Photochemical Mechanisms Research Articles

Mechanism of Dinitrogen Photoactivation by P2PPhFe Complexes: Thermodynamic and Kinetic Computational Studies.

The P2PPhFe(N2)(H)2 catalyst showed a significant ammonia yield under light irradiation. However, under thermal conditions, the hydrogen evolution reaction (HER) is favored over the nitrogen reduction reaction (N2RR), making P2PPhFe(N2)(H)2 an ideal system for studying the competition between both reactions. In this study, we used a series of computational tools to elucidate the photochemical reaction mechanism for the N2RR and thermal pathways leading to the HER with this catalyst. We calculated the energy profile for each transformation and estimated the rate constants for each step. Our results, which are consistent with experimental observations, indicate that photoinduced H2 elimination from P2PPhFe(N2)(H)2 promotes the formation of P2PPhFe(N2)2, which is on-path for N2RR. However, this elimination process is kinetically hindered due to high-energy barriers. Furthermore, our calculations reveal enhanced dinitrogen activation upon the conversion of P2PPhFe(N2)(H)2 to P2PPhFe(N2)2.

Ultrafast transient absorption spectra and kinetics of human blue cone visual pigment at room temperature

The ultrafast photochemical reaction mechanism, transient spectra, and transition kinetics of the human blue cone visual pigment have been recorded at room temperature. Ultrafast time-resolved absorption spectroscopy revealed the progressive formation and decay of several metastable photo-intermediates, corresponding to the Batho to Meta-II photo-intermediates previously observed with bovine rhodopsin and human green cone opsin, on the picosecond to millisecond timescales following pulsed excitation. The experimental data reveal several interesting similarities and differences between the photobleaching sequences of bovine rhodopsin, human green cone opsin, and human blue cone opsin. While Meta-II formation kinetics are comparable between bovine rhodopsin and blue cone opsin, the transition kinetics of earlier photo-intermediates and qualitative characteristics of the Meta-I to Meta-II transition are more similar for blue cone opsin and green cone opsin. Additionally, the blue cone photo-intermediate spectra exhibit a high degree of overlap with uniquely small spectral shifts. The observed variation in Meta-II formation kinetics between rod and cone visual pigments is explained based on key structural differences.

Light‐Driven Site‐Selective Glycosylation of Native Carbohydrates

Carbohydrates constitute the largest source of biomass on Earth, but their synthetic modification is highly challenging due to their high content of oxygen functionalities. The site‐ and stereoselective modification of native sugars is a definitive goal of glycochemistry research. Recent efforts to bypass the need for protecting groups, leveraging selective activation through photochemical mechanisms for site‐selective C‐C bond formation from native sugars, are likely to largely impact all glycochemistry‐related areas. Davis, Koh, and co‐workers have recently presented their use of photocatalysis to develop a “cap and glycosylate” approach for the site‐ and stereoselective C‐glycosylation of native sugars. The modernization of a direct radical functionalization of in situ formed thioglycoside using photocatalysis was used in the synthetic manipulation of unprotected carbohydrates. This allowed reaching complex saccharides, and post‐translational modification of proteins.

Light-Driven Site-Selective Glycosylation of Native Carbohydrates.

Carbohydrates constitute the largest source of biomass on Earth, but their synthetic modification is highly challenging due to their high content of oxygen functionalities. The site- and stereoselective modification of native sugars is a definitive goal of glycochemistry research. Recent efforts to bypass the need for protecting groups, leveraging selective activation through photochemical mechanisms for site-selective C-C bond formation from native sugars, are likely to largely impact all glycochemistry-related areas. Davis, Koh, and co-workers have recently presented their use of photocatalysis to develop a "cap and glycosylate" approach for the site- and stereoselective C-glycosylation of native sugars. The modernization of a direct radical functionalization of in situ formed thioglycoside using photocatalysis was used in the synthetic manipulation of unprotected carbohydrates. This allowed reaching complex saccharides, and post-translational modification of proteins.

