Photolysis Yield Research Articles

Mechanism of UV photodegradation of fluoroquinolone antibiotic ciprofloxacin in aqueous solutions.

Mechanism of direct UV photolysis of the zwitterionic and anionic forms of the quinolone antibiotic ciprofloxacin (CIP) was revealed by combination of nanosecond laser flash photolysis, steady-state photolysis coupled with high resolution LC-MS and DFT quantum-chemical calculations. For both forms, the main intermediate is a dissociative triplet state, which loses a fluorine ion to form a triplet carbocation; subsequent solvent attack of the latter leads to the formation of products of hydroxylation both the aromatic ring and the piperazyl substituent. Correspondingly, the quantum yield of photolysis of both CIP forms does not depend on the excitation wavelength, but depends on the concentration of dissolved oxygen. Secondary photolysis leads to a number of products of oxidation of the aromatic system, as well as oxidation, opening and full destruction of the piperazinyl substituent. The results obtained may be important for understanding the fate of quinolone antibiotics in UVC disinfection processes and in natural waters under the action of sunlight.

Laser flash photolysis and quantum chemical studies of UV degradation of pharmaceutical drug chloramphenicol: Short-lived intermediates, quantum yields and mechanism of photolysis

Using methods of time-resolved and stationary photolysis, HPLC-MS and quantum-chemical calculations by the DFT method, the mechanism of direct UV photolysis of the antibiotic chloramphenicol (CAP) was established. For the first time, short-lived intermediates formed during photolysis were detected. The primary photoprocess is the cleavage of the β-C-C bond relative to the aromatic system with the formation of 4-nitrobenzylalcohol radical and residual aliphatic radical. The first radical in deoxygenated solutions predominantly transforms into para-nitrobenzaldehyde and its secondary photolysis products. In the presence of oxygen, the aromatic radical and para-nitrobenzaldehyde are transformed into para-nitrosobenzoic and para-nitrobenzoic acids as a result of reaction with reactive oxygen species (ROS). Formation of ROS is provoked by reactions of aliphatic radical with dissolved oxygen, so this radical is very important for CAP degradation. The quantum yield of direct photolysis of CAP is ∼3% and does not depend on the presence of dissolved oxygen and on the change of the excitation wavelength in the range of 254–308 nm. Obtained data are important for further understanding of the transformation pathways of CAP and similar PPCP in natural and wastewaters under the action of sunlight and artificial UV radiation.

Cation delocalization and photo-isomerization enhance the uncaging quantum yield of a photocleavable protecting group.

Photocleavable protecting groups (PPGs) enable the light-induced, spatiotemporal control over the release of a payload of interest. Two fundamental challenges in the design of new, effective PPGs are increasing the quantum yield (QY) of photolysis and red-shifting the absorption spectrum. Here we describe the combination of two photochemical strategies for PPG optimization in one molecule, resulting in significant improvements in both these crucial parameters. Furthermore, we for the first time identify the process of photo-isomerization to strongly influence the QY of photolysis of a PPG and identify the cis-isomer as the superior PPG.

Loss of toxicity of microcystins in UV/H2O2 and UV/Cl2 treatment

Advanced oxidation processes are one of the best treatments for algal toxins in drinking water, but because it involves chemical transformation of microcystins, there is a concern that the products may retain toxicity. An established advanced oxidation process (UV/H2O2) was compared with an emerging one (UV/Cl2) in terms of effectiveness for detoxifying the two most common variants of algal hepatotoxin microcystin (LR and YR). LC-MS/MS analysis of the products indicated that the ADDA group of the molecule responsible for their toxicity is susceptible to damage in the UV/H2O2 and UV/Cl2 process, suggesting that the transformation products lose their toxicity. This was confirmed by ADDA-specific ELISA analysis of treated samples of the LR variant. ELISA-measured concentrations of microcystins in the treated samples matched the LC-MS/MS-measured concentrations of the parent molecule remaining after treatment, which confirmed that the ADDA group is damaged during the UV/H2O2 and UV/Cl2 treatment. Microcystin-LR was also highly susceptible to a reaction with chlorine (>99 % decrease at 60 min·mg/L) and direct UV photolysis, with quantum yield of photolysis of 0.032 ± 0.007 for 254 nm UV wavelength, and 0.11 ± 0.00 for the 200–300 nm spectrum.

