Photobleaching Reactions Research Articles

Chlorogenic acids protect organic dyes against self-catalyzed photobleaching

An environmentally benign method for preventing the photofading of organic dyes was developed using chlorogenic acids, also known as caffeoylquinic acids (CQAs), which can be obtained in large quantities from agricultural wastes. In this study, 5-caffeoylquinic acid (5-CQA) and 4,5-dicaffeolyquinic acid (4,5-diCQA) were used as singlet oxygen (1O2) quenchers, and their suppression mechanisms were evaluated by the self-catalyzed photobleaching reactions of methylene blue (MB) and rose bengal (RB) in aqueous solutions at different pH values. Light irradiation experiments, half-wave oxidation potentials measurements, LC-MS, and NMR demonstrated that CQAs effectively inhibited the photofading of the dyes in the pH range of 4–9 with two independent reaction mechanisms. 4,5-diCQA was more effective in preventing the photofading than 5-CQA. The kinetic analysis estimated the 1O2 quenching constants of CQAs (kq = 7.22 × 107 M−1 s−1 and 9.33 × 107 M−1 s−1 for 5-CQA and 4,5-diCQA) and the bimolecular rate constant for the chemical quenching of 1O2 by MB (kf = 2.88 × 104 M−1 s−1). Comparing these rate constants revealed that the quenching reactions of 1O2 by CQAs were more predominantly than the photofading reaction of MB.

Extreme-Ultraviolet Excited Scintillation of Methylammonium Lead Bromide Perovskites

Inorganic–Organic lead halide materials have been recognized as potential high-energy X-ray detectors because of their high quantum efficiencies and radiation hardness. Surprisingly little is known about whether the same is true for extreme-ultraviolet (XUV) radiation, despite applications in nuclear fusion research and astrophysics. We used a table-top high-harmonic generation setup in the XUV range between 20 and 45 eV to photoexcite methylammonium lead bromide (MAPbBr3) and measure its scintillation properties. The strong absorbance combined with multiple carriers being excited per photon yield a very high carrier density at the surface, triggering photobleaching reactions that rapidly reduce the emission intensity. Concurrent to and in spite of this photobleaching, a recovery of the emission intensity as a function of dose was observed. X-ray photoelectron spectroscopy and X-ray diffraction measurements of XUV-exposed and unexposed areas show that this recovery is caused by XUV-induced oxidation of MAPbBr3, which removes trap states that normally quench emission, thus counteracting the rapid photobleaching caused by the extremely high carrier densities. Furthermore, it was found that preoxidizing the sample with ozone was able to prolong and improve this intensity recovery, highlighting the impact of surface passivation on the scintillation properties of perovskite materials in the XUV range.

Chitosan films containing TiO2 nanoparticles modified with tungstophosphoric acid for the photobleaching of malachite green in solid-gas interfaces upon different wavelengths

Chitosan (CS) films containing visible light absorbing TiO2 nanoparticles modified with tungstophosphoric acid (TPA-TiO2) at different TPA-TiO2:CS ratios (10%, 20%, 30% and 40% (w/w) called CS/TPA-TiO2-10, CS/TPA-TiO2-20, CS/TPA-TiO2-30, and CS/TPA-TiO2-40 respectively) were prepared. Films were characterized by DRS, SEM-EDS, XRD, ATR-FT-IR, and contact angle (CA). Materials were evaluated in malachite green (MG) photobleaching solid-gas interface reactions upon different wavelengths.SEM-EDS micrographs showed that TPA-TiO2 nanoparticles were well dispersed through the CS film. According to ATR-FT-IR results, the interactions between TPA-TiO2 nanoparticles and the CS functional groups were weak. Regarding the MG photobleaching reactions, CS/TPA-TiO2-30 film showed the highest activity upon UV-A + visible irradiation, which was even higher than that of chitosan films containing the same amount of Evonik P-25 (CS/P25-30). It was also observed by DRS that the main absorption band of malachite green underwent a hypsochromic shifting caused by oxidative reactions of N-demethylation. On the other hand, at wavelengths higher than 450 nm (Vis-1), CS/TPA-TiO2-30 films showed a marked dye photobleaching. Finally, upon irradiation at wavelengths above 590 nm (Vis-2), a slight color fading was observed. These results revealed that CS/TPA-TiO2-30 films were efficient in the photobleaching of malachite green in solid-gas interfaces through an oxidative path by photocatalytic and photosensitized routes. CS/TPA-TiO2-30 films showed high stability and MG photobleaching capacity after 5 reuse cycles.

