New High‐Power LED Opens Photochemistry for Near Infrared Sensitized Radical and Cationic Photopolymerization

Cyanines derived from heptamethines were investigated in combination with iodonium salts regarding their capability to initiate radical polymerization of tripropylene glycol diacrylate and epoxides derived from bisphenol‐A‐diglycidylether. A new Near Infrared (NIR) LED prototype emitting at 805 nm with an exposure density of 1.2 W·cm‐2 facilitated initiation of both radical and cationic polymerization using sensitizers derived from cyanines. This new light‐emitting device has brought photochemistry of cyanines with the general structure 1 into a new perspective because a combination of photonic and thermal processes responsibly influences reaction pathways. Particular, cationic cyanines comprising a cyclopentene moiety and diphenylamino group in the center initiated cationic polymerization of epoxides as reported for the first time in this communication. Selective oxidation of this unit to a fulvene pattern explains why particular these derivatives may function as initiator for cationic polymerization because formation of nucleophilic photoproducts occurs with minor efficiency. On the other hand, replacement of the diphenylamino group by a barbital group at the meso‐position failed to initiate cationic polymerization of epoxides.

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