Photodegradation Quantum Yields Research Articles

Competing Nonadiabatic Relaxation Pathways for Near-UV Excited ortho-Nitrophenol in Aqueous Solution.

Nitrophenols are atmospheric pollutants found in brown carbon aerosols produced by biomass burning. Absorption of solar radiation by these nitrophenols contributes to atmospheric radiative forcing, but quantifying this climate impact requires better understanding of their photochemical pathways. Here, the photochemistry of near-UV (λ = 350 nm) excited ortho-nitrophenol in aqueous solution is investigated using transient absorption spectroscopy and time-resolved infrared spectroscopy over the fs to μs time scale to characterize the excited states, intermediates, and photoproducts. Interpretation of the transient spectroscopy data is supported by quantum chemical calculations using linear-response time-dependent density functional theory (LR-TDDFT). Our results indicate efficient nonradiative decay via an S1(ππ*)/S0 conical intersection leading to hot ground state ortho-nitrophenol which vibrationally cools in solution. A previously unreported minor pathway involves intersystem crossing near an S1(nπ*) minimum, with decay of the resulting triplet ortho-nitrophenol facilitated by deprotonation. These efficient relaxation pathways account for the low quantum yields of photodegradation.

Insight into the photodegradation of methylisothiazolinone and benzoisothiazolinone in aquatic environments

Methylisothiazolinone (MIT) and Benzisothiazolinone (BIT) are two widely used non-oxidizing biocides of isothiazolinones. Their production and usage volume have sharply increased since the pandemic of COVID-19, inevitably leading to more release into water environment. However, their photochemical behaviors in water environment are still unclear. Therefore, this study investigated photodegradation properties of MIT and BIT in natural water under simulated sunlight. The results demonstrated that direct photolysis was mainly responsible for their photodegradation which occurred through their excited singlet states rather than triplet states. The quantum yields of MIT and BIT photodegradation were 11 - 13.6 × 10−4 and 2.43 - 5.79 × 10−4, respectively. pH had almost no effect on the photodegradation of MIT, while the photodegradation of BIT was significantly promoted under alkaline condition due to abundance of BIT in its deprotonated form (BIT-N−). Cl−, NO3− and dissolved organic matter (DOM) in natural water inhibited the photodegradation of both MIT and BIT, with the light screening effect of DOM being the most significantly inhibitory factor. The addition of other isothiazolinones, which possibly coexisted with MIT and BIT in actual condition, slightly inhibited the photodegradation of MIT and BIT. The estimated half-life under natural sunlight at a 30°N latitude was estimated to be approximately 1.1 days. The photodegradation pathways of MIT and BIT are similar, primarily initiated from the ring-opening at the N-S bond, with Frontier electron densities (FED) calculations suggesting the likelihood of oxidation and ·OH addition reactions at the O, N, and S sites. While the photodegradation products exhibited significantly reduced acute toxicity compared to their parent compounds, they nonetheless posed substantial chronic toxicity. These insights are vital for assessing the ecological impacts of MIT and BIT in aquatic environments.

Determining wavelength-dependent quantum yields of photodegradation: importance of experimental setup and reference values for actinometers.

Accurate quantum yields are crucial for modeling photochemical reactions in natural and engineered treatment systems. Quantum yields are usually determined using a single representative light source such as xenon lamps to mimic sunlight or UVC light for water treatment. However, photodegradation modeling can be improved by understanding the wavelength dependence of quantum yields and the potential errors introduced by the experimental setup. In this study, we investigated the effects of experimental setup on measured quantum yields using four photoreactor systems and up to 11 different light sources. When using a calibrated spectroradiometer to measure incident irradiance on an open solution surface, apparent quantum yields were up to two times higher if light reflection and light screening were not accounted for in the experimental setup. When the experimental setup was optimized to allow for accurate irradiance measurements, quantum yields were reproducible across photoreactors. The optimized experimental setup was then used to determine quantum yields of uridine, atrazine, p-nitroanisole (PNA), sulfamethoxazole, and diclofenac across the UV spectrum. No significant wavelength dependence of quantum yields was observed for sulfamethoxazole and diclofenac, in contrast to wavelength-dependent quantum yields for uridine, atrazine, and PNA. These reference values can be used for determining wavelength-dependent quantum yields of other compounds of interest. Additionally, more accurate results can be obtained when using (1) an actinometer with similar light absorption and photoreactivity compared to that of the target chemical, (2) optically transparent actinometer solutions that can account for light reflection within reaction vessels, and (3) a quantum yield that corresponds to the spectrum of the selected light source.

