Dissolved Black Carbon Facilitates the Photodegradation of Microplastics via Molecular Weight-Dependent Generation of Reactive Intermediates.

Photodegradation of microplastics (MPs) induced by sunlight plays a crucial role in determining their transport, fate, and impacts in aquatic environments. Dissolved black carbon (DBC), originating from pyrolyzed carbon, can potentially mediate the photodegradation of MPs owing to its potent photosensitization capacity. This study examined the impact of pyrolyzed wood derived DBC (5 mg C/L) on the photodegradation of polystyrene (PS) MPs in aquatic solutions under UV radiation. It revealed that the photodegradation of PS MPs primarily occurred at the benzene ring rather than the aliphatic segments due to the fast attack of hydroxyl radical (•OH) and singlet oxygen (1O2) on the benzene ring. The photosensitivity of DBC accelerated the degradation of PS MPs, primarily attributed to the increased production of •OH, 1O2, and triplet-excited state DBC (3DBC*). Notably, DBC-mediated photodegradation was related to its molecular weight (MW) and chemical properties. Low MW DBC (<3 kDa) containing more carbonyl groups generated more •OH and 1O2, accelerating the photodegradation of MPs. Nevertheless, higher aromatic phenols in high MW DBC (>30 kDa) scavenged •OH and generated more O2•-, inhibiting the photodegradation of MPs. Overall, this study offered valuable insights into UV-induced photodegradation of MPs and highlighted potential impacts of DBC on the transformation of MPs.

Role of Direct and Sensitized Photolysis in the Photomineralization of Dissolved Organic Matter and Model Chromophores to Carbon Dioxide.

This study addresses the fundamental processes that drive the photomineralization of dissolved organic matter (DOM) to carbon dioxide (CO2), deconvoluting the role of direct and sensitized photolysis. Here, a suite of DOM isolates and model compounds were exposed to simulated sunlight in the presence of various physical and chemical quenchers to assess the magnitude, rate, and extent of direct and sensitized photomineralization to CO2. Results suggest that CO2 formation occurs in a biphasic kinetic system, with fast production occurring within the first 3 h, followed by slower production thereafter. Notably, phenol model chromophores were the highest CO2 formers and, when conjugated with carboxylic functional groups, exhibited a high efficiency for CO2 formation relative to absorbed light. Simple polycarboxylated aromatic compounds included in this study were shown to be resistant to photomineralization. Quencher results suggest that direct photolysis and excited triplet state sensitization may be largely responsible for CO2 photoproduction in DOM, while singlet oxygen and hydroxyl radical sensitization may play a limited role. After 3 h of irradiation, the CO2 formation rate significantly decreased, and the role of sensitized reactions in CO2 formation increased. Together, the results from this study advance the understanding of the fundamental reactions driving DOM photomineralization to CO2, which is an important part of the global carbon cycle.

The Unimolecular Decomposition Mechanism of Trimethyl Phosphate.

Trimethyl phosphate (TMP), an organophosphorus compound (OPC), is a promising fire-retardant candidate for lithium-ion battery (LIB) electrolytes to mitigate fire spread. This study aims to understand the mechanism of TMP unimolecular thermal decomposition to support the integration of a TMP chemical kinetic model into a LIB electrolyte surrogate model. Reactive intermediates and products of TMP thermal decomposition were experimentally detected using vacuum ultraviolet (VUV) synchrotron radiation and double imaging photoelectron photoion coincidence (i2PEPICO) spectroscopy. Phosphorus-containing intermediates such as PO, HPO and HPO2 were identified. Sampling effects could successfully be obviated thanks to photoion imaging, which also showed evidence for isomerization reactions upon wall collisions in the ionization chamber. Quantum chemical calculations performed for the unimolecular decomposition of TMP revealed for the first time that isomerization channels via hydrogen and methyl transfer (barrier heights of 65.9 and 72.6 kcal/mol, respectively) are the lowest-energy primary steps of TMP decomposition followed by CH3OH/CH3/CH2O or dimethyl ether (DME) production, respectively. We found an analogous DME production channel in the unimolecular decomposition of dimethyl methylphosphonate (DMMP), another important OPC fire-retardant additive with a similar molecular structure to TMP, which are not included in currently available chemical kinetic models.

