics in water. In this work, such interactions were investigated through adsorption experiments, instrumental analyses and theoretical DFT calculations. Three typical antibiotics (norfloxacin (NOR), sulfadiazine (SDZ) and tetracycline (TC)) and different graphene-based materials (divided into two groups: graphene oxides-based ones (GOs) and reduced GOs (RGOs)) were employed. Optimal adsorption pHs for NOR, SDZ, and TC are 6.2, 4.0, and 4.0, respectively. At corresponding optimal pHs, NOR favored RGOs (adsorption capability: ∼50 mg/g) while SDZ preferred GOs (∼17 mg/g); All adsorbents exhibited similar uptake of TC (∼70 mg/g). Similar amounts of edge carboxyls of both GOs and RGOs wielded electrostatic attraction with NOR and TC, but not with SDZ. According to DFT-calculated most-stable-conformations of antibiotics-adsorbents complexes, the intrinsic distinction between GOs and RGOs was the different amounts of sp² and sp³ hybridization regions: π–π electron donor–acceptor effect of antibiotic-sp²/sp³ and H-bonds of antibiotic-sp³ coexisted. Binding energy (BE) of the former was larger for NOR; the latter interaction was stronger for SDZ; two species of TC at the optimal pH, i.e., TC⁺ and TC⁰, possessed larger BE with sp³ and sp² regions, respectively

Dialytic Synthesis of Two-Dimensional Cu-Based Metal–Organic Frameworks for Gas Separation: Designable MOF–Polymer Interface

We demonstrate a dialytic strategy for the synthesis of congeneric two-dimensional metal–organic framework (2D MOF) nanosheets with a dialysis membrane using 1,4-benzenedicarboxylic acid (BDC), 1,4-naphthalenedicarboxylic acid (NDC), and 9,10-anthracenedicarboxylic acid (ADC) as organic linkers and copper(II) as a metal precursor, respectively. Polyimide (PI) membranes containing these empty 2D MOF nanosheets exhibit distinct molecular sieve effects. Molecular dynamic simulation results reveal that the structures of MOF–polymer interfaces are designable by modifying the MOF interlayer distance and aperture size, which has significant influences on gas permeability and selectivity. As a result, Cu-NDC/PI with the moderate composite interface structure shows superior performance toward H2/CH4 and CO2/CH4 separations with a selectivity of 199 and 63 over Cu-BDC (121 and 53) and Cu-ADC (135 and 54), respectively