Showing papers on "Flash photolysis published in 2022"

Photoaging of Baby Bottle-Derived Polyethersulfone and Polyphenylsulfone Microplastics and the Resulting Bisphenol S Release.

Abstract: This study evaluated the release of bisphenol S (BPS) from polyethersulfone (PES) and polyphenylsulfone microplastics (MPs) derived from baby bottles under UV irradiation. Released BPS fluctuates over time because it undergoes photolysis under UV254 irradiation. Under UV365 irradiation, the highest released concentration at 50 °C was 1.7 and 3.2 times that at 35 and 25 °C, respectively, as the activation energy of the photochemical reactions responsible for MP decay was reduced at high temperatures. Low concentrations of humic acid (HA, ≤10 mg·L-1) promote BPS release because HA acts as a photosensitizer. A high concentration of HA (10∼50 mg·L-1) decreases the BPS release because HA shields MPs from light and scavenges reactive radicals that are produced via photochemical reactions. For example, under UV irradiation, hydroxyl radicals (•OH) attack results in the breakage of ether bonds and the formation of phenyl radicals (Ph•) and phenoxy radicals (Ph-O•).The•OH addition and hydrogen extractions further produce BPS from the decayed MPs. A leaching kinetics model was developed and calibrated by the experimental data. The calibrated model predicts the equilibrium level of BPS release from MPs that varies with the surface coverage density of BPS and leaching rate constants. This study provides groundwork that deepens our understanding of environmental aging and the chemical release of MPs.

Molecular Rubies in Photoredox Catalysis

Abstract: The molecular ruby [Cr(tpe) 2 ] 3+ and the tris(bipyridine) chromium(III) complex [Cr(dmcbpy) 3 ] 3+ as well as the tris(bipyrazine)ruthenium(II) complex [Ru(bpz) 3 ] 2+ were employed in the visible light-induced radical cation [4+2] cycloaddition (tpe = 1,1,1-tris(pyrid-2-yl)ethane, dmcbpy = 4,4′-dimethoxycarbonyl-2,2′-bipyridine, bpz = 2,2′-bipyrazine), while [Cr(ddpd) 2 ] 3+ serves as a control system (ddpd = N,N′-dimethyl-N,N′-dipyridin-2-ylpyridine-2,6-diamine). Along with an updated mechanistic proposal for the CrIII driven catalytic cycle based on redox chemistry, Stern-Volmer analyses, UV/Vis/NIR spectroscopic and nanosecond laser flash photolysis studies, we demonstrate that the very weakly absorbing photocatalyst [Cr(tpe) 2 ] 3+ outcompetes [Cr(dmcbpy) 3 ] 3+ and even [Ru(bpz) 3 ] 2+ in particular at low catalyst loadings, which appears contradictory at first sight. The high photostability, the reversible redoxchemistry and the very long excited state lifetime account for the exceptional performance and even reusability of [Cr(tpe) 2 ] 3+ in this photoredox catalytic system. Graphical Abstract The molecular ruby [Cr(tpe) 2 ] 3+ catalyzes the visible light-induced radical cation [4+2] cycloaddition with unprecedented photostability and recyclability. Detailed mechanistic studies reveal that reductive quenching of [Cr(tpe) 2 ] 3+ is fast while the first C-C formation is the slowest step and triplet oxygen is required to rapidly regenerate [Cr(tpe) 2 ] 3+ and prevent detrimental back-electron transfer.

Photo- and Radiation-Induced One-Electron Oxidation of Methionine in Various Structural Environments Studied by Time-Resolved Techniques

