81 research outputs found
Is Single-Molecule Fluorescence Spectroscopy Ready To Join the Organic Chemistry Toolkit? A Test Case Involving Click Chemistry
Single molecule spectroscopy (SMS) has matured to a point where it can be used as a convenient tool to guide organic synthesis and drug discovery, particularly applicable to catalytic systems where questions related to homogeneous vs heterogeneous pathways are important. SMS can look at intimate mechanistic details that can inspire major improvements of the catalyst performance, its recovery, and reuse. Here, we use the click reaction between alkynes and azides as an example where improvements at the bench have been inspired and validated using single-molecule fluorescence spectroscopy
Expanding the Color Space in the Two-Color Heterogeneous Photocatalysis of Ullmann C–C Coupling Reactions
The Ullmann reaction can be heterogeneously photocatalyzed by UV excitation of Pd@TiO2. While a similar dose of visible light does not initiate the reaction, combination with UV light enhances activity and selectivity toward cross-combination products. The reaction involves photoresponsive intermediates, likely associated with the catalyst, where Pd nanostructures can absorb visible radiation and TiO2, the UV component. Hence, a single hybrid-photoresponsive material can have different reactivity upon two-wavelength irradiation enabling direct tuning of yields and selectivity. This color-space expansion promises unprecedented uses in organic chemistry, where improvement in catalytic activity and selectivity will impact the development of catalytic processes with long-term multicycle performance
Spectroscopic and Time-Dependent DFT Study of the Photophysical Properties of Substituted 1,4-Distyrylbenzenes
Thiol-Stabilized Gold Nanoparticles: New Ways To Displace Thiol Layers Using Yttrium or Lanthanide Chlorides
We use the aurophilic interactions shown by lanthanides to overcome the sulfur–gold interaction. UV–vis and X-ray photoelectron spectroscopy confirm that yttrium or lanthanide chlorides easily displace sulfur ligands from the surface of thiol-stabilized gold nanoparticles
Improving the Sunscreen Properties of TiO2 through an Understanding of Its Catalytic Properties
The use of particulate titanium dioxide (TiO2) as an active sunscreen ingredient has raised concerns about potential risks from TiO2-mediated free radical formation. To date, remediation attempts have concentrated on reducing the yield of free radical generation by TiO2 upon sunlight exposure. The problem with this approach is that given the band gap in TiO2, production of radical and the ensuing reactive oxygen species (ROS) is completely normal. Our strategy is based on a nontoxic, biocompatible shell that neutralizes the free radicals by scavenging them with natural antioxidants before they exit the particle. The new lignin@TiO2 composites preserve the scattering and absorption properties of TiO2 because the particles retain their nanoscale dimensions as preferred by the cosmetic industry. Although the target properties for photocatalysis and sun-protection applications are opposite, we argue that exactly the same knowledge is required to optimize either one
Catalytic farming: reaction rotation extends catalyst performance
The use of heterogeneous catalysis has key advantages compared to its homogeneous counterpart, such as easy catalyst separation and reusability. However, one of the main challenges is to ensure good performance after the first catalytic cycles. Active catalytic species can be inactivated during the catalytic process leading to reduced catalytic efficiency, and with that loss of the advantages of heterogeneous catalysis. Here we present an innovative approach in order to extend the catalyst lifetime based on the crop rotation system used in agriculture. The catalyst of choice to illustrate this strategy, Pd@TiO2, is used in alternating different catalytic reactions, which reactivate the catalyst surface, thus extending the reusability of the material, and preserving its selectivity and efficiency. As a proof of concept, different organic reactions were selected and catalyzed by the same catalytic material during target molecule rotation
Visible Light Production of Hydrogen by Ablated Graphene: Water Splitting or Carbon Gasification?
Reduced graphene oxide modified by pulsed laser ablation causes water splitting under visible light illumination (532 nm). When the light source is a pulsed laser, water splitting is accompanied by carbon gasification (CO formation); however, conventional (LED) light sources produce water splitting exclusively
Photoinduced Hydrogen Fuel Production and Water Decontamination Technologies. Orthogonal Strategies with a Parallel Future?
Decorated titania fibers as photocatalysts for hydrogen generation and organic matter degradation
Heterogenous photocatalysts based on electrospun fibers composed of polyvinylpyrrolidone and titanium propoxide were prepared and heated at 500, 750 and 950 °C to obtain anatase and rutile fibers. The fibers were then decorated with Pd and Co nanoparticles as well as a symmetrical zinc phthalocyanine (Pc). The fibrous materials obtained have a paper-like macroscopic appearance allowing for easy handling and separation. The photocatalytic activities of the new materials were evaluated for the generation of H2 upon UV (368 nm) or visible (630 nm) light excitation. Depending on the heat treatment or the post-synthetic decoration method, the materials show higher, or similar, activity compared to P25-TiO2, with superior ease of separation. The catalysts showed ability to degrade organic matter, with MeOH used as a model compound. This is of considerable importance for potential water treatment applications that will require flow-compatible materials
A Mechanistic Study of Halogen Addition and Photoelimination from ?-Conjugated Tellurophenes
The ability to drive
reactivity using visible light is of importance
for many disciplines of chemistry and has significant implications
for sustainable chemistry. Identifying photochemically active compounds
and understanding photochemical mechanisms is important for the development
of useful materials for synthesis and catalysis. Here we report a
series of photoactive diphenyltellurophene compounds bearing electron-withdrawing
and electron-donating substituents synthesized by alkyne coupling/ring
closing or palladium-catalyzed ipso-arylation chemistry. The redox
chemistry of these compounds was studied with respect to oxidative
addition and photoelimination of bromine, which is of importance for
energy storage reactions involving X<sub>2</sub>. The oxidative addition
reaction mechanism was studied using density functional theory, the
results of which support a three-step mechanism involving the formation
of an initial η<sup>1</sup> association complex, a monobrominated
intermediate, and finally the dibrominated product. All of the tellurophene
derivatives undergo photoreduction using 430, 447, or 617 nm light
depending on the absorption properties of the compound. Compounds
bearing electron-withdrawing substituents have the highest photochemical
quantum efficiencies in the presence of an alkene trap, with efficiencies
of up to 42.4% for a pentafluorophenyl-functionalized tellurophene.
The photoelimination reaction was studied in detail through bromine
trapping experiments and laser flash photolysis, and a mechanism is
proposed. The photoreaction, which occurs by release of bromine radicals,
is competitive with intersystem crossing to the triplet state of the
brominated species, as evidenced by the formation of singlet oxygen.
These findings should be useful for the design of new photochemically
active compounds supported by main-group elements
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