Construction of Embedded Sulfur-Doped g-C3N4/BiOBr S-Scheme Heterojunction for Highly Efficient Visible Light Photocatalytic Degradation of Organic Compound Rhodamine B.

Constructing S-scheme heterojunction catalysts is a key challenge in visible-light catalysed degradation of organic pollutants. Most heterojunction materials are reported to face significant obstacles in the separation of photogenerated electron-hole pairs owing to differences in the material size and energy barriers. In this study, sulfur-doped g-C3N4 oxidative-type semiconductor materials are synthesized and then coupled with BiOBr reductive-type semiconductor to form S-g-C3N4/BiOBr S-scheme heterojunction. A strong and efficient internal electric field is established between the two materials, facilitating the separation of photogenerated electron-hole pairs. Notably, in situ XPS proved that after visible light irradiation, Bi3+ is converted into Bi(3+ɑ)+, and a large number of photogenerated holes are produced on the surface of BiOBr, which oxidized and activated H2O into •OH.  â€¢OH cooperated with •O2 - and 1O2 to attack Rhodamine B (RhB) molecules to achieve deep oxidation mineralization. The composite material is designed with a LUMO energy level higher than that of RhB, promoting the sensitization of RhB by injecting photogenerated electrons into the heterojunction, thereby enhancing the photocatalytic performance to 22.44 times that of pure g-C3N4. This study provides a new perspective on the efficient degradation of organic molecules using visible light catalysis.

Photogeneration and quenching of singlet molecular oxygen by bacterial C40 carotenoids with long chain of conjugated double bonds.

Measurement of photosensitized luminescence of singlet oxygen has been applied to studies of singlet oxygen generation and quenching by C40 carotenoids (neurosporene, lycopene, rhodopin, and spirilloxanthin) with long chain of conjugated double bonds (CDB) using hexafluorobenzene as a solvent. It has been found that neurosporene, lycopene, and rhodopin are capable of the low efficient singlet oxygen generation in aerated solutions upon photoexcitation in the spectral region of their main absorption maxima. The quantum yield of this process was estimated to be (1.5-3.0) × 10-2. This value is near the singlet oxygen yields in solutions of ζ-carotene (7 CDB) and phytoene (3 CDB) and many-fold smaller than in solutions of phytofluene (5 CDB) (Ashikhmin et al. Biochemistry (Mosc) 85:773-780, https://doi.org/10.1134/S0006297920070056 , 2020, Biochemistry (Mosc) 87:1169-1178, 2022, https://doi.org/10.1134/S00062979221001082022 ). Photogeneration of singlet oxygen was not observed in spirilloxanthin solutions. A correlation was found between the singlet oxygen yields and the quantum yields and lifetimes of the fluorescence of the carotenoid molecules. All carotenoids were shown to be strong physical quenchers of singlet oxygen. The rate constants of 1O2 quenching by the carotenoids with long chain of CDB (9-13) were close to the rate constant of the diffusion-limited reactions ≈1010 M-1 s-1, being many-fold greater than the rate constants of 1O2 quenching by the carotenoids with the short chain of CDB (3-7) phytoene, phytofluene, and ζ-carotene studied in prior papers of our group (Ashikhmin et al. 2020, 2022). To our knowledge, the quenching rate constants of rhodopin and spirilloxanthin have been obtained in this paper for the first time. The mechanisms of 1O2 photogeneration by carotenoids in solution and in the LH2 complexes of photosynthetic cells, as well as the efficiencies of their protective action are discussed.

Visible Light-sensitized CO2 Methanation along a Relaxed Heat Available Route.

In photocatalysis, the resulted heat by the relaxation of most of incident light no longer acts as the industrially favorite driving force back to the target photo-reaction due to more or less the negative relation between photocatalytic efficiency and temperature. Here, we reported a visible light-sensitized protocol that completely reversed the negatively temperature-dependent efficiency in photo-driven CO2 methanation with saturated water vapor. Uniform Pt/N-TiO2/PDI self-assembly material decisively injects the excited electron of PDI sensitizer into N-TiO2 forming Ti-H hydride which is crucially temperature-dependent nucleophilic species to dominate CO2 methanation, rather than conventionally separated and trapped electrons on the conductor band. Meanwhile, the ternary composite lifts itself temperature from room temperature to 305.2 °C within 400s only by the failure excitation upon simulated sunlight of 2.5 W/cm2, and smoothly achieves CO2 methanation with a record number of 4.98 mmol g-1 h-1 rate, compared to less than 0.02 mmol g-1 h-1 at classic Pt/N-TiO2/UV photocatalysis without PDI sensitization. This approach can reuse ~53.9% of the relaxed heat energy from the incident light thereby allow high-intensity incident light as strong as possible within a flowing photo-reactor, opening the most likely gateways to industrialization.

