Observed Quantum Yield Research Articles

Overlooked Generation of Reactive Oxidative Species from Water and Dioxygen by Far UV Light.

Far UV light at 222 nm (UV222) is gaining much attention for efficient water purification in UV222 irradiation and UV222-based advanced oxidation processes (AOPs). The direct photolysis of pollutants is regraded to be their major removal mechanism by a sole UV222 treatment. However, this paper reports the important roles of reactive oxidative species (ROS) generated from dioxygen and water under only UV222 radiation. Multiple ROSs are identified, including hydroxyl radical (HO·), singlet oxygen (1O2), superoxide radical anion (·O2-), and ozone (O3). HO· is the major ROS for the degradation of 18 organic micropollutants under UV222 radiation, with an observed quantum yield of 0.447 and the concentration of 10-13 M at pH 7. Dioxygen is the initial source of ROS, while water mainly serves as a medium to react with the photolytic intermediate of O3 (i.e., O(1D)) to form HO·. Water matrix components of HCO3- and natural organic matter can inhibit the HO· concentration, whereas NO3- significantly enhances it. In drinking water, UV222 alone removes 18 micropollutants more efficiently than the typical UV254/H2O2 AOP (150 μM), with reduced energy consumption. This study discloses a novel mechanism of ROS generation in UV222 irradiation and underscores UV222 as an emerging chemical-free AOP for water purification.

Synthesis, characterization, and in-vitro bioimaging applications of green emissive AgInS2-based core multi and alloyed shell (AgInS2/CdS/ZnS and AgInS2/ZnS/CdS) nanocrystals

This study explores the synthesis, characterization, and preliminary bioimaging applications of green-emissive AgInS2-based nanocrystals (NCs) featuring core-multi and alloyed shell structures, specifically AgInS2/CdS/ZnS and AgInS2/ZnS/CdS. The nanocrystals were synthesized using a controlled hot-injection method, which ensured the formation of uniform core–shell structures. Detailed characterization through X-ray diffraction (XRD), transmission electron microscopy (TEM), and photoluminescence spectroscopy confirmed the successful development of these core-multi and alloyed shell architectures. The photoluminescence quantum yields (QY) were determined to be approximately 25 % for AgInS2/CdS/ZnS and 30 % for AgInS2/ZnS/CdS, highlighting their efficient luminescent properties. In preliminary bioimaging studies, these nanocrystals exhibited effective cellular imaging capabilities. They showed pronounced fluorescence with minimal background interference in A549 and U87MG cells, suggesting their potential for precise and targeted imaging applications. Despite these promising results, further investigation is needed to fully understand the specificity of these nanocrystals and their interaction mechanisms with various cell types. The observed quantum yields and targeted imaging capabilities indicate that AgInS2-based nanocrystals are promising candidates for advanced bioimaging technologies.

Europium‐Activated Perovskite Cubical SrZrO3 Red Phosphor: Synthesis, Characterization, and Sustainable Applications in PC‐LEDs and Environmental Forensic Analysis

AbstractA series of SrZrO3 : Eu3+ nano phosphors synthesized by a modified high‐temperature solid‐state reaction method. The structural, functional vibration groups, optical, morphological, luminescence and quantum yield studies were characterized through XRD, FTIR, UV, ‘SEM & HR‐TEM’ and PL analyses respectively. The SrZrO3 : Eu3+ phosphor has a perovskite orthorhombic structure and emits intense red emission around 612 nm when it is excited under 465 nm and covers NUV light range. The calculated average lifetime indicates the nature of the allowed emission transition in Eu3+ (2.44 μs). The calculated colour purity and Commission International del’ Eclairagethe (CIE) chromaticity coordinates are 97.9 % and (0.5894, 0.4031) respectively. The observed quantum yield and thermal stability for the SrZrO3 : Eu3+ (9 mol %) phosphor are 22 % and 87 % at 423 K, respectively. These obtained results suggest that SrZrO3 : Eu3+ (9 mol %) phosphor would meliorate for red‐emitting phosphor in profound RGB‐based pc‐WLED applications.

