Norrish Reactions Research Articles

UV LED ageing of polymers for PV cell encapsulation

Encapsulation polymers in terrestrial solar modules degrade due to ultraviolet radiation from the sun. To assess a polymer’s durability under UV light, accelerated aging tests can be conducted. A new LEDs device allows us to investigate the effects of temperature, irradiation, and UV source spectrum on the photooxidation mechanism and kinetics of two polyethylene-based commercial encapsulants, differentiated by the presence or absence of UV absorbers. The photooxidation rate of the polymer matrix increases as the temperature and irradiance increase between 62 and 82 °C, and 12 and 28 W.m−2, respectively. In the last case, the photooxidation rate is not proportional to the number of photons. Finally, we observed different distributions of degradation products under UVB radiation at 305 nm compared to those under UVA radiation at 365 nm. UVB photons enable Norrish reactions that are not possible with UVA alone. Special care is needed to maintain a balance between UVA and UVB photons to ensure representative durability tests. With a few adjustments to their emission spectrum, UV LED devices appear to be good candidates for accelerated aging of encapsulation polymers.

Continuous Flow Synthesis of Benzotriazin-4(3H)-ones via Visible Light Mediated Nitrogen-Centered Norrish Reaction.

We report a new protocol for the synthesis of substituted benzotriazin-4(3H)-ones which are underrepresented heterocyclic scaffolds with important pharmacological properties. Our method exploits acyclic aryl triazine precursors that undergo a photocyclization reaction upon exposure to violet light (420 nm). Continuous flow reactor technology is exploited to afford excellent yields in only 10 min residence time with no additives or photocatalysts needed. The underlying reaction mechanism appears to be based on an unprecedented variation of the classical Norrish type II reaction with concomitant fragmentation and formation of N-N bonds. Scalability, process robustness, and green credentials of this intriguing transformation are highlighted.

Unveiling microplastic spectral signatures under weathering and digestive environments through shortwave infrared hyperspectral sensing

Microplastic (MP) pollution presents a novel challenge for marine environmental protection, necessitating comprehensive and long-term monitoring and assessment approaches. Environmental MPs can undergo weathering and microorganism-related digestive processes, altering their original surface properties and chemical structure, thus complicating their quantification and identification. This study aims to establish a comprehensive hyperspectral database for weathered and digestion-degraded MPs, using a wide variety of polymer types collected as either virgin particles or commercial products (within a size range of approximately 3 mm), and to investigate the impact of these processes on their spectral characteristics. Polypropylene (PP) and polyethylene (PE) MPs exhibited significant responses to weathering treatment, as indicated by the formation of new characteristic peaks or slight peak shifts around 1679–1705 nm, which can be attributed to the formation of carbonyl and vinyl functional groups through Norrish reactions. Similarly, polyethylene terephthalate (PET), acrylonitrile butadiene styrene (ABS), and polystyrene (PS) MPs demonstrated notable degradation following digestive treatment, as evidenced by the emergence of new absorption peaks at approximately 1135–1165 nm, possibly associated with alterations involving carbonyl and vinyl functional groups. The results were further validated based on their comparable spectral characteristics of the resultant MPs to reference polymers and possible additives, considering a reasonably accurate match of approximately 80% for the studied MP samples. This study showcases the significant advantage of using shortwave infrared hyperspectral sensing for rapid identification of virgin and exposed MPs with a relatively large scan area after a simple sample preparation. This approach, combined with other complementary characterization techniques, shall provide highly throughput results for MPs identification. This research provides valuable insights into the features extracted from environmental MPs and establishes a foundation for improving their classification efficiency for environmental applications.

Wavelength-dependence of Norrish type I cleavage reactions in carbonyl groups of dammar resin triterpenoids and its implications for accelerated aging of organic materials

Norrish Type I cleavage reactions of carbonyl groups in dammar triterpenoids were studied using different wavelengths of UV exposure. Films of dammar resin were exposed to xenon arc irradiance, using longpass filters with varying cut-on wavelengths. The results were studied using gas chromatography mass spectrometry of the triterpenoid fraction. It was shown that α-cleavage of carbonyl groups, leading to formation of A-seco compounds, takes place at wavelengths present in daylight filtered by window glass, thus validating the aging method for the study of the stability of materials during indoor exposure. Claims that xenon arc exposure leads to abnormal degradation appear incorrect. By eliminating UV radiation entirely below 400 nm, Norrish reactions are inhibited. These results are of particular relevance in accelerated aging studies in heritage science but also in other fields.

