NIR phosphorescence from decatungstate anions allows the conclusive characterization of its elusive excited triplet behaviour and kinetics.

The characterization of the triplet state of decatungstate (3DT*) and its NIR phosphorescence with lifetimes ∼100 ns in acetonitrile allow the easy determination of rate constants that are key to understanding its role in catalysis. The absence of oxygen quenching can now be understood as the excitation energy of 3DT* is lower than the energy of singlet oxygen.

The nitro to amine reduction: from millions of tons to single molecule studies.

Palladium nanostructures are interesting heterogeneous catalysts because of their high catalytic activity in a vast range of highly relevant reactions such as cross couplings, dehalogenations, and nitro-to-amine reductions. In the latter case, the catalyst Pd@GW (palladium on glass wool) shows exceptional performance and durability in reducing nitrobenzene to aniline under ambient conditions in aqueous solutions. To enhance our understanding, we use a combination of optical and electron microscopy, in-flow single molecule fluorescence, and bench chemistry combined with a fluorogenic system to develop an intimate understanding of Pd@GW in nitro-to-amine reductions. We fully characterize our catalyst in situ using advanced microscopy techniques, providing deep insights into its catalytic performance. We also explore Pd cluster migration on the surface of the support under flow conditions, providing insights into the mechanism of catalysis. We show that even under flow, Pd migration from anchoring sites seems to be minimal over 4 h, with the catalyst stability assisted by APTES anchoring.

Photocatalytic Semi-Hydrogenation of Alkynes: A Game of Kinetics, Selectivity and Critical Timing.

The semi-hydrogenation reaction of alkynes is important in the fine chemicals and pharmaceutical industries, and it is thus important to find catalytic processes that will drive the reaction efficiently and at a low cost. The real challenge is to drive the alkyne-to-alkene reaction while avoiding over-hydrogenation to the saturated alkane moiety. The problem is more difficult when dealing with aromatic substitution at the alkyne center. Simple photocatalysts based on Palladium tend to proceed to the alkane, and stopping at the alkene with good selectivity requires very precise timing with basically no timing tolerance. We report here that the goal of high conversion with high selectivity could be achieved with TiO2-supported copper (Cu@TiO2), although with slower kinetics than for Pd@TiO2. A novel bimetallic catalyst, namely, CuPd@TiO2 (0.8% Cu and 0.05% Pd), with methanol as the hydrogen source could improve the kinetics by 50% with respect to Cu@TiO2, while achieving selectivities over 95% and with exceptional timing tolerance. Further, the low Palladium content minimizes its use, as Palladium is regarded as an element at risk of depletion.

A laser flash photolysis study of the free radical chemistry of lipoic acid and dihydrolipoic acid.

The free radical chemistry of lipoic acid (LA) and dihydrolipoic acid (DHLA) intersect at the point where DHLA loses hydrogen to a good hydrogen abstracting radical, while LA reacts with strongly reducing ketyl radicals capable of donating a hydrogen atom. While aliphatic thiyl radicals have an absorbance at ~ 330 nm, the resulting radical, formally also a thiyl radical has distinct spectroscopic properties with a maximum at 385 nm, suggesting that the two sulphur centres interact strongly with each other as part of the chromophore. The reactions that form these radicals were studied by laser flash photolysis that revealed DHLA as an excellent hydrogen donor, while LA is an excellent hydrogen acceptor. The results support earlier evidence that the real antioxidant is DHLA, while LA is not; yet, the reported facile interconversion of the two molecules suggests that LA may be a better supplement, given its shelf stability, compared with a far more difficult-to-handle DHLA.

A beginners guide to understanding the mechanisms of photochemical reactions: things you should know if light is one of your reagents.

The increasing popularity of applied photochemistry has changed the composition of the practitioners of photochemistry, from traditional specialists, to users whose expertise lies elsewhere, yet they find light as a useful and powerful reagent. I introduce Kasha's rule very early in this tutorial; this unconventional approach allows me to bypass information about high electronic states in favor of the lowest singlet and triplet excited states. Doing this I try to provide a fast entry enabling newcomers in the field of applied photochemistry to have a taste of what the field has to offer, in the hope that they will like what they see, and venture further into the many resources available to go deeper into the fascinating field of organic photochemistry.

Silver nanoparticles with exceptional near-infrared absorbance for photoenhanced antimicrobial applications.

