13 research outputs found
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 (NH<sub>2</sub>) and withdrawing (NO<sub>2</sub>) 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
ultravioletâvisible 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 âonâ
(<i>S</i> = 1) and âoffâ (<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)âSAM<sub>AZO</sub>//SAM<sub>C12</sub>âHg 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 CO<sub>2</sub>
We
propose the concept of site partition to explain the role of
guest molecules in increasing CO<sub>2</sub> uptake in metalâorganic
frameworks and to design new covalent porous materials for CO<sub>2</sub> capture. From grand canonical Monte Carlo simulations of
CO<sub>2</sub> 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 CO<sub>2</sub> 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 CO<sub>2</sub> 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 sp<sup>2</sup>-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 Clarâs 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<sub>N</sub>2 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<sub>N</sub>2 reactions in three solvents that lead to endothermic
and exothermic reaction processes, and we show that within the hybridization
reaction coordinate the S<sub>N</sub>2 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
3â5 Ă
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 Ă
. We show from molecular dynamics
simulations that CO<sub>2</sub>, N<sub>2</sub>, and CH<sub>4</sub> all can permeate through a 6 Ă
nanopore in graphene without
any selectivity. But when a monolayer of [emim]Â[BF<sub>4</sub>] ionic
liquid (IL) is deposited on the porous graphene, CO<sub>2</sub> 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 CO<sub>2</sub>, 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 CO<sub>2</sub>/CH<sub>4</sub> separation, yielding a CO<sub>2</sub>/CH<sub>4</sub> selectivity of about 42 and CO<sub>2</sub> permeance of âź10<sup>5</sup> 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 HuĚckel and MoĚbius Macrocyclic Topologies: A Theoretical Prediction
Several Pd and/or Au bis-metalated [32]ÂoctaphyrinsÂ(1,0,1,0,1,0,1,0)
were theoretically designed with rich conformations of HuĚckel
or MoĚbius topology. The conformations and hence properties
of macrocycles were tuned by twisting the active pyrrolic ring either
clockwise (through multistep reactions with several HuĚckel
and MoĚbius macrocyclic intermediates) or anticlockwise (via
a direct HuĚckelâHuĚckel chiral interconversion).
The encapsulated metal atoms, M<sub>1</sub>, M<sub>2</sub> = 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 (M<sub>1</sub> =
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]ÂoctaphyrinsÂ(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., M<sub>1</sub> â M<sub>2</sub>) show stronger
and more splitting absorptions than the homometal-coordinated ones
with M<sub>1</sub> = M<sub>2</sub>. The hyperpolarizabilities sharply
augment with the metalation in HuĚckel 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 CuÂ(II) from water. High synergistic
removal rates of both TC and CuÂ(II) from water (>90%) were reached.
Interactive effects among targeted water temperature (<i>T</i><sub>1</sub>), stock solution temperature (<i>T</i><sub>2</sub>), and flocculant dosage on flocculation performance were
assessed using response surface methodology. To optimize flocculation,
operation strategies of adjusting <i>T</i><sub>2</sub> and
dosage according to <i>T</i><sub>1</sub> 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 CuÂ(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 CuÂ(II) with TC and the flocculant
promoted flocculation; switchable interactions (H bonds and hydrophobic
association) of the TCâflocculant at different temperatures
were key factors affecting operation strategies. When these interactions
were weakened step by step, TC and CuÂ(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 CO<sub>2</sub>
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 â˘CO2â 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: âź50 mg/g) while SDZ
preferred GOs (âź17 mg/g); All adsorbents exhibited similar
uptake of TC (âź70 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 sp<sup>2</sup> and sp<sup>3</sup> hybridization regions: ĎâĎ electron donorâacceptor
effect of antibiotic-sp<sup>2</sup>/sp<sup>3</sup> and H-bonds of
antibiotic-sp<sup>3</sup> 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<sup>+</sup> and TC<sup>0</sup>, possessed larger BE with sp<sup>3</sup> and sp<sup>2</sup> regions,
respectively
Dialytic Synthesis of Two-Dimensional Cu-Based MetalâOrganic Frameworks for Gas Separation: Designable MOFâPolymer Interface
We demonstrate a dialytic strategy for the synthesis
of congeneric
two-dimensional metalâorganic 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 MOFâpolymer
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