New thermally stable photochromic di(het)arylethenes of the cyclopentenone series

Oxidation of 2,3-diarylcyclopent-2-en-1-ones with m-chloroperoxybenzoic acid gave new thermally stable sulfone derivatives of photochromic diarylethenes. The spectral properties of the compounds obtained (the wavelengths of the maxima of the absorption bands of their initial and cyclic forms, the quantum yields of photocyclization and photobleaching reactions) as well as their thermal stability and fatigue resistance were examined. The relationship between the structures of the synthesized compounds and their photochromic properties was determined. The energy differences between the ground-state molecules of the starting and photoinduced isomers of 2,3-diarylcyclopent-2-en-1-ones were calculated by the DFT/B3LYP1 method with the 6-31G(d) basis set. The calculated energy differences can be used to predict and explain such spectral characteristics of photochromic diarylethenes as the thermal stability of photoinduced isomers and the quantum yields of cycloreversion reactions.

Photochromic properties of 7′,8′‐dichloro‐1,3,3‐trimethylspiro[indoline‐2,3′‐[3H]benzo[b][1,4]oxazine] doped in PMMA and epoxy resin thin films

PurposeThe purpose of this paper is to evaluate the photochromic performance of photochromic compounds in polymer matrices.Design/methodology/approachThe poly(methyl methacrylate) PMMA and epoxy resin doped with photochromic spirooxazine (SO) are prepared and the effects of ultraviolet (UV) irradiation are studied using spectrophotometer. The reversible reaction is effected using white light. Photochemical fatigue resistance of these films is also studied.FindingsIrradiation of colourless 7′,8′‐dichloro‐1,3,3‐trimethylspiro[indoline‐2,3′‐[3H]benzo[b][1,4]oxazine] (SO) doped in PMMA and epoxy resin with UV light (366 nm) results in the formation of an intense purple‐red coloured zwitterionic photomerocyanine (PMC). The reverse reaction is photochemically induced by irradiation with white light. Photocolouration and photobleaching reactions follow a first‐order rate equation. It is found that photocoloration rate constant of (SO) in both matrices is almost the same, which is unexpected. On the other hand, the rate of photobleaching reaction of (PMC) in PMMA is twice slower than that in the epoxy resin. It seems that the presence of the two chlorine atoms at positions 7′ and 8′ of the benzooxazine moiety destabilise the PMC in epoxy resin film and results in speeding up the fading process compared to that in PMMA. SO doped in epoxy resin shows much better fatigue resistance than that doped in PMMA.Research limitations/implicationsThe PMMA and epoxy resin polymers doped photochromic spirobenzooxazine described in this paper were prepared and studied. The principle of study established can be applied to any type of polymer or to any type of photochromic compounds.Practical implicationsThe photochromic materials developed can be used for different applications, such as coatings and holography.Originality/valueThe method developed may be used to enhance the performance of photochromic materials.

Photochromic properties of 1,3,3-trimethylspiro[indoline-2,3′-[3H]naphtho[2,1-b][1,4]oxazine] doped in PMMA and epoxy resin thin films

Irradiation of colorless 1,3,3-trimethylspiro[indoline-2,3′-[3H]naphtho[2,1-b][1,4]oxazine] SO doped in PMMA and epoxy resin with UV light (at 366nm) results in the formation of an intensely colored zwitterionic photomerocyanine PMC. The reverse reaction was photochemically induced by irradiation with white light. Photocoloration and photobleaching reactions follow a first-order rate equation. It was found that photocoloration rate constant of SO in PMMA film is greater than that in epoxy resin. On the other hand, the photobleaching rate constant is almost identical in both matrices. Spirooxazine doped in epoxy resin shows much better fatigue resistance than that doped in PMMA.

