Determination of Astaxanthin and Canthaxanthin Triplet Properties in Different Polarities of the Solvent by Laser Flash Photolysis

Triplet‐triplet extinction coefficients for astaxanthin ( I ) and canthaxanthin ( II ) in different deaerated polarity solutions of MeCN and benzene were evaluated by laser flash photolysis at 298 K in the spectral region from 350 to 650 nm by energy transfer method, employing 2‐acetonaphthone as sensitizer. The triplet‐triplet extinction coefficients in MeCN and benzene were different in terms of the carotenoid present. The maximum triplet‐triplet extinction coefficient was 0.1–1.7×10⁵ L·mol⁻¹·cm⁻¹ in different solvents. The rate constants of triplet decay were I : 1.25×10¹⁰ L·mol⁻¹·s⁻¹, II : 1.12×10¹⁰ L·mol⁻¹·s⁻¹ in MeCN; and I : 1.75×10¹⁰ L·mol⁻¹·cm⁻¹, II : 3.27×10¹⁰ L·mol⁻¹·s⁻¹ in benzene. The bimolecular rate constants of energy transfer from triplet excited 2‐acetonaphthone to carotenoids were determined from the linear regression of the decay rate constant of 2‐acetonaphthone triplet at varying carotenoid concentrations. The triplet lifetimes of ³AST* and ³CAN* in different solvents were also determined. The results indicated that triplet energy transfer was nearly diffusion‐controlled.     

Photopolymerization of acrylamide initiated by the three‐component system safranine/triethanolamine/diphenyliodonium chloride: The effect of the aggregation of the salt

The effects of diphenyliodonium chloride (DPI) on the polymerization of acrylamide photoinitiated by the dye safranine with triethanolamine as a coinitiator were investigated in aqueous solutions. The salt notably increased the polymerization rate. This accelerating effects increased appreciably at concentrations of DPI at which the light scattering of the solutions became important. The effects of DPI on the photophysical properties of the dye were also investigated. Although the absorption and fluorescence were scarcely affected, the triplet yield increased by 60%. However, the polymerization rate increased by a factor much higher than that of the triplet yield. The results obtained at high concentrations of the salt could be ascribed to the presence of aggregates of the hydrophobic cations. The lower limit established for the formation of the aggregates was a DPI concentration of approximately 1 × 10⁻³ M. Possible mechanisms for the action of the salt were examined. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 4916–4920, 2004     

Absorption spectrum,lifetime and reactivity of the excited triplet state of dimesitylcarbene

The first excited triplet state of dimesitylcarbene has been generated in solution at room temperature. It has a lifetime of 60 ns and shows fluorescence with λ_max = 501 nm and absorption with λ_max = 360 nm. This species is quenched by oxygen and carbon tetrachloride with rate constants of (4.4 ± 0.8) × 10¹⁰ and (7.3 ± 0.6) × 10⁹ M⁻¹ s⁻¹, respectively.     

Glow discharge source atomization for the laser-excited atomic fluorescence spectrometric studies of indium

A demountable glow discharge source has been used for the atomization of the analyte solutions deposited on graphite and copper rod cathodes. Indium atoms are sputtered-atomized from the cathode surface and are excited by a pulsed, frequency-doubled dye laser pumped by the nitrogen laser. Atomic fluorescence measurements were performed using the non-resonance fluorescence transition. The detection limits of indium in aqueous solutions (10μl) deposited on graphite and copper electrodes were 8 × 10⁻⁹ and 11 × 10⁻⁹g, respectively (8 ng and 11ng).     

Photolysis of acylphosphine oxides II: The influence of methyl substitution in benzoyldiphenylphosphine oxides

Argon-saturated dilute solutions of 2-methyl, 4-methyl-, 2,4,6-trimethyl- and non-substituted benzoyldiphenylphosphine oxide (OTDPO, PTDPO, TMDPO and BDPO respectively) in various solvents were irradiated with flashes (duration, 20 ns) of 347 nm light. All compounds underwent α scission in times much shorter than the flash time: φ(α) = 0.5 – 0.6 (OTDPO, TMDPO and BDPO) and φ(α) - 1.0 (PTDPO). Diphenylphosphonyl radicals generated in this way characteristically absorb light around 330 nm. In the case of OTDPO, photoenolization competes with α scission, and transient spectra were observed which were assigned to the excited triplet (k = 3 × 10⁷ s⁻¹) and to the ground state (k = (3 × 10⁵) - (1.5 × 10⁴) s⁻¹) of the enols. TMDPO did not undergo enolization. It is concluded that enolization involves triplets and that a triplet lifetime longer than a few nanoseconds is a prerequisite for its occurrence.     

