Quantum Yield Of Degradation Research Articles

Novel Functional Axially Substituted Silicon (IV) Phthalocyanine Derivative and its Photochemical Properties

Photodynamic therapy is a highly specific and clinically approved method in which a non-toxic photosensitizer drug is administered to the patient for cancer treatment. Phthalocyanines with their long wavelength absorption and fluorescence from 650 to 800 nm can be used as photosensitizers for photodynamic therapy and are used in clinical trials. In this study, functional novel axially substituted silicon (IV) phthalocyanine (PS-2) was synthesized. Unsubstituted dichlorosilicon (IV) phthalocyanine was synthesized from 1,3-diiminoisoindoline via cyclotetramerization. The axial substitution reaction was carried out using dichlorosilicon (IV) phthalocyanine and excess of N-Boc-ethanolamine. Structural characterization of this novel PS-2 by FT-IR, mass, 1H NMR and UV-Vis spectroscopy were performed. Photochemical properties (photo degradation quantum yields (d) and singlet oxygen quantum yield () of PS-2, which are the first steps for cancer treatment, were investigated.

In vitro antiproliferative effect of four ball-type phthalocyanines linked by t-butylcatechol and high singlet oxygen production

Four ball-type phthalocyanines (H2Pc, MgPc, CoPc, and ZnPc) containing 4-tert-butylcatechol substituents at alpha-positions were synthesized from 3,3′-(4-tert-butyl-1,2-phenylene)bis(oxy)diphthalonitrile. The structures were elucidated by elemental analysis and ultraviolet-visible, Fourier-transform infrared, proton nuclear magnetic resonance, and Maldi-Tof mass spectroscopies. The photophysicochemical properties of H2Pc, MgPc, and ZnPc were investigated in dimethylsulphoxide. The fluorescence quantum yields (ΦF), triplet state quantum yields (ΦT), and singlet oxygen quantum yields (ΦΔ) with respect to H2Pc, MgPc and ZnPc were experimentally obtained. Furthermore, the degradation quantum yield (Φd) was evaluated for all complexes. Finally, the magnetic circular dichroism spectra of the four Pcs were obtained. Among these ball-type phthalocyanines, MgPc had the highest triplet lifetime and ZnPc had the highest singlet oxygen quantum yield. Moreover, the in vitro antiproliferative effect of ball-type phthalocyanines (H2Pc, MgPc, CoPc, and ZnPc) was evaluated on MCF-7 cell line originated from luminal A breast cancer and MDA-MB-231 cell line originated from triple negative breast cancer. For this purpose, cell viability MTT (3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide) assay was used. For all compounds, 0.2, 0.4, 0.8, 1.6, 3.2, 6.4, 12.8, 25.6, 51.2, 102.4 µM concentrations were applied to both cell lines. There was a significant decrease in cell viability for MCF-7 and MDA-MB-231 cells (p < 0.05).

UV-A induced damage to lysozyme via Type I photochemical reactions sensitized by kynurenic acid

In this work we studied the mechanisms of Type I photodamage to a model protein, hen egg white lysozyme (HEWL), sensitized by kynurenic acid (KNA) – one of the most efficient photosensitizers of the human eye lens present in trace amounts within tissue. The kynurenic acid radical, KNA•−, formed in the quenching of triplet KNA by HEWL, can be readily oxidized by molecular oxygen with the formation of superoxide anion radical O2•−. This leads to two ways of damage to proteins: either via the direct reactions between KNA•− and HEWL• radicals (Type Ia) or via the reactions between superoxide anion O2•− and HEWL• radicals (Type Ib). Our results demonstrate significant degradation of the protein during Type Ia photolysis with the formation of various oligomeric and oxygenated forms of HEWL and several deoxygenated products of KNA. Liquid chromatography-mass spectrometry analysis revealed the cross-linking of HEWL via tryptophan (Trp62) and tyrosine (Tyr23) residues and, for the first time, the covalent binding of KNA to protein via tryptophan (Trp62 and Trp123) residues. It was found that Type Ib reactions lead to substantially smaller damage to HEWL; the degradation quantum yields (Φdeg) of HEWL are 1.3 ± 0.3% and 0.12 ± 0.03% for Type Ia and Ib photolyses, respectively. Low Φdeg values for both types of photolysis indicate the Back Electron Transfer (BET) with the restoration of initial reagents as the main radical decay path with significantly higher BET efficiency in the case of Type Ib reactions. Therefore, in essentially oxygen-free tissues like the eye lens, the direct radical reactions via Type Ia mechanism could induce significantly larger damage to proteins, leading to their cross-linking and oxidation. The accumulation of these modifications can cause the development of various diseases, in particular, cataracts in the eye lens.