Structure-Activity Relationship of Benzophenazine Derivatives for Homogeneous and Heterogenized Photooxygenation Catalysis.

Singlet oxygen is a powerful oxidant used in various applications, such as organic synthesis, medicine, and environmental remediation. Organic and inorganic photosensitizers are commonly used to generate this reactive species through energy transfer with the triplet ground state of oxygen. We describe here a series of novel benzophenazine derivatives as a promising class of photosensitizers for singlet oxygen photosensitization. In this study, we investigated the structure-activity relationship of these benzophenazine derivatives. Akin to a molecular compass, the southern fragment was first functionalized with either aromatic tertiary amines, alkyl tertiary amines, aromatic sulfur groups, alkyl sulfur groups, or cyclic ethers. Enhanced photophysical properties (in terms of triplet excited-state lifetime, absorption wavelength, triplet state energy, and O2 quenching capabilities) were obtained with cyclic ether and sulfur groups. Conversely, the presence of an amine moiety was detrimental to the photocatalysts. The western and northern fragments were also investigated and slightly undesirable to negligible changes in photophysical properties were observed. The most promising candidate was then immobilized on silica nanoparticles and its photoactivity was evaluated in the citronellol photooxidation reaction. A high NMR yield of 97 % in desired product was obtained, with only a slight decrease over several recycling runs (85 % in the fourth run). These results provide insights into the design of efficient photosensitizers for singlet oxygen generation and the development of heterogeneous systems.

Thiocarbonyl-Bridged N-Heterotriangulenes for Energy Efficient Triplet Photosensitization: A Theoretical Perspective.

Structurally-rigid metal-free organic molecules are of high demand for various triplet harvesting applications. However, inefficient intersystem crossing (ISC) due to large singlet-triplet gap ( Δ E S - T ${\Delta {E}_{S-T}}$ ) and small spin-orbit coupling (SOC) between lowest excited singlet and triplet often limits their efficiency. Excited electronic states, fluorescence and ISC rates in several thiocarbonyl-bridged N-heterotriangulene ( m ${m}$ S-HTG) with systematically increased thione content ( m = ${m=}$ 0-3) are investigated implementing polarization consistent time-dependent optimally-tuned range-separated hybrid. All m ${m}$ S-HTGs are dynamically stable and also thermodynamically feasible to synthesize. Relative energies of several low-lying singlets ( S n ${{S}_{n}}$ ) and triplets ( T n ${{T}_{n}}$ ), and their excitation nature (i. e., n π * ${n{\pi }^{^{\ast}}}$ or π π * ${\pi {\pi }^{^{\ast}}}$ ) and SOC are determined for these m ${m}$ S-HTGs in dichloromethane. Low-energy optical peak displays gradual red-shift with increasing thione content due to relatively smaller electronic gap resulted from greater degree of orbital delocalization. Significantly large SOC due to different orbital-symmetry and heavy-atom effect produces remarkably high ISC rates ( k I S C ${{k}_{ISC}}$ ~1012 s-1) for enthalpically favoured S 1 n π * → T 2 ${{S}_{1}\left(n{\pi }^{^{\ast}}\right)\to {T}_{2}}$ ( π π * ${\pi {\pi }^{^{\ast}}}$ ) channel in these m ${m}$ S-HTGs, which outcompete radiative fluorescence rates (~108 s-1) even directly from higher lying optically bright π π * ${\pi {\pi }^{^{\ast}}}$ singlets. Importantly, high energy triplet excitons of ~1.7 eV resulting from such significantly large ISC rates from non-fluorescent S 1 n π * ${{S}_{1}\left(n{\pi }^{^{\ast}}\right)}$ make these thiocarbonylated HTGs ideal candidates for energy efficient triplet harvest including triplet-photosensitization.

Cefadroxil photodegradation processes sensitized by natural pigments: mechanistic aspects and impact on the antimicrobial function.

