NIR‐Sensitized Activated Formation of Lophyl Radicals by Heptamethine Cyanines Enables Dry Film Photoresists

Abstract

Abstract 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 ( HABI s) 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 S 1 to S 0 additionally influenced the endothermic reaction between L· and NPG . These systems worked in practical applications for dry film photoresists ( DFR s), reported here for the first time.