Investigation of photoreaction for 2-nitrobenzofuran and its 3-methyl derivative

Abstract Aromatic nitro compounds undergo various photoreactions such as photosubstitution, photoredox reactions, photodissociation, and photoinduced nitro–nitrite rearrangements. The reaction mechanisms for these photoreactions, especially nitro–nitrite rearrangement reactions have never been completely understood. It was originally assumed that the potential cause for a nitro–nitrite rearrangement originates from the twisted dihedral angle between the plane of the nitro group and the plane of the aromatic ring [O.L. Chapman et al., J. Am. Chem. Soc. 88 (1966) 5550–5554]. Although the majority of aromatic nitro compounds have coplanar conformation and undergo nitro–nitrite rearrangement, no studies clearly substantiate this theory. This study reports on the photochemical reactions of 2-nitrobenzofuran and the effects of 3-methyl substituent on the compound. The results show that a nitro–nitrite rearrangement selectively occurs in deaerated acetonitrile solution for the photoreactions of 2-nitrobenzofuran. Moreover, nitro–nitrite rearrangement might occur in the photoreaction of 3-methyl-2-nitrobenzofuran. The quantum yield of degradation for 3-methyl-2-nitrobenzofuran was higher than that for 2-nitrobenzofuran, indicating that the methyl substituent at the 3-position changes the photoreactivity of 2-nitrobenzofuran.

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.

Effects of Dissolved Water Constituents on the Photodegradation of Fenitrothion and Diazinon

The photochemical degradation of two widely used organophosphorothioate insecticides, fenitrothion and diazinon, was investigated in aqueous solutions containing three separate dissolved constituents commonly found in natural waters (NO 3 − , CO 3 2− and dissolved organic matter (DOC)). The effect of these constituents on pesticide photodegradation was compared to degradation in “constituent-free” pure water. Solutions were irradiated in an Atlas solar simulator fitted with a UV-filtered Xenon arc lamp with light irradiances (500 W m−2) measured using a spectral radiometer to allow derivation of quantum yields of degradation. Fenitrothion absorbs light within the solar UV range (λ, 295–400 nm) and underwent direct photolysis in pure water whereas diazinon (λ max ∼250 nm) showed no observable loss over the experimental period. However, photodegradation conforming to pseudo-first-order kinetics was observed for both chemicals in the presence of the dissolved constituents (at concentrations typically observed in natural waters), with the rates of photodecay observed in the order of NO 3 − > CO 3 2− ≅ DOC, with the highest rates observed in the 3 mM NO 3 − solutions (k Fen = 0.155 ± 0.041 h−1; k Dia = 0.084 ± 0.0007 h−1). For diazinon this rate was comparable to fenitrothion photolysis in pure water (k fen 0.072 ± 0.0078 h−1), highlighting the importance of NO 3 − on a non-photolabile pesticide, with indirect photodegradation probably attributable to the light-induced release of aqueous hydroxyl radicals (·OH) from NO 3 − . Suwannee river fulvic acid (serving as DOC) did not statistically affect the rate of photodecay for fenitrothion relative to its photolysis in MilliQ water, although measured rates in DOC solutions were slightly lower. However, measurable rates of photodecay were apparent for diazinon in the DOC solutions, indicating that fulvic acid, possibly in the form of “excited” triplet-state-DOC plays a role in diazinon transformation. Hydrolysis was not apparent for fenitrothion (in buffered solutions of pH 5–9) but was notable for diazinon at the lower pHs of 5 and 3 (k Dia-hyd 0.3414 h−1 at pH 3 and 0.228 h−1 at pH 5), resulting in the formation of the degradate, 2-isopropyl–6-methyl–4-pyrimidinol. This work highlights the importance of dissolved constituents on abiotic photodegradation of pesticides and it is recommended that these constituents be incorporated into laboratory-based fate-testing regimes.

