Photochemical Initiation Research Articles

Photochemical Initiation and Reactions of Thiyl Radicals Studied with SH2' Radical Traps.

A radical trapping method based on an SH2' homolytic substitution reaction was applied to study the mechanism of a photochemical spirocyclisation of indole-ynones in the presence of thiols. Starting material, products and a range of trapped radical intermediates were simultaneously detected in reaction mixtures by mass spectrometry (MS). The trapped intermediates included both initiating and main chain propagating radicals. These data made it possible to propose a self-initiation mechanism consistent with the originally postulated photoexcitation of an intramolecular electron donor-acceptor complex of the substrate. The effect of thiol structure on the MS peak intensity of the reaction components was rationalised in terms of the relative stability of the radical intermediates. The results were compared to a simpler related reaction, a photochemical thiol-ene addition where reagents, products and trapped intermediate radicals were also detected by MS. Relative MS peak intensities were again explained by a combination of electronic and steric effects on the stability of intermediate radicals. Overall, SH2' radical trapping was demonstrated to be a powerful experimental technique for providing mechanistic evidence on photochemical and other organic radical reactions.

Photochemical initiation of polariton-mediated exciton propagation

Abstract Placing a material inside an optical cavity can enhance transport of excitation energy by hybridizing excitons with confined light modes into polaritons, which have a dispersion that provides these light–matter quasi-particles with low effective masses and very high group velocities. While in experiments, polariton propagation is typically initiated with laser pulses, tuned to be resonant either with the polaritonic branches that are delocalized over many molecules, or with an uncoupled higher-energy electronic excited state that is localized on a single molecule, practical implementations of polariton-mediated exciton transport into devices would require operation under low-intensity incoherent light conditions. Here, we propose to initiate polaritonic exciton transport with a photo-acid, which upon absorption of a photon in a spectral range not strongly reflected by the cavity mirrors, undergoes ultra-fast excited-state proton transfer into a red-shifted excited-state photo-product that can couple collectively with a large number of suitable dye molecules to the modes of the cavity. By means of atomistic molecular dynamics simulations we demonstrate that cascading energy from a photo-excited donor into the strongly coupled acceptor-cavity states via a photo-chemical reaction can indeed induce long-range polariton-mediated exciton transport.

Group transfer polymerization in bulk methacrylates

AbstractGroup transfer polymerization (GTP) is a polymerization method developed to obtain targeted (meth)acrylic polymers in solution at ambient temperatures. In this work, it is employed with methacrylic monomers to obtain low Ð polymers in bulk. In this regard, different initiator systems that exhibit different mechanisms are compared concerning the molecular weight, the polydispersity and double bond conversion of the resulting bulk polymers. The respective systems were chosen carefully to give a broad overview of earlier developed initiator‐catalyst combinations 1‐methoxy‐1‐(trimethylsiloxy)‐2‐methylprop‐1‐ene (MTS) & tetrabutylammonium cyanide (TBACN) and more recently investigated initiating systems MTS & trityl tetrakis(pentafluorophenyl)borate (TTPB) or dimethyl phenyl silane (DMPS) & tris(pentafluorophenyl)borate (BCF). The described initiating systems are applied as a two‐component (2K) system to ensure a homogeneous distribution of the respective initiator and catalyst in the bulk monomer. In addition to the 2K experiments, photochemical initiation is also applied to bulk formulations. Therefore, a photoacid generator (PAG) and MTS is used to trigger the polymerization reaction by irradiation with UV light. A highly controlled photopolymerization method in bulk was developed that way achieving a low polydispersity polymer with high double bond conversion.

Substituent effect on the visible light initiating ability of chalcones

Chalcones (Chals) are considered as attractive photoinitiators due to their large-scale conjugated structure and the ease of modification to the substituent structure on both sides. However, there is still a lack of full understanding as to the effects of substituents and their positions on the photophysical and photochemical properties and initiation efficiency of chalcones. In the present study, eight types of chalcone were designed and synthesized from the aromatic aldehydes and ketones containing four different substituents through para-substitution. These chalcones exhibit varying degrees of absorption in the visible region, and their maximum absorption wavelength increasing with the electron donating ability of the substituents when the substituents are positioned on the same side. At the aldehyde side, substitution exerts a more significant effect on their absorption when the substituents are identical. According to photopolymerization and steady-state photolysis, C4S performs best in the efficiency of photochemical hydrogen atom transfer, which leads to the high polymerization efficiency. Furthermore, it shows a great potential for use in the deep curing and curing of light-colored materials due to the lighter color of C4S and its smaller visible light absorption.