Wavelength effect on polymer photooxidation under LED weathering conditions

This paper is devoted to the investigation of the photooxidation of poly(N-vinylcarbazole (PVK) and polyethylene (LDPE) by exposure to λ ≥ 300 nm polychromatic light (SEPAP 12.24) or UV light produced by a new setup equipped with LEDs emitting at 365 nm (LED 365), both at a temperature of 60°C. Photooxidation was studied by infrared and UV-visible spectroscopies and emission spectroscopy in the case of PVK. The mechanisms by which the oxidation products were formed in both polymers were recalled. Depending on the conditions of ageing, i.e., polychromatic light or exposure to LED 365, the same oxidation products were obtained but with different rates and different relative amounts. This result reflected the influence of the wavelengths on the mechanisms. In the case of PVK, the formation of oxidation photoproducts and yellowing were not impacted, but the kinetics of the decrease in excimer fluorescence, which is linked to the crosslinking of the polymer, were dramatically modified. In the case of LDPE, wavelength effects on the photolysis of ketones by Norrish reactions were observed. As a consequence, irradiations with the LED 365 setup could not be claimed to be representative of polychromatic irradiations because the shortest wavelengths (300 nm < λ < 350 nm) were missing and could not reproduce natural outdoor conditions of weathering.

A vinylogous Norrish reaction as a strategy for light-mediated ring expansion

Photo-mediated ring expansion using UV-A light. 22 examples, 75–96% yield. The mechanism was investigated by DFT and Transient Absorption Spectroscopy.

Applications of Norrish type I and II reactions in the total synthesis of natural products: a review.

Natural products and their analogue have played a key role in the drug discovery and development process. In the laboratory, the total synthesis of secondary metabolites is very useful in ascertaining the hypothetical complex structure of molecules of natural origin. Total synthesis of natural products using Norrish type I and II reactions as a crucial step has been explored in this overview. Norrish reactions are important photo-induced transformations of carbonyl compounds in organic synthetic chemistry and are connected in numerous industrially and biologically relevant procedures and the processing of carbonyl compounds in the atmosphere. The present review tries to focus on the brilliant applications of Norrish type I and II photochemical reactions as a key step in the total synthesis of natural products and highlights on natural sources, structures, and biological activities of the promising natural products for the first time elegantly.

Use of fluorescent probes in supporting functional group analysis resulting from polymer ageing

Based on the determination of the solvatochromic parameters of a series of polymers with various functional groups containing oxygen atoms, a method was proposed which allowed for discriminating the oxidation products formed by exposure of polymers to photo- and/or thermo-ageing. The polymers used to determine the solvatochromic parameters contained polar groups such as chain-pendant ester (EVA, PMMA, PVAc), chain ester (PLA), alcohol (EVOH, PVA), chain-pendant acid (PAA), ketone (PK), ether (PEO). The solvatochromic parameters π∗, α and β were determined following procedures reported in the literature, and it was shown that the most apolar polymer PE, displays the lowest π∗ value; the presence of acetate groups in PE makes the π∗ value increase. Polyesters (PLA, PMMA, PVAc) have similar π∗ values. Then, alcohols and ether groups (EVOH, PVA, PEO) have higher π∗ values. Finally, the highest π∗ values are obtained for PAA and PK. Concerning the β value, the highest value was obtained for (PE, EVOH, PEO), which have the highest capacity to accept a proton, and are close to polyethylene in terms of chemical structure. On the contrary, the polymers with the lowest β value are the ones which have acid/ester/ketone functions (PK, PAA, PLA), reflecting their lowest capacity to accept a proton. This method was successfully applied to discriminate the oxidation products formed in thermo- and photo-oxidation of polyethylene which are known to be slightly different due to the Norrish reactions that do not occur in thermooxidation.

Exploring Norrish type I and type II reactions: an ab initio mechanistic study highlighting singlet-state mediated chemistry.