Long-lived Triplet Excited State of Perylenemonoimide (PMI) in a Diimine Pt(II) Bis-acetylide Complex: Preparation and Study of the Photophysical Property with Transient Spectra

We prepared a N^N Pt(II) bisacetylide complex that has strong absorption of visible light (molar absorption coefficients ϵ=6.7×10⁴ M⁻¹ cm⁻¹ at 570 nm), and the singlet oxygen quantum yield (Φ_Δ) is up to 78 %. Femtosecond transient absorption spectra show the intersystem crossing (ISC) of the complex takes 81.8 ps, nanosecond transient absorption spectra show the triplet excited state lifetime is 7.6 μs. Density functional theory (DFT) computation demonstrated that the S₁ and T₁ states are mainly localized on the perylenemonoimide (PMI) ligands, although the involvement of the Pt(II) centre is noticeable. The complex was used as triplet photosensitizer to generate delayed fluorescence with perylenebisimide (PBI) as the triplet state energy acceptor and emitter, via the intermolecular triplet-triplet energy transfer (TTET) and triplet-triplet annihilation (TTA), the delayed fluorescence lifetime is up to 52.5 μs under the experimental conditions.     

The fluorescence quenching of 1,4-bis[2-(5-phenyl-oxazolyl)]-benzene by bromomethanes

The interaction between the ground and excited states of 1,4-bis[2-(5-phenyloxazolyl)]-benzene and bromomethanes such as CBr₄, CHBr₃ and CH₂Br₂ were investigated in benzene. Distinct complex formation was not observed either in the ground state or in the excited states. The excited singlet and triplet states are deactivated by these bromomethanes. The triplet yield is increased on the addition of CHBr₃ or CH₂Br₂, whereas it is decreased on the addition of CBr₄. The fluorescence quenching rate constants k_q at 23 °C were determined to be 1.6 × 10¹⁰ M⁻¹ s⁻¹, 3.6 × 10⁸M⁻¹s⁻¹ and 2.4 × 10⁷M⁻¹s⁻¹ for CBr₄, CHBr₃ and CH₂Br₂ respectively. The rate constants k_ST′ of the enhanced intersystem crossing associated with the fluorescence quenching were evaluated from emission—absorption flash photolysis experiments as 3.0 × 10⁸ M⁻¹s⁻¹, 1.9 × 10⁸ M⁻¹s⁻¹ and 5.1 × 10⁷ M⁻¹s⁻¹ for CBr₄, CHBr₃ and CH₂Br₂ respectively. k_ST′ increases with increasing number of bromine atoms contained in the quencher, so that the enhanced intersystem crossing is due to the external heavy-atom effect of the quencher. The apparent triplet yield for the quenching system depends not only on k_ST′ but also on the rates of the other non-radiative processes. This is the reason why the apparent triplet yield does not necessarily increase on fluorescence quenching by bromomethanes.     

Photochemical reactions of Nickel(II)bis(ethyl-3,5-di-t-butyl-4-hydroxybenzyl phosphonate) in hexane solution

The photochemical processes of Ni(II)bis(ethyl-3,5-di-t-butyl-4-hydroxybenzyl phosphonate) (I) in an inert solvent were investigated. The 2,6-di-t-butyl-4-methylphenol (II) and 3,3′,5,5′-tetra-t-butyl-4,4′-stilbenequinone (III) were found in the solution irradiated with light absorbed by (I). The quantum yield of the process for the decomposition of I is low (Φ=1×10⁻³ mol einstein⁻¹).It was found that I quenches singlet oxygen in the anthracene +I+ hexane system irradiated with light at λ = 365 nm. The rate constant of the process was determined as equal to 2×10⁸ dm³ mol⁻¹ s⁻¹. It was found that the process of singlet oxygen deactivation by I consists mainly of the physical interaction between these two molecules.     