Photodegradation of free base and zinc porphyrins in the presence and absence of oxygen

Comparison of photostability in degassed and aerated toluene solutions is reported for 5,10,15,20-tetraphenylporphyrin, 5,10,15-tri(p-tolyl)porphyrin, and their zinc analogues. After degassing, quantum yields of photodegradation are higher, but the photodecomposition rates decrease. Lower stability in deoxygenated solutions is due to much longer triplet lifetimes: 200–300 microseconds, compared to 200–360 ns in non-degassed toluene. For the zinc porphyrins, the LC–MS results show that the initial photoproduct contains two oxygen atoms. Based on electronic absorption and calculations, it is assigned to dehydrated zinc biladienone structure, relatively stable in toluene, but readily demetallated in dichloromethane. A similar species is formed also in the case of free bases, but it then undergoes hydration due to traces of water present in the solvent. Zinc derivatives were found to form biladienones even in degassed solutions. To explain this observation, we postulate formation of a complex with remaining oxygen or oxygen-containing species which is not removed by freeze–thaw procedure. This hypothesis is confirmed by MS results and by the analysis of photodegradation products obtained when zinc porphyrin is complexed with dimethylsulfoxide (DMSO). Under these circumstances, changes in absorption are the same as in the absence of DMSO when non-degassed toluene is used, but irradiation of deoxygenated solutions leads to a different photoproduct. For both degassed and non-degassed solvents, complexation with DMSO results in the enhancement of photostability.Graphical abstract

Glycosylated zinc(II) phthalocyanine photosensitizer: Synthesis, photophysical properties and in vitro photodynamic activity on breast cancer cell line

Breast cancer is a common type of cancer among women worldwide. Triple negative breast cancer is a highly aggressive subtype of breast cancer and accounts for approximately 24% of all identified breast cancer cases. In triple negative breast cancer, the therapeutic approach options are very limited. In this study, 4-[2′,3′,5′,6′-tetrafluoro-4′-(1,2:5,6-di-O-isopropylidene-α-d-glucofuranosyl)benzyloxy]phthalonitrile and its peripherally tetra substituted zinc phthalocyanine derivatives are reported for the first time. Photophysical and photochemical properties of the glycosylated zinc(II) phthalocyanine derivative were investigated for its suitability for use as a photosensitizer in photodynamic therapy. The fluorescence (ΦF: 0.089), singlet oxygen (Φd: 0.45) and photodegradation quantum yields (Φ∆: 6.07×10−5) of this compound were carried out in N,N-dimethylformamide. The in vitro photodynamic activities of this compound against MDA-MB-231 cells were also studied. Cytotoxicity analysis in this cell lines was performed by MTT (3-(4,5-dimethylthiazol-2-yl) method. Cytotoxicity test was applied on cell lines of MDA-MB-231 and HUVEC. The IC50 value is determined for glycosylated zinc(II) Pc derivative.

Mechanistic investigation of direct photodegradation of chloroquine phosphate under simulated sunlight

Chloroquine phosphate (CQ) is an antiviral drug for Coronavirus Disease 2019 and an old drug for treatment of malaria, which has been detected in natural waters. Despite its prevalence, the environmental fate of CQ remains unclear. In this study, the direct photodegradation of CQ under simulated sunlight was investigated. The effect of various parameters such as pH, initial concentration and environmental matrix were examined. The photodegradation quantum yield of CQ (4.5 × 10−5−0.025) increased with the increasing pH value in the range of 6.0–10.0. The electron spin resonance (ESR) spectrometry and quenching experiments verified that the direct photodegradation of CQ was primarily associated with excited triplet states of CQ (3CQ*). The common ions had negligible effect and humic substances exhibited a negative effect on CQ photodegradation. The photoproducts were identified using high-resolution mass spectrometry and the photodegradation pathway of CQ was proposed. The direct photodegradation of CQ involved the cleavage of the C–Cl bond and substitution of the hydroxyl group, followed by further oxidation to yield carboxylic products. The photodegradation processes were further confirmed by the density functional theory (DFT) computation for the energy barrier of CQ dichlorination. The findings contribute to the assessment of the ecological risk associated with the overuse of Coronavirus drugs during global public health emergencies.