Enhancing Fluorescence Brightness: Effect of Reverse Intersystem Crossing Studied by Fluorescence Fluctuation Spectroscopy

Experiments based on fluorescence detection are limited by the population of the fluorescence marker's long-lived dark triplet state, leading to pronounced photobleaching reactions and blinking which reduces the average fluorescence signal obtained per time interval. By irradiation with a second, red-shifted laser line, we initiate reverse intersystem crossing (ReISC) which enhances the fluorescence signal of common fluorophores up to a factor of 14. The reverse intersystem crossing from the triplet state back to the singlet system is achieved by photoexcitation to higher-excited triplet states, which are, however, prone to photobleaching. We gain insights into the competing pathways of ReISC and photobleaching. The relative efficiencies of these two pathways and the triplet lifetime determine the achievable fluorescence enhancement, which varies strongly with the choice of dye, excitation irradiance and wavelength, and with environmental conditions. The study of ReISC not only results in a better understanding of a fluorescent label's photophysics, but the method is a possible approach to optimize fluorescence emission in experiments, where signal strength is a critical parameter.

Single‐crystalline photochromism of diarylethene dimers bridged by a spiro structure

AbstractNew types of diarylethene dimers bridged by a spiro structure were synthesized. One of their dimers formed three kinds of polymorphic forms by recrystallization from different solvents: hexane (1a‐α), acetone (1a‐β), and acetonitrile (1a‐γ). In crystals of 1a‐α and 1a‐β the molecules are packed in solvent‐free crystal structures, whereas crystal 1a‐γ includes acetonitrile molecules in the crystal. The difference of significant photochromic reactivities in their polymorphic crystals was observed for photocoloration reactions. Crystals 1a‐β and 1a‐γ showed photochromism in the single‐crystal phase, but 1a‐α did not do so. Their photochromic reactivity was found to depend on the distance between the reactive carbon atoms by X‐ray crystallographic analysis. When the distance is less than 4.2 Å in the antiparallel conformation, the molecule can undergo photochromism in the crystal. On the other hand, crystals of bromo‐substituted diarylethene dimer (2a) cannot be suitable for X‐ray crystallographic analysis when it was recrystallized from hexane, acetone, and acetonitrile. However, it formed single crystal by recrystallization from p‐xylene, which showed photochromism in the crystalline phase. The edge‐to‐face aromatic interaction between p‐xylene molecules assisted the crystal growth of the diarylethene dimer. Upon irradiation with ultraviolet light, two kinds of the enantiomers, (M,SS)‐ and (P,RR)‐2b, are produced. Partial photobleaching reactions of either of the two enantiomers by irradiation with linearly polarized visible light were confirmed by polarized absorption spectroscopy. Copyright © 2007 John Wiley & Sons, Ltd.

Photochromic properties of (E)‐dicyclopropylmethylene‐(2,5‐dimethyl‐3‐furylethylidene)‐succinic anhydride doped in epoxy polymer film

PurposeTo evaluate the photochromic performance of photochromic compounds in polymer matrices.Design/methodology/approachThe epoxy resin doped with photochromic fulgide were prepared and the effect of UV irradiation were studied using spectrophotometer. The reversible reaction was effected using white light. The effect of heat was also determined.FindingsA film of fulgide 1‐E doped in epoxy polymer was irradiated with UV light (366 nm), the film turned pink. The later colour was switched back to the original colour when the film was irradiated with a white light. The photocoloration and photobleaching obeyed first order rate equations with rate constants being 4.19×10−3 s−1 and 2.86×10−2 s−1, respectively. It was found that the film showed a good fatigue resistance. Another film was preheated at 80°C for 1‐4 h. No change in the UV absorption spectra of the film was observed. Similarly, the photocoloration and photobleaching of the annealed film showed first order rate equations with rate constants being 8.77×10−3 s−1 and 4.02×10−2s−1, respectively. Interestingly, the photocoloration and photobleaching reactions of the annealed film were faster than those of the non‐annealed film.Research limitations/implicationsThe epoxy resin doped photochromic fulgides described in the present paper was prepared and studied. The principle of study established can be applied to any type of resin or to any type of photochromic compounds.Practical implicationsThe photochromic materials developed can be used for different applications, such as coatings and holography.Originality/valueThe method developed may be used to enhance the performance of photochromic materials.