Abstract: Oxidation of methionine (Met) is an important reaction that plays a key role in protein modifications during oxidative stress and aging. The first steps of Met oxidation involve the creation of very reactive and short-lived transients. Application of complementary time-resolved radiation and photochemical techniques (pulse radiolysis and laser flash photolysis together with time-resolved CIDNP and ESR techniques) allowed comparing in detail the one-electron oxidation mechanisms initiated either by ●OH radicals and other one-electron oxidants or the excited triplet state of the sensitizers e.g., 4-,3-carboxybenzophenones. The main purpose of this review is to present various factors that influence the character of the forming intermediates. They are divided into two parts: those inextricably related to the structures of molecules containing Met and those related to external factors. The former include (i) the protection of terminal amine and carboxyl groups, (ii) the location of Met in the peptide molecule, (iii) the character of neighboring amino acid other than Met, (iv) the character of the peptide chain (open vs cyclic), (v) the number of Met residues in peptide and protein, and (vi) the optical isomerism of Met residues. External factors include the type of the oxidant, pH, and concentration of Met-containing compounds in the reaction environment. Particular attention is given to the neighboring group participation, which is an essential parameter controlling one-electron oxidation of Met. Mechanistic aspects of oxidation processes by various one-electron oxidants in various structural and pH environments are summarized and discussed. The importance of these studies for understanding oxidation of Met in real biological systems is also addressed.

Silyl glyoximides: Toward a New Class of Visible Light Photoinitiators

Abstract: Many academic and industrial works are carried out for the search of new classes of visible light photoinitiators. In the last decade, new Type I photoinitiator generating silyl, germyl, or stannyl radicals is elegantly reported; the chemical mechanisms being often associated with the cleavage of the C(═O)Si, C(═O)Ge, or C(═O)Sn bonds. In this context, silyl glyoxylates are also reported as dual Type I and Type II photoinitiators. Silyl glyoximides are proposed here as new near-UV and blue light sensitive photoinitiators. The synthesis of such compounds as well as their light absorption properties is discussed. Their photochemical properties are studied through steady state photolysis experiments and molecular modeling data (i.e., through the calculations of the frontier orbitals, CC bond dissociation energies and triplet state energy levels). To finish, their photoinitiating ability upon near-UV or blue light emitting diode light is examined in a benchmark methacrylate monomer blend (BisGMA/TEGDMA). Markedly, a Type I photoinitiator behavior is highlighted but better initiating properties are found in multicomponent systems in combination with iodonium salt and amine.

Photochemistry of Sodium Thiosulfate in Aqueous Solutions Revisited

Abstract: A mechanistic study of sodium thiosulfate (S2O32−) photolysis in aqueous solutions at neutral pH was performed using stationary and nanosecond laser flash photolysis. S2O32− photochemistry was found to be oxygen-dependent. Quantum yield of S2O32− disappearance was measured; it was found to depend on irradiation wavelength and presence of dissolved oxygen in solutions. The photochemical properties were explained by means of model containing two primary reactions, namely photoionization and photodissociation of thiosulfate to S•− and SO3•− radical anions. In the presence of dissolved oxygen we were forced to propose the formation of a weak complex formed by thiosulfate and dioxygen, [S2O32−…O2]. The photoexcitation of this complex directly yields S2O3•− and O2•−. Kinetic behavior of intermediate absorption in laser flash photolysis experiments was found to be rather complicated because of the presence of an aquated electron and several sulfur-containing radical anions. In oxygen-free solutions these radicals are S2O3•−, •S4O63−, S•−, and SO3•−. In the presence of dissolved oxygen SO2•−, SO5•− and S2O5•− radicals as well as superoxide anion radical should be added to explain the kinetics of intermediate absorption. The existence of the S2O5•− radical anion was proposed for the first time. Several rate constants of these radicals’ reactions were measured.

The Photoinduced Electrocyclization Reaction of Triphenylamine (TPA) in Sustainable and Confined Micellar Solutions: A Steady‐State and Laser Flash Photolysis Approach

Abstract: The irradiation of triphenylamine (TPA) under homogeneous (cyclohexane, acetonitrile and methanol) and micellar [sodium dodecyl sulfate (SDS), cetyltrimethylammonium chloride (CTAC) and polyethylene glycol monododecyl ether (Brij-P35)] conditions has been investigated through a combined steady-state and time-resolved spectroscopic approach. Photolysis of TPA at 254 nm in different media afforded N-phenylcarbazole (N−PhCA) as the main photoproduct, and when the photoreaction of TPA was carried out in micellar solution the relative rate of formation of N−PhCA was found to be faster than in homogeneous media due to the environmental confined and hydrophobic micellar core. On the other hand, the transient N-phenyl-4a,4b-dihydrocarbazole (DHC0) was detected and fully characterized via laser flash photolysis. Finally, the location of TPA within the hydrophobic core of the micelle was investigated by 1D and 2D NMR spectroscopic analyses.

