Photosensitized reaction

The excited-state species can also reduce its energy by transfer to another molecule (quencher), which may diffuse it or change it into chemical energy. The process by which a photochemical or photophysical alteration occurs in one molecular entity, as a result of initial absorption of radiation by another molecular entity, is called photosensitization. 

The species absorbing and transferring the radiant energy is called the photosensitizer. In mechanistic photochemistry the term is limited to cases in which the photosensitizer is not consumed in the reaction [29]. 

Excitation of a photosensitizer molecule (Sens) by light absorption  

Quenching of the sensitizer excited state by a quencher molecule (Q)  

When the reaction of a non-absorbing quencher is induced by electron transfer (not energy transfer) using an excited light-absorbing sensitizer, the process is called electron transfer photosensitization. Then interaction between sensitizer and quencher consists of photoinduced electron transfer  

The excited triplet sensitizer can undergo its primary reaction with molecules in its vicinity by (Spikes, 1989)  

Electron transfer, including simultaneous transfer of a proton corresponding to the transfer of a hydrogen atom, resulting in free radical reactions (Equation 2.10 to Equation 2.12) — termed type I, or free radical, reaction 

Energy transfer, with spin conservation, to ground state molecular oxygen ( ,02) to form singlet oxygen (Equation 2.14) — termed type II reaction 

Type I and type II processes can take place simultaneously in a competitive 

As the intensity increases, a greater proportion of the bromine atoms formed are converted to Br2 instead of entering the chain most of the additional quanta therefore are wasted, and the process is less efficient. Because /c2 is very small, the quantum yield is less than unity at room temperature in spite of the fact that the HBr is formed in a chain reaction. As the temperature increases, the increase in /c2 increases the quantum yield (k is nearly independent of temperature). 

Photosensitized reactions make up an important class of photochemical reactions. In these reactions the reactants are mixed with a foreign gas mercury or cadmium vapor are often used. The primary photochemical act is the absorption of the quantum by the foreign atom or molecule. 

If a mixture of hydrogen, oxygen, and mercury vapor is exposed to ultraviolet light, the mercury vapor absorbs strongly at 253.7 nm with the formation of an excited mercury atom, Hg  

The importance of photosensitization derives f rom the f act that reaction is produced in the presence of the sensitizer in circumstances where the direct photochemical dissociation is not possible. The example just cited is a case in point. Radiation of wavelength 253.7 nm was absorbed by a mercury atom. The excited mercury atom dissociated a molecule of hydrogen by transferring the excitation energy in a collision. The mercury atom had 471.5 kJ of this 432.0 kJ were needed for the dissociation 39.5 kJ are left over and go into additional translational energy of the two hydrogen atoms and the mercury atom. If the attempt is made to dissociate H2 directly by the process 

The decomposition of ozone sensitized by chlorine is another example of photosensitization. Ozone is stable under irradiation by visible light. The absorption continuum begins at about 290 nm. In the presence of a little chlorine, ozone decomposes rapidly. The chlorine absorbs continuously below 478.5 nm  

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P-Carotene is prescribed in the treatment of the inherited skin disorder erythropoietic protoporphyria (EPP) to reduce the severity of photosensitivity reactions in such patients. The essential theoretical background relevant to the role of carotenoids as photoconductors has been reviewed (211). P-Carotene has also been used as a photoconductor in recording-media film. 

Photosensitive Reactions. The reduction of chromium (VI) by organic compounds is highly photosensitive, and this property is used ia photosensitive dichromate-coUoid systems. 

The detection of spectral sensitizing action often depends on amplification methods such as photographic or electrophotographic development or, alternatively, on chemical or biochemical detection of reaction products. Separation of the photosensitization reaction from the detection step or the chemical reaction allows selection of the most effective spectral sensitizers. Prime considerations for spectral sensitizing dyes include the range of wavelengths needed for sensitization and the absolute efficiency of the spectrally sensitized process. Because both sensitization wavelength and efficiency are important, optimum sensitizers vary considerably in their stmctures and properties. 

Photosensitized Reactions for Polymers. The economic and technical features for photocross-linking, photosolubilizafion, and photopolymerization reactions have been reviewed (55). The widely used poly(vinyl ciunamates) (PVCN) photocross-link by a photodimerization reaction. 

Complex reactions occur on the autoxidation of pyrroles (see Section 3.05.1.4) predictably, susceptibility to autoxidation increases with increasing alkyl substitution, llie photosensitized reaction of pyrrole and oxygen yields 5-hydroxy-A -pyrrolin-2-one, probably by way of an intermediate cyclic peroxide (Scheme 28) (76JA802). 

