Photoreduction Products Research Articles

Photochemistry of 2′-halobenzanilides: reaction mechanism

The UV photolysis of acetonitrile solutions of 2′-halobenzanilide were followed by UV-spectroscopy, HPLC and time resolved laser flash photolysis. The chemical yields of the major products produced from the irradiation with medium pressure Hg-lamp were measured. The chemical yields of the photosubstitution product ranged from 3 to 45% (the yield was 3% when the starting material was a solution of 2′-chlorobenzanilide in CH 3CN and was 45% when the starting material was a solution of 2′-bromobenzanilide in 10% 1 mol NaOH:CH 3CN). The quantum yield measurements of the photoproducts were carried out in N 2 and O 2 atmosphere in the presence and absence of OH ions. The results showed an enhancement in the yield of the photosubstituted product in the presence of OH ions and in switching from chloro- to bromo-substituent. The presence of O 2 lowered the yield of photosubstitution and photoreduction products. Time resolved studies showed two peaks, one appeared at 395 nm and the second centred around 315 nm. These two peaks were identical in both cases of 2′-halobenzanilides (halo = Cl or Br).

Femtosecond Dynamics of Flavin Cofactor in DNA Photolyase: Radical Reduction, Local Solvation, and Charge Recombination

We report here our femtosecond studies of the photoreduction dynamics of the neutral radical flavin (FADH) cofactor in E. coli photolyase, a process converting the inactive form to the biologically active one, a fully reduced deprotonated flavin FADH(-). The observed temporal absorption evolution revealed two initial electron-transfer reactions, occurring in 11 and 42 ps with the neighboring aromatic residues of W382 and F366, respectively. The new transient absorption, observed at 550 nm previously in photolyase, was found from the excited-state neutral radical and is probably caused by strong interactions with the adenine moiety through the flavin U-shaped configuration and the highly polar/charged surrounding residues. The solvation dynamics from the locally ordered water molecules in the active site was observed to occur in approximately 2 ps. These ultrafast ordered-water motions are critical to stabilizing the photoreduction product FADH(-) instantaneously to prevent fast charge recombination. The back electron-transfer reaction was found to occur in approximately 3 ns. This slow process, consistent with ultrafast stabilization of the catalytic cofactor, favors photoreduction in photolyase.

９，１０‐アントラキノンを溶解した各種高分子のフォトクロミズムとＥＤＡ錯体

Some polymers were mixed with 9, 10-anthraquinone (AQ); their photochromism and photoreduction products were investigated by using electronic absorption. For the films that contained an ester group in a structural unit, the colors of these films when irradiated with ultraviolet ray (380 nm) have been confirmed to change from pale yellow to vermilion. The reverse color change has been observed after the irradiation with the heating from room temperature up to glass transition temperature (Tg) of the films. The absorptions appeared around 380 nm and 500 nm. After the irradiation, the absorption band at 380 nm is characteristic of anthrahydroquinone (AQH2), The broad band around 500 nm is assigned to the electron donor-acceptor (EDA) because of its characteristics. These phenomena are because of the involvement of the interactions between polymers with ester groups and AQ and AQH2.

Further solid-phase microextraction–gas chromatography–mass spectrometry applications: “on-fibre” and aqueous photodegradation of nitro musks☆

Photodegradation kinetics of four nitro musks (musk xylene, musk tibetene, musk ketone and musk moskene) in solid-phase microextraction (SPME) fibres (“photo-SPME”) and in an aqueous system were studied by means of gas chromatography–mass spectrometry (MS) detection. Musks fragrances were extracted from aqueous solutions using SPME fibres that subsequently were exposed to different UV irradiation times, generating photoproducts easily characterized by their mass spectra. Aqueous photodegradation studies followed by SPME were also performed and compared to photo-SPME. The same fourteen photoproducts have been detected in both media. The potential of this approach to readily calculate apparent rate constants (from 10−4 to 10−3 s−1), half-life times (from 3 to 32 min) and reaction orders (n = 1) in both media is stated. The main photoreaction determined was photocyclization, but products of photoreduction and photorearrangement have also been detected. Tentative photodegradation pathways for musk moskene and musk tibetene are proposed for the first time.

