Near-Infrared Photoinitiators: Synthesis, Photophysics, and Study on the Photocuring Properties

Near-infrared (NIR) light has lower energy consumption, reduced radiation, and deeper penetration, making it more promising for application in polymerization. In this work, we designed and synthesized four NIR photoinitiators. Cy-BP-1 and Cy-DMB-1 exhibit maximum absorption wavelengths at approximately 783 nm, while Cy-BP-2 and Cy-DMB-2 absorb maximally at approximately 823 nm. All photoinitiators exhibit high molar extinction coefficients (ε > 3 × 105 L mol–1 cm–1), facilitating efficient NIR light absorption and excited-state transition. Photopolymerization experiments demonstrated the efficacy of cyanine photoinitiators as efficient single- and multicomponent photoinitiation systems for acrylate monomer polymerization. Upon excitation, cyanines undergo self-cleavage to generate free radicals and participate in electron transfer reactions with a co-initiator (Iod). When the doping concentration of the cyanine photoinitiator was 0.1 wt %, the resin with a thickness of 2.0 cm could be cured after 36 min of irradiation. Notably, even at a low concentration (0.01 wt %), Cy-BP-1, when used as a single-component photoinitiator, exhibited the fastest initiation rate and highest monomer conversion (FC = 72.2%), resulting in a solid sample with optimal color and transparency after polymerization. Consequently, Cy-BP-1 was successfully used to prepare resin lenses, which exhibited a high polymerization degree (FC = 86.6%) and excellent optical properties, including high transmittance in the visible region (>90%) and a high refractive index of 1.53 at 598 nm. The successful design and synthesis of the cyanine photoinitiator not only fills the gap in the market for NIR photoinitiators but also broadens the application field of NIR light, providing a convenient and rapid method for preparing resin lenses.