Highly Sensitive, Easy-to-Use, One-Step Detection of Peroxide-, Nitrate- and Chlorate-Based Explosives with Electron-Rich Ni Porphyrins.

Homemade explosives, such as peroxides, nitrates, and chlorates, are increasingly abused by terrorists, criminals, and amateur chemists. The starting materials are easily accessible and instructions on how to make the explosives are described on the Internet. Safety considerations raise the need to detect these substances quickly and in low concentrations using simple methods. Conventional methods for the detection of these substances require sophisticated, electrically operated, analytical equipment. The simpler chemical detection methods are multistep and require several chemicals. We have developed a simple, one-step method that works similarly to a pH test strip in terms of handling. The analytical reaction is based on an acid-catalyzed oxidation of an electron-rich porphyrin to an unusually stable radical cation and dication. The detection limit for the peroxide-based explosive triacetone triperoxide (TATP), which is very frequently used by terrorists, is 40 ng and thus low enough to detect the substance without direct contact via the gas phase. It is sufficient to bring the stick close to the substance to observe a color change from red to green. Nitrates and chlorates, such as ammonium nitrate, urea nitrate, or potassium chlorate, are detected by direct contact with a sensitivity of 85-350 ng. A color change from red to dark brown is observed. The test thus detects all homemade explosives and distinguishes between the extremely impact-, shock-, and friction-sensitive peroxides and the less sensitive nitrates and chlorates by color change of a simple test strip.

To Isomerize or not to Isomerize? E/Z Isomers of Cyclic Azobenzene Derivatives and Their Reactivity Upon One-Electron Reduction.

Azo compounds are efficient electron acceptors. Upon one-electron reduction they generally isomerize forming the thermodynamically most stable radical anion. Herein we show that the size of the central ring in 1,2-diazocines and diazonines has a ruling influence on the configuration of the one-electron reduced species. Markedly, diazonines, which bear a central nine membered heterocycle, show light-induced E/Z isomerization, but retain the configuration of the diazene N=N moiety upon one-electron reduction. Accordingly, E/Z isomerization is not induced by reduction.

Symmetrical and Mixed Tris(acyl)phosphines: Synthesis, Oxidation and Photochemistry.

Herein, we present a convenient synthesis for symmetrical and mixed substituted tris(acyl)phosphines (TAPs) starting from red phosphorus. All TAPs exhibit a phosphaalkene-acylphosphine equilibrium, which was investigated in detail by variable-temperature (VT) NMR spectroscopy supported by density-functional theory (DFT) calculations. Depending on the substituents, two phosphaalkene derivatives and ten acylphosphine derivatives could be isolated. NMR spectroscopy and single-crystal X-ray crystallography enabled a clear structural assignment of these compounds. Oxidation of selected TAPs led to the formation of the corresponding tris(acyl)phosphine oxides (TAPOs). Furthermore, their spectroscopic properties as well as their photochemistry was investigated. Especially, the TAPO compounds were evaluated for their suitability as photoinitiators by CIDNP spectroscopy, photobleaching measurements and by storage stability tests.

Symmetrical and Mixed Tris(acyl)phosphines: Synthesis, Oxidation and Photochemistry.

Invited for the cover of this issue are the groups of M. Haas, G. Gescheidt and H. Grützmacher from the Graz University of Technology and the ETH Zürich. The image depicts a phosphorus mine, where the workers are acid chlorides using their shovels and red phosphorus to provide the chemicals necessary to produce novel reagents. Read the full text of the article at 10.1002/chem.202302535.

Direct detection of photo-induced reactions by IR: from Brook rearrangement to photo-catalysis.

In situ IR detection of photoreactions induced by the light of LEDs at appropriate wavelengths provides a simple, cost-effective, and versatile method to get insight into mechanistic details. In particular, conversions of functional groups can be selectively followed. Overlapping UV-Vis bands or fluorescence from the reactants and products and the incident light do not obstruct IR detection. Compared with in situ photo-NMR, our setup does not require tedious sample preparation (optical fibers) and offers a selective detection of reactions, even at positions where 1H-NMR lines overlap or 1H resonances are not clear-cut. We illustrate the applicability of our setup following the photo-Brook rearrangement of (adamant-1-yl-carbonyl)-tris(trimethylsilyl)silane, address photo-induced α-bond cleavage (1-hydroxycyclohexyl phenyl ketone), study photoreduction using tris(bipyridine)ruthenium(II), investigate photo-oxygenation of double bonds with molecular oxygen and the fluorescent 2,4,6-triphenylpyrylium photocatalyst, and address photo-polymerization. With the LED/FT-IR combination, reactions can be qualitatively followed in fluid solution, (highly) viscous environments, and in the solid state. Viscosity changes during the reaction (e.g., during a polymerization) do not obstruct the method.