Evaluation of photostability, cellular uptake, photochemical mechanism, and photodynamic efficacy of rose bengal-incorporated mesoporous silica nanoparticles

Evaluation of photostability, cellular uptake, photochemical mechanism, and photodynamic efficacy of rose bengal-incorporated mesoporous silica nanoparticles

Impact of Lockdowns on Air Pollution: Case Studies of Two Periods in 2022 in Guangzhou, China

The photochemical mechanisms of ozone (O3) formation are complex, and simply reducing nitrogen oxide (NOx) emissions is insufficient to reduce O3 concentrations. The lockdown due to the Coronavirus Disease 2019 (COVID-19) pandemic provided a rare opportunity to explore the mechanisms of O3 formation and evaluate the performance of NOx emission control strategies through practical observations. This study integrates data from ground stations with observations from the TROPOMI sensor on the Sentinel-5P satellite to analyze air quality changes during the two one-month lockdown periods in Guangzhou, China, in March and November 2022. Our analysis particularly focuses on the impact of these lockdowns on O3 and NO2 concentrations, along with shifts in the sensitivity of ozone formation. Furthermore, we have assessed concentration changes of four major pollutants: PM2.5, PM10, SO2, and CO. The results show that the average O3 concentration in Guangzhou decreased during the March lockdown, while the average O3 concentration at three stations in the western part of Guangzhou increased during the November lockdown. The western part of Guangzhou is a VOCs (volatile organic compounds)-limited zone, and the NO2 emission reduction from the lockdown reduced the titration effect on O3, which led to the increase in O3 concentration. Overall, the impact of COVID-19 lockdowns on O3 concentrations depended on the local O3 producing sensitive system, and emissions of other major pollutants were reduced substantially, as reported in many other cities around the world.

Regional source contributions to summertime ozone in the Yangtze River Delta

Ozone (O3) pollution is increasing in the Yangtze River Delta (YRD), with significant influences from regional transport during pollution events. However, the specific contributions of different regions remain imprecisely quantified. This study estimated the regional contributions of eight regions to summer O3 in the YRD using the Community Multiscale Air Quality (CMAQ) model with a source-tagged photochemical mechanism. Non-background O3 is attributed to nitrogen oxides (NOX) and volatile organic compounds (VOCs) from different regions. Background O3 is the predominant contributor with average ratios of 75.0%, 65.3%, 64.8% and 63.0% for Shanghai, Nanjing, Hangzhou and the entire YRD, respectively. Locally formed O3 are 39.9, 42.0, 39.6, and 36.9 parts per billion (ppb) by volume in the above areas. North Zhejiang and south Jiangsu are the primary sources of heavy pollution episodes as the maximum daily 8-h average (MDA8) O3 concentrations in these regions increase the most. NOX is the predominant contributor to heavy pollution episodes with the maximum increment attributed to NOX (O3N) of 14.3 ppb, while VOCs only contribute to 2.1 ppb (O3V) in Jiangsu. Relative humidity is crucial in heavy pollution episodes while high temperature, low planetary boundary layer (PBL) height and the wind field are associated with regional transport. Diurnal variation underscores the importance of understanding O3 formation in the afternoon (12:00–16:00), which is essential for devising effective mitigation policies.

Photochemical Mechanism of Co-C Bond Activation via Triplet Energy Transfer in Ethyl(aqua)cobaloxime.

The photochemical decomposition of ethyl(aqua)cobaloxime, [EtCoIII(dmgH)2H2O], a vitamin B12 derivative model complex, was investigated to understand the mechanism of the Co-CEt bond scission induced by light. Upon irradiation of [EtCoIII(dmgH)2H2O], the Co-CEt bond undergoes homolytic scission, resulting in Et/Co(II) radical pair (RP) formation in a similar fashion observed in alkylcobalamins. The [EtCoIII(dmgH)2H2O] complex acts as a potent quencher of a wide variety of excited states in the presence of organic molecules such as benzophenone. It has been proposed that the reaction mechanism involves Dexter energy transfer, resulting in the bond dissociation process. Two issues associated with the proposed mechanism have been investigated, namely, (i) how the energy transfer occurs from benzophenone to cobaloxime and (ii) how the Co-CEt bond is activated and cleaved. Both TD-DFT and CASSCF/NEVPT2 methods have been applied to show the feasibility of the energy transfer reaction via the triplet pathway from photocatalyst to substrate.