Photophysical and photochemical properties of the pharmaceutical compound salbutamol in aqueous solutions

Salbutamol is a potent β 2-adrenergic receptor agonist widely used in the treatment of bronchial asthma and chronic obstructive pulmonary disease. An increasing number of studies have detected salbutamol in natural water systems worldwide. Studies have shown that sunlight degrades salbutamol resulting in the formation of products; some showing higher toxicity to bacteria Vibrio fischeri than the parent compound. In this contribution, steady-state absorption and emission techniques, high-performance liquid chromatography, and transient absorption spectroscopy are used to investigate the photochemistry of salbutamol in aqueous buffer solutions at controlled pH values. Ground- and excited-state calculations that include solvent effects are performed to guide the interpretation of the experimental results. Salbutamol is sensitive to UVB light absorption in the pH range from 3 to 12, forming products that absorb light at longer wavelengths than the parent compound. Quantum yields of degradation reveal that the deprotonated species is 10-fold more photo-active than the protonated species. In line with this result, the fluorescence quantum yield of the protonated species is more than an order of magnitude higher than that of the deprotonated species. Transient absorption spectroscopy shows that population of the triplet state occurs with a rate constant of 7.1 × 10 8 s −1 in the protonated species, while a rate constant of 1.7 × 10 10 s −1 is measured for the deprotonated species. While degradation of the deprotonated species is not affected by the presence of molecular oxygen, a twofold increase in the photodegradation yield of the protonated species in air-saturated conditions is observed.

Investigation of the solution initial pH effects on the performance of UV/Fe-ZSM5/H2O2 process

Effect of the solution initial pH was investigated on the performance of photo-Fenton process using Fe-ZSM5 zeolite as a heterogeneous catalyst for degradation of azo dye Acid Red 14. All experiments were performed in a 1.0 L tubular closed circulation batch photo-reactor. The dye degradation quantum yield was maximal when the solution initial pH was about 5. An amount of about 0.3 mg L(-1) of Fe ions leached out of the zeolite. However, monitoring Fe ion concentration during the experiments showed that the efficiency of the process is not due to the leached Fe ions. The effect of pH on decomposition rate of hydrogen peroxide in different systems such as H(2)O(2) alone, UV/H(2)O(2), UV/Zeolite and UV/Zeolite/H(2)O(2) was explored in detail to explain the results obtained.

Reaction Pathways for the Photodegradation of the Organophosphorus Cyanophos in Aqueous Solutions

Photodegradation in aqueous solutions is an important pathway for many agrochemicals such as pesticides. In the present work, the photochemical transformation of cyanophos (CYA) was investigated in aqueous solutions using UV light within the 254-313 nm range as well as solar light. The study was performed in order to have a deep insight into the mechanistic pathways for the photochemical disappearance of CYA. Upon UV irradiation of an aerated solution of CYA, the degradation quantum yield was found equal to 1.8 x 10(-2). It is independent of the excitation wavelength but varies with oxygen concentration. It increased by a factor of 2 from oxygen-saturated to oxygen-free solution. Photosensitized experiments were performed using acrylamide and hydroquinone as energy acceptor and energy donor substrates, respectively. They show that both singlet and triplet excited states were involved in the photochemical behavior of CYA. The laser flash photolysis experiments clearly showed the involvement of the triplet excited state which was efficiently quenched by molecular oxygen and acrylamide with the rate constants 1.97 x 10(9) and 2.71 x 10(9) mol(-1) L s(-1), respectively. The photoproducts structures were proposed according to the mass spectral data using the LC/MS technique. The analytical study shows that various processes such as hydrolysis, homolytic bond dissociations and Photo-Fries process occur.

Photolytic degradation of methyl-parathion and fenitrothion in ice and water: Implications for cold environments

Here we investigate the photodegradation of structurally similar organophosphorus pesticides; methyl-parathion and fenitrothion in water (20 °C) and ice (−15 °C) under environmentally-relevant conditions with the aim of comparing these laboratory findings to limited field observations. Both compounds were found to be photolyzed more efficiently in ice than in aqueous solutions, with quantum yields of degradation being higher in ice than in water (fenitrothion > methyl-parathion). This rather surprising observation was attributed to the concentration effect caused by freezing the aqueous solutions. The major phototransformation products included the corresponding oxons (methyl-paraoxon and fenitroxon) and the nitrophenols (3-methyl-nitrophenol and nitrophenol) in both irradiated water and ice samples. The presence of oxons in ice following irradiation, demonstrates an additional formation mechanism of these toxicologically relevant compounds in cold environments, although further photodegradation of oxons in ice indicates that photochemistry of OPs might be an environmentally important sink in cold environments.

