Light-induced Decomposition Research Articles

Light-induced phase transformation to two-dimensional amorphous spinel CoAlOx nanosheets as a tandem photothermal catalyst for efficient and durable toluene oxidation

Low temperature fabrication of spinel oxides and development of efficient and durable toluene photothermal catalysts remain challenging. Herein, two-dimensional amorphous spinel CoAlOx nanosheets (CoAl-LDH-L2) have been fabricated via a light-induced phase transformation strategy using intense photooxidation. Light-induced decomposition and phase transformation process have been used to elucidate the growth mechanism. The CoAl-LDH-L2 exhibits improved toluene adsorption and photothermal conversion, with the highest photothermal catalytic toluene conversion (∼100 % at 500 ppm) at 90 °C, outperforming most previously reported catalysts. Irradiation with mild external heating significantly increases the mineralization rate (∼90 %) and improves the long-term and cycling stability (∼5% losses within 39 h) relative to that under single-light-driven conditions. In-situ DRIFT spectroscopy was also performed for revealing the “tandem photothermal catalysis” mechanism for toluene conversion. This work offers a novel strategy for developing spinel oxides as low-temperature photothermal catalyst in a green and sustainable way.

The effect of ancillary ligands on hydrocarbon C-H bond functionalization by uranyl photocatalysts.

The aqueous uranyl dication has long been known to facilitate the UV light-induced decomposition of aqueous VOCs (volatile organic compounds), via the long-lived highly efficient, uranyl excited state. The lower-energy visible light excited uranyl ion is also able to cleave unactivated hydrocarbon C-H bonds, yet the development of this reactivity into controlled and catalytic C-H bond functionalization is still in its infancy, with almost all studies still focused on uranyl nitrate as the precatalyst. Here, hydrocarbon-soluble uranyl nitrate and chloride complexes supported by substituted phenanthroline (Ph2phen) ligands are compared to each other, and to the parent salts, as photocatalysts for the functionalization of cyclooctane by H atom abstraction. Analysis of the absorption and emission spectra, and emission lifetimes of Ph2phen-coordinated uranyl complexes demonstrate the utility of the ligand in light absorption in the photocatalysis, which is related to the energy and kinetic decay profile of the uranyl photoexcited state. Density functional theory computational analysis of the C-H activation steps in the reaction show how a set of dispersion forces between the hydrocarbon substrate and the Ph2phen ligand provide control over the H atom abstraction, and provide predictions of selectivity of H atom abstraction by the uranyl oxo of the ring C-H over the ethyl C-H in an ethylcyclohexane substrate.

Size-Dependent Effect of Indocyanine Green Nanoimaging Agent for Metastatic Lymph Node Detection.

Identification of metastatic lymph nodes is a crucial step in lymph node dissection to prevent further cancer spread and recurrence. However, the current limitations in metastatic lymph node detection often result in extensive resection of normal lymph nodes, leading to serious complications. The clinical application of indocyanine green (ICG) as a tool for lymph node detection is challenging because of its short plasma half-life and rapid light-induced decomposition and clearance. To overcome this limitation, we used polydopamine nanoparticles (PNs) as carriers for ICG and screened for the optimal particle size for detecting metastatic lymph nodes. ICG/PNs with sizes of 80, 160, 300, and 600 nm were synthesized, and their ICG loading efficiency, physical stability, and lymph node distribution were evaluated. The ICG absorbed on the PNs was found to be protected from light degradation, and its retention at the lymph nodes was improved. Notably, the ICG/PNs favored the fluorescence signal at the metastatic lymph nodes compared to the nonmetastatic lymph nodes. Among the tested particle sizes, the 80-nm ICG/PN showed a higher distribution in the metastatic lymph nodes. This study suggests that the 80-nm ICG/PN is a potentially valuable reagent for the detection and diagnosis of lymph node metastasis.

Histidine-assisted reduction of arylnitrenes upon photo-activation of phenyl azide chromophores in GFP-like fluorescent proteins.

The photochemically active sites of the proteins sfGFP66azF and Venus66azF, members of the green fluorescent protein (GFP) family, contain a non-canonical amino acid residue p-azidophenylalanine (azF) instead of Tyr66. The light-induced decomposition of azF at these sites leads to the formation of reactive arylnitrene (nF) intermediates followed by the formation of phenylamine-containing chromophores. We report the first study of the reaction mechanism of the reduction of the arylnitrene intermediates in sfGFP66nF and Venus66nF using molecular modeling methods. The Gibbs energy profiles for the elementary steps of the chemical reaction in sfGFP66nF are computed using molecular dynamics simulations with quantum mechanics/molecular mechanics (QM/MM) potentials. Structures and energies along the reaction pathway in Venus66nF are evaluated using a QM/MM approach. According to the results of the simulations, arylnitrene reduction is coupled with oxidation of the histidine side chain on the His148 residue located near the chromophore.