The complexometric behavior of selected aroyl-S,N-ketene acetals shows that they are more than AIEgens

Using the established synthetic methods, aroyl-S,N-ketene acetals and subsequent bi- and multichromophores can be readily synthesized. Aside from pronounced AIE (aggregation induced emission) properties, these selected examples possess distinct complexometric behavior for various metals purely based on the underlying structural motifs. This affects the fluorescence properties of the materials which can be readily exploited for metal ion detection and for the formation of different metal-aroyl-S,N-ketene acetal complexes that were confirmed by Job plot analysis. In particular, gold(I), iron(III), and ruthenium (III) ions reveal complexation enhanced or quenched emission. For most dyes, weakly coodinating complexes were observed, only in case of a phenanthroline aroyl-S,N-ketene acetal multichromophore, measurements indicate the formation of a strongly coordinating complex. For this multichromophore, the complexation results in a loss of fluorescence intensity whereas for dimethylamino-aroyl-S,N-ketene acetals and bipyridine bichromophores, the observed quantum yield is nearly tripled upon complexation. Even if no stable complexes are formed, changes in absorption and emission properties allow for a simple ion detection.

Carbazole appended trans-dicationic pyridinium porphyrin finds supremacy in DNA binding/photocleavage over a non-carbazolyl analogue.

A carbazolyl appended trans-pyridyl porphyrin (1) was synthesized and its dicationic form 2 was obtained by methylation of the pyridyl group. Copper and zinc complexes of porphyrin 2 (Cu(II), 3; Zn(II), 4) were isolated and characterized by various modern spectroscopic techniques. The DNA binding properties of 2, 3, and 4 have been explored against calf thymus-DNA (CT-DNA). DNA binding was quantized using the intrinsic binding constant (Kb) that was calculated by UV-visible absorption spectroscopy, and the value Kb = 1.6 × 106 M-1 for compound 2 reveals a better interaction of 2 towards CT-DNA than those of 3 (3.1 × 105 M-1) and 4 (3.4 × 105 M-1), which follows the order 2 > 4 > 3. The fluorescence quenching efficiency and ethidium bromide quenching assay also indicated a good binding affinity of all the compounds towards CT-DNA. Furthermore, the spectroscopic data suggest that the possible mode of interaction is intercalation. The docking studies were in accordance with the experimental results. Notably, DNA cleavage studies reveal that 2 shows better damage than 3 and 4 which is in accordance with the binding affinity order 2 > 4 > 3. The observed quantum yield (2: 0.65, 3: 0.33, and 4: 0.97) and no change in DNA cleavage in the presence of NaN3 reveal the involvement of singlet oxygen. The singlet excited state lifetimes were in the range of 6.3-1.2 ns. Furthermore, these porphyrins can be investigated as interesting photosensitizers in photodynamic therapy and photochemotherapy.

Photoreductive dissolution of cerium oxide nanoparticles and their size-dependent absorption properties.

Cerium oxide has attracted attention recently for its photocatalytic properties, but there are gaps in understanding its performance, especially at low and high pH. UV irradiation of ceria nanoparticles causes electrons from photogenerated electron-hole pairs to localize as small polarons, yielding Ce3+ ions. In pH 10 solution, ceria nanoparticles capped with polyacrylic acid ligands can accumulate large numbers of Ce3+ defects as revealed by strong bleaching of the absorption onset. In contrast, we show that UV irradiation of several-nanometer diameter ceria nanoparticles in acidic (pH < 3) aqueous solution releases Ce3+ ions into solution with a quantum yield that approaches 70% and that varies with excitation wavelength, particle size, and the presence of a hole scavenger (glycerol) on the nanoparticle surface. The instability of Ce3+ at the nanoparticle surface and the ability of electron small polarons to migrate to the surface by hopping strongly suggest that nanoceria is fully oxidized and essentially free of Ce3+ centers at pH < 3. Efficient photoreduction and the excellent stability of unirradiated nanoparticles make it easy to shrink the nanoparticles using only light, while maintaining them in a fully oxidized state. This enables study of the size-dependent absorption properties of ceria nanoparticles that are free of Ce3+ defects. No evidence of quantum confinement is observed, consistent with highly localized excited states. The observed quantum yields of photoreduction are higher than reported for other metal oxides, revealing that a significant fraction of electron-hole pairs are available for driving surface redox reactions, even in fully oxidized particles.

Comparing the UV/Monochloramine and UV/Free Chlorine Advanced Oxidation Processes (AOPs) to the UV/Hydrogen Peroxide AOP Under Scenarios Relevant to Potable Reuse.