A laser photolysis study of the quenching kinetics of triplet-state all-<Emphasis Type="Italic">trans</Emphasis>-retinal by oxygen in aqueous albumin and liposome solutions

The kinetics of quenching of the all-trans-retinal triplet state by air oxygen in aqueous solutions of bovine serum albumin and in a cardiolipin liposome suspension was investigated by nanosecond laser photolysis. It was established that the quenching reaction rate constant in the albumin solution (1.8 × 10⁸ l mol⁻¹ s⁻¹) was an order of magnitude less than in liposomes (3.1 × 10⁹ l mol⁻¹ s⁻¹). These constants were 5.0 × 10⁹ and 1.1 × 10⁹ l mol⁻¹ s⁻¹ in methanol and aqueous solutions containing 10 vol % methanol, respectively. The effect of hindered oxygen access to the Lall-trans-retinal attached to albumin is discussed in terms of its influence on the photooxidation processes in the retina.     

Thioxanthone–anthracene‐9‐carboxylic acid as radical photoinitiator in the presence of atmospheric air

Thioxanthone–anthracene‐9‐carboxylic acid (TX‐ANCA) namely 14‐oxo‐14H‐naphthol [2,3‐b]thioxanten‐12‐carboxylic acid, is synthesized and characterized as part of our continuing interest for syntheses of polyaromatic initiators. Photoinitiator, TX‐ANCA have good absorption properties in the UV and visible region of the electromagnetic spectrum (ɛ₃₇₀: 9080 M⁻¹cm⁻¹, ɛ₄₃₀: 6151 M⁻¹ cm⁻¹). The fluorescence quantum yield is calculated as 0.1 which is slightly higher than of the parent thioxanthone compound (φ_f: 0.07). The phosphorescence lifetime is found to be 39 ms. The possible initiating mechanism of TX‐ANCA is based on photoexcitation of TX‐ANCA and quenching of triplet excited states of TX‐ANCA by molecular oxygen generates singlet oxygen. Singlet oxygen reacts with the anthracene moiety of TX‐ANCA possibly forms an endoperoxide. The endoperoxides undergoes photochemical or thermal decomposition to form radicals which are able to initiate free radical polymerization. © 2018 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2018 , 56, 1878–1883     

The mechanisms of removal of heavy metals from water by ionizing radiation

The removal of heavy metal ions from water using electron beam and gamma irradiation has been investigated for the cases of Pb²⁺ and Hg²⁺ ions. These metal ions are reduced by hydrated electrons and hydrogen atoms to lower or zero valence state and eventually precipitate out of solution. Ethanol is applied as a relatively non-toxic additive to scavenge ^·OH radicals, to enhance reduction and inhibit oxidation. Mercury can be completely (>99.9%) removed from aqueous solution of 1×10⁻³ mol L⁻¹ mercury (II) chloride by using a 3 kGy dose. However, a 40 kGy dose is required to remove 96% of lead ions from a 1×10⁻³ mol L⁻¹ of PbCl₂ solution. The effect of dissolved oxygen and carbonate were also investigated. E-beam irradiation of 1×10⁻³ mol L⁻¹ lead ions complexed with ethylenediamine tetraacetic acid (EDTA) in deoxygenated as well as air-saturated solutions in the absence of ethanol resulted in removal of about 97% of the lead.     

Photochemical Degradation of the Plant Growth Regulator 2‐(1‐Naphthyl) acetamide in Aqueous Solution Upon UV Irradiation

The photochemical degradation of 2‐(1‐naphthyl) acetamide (NAD) in aqueous solution using simulated sunlight excitation as well as UV light within the 254–300 nm range was investigated to obtain an insight into the transformation mechanism that could occur under environmental conditions. Several photoproducts were identified using HPLC/MS/MS techniques. The degradation quantum yield was found to be independent of the excitation wavelength, but showed a dependence of oxygen concentration. This increased by a factor of approximately 3 from aerated to oxygen‐free solutions. There is a clear involvement of both triplet and singlet excited states in NAD photoreactivity. The participation of singlet oxygen as a significant route in NAD degradation was ruled out by comparison with the behavior using Rose Bengal as a photosensitizer. A mechanistic pathway implying hydroxylation process through NAD radical cation species as well as an oxidation reaction by molecular oxygen is proposed. The photochemical behavior of NAD appears to mainly involve the aromatic moieties without any participation of the amide side chain. Toxicity tests clearly show that the generated primary photoproducts are responsible for a significant increase in the toxicity. However, upon prolonged irradiation this toxicity tends to decrease.     

