Laser Flash Photolysis Research Articles

Quenching Rate Constants of Lewis Base‐Boryl Radical by Substrates: a Laser Flash Photolysis Study

The advanced strategy using Lewis base‐boryl radicals (LBRs) has recently been proposed for the addition of alkyl substituents to the full‐carbon quaternary center of an organic molecule. However, as the rate‐determining step in the whole route, reaction rate constants of LBRs with substrates are extremely lacking. In this paper, 4‐dimethylaminopyridine (DMAP)‐BH2• was selected as a representative of LBRs, and its reactions with six monochloro‐substituted substrates, including three methyl chlorobenzoates and three chlorinated acetanilides were studied in experiments and theoretical calculations. The bimolecular reaction rate constants, kq, were determined using laser flash photolysis approach. By comparing activation energies along the two addition pathways, we have clarified the rate‐determining step as the attacking to carbonyl oxygen instead of chlorine atom. Furthermore, noncovalent interaction (NCI) analyses on these substrates indicate that weak interactions, such as hydrogen‐bonding and van der Waals interactions, have significant influence on the reactivity of these substrates. Our study provides concrete clues to extend this synthetic strategy.

Faster Excited-State Intramolecular Electron Transfer from Perylenediimide Dianion Compared to Its Radical Anion.

The properties of the excited perylenediimide dianion (PDI2-∗) and its intramolecular electron transfer (ET) behavior were examined using femtosecond laser flash photolysis on PDI2- and PDI2--acceptor (A) dyads. Upon laser excitation, the dianion of PDI first generated a singlet (S1) state, followed by a triplet (T1) state. In all of the dyads, rate constants of the intramolecular ET from PDI2-∗ (S1) varied with the driving forces. Comparison of the PDI2-∗-A and PDI•-∗-A dyad systems revealed higher rate constants for the ET from PDI2-∗ than from PDI•-∗, which can be attributed to the difference in electronic coupling for the ET. These findings elucidate the previously unexplored photoinduced ET characteristics of PDI2-, thereby advancing the groundwork for photochemical studies of excited multi-ions and their applications in related materials.

Potential of homogeneous persulfate activation as a strategy for the oxidative treatment of tetramethylammonium hydroxide: Superiority of sulfate radical over hydroxyl radical in the oxidation of methylammoniums

Tetramethylammonium hydroxide (TMAH) as the etching and chelating agent exhibits the extreme recalcitrance to advanced oxidation processes utilizing hydroxyl radical (OH). This study showed that based on the relative position of PDS/heat and PDS/UV against H2O2/UV in terms of TMAH degradation efficiency, sulfate radical (SO4−) was superior to OH in oxidizing TMAH. PDS/heat (at 60 °C) and PDS/UV at the initial pH = 7 achieved TMAH degradation efficiency of ∼ 80–90 % within 1 h whereas H2O2/UV barely eliminated TMAH, which contrasted with H2O2/UV decomposing phenol faster than the activated PDS. The high susceptibility of TMAH (in which four methyl groups bonded to nitrogen removed the lone pair of electrons) to oxidation by SO4− aligned with the pH–dependent efficiency of PDS/heat for the treatment of methylamines. PDS/heat effectively degraded methylammoniums (lacking nitrogen lone pair available for electron transfer due to amine protonation) under non–alkaline conditions, whereas acidification caused the efficiency loss of ∼ 90 % for H2O2/UV. Methylammoniums as radical scavengers retarded benzoic acid degradation by PDS/heat to a more pronounced extent than counterparts with deuterated N–methyl groups, which implied that hydrogen–atom abstraction primarily contributed to the SO4−–induced oxidation of methylammoniums. Estimation of the bimolecular reaction rate constants using laser flash photolysis and electron paramagnetic resonance spectroscopic detection of TMAH–derived carbon–centered radical supported the superiority of SO4− over OH in TMAH oxidation. These findings suggested the potential of homogeneous persulfate activation in the pH-insensitive oxidative treatment of alkylamines and quaternary ammonium cations structurally analogous to TMAH.

Mechanism of UV photodegradation of fluoroquinolone antibiotic ciprofloxacin in aqueous solutions.