In this work, we outline a simple method for synthesizing decahedral and triangular silver nanoparticles using light to tune particle shape and spectral characteristics. Notably, we were able to generate triangular silver nanoparticles with exceptional absorbance in the near-infrared (NIR) region, with high spectral overlap with the biological window, making them particularly promising for biological applications. We further demonstrate that under complementary LED illumination, these excitable plasmonic particles display exceptional antibacterial properties, several orders of magnitude more potent than similar particles under dark conditions or under illumination that does not match particle absorbance. This work demonstrates the powerful effects that LED lights can have on the antibacterial activity of AgNPs, providing an inexpensive and easily implemented route to unlocking the full potential of AgNPs in photobiological applications.

Facile Scale-Up of the Flow Synthesis of Silver Nanostructures Based on Norrish Type I Photoinitiators.

Silver nanoparticles have become one of the most commercially and industrially relevant nanomaterials of the 21st century, owing to their potent antibacterial properties, as well as their useful catalytic and optical properties. Although many methods have been explored to produce AgNPs, we favor the photochemical approach using photoinitiators to produce AgNPs, owing to the high degree of control over reaction conditions, and the generation of so-called AgNP 'seeds' that can be used as-is, or as precursors for other silver nanostructures. In this work, we explore the scale-up of AgNP synthesis using flow chemistry and assess the usefulness of a range of industrial Norrish Type 1 photoinitiators in terms of flow compatibility and reaction time, as well as the resulting plasmonic absorption and morphologies. We establish that while all the photoinitiators used were able to generate AgNPs in a mixed aqueous/alcohol system, photoinitiators that generate ketyl radicals showed the greatest promise in terms of reaction times, while also showing greater flow compatibility compared to photoinitiators that generate 𝛼-aminoalkyl and α-hydroxybenzyl radicals. These findings help to establish a guideline for adapting photochemical AgNP syntheses to flow systems, helping to improve the scalability of the method in one of the largest industries in nanomaterial chemistry.

Fibrous TiO2 Alternatives for Semiconductor-Based Catalysts for Photocatalytic Water Remediation Involving Organic Contaminants.

Water decontamination remains a challenge in several developed and developing countries. Affordable and efficient approaches are needed urgently. In this scenario, heterogeneous photocatalysts appear as one of the most promising alternatives. This justifies the extensive attention that semiconductors, such as TiO2, have gained over the last decades. Several studies have evaluated their efficiency for environmental applications; however, most of these tests rely on the use of powder materials that have minimal to no applicability for large-scale applications. In this work, we investigated three fibrous TiO2 photocatalysts, TiO2 nanofibers (TNF), TiO2 on glass wool (TGW), and TiO2 in glass fiber filters (TGF). All materials have macroscopic structures that can be easily separated from solutions or that can work as fixed beds under flow conditions. We evaluated and compared their ability to bleach a surrogate dye molecule, crocin, under batch and flow conditions. Using black light (UVA/visible), our catalysts were able to bleach a minimum of 80% of the dye in batch experiments. Under continuous flow experiments, all catalysts could decrease dye absorption under shorter irradiation times: TGF, TNF, and TGW could, respectively, bleach 15, 18, and 43% of the dye with irradiation times as short as 35 s. Catalyst comparison was based on the selection of physical and chemical criteria relevant for application on water remediation. Their relative performance was ranked and applied in a radar plot. The features evaluated here had two distinct groups, chemical performance, which related to the dye degradation, and mechanical properties, which described their applicability in different systems. This comparative analysis gives insights into the selection of the right flow-compatible photocatalyst for water remediation.

A simple Norrish Type II actinometer for flow photoreactions.

This contribution addresses a frequent problem in flow photochemistry, where methodologies to determine the quantum efficiency of photoreactions are totally lacking. In spite of numerous studies being available in the literature, product reaction yields are never accompanied by measurements to determine their quantum yields. Basically, the key reagent in the reaction, light, is not measured under the experimental conditions of exposure. We report here a flow actinometer based on the photochemistry of valerophenone that can be readily implemented in the organic laboratory for irradiations in the UV region. For example for UVB lamps used in our work, the irradiance was measured as 1.1 × 10-4 einstein l-1 s-1. Our photoreactor design involves wrapping low-pressure lamps with Teflon tubbing, where the photochemistry takes place. Similar strategies could be implemented with other geometries or with lamps (e.g. LED) and actinometers with sensitivity in other spectral regions.

