Photodecomposition

Photodecomposition. The photochemistry of acetaldehyde has been studied by several investigators in the past. Both the primary processes and the free-radical chain processes have been studied, but it has not been until recently that a reasonable understanding of the primary processes has been achieved. If free-radical chain reactions are assumed to be secondary processes, three decomposition pathways can be envisaged  

In a pair of complementary studies, Archer et al. (9,10) have made observations that agree with the previous studies but differ somewhat in interpretation. Their work interprets the decomposition mechanism in terms of several species of electronically excited singlet states. These studies agree with the previously discussed studies, if the molecular products formed via process 47 are assumed to be due to singlet decomposition only and if 4 isc = 1 - I CO The yield of intersystem 

At low excitation energies, the Si - A- Ti process appears to be almost totally dominant. Archer et al. (9,10) found that at 313 nm, 80% of the Si states depopulate by this process, which is in good agreement with Parmenter and Noyes (184) and with Calvert and Layne (45), who found a yield of 0.81 for this process. 

The fate of the singlet state is somewhat less clear. Parmenter and Noyes (184) and Archer et al. (9,10) have proposed two distinct dissociative processes which can account for molecular products (1) S- — internal conversion followed by unimolecular decomposition to CH4 + CO, and (2) a very fast process, probably a predissociation from Si to form the same products. The findings of Parmenter and Noyes appear to assign exactly the opposite relative importance to these two processes, as do the findings of Archer et al. 

On the basis of the work discussed above, the following Si depopulation mechanism is proposed  

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Another method, called photobleaching, works on robust soHds but may cause photodecomposition in many materials. The simplest solution to the fluorescence problem is excitation in the near infrared (750 nm—1.06 pm), where the energy of the incident photons is lower than the electronic transitions of most organic materials, so fluorescence caimot occur. The Raman signal can then be observed more easily. The elimination of fluorescence background more than compensates for the reduction in scattering efficiency in the near infrared. Only in the case of transition-metal compounds, which can fluoresce in the near infrared, is excitation in the midvisible likely to produce superior results in practical samples (17). 

Photochemical decomposition of riboflavin in neutral or acid solution gives lumichrome (3), 7,8-dimethyl all oxazine, which was synthesized and characterized by Karrer and his co-workers in 1934 (11). In alkaline solution, the irradiation product is lumiflavin (4), 7,8,10-trimethyhsoalloxazine its uv—vis absorption spectmm resembles that of riboflavin. It was prepared and characterized in 1933 (5). Another photodecomposition product of riboflavin is 7,8-dimethy1-10-foTmylmethy1isoa11oxazine (12). 

Photodecomposition of A -l,2,3-triazolines gives aziridines. In cyclohexane the cis derivative (304) gives the cis product (305), whereas photolysis in benzene in the presence of benzophenone as sensitizer gives the same ratio of cis- and trans-aziridines from both triazolines and is accounted for in terms of a triplet excited state (70AHC(ll)i). A -Tetrazo-lines are photolyzed to diaziridines. 

The photodecomposition of 2,1-benzisoxazolium salts gave iV-substituted phenones (Scheme 22). In one case the l-(adamantyl)-3-phenyl-2,l-benzisoxazolium cation (51) did not generate a substituted phenone with reductive ring substitution. Rather, adamantyl ring rupture occurred to produce (52) (Scheme 22) (78JOC123.3, 77JOC3929). 

The photodecomposition of benzotriazine A/ -oxides produced 3-substituted 2,1-benzisoxazoles (Scheme 183) (73JA2390). The photolysis of cinnoline 1-oxides produced 3-methyl-2,1-benzisoxazoles (Scheme 183) (74TL2643, 74T2645). 