Probing reactions between imipramine and hydroxyl radical with the photolysis of iron(III) oxalate: Implications for the indirect photooxidation of tricyclic antidepressants in waters

Abstract: Photodegradation of imipramine (IMI), a widely used tricyclic antidepressant, in the presence of Fe(III) oxalate and potassium persulfate (PS) was studied for the first time by combination of steady state and laser flash photolysis. It was demonstrated that Fe(III) oxalate system exhibits an effective photooxidation of IMI due to the formation of reactive oxygen species (ROS) with a very high quantum yield, φROS(308 nm) ∼0.3. The quantum yield of the most important ROS, hydroxyl radical, was also measured for the first time (φOH(308 nm) = 0.25 ± 0.02) as well as the rate constant of the reaction between •OH radical and IMI (kOH = (1.5 ± 0.1) ×1010 M−1s−1). Nature and spectral properties of short lived organic radicals formed in this reaction were also determined experimentally and proved by quantum-chemical calculations. At a IMI concentration of less than 10 µM it is possible to achieve full disappearance of the initial compound in Fe(III)-oxalate system. Main aromatic by-products of the IMI oxidation are connected with its consequent hydroxylation and elimination of aliphatic substituent. Complete degradation of both IMI and all aromatic byproducts needs joint application of Fe(III) oxalate and PS, as additional oxidizing agent. Combined Fe(III) oxalate – PS system demonstrates high photoactivity under UV irradiation and could be used for the effective degradation of IMI and its close analogues in aqueous media.

Factors Governing Reactivity and Selectivity in Hydrogen Atom Transfer from C(sp³)–H Bonds of Nitrogen-Containing Heterocycles to the Cumyloxyl Radical

Abstract: A kinetic study of the hydrogen atom transfer (HAT) reactions from nitrogen-containing heterocycles (secondary and tertiary lactams, 2-imidazolidinones, 2-oxazolidinones, and succinimides) to the cumyloxyl radical has been carried out employing laser flash photolysis with ns time resolution. HAT occurs from the C–H bonds that are α to nitrogen, activated by hyperconjugative overlap with the N–C═O π system. In the lactam series, the second-order HAT rate constant (kH) was observed to decrease by a factor of ∼4 going from the five- and six-membered ring derivatives to the eight-membered ones, a behavior that was rationalized on the basis of a reduced extent of hyperconjugative activation associated to the greater flexibility of the larger rings compared to the smaller ones. In the five-membered-ring substrate series, the kH values were observed to increase by >3 orders of magnitude on going from succinimide to 2-imidazolidinones, a behavior that was explained in terms of the divergent contribution of hyperconjugative activation and deactivating electronic effects determined by ring functionalities. The results are discussed in the framework of the development of HAT-based C–H bond functionalization procedures.

Probing reactions between imipramine and hydroxyl radical with the photolysis of iron(III) oxalate: Implications for the indirect photooxidation of tricyclic antidepressants in waters

Abstract: Photodegradation of imipramine (IMI), a widely used tricyclic antidepressant, in the presence of Fe(III) oxalate and potassium persulfate (PS) was studied for the first time by combination of steady state and laser flash photolysis. It was demonstrated that Fe(III) oxalate system exhibits an effective photooxidation of IMI due to the formation of reactive oxygen species (ROS) with a very high quantum yield, φROS(308 nm) ∼0.3. The quantum yield of the most important ROS, hydroxyl radical, was also measured for the first time (φOH(308 nm) = 0.25 ± 0.02) as well as the rate constant of the reaction between •OH radical and IMI (kOH = (1.5 ± 0.1) ×1010 M−1s−1). Nature and spectral properties of short lived organic radicals formed in this reaction were also determined experimentally and proved by quantum-chemical calculations. At a IMI concentration of less than 10 µM it is possible to achieve full disappearance of the initial compound in Fe(III)-oxalate system. Main aromatic by-products of the IMI oxidation are connected with its consequent hydroxylation and elimination of aliphatic substituent. Complete degradation of both IMI and all aromatic byproducts needs joint application of Fe(III) oxalate and PS, as additional oxidizing agent. Combined Fe(III) oxalate – PS system demonstrates high photoactivity under UV irradiation and could be used for the effective degradation of IMI and its close analogues in aqueous media.