Mechanism of direct UV photolysis of the zwitterionic and anionic forms of the quinolone antibiotic ciprofloxacin (CIP) was revealed by combination of nanosecond laser flash photolysis, steady-state photolysis coupled with high resolution LC-MS and DFT quantum-chemical calculations. For both forms, the main intermediate is a dissociative triplet state, which loses a fluorine ion to form a triplet carbocation; subsequent solvent attack of the latter leads to the formation of products of hydroxylation both the aromatic ring and the piperazyl substituent. Correspondingly, the quantum yield of photolysis of both CIP forms does not depend on the excitation wavelength, but depends on the concentration of dissolved oxygen. Secondary photolysis leads to a number of products of oxidation of the aromatic system, as well as oxidation, opening and full destruction of the piperazinyl substituent. The results obtained may be important for understanding the fate of quinolone antibiotics in UVC disinfection processes and in natural waters under the action of sunlight.

Photochemical generation of ketenes from phenanthrene-based cyclobutanones

Various 9,10-cyclopropanated phenanthrenes have been shown previously to be viable photochemical precursors to many different types of carbenes. There is also one known example in the literature of a related approach used to generate diphenylketene from a phenanthrene-derived cyclobutanone in a laser flash photolysis study. In an effort to broaden the scope of this promising yet under-utilized method of ketene generation, the synthesis and characterization of several phenanthrene-based cyclobutanones, and their subsequent photolysis to produce a variety of ketenes, are presented in this report. The ketenes are produced, along with phenanthrene, by a formal 2 + 2 cyclo-reversion of the cyclobutanone moiety and have been intercepted by benzyl alcohol and benzyl amine to form the corresponding benzyl esters and amides respectively.

Riboflavin‐Catalyzed Photoinduced Atom Transfer Radical Polymerization

AbstractThe photoATRP of methyl acrylate (MA) is investigated using riboflavin (RF) and CuBr2/Me6TREN as a dual catalyst system under green LED irradiation (λ ≈ 525 nm). Both RF and CuBr2/Me6TREN enhanced oxygen tolerance, enabling effective ATRP in the presence of residual oxygen. High molar mass polymers (up to Mn ≈ 129 000 g·mol−1) with low dispersity (Đ ≤ 1.16) are prepared, and chain‐end fidelity is confirmed through successful chain extension. The molecular masses of the obtained polymer increased linearly with conversion and showed high initiation efficiency. Mechanistic studies by laser flash photolysis reveal that the predominant activator generation mechanism is reductive quenching of RF by Me6TREN (83%, under [CuBr2]/[Me6TREN] = 1/3 condition), supported by polymerization kinetics and thermodynamic calculations.

ATRP with ppb Concentrations of Photocatalysts.

In atom transfer radical polymerization (ATRP), dormant alkyl halides are intermittently activated to form growing radicals in the presence of a CuI/L/X-CuII/L (activator/deactivator) catalytic system. Recently developed very active copper complexes could decrease the catalyst concentration to ppm level. However, unavoidable radical termination results in irreversible oxidation of the activator to the deactivator species, leading to limited monomer conversions. Therefore, successful ATRP at a low catalyst loading requires continuous regeneration of the activators. Such a regenerative ATRP can be performed with various reducing agents under milder reaction conditions and with catalyst concentrations diminished in comparison to conventional ATRP. Photoinduced ATRP (PhotoATRP) is one of the most efficient methods of activator regeneration. It initially employed UV irradiation to reduce the air-stable excited X-CuII/L deactivators to the activators in the presence of sacrificial electron donors. Photocatalysts (PCs) can be excited after absorbing light at longer wavelengths and, due to their favorable redox potentials, can reduce X-CuII/L to CuI/L. Herein, we present the application of three commercially available xanthene dyes as ATRP PCs: rose bengal (RB), rhodamine B (RD), and rhodamine 6G (RD-6G). Even at very low Cu catalyst concentrations (50 ppm), they successfully controlled PhotoATRP. Well-defined polymers with preserved livingness were prepared under green LED irradiation, with subppm concentrations ([PC] ≥ 10 ppb) of RB and RD-6G or 5 ppm of RD. Interestingly, these PCs efficiently controlled ATRP at wavelengths longer than their absorption maxima but required higher loadings. Polymerizations proceeded with high initiation efficiencies, yielding polymers with narrow molecular weight distributions and high chain-end fidelity. UV-vis, fluorescence, and laser flash photolysis studies helped to elucidate the mechanism of the processes involved in the dual-catalytic systems, comprising parts per million of Cu complexes and parts per billion of PCs.

