Long-wavelength absorbing photoinitiators based phenothiazine acyldiphenylphosphine oxides for free radical photopolymerization

With the development of thick film curing in the field of photocuring, the design and preparation of visible light photoinitiators (PIs) have become particularly important. Based on the phenothiazine group, we designed and synthesized four new visible light photoinitiators (diphenylphosphoryl)(10-methyl-10H-phenothiazin-2-yl) methanone (QTPO-1), (diphenylphosphoryl)(10-ethyl-10H-phenothiazin-2-yl)methanone (QTPO-2), (diphenylphosphoryl)(10-phenyl-10H-phenothiazin-2-yl)methanone (QTPO-3), and (10-benzoyl-3,7-bis(dimethylamino)10H-phenothiazin-2-yl)(diphenylphosphoryl) methanone (QTPO-4). QTPOs are well soluble in solvents and monomers. Among them, the absorption peak of QTPO-4 red-shifted to 667 nm with the molar absorption coefficient is 15,530 M-1 cm -1, indicating that it can be used for Near -IR light polymerization. The double bond conversion (DC) rates of QTPO-1, QTPO-2, and QTPO-3 in Poly (ethylene glycol) diacrylate (PEGDA) under the irradiation of Light-emitting diode (LED)@365 nm were high up to 97 %, 90 %, and 93 %, and after addition of N-phenylglycine (NPG), the DC increased to 99 %, 93 %, and 98 % respectively, while the DC rate of 2,4,6-Trimethyl benzoyldiphenyl phosphine oxide (TPO) is 94 %. The DC rate of QTPO-4/NPG in (PEGDA) reaches 84 % under the irradiation of LED@660 nm. Moreover, the curing depth of QTPO-4/NPG/TMPTA under LED@660 nm was 1.66 cm which was almost 1.9 times that of TPO/TMPTA under LED@405 nm (0.85 cm), and the migration ratio of QTPO-4 low to 0.22 %. Meanwhile, the polymerization mechanism of QTPOs was studied and verified through steady -state photolysis and electron spin resonance spin-trapping experiment (ESR-ST). Additionally, the exhibit superior biocompatibility performance and thermal stability of QTPOs are illustrated by cytotoxicity assays and thermal stability tests.