Utilities incorporating the potable reuse of municipal wastewater are interested in converting from the UV/H2O2 to the UV/free chlorine advanced oxidation process (AOP). The AOP treatment of reverse osmosis (RO) permeate often includes the de facto UV/chloramine AOP because chloramines applied upstream permeate RO membranes. Models are needed that accurately predict oxidant photolysis and subsequent radical reactions. By combining radical scavengers and kinetic modeling, we have derived quantum yields for radical generation by the UV photolysis of HOCl, OCl-, and NH2Cl of 0.62, 0.55, and 0.20, respectively, far below previous estimates that incorporated subsequent free chlorine or chloramine scavenging by the •Cl and •OH daughter radicals. The observed quantum yield for free chlorine loss actually decreased with increasing free chlorine concentration, suggesting scavenging of radicals participating in free chlorine chain decomposition and even free chlorine reformation. Consideration of reactions of •ClO and its daughter products (e.g., ClO2-), not included in previous models, were critical for modeling free chlorine loss. Radical reactions (indirect photolysis) accounted for ∼50% of chloramine decay and ∼80% of free chlorine loss or reformation. The performance of the UV/chloramine AOP was comparable to the UV/H2O2 AOP for degradation of 1,4-dioxane, benzoate and carbamazepine across pH 5.5-8.3. The UV/free chlorine AOP was more efficient at pH 5.5, but only by 30% for 1,4-dioxane. At pH 7.0-8.3, the UV/free chlorine AOP was less efficient. •Cl converts to •OH. The modeled •Cl:•OH ratio was ∼20% for the UV/free chlorine AOP and ∼35% for the UV/chloramine AOP such that •OH was generally more important for contaminant degradation.

Hydrogen Production via Water Dissociation Using Pt–TiO2 Photocatalysts: An Oxidation–Reduction Network

Several TiO2 based semiconductors with different Pt loadings are prepared using incipient impregnation, wet impregnation and the sol-gel method. These photocatalysts are evaluated in the Photo-CREC-Water II Photoreactor for hydrogen production via water dissociation, using an organic renewable scavenger (ethanol). Results obtained show the influence of the photocatalyst preparation in the production of hydrogen and in the observed quantum yields. Furthermore, it is established that the reaction networks leading to hydrogen production, using various photocatalysts, share common features. This analysis is developed by both identifying and quantifying different chemical species and their changes with irradiation time. Key species in this oxidation–reduction network are hydrogen, hydrogen peroxide, ethanol, methane, ethane, acetaldehyde and carbon dioxide. On this basis, it is shown that under an inert gas atmosphere, ethanol consumption is sub-stoichiometric. This points towards simultaneous ethanol consumption and the formation of the ethanol scavenger.

Changing the Emission Properties of Phosphorescent C^C*-Cyclometalated Thiazol-2-ylidene Platinum(II) Complexes by Variation of the β-Diketonate Ligands.

Cyclometalated thiazol-2-ylidene platinum(II) complexes based on the N-phenyl-4,5-dimethyl-1,3-thiazol-2-ylidene N-heterocyclic carbene (NHC) ligand and seven different β-diketonate ligands have been synthesised and investigated for their structural and photophysical properties. The complexes were synthesised in a one-pot procedure starting with the in situ formation of the corresponding silver(I) carbene and transmetalation to platinum, followed by the reaction with the respective β-diketonate under basic conditions. All the compounds were fully characterised by standard techniques, including 195 Pt NMR spectroscopy. Three solid-state structures revealed quite different aggregation behaviour depending on the β-diketonate architecture. The reported complexes showed strong phosphorescence at room temperature in amorphous poly(methyl methacrylate) films. The emission wavelengths (ca. 510 nm) were found to be independent of the β-diketonate ligand, but the electronically diverse β-diketonates strongly influence the observed quantum yields (QY) and decay lifetimes. The results of theoretical studies employing density functional theory (DFT) methods support the conclusion of a metal-to-ligand charge transfer (3 MLCT) as the main emission process, in accordance with the reported photophysical properties. Standard organic light-emitting diodes (OLEDs) prepared with unoptimised matrix materials using one of the complexes showed values of 12.3 % external quantum yield, 24.0 lm W-1 luminous efficacy and 37.8 cd A-1 current efficiency at 300 cd m-2 .

Computational study with DFT and kinetic models on the mechanism of photoinitiated aromatic perfluoroalkylations.

A combination of DFT calculations and kinetic models is applied to fully elucidate the seemingly complex reactivity of α-cyano arylacetates toward metal-free photoinitiated aromatic perfluoroalkylation. The resulting mechanistic framework rationalizes the observed quantum yield as well as the differences in reactivity and/or selectivity of seemingly similar substrates. The use of a kinetic model for the chemical interpretation of the DFT-computed reaction constants is shown to be critical.