Vibrational Circular Dichroism of a Chiral Triplet Nitrene Investigated Under Matrix-Isolation Conditions in Parahydrogen.

Vibrational circular dichroism (VCD) spectra of chiral high-spin organic radicals are expected to show a strong intensity enhancement and are thought to be difficult to predict using state-of-the-art theoretical methods. Herein we show that the chiral triplet nitrene obtained from photochemical cleavage of N2 from enantiopure 2-azido-9H-fluorenol does not feature extraordinarily strong intensities and that the experimental spectra match nicely with calculated ones. Thereby, this study demonstrates the general feasibility of studies on chiral high-spin organics by matrix-isolation VCD.

Hydrogen Tunneling Exhibiting Unexpectedly Small Primary Kinetic Isotope Effects.

Probing quantum mechanical tunneling (QMT) in chemical reactions is crucial to understanding and developing new transformations. Primary H/D kinetic isotopic effects (KIEs) beyond the semiclassical maximum values of 7-10 (room temperature) are commonly used to assess substantial QMT contributions in one-step hydrogen transfer reactions, because of the much greater QMT probability of protium vs. deuterium. Nevertheless, we report here the discovery of a reaction model occurring exclusively by H-atom QMT with residual primary H/D KIEs. 2-Hydroxyphenylnitrene, generated in N2 matrix, was found to isomerize to an imino-ketone via sequential (domino) QMT involving anti to syn OH-rotamerization (rate determining step) and [1,4]-H shift reactions. These sequential QMT transformations were also observed in the OD-deuterated sample, and unexpected primary H/D KIEs between 3 and 4 were measured at 3 to 20 K. Analogous residual primary H/D KIEs were found in the anti to syn OH-rotamerization QMT of 2-cyanophenol in a N2 matrix. Evidence strongly indicates that these intriguing isotope-insensitive QMT reactivities arise due to the solvation effects of the N2 matrix medium, putatively through coupling with the moving H/D tunneling particle. Should a similar scenario be extrapolated to conventional solution conditions, then QMT may have been overlooked in many chemical reactions.

Inferring the Molecular Basis for Dissolved Organic Matter Photochemical and Optical Properties.

Despite the widespread use of photochemical and optical properties to characterize dissolved organic matter (DOM), a significant gap persists in our understanding of the relationship among these properties. This study infers the molecular basis for the optical and photochemical properties of DOM using a comprehensive framework and known structural moieties within DOM. Utilizing Suwannee River Fulvic Acid (SRFA) as a model DOM, carboxylated aromatics, phenols, and quinones were identified as dominant contributors to the absorbance spectra, and phenols, quinones, aldehydes, and ketones were identified as major contributors to radiative energy pathways. It was estimated that chromophores constitute ∼63% w/w of dissolved organic carbon in SRFA and ∼47% w/w of overall SRFA. Notably, estimations indicate the pool of fluorescent compounds and photosensitizing compounds in SRFA are likely distinct from each other at wavelengths below 400 nm. This perspective offers a practical tool to aid in the identification of probable chemical groups when interpreting optical and photochemical data and challenges the current "black box" thinking. Instead, DOM photochemical and optical properties can be closely estimated by assuming the DOM is composed of a mixture of individual compounds.

Characterizing the Impact of Cyanobacterial Blooms on the Photoreactivity of Surface Waters from New York Lakes: A Combined Statewide Survey and Laboratory Investigation.