Kinetic modeling and mechanisms of parahalogenated phenols degradation by UV222/H2O2

Advanced oxidation technology of KrCl excimer lamp activated H2O2 (UV222/H2O2) have been received widespread attention in the field of water treatment recently, due to the higher radical yield and environmentally friendly nature compared to UV254/H2O2. In this investigation, four representative parahalogenated phenols (HPs) degradation kinetic and mechanisms by UV222 alone and UV222/H2O2 were studied, respectively. The results showed that the photolysis rate constants (kobs) under UV222 followed the order of p-Chlorophenol (p-CP) > p-Fluorophenol (p-FP) > p-Bromophenol (p-BP) > p-Iodophenol (p-IP), and the degradation rate constants (kobs) by UV222/H2O2 followed the order of p-FP > p-CP > p-BP > p-IP. The molar absorption coefficient (ε222nm) and quantum yield (Φ222nm) of HPs photolysis were measured. Kinetic model simulating results are in fair agreement with the experimental results. Through HPLC-MS analysis and DFT calculation, the impact of halogen ions on the degradation kinetics of HPS by UV222 and UV222/H2O2 was found, the possible reaction pathways and mechanisms were proposed. Compared with UV254/H2O2 for removing p-CP, UV222/H2O2 was much more energy efficient.

Strategies to develop α-aminoketone derivatives photoinitiators with low migration ability for UV–vis LED photopolymerization

In this work, a polymerizable vinyl sulfide modified 907 derivative (PI-1) and four macromolecular photoinitiators (PI-2–5) based on PI-1 were designed and synthesized to realize low migration together with a high photoactivity. The structures of these photoinitiators were confirmed by 1H NMR, 13C NMR, HRMS, and GPC analysis. Time-resolved 1H NMR, photolysis, and quantum yield experiments illustrated that the rate of the photogenerated free radicals from PI-1 was slower than that of Irgacure 907. The macromolecular strategies of PI-1 successfully improved the photoactivity of PI-2–5 compared with PI-1. Moreover, PI-4 prepared by copolymerization between PI-1 and methyl methacrylate demonstrated a higher quantum yield of free radical generation than Irgacure 907 and better photoinitiating performance as evaluated by bulk and solution photopolymerization. Furthermore, the migration stabilities of these photoinitiators in the photocured films were better than those of 907. The high efficiency and low migration of the above photoinitiators make them potential applications in food packaging and biological materials.

Synergistic approach of type I hybrid complexes for highly efficient metal-based initiating strategies: Toward low energy-consuming polymerization for thermoplastic composite implementation

From a circular plastic economy perspective, a new configuration of Metal Acetylacetonate Bidentate Ligand Interaction (MABLI) mechanism adapted to the chemistry of the recyclable Elium® thermoplastic resins has been recently discovered to initiate the free radical polymerization of these resins under mild conditions (under air, at room temperature, non-purified monomers, absence of hazardous chemical substances). This initiating strategy is based on the synergy between the initial MABLI couple and a phosphine oxide (Type I photoinitiator). Despite the absence of irradiation, the mere presence of the phosphine oxide reduced significantly the curing time and the final conversion obtained is nearly 100 %. In order to dismantle the role of each component and to predict new high-performance MABLI redox initiating systems based on a similar configuration, a mechanistic study was pursued. It has resulted in the identification of new hybrid complexes obtained by ligand exchange between the photoinitiator and the metallic complex. On the other hand, presence of the Type I photoinitiator can level up the initiating strategy by combining MABLI and photoactivation into a dual cure approach: thereby, the photolysis yield and the polymerization efficiency of the complexed form of the phosphine oxide was studied and compared to the free form of the photoinitiator.

Photolysis of Fe(III) complex with ethylenediamine-N,N′-disuccinic acid and its efficiency in generation of •OH radical

The mechanism of photolysis of the Fe(III) complex with ethylenediamine-N,N′-disuccinic acid ([FeEDDS]-) was revealed using a combination of time resolved and stationary photochemical methods. Using laser flash photolysis (λex = 355 nm), the formation of the primary intermediate, the radical complex of Fe(II) with quantum yield (φ0 = 0.21) was detected for the first time. The lifetime (1.8 ms) and the spectral characteristics (λmax = 520 nm, ε520 nm = 160 M−1cm−1) of this intermediate were also determined. The dependence of the quantum yield of photolysis of the [FeEDDS]- complex (φFeEDDS) and the hydroxyl radical quantum yield (φOH) on the excitation wavelength, pH, and concentrations of the ligand and iron ions were obtained for the first time. It has been established that under optimal conditions at neutral pH, the value of φFeEDDS is about 0.8, and φOH is about 0.15. It was found that φFeEDDS does not depend on the initial concentrations of Fe(III), EDDS, but depends on pH, the excitation wavelength and the presence of oxygen. φOH does not depend on the initial concentrations of Fe(III), EDDS, but depends on pH and the excitation wavelength. The high φOH values make the [FeEDDS]- complex a suitable system for the generation of •OH radical at neutral pH under UV radiation.