Forced Solid-State Oxidation Studies of Nifedipine-PVP Amorphous Solid Dispersion.

In the present study, the oxidative degradation behavior of nifedipine (NIF) in amorphous solid dispersions (ASDs) prepared with poly(vinyl pyrrolidone) (PVP) with a short (K30) and a long (K90) chain length was investigated. The ASDs were prepared via dry ball-milling and analyzed using Fourier transform infrared (IR) spectroscopy, X-ray scattering, and differential scanning calorimetry. The ASDs were exposed to accelerated thermal-oxidative conditions using a pressurized oxygen headspace (120 °C for 1 day) and high temperatures at atmospheric pressure (60-120 °C for a period of 42 days). Additionally, solution-state oxidative degradation studies showed that pure NIF degrades to a greater extent than in the presence of PVP. Electronic structure calculations were performed to understand the impact of drug-polymer intermolecular interactions on the autoxidation of drugs. While no drug degradation was observed in freshly prepared ASD samples, alkyl free radicals were detected via electron paramagnetic resonance (EPR) spectroscopy. The free radicals were found to be consumed to a greater extent by PVP K30- than PVP K90-based ASDs upon exposure to high oxygen pressures. This was consistent with the greater solid-state oxidative degradation of NIF observed in ASDs with PVP K30 than with PVP K90. As no drug recrystallization occurred during this study period, the lower glass-transition temperature and presumed greater molecular mobility of PVP K30 and its ASD as compared to the PVP K90 system appear to contribute to the greater drug degradation in PVP-K30-based ASDs. The extent and the rate of oxidative degradation were higher in the case of PVP-K30-based ASD as compared to that in PVP-K90-based ASD, and the overall degradation increased with an increase in temperature. IR spectral analysis of drug-polymer interactions supports the electronic calculations of the oxidation process. We infer that, apart from the initial free radical content, the difference in the extent of drug-polymer intermolecular interactions in ASDs and amorphous stabilization during the forced oxidation experiments contribute to the observed differences in the autoxidative reactivity of the drug in ASDs with different PVP chain lengths. Overall, the chemical degradation of NIF in ASDs with two PVP chain lengths obtained from accelerated solid-state oxidation studies was in qualitative agreement with that obtained from long-term (3 years) storage under ambient conditions. The study highlights the ability of accelerated processes to determine the oxidative degradation behavior of polymeric ASDs and suggests that the polymer chain length could factor into chemical as well as physical stability considerations.

Synthesis of Mixed-Functionalized Tetraacylgermanes.

Tetraacylgermanes are known as highly efficient photoinitiators. Herein, the synthesis of mixed tetraacylgermanes 4 a-c and 6 a-e with a nonsymmetric substitution pattern is presented. Germenolates are crucial intermediates of these new synthetic protocols. The synthesized compounds show increased solubility compared with symmetrically substituted tetraacylgermanes 1 a-d. Moreover, these mixed derivatives reveal broadened n-π* absorption bands, which enhance their photoactivity. Higher absorption of these new compounds at wavelengths above 450 nm causes efficient photobleaching when using an LED emitting at 470 nm. The quantum yields are in the range of 0.15-0.57, depending on the nature of the aroyl substituents. On the basis of these properties, mixed-functionalized tetraacylgermanes serve as ideal photoinitiators in various applications, especially in those requiring high penetration depth. The synthesized compounds were characterized by elemental analysis, IR spectroscopy, NMR and CIDNP spectroscopy, UV/Vis spectroscopy, photolysis experiments, and X-ray crystallography. The CIDNP data suggest that the germyl radicals generated from the new tetraacylgermanes preferentially add to the tail of the monomer butyl acrylate. In the case of 6 a-e only the mesitoyl groups are cleaved off, whereas for 4 a-c both the mesitoyl and the aroyl group are subject to α-cleavage.

Photoswitching of Diazocines in Aqueous Media.