Norrish reactions are important photo-induced reactions in mainstream organic chemistry and are implicated in many industrially and biologically relevant processes and in the processing of carbonyl molecules in the atmosphere. The present study reports multi-reference electronic structure calculations designed to assess details of the potential energy profiles associated with the Norrish type-I and type-II reactions of a prototypical ketone 5-methyl-hexan-2-one. We show that the well-established 'triplet state mediated' reaction pathways following initial population of a singlet excited state can be complemented by (hitherto rarely recognized) 'singlet state only' Norrish type-I and type-II reaction mechanisms that involve no spin-forbidden transitions along the respective reaction paths, and suggest how the efficiencies of such reactions might be affected by strategic substitutions at selected sites within the parent ketone.

Selective surface modification of TiO2-coated polypropylene by photodegradation

It is known that TiO2 coating of polymers protects against UVC light, despite degrading when under UVB or UVA light. We studied carefully polypropylene coated with TiO2 under UVC light, and we found the formation of hydroxyls and no formation of carbonyls, being this technique eligible for selective modification of polypropylene surfaces with hydroxyls. Besides that, we also found the formation of insaturation between carbons produced by the Norrish reaction in pristine polypropylene, thanks to a simple experimental technique proposed in this paper based on FTIR spectral subtraction. The samples were thermal-pressed to simulate a thermoforming processing and verify whether there was any influence on the photostabilization provided by TiO2 coating.

Alternating Sulfone Copolymers Depolymerize in Response to Both Chemical and Mechanical Stimuli.

This work describes the depolymerization of poly(vinyl acetate-alt-sulfur dioxide) (PVAS) as initiated by chemical and mechanical stimuli. In recent years, macromolecules that are able to depolymerize in response to specific stimuli have been highly sought because of their ability to amplify signal for sensing and drug delivery. Examples include self-immolative polymers from alkoxyphenol derivatives and polyaldehydes. We show here that alternating copolymers of sulfur dioxide and vinyl acetate are able to undergo similar depolymerization into their monomer components in response to various chemical and mechanical stimuli. Certain vinyl monomers such as vinyl acetate are able to polymerize with sulfur dioxide in a perfectly alternating manner, and the resulting copolymer possesses a low ceiling temperature. We show that this polymer is able to break down into its monomer components when subjected to UV/acetone, various Reactive Oxygen Species (ROS), and ultrasonication. In the case of UV, the acetone reacted via a Norrish reaction to produce free radicals that caused clean monomer production. For ROS, the polymer showed reactivity to both oxidizing and radical-containing ROS. Through kinetic studies, these polymers were shown to proceed via a two-part, first-order kinetic model with a fast initiation phase and a slow depolymerization phase. Finally, the polymers were subjected to probe ultrasonication, and depolymerization occurred as well. Most tellingly, the polymer again showed a fast initiation step and continued to depolymerize even after ultrasonication stopped. This class of polymers shows potential for drug delivery in response to both endogenous chemical and externally-applied mechanical cues.

Photo- and thermal-oxidation of polyethylene: Comparison of mechanisms and influence of unsaturation content

The behaviour of polyethylene with different contents in vinyl and t-vinylene groups have been studied by photooxidation with λ ≥ 300 nm light or by thermooxidation at a temperature of 100 °C. The oxidation was studied by infrared spectroscopy and it was shown that the same oxidation products were obtained, but with different relative concentrations depending on the conditions of ageing, i.e. photochemical or thermal conditions. The mechanisms by which the oxidation products are formed were recalled. The differences between photo- and thermo-oxidation were evidenced on the basis of the stability of ketones that do not accumulate in photochemical conditions, as a result of Norrish reactions. The influence of the initial amount of unsaturated groups on the rates of oxidation was characterized. It was shown that the concentration of unsaturations had no effect on the rate of photooxidation but dramatically influenced the stability in thermooxidative conditions.

Dependence of ultraviolet absorbers' performance on ultraviolet wavelength

So far, ultraviolet absorbers (UVA) have been discussed mainly on the basis of the extinction coefficient ( ɛ), and the light degradation of polymeric materials is explained by photolysis of hydroperoxides and Norrish reactions of ketones. This study focuses on the photo-antioxidant ability as characteristic which UVA must possess for the light stabilization, and examines the dependence of the photo-initiation by oxidation products on UV wavelength range. As a result, it has been made clear that UV rays ranging 290–320 nm trigger photo-oxidation (degradation) by photolysis of hydroperoxides and Norrish reactions of ketones, and α-diketone derived from further oxidation of the ketones, for example, initiates photo-oxidation at a surprisingly higher velocity under UV rays above 350 nm. UV rays in the range of 320–350 nm participate in the photo-initiation by an enone ( C C CO ) structure in addition to the above-mentioned oxidation products, in which the initiation velocity is the medium between the velocities for the above two wavelength ranges. That is to say, the longer the UV rays wavelength range, the faster the photo-initiation velocity. Finally, it is concluded that excellent UVA are judged based on λ max rather than ɛ, and that UVA absorbing longer wavelength UV rays can more control the photo-oxidation.