Jap-KIingermarm reactions, 4, 301 oxidation, 4, 299 reactions, 4, 299 synthesis, 4, 362 tautomerism, 4, 38, 200 Indole, 5-amino-synthesis, 4, 341 Indole, C-amino-oxidation, 4, 299 tautomerism, 4, 298 Indole, 3-(2-aminobutyl)-as antidepressant, 4, 371 Indole, (2-aminoethyl)-synthesis, 4, 278 Indole, 3-(2-aminoethyl)-synthesis, 4, 337 Indole, aminomethyl-reactions, 4, 71 Indole, 4-aminomethyl-synthesis, 4, 150 Indole, (aminovinyl)-synthesis, 4, 286 Indole, 1-aroyl-oxidation, 4, 57 oxidative dimerization catalysis by Pd(II) salts, 4, 252 Indole, 1-aroyloxy-rearrangement, 4, 244 Indole, 2-aryl-nitration, 4, 211 nitrosation, 4, 210 synthesis, 4, 324 Indole, 3-(arylazo)-rearrangement, 4, 301 Indole, 3-(arylthio)-synthesis, 4, 368 Indole, 3-azophenyl-nitration, 4, 49 Indole, 1-benzenesulfonyl-by lithiation, 4, 238 Indole, 1-benzoyl photosensitized reactions with methyl acrylate, 4, 268 Indole, 3-benzoyl-l,2-dimethyl-reactions... 

The direct irradiation of A gives predominantly B, but the photosensitized reaction gives more C. Explain. 

Prolonged exposure to sunlight may result in skin reactions similar to a severe sunburn (photosensitivity reactions). When going outside, cover exposed areas of the skin or apply a protective sunscreen to exposed areas. 

The more common adverse effects seen with the administration of these dm include nausea, diarrhea, headache, abdominal pain or discomfort, and dizziness. A more serious adverse reaction seen with the administration of the fluoroquinolones, especially lomefloxacin and sparfloxacin, is a photosensitivity reaction. This is manifested by an exaggerated sunburn reaction when the skin is exposed to the ultraviolet rays of sunlight or sunlamps. 

All fluoroquinolone drugs can cause pain, inflammation, or rupture of a tendon. The Achilles tendon is particularly vulnerable. This problem can be so severe tiiat prolonged disability results, and, at times, surgical intervention may be necessary to correct die problem, hi addition, the fluoroquinolone drugs, particularly sparfloxacin and lomefloxacin, cause dangerous photosensitivity reactions. Fhtients have experienced severe reactions even when sunscreens or sunblocks were used. 

Adverse reactions to the gold compounds may occur any time during therapy, as well as many months after therapy has been discontinued. Dermatitis (inflammation of the skin) and stomatitis (inflammation of mucosa of the mouth, gums, and possibly the tongue) are the most common adverse reactions seen. Pruritus (itching) often occurs before the skin eruption becomes apparent. Photosensitivity reactions (exaggerated sunburn... 

Electrolyte imbalances, anorexia, nausea, vomiting, dizziness, rash, photosensitivity reactions, postural or orthostatic hypotension, glycosuria Electrolyte imbalances, anorexia, nausea, vomiting, fever, chills, anxiety, confusion, hematologic changes me as bumetanide... 

Thiazide and related diuretics, loop diuretics, potassium-sparing diuretics, carbonic anhydrase inhibitors, triamterene Avoid exposure to sunlight or ultraviolet light (sunlamps, tanning beds) because exposure may cause exaggerated sunburn (photosensitivity reaction). Wear sunscreen and protective clothing until tolerance is determined. 

Abdominal pain, nausea, vomiting, anorexia, diarrhea, rash, drowsiness, dizziness, photosensitivity reactions, blurred vision, weakness, and headache may occur with the administration of nalidixic acid. Visual disturbances, when they occur, are noted after each dose and often disappear after a few days of therapy. 

The experimental results described in this work64 concerning the sulfoxide and the sulfones can also be explained by a two-electron process in agreement with electrochemical evidence and photosensitized reactions (see the previous section), using the successive reactions1 ... 

For a review of other complications that can take place in photosensitized reactions, see Engel, PS. Monroe, B.M. Adv. Photochem., 1971, 8, 245-313. 

Katsumura, Kitaura and their coworkers [74] found and discussed the high reactivity of vinylic vs allylic hydrogen in the photosensitized reactions of twisted 1,3-dienes in terms of the interaction in the perepoxide structure. Yoshioka and coworkers [75] investigated the effects of solvent polarity on the product distribution in the reaction of singlet oxygen with enolic tautomers of 1,3-diketones and discussed the role of the perepoxide intermediate or the perepoxide-Uke transition state to explain their results. A recent review of the ene reactions of was based on the significant intervention of the perepoxide structure [76], which can be taken as a quasi-intermediate. 