Direct UV photodegradation of nalidixic acid in aqueous solutions: A mechanistic study

Mechanism of direct UV photolysis of nalidixic acid (NA), a model quinolone antibiotic, was revealed using a combination of steady-state photolysis coupled with high resolution LC-MS and DFT quantum-chemical calculations. Both quantum yields of photodegradation and detailed identification of final products were performed for the first time for two main forms of NA: neutral and anionic. The quantum yield of NA photodegradation is 0.024 and 0.0032 for the neutral and anionic forms in the presence of dissolved oxygen and 0.016/0.0032 in deoxygenated solutions, respectively. The main process is photoionization with the formation of a cation radical, which undergoes transformation into three different neutral radicals and further into final photoproducts. It is shown that the triplet state does not play a role in the photolysis of this compound. The main products of photolysis are the products of the loss of carboxyl, methyl and ethyl groups in the NA molecule, as well as the dehydrogenation of the ethyl group. The results obtained may be important for understanding the fate of pyridine herbicides in the processes of disinfection by UV and in natural waters under the action of sunlight.

The synthesis of novel water-soluble zinc (II) phthalocyanine based photosensitizers and exploring of photodynamic therapy activities on the PC3 cancer cell line.

In this study, Schiff base substituted phthalocyanine complexes (Zn1c, Zn2c) and their quaternized derivatives (Q-Zn1c, Q-Zn2c) were synthesized for the first time. Their structures have been characterized by FT-IR, 1H-NMR, UV-Vis, mass spectrometry and elemental analysis as well as. The photophysicochemical properties (fluorescence, singlet oxygen and photodegradation quantum yield) of these novel complexes were investigated in dimethylsulfoxide (DMSO) for both non-ionic and quaternized cationic phthalocyanine complexes and in aqueous solution for quaternized cationic phthalocyanine complexes. Water soluble cationic phthalocyanine compounds gave good singlet oxygen quantum yield (0.65 for Q-Zn1c, 0.66 for Q-Zn2c in DMSO; 0.65 for Q-Zn2c in aqueous solution). The binding of Q-Zn1c and Q-Zn2c to BSA/DNA was studied by using UV-Vis and fluorescence spectroscopy and these. Studies indicate that the mechanism of BSA quenching by quaternized zinc(II) phthalocyanines was static quenching. Quaternized zinc(II) phthalocyanines interacted with ct-DNA by intercalation. Quaternized zinc(II) phthalocyanines caused a decrease in cell viability and triggered apoptotic cell death after PDT was applied at a concentration that did not have a toxic effect on their own. Q-Zn1c and Q-Zn2c mediated PDT reduced the activity of SOD, CAT, GSH while increased MDA level in the prostate cancer cells. Furthermore, expression of apoptotic proteins after PDT was examined. The results revealed that the synthesized water soluble quaternized zinc(II) phthalocyanine complexes (Q-Zn1c and Q-Zn2c) are promising potential photosensitizers for PDT.

Influence of different visible LED light sources on photo-degradation of red cabbage extract

Abstract This study elucidated the effect of wavelength, radiant fluence, and pH on the photobleaching of red cabbage extracts (RCE). Light-induced degradation of anthocyanins (ACN), color, total phenolic content (TPC), and antioxidant activity of RCE was evaluated. Blue light was found to have the most detrimental effect on red cabbage ACN, followed by white and green lights. The photo-degradation increased with pH and radiant fluence. The highest loss of ACN was estimated at pH 7, being about 91 %, 76 %, and 54 % under blue, white, and green light, respectively. Color attributes and antioxidant activity of the samples showed similar trends. The photo-degradation quantum yield was higher under blue light (pH 3: 0.46 × 10−4 mol einstein−1; pH 5: 4.5 × 10−4 mol einstein−1; pH 7: 4.5 × 10−4 mol einstein−1) than under other irradiations. The findings can help industrial processors devise suitable strategies to minimize the degradation of ACN under LED illumination.