Probing zinc oxide as a semiconductor photosensitizer of energetic materials to laser radiation

The development of methods for laser ignition of energetic materials opens up significant prospects for improving the safety of their application. The photochemical processes can increase the selective sensitivity of energetic material to laser radiation and make the most of its capabilities. The paper considers the application of zinc oxide as a photosensitizing additive capable of significantly reducing the laser ignition threshold of pentaerythritol tetranitrate (PETN). The dependencies of the laser initiation threshold by the fundamental frequency (1064 nm), second (532 nm), and third harmonic (355 nm) of an Nd:YAG laser on the concentration of zinc oxide were studied. The ignition threshold energy density is reduced by a factor from 2 to 5, depending on the oxide concentration and the laser radiation wavelength. The proposed initiation mechanisms based on the results of the several spectroscopy methods were discussed for each laser harmonic. Laser ignition of the PETN-ZnO composite through the ultraviolet laser radiation combined the thermal and photochemical mechanism. Visible laser harmonic (532 nm) drives a purely photochemical initiation of the PETN. The obtained results indicate that a mechanism of initiation by visible light should be caused by the charge transfer in the ZnO-PETN interface. The reported experimental result does not only serve as an example of the successful photochemical ignition of energetic material but also shows that processes, techniques, and materials of semiconductor photocatalysis are applicable to laser initiation.

Study on Characteristics of the Light-Initiated High Explosive-Based Pulse Laser Initiation

The silver acetylene silver nitrate loading technology of the light initiated high explosive, as one of important means to simulate the structural response of powerful pulsed X-ray, adopts the pulse laser initiation. It has advantages of improvement of practical control, heterogenous loading realization and simultaneous loading timeliness. In this paper, the physical and mathematical models of hot spot initiation and photochemical initiation of energetic materials under the action of laser are firstly established, and then the laser initiation mechanism of the light initiated high explosive is specifically analyzed, and the laser initiation experiment is conducted based on the optical adsorption property of the light initiated high explosive. From this study, the laser initiation thresholds of 193 nm, 266 nm, 532 nm, 1064 nm wavelengths are given, and they are 5.07 mJ/mm2, 6.77 mJ/mm2, 7.21 mJ/mm2 and 10.61 mJ/mm2, respectively, and the complete detonation process is verified by detonation velocity. This work technically supports the study of pulse laser initiation process, mechanism and explosion loading rule as well as the loading technology of the light initiated high explosive to simulate the structural response of X ray.

Effects of Monomer Composition of Urethane Methacrylate Based Resins on the C=C Degree of Conversion, Residual Monomer Content and Mechanical Properties.

(1) Background: This study investigated the influence of Bis-GMA, TEGDMA, UDMA, and two different polyethylene glycol (PEG)-containing, UDMO-based co-monomers on the Young’s modulus and flexural strength, degree of methacrylate C=C double bond conversion and residual monomer elution of experimental dental resins. (2) Methods: Urethane methacrylate-based monomer was synthesised via a radical chain growth polymerization mechanism using PEG in order to improve the mechanical properties. Dental resins were formulated using Bis-GMA, UDMA, or UDMO as base monomers combined with TEGDMA as a dilution monomer and DMAEM + CQ as the photo-initiator system. Degree of conversion (DC), mechanical properties, and residual monomer content of light-activated methacrylate resin formulations were evaluated and statistically analysed by ANOVA and a Tukey’s test. (3) Results: PEG-containing UDMO resins had lower Young’s modulus and elastic strength than UDMA-derived resin for all irradiation times. The highest DC (67,418%) was observed for the PEG-containing UDMO-based resin formulation when light cured for 40 s. For all samples, DC increased with the photo-polymerization time. The amount of residual monomer decreased after increasing the light-curing period from 20 to 40 s, resin with UDMO content 0.01 mol of PEG having the smallest amount of free eluted monomer. (4) Conclusions: A strong structure–property relationship exists in photo-cured dimethacrylate-based dental resins. The time and quantity of the photochemical initiation system can influence the physical–mechanical properties of the resins but also the monomers in their composition.