Structural, optical spectroscopic, and density functional theory calculations of squaraine dye for organic electronic applications

The structural, optical spectroscopic, and density functional theory calculations of 2,4-Bis[4-(N,N-diisobutylamino)−2,6-dihydroxyphenyl] squaraine dye (SQ) are promising investigations that can be useful in photochemical mechanisms explanation and in designing promising chemosensors. The nature of the bonds, the chemical composition of SQ, crystallographic structure, morphology, and quality characteristics were explored by FTIR, XRD, and SEM investigations. The crystallite size, the dislocation density, and the lattice strain were found to be 57.3 nm, 3.045 × 10−4 nm−2, and 3.065 × 10−3, respectively. The spectrum behavior of the absorbance and molar absorption coefficient spectra of SQ in DCM with three concentrations were analyzed to get some important optical properties of SQ dye. The determined optical gap transition equals Egapopt=1.81eV. Quantum chemical computations and optimized molecular geometries identified where electrophilic attacks are likely and analyzed the molecule’s electrostatic potential. Using DFT calculations at the B3LYP/6-31+G(d,p) level, the electronic structure of the squaraine molecule was confirmed, showing a HOMO–LUMO gap of 1.73 eV. MEP analysis indicated that the oxygen atoms are electron-rich, suggesting potential for hydrogen bonding and electrophilic interactions.

Photocatalytic degradation of non-steroidal anti-inflammatory drugs with ketoprofen as model compound. Intermediates and total reaction mechanism

Ketoprofen KET and other non-steroidal anti-inflammatory drugs NSAIDs are extensively used throughout the world to reduce pain, fever, and inflammation. As a result of this, they have been detected in environmental waters and represent a potential health risk. There are several reports about KET photocatalytic degradation but most of them deal with experimental conditions or catalytic materials to perform this process. In this study, the mechanisms and intermediates involved in KET photocatalysis with TiO2 in aqueous media were investigated. First, the mineralization rate was assessed by TOC measurements. The formation and eventual degradation of intermediate organic compounds were investigated by HPLC, UV–Vis, IR, 1H NMR and HPLC-MS studies. TOC analysis indicate that KET is quickly transformed into other compounds that eventually are degradated and 70 % mineralization was achieved after five hours of irradiation. UV–Vis and HPLC studies indicate that KET is transformed into some other aromatic compounds within minutes. IR studies demonstrate the conversion of KET into several aromatic compounds which in turn degradate into low molecular saturated and unsaturated acids. 1H NMR studies indicate KET is transformed into several aromatic compounds such as 3-hydroxy-benzophenone, phenol, 1,4-hydroquinone, 1,2,4-benzenetriol, and catechol. HPLC-MS studies indicate KET is degradated by several photochemical and photocatalytic parallel mechanisms. Based on this and previous studies, a unified and complete mechanism for KET photocatalytic degradation is presented.

Phenothiazine-carbazole-based bis oxime esters (PCBOEs) for visible light polymerization

In this study, a series of seven PCBOEs was innovatively designed and successfully synthesized for the first time by bridging a phenothiazine and a carbazole unit by mean of an acetylene spacer and bearing oxime ester groups as end groups on both sides. Structures of the different difunctional oxime esters were confirmed through NMR and HRMS. All PCBOEs exhibited excellent light absorption properties in the visible range. Upon excitation with a 405 nm LED for 60 s, a complete photolysis of the oxime esters could be achieved. Parameters such as the singlet state energy and the triplet state energy demonstrated the facile generation of the corresponding radicals upon light absorption by PCBOEs. Photopolymerization kinetics in TMPTA confirmed the homolytic cleavage of oxime esters and the generation of radicals. CO2 absorption peaks were detected when the PCBOEs/TMPTA systems were excited with a 405 nm LED. Based on the positive results, the photochemical mechanisms of polymerization with PCBOEs was proposed. PCBOE3 was identified as the best candidate of the series and this bifunctional oxime ester was successfully used for the 3D-printing of the “IS2M” logo via DLW. Experimental results from DSC also confirmed that PCBOEs not only serve as photoinitiators but also possess high reactivity as thermal initiators.