Sulfate Radical Anion: Laser Flash Photolysis Study and Application in Water Disinfection and Decontamination

Abstract: Sulfate radicals (SO 4 •- ) reactivity against gram-negative ( E. coli ) and gram-positive ( E. faecalis ) bacteria and Contaminants of Emerging Concern (CECs) (Diclofenac-DCF, Sulfamethoxazole-SMX and Trimethoprim-TMP) was investigated through laser flash photolysis (LFP) technique. Analysis of the lifetime of SO 4 •- in presence of cell-wall compounds of bacteria and CECs allowed determining reactivity of SO 4 •- towards these compounds. Results showed that SO 4 •- reacts with common cell-wall components through H-abstraction mechanism ( k SO4•− < 10 8 M -1 s -1 ). By contrast, k SO4•− > 10 9 M -1 s -1 were found using aromatic amino acids (AAA) only present in Porins of the gram-negative outer-membrane. The intermediates detected from the reaction of SO 4 •- with the AAA confirmed the involvement of electron transfer processes. Moreover, k SO4•− values determined for DCF, TMP and SMX also agreed with an electron transfer mechanism. Interestingly, bacteria and CECs removal at pilot plant scale by UV-C/SO 4 •- is in accordance with the k SO4•− obtained using the LFP: E. coli > E. faecalis and DCF > TMP ≅ SMX. • LFP as a tool to investigate SO 4 •- reactivity towards biological and chemical targets. • SO 4 •- reacts through H abstraction with the cell wall constituents with k SO4•−,Q of 107 M -1 s -1 . • SO 4 •- reacts by e - transfers with amino acids, making gram negative bacteria more susceptible to SR-AOP. • CECs quickly reacted with SO 4 •- , showing k SO4•− of 10 9 M -1 s -1 through e- transfer mechanism. • UVC/PS results at pilot plant scale confirm feasibility of SR-AOP to decontaminate and disinfect water.

Substituents affect the mechanism of photochemical E-Z isomerization of diarylethene triazoles via adiabatic singlet excited state pathway or via triplet excited state

Abstract: Photochemical reactivity in the Z-E isomerization for two heterostilbene derivatives containing 1,2,3-triazole unit were investigated theoretically and experimentally by irradiation experiments, fluorescence and laser flash photolysis (LFP). The molecules were designed to probe the effect of the para-nitro group in 1 on the photochemical E-Z pathways, as well as to investigate the steric effect of the ortho-methyl group in 2. The quantum yield for the Z → E isomerization for both cis-isomers is 0.42, and for the E → Z is somewhat lower 0.16 and 0.12, respectively. Furthermore, fluorescence measurements for the ortho-methyl derivative indicated that the Z → E isomerization takes place in an adiabatic reaction on the potential energy surface of the S1 state. On the contrary, the para-nitro derivative undergoes the Z → E isomerization via a triplet excited state, which was detected by LFP. For both cis- and trans-isomers of the nitro derivative a transient was detected absorbing with a maximum at 520 nm, which was assigned to the triplet excited state of the trans-isomer. All experimental observations were corroborated by computations. The stationary points were computed at the PBE50/6-31++G** level of theory, whereas potential energy surfaces were obtained by linear interpolation and computations at the SF-TDDFT/PBE50/6-31++G** level of theory. The mechanistic investigation presented gives insight in the fundamental and simple Z → E isomerization and provides new findings which are important in the rational design of different photoreactive diarylethene derivatives used in different fields of science.

