NIR-Sensitized Activation of Lophyl Radicals by Heptamethine Cyanines Enables Dry Film Photoresists.

The near-infrared (NIR) sensitized generation of lophyl radicals (L∙) by heptamethine cyanine led to initiation of radical photopolymerization of multi-functional acrylates when different hexaarylbisimidazoles (HABIs) and N-phenylglycine (NPG) operated coinitiator. The latter functioned as donor. For a deeper understanding, heptamethine cyanines were used following a photoinduced electron transfer (PET). HABI derivatives with electron-donating and -withdrawing substituents demonstrated that those with electron acceptors resulted in a higher photopolymerization efficiency of multifunctional acrylates. Tri-(propylene glycol) diacrylate (TPGDA) and tri-methylolpropane triacrylate (TMPTA) served as the monomers. Sensitizers (Sens) exposed with a high intense NIR-light source at 808 nm exhibiting a positive charge in the cyanine pattern significantly operate more efficient for radical photopolymerization than a Sens without a positive charge. Differences in efficiency of PET can give an explanation for these differences. The heat generated by the cyanine's internal conversion from S1 to S0 additionally influenced the slightly endothermic reaction between L∙ and NPG. These systems worked in practical applications for dry film photoresists (DFRs), reported here for the first time.