Room-temperature solution processed silicon nanocluster: Theory prediction and experiment achievement

We have presented in this work that silicon nanoclusters (Si NCs) embedded in silicon nitride (▪) can be synthesized by vacuum ultra-violet irradiation of perhydropolysilazane (PHPS) at room temperature. The formation of Si NCs is first predicted by reactive molecular dynamics (ReaxFF-MD) simulations based on a self-developed Si/N/H force field. Then the existence of Si NCs is further verified by photoluminescence (PL) spectroscopy and Raman spectroscopy characterization. Additionally, the photochemical conversion mechanism of PHPS is examined. Moreover, it has been found based on Fourier transform infrared (FT-IR) measurements that ▪ samples prepared with an intermediate irradiation time are the most readily to be oxidized, which can be attributed to the high concentration of the active group -▪, being consistent with the reaction mechanism. We have demonstrated that ReaxFF-MD simulation can be a powerful complement to experimental investigations in revealing the conversion mechanism of polymer precursor to ceramics at the atomic level.

Effects of heat waves on ozone pollution in a coastal industrial city: Meteorological impacts and photochemical mechanisms

Surface ozone (O3) pollution triggered by extreme weather conditions is attracting increasing attention. Heat waves in summer of 2022 were chosen to explore the photochemical and meteorological impacts on O3 formation in the southeastern coastal industrial city of Quanzhou in China. The machine-learning (ML)-based weather normalization showed that the de-weathered O3 concentrations (increased by 6.69 μg m−3) in 2022 were higher than those from 2015 to 2021. Temperature is the most important variable (33.5%), followed by solar radiation (23.5%) and RH (10.1%), differing from the O3 pollution in inland cities. The Observation-Based Model (OBM) analysis results showed that hydroxyl radical (OH) was the predominant oxidant for daytime atmospheric oxidation capacity (AOC). And oxygenated volatile organic compounds (OVOCs), NO2, and CO were the dominant contributors to OH reactivity, accelerating the recycling of ROx radicals and O3 formation. The daytime reaction rate of HO2+NO during the O3 pollution episodes was 20.0 ppb h−1, accounting for 65% of the total O3 production. The contribution of RO2+NO to the O3 production enhanced the possibility of the MDA8h O3 exceeding the Air Quality Standard of China. This study improves the understanding of O3 formation mechanisms in a coastal industrial city during heat waves, and the elevated contributions of meteorological conditions to O3 pollution become more challenge for the reduction of anthropogenic emissions controll.

Elucidating and Optimizing the Photochemical Mechanism of Coumarin-Caged Tertiary Amines.

Photoactivatable or "caged" pharmacological agents combine the high spatiotemporal specificity of light application with the molecular specificity of drugs. A key factor in all optopharmacology experiments is the mechanism of uncaging, which dictates the photochemical quantum yield and determines the byproducts produced by the light-driven chemical reaction. In previous work, we demonstrated that coumarin-based photolabile groups could be used to cage tertiary amine drugs as quaternary ammonium salts. Although stable, water-soluble, and useful for experiments in brain tissue, these first-generation compounds exhibit relatively low uncaging quantum yield (Φu < 1%) and release the toxic byproduct formaldehyde upon photolysis. Here, we elucidate the photochemical mechanisms of coumarin-caged tertiary amines and then optimize the major pathway using chemical modification. We discovered that the combination of 3,3-dicarboxyazetidine and bromine substituents shift the mechanism of release to heterolysis, eliminating the formaldehyde byproduct and giving photolabile tertiary amine drugs with Φu > 20%─a 35-fold increase in uncaging efficiency. This new "ABC" cage allows synthesis of improved photoactivatable derivatives of escitalopram and nicotine along with a novel caged agonist of the oxytocin receptor.