Further solid-phase microextraction–gas chromatography–mass spectrometry applications: “on-fibre” and aqueous photodegradation of nitro musks

Photodegradation kinetics of four nitro musks (musk xylene, musk tibetene, musk ketone and musk moskene) in solid-phase microextraction (SPME) fibres (“photo-SPME”) and in an aqueous system were studied by means of gas chromatography–mass spectrometry (MS) detection. Musks fragrances were extracted from aqueous solutions using SPME fibres that subsequently were exposed to different UV irradiation times, generating photoproducts easily characterized by their mass spectra. Aqueous photodegradation studies followed by SPME were also performed and compared to photo-SPME. The same fourteen photoproducts have been detected in both media. The potential of this approach to readily calculate apparent rate constants (from 10 −4 to 10 −3 s −1), half-life times (from 3 to 32 min) and reaction orders ( n = 1) in both media is stated. The main photoreaction determined was photocyclization, but products of photoreduction and photorearrangement have also been detected. Tentative photodegradation pathways for musk moskene and musk tibetene are proposed for the first time.

Confirming the pinking mechanism of PVC profiles in mild climatic conditions

AbstractPinking of TiO2‐pigmented PVC can be observed for profiles exposed to the mild environmental conditions of northern European countries. In a previous paper, the pinking phenomenon was accounted for at the molecular level by the formation of polyene sequences, which are not completely photooxidatively bleached in conditions of oxygen starvation. This causation has been confirmed by exposing to visible light in a SEPAP 12.24 apparatus profiles in which pinking was produced during 4 year‐weathering in N.W. France. The fast phototransformations in visible light of chloropolyenes in trans‐configurations leading to α‐chloropolyenes in cis‐configurations accounts for the conversion of pinking into limited graying. It appears that the first and more intense graying observed after the initial exposure to UV and visible light of profiles in the SEPAP 12.24 can be essentially attributed to the photoreduction products of lead stabilizers. Graying could be bleached through thermooxidation at 65°C, could not be converted into pinking, and has no environmental relevance.

Identification of new photoreduction products of pentachlorophenol.

Pentachlorophenol (PCP), a fungicide and insecticide, leaves behind considerable residue in the environment in the form of unreacted molecule, metabolites and photoreduction products. Most of the previously reported photoproducts have resulted from dechlorination of the parent molecule. We have shown that photoreduction of PCP also proceeds by photoreduction of the parent molecule at carbons 2 and 4. These products were identified and characterized as 2-dehydro-2,3,4,5,6-pentachlorocyclohexanone (II) and 2,3,4,5,6-pentachlorocyclohexanone (IV). Both of them display keto-enol tautomerism. Identification and characterization of new photoproducts of PCP is significant for determining its overall environmental impact in the ecosystem.

Photoreaction of 2′-chloro-4-R-benzanilide: synthesis of 2-(4-R-phenyl)-1,3-benzoxazole and 9-R-phenanthridin-6(5H)-one (R=H, CH3, CH3O)

The UV photolysis of 2′-chloro-4-R-benzanilides in acetonitrile solution under nitrogen atmosphere leads to the formation of intramolecular photocyclization products (9-R-phenanthridin-6(5)-one, 20–30%), along with the minor photoreduction, photo-Fries, and intramolecular photosubstitution products. The photoreaction of 2′-chloro-4-R-benzanilide in acetonitrile solution containing 10% of water or aqueous sodium hydroxide however gives 2-(4-R-phenyl)-1,3-benzoxazole as a major product (∼30%) along with the photoreduction, photo-Fries and intramolecular photocyclization products.

Solar and ultraviolet N-dealkylation of N, N-dimethylaminobenzylidene malonic acid derivatives via photoexcited polycyclic nitroaromatic compounds

Broadband irradiation of N, N-dimethylaminobenzylidene malonic acid derivatives in benzene solution with polycyclic nitroaromatic compounds, namely 1-nitronaphthalene, 9-nitroanthracene and 1-nitropyrene leads to mono-alkylated products predominant, accompanied by the corresponding photoreduction products of the nitroaromatics. Lower conversion was observed in acetonitrile concomitant with 2–4% of the mono-alkylated products, however, no conversion was observed in methanol as solvent. The structures of the products were assigned based on IR, 1H-, 13C NMR and mass spectra, elemental analyses and comparison with authentic samples.