Butan-3-one, 2-hydroxy-2-(indolyl)-photodecomposition, 4, 233 Butan-3-one, 2-hydroxy-2-(pyrrolyl)-photodecomposition, 4, 233 Butazolidines applications, 5, 782 Butazone, y-hydroxyphenyl-antiinflammatory agents, 5, 296 Butazone, phenyl-metabolism, 1, 239, 5, 301 synthesis, 5, 230 But-l-ene, 1-morpholino-polymers, 1, 291... 

Anotlrer consideration in the production of thin fllms by photochemical processes is that the fraction of the beam which is not used in photodecomposition will heat any substrate on which it is desired to form the fllm. The power of tire light source which can be used for photodecomposition in the gaseous phase only is therefore limited by the transmission of energy. Clearly this transmitted beam represents a constant source of energy which... 

Alkyl radicals, R, react very rapidly with O2 to form alkylperoxy radicals. H reacts to form the hydroperoxy radical HO2. Alkoxy radicals, RO, react with O2 to form HO2 and R CHO, where R contains one less carbon. This formation of an aldehyde from an alkoxy radical ultimately leads to the process of hydrocarbon chain shortening or clipping upon subsequent reaction of the aldehyde. This aldehyde can undergo photodecomposition forming R, H, and CO or, after OH attack, forming CH(0)00, the peroxyacyi radical. 

Because the laser beam is focused on the sample surface the laser power is dissipated in a very smaU area which may cause sample heating if the sample is absorbing and may cause break-down if the sample is susceptible to photodecomposition. This problem sometimes may be avoided simply by using the minimum laser power needed to observe the spectrum. If that fails, the sample can be mounted on a motor shaft and spun so that the power is dissipated over a larger area. Spinners must be adjusted carefully to avoid defocusing the laser or shifting the focal spot off the optic axis of the monochromator system. 

CIO2 dissolves exothermically in water and the dark-green solutions, containing up to 8g/l, decompose only very slowly in the dark. At low temperatures crystalline clathrate hydrates, C102.nH20, separate (n 6-10). Illumination of neutral aqueous solutions initiates rapid photodecomposition to a mixture of chloric and hydrochloric acids ... 

Studies in the photoinitiation of polymerization by transition metal chelates probably stem from the original observations of Bamford and Ferrar [33]. These workers have shown that Mn(III) tris-(acety]acetonate) (Mn(a-cac)3) and Mn (III) tris-(l,l,l-trifluoroacetyl acetonate) (Mn(facac)3) can photosensitize the free radical polymerization of MMA and styrene (in bulk and in solution) when irradiated with light of A = 365 at 25°C and also abstract hydrogen atom from hydrocarbon solvents in the absence of monomer. The initiation of polymerization is not dependant on the nature of the monomer and the rate of photodecomposition of Mn(acac)3 exceeds the rate of initiation and the initiation species is the acac radical. The mechanism shown in Scheme (14) is proposed according to the kinetics and spectral observations ... 

The quantum yield of the initiation process (<, ) is quite low 8 x 10, indicating the great stability of the chelate ring toward photolysis. However, the quantum yield of photodecomposition 4>d) under similar condition is 2 X 10, which is higher than It is clear, therefore, that not every molecule of Mn(acac)3 that is decomposed initiates polymerization apparently, ex-... 

Photoinitiation of polymerization of MMA and styrene by Mn(facac)3 was also investigated, and it was shown that the mechanism of photoinitiation is different [33] from that of Mn(acac)3 and is subject to the marked solvent effect, being less efficient in benzene than in ethyl acetate solutions. The mechanism shown in Schemes (15) and (16) illustrate the photodecomposition scheme of Mn(facac)3 in monomer-ethyl acetate and monomer-benzene solutions, respectively. (C = manganese chelate complex.)... 