Role of H-bond formation in the photoreactivity of curcumin

Curcumin is the main constituent of curry. In its ground state it shows chemo-preventive, chemo-therapeutic and anti-inflammatory effects. For its immunostimulating action it has been considered for the development of drugs suitable for treating AIDS and cystic fibrosis. Further biological action is induced in curcumin by photoactivation: in suitable environmen- tal conditions electronically excited curcumin can act as a singlet oxygen generator. Moreover, cytotoxicity is enhanced by light exposure and antibacterial effects are photosensitized. This work is aimed to understand the photobiological action of curcumin by elucidating the deactivation mechanisms of its first excited singlet state. In particular we find evidence of the role of tautomerization in the excited state by measuring fluorescence lifetimes and quantum yields for such compound dissolved in solvents of different polarity and H-bonding capability. Degradation quantum yield and singlet oxygen generation efficiency were also measured in acetonitrile and methanol. The results emphasize the strong dependence of the deactivation processes from the environment. The deactivation phenomenology can be fully explained by postulating intramolecular proton transfer in the cis enol conformer to be the leading non-radiative deactivation pathway.

Role of H-bond formation in the photoreactivity of curcumin

Curcumin is the main constituent of curry. In its ground state it shows chemo-preventive, chemo-therapeutic and anti-inflammatory effects. For its immunostimulating action it has been considered for the development of drugs suitable for treating AIDS and cystic fibrosis. Further biological action is induced in curcumin by photoactivation: in suitable environmental conditions electronically excited curcumin can act as a singlet oxygen generator. Moreover, cytotoxicity is enhanced by light exposure and antibacterial effects are photosensitized. This work is aimed to understand the photobiological action of curcumin by elucidating the deactivation mechanisms of its first excited singlet state. In particular we find evidence of the role of tautomerization in the excited state by measuring fluorescence lifetimes and quantum yields for such compound dissolved in solvents of different polarity and H-bonding capability. Degradation quantum yield and singlet oxygen generation efficiency were also measured in acetonitrile and methanol. The results emphasize the strong dependence of the deactivation processes from the environment. The deactivation phenomenology can be fully explained by postulating intramolecular proton transfer in thecisenol conformer to be the leading non-radiative deactivation pathway.

1-Oxoindan-2-yl and 1,3-dioxoindan-2-yl esters as photoremovable protecting groups

1-Oxoindan-2-yl and 1,3-dioxoindan-2-yl carboxylic acid esters react with an excess of hydrogen atom or electron donors to release the corresponding acids, in addition to indan-1-one and indan-1,3-dione, respectively, as by-products. The maximum degradation quantum yields of 1-oxoindan-2-yl esters in H-donating propan-2-ol were found to approach 10, indicating that a chain reaction process, involving hydrogen transfer from the ketyl radical intermediates formed from an excited ester by hydrogen abstraction from an alcohol, participates. Such a cleavage mechanism parallels that observed earlier in photolysis of phenacyl esters. The corresponding 4,7-dimethyl substituted derivatives showed no contribution of the photoenolization mechanism apparently because of electronic and geometric reasons. Both 1-oxoindan-2-yl and 1,3-dioxoindan-2-yl chromophores are proposed to be utilized as photoremovable protecting groups in applications when higher concentrations of the hydrogen/electron donors are experimentally feasible.