Oxybenzone-loaded Polymeric Nanocapsule Gel: Strategy to Reduce Skin Penetration and Drug-induced Irritation

Aim: This work aimed to determine whether the oxybenzone-loaded nanocapsule gel (NC gel) can reduce the degree of skin penetration and, thereby, the skin irritation caused by the drug. Background: Sunscreens must be retained on the uppermost skin surface, forming a protective film throughout the entire UV exposure period. Skin penetration and/or light-induced decomposition of sunscreens is undesirable as this would decrease the expected UV protection. The changes in colour and appearance of the sunscreen, as well as the possible increase in the phototoxicity and allergenic potential of sunscreen breakdown products, are also of concern. : Furthermore, exposure to UVB radiation leads to impairment in the stratum corneum and hence enhances the skin's penetration of sunscreen actives, leading to systemic absorption. Currently, available sunscreen agents are low-molecular-weight lipophilic molecules, which make them capable of penetrating the skin and being systemically absorbed. : Oxybenzone is an organic compound widely used in commercial sunscreen preparations. The low molecular weight and lipophilic nature of oxybenzone facilitate penetration through the stratum corneum, leading to irritation and allergic reactions. : Polymeric nanocapsules (NCs) have been developed to ensure a controlled release of pharmaceutical and cosmetic agents and/or exert a preventive effect against the harmful side effects induced by direct contact of tissues with high amounts of the drug. Hence, we hypothesised that encapsulating the oxybenzone polymeric nanocapsule system would reduce the irritation and its allergic potential upon topical application. Objective: The current study aimed to develop a polymeric nanocapsule-based gel of oxybenzone and investigate its skin deposition by ex vivo permeation studies. Methods: The polymeric nanocapsules were prepared by the solvent displacement method using a combination of Eudragit RSPO and Eudragit RLPO polymers. Further nanocapsules were incorpo-rated into the xanthan gum gel base. It was evaluated for particle size, PDI, zeta potential, drug con-tent, in vitro release, ex vivo permeation, sunscreen efficacy and skin irritation potential by the HET CAM test. Results: Particle size, PDI, zeta potential, percent encapsulation, and loading of drug-loaded nanocap-sule suspension were found to be 237.1 nm, 0.189 + 58.4 mV, 852 + 0.5%, and 19.45 + 0.94%, re-spectively. DSC results indicate the suitability of selected excipients. In vitro release studies indicated 68.99% oxybenzone release from NC suspension, while NC gel showed 58.87% at the end of 24 hrs. Ex vivo permeation results showed significantly higher drug permeation from plain oxybenzone gel than NC gel. Permeation flux and reduction ratio suggest a three-fold reduction in skin permeation by NC gel. The gel offered medium-level sun protection with an SPF value of 22.12. According to the HET CAM study, there was no evidence of skin irritation caused by NC gel. Conclusion: It is concluded that the encapsulation of oxybenzone prevents direct interaction with the skin, reducing the chances of skin irritation and penetration. Results suggest that nanocapsules in gel bases can provide a promising formula for safer topical delivery of sunscreen.

Light-driven biosynthesis of volatile, unstable and photosensitive chemicals from CO2

Directing CO2 conversion using photoautotrophic microbes offers a promising route to coupling carbon mitigation with petrochemical replacement. However, solar-based biomanufacturing is hampered by inefficient genetic manipulation, narrow product scope and light-induced decomposition. Here we report a spatiotemporally separated modular strategy to realize CO2-to-molecule conversion by sequentially linking carbon sequestration and cellular catalysis via stable mediator compounds. The carbon fixation rate of the sequestration module was improved by approximately 50% through metabolic network remodelling, while biphasic catalysis, multiple gene editing and high-throughput workflow were applied to the catalysis modules to produce olefins, cinnamaldehyde and curcumin. The catalytic efficiency was notably enhanced by up to 114-fold compared with the monoculture. This modular design approach enables the rapid development of sustainable biorefineries in a plug-and-play fashion, as evidenced by the production of various chemicals at the gram-per-litre level through scaled-up fermentation. This carbon-negative flexible platform notably widens the applicability of light-driven biosynthesis and may boost the bioindustry of CO2 reduction in a sustainable future. Carbon-negative biomanufacturing is typically hampered by narrow product scope and light-induced decomposition. Now, an integrated photosynthetic carbon sequestration and cellular catalysis strategy for CO2 valorization are reported. Multiple gene editing and a high-throughput workflow have facilitated its application to the biocatalytic production of styrene, intracellular unstable aldehydes and photosensitive molecules from CO2.