A new view on the mechanism of UV photodegradation of the tricyclic antidepressant carbamazepine in aqueous solutions

Mechanism of direct UV photolysis of the tricyclic antidepressant carbamazepine (CBZ) at neutral pH was revealed by a combination of nanosecond laser flash photolysis, steady-state photolysis combined with high resolution LC-MS and DFT quantum-chemical calculations. The detection of short-lived intermediates and the detailed identification of final products were performed for the first time. The quantum yield of CBZ photodegradation (282 nm) is about 0.1% and 0.18% in air-equilibrated and argon-saturated solutions. The primary stage is photoionization with the formation of CBZ cation radical followed by a rapid nucleophilic attack by a solvent molecule. The primary photoproducts are 10-oxo-9-hydro-carbamazepine, 9-formylacridine-10(9H)-carboxamide (a result of ring contraction) and various isomers of hydroxylated CBZ. Prolonged irradiation results to accumulation of acridine derivatives, which should lead to an increase of the toxicity of photolyzed CBZ solutions. The obtained results may be important for understanding the fate of tricyclic antidepressants in processes of UVC disinfection and in natural waters under action of sunlight.

Water-soluble meso-thienyl BODIPY therapeutics: Synthesis, characterization, exploring photophysicochemical and DNA/BSA binding properties

Recently, there is rising attention in photodynamic therapy (PDT) compared to traditional methods (immunotherapy, surgery, chemotherapy or radiotherapy) in the treatment of cancer. This method includes the sensitizers which are activated to generate cytotoxic reactive oxygen species (ROS). BODIPYs are known as one of the sensitizers used in PDT applications. Encouraged by these fact in this study, we present the synthesis of novel BODIPY derivatives with meso-thienyl segment, diiodine moieties at 2-,6-positions and distyryl structure at 3-,5-positions. The characterization of all compounds was performed by using various spectroscopic methods (UV–Vis, FT-IR, Mass, 1H-NMR and 13C-NMR spectroscopy). To assess the potentials of the synthesized compounds in PDT applications, fluorescence, singlet oxygen and photodegradation quantum yields were calculated. Singlet oxygen quantum yields are 0.03, 0.84, 0.13, 0.32 and 0.76 for BODIPY compounds 1–5, respectively. Besides the photophysicochemical studies, DNA/BSA binding experiments for water-soluble BODIPY derivative (5) were examined. For water-soluble BODIPY derivative 5, the DNA binding constant (Kb) is 1.75 × 105 and the BSA binding constant (KBSA) is 4.15 × 104.

Photophysical and photochemical properties and comparison of tolyl and tosyl coumarin-bearing phthalocyanines

In this study, peripheral and non-peripherally substituted Zn(II) phthalocyanine complexes were synthesized from 7‑hydroxy-3-(p-tolyl)coumarin and 7‑hydroxy-3-(p-tosyl)coumarin compounds. The synthesized new compounds were characterized using elemental analysis, FT-IR, UV–Vis, Fluorescence 1HNMR spectroscopy and MALDI-TOF mass spectrometry. All the synthesized phthalocyanine complexes showed good solubility in organic solvents such as acetone, dichloromethane, chloroform, pyridine, and ethyl acetate. Fluorescent quenching behavior was investigated using 1,4-benzoquinone and potassium iodide as a quencher. The photophysical (fluorescent quantum yields and lifetimes) and photochemical (single oxygen and photodegradation quantum yields) properties of these new phthalocyanines were examined in dimethyl sulfoxide. Phthalocyanine complexes containing 7‑hydroxy-3-(p-tolyl)coumarin had higher singlet oxygen quantum yields than phthalocyanine complexes containing 7‑hydroxy-3-(p-tosyl)coumarin. Phthalocyanines to which coumarins are peripherally bound were more advantageous than their non-peripherally bound derivatives. As a result of their photophysical and photochemical properties, coumarin-phthalocyanine complexes containing tolyl-/tosyl- groups can be used as photosensitizing candidates in photodynamic therapy and can be developed with targeted modifications.

Synthesis, photophysical and photochemical properties of unsymmetrical zinc(II) phthalocyanines bearing 8-hydroxyquinoline unit

8-Hydroxyquinolines (8-HQs) are very important molecules in the field of biochemistry. They may play different roles due to their chelating ability. Especially, its interaction with different metals such as iron and copper, which have important functions in the growth of cancer cells, helps inhibit proliferation. Moreover, the metal complexes of 8-HQ derivatives contribute to the generation of singlet oxygen species. On the other hand, phthalocyanines are macrocyclic compounds that have the capacity to generate singlet oxygen species and anticancer activity by photodynamic therapy. In this study, unsymmetrical zinc(II) phthalocyanine derivatives bearing one 8-HQ group and three tert-butyl groups (8-MeQ-ZnPc and 8-HQ-ZnPc) were prepared. For the synthesis of target molecules, unsymmetrical zinc(II) phthalocyanine containing methoxyquinoline unit (8-MeQ-ZnPc) was prepared first, and then the demethylation reaction was successfully carried out with BBr3 for obtaining hydroxyquinoline counterpart (8-HQ-ZnPc). The fluorescence, singlet oxygen, and photodegradation quantum yields of 8-MeQ-ZnPc and 8-HQ-ZnPc were measured in DMSO, and the results show that the photosensitizers can be a promising anticancer agent candidates.

