Photochemical Generation Research Articles

Diels-Alder Reaction of Photochemically Generated (E)-Cyclohept-2-enones: Diene Scope, Reaction Pathway, and Synthetic Application.

Upon irradiation at λ = 350 nm, cyclohept-2-enone undergoes an isomerization to the strained (E)-isomer. The process was studied by XMS-CASPT2 calculations and found to proceed by two competitive reaction channels on either the singlet or the triplet hypersurface. (E)-Cyclohept-2-enone is a reactive dienophile in thermal [4 + 2] cycloaddition reactions with various dienes. Ten different dienes were probed, most of which─except for 1,3-cyclohexadiene─underwent a clean Diels-Alder reaction and gave the respective trans-fused six-membered rings in good yields (68-98%). The reactions with furan were studied in detail, both experimentally and by DLPNO-CCSD(T) calculations. Two diastereoisomers were formed in a ratio of 63/35 with the exo-product prevailing, and the configuration of both diastereoisomers was corroborated by single crystal X-ray crystallography. The outcome of the photoinduced Diels-Alder reaction matched both qualitatively and quantitatively the calculated reaction pathway. Apart from cyclohept-2-enone, five additional cyclic hept-2-enones and cyclooct-2-enone were employed in their (E)-form as dienophiles in the Diels-Alder reaction with 1,3-cyclopentadiene (80-98% yield). The method was eventually applied to a concise total synthesis of racemic trans-α-himachalene (four steps, 14% overall yield).

Vanadium Species-Assisted Photochemical Vapor Generation for Direct Detection of Trace Tellurium with Inductively Coupled Plasma Mass Spectrometry.

Photochemical vapor generation (PVG) is emerging as an alternative sample introduction method in the field of atomic spectrometry. The addition of transition metals can largely improve the PVG yields of elements with the enhancement of 1.4 to 30 000-fold, based on previous reports. In this work, the use of vanadium species as novel "sensitizers" in PVG was first reported, tellurium (Te) was selected as the target. The efficient photochemical reduction of Te was observed in the presence of 9% (v/v) formic acid (FA), 20%(v/v) acetic acid (AA), and 40 mg L-1 of V(V) (existing as VO3-) with the conversion efficiency of 87 ± 3%. Under the selected conditions, there was no significant difference in analytical sensitivity between Te(IV) and Te(VI), making the direct detection of total Te possible. The limit of detection (LOD, 3σ) was 2.9 ng L-1 for Te with inductively coupled plasma mass spectrometry (ICP MS) measurement. Good precisions of 2.3% and 2.2% (relative standard deviations, RSD) for seven times replicate measurement of 0.5 μg L-1 Te(IV) and Te(VI) standard solutions were obtained. The sensitivity was enhanced about 55-fold compared to that using traditional direct solution nebulization. The method was applied for the determination of trace Te in three water samples and two certified reference materials of sediment with satisfactory results. The possible mechanism was investigated. The generation of volatile vanadium along with (CH3)2Te was found in PVG for the first time. The new findings in this work will be helpful for exploration of efficient "sensitizers" in PVG and further expanding the scope of elements amenable to PVG as well.

Mechanistic study on photochemical generation of I•/I2•− radicals in coastal atmospheric aqueous aerosol

The reactive iodine species may exhibit significant impacts on many global atmospheric issues and the I•/I2•− radicals play key roles for inducing the formation of these reactive iodine species. However, the current understanding on the formation of I•/I2•− radicals in atmospheric aqueous aerosol is still quite limited. The results reported herein suggest that I•/I2•− can be produced simultaneously in aqueous aerosol by several sunlight-driven photochemical pathways including direct photo-dissociation of soluble organic iodine (SOI) at rates of 0.10–1.34 × 10−9 M ns−1 and 0.99–5.68 × 10−7 M μs−1, •OH-mediated oxidation of I− at 0.03–1.41 × 10−8 M ns−1 and 0.05–4.10 × 10−6 M μs−1, and 3DOM⁎-induced oxidation of I− at 1.57–1.65 × 10−9 M ns−1 and 0.99–5.68 × 10−7 M μs−1 for generation of I• and I2•−, respectively. Meanwhile, the pathway of eaq−-initiated stepwise reduction of IO3− to I2(aq) and further photolyzed into I• plays negligible role in formation of I•/I2•− due to the low reaction rates and severe quenching effect of eaq− by dissolved O2. Our work presented the new data on mechanism and kinetics for comprehensive elucidation of I•/I2•− formation in coastal atmospheric aqueous aerosol and would help to better understand the transformation mechanism of iodine species, pathways of iodine cycling and the associated environmental impacts involving atmospheric reactive iodine radicals.