Fluence-based photo-reductive decomposition of PFAS using vacuum UV (VUV) irradiation: Effects of key parameters and decomposition mechanism

PFAS are a group of anthropogenic chemicals capturing global attention due to their numerous applications, ubiquitous detection, and toxicity. PFOS, a well-known compound of this class, is the most frequently detected PFAS in drinking water and human blood samples of several countries. In this study, a fluence-based approach using well-controlled quasi-collimated beam of UV (254 nm) and UV/VUV (254/185 nm) irradiations was followed to investigate the photodegradation of PFOS. Very low ε185 nm of PFOS, 57.2 ± 6.4 M-1 cm-1, implied the negligible contribution of direct photolysis in its decomposition, whereas the use of different scavengers indicated the prominent role of eaq−. Compared with other photo-reductive mediators, the addition of sulfite led to greater decomposition of PFOS which favored both UV and UV/VUV processes. Greater molar absorption coefficient and quantum yield of sulfite photolysis in 185 nm rather than corresponding values in 254 nm resulted in greater decomposition of PFOS in UV/VUV/sulfite process. Fluence-based decomposition rates of PFOS isomers (branched: 2.044 ± 0.336 J-1 cm2 and linear: 0.053 ± 0.003 J-1 cm2) were independent of fluence rate, while time-based rates were directly dependent on the fluence rate. Besides, the effects of key parameters including sulfite dosage, solution pH, temperature, initial concentration of PFOS, and dissolved oxygen were investigated. Defluorination and mass spectrometry results showed faster evolution of intermediates, i.e., their formation and consequent degradation, in the UV/VUV/sulfite process compared with the UV/sulfite process. H/F exchange with consequent C-C bond scission, along with desulfonation with consequent PFCAs formation are proposed decomposition pathways of PFOS upon reaction with eaq−.

Photochemical Degradation of 4-Nitrocatechol and 2,4-Dinitrophenol in a Sugar-Glass Secondary Organic Aerosol Surrogate.

The roles that chemical environment and viscosity play in the photochemical fate of molecules trapped in atmospheric particles are poorly understood. The goal of this work was to characterize the photolysis of 4-nitrocatechol (4NC) and 2,4-dinitrophenol (24DNP) in semisolid isomalt as a new type of surrogate for glassy organic aerosols and compare it to photolysis in liquid water, isopropanol, and octanol. UV/vis spectroscopy was used to monitor the absorbance decay to determine the rates of photochemical loss of 4NC and 24DNP. The quantum yield of 4NC photolysis was found to be smaller in an isomalt glass (2.6 × 10-6) than in liquid isopropanol (1.1 × 10-5). Both 4NC and 24NDP had much lower photolysis rates in water than in organic matrices, suggesting that they would photolyze more efficiently in organic aerosol particles than in cloud or fog droplets. Liquid chromatography in tandem with mass spectrometry was used to examine the photolysis products of 4NC. In isopropanol solution, most products appeared to result from the oxidation of 4NC, in stark contrast to photoreduction and dimerization products that were observed in solid isomalt. Therefore, the photochemical fate of 4NC, and presumably of other nitrophenols, should depend on whether they undergo photodegradation in a liquid or semisolid organic particle.

Aqueous photodegradation of the benzophenone fungicide metrafenone: Carbon-bromine bond cleavage mechanism

Metrafenone (MF), as a new type of benzophenone fungicide, has been widely used in agriculture and is persistent in the environment. Understanding its photochemical fate is essential for the comprehensive evaluation of its ecological risk. In the present work, we reported a detailed study on the photochemical transformation of MF in aqueous solution under irradiation (at λ > 290 nm using a high pressure mercury lamp). MF was relatively photo-reactive showing a low polychromatic quantum yield of photolysis (1.06 × 10−4, 20 µM) counterbalanced by a significant light absorption above 290 nm. A series of photoproducts were identified by high resolution mass spectrometry (HR-MS) analysis, and three different pathways, including oxidation of the methyl group, debromination and replacement of bromine by hydroxyl group were proposed. Among them, debromination was identified as the dominating process that could be achieved via homolytic C-Br bond cleavage from singlet and triplet MF, as confirmed by laser flash photolysis (LFP) experiments and density functional theory (DFT) calculations. Toxicity assessment revealed that photochemical degradation reduced the ecotoxicity of MF efficiently. Nitrate ions and humic acid promoted the MF photolysis, while bicarbonate exhibited no effect. Results obtained in this work would increase our understanding on the environmental fate of MF in sunlit surface waters.