Photoreaction of 2-halo-N-pyridinylbenzamide: intramolecular cyclization mechanism of phenyl radical assisted with n-complexation of chlorine radical.

The photochemical behavior of 2-halo-N-pyridinylbenzamide (1-4 in Chart 1) was studied. The photoreaction of 2-chloro-N-pyridinylbenzamides 1a, 2a, 3a, and 4 afforded photocyclized products, benzo[c]naphthyridinones (6-9 and 16), in high yield, whereas the bromo analogues 1b, 2b, and 3b produced extensively photoreduced products, N-pyridinylbenzamides (1c, 10, and 11), with minor photocyclized product. Since the photocyclization reaction of 2-chloro-N-pyridinylbenzamide is retarded by the presence of oxygen and sensitized by the presence of a triplet sensitizer, acetone or acetophenone, a triplet state of the chloro analogue is involved in the reaction. Since several radical intermediates, particularly n-complexes of chlorine radical, are identified in the laser flash photolysis of 2-chloro-N-pyridinylbenzamide, an intramolecular cyclization mechanism of phenyl radical assisted with n-complexation of chlorine radical for the cyclization reaction is proposed: the triplet state (78 kcal/mol) of the chloro analogue (1a), which is populated by the excitation of 1a undergoes a homolytic cleavage of the C-Cl bond to give phenyl and chlorine radicals; while chlorine radical holds the neighbor pyridinyl ring with its n-complexation, the intramolecular arylation of the phenyl radical with the pyridinyl ring proceeds to produce a conjugated 2,3-dihydropyridinyl radical and then the conjugated radical aromatizes to afford a cyclized product, benzo[c]naphthyridinone by ejecting a hydrogen. The photoreduction product can be formed by hydrogen atom abstraction of the phenyl sigma radical from the environment.

Photochemistry and phototoxicity studies of flutamide, a phototoxic anti-cancer drug

The phototoxic anti-cancer drug flutamide is photolabile under UV-B light in either aerobic or anaerobic conditions. Irradiation of a methanol solution of this drug produces several photoproducts, one by photoreduction of the nitro group, one by rupture of the aromatic-NO 2 bond of the parent compound, two as a result of the rupture of the CO–NH bond and one derived from the photoreduction product by scission of the aromatic-NH 2 bond. Flutamide shows a photohemolytic effect on human erythrocytes and photoinduces lipid peroxidation. Studies on peripheral blood polymorphonuclear cells (neutrophils) demonstrated the phototoxicity of flutamide as well as inhibition of the cytotoxicity respiratory burst by the photoproduct derived from its photoreduction. The results suggest that the inhibition of the respiratory burst observed in phorbol myristate acetate (PMA)-activated cells is mediated by photosensitization and concomitant singlet oxygen production and/or formation of toxic photoproducts.

New phototriggers: extending the p-hydroxyphenacyl pi-pi absorption range.

[equation--see text] Introducing 3-methoxy or 3,5-dimethoxy substituents on the 4-hydroxyphenacyl (pHP) photoremovable protecting group has been explored with two excitatory gamma-amino acids, L-glutamic acid and gamma-amino butyric acid (GABA). These substituents significantly extend the absorption range of the pHP chromophore, e.g., the tail of absorption bands of 2a,b extend above 400 nm, well beyond the absorptions of aromatic amino acids and nucleotides. Irradiation releases the amino acids with rate constants of approximately 10(7) s(-)(1) and appearance efficiencies (Phi(app)) of 0.03-0.04. The photoproducts are formed through the pHP excited triplet and are primarily products of photoreduction and photohydrolysis. 1a,b also rearranged to the phenylacetic acid 3.