Ultra-sensitive speciation analysis of tellurium by manganese and iron assisted photochemical vapor generation coupled to ICP-MS/MS

Photochemical vapor generation (PVG) of Te4+ was undertaken with a simple reactor consisting of a polytetrafluoroethylene reaction coil wrapped around a low-pressure mercury tube lamp and using a flow-injection for sample delivery. The composition of a reaction medium, the influence of irradiation time and the effect of added sensitizers and interferents were investigated using high-resolution continuum source atomic absorption spectrometry and a miniature diffusion flame atomizer. A mixture of 5 M acetic acid and 3.5 M formic acid and sample flow rate of 4 mL min−1 permitting a 36 s irradiation time were found optimal for PVG of Te4+. The addition of 250 mg L−1 Mn2+ and 15 mg L−1 Fe2+ ions as sensitizers enhanced the overall PVG efficiency 2.75-fold to 50 ± 2%. In order to achieve higher sensitivity necessary for determination of Te in real environmental samples, PVG was coupled to inductively coupled plasma triple quadrupole mass spectrometer and detection was performed with O2 in the reaction cell utilizing a mass shift mode of measurement (m/z 128 → m/z 144) to ensure interference free ion detection. A limit of detection 1.3 ng L−1 and repeatability (RSD) 0.9% at 250 ng L−1 were achieved. This ultrasensitive methodology was validated for speciation analysis of Te in water samples of various matrix complexities (fresh water, well water, seawater and contaminated water). Since no response was observed from Te6+ under optimal PVG conditions, Te4+ was selectively determined by direct PVG. The sum of Te4+ and Te6+ was determined after pre-reduction of Te6+ in 6 M HCl (95 °C), evaporation to dryness and reconstitution in the reaction medium containing sensitizers. Very good accuracy was demonstrated by spiked recoveries for both Te4+ and total Te in water samples and also by total Te determination in fresh water Standard Reference Material NIST 1643f.

Selenonium ylides: synthesis, characterization, and applications to photo-induced cyclopropanation reactions.

Carbenes are important and highly reactive intermediates for the synthesis of various complex molecules. They are now an indispensable chemical species in organic chemistry and are used frequently to synthesize complex compounds in drug discovery chemistry. In general, carbenes are synthesized by a combination of transition metal catalysts and diazo compounds or by the decomposition reactions of diazo compounds. This paper reports the development of the visible light for the photochemical generation of carbenes from a novel C,Se-selenonium ylide. Overall, this photochemical carbene generation method using C,Se-selenonium ylide does not require a catalyst, is simple to perform, and enables highly efficient cyclopropanation reactions with alkenes.

MoS2–Covalent Organic Framework Composite as a Bifunctional Supporter for the Determination of Trace Nickel by Photochemical Vapor Generation–Microplasma Optical Emission Spectrometry