We present a systematic investigation of the photophysical properties of diazocines in aqueous media. The Z-E photoconversion yields of CH2CH2- and CH2S-bridged diazocines decrease with increasing water content in acetonitrile. However, there is one exception. A CH2-NAc-bridged diazocine mostly retains its photostationary state in water (85 to 72%) because of the high quantum yields for the Z → E conversion. Moreover, it is water-soluble without further substitution and is therefore ideally suited as a photoswitch in biological (aqueous) environments.

Isolable Geminal Bisgermenolates: A New Synthon in Organometallic Chemistry.

We have synthesized the first isolable geminal bisenolates L2 K2 Ge[(CO)R]2 (R=2,4,6-trimethylphenyl (2 a,b), L=THF for (2 a) or [18]-crown-6 for (2 b)), a new synthon for the synthesis of organometallic reagents. The formation of these derivatives was confirmed by NMR spectroscopy and X-ray crystallographic analysis. The UV/Vis spectra of these anions show three distinct bands, which were assigned by DFT calculations. The efficiency of 2 a,b to serve as new building block in macromolecular chemistry is demonstrated by the reactions with two different types of electrophiles (acid chlorides and alkyl halides). In all cases the salt metathesis reaction gave rise to novel Ge-based photoinitiators in good yields.

Systematic Quantification of Electron Transfer in a Bare Phospholipid Membrane Using Nitroxide-Labeled Stearic Acids: Distance Dependence, Kinetics, and Activation Parameters.

In this report, we present a method to characterize the kinetics of electron transfer across the bilayer of a unilamellar liposome composed of 1,2-dimyristoyl-sn-glycero-3-phosphocholine. The method utilizes synthetic phospholipids containing noninvasive nitroxide spin labels having the >N-O• moiety at well-defined distances from the outer surface of the liposome to serve as reporters for their local environment and, at the same time, permit measurement of the kinetics of electron transfer. We used 5-doxyl and 16-doxyl stearic acids. The paramagnetic >N-O• moiety is photo-oxidized to the corresponding diamagnetic oxoammonium cation by a ruthenium electron acceptor formed in the solution. Electron transfer is monitored by three independent spectroscopic methods: by both steady-state and time-resolved electron paramagnetic resonance and by optical spectroscopy. These techniques allowed us to differentiate between the electron transfer rates of nitroxides located in the outer leaflet of the phospholipid bilayer and of those located in the inner leaflet. Measurement of electron transfer rates as a function of temperature revealed a low-activation barrier (ΔG‡ ∼ 40 kJ/mol) that supports a tunneling mechanism.

The Chemistry of Acylgermanes: Triacylgermenolates Represent Valuable Building Blocks for the Synthesis of a Variety of Germanium-Based Photoinitiators.

The formation of a stable triacylgermenolate 2 as a decisive intermediate was achieved by using three pathways. The first two methods involve the reaction of KOtBu or alternatively potassium with tetraacylgermane 1 yielding 2 via one electron transfer. The mechanism involves the formation of radical anions (shown by EPR). This reaction is highly efficient and selective. The third method is a classical salt metathesis reaction toward 2 in nearly quantitative yield. The formation of 2 was confirmed by NMR spectroscopy, UV-vis measurements, and X-ray crystallography. Germenolate 2 serves as a starting point for a wide variety of organo-germanium compounds. We demonstrate the potential of this intermediate by introducing new types of Ge-based photoinitiators 4b-4f. The UV-vis absorption spectra of 4b-4f show considerably increased band intensities due to the presence of eight or more chromophores. Moreover, compounds 4d-4f show absorption tailing up to 525 nm. The performance of these photoinitiators is demonstrated by spectroscopy (time-resolved EPR, laser flash photolysis (LFP), photobleaching (UV-vis)) and photopolymerization experiments (photo-DSC measurements).

Holistic approach to chemical degradation of Nafion membranes in fuel cells: modelling and predictions.