Cefadroxil is a widely used antibiotic with a low elimination efficiency in wastewater treatments plants, so it represents a contaminants of emerging concern that should be removed. The photosensitization process that involves natural pigments and visible sunlight can be offered as an environmentally friendly alternative to be considered for Cefadroxil degradation. In this investigation, we carried out a mechanistic and kinetic approach to Cefadroxil photodegradation sensitized by Riboflavin and Humic Acid, in individual and combined processes. Our experiments indicate that Cefadroxil is able to interact with the excited states of Riboflavin as well as with the photogenerated reactive oxygen species, with an important contribution of singlet oxygen. The antibiotic was less sensitive to the photodegradation in the presence of Humic Acids and in the mixture of Riboflavin and Humic Acids. Self-sensitization processes and internal filter effects are proposed as possible explanations for the observed phenomenon. The reaction between Cefadroxil and singlet oxygen showed a dependence with the pH of the medium, the photodegradation kinetic constants are greater at alkaline pH compared to neutral pH. The reaction is favored when the anionic species of the antibiotic is present. Microbiological tests on S. aureus indicated that the antibiotic reduce its antimicrobial activity as a consequence of the photooxidative process mediated by singlet oxygen. We believe that the results are relevant since, the sensitized photodegradation process could lead to the oxidation of Cefadroxil and to the progressive loss of its antimicrobial function, this fact could contribute to the decrease in the generation of bacterial multi-resistance to antibiotics in the environment.

BODIPY-fused uracil: synthesis, photophysical properties, and applications.

Fluorescent nucleobase and nucleic acid analogs are important tools in chemical and molecular biology as fluorescent labelling of nucleobases has applications in cellular imaging and anti-tumor activity. Boron-dipyrromethene (BODIPY) dyes exhibiting high brightness and good photostability are extensively used as fluorescent labelling agents and as type II photosensitizers for photodynamic therapy. Thus, the combination of nucleobases and BODIPY to obtain new compounds with both anti-tumor activity and fluorescent imaging functions is the focus of our research. We synthesized two new nucleobase analogs 1 and 2 by fusing the BODIPY core directly with uracil which resulted in favorable photophysical properties and high emission quantum efficiencies particularly in organic solvents. Further, we explored the newly synthesized derivatives, which possessed good singlet oxygen generation efficiencies and bio-compatibility, as potential PDT agents and our results show that they exhibit in vitro anti-tumor activities.

Evaporation-Induced Transformations in Volatile Chemical Product-Derived Secondary Organic Aerosols: Browning Effects and Alterations in Oxidative Reactivity.

Volatile chemical products (VCPs) are increasingly recognized as significant sources of volatile organic compounds (VOCs) in urban atmospheres, potentially serving as key precursors for secondary organic aerosol (SOA) formation. This study investigates the formation and physicochemical transformations of VCP-derived SOA, produced through ozonolysis of VOCs evaporated from a representative room deodorant air freshener, focusing on the effects of aerosol evaporation on its molecular composition, light absorption properties, and reactive oxygen species (ROS) generation. Following aerosol evaporation, solutes become concentrated, accelerating reactions within the aerosol matrix that lead to a 42% reduction in peroxide content and noticeable browning of the SOA. This process occurs most effectively at moderate relative humidity (∼40%), reaching a maximum solute concentration before aerosol solidification. Molecular characterization reveals that evaporating VCP-derived SOA produces highly conjugated nitrogen-containing products from interactions between existing or transformed carbonyl compounds and reduced nitrogen species, likely acting as chromophores responsible for the observed brownish coloration. Additionally, the reactivity of VCP-derived SOA was elucidated through heterogeneous oxidation of sulfur dioxide (SO2), which revealed enhanced photosensitized sulfate production upon drying. Direct measurements of ROS, including singlet oxygen (1O2), superoxide (O2•-), and hydroxyl radicals (•OH), showed higher abundances in dried versus undried SOA samples under light exposure. Our findings underscore that drying significantly alters the physicochemical properties of VCP-derived SOA, impacting their roles in atmospheric chemistry and radiative balance.

Aqueous-Phase Photoreactions of Mixed Aromatic Carbonyl Photosensitizers Yield More Oxygenated, Oxidized, and less Light-Absorbing Secondary Organic Aerosol (SOA) than Single Systems.