Laboratory study of nitrate photolysis in Antarctic snow. II. Isotopic effects and wavelength dependence.

Atmospheric nitrate is preserved in Antarctic snow firn and ice. However, at low snow accumulation sites, post-depositional processes induced by sunlight obscure its interpretation. The goal of these studies (see also Paper I by Meusinger et al. ["Laboratory study of nitrate photolysis in Antarctic snow. I. Observed quantum yield, domain of photolysis, and secondary chemistry," J. Chem. Phys. 140, 244305 (2014)]) is to characterize nitrate photochemistry and improve the interpretation of the nitrate ice core record. Naturally occurring stable isotopes in nitrate ((15)N, (17)O, and (18)O) provide additional information concerning post-depositional processes. Here, we present results from studies of the wavelength-dependent isotope effects from photolysis of nitrate in a matrix of natural snow. Snow from Dome C, Antarctica was irradiated in selected wavelength regions using a Xe UV lamp and filters. The irradiated snow was sampled and analyzed for nitrate concentration and isotopic composition (δ(15)N, δ(18)O, and Δ(17)O). From these measurements an average photolytic isotopic fractionation of (15)ɛ = (-15 ± 1.2)‰ was found for broadband Xe lamp photolysis. These results are due in part to excitation of the intense absorption band of nitrate around 200 nm in addition to the weaker band centered at 305 nm followed by photodissociation. An experiment with a filter blocking wavelengths shorter than 320 nm, approximating the actinic flux spectrum at Dome C, yielded a photolytic isotopic fractionation of (15)ɛ = (-47.9 ± 6.8)‰, in good agreement with fractionations determined by previous studies for the East Antarctic Plateau which range from -40 to -74.3‰. We describe a new semi-empirical zero point energy shift model used to derive the absorption cross sections of (14)NO3 (-) and (15)NO3 (-) in snow at a chosen temperature. The nitrogen isotopic fractionations obtained by applying this model under the experimental temperature as well as considering the shift in width and center well reproduced the values obtained in the laboratory study. These cross sections can be used in isotopic models to reproduce the stable isotopic composition of nitrate found in Antarctic snow profiles.

Laboratory study of nitrate photolysis in Antarctic snow. I. Observed quantum yield, domain of photolysis, and secondary chemistry

Post-depositional processes alter nitrate concentration and nitrate isotopic composition in the top layers of snow at sites with low snow accumulation rates, such as Dome C, Antarctica. Available nitrate ice core records can provide input for studying past atmospheres and climate if such processes are understood. It has been shown that photolysis of nitrate in the snowpack plays a major role in nitrate loss and that the photolysis products have a significant influence on the local troposphere as well as on other species in the snow. Reported quantum yields for the main reaction spans orders of magnitude - apparently a result of whether nitrate is located at the air-ice interface or in the ice matrix - constituting the largest uncertainty in models of snowpack NOx emissions. Here, a laboratory study is presented that uses snow from Dome C and minimizes effects of desorption and recombination by flushing the snow during irradiation with UV light. A selection of UV filters allowed examination of the effects of the 200 and 305nm absorption bands of nitrate. Nitrate concentration and photon flux were measured in the snow. The quantum yield for loss of nitrate was observed to decrease from 0.44 to 0.003 within what corresponds to days of UV exposure in Antarctica. The superposition of photolysis in two photochemical domains of nitrate in snow is proposed: one of photolabile nitrate, and one of buried nitrate. The difference lies in the ability of reaction products to escape the snow crystal, versus undergoing secondary (recombination) chemistry. Modeled NOx emissions may increase significantly above measured values due to the observed quantum yield in this study. The apparent quantum yield in the 200nm band was found to be ∼1%, much lower than reported for aqueous chemistry. A companion paper presents an analysis of the change in isotopic composition of snowpack nitrate based on the same samples as in this study.

