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

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

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

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

Perylene-Grafted Silicas: Mechanistic Study and Applications in Heterogeneous Photoredox Catalysis

This is the peer reviewed version of the following article: Chem. Eur. J. 2019, 25, 14928 14934, which has been published in final form at https://doi.org/10.1002/chem.201903539. This article may be used for non-commercial purposes in accordance with Wiley Terms and Conditions for Self-Archiving[EN] A mechanistic study is herein presented for the use of heterogeneous photocatalysts based on perylene moieties. First, the successful immobilization of perylene diimides (PDI) on silica matrices is demonstrated, including their full characterization by means of electronic microscopy, surface area measurements, powder XRD, thermogravimetric analysis, and FTIR, Si-29 and C-13 solid-state NMR, fluorescence, and diffuse reflectance spectroscopies. Then, the photoredox activity of the material was tested by using two model reactions, alkene oxidation and 4-nitrobenzylbromide reduction, and mechanistic studies were performed. 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Nitro to amine reductions using aqueous flow catalysis under ambient conditions

A catalyst based on Pd on glass wool (Pd@GW) shows exceptional performance and durability for the reduction of nitrobenzene to aniline at room temperature and ambient pressure in aqueous solutions. The reaction is performed in a flow system and completed with 100% conversion under a variety of flow rates, 2 to 100 mLmin−1 (normal laboratory fast flow conditions). Sodium borohydride or dihydrogen perform well as reducing agents. Scale-up of the reaction to flows of 100 mLmin−1 also shows high conversions and robust catalytic performance. Catalyst deactivation can be readily corrected by flowing a NaBH4 solution. The catalytic system proves to be generally efficient, performing well with a range of nitroaromatic compounds. The shelf life of the catalyst is excellent and its reusability after 6-8 months of storage showed the same performance as for the fresh catalyst