A microplasma-based optical emission spectrometry (OES) system has emerged as a potential tool for field analysis of heavy metal pollution due to its features of portability and low energy consumption, while the development of an efficient sample introduction approach against matrix interference is crucial to meet the requirements of complex sample analysis. Herein, a MoS2-covalent organic framework (COF) composite serves as a bifunctional supporter for efficient sample separation/enrichment and photochemical vapor generation (PVG) enhancement, thereby achieving highly selective and sensitive detection of heavy metals in environmental water by dielectric barrier discharge (DBD) microplasma-OES. With trace nickel analysis as a model, the MoS2-COF composite with a large specific surface area and a porous structure can not only efficiently separate and enrich nickel ions from a sample matrix through electrostatic interaction and coordination to reduce the interference of coexisting ions but also significantly improve the subsequent PVG efficiency due to the formed heterojunction and more negative reduction potential. Under optimized conditions, a linear range of 0.1-10 μg L-1 along with a detection limit of 0.03 μg L-1 is obtained for nickel. Compared with direct PVG, the tolerance to coexisting ions is greatly enhanced, and the detection limit is also improved by 43-fold. The accuracy and practicability of the present PVG-DBD-OES system are verified by measuring several certified reference materials and real water samples. MoS2-COF as a bifunctional supporter promotes the performance of the PVG-DBD-OES system in terms of anti-interference ability and detection sensitivity, especially for robust and efficient on-site analysis of complex samples.

GC-MS exploration of photochemically generated species of Os, W and Ru from reductive and oxidative media

GC-MS identification of W(CO)6 and OsO4 confirms these products of photochemical vapor generation from W(vi) and Os(iv) in reductive and oxidative media, respectively.

Model Analyses of Changes in Spring Surface Ozone Concentrations over Shandong Province in the Period of 2014‒2017

The concentrations of surface ozone (O3) in eastern China have increased significantly in recent years, resulting in the earlier appearance of more serious O3 pollution. Measurements at 14 stations in Shandong Province showed that the monthly mean O3 concentrations in the late spring (May) increased by 22.2 µg m−3 from 2014 to 2017. To investigate the reasons of the increase of O3 in springtime from 2014 to 2017, the changes of O3 concentrations due to meteorological conditions and emissions in May were studied based on ambient measurements and simulations with the RAMS-CMAQ modeling system. By analyzing the observed data, it was found that the variations in wind field were conducive to the accumulation of O3, while the effects of other meteorological parameters on O3 concentrations were different at the same site between years. Further to perform a series of simulations with only the meteorological conditions changed in May from 2014 to 2017, the results showed that the effects of variations in meteorological conditions had become more and more important in the changes of O3 concentrations in May between years, especially the factors that affected the photochemical generation of O3. For example, the percentage of the sites where the changes of O3 concentrations were dominated by the variations of the meteorological conditions increased from 28.6% to 78.6% over the region in May between years. Besides, the changes in NOx emissions had a close relationship with the variations in O3 concentrations when the changes of O3 were dominated by the emission variations.

Red light induced folding of single polymer chains

We pioneer the photochemical generation of single chain nanoparticles (SCNPs) at the to-date mildest reported wavelength of 625 nm by exploiting the photochemical uncaging of methylene blue protected amines. The protected amines are tethered to polymers prepared via reversible addition-fragmentation chain transfer (RAFT) polymerisation, and subsequently undergo intrachain crosslinking by amide formation.

Controlling Cell Shape and Microtubule Organization by Extracellular Matrix Micropatterning.

Micropatterning of extracellular matrix proteins enables defining cell position and shape in experiments investigating intracellular dynamics and organization. While such standardization is advantageous in automated and quantitative analysis of many cells, the original methods generating such patterns are cumbersome and inflexible. However, recent development of contact-less methods that allow photochemical generation of protein patterns robustly and rapidly is boosting the broader availability of micropatterning approaches. Here, we describe an optimized protocol to achieve large micropatterned areas with high fidelity using a commercially available microscope-mounted UV projection system.

Simultaneous Detection of Ruthenium and Osmium by Photochemical Vapor Generation-Inductively Coupled Plasma-Mass Spectrometry

Efficient simultaneous photochemical vapor generation (PVG) of ruthenium (Ru) and osmium (Os) in the medium of formic acid was demonstrated. A flow-through photoreactor hyphenated to an inductively coupled plasma-mass spectrometer (ICP-MS) was used for the PVG and subsequent detection of the two elements. A similar synergistic enhancement from cobalt and cadmium ions on the PVG efficiency of both Ru and Os was discovered. Following the critical evaluation of the impacts of various transition metal ions, the concentrations of formic acid, cobalt, and cadmium ions, the flow rate of carrier gas, and the UV irradiation time, impressive limits of detection (LODs) of 5 and 0.5 ng L-1 were achieved for Ru and Os, respectively. The accuracy of the proposed PVG-ICP-MS method was validated by the analysis of several water samples with desirable spike recoveries obtained. Furthermore, the volatile compounds of Ru were directed and cryogenically trapped in acetonitrile and generation of carbonyls of Ru was verified by high-resolution electrospray ionization-mass spectra (ESI-MS).