Improvement of photochemical and enzyme inhibition properties of new BODIPY compound by conjugation with cisplatin

In this study, new BODIPY complexes were synthesized, characterized and investigated for their some metabolic enzyme inhibition and photophysicochemical properties. BODIPY central core unit was synthesized by using conventional method followed by Suzuki-Miyaura Cross-Coupling reaction. After converting the obtained compound into the diiodinated derivative in the presence of HIO3/I2, quaternization of BODIPY derivative was achieved with cisplatin [cis-PtCl(NO3)(NH3)2] over the pyridine segment. Characterizations of all intermediates and the target compound were determined by 1H-NMR, 13C-NMR and MS spectroscopic methods. The potentials of the synthesized compounds in photodynamic therapy applications were determined by calculating the fluorescence, singlet oxygen and photodegradation quantum yields. Also, all complexes were screened for human carbonic anhydrase I, II (hCAs I and II), acetylcholinesterase (AChE), butyryl cholinesterase (BChE), and α-glucosidase inhibitory activities. Results indicated that all the synthesized complexes exhibited potent inhibitory activities against standard inhibitors, revealed by IC50 values.

Metal or metal-free phthalocyanines containing morpholine substituents: synthesis, spectroscopic and photophysicochemical properties

The phthalonitrile (3) and its peripherally tetra 4-(3-morpholinophenoxy) substituted metal-free (4), lead(II) (5) and zinc(II) (6) phthalocyanine derivatives were synthesized. The structures of 3-6 were confirmed by FT-IR, 1H-NMR, MALDI-TOF mass, UV-vis and fluorescence spectral data. The solubility and aggregation behaviors of 4-6 were determined in dimethyl sulfoxide (DMSO), N,N-dimethylformamide (DMF), tetrahydrofuran (THF), chloroform (CHCl3) and dichloromethane (CH2Cl2). The observed sharp absorptions were evidence of formation of non-aggregated phthalocyanine species at the studied concentration. The effects of the substituent and central metal ions (metal-free, zinc or lead) on spectroscopic and photophysicochemical properties were determined. Their photophysicochemical properties such as fluorescence quantum yields and lifetimes, singlet oxygen generation and photodegradation quantum yields were investigated in DMSO. The fluorescence spectra of metal-free (4) and lead(II) (5) phthalocyanines were not recorded due to negligible fluorescence emissions of these phthalocyanines. The zinc(II) phthalocyanine (6) had 0.17 fluorescence quantum yield (ΦF) and 1.42 fluorescence lifetime (τF). The phthalocyanines (4-6) had 0.18, 0.41, 0.52 singlet oxygen quantum yields (ΦΔ) and 0.51, 3.90, 0.46 photodegradation quantum yields (ΦΔ), respectively. The zinc(II) phthalocyanine (6) in particular could be a potential Type II photosensitizer candidate for photodynamic therapy in cancer treatment.

Large-molecular-weight acyldiphenylphosphine oxides as low-mobility type I photoinitiator for radical polymerization

In recent years, the light source for photopolymerization technology is evolving from energy-hungry mercury lamps to the safer light-emitting diodes (LEDs), and it is urgent to develop new photoinitiator adapted to the photopolymerization system with LED light source. Some acyldiphenylphosphine oxide photoinitiators have been designed and developed for LED light source, but their application is severely limited in the fields of food packing, medical equipment, environmentally-friendly product etc. due to their high mobility in photocured polymers. In this study, eight acyldiphenylphosphine oxide molecules (DAPO-1 ∼ DAPO-8) were synthesized by introducing large molecular weight groups and their UV–visible absorption, photodegradation quantum yield, photopolymerization properties, mobility and solubility in monomer were determined. Thereinto, photoinitiators show lower mobility than TPO, the photoinitiator commonly used in industry, as well as good solubility in monomers and excellent photopolymerization performance. Therefore, they can serve as potential substitutes for commercial TPO, especially in the application of low-migration photocurable system.