Practical and Selective sp3 C-H Bond Chlorination via Aminium Radicals.

The introduction of chlorine atoms into organic molecules is fundamental to the manufacture of industrial chemicals, the elaboration of advanced synthetic intermediates and also the fine‐tuning of physicochemical and biological properties of drugs, agrochemicals and polymers. We report here a general and practical photochemical strategy enabling the site‐selective chlorination of sp3 C−H bonds. This process exploits the ability of protonated N‐chloroamines to serve as aminium radical precursors and also radical chlorinating agents. Upon photochemical initiation, an efficient radical‐chain propagation is established allowing the functionalization of a broad range of substrates due to the large number of compatible functionalities. The ability to synergistically maximize both polar and steric effects in the H‐atom transfer transition state through appropriate selection of the aminium radical has provided the highest known selectivity in radical sp3 C−H chlorination.

Practical and Selective sp3C−H Bond Chlorination via Aminium Radicals

AbstractThe introduction of chlorine atoms into organic molecules is fundamental to the manufacture of industrial chemicals, the elaboration of advanced synthetic intermediates and also the fine‐tuning of physicochemical and biological properties of drugs, agrochemicals and polymers. We report here a general and practical photochemical strategy enabling the site‐selective chlorination of sp3C−H bonds. This process exploits the ability of protonatedN‐chloroamines to serve as aminium radical precursors and also radical chlorinating agents. Upon photochemical initiation, an efficient radical‐chain propagation is established allowing the functionalization of a broad range of substrates due to the large number of compatible functionalities. The ability to synergistically maximize both polar and steric effects in the H‐atom transfer transition state through appropriate selection of the aminium radical has provided the highest known selectivity in radical sp3C−H chlorination.

Photochemical Selenosulfonation of Tricyclo[4.1.0.02,7]heptane Derivatives

Se-Phenyl methane-, benzene-, and 4-methylbenzenesulfonoselenoates PhSeSO2R (R = Me, Ph, 4-MeC6H4) react with 1-X-tricyclo[4.1.0.02,7]heptanes (X = Me, Ph, SiMe3) under photochemical initiation to give products of addition at the C1–C7 central bicyclobutane bond with a bicyclo[3.1.1]heptane (norpinane) skeleton, where the R-sulfonyl group is attached to C7 and is oriented syn and the phenylselanyl group on C6 can occupy both syn and anti positions. Radical mechanism of the reaction and its stereochemistry are discussed.

Photosensitive Hypervalent Fluorinated Sulfur Containing Polymers for Light Sensitive Applications

Photochemical initiation of polymerization by visible or broadband ultraviolet irradiation of a mixture of 1,4-(bis-chlorotetrafluoro-λ6-sulfanyl) benzene and 1,4-diethynyl benzene forms light responsive polymers. The novel polymers have a trans-tetrafluoro-λ6-sulfanyl (tetrafluorosulfanyl, SF4) group backbone and are sensitive to electron beam (EB) and extreme ultraviolet (EUV) irradiation. Both EB lithography and EUV contrast experiments demonstrate the sensitivity of the polymer films to short wavelength irradiation.

