Novel benzophenone derivatives photoinitiators based on carbazole group for deep curing and fast 3D printing

With the development of photopolymerization in thick film curing, the design and synthesis of effective photoinitiators are becoming increasingly critical. A series of photoinitiators (4-(dimethylamino)phenyl)(9-ethyl-9H-carbazol-3-yl)methanone (PECM), (4-(dimethylamino)phenyl)(9-phenyl-9H-carbazol-3-yl)methanone (PPCM), (9-allyl-9H-carbazol-3-yl)(4-(dimethylamino)phenyl)methanone (PACM), and (4-(dimethylamino)phenyl)(9-(hex-5-en-1-yl)-9H-carbazol-3-yl)methanone (PHCM) based on carbazole group were synthesized for thick film curing. These photoinitiators exhibited the maximum absorption wavelength red-shifted to 353 nm compared to BP and had high molar absorption coefficients (over 27,000 L·mol⁻¹·cm⁻¹). Under the irradiation of LED, the three-component system composed of photoinitiator, diphenyliodonium hexafluorophosphate, and triethanolamine could initiate the rapid photopolymerization of polyethylene glycol diacrylate (PEGDA 400). Under low intensity LED@405 nm (5 mW/cm²) irradiation, PHCM achieved a final double bond conversion (DC) of 98 % within 20 s, significantly higher than BP (65 %). Under the irradiation of LED@365 nm (5 mW/cm²), PHCM reached 99 % DC within 8 s, while BP reached 97 % within 20 s. This performance was attributed to the fact that the molar absorption coefficients of PHCM at 365 nm (30010 L·mol⁻¹·cm⁻¹) and 405 nm (746 L·mol⁻¹·cm⁻¹) were much higher than those of BP (71 L·mol⁻¹·cm⁻¹, 46 L·mol⁻¹·cm⁻¹). Additionally, curing depth of PHCM reached 8.4 cm after 100 s irradiation under LED@405 nm. PHCM also demonstrated excellent cell compatibility and migration ratio as low as 1.8 %, which was 1/5 of that of BP (9.5 %). Furthermore, PHCM was suitable for 3D printers with 4 s/layer. In addition, the initiation mechanism of the photoinitiator was studied by photodegradation and ESR tests.