Effective Singlet Oxygen Sensitizers Based on the Phenazine Skeleton as Efficient Light Absorbers in Dye Photoinitiating Systems for Radical Polymerization of Acrylates

A series of dyes based on the phenazine skeleton were synthesized. They differed in the number of conjugated double bonds, the arrangement of aromatic rings (linear and/or angular system), as well as the number and position of nitrogen atoms in the molecule. These compounds were investigated as potential singlet oxygen sensitizers and visible light absorbers in dye photoinitiating systems for radical polymerization. The quantum yield of the singlet oxygen formation was determined by the comparative method based on the ¹H NMR spectra recorded for the tested dyes in the presence of 2,3-diphenyl-p-dioxene before and after irradiation. The quantum yield of the triplet state formation was estimated based on the transient absorption spectra recorded using the nanosecond flash photolysis technique. The effectiveness of the dye photoinitiating system was characterized by the initial rate of trimethylolpropane triacrylate (TMPTA) polymerization. In the investigated photoinitiating systems, the sensitizer was an electron acceptor, whereas the co-initiator was an electron donor. The effectiveness of TMPTA photoinitiated polymerization clearly depended on the arrangement of aromatic rings and the number of nitrogen atoms in the modified phenazine structure as well as the quantum yield of the triplet state formation of the photosensitizer in the visible light region.