Laser flash photolysis of titanium dioxide suspensions for the evaluation of solvent-mediated radical reactions.

The chemistry originating from the scavenging of the highly electrophilic hole in TiO2 can be readily monitored using laser flash photolysis techniques. Dilute suspensions are sufficiently transparent in the UV region that long lived signals from reactions of solvent radicals with 1,1-diphenylethylene can be readily monitored. Transient signals originating from hole, electron and trapped radicals are extremely long lived showing stretched exponentials (nanoseconds to milliseconds), adequately described by fractal models.

Understanding α-lipoic acid photochemistry helps to control the synthesis of plasmonic gold nanostructures.

We propose the photopolymerization of lipoic acid (LA) as an novel approach to produce a cross-linked polymeric matrix of lipoic acid monomers (PALA) which helps to control the size of plasmonic gold nanostructures when using 3,3,6,8-tetramethyl-1-tetralone as the photo-initiator for the reduction of Au(III) to Au0. A complete characterization of the polymer is included, and the dual behaviour of LA as an in situ stabilizer and reducing agent is investigated. These findings are relevant to the understanding of the photochemical transformation of this biologically relevant compound and would benefit the increasing use of LA and PALA for the synthesis of various nanomaterials.

Exploring the Antibacterial Properties of Lignin-coated Magnetic Nanoparticles Synthesized in a One-pot Process†.

Given the current grave problems with antibiotic resistance, the discovery of novel, unconventional antibacterial drugs is not just important but also urgent. In this contribution, we report on the synthesis and testing of several composite nanomaterials that may find applications as therapeutic drugs or surface disinfectants. These materials are based on magnetic nanostructures coated with lignin, for example, lignin@FeCo. The magnetic properties of these nanocomposites facilitate removal or localization, whereas the lignin shell provides biocompatibility. These nanomaterials are mild antibacterials in the absence of light, but when illuminated become powerful antibacterial agents with typically ≥6 log units of bacterial reduction in 1-5 min of irradiation. These materials are strongly absorbing, including in the very useful NIR biological window, which we illustrate using 810 nm LED irradiation. We also show that in the short time required for antibacterial action, thermal changes are very small (≤5°C). Further, biocompatibility tests using fibroblasts show very limited cell damage and no enhanced adverse effect during 810 nm NIR illumination. As a surface coating for the active material, lignin provides a "trojan horse" strategy to facilitate the antibacterial action.

Photosensitized selective semi-oxidation of tetrahydroisoquinoline: a singlet oxygen path.

Selective semi-oxidation of tetrahydroisoquinoline (THIQ) leads to a valuable dihydroisoquinoline (DHIQ) derivative via singlet oxygen photooxidation process. Typical photosensitisers (i.e., Ru complexes) can activate the reaction even under heterogeneous conditions that facilitate catalyst separation and reusability. In contrast to DHIQ, THIQ acts as an efficient singlet oxygen quencher driving the reaction selectivity. The reaction can also be facilitated by semiconductor catalysts such as MoCo@GW, a glass wool-based catalyst that is easy to separate and reuse and compatible with flow photochemistry. Its role is to mediate the formation of isoquinoline (IQ) and thus an in situ-generated singlet oxygen catalyst. Laser flash photolysis with NIR detection provides proof of the singlet oxygen mechanism proposed and rate constants for the key steps that mediate the oxidation.

Unveiling the Mechanism for the Photochemistry and Photodegradation of Vanillin.

The photolysis of vanillin produces a short-lived triplet state where its lifetime is controlled by efficient self-quenching (kSQ ~ 2 × 109 m-1 s-1 ) which also generates radicals. Free radical reactions, including vanillin dimer formation, are responsible for the degradation of vanillin and is accompanied by yellowing of the acetonitrile solutions. Laser flash photolysis studies reveal a triplet absorbing at 390 nm, readily quenched by naphthalenes, conjugated dienes and oxygen. Vanillin is also a good singlet oxygen sensitizer as revealed by its characteristic NIR emission at 1270 nm.

Photoprotection and Photostability of a New Lignin-Gelatin-Baccharis antioquensis-Based Hybrid Biomaterial.

