Dye Photoinitiators-mechanisms

Acid-catalyzed photoresist films acid diffusion, 35 acid generation, 303233/341 advantages, 28 catalytic chain length, 3435r development of classes of cationic photoinitiators, 28 experimental procedure, 35-36 generation mechanism from irradiation of triphenylsulfonium salts, 28-29 merocyanine dye method for acid analysis, 30,31/33/... 

Scheme 9 presents a general mechanism consistent with the photopolymerization results we observe with our photoinitiators and electron donors. Table 14 shows relative values of termination rate constants for the radicals generated by cleavage of semireduced dye. Most of the radicals are less effective terminators than acetyl with the exception of... 

Sulfinates. Arylsulfinate anions were shown to be effective activators for photoreducible dye polymerization systems by Hiller, Margerum, and Rust (41). The work is described with applications in a number of patent publications (20,41a,42). These workers also studied the mechanism of the photoinitiation process (43). 

Sensitized photooxidation of sulfur-containing amino acids in neutral aqueous solution occurs via electron transfer from the sulfur to the triplet state of the sensitizer (the mechanism can be adapted to other dye-sensitized photoinitiated polymerization). Electron transfer quenching in the system is followed mainly by diffusion of the sulfur-centered radical cation and benzophenone radical anion. Laser... 

Illustrated in the following reaction sequence is a description of the mechanism of dye-sensitized photoinitiation. Here, Q stands for the dye (a quinonoid structure) and HD represents the hydrogen or electron donor ... 

J. V. Crivello The amino groups present in the photosensitizers do not substantially inhibit polymerization for two reasons. Firsts their concentration is very low, approximately 0.1 mol photosensitizer per mol photoinitiator is necessary in such systems. Second, the mechanism of photosensitization as discussed in my talk involves electron transfer in which the photosensitizer cation-radical is the initiator of cationic polymerization. The aminium cation-radicals derived from the dyes containing dimethylamino groups are, thus initiators and not inhibitors. 

Dyes comprise a large fraction of visible light photoinitiators because their excited electronic states are more easily attained. Co-initiators, such as tertiary amines, iodonium salts, triazines, or hexaarylbisimidazoles, are required since dye photochemistry entails either a photoreduction or photo-oxidation mechanism. Numerous dye families are available for selection of an appropriate visible initiation wavelength an example of a thiazine dye (with an absorption peak aroimd 675 nm) is methylene blue (11). 

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