Aromatic carbonyls have been mainly probed as photosensitizers for aqueous secondary organic aerosol (aqSOA) and light-absorbing organic aerosol (i.e., brown carbon or BrC) formation, but due to their organic nature, they can also undergo oxidation to form aqSOA and BrC. However, photochemical transformations of aromatic carbonyl photosensitizers, particularly in multicomponent systems, are understudied. This study explored aqSOA formation from the irradiation of aromatic carbonyl photosensitizers in mixed and single systems under cloud/fog conditions. Mixed systems consisting of phenolic carbonyls only (VL + ActSyr + SyrAld: vanillin [VL] + acetosyringone [ActSyr] + syringaldehyde [SyrAld]) and another composed of both nonphenolic and phenolic carbonyls (DMB + ActSyr + SyrAld: 3,4-dimethoxybenzaldehyde [DMB], a nonphenolic carbonyl, + ActSyr + SyrAld) were compared to single systems of VL (VL*) and DMB (DMB*), respectively. In mixed systems, the shorter lifetimes of VL and DMB indicate their diminished capacity to trigger the oxidation of other organic compounds (e.g., guaiacol [GUA], a noncarbonyl phenol). In contrast to the slow decay and minimal photoenhancement for DMB*, the rapid photodegradation and significant photoenhancement for VL* indicate efficient direct photosensitized oxidation (i.e., self-photosensitization). Relative to single systems, the increased oxidant availability promoted functionalization in VL + ActSyr + SyrAld and accelerated the conversion of early generation aqSOA in DMB + ActSyr + SyrAld. Moreover, the increased availability of oxidizable substrates countered by stronger oxidative capacity limited the contribution of mixed systems to aqSOA light absorption. This suggests a weaker radiative effect of BrC from mixed photosensitizer systems than BrC from single photosensitizer systems. Furthermore, more oxygenated and oxidized aqSOA was observed with increasing complexity of the reaction systems (e.g., VL* < VL + ActSyr + SyrAld < VL + ActSyr + SyrAld + GUA). This work offers new insights into aqSOA formation by emphasizing the dual role of organic photosensitizers as oxidant sources and oxidizable substrates.

Highways and Detours in the Realm of Photodynamic Therapy.

Photodynamic therapy (PDT) has been a topic of interest since the first report in 1900 but has yet to become a 'mainstream' treatment protocol in the medical field. There are clear indications for which PDT might be the 'method of choice', but it is unlikely that there will be protocols for the treatment of systemic disease. This report discusses recent developments for promoting PDT efficacy, in the context of what is already known. Factors that can limit the scope of these applications are also indicated. Among the more interesting of these developments is the use of formulation techniques to target specific organelles for photodamage. This can enhance responses to PDT and circumvent situations where an impaired death pathway interferes with PDT efficacy.

Carbon-TiO2 Hybrid Quantum Dots for Photocatalytic Inactivation of Gram-Positive and Gram-Negative Bacteria.

Carbon-semiconductor hybrid quantum dots are classical carbon dots with core carbon nanoparticles doped with a selected nanoscale semiconductor. Specifically, on those with the nanoscale TiO2 doping, denoted as CTiO2-Dots, their synthesis and thorough characterization were reported previously. In this work, the CTiO2-Dots were evaluated for their visible light-activated antibacterial function, with the results showing the effective killing of not only Gram-positive but also the generally more resistant Gram-negative bacteria. The hybrid dots are clearly more potent antibacterial agents than their neat carbon dot counterparts. Mechanistically, the higher antibacterial performance of the CTiO2-Dots is attributed to their superior photoexcited state properties, which are reflected by the observed much brighter fluorescence emissions. Also considered and discussed is the possibility of additional contributions to the antibacterial activities due to the photosensitization of the nanoscale TiO2 by its doped core carbon nanoparticles.

Using Instanton Theory to Study Quantum Effects in Photosensitization.

Electronic excitation is usually accomplished using light (photoexcitation) and is a key step in a vast number of important physical and biological processes. However, in instances where photoexcitation is not possible, a photosensitizer can excite the target molecule in a process called photosensitization. Unfortunately, full details of its mechanism are still unknown. This perspective gives an overview of the current understanding of photosensitization and describes how instanton theory can be used to fill the gaps, especially with regard tothe importance of quantum tunnelling effects.

Employing singlet-singlet energy transfer for boosting the reactivity of Type I photoinitiators in radical photopolymerization.

A remarkable and unexpected increase in the photopolymerization efficiency of an acrylic resin by a bisacylphosphine oxide photoinitiator was observed when an optical brightener was present in the medium. High values for the maximal rates of photopolymerization were obtained by RT-FTIR at 365 nm under a very low irradiance of 1 mW/cm2. Fluorescence studies revealed that the quenching process was attributed to singlet-singlet energy transfer between the first singlet excited state of the optical brightener and the ground state photoinitiator. This mechanism acts as an additional pathway for the excitation of the photoinitiator, thereby increasing the total amount of initiating radicals. Using the Förster resonance energy transfer model, we calculated the relative efficiency of the photosensitization process compared to the direct excitation efficiency of the photoinitiator. The results demonstrate that the photosensitization process can be predicted, paving the way for further improvements in photoinitiating systems.