Resolving the overlooked photochemical nitrophenol transformation mechanism induced by nonradical species under visible light

Nitrophenols present on the surface of particulates are ubiquitous in the atmosphere. However, its atmospheric photochemical transformation pathway remains unknown, for which the crucial effect of visible light is largely overlooked, resulting in an incomplete understanding of the effects of nitrophenols in the atmospheric environment. This study delves into the photolysis mechanism of 4-nitrophenol (4NP), one of the most abundant atmospheric nitrophenol compounds, on the surface of photoactive particulates under visible light irradiation. Unexpectedly, the nonradical species (singlet oxygen, 1O2) was identified as a dominant factor in driving the visible photolysis of 4NP. The pathways of HONO and p-benzoquinone (C6H4O2) generation were clarified by acquiring direct evidence of C-N and O-H bond breakage in the nitro (-NO2) and hydroxyl (-OH) groups of 4NP. The further decomposition of HONO results in the generation of NO and hydroxyl radicals, which could directly contribute to atmospheric oxidizing capacity and complicate the PM2.5 composition. Significantly, the behavior of 1O2-induced visible photolysis of 4NP was universal on the surface of common particulates in the atmosphere, such as A1 dust and Fe2O3. This work advances the understanding of the photochemical transformation mechanism of particulate-phase atmospheric nitrophenols, which is indispensable in elucidating the role of nitrophenols in atmospheric chemistry.

Design, Synthesis, and Photochemical Properties of Gene‐directed Caged RyR Probes for Photopharmacological Studies

AbstractRyanodine receptor (RyR) is a crucial intracellular Ca2+ channel involved in various physiological processes associated with several diseases. This paper presents the synthesis and evaluation of novel photoactivatable caged compounds for the RyR. We used (6‐bromo‐7‐hydroxycoumarn‐4‐yl)methyl (Bhc) as the photolabile protecting group to develop caged versions of 4‐CmC, a broad‐spectrum RyR agonist, and FLA365, an antagonist. The synthesized compounds, Bhcmoc‐4‐CmC and Bhcmoc‐FLA365, exhibited photoreactivity that enabled spatiotemporal control over the RyR activity. Bhcmoc‐4‐CmC presents a quantum yield of 25 % and a photolysis efficiency of 1,075 M−1 cm−1 under 405 nm light, triggering Ca2+ release from the endoplasmic reticulum. Conversely, Bhcmoc‐FLA365 presents a reduced quantum yield of 0.61 %, which can be attributed to the quenching effects of its tertiary amine structure. We also applied gene‐directed caging, synthesizing β‐galactosidase (β‐Gal) activatable Gal‐Bhcmoc‐4‐CmC and Gal‐Bhcmoc‐FLA365, and porcine liver esterase (PLE)‐activatable CM‐Bhcmoc‐FLA365, to target these probes to the specific cell types. These caged compounds contribute significantly to photopharmacology, providing tools for the precise analysis and manipulation of the RyR functions. Future research objectives involve further optimization and analysis of the photochemical mechanisms of these caged compounds to enhance their applications in biological and medical research.

Theoretical and Experimental Evaluation of the Electronic Relaxation Mechanisms of 2‐Pyrimidinone: The Primary UVA Absorbing Moiety of the DNA and RNA (6‐4) Photolesion

The (6‐4) photolesion is a key photodamage that occurs when two adjacent pyrimidine bases in a DNA strand bond together. To better understand how the absorption of UVB and UVA radiation by the 2‐pyrimidinone moiety in a (6‐4) lesion can damage DNA, it is important to study the electronic deactivation mechanism of its 2‐pyrimidinone chromophore. This study employs theoretical (MS‐CASPT2/cc‐pVDZ level) and experimental (steady state and femtosecond broadband spectroscopic) methods to elucidate the photochemical relaxation mechanisms of 2‐(1H)‐pyrimidinone and 1‐methyl‐2‐(1H)‐pyrimidinone in aqueous solution (pH 7.4). In short, excitation at 320 nm leads to the population of the S11(pp*) state with excess vibrational energy, which relaxes to the S11(pp*) minimum in one picosecond or less. A trifurcation event in the S11(pp*) minimum ensued, leading to radiative and nonradiative decay of the population to the ground state or the population of the long‐lived and reactive T13(pp*) state in hundreds of picoseconds. Collectively, the theoretical and experimental results the idea that in DNA and RNA, the T13(pp*) state of the 2‐pyrimidinone moiety in the (6‐4) lesion can further participate in photosensitized chemical reactions increasing DNA and RNA damage.

