Photochemical aspects of photoinitiation using NIR lasers

Triazines (VI) are alternative electron acceptor materials [HAU 00a] with comparable electron affinity as compared to iodonium salts (IV). Electron transfer reduces VI while the anion radical formed fast decomposes into the highly reactive halogenated radical derived from VI that efficiently initiates radical polymerization. Structure VI possesses the possibility to initiate pol5Tneiization in two positions, which makes them attractive as bivalent radical initiators. 

The mechanism shown in Figme 7.2 requires adding of a donor D to avoid electron back-transfer from A to Sens d, which would be applicable in case of positively charged cyanine dyes. Tetraarylborates VII [YU 08], (aniline)acetic acids VIII [PHO 14] or heterocyclic sulfur compounds IX [WIT 04] represent some representative materials with electron donating properties that can be added as donor. 

Structure IX exists in a tautomeric equilibrium with the mercapto IX(SH) and thione IX(=S) structure as shown in equation [7.4]. NMR experiments demonstrated a major contribution of the thione (IX) in this equilibrium in model coatings [BER 14] proving that particularly the thione responsibly contributes to initiator radieal formation. Nevertheless, the thiol tautomer IX(SH) possesses some stabihzing functions. 

Dye-borate systems were disclosed to initiate radical polymerization [P C 08, PAC 01, KAB 98] but the sensitivity significantly increases by addition of iodonium borates to a dye-borate system [SIM 09]. This combination avoids ion exchange in systems comprising a cationic polymethine dye and an iodonium ion because both comprise the same counter ion. Ion exchange can lead to undesired events (i.e. crystallization) affecting sensitivity of the lithographic material. Further studies on dye-borate systems discuss the efficiency of electron transfer from the borate to the acceptor excited [SCH 90]. Moreover, oxidation of the (anilino)acetic acid results in fast generation of the aminyl radical and the release of CO2 as shown in equation [7.5]. 

Moreover, the thione should also play a major role in the oxidation mechanism of the heterocycle resulting in generation of thiyl radicals as shown in equation [7.6]. 

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