Bithiophene triarylborane dyad: An efficient material for the selective detection of CN− and F− ions

A new fluorescent chemosensor based on bithiophene coupled dimesitylborane (BMB-1) was synthesized and characterized. BMB-1 was used for colorimetric and turn-on fluorescent sensing of cyanide (CN⁻) and fluoride (F⁻) ions, in the presence of other competitive anions in an aqueous (CH₃CN–H₂O) medium. BMB-1 showed a hypsochromic shift (blue shift) with addition of CN⁻ and F⁻ ions in absorption studies. The lower detection level of CN⁻ and F⁻ ions is 1.37 × 10⁻⁹ and 1.75 × 10⁻⁹ M, respectively. The BMB-1 binding mechanism is based on the nucleophilic addition of CN⁻ and F⁻ ions in the internal charge transfer transition of bithio moiety to the boranylmesitylene unit, and the color changes were observed under UV light. This result is further confirmed by Fourier transform infrared spectroscopy, mass spectrometry and density functional theory calculations. Also, the BMB-1 probe is found to be a good adsorbent for the removal of F⁻ ions in real water samples using the adsorption technique.     

Mechanistic studies of the sensitized photoreduction of bis(acetylacetonato)copper(II) by phenylalkyl ketones

Bis(acetylacetonato)copper(II) (Cu(acac)₂) interacts with both the triplet excited state and the triplet biradical of phenylalkyl ketones which undergo the Norrish type II reaction. Mechanistic studies by static quenching methods show that the triplet biradicals interact with the paramagnetic copper species, leading to the preferential formation of cyclobutanols without the formation of new products; in the presence of Ph₃P the former interaction causes the known reduction of Cu(acac)₂) to Cu(acac)(Ph₃P)₂, with a rate constant of about 6 × 10⁹ M⁻¹ s⁻¹. It is shown that Ph₃P interacts with one reactive intermediate, the triplet excited state ketone. The results of extensive kinetic analysis strongly support the proposed reaction mechanism.     

CHLOROPHYLL PHOTOCHEMISTRY IN CONDENSED MEDIA—I. TRIPLET STATE QUENCHING AND ELECTRON TRANSFER TO QUINONE IN CELLULOSE ACETATE FILMS*

Chlorophyll-a was incorporated into cellulose acetate films and the triplet state decay kinetics and electron transfer from triplet to p-benzoquinone in aqueous solution was studied using laser flash photolysis and EPR. The triplet was found to decay by first order kinetics with a rate constant which was independent of Chl concentration. The triplet yield, however, was concentration dependent. These properties are due to quenching which occurs only at the singlet state level. In the presence of quinone, the triplet is quenched and, when the quinone is in an aqueous solution in contact with the film, Chl cation radical (C^±) as well as the semiquinone anion radical (Q^±) can be observed. The C decays by second order kinetics with a rate constant of 1.5 × 10⁶M^-1 s^-1. Although triplet conversion to radicals is slightly lower in the films as compared to fluid solutions (? 3 times), the lifetimes of the radicals are greatly increased (? 10³ times).     

An Anthracene-Based Metal-Organic Framework for Selective Photo-Reduction of Carbon Dioxide to Formic Acid Coupled with Water Oxidation

A Zr-based metal-organic framework has been synthesized and employed as a catalyst for photochemical carbon dioxide reduction coupled with water oxidation. The catalyst shows significant carbon dioxide reduction property with concomitant water oxidation. The catalyst has broad visible light as well as UV light absorption property, which is further confirmed from electronic absorption spectroscopy. Formic acid was the only reduced product from carbon dioxide with a turn-over frequency (TOF) of 0.69 h⁻¹ in addition to oxygen, which was produced with a TOF of 0.54 h⁻¹. No external photosensitizer is used and the ligand itself acts as the light harvester. The efficient and selective photochemical carbon dioxide reduction to formic acid with concomitant water oxidation using Zr-based MOF as catalyst is thus demonstrated here.     