Cyanobacterial blooms introduce autochthonous dissolved organic matter (DOM) into aquatic environments, but their impact on surface water photoreactivity has not been investigated through collaborative field sampling with comparative laboratory assessments. In this work, we quantified the apparent quantum yields (Φapp,RI) of reactive intermediates (RIs), including excited triplet states of dissolved organic matter (3DOM*), singlet oxygen (1O2), and hydroxyl radicals (•OH), for whole water samples collected by citizen volunteers from more than 100 New York lakes. Multiple comparisons tests and orthogonal partial least-squares analysis identified the level of cyanobacterial chlorophyll a as a key factor in explaining the enhanced photoreactivity of whole water samples sourced from bloom-impacted lakes. Laboratory recultivation of bloom samples in bloom-free lake water demonstrated that apparent increases in Φapp,RI during cyanobacterial growth were likely driven by the production of photoreactive moieties through the heterotrophic transformation of freshly produced labile bloom exudates. Cyanobacterial proliferation also altered the energy distribution of 3DOM* and contributed to the accelerated transformation of protriptyline, a model organic micropollutant susceptible to photosensitized reactions, under simulated sunlight conditions. Overall, our study provides insights into the relationship between the photoreactivity of surface waters and the limnological characteristics and trophic state of lakes and highlights the relevance of cyanobacterial abundance in predicting the photoreactivity of bloom-impacted surface waters.

Attenuation photochemical potential of Pontal of Paranapanema river waters (Brazil) from agrochemicals: geographical and temporal assessment.

Knowledge of the environmental photochemical fate of pesticides is essential to assess their potential impacts. However, there are few studies in the literature focused on the photochemical attenuation of micropollutants in Brazilian rivers. In this context, this study characterized the surface waters of the Pontal of Paranapanema region (region which concentrates more than 80% of Brazilian sugarcane cultivations), in order to determine its photochemical attenuation potential against micropollutants in different seasons. Thus, the steady-state concentrations of the photochemically produced reactive intermediates (PPRIs) (hydroxyl radical, HO•; singlet oxygen, 1O2, and triplet-excited state chromophoric dissolved organic matter, 3CDOM*), formed in the rivers, were simulated by using the APEX model (Aquatic Photochemistry of Environmentally-occurring Xenobiotics), considering the sunlight irradiance, water chemistry, and depth. Based on our simulations, these concentrations vary between 0.35 × 10-15 and 4.52 × 10-14 mol L-1 for HO•, 1.3 × 10-15 and 1.2 × 10--14 mol L-1 for 3CDOM*, and 2.5 × 10-15 and 2.5 × 10-14 mol L-1 for 1O2. Finally, mathematical simulations were used for predicting persistence of pesticides atrazine (ATZ) and diuron (DIR) in Pontal of Paranapanema surface waters and the half-life times (t1/2) of the pollutants ranged from a few hours to one week.

Effects of dissolved organic matter and halogen ions on phototransformation of pharmaceuticals and personal care products in aquatic environments.

Photochemical reactions contribute to the attenuation and transformation of pharmaceuticals and personal care products (PPCPs) in surface natural waters. Nevertheless, effects of DOM and halogen ions on phototransformation of PPCPs remain elusive. This work selected disparate PPCPs as target pollutants to investigate their aquatic phototransformation processes. Results show that PPCPs containing multiple electron-donating groups (-OH, -NH2, -OR, etc.) are more reactive with photochemically produced reactive intermediates (PPRIs) such as triplet DOM (3DOM*), singlet oxygen (1O2), and reactive halogen species (RHSs), relative to PPCPs containing electron-withdrawing groups (-NOR, -COOR, -OCR, etc.). The generation of RHSs as a result of the coexistance of DOM and halide ions changed the contribution of PPRIs to the photochemical conversion of PPCPs during their migration from fresh water to seawater. For PPCPs (AMP, SMZ, PN, NOR, CIP, etc) with highly reactive groups toward RHSs, the generation of RHSs facilitated their photolysis in halide ion-rich waters, where Cl- plays a critical role in the photochemical transformation of PPCPs. Density functional theory (DFT) calculations showed that single electron transfer and H-abstraction are main reaction pathways of RHSs with the PPCPs. These results demonstate the irreplaceable roles of PPRIs and revealing the underlying reaction mechanisms during the phototransformation of PPCPs, which contributes to a better understanding of the environmental behaviors of PPCPs in complex aquatic environments.

In situ Diazonium Salt Formation and Photochemical Aryl-Aryl Coupling in Continuous Flow Monitored by Inline NMR Spectroscopy.