Proton-controlled Action of an Imidazole as Electron Relay in a Photoredox Triad

Abstract: Electron relays play a crucial role for efficient light-induced activation by a photo-redox moiety of catalysts for multi-electronic transformations. Their insertion between the two units reduces detrimental energy transfer quenching while establishing at the same time unidirectional electron flow. This rectifying function allows charge accumulation necessary for catalysis. Mapping these events in photophysical studies is an important step towards the development of efficient molecular photocatalysts. Three modular complexes comprised of a Ru-chromophore, an imidazole electron relay function, and a terpyridine unit as coordination site for a metal ion were synthesized and the light-induced electron transfer events studied by laser flash photolysis. In all cases, formation of an imidazole radical by internal electron transfer to the oxidized chromophore was observed. The effect of added base evidenced that the reaction sequence depends strongly on the possibility for deprotonation of the imidazole function in a proton-coupled electron transfer process. In the complex with MnII present as a proxy for a catalytic site, a strongly accelerated decay of the imidazole radical together with a decreased rate of back electron transfer from the external electron acceptor to the oxidized complex was observed. This transient formation of an imidazolyl radical is clear evidence for the function of the imidazole group as an electron relay. The implication of the imidazole proton and the external base for the kinetics and energetics of the electron trafficking is discussed.

Proton-controlled Action of an Imidazole as Electron Relay in a Photoredox Triad

Abstract: Electron relays play a crucial role for efficient light-induced activation by a photo-redox moiety of catalysts for multi-electronic transformations. Their insertion between the two units reduces detrimental energy transfer quenching while establishing at the same time unidirectional electron flow. This rectifying function allows charge accumulation necessary for catalysis. Mapping these events in photophysical studies is an important step towards the development of efficient molecular photocatalysts. Three modular complexes comprised of a Ru-chromophore, an imidazole electron relay function, and a terpyridine unit as coordination site for a metal ion were synthesized and the light-induced electron transfer events studied by laser flash photolysis. In all cases, formation of an imidazole radical by internal electron transfer to the oxidized chromophore was observed. The effect of added base evidenced that the reaction sequence depends strongly on the possibility for deprotonation of the imidazole function in a proton-coupled electron transfer process. In the complex with MnII present as a proxy for a catalytic site, a strongly accelerated decay of the imidazole radical together with a decreased rate of back electron transfer from the external electron acceptor to the oxidized complex was observed. This transient formation of an imidazolyl radical is clear evidence for the function of the imidazole group as an electron relay. The implication of the imidazole proton and the external base for the kinetics and energetics of the electron trafficking is discussed.

Accumulation of mono-reduced [Ir(piq)_2(LL)] photosensitizers relevant for solar fuels production

Abstract: A series of nine [Ir(piq)2(LL)]+.PF6- photosensitizers, where piqH = 1-phenylisoquinoline, was developed and investigated for excited-state electron transfer with sacrificial electron donors that included triethanolamine (TEOA), triethylamine (TEA) and 1,3-dimethyl-2-phenyl-2,3-dihydro-1H-benzo[d]imidazole (BIH) in acetonitrile. The photosensitizers were obtained in 57-82% yield starting from the common [Ir(piq)2µ-Cl]2 precursor and were all characterized by UV-Vis absorption as well as by steady-state, time-resolved spectroscopies and electrochemistry. The excited-state lifetimes ranged from 250 to 3350 ns and excited-state electron transfer quenching rate constants in the 109 M-1 s-1 range were obtained when BIH was used as electron donor. These quenching rate constants were three orders of magnitude higher than when TEA or TEOA was used. Steady-state photolysis in the presence of BIH showed that the stable and reversible accumulation of mono-reduced photosensitizers was possible, highlighting the potential use of these Ir-based photosensitizers in photocatalytic reactions relevant for solar fuels production.

Accumulation of mono-reduced [Ir(piq)2(LL)] photosensitizers relevant for solar fuels production

Abstract: A series of nine [Ir(piq)2(LL)]+.PF6- photosensitizers, where piqH = 1-phenylisoquinoline, was developed and investigated for excited-state electron transfer with sacrificial electron donors that included triethanolamine (TEOA), triethylamine (TEA) and 1,3-dimethyl-2-phenyl-2,3-dihydro-1H-benzo[d]imidazole (BIH) in acetonitrile. The photosensitizers were obtained in 57-82% yield starting from the common [Ir(piq)2µ-Cl]2 precursor and were all characterized by UV-Vis absorption as well as by steady-state, time-resolved spectroscopies and electrochemistry. The excited-state lifetimes ranged from 250 to 3350 ns and excited-state electron transfer quenching rate constants in the 109 M-1 s-1 range were obtained when BIH was used as electron donor. These quenching rate constants were three orders of magnitude higher than when TEA or TEOA was used. Steady-state photolysis in the presence of BIH showed that the stable and reversible accumulation of mono-reduced photosensitizers was possible, highlighting the potential use of these Ir-based photosensitizers in photocatalytic reactions relevant for solar fuels production.

