Polychrome luminescence and photochromic transformations in anthrone solutions

Polychrome luminescence was observed in the UV irradiation of an aerated anthrone solution in ethanol or isopropanol: the initially blue luminescence of the solution changed to indigo blue and then to green. Simultaneously with the luminescence changes, the colorless solution became yellow. The appearance of green luminescence was preceded by a period during which the solution almost did not luminesce. It was found that the photooxidation of ethanol (isopropanol) with oxygen sensitized by anthrone (KH) occurred during irradiation. Green luminescence appeared at the time when the concentration of dissolved oxygen decreased below a critical value. The green luminescence spectrum with a maximum at 495 nm and a shoulder at ～475 nm was ascribed to the oxonium form of anthrone (KH ₂ ⁺ ). The possibility of using the oxygen quenching of the green fluorescence of anthrone solutions for analytical purposes and the use of the anthrone-sensitized photooxidation of ethanol with oxygen for the deoxygenation of solutions in laboratory practice are discussed.     

Effect of Nitrene Multiplicity on Product Yields upon Photooxidation of Organic Azides

The effect of the spin state of intermediate species on the product yields upon photooxidation of 4,4"-diazidodiphenyl was studied. It was found that addition of triplet photosensitizers to the reaction mixture resulted in a substantial increase in the yield of nitroso compounds. Maximum sensitizing effect is observed for the photosensitizers with a triplet level energy of 250 kJ/mol or higher. Using the solvents stabilizing the nitrene adduct with oxygen RNOO^Sbrings about an increase in the yields of both nitro and nitroso compounds. This result indicates that the nitrene adduct is the precursor of the above-mentioned compounds.     

Reactivity of singlet oxygen generated by the photosensitization of tetraphenylporphyrin in liposomes

A hydrophobic porphyrin derivative, tetraphenylporphyrin (TPP), was used as a sensitizer, and an anionic dye, methyl orange (MO), was employed as a substrate of photooxidation. TPP was incorporated into the hydrophobic environment of phosphatidylcholine (PC) bilayer membranes, liposomes. When oxygen was purged out of the liposome suspension by nitrogen bubbling, the degradation of MO was completely inhibited. A specific superoxide scavenger, superoxide dismutase, had no effect on the MO degradation. The replacement of H₂O by D₂O resulted in a 10 times enhancement in the photodegradation of MO. These results suggested that singlet oxygen was generated by the TPP photosensitization and worked as the mediator of the photoreaction from TPP. Trisulphonated TPP,-phenyl-,, -tri(p-sulphonyl)porphyrin (TPPS), is soluble in aqueous solution. The light irradiation to an aqueous solution of TPPS gave rise to the rapid bleaching (decomposition) of the sensitizer itself. On the other hand, TPP in the hydrophobic environment of liposomes was stable during light irradiation and worked as a sensitizer for the continuous photoreaction. Maximum reactivity was observed at the PC/TPP mole ratio of 50. When TPP molecules were incorporated into liposomes at larger concentrations (PC/TPP<50), a part of the excitation energy of the sensitizer molecules was nonradiatively converted into the lattice energy by the resonance between the closely located TPP molecules. This led to lower efficiency for the photoactivation of oxygen. On the other hand, the increase in liposome concentration resulted in the enhancement of the MO binding to lipid membranes and the retardation of MO degradation. Also, the electrostatic attraction and repulsion between the membrane and the substrate influenced the reaction rate greatly. The oxidative degradations of the substrate by singlet oxygen were considered to be much faster in the polar environment than in the less polar environment. The charge transfer or the polarized transition complex of singulet oxygen and MO are presumed to be stabilized in the polar environment. The distribution of substrate between the less polar membrane surface and the polar bulk aqueous solution was another important factor in the photooxidation.     

ESR study of radical intermediates formed upon photooxidation of 4,4"-diazidodiphenyl

Radical intermediates (phenyl and phenylcarbonyl radicals) formed in photooxidation of 4,4"-diazidodiphenyl in benzene and toluene have been studied by ESR spectroscopy. These radicals are formed as a result of abstraction of a hydrogen atom from a solvent molecule by the triplet nitrene—dioxygen complex.     

