Synthesis and Properties of Visible Radical Photoinitiators Based on Semi‐Indigo

Abstract Nowadays, with the development of light‐emitting diode (LED), developing photoinitiators compatible with LED has emerged as a critical area of research. The maximum absorption peak of existing photoinitiators is between 200–400 nm, which is not compatible with LED light sources. In this study, four LED hydrogen‐abstraction free radical photoinitiators (PIs) were synthesized by introducing aldehyde groups directly into semi‐indigo by a one‐step method at room temperature. Compared to the absorption peak of benzophenone (BP) at 257 nm, the maximum absorption wavelengths of PI‐1, PI‐2, PI‐3, and PI‐4 exhibited a redshift to 507 nm, 523 nm, 496 nm, and 494 nm, respectively. These PIs demonstrated a significant redshift, with their absorption extending beyond 600 nm. Meanwhile, under the irradiation of LED@ 520 nm (5 mW/cm 2 ), the double bond conversion rate of PI‐3 and PI‐4 exceeds 98%. On the other hand, the migration stability of the curing system was evaluated employing an immersion extraction technique. The mobility values recorded for PI‐1, PI‐2, PI‐3, and PI‐4 were 0.74%, 0.71%, 1.30%, and 0.76%, respectively, which were substantially lower in comparison to that of BP at 22.5%. Moreover, PIs have excellent cell compatibility and can be used in 3D printing. These findings highlight that PI1–PI4 possesses long‐wavelength absorption, low migration, and is synthesized via a straightforward process.