The state of health of polyfluorinated sulfonic-acid ionomer membranes (e.g. Nafion®) in low-temperature proton exchange membrane fuel cells (LT-PEMFCs) is negatively influenced by degradation phenomena occurring during their operation. As a consequence, the performance and durability of the membrane are decreased. In this article, we focus on simulating and predicting chemical membrane degradation phenomena using a holistic zero-dimensional kinetic framework. The knowledge of chemical degradation mechanisms is widely spread. We have collected and evaluated an extensive set of chemical mechanisms to achieve a holistic approach. This yields a set of 23 coupled chemical equations, which provide the whole cause and effect chain of chemical degradation in LT-PEMFCs (based on the Fenton reaction between Fe2+ and H2O2via the attack of hydroxyl radicals on the membrane, loss of ionomer moieties and emission of fluoride). Our kinetic framework allows the reproduction of experimentally accessible data such as fluoride emission rates and concentrations of ionomer moieties (from both in situ and ex situ tests). We present an approach, which allows estimations of the membrane lifetime based on fluoride emission rates. In addition, we outline the demetallation of Fe-N-C catalysts as a source of additional harmful iron species, which accelerate chemical membrane degradation. To demonstrate the expandability and versatility of the kinetic framework, a set of five chemical equations describing the radical scavenging properties of cerium agents is coupled to the main framework and its influence on membrane degradation is analysed. An automated solving routine for the system of coupled chemical equations on the basis of the chemical kinetic simulation tool COPASI has been developed and is freely accessible online ().

Efficient Conversion of Light to Chemical Energy: Directional, Chiral Photoswitches with Very High Quantum Yields.

Photochromic systems have been used to achieve a number of engineering functions such as light energy conversion, molecular motors, pumps, actuators, and sensors. Key to practical applications is a high efficiency in the conversion of light to chemical energy, a rigid structure for the transmission of force to the environment, and directed motion during isomerization. We present a novel type of photochromic system (diindane diazocines) that converts visible light with an efficiency of 18 % to chemical energy. Quantum yields are exceptionally high with >70 % for the cis-trans isomerization and 90 % for the back-reaction and thus higher than the biochemical system rhodopsin (64 %). Two diastereomers (meso and racemate) were obtained in only two steps in high yields. Both isomers are directional switches with high conversion rates (76-99 %). No fatigue was observed after several thousands of switching cycles in both systems.

Nitrogen Bridged Diazocines: Photochromes Switching within the Near-Infrared Region with High Quantum Yields in Organic Solvents and in Water.

Diazocines are bridged azobenzenes with superior photophysical properties. In contrast to azobenzenes the Z configuration is thermodynamically stable and the E isomer is metastable. We present a new class of nitrogen bridged diazocines with bathochromically shifted switching wavelengths and remarkably high quantum yields (-NH-CH2- bridged diazocine: ΦZ→E = 0.57, ΦE→Z = 0.8). Z to E isomerization is induced by irradiation with blue light, whereas switching back to the Z isomer is accomplished with light in the near-infrared window (up to 740 nm), which is important for medical applications like photopharmacology (deep tissue penetration). Furthermore, substitution at the bridging nitrogen should provide access to widely applicable tricyclic, photoswitchable pharmacophores. The -NAc-CH2- bridged derivative is soluble in water, and all photophysical properties (conversion rates, quantum yields, and thermal half-lives) are largely retained. Hence, this diazocine is an ideal photoswitch for applications in biochemical systems and in photopharmacology.

In Situ Observation of Photoswitching by NMR Spectroscopy: A Photochemical Analogue to the Exchange Spectroscopy Experiment.

We present 1D and 2D NMR experiments that provide in situ insights into photoinduced isomerizations. Irradiation during the mixing period of an exchange spectroscopy (EXSY) experiment leads to characteristic cross peaks in 2D spectra. The phototriggered exchange of magnetization occurring in photoswitchable (Z)- and (E)-isomers of three selected azo compounds provides information on the dynamic E/Z equilibria. We report the dependence of the diagonal-to-cross-peak ratio on concentration, light intensity, and mixing time. In analogy to exchange spectroscopy, this ratio mirrors the efficiency of light induced molecular transformations. Furthermore, we present a time-saving 1D version and a combined light/phase cycle scheme for enhanced detectability of photoinduced changes in the spectrum. This insight into light-induced structural changes is highly suited to study macromolecules, in which photoswitchable units trigger conformational changes.

Unprecedented Bifunctional Chemistry of Bis(acyl)phosphane Oxides in Aqueous and Alcoholic Media.