An AIE singlet oxygen generation system based on supramolecular strategy

The design of supramolecular system s with efficient singlet oxygen generation has attracted considerable interests.Herein,an AIE-based singlet oxygen generation system with chemiluminescence properties is reported in aqueous media based on supramolecular host-guest assembly between a water-soluble pillar[5]arene(WP5) and an AIE photosensitizer(TPEDM).The formed supramolecular nanoparticles exhibit significant singlet oxygen generation ability as well as enhanced fluorescence.In addition,by introducing catalase,this H_2 O_2-responsive supramolecular system shows increased ~1 O_2 generation efficiency compared with the blank nanoparticles.An efficient chemiluminescence system can also be achieved by entrapping an energy donor adamantane derivative(AMPPD).Moreover,the present system can function as nanoreactors to perform the photooxidation of dopamine to form polydopamine with visible light irradiation.This wo rk provides a new strategy for the construction of ~1 O_2 generation system based on supramolecular nanomaterials,which has potential applications in the fields such as chemiluminescence imaging and controlled photocatalysis.     

LONG-LIVED AFTERGLOW OF PHOTOSYNTHETIC PIGMENTS IN SOLUTION: PHOTOCHEMILUMINESCENCE

Abstract —Our recent research on photochemiluminescence (PCL) of pigments in solutions is reviewed. PCL was observed in the course of photooxidation by oxygen of chlorophyll a , bacteriochlorophyll, protochlorophyll, their analogs, synthetic dyes and aromatic hydrocarbons. The PCL of chlorophyll was studied in detail. It depends on oxygen concentration, intensity of exciting light, pH, nature of pigments, solvents etc. The thermochemiluminescence was observed after illumination of liquid and solid pigment solutions at low temperature (down to - 170C). The excitation spectra of PCL coincide with the pigment absorption spectra. The PCL emission spectra in most cases differ from those of pigment fluorescence. Electron acceptors, electron donors, radical inhibitors and β-carotene quench PCL. The quenching efficiency of electron acceptors is similar to their action on the chlorophyll triplet state. The quenching effect of radical inhibitors and β-carotene correlates with their activity in reaction with singlet oxygen. The effect of quenchers on the chlorophyll fluorescence, photobleaching and pigment sensitized oxygenation was studied. Analysis of experimental data allowed the assumption that chemiluminescence accompanies the decomposition of labile pigment peroxides. The accumulation of peroxides is probably due to the reaction in the complex of pigment and singlet oxygen, formed as a result of energy transfer from photoexcited (triplet) pigment molecules to oxygen. The terminal chemiluminescence emission proceeds from the singlet excited states of molecules of pigments and products of their oxidation.     

Photooxidation of Ethylbenzene with TiO<Subscript>2</Subscript> and Metal Coated TiO<Subscript>2</Subscript> and its Kinetics

Summary. Photooxidation of ethylbenzene with oxygen to give ethylbenzene hydroperoxide has been achieved in a stirred photochemical reactor that was cooled by a water system by irradiation with a 400W high-pressure mercury lamp and using TiO₂ powder and metal coated TiO₂. The effects of the amount of copper or silver coated on TiO₂ and of the temperature on the rate of oxidation have been investigated. It is suggested that thermal cleavage of the O–O bond and photochemically generated singlet oxygen should be considered as the initiating step in a radical chain mechanism. An optimum loading of 6% Ag or 4–5% Cu was observed for photooxidation of ethylbenzene.     

Photooxidation of Ethylbenzene with TiO 2 and Metal Coated TiO 2 and its Kinetics

Photooxidation of ethylbenzene with oxygen to give ethylbenzene hydroperoxide has been achieved in a stirred photochemical reactor that was cooled by a water system by irradiation with a 400W high-pressure mercury lamp and using TiO₂ powder and metal coated TiO₂. The effects of the amount of copper or silver coated on TiO₂ and of the temperature on the rate of oxidation have been investigated. It is suggested that thermal cleavage of the O–O bond and photochemically generated singlet oxygen should be considered as the initiating step in a radical chain mechanism. An optimum loading of 6% Ag or 4–5% Cu was observed for photooxidation of ethylbenzene.     

THE REACTIVITY OF CITRONELLOL and α-THUJENE WITH SINGLET OXYGEN. RATE CONSTANTS OF CHEMICAL REACTION and PHYSICAL QUENCHING*

The quantum yields of Rose Bengal sensitized photooxidation of citronellol and α-thujene have been determined as a function of added acceptor and compared with those of furfuryl alcohol as a standard. The results permitted the calculation of the corresponding rate constants of chemical reaction (k_T) and physical quenching (K_q) of singlet oxygen. The sum (k_T+ k_q) has been verified independently by a Stern-Volmer analysis of the singlet oxygen luminescence quenching. α-Thujene reacts faster with singlet oxygen than citronellol, physical quenching being negligible in both cases.     