Not all sensitized photochemical reactions occur by electronic energy transfer. Schenck<77,78) has proposed that many sensitized photoreactions involve a sensitizer-substrate complex. The nature of this interaction could vary from case to case. At one extreme this interaction could involve a-bond formation and at the other extreme involve loose charge transfer or exciton interaction (exciplex formation). The Schenck mechanism for a photosensitized reaction is illustrated by the following hypothetical reaction ... 

The iodine-catalyzed photoisomerization of all-trans- a- and (3-carotenes in hexane solutions produced by illumination with 20 W fluorescence light (2000 lux) and monitored by HPLC with diode-array detection yielded a different isomer distribution (Chen et al. 1994). Four cis isomers of [3-carotene (9-cis, 13-cis, 15-cis, and 13,15-cli-r/.v) and three cis isomers of a-carotene (9-cis, 13-cis, and 15-ri.v) were separated and detected. The kinetic data fit into a reversible first-order model. The major isomers formed during the photosensitized reaction of each carotenoid were 13,15-di-d.v- 3-carotene and 13-ds-a-carotene (Chen et al. 1994). 

Pathak, M.A. (1984) Mechanisms of psoralen photosensitization reactions./. Natl. Cancer Inst. Monogr. 66, 41-46. 

There are two possible initial steps of photosensitized reactions, leading to the formation of superoxide radical ... 

Photosensitization of diaryliodonium salts by anthracene occurs by a photoredox reaction in which an electron is transferred from an excited singlet or triplet state of the anthracene to the diaryliodonium initiator.13"15,17 The lifetimes of the anthracene singlet and triplet states are on the order of nanoseconds and microseconds respectively, and the bimolecular electron transfer reactions between the anthracene and the initiator are limited by the rate of diffusion of reactants, which in turn depends upon the system viscosity. In this contribution, we have studied the effects of viscosity on the rate of the photosensitization reaction of diaryliodonium salts by anthracene. Using steady-state fluorescence spectroscopy, we have characterized the photosensitization rate in propanol/glycerol solutions of varying viscosities. The results were analyzed using numerical solutions of the photophysical kinetic equations in conjunction with the mathematical relationships provided by the Smoluchowski16 theory for the rate constants of the diffusion-controlled bimolecular reactions. 

The fluorescence decrease in Figure 1 can be attributed to the consumption of the anthracene photosensitizer during the photosensitization reaction. The photosensitization proceeds by an electron transfer reaction from the anthracene to the initiator, resulting in loss of aromaticity of the of the central ring.17 Therefore, the photosensitization reaction leads to a disruption in the n electron structure of the anthracene, and the resulting molecule does not absorb at 364 nm (nor fluoresce in the 420 - 440 nm region). Hence, the steady-state fluorescence measurements allow the anthracene concentration to be monitored in situ while the photosensitization reaction takes place. 

A plot of the anthracene fluorescence intensity at 425 nm as a function of the reaction time is shown in Figure 2. Again, this figure exhibits the effect of the instrumental artifact in the initial fluorescence data however, examination of the final 90% of the profile reveals that the anthracene concentration profile closely follows a first order exponential decay. Although the photosensitization reaction is bimolecular, the anthracene concentration follows a pseudo-first-order profile since the initiator is present in excess (i.e. r = = kjCA where r, Q and CA represent... 

Figure 5. Simulation results for the anthracene concentration as a function of time during the photosensitization reaction at different viscosities.

The experimental and simulation results presented here indicate that the system viscosity has an important effect on the overall rate of the photosensitization of diary liodonium salts by anthracene. These studies reveal that as the viscosity of the solvent is increased from 1 to 1000 cP, the overall rate of the photosensitization reaction decreases by an order of magnitude. This decrease in reaction rate is qualitatively explained using the Smoluchowski-Stokes-Einstein model for the rate constants of the bimolecular, diffusion-controlled elementary reactions in the numerical solution of the kinetic photophysical equations. A more quantitative fit between the experimental data and the simulation results was obtained by scaling the bimolecular rate constants by rj"07 rather than the rf1 as suggested by the Smoluchowski-Stokes-Einstein analysis. These simulation results provide a semi-empirical correlation which may be used to estimate the effective photosensitization rate constant for viscosities ranging from 1 to 1000 cP. 

The word dermatitis denotes an inflammatory erythematous rash. The disorders discussed in this chapter include contact dermatitis, seborrheic dermatitis, diaper dermatitis, and atopic dermatitis. Drug-induced skin disorders have been associated with most commonly used medications and may present as maculopapular eruptions, fixed-drug eruptions, and photosensitivity reactions. 

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