Enhancing Singlet Oxygen Photocatalysis with Plasmonic Nanoparticles.

Photocatalysts able to trigger the production of singlet oxygen species are the topic of intense research efforts in organic synthesis. Yet, challenges still exist in improving their activity and optimizing their use. Herein, we exploited the benefits of plasmonic nanoparticles to boost the activity of such photocatalysts via an antenna effect in the visible range. We synthesized silica-coated silver nanoparticles (Ag@SiO2 NPs), with silica shells which thicknesses ranged from 7 to 45 nm. We showed that they served as plasmonically active supports for tris(bipyridine)ruthenium(II), [Ru(bpy)3]2+, and demonstrated an enhanced catalytic activity under white light-emitting diode (LED) irradiation for citronellol oxidation, a key step in the commercial production of rose oxide fragrance. A maximum enhancement of the plasmon-mediated reactivity of approximately 3-fold was observed with a 28 nm silica layer along with a 4-fold enhancement in the emission intensity of the photocatalyst. Using electron energy loss spectroscopy (EELS) and boundary element method simulations, we mapped the decay of the plasmonic signal around the Ag core and provided a rationale for the observed catalytic enhancement. This work provides a systematic analysis of the promising properties of plasmonic NPs used as catalysis-enhancing supports for common homogeneous photocatalysts and a framework for the successful design of such systems in the context of organic transformations.

Applications of Plasmon-Enhanced Nanocatalysis to Organic Transformations.

Localized surface plasmon resonance (LSPR) is a physical phenomenon exhibited by nanoparticles of metals including coinage metals, alkali metals, aluminum, and some semiconductors which translates into electromagnetic, thermal, and chemical properties. In the past decade, LSPR has been taken advantage of in the context of catalysis. While plasmonic nanoparticles (PNPs) have been successfully applied toward enhancing catalysis of inorganic reactions such as water splitting, they have also demonstrated exciting performance in the catalysis of organic transformations with potential applications in synthesis of molecules from commodity to pharmaceutical compounds. The advantages of this approach include improved selectivity, enhanced reaction rates, and milder reaction conditions. This review provides the basics of LSPR theory, details the mechanisms at play in plasmon-enhanced nanocatalysis, sheds light onto such nanocatalyst design, and finally systematically presents the breadth of organic reactions hence catalyzed.

Understanding Color Tuning and Reversible Oxidation of Conjugated Azomethines.

With the aim of achieving reversible oxidation and color tuning, the effect of the central aromatic on the spectroscopic, electrochemical, and spectrochemical properties of a series of electrochromic azomethine triads was investigated. The absorption of the alkylated thiophene derivatives was blue-shifted relative to their unalkylated counterparts when the central aromatic was either a bi- or terthiophene. It was further found that the alkylated thiophene derivatives had larger Stokes shifts than their unsubstituted counterparts. Theoretical calculations demonstrated that the torsion angles of these alkylated cores with respect to the flanking azomethines were responsible for the spectroscopic effects. While the electrochemical oxidation potential of the triads varied by only 100 mV, the reversibility of their anodic process was contingent on the central aromatic. The absorption of the electrochemically produced state red-shifted between 165 and 280 nm from its corresponding neutral state, leading to perceived color changes between orange and blue. Reversible color changes were chemically mediated with ferric chloride/hydrazine. The absorption of the chemically oxidized state shifted between 155 and 220 nm from the corresponding neutral state, contingent on the central aromatic. The palette of perceived colors that was possible with oxidation included orange, yellow, blue, and gray.

Electron energy-loss spectroscopy (EELS) with a cold-field emission scanning electron microscope at low accelerating voltage in transmission mode.

A commercial electron energy-loss spectrometer (EELS) attached to a high-resolution cold-field emission scanning electron microscope in transmission mode (STEM) is evaluated and its potential for characterizing materials science thin specimens at low accelerating voltage is reviewed. Despite the increased beam radiation damage at SEM voltages on sensitive compounds, we describe some potential applications which benefit from lowering the primary electrons voltage on less-sensitive specimens. We report bandgap measurements on several dielectrics which were facilitated by the lack of Cherenkov radiation losses at 30 kV. The possibility of volume plasmon imaging to probe local composition changes in complex materials was demonstrated using energy-filtered STEM, either via spectrum imaging or elemental mapping using the "three-windows" method. As plasmonic materials are increasing used for energy, electronics or biomedical applications, the ability of reliably evaluate their properties at low accelerating voltage in a SEM is very appealing and is demonstrated. The energy resolution of the spectrometer, taken as the full width at half maximum of the zero-loss peak, was routinely measured at around 0.55 eV and it is demonstrated that t/λ ratios up to 1.5 allowed practical EEL spectroscopy at 30 kV.