Ab Initio and RRKM/Master Equation Analysis of the Photolysis and Thermal Unimolecular Decomposition of Bromoacetaldehyde

Bromoacetaldehyde (BrCH2CHO) is a major stable brominated organic intermediate of the bromine-ethylene addition reaction during the arctic bromine explosion events. Similar to acetaldehyde, which has been recently identified as a source of organic acids in the troposphere, it may be subjected to photo-tautomerization initially forming brominated vinyl compounds. In this study, we investigate the unimolecular reactions of BrCH2CHO under both photolytic and thermal conditions using high-level quantum chemical calculations and Rice-Ramsperger-Kassel-Marcus (RRKM)/master equation analysis. The unimolecular decomposition of BrCH2CHO takes place through 14 dissociation and isomerization channels along a potential energy surface involving eight wells. Under the assumption of singlet ground-state potential energy surface-dominated photodynamics, the primary photodissociation yields of BrCH2CHO are investigated under both collision-free and collision energy transfer conditions. At atmospheric pressure and under tropospheric actinic flux conditions at ground level, depending on the assumed collisional energy transfer parameter, 150 cm-1 < ⟨ΔEdown⟩ < 450 cm-1, 78-33% of BrCH2CHO undergoes direct photodissociation instead of collisional deactivation at an excitation wavelength of 320 nm. This is significantly higher than the 14% reported for acetaldehyde, hence indicating a strong effect of bromine substitution on the product photolysis yield that is related to additional favorable Br and HBr forming dissociation channels. In contrast to the overall photodissociation quantum yield, the relative branching fractions of the photodissociation products are less dependent on the collisional energy transfer parameter. For a representative value of ⟨ΔEdown⟩ = 300 cm-1 and an excitation wavelength of 320 nm, with 27% for C-C bond fission, 11% for C-Br bond fission, 7% for HBr elimination, and only below 2% each for a consecutive O-Br fission reaction and the photo-tautomerization channel yielding brominated vinyl alcohol, the photodissociation is markedly different from the acetaldehyde case. Finally, as brominated halogenated compounds are of interest for flame inhibition purposes, thermal multichannel unimolecular rate constants were calculated for temperatures in the range from 500 to 2000 K. At a temperature of 2000 K and ambient pressure, the two main reaction channels are the C-Br and C-C bond fissions, contributing 35 and 43% to the total reaction flux, respectively.

Chemical and photochemical error rates in light-directed synthesis of complex DNA libraries.

Nucleic acid microarrays are the only tools that can supply very large oligonucleotide libraries, cornerstones of the nascent fields of de novo gene assembly and DNA data storage. Although the chemical synthesis of oligonucleotides is highly developed and robust, it is not error free, requiring the design of methods that can correct or compensate for errors, or select for high-fidelity oligomers. However, outside the realm of array manufacturers, little is known about the sources of errors and their extent. In this study, we look at the error rate of DNA libraries synthesized by photolithography and dissect the proportion of deletion, insertion and substitution errors. We find that the deletion rate is governed by the photolysis yield. We identify the most important substitution error and correlate it to phosphoramidite coupling. Besides synthetic failures originating from the coupling cycle, we uncover the role of imperfections and limitations related to optics, highlight the importance of absorbing UV light to avoid internal reflections and chart the dependence of error rate on both position on the array and position within individual oligonucleotides. Being able to precisely quantify all types of errors will allow for optimal choice of fabrication parameters and array design.