Differences in the Reaction Mechanisms of Chlorine Atom and Hydroxyl Radical with Organic Compounds: From Thermodynamics to Kinetics.

Hydroxyl radical (HO•) and chlorine atom (Cl•) are common reactive species in aqueous environments. However, the intrinsic difference in their reactions with organic compounds has not been revealed. This study compared the reaction mechanisms of HO• and Cl• with 13 aromatic and 11 aliphatic compounds by quantum chemical calculation and laser flash photolysis. Both HO• and Cl• can spontaneously react with aromatic compounds via radical adduct formation (RAF), hydrogen atom transfer (HAT), and single electron transfer (SET) pathways. The SET reactions of Cl• were more thermodynamically favorable than HO•, but contrary results were obtained for HAT reactions. According to the free energy of activation (ΔGaq‡), the dominant oxidation mechanisms of aromatic compounds were RAF and SET by HO• and SET by Cl•. The important role of SET in the HO• reactions with aromatic compounds was further verified by accurately calculating the solvation free energy of HO•/HO- and experimentally tracking the radical cations, which were generally neglected in previous studies. Meanwhile, the ΔGaq‡ value of each reaction pathway of Cl• was lower than that of HO•, resulting in higher rate constants of Cl• with aromatic compounds than HO•. For saturated aliphatic compounds, HAT was found to be the only mechanism accounting for their transformation by HO• and Cl•. This study proposed general rules for the reaction mechanisms of HO• and Cl• and unraveled their differences in the aspects of thermodynamics and kinetics, providing fundamental information for understanding contaminant transformation in processes involving HO• and Cl•.

Spectroscopic insights into BSA-mediated deaggregation of m-THPC

Meta-tetra(hydroxyphenyl)chlorin (m-THPC) is among the most potent photosensitizers, known for its high singlet oxygen generation efficiency. However, its clinical effectiveness in photodynamic therapy (PDT) is compromised by its propensity to aggregate in aqueous solutions, adversely affecting its photophysical properties and therapeutic potential. A series of spectroscopic techniques, including UV-Vis absorption, fluorescence spectroscopy, and laser flash photolysis, revealed that m-THPC exhibits significant aggregation, particularly in MeOH-PBS mixtures with MeOH content below 30%. This aggregation adversely affects its photophysical properties leading to reduced fluorescence quantum yield and most importantly reducing its singlet oxygen quantum yield. This study introduces the use of bovine serum albumin (BSA) to counteract the aggregation of m-THPC, aiming to enhance its solubility, stability, and efficacy in physiological settings. Through advanced spectroscopic analyses we demonstrated that the m-THPC@BSA complex exhibits restored photophysical properties characteristic for monomeric form. Notably, the complex showed a significant restoration of the singlet oxygen quantum yield (ΦΔ = 0.21) compared to aggregated m-THPC. These results underscore the potential of BSA to preserve the monomeric form of m-THPC, mitigating aggregation-induced losses in singlet oxygen production. Our findings suggest that BSA-mediated delivery systems could play a crucial role in optimizing the clinical utility of hydrophobic photosensitizers like m-THPC.

Chloride-mediated transformation of sulfate radicals to hydroxyl radicals for enhanced contaminant degradation in high-temperature wastewater

Using the excess heat in high-temperature industrial wastewater to activate oxidant precursors for organic contaminant degradation would make advanced oxidation processes more sustainable in the context of carbon neutrality, while the salt-mediated radical transformation chemistry in such wastewater is poorly understood. We herein demonstrate that chloride (Cl–) in the range of 0.28–56.34 mM accelerates the degradation of organic contaminants in the heat activated persulfate process in both synthetic and authentic wastewater. We use laser flash photolysis technique together with kinetic modeling to directly determine the bimolecular rate constants of radical–radical (e.g., kSO4·--SO4·- = 2.50 × 109 M−1 s−1), Cl–-radical (e.g., kSO4·--Cl- = 3.75 × 108 M−1 s−1), and radical-contaminant (e.g., kHO·-BA = 1.02 × 1010 M−1 s−1) reactions at 55 °C, which are substantially higher than those at ambient water temperature (25 °C). We also combine experimental and modelling results to calculate the contribution of different radicals to contaminant removal at 55 °C. The results clearly demonstrate that Cl– in the range of 0.28–56.34 mM promotes the transformation of SO4– to HO at 55 °C, with reactive chlorine species involved as the intermediates. The findings improve fundamental understanding of Cl–-mediated radical chemistry in high-temperature water and provide an engineering strategy to recycle the heat in wastewater to enhance contaminant remediation.