Kaeriyama and Shimura [34] have reported the photoinitiation of polymerization of MMA and styrene by 12 metal acetylacetonate complex. These are Mn(acac)3, Mo02(acac)2, Al(acac)3, Cu(bzac)2, Mg(acac)2, Co(a-cac)2, Co(acac)3, Cr(acac)3, Zn(acac)2, Fe(acac)3, Ni(a-cac)2, and (Ti(acac)2) - TiCU. It was found that Mn(a-cac)3 and Co(acac)3 are the most efficient initiators. The intraredox reaction with production of acac radicals is proposed as a general route for the photodecomposition of these chelates. 

The polymerization of MMA photoinitiated by al-koxo-oxo-bis(8-quinolyloxo) vanadium (V) complex [VOQ2 OR] has also been studied [38,39]. The alkyloxo radical ( OR) formed from the photodecomposition of the chelate (A = 365) nm at 25°C) was found to be the initiating species ... 

More recently, self-assembling 3//-azepine monolayers on a gold surface have been obtained by the photodecomposition of bis ll-[(4-azidobenzoyl)oxy]undec-l-yl disulfide.284 Other than some alkyl and aryl 177-azepine-l-carboxylates, which possess fungicidal activity, particularly against Sclerotium rolfsii,104 the unsaturated azepine systems surveyed in this section do not have any notable biological activity. 

There has been intense study of the complexes of bi- and polydentate ammines since the mid-1970s, driven by interest in the catalytic photodecomposition of water using the excited states of Ru(bipy)g+ (n = 2,3) and related systems (Figure 1.18) [5, 7, 8, 71]. 

The photopolymerization of this monomer with a mercury arc89,9°) produces small yields of low molecular-weight products. In the presence of oxygen an induction period is noted and the polymers contain an appreciable amount of peroxide units in the chains9 ). The photolysis of 2-vinylfuran was briefly described by Hiraoka92 cyclopentadiene and CO were reported as products. It is not certain if free radicals are involved in this photodecomposition, but presumably they are. 

Dacarbazine is activated by photodecomposition (chemical breakdown caused by radiant energy) and by enzymatic N-demethylation. Formation of a methyl carbonium ion results in methylation of DNA and RNA and inhibition of nucleic acid and protein synthesis. Cells in all phases of the cell cycle are susceptible to dacarbazine. The drug is not appreciably protein bound, and it does not enter the central nervous system. 

Azo-compounds and peroxides undergo photodecomposition to radicals when irradiated with light of suitable wavelength. The mechanism appears similar to that of thermal decomposition to the extent that it involves cleavage of the same bonds. The photodecomposition of azo-compounds is discussed in Section 3.3.1.1.2 and peroxides in Sections 3.3.2.1.2 (diacyl peroxides) and 3.3.2.3.2 (peroxyesters). Specific photoinitiators are discussed in Section 3.3.4. It is also worth noting that certain monomers may undergo photochemistry and direct photoinitiation on irradiation of monomer is possible. 

Diacyl peroxides have continuous weak absorptions in the UV to ca 280 nm (e ca 50 M cm 1 at 234 nm),147 Although the overall chemistry in thermolysis and photolysis may appear similar, substantially higher yields of phenyl radical products are obtained when BPO is decomposed photochemically. It has been suggested that, during the photodecomposition of BPO, (3-scission may occur in... 

Pcroxycstcrs seldom find use as photoinitiators since photodecomposition requires light of 250-300 nm, a region where many monomers also absorb. This situation may be improved by the introduction of a suitable chromophore into the molecule or through the use of sensitizers.201 "02 The pero wester (50) is reported to have kimi, 366 nm and < i near unity.2,01... 

Phenacyl radicals are produced by photodecomposition of initiators containing the phenone moiety (Scheme 3.74). These initiators include benzoin derivatives and acylphosphine oxides (see 3.3.4.1.1). Acyl radicals can be formed by... 

Phosphinyl radicals (e.g. 103-107) arc generated by photodecomposition of acyl phosphinates or acyl phosphine oxides (see 3.3.4.LI)282,466 474,473 or by hydrogen abstraction from the appropriate phosphine oxide.467... 

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