Sensitized Photooxidation of Ortho-Prenyl Phenol: Biomimetic Dihydrobenzofuran Synthesis and Total 1O2 Quenching†

The sensitized photooxidation of ortho-prenyl phenol is described with evidence that solvent aproticity favors the formation of a dihydrobenzofuran [2-(prop-1-en-2-yl)-2,3-dihydrobenzofuran], a moiety commonly found in natural products. Benzene solvent increased the total quenching rate constant (k_T) of singlet oxygen with prenyl phenol by ~10-fold compared to methanol. A mechanism is proposed with preferential addition of singlet oxygen to prenyl site due to hydrogen bonding with the phenol OH group, which causes a divergence away from the singlet oxygen ‘ene’ reaction toward the dihydrobenzofuran as the major product. The reaction is a mixed photooxidized system since an epoxide arises by a type I sensitized photooxidation.     

Photochemistry of A1E, a retinoid with a conjugated pyridinium moiety: competition between pericyclic photooxygenation and pericyclization

The photochemistry of the retinoid analogue A1E shows an oxygen and solvent dependence. Irradiation of A1E with visible light (lambda(irr) = 425 nm) in methanol solutions resulted in pericyclization to form pyridinium terpenoids. Although the quantum yield for this cyclization is low (approximately 10(-4)), nevertheless the photochemical transformation occurs with quantitative chemical yield with remarkable chemoselectivity and diastereoselectivity. Conversely, irradiation of A1E under the same irradiation conditions in air-saturated carbon tetrachloride or deuterated chloroform produced a cyclic 5,8-peroxide as the major product. Deuterium solvent effects, experiments utilizing endoperoxide, phosphorescence, and chemiluminescence quenching studies strongly support the involvement of singlet oxygen in the endoperoxide formation. It is proposed that, upon irradiation, in the presence of oxygen, A1E acts as a sensitizer for generation of singlet oxygen from triplet oxygen present in the solution; the singlet oxygen produced reacts with A1E to produce cyclic peroxide. Thus, the photochemistry of A1E is characterized by two competing reactions, cyclization and peroxide formation. The dominant reaction is determined by the concentration of oxygen, the concentration of A1E, and the lifetime of singlet oxygen in the solvent employed. If the lifetime of singlet oxygen in a given solvent is long enough, then oxidation (peroxide formation) is the major reaction. If the singlet oxygen produced is quenched by the protonated solvent molecules faster than singlet oxygen reacts with A1E, then cyclization dominates.     

CHEMILUMINESCENCE IN THE PHOTOOXIDATION OF HUMIC ACIDS*

Abstract— Photo-irradiation of aqueous basic solutions of soil humic acids and synthetic melanins with UV and visible light (Λ > 320 nm) under oxygen or nitrogen atmospheres generates electronic exicted states and radicals. These processes give rise to a long-lived chemiluminescence with emission maxima at 480–500, 570 and 615–650 nm, as well as a paramagnetic resonance (EPR) signal with g-value at 2.006 and ΔH～ 3Gs. Chemiluminescence intensity and EPR signals follow multistep kinetics. An increase of the ratio of OD at 260/400 nm and 400/600 nm and a decrease of amplitude of an EPR signal after prolonged photo-irradiation were observed. Long irradiation also causes a decrease of fluorescence intensity bands with maxima at 535 nm and 495 nm (Λ_ex 480 and 400 nm, respectively), and an increase of the short wavelength band with a maximum at 450 nm (Λ_ex 260 nm). The data indicate that a complex chain of reactions initiated by reactive species leads to the degradation of the aromatic core of the polymers. Oxygen efficiently enhances the chemiluminescence intensity and the rate of photodegradation. The mechanism of photodegradative oxidation and chemiluminescence probably involves an energy transfer process and singlet oxygen formation. The possibility of its occurrence in nature and its significance are discussed.     