Photochemical dynamics of E-methylfurylfulgide—kinematic effects on photorelaxation dynamics of furylfulgides

With the present theoretical study of the photochemical switching of E-methylfurylfulgide we contribute an important step towards the understanding of the photochemical processes in furylfulgide-related molecules. We have carried out large-scale, full-dimensional direct semiempirical configuration-interaction surface-hopping dynamics of the photoinduced ring-closure reaction. Simulated static and dynamical UV/Vis-spectra show good agreement with experimental data of the same molecule. By a careful investigation of our dynamical data, we were able to identify marked differences to the dynamics of the previously studied E-isopropylfurylfulgide. With our simulations we can not only reproduce the experimentally observed quantum yield differences qualitatively but we can also pinpoint two reasons for them: kinematics and pre-orientation. With our analysis, we thus offer straightforward molecular explanations for the high sensitivity of the photodynamics towards seemingly minor changes in molecular constitution. Beyond the realm of furylfulgides, these insights provide additional guidance to the rational design of photochemically switchable molecules.

Aquatic photochemistry of the sulfonamide antibiotic sulfapyridine

Abstract The photolytic behavior of the sulfonamide antibiotic sulfapyridine in water was investigated using a laboratory photoreactor approximating full-spectrum sunlight. Direct photolysis of sulfapyridine was rapid, with a half-life of 2.6 h and 31 min, and an observed quantum yield of 0.0013 ± 0.0002 and 0.013 ± 0.001, for the neutral species and fully deprotonated species, respectively. Direct photolysis rates increased dramatically with degree of deprotonation, with measured p K a 1 and p K a 2 values of 2.22 ± 0.03 and 8.58 ± 0.02, respectively. Indirect photolysis was assessed using water from constructed wetland mesocosms. A four-fold increase in the dissipation rate of sulfapyridine was observed due to the influence of high levels of dissolved organic carbon, after accounting for light screening by such materials. Nitrates had no observable effect on indirect photolysis rates. Major photoproducts identified were SO 2 extrusion and OH addition products. These results show that photolytic processes are a major removal mechanism of sulfonamide drugs in aquatic systems.

A Highly Stable Rhenium−Cobalt System for Photocatalytic H2 Production: Unraveling the Performance-Limiting Steps

Increased long-term performance was found for photocatalytic H(2) production in a homogeneous combination of [Re(NCS)(CO)(3)bipy] (1; bipy = 2,2'-bipyridine), [Co(dmgH)(2)] (dmgH(2) = dimethylglyoxime), triethanolamine (TEOA), and [HTEOA][BF(4)] in N,N-dimethylformamide, achieving TON(Re) up to 6000 (H/Re). The system proceeded by reductive quenching of *1 by TEOA, followed by fast (k(1) = 1.3 x 10(8) M(-1) s(-1)) electron transfer to [Co(II)(dmgH)(2)] and subsequent protonation (K(2)) and elimination (k(3), second-order process in cobalt) of H(2). Observed quantum yields were up to approximately 90% (H produced per absorbed photon). The type of acid had a substantial effect on the long-term stability. A decomposition pathway involving cobalt is limiting the long-term performance. Time-resolved infrared (IR) spectroscopy confirmed that photooxidized TEOA generates a second reducing equivalent, which can be transferred to 1 (70%, k(2e)(-) = 3.3 x 10(8) M(-1) s(-1)) if no [Co(II)(dmgH)(2)] is present.

Requisites to realize high conversion efficiency of solar cells utilizing carrier multiplication

We have calculated the limiting conversion efficiency of solar cells utilizing carrier multiplication (CM), using the detailed balance theory. The solar cells were assumed to comprise quantum dots (QDs) embedded in another material. It has been elucidated that three requisites must be fulfilled, so that a sufficient number of photons in the solar spectrum contribute to CM, resulting in significantly higher conversion efficiency than the values of conventional cells. These requisites are as follows: (1) the effective mass of electrons in the QDs should be much lighter than that of holes, so that the threshold photon energy above which CM can occur is close to the energy gap of the QDs. In this respect, InAs is a promising candidate for the QD material, but PbSe and Si are not. (2) The potential barrier height for electrons in the QDs, which determines the upper limit of the quantum yield of photon-to-carrier conversion ( γ limit), should be slightly larger than the energy gap of the QDs to achieve a γ limit value of 2, when the solar cells are used under the non-concentrated insolation. InAs QDs embedded in Al x Ga 1− x As y Sb 1− y is a possible candidate to fulfill these two criteria. A higher barrier does not contribute to generation of more carriers, but likely disturbs electron transport. In contrast, under the concentrated insolation, a potential barrier slightly higher than twice the energy gap to achieve a γ limit value of 3 leads to higher conversion efficiency. (3) The quantum yield of photon-to-carrier conversion as a function of photon energy should rise as steeply as possible at the threshold photon energy. The experimentally observed quantum yield with a sloping rise leads to little improvement in conversion efficiency due to CM, under the non-concentrated insolation. Although it could be improved under the concentrated insolation, the conversion efficiency cannot reach the limiting value for triple-junction solar cells.