Photochromic properties of (E)-dicyclopropyl-methylene-(2,5-dimethyl-3-furylethylidene)-succinic anhydride doped in PMMA polymer film

A film of fulgide 1-E doped in a PMMA polymer film and spread over a quartz plate was irradiated with ultraviolet light (366 nm), the film was turned pink. The later color was switched back to the original color when the film was irradiated with a white light. The photocoloration and photobleaching reactions obeyed first order rate equations with rate constants equal 1.49 × 10−3 s−1 and 5.75 × 10−2 s−1, respectively. Another two films were heated for four hours, one at 62 °C and the other at 82 °C. The fatigue resistant of the film was greatly improved when the annealing temperature increased to 82 °C. The UV-spectra of the films show two absorption maxima at λmax = 360 and 300 nm. The UV-spectra of the annealed film at 82 °C show a decrease in the height of the second maximum absorbance band at λmax = 300 nm. Similarly, the photocoloration and photobleaching reactions of the annealed films show first order rate equations. Interestingly, the rate of the photocoloration reaction increases with increasing the annealing temperature. On the other hand, the rate of the photobleaching reaction is almost similar for the three films. In conclusion, the Photochromic properties of fulgide 1-E doped in PMMA polymer film was improved significantly upon annealing at 82 °C, compared with other epoxy films we studied previously.

PHOTOYELLOWING OF UNTREATED AND ACETYLATED ASPEN CHEMITHERMOMECHANICAL PULP UNDER ARGON, AMBIENT, AND OXYGEN ATMOSPHERES

Untreated and acetylated hydrogen peroxide-bleached aspen chemithermomechanical pulp (CTMP) was subjected to accelerated light-induced aging in various atmospheres (argon, ambient, or oxygen). The photochemical changes that took place during irradiation were followed by solid-state UV/VIS diffuse reflectance spectroscopy. The degree of photoyellowing of the untreated CTMP decreased when the air in the surrounding atmosphere was replaced with oxygen-free argon. The decrease was only moderate, indicating that atmospheric oxygen is not of sole importance for the light-induced discoloration or that only a trace amount of oxygen (strongly adsorbed to the fiber material) is necessary to cause discoloration. Acetylation clearly diminished the kinetics of photoyellowing in all atmospheres resulting in substantially less absorption in the entire visible range (λ > 400 nm). However, the development of chromophores was attenuated in an oxygen-enriched atmosphere, suggesting that oxygen is important for the color-retarding or photobleaching reactions of acetylated lignocellulosic materials. Independently of the degree of acetylation and the surrounding atmosphere, irradiation with UV/VIS fluorescent lamps generated an apparent absorption maximum around 360 nm with a shoulder at approximately 420 nm.

The role of the triplet state on the photobleaching processes of xanthene dyes in a poly(vinyl alcohol) matrix

This work presents the results for the temperature dependence of the photobleaching reactions of Fluorescein and Rose Bengal dissolved in poly(vinyl alcohol) in the presence of diethylenetriamine, potassium dichromate and 2-aminoethanethiol hydrochloride. These samples were subjected to a continuous irradiation with a 150 W mercury high pressure arc lamp at a wavelength coincident with the maximum of the dye absorption. The photobleaching processes were described by a double exponential and a mono-exponential function for Fluorescein and Rose Bengal, respectively. Apparent activation energies were calculated using the Arrhenius plots and these values were strongly affected by the presence of additives. Different additives are employed in the studies of the photobleaching reactions of Fluorescein and Rose Bengal in poly(vinyl alcohol) matrices: (a) diethylenetriamine is a quencher of all excited states; (b) potassium dichromate is an electron acceptor complexed with the polymer chains and inhibited the photobleaching; (c) 2-aminoethanethiol hydrochloride is an efficient quencher of the triplet state. The role of these additives upon the photoreaction is discussed.

Photothermal stability of encapsulated Si solar cells and encapsulation materials upon accelerated exposures

We have conducted a series of accelerated exposure test (AET) studies for various samples of crystalline-Si (c-Si) and amorphous-Si (a-Si) cells that were encapsulated with different superstrates, pottants, and substrates. Transmittance, fluorescence, color indices, impedance spectroscopy, laser optical beam induced current (OBIC), and light and dark current–voltage ( I– V) measurements were used to characterize the samples. Nonuniform browning patterns of ethylene vinyl acetate (EVA) pottants were observed for glass/EVA/glass-encapsulated c-Si cell samples under solar simulator exposures at elevated temperatures. The EVA discolored gradually into a yellow–brown color when exposed under ∼7.5 ultraviolet (UV) suns at a black panel temperature (BPT) of ∼85°C, and very rapidly into a dark brown color when exposed under ∼9.0 UV suns at a BPT of ∼145°C. The latter conditions also caused extensive delamination of a polyolefin-based thermoplastic pottant laminated between two borosilicate plates. No visible color change of EVA could be observed for the c-Si cell samples heated in an oven at 85°C or exposed under ∼1.2 UV suns at BPT=60–65°C or 80–85°C. Under ∼1.2 UV suns exposure at a BPT of 60–65°C, the thin layers of EVA or silicone in the lightweight c-Si cell samples, which were laminated in a polymer/solar cell/polymer configuration with Tedlar or Tefzel films, did not discolor because of photobleaching reactions. In comparison, the polyester or nylon superstrate films turned yellow on laminates of polymer/a-Si/polymer minimodules, and significant delamination was observed for the polyester/EVA layers. For all c-Si cell samples tested, irregular changes in the I– V parameters were observed that could not be attributed only to transmittance changes of the superstrate/pottant layers. Under UV-transmitting polymer superstrate films, current/efficiency losses were greater for the c-Si and a-Si cells that were laminated with silicone-type adhesives than with EVA or polyethylene pottants.

