Photolysis thin film studies

The identity of methoprene photoproducts has been studied from aqueous emulsions, thin films on glass or silica gel, and in methanolic solution (Figures 3 and 4, 40). As a thin film (0.1 ym) on glass, the half-life of methoprene was about 6 hr. After 93% degradation of parent, more than 50 photoproducts were observed, only five of these present in 3% or higher yield 7-methoxycitronellic acid (4%), 7-methoxycitronellal (4%), the 4,5-epoxide of methoprene (6%), a C12 methyl ketone (3%), and 14C02 (6%). Similar products were encountered on photolysis of a 100 ppm aqueous emulsion of methoprene, except that methoxy-citronellal was isolated only as its dimethyl acetal (9% yield), a presumed artifact of work-up. In addition to the same products identified from thin film studies, at least forty-six other discrete products were detected, but not identified (40). 

Most photolysis studies involving triazine herbicides have been carried out in aqueous solutions of these compounds. These studies have also been carried out in greatest detail with respect to identification of photolysis products, delineation of photolysis mechanisms, and rates of photolysis. The photolysis of thin films of the triazine herbicides has been studied less frequently and in much less detail. There have been no reports of vapor-phase photolysis studies however, there have been two studies investigating the photolysis of a triazine herbicide sorbed to an aerosol. Only photolysis studies reported in 1970 or later have been included in the following discussion. Earlier photolysis studies of the triazine herbicides have been reviewed by Jordan et al. (1970). 

Nearly all thin film photolysis studies involving triazine herbicides have utilized model surfaces such as filter paper (Jordan et al., 1964 Morita et al., 1988), aluminum (Jordan et al., 1965), glass (Pape and Zabik, 1972 Chen et al., 1984 Hubbs and Lavy, 1990), and silica gel (Lotz et al., 1983). A shortcoming of the use of model surfaces is that herbicide dissipation due to volatility losses is often not accounted for (Hubbs and Lavy, 1990). Konstantinou et al. (2001) studied the sunlight photolysis of atrazine, propazine, and prometryn on soil (sandy clay loam, clay loam, and... 

Figure 2.9. The N-alkyldiazopiperidinedione structure is typical of the 1,3-diacyl-2-diazo compounds studied at IBM. Photolysis produces a carboxylic acid analogous to the chemistry of diazonaphthoquinones. These materials absorb strongly in the DUV region, but bleach completely as indicated in the spectra of a methanolic solution bottom). Resists formulated from these materials in novolac show residual unbleachable absorbance due to the resin as shown in the spectra of thin films (top).

Polymers are sometimes studied in solution. This has the advantage that the systems are free from kinetic and mechanistic complexities typical of solid state reactions. However, the conditions are far from natural. When solid polymers are to be studied, reliable results are most easily obtained by irradiating thin films cast from solutions. The use of thin films ensures homogeneous dissipation of energy into the sample, and the solvent casting technique does not require high temperatures which are sometimes responsible for partial oxidation or degradation of the polymer prior to the photolysis. 

I < 10" Io k = 3/2 for 10" /q rate depends on the magnitude of the photon flux. This makes interpretations of kinetic data based on intensity effects tenuous, since the light intensity within a sample is not constant but decays exponentially. The exceptions here involve thin films or photolysis studies at longer wavelengths having small absorption coefficients. 

Varma and Williams developed kinetic equations that agree in certain limiting cases with observed decomposition curves [121]. Their model was based on IR absorption studies on thin films and evidence that UV photolysis occurs in the region of absorption rather than in layers on the surface or homogeneously through the film. 

The photochemical reactions of the ketones (12) which are used as sunscreens in cosmetics has shown that degradation is considerable when they are irradiated as thin films. Norrish Type I reactions dominate affording benzoic acid derivatives." A study of the biradicals formed on flash photolysis of the ketone (13) has been reported. The reaction involves a Norrish Type I process and yields an acyl-ketyl biradical that transforms into an alkyl-ketyl biradical by decarbonyla-tion. 