Hierarchical polymeric architectures through molecular imprinting in liquid crystalline environments

The use of liquid crystalline (LC) media as sacrificial templates during the polymer synthesis has been explored. The LC-media introduce morphological features into resultant polymers which when used together with molecular imprinting can produce materials with hierarchical architectures. Bupivacaine (1) imprinted co-polymers of 2-hydroxyethylmethacrylate (HEMA) (2a) and 1,4-divinylbenzene (DVB) (3a) were synthesized using photochemical initiation in lyotrophic liquid crystalline phases of AOT (5) in water/p-xylene and Triton X-100 (6) /water systems. SEM studies revealed the impact of the LC-media on polymer morphology, with polymer brush-like structures, with bristles of ≈30 nm diameter. The polymer morphology reflects that of the hexagonal phase of the LC medium. The rebinding characteristics of polymer films were evaluated quartz crystal microbalance (QCM, under FIA conditions). The influence of the presence of imprinting-derived recognition sites in AOT (5) in water/p-xylene polymer film induced brush-like features which provided a 25-fold enhancement of sensor sensitivity. This chemosensor was shown to be selective for the local anesthetic template, bupivacaine, through studies using the structural analogues ropivacaine and mepivacaine.

Designing Homogeneous Bromine Redox Catalysis for Selective Aliphatic C-H Bond Functionalization.

The potential of homogeneous oxidation catalysis employing bromine has remained largely unexplored. We herein show that the combination of a tetraalkylammonium bromide and meta-chloroperbenzoic acid offers a unique catalyst system for the convenient and selective oxidation of saturated C(sp3 )-H bonds upon photochemical initiation with day light. This approach enables remote, intramolecular, position-selective C-H amination as demonstrated for 20 different examples. For the first time, an N-halogenated intermediate was isolated as the active catalyst state in a catalytic Hofmann-Löffler reaction. In addition, an expeditious one-pot synthesis of N-sulfonyl oxaziridines from N-sulfonamides was developed and exemplified for 15 transformations. These pioneering examples provide a change in paradigm for molecular catalysis with bromine.

Laser initiation of RDX crystal slice under ultraviolet and near-infrared irradiations

Interactions between energetic materials and laser beams of specific frequency are of both scientific and engineering importance for understanding and manipulating the laser-induced ignition of energetic crystals. In this work, we investigate the effects of laser irradiation of variable energy densities from ultraviolet (355 nm) to near-infrared (1064 nm) on the ignition properties of a well-treated cyclotrimethylenetrinitramine (RDX) crystal slice (9.3 mm × 9.1 mm × 2.9 mm). The laser-induced damage and initiation dynamics were characterized in detail by using optical microscopy as well as an ultrafast pump-probe imaging technique at nanoseconds. It discloses that both ignition probabilities (p) of 355 and 1064 nm change exponentially with increasing laser fluence (H, J/cm2): p(355)=1−e−1.72*(H−3.981), p(1064)=1−e−0.74*(H−7.898). RDX crystals can be more easily ignited under ultraviolet laser irradiation than by near-infrared one due to its different absorptions (α(355) = 1.6306 cm−1, and α(1064) = 0.5313 cm−1) and the effects of photochemical initiation mechanism. The damage induced by either 355 nm or 1064 nm laser exhibits three typical morphologies varied with laser exposure. In addition, damage generated by ultraviolet laser appears on the incident surface of the crystal slice, while it is mainly located on the exit surface when being ignited by near-infrared laser. Our work sheds light on the dedicated interaction mechanism between energetic crystals and laser beams of various frequency and energy density.

Size and Surface Chemistry Tuning of Silicon Carbide Nanoparticles.

Chemical transformations on the surface of commercially available 3C-SiC nanoparticles were studied by means of FTIR, XPS, and temperature-programmed desorption mass spectrometry methods. Thermal oxidation of SiC NPs resulted in the formation of a hydroxylated SiO2 surface layer with C3Si-H and CHx groups over the SiO2/SiC interface. Controllable oxidation followed by oxide dissolution in HF or KOH solution allowed the SiC NPs size tuning from 17 to 9 nm. Oxide-free SiC surfaces, terminated by hydroxyls and C3Si-H groups, can be efficiently functionalized by alkenes under thermal or photochemical initiation. Treatment of SiC NPs by HF/HNO3 mixture produces a carbon-enriched surface layer with carboxylic acid groups susceptible to amide chemistry functionalization. The hydroxylated, carboxylated, and aminated SiC NPs form stable aqueous sols.