A novel class of diazonium salts is introduced for the photochemical aryl-aryl coupling to produce (substituted) biphenyls. As common diazonium tetrafluoroborate salts fail, soluble and safe aryl diazonium trifluoroacetates are applied. In this mild synthesis route no catalysts are required to generate an aryl-radical by irradiation with UV-A light (365 nm). This reactive species undergoes direct C-H arylation at an arene, forming the product in reasonable reaction times. With the implementation of a continuous flow setup in a capillary photoreactor 13 different biphenyl derivatives are successfully synthesized. By integrating an inline 19F-NMR benchtop spectrometer, samples are reliably quantified as the fluorine-substituents act as a probe. Here, real-time NMR spectroscopy is a perfect tool to monitor the continuously operated system, which produces fine chemicals of industrial relevance even in a multigram scale.

Photochemical processes transform dissolved organic matter differently depending on its initial composition.

Dissolved organic matter (DOM) is one of the most important fluxes in the global carbon cycle but its response to light exposure remains unclear at a molecular-level. The chemical response of DOM to light should vary with its molecular composition and environmental conditions while some basic hypotheses are still unclear, such as the balance between photobleaching and photo-humification and the question of oxidative properties. Here we exposed aquatic DOM from diverse freshwaters impacted by different levels of anthropogenic activity and algal exudates to environmentally-realistic light conditions. We found that photobleaching occurred in DOM with relatively high initial humic content producing low H/C molecules, whereas DOM with low initial humic content was humified. DOM pools with relatively high initial saturation and low aromaticity were prone to transform towards more unsaturated molecular formulae and high H/C molecules with a distinct decrease of bioavailability. Photo-transformation was mainly influenced by reactive intermediates, with reactive oxygen species (ROS) playing a dominant role in humification when the initial humus content of DOM was high. In contrast, for algal DOM with high protein content, it was likely that the autoxidation of excited state DOM was more important than indirect oxidation involving ROS. Our results reveal how photo-transformation patterns depend on the initial composition of DOM and provide new insights into the role of photochemical processes in biogeochemical cycling of DOM.

Hydrogen-Bonded Complex of the Parent Phosphinidene.

The diatomic molecule PH is very reactive, and it serves as the parent compound for phosphinidenes featuring a monovalent phosphorus atom. Herein, we report the characterization and reactivity of a rare hydrogen-bonded complex of PH. Specifically, the molecular complex between PH and HCl has been generated by photolysis of chlorophosphine (H2PCl) at 254 nm in a solid Ar-matrix at 10 K. The IR spectrum of the complex HP⋅⋅⋅HCl and quantum chemical calculations at the UCCSD(T)-F12a/haTZ level consistently prove that the phosphorus atom acts as a hydrogen bond acceptor with a binding energy (D0) of -0.6 kcal mol-1. In line with the observed absorption at 341 nm for the binary complex, the triplet phosphinidene PH undergoes prototype H-Cl bond insertion by reformation of H2PCl upon photoexcitation at 365 nm. However, this hydrogen-bonded complex is unstable in the presence of N2 and HCl, as both molecules prefers stronger interactions with HCl than PH in the observed complexes HP⋅⋅⋅HCl⋅⋅⋅N2 and HP⋅⋅⋅2HCl.

Photoproduction of reactive intermediates from dissolved organic matter in coastal seawater around an urban metropolis in South China: Characterization and predictive modeling.