A low temperature spectrophotometric study of the photomerocyanine form of spirooxazine doped in polystyrene film

The photocoloration and photobleaching reactions of spironaphthooxazine doped in polystyrene have been studied by using low-temperature absorption spectroscopy. The absorption spectra of PMC produced at 25 K are remarkably different from those obtained at 77–200 K with regard to the broad absorption gains at around 670 and 485 nm. Upon visible irradiation, PMC undergoes photobleaching reaction whose efficiency varies with the irradiation time and wavelengths. Regardless of the irradiation wavelength, the broad holes were initially produced at 25 K on both sides of the absorption spectra while the antihole is obtained at around 530–600 nm. As time passes, the centroid of the hole gradually shifts toward the burn wavelength. From experimental observation, the broad absorption located at around 485 and 670 nm are attributed to the nonplanar photoproduct X trapped in the polymer matrix. Therefore the hole and antihole observed at the very early stage of the visible irradiation are interpreted as being due to the photochemical transformation of X to transoids.

Photostability of xanthene molecules trapped in poly(vinyl alcohol) (PVA) matrices

Photobleaching reactions of fluorescein and Rose Bengal molecules dissolved in PVA matrices were studied as a function of the temperature. We used and compared two theoretical approaches to describe these photobleaching kinetics. Photobleaching quantum yield calculations were done using an adapted model (B-model) for polychromatic excitation that allowed us to estimate the average number of photons absorbed by each dye molecule before its bleaching. The other model uses exponential functions allowing us to calculate rate constants and activation energy. A slower photobleaching process was obtained for Rose Bengal sorbed in PVA matrices compared with fluorescein. We also obtained that the Rose Bengal photobleaching process is always single-exponential kinetic independent of the polymer relaxation process. Nevertheless, fluorescein photobleaching reaction is bi-exponential process at higher concentrations and it is strongly dependent on the polymer relaxation processes. These results were interpreted in terms of the different efficiencies among the processes involving quenching of the lowest triplet state.

Dye-polymer interactions controlling the kinetics of fluorescein photobleaching reactions in poly(vinyl alcohol)

This work presents results for the temperature dependence of the photobleaching reactions of Fluorescein dissolved in poly (vinyl alcohol) (PVA) irradiated by a continuous 150 W Hg high pressure arc lamp. We observed that the photobleaching process efficiency is dependent on: lamp power, dye concentration and temperature. Although the photobleaching processes can only be described by a single-exponential function for lower concentration samples, a biphasic model must be employed for intermediate concentrations, resulting in one slower and one faster processes, while, for higher concentration samples, even multiple-exponential functions are unable to tit the experimental data. This complex behavior is probably due to the co-existence of many different types of energy transfer and photochemical processes with different rate constants. Therefore, a model based on a sequential distribution of site occupation by the dye molecules in the matrix, controlled by dyepolymer interaction strengths, is proposed to explain the complex-exponential kinetic behavior. Moreover, photobleaching process efficiency- is somewhat increased by temperature and, above the PVA glass transition temperature, T g, (350 K), it follows a first order kinetics, fitted by a mono-exponential function.

Optical Density Dependence of Write/Read Characteristics in Photon-Mode Photochromic Memory

When photobleaching reactions are used for writing in a photochromic optical memory, the initial optical density in the recording layer plays an important role in the various write/read characteristics; i.e., secondary harmonics, repeatedly readout characteristics, and crosstalks in a multi-wavelength recording. Better characteristics, that is, lower secondary harmonics, higher readout repeatability and lower nonlinear component of crosstalks, were obtained for media with higher optical density.