Figure 8.4 shows the importance of the coordination mode around the metal ion for the electrochemical properties of the layer. In Fig. 8.4 the mediation of the Fe(II)/(III) oxidation is studied by using a rotating disk electrode. Initially a thin film of [Ru(bipy)2(PVP)5Cl] is used and with this coating the current potential curve I is obtained (see Fig. 8.4b). On photolysis of the coating and formation of the aquocomplex (according to Reaction 5) curve II is obtained. Rotating disk behavior very clearly shows that the redox potential of the modifying layer is of prime importance to the electrochemical properties of the modified electrode.

Carbonyl complexes in thin films of polystyrene and other polymers have been studied by Butler et. Polymer films arise again in a paper by Bronshtein et al dealing with the general issue of the photolysis of metal carbonyls in thin films. This technique has become a common and reliable method for exploring metal carbonyl photochemistry. 

Itaya, A., Yamada, T., and Masuhara, H., Laser photolysis study of photoinduced charge separation in poly(Ai-vinylcarbazole) thin films, Chem. Phys. Lett., 174, 145, 1990. 

Photolysis experiments carried out with thin films gave essentially same results as those with solution NMR study. IH NMR spectrum of a reaction mixture after irradiation of a resist film provided evidence supporting our proposed imaging mechanism that only mass loss firom the deprotection was acetaldehyde. 

A separate development of the TRMC techniques was their application to the study of dipolar and excitonic species formed on flash-photolysis of dilute solutions and, more recently, to charge transport and charge separation in thin (aligned) solid films. In the present review we restrict ourselves to results that we have obtained on pulse-irradiated materials, for which the method has become known as the pulse-radiolysis time-resolved microwave conductivity or PR-TBAIC technique. 

One important exception to this rule is vinylketone polymers in which the Norrish type II reaction is responsible for the decrease of the molecular weight of irradiated polymers (see section 4). In contrast to polymethylmethacrylate, polymethylacrylate becomes insoluble on irradiation with 253.7 nm in vacuo [82]. In air, no visible insoluble material is formed and an apparent quantum yield of chain scission of 1.3 x 10-2 has been determined by viscosity measurements [82]. However, a qualitative comparison of sedimentation patterns of initial and irradiated samples indicates that crosslinking also occurs in the presence of air even if gelation is retarded by oxygen. This makes the above-mentioned value meaningless. Photolysis of thin polymethylacrylate films at 253.7 nm in vacuo has also been studied by measuring the insoluble fraction as a function of dose as described in section 2. Quantum yields of 1.9 x 10-3 have been estimated for both the chain scission and the crosslinking processes [83]. 

All of the studies to date have consistently demonstrated that the photolysis rate of 2,3,7,8-TCDD is dependent on the chemical environment in which it resides. Organic solvents dramatically accelerate the photolysis rate. Crosby and coworkers (38) found that, although the half-life of 2,3,7,8-TCDD in methanol under fluorescent lamp irradiation was approximately 3 hours, irradiation on either a Norfolk sandy soil for 96 hours, or as a thin dry film on glass for 14 days showed no loss of 2,3,7,8-TCDD Wipf and coworkers (39) state that the photolysis rate of 2,3,7,8-TCDD on vegetation is Increased by a factor of 25 when olive oil or arachis oil is used as a hydrogen source. 

Further confirmation of the important effect of solid-phase transitions in polymer photochemistry was reported by Dan and Guillet (29). They studied the quantum yields of chain scission, c >s, as a function of temperature in thin solid films of vinyl ketone homo- and copolymers. For polymers where the Norrish type-II mechanism was possibici large increases in n were observed at and above the glass transition T. Figure 8 shows this effect in a styrene copolymer containing about 5% phenyl vinyl ketone (PVK). Below Tg, )s is about 0.07, but at Tg it rises to about 0.3, a value similar to that observed for photolysis in solution at 2S°C. A similar effect was observed with poly (methyl methacrylate-co-methylvinyl ketone) (PMMA-MVK) and PVK homopolymer. 

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