Laser photolysis study of the triplet states of fulvic acids in aqueous solutions

The spectral and kinetic characteristics of short-lived intermediates formed during the photolysis of aqueous and alkaline (0.1 mol l^?1 NaOH) solutions of fulvic acids (FA) were studied by the nanosecond laser photolysis technique. Laser photolysis of FA at 337 nm leads to the formation of short-lived triplet states of FA (³FA) with a quantum yield of about 1% and different relatively long-lived intermediates (with decay rate constants in deoxygenated solutions of 1.8 × 10³–2.1 × 10⁵ and 80–160 s^?1, respectively), which are characterized by absorption spectra with maximums at λ ≤ 400 nm. ³FA are quenched by atmospheric oxygen with rate constants of 5.4 × 10⁸–1.1 × 10⁹ l mol^?1 s^?1. Introduction of phenols into the solutions at concentrations up to 0.1 mol l^?1 has no appreciable effect on the decay kinetics of the detected intermediate products of FA photolysis.     

A FLASH-PHOTOLYSIS INVESTIGATION OF FLAVIN PHOTOSENSITIZATION OF PURINE NUCLEOTIDES

Evidence for the existence of a reactive triplet excited state of lumiflavin has been obtained by the flash-photolysis technique. The triplet state is formed in high yield on the irradiation of flavin solutions in water or chloroform by visible light, and it has been demonstrated that it can transfer its energy to a second molecular species. The flavin-sensitised oxidation of two purine nucleotides, adenylic and guanylic acids, has been studied by flash-photolysis and by long-term irradiation, and the results suggest a triplet-triplet mechanism for the transfer of energy from the excited flavin to the nucleotide. Approximate absorption spectra of the triplet state and of a semiquinone of the flavin have been calculated from the complex transient absorption curves observed on flashing the flavin solution. The triplet decays by a first-order process where k₁= 1·1 × 10^-3. The chemiluminescence spectrum of skatole is identical with the fluorescence spectrum of o-formamidoacetophenone in the same environment Similar results for 2,3-dimethylindole lead to the identification of the acylamide anion as the emitter in indole chemiluminescence. A peroxide ring cleavage excitation mechanism is proposed. 10⁴ sec^-1 and the semiquinone by a second-order process where k₂= 0·75 × 10⁹ 1.m^-1 sec^-1. The rate constants and extinction coefficients obtained enable decay curves to be calculated which fit satisfactorily those measured with the kinetic-flash apparatus.     

Photochemical Reaction Between 1,2‐Naphthoquinone and Adenine in Binary Water‐Acetonitrile Solutions

The photochemical reaction between 1,2‐naphthoquinone (NQ ) and adenine was investigated using nanosecond time‐resolved laser flash photolysis. With photolysis at 355 nm, the lowest triplet state T₁ of NQ was produced via intersystem crossing from its singlet excited state. The triplet‐triplet absorption of the state contributes three bands of transient spectra at 374, 596 and 650 nm, respectively, in pure acetonitrile and binary water‐acetonitrile solutions. In the presence of adenine, the observation of ⁺ (at 363 nm) and radical (at 343 and 485 nm) indicates a multistep mechanism of electron transfer process followed by a proton transfer between ³NQ * and adenine. By fitting with the Stern‐Volmer relationship, the quenching rate constant k _q of ³NQ * by adenine in binary water‐acetonitrile solutions (4/1, volume ratio, v/v) is determined as 1.66 × 10⁹ m ⁻¹ s⁻¹. Additionally, no spectral evidence confirms the existence of electron transfer between ³NQ * with thymine, cytosine and uracil.     

Extremely fast radiationless transition from the triplet state of 9,10-diazaphenanthrene

The decay of the lowest excited triplet state of 9,10-diazaphenanthrene (DAP) was investigated. A surprisingly high sublevel decay rate (k_y = 3.3 × 10⁷ s⁻¹) in biphenyl and fluorene hosts was found at 3.0 K from the decay of a time-resolved EPR (TREPR) signal. This high decay rate was confirmed by a lifetime measurement of the delayed fluorescence using a picosecond laser and photon counting system. The nature of the fast radiationless transition is discussed.