Factors that affect the EVA encapsulant discoloration rate upon accelerated exposure

Several factors that affect the discoloration rate of the ethylene-vinyl acetate ( EVA) copolymer encapsulants used in crystalline-Si photovoltaic ( PV) modules upon accelerated exposure have been investigated primarily by employing UV-visible spectrophotometry, spectrocolorimetry, and fluorescence analysis. A variety of film samples including the two typical ( unprimed) EVA formulations, A9918 and 15295, were studied. The films were laminated, cured, and exposed to either a concentrated 1-kW Xe or an enhanced-UV light source. The results indicate that the extent of EVA discoloration can be affected by factors of two general categories: chemical and physical. In the chemical category, the degradative factors include (1) EVA formulation, (2) presence and concentration of curing-generated, UV-excitable chromophores that depend on the type of curing agent used, (3) loss rate of the UV absorber, Cyasorb UV 531 TM, (4) curing agent and curing conditions, and (5) photobleaching reactions due to diffusion of air into the laminated films. In the physical category, the factors involve (6) UV light intensity, (7) UV-filtering effect of glass superstrates, (8) gas permeability of polymeric superstrates, (9) film thickness, and (10) lamination-delamination (maybe chemical and/or mechanical effect, too). Photodecomposition of the Cyasorb was first verified in cyclohexane solutions and then in Elvax 150 TM ( EVX) films (the copolymer without any additives and curing agent). Cyasorb decomposition rates in cyclohexane solutions are exponentially proportional to the light intensity, but can be greatly reduced by a free-radical scavenger, Tinuvin 770 TM, and furthermore by an antioxidant, Naugard P TM. The discoloration rate of EVA increases with increasing loss of Cyasorb UV 531 and is faster for the EVA A9918 films that have a greater concentration of UV-excitable chromophores generated from a slower curing than for the EVA 15295 films that are fast cured. In general, the loss rate of the UV absorber and the rate of discoloration from light yellow to brown follow a sigmoidal pattern. A reasonably good correlation for changes in transmittance at 420 nm, yellowness index, and fluorescence peak area (or intensity ratio) is obtained as the extent of EVA discoloration progresses. No discoloration was observed for the laminated EVX films that contain no stabilizers and curing-generated chromophores. The discoloration rate of both types of EVA can be largely reduced by UV-filtering glass superstrates that remove UV< 320nm. Photobleaching reactions are responsible for the non-discoloration of unlaminated EVA, the visually clear perimeter around the edges of laminated samples, and the EVA films laminated with gas-permeable polymer film superstrates. Delamination of EVA films from the top glass superstrate was observed after prolonged UV exposure.

Optical sensing of oxygen using phosphorescence quenching

Optical measurements that exploit the ability of oxygen to quench the photoexcited states of other molecules provide an alternative to the standard electrochemical and paramagnetic methods of measuring molecular oxygen concentration. In the present work we have re-examined the phosphorescence quenching technique as a means of monitoring oxygen concentrations in a gas stream at room temperature. Camphorquinone (CQ) is selected because it has strong absorption and emission bands in the visible region, enabling the use of inexpensive silica fibre-optic light guides. Determination of oxygen in the range 0.1–25% is easily achievable and, for a 10 μm thick PVC film containing CQ, the time to attain 90% response is of the order of 10 s. Photooxidation and photoreduction account for a significant loss of material under certain conditions of illumination and film composition. The use of triplet lifetime as a measure of oxygen concentration offers several advantages. In particular, by using an intrinsic property of the sensing molecule, variations in lamp intensity and photomultiplier sensitivity are avoided and the effects of photobleaching reactions are minimized.

Photochemistry in strongly absorbing media

The authors report the solution to the coupled nonlinear first-order partial differential equations representing the dynamics of photochemical reactions in systems essentially free from diffusion. No limitation regarding absorption of light by products of inert medium is assumed. These equations have been solved with the boundary condition that the species be uniformly distributed in the material prior to irradiation; the incident light may have an arbitrary time dependence. This analytical method of solution has been applied to photobleaching reactions described by various rate laws, where both reactants and products can absorb light. In addition, model systems in which the products absorb more than the reactants (photoantibleaching reactions) are also studied. An empirical procedure is introduced that accounts for changes in surface reflectance during the photoreaction in a satisfactory manner. Finally, a comparison is made with the corresponding equations describing photochemistry in perfectly stirred media.