A new mechanism of benzophenone photoreduction in photoinitiated crosslinking of polyethylene and its model compounds

The photolytic products and a new photoreduction mechanism of benzophenone (BP) as a photoinitiator in the photocrosslinking of polyethylene (PE) and its model compounds (MD) have been studied by means of fluorescence, ESR, 13C and 1H NMR spectroscopy. The fluorescence spectra from the PE and MD systems demonstrate that the main photoreduction product of BP (PPB) is benzpinacol formed by the recombination of two diphenylhydroxymethyl (K•) radical intermediates. The ESR spectrum obtained from the UV irradiation of the MD/BP system gives positive evidence of K• radicals. Two new PPB products: an isomer of benzpinacol with quinoid structure, 1-phenyl-hydroxymethylene-4-diphenyl-hydroxymethyl-2,5-cyclohexa-diene and three kinds of α-alkylbenzhydrols have been detected and identified for the first time by 13C and 1H NMR spectroscopy from the MD systems. The latter could be formed by the reactions of K• radicals with alkyl radicals produced by hydrogen abstraction of the excited triplet state 3(BP)* from polyethylene or its model compounds. These results provide new experimental evidence for elucidating the photoreduction mechanism of BP in the photoinitiated crosslinking of polyethylene. © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 38: 999–1005, 2000

Photoreduction of Polycyclic DBO-Type Azoalkanes with Amines to Hydrazine Products

The n,π*-singlet-excited polycyclic, α-substituted DBO-type azoalkanes 1 are reduced by amines to form the corresponding hydrazines 2 as photoreduction products in competition to denitrogenation to the bicyclo[2.2.0]hexane photoproducts 3. The photophysical, photochemical, and electrochemical properties of the azoalkane determine the product ratio of the hydrazine 2 versus the bicyclo[2.2.0]hexane 3. The quantum yields of the photoreduction are quite low (<0.1), for which efficient deactivation of the singlet excited state through charge transfer is postulated. The charge transfer in the exciplex between the azoalkane and the amine promotes the hydrogen abstraction of the α-CH bond from the amine by the azoalkane and, thus, facilitates its photoreduction, a general photochemical process applicable to singlet- as well as triplet-excited azoalkanes.

Detection of Human Serum Albumin by a Photoinduced Fluorogenic Reaction

ABSTRACT 9,10-anthraquinone (AQ), a photochemically reactive substrate, has been conjugated together with human serum albumin (HSA), and the photoinduced fluorogenic reaction of the HSA-AQ conjugate was studied. This conjugate shows a greatly enhanced photochemical fluorimetric activity compared with that of free AQ. The spectral characteristics of the photoreduction product of the HSA-AQ conjugate was investigated and large blue shifts in both the excitation and emission bands, compared with those of free AQ, were observed. The kinetic behavior of the photoinduced fluorogenic reaction and the effects of some experimental conditions were also investigated. The enhancement in the photoinduced fluorogenic reactivity of HSA-AQ was utilized in the sensitive detection of HSA by a simple in-situ photoinduced kinetic fluorimetric method. The calibration graph was linear over the range 0-1.0x10−6 mol/L HSA. The detection limit was 6.5x10−10 mol/L HSA and the relative standard deviation was 2.7% for 6.0x10−8 mol/L HSA (n=6).

Photochemical Rearrangement of Enediynes: Is a “Photo-Bergman” Cyclization a Possibility?

The enediynes 1 and 2 were photolyzed in 2-propanol to yield products akin to those analogous to a thermal Bergman cyclization mechanism. In addition, products resulting from photoreduction of one of the triple bonds of the enediyne functionality are also formed. The product distributions and yields were found to be dependent on the substituents on the triple bonds and on the nature of the double bond; in particular, phenyl substituents on the triple bonds eliminated the photoreduction products, and overall yields were higher when the double bond of the enediyne was of aromatic character. Triplet sensitization studies and laser flash photolysis experiments point toward radical mechanisms taking place during formation of both classes of products, with the photoreduction products forming from the excited triplet state and the cyclized naphthyl products forming from either the singlet or the triplet states. For the cyclization reaction, a substituted 1,4-dehydronaphthalene biradical species, structurally identical to a hypothetical Bergman biradical, is suggested to be the most likely intermediate.