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.

New Metallophthalocyanines Bearing 2-Methylimidazole Moieties-Potential Photosensitizers against Staphylococcus aureus.

Newly developed tetra- and octasubstituted methimazole-phthalocyanine conjugates as potential photosensitizers have been obtained. Synthesized intermediates and final products were characterized by the MALD-TOF technique and various NMR techniques, including 2D methods. Single-crystal X-ray diffraction was used to determine the crystal structures of dinitriles. The studied phthalocyanines revealed two typical absorption bands—the Soret band and the Q band. The most intense fluorescence was observed for octasubstituted magnesium(II) phthalocyanine in DMF (ΦFL = 0.022). The best singlet oxygen generators were octasubstituted magnesium(II) and zinc(II) phthalocyanines (Φ∆ 0.56 and 0.81, respectively). The studied compounds presented quantum yields of photodegradation at the level between 10−5 and 10−6. Due to their low solubility in a water environment, the liposomal formulations were prepared. Within the studied group, octasubstituted zinc(II) phthalocyanine at the concentration of 100 µM activated with red light showed the highest antibacterial activity against S. aureus equal to a 5.68 log reduction of bacterial growth.

Direct UV photodegradation of herbicide triclopyr in aqueous solutions: A mechanistic study

Mechanism of direct UV photolysis of pyridine herbicide triclopyr (TRI) was revealed by the combination of nanosecond laser flash photolysis, steady-state photolysis coupled with high resolution LC-MS and DFT quantum-chemical calculations. Both the detection of short-lived intermediates and the detailed identification of final products were done for the first time. The quantum yield of TRI photodegradation is about 4% at both UVC (254 nm) and UVB (308 nm) excitation. The primary stage is the heterolytic cleavage of C–Cl bond in dissociative triplet state of TRI with the formation of phenyl cation followed by a fast nucleophilic attack by a solvent molecule. The minor channel is the photohydrolysis leading to the formation of 3,5,6-trichloropyridin-2-ol. Primary photoproducts undergo secondary photolysis by the mechanism similar to initial TRI with the formation of products of acetic group elimination, sequential substitution of chlorine atoms to hydroxyl groups and, finally, oxidation and opening of the pyridine ring. Obtained results can be important for understanding the fate of pyridine herbicides in the processes of UVC disinfection and in natural waters under action of the sunlight.

Photoreactivity and stability of flavonoid yellows used in cultural heritage

Flavonoid yellows are historical dyes widely found in cultural heritage, in particular those based on the glycosides of luteolin, quercetin and kaempferol. In this research, for the first time, the photodegradation quantum yields (ΦR) were calculated for luteolin, quercetin and kaempferol as well as their glycosides Lut-7-O-Glc, Que-3-O-Glr, Kaem-3-O-Glc and compared to the reactivity of eriodictyol. The results are discussed within the current state of the art on the mechanisms of degradation for these antioxidant compounds, which are reviewed in the introduction. ΦR values were obtained in solution irradiating at 313 nm and 366 nm, providing the quantification of the stability of flavonoid yellows. In particular, by irradiating at 366 nm, a very good stability scale is obtained, with ΦR values for luteolin and the 3-O-glycosides of quercetin and kaempferol on the order of 10−6. The more reactive quercetin and kaempferol are characterized by ΦR values of ca. 3 × 10−5. The relative stability of these yellows can be explained by a photoprotective mechanism based on excited state proton transfer, which is more efficient when there is an OH in the C5 position. In addition, electron transfer, and consequently oxidation, is also enhanced through the OH in position C3 when compared to C5.Solvents effects were also studied and to mimic the environment of the dye in wool, irradiation at 366 nm in a proteinaceous gel was carried out. In this gel, the ΦR values increased for all the molecules studied, reaching 1 × 10−4 and 2 × 10−4 for kaempferol and quercetin, respectively. This drastic enhancement shows that the environment has a major impact on stability. The main degradation products were also studied by irradiation with a xenon source (λirr ≥ 300 nm), and characterized by HPLC-DAD-MS and LC-HRMS/MS. These data, on the reaction mechanism, show that the double bond between C2 and C3, in the C ring, is the most reactive point of the molecules, as in all the flavonoids degradation started by solvent attack, followed in some cases by ring opening and the formation of low molecular weight hydroxyl compounds such as benzoic acids, in agreement with the general reaction scheme proposed in literature.