NIR‐Sensitized Activated Formation 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 multifunctional acrylates when different hexa-arylbisimidazoles (HABIs) and N-phenylglycine (NPG) operated as 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 efficiently 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 endothermic reaction between L· and NPG. These systems worked in practical applications for dry film photoresists (DFRs), reported here for the first time.