Characterization of a Triplet Vinylidene

Singlet vinylidenes (R2C=C:) are proposed as intermediates in a series of organic reactions, and very few have been studied by matrix isolation or gas-phase spectroscopy. Triplet vinylidenes, however, featuring two unpaired electrons at a monosubstituted carbon atom are thus far only predicted as electronically excited-state species and represent an unexplored class of carbon-centered diradicals. We report the photochemical generation and low-temperature EPR/ENDOR characterization of the first ground-state high-spin (triplet) vinylidene. The zero-field splitting parameters (D = 0.377 cm–1 and |E|/D = 0.028) were determined, and the 13C hyperfine coupling tensor was obtained by 13C-ENDOR measurements. Most strikingly, the isotropic 13C hyperfine coupling constant (50 MHz) is far smaller than the characteristic values of triplet carbenes, demonstrating a unique electronic structure which is supported by quantum chemical calculations.

Highly sensitive determination of trace antimony in water samples by cobalt ion enhanced photochemical vapor generation coupled with atomic fluorescence spectrometry or ICP-MS

A method for highly sensitive determination of trace antimony was proposed by using cobalt ion enhanced photochemical vapor generation (PCVG) for sample introduction into atomic fluorescence spectrometry (AFS) or inductively coupled plasma-mass spectrometry (ICP-MS) for elemental detection. During the PCVG process, the sample introduction efficiency of Sb could be significantly improved by addition of 5 mg L−1 Co2+ in the mixed acid medium of 10% (v/v) formic acid and 20% (v/v) acetic acid, with a final 12-fold and 133-fold enhancement of AFS and ICP-MS intensity, respectively. The experimental conditions including enhancement ions, acid medium, UV irradiation, working gas as well as potential interference were investigated in detail. Under the optimal experimental conditions, the limit of detection (LOD) for Sb was 0.05 and 0.001 μg L−1 by using AFS and ICP-MS determination, respectively. The method was successfully used for analysis of real water samples, with satisfactory recoveries of 92–94%.

Smart Solar-Metal-Air Batteries Based on BiOCl Photocorrosion for Monolithic Solar Energy Conversion and Storage.

Herein, a BiOCl hydrogel film electrode featuring excellent photocorrosion and regeneration properties acts as the anode to construct a novel type of smart solar-metal-air batteries (SMABs), which combines the characteristics of solar cells (direct photovoltaic conversion) and metal-air batteries (electric energy storage and release interacting with atmosphere). The cyclic photocorrosion processes between BiOCl (Bi3+ ) and Bi can simply be achieved by solar light illumination and standing in the dark. Upon illumination, the device takes open-circuit configuration to charge itself from the sunlight. Notably, in this system, the converted solar energy can be stored in the SMABs without the need of external assistance. In the discharging process in the dark, Bi0 spontaneously turns back to Bi3+ producing electrons to induce the oxygen reduction reaction. With an illumination of 15 min, the battery with an electrode area of 1 cm2 can be continuously discharged for ≈3000 s. Taking elemental Bi as the calculation object, the theoretical capacity of the SMABs is 384.75mAh g-1 , showing its potential application in energy storage. This novel type of SMABs is developed based on the unique photocorrosive and self-oxidation reaction of BiOCl to achieve photochemical energy generation and storage.