Visible Light-Induced Decomposition of Acyl Peroxides Using Isocyanides: Synthesis of Heteroarenes by Radical Cascade Cyclization.

Visible light-mediated facile synthesis of heteroarenes, namely, isoquinolines, benzothiazoles, and quinazolines, is demonstrated by employing isocyanides and inexpensive acyl peroxides. It is shown for the first time that singlet-excited isocyanides decompose acyl peroxides into aryl/alkyl radicals. The latter attack isocyanides, yielding imidoyl radicals that subsequently cyclize to afford heteroarene products. The protocol involving radical cascade reactions obviates the requirement of any external photocatalyst, oxidant, additive, and base.

Isocyanide Ligands Promote Ligand-to-Metal Charge Transfer Excited States in a Rhenium(II) Complex.

A metal-to-ligand charge transfer with mixed intraligand character is observed for the rhenium hexakisarylisocyanide complex [Re(CNAr)6]PF6 (CNAr = 2,6-dimethylphenylisocyanide, λmax = 300 nm). Upon oxidation to [Re(CNAr)6](PF6)2, the dominant low energy optical transition is a ligand-to-metal charge transfer (LMCT) mixed with intraligand transitions (λmax = 650 nm). TD-DFT was used to identify the participating ligand-based orbitals in the LMCT transition, revealing that the majority of the donor orbital is based on the aryl ring (85%) as opposed to the CN bond (14%). For both [Re(CNAr)6]+ and [Re(CNAr)6]2+, structural characterization by X-ray diffraction reveals deviations from Oh geometry at the central Re ion, with larger reduction in symmetry observed for Re(II). For [Re(CNAr)6]+, these structural changes lead to a broadening of the strong ν(C≡N) stretch (2065 cm-1), as the degeneracy of the T1u IR-active mode is broken. Furthermore, a shoulder is observed for this ν(C≡N) stretch, resulting from deviation of the C-N-Ar bond from linearity. By contrast, [Re(CNAr)6]2+ has two weak bands in the ν(C≡N) region (2065 and 2121 cm-1). DFT calculations indicate that reduction of symmetry at the central rhenium ion manifests in the decrease in intensity and the observed split of the ν(C≡N) band. Stability of both complexes are limited by light-induced decomposition where Re(I) dissociates a isocyanide ligand upon irradiation and Re(II) absorbance decays under ambient light. These data provide new insights to the electronic structure of [Re(CNAr)6]2+, enhancing our understanding of LMCT excited states and the versatility of isocyanide ligands.

Ultraviolet light-induced decomposition of benzothiophene and dibenzothiophene derivatives for efficient sulfur removal without additives and catalysts

When producing liquid fuels from petroleum, hydrodesulfurization methods reduce the concentration of sulfur to ≤10 mg(S)/L (weight/volume concentration of sulfur), namely, ≤ 0.31 mmol/L. Dibenzothiophene derivatives (DBTs), which are known as particularly difficult desulfurizing substances, have been decomposed reductively in severe conditions of high temperatures (270–372 °C) and high pressure (50–102 atm of H2). In this study, we developed a UV light irradiation-based desulfurization method for aromatic sulfur compounds such as benzothiophene derivatives (BTs) and DBTs under room temperature and atmospheric pressure without the use of catalysts or additives. This method is simple, location-independent, and low-cost, and has low environmental impact. BTs and DBTs completely decomposed in approximately 8 h and 16 h, respectively, under ultraviolet (UV) light irradiation (λ = 254 nm) from a 8 W lamp. The yellow precipitates that were produced upon decomposition were confirmed to be sulfur allotropes (Sn). The residual hydrocarbon portion of DBT after sulfur removal was determined to be benzene. The decomposition reaction was determined to exhibit pseudo-first-order reaction. DFT calculations confirmed the degradation mechanism as follows: UV light irradiation induces a photochemically excited triplet state of DBTs. The excited DBTs reacts with O2 to form a π-complex, which isomerizes to a more stable σ-complex. The DBTs-O2 (σ-complex) then reacts with free DBTs to afford two molecules of dibenzothiophene-5-oxide derivatives (DBTOs), which are excited to singlet states on photoirradiation. The excited DBTOs isomerize through a minimum energy intersection eventually to dibenzofuran episulfides from which sulfur extrusion occurs.

