In this work, we employ phenanthroimidazole (PAI), a chromophore with high energy gap and superior light-absorbing ability as a building block to construct photoinitiators (PIs). Four PAI-based PIs, PAI-DBF, PAI-DBT, PAI-TPA and PAI-tCz, are designed and modulated by a frontier molecular orbital engineering by combining groups with different electron push-pull abilities to PAI moiety, leading to the highest occupied molecular orbital (HOMO) distribution localized on the PAI and arylamine, respectively. The new PIs can initiate free radical and cationic photopolymerization with diphenyl iodonium hexafluorophosphate under 365/405 nm LED irradiation. It is found that PAI-DBF and PAI-DBT show much higher photoinitiating performance because the PAI moiety is the main light absorber and electron-donating (HOMO-controlling) segment, leading to much higher photon utilization efficiency. In particular, the new PAI-based PIs are of large energy gaps without using a strong pull-push structure, which can be used to fabricate high-optical-quality products via photolithography. This work provides a guideline for constructing novel PIs for free radical and cationic photopolymerization.