MIL-125-NH2 catalyzed photochemical vapor generation coupled with HPLC-ICPMS for speciation analysis of selenium

An on-line photochemical vapor generation (PCVG) method for highly sensitive speciation analysis of selenium was proposed by using metal organic framework (MOF) MIL-125-NH2 as a catalyst. When it was coupled with HPLC (high performance liquid chromatography)-ICPMS (inductively coupled plasma mass spectrometry) for Se separation and detection, inorganic selenium species (Se(IV) and Se(VI)) were firstly separated in HPLC with Na2CO3-NaHCO3 as an eluent, and then mixed with MIL-125-NH2 nanoparticles and formic acid in the PCVG reactor to form gaseous selenium-containing species under ultraviolet irradiation, which was introduced into ICPMS for selenium measurement after gas–liquid separation from the reaction solution. Experimental factors of HPLC separation, PCVG reaction and ICPMS detection were carefully investigated. Under the optimized conditions, the limits of detection (LOD) for Se(IV) and Se(VI) were found to be both as low as 0.8 ng/mL. The relative standard deviation (RSD) of Se(IV) and Se(VI) were 4.6% and 3.7% at 20 ng/mL (n = 11), respectively. Certified reference materials and bottled natural mineral water samples were successfully analyzed by the proposed method.

Ultrasensitive Detection of Ruthenium by Coupling Cobalt and Cadmium Ion-Assisted Photochemical Vapor Generation to Inductively Coupled Plasma Mass Spectrometry

An extremely sensitive methodology for the determination of Ru was developed by coupling photochemical vapor generation (PVG) analyte introduction with inductively coupled plasma mass spectrometry (ICPMS). PVG was undertaken with a thin-film flow-through photoreactor in a medium comprising 8 M formic acid in the presence of 10 mg L-1 Co2+ and 25 mg L-1 Cd2+. The volatile product (presumably ruthenium pentacarbonyl) was generated in a flow injection mode, yielding an overall efficiency of 29% at a sample flow rate of 1.4 mL min-1. The presence of both Co2+ and Cd2+ sensitizers enhanced PVG efficiency by 3,200-fold, permitting a 31 s irradiation time. Although enhanced efficiency (≈40%) could be obtained with increased Co2+ concentration, this was not suitable for routine use due to co-generation of cobalt carbonyl. Excellent repeatability (<2.5%) and reproducibility (4%) were achieved for 200 ng L-1 Ru3+. Limits of detection ranged from 20 to 42 pg L-1 (10-21 fg absolute) depending on the measured isotope and operational mode of the ICPMS reaction/collision cell. Interferences from inorganic acids and their anions, several transition metals, and metalloids were investigated. Practical application of the methodology was demonstrated by the analysis of seven water samples of various matrix complexities (well water, spring water, contaminated water, and seawater).

Plastic-free silica-titania-polyphenol heterojunction hybrids for efficient UV-to-blue light blocking and suppressed photochemical reactivity

The photocatalytic activity of ultraviolet (UV) filters, generating reactive oxygen species (ROS), must be minimized in skincare products to avoid adverse biological and chemical effects. Unfortunately, the screening effect of UV filters is intrinsically correlated with their photocatalytic activity in the presence of oxygen and water. Polymeric encapsulation has been widely employed to inhibit photochemical ROS generation; however, it can cause serious microplastic issues because UV filters act as photocatalysts under sunlight irradiation. Here we present plastic-free silica-titania-tannin (SiO2-TiO2-TA) heterojunction hybrids to increase UV-to-blue light blocking effects while suppressing UV-induced ROS generation by altering their photochemical routes. Mesoporous SiO2-TiO2 particles were prepared through the in situ synthesis of TiO2 nanocrystals within SiO2 particles, followed by the spontaneous deposition of TA via Ti-pyrogallol and Ti-catechol coordination. Ligand-to-metal charge transfer (LMCT) from TA to the conduction band of TiO2, demonstrated by theoretical calculations, extends the absorption spectrum of TiO2 over the blue wavelength regime, increasing a specific sun protection factor ∼ 1.8 times. The TA layer efficiently suppressed the UV-induced ROS generation: the SiO2-TiO2-TA produced only < 1 % hydroxyl radical ions compared to pristine TiO2 under simulated solar irradiation. Accordingly, the UV and blue light-induced cytotoxicity of fibroblasts was significantly alleviated by the ROS stabilization effect of TA complexed with TiO2, while pristine TiO2 exacerbated the cytotoxic effect via ROS generation in the cytoplasm. This work demonstrates that polyphenol coatings are a promising method to suppress the ROS generation by inorganic UV filters while increasing their screening effects through LMCT.