Femtochemistry of Norrish Type-I Reactions: IV. Highly Excited Ketones—Experimental

Femtosecond dynamics of Norrish type-I reactions of cyclic and acyclic ketones have been investigated in real time for a series of 13 compounds using femtosecond-resolved time-of-flight mass spectrometry. A general physical description of the ultrafast processes of ketones excited into a high-lying Rydberg state is presented. It accounts not only for the results that are presented herein but also for the results of previously reported studies. For highly excited ketones, we show that the Norrish type-I reaction is nonconcerted, and that the first bond breakage occurs along the effectively repulsive S2 surface involving the C-C bond in a manner which is similar to that of ketones in the S1 state (E. W.-G. Diau et al. ChemPhysChem 2001, 2, 273-293). The experimental results show that the wave packet motion out of the initial Franck-Condon region and down to the S2 state can be resolved. This femtosecond (fs) internal conversion from the highly excited Rydberg state to the S2 state proceeds through conical intersections (Rydberg-valence) that are accessed through the C=O stretching motion. In one of these conical intersections, the internal energy is guided into an asymmetric stretching mode. This explains the previously reported pronounced nonstatistical nature of the reaction. The second bond breakage involves an excited-state acyl radical and occurs on a time scale that is up to one order of magnitude longer than the first. We discuss the details regarding the ion chemistry, which determines the appearance of the mass spectra that arise from ionization on the fs time scale. The experimental results presented here, aided by the theoretical work reported in paper III, provide a unified picture of Norrish reactions on excited states and on the ground-state potential energy surfaces.

Femtochemistry of Norrish type-I reactions: IV. Highly excited ketones--experimental.

Femtosecond dynamics of Norrish type-I reactions of cyclic and acyclic ketones have been investigated in real time for a series of 13 compounds using femtosecond-resolved time-of-flight mass spectrometry. A general physical description of the ultrafast processes of ketones excited into a high-lying Rydberg state is presented. It accounts not only for the results that are presented herein but also for the results of previously reported studies. For highly excited ketones, we show that the Norrish type-I reaction is nonconcerted, and that the first bond breakage occurs along the effectively repulsive S2 surface involving the C-C bond in a manner which is similar to that of ketones in the S1 state (E. W.-G. Diau et al. ChemPhysChem 2001, 2, 273-293). The experimental results show that the wave packet motion out of the initial Franck-Condon region and down to the S2 state can be resolved. This femtosecond (fs) internal conversion from the highly excited Rydberg state to the S2 state proceeds through conical intersections (Rydberg-valence) that are accessed through the C=O stretching motion. In one of these conical intersections, the internal energy is guided into an asymmetric stretching mode. This explains the previously reported pronounced nonstatistical nature of the reaction. The second bond breakage involves an excited-state acyl radical and occurs on a time scale that is up to one order of magnitude longer than the first. We discuss the details regarding the ion chemistry, which determines the appearance of the mass spectra that arise from ionization on the fs time scale. The experimental results presented here, aided by the theoretical work reported in paper III, provide a unified picture of Norrish reactions on excited states and on the ground-state potential energy surfaces.

Femtosecond elementary dynamics of transition states and asymmetric α-cleavage in Norrish reactions

In this Letter, we report studies of the femtosecond dynamics of transition states and asymmetric α-cleavage in Norrish type-I reactions. Systematically, we consider the dynamics of dissociation along two reaction coordinates in methyl ethyl ketone and diethyl ketone, and compare these results with those of dimethyl ketone and methyl chloro ketone. The concept of concertedness as the system moves away from the transition-state region is examined and the primary and secondary processes involved in the α-cleavage are resolved. We compare experimental results with theoretical predictions of impulsive and statistical models.