Chromophoric dissolved organic matter (CDOM) is an important photochemical precursor to reactive intermediates (RIs) (e.g., excited triplet states of chromophoric dissolved organic matter (3CDOM⁎), hydroxyl radicals (·OH), and singlet oxygen (1O2)) in aquatic systems to drive the photodegradation of contaminants. There have been limited studies on the photoproduction of RIs in coastal seawater CDOM in Asia, which impedes our ability to model the lifetimes and fates of contaminants in these coastal seawater systems. Hong Kong is an urban metropolis in South China, whose coastal seawater is susceptible to anthropogenic activities from the surrounding areas and the nearby Pearl River. We investigated the photoproduction of RIs in seawater around Hong Kong during the wet vs. dry season. Higher intensities of fluorescent components, dissolved organic carbon concentration ([DOC]), apparent quantum yields of RIs (ΦRIs), and steady-state concentrations of photogenerated RIs ([RIs]ss) were observed for samples collected in the areas closest to the Pearl River during the wet season. Lower humification degrees and ΦRIs but higher intensities of fluorescent components and [RIs]ss were generally observed for the wet season samples compared to the dry season samples. Statistical analysis revealed strong significant correlations (Spearman |r| > 0.6, p < 0.05) between ΦRIs and the absorbance properties (including the absorbance ratio E2:E3, spectral slope coefficients S350-400, and spectral slope ratio SR) of CDOM, and between [RIs]ss and the quantity-reflected properties (including the fluorescence intensity of humic-like components) of CDOM. Our modeling analyses combining orthogonal partial least squares and stepwise multiple linear regression showed excellent prediction strengths for [1O2]ss and [3CDOM⁎]ss (R2adj > 0.7) when [DOC] and the chemical and optical properties of CDOM were used as predictor variables. These modeling results demonstrate the feasibility of predicting the concentrations and quantum yields of RIs in seawater around Hong Kong, and potentially other coastal cities in South China, from easily measurable chemical and optical properties.

Elucidating the inhibitory effects of natural organic matter on the photodegradation of organic micropollutants: Atrazine as a probe compound.

Natural organic matter (NOM) is a complex mixture of heterogeneous compounds with varying functional groups and molecular sizes. Understanding the impact of NOM on the generation of photochemically produced reactive intermediates (PPRIs) and their potential inhibitory effects on photolysis has remained challenging due to the variations in the reactivities and concentrations of these functional groups. To address this gap, tannic acid (TA), gallic acid (GA), catechin (CAT), and tryptophan (Trp), were chosen as potential substitutes for NOM. Their effects on the photochemical transformation process were evaluated and compared with the widely used Suwannee River NOM (SRNOM). Atrazine (ATZ) was selected as a probe organic micropollutant (OMP). In this investigation, a significantly higher concentration of HO• was observed compared to O21, and the triplet excited state ( NOM*3). The findings suggest that the substituted phenols, particularly those with carboxylate-substitutions, played a substantial role in HO• formation, while electron-rich moieties acted as antioxidants, consuming NOM*3. Hydroxyl, carboxylic, and amino acid were the active groups for O21 formation. However, the inhibitory effects induced by the NOM surrogates were significant and mainly attributed to the direct photolysis inhibition caused by the inner filter effect. The scope of this work was further extended to include SRNOM, where similar trends with less pronounced formation of PPRIs and inner filter effects were observed. Therefore, this study sheds some light on the role of the functional groups in NOM during photochemical transformations of OMPs, thereby deepening our understanding of their fate in aqueous systems.

Altering the Localization of an Unpaired Spin in a Formal Ni(V) Species.

The participation of both ligand and the metal center in the redox events has been recognized as one of the ways to attain the formal high valent complexes for the late 3d metals, such as Ni and Cu. Such an approach has been employed successfully to stabilize a Ni(III) bisphenoxyl diradical species in which there exist an equilibrium between the ligand and the Ni localized resultant spin. The present work, however, broadens the scope of the previously reported three oxidized equivalent species by conveying the approaches that tend to affect the reported equilibrium in CH3 CN at 233 K. Various spectroscopic characterization revealed that employing exogenous N-donor ligands like 1-methyl imidazole and pyridine favors the formation of the Ni centered localized spin though axial binding. In contrast, due to its steric hinderance, quinoline favors an exclusive ligand localized radical species. DFT studies shed light on the novel intermediates' complex electronic structure. Further, the three oxidized equivalent species with the Ni centered spin was examined for its hydrogen atom abstraction ability stressing their key role in alike reactions.

Polyhedral Oligomeric Silsesquioxane D3h-(RSiO1.5)14.