Synergistic effects of UVC and oxidants (PS vs. Chlorine) on carbamazepine attenuation: Mechanism, pathways, DBPs yield and toxicity assessment

The environmental impact and health risk of carbamazepine (CBZ) in water environment has been continuously reported. Ultraviolet C (UVC)/persulfate (PS) and UVC/chlorine (NaClO) processes were confirmed to be able to efficiently eliminate CBZ through the oxidation by reactive radicals (52.82% SO4−· and 44.21% ·OH in UVC/PS system, 53.47% RCS and 43.84% ·OH in UVC/chlorine process, pH 7.0). The second-order rate constants of CBZ reacted with ·OH, SO4−· and RCS were determined to be 8.2 × 109, 9.1 × 108 and 4.2 × 1010 M−1 s−1, respectively. In both processes, increasing oxidants dosage and rising temperature was favorable for elimination, while increasing CBZ concentration retarded its degradation rate. The coexisting of NOM, SO42− and CO32–/HCO3– presented the inhibitory effect, while NO3– promoted the removal. NH4+ exerted an inhibitory effect in UVC/chlorine process, while exerted no obvious influence in UVC/PS system. Cl− posed a promoting role in UVC/PS treatment and a dual effect in UVC/chlorine process. pH affected the quantum yield of chlorine photolysis and the formation of RCS and ·OH. The degradation of CBZ oxidation via UVC/chlorine process was more pH dependent. 12 and 14 CBZ oxidation products were identified in UVC/PS and UVC/chlorine systems, respectively. SO4−·, RCS and ·OH were readily to attack C = C in CBZ molecule, mainly yielding hydroxylated CBZ, epoxy CBZ, pyridine aldehydes and ketones. Of the disinfection by-products (DBPs) assessed, both processes controlled the formation of haloacetic acids, while increased the generation of trichloromethane, especially via UVC/chlorine process. UVC/chlorine process was more economical and effective (EEO UVC/chlorine = 0.2213 kWh L−1), whereas UVC/PS process was safer in terms of DBPs and toxicity control.

FC(O)C(O)F, FC(O)CF2C(O)F, and FC(O)CF2CF2C(O)F: Ultraviolet and Infrared Absorption Spectra and 248 nm Photolysis Products.

Perfluorodicarbonyl (PFDC) compounds may be emitted directly into the atmosphere or formed in the atmospheric degradation of trace fluorinated gases, such as unsaturated perfluoro cyclic compounds. A potential atmospheric removal process for PFDCs is UV photolysis, which is presently not well-characterized. In this work, UV and infrared absorption spectra of FC(O)C(O)F, FC(O)CF2C(O)F, and FC(O)CF2CF2C(O)F (three of the simplest PFDCs) and their 248 nm photolysis products are reported. UV spectra were measured at 296 K between 190 and 320 nm using single wavelength and broadband diode array spectroscopic measurement techniques. Infrared absorption spectra were measured at 296 K using Fourier transform infrared spectroscopy between 500 and 4000 cm-1. The PFDCs are shown to be potent greenhouse gases with radiative efficiencies (well-mixed) of 0.142, 0.218, and 0.293 W m-2 ppb-1 for FC(O)C(O)F, FC(O)CF2C(O)F, and FC(O)CF2CF2C(O)F, respectively. Photolysis product yields (248 nm) were measured using pulsed laser photolysis combined with infrared absorption detection of radical products scavenged to stable bromides by reaction with Br2. BrC(O)F was identified as a major stable end product in all systems with a yield greater than ∼90%. The infrared spectrum of BrC(O)F is reported as part of this study. FC(O)CBrF2 and FC(O)CF2CBrF2 were also identified as products in the photolysis of FC(O)CF2C(O)F and FC(O)CF2CF2C(O)F, respectively, by comparison with theoretically calculated infrared absorption spectra. A carbonyl difluoride (CF2O) primary photolysis yield of ∼10% was measured in the photolysis of FC(O)C(O)F.

Investigation into the Direct Photolysis Process of Photo-Induced RAFT Polymerization by ESR Spin Trapping.

The direct photolysis of reversible addition fragmentation chain transfer (RAFT) agents under visible light was demonstrated by electron spin resonance (ESR) using 5,5-dimethyl-1-pyrroline N-oxide as a typical spin trap. The hyperfine coupling lines obtained by ESR spectroscopy showed the successful capture of the carbon-centered and the sulfur-centered radical. Photo-polymerization of vinyl acetate under different wavelengths was performed to verify the effects of wavelength on the process. The effect of the R group of RAFT agents on the photolysis was investigated by spin-trapping experiments using poly (butyl acrylate) and poly (vinyl acetate) as macroRAFT agents. The quantitative experiment showed the yield of photolysis of a xanthate to be only 0.023% under λ > 440 nm.

Structure and Function of Tryptophan-Tyrosine Dyads in Biomimetic β Hairpins.