Dual behavior of histidine during sensitized photo-oxidation of model compounds and proteins

Histidine (His) photo-oxidation has been widely investigated with several transient and stable products characterized, especially for aerobic conditions. Due to its role and structure, His-side chain can be a key player in the quenching of excited states such as the triplet state of the photosensitizer 3-carboxybenzophenone (3CB*). The capacity of His and its derivatives to quench 3CB* under anaerobic conditions are characterized in the current study by laser flash photolysis, with the resulting oxidation products examined by mass spectrometry to determine the reaction mechanism. The latter include adducts of the 3-carboxybenzophenone ketyl radical (CBH•) to the imidazole ring (Imid-CH2-CBH), His-His dimers, and other products with lower yields. The data obtained with model compounds are compared to those obtained with more complicated systems, including the peptide Exendin-4, and the protein MtHpt1. The data obtained from transient spectroscopy and product analyses indicate that two CB* quenching mechanisms occur: (i) proton-coupled electron transfer (as reported previously) yielding radicals that can recombine to give His-His dimers and CBH-adducts, and (ii) energy transfer yielding 3His* undergoing further reaction leading to formation of Imidazyl-CH2-CBH adduct. The latter, unexpected process only occurs when His and its derivatives have a free α-amino group. This process yielded a novel adduct between the imidazole ring and the CBH• formed by sensitizer reduction.

Reinvestigation on the UV/sulfite oxidation of organic pollutants: The overlooked role of excited states and bromate control

In recent years, the UV/sulfite (S(IV)) has been proposed as a promising advanced oxidation process for water treatment in the presence of oxygen. However, the mechanism of S(IV) activation is not fully understood, as the interaction between S(IV) and photo-excited pollutants has been underexplored. In this work, an aerated UV/S(IV) system was established, and propranolol (PRO) was selected as a target pollutant. Under the irradiation at 254 nm, this system could degrade 95.6 % of PRO with 2 mM S(IV) in 60 min. The removal of PRO was positively related to the S(IV) dose, while an increasing pH exhibited a negative effect. Direct photolysis, SO4•−, and HO• collaboratively contributed to PRO removal, with SO4•− playing a primary role. Laser flash photolysis reveals that both radical cation of PRO (product of biphotonic process) and the triplet state (T1PRO) can react with HSO3− at rate constants of 5.2 × 108 M−1 s−1 and 2.1 × 107 M−1 s−1, accordingly. These reactions may offset the deficient photoactivation of S(IV) under acidic conditions. Apart from PRO, other organic pollutants could also be effectively degraded. Furthermore, this system has a great advantage over UV/persulfate due to the absence of transformation of background bromide ions to hazardous bromate. Our study significantly updates the decontaminating mechanism of organic pollutants in the aerated UV/sulfite system.

Elusive Arylalkylcarbenes in Solution and in Crystals: Facile 1,2-R (R = H, Ph) Migrations of 1,2,2-Triphenylethylidene.

Highly reactive arylalkylcarbenes generated in solution by photolysis of their aryldiazoalkane precursors tend to undergo competing inter- and intramolecular reactions to yield a complex mixture of products. Having previously shown the use of crystals to effectively control the reactivity of arylalkylcarbenes to afford high yields of a single product, it was of interest to investigate whether the crystalline environment could also enable spectroscopic detection of these intermediates en route to the photoproduct. Using 1,2,2-triphenyldiazoethane (3) as a model substrate to probe the effect of alternative reaction trajectories that yield triphenylethylene (5) by competing 1,2-H shift or 1,2-Ph migration, we report selectivities consistent with reaction from a spin-equilibrated carbene 4 in solution, while reactions in crystals primarily afford alkene 5 via a lattice-controlled 1,2-H shift. Attempts to detect 1,2,2-triphenylethylidene 4 in crystals by nanosecond laser-flash photolysis or by triplet-triplet fluorescence at 77 K were unsuccessful, indicating that arylalkylcarbenes possessing α-H substituents undergo facile 1,2-H shifts both in solution and in the solid state. However, related tert-butylphenylmethylene with no α-H substituents could be observed by triplet-triplet fluorescence at 77 K in glassy matrices.