Factors Controlling the Rate of Photodegradation in Polymers: The Effect of Temperature on the Photodegradation Quantum Yield in a PVC Polymer Containing Metal–Metal Bonds in the Polymer Chain

The effect of temperature on the degradation quantum yield of a poly(vinyl chloride) polymer with Cp2Mo2(CO)6 units incorporated into its chains was studied (Cp = cyclopentadienyl). The polymer is photochemically reactive in the absence of oxygen because the CpMo(CO)3 radicals formed by photolysis of the Mo–Mo bonds react with C–Cl bonds to form CpMo(CO)3Cl units. Quantum yields as a function of temperature were obtained for this polymer and for two control systems, Cp′2Mo2(CO)6 dispersed in PVC and Cp′2Mo2(CO)6 in hexane/CCl4 solution (Cp′ = η5-C5H4CH3). The quantum yields of the two control systems showed only slight increases with an increase in temperature. For the reaction in hexane/CCl4, this temperature dependence is attributed to the decrease in viscosity of the solution and the subsequent decrease in the radical-radical recombination efficiency. For the Cp′2Mo2(CO)6 dispersed in PVC, the small temperature dependence is attributed to an increase in free volume as the temperature increases. In contrast to these results, the temperature dependence of the quantum yield of the PVC polymer with Cp2Mo2(CO)6 units along its chains is relatively large. It is proposed that an increase in temperature facilitates the polymer chain relaxation processes (involving recoil and rotation) following photolysis of the Mo–Mo bond. The radical–radical recombination efficiency is subsequently decreased, which leads to a net increase in chain cleavage and degradation efficiency.

Mechanistic Aspects of the Effects of Stress on the Rates of Photochemical Degradation Reactions in Polymers

This review examines the mechanistic origins of the effects of stress on the photochemical degradation rates of polymers. Recent studies have shown that tensile and shear stresses accelerate the rate of the photochemical degradation of polymers. Conversely, compressive stress generally retards the rate of photochemical degradation. After an initial discussion of the photochemical auto‐oxidation mechanism, the three primary hypotheses that purport to explain how stress affects photochemical degradation are examined. The first hypothesis is attributed to Plotnikov, who proposed that stress changes the quantum yields of the reactions that lead to bond photolysis. The second hypothesis, attributed to a number of researchers, says that stress affects the ability of the geminate radical pairs, formed in the photochemical bond cleavage reactions, to recombine. The third hypothesis proposes that stress changes the rates of radical reactions subsequent to radical formation. A further attempt to account for the effects of stress on degradation rates is a modification of the so‐called Zhurkov equation that has been used rather successfully to predict the effects of stress on degradation rates in thermal reactions. This empirical equation relates the quantum yield of degradation to a composite activation barrier for the overall photochemical reaction. Following the discussion of these hypotheses, experimental mechanistic studies of stress effects are summarized, and what little data there is is shown to be consistent with the hypothesis that proposes that stress primarily affects the ability of photochemically generated radical pairs to recombine. By decreasing the efficiency of radical–radical recombination, the effect is to increase the relative efficiencies of the radicals' other reactions and hence the rate of degradation. In addition to stress, other factors can affect the rates of polymer photodegradation. These factors include the absorbed light intensity, the polymer morphology, the rate of oxygen diffusion in the polymer, and the chromophore concentration. Each of these parameters must be carefully controlled in mechanistic studies that probe the effects of stress on degradation rates.

Photocatalyzed oxidation and mineralization of C1–C5 linear aliphatic acids in UV-irradiated aqueous titania dispersions—kinetics, identification of intermediates and quantum yields

The photocatalyzed degradation and mineralization of the linear carboxylic acid C1–C5 series (namely, formic acid, acetic acid, propanoic acid, butanoic acid, and valeric acid) were examined in UV-irradiated air-equilibrated aqueous TiO 2 dispersions: (a) to assess the extent of (dark) adsorption and photoadsorption; (b) to determine their relationship with degradation; (c) to elucidate the dynamics of the degradative and mineralization processes (as loss of total organic carbon); (d) to identify intermediates by liquid chromatography (HPLC), gas chromatography (GC–FID) and gas chromatography coupled to a mass detector (GC–MS), and (e) to determine the quantum yields of degradation of this linear acid series based on formic acid ( Φ = 0.12 ± 0.02) as a secondary actinometer proposed for aliphatic substrates. At low catalyst loading (2.0 g/L) and except for formic acid, adsorption of the other four C2–C5 acids increased with chain length, although the overall level was rather small, contrary to high catalyst loading (20 g/L) where all four C2–C5 acids adsorbed to nearly the same extent (15–19%) with adsorption of formic acid on TiO 2 being two-fold greater (34%). The dynamics of degradation and mineralization decreased with increase in chain length, i.e. formic acid degraded and mineralized faster than its longer chain congeners. Several intermediates were identified in the degradation of the C3–C5 acids; in particular, 21 species were positively identified in the degradation of valeric acid by the techniques used. On the basis of partial charges and frontier electron densities of all atoms of the carboxylic acids, together with known species formed a route is proposed for valeric acid degradation. The quantum yields of degradation for the C2–C5 acid series increased with chain length from Φ = 0.010 for acetic acid to Φ = 0.067 for valeric acid.