Spectral Investigations of Solvatochromism and Preferential Solvation on 1,4-Dihydroxy-2,3-Dimethyl-9,10-Anthraquinone

Solvatochromic and preferential solvation of 1,4-dihydroxy-2,3-dimethyl-9,10-anthraquinone (DHDMAQ) have been investigated using optical absorption and fluorescence emission techniques. Optical absorption spectra of DHDMAQ in different solvents show the intra molecular charge transfer band in the region 400-550 nm. The observed blue shift with solvent polarity indicates the delocalisation of the excited state, owing to reduction in quasiaromaticity of the chelate rings formed by intra molecular hydrogen bonds, due to electrostatic or hydrogen bonding interaction. This is also confirmed by the observed low oscillator strength and the transition dipole moment. The observed quantum yield of DHDMAQ in different solvents is due to the inter molecular hydrogen bond in the excited state in addition to the intra molecular hydrogen bond. It also reveals from the low oscillator strength, which indicates that the radiative decay is low. Excited state dipole moment of DHDMAQ is calculated by solvatochromic data and it shows a lower value than ground state dipole moment. The preferential solvation parameter shows that in dimethyl formamide (DMF)+ethanol mixture, the DHDMAQ is preferentially solvated by ethanol in DMF rich region and by DMF in ethanol rich region. In the case of DMF+dichloromethane mixture DHDMAQ is preferentially solvated by DMF.

Structure and Dynamics of Poly(T) Single-Strand DNA: Implications toward CPD Formation

The formation of cyclobutane pyrimidine dimers between adjacent thymines by UV radiation is thought to be the first event in a cascade leading to skin cancer. Recent studies showed that thymine dimers are fully formed within 1 ps of UV irradiation, suggesting that the conformation at the moment of excitation is the determining factor in whether a given base pair dimerizes. MD simulations on the 50 ns time scale are used to study the populations of reactive conformers that exist at any given time in T18 single-strand DNA. Trajectory analysis shows that only a small percentage of the conformations fulfill distance and dihedral requirements for thymine dimerization, in line with the experimentally observed quantum yield of 3%. Plots of the pairwise interactions in the structures predict hot spots of DNA damage where dimerization in the ssT18 is predicted to be most favored. The importance of hairpin formation by intra-strand base pairing for distinguishing reactive and unreactive base pairs is discussed in detail. The data presented thus explain the structural origin of the results from the ultrafast studies of thymine dimer formation.

Self-absorption correction for solid-state photoluminescence quantum yields obtained from integrating sphere measurements

A new method is presented for analyzing the effects of self-absorption on photoluminescence integrating sphere quantum yield measurements. Both the observed quantum yield and luminescence spectrum are used to determine the self-absorption probability, taking into account both the initial emission and subsequent absorption and reemission processes. The analysis is experimentally validated using the model system of the laser dye perylene red dispersed in a polymer film. This approach represents an improvement over previous methods that tend to overestimate the true quantum yield, especially in cases with high sample absorbance or quantum yield values.

Environmental Photochemistry of Tylosin: Efficient, Reversible Photoisomerization to a Less-Active Isomer, Followed by Photolysis

The environmental photochemical kinetics of tylosin, a common veterinary macrolide antibiotic and growth promoter, were investigated under simulated sunlight. An efficient, reversible photoisomerization was characterized using kinetic, mass spectrometry, and proton nuclear magnetic resonance data. The photoisomerization was confirmed to occur by a rotation about the distal alkene of the ketodiene functionality. Concurrent forward (quantum yield = 0.39 +/- 0.09) and back (quantum yield = 0.32 +/- 0.08) reactions lead to a photochemical equilibrium near a tylosin/photoisomer ratio of 50:50, completed in less than 2 min under a spectrum equivalent to noontime, summer sunlight. The activity of the isomer for the inhibition of Escherichia coli DH5alpha growth was observed to be less than that of tylosin. On a longer time scale than that of isomerization, the isomer mixture undergoes photolysis with a quantum yield of (1.4 +/- 0.3) x 10(-3). The observed quantum yields and UV-vis absorbance data allow for the prediction of the photochemical behavior of tylosin in most environmental systems. Indirect photosensitization was not a significant loss process in solutions of Suwannee River fulvic acid with concentrations from 1 to 20 mg L(-1).