Simulation of Regional Secondary Organic Aerosol Formation From Monocyclic Aromatic Hydrocarbons Using a Near‐Explicit Chemical Mechanism Constrained by Chamber Experiments

AbstractThe formation of secondary organic aerosol (SOA) is inextricably linked to the photo‐oxidation of aromatic hydrocarbons. However, models still exhibit biases in representing SOA mass and chemical composition. We implemented a box model coupled with a near‐explicit photochemical mechanism, the Master Chemical Mechanism (MCMv3.3.1), to simulate a series of chamber studies and assess model biases in simulating SOA from representative monocyclic aromatic hydrocarbons, that is, toluene and three xylene isomers (TX SOA). The box model underpredicted SOA yields of toluene and xylenes by 4.7%–100%, which could be improved by adjusting the saturation vapor pressure (SVP) of their oxidation products. After updating the SVP values, the mass concentration of TX SOA in the Yangtze River Delta region during summer doubled, and there was also an approximate 3% enhancement in the total SOA. Compared to a lumped mechanism used for simulating TX SOA, MCM predicted comparable mass concentrations but exhibited different volatility distributions and oxidation states.

Environmental significance of PAH photoproduct formation: TiO2 nanoparticle influence, altered bioavailability, and potential photochemical mechanisms

Interactions between polycyclic aromatic hydrocarbons (PAHs) and titanium dioxide (TiO2) nanoparticles (NPs) can produce unforeseen photoproducts in the aqueous phase. Both PAHs and TiO2-NPs are well-studied and highly persistent environmental pollutants, but the consequences of PAH-TiO2-NP interactions are rarely explored. We investigated PAH photoproduct formation over time for benzo[a]pyrene (BaP), fluoranthene (FLT), and pyrene (PYR) in the presence of ultraviolet A (UVA) using a combination of analytical and computational methods including, identification of PAH photoproducts, assessment of expression profiles for gene indicators of PAH metabolism, and computational evaluation of the reaction mechanisms through which certain photoproducts might be formed. Chemical analyses identified diverse photoproducts, but all PAHs shared a primary photoproduct, 9,10-phenanthraquinone (9,10-PQ), regardless of TiO2-NP presence. The computed reaction mechanisms revealed the roles photodissociation and singlet oxygen chemistry likely play in PAH mediated photochemical processes that result in the congruent production of 9,10-PQ within this study. Our investigation of PAH photoproduct formation has provided substantial evidence of the many, diverse and congruent, photoproducts formed from physicochemically distinct PAHs and how TiO2-NPs influence bioavailability and time-related formation of PAH photoproducts.

Zn-Ti oxo cluster photoresists for EUV Lithography: Cluster structure and lithographic performance

Low reflectivity of multilayer mirror–induced low efficiency of existing extreme ultraviolet lithography (EUVL) has triggered the search for photoresists with heightened sensitivity. For this purpose, considerable research has focused on elements with high extreme ultraviolet (EUV) absorption, leading to considerable advancements. Solubility alteration, often resulting from ligand variations after exposure, is influenced by the cluster structure, markedly impacting lithography sensitivity. Herein, three Ti-based metal clusters, Ti6O4(OEt)8(OMc)8 (T-2), Zn2Ti4O4(OiPr)2(OMc)10 (TZ-1), and Zn4Ti2O2(OAc)2(OBu)2(OMc)10 (TZ-2), are designed and synthesized to elucidate the effects of the structure on lithography. These mixed metal–oxo clusters derived from Ti6O4(OR)8(OOCR)8 and stabilized by identical ligands exhibit structural diversity. The increase in the number of zinc atoms results in marked alterations in the oxo core, thereby triggering alterations in ligand coordination modes. These variations confer metal clusters with different solubility and lithographic properties. Analysis of the photochemical reaction mechanism and theoretical calculations reveal a reactive tridentate ligand coordination mode that considerably improves lithography sensitivity. Therefore, changing the ligand coordination mode by designing a mixed metal–oxo core holds promise for discovering new photoresists for EUV lithography.