Origin of Tensile Stress-Induced Rate Increases in the Photochemical Degradation of Polymers

A novel poly(vinyl chloride)-based polymer (1) containing Mo−Mo bonds along the backbone was prepared by the reaction of carboxylated PVC (ca. 1.8 wt % −COOH) with (HOCH2CH2Cp)2Mo2(CO)6. When irradiated with visible light, this polymer photodegrades, even in the absence of oxygen. Infrared spectroscopic analysis demonstrated that the chlorine atoms along the polymer backbone act as built-in traps for Mo-centered radicals formed by photolysis of the Mo−Mo bonds. The effect of stress on the degradation quantum yield of 1 and its plasticized analogue (2), made by addition of 20 wt % dioctyl phthalate (DOP) to 1, was studied. The presence of the internal radical trap permitted the polymer samples to be irradiated in the absence of oxygen, thus eliminating the kinetically complicating effects of rate-limiting oxygen diffusion. Results from both polymers showed that stress initially increased the quantum yields for degradation, but the quantum yields reached a maximum value and then decreased with higher stress...

On the photochemical stability of dialkoxy–PPV; a quantitative approach

In this paper we describe a quantitative approach to the photochemical degradation of a soluble dialkoxy–PPV that was also used to make polymer light–emitting diodes. We have defined a quantum yield for degradation (γ) and investigated the dependence (γ) on various experimental parameters, e.g. wavelength and power of incident light, and the dependence on the oxygen concentration. In the presence of oxygen the main degradation mechanism is photo–oxidation.

Degradation of Fluorescent Whitening Agents in Sunlit Natural Waters

Stilbene-type fluorescent whitening agents (FWAs), such as the distyryl biphenyl (DSBP) and the diaminostilbene types (DAS 1 and DAS 2), are commonly used in detergents and papers. They are not readily biodegradable, but due to their ability to absorb part of the terrestial sunlight, they can be photochemically degraded in natural surface waters. Following a fast preceding photoisomerization, the three compounds are degraded by direct photochemical processes yielding mainly aldehydes and alcohols. Their degradation quantum yields are similar, about 10-4. Nevertheless, in samples of a eutrophic Swiss lake water, DSBP is photochemically degraded three times faster (t1/2 = 87 min) in summer noon terrestrial sunlight at 25 °C than DAS 1 and DAS 2 (t1/2 = 278 and 313 min) because of a higher rate of sunlight absorption by the DSBP isomer mixture. All FWAs are degraded faster if the oxygen concentration is increased. Dissolved natural organic material partly inhibits the degradation of DSBP in the reaction with...

Degradation of Fluorescent Whitening Agents in Sunlit Natural Waters

Stilbene-type fluorescent whitening agents (FWAs), such as the distyryl biphenyl (DSBP) and the diaminostilbene types (DAS 1 and DAS 2), are commonly used in detergents and papers. They are not readily biodegradable, but due to their ability to absorb part of the terrestial sunlight, they can be photochemically degraded in natural surface waters. Following a fast preceding photoisomerization, the three compounds are degraded by direct photochemical processes yielding mainly aldehydes and alcohols. Their degradation quantum yields are similar, about 10-4. Nevertheless, in samples of a eutrophic Swiss lake water, DSBP is photochemically degraded three times faster (t1/2 = 87 min) in summer noon terrestrial sunlight at 25 °C than DAS 1 and DAS 2 (t1/2 = 278 and 313 min) because of a higher rate of sunlight absorption by the DSBP isomer mixture. All FWAs are degraded faster if the oxygen concentration is increased. Dissolved natural organic material partly inhibits the degradation of DSBP in the reaction with molecular oxygen. The behavior of these compounds illustrates the influence of a preceding isomer equilibrium on degradation rate coefficients.