Tyrosine–tryptophan (YW) dyads are ubiquitous structural motifs in enzymes and play roles in proton-coupled electron transfer (PCET) and, possibly, protection from oxidative stress. Here, we describe the function of YW dyads in de novo designed 18-mer, β hairpins. In Peptide M, a YW dyad is formed between W14 and Y5. A UV hypochromic effect and an excitonic Cotton signal are observed, in addition to singlet, excited state (W*) and fluorescence emission spectral shifts. In a second Peptide, Peptide MW, a Y5–W13 dyad is formed diagonally across the strand and distorts the backbone. On a picosecond timescale, the W* excited-state decay kinetics are similar in all peptides but are accelerated relative to amino acids in solution. In Peptide MW, the W* spectrum is consistent with increased conformational flexibility. In Peptide M and MW, the electron paramagnetic resonance spectra obtained after UV photolysis are characteristic of tyrosine and tryptophan radicals at 160 K. Notably, at pH 9, the radical photolysis yield is decreased in Peptide M and MW, compared to that in a tyrosine and tryptophan mixture. This protective effect is not observed at pH 11 and is not observed in peptides containing a tryptophan–histidine dyad or tryptophan alone. The YW dyad protective effect is attributed to an increase in the radical recombination rate. This increase in rate can be facilitated by hydrogen-bonding interactions, which lower the barrier for the PCET reaction at pH 9. These results suggest that the YW dyad structural motif promotes radical quenching under conditions of reactive oxygen stress.

Modeling and validation of a modular multi-lamp photo-reactor for cetylpyridinium chloride degradation by UV and UV/H2O2 processes

UV-C assisted disinfection processes are among the well-known tertiary processes for water reuse, wastewater reclamation and domestic water disinfection. However, treated wastewaters normally contain several organic compounds and pollutants that can be partially degraded under UV-C irradiation to different by-products, often more toxic than their parent compounds, and/or, for longer treatment times, completely mineralized. For this reason, kinetic modeling of pollutants degradation in UV-C assisted processes is of crucial importance in plants design. This work aims to the characterization of a reactor configuration consisting of 6 lamps placed following a symmetric hexagonal geometry. Specifically, a mathematical model is proposed to estimate the mean optical length and degradation kinetics for UV-C and UV-C/H2O2 processes. The model has been tested with different reference organic compounds and represents a significant characterization of a modular geometric unit used in a great variety of applications. Finally, the characterized reactor was used to estimate the quantum yield of direct photolysis at 254 nm (Φ254CPC=(8.04±0.36)·10-3mol·ein-1) and kinetic constant of reaction with hydroxyl radical of cetylpyridinium chloride (CPC) ((1.11±0.0778)·109L·mol-1·s-1), an emerging pollutant of wide interest.

Release of Terminal Alkynes via Tandem Photodeprotection and Decarboxylation of o-Nitrobenzyl Arylpropiolates in a Flow Microchannel Reactor

Photocleavable protecting groups (PPGs) offer a complementary protection paradigm compared to traditional protection groups. Herein, an o-nitrobenzyl (NB) PPG was employed to protect a variety of arylpropiolic acids. Upon a cascade of light-triggered photodeprotection in a microchannel reactor (residence times of 100-500 s), followed by Cu-catalyzed decarboxylation at 60 °C, the NB-protected arylpropiolic acid afforded a terminal alkyne. This terminal alkyne was further reacted in situ with an azide via click chemistry to yield a 1,2,3-triazole in a one-pot reaction. Furthermore, the effect of different substituents (methyl, vinyl, allyl, and phenyl) at the benzylic position on the rate of photodeprotection was studied. The quantum yields of photolysis for the benzylic-substituted esters were determined to be as high as 0.45 compared to the unsubstituted ester with a 0.08 quantum yield of photolysis.

The importance of a photon-based approach to quantum yield determinations

It was recently reported that when using polychromatic light sources, a photon-based approach should be used to determine photochemical parameters instead of an energy-based approach, and that failure to do so would lead to errors. To illustrate whether the errors could be significant, the quantum yield of atrazine photolysis was determined by measuring its decomposition rate using a medium pressure UV lamp. The results showed that the use of the previously-reported method to calculate quantum yield introduced a 9% error. Furthermore, simplification of the equation via expanding the fraction of light absorbed as a Taylor series as previously-reported introduced a 3% error. The errors would be more significant when the photolyzed compound has a higher concentration or has a broader absorption spectrum.