Cyclometalated Spirobifluorene Imidazolylidene Platinum(II) Complexes with Predominant 3LC Emissive Character and High Photoluminescence Quantum Yields.

Abstract: Two novel bidentate C^C*spiro cyclometalated platinum(II) complexes comprising a spiro-conjugated bifluorene ligand and different β-diketonate auxiliary ligands are synthesized and characterized. Their preparation employs a robust and elaborate synthetic protocol commencing with an N-heterocyclic carbene precursor. Structural characterization by means of NMR techniques and solid-state structures validate the proposed and herein presented molecular scaffolds. Photophysical studies, including laser flash photolysis methods, reveal an almost exclusively ligand-centered triplet state, governed by the C^C*spiro-NHC ligand. The high triplet energies and the long triplet lifetimes in the order of 30 μs in solution make the complexes good candidates for light-emitting diode-driven photocatalysis, as initial energy transfer experiments reveal. In-depth time-dependent density functional theory investigations are in excellent accordance with our spectroscopic findings. The title compounds are highly emissive in the bluish-green color region with quantum yields of up to 87% in solid-state measurements.

Probing the Role of Murine Neuroglobin CDloop–D-Helix Unit in CO Ligand Binding and Structural Dynamics

Abstract: We produced a neuroglobin variant, namely, Ngb CDless, with the excised CDloop- and D-helix, directly joining the C- and E-helices. The CDless variant retained bis-His hexacoordination, and we investigated the role of the CDloop–D-helix unit in controlling the CO binding and structural dynamics by an integrative approach based on X-ray crystallography, rapid mixing, laser flash photolysis, resonance Raman spectroscopy, and molecular dynamics simulations. Rapid mixing and laser flash photolysis showed that ligand affinity was unchanged with respect to the wild-type protein, albeit with increased on and off constants for rate-limiting heme iron hexacoordination by the distal His64. Accordingly, resonance Raman spectroscopy highlighted a more open distal pocket in the CO complex that, in agreement with MD simulations, likely involves His64 swinging inward and outward of the distal heme pocket. Ngb CDless displays a more rigid overall structure with respect to the wild type, abolishing the structural dynamics of the CDloop–D-helix hypothesized to mediate its signaling role, and it retains ligand binding control by distal His64. In conclusion, this mutant may represent a tool to investigate the involvement of CDloop–D-helix in neuroprotective signaling in a cellular or animal model.

Solvent-Modulated Self-Assembly of Naphthalenediimide-Based Cd(II) Complexes and the Controllable Photochromism via Conformational Isomerization.

Abstract: Rational regulation of the properties of photochromic materials is a challenging and meaningful work. In the present work, NDI-based complexes, namely, [Cd0.5(NDI)(HBDC)]·H2O (1) and a series of conformational isomers of {[Cd(NDI)0.5(BDC)]·MeCN}n (2), were synthesized by varying the solvent conditions (H2BDC = terephthalic acid, NDI = N,N'-bis(3-pyridylcarbonylhydrazine)-1,4,5,8-naphthalene diimide). Complex 1 exhibits a 0D mononuclear structure without photochromic behavior due to the bad conjugation of the naphthalene diimide moiety. The conformational isomers of complex 2 manifest a 3D network, showing ultra-fast photo-induced intermolecular electron transfer photochromic behavior under X-ray, UV, and visible light. However, they show different photochromic rates and coloring contrast upon photoirradiation, which originates from their difference in the distances of lone pair(COO)···π(NDI). This was realized via controlling the solvent ratio in the reaction system. In addition, compared to UV/X-ray light, 2 exhibits greater sensitivity to visible light and is an organic-inorganic hybrid material with photomodulated luminescence. Based on the excellent performance, complex 2 can be applied to filter paper, showing potential applications as an inkless printing medium and selective perception of ammonia and amine vapors in the solid state via different visual color changes.