Reactivity of the phosphaethynolate anion with stabilized carbocations: mechanistic studies and synthetic applications.

The reactivity of sodium phosphaethynolate Na(OCP) towards various Mayr's reference electrophiles was investigated using conventional UV-visible and laser-flash photolysis techniques. The kinetic data, along with density functional theory (DFT) calculations, enabled the first experimental quantification of the phosphorus nucleophilicity of [OCP]-. Product studies of these reactions demonstrate the formation of secondary as well as tertiary phosphines. The mechanism of this unprecedented phosphorus-atom transfer reaction is thoroughly discussed, with key intermediates successfully isolated and characterized. Importantly, some bulky secondary phosphine oxides synthesized using this approach, have demonstrated high efficiency as ligands in the Suzuki coupling reaction.

Unveiling the environmental significance of acetylperoxyl radical: Reactivity quantification and kinetic modeling.

Acetylperoxyl radical (CH3C(O)OO•) is among highly reactive organic radicals which are known to play crucial roles in atmospheric chemistry, aqueous chemistry and, most recently, peracetic acid (PAA)-based advanced oxidation processes. However, fundamental knowledge for its reactivity is scarce and severely hampers the understanding of relevant environmental processes. Herein, three independent experimental approaches were exploited for revelation and quantification of the reaction rates of acetylperoxyl radical. First, we developed and verified laser flash photolysis of biacetyl, ultraviolet (UV) photolysis of biacetyl, and pulse radiolysis of acetaldehyde, each as a clean source of CH3C(O)OO•. Then, using competition kinetics and selection of suitable probe and competitor compounds, the rate constants between CH3C(O)OO• and compounds of diverse structures were determined. The three experimental approaches complemented in reaction time scale and ease of operation, and provided cross-validation of the rate constants. Moreover, the formation of CH3C(O)OO• was verified by spin-trapped electron paramagnetic resonance, and potential influence of other reactive species in the systems was assessed. Overall, CH3C(O)OO• displays distinctively high reactivity and selectivity, reacting especially favorably with naphthyl and diene compounds (k ∼ 107-108 M-1 s-1) but sluggishly with N- and S-containing groups. Significantly, we demonstrated that incorporating acetylperoxyl radical-oxidation reactions significantly improved the accuracy in modeling the degradation of environmental micropollutants by UV/PAA treatment. This study is among the most comprehensive investigation for peroxyl radical reactivity to date, and establishes a robust methodology for investigating organic radical chemistry. The determined rate constants strengthen kinetic databases and improve modeling accuracy for natural and engineered systems.

Photophysical properties of benzo[i]dipyrido[3,2-a:2′,3′-c] phenazine (dppn) – A prospective ligand for light-activated anticancer complexes

Benzo[i]dipyrido[3,2-a:2′,3′c]phenazine (dppn) is a prospective ligand for constructing light-activated anticancer complexes of platinum metals. In spite of practical importance, only scarce information on the dppn photophysics and photochemistry was found in the literature. In this work photophysical properties of dppn in acetonitrile solutions were studied by means of stationary and laser flash photolysis, time-resolved luminescence, time-resolved singlet oxygen detection and ultrafast TA spectroscopy. Quantitative characteristics of luminescence (quantum yield 0.26, lifetime 52 ns in deaerated solutions) were measured. Triplet state of the initial compound 3dppn* is formed with the quantum yield estimated as 0.4 from the singlet oxygen production; the main channels of 3dppn* are quenching by dissolved oxygen and self-quenching by the ground state of dppn. Combining the data of ultrafast TA absorption and time-resolved luminescence, we detected the formation of “dark” (not emissive) excited state of dppn with the decay time ca. 150 ps. Two hypotheses on the nature of the dark state were put forward.

Atmospheric Intermediates at the Air-Water Interface.