Aggregates of Ionic-Bonds Coupled Polymer and Their Photosensitization Enhancement Effect.

The formation of nanoaggregates makes a great difference to the improvement of photodynamic therapy (PDT) performance to some extent, but constructing stable aggregates with a clear structure is simultaneously a big challenge for us. Herein, just by electrostatic interaction, cationic 2PAHs and anionic FBA351, regarded as acceptor (A) and donor (D), respectively, are utilized to prepare stable aggregate of ionic-bonds coupled polymer (ICP) with repeated "D-A" structure, which is fully characterized by nuclear magnetic resonance (NMR), time-of-flight mass spectrometry, X-ray photoelectron spectroscopy (XPS), and scanning electron microscopy (SEM). Remarkably, aggregate ICP with multiple "D-A" structures showed enhanced photosensitization efficiency over its precursor 2PAHs and FBA351, which is in accord with the image-guided photodynamic anticancer therapy. Such results not only offer a simple way to obtain stable aggregate but also give us a guideline to design efficient photosensitizers.

Second Generation of Near-Infrared Cyanine-Based Photocatalysts for Faster Organic Transformations.

A second generation of cyanine-based near-infrared photocatalysts has been developed to accelerate organic transformations. Cyanines were prepared and fully characterized prior to evaluation of their photocatalytic activities. Catalyst efficiency was determined by using two model oxidation and reduction reactions. For the aza-Henry reaction, cyanines bearing an amino group on the heptamethine chain led to the best results. For trifluoromethylation, the stability of the photocatalyst was found to be the key parameter for efficient and rapid conversion.

The cysteine residue in beta-lactoglobulin reacts with oxidized tyrosine residues in beta-casein to give casein-lactoglobulin dimers.

Proteins are modified during milk processing and storage, with sidechain oxidation and crosslinking being major consequences. Despite the prevalence and importance of proteins in milk, and particularly caseins (∼80% of total content), the nature of the cross-links formed by oxidation, and their mechanisms of formation, are poorly characterized. In this study, we investigated the formation and stability of cross-links generated by the nucleophilic addition of Cys residues to quinones generated on oxidation of Tyr residues. The mechanisms and stability of these adducts was explored using ubiquitin as a model protein, and ß-casein. Ubiquitin and ß-casein were oxidized using a rose Bengal/visible light/O2 system, or by the enzyme tyrosinase. The oxidized proteins were incubated with glutathione or ß-lactoglobulin (non-oxidized, but unfolded by treatment at 70 °C), before analysis by SDS-PAGE, immunoblotting and LC-MS. Our data indicate that Cys-quinone adducts are readily-formed, and are stable for >48 h. Thus, oxidized ß-casein reacts efficiently with the thermally unfolded ß-lactoglobulin, likely via Michael addition of the exposed Cys to a Tyr-derived quinone. These data provide a novel, and possibly general, mechanism of protein cross-link formation, and provides information of the stability of these species that have potential as markers of protein quality.

Freezing-Enhanced Photoreduction of Iodate by Fulvic Acid.

Iodate is a stable form of iodine species in the natural environment. This work found that the abiotic photosensitized reduction of iodate by fulvic acid (FA) is highly enhanced in frozen solution compared to that in aqueous solution. The freezing-induced removal of iodate by FA at an initial pH of 3.0 in 24 h was lower than 10% in the dark but enhanced under UV (77.7%) or visible light (31.6%) irradiation. This process was accompanied by the production of iodide, reactive iodine (RI), and organoiodine compounds (OICs). The photoreduction of iodate in ice increased with lowering pH (pH 3-7 range) or increasing FA concentration (1-10 mg/L range). It was also observed that coexisting iodide or chloride ions enhanced the photoreduction of iodate in ice. Fourier transform ion cyclotron resonance mass spectrometric analysis showed that 129 and 403 species of OICs (mainly highly unsaturated and phenolic compounds) were newly produced in frozen UV/iodate/FA and UV/iodate/FA/Cl- solution, respectively. In the frozen UV/iodate/FA/Cl- solution, approximately 97% of generated organochlorine compounds (98 species) were identified as typical chlorinated disinfection byproducts. These results call for further studies of the fate of iodate, especially in the presence of chloride, which may be overlooked in frozen environments.