Low-intensity-low-temperature stability assessment of perovskite solar cells operating on simulated Martian surface conditions.

Immigration to Mars, which is expected to be powered mainly by photovoltaics, is one of the greatest dreams of humanity. However, the extreme temperature difference and high-energy cosmic radiation on the surface of Mars make it difficult for conventional photovoltaics to operate steadily over time. With their advantages of being lightweight, having a high irradiation tolerance, and an outstanding power conversion efficiency (PCE), perovskite solar cells (PSCs) have shown themselves to be a promising candidate for Martian applications. In this study, we simulated the low-intensity-low-temperature (LILT) environment of the Mars surface, and monitored the in situ device performance of PSCs. Surprisingly, the device PCE was not only maintained at a high level but was even improved slightly. Further investigation revealed that the self-healing effect of perovskites under LILT conditions could be attributed to the light-induced decomposition of the perovskite film and the β-phase perovskite recrystallization process at the perovskite/hole transport layer interface. Interfacial β-phase perovskites are stable at low temperatures, which can facilitate charge extraction and protect the perovskite bulk from long-term light damage. This study demonstrated the feasibility of PSCs and provides a reference for Martian applications.

Temperature Dynamics of MAPbI3 and PbI2 Photolysis: Revealing the Interplay between Light and Heat, Two Enemies of Perovskite Photovoltaics

Regardless of the impressive photovoltaic performances demonstrated for lead halide perovskite solar cells, their practical implementation is severely impeded by the low device stability. Complex lead halides are sensitive to both light and heat, which are unavoidable under realistic solar cell operational conditions. Suppressing these intrinsic degradation pathways requires a thorough understanding of their mechanistic aspects. Herein, we explored the temperature effects in the light-induced decomposition of MAPbI3 and PbI2 thin films under anoxic conditions. The analysis of the aging kinetics revealed that MAPbI3 photolysis and PbI2 photolysis have quite high effective activation energies of ∼85 and ∼106 kJ mol-1, respectively, so decreasing the temperature from 55 to 30 °C can extend the perovskite lifetime by factors of >10-100. These findings suggest that controlling the temperature of the perovskite solar panels might allow the long operational lifetimes (>20 years) required for the practical implementation of this promising technology.

When iodide meets bromide: Halide mixing facilitates the light-induced decomposition of perovskite absorber films

APbX3 lead perovskites, where A is either an organic (methylammonium MA+ and formamidinium FA+) or an inorganic (Cs+) species, have recently emerged as highly promising photovoltaic materials. This interest is related to the exceptionally high power conversion efficiency (> 25%) demonstrated recently in the case of mixed-cation and mixed-halide perovskites. However, the poor intrinsic stability of complex lead halides remains a major hindrance in the commercialization of this emerging photovoltaic technology. An intense research effort is currently focused on revealing the origins and mechanistic aspects of the various pathways of degradation occurring in perovskite solar cells. In this paper, we present a systematic comparative study of a series of mixed-cation perovskite systems: MA0.15FA0.85PbI2.55Br0.45, Cs0.1MA0.15FA0.75PbI2.55Br0.45, Cs0.15FA0.85PbI2.55Br0.45, MA0.15FA0.85PbI3, Cs0.1MA0.15FA0.75PbI3, and Cs0.15FA0.85PbI3 all of which deliver high photovoltaic efficiencies in devices. Using a set of complementary analytical techniques, we demonstrate that bromide-containing mixed-halide perovskites have much lower photostability when compared to the equivalent iodide-based materials. The light-induced photochemical aging produced metallic lead as one of the final decomposition products in the case of all the studied complex lead halides except for Cs0.15FA0.85PbI3, which demonstrated outstanding stability under white light exposure. Theoretical calculations of the bromide-containing mixed-halide perovskites indicated that hole-coupling drives the formation of interstitial-vacancy halide pair defects to become more thermodynamically favorable, thus leading to the accelerated degradation of the halide-mixed perovskites. The obtained results demonstrate the usefulness of compositional engineering as a promising approach to boost the operational stability of perovskite solar cells and pave the way towards their successful commercialization.