While smaller polyhedral oligomeric silsesquioxanes TnRn (POSS) are readily accessible or even commercially available, unambiguously authenticated larger systems (n>12) have barely been reported. Synthesis and isolation procedures are lengthy, and yields are often very low. Herein, we present the surprisingly straightforward and high-yielding access to the phenyl-substituted derivative of a so far only postulated second D3h-symmetric T14 isomer and with that the largest crystallographically characterized POSS cage with organic substituents. Treatment of the commercially available incompletely condensed T7Ph7(OH)3 silsesquioxane with catalytic amounts of trifluoromethanesulfonic acid results in high yields of the T14Ph14 framework, which is isolated in crystalline form by a simple work-up. D3h-T14Ph14 was analyzed by single crystal X-ray diffraction, multinuclear NMR spectroscopy and thermal analysis. The relative energies of all four theoretically possible T14Ph14 isomers were determined by optimization of the corresponding structure using DFT methods.

Phosphinidenes: Fundamental Properties and Reactivity.

Phosphinidenes are heavy congeners of nitrenes that have been broadly used as in situ reagents in synthetic phosphorus chemistry and also serve as versatile ligands in coordination with transition metals. However, the detection of free phosphinidenes is largely challenged by their high reactivity and also the lack of suitable synthetic methods, rendering the knowledge about the fundamental properties of this class of low-valent phosphorus compounds limited. Recently, an increasing number of free phosphinidenes bearing prototype structural and bonding properties have been prepared for the first time, thus enabling the exploration of their distinct reactivity from the nitrene analogues. This Concept article will discuss the experimental approaches for the generation of the highly unstable phosphinidenes and highlight their distinct reactivity from the nitrogen analogues so as to stimuate future studies about their potential applications in phosphorus chemistry.

On-Surface Synthesis and Real-Space Visualization of Aromatic P3 N3.

On-surface synthesis is at the verge of emerging as the method of choice for the generation and visualization of unstable or unconventional molecules, which could not be obtained via traditional synthetic methods. A case in point is the on-surface synthesis of the structurally elusive cyclotriphosphazene (P3 N3 ), an inorganic aromatic analogue of benzene. Here, we report the preparation of this fleetingly existing species on Cu(111) and Au(111) surfaces at 5.2 K through molecular manipulation with unprecedented precision, i.e., voltage pulse-induced sextuple dechlorination of an ultra-small (about 6 Å) hexachlorophosphazene P3 N3 Cl6 precursor by the tip of a scanning probe microscope. Real-space atomic-level imaging of cyclotriphosphazene reveals its planar D3h -symmetric ring structure. Furthermore, this demasking strategy has been expanded to generate cyclotriphosphazene from a hexaazide precursor P3 N21 via a different stimulation method (photolysis) for complementary measurements by matrix isolation infrared and ultraviolet spectroscopy.

Photobleaching-induced changes in the optical and photochemical properties of algal organic matter.

Algal organic matter (AOM), a significant source of endogenous dissolved organic matter (DOM) is released in high concentrations during cyanobacterial blooms, along with cyanotoxins. Subsequent photobleaching of AOM is an important phenomenon to investigate. In this study, intracellular organic matter (IOM) and extracellular organic matter (EOM) were extracted from cultured cyanobacteria taken from Taihu Lake in China. The formation of photochemically produced reactive intermediates in different stages of IOM and EOM photobleaching was compared to Suwannee River DOM (SRDOM, reference standard DOM). Results revealed notable differences influenced by the pigment component among IOM, EOM, and SRDOM. The pigment in IOM contributed to a triplet state pool with strong energy-transfer but limited electron-transfer capabilities. Notably, IOM exhibited the highest triplets state quantum yield value in the visible region, suggesting its potential significance in pollutant degradation in deeper water layers. For EOM, one of the pools exhibits photolability and remarkable electron-transfer capability, indicating it as a high-energy triplet state component. Moreover, three cyanotoxins (MC-LR, ACA, and ATX-a) were detected in the extracted AOM, and their photodegradation was monitored during the AOM photobleaching process. This highlights the potential role of AOM as a photosensitizer in the natural self-cleaning mechanisms of water bodies, facilitating the degradation of organic pollutants through photochemical reactions. The findings of this study contribute to understanding the dynamic nature of AOM and its implications in environmental processes.