The air-water interface (AWI) is a ubiquitous reaction field different from the bulk phase where unexpected reactions and physical processes often occur. The AWI is a region where air contacts cloud droplets, aerosol particles, the ocean surface, and biological surfaces such as fluids that line human epithelia. In Earth's atmosphere, short-lived intermediates are expected to be generated at the AWI during multiphase reactions. Recent experimental developments have enabled the direct detection of atmospherically relevant, short-lived intermediates at the AWI. For example, spray ionization mass spectrometric analysis of water microjets exposed to a gaseous mixture of ozone and water vapor combined with a 266 nm laser flash photolysis system (LFP-SIMS) has been used to directly probe organic peroxyl radicals (RO2·) produced by interfacial hydroxyl radicals (OH·) + organic compound reactions. OH· emitted immediately after the laser flash photolysis of carboxylic acid at the gas-liquid interface have been directly detected by time-resolved, laser-induced florescence techniques that can be used to study atmospheric multiphase photoreactions. In this Featured Article, we show some recent experimental advances in the detection of atmospherically important intermediates at the AWI and the associated reaction mechanisms. We also discuss current challenges and future prospects for atmospheric multiphase chemistry.

Absolute Rate Constants for the Reaction of Benzil and 2,2′-Furil Triplet with Substituted Phenols in the Ionic Liquid 1-Butyl-3-methylimidazolium Hexafluorophosphate: A Nanosecond Laser Flash Photolysis Study

Absolute Rate Constants for the Reaction of Benzil and 2,2′-Furil Triplet with Substituted Phenols in the Ionic Liquid 1-Butyl-3-methylimidazolium Hexafluorophosphate: A Nanosecond Laser Flash Photolysis Study

Unusual Photochemistry in Aromatic Dithioimides: Quantitative Thione Reduction Promoted by Ether Solvents.

We report a mechanistic investigation of an aromatic dithioimide (2SS) displaying puzzling yet efficient photochemistry in ether solvents. Perplexingly, 2SS dissolved in ether solvents in a sealed and degassed vial was photochemically converted to the corresponding diimide (2OO), as determined by 1H NMR following product extraction. With no external sources of oxygen in the sample, could the oxygen in 2OO be from the ether itself? To study this unprecedented proposition, we attempt to uncover the ether's involvement in this reaction. As seen by laser-flash photolysis, 2SS appears to first react with the solvent from its singlet excited state. Following the reaction by NMR under rigorously oxygen- and water-free conditions led to the identification of a photoreductive pathway that quantitatively transformed one thione into a methylene to yield 2SH2. Subsequent oxidation of 2SH2 or irradiation of 2SS under air proved that molecular oxygen was indeed necessary to observe an oxidative pathway leading to 2OO, ruling out the initially proposed involvement of an ether oxygen. An explanation of 2SS desulfurization was further revealed through the study of solvent by-products by GC-MS analysis. Supported by DFT calculations, a mechanism is proposed to involve a chain reaction initiated by photochemically generated ether radical.

Mechanistic investigations of polyaza[7]helicene in photoredox and energy transfer catalysis.

Organic photocatalysts frequently possess dual singlet and triplet photoreactivity and a thorough photochemical characterization is essential for efficient light-driven applications. In this article, the mode of action of a polyazahelicene catalyst (Aza-H) was investigated using laser flash photolysis (LFP). The study revealed that the chromophore can function as a singlet-state photoredox catalyst in the sulfonylation/arylation of styrenes and as a triplet sensitizer in energy transfer catalysis. The singlet lifetime is sufficiently long to exploit the exceptional excited state reduction potential for the activation of 4-cyanopyridine. Photoinduced electron transfer generating the radical cation was directly observed confirming the previously proposed mechanism of a three-component reaction. Several steps of the photoredox cycle were investigated separately, providing deep insights into the complex mechanism. The triplet-excited Aza-H, which was studied with quantitative LFP, is formed with a quantum yield of 0.34. The pronounced triplet formation was exploited for the isomerization reaction of (E)-stilbene to the Z-isomer and the cyclization of cinnamyl chloride. Catalyst degradation mainly occurs through the long-lived Aza-H triplet (28 µs), but the photostability is greatly increased when the triplet efficiently reacts in a catalytic cycle such that turnover numbers exceeding 4400 are achievable with this organocatalyst.