Stability of 2D and 3D Perovskites Due to Inhibition of Light-Induced Decomposition

This review investigates the stability of 2D and 3D perovskite materials for energy conversion from solar radiation. The lower tolerance factor with absorption wavelength at 8260 A made the $${\rm{MAPbI}}_{3}$$ the most popular in perovskite solar cell applications. However, these materials are affected by a range of stability issues. The combined effect of light, moisture and oxygen can lead to the chemical degradation of $${\rm{MAPbX}}_{3}$$ perovskites. Hence, ternary antimony/bismuth halide came into the picture in the search for stable and nontoxic material for solar energy applications, but this material is characterized by poor photovoltaic behavior, with a higher band gap, lower dielectric constant and higher effective mass. Another strategy by atomic transmutation thus suggested double perovskites as potential material, with better stability, but the band gap energy (3.3 eV) of these material is situated near the ultraviolet (UV) range. Therefore, recent reports suggest $$\rm{ABS}_{3}$$ chalcogenides as a potential replacement without such drawbacks. Another alternative is to use 2D $$A_{2}A_{n-1}^{'}B_{n}X_{3n+1}$$ Ruddlesden–Popper perovskite materials, which show better stability with higher efficiency than their 3D counterparts.

Light-induced degradation and self-healing inside CH3NH3PbI3-based solar cells

CH3NH3PbI3 (MAPbI3)-based perovskite solar cells (PSCs) with special hole and electron transport layers (HTL and ETL) were prepared to study their light-induced degradation. Obvious degradation was observed under initial light exposure not only at the device level but also at the film morphology and electronic structure level. Device performance parameters, such as short-circuit current (JSC), power conversion efficiency, fill factor, and hysteresis effect, were aggravated with an initial light exposure of less than ∼8 h at 1 sun intensity. Meanwhile, the deteriorated crystallinity and electronic structure of the MAPbI3 film were also detected with x-ray diffraction, ultraviolet photoelectron spectroscopy, and UV-Visible absorption spectroscopy. The observed degradation is rationally related to the light-induced decomposition of MAPbI3. However, the degradation can be partly recovered with the following light exposure resulting in self-healing of the devices and MAPbI3 films. The self-healing behavior should be ascribed to the conversion of decomposition products back to MAPbI3, because the intermediates are wrapped tightly in the photoactive layer by the compact coverlayers of HTLs and ETLs and some reversible reactions occur consequently. The mechanism of self-healing is discussed by introducing the trapped states derived from ion migration. The PSCs prepared here imply a good optical stability and thus a good performance facilitated by tight wrapping of the active MAPbI3.

UV degradation of the interface between perovskites and the electron transport layer.

The stability of the perovskite/electron transport layer (ETL) interface is critical for perovskite solar cells due to the presence of ultraviolet (UV) light in the solar spectrum. Herein, we have studied the decomposition process and performance evolution of the perovskite layer in contact with different ETLs under strong ultraviolet irradiation. The normally used SnO2 layer has lower photocatalytic activity in comparison with the TiO2 layer, but the perovskite/SnO2 interface is still severely decomposed along with the formation of hole structures. Such UV light-induced decomposition, on the one hand, leads to the decomposition of the perovskite phase into PbI2 and more seriously, the formed hole structure significantly limits the carrier injection at the interface owing to the separation of the perovskite active layer from ETLs. Under the same conditions, the perovskite/PCBM interface is very stable and maintains a highly efficient carrier injection. There is no significant efficiency degradation of the encapsulated PCBM-based devices measured outdoors for about three months.

S-Nitrosoglutathione exhibits greater stability than S-nitroso-N-acetylpenicillamine under common laboratory conditions: A comparative stability study

