Isolated Neutral [4]Helicene Radical Provides Insight into Consecutive Two-Photon Excitation Photocatalysis

Direct activation of strong bonds in readily available, benchtop substrates offer a straightforward simplification, albeit in most cases existing catalytic systems are limited to unlock such activation. In recent years, a surge of in-situ generated organic radicals that can act as potent photoinduced electron transfer (PET) agents have proved to be a powerful manifold for the activation of remarkably stable bonds. Herein we document the use of N,N′-di-n-propyl-1,13-dimethoxyquinacridine (nPr-DMQA•), an isolated and stable neutral helicene radical, as a highly photoreducing species. This isolable doublet state open shell radical offers a unique opportunity to shed light on the mechanism behind PET reactions of organic radicals. Experimental and spectroscopic studies revealed that this doublet radical has a long lifetime of 4.6 ± 0.2 ns, an estimated excited state oxidation potential of -3.31 V vs SCE, and can undergoes PET with organic substrates. The strongly photoreducing nature of the nPr-DMQA• was experimentally confirmed by the demonstration of photo activation of electron rich aryl bromides and chlorides. We further demonstrated that nPr-DMQA• can be photochemically generated from its cation analog (nPr-DMQA+) allowing catalytic functionalization of aryl halide via a consecutive photoexcitation mechanism (ConPET). Dehalogenation, photo-Arbuzov, photo-borylation and C-C bond formation reactions with aryl chlorides and bromides are reported herein, as well as the α-arylation of carbonyl using cyclic ketones. The latter transformation exhibits the facile synthesis of α-arylated cyclic ketones as critical feedstock chemical for diverse useful molecules, especially in the biomedical enterprises.

Thermal behaviour of organic radicals and paramagnetic centres in chert: A case study of Bergeracois brown chert, Dordogne, France

In order to successfully understand the complex evolution of prehistoric societies, archaeologists require absolute dating tools, which are not only accurate but also widely applicable. Thermoluminescence (TL) dating is one such approach that has been successfully used to establish a general chronological framework for prehistoric sites and is particularly suited for use on heated lithic artefacts. Experiments conducted in this study have clearly shown the applicability of Electron Spin Resonance (ESR) isothermal modelling in combination with TL dating to constrain firing temperature. This expands the potential application for TL dating to include artefacts treated at low firing temperatures. The present study shows potential in terms of precision and accuracy for framing the “equivalent firing temperature”. At the same time, the comparison of the TL signal with the lattice-defects and aluminium centres invigorate the use of ESR dating on heated flint, especially with samples that have received low thermal treatment. The presence of organic matter in large quantity raises concern on the pyrolysis effect on the luminescence signal; however, the use of ESR isothermal and isochronal modelling could potentially lead to the ability to overcome current interferences of the organic radicals within the dating signal of TL.

A crystalline radical cation derived from Thiele’s hydrocarbon with redox range beyond 1 V

Thiele’s hydrocarbon occupies a central role as an open-shell platform for new organic materials, however little is known about its redox behaviour. While recent synthetic approaches involving symmetrical carbene substitution of the CPh2 termini yield isolable neutral/dicationic analogues, the intervening radical cations are much more difficult to isolate, due to narrow compatible redox ranges (typically < 0.25 V). Here we show that a hybrid BN/carbene approach allows access to an unsymmetrical analogue of Thiele’s hydrocarbon 1, and that this strategy confers markedly enhanced stability on the radical cation. 1•+ is stable across an exceptionally wide redox range (> 1 V), permitting its isolation in crystalline form. Further single-electron oxidation affords borenium dication 12+, thereby establishing an organoboron redox system fully characterized in all three redox states. We perceive that this strategy can be extended to other transient organic radicals to widen their redox stability window and facilitate their isolation.

Comment on “Isotopic evidence for dominant secondary production of HONO in near-ground wildfire plumes” by Chai et al. (2021)

Chai et al. (2021) recently published measurements of wildfire-derived (WF) oxides of nitrogen (NOx) and nitrous acid (HONO) and their isotopic composition. The method used to sample NOx, collection in alkaline solution, has a known 1:1 interference from another reactive nitrogen compound, acetyl peroxynitrate (PAN). Although PAN is thermally unstable, subsequent reactions with nitrogen dioxide (NO2) in effect extend the lifetime of PAN many times longer than the initial decomposition reaction would indicate. This, coupled with the rapid and efficient formation of PAN in WF plumes, means the NOx measurements reported by Chai et al.​​​​​​​ were severely impacted by PAN. In addition, the model reactions in the original paper included neither the reactions of NO2 with hydroxyl radical (OH) to form nitric acid nor the efficient reaction of larger organic radicals with nitric oxide to form organic nitrates (RONO2).

Comment on “Isotopic evidence for dominant secondary production of HONO in near-ground wildfire plumes.”

Chai et al. recently published measurements of wild fire (WF) derived oxides of nitrogen (NOx) and nitrous acid (HONO) and their isotopic composition. The method used to sample NOx, collection in alkaline solution, has a known 1:1 interference from another reactive nitrogen compound, acetyl peroxynitrate (PAN). Although PAN is thermally unstable, subsequent reactions with nitrogen dioxide (NO2) in effect extend the lifetime of PAN many times longer than the initial decomposition reaction would indicate. This, coupled with the rapid and efficient formation of PAN in WF plumes, means the NOx measurements reported by Chai et al. were severely impacted by PAN. In addition, the model reactions in the original paper did not include the reactions of NO2 with hydroxyl radical (OH) to form nitric acid, nor the efficient reaction of larger organic radicals with nitric oxide to form organic nitrates (RONO2).

A New Mechanism of the Selective Photodegradation of Antibiotics in the Catalytic System Containing TiO2 and the Inorganic Cations

The mechanism of sulfisoxazole (SFF) selective removal by photocatalysis in the presence of titanium (IV) oxide (TiO2) and iron (III) chloride (FeCl3) was explained and the kinetics and degradation pathways of SFF and other antibiotics were compared. The effects of selected inorganic ions, oxygen conditions, pH, sorption processes and formation of coordination compounds on the photocatalytic process in the presence of TiO2 were also determined. The Fe3+ compounds added to the irradiated sulfonamide (SN) solution underwent surface sorption on TiO2 particles and act as acceptors of excited electrons. Most likely, the SFF degradation is also intensified by organic radicals or cation organic radicals. These radicals can be initially generated by reaction with electron holes, hydroxyl radicals and as a result of electron transfer mediated by iron ions and then participate in propagation processes. The high sensitivity of SFF to decomposition caused by organic radicals is associated with the steric effect and the high bond polarity of the amide substituent.