Unambiguous evidence for the participation of singlet oxygen ( 1 ) in photodynamic oxidation of amino acids

Abstract— A variety of experimental tests have been applied to the methylene-blue-sensitized photooxidation of amino acids to distinguish between singlet oxygen and non-singlet oxidation mechanisms. Conventional flash photolysis and laser photolysis were used to measure the rate constants for the quenching of excited triplet sensitizer and singlet oxygen by the amino acids histidine. tryptophan and methionine and the nucleotide guanosine-5′-monophosphate. In the case of histidine, the rate constants alone rule out an oxidation mechanism involving direct reaction with excited dye. With the other amino acids, and with guanosine monophosphate, the oxidation rates might be accounted for by either mechanism. The inhibition of the photo-oxidation of both tryptophan and methionine as well as histidine by the singlet-oxygen quenchers N₃^? and tetramethylethylene suggests that these reactions occur via a singlet-oxygen mechanism. A newly developed test of singlet oxygen reactions involving a comparison of photooxidation rates in normal and perdeuterated solvents has been used to establish that the photooxidation of tryptophan proceeds primarily by a singlet-oxygen mechanism. These experiments appear to constitute the first proof that singlet oxygen is involved in the photooxidation of the three amino acids tryptophan, methionine and histidine.     

Photoinduced Formation of Reactive Oxygen Species from the Acid Form of 6‐(Hydroxymethyl)pterin in Aqueous Solution

The photochemistry of 6‐(hydroxymethyl)pterin (HPT; 1 ) in aqueous solution (pH 5–6) was investigated by irradiation at 350 nm at room temperature. The photochemical reactions of the acidic form 1a were followed by UV/VIS spectrophotometry, thin‐layer chromatography (TLC), high‐performance liquid chromatography (HPLC), and enzymatic methods for the determination of the superoxide anion radical (O ) and hydrogen peroxide (H₂O₂). When 1a is exposed to UV‐A radiation, the intermediates 4 and 4′ are formed reacting with O₂ to yield 6‐formylpterin (FPT; 5 ) and 6‐carboxypterin (CPT; 6 ) under formation of O and H₂O₂ (Scheme 3). The quantum yields of the disappearance of HPT ( 1a ) and of the formation of the photoproducts 5 and 6 were determined. HPT was investigated for its efficiency in singlet‐oxygen (¹O₂) production in acidic aqueous solution. The corresponding quantum yield of ¹O₂ production (Φ_Δ) was 0.15 ± 0.02, as measured by the ¹O₂ luminescence in the near‐IR (1270 nm) upon continuous excitation of the sensitizer. However, ¹O₂ does not participate in the actual photooxidation of HPT ( 1a ) to FPT ( 5 ) and CPT ( 6 ).     

PHOTOGENERATION OF SUPEROXIDE ION and HYDROGEN PEROXIDE FROM TRYPTOPHAN and ITS PHOTOOXIDATION PRODUCTS: THE ROLE OF 3a-HYDROPEROXYPYRROLIDINOINDOLE*

Abstract— Mechanism of the photogeneration of hydrogen peroxide and superoxide ion from tryptophan (Trp) and its photooxidation products was investigated. Near-ultraviolet irradiation of 3a-hydro-peroxypyrrolidinoindole, an intermediate in the photooxidation of Trp, has been shown to generate hydrogen peroxide efficiently under aerobic conditions. Irradiation of N-formylkynurenine in the presence of 3α-hydroxypyrrolidinoindole also produced hydrogen peroxide. The formation of superoxide ion in both reactions has been confirmed, whereas the reaction of Trp with chemically generated singlet oxygen did not produce any detectable amount of superoxide ion.     

PHOTOOXIDATION OF XANTHOBILIRUBIC ACID IN AQUEOUS SOLUTION: PRODUCT AND MECHANISM STUDIES

Abstract— Xanthobilirubic acid, an oxodipyrrylmethene with a chromophore very similar to that of bilirubin, was aerobically irradiated as its sodium salt in borate buffer at pH 8. Two photooxidation products, methylethylmaleimide and 5-formyl-2,4-dimethyl-1 H -pyrrole-3-propanoic acid, were isolated. These products can be rationalized on the basis of either a Type I or a Type II (singlet oxygen) photooxygenation mechanism. The reaction is inhibited by azide, a singlet oxygen quencher, and sensitized by methylene blue. However, both the self-sensitized and the methylene blue-sensitized reactions are enhanced, rather than inhibited, by high concentrations of 1,4-diazabicyclo[2.2.2]octane, another known singlet oxygen quencher. It is therefore proposed that the diazabicyclooctane can also act as an electron donor.
The self-sensitized photooxidation of xanthobilirubic acid is an autocatalytic reaction. This is supported by the fact that addition of a previously irradiated solution to a freshly-prepared solution significantly increases the rate of photodegradation. A similar catalyst is formed, but much more slowly, from xanthobilirubic acid in the dark in the presence of oxygen.     