S-Nitrosothiols (RSNOs) such as S-nitrosoglutathione (GSNO) and S-nitroso-N-acetylpenicillamine (SNAP) are susceptible to decomposition by stimuli including heat, light, and trace metal ions. Using stepwise isothermal thermogravimetric analysis (TGA), we observed that NO-forming homolytic cleavage of the S–N bond occurs at 134.7 ± 0.8 °C in GSNO and 132.8 ± 0.9 °C in SNAP, contrasting with the value of 150 °C that has been previously reported for both RSNOs. Using mass spectrometry (MS), nuclear magnetic resonance (NMR), and attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR), we analyzed the decomposition products from TGA experiments. The organic product of GSNO decomposition was glutathione disulfide, while SNAP decomposed to form N-acetylpenicillamine disulfide as well as other products, including tri- and tetrasulfides. In addition, we assessed the relative solution stabilities of GSNO and SNAP under common laboratory conditions, which include variable temperature, pH, and light exposure with rigorous exclusion of trace metal ions by chelation. GSNO exhibited greater stability than SNAP over a 7-day period except in one instance. Both RSNOs demonstrated an inverse relationship between solution stability and temperature, with refrigeration considerably extending shelf life. A decrease in pH from 7.4 to 5.0 also enhanced the stability of both RSNOs. A further decrease in pH from 5.0 to 3.0 resulted in decreased stability for both RSNOs, and is notably the only occasion in which SNAP proved more stable than GSNO. After 1 h of exposure to overhead fluorescent lighting, both RSNOs displayed high susceptibility to light-induced decomposition. After 7 h, GSNO and SNAP decomposed 19.3 ± 0.5% and 30 ± 2%, respectively.

Improving the Stability of Metal Halide Perovskite Quantum Dots by Encapsulation.

Metal halide perovskite quantum dots (PQDs), with excellent optical properties and spectacular characteristics of direct and tunable bandgaps, strong light-absorption coefficients, high defect tolerance, and low nonradiative recombination rates, are highly attractive for modern optoelectronic devices. However, the stability issue of PQDs remains a critical challenge of this newly emerged material despite the recent rapid progress. Here, the encapsulation strategies to improve the stability of PQDs are comprehensively reviewed. A special emphasis is put on the effects of encapsulation, ranging from the improvement of chemical stability, to the inhibition of light-induced decomposition, to the enhancement of thermal stability. Particular attention is devoted to summarizing the encapsulation approaches, including the sol-gel method, the template method, physical blending, and microencapsulation. The selection principles of encapsulation materials, including the rigid lattice or porous structure of inorganic compounds, the low penetration rate of oxygen or water, as well as the swelling-deswelling process of polymers, are addressed systematically. Special interest is put on the applications of the encapsulated PQDs with improved stability in white light-emitting diodes, lasers, and biological applications. Finally, the main challenges in encapsulating PQDs and further investigation directions are discussed for future research to promote the development of stable metal halide perovskite materials.

Laser Irradiation of Organic Tattoo Pigments Releases Carcinogens with 3,3′-Dichlorobenzidine Inducing DNA Strand Breaks in Human Skin Cells

Laser Irradiation of Organic Tattoo Pigments Releases Carcinogens with 3,3′-Dichlorobenzidine Inducing DNA Strand Breaks in Human Skin Cells

The Role of Excitation Energy in Photobrightening and Photodegradation of Halide Perovskite Thin Films.

We study the impact of excitation energy on the photostability of methylammonium lead triiodide (CH3NH3PbI3 or MAPI) perovskite thin films. Light soaking leads to a transient increase of the photoluminescence efficiency at excitation wavelengths longer than 520 nm, whereas light-induced degradation occurs when exciting the films with wavelengths shorter than 520 nm. X-ray diffraction and extinction measurements reveal the light-induced decomposition of CH3NH3PbI3 to lead iodide (PbI2) for the high-energy excitation regime. We propose a model explaining the energy dependence of the photostability that involves the photoexcitation of residual PbI2 species in the perovskite triggering the decomposition of CH3NH3PbI3.

Study of degradation products and degradation pathways of sulfonated azo dyes under ultraviolet irradiation

Monitoring the light-induced decomposition course of azo dyes is essential to understand their degradation pathways and mechanisms. In this study, two model dyes are synthesized and used for stimulating the photodegradation processes of azo and hydrazone dyes, respectively. Their intermediates formed during initial and final fading processes are characterized by high-performance liquid chromatography-electrospray ionization-mass spectrometry, gas chromatography-mass spectrometry and ion chromatography. Results reveal that no dramatic differences are observed between the two dyes, although hydrazone dye would undergo a more complicated degradation process. Hydroxyl radicals are the dominant reactive species involved in the photodegradation of both model dyes under ultraviolet irradiation. In the initial steps, the intermediates are almost hydroxylated derivatives, while low-molecular-weight dicarboxylic acids and their hydroxylated and esterified derivatives, as well as non-volatile inorganic ions, are detected and evidenced in the final steps. Furthermore, photodegradation pathways and mechanisms for the two model dyes are proposed accordingly.