In-situ photochemical spectrofluorimetric detection of 9,10-anthraquinone-labeled bovine serum albumin

Bovine serum albumin (BSA) labeled with 9,10-anthraquinone (AQ) shows a greatly enhanced photochemical luorimetric activity compared with that of free AQ. The spectral characteristics of the photoreduction product of conjugated AQ were investigated and large blue shifts in the excitation and emission bands compared with those of free AQ were observed. The enhancement in the photochemical reactivity can be employed for sensitive detection of labeled BSA by a simple in-situ photochemical kinetic fluorimetric method. The kinetic behavior of the photochemical reaction and the effects of some experimental conditions were investigated. The calibration graph was linear over the range 0–1.8 × 10 −7 M BSA. The detection limit was 1.2 × 10 −10 M BSA and the relative standard deviation was 2.24% for 1.42 × 10 −8 M BSA ( n = 7).

In-situ photochemical spectrofluorimetric detection of 9,10-anthraquinone-labeled bovine serum albumin

Bovine serum albumin (BSA) labeled with 9,10-anthraquinone (AQ) shows a greatly enhanced photochemical fluorometric activity compared with that of free AQ. The spectral characteristics of the photoreduction product of conjugated AQ was investigated and large blue shifts in the excitation and emission bands compared with those of free AQ were observed. The enhancement in the photochemical reactivity can be employed for sensitive detection of labeled BSA by a simple in-situ photochemical kinetic fluorimetric method. The kinetic behavior of the photochemical reaction and the effects of some experimental conditions were investigated. The calibration graph was linear over the range 0–1.8 × 10 −7 M BSA. The detection limit was 1.2 × 10 −10 M BSA and the relative standard deviation was 2.24% for the 1.42 × 10 −8 M BSA ( n = 7).

Photophysics and photochemistry of p-nitroaniline as photoinitiator

Steady-state (313 and 365 nm) and laser (308 and 337 nm) photolysis have been employed to carry out a structure-reactivity investigation on p-nitroaniline as photoinitiator of polymerization. Detailed studies on the spectroscopy of the molecule were accomplished. Quantum yields of p-nitroaniline photoreduction induced by a tertiary amine, 2-( N,N-diethylamino)ethanol have been determined. The dependence of the photoreduction behaviour on several factors, such as photoinitiator concentration, proportion of the aliphatic amine added to p-nitroaniline as reducing agent, nature of the coinitiator, irradiation energy, and wavelength has been studied. The analysis by gas chromatography-mass spectroscopy technique of the photolysed solutions allowed 1,4-benzenediamine to be identified as the main photoreduction product. A mechanism for the p-nitroaniline photoreduction induced by the aliphatic amine is suggested.

Charge-Transfer-Induced Photoreduction of Azoalkanes by Amines

The unique DBH-type azoalkanes 1, which exhibit high intersystem crossing quantum yields, have made possible the exploration of the bimolecular photoreduction of the n,π* triplet-excited azo chromophore. In the laser-flash photolysis, amines were found to quench the triplet azoalkane 1a with high rate constants (kq ca. 108 M-1 s-1). Steady-state photolysis of the azoalkanes 1a and 1b (φISC ca. 0.5) in the presence of primary, secondary, and tertiary aliphatic amines gave high chemical yields of the corresponding hydrazines 4a and 4b in competition with the unimolecular products, namely the housanes 2 and the aziranes 3. In contrast, the azoalkane 1c undergoes appreciable photoreduction only in neat amines, while the azoalkane 1d (φISC ca. 0.10) is not reduced even under such conditions. Except for N,N-dimethylbenzylamine, the amine oxidation products of the azoalkane photoreduction are analogous to those obtained from the reactions of amines with triplet benzophenone. In marked contrast, the absolute quantum yields of photoreduction for azoalkanes 1 are substantially lower (0.01−0.06) than for benzophenone (0.3−1.0). Efficient deactivation of the triplet-excited states by charge-transfer ( ), which competes with hydrogen atom abstraction ( ), is postulated to account for the low quantum yields. The efficiencies of photoreduction follow the trend primary ≈ tertiary ≫ secondary amines observed with benzophenone, for which secondary amines also display the poorest efficiency. Electron transfer to triplet-excited azoalkanes, analogous to benzophenone, is observed for amines with low oxidation potentials. Thus, when triphenylamine (Eox = 0.85 V versus SCE) is used, the formation of its radical cation can be readily detected by laser-flash photolysis.