The aim of this study was to develop a new hybrid biomaterial that could photo-stabilize and improve the photoprotective capacity of a Baccharis antioquensis extract. Different combinations of lignin/gelatin/natural extract were applied to prepare hybrid biomaterial nanoparticles (NPs), which were then incorporated into an emulsion. The in vitro photoprotection and photostability were evaluated. The methanolic extract showed high phenolic content (646.4 ± 9.5 mg GAE/g dry extract) and a DPPH radical assay revealed that the antiradical capacity of the extract (0.13 to 0.05 g extract/mmol DPPH) was even better than that of BHT. The particle size of the hybrid biomaterial ranged from 100 to 255 nm; a polydispersity index (PdI) between 0.416 and 0.788 is suitable for topical use in dermocosmetic products. The loading capacity of the extract ranged from 27.0 to 44.5%, and the nanoparticles (NPs) showed electrostatic stability in accordance with the zeta potential value. We found that the formulation based on lignin: extract (1:1 ratio) and gelatin: lignin: extract (0.5:0.5:1 ratio) demonstrated photoprotection qualities with a sun protection factor (SPF) ranging from 9.4 to 22.6. In addition, all the hybrid NP-formulations were time-stable with %SPFeff and %UVAPFeff greater than 80% after exposure to 2 h of radiation. These results suggest that the hybrid biopolymer-natural extract improved the photoprotection and photostability properties, as well as the antiradical capacity, of the B. antioquensis extract, and may be useful for trapping high polyphenol content from natural extracts, with potential application in cosmeceutical formulations.

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.

Photoenolization as a convenient driver for the synthesis of plasmonic nanostructures.

Substituted tetralones such as 3,3,6,8-tetramethyl-1-tetralone undergo photoenolization to produce a photoenol excited state with a lifetime around ~ 3 µs, which involves the carbonyl triplet state of the ketone (τ ~ 1.9 ns), as a precursor; the excited photoenol also has biradical character and is useful for the fast synthesis of gold nanostructures. In the case of excited photoenols like this one, if metal ion trapping fails, they return to the original ketone precursor and remain available for future events that can lead to the target nanoparticles. This study includes the characterization of the photochemistry of the substituted tetralone, and the dual behavior of reaction intermediates, as biradicals and excited states, in energy and electron transfer processes.

Solar Driven Photocatalytic Activity of Porphyrin Sensitized TiO2: Experimental and Computational Studies.

The absence of a secure long-term sustainable energy supply is recognized as a major worldwide technological challenge. The generation of H2 through photocatalysis is an environmentally friendly alternative that can help solve the energy problem. Thus, the development of semiconductor materials that can absorb solar light is an attractive approach. TiO2 has a wide bandgap that suffers from no activity in the visible spectrum, limiting its use of solar radiation. In this research, the semiconductor absorption profile was extended into the visible region of the solar spectrum by preparing porphyrin-TiO2 (P-TiO2) composites of meso-tetra(4-bromophenyl)porphyrin (PP1) and meso-tetra(5-bromo-2-thienyl)porphyrin (PP2) and their In(III), Zn(II) and Ga(III) metal complexes. Density functional theory (DFT) and time-dependent density functional theory (TD-DFT) calculations were performed on the porphyrins to gain insight into their electron injection capability. The results demonstrate that P-TiO2 systems merit further in-depth study for applications that require efficient photocatalytic H2 generation.

Heterogeneous photocatalysis of azides: extending nitrene photochemistry to longer wavelengths.

The photodecomposition of azides to generate nitrenes usually requires wavelengths in the <300 nm region. In this study, we show that this reaction can be readily performed in the UVA region (368 nm) when catalyzed by Pd-decorated TiO2. In aqueous medium the reaction leads to amines, with water acting as the H source; however, in non-protic and non-nucleophilic media, such as acetonitrile, nitrenes recombine to yield azo compounds, while azirine-mediated trapping occurs in the presence of nucleophiles. The heterogeneous process facilitates catalyst separation while showing great chemoselectivity and high yields.

Photochemical benzylic radical arylation promoted by supported Pd nanostructures.

We report a novel way to promote photochemical benzylic radical arylations using Pd nanostructures. Traditional benzylic radical reaction pathways are challenged by the presence of metal centres that provoke unprecedented regioselectivity towards more synthetically relevant C(sp3)-C(sp2) couplings. This new C-H activation pathway is rationalised by means of a pseudo-persistent radical effect facilitated by metal centres. We show the mechanistic and computational aspects of the heterogeneous photocatalytic processes that are the root of this drastic change in reactivity.