Sulfate Formation by Photosensitization in Mixed Incense Burning-Sodium Chloride Particles: Effects of RH, Light Intensity, and Aerosol Aging.

Elevated particulate sulfate concentrations have been frequently observed in coastal areas when air masses are influenced by continental emissions, especially combustion sources like biomass burning. We studied the SO2 uptake by laboratory-generated droplets containing incense smoke extracts and sodium chloride (IS-NaCl) under irradiation and found enhanced sulfate production over pure NaCl droplets, attributable to photosensitization induced by constituents in IS. Low relative humidity and high light intensity facilitated sulfate formation and increased the SO2 uptake coefficient by IS-NaCl particles. Aging of the IS particles further enhanced sulfate production, attributable to the enhanced secondary oxidant production promoted by increased proportions of nitrogen-containing CHN and oxygen- and nitrogen-containing CHON species under light and air. Experiments using model compounds of syringaldehyde, pyrazine, and 4-nitroguaiacol verified the enhancements of CHN and CHON species in sulfate formation. This work provides experimental evidence of enhanced sulfate production in laboratory-generated IS-NaCl droplets via enhanced secondary oxidant production triggered by photosensitization in multiphase oxidation processes under light and air. Our results can shed light on the possible interactions between sea salt and biomass burning aerosols in enhancing sulfate production.

Overlooked Transformation of Nitrated Polycyclic Aromatic Hydrocarbons in Natural Waters: Role of Self-Photosensitization.

Photochemical transformation is an important process that involves trace organic contaminants (TrOCs) in sunlit surface waters. However, the environmental implications of their self-photosensitization pathway have been largely overlooked. Here, we selected 1-nitronaphthalene (1NN), a representative nitrated polycyclic aromatic hydrocarbon, to study the self-photosensitization process. We investigated the excited-state properties and relaxation kinetics of 1NN after sunlight absorption. The intrinsic decay rate constants of triplet (31NN*) and singlet (11NN*) excited states were estimated to be 1.5 × 106 and 2.5 × 108 s-1, respectively. Our results provided quantitative evidence for the environmental relevance of 31NN* in waters. Possible reactions of 31NN* with various water components were evaluated. With the reduction and oxidation potentials of -0.37 and 1.95 V, 31NN* can be either oxidized or reduced by dissolved organic matter isolates and surrogates. We also showed that hydroxyl (•OH) and sulfate (SO4•-) radicals can be generated via the 31NN*-induced oxidation of inorganic ions (OH- and SO42-, respectively). We further investigated the reaction kinetics of 31NN* and OH- forming •OH, an important photoinduced reactive intermediate, through complementary experimental and theoretical approaches. The rate constants for the reactions of 31NN* with OH- and 1NN with •OH were determined to be 4.22 × 107 and 3.95 ± 0.01 × 109 M-1 s-1, respectively. These findings yield new insights into self-photosensitization as a pathway for TrOC attenuation and provide more mechanistic details into their environmental fate.

Humic Substance Photosensitized Degradation of Phthalate Esters Characterized by 2H and 13C Isotope Fractionation.

The photosensitized transformation of organic chemicals is an important degradation mechanism in natural surface waters, aerosols, and water films on surfaces. Dissolved organic matter including humic-like substances (HS), acting as photosensitizers that participate in electron transfer reactions, can generate a variety of reactive species, such as OH radicals and excited triplet-state HS (3HS*), which promote the degradation of organic compounds. We use phthalate esters, which are important contaminants found in wastewaters, landfills, soils, rivers, lakes, groundwaters, and mine tailings. We use phthalate esters as probes to study the reactivity of HS irradiated with artificial sunlight. Phthalate esters with different side-chain lengths were used as probes for elucidation of reaction mechanisms using 2H and 13C isotope fractionation. Reference experiments with the artificial photosensitizers 4,5,6,7-tetrachloro-2',4',5',7'-tetraiodofluorescein (Rose Bengal), 3-methoxy-acetophenone (3-MAP), and 4-methoxybenzaldehyde (4-MBA) yielded characteristic fractionation factors (-4 ± 1, -4 ± 2, and -4 ± 1‰ for 2H; 0.7 ± 0.2, 1.0 ± 0.4, and 0.8 ± 0.2‰ for 13C), allowing interpretation of reaction mechanisms of humic substances with phthalate esters. The correlation of 2H and 13C fractions can be used diagnostically to determine photosensitized reactions in the environment and to differentiate among biodegradation, hydrolysis, and photosensitized HS reaction.