在非质子溶剂中竹红菌甲素敏化光氧化胆红素

胆红素IX_α(胆红素)是哺乳动物体内血红素的代谢产物,由于它与小儿黄疸病光疗的直接关系,以及近来发现它在体内可能作为生物抗氧剂,所以一直受到科学家的重视。在非质子溶剂中胆红素的光氧化可以生成胆绿素IX_α(胆绿素)及一系列单吡咯、双吡咯的衍生物。对于生成单、双吡咯小分子衍生物,用类型Ⅱ(¹O₂为中间体)的光氧化机制很好解释,但对胆绿素的生成到底是经过单重态氧途径,还是离子自由基途径尚不完全清楚。     

Studies on the photooxidation mechanism of polymers. I. Photolysis and photooxidation of polystyrene

A new mechanism has been proposed for the photooxidation of polystyrene as film and in benzene. The initial stage of the photooxidative degradation may involve reactions of singlet oxygen with polystyrene molecules. Singlet oxygen may be formed in the reaction between excited benzene ring in polystyrene molecule and molecular oxygen. The addition of singlet oxygen quenchers such as 1,3-cyclohexadiene or β-carotene reduces the rate of polymer degradation in benzene solution. The mechanisms of the photolysis of polystyrene as film and in benzene solution, in vacuo and in the presence of oxygen, are discussed and interpretations proposed. The pronounced yellowing of polystyrene during the photooxidation process is interpreted as a reaction involving benzene ring-opening photooxidation in polystyrene molecule. These results were obtained by comparing ultraviolet and infrared spectra in experiments of photooxidation of pure liquid benzene and polystyrene film.     

Light Emission Resulting from Hydroxylamine-Induced Singlet Oxygen Formation of Oxidizing LDL Particles

Abstract— Oxidation of low-density lipoprotein (LDL) by low amounts of cupric ions resulted in the formation of singlet oxygen (₁O₂, ₁DL_g) when hydroxylamine (NH2OH) was added. Direct evidence on this excited species came from partial spectral resolution of the emitted light in the red spectral region (634 nm and 703 nm), which can be attributed to the dimol decay of singlet oxygen. Additional evidence for the existence of singlet oxygen came from the enhancing effect of deuterium oxide buffer (D20) on chemiluminescence intensity and the quenching effect of sodium azide. A linear correlation between NH2OH-de-pendent chemiluminescence intensity and the amount of diene conjugates (DC) formed in this reaction was observed. Removal of adventitious transition metals by adequate chelators prevented chemiluminescence in this system; NH2OH was also found to efficiently decrease metabolites of lipid peroxidation (LPO). Our findings are consistent with a sequence of reactions in which NH2OH first converts transition metals to their reduced state, thereby stimulating the formation of alkoxy- and peroxy-radicals. Peroxyradicals decompose in a bimolecular Russel reaction to hydroxyl compounds and singlet oxygen while the majority of alkoxy radicals are eliminated by a secondary reaction with NH2OH. Identical effects were observed when reducing antioxidants such as ascorbic acid or trolox C were used instead of hydroxylamine.     

BIOLOGICAL ACTIVITIES OF PHTHALOCYANINES—IV. TYPE II SENSITIZED PHOTOOXIDATION OF L-TRYPTOPHAN AND CHOLESTEROL BY SULFONATED METALLO PHTHALOCYANINES

Abstract— Sulfonated phthalocyanine and a series of its metal chelates in combination with red light irradiation led to the degradation of L-tryptophan in oxygenated aqueous solution. The photoproducts and the rate of transformation of L-tryptophan are compared with hematoporphyrin and rose bengal sensitized photooxidation. In all cases the primary photoproducts are characterized as cis and trans -3a-hydroperoxy-l,2,3,3a,8,8a-hexahydropyrrolo[2,3-b]indole-2-carboxylic acid. Support for the involvement of singlet excited oxygen is obtained from azide inhibition and the formation of the specific singlet oxygen product with cholesterol. We observed the contribution of another pathway in the case of the manganese complex.