Tailor-made photoinitiators play an important role for efficient radical polymerisations in aqueous media, especially in hydrogel manufacturing. Bis(acyl)phosphane oxides (BAPOs) are among the most active initiators. Herein, we show that they display a remarkable photochemistry in aqueous and alcoholic media: Photolysis of BAPOs in the presence of water or alcohols provides a new delocalized π-radical, which does not participate in the polymerization. It either converts into a monoacylphosphane oxide acting as a secondary photoactive species or it works as a one-electron reducing agent. Upon the electron-transfer process, it again produces a dormant photoinitiator. We have established the structure and the chemistry of this π radical using steady-state and time-resolved (CIDEP) EPR together with ESI-MS, NMR spectroscopy, and DFT calculations. Our results show that bis(acyl)phosphane oxides act as bifunctional reagents when applied in aqueous and alcoholic media.

Hydrogen Abstraction from the C15 Position of the Cholesterol Skeleton.

Cholesterol (Ch) is an integral part of cell membrane, where it is prone to oxidation. In humans, oxidation of Ch is commonly linked to various pathologies like Alzheimer's disease, atherosclerosis, and even cancer, which proceed via mechanisms involving enzymatic and free radical pathways. The latter begin with hydrogen abstraction (HA) from Ch by a reactive free radical. It has been established that the most efficient HA from Ch occurs at C7, although HA from C4 by peroxyl radicals has recently been observed. Conversely, HA from Ch positions other than the thermodynamically preferred C7 or C4 has never been reported. We have designed a Ch derivative where a benzophenone moiety is linked to C7 by a covalent bond. This mirrors a specific orientation of Ch within a confined environment. Product analysis and time-resolved spectroscopic studies reveal an unprecedented HA from C15, which is a thermodynamically unfavorable position. This indicates that a specific topology of reactants is crucial for the reactivity of Ch. The relative orientation of the reactants can also be relevant in biological membranes, where Ch, polyunsaturated fatty acids, and numerous oxidizing species are confined in highly restricted and anisotropic environments.

Recent Advances in Germanium-Based Photoinitiator Chemistry.

Acylgermanes provide an outstanding photoinduced reactivity at very useful absorption wavelengths. This encouraged multidisciplinary research groups to utilize them as highly effective and non-toxic photoinitiators particularly for medical applications. In this Minireview, we present the most recent breakthroughs to synthesize acylgermanes. We also outline mechanistic aspects of photoinduced reactions of several acylgermane derivatives based on fundamental spectroscopic insights. These studies may aid future developments for tailor-made photoinitiators.

Charge-Transfer Salts of 6,6-Dicyanopentafulvenes: From Topology to Charge Separation in Solution.

6,6-Dicyanopentafulvene derivatives and metallocenes with redox potentials appropriate for forming their radical anions form highly persistent donor-acceptor salts. The charge-transfer salts of 2,3,4,5-tetraphenyl-6,6-dicyanofulvene with cobaltocene (1⋅Cp2 Co) and 2,3,4,5-tetrakis(triisopropylsilyl)-6,6-dicyanofulvene with decamethylferrocene (2⋅Fc*) have been prepared. The X-ray structures of the two salts, formed as black plates, were obtained and are discussed herein. Compared with neutral dicyanopentafulvenes, the chromophores in the metallocene salts show substantial changes in bond lengths and torsional angles in the solid state. EPR, NMR, and optical spectroscopy, as well as superconducting quantum interference device (SQUID) measurements, reveal that charge-separation in the crystalline states and in frozen and fluid solutions depends on subtle differences of redox potentials, geometry, and on ion pairing. Whereas 1⋅Cp2 Co reveals paramagnetic character in the crystalline state and in solution, compound 2⋅Fc* shows a delicate balance between para- and diamagnetism, depending on the temperature and solvent characteristics.

Tetraacylstannanes as Long-Wavelength Visible-Light Photoinitiators with Intriguing Low Toxicity.

The first tetraacylstannanes Sn[(CO)R]4 (R=2,4,6-trimethylphenyl (1 a) and 2,6-dimethylphenyl (1 b)), a class of highly efficient Sn-based photoinitiators, were synthesized. The formation of these derivatives was confirmed by NMR spectroscopy, mass spectrometry, and X-ray crystallography. The UV/Vis absorption spectra of 1 a, b reveal a significant redshift of the longest wavelength absorption compared to the corresponding germanium compounds. In contrast to the known toxicity of organotin derivatives, the AMES test and cytotoxicity studies reveal intriguing low toxicity. The excellent performance of 1 as photoinitiators is demonstrated by photobleaching (UV/Vis) and NMR/CIDNP investigations, as well as photo-DSC studies.