Aryl-Cl vs heteroatom-Si bond cleavage on the route to the photochemical generation of σ,π-heterodiradicals.

The photochemistry of aryl chlorides having a X-SiMe3 group (X = O, NR, S, SiMe2) tethered to the aromatic ring has been investigated in detail, with the aim to generate valuable ϭ,π-heterodiradicals. Two competitive pathways arising from the excited triplet state of the aromatics have been observed, namely heterolysis of the aryl-chlorine bond and homolysis of the X-silicon bond. The former path is found in chlorinated phenols and anilines, whereas the latter is exclusive in the case of silylated thiophenols and aryl silanes. A combined experimental/computational approach was pursued to explain such a photochemical behavior.Graphical abstract.

Detailed evaluation of conditions of photochemical vapor generation for sensitive determination of nickel in water samples by ICP-MS detection

Photochemical vapor generation (PVG) of Ni was optimized using inductively coupled plasma mass spectrometry detection and a 19 W high-efficiency flow-through photoreactor operated in a flow injection mode (0.49 mL sample volume). The main aim was to achieve very high sensitivity and low limits of detection (LOD) for determination of Ni in fresh water samples, which necessitated to keep low Ni contamination and rather low dilution of samples by added formic acid as a photochemical agent. With respect to that a reaction medium composed of 30% (m/v) formic acid and a flow rate through the photoreactor of 1.5 mL min−1, which corresponds to an irradiation time of 29 s, were chosen as optimal. These PVG conditions were characterized by 42% overall PVG efficiency and by relative and absolute LOD of 1.9 ng L–1 and 0.9 pg, respectively. Almost double efficiency was achievable when pH was adjusted by an addition of 1.5 M ammonium formate, reflected in the proportional increase in sensitivity. However, no significant benefit to analytical performance was found at those conditions. Interferences from inorganic anions likely to be encountered during analytical application to real samples (i.e., NO3−, Cl−, SO42−) were investigated in detail as well as interferences from several transition metals and metalloids. The accuracy was successfully verified by an analysis of the standard reference material NIST 1643f (fresh water) and certified reference materials SLRS-6 (river water), ERM-CA713 (wastewater) and CTA-FFA-1 (fine fly ash), the last one after simple acid digestion. The applicability of this sensitive methodology was further demonstrated by a direct analysis of three water samples (river and well) that were collected and only acidified with formic acid to 30% (m/v).

Three-Dimensional Printed Dual-Mode Chemical Vapor Generation Point Discharge Optical Emission Spectrometer for Field Speciation Analyses of Mercury and Inorganic Selenium.

Due to the large size and high energy consumption of instruments, field elemental speciation analysis is still challenging so far. In this work, a portable and compact system device (230 mm length × 38 mm width × 84 mm height) was fabricated by using three-dimensional (3D) printing technology for the field speciation analyses of mercury and inorganic selenium. The device comprises a cold vapor generator, photochemical vapor generator, and miniaturized point discharge optical emission spectrometer (μPD-OES). For mercury, inorganic mercury (IHg) was selectively reduced to Hg0 by cold vapor generation, whereas the reductions of both IHg and methylmercury (MeHg) were obtained by photochemical vapor generation (PVG) in the presence of formic acid. For selenium, Se(IV) and total inorganic selenium were converted to their volatile species by PVG in the presence and the absence of nano-TiO2, respectively. The generated volatile species were consequently detected by μPD-OES. Limits of detection of MeHg, IHg, Se(IV), and Se(VI) were 0.1, 0.1, 5.2, and 3.5 μg L-1, respectively. Precision expressed as the relative standard deviations (n = 11) were better than 4.5%. The accuracy and practicality of the proposed method were evaluated by the analyses of Certified Reference Materials (DORM-4, DOLT-5, and GBW(E)080395) and several environmental water samples with satisfactory recoveries (95-103%). This work confirms that 3D printing has great potential to fabricate a simple, miniaturized, easy-to-operate, and low gas and power consuming atomic spectrometer for field elemental speciation analysis.