Investigation of the Bromination/Dehydrobromination of Long Chain Alkanes

The partial oxidation of dodecane and paraffin wax with bromine at relatively low temperatures (≤120 °C) and the dehydrobromination of the subsequent alkyl bromide intermediates at higher temperatures (>175 °C) to form higher alkenes were investigated. Products were analyzed using nuclear magnetic resonance spectroscopy, gas chromatography, and mass spectrometry. Bromination favored the formation of alkyl bromides on internal carbons of the model long chain alkanes over terminal sites. At lower temperatures (∼90 °C), ultraviolet photochemical initiation of bromination was necessary, while above 105 °C, thermally initiated bromination was observed. Photochemical bromination was found to be inhibited by water and oxygen. The formation of alkenes by the dehydrobromination of the alkyl bromides was observed by temperature-programmed reaction to occur at higher temperatures for n-dodecane (∼190 °C) than for the mixed alkanes in paraffin wax (∼175 °C).

Versatility of Pyrylium Salt/Vinyl Ether Initiating System for Epoxide Dual-Cure Polymerization: Kick-Starting Effect of the Coinitiator.

Pyrylium salts combined with vinyl ethers are shown to act as new versatile dual-cure initiating systems for both photochemical and thermal initiation of oxirane monomers. The combination of both possibilities allows the curing of thick samples through photoinduced frontal polymerization. On the basis of quantum calculations and photochemical experiments, some clues are given about the reaction mechanisms involved. Interestingly, a sequential kick-starting effect is observed in the presence of vinyl ether enabling the curing of oxetane monomers. Thereby, this communication presents a short overview of potential of pyrylium salts in cationic polymerization of oxiranes.

A highly selective protein adsorber via two-step surface-initiated molecular imprinting utilizing a multi-functional polymeric scaffold on a macroporous cellulose membrane

Protein-imprinted cellulose membranes with tailored binding selectivity have been prepared by two-step surface grafting based on an orthogonal photochemical initiation.

Окисление пероксидом водорода как эффективный способ повышения реакционной способности ПЭТФ трековых мембран в реакциях фотоинициированной полимеризации

In this article, we report on functionalization of track-etched membrane based on poly(ethylene terephthalate) (PET TeMs) oxidized by advanced oxidation systems and by grafting of acrylic acid using photochemical initiation technique for the purpose of increasing functionality thus expanding its practical application. Among advanced oxidation processes (H2O2/UV) system had been chosen to introduce maximum concentration of carboxylic acid groups. Benzophenone (BP) photo-initiator was first immobilized on the surfaces of cylindrical pores which were later filled with aq. acrylic acid solution. UV-irradiation from both sides of PET TeMs has led to the formation of grafted poly(acrylic acid) (PAA) chains inside the membrane nanochannels. Effect of oxygen-rich surface of PET TeMs on BP adsorption and subsequent process of photo-induced graft polymerization of acrylic acid (AA) were studied by ESR. The surface of oxidized and AA grafted PET TeMs was characterized by UV-vis, ATR-FTIR, XPS spectroscopies and by SEM.

Chromatographic efficiency comparison of polyhedral oligomeric silsesquioxanes-containing hybrid monoliths via photo- and thermally-initiated free-radical polymerization in capillary liquid chromatography for small molecules

Monolithic poly(methacrylate epoxy cyclosiloxane-co-polyhedral oligomeric silsesquioxanes) (epoxy-MA–POSS) capillary columns have been prepared via either photo- or thermally-initiated polymerization of the corresponding monomers using a 1-propanol/PEG 400 mixture as porogens. Photochemical polymerization was accomplished by irradiation of the UV-transparent capillary for 10min at room temperature, while thermal polymerization was performed at 55°C, 60°C or 65°C for 18h. The evaluation of chromatographic property for two hybrid epoxy-MA–POSS monoliths was carried out. The results indicate that hybrid monoliths fabricated by photochemical initiation exhibit higher column efficiency (97,000–98,400plates/m) than those synthesized by thermal polymerization (41,100–48,000plates/m) in cLC. The higher efficiency of photo-initiated hybrid monoliths is closely related to lower eddy dispersion (A-term) and mass transfer resistance (C-term).