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Mechanism, photochemical

Dichromated Resists. The first compositions widely used as photoresists combine a photosensitive dichromate salt (usually ammonium dichromate) with a water-soluble polymer of biologic origin such as gelatin, egg albumin (proteins), or gum arabic (a starch). Later, synthetic polymers such as poly(vinyl alcohol) also were used (11,12). Irradiation with uv light (X in the range of 360—380 nm using, for example, a carbon arc lamp) leads to photoinitiated oxidation of the polymer and reduction of dichromate to Ct(III). The photoinduced chemistry renders exposed areas insoluble in aqueous developing solutions. The photochemical mechanism of dichromate sensitization of PVA (summarized in Fig. 3) has been studied in detail (13). [Pg.115]

Fig. 20. Proposed photochemical mechanisms for the generation of acid from sulfonium salt photolysis. Shown ate examples illustrating photon absorption by the onium salt (direct irradiation) as well as electron transfer sensitization, initiated by irradiation of an aromatic hydrocarbon. Fig. 20. Proposed photochemical mechanisms for the generation of acid from sulfonium salt photolysis. Shown ate examples illustrating photon absorption by the onium salt (direct irradiation) as well as electron transfer sensitization, initiated by irradiation of an aromatic hydrocarbon.
Fig. 21. Representative nonionic photoacid generators. A variety of photochemical mechanisms for acid production ate represented. In each case a sulfonic acid derivative is produced (25,56,58—60). (a) PAG that generates acid via 0-nitrobenzyl rearrangement (b) PAG that generates acid via electron transfer with phenohc matrix (c) PAG that is active at long wavelengths via electron-transfer sensitization (d) PAG that generates both carboxylic acid and... Fig. 21. Representative nonionic photoacid generators. A variety of photochemical mechanisms for acid production ate represented. In each case a sulfonic acid derivative is produced (25,56,58—60). (a) PAG that generates acid via 0-nitrobenzyl rearrangement (b) PAG that generates acid via electron transfer with phenohc matrix (c) PAG that is active at long wavelengths via electron-transfer sensitization (d) PAG that generates both carboxylic acid and...
In summary, the production of substituted silylenes and silyl radicals upon exhaustive irradiation at 254 nm of polysilane high polymers suggested that the polymer photochemistry resembled that previously reported for short chain acyclic and cyclic oligomers (39). More recent experiments, however, have suggested that the photochemical mechanism for the degradation of the high polymers is more complex than first envisioned (vide infra) (48). [Pg.119]

Rettig W, Rurack K, Sczepan M (2001) From cyanines to styryl bases - photophysical properties, photochemical mechanisms, and cation sensing abilities of charged and neutral polymethinic dyes. In Valeur B, Brochon JC (eds) New trends in fluorescence spectroscopy applications to chemical and life sciences. Springer, Berlin, pp 125-155... [Pg.98]

From Cyanines to Styryl Bases -Photophysical Properties, Photochemical Mechanisms, and Cation Sensing Abilities of Charged and Neutral Polymethinic Dyes, in Valeur B. and Brochon J. C. (Eds),... [Pg.349]

Fig. 4.4 Schematic representation of Type I, Type II, and Type III photochemical mechanisms thought to operate in PDT (See Color Plates)... Fig. 4.4 Schematic representation of Type I, Type II, and Type III photochemical mechanisms thought to operate in PDT (See Color Plates)...
Mroz P, Pawlak A, Satti M, Lee H, Wharton T, Gali H, Sarna T, Hamblin MR (2007a) Functionalized fullerenes mediate photodynamic killing of cancer cells Type I versus Type II photochemical mechanism. Free Radic Biol Med 43 711-9. [Pg.104]

Some speculation is inevitable in proposing any photochemical mechanism and some liberty has been taken in this regard in the present discussion. Areas of boron photochemistry are indicated where proposed mechanisms are controversial and where further experimental work is necessary. For example, very little is known about triplet states of boron molecules. Some of these questions may be answered in the future by non-photochemical techniques... [Pg.6]

Several types of photochemical mechanisms in borazene systems have been observed. In oxygen free CCI4 solution VIII is converted to... [Pg.24]

A third photochemical mechanism observed by Hancock and Kramer involves methyl migration,... [Pg.25]

In recent years luminescence nomenclature has become confusing within the literature and in practice. Luminescence involves both phosphorescence and fluorescence phenomena. While luminescence is the appropriate term when the specific photochemical mechanism is unknown, fluorescence is far more prevalent in practice. Moreover, the acronym LIE has historically inferred laser -induced fluorescence however, in recent years it has evolved to the more general term light -induced fluorescence due to the various light sources found within laboratory and real-time instruments. Within this chapter fluorescence and LIE are interchangeable terms. [Pg.338]

There are always products present that are formed through the combination of two RfS radicals. For example, the reaction of CSF2 with CF3SCI proceeds by the following photochemical mechanism (84) ... [Pg.165]

Photochemical Mechanisms, Highly Complex (Johnston and Cramarossa) Photochemical Oxidation of Aldehydes by Molecular Oxygen, Kinetics... [Pg.180]

Single Crystals, Photochemical Mechanism in FTIR Studies of Diacyl... [Pg.181]

Irradiation of ethyl or isopropyl pyruvate in the vapor phase produces a complex mixture of products which does not fit any simple photochemical mechanism.93 The initial step of the reaction appears to be formation of radicals by a carbon-carbon bond cleavage. [Pg.99]

Sulfur isotopes also show mass-independent effects that are probably produced by the same photochemical mechanism as oxygen effects in the Earth s upper atmosphere. Mass independent variations in sulfur from Martian meteorites have been interpreted to result from volcanic injections of SO2 and H2S into the Martian atmosphere followed by photolysis, which fractionates the sulfur isotopes. There is also evidence from ancient terrestrial sediments that the same photo lytic process was operating on sulfur in the Earth s atmosphere prior to 2.4 Ga, before oxygen began to accumulate in the atmosphere (see review by Thiemens, 2006). [Pg.224]

The beginning and end points of a photochemical reaction pathway are the structures of the starting materials (substrates) and the isolated products. Elucidation of product structures can be carried out by conventional methods. Structure determination for products derived from labelled substrates, such as those with isotopic labels or with extra substituents, or from substrates with distinctive stereochemical features, can result in the elimination of certain mechanistic possibilities and provide support for others. Two key questions for photochemical mechanisms, as for thermal mechanisms, are whether or not a reactive intermediate (such as a biradical) lies on the reaction pathway, and if so, what are the rate constants for reaction steps subsequent to its formation. Questions that are peculiar to photochemical mechanisms mav be expressed ... [Pg.31]

One of the most characteristic types of ground-state reaction for alkenes is electrophilic addition, often involving a proton acid as addend or catalyst. In the excited state similar reactions can occur, with water, alcohols or carboxylic acids as commonly encountered addends. However, there is a variety of photochemical mechanisms according to the conditions or substrate used. In a few instances it is proposed that the electronically excited state is attacked directly by a proton from aqueous acid, for example when styrenes are converted to l-arylethanols (2.47 the rate constant for such attack is estimated to be eleven to fourteen orders of magnitude greater than that for attack on the ground state, and the orientation of addition is that expected on the basis of relativecarbonium ion stabilities (Markowni-kov addition). [Pg.58]


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See also in sourсe #XX -- [ Pg.247 ]

See also in sourсe #XX -- [ Pg.30 ]

See also in sourсe #XX -- [ Pg.108 ]

See also in sourсe #XX -- [ Pg.30 ]




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