Flash Photolysis Studies 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.

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.

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.

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

Laser flash photolysis and quantum chemical studies of UV degradation of pharmaceutical drug chloramphenicol: Short-lived intermediates, quantum yields and mechanism of photolysis

Using methods of time-resolved and stationary photolysis, HPLC-MS and quantum-chemical calculations by the DFT method, the mechanism of direct UV photolysis of the antibiotic chloramphenicol (CAP) was established. For the first time, short-lived intermediates formed during photolysis were detected. The primary photoprocess is the cleavage of the β-C-C bond relative to the aromatic system with the formation of 4-nitrobenzylalcohol radical and residual aliphatic radical. The first radical in deoxygenated solutions predominantly transforms into para-nitrobenzaldehyde and its secondary photolysis products. In the presence of oxygen, the aromatic radical and para-nitrobenzaldehyde are transformed into para-nitrosobenzoic and para-nitrobenzoic acids as a result of reaction with reactive oxygen species (ROS). Formation of ROS is provoked by reactions of aliphatic radical with dissolved oxygen, so this radical is very important for CAP degradation. The quantum yield of direct photolysis of CAP is ∼3% and does not depend on the presence of dissolved oxygen and on the change of the excitation wavelength in the range of 254–308 nm. Obtained data are important for further understanding of the transformation pathways of CAP and similar PPCP in natural and wastewaters under the action of sunlight and artificial UV radiation.

Nanosecond Laser Flash Photolysis Study of the Photochemistry of 2‑Alkoxy Thioxanthones

Laser flash photolysis studies (l = 355 nm) on the characterization and reactivity for the triplet excited state of 2-methoxythioxanthone, 2-benzyloxythioxanthone, and 2-(n-propoxy) thioxanthone revealed that the transients generated in their excitation, in acetonitrile, have absorption bands with maxima at 310 and 620 nm. These transients decayed by a mixed 1st and 2nd order kinetics, with a lifetime of 5.3 ms for 2-methoxythioxanthone and benzyloxythioxanthone and 3.9 ms for 2-(n-propoxy) thioxanthone. In methanol, ethanol, and 2-propanol, a new absorption at 430-460 nm was observed, which was attributed to their corresponding ketyl radical (the second-order quenching rate constant (kq) is ca. 105 L mol-1 s-1). The energy transfer rate constants for trans-stilbene, 1-methylnaphthalene, and 1,3-cyclohexadiene were diffusion-controlled, indicating a triplet energy for these alkoxy thioxanthones higher than 61 kcal mol-1. The quenching rate constant observed for 1,4-cyclohexadiene (ca.109 L mol-1 s-1) suggests that the lowest energy triplet excited state for these thioxanthones must have nπ* character. For phenols and indole (kq ca. 109 L mol-1 s-1), the reaction mechanism must involve the initial formation of a triplet exciplex, followed by a coupled electron/proton transfer. For triethylamine and 1,4-diazabicyclo[2.2.2]octane (kq ca. 109 L mol-1 s-1) it was observed the formation of the radical anion of the alkoxy thioxanthones formed from electron transfer (l = 410 nm).

Aerobic self-photooxidation of arylphosphines studied by steady state and laser flash photolyses

Arylphosphines (ArP) undergo UV-photodecomposition with quantum yields of 2 in aerated acetonitrile to form the corresponding oxides (ArPO). Laser flash photolysis studies showed that triplet ArP was quenched by the dissolved oxygen. Based on these results, it was concluded that singlet oxygen produced by triplet energy transfer from triplet ArP is involved as the primary oxidant in the formation of ArPO upon photolysis of ArP. We discuss the mechanism for forming ArPO upon photolysis of ArP with the over-unity quantum yield by considering phosphadioxiranes of ArP as the secondary oxidant.

A laser flash photolysis study of the free radical chemistry of lipoic acid and dihydrolipoic acid.

The free radical chemistry of lipoic acid (LA) and dihydrolipoic acid (DHLA) intersect at the point where DHLA loses hydrogen to a good hydrogen abstracting radical, while LA reacts with strongly reducing ketyl radicals capable of donating a hydrogen atom. While aliphatic thiyl radicals have an absorbance at ~ 330nm, the resulting radical, formally also a thiyl radical has distinct spectroscopic properties with a maximum at 385nm, suggesting that the two sulphur centres interact strongly with each other as part of the chromophore. The reactions that form these radicals were studied by laser flash photolysis that revealed DHLA as an excellent hydrogen donor, while LA is an excellent hydrogen acceptor. The results support earlier evidence that the real antioxidant is DHLA, while LA is not; yet, the reported facile interconversion of the two molecules suggests that LA may be a better supplement, given its shelf stability, compared with a far more difficult-to-handle DHLA.

Internal Proton Transfer in the Activation of Heliorhodopsin

Heliorhodopsin (HeR), a recently discovered new rhodopsin family, contains a single counterion of the protonated Schiff base, E108 in HeR from Thermoplasmatales archaeon SG8-52-1 (TaHeR). Upon light absorption, the M and O intermediates form in HeRs, as well as type-1 microbial rhodopsins, indicating that the proton transfer from the Schiff base leads to the activation of HeRs. The present flash photolysis study of TaHeR in the presence of a pH-sensitive dye showed that TaHeR contains a proton-accepting group (PAG) inside protein. Comprehensive mutation study of TaHeR found the E108D mutant abolishing the M formation, which is not only at pH 8, but also at pH 9 and 10. The lack of M observation does not originate from the short lifetime of the M intermediate in E108D, as FTIR spectroscopy revealed that a red-shifted K-like intermediate is long lived in E108D. It is likely that the K-like intermediate returns to the unphotolyzed state without internal proton transfer in E108D. E108 and D108 are the Schiff base counterions of the wild-type and E108D mutant TaHeR, respectively, whereas small difference in length of side chains determine internal proton transfer reaction from the Schiff base. Based on the present finding, we propose that the internal water cluster (four water molecules) constitutes PAG in the M intermediate of TaHeR. In the wild type TaHeR, a protonated water cluster is stabilized by forming a salt bridge with E108. In contrast, slightly shortened counterion (D108) cannot stabilize the protonated water cluster in E108D, and thus impairs internal proton transfer from the Schiff base.

Laser flash photolysis study of Nb2O5/g-C3N4 heterostructures as efficient photocatalyst for molecular H2 evolution

Improvements of visible light activity, slow recombination rate, stability, and efficiency are major challenges facing photocatalyst technologies today. Utilizing heterostructures of g-C3N4 (bandgap ∼2.7eV) with Nb2O5 (bandgap ∼3.4eV) as an alternative materials for the first time, we tried to overcome such challenges in this work. Heterostructures of Nb2O5/g-C3N4 have been synthesized via hydrothermal technique. And then a time-resolved laser flash photolysis of those heterostructures has been analyzed, focusing on seeking how to improve photocatalytic efficiency for molecular hydrogen (H2) evolution. The transient absorption spectra and the lifetime of charge carriers at different wavelengths have been observed for Nb2O5/g-C3N4, where g-C3N4 was used for a control. The role of hole scavenger (methanol) has also been investigated for the purpose of boosting charge trapping and H2 evolution. The long lifetime of Nb2O5/g-C3N4 heterostructures (6.54165 μs) compared to g-C3N4 (3.1651897 μs) has successfully supported the increased H2 evolution of 75 mmol/h.g. An enhancement in the rate of H2 evolution (160 mmol/h.g) in the presence of methanol has been confirmed. This study not only deepens our understanding of the role of scavenger, but also enables a rigorous quantification of the recombination rate crucial for photocatalytic applications in relation with efficient H2 production.

Impact of protein-chromophore interaction on the retinal excited state and photocycle of Gloeobacter rhodopsin: role of conserved tryptophan residues.

The function of microbial as well as mammalian retinal proteins (aka rhodopsins) is associated with a photocycle initiated by light excitation of the retinal chromophore of the protein, covalently bound through a protonated Schiff base linkage. Although electrostatics controls chemical reactions of many organic molecules, attempt to understand its role in controlling excited state reactivity of rhodopsins and, thereby, their photocycle is scarce. Here, we investigate the effect of highly conserved tryptophan residues, between which the all-trans retinal chromophore of the protein is sandwiched in microbial rhodopsins, on the charge distribution along the retinal excited state, quantum yield and nature of the light-induced photocycle and absorption properties of Gloeobacter rhodopsin (GR). Replacement of these tryptophan residues by non-aromatic leucine (W222L and W122L) or phenylalanine (W222F) does not significantly affect the absorption maximum of the protein, while all the mutants showed higher sensitivity to photobleaching, compared to wild-type GR. Flash photolysis studies revealed lower quantum yield of trans-cis photoisomerization in W222L as well as W222F mutants relative to wild-type. The photocycle kinetics are also controlled by these tryptophan residues, resulting in altered accumulation and lifetime of the intermediates in the W222L and W222F mutants. We propose that protein-retinal interactions facilitated by conserved tryptophan residues are crucial for achieving high quantum yield of the light-induced retinal isomerization, and affect the thermal retinal re-isomerization to the resting state.

Phenacyl Bromide as a Single‐Component Photoinitiator: Photoinduced Step‐Growth Polymerization of N ‐Methylpyrrole and N ‐Methylindole

The synthesis of step-growth polymers by photoinduced methods is a challenging issue in synthetic chemistry. Here, we report a single component near UV responsive photopolymerization system for step-growth polymerization of N -methylpyrrole (MPyr) and N -methylindole (MIn) by using phenacyl bromide (PAB). The obtained high molecular weight conjugated polymers were characterized by spectral and chromatographic methods. Detailed laser flash photolysis and spectroscopic studies revealed that polymerization proceeds by successive photoinduced cleavage of PAB followed by electron transfer, proton release and coupling processes. After photolysis, chain growth continues also in daylight or dark by acidic oxidation. The structural features of the polymers were investigated. PAB single component photoinitiator enables an efficient, rapid, room temperature step-growth polymerization process of MPyr and MIn that can be applied to other conjugated monomers.

Phenacyl Bromide as a Single-Component Photoinitiator: Photoinduced Step-Growth Polymerization of N-Methylpyrrole and N-Methylindole.

The synthesis of step-growth polymers by photoinduced methods is a challenging issue in synthetic chemistry. Here, we report a single component near UV responsive photopolymerization system for step-growth polymerization of N-methylpyrrole (MPyr) and N-methylindole (MIn) by using phenacyl bromide (PAB). The obtained high molecular weight conjugated polymers were characterized by spectral and chromatographic methods. Detailed laser flash photolysis and spectroscopic studies revealed that polymerization proceeds by successive photoinduced cleavage of PAB followed by electron transfer, proton release and coupling processes. After photolysis, chain growth continues also in daylight or dark by acidic oxidation. The structural features of the polymers were investigated. PAB single component photoinitiator enables an efficient, rapid, room temperature step-growth polymerization process of MPyr and MIn that can be applied to other conjugated monomers.

Sulfate radical anion: Laser flash photolysis study and application in water disinfection and decontamination

Sulfate radicals (SO4•-) 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 SO4•- in presence of cell-wall compounds of bacteria and CECs allowed determining reactivity of SO4•- towards these compounds. Results showed that SO4•- reacts with common cell-wall components through H-abstraction mechanism (kSO4•− < 108 M-1s-1). By contrast, kSO4•− > 109 M-1s-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 SO4•- with the AAA confirmed the involvement of electron transfer processes. Moreover, kSO4•− 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/SO4•- is in accordance with the kSO4•− obtained using the LFP: E. coli > E. faecalis and DCF > TMP ≅ SMX.

Molecular Rubies in Photoredox Catalysis.

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.

Photocatalytic generation of a non-heme Fe(iii)-hydroperoxo species with O2 in water for the oxygen atom transfer reaction.

Coupling a photoredox module and a bio-inspired non-heme model to activate O2 for the oxygen atom transfer (OAT) reaction requires a vigorous investigation to shed light on the multiple competing electron transfer steps, charge accumulation and annihilation processes, and the activation of O2 at the catalytic unit. We found that the efficient oxidative quenching mechanism between a [Ru(bpy)3]2+ chromophore and a reversible electron mediator, methyl viologen (MV2+), to form the reducing species methyl viologen radical (MV˙+) can convey an electron to O2 to form the superoxide radical and reset an Fe(iii) species in a catalytic cycle to the Fe(ii) state in an aqueous solution. The formation of the Fe(iii)-hydroperoxo (FeIII-OOH) intermediate can evolve to a highly oxidized iron-oxo species to perform the OAT reaction to an alkene substrate. Such a strategy allows us to bypass the challenging task of charge accumulation at the molecular catalytic unit for the two-electron activation of O2. The FeIII-OOH catalytic precursor was trapped and characterized by EPR spectroscopy pertaining to a metal assisted catalysis. Importantly, we found that the substrate itself can act as an electron donor to reset the photooxidized chromophore in the initial state closing the photocatalytic loop and hence excluding the use of a sacrificial electron donor. Laser Flash Photolysis (LFP) studies and spectroscopic monitoring during photocatalysis lend credence to the proposed catalytic cycle.

Unveiling the Mechanism for the Photochemistry and Photodegradation of Vanillin.

The photolysis of vanillin produces a short-lived triplet state where its lifetime is controlled by efficient self-quenching (kSQ ~ 2 × 109 m-1 s-1 ) which also generates radicals. Free radical reactions, including vanillin dimer formation, are responsible for the degradation of vanillin and is accompanied by yellowing of the acetonitrile solutions. Laser flash photolysis studies reveal a triplet absorbing at 390 nm, readily quenched by naphthalenes, conjugated dienes and oxygen. Vanillin is also a good singlet oxygen sensitizer as revealed by its characteristic NIR emission at 1270 nm.

Phthalocyanine as a Bioinspired Model for Chlorophyll f-Containing Photosystem II Drives Photosynthesis into the Far-Red Region.

The textbook explanation that P680 pigments are the red limit to drive oxygenic photosynthesis must be reconsidered by the recent discovery that chlorophyll f (Chlf)-containing Photosystem II (PSII) absorbing at 727 nm can drive water oxidation. Two different families of unsymmetrically substituted Zn phthalocyanines (Pc) absorbing in the 700-800 nm spectral window and containing a fused imidazole-phenyl substituent or a fused imidazole-hydroxyphenyl group have been synthetized and characterized as a bioinspired model of the Chlf/TyrosineZ /Histidine190 cofactors of PSII. Transient absorption studies in the presence of an electron acceptor and irradiating in the far-red region evidenced an intramolecular electron transfer process. Visible and FT-IR signatures indicate the formation of a hydrogen-bonded phenoxyl radical in ZnPc II-OH. This study sets the foundation for the utilization of a broader spectral window for multi-electronic catalytic processes with one of the most robust and efficient dyes.

Singlet fission in nanoaggregate of bis(phenylethynyl) derivative of benzene (BPEB): High energy triplet exciton generation with >100 % yield

1,4-bis(phenylethynyl)benzene (BPEB) is observed to show a prominent ∼0.17 eV red shift of absorption band due to monomer-aggregate conversion. Along with the absorption band shift it also causes nearly one order reduction in both emission yield and singlet lifetime. The ultrafast transient absorption measurements show annihilation free decay of singlet excitons with ∼40 ps lifetime for this nanoaggregate. Concomitant growth of triplet excitonic absorption and ground state bleach signal with similar time constant ensures presence of singlet fission (SF) process in the present system. Flash photolysis study indicates separated triplet exciton yield of ∼139 % and its decay with ∼4.4 ms lifetime. Observation of triplet-triplet annihilation (TTA) and resultant delayed fluorescence process in this NA also confirms the near-isoergic or endoergic nature of SF process. It also indicates the presence of high energy triplet excitonic state which can be utilized to boost the silicon solar cell efficiency or sensitize other materials having application in photodynamic therapy or near-IR phosphorescence based bio-imaging.

Synthesis, theoretical calculations and laser flash photolysis studies of selected amphiphilic porphyrin derivatives used as biofilm photodegradative materials

Two positively charged metalloporphyrins were synthesized and theoretical studies were carried out. The complexes